Removal of hexavalent chromium from aqueous systems using jujube tree branch biochar-loaded nano zero-valent iron
-
,
Abstract
This study developed a composite material (JBC-nZVI) using jujube tree branch biochar (JBC) to support nano zero-valent iron (nZVI) for hexavalent chromium (Cr(VI)) removal from aqueous solutions. The porous structure of JBC improved nZVI dispersion, reducing aggregation and enhancing reactivity and stability. The composite was characterized using SEM-EDS, BET, FT-IR, and particle size distribution analysis. Under optimized conditions (50 mg/L Cr(VI), pH 3, 1 g/L dosage), JBC-nZVI achieved 88.7 % Cr(VI) removal in 120 min. Kinetic studies showed the removal followed a pseudo-second-order model, with chemical adsorption as the main process. JBC-nZVI facilitated electron transfer between Cr(VI) and nZVI, promoting the reduction process. XRD and XPS analyses revealed that Cr(VI) removal involved adsorption, reduction, and coprecipitation. This composite not only provides an efficient method for Cr(VI) remediation but also valorizes jujube tree branch biomass waste, enhancing nZVI stability and offering a novel approach for environmental cleanup.
1 Introduction
Chromium (Cr), a prevalent heavy metal contaminant, is introduced into the environment primarily through industrial effluents from sectors such as electroplating, dye manufacturing, papermaking, leather tanning, and paint synthesis [1]. Chromium contamination in aquatic and terrestrial environments has become a global concern due to its persistence, bioaccumulation, and severe toxicity to both ecosystems and human health [2]. Of the two predominant oxidation states, hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)), Cr(VI) is more toxic than hexavalent chromium because of its enhanced solubility, mobility, and inherent toxicity. Numerous studies have highlighted the carcinogenic, mutagenic, and teratogenic effects of Cr(VI), necessitating stringent regulatory limits worldwide [3], 4]. The carcinogenic potential of Cr(VI), particularly its links to lung and nasopharyngeal cancer [5], underscores the need for effective remediation strategies. According to the World Health Organization (WHO, 2011), the permissible limit for Cr(VI) in drinking water is 0.05 mg/L. Consequently, significant research efforts are directed toward developing innovative and efficient technologies for Cr(VI) removal [6], 7].
Nano zero-valent iron (nZVI) has garnered significant attention because of its strong reductive capabilities and potential application in the remediation of Cr(VI)-contaminated water [8]. However, the practical use of nZVI is hindered by its tendency to rapidly oxidize and aggregate during the reactions, which significantly reduces its reactivity. To mitigate these limitations, various modification strategies have been investigated, including the incorporation of inactive metals, surfactants, inorganic minerals, and organic materials [9], [10], [11], [12]. However, these approaches may introduce secondary environmental pollutants, raising concerns about their long-term sustainability and safety.
To address the limitations of nZVI, the use of biochar as a support material has been proposed to mitigate aggregation and enhance removal efficiency [13]. Biochar, a byproduct of biomass pyrolysis under limited oxygen conditions, exhibits a large specific surface area and a porous structure conducive to nanomaterial loading. The valorization of agricultural residues into functional biochar not only addresses waste management issues but also provides a sustainable material for environmental remediation [14]. Furthermore, its abundant surface functional groups enhance electron transfer during redox reactions, making it highly promising for wastewater treatment applications. Numerous studies have demonstrated that biochar derived from various biomass sources, including sugarcane bagasse [15], eggplant stems [15], citrus peels [15], and straw [16], can effectively support nZVI for the removal of heavy metal contaminants from aqueous solutions.
In comparison to other adsorbents reported in the literature, the Cr(VI) adsorption capacity of JBC-nZVI demonstrates competitive performance. For example, biochar derived from sugarcane bagasse has a maximum adsorption capacity of 29.08 mg/g [17], while biochar loaded with nZVI using rice straw as a support exhibits an adsorption capacity of approximately 40 mg/g [18]. In comparison, our JBC-nZVI material achieved an adsorption capacity of 43.5 mg/g under optimal conditions, which is comparable to or better than many of these materials. This indicates that JBC-nZVI not only offers a sustainable and environmentally friendly option but also demonstrates effective removal efficiency for hexavalent chromium.
Jujube twigs (JTB) are a common agricultural waste product. Current disposal practices, primarily involving open pile storage or incineration, are not only detrimental to the resource valorization of JTB but also pose a risk of secondary pollution. However, the high cellulose content of discarded JTB makes it a viable feedstock for biochar production via pyrolysis [19]. Converting discarded jujube twigs into environmentally functional materials, such as jujube twig-derived biochar (JBC), and using them as a support for nZVI offers a sustainable and environmentally friendly remediation approach [20]. This strategy not only mitigates the disposal burden of jujube twigs but also enhances pollutant removal from aqueous environments, effectively reducing heavy metal concentrations. Consequently, the valorization of discarded jujube tree branches into biochar has significant environmental implications. Furthermore, the integration of JBC with nZVI (JBC-nZVI) presents a promising synergistic approach, imparting robust reductive capacity to the biochar while simultaneously providing structural stability to nZVI [21].
Notably, jujube tree branches (JTB) have a distinct compositional advantage as JBC raw material: their elemental analysis (Table 1) shows a high cellulose content of 47.21 wt% and low ash content of 5.1 wt%, which differs significantly from other common biomass. During pyrolysis (600 °C) and KOH activation (800 °C), the high cellulose content of JTB promotes the formation of a hierarchical porous structure in JBC, and the low ash content avoids pore blockage by inorganic impurities. This structural feature provides sufficient ‘nanoparticle nests’ for subsequent nZVI loading, which is a key prerequisite for the uniform dispersion of nZVI in the composite [22].
Element composition of jujube twigs and BC.
| Element (wt%) | C | H | O | N | Other |
|---|---|---|---|---|---|
| Jujube twiga | 47.21 | 5.91 | 41.36 | 0.42 | 5.1 |
| BC | 82.45 | 1.37 | 7.84 | 0.68 | 7.66 |
-
aThe elemental composition parameters of the jujube branches used in this table have been reported in our previous work [23].
The removal of Cr(VI) is a complex process involving multiple mechanisms. Due to the differing reaction kinetics of individual processes, such as adsorption and reduction, and the varying contributions of composite components like JBC and nZVI, the predominant removal mechanism is expected to change under different conditions [24]. However, detailed investigations into the Cr(VI) removal process using JBC-nZVI have been limited. Most previous studies have emphasized the role of nZVI in Cr(VI) remediation, while relatively few explore the specific contribution of biochar. In particular, the potential of biochar as an electron mediator in enhancing Cr(VI) reduction has not been thoroughly examined.
Although some studies have focused on modification strategies for nZVI, there is still a lack of systematic research on biochar derived from agricultural waste (such as jujube twigs) as a supporting material, particularly regarding its environmental sustainability and resource recycling potential. Despite the progress, few studies have systematically investigated the use of jujube twig-derived biochar as a support for nZVI, particularly in terms of its environmental sustainability and resource recycling potential. Therefore, this study comprehensively investigated the Cr(VI) removal performance and mechanisms of JBC-supported nZVI (JBC-nZVI) in aqueous solutions. The specific objectives are: (1) to evaluate the Cr(VI) removal efficiency of JBC-nZVI under varying reaction conditions, including initial pH, dosage, temperature, reaction time, and the presence of coexisting ions; (2) to explore the role of JBC in the Cr(VI) removal process; (3) to characterize the transformation of nZVI and Cr species on the biochar surface and propose a detailed mechanistic model for Cr(VI) removal by JBC-nZVI; and (4) to assess the regeneration and reusability potential of JBC-nZVI.
2 Experiment
2.1 Materials and chemicals
Potassium dichromate (K2Cr2O7, >99 %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used as the source of Cr(VI). The JBC-nZVI composite was synthesized using ferrous sulfate heptahydrate (FeSO4·7H2O, >99 %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) as the iron source and sodium borohydride (NaBH4, >98 %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) as the reducing agent. Coexisting ions were introduced using analytical-grade calcium chloride (CaCl2, >96 %, Tianjin Hongyan Reagent Factory, Hedong District, Tianjin, China), magnesium chloride (MgCl2, >99 %, Tianjin Shengao Chemical Reagent Co., Ltd., Tianjin, China), zinc chloride (ZnCl2, >98 %, Tianjin Hongyan Reagent Factory, Hedong District, Tianjin, China), sodium chloride (NaCl, >99 %, Tianjin Hongyan Reagent Factory, Hedong District, Tianjin, China), sodium carbonate (Na2CO3, >99.8 %, Tianjin Hongyan Reagent Factory, Hedong District, Tianjin, China), and sodium nitrate (NaNO3, >99 %, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China).
2.2 Preparation of nZVI, JBC and JBC-nZVI
2.2.1 Preparation of JBC
The discarded jujube tree branches used as biochar feedstock were sourced from Jiaxian County, Yulin City, Shaanxi Province, China. The branches were thoroughly washed, dried at 105 °C for 24 h, and sieved through a 120 mesh screen. Biochar (BC) was synthesized using a high-temperature carbonization method. Specifically, the dried branches were pyrolyzed at 600 °C under a nitrogen atmosphere in a tube furnace with a heating rate of 5 °C/min for 2 h. To enhance the porosity and surface activity, the resulting BC was chemically activated by mixing with KOH at a 1:2 mass ratio and ground to homogeneity. The mixture was subjected to a second thermal treatment at 800 °C for 2 h in a tube furnace. The resulting pyrolyzed material was washed with 1 M hydrochloric acid (HCl), to remove residual inorganic components, filtered until a neutral pH was achieved, and subsequently dried at 105 °C to obtain jujube branch biochar (JBC). The elemental compositions of the jujube branches and BC were determined using an elemental analyzer. The results are presented in Table 1.
2.2.2 Preparation of biochar-supported nano zero-valent iron (JBC-nZVI)
JBC-nZVI was synthesized using a liquid-phase reduction method. A predetermined mass of JBC was introduced into a completely dissolved solution of FeSO4·7H2O under continuous stirring for 30 min while maintaining a biochar-to-iron mass ratio of 1:2. The mixture was then purged with nitrogen gas (N2) for 15 min to eliminate dissolved oxygen (O2) and to maintain an inert atmosphere. Subsequently, 100 mL of freshly prepared 1 M NaBH4 solution was gradually introduced into the suspension while stirring vigorously for 2 h, ensuring a complete reduction of Fe2+ to nZVI. This step was conducted under nitrogen to prevent oxidation. The resulting suspension was then centrifuged at 2,000 rpm and washed three times with distilled water to remove any residues [25], 26]. Finally, the JBC-nZVI composite was freeze-dried for 8 h and stored in an airtight container to prevent exposure to air. The synthesis parameters of JBC-nZVI, including the pyrolysis temperature (600 °C), activation temperature (800 °C), pyrolysis duration (2 h), and JBC-to-Fe mass ratio (1:2), were optimized based on prior foundational research.
2.2.3 Preparation of nZVI
For comparison, nZVI was synthesized following the same liquid-phase reduction procedure described in Section 2.2.2, except that JBC was not included in the reaction system.
A schematic representation of the JBC-nZVI preparation process is illustrated in Figure 1.
Experimental flow chart.
2.3 Adsorption experiment
A 1,000 mg/L Cr(VI) stock solution was prepared by dissolving potassium dichromate (K2Cr2O7) in distilled water. Before use, the stock solution was diluted to the desired concentrations using deionized water, and the initial pH of the solution was adjusted using 0.1 M sodium hydroxide (NaOH) or 0.1 M nitric acid (HNO3). Batch adsorption experiments were conducted in 250 mL Erlenmeyer flasks, in which a predetermined mass of the adsorbent was added to 100 mL of the Cr(VI) solution. The flasks were then placed on a dual-function constant-temperature water bath shaker at 150 rpm to ensure uniform mixing and contact between the adsorbent and solution. At predetermined intervals, aliquots of the solution were withdrawn, and the supernatant was filtered through a 0.45 μm membrane filter for subsequent Cr(VI) concentration analysis. The Cr(VI) concentration was measured, and all experiments were performed in triplicate to ensure the reliability of the results.
The influence of five key factors on the Cr(VI) removal rate and adsorption capacity was investigated under varying experimental conditions: (1) initial pH (3–11); (2) material dosage (0.25–2.5 g/L); (3) initial Cr(VI) concentration (10–125 mg/L); (4) reaction time (5–150 min); and (5) temperature (15–45 °C). Additionally, batch experiments were conducted to assess the effect of coexisting ions, including chloride (Cl−), carbonate (CO3 2−), nitrate (NO3 −), calcium (Ca2+), magnesium (Mg2+), and zinc (Zn2+), on the Cr(VI) removal efficiency.
The Cr(VI) removal efficiency (adsorption rate, %) and adsorption capacity (mg/g) were calculated using eqs. (1) and (2):
where:
R is the Cr(VI) removal efficiency (%)
C 0 is the initial Cr(VI) concentration (mg/L)
C t is the Cr(VI) concentration at time t (mg/L).
where:
q e is the adsorption capacity at equilibrium (mg/g)
Ce is the equilibrium concentration of Cr(VI) (mg/L)
V is the solution volume (L)
m is the mass of the adsorbent (g).
In the kinetic experiment, a Cr(VI) solution with a volume of 250 mL and concentrations ranging from 10 to 200 mg/L was prepared. A fixed mass of 0.5 g of the JBC-nZVI composite material (equivalent to a dosage of 2 g/L) was added to the solution. The resulting mixture was stirred for a duration ranging from 0 to 2 h. After the reaction, the mixture was subjected to filtration using a 0.22 μm syringe filter to ensure the complete removal of any residual particulate matter. The Cr(VI) concentration was determined using the 1,5-diphenylcarbazide method, and the absorbance was analyzed using a UV spectrophotometer of 540 nm wavelength.
To elucidate the adsorption kinetics, the experimental data were fitted to the pseudo-first-order (PFO) kinetic model (eq. (3)), the pseudo-second-order (PSO) (eq. (4)) kinetic model, and the intra-particle diffusion model (eq. (5)). These models were used to determine the adsorption rate and the controlling mechanism of the process.
In these eqs.:
q t (mg/g) represents the adsorption capacity at time t.
q e (mg/g) is the adsorption capacity at equilibrium.
t (min) denotes the adsorption time.
k 1 (min−1) is the pseudo-first-order adsorption rate constant.
k 2 (g·mg−1·min−1) is the pseudo-second-order adsorption rate constant.
k d (mg·g−1·min0.5) is the intra-particle diffusion rate constant.
t 0.5 is the square root of adsorption time.
C (mg/g) represents the boundary layer thickness.
If the linear plot of q t versus t 0.5 passes through the origin (C = 0), it indicates that intra-particle diffusion is the sole rate-limiting step of the adsorption process. Conversely, if the plot does not pass through the origin (C ≠ 0), it indicates that additional mechanisms, beyond intra-particle diffusion, influence the adsorption process.
Thermodynamics is a crucial model for studying the behavior of adsorbents. By employing thermodynamic models, the removal of Cr(VI) by the material was analyzed to elucidate the adsorption process and driving forces, as well as to deeply investigate the factors influencing adsorption efficiency. In this field, parameters such as the Gibbs free energy change (ΔG 0), enthalpy change (ΔH 0), and entropy change (ΔS 0) are commonly used to assess the spontaneity of adsorption, thermal effects, and the disorder of the system. Based on this, these thermodynamic parameters can be calculated using the following eqs.:
In these eqs.:
ΔG 0 (KJ/mol) represents the change in Gibbs free energy.
ΔH 0 (KJ/mol) represents the enthalpy change.
ΔS 0 (KJ/mol) represents the entropy change.
R (8.314 J/(mol·K)) is the ideal gas constant.
T (K) represents the thermodynamic temperature.
K 0 denotes the adsorption equilibrium constant.
q e (mg/g) is the theoretical equilibrium removal amount.
C e (mg/L) is the equilibrium concentration of Cr(VI).
2.4 Regeneration and reusability experiments
To assess the regeneration and reusability of the JBC-nZVI composite, a series of adsorption-desorption cycles were conducted. Following the initial Cr(VI) adsorption, the spent JBC-nZVI composite was separated from the solution via filtration and subsequently regenerated using a 0.1 M NaOH solution. Specifically, the Cr(VI)-laden JBC-nZVI composite was mixed with the 0.1 M NaOH solution and agitated for 24 h to facilitate Cr(VI) desorption. This procedure was replicated over five cycles to determine the composite’s regenerative capacity and sustained performance.
2.5 Characterization analysis
The elemental compositions of the samples, including carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), were determined using a German Elementar UNICUBE elemental analyzer. To examine the crystal structure, X-ray diffraction (XRD) analysis was performed using a German Bruker Advance D8 X-ray diffractometer equipped with a Cu target Kα radiation source and a LynxEye detector. Additionally, the structural properties and functional groups of the composite material were characterized using a German Bruker TENSOR 27 Fourier transform infrared spectrometer (FTIR). The surface morphology and elemental composition of the materials were examined using a German ZEISS Sigma 300 field emission scanning electron microscope (SEM) equipped with an Oxford X-Max extreme SDD silicon drift detector for energy dispersive X-ray spectroscopy (EDS). To further investigate the surface functional groups and elemental composition of the composite material before and after the reaction, X-ray photoelectron spectroscopy (XPS) was conducted using a thermo electron thermo escalab 250Xi spectrometer. The specific surface area was determined by N 2 adsorption/desorption using a micromeritics ASAP 2460 physical adsorption analyzer (BET). Additionally, the zeta potential of the sample under varying pH conditions (3.0–11.0) was measured using a UK Zetasizer nano ZS90 100 zeta potential analyzer.
3 Results and discussion
3.1 Characterization
The XRD analysis provided insight into the crystal structure of the samples. As shown in Figure 2, the XRD pattern of JBC exhibits two weak diffraction peaks at approximately 26° and 43°, corresponding to the (002) and (100) planes of graphitic microcrystals in carbon-based materials [27]. The XRD pattern of JBC-nZVI shows a distinct peak at a diffraction angle of 44.6°, confirming the successful deposition of Fe0 nanoparticles onto the JBC surface. After Cr(VI) removal, a significant decrease in the intensity of the Fe0 diffraction peak was noted, likely attributed to partial oxidation of Fe0. Additionally, diffraction peaks corresponding to iron oxides, specifically Fe2O3 and Fe3O4, were observed, indicating the formation of these oxides on the material’s surface.
XRD patterns of JBC, JBC-nZVI, and JBC-nZVI-Cr.
SEM analysis revealed the surface morphology of the synthesized materials. As shown in Figure 3(a), nZVI exhibited a spherical morphology but formed larger chain-like aggregates due to van der Waals forces and surface magnetic interactions. Image analysis using the nano measurer software determined the average particle size of nZVI to be 117.77 nm (Figure 3(b)). Figure 3(c) presents the SEM image of the JBC-nZVI composite material, demonstrating a more uniform dispersion of nZVI particles. The average particle size of nZVI within the composite was 51.9 nm, as shown in Figure 3(d), indicating a significant reduction in particle size and improved dispersion compared to the pure nZVI aggregates. The spherical nZVI particles were uniformly distributed on the JBC surface with minimal aggregation, thereby increasing the available contact area between nZVI and Cr(VI) ions. This enhanced interaction contributes to the higher removal efficiency of the composite material. Furthermore, the JBC matrix provides a sufficient number of active surface sites, which facilitates the even dispersion of nZVI and contributes to its reduced particle size [28]. Additionally, as shown in Figure 3(e), elemental mapping analysis revealed that the C was homogeneously distributed across the carbon framework. The Fe element, excluding the O-containing regions was also evenly dispersed within the channels and pores of JBC, confirming the uniform loading of nZVI onto the JBC structure [23]. A minor amount of Na was detected, which likely originated from residual NaBH4 used during the synthesis.
Morphology and particle size distribution of nZVI and JBC-nZVI composite. (a, b) SEM and particle size distribution of nZVI; (c, d) SEM and particle size distribution of JBC-nZVI; (e) EDS for JBC-nZVI.
The Brunauer–Emmett–Teller (BET) analysis presented in Table 2 revealed that the specific surface area of JBC-nZVI was 512.23 m2/g, which is significantly lower than that of JBC (1,987.57 m2/g). This reduction in the specific surface area is attributed to the deposition of nZVI particles on the JBC surface through the reduction process, where the particles either fill or attach to the pores of JBC, leading to a substantial decrease in the specific surface area [29]. Furthermore, partial oxidation of nZVI during storage or use may result in the formation of iron oxides, such as Fe2O3 and Fe3O4. These oxides can also attach to the surface of JBC pores or within their channels, resulting in pore narrowing. These findings indicate that the porous structure of JBC effectively mitigates nZVI aggregation and promotes its uniform dispersion. Additionally, the average pore diameter of JBC-nZVI was determined to be 2.72 nm, which is larger than that of JBC (1.73 nm). This increase in pore diameter can be attributed to the corrosive effect of the liquid-phase reduction process on the surface and pore structure of JBC. This effect may lead to the expansion or partial destruction of fine pores, thereby increasing the overall pore diameter [30]. The larger pore diameter of JBC-nZVI facilitates enhanced contact between JBC-nZVI and Cr(VI) ions, thereby improving the adsorption efficiency and promoting Cr(VI) removal.
Specific surface area and pore structure parameters of KBC and KBC-nZVI.
| Sample | BET (m2/g) |
BETmic
(m2/g) |
Volume(cm3/g) |
V
average
(nm) |
|
|---|---|---|---|---|---|
| V total | V mic | ||||
| JBCa | 1,987.57 | 1,699.54 | 0.86 | 0.66 | 1.73 |
| JBC-nZVI | 512.23 | 459.11 | 0.36 | 0.19 | 2.72 |
-
aThe BET and pore structure parameters of JBC in this table were reported in our own previous work [23].
Figure 4(a) displays the infrared spectra of nZVI, JBC, and JBC-nZVI composite materials before and after the reaction. After the reaction, characteristic peaks at 3,430 cm−1, 1,630 cm−1, 1,079–1,113 cm−1, 560 cm−1, and 477 cm−1, corresponding to -OH, C=O, C–O, and Fe–O, respectively, were observed in the JBC-nZVI-Cr spectrum, which was consistent with those observed in the JBC-nZVI spectrum before the reaction, indicating the preservation of key functional groups. However, in JBC-nZVI-Cr composites, the intensity of these peaks was significantly enhanced compared to JBC-nZVI, which can be attributed to the acidic reaction conditions, specifically pH 3, under which the Cr(VI) removal process was conducted. This intensity enhancement suggests that nZVI underwent corrosion under acidic conditions, leading to increased exposure of functional groups [31]. Furthermore, JBC, derived from jujube tree branches, may experience the dissolution of inorganic components under acidic conditions, exposing additional functional groups and contributing to the enhanced peak intensity observed in JBC-nZVI-Cr.
Spectroscopic and XPS characterization of JBC, JBC-nZVI, and reaction products. (a) FT-IR spectra of JBC, JBC-nZVI, and JBC-nZVI-Cr; (b) Raman spectra of JBC, JBC-nZVI, JBC-nZVI-Cr, and nZVI; (c) O 1s spectra of JBC and JBC-Cr; (d, f) Cr 2p spectra of JBC-Cr and JBC-nZVI-Cr; (e) Fe 2p spectra of JBC-nZVI and JBC-nZVI-Cr.
Raman spectroscopy was used to investigate the crystalline structure and graphitization degree of the materials. As depicted in Figure 4(b), the Raman spectrum of the nZVI sample exhibited a peak at 689 cm−1, which corresponds to magnetite (Fe3O4), and a peak at 1,399 cm−1, indicative of hematite (α-Fe2O3). Following the loading of nZVI onto JBC, a reduction in the intensity of the Fe3O4 peak at 689 cm−1 was observed, and other iron-related peaks were strongly masked by carbon signals. The Raman spectra of the biochar, as shown in Figure 4(b), displayed characteristic D-band and G-band peaks at 1,351 cm−1 and 1,600 cm−1, respectively. The D-band corresponds to disordered graphite, while the G-band is associated with in-plane vibrations of sp2-bonded carbon [32]. The intensity ratio of the D-band to the G-band (ID/IG) reflects the graphitization degree of the biochar. Following nZVI loading, a slight decrease in the ID/IG value was observed, which can be attributed to the masking of surface functional groups by nZVI particles.
Given that both Cr(VI) and Cr(III) can adsorb onto the surfaces of JBC and JBC-nZVI, an XPS analysis was conducted to further investigate the reduction contribution of Cr(VI). As illustrated in Figure 4(d)–(f), the Cr 2p spectra exhibited two characteristic peaks corresponding to Cr 2p3/2 and Cr 2p1/2, respectively. The binding energies of Cr(III) were determined to be 577.2 eV and 586.5 eV, while those of Cr(VI) were 578.7 eV and 588 eV. The XPS results indicate that the percentages of Cr(VI) and Cr(III) on the JBC surface were 47.8 % and 52.2 %, respectively. In contrast, on the JBC-nZVI surface, the percentages were 26.9 % and 73.1 %, respectively. Compared with JBC, the incorporation of nZVI significantly increases the proportion of Cr(III), demonstrating the strong reduction capacity of nZVI for Cr(VI) [33]. The primary mechanism involves the reduction of Cr(VI) to the less toxic Cr(III) by Fe0 and Fe2+, while a minor fraction of Cr(VI) remains physically adsorbed on the material surface [23]. Overall, the Cr(VI) removal mechanism of the JBC-nZVI composite is a combination of physical adsorption and reduction of Cr(VI), which substantially enhances the Cr(VI) removal efficiency.
The reduction capacity of biochar is reportedly correlated with its functional groups and structural properties. The oxygen-containing functional groups and conjugated aromatic structures in biochar can act as electron donors. Previous studies have demonstrated that oxygen-containing functional groups, particularly C–O, play key roles in electron donation. This observation was further supported by the FTIR spectroscopy results presented in Figure 4(a), which indicated the presence of these functional groups. Furthermore, the XPS analysis of O 1s (Figure 4(c)) revealed significant changes in the functional group distribution after JBC reacted with Cr(VI). Specifically, the peak areas of O–C=O and C–O decrease by 17.7 % and 2.9 %, respectively, while the C=O peak increases by 2.4 %. These alterations in functional group distribution may be attributed to the oxidation of C–O groups by Cr(VI), resulting in the formation of C=O [34], providing further evidence of JBC’s reduction effect on Cr(VI). Additionally, after adsorption, a new Cr–O characteristic peak emerges at 530.5 eV (18.2 %), indicating that Cr(VI) was adsorbed onto the JBC surface.
In the JBC-nZVI system, JBC functions as both a structural support for nZVI and an electron transfer medium in Cr(VI) reduction reactions. Although biochar possesses the inherent capability to directly donate electrons for Cr(VI) reduction, previous studies have demonstrated a reduction in its electron-donation capacity in the presence of external reducing agents. This phenomenon suggests that JBC primarily acts as an “electron shuttle,” facilitating electron transfer between nZVI and Cr(VI) [35]. Biochar, being a carbonaceous material with favorable electron transfer potential, utilizes its oxygen-containing functional groups, such as hydroxyl and carbonyl groups, to play a critical role in the electron transfer process. Previous research has demonstrated that biochar facilitates electron transfer through its redox-active functional groups, particularly in the presence of external reducing agents [36]. The “electron medium” function of biochar is mainly achieved through its reversible redox functional groups. In the JBC-nZVI system, nZVI particles are immobilized on the biochar surface, potentially reducing the accessibility of some functional groups. However, the presence of nZVI enhances biochar’s adsorption capacity, likely due to increased surface interactions and synergistic effects between biochar and nZVI. The conjugated structure and adsorption capability of biochar facilitate electron transfer and Cr(VI) reduction. In scenarios in which Cr(VI) cannot directly interact with nZVI, JBC adsorbs Cr(VI) and accepts electrons from nZVI via its oxygen-containing functional groups, such as hydroxyl and carbonyl groups. These functional groups accept electrons from nZVI and subsequently transfer them to Cr(VI), effectively reducing it. This indirect reduction mechanism is known as “orthogonal reduction” [37], where the biochar functions as an electron mediator between nZVI and Cr(VI), ensuring efficient Cr(VI) removal even in cases of limited direct interaction between nZVI and Cr(VI). The FT-IR analysis results (Figure 4(a)) further confirm that the biochar surface is rich in hydroxyl and carbonyl groups, which enhance Cr(VI) adsorption and facilitate its reduction via coordination mechanisms. Furthermore, this coordination ability of biochar effectively mitigates the passivation of nZVI, thereby extending its reactivity. In summary, JBC in this system plays multiple roles in Cr(VI) removal. First, JBC functions as a support material and exhibits good stability and dispersion [38]. Second, JBC efficiently adsorbed Cr(VI), acting as an adsorption medium. Third, JBC directly participates in the Cr(VI) reduction reaction as an electron donor [39]. Additionally, JBC functions as an electron transfer medium between adsorbed Cr(VI) and nZVI, enabling efficient electron transfer and promoting Cr(VI) reduction through a neighboring reduction mechanism.
To further elucidate the transformation of nZVI and Cr, an XPS analysis of Fe 2p in JBC-nZVI was conducted to investigate the changes in the iron oxidation states before and after the reaction with Cr(VI). As shown in Figure 4(e), the Fe 2p spectrum reveals peaks corresponding to Fe0, Fe(II), and Fe(III). Before the reaction, a characteristic peak indicative of Fe0 was observed on the JBC-nZVI surface, confirming the successful loading of nZVI onto the JBC. After the reaction, the characteristic peak of Fe0 was no longer detectable on the JBC-nZVI-Cr surface, indicating that nZVI was completely oxidized during the Cr(VI) adsorption process [40]. Furthermore, before the reaction, the Fe(II) to Fe(III) ratio on the JBC-nZVI surface was 45.5 %–48.1 %, whereas after the reaction, the ratio shifted to 34.4 %–65.6 %. This represents an 11.1 % decrease in the proportion of Fe(II) and a 17.5 % increase in Fe(III), indicating the formation of iron oxides and the oxidation of some Fe(II) to Fe(III) by Cr(VI). These findings confirm that during the JBC-nZVI and Cr(VI) adsorption and reduction process, Fe(II), either directly or through conversion/oxidation by nZVI, plays a significant role in reducing most of the Cr(VI) to Cr(III).
Additionally, the XRD analysis (Figure 2) revealed the emergence of new diffraction peaks corresponding to FeO(OH), FeCr2O4, and CrO(OH) on the material’s surface after the reaction with Cr(VI). This observation further supports the presence of Fe(II), Fe(III), and Cr(III) species in JBC-nZVI-Cr, indicating that a redox reaction occurred between Cr(VI) and nZVI. The primary redox process involves the reduction of Cr(VI) to Cr(III), which precipitates as CrO(OH) and FeCr2O4, while Fe0 undergoes oxidation to form FeO(OH), Fe2O3, and Fe3O4 [41]. The XRD characterization results confirmed that JBC-nZVI participates in the adsorption and reduction of Cr(VI).
As shown in the CV curves (Figure 5), in the potential range of −0.9 to −0.7 V, the intensity of the oxidation peak for JBC-nZVI-Cr (post-reaction) is significantly attenuated compared to that of pristine JBC-nZVI (pre-reaction), which provides direct evidence for the occurrence of electron transfer during the reaction [42]. Moreover, the characteristic oxidation peak observed in the curve of pristine JBC-nZVI corresponds to the stepwise electron release processes of Fe0 (Fe0→Fe2+) and Fe2+ (Fe2+→Fe3+), confirming that JBC-nZVI possesses high-efficiency electron donor potential. In contrast, the marked attenuation of this oxidation peak intensity and the overall reduction in electrochemical activity in the curve of JBC-nZVI-Cr (post-reaction) reveal two key phenomena: (1) the reductive Fe0/Fe2+ species, which act as the primary electron donors, are consumed during the reaction; (2) a passivation layer composed of Fe–Cr oxides/hydroxides forms on the material surface, impeding subsequent electron transfer processes.
CV curves of JBC-nZVI before and after Cr(VI) removal.
Specifically, these XRD, XPS, and CV results directly verify the reduction-coprecipitation step in the Cr(VI) removal mechanism: the disappearance of the Fe0 diffraction peak (Figure 2), the decrease in Fe(II) proportion from 45.5 % to 34.4 % (Figure 4e), and the significant attenuation of the oxidation peak at −0.9 ∼ −0.7 V in the post-reaction CV curve (Figure 5, corresponding to Fe0→Fe2+ and Fe2+→Fe3+ electron release) collectively confirm that Fe0/Fe(II) acts as electron donors to reduce Cr(VI); meanwhile, the increase in Cr(III) proportion to 73.1 % (Figure 4f), the emergence of FeCr2O4 diffraction peaks (Figure 2), and the reduced overall electrochemical activity of JBC-nZVI-Cr in CV tests (attributed to surface passivation by Fe–Cr oxides/hydroxides) prove that reduced Cr(III) coprecipitates with Fe(III) to form stable Fe–Cr minerals, avoiding secondary release of Cr(III).
3.2 Influence factor
3.2.1 Effect of initial pH
As shown in Figure 6(c), the adsorption efficiency of Cr(VI) by the JBC-nZVI composite decreases as the pH increases from 3 to 11, with the removal efficiency decreasing from 88.4 % to 40.6 %. This trend suggests that lower pH conditions favor Cr(VI) removal. Under acidic conditions, HCrO4 − in the solution is readily absorbed by the JBC-nZVI composite material. Additionally, the reduction of Cr(VI) consumes a substantial quantity of H+, which enhances the dissolution of the iron passivation layer, thereby increasing the exposure of active sites and improving the contact between Cr(V) and iron [43]. The point of zero charge (pHpzc) of the JBC-nZVI composite material, as depicted in Figure 6(d), was determined to be 8.21. Under acidic conditions, the surface of JBC-nZVI is positively charged, facilitating the electrostatic attraction between Cr(VI) anions and enhancing the removal efficiency [44]. The pHpzc of JBC, as shown in Figure 6(b) is 5.12. The incorporation of nZVI increased the pHpzc to 8.21, broadening the pH range in which strong electrostatic attraction occurs. This modification strengthens the electrostatic attraction between the composite and Cr(VI) anions, thereby improving Cr(VI) removal efficiency across a wider pH spectrum. Under alkaline conditions, Cr(VI) predominantly exists as CrO4 2−, which has higher adsorption-free energy, making it more difficult to adsorb. When the pH exceeds 8.21, the surface of the JBC-nZVI composite material becomes negatively charged, resulting in electrostatic repulsion with Cr(VI) anions, thereby impeding both adsorption and reduction [45]. Furthermore, under alkaline conditions, the formation of hydroxide precipitates on the JBC-nZVI surface hinders electron transfer, further reducing the adsorption efficiency.
Effect of pH on Cr(VI) adsorption and surface charge properties. (a) Effect of pH on Cr(VI) adsorption by JBC; (b) zeta potential of JBC at different pH values; (c) effect of pH on Cr(VI) adsorption by JBC-nZVI; (d) zeta potential of JBC-nZVI at different pH values.
3.2.2 Effect of dosage, initial concentration, time, and temperature
The influence of the JBC-nZVI composite material dosage on Cr(VI) adsorption was investigated under the following controlled conditions: pH 3, Cr(VI) concentration of 50 mg/L, adsorption time of 120 min, temperature of 25 °C, and stirring speed of 150 rpm. As illustrated in Figure 7(a), an increase in the Cr(VI) dosage from 0.25 g/L to 2.5 g/L enhanced the Cr(VI) removal efficiency from 65.8 % to 88.9 %, accompanied by a decrease in the adsorption capacity from 134.63 mg/g to 18.2 mg/g. The observed increase in the removal efficiency can be attributed to the greater availability of active sites provided by the higher dosage of the composite material [46]. However, upon reaching adsorption saturation, the removal efficiency stabilized, and the adsorption capacity declined due to the decreased number of adsorption sites per unit mass. Therefore, based on these findings, the optimal dosage of the JBC-nZVI composite material for Cr(VI) removal was determined to be 1 g/L.
Effects of various parameters on Cr(VI) removal: (a) Initial concentration, (b) reaction time, (c) dosage, and (d) reaction temperature at different times.
The influence of the initial Cr(VI) concentration on the adsorption performance of the JBC-nZVI composite material was investigated under the following conditions: pH 3, dosage of 1 g/L, adsorption time of 120 min, and temperature of 25 °C, as illustrated in Figure 7(b) The results demonstrated that as the initial Cr(VI) concentration increased from 10 mg/L to 125 mg/L, the Cr(VI) removal efficiency gradually decreased, while the adsorption capacity initially increased and subsequently decreased. At low concentrations, the high availability of adsorption sites allowed for increased adsorption capacity as the Cr(VI) ions occupied the available sites. However, as the concentration further increased, the adsorption sites became saturated, resulting in a decrease in the adsorption capacity. Additionally, the formation of a Fe(III)/Cr(III) oxide layer on the composite surface hindered the electron transfer and reduction of Cr(VI), contributing to the decline in adsorption efficiency [47]. Consequently, a Cr(VI) concentration of 50 mg/L was selected for subsequent experiments to ensure optimal adsorption performance.
Under the conditions of pH = 3, Cr(VI) concentration of 50 mg/L, dosage of 1 g/L, temperature of 25 °C, and stirring speed of 150 rpm, the effect of reaction time of JBC-nZVI composite material on Cr(VI) adsorption was investigated, as shown in Figure 7(c). The results demonstrated a progressive increase in both Cr(VI) removal efficiency and adsorption capacity as the reaction time was extended from 5 min to 150 min. After 30 min, the removal efficiency reached 83 %. Adsorption equilibrium was attained at 120 min, with a corresponding removal efficiency of 85 % and adsorption capacity of 43.47 mg/g. The initial rapid adsorption phase was attributed to the high availability of active sites, while the subsequent slower adsorption was due to the progressive occupation of adsorption sites, reducing the driving force for Cr(VI) uptake [48]. Thus, a final adsorption time of 120 min was selected for optimal adsorption efficiency in subsequent experiments.
Under the conditions of pH = 3, Cr(VI) concentration of 50 mg/L, dosage of 1 g/L, and stirring speed of 150 rpm, the effect of reaction temperature on the adsorption of Cr(VI) by the JBC-nZVI composite material at different times was investigated, as shown in Figure 7(d). During the initial stage of adsorption (5 min), increasing the temperature from 15 °C to 45 °C significantly promotes the migration and collision frequency of Cr(VI) ions, thereby accelerating the adsorption rate. At this stage, adsorption is primarily driven by the rapid occupation of surface active sites. Higher temperatures enhance the diffusion of Cr(VI) ions and the reaction rate on the surface of the adsorbent. When the temperature is 45 °C, the Cr(VI) removal efficiency within 5 min is markedly higher than at 15 °C. At a reaction time of 120 min, as the temperature increases, both the removal rate and adsorption capacity of Cr(VI) show an upward trend, reaching their peak at 45 °C, with values of 88.6 % and 45.3 mg/g, respectively [49]. The observed improvement in adsorption performance can be attributed to the enhanced migration and collision probability of Cr(VI) ions at higher temperatures, which favors the adsorption process. After a longer duration (150 min), the adsorption process approaches saturation or equilibrium. At this stage, changes in temperature have a minimal impact on the adsorption capacity of Cr(VI), and may even have adverse effects. However, considering energy consumption and economic feasibility, room temperature is regarded as the optimal temperature for practical applications.
Compared with peanut shell biochar-supported nano-cerium oxide (BC–Ce, maximum adsorption capacity 47.83 mg/g, optimal dosage 2 g/L, 6 h equilibrium) and acid-modified biochar-supported nZVI (nZVI@HCl-BC, equilibrium adsorption capacity 21.21 mg/g), JBC-nZVI (1 g/L dosage, pH 3, 120 min) achieves a competitive 43.47 mg/g adsorption capacity and 85 % removal efficiency with lower dosage and faster equilibrium. While glucose-derived carbon-modified nZVI (nZVI@GC) has higher capacity (178.6 mg/g), it requires 24 h to reach equilibrium – JBC-nZVI’s 120 min equilibrium and balanced performance make it more practical for moderate-concentration Cr(VI) treatment [50].
3.2.3 Influence of co-existing ions
The complex composition of real industrial wastewater introduces coexisting ions that can compete for adsorption sites or react with Cr(VI), thereby influencing its removal efficiency. To evaluate these effects, three common anions (Cl−, CO3 2−, NO3 −) and three metal cations (Ca2+, Mg2+, Zn2+) were selected as representative coexisting ions to investigate their effects on Cr(VI) removal by the JBC-nZVI composite material at three different reaction times. As shown in Figure 8, at a reaction time of 5 min representing the early adsorption stage the process is mainly driven by high concentrations of active sites, with ion competition effects not yet fully manifested. At this stage, Cl−, Ca2+, Mg2+, and Zn2+ have minimal impact on Cr(VI) removal, indicating limited influence over short periods due to their inability to compete effectively for adsorption sites or alter the process significantly. NO3 −, with strong electron acceptance capacity, may slightly inhibit Cr(VI) reduction initially, but the effect remains limited, with little change in removal efficiency. CO3 2− has a minor effect in this short timeframe but could increase pH, potentially hindering reduction later. At 120 min, the presence of Cl−, Ca2+, Mg2+, and Zn2+ shows no significant impact on Cr(VI) removal, likely due to equilibrium conditions. However, NO3 −, a common soluble electron acceptor, reacts with nZVI even at low concentrations, significantly inhibiting Cr(VI) removal reducing efficiency by 16.4 % [51]. This is due to competition for electrons, forming intermediates like NO2 −, NH4 +, and N 2, which can passivate the nZVI surface and decrease reactivity. Similarly, CO3 2− causes a 49.2 % reduction, owing to competitive adsorption, pH increase from hydrolysis, and formation of iron carbonate or bicarbonate precipitates that block active sites [52]. Additionally, reactions between Fe2+ and HCO3 − produce precipitates such as FeCO3, further hindering reduction. At 150 min, adsorption reaches equilibrium, and ion competition becomes more pronounced. While Cl−, Ca2+, Mg2+, and Zn2+ still exert minimal influence, NO3 − significantly decreases Cr(VI) removal efficiency indicating strong competitive effects and formation of inhibitory intermediates. CO3 2− remains the most impactful, potentially reducing removal efficiency by over 50 % due to combined effects of pH elevation, carbonate precipitation, and surface passivation.
The influence of interfering ions on the adsorption of Cr (VI) by JBC-nZVI at different times.
3.3 Kinetic study
To investigate the adsorption kinetics of Cr(VI) removal, by JBC and JBC-nZVI composite materials, three kinetic models: pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, were employed to simulate the adsorption behavior under controlled conditions of 25 °C, pH 3, and an initial Cr(VI) concentration of 50 mg/L. The fitting results from these models are presented in Tables 3 and 4, and the corresponding kinetic profiles are depicted in Figure 9.
Fitting parameters for the quasi-first-order and quasi-second-order kinetic models.
| Sample | Pseudo-first-order | Pseudo-second-order | ||||
|---|---|---|---|---|---|---|
| K 1(min−1) | q e (mg/g) | R 2 | K 2(g·mg−1·min−1) | qe (mg/g) | R 2 | |
| JBC | 0.0485 | 14.95 | 0.869 | 0.0067 | 20.55 | 0.994 |
| JBC-nZVI | 0.0345 | 3.18 | 0.925 | 0.0352 | 43.61 | 0.999 |
Fitting parameters for the intraparticle diffusion model.
| Sample | Intra-particle diffusion | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| K d1 (mg·g−1 min0.5) | C 1 (mg/g) | R 1 2 | K d2 (mg·g−1 min0.5) | C 2 (mg/g) | R 2 2 | K d3 (mg·g−1 min0.5) | C 3 (mg/g) | R 3 2 | |
| JBC | 1.85 | 6.42 | 0.986 | 0.739 | 12.29 | 0.887 | 0.052 | 19.01 | 0.939 |
| JBC-nZVI | 0.71 | 38.59 | 0.998 | 0.193 | 41.56 | 0.812 | 0.087 | 42.48 | 0.921 |
Adsorption kinetics models for Cr(VI) removal by JBC and JBC-NZVI: (a) Quasi-first-order and quasi-second-order kinetic models, and (b) intraparticle diffusion models.
The kinetic data presented in Figure 9(a) and Table 3 indicate that the Cr(VI) removal process by JBC is best described by the pseudo-second-order kinetic model, with a correlation coefficient (R 2) of 0.994. This value is significantly higher than the R 2 0.869 obtained for the pseudo-first-order kinetic model, indicating a superior fit. Additionally, the calculated adsorption capacity derived from the pseudo-second-order model was 20.55 mg/g, which closely aligns with the experimentally determined value of 19.6 mg/g. This agreement indicates that the pseudo-second-order model accurately represents the adsorption process of JBC, highlighting the dominance of chemical adsorption in Cr(VI) removal [53]. Similarly, for the Cr(VI) removal process using JBC-nZVI composite material, the pseudo-second-order model exhibited a significantly higher correlation coefficient (R 2 = 0.999) compared to the pseudo-first-order kinetic model (R 2 = 0.925). The adsorption capacity calculated from the pseudo-second-order model (43.61 mg/g) closely aligns with the experimental value (43.5 mg/g). In contrast, the adsorption capacity calculated by the pseudo-first-order kinetic model (3.18 mg/g) deviates substantially from the experimental value, further confirming the superiority of the pseudo-second-order model in describing the adsorption process. Therefore, the pseudo-second-order kinetic model is more suitable for describing the Cr(VI) adsorption behavior of the JBC-nZVI composite material, further confirming the dominant role of chemical adsorption in this process [54]. In summary, the Cr(VI) removal processes for both JBC and JBC-nZVI composite materials are characterized by chemical adsorption as the primary mechanism. However, the JBC-nZVI composite material demonstrates a significantly higher adsorption capacity than JBC alone, highlighting its enhanced performance in Cr(VI) removal.
To further elucidate the rate-limiting steps in the Cr(VI) adsorption process by JBC and JBC-nZVI composite materials, the intraparticle diffusion model was employed to fit the experimental data. The results indicate that the Cr(VI) adsorption process by JBC can be divided into three distinct stages. The initial stage is characterized by surface diffusion, where the high Cr(VI) concentration and abundance of active sites on the adsorbent surface result in a rapid adsorption rate. Consequently, surface diffusion becomes the predominant process, as evidenced by the maximum slope (K d1) [55]. This is followed by the intraparticle diffusion stage, where the progressive occupation of surface sites decreases the adsorption rate. During this stage, Cr(VI) ions slowly diffuse into the pores within the JBC matrix, resulting in a reduced slope (K d2). The final stage represents the equilibrium stage, where the adsorption process reaches a steady state, and the slope (K d3) is minimal [56]. The non-zero intercept values (C) observed for each stage of the process indicate that intraparticle diffusion is not the sole rate-limiting step. Furthermore, the surface diffusion rate was observed to be significantly higher than the intraparticle diffusion rate, indicating that Cr(VI) removal by JBC is governed by a combination of surface diffusion and intraparticle diffusion mechanisms. Similarly, the Cr(VI) adsorption process of the JBC-nZVI composite material also exhibited three distinct stages: an initial surface diffusion stage, characterized by a rapid adsorption rate due to the high Cr(VI) concentration and abundance of surface active sites; an intraparticle diffusion stage, where Cr(VI) gradually occupies active sites and diffuses into the pores of the adsorbent; and a final equilibrium stage, where the intraparticle diffusion further slows down, and the adsorption process stabilizes, reaching a steady-state [57]. Similar to JBC, the non-zero intercept values (C) for each stage in the JBC-nZVI composite material also indicate that intraparticle diffusion is not the sole rate-limiting step. Consequently, the Cr(VI) removal process by the JBC-nZVI composite material is governed by both surface diffusion and intraparticle diffusion. However, due to the increased availability of surface active sites and enhanced diffusion properties of the JBC-nZVI composite material, the adsorption process exhibited greater adsorption capacity and higher removal efficiency.
3.4 Adsorption isotherm and thermodynamics
To gain deeper insights into the adsorption mechanism of Cr(VI) by JBC and JBC-nZVI composite materials, the Langmuir model, Freundlich model, and thermodynamic model were employed. The isotherm and thermodynamic model analyses of Cr(VI) were conducted under controlled experimental conditions of 25 °C and pH 3. The results and associated parameters derived from these models are presented in Figure 10 and Tables 5 and 6.
Adsorption isotherms and thermodynamic analysis of Cr(VI) removal. (a, b) Adsorption isotherms of Cr(VI) on JBC and JBC-nZVI; (c, d) thermodynamic plots of Cr(VI) adsorption by JBC and JBC-nZVI.
Fitting parameters for isotherm models.
| Sample | Langmuir | Freundlich | ||||
|---|---|---|---|---|---|---|
| K L (L/mg) | q m (mg/g) | R 2 | K F (mg/g) | n | R 2 | |
| JBC | 0.239 | 30.836 | 0.981 | 9.425 | 3.525 | 0.996 |
| JBC-nZVI | 1.369 | 65.445 | 0.996 | 34.647 | 5.911 | 0.86 |
Fitting parameters for thermodynamic models.
| Sample | T | △G 0 (KJ/mol) | △H 0 (KJ/mol) | △S 0 (KJ/(mol·K)) |
|---|---|---|---|---|
| JBC | 288 | −0.811 | 12.541 | 0.046 |
| 298 | −1.299 | |||
| 308 | −1.737 | |||
| JBC-nZVI | 288 | −3.831 | 15.011 | 0.065 |
| 298 | −4.304 | |||
| 308 | −5.144 |
As shown in Figure 10(a), (b) and Table 5, the adsorption characteristics of JBC and JBC-nZVI composite materials differ significantly under the Langmuir and Freundlich models. For JBC, the Freundlich model demonstrated a higher correlation coefficient (R 2 = 0.996) compared with the Langmuir model (R 2 = 0.981), indicating that the Freundlich model provides a better fit for describing Cr(VI) adsorption by JBC [58]. The Freundlich model indicates that the JBC adsorbent surface is heterogeneous, characterized by active sites with varying affinities for Cr(VI) and that the adsorption process involves multilayer formation rather than monolayer adsorption. The results further revealed that the n-value of JBC is 3.525, which falls within the range of 1 and 10, indicating that the adsorption of Cr(VI) is both efficient and highly heterogeneous [59].
For the JBC-nZVI composite material, the data presented in Table 5 demonstrate that the Langmuir isotherm model exhibits a higher correlation coefficient (R 2 = 0.996) than the Freundlich model (R 2 = 0.86). This indicates that the Langmuir model better describes the Cr(VI) adsorption process by JBC-nZVI, implying that adsorption primarily occurs through a monolayer mechanism. The adsorption performance of the JBC-nZVI composite material is primarily governed by the availability of adsorption sites within its porous structure. According to the Langmuir model, the maximum adsorption capacity (qm ) of JBC-nZVI for Cr(VI) was determined to be 65.445 mg/g, with an n-value of 5.911, indicating that the adsorption process is thermodynamically favorable and that the binding strength between the adsorbent and Cr(VI) ions is relatively high [60]. In summary, the applicability and underlying adsorption mechanisms of the Langmuir and Freundlich models differ in describing the Cr(VI) adsorption behavior of JBC and JBC-nZVI composite materials. The Freundlich model provides a better description of Cr(VI) adsorption by JBC, whereas the Langmuir model aligns more closely with the adsorption behavior of the JBC-nZVI composite material.
The thermodynamic parameters presented in Figure 10(c), (d) and Table 6 indicate that the adsorption process for both JBC and JBC-nZVI composite materials is spontaneous. The negative ΔG° values confirm the spontaneity of the adsorption process. Furthermore, an increase in temperature correlates with an increase in the absolute value of ΔG°, indicating that higher temperatures favor the adsorption of Cr(VI). For JBC, the negative ΔG° value and decreasing trend with increasing temperature indicate that higher temperature enhances the spontaneity of the adsorption reaction, facilitating the adsorption of Cr(VI) [61]. Compared with JBC, the lower ΔG° value observed for JBC-nZVI composite material indicates that the adsorption process was more spontaneous and effective. Additionally, the positive ΔH o values indicate that the adsorption process is endothermic, requiring thermal energy input for the reaction to proceed. As the temperature increases, the molecular activity intensifies, thereby promoting the adsorption of Cr(VI) onto the surfaces of JBC and JBC-nZVI composite materials. The positive ΔS o values indicate an increase in randomness at the solid/liquid interface during the adsorption process, suggesting a certain degree of disorder. The increase in randomness is more pronounced for JBC-nZVI composite material [62], reflecting its stronger affinity for Cr(VI). In conclusion, JBC-nZVI composite material exhibits superior adsorption performance compared to JBC. The adsorption process is more spontaneous and thermodynamically favorable, particularly at elevated temperatures.
3.5 Regeneration and reusability research
The data in Figure 11 demonstrated that the Cr(VI) removal efficiency of the JBC-nZVI composite material progressively decreased with an increasing number of reuse cycles. The removal efficiencies were 88.4 %, 72.1 %, 60.4 %, 58.5 %, and 49.1 % for the consecutive cycles, respectively. The experimental results indicate that the reusability of the JBC-nZVI composite material was significantly enhanced, primarily due to the loading of nZVI onto the JBC matrix. The JBC structure effectively facilitated the dispersion of nZVI and mitigated particle aggregation, which prolongs nZVI activity and stability during repeated cycles of use. As a result, the material maintains a higher sustained Cr(VI) removal capacity compared to unmodified nZVI [63].
Reusability and stability of JBC-nZVI for Cr(VI) removal.
3.6 Removal mechanism
The Cr(VI) removal by JBC-nZVI is a synergistic process involving adsorption-reduction-co-precipitation, with each step playing a sequential and complementary role supported by existing characterization data. Adsorption serves as a “preliminary enrichment” step: JBC-nZVI’s point of zero charge (pHpzc = 8.21, Figure 6d) ensures a positively charged surface under optimal pH 3, enabling electrostatic attraction of HCrO4 −, while its porous structure (average pore diameter 2.72 nm, Table 2) provides mass transfer channels for Cr(VI) ions – this concentration of Cr(VI) on the composite surface primes the subsequent reduction reaction. Reduction acts as the “toxicity mitigation” core step: XRD analysis (Figure 2) shows the disappearance of the Fe0 diffraction peak (2θ = 44.6°) and emergence of Fe2O3/Fe3O4 peaks, confirming Fe0 oxidation to Fe2+/Fe3+ (electron donors), while XPS (Figure 4f) reveals that 73.1 % of surface Cr exists as Cr(III)-verifying efficient conversion of toxic Cr(VI) to low-toxicity Cr(III). Co-precipitation functions as the “permanent stabilization” step: XRD (Figure 2) detects new FeCr2O4 diffraction peaks (2θ = 35.5°), confirming Cr(III) coprecipitates with Fe(III) to form insoluble minerals, avoiding secondary Cr(III) release [64].
The synergy between JBC and nZVI further reinforces this removal process. First, JBC mitigates nZVI aggregation: SEM and particle size analysis (Figure 3b/d) show nZVI in the composite has an average size of 51.9 nm, far smaller than the 112.98 nm aggregates of pure nZVI-JBC’s hierarchical pores act as “nanoparticle nests” to increase nZVI active site exposure and reactivity. Second, JBC acts as an “electron shuttle”: Raman spectra (Figure 4b) show JBC has a low ID/IG ratio (0.786) indicating high graphitization, and XPS O 1s analysis (Figure 4c) confirms 18.7 % C–O groups – these features facilitate electron transfer from nZVI to adsorbed Cr(VI), even for Cr(VI) not in direct contact with nZVI. Third, JBC promotes stable Fe–Cr precipitate formation: FTIR (Figure 4a) shows abundant –OH groups on JBC, which complex with Fe(III)/Cr(III) to induce ordered growth of FeCr2O4, enhancing long-term Cr(III) stabilization [65]. Collectively, these interactions between JBC and nZVI, coupled with the sequential adsorption-reduction-co-precipitation steps, drive efficient and stable Cr(VI) removal (Figure 12).
Mechanism diagram of Cr(VI) removal by JBC-nZVI.
4 Conclusions
In this study, Jujube tree branch biochar (JBC) was used as a support matrix to successfully synthesize Jujube tree branch biochar-supported nanoscale zero-valent iron (JBC-nZVI) composite materials. The JBC-nZVI composite material demonstrated effective Cr(VI) removal efficiency. The nZVI particles were uniformly dispersed with a small particle size within the composite. The incorporation of nZVI onto JBC significantly enhances the reaction efficiency and Cr(VI) removal capacity. The optimal removal conditions were determined through single-factor experiments: pH 3, dosage 1 g/L, initial Cr(VI) concentration 50 mg/L, reaction time 120 min, and temperature 25 °C. Zeta potential measurements indicated that electrostatic attraction for Cr(VI) ions was enhanced over a broad pH range. The removal efficiency reached 88.4 %, which was 11.2 % higher than that of raw biochar. XRD and XPS analyses revealed that surface Fe0 was oxidized to Fe2+ and Fe3+, while Cr(VI) was reduced to Cr(III). Kinetic and thermodynamic modeling demonstrated that chemisorption predominantly governs the process, with the Langmuir isotherm providing the best fit, indicating monolayer adsorption. The process was spontaneous and endothermic, with an increase in system disorder during adsorption. In summary, the removal of Cr(VI) by JBC-nZVI involves a synergistic effect of adsorption, reduction, and co-precipitation, highlighting its potential for practical water treatment applications.
Acknowledgments
The authors extend their appreciation to the Yulin City, Shaanxi Province Science and Technology Plan Project, China [grant numbers CXY-2021-101-03]; General Projects of Shaanxi Provincial Science and Technology Department, China [grant number 2023-JC-YB-143]; the College Students’ Innovative Training Program, China [grant number 202411395025]; and the College Students’ Innovative Training Program, China [grant number S202411395116]; Shaanxi Provincial Intellectual Property Office patent transformation supply-side capacity improvement project, P.R. China [grant number 22ZXJH048]; New water-retaining material patent application promotion and cultivation project, P.R. China [grant number 2024ZSCQ001]. The Yulin City, Shaanxi Province Science and Technology Plan Project, China [grant numbers KJZG-2025-2Q-09]; Yulin University Graduate Innovation and Entrepreneurship Fund Project [2025YLYCX12].
-
Funding information: This work was supported by the Yulin City, Shaanxi Province Science and Technology Plan Project, China [grant numbers CXY-2021-101-03]; General Projects of Shaanxi Provincial Science and Technology Department, China [grant number 2023-JC-YB-143]; the College Students’ Innovative Training Program, China [grant number 202411395025]; and the College Students’ Innovative Training Program, China [grant number S202411395116]; Shaanxi Provincial Intellectual Property Office patent transformation supply-side capacity improvement project, P.R. China [grant number 22ZXJH048]; New water-retaining material patent application promotion and cultivation project, P.R. China [grant number 2024ZSCQ001]. The Yulin City, Shaanxi Province Science and Technology Plan Project, China [grant numbers KJZG-2025-2Q-09]; Yulin University Graduate Innovation and Entrepreneurship Fund Project [2025YLYCX12].
-
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
-
Conflict of interest statements: The authors state no conflict of interest.
-
Data availability statement: All data generated or analysed during this study are included in this published article.
References
1. Li, T, Zhu, F, Gao, YQ, Iribagiza, MR, Hu, GY, Guan, J. Efficient elimination of Cr(VI) in groundwater using nano zero-valent iron synthesized with Ginkgo biloba extracts: enhanced mechanism and reduced toxicity. Environ Sci Wat Res 2024;10:339–52. https://doi.org/10.1039/d3ew00479a.Search in Google Scholar
2. Nadumane, SS, Biswas, R, Mazumder, N. Eco-friendly colorimetric detection of lead and Mercury using l-cysteine-functionalized gold nanoparticles: a step towards greening the exposome. RSC Adv 2025;15:11573–9. https://doi.org/10.1039/d5ra01445j.Search in Google Scholar PubMed PubMed Central
3. Netzahuatl-Muñoz, AR, Aranda-Garcia, E, Cristiani-Urbina, E. Chromium recovery from chromium-loaded Cupressus lusitanica bark in two-stage desorption processes. Plants 2023;12:3222. https://doi.org/10.3390/plants12183222.Search in Google Scholar PubMed PubMed Central
4. An, QY, Zhang, M, Guo, DB, Wang, GS, Xu, H, Fan, C, et al.. Cr(VI) removal by recombinant Escherichia coli harboring the main functional genes of sporosarcina saromensis M52. F Microbiol 2022;13:820657. https://doi.org/10.3389/fmicb.2022.820657.Search in Google Scholar PubMed PubMed Central
5. Yang, Y, Huang, J, Sun, QM, Wang, JQ, Huang, LC, Fu, SY, et al.. microRNAs: key players in plant response to metal toxicity. Int J Mol Sci 2022;23:8642. https://doi.org/10.3390/ijms23158642.Search in Google Scholar PubMed PubMed Central
6. Wei, YZ, Yu, JL, Haider, FU, Zhang, QH, Chu, R, Cai, LQ. Integrated removal of chromium, lead, and cadmium using nano-zero-valent iron-supported biochar: mechanistic insights and eco-toxicity assessment. Ecotox Environ Safe 2025;289:117532. https://doi.org/10.1016/j.ecoenv.2024.117532.Search in Google Scholar PubMed
7. Lu, HJ, Zhu, F, Xu, HS, Liu, JX, Wu, YY. Insights of mechanism into enhanced removal of Cr(VI) by Ginkgo biloba leaves synthesized bimetallic nano-zero-valent iron/copper. Colloids Surf 2023;675:132094. https://doi.org/10.1016/j.colsurfa.2023.132094.Search in Google Scholar
8. Zhu, F, He, SY, Liu, T. Effect of pH, temperature and co-existing anions on the removal of Cr(VI) in groundwater by green synthesized nZVI/Ni. Ecotox Environ Safe 2018;163:544–50. https://doi.org/10.1016/j.ecoenv.2018.07.082.Search in Google Scholar PubMed
9. Tan, XP, Shaaban, M, Yang, JW, Cai, YJ, Wang, BY, Peng, QA. Efficient removal of hexavalent chromium from an aquatic system using nanoscale zero-valent iron supported by ramie biochar. Nanomaterials 2021;11:2698. https://doi.org/10.3390/nano11102698.Search in Google Scholar PubMed PubMed Central
10. Yu, P, Hu, T, Chen, HH, Wu, FF, Liu, H. Effective removal of congo red by triarrhena biochar loading with TiO2 nanoparticles. Scanning 2018;2018:7670929–7. https://doi.org/10.1155/2018/7670929.Search in Google Scholar PubMed PubMed Central
11. Zhu, F, Liu, JX, Liu, T. Enhanced immobilization mechanisms and transport of simulated acid rain on chromium in contaminated soil mixed with nZVI/Ni. Environ Geochem Health 2025;47:1–18. https://doi.org/10.1007/s10653-025-02478-w.Search in Google Scholar PubMed
12. Lou, ZM, Song, LD, Zhang, XL, Wen, XF, Cheng, D, Yu, JM, et al.. Pre-oxidized zerovalent iron creates reactive Fe (II) species for continuous self-driven depassivation: implications for contaminant removal. Chem Eng J 2025;507:160770. https://doi.org/10.1016/j.cej.2025.160770.Search in Google Scholar
13. Chaudhary, H, Dinakaran, J, Notup, T, Vikram, K, Rao, KS. Comparison of adsorption performance of biochar derived from urban biowaste materials for removal of heavy metals. Environ Manag 2024;73:408–24. https://doi.org/10.1007/s00267-023-01866-1.Search in Google Scholar PubMed
14. Liang, WJ, Tang, JX, Li, Y, Song, SZ, Jiang, XY, Hao, LY, et al.. Biochar and bentonite application improves Aeolian sandy soil health and enhances soil carbon sequestration and emission reduction potential. Sci Rep 2025;15:2205.10.1038/s41598-025-86636-1Search in Google Scholar PubMed PubMed Central
15. Wang, YP, Liu, YL, Tian, SQ, Yang, JJ, Wang, L, Ma, J. Straw biochar enhanced removal of heavy metal by ferrate. J Hazard Mater 2021;416:126128. https://doi.org/10.1016/j.jhazmat.2021.126128.Search in Google Scholar PubMed
16. Xu, S, Yu, WG, Liu, S, Xu, CY, Li, JH, Zhang, YC. Adsorption of hexavalent chromium using banana pseudostem biochar and its mechanism. Sustainability 2018;10:4250. https://doi.org/10.3390/su10114250.Search in Google Scholar
17. Liang, MN, Ding, YM, Zhang, Q, Wang, DQ, Li, HH, Lu, L. Removal of aqueous Cr(VI) by magnetic biochar derived from bagasse. Sci Rep 2020;10:21473. https://doi.org/10.1038/s41598-020-78142-3.Search in Google Scholar PubMed PubMed Central
18. Qian, LB, Zhang, WY, Yan, JC, Han, L, Chen, Y, Ouyang, D, et al.. Nanoscale zero-valent iron supported by biochars produced at different temperatures: synthesis mechanism and effect on Cr(VI) removal. Environ Pollut 2017;223:153–60. https://doi.org/10.1016/j.envpol.2016.12.077.Search in Google Scholar PubMed
19. Deng, YX, Wang, XY, Lynch, I, Guo, ZL, Zhang, P, Wu, LS, et al.. Homogeneous dispersion of amorphous nanoscale zero-valent iron supported on chlorella-derived biochar: in-situ synthesis and application mechanism for Cr(VI) removal over a wide pH range. Sep Purif Technol 2024;330:125207. https://doi.org/10.1016/j.seppur.2023.125207.Search in Google Scholar
20. Li, H, Ren, ZY, Huang, D, Jing, Q, Tang, HK. Removal of hexavalent chromium in aqueous solution by cellulose filter paper loaded with nano-zero-valent iron: performance investigation and numerical modeling. Int J Env Res He 2023;20:1867. https://doi.org/10.3390/ijerph20031867.Search in Google Scholar PubMed PubMed Central
21. Giri, PM, Parathasarathy, DP. Adsorption study on hexavalent chromium removal using magnetic biochar from ziziphus jujube seed. Chem Eng Technol 2025;48:e70003. https://doi.org/10.1002/ceat.70003.Search in Google Scholar
22. Rao, WX, Liu, H, Lv, GC, Wang, DY, Liao, LB. Effective degradation of Rh 6G using montmorillonite-supported nano zero-valent iron under microwave treatment. Materials 2018;11:2212. https://doi.org/10.3390/ma11112212.Search in Google Scholar PubMed PubMed Central
23. Wang, XX, Tan, Z, Shi, S, Zhang, SY, Yang, S, Zhang, XY, et al.. Preparation of cellulose-grafted acrylic acid stabilized Jujube branch biochar-supported nano zero-valent iron composite for Cr(VI) removal from water. Nanomaterials 2025;15:441. https://doi.org/10.3390/nano15060441.Search in Google Scholar PubMed PubMed Central
24. Deng, TT, Li, HS, Ding, S, Chen, F, Fu, JB, Zhao, JW. Enhanced adsorptivity of hexavalent chromium in aqueous solutions using CTS@nZVI modified wheat straw-derived porous carbon. Nanomaterials 2024;14:973. https://doi.org/10.3390/nano14110973.Search in Google Scholar PubMed PubMed Central
25. Su, HJ, Fang, ZQ, Tsang, PE, Zheng, LC, Cheng, W, Fang, JZ, et al.. Remediation of hexavalent chromium contaminated soil by biochar-supported zero-valent iron nanoparticles. J Hazard Mater 2016;318:533–40. https://doi.org/10.1016/j.jhazmat.2016.07.039.Search in Google Scholar PubMed
26. Chen, X, Fan, GJ, Zhu, XX, Li, HB, Li, YH, Li, H, et al.. The remediation of hexavalent chromium-contaminated soil by nanoscale zero-valent iron supported on sludge-based biochar. J Soils Sediments 2023;23:1607–16. https://doi.org/10.1007/s11368-023-03433-x.Search in Google Scholar
27. Mosaferi, M, Nemati, S, Khataee, A, Nasseri, S, Hashemi, AA. Removal of arsenic (III, V) from aqueous solution by nanoscale zero-valent iron stabilized with starch and carboxymethyl cellulose. J Environ Health Sci 2014;12:74. https://doi.org/10.1186/2052-336x-12-74.Search in Google Scholar PubMed PubMed Central
28. Meng, TT, Sun, P. Variations of deep soil moisture under different vegetation restoration types in a watershed of the Loess Plateau, China. Sci Rep 2023;13:4957. https://doi.org/10.1038/s41598-023-32038-0.Search in Google Scholar PubMed PubMed Central
29. Wei, SX, Du, G, Li, CH, Zhang, LY, Li, JM, Mao, AQ, et al.. Removal mechanism of Pb (ii) from soil by biochar-supported nanoscale zero-valent iron composite materials. RSC Adv 2024;14:18148–60. https://doi.org/10.1039/d4ra03357d.Search in Google Scholar PubMed PubMed Central
30. Li, J, Fan, MJ, Yuan, ZT, Liu, F, Li, M. One-pot synthesis of lamellar Fe-Cu bimetal-decorated reduced graphene oxide and its enhanced removal of Cr(VI) from water. Nanomaterials 2023;13:2745. https://doi.org/10.3390/nano13202745.Search in Google Scholar PubMed PubMed Central
31. Qiu, Y, Zhang, Q, Gao, B, Li, M, Fan, ZX, Sang, WJ, et al.. Removal mechanisms of Cr(VI) and Cr(III) by biochar supported nanosized zero-valent iron: synergy of adsorption, reduction, and transformation. Environ Pollut 2020;265:115018. https://doi.org/10.1016/j.envpol.2020.115018.Search in Google Scholar PubMed
32. Zhang, T, Hu, C, Li, Q, Chen, CX, Hu, JH, Xiao, XY, et al.. Hydrogen peroxide activated by biochar-supported sulfidated nano zerovalent iron for removal of sulfamethazine: response surface method approach. Int J Environ Res Publ Health 2022;19:9923. https://doi.org/10.3390/ijerph19169923.Search in Google Scholar PubMed PubMed Central
33. Zhang, M, Li, JH, Wang, YC. Impact of biochar-supported zerovalent iron nanocomposite on the anaerobic digestion of sewage sludge. Environ Sci Pollut Res 2019;26:10292–305. https://doi.org/10.1007/s11356-019-04479-6.Search in Google Scholar PubMed PubMed Central
34. Chen, XL, Li, F, Xie, XJ, Li, Z, Chen, L. Nanoscale zero-valent iron and chitosan functionalized Eichhornia crassipes biochar for efficient hexavalent chromium removal. Int J Environ Res He 2019;16:3046. https://doi.org/10.3390/ijerph16173046.Search in Google Scholar PubMed PubMed Central
35. Feng, ZY, Zhu, JQ, Zhuo, S, Chen, J, Huang, WY, Cheng, H, et al.. Magnetic/Zeolitic imidazolate framework-67 nanocomposite for magnetic solid-phase extraction of five flavonoid components from Chinese herb Dicranopteris pedata. Molecules 2023;28:702. https://doi.org/10.3390/molecules28020702.Search in Google Scholar PubMed PubMed Central
36. Ma, FF, Zhao, BW, Diao, JR, Jiang, YF, Zhang, J. Mechanism of phosphate removal from aqueous solutions by biochar-supported nanoscale zero-valent iron. RSC Adv 2020;10:39217–25. https://doi.org/10.1039/d0ra07391a.Search in Google Scholar PubMed PubMed Central
37. Siciliano, A. Removal of Cr(VI) from water using a new reactive material: magnesium oxide supported nanoscale zero-valent iron. Materials 2016;9:666. https://doi.org/10.3390/ma9080666.Search in Google Scholar PubMed PubMed Central
38. Zhao, WM, Zhang, TM, Wang, YL, Qiao, JH, Wang, ZR. Corrosion failure mechanism of associated gas transmission pipeline. Materials 2018;11:1935. https://doi.org/10.3390/ma11101935.Search in Google Scholar PubMed PubMed Central
39. Salama, E, Samy, M, Shokry, H, El-Subruiti, G, El-Sharkawy, A, Hamad, H, et al.. The superior performance of silica gel supported nano zero-valent iron for simultaneous removal of Cr(VI). Sci Rep 2022;12:22443. https://doi.org/10.1038/s41598-022-26612-1.Search in Google Scholar PubMed PubMed Central
40. Hilali, N, Mohammadi, H, Amine, A, Zine, N, Errachid, A. Recent advances in electrochemical monitoring of chromium. Sensors 2020;20:5153. https://doi.org/10.3390/s20185153.Search in Google Scholar PubMed PubMed Central
41. Yang, KY, Bu, HJ, Zhang, Y, Yu, HX, Huang, SN, Ke, LX, et al.. Efficacy of simultaneous hexavalent chromium biosorption and nitrogen removal by the aerobic denitrifying bacterium Pseudomonas stutzeri YC-34 from chromium-rich wastewater. Front Microbiol 2022;13:961815. https://doi.org/10.3389/fmicb.2022.961815.Search in Google Scholar PubMed PubMed Central
42. Yang, C, Hao, H, Jia, R, He, Y, Lu, CY, Xue, CH, et al.. Coal tar pitch preoxidation-impregnation regulates pore distribution to prepare porous carbon for electrochemistry. ACS Omega 2025;18.10.1021/acsomega.5c09681Search in Google Scholar PubMed PubMed Central
43. Venkatraman, Y, Arunkumar, P, Kumar, NS, Osman, AI, Muthiah, M, Al-Fatesh, AS, et al.. Exploring modified rice straw biochar as a sustainable solution for simultaneous Cr(VI) and Pb(II) removal from wastewater: characterization, mechanism insights, and application feasibility. ACS Omega 2023;8:38130–47. https://doi.org/10.1021/acsomega.3c04271.Search in Google Scholar PubMed PubMed Central
44. Fernández-López, JA, Angosto, JM, Avilés, MD. Biosorption of hexavalent chromium from aqueous medium with opuntia biomass. Sci World 2014;2014:670249–8. https://doi.org/10.1155/2014/670249.Search in Google Scholar PubMed PubMed Central
45. Liang, MN, Xu, SP, Zhu, YN, Chen, X, Deng, ZL, Yan, LL, et al.. Preparation and characterization of Fe-Mn binary oxide/mulberry stem biochar composite adsorbent and adsorption of Cr(VI) from aqueous solution. Int J Environ Res He 2020;17:676. https://doi.org/10.3390/ijerph17030676.Search in Google Scholar PubMed PubMed Central
46. Zhuang, M, Shi, W, Wang, H, Cui, LQ, Quan, GX, Yan, JL. Carbothermal synthesis of Ni/Fe bimetallic nanoparticles embedded into graphitized carbon for efficient removal of chlorophenol. Nanomaterials 2021;11:1417. https://doi.org/10.3390/nano11061417.Search in Google Scholar PubMed PubMed Central
47. Gómez-Carnota, D, Barriada, JL, Herrero, R. Towards the development of sustainable hybrid materials to achieve high Cr(VI) removals in a one-pot process. Nanomaterials 2022;12:3952. https://doi.org/10.3390/nano12223952.Search in Google Scholar PubMed PubMed Central
48. Gao, N, Guan, QQ, Kong, ZN. A comparative study of confinement and layer-modified Zr-based MOFs for the efficient removal of Cr(VI) from wastewater. RSC Adv 2023;13:15041–54. https://doi.org/10.1039/d3ra01308a.Search in Google Scholar PubMed PubMed Central
49. Liu, F, Wang, XH, Chen, BY, Zhou, SL, Chang, CT. Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers. J Taiwan Inst Chem E 2017;70:401–10. https://doi.org/10.1016/j.jtice.2016.10.043.Search in Google Scholar
50. Luo, K, Lin, NF, Li, X, Pang, Y, Wang, LP, Lei, M, et al.. Efficient hexavalent chromium removal by nano-cerium oxide functionalized biochar: insight into the role of reduction. Environ Chem Eng 2023;11:110004. https://doi.org/10.1016/j.jece.2023.110004.Search in Google Scholar
51. Liu, MY, Chen, G, Xu, LL, He, ZC, Ye, YY. Environmental remediation approaches by nanoscale zero-valent iron (nZVI) based on its reductivity: a review. RSC Adv 2024;14:21118–38. https://doi.org/10.1039/d4ra02789b.Search in Google Scholar PubMed PubMed Central
52. Tang, C, Hu, T, Du, CZ, Liao, ZQ, Cheng, WY, Wang, F, et al.. Fe-N-Doped conjugated organic polymer efficiently enhanced the removal rate of Cr (VI) from water. Polymers 2023;15:2918. https://doi.org/10.3390/polym15132918.Search in Google Scholar PubMed PubMed Central
53. Li, QM, Liu, ML, Qiu, XC, Liu, X, Dapaah, MF, Niu, QJ, et al.. Removal of chromium(VI) by nanoscale zero-valent iron supported on melamine carbon foam. Nanomaterials 2022;12:1866. https://doi.org/10.3390/nano12111866.Search in Google Scholar PubMed PubMed Central
54. Jiao, KR, Deng, B, Song, P, Ding, HL, Liu, HL, Lian, B. Difference analysis of the composition of iron (Hydr) oxides and dissolved organic matter in pit mud of different pit ages in Luzhou Laojiao and its implications for the ripening process of pit mud. Foods 2023;12:3962. https://doi.org/10.3390/foods12213962.Search in Google Scholar PubMed PubMed Central
55. Xie, SJ, Xiao, Y, Huang, L, Li, JX, Yan, J, Li, Q, et al.. The constructing of the oxide phase diagram for fluoride adsorption on La-Fe-Al: a collaborative study of density functional calculation and experimentation. Nanomaterials 2024;14:619. https://doi.org/10.3390/nano14070619.Search in Google Scholar PubMed PubMed Central
56. Du, LL, Gao, P, Liu, YL, Minami, T, Yu, CB. Removal of Cr (VI) from aqueous solution by polypyrrole/hollow mesoporous silica particles. Nanomaterials 2020;10:686. https://doi.org/10.3390/nano10040686.Search in Google Scholar PubMed PubMed Central
57. Yu, CY, Yang, JY. Removal of Cr(VI) in wastewater by Fe–Mn oxide loaded sludge biochar. RSC Adv 2024;14:11746–57. https://doi.org/10.1039/d4ra00169a.Search in Google Scholar PubMed PubMed Central
58. Guo, XY, Zhao, ZL, Gao, XL, Fu, HL, Hu, ZY, Zhang, XY, et al.. Study on the adsorption performance of fly ash loaded on nano-FeS for chromium-containing wastewater treatment. Heliyon 2024;10:e34661. https://doi.org/10.1016/j.heliyon.2024.e34661.Search in Google Scholar PubMed PubMed Central
59. Mokokwe, G, Letshwenyo, MW. Investigation of clay brick waste for the removal of copper, nickel, and iron from aqueous solution: batch and fixed-bed column studies. Heliyon 2022;8:e09963. https://doi.org/10.1016/j.heliyon.2022.e09963.Search in Google Scholar PubMed PubMed Central
60. Fang, XL, Wang, C, Zhou, SW, Cui, PF, Hu, HAZ, Ni, XY, et al.. Hydrogels for antitumor and antibacterial therapy. Gels 2022;8:315. https://doi.org/10.3390/gels8050315.Search in Google Scholar PubMed PubMed Central
61. Qi, JM, Li, B, Zhou, PX, Su, XT, Yang, D, Wu, JX, et al.. Study on adsorption of hexavalent chromium by composite material prepared from iron-based solid wastes. Sci Rep 2023;13:135. https://doi.org/10.1038/s41598-023-27414-9.Search in Google Scholar PubMed PubMed Central
62. Wei, YZ, Chu, R, Zhang, QH, Usman, M, Haider, FU, Cai, LQ. Nano zero-valent iron loaded corn-straw biochar for efficient removal of hexavalent chromium: remediation performance and interfacial chemical behaviour. RSC Adv 2022;12:26953–65. https://doi.org/10.1039/d2ra04650d.Search in Google Scholar PubMed PubMed Central
63. Lu, H, Wang, XY, Cong, Q, Chen, XL, Li, QP, Li, XQ, et al.. Research progress on the degradation of organic pollutants in water by activated persulfate using biochar-loaded nano zero-valent iron. Molecules 2024;29:1130. https://doi.org/10.3390/molecules29051130.Search in Google Scholar PubMed PubMed Central
64. Ji, X, Zhou, CY, Chen, LX, Li, YZ, Hua, TC, Li, Y, et al.. Reduction, mineralization, and magnetic removal of chromium from soil by using a natural mineral composite. Environ Sci Ecotechnol 2022;11:100181. https://doi.org/10.1016/j.ese.2022.100181.Search in Google Scholar PubMed PubMed Central
65. Shu, YR, Ji, B, Cui, BH, Shi, YT, Wang, J, Hu, M, et al.. Almond shell-derived, biochar-supported, nano-zero-valent iron composite for aqueous hexavalent chromium removal: performance and mechanisms. Nanomaterials 2020;10:198. https://doi.org/10.3390/nano10020198.Search in Google Scholar PubMed PubMed Central
© 2026 the author(s), published by De Gruyter, Berlin/Boston
This work is licensed under the Creative Commons Attribution 4.0 International License.
Articles in the same Issue
- Green fabrication of γ- Al2O3 bionanomaterials using Coriandrum sativum plant extract and its photocatalytic and antibacterial activity
- Research Articles
- Dual nanofiller strategy for enhanced mechanical properties in carbon fiber reinforced polyamide blends
- Hydrothermal synthesis of Ricinus communis seed extract-mediated CQDs and their applications as antimicrobial, antioxidant, anti-inflammatory, and photocatalytic agents
- Mechanical and thermal characterization of Kevlar–Basalt hybrid composites reinforced with Al2O3 nanoparticles
- A dual-responsive Luteolin nanoparticles suppresses colorectal cancer growth by targeting tumor angiogenesis
- Heat and mass transfer in MHD-UCM nanofluid flow past a porous nonlinear stretching sheet with chemical reaction and bioconvection: a dual-method approach
- Orally administered dual-responsive insulin nanomedicine for diabetes and diabetic retinopathy
- Comparative RSM-based study of nano, hybrid, and ternary nanofluid heat transfer with Cattaneo–Christov flux in porous media
- Yttrium-doped copper oxide nanoparticles: potential therapeutic applications in metabolic, neurodegenerative, arthritic, and diabetic disorders
- Green synthesis of alginate-encapsulated silver nanoparticles and its utilization as anticancer and antimicrobial therapeutics
- Improving bending fatigue lifetime and interfacial adhesion of Cu–Mn flexible interconnects using self-forming nanolayer
- Fe-doped ZIF-8 for sunlight-driven photocatalytic degradation of methylene blue: synthesis, characterization, and AI-based process modeling
- Synthesis, characterization and biological activity of metal nanoparticles with green method using apple industry waste
- Synthesis of multifunctional chitosan-zinc oxide composite film with antimicrobial activity for packaging applications
- Drop-casted Ag2O:MnO2-nanocomposite thin-films-based gas sensors for improved sensitivity CH4 and NH3 gases: fabrication and unveiling of microstructural morphology, optical, and gas sensing characterizations
- Removal of hexavalent chromium from aqueous systems using jujube tree branch biochar-loaded nano zero-valent iron
- Numerical study of thermal and solutal stratification via Williamson nanofluid model with Cattaneo–Christove heat flux
- Bioinspired synthesis of AgO-ZnO nanocomposite via Cucumis melo leaves: a green catalyst for efficient malachite green degradation and antibacterial applications
- Review Articles
- Integrating spatial analysis in the study of silver nanoparticles for radiotherapy: from molecular mechanisms to bio distribution in biological systems
- Review of thermal conductivity of gold nanoparticle composites
- Nanogenerators for energy harvesting: a holistic review of mechanisms, materials, and emerging applications
- Nanoparticles as SERS substrates: recent progress in hormone sensing
- Special Issue on Green Nanotechnology and Nano-materials for Environ. Sust.
- Green synthesis of metal and metal oxide nanoparticles: a pathway to sustainable energy and sensing applications
Articles in the same Issue
- Green fabrication of γ- Al2O3 bionanomaterials using Coriandrum sativum plant extract and its photocatalytic and antibacterial activity
- Research Articles
- Dual nanofiller strategy for enhanced mechanical properties in carbon fiber reinforced polyamide blends
- Hydrothermal synthesis of Ricinus communis seed extract-mediated CQDs and their applications as antimicrobial, antioxidant, anti-inflammatory, and photocatalytic agents
- Mechanical and thermal characterization of Kevlar–Basalt hybrid composites reinforced with Al2O3 nanoparticles
- A dual-responsive Luteolin nanoparticles suppresses colorectal cancer growth by targeting tumor angiogenesis
- Heat and mass transfer in MHD-UCM nanofluid flow past a porous nonlinear stretching sheet with chemical reaction and bioconvection: a dual-method approach
- Orally administered dual-responsive insulin nanomedicine for diabetes and diabetic retinopathy
- Comparative RSM-based study of nano, hybrid, and ternary nanofluid heat transfer with Cattaneo–Christov flux in porous media
- Yttrium-doped copper oxide nanoparticles: potential therapeutic applications in metabolic, neurodegenerative, arthritic, and diabetic disorders
- Green synthesis of alginate-encapsulated silver nanoparticles and its utilization as anticancer and antimicrobial therapeutics
- Improving bending fatigue lifetime and interfacial adhesion of Cu–Mn flexible interconnects using self-forming nanolayer
- Fe-doped ZIF-8 for sunlight-driven photocatalytic degradation of methylene blue: synthesis, characterization, and AI-based process modeling
- Synthesis, characterization and biological activity of metal nanoparticles with green method using apple industry waste
- Synthesis of multifunctional chitosan-zinc oxide composite film with antimicrobial activity for packaging applications
- Drop-casted Ag2O:MnO2-nanocomposite thin-films-based gas sensors for improved sensitivity CH4 and NH3 gases: fabrication and unveiling of microstructural morphology, optical, and gas sensing characterizations
- Removal of hexavalent chromium from aqueous systems using jujube tree branch biochar-loaded nano zero-valent iron
- Numerical study of thermal and solutal stratification via Williamson nanofluid model with Cattaneo–Christove heat flux
- Bioinspired synthesis of AgO-ZnO nanocomposite via Cucumis melo leaves: a green catalyst for efficient malachite green degradation and antibacterial applications
- Review Articles
- Integrating spatial analysis in the study of silver nanoparticles for radiotherapy: from molecular mechanisms to bio distribution in biological systems
- Review of thermal conductivity of gold nanoparticle composites
- Nanogenerators for energy harvesting: a holistic review of mechanisms, materials, and emerging applications
- Nanoparticles as SERS substrates: recent progress in hormone sensing
- Special Issue on Green Nanotechnology and Nano-materials for Environ. Sust.
- Green synthesis of metal and metal oxide nanoparticles: a pathway to sustainable energy and sensing applications
