Activated carbon derived from spherical hydrochar functionalized with triethylenetetramine: synthesis, characterizations, and adsorption application

3.1.1. Textural properties

Figure S2 depicts the variation of N₂ adsorption-desorption isotherms for GACs synthesized with various glucose-TETA ratios, clearly indicating that the adsorption isotherms of all the GACs generated hysteresis loops. Isotherms are typically characterized by mesopores and macropores, with a small external surface area [15]; moreover, a wide-knee hysteresis loop was present in both the adsorption and desorption isotherms, appearing in the multilayer range of physical adsorption isotherms. According to the nomenclature of the International Union of Pure and Applied Chemistry, porous carbon materials exhibit an H4-type hysteresis loop, which is associated with narrow, slit-like pores. In addition, the AC sample without TETA (GAC_0%) exhibits a relatively higher non-micropore volume (0.167 cm³/g) than the ACs samples modified with TETA (GAC_1%−5%; 0.033–0.108 cm³/g). The reason is ascribed to TETA fills in the pores. The addition of TETA can alter the surface properties of GAC_1%−5%.

Table 1 provides a summary of the corresponding textural parameters of the GACs. Increasing the TETA ratios resulted in a decreased surface area (S_BET), non-micropore volume, and total pore volume, but an increase of the micropore volume. Furthermore, the average pore width decreased as the ratio of TETA increased, indicating that the GAC pore size was determined by the amount of TETA in the sample. As expected, the average pore size of GACs was a function of the proportion of the non-micropore (mesopore and macropore) volume.

3.1.2. Morphological properties

The morphologies of GH_0% and GAC_0% are portrayed in Figure 2. The SEM images of both GH_0% and GAC_0% reveal interconnected spheres with relatively uniform sizes, smooth outer surfaces, and regular spherical shapes. According to a study by Sevilla and Fuertes [17], the formation of a carbon-rich solid through the hydrothermal carbonization of glucose is attributed to dehydration, condensation, or polymerization and aromatization reactions. In short, the carbon-rich hydrochar particle consists of two parts: a hydrophobic core comprising a highly aromatic nucleus, and a hydrophilic shell comprising a high concentration of reactive oxygen functional groups such as hydroxyl-phenolic, carbonyl, or carboxyl [17], [18].

Figure 2:

Scanning electron microcopy-energy dispersive X-ray (SEM-EDX) spectra of the (A) glucose-derived hydrochar (GH)_0% and (B) glucose-activated carbon (GAC)_0% samples.

Figure 2 also indicates that GH_0%exhibited a higher average diameter (0.82–0.92 μm) than that of GAC_0% (0.38–0.43 μm), suggesting that the size of carbon spheres is narrowly affected by the chemical activation process; the SEM results also revealed that the spherical morphology of GAC_0% possesses a similar particle size. This means that the regular shape of carbon spheres was still maintained after chemical activation. Interconnected particle properties of GH_0%and GAC_0% afforded more facile adsorbent separation from aqueous solutions (i.e. filtration with a microfilter) because of the overall increase in particle size [19].

As displayed in Figure 3, there were substantial changes to the surface morphology of the samples following TETA modification. The samples initially possessed extremely rough surfaces, and the results demonstrated that TETA was successfully grafted onto the surface of GAC during our experiment. Moreover, the morphologies of GH_1% and GAC_1% were nearly identical, again confirming that chemical activation with NaOH did not cause noticeable alterations to the morphology of GACs. Therefore, NaOH activation only modified the surface of GACs, without affecting the morphology and the bulk carbon core. This conclusion is consistent with findings in the extant literature [20].

Figure 3:

Scanning electron microcopy-energy dispersive X-ray (SEM-EDX) spectra of the (A) glucose-derived hydrochar (GH)_1% and (B) glucose-activated carbon (GAC)_1% samples.

The EDX spectra displayed in Figures 2 and 3 reveal that elements containing C and O are primary elements; for example, hydochar is a carbon-rich solid that is approximately 93–96% carbon. The higher percentage of Na in the GAC samples indicated that the presence of neither the functional hydrolyzed-carboxylic acid group (–COO–Na⁺ or –COO⁻ under aqueous conditions) nor the functional hydrolyzed-phenolic group (–O–Na⁺ or –O⁻ under aqueous conditions) was strongly correlated with the high cationic affinity of the GAC samples [19]. The presence of carboxylic and phenolic groups on the surface of GHs and GACs was additionally confirmed by the FTIR analysis results (Figure 4). Moreover, the higher percentage of oxygen in the GAC samples suggests that the GAC samples had more acidic functional groups than the GH samples. Noticeably, the percentage of N element is lower than the detection limitation because of low impregnation of TETA (only 1%); therefore it cannot be identified in EDX analysis.

Figure 4:

Fourier transform infrared (FTIR) spectra of the glucose activated carbon (AC) samples at various triethylenetetramine (TETA) impregnation ratios.

3.1.2. Functional group characterizations

The functional groups of GAC_0% and its precursor (GH_0%) were identified using FTIR (Figure S3). For the GH_0% sample, two peaks identified at 1965 cm⁻¹ and 1200 cm⁻¹ were involved in the stretching of the C=O and C–O bonds in “pristine” carboxyl groups, respectively (Figure S3). Furthermore, the band at 1600 cm⁻¹ is related to the aromatic C=C ring stretching motion, and the band at 3400 cm⁻¹ confirms the presence of oxygen in hydrochar in the form of –OH. The presence of aromatic C–H out-of-plane bending vibrations is evident by the peak at 798 cm⁻¹ (including aldol condensation and dehydration) [7]. As researchers have previously noted, the appearance of C=O groups on the surface of glucose hydrochar is due to the dehydration of equatorial hydroxyl groups [3], [17].

The FTIR spectrum of GAC_0%(Figure S3) revealed similar peaks to those of GH_0% (i.e. C=O, C=C, C–O, –OH, and C–H). New vibration bands at 2919 cm⁻¹ and 2850 cm⁻¹ corresponded to asymmetrical and symmetrical C–H, respectively. The peaks related to the carboxyl groups in GAC_0% noticeably shifted toward lower wavelengths after the sample underwent chemical activation with NaOH, compared with the peaks in GH_0% groups; specifically, the C=O and C–O peaks decreased from 1659 cm⁻¹ to 1590 cm⁻¹ and from 1200 cm⁻¹ to 1030 cm⁻¹, respectively. In addition, the marked increase in band intensity at 3390 cm⁻¹ indicated that new –OH groups can form during chemical activation.

Qualitative information on the functional groups present on the TETA-modified GAC_x% surfaces and their spectroscopic assignments is displayed in Figure 4. The bands observed at approximately 3370 cm⁻¹ are attributed to the stretching vibrations of the primary amine (–NH₂) group overlapped with the stretching band in the hydroxyl groups (O–H) centered at 3400–3500 cm⁻¹ [21]. The bands found in the 2800–3000 cm⁻¹ range belong to asymmetrical and symmetrical C–H stretching vibrations of the methyl (–CH₃–) and methylene (–CH₂–) groups. The presence of a carbon-carbon triple bone (C≡C) in disubstituted alkynes can also be inferred from the bands in the range of 2250–2400 cm⁻¹. The sharp peaks recognized between 1500 cm⁻¹ and 1690 cm⁻¹ are attributed to the stretching vibration of C=O in the secondary amide group [22] or –NH₂ scissor frequencies [21], [23]. The C=O bond in the carboxyl group clearly shifted to lower wavelengths (i.e. deformation vibrations) after reaction with TETA. Specifically, the C=O bond in the pristine carboxyl group peaked at 1590 cm⁻¹ (Figure S3), but after TETA modification, the peak decreased to approximately 1580 cm⁻¹ (Figure 4). The bands observed at around 1400 cm⁻¹ also reveal the aromatization (C=C) of the GAC_x% samples. Finally, the bands located between 960 cm⁻¹ and 1130 cm⁻¹ are characteristic of stretching C–O groups; the low intensity in these bands confirms the existence of strong reactions between pristine carboxyl groups and TETA. The reaction between the carboxyl groups and TETA, which converts the C=O in carboxyl groups into C=O in the amide groups, was introduced by Brady and Duncan [24] and occurs as follows:

3.2.1. Adsorption isotherms for lead and copper

Figures 5 and 6 show the adsorption isotherms of Pb²⁺ and Cu²⁺ ions on the hydrochars and ACs at various impregnation ratios of TETA and glucose. After the surfaces of the GH samples were modified with TETA, the Q^o_max values of Pb²⁺ and Cu²⁺ increased by 29%–114% and 15%–245%, respectively. This result indicates that the TETA surfactant effectively bonds metal ions. The maximum adsorptive amounts of the Pb²⁺ions were ordered as follows: GH_1%>GH_3%>GH_2%>GH_4%>GH_5%>GH_0%. Conversely, the ordering of Cu²⁺adsorption was GH_5%>GH_1%≈GH_3%>GH_2%>GH_0%. If the oxygen-containing groups on the surface of GH_0% (–COOH and –OH) are assumed to take primary responsibility for binding Cu²⁺ and Pb²⁺ ions, the enhanced adsorption capacities of the metal cations for GH_1%−5%are mainly attributable to the complex reactions between Cu²⁺ and Pb²⁺ ions with the amino groups (–NH₂); other scholars have confirmed this finding [26], [27]. As expected, the GH samples exhibited higher adsorption capacities of Pb²⁺ and Cu²⁺ than the commercial AC did (Table 2).

Figure 5:

Adsorption isotherms of Pb²⁺ ions by various (A) glucose hydrochar and (B) activated carbon (AC) samples.

Figure 6:

Adsorption isotherms of Cu²⁺ ions by various (A) glucose hydrochar and (B) activated carbon (AC) samples.

The GAC_1%−5% samples with TETA-modified surfaces possessed higher adsorption capacities toward heavy metals compared with their precursors (GH_1%−5%), with increasing ratios of approximately 189–316% for Pb²⁺ and 160–720% for Cu²⁺ (Table 2). Notably, the adsorption capacity of GAC_0% for Pb²⁺ and Cu²⁺ was greatly enhanced, compared with that of GH_0%, increasing to 391% and 704%, respectively. The result demonstrates that chemical activation with NaOH can effectively increase the cation exchange capacity. Furthermore, other adsorption mechanisms – complex reactions with oxygen functional groups (–COOH and –OH) and pore filling – might improve the binding capacities for GAC_x% samples.

The Q^o_maxvalues of GACs modified with TETA are slightly higher than those without TETA, appropriately 6%–31% for Pb²⁺and 2%–20% for Cu²⁺. Therefore, we can conclude that neither amine groups had minor contributions in the adsorption mechanisms of the surface modified GACs samples nor several amine groups might be destroyed during the carbonization at 800°C. The maximum diminished theoretical adsorption capacities for Pb²⁺were ordered as follows: GAC_3%>GAC_1%>GAC_4%>GAC_5%>GAC_2%>GAC_0%; by contrast, the ordering of Cu²⁺ was GAC_5%>GAC_4%=GAC_2%>GAC_1%> GAC_3%>GAC_0%.

Generally, the adsorption efficiencies (Q^o_max; mmol/g) of Cu²⁺ were remarkably higher than those of Pb²⁺, in both GH_x% and GAC_x% (Table 2). The adsorption amount order can be explained by the chemical properties of the ions (i.e. hydrated ionic radius, ionic potential, electronegativity, charge density, first hydrolysis equilibrium constant, and ionic radius) [28], which are summarized in Table S1.

As expected, most of the GH and GAC samples that were functionalized with TETA in this study provided greater adsorption capacities for Cu²⁺ and Pb²⁺ compared with commercial AC (Table 2) under the same experimental conditions, indicating that TETA is efficient in binding metal cations in the solutions.

3.2.2. Adsorption isotherms for phenol, MG5, and AR1

The adsorption isotherms of phenol, MG5, and AR1 on the GH and GAC samples are depicted in Figures 7–9. For the samples modified with TETA (GH_1%−5% and GAC_1%−5%), their adsorption capacities toward phenol, MG5, and AR1 were inversely proportional to the glucose-TETA mass ratios (Table 2). The adsorption capacities of TETA-modified GACs decreased in the following order: GAC_1%>GAC_2%> GAC_4%>GAC_3%>GAC_5% for phenol, GAC_1%>GAC_2%>GAC_4%> GAC_5%>GAC_3% for AR1, and GAC_1%>GAC_2%>GAC_3%>GAC_4%>GAC_5%for MG5. A similar decreasing tendency was found in the TETA-modified GH samples.

Figure 7:

Adsorption isotherms of phenol by various (A) glucose hydrochar and (B) activated carbon (AC) samples.

Figure 8:

Adsorption isotherms of methylene green 5 (MG5) by various (A) glucose hydrochar and (B) activated carbon (AC) samples.

Figure 9:

Adsorption isotherms of acid red 1 (AR1) by various (A) glucose hydrochar and (B) activated carbon (AC) samples.

Several possible mechanisms of the phenol compound adsorption have been postulated in the literature [29], [30], [31]: (1) electrostatic attractions, (2) π–π dispersion interactions, (3) hydrogen bonding, and (4) pore filling.

The mechanisms of electrostatic attractions between the positive protonated amine groups (–NH₃⁺) on the surface of adsorbents and phenolate anions (C₆H₅O⁻) are not relevant to this study, and thus are not discussed. This is because the amine groups on the surface of adsorbents can be protonated to –NH₃⁺ under an acidic condition (solution pH controlled at approximately 5.0), but the phenol, a very weak acid, exists primarily in a nondissociated form in water because its pK_ais approximately 9.89.

Dispersion interactions between the π-electron in phenol and the π-electron in carbon can be the dominant mechanism of adsorption in aromatic inorganic compounds. However, the π–π interaction is greatly affected by the π-electron density in the basal plane of the carbonous materials. The abundance of oxygen- and nitrogen-containing functional groups (i.e. the electron-withdrawing groups) on the surface of carbonous materials can result in a drop in π-electron density and adsorption sites in the basal planes (graphene layers) because of π-electron localization and withdrawal [31]. This explanation is valid for the GH_0% sample, which has rich oxygen-bearing groups on its surface and possesses the lowest adsorption capacity (approximately 11 mg/g), and is consistent with the findings of other researchers [31], [32], [33]. Furthermore, increasing TETA ratios reduces adsorption capacities (Table 2 and Figure 7) because of an overabundance of amino groups linked to a smaller surface, which can weaken the π–π interactions between the aromatic rings of the adsorbate and the adsorbent matrix [34].

For hydrogen-bonding interactions (i.e. physical adsorption), it is necessary to classify the types of H-bonding formation. First, hydrogen bonding occurs between hydrogen from the hydroxyl group of phenol, and the oxygen complex on the surface of the adsorbent. Lorenc-Grabowska and colleagues [29] proposed that if hydrogen bond formation is the mechanism of phenol adsorption, a strong competition between water and phenol molecules at low phenol concentration leads to reduced adsorption capacity [30]; thus, water molecules are much more competitive in adsorbing on these groups compared with the hydrophobic phenol molecules [35]. However, concave downward adsorption isotherms were obtained for all the GH and GAC samples (Figure 7), indicating only a weak competition between phenol and water adsorption. Therefore, the hydrogen bond interaction in this study is negligible, evidenced in particular by the low adsorption capacity of GH_0% (approximately 11 mg/g). Second, hydrogen bonding occurs between the nitrogen groups on the adsorbent surface and the hydroxyl group of the phenol; similar results are available in the literature [34]. Additionally, amine groups have a dominant role in the adsorption of phenolic compounds onto amine-modified hyper-crosslinked polymeric resin through hydrogen bonding. This mechanism can explain why the adsorption capacities of GH_1%−2%and GAC_1%−2%were higher than GH_0% and GAC_0%(Table 2 and Figure 7), respectively.

The phenol adsorption capacity of GAC_0% (approximately 122 mg/g) was extremely higher than that of GH_0% because of the pore filling mechanism, indicating that the micropore filling was the other central mechanism of phenol adsorption onto GAC_0%. Phenol adsorption mainly occurs in micropores smaller than 1.4 nm, and the average pore width of the GAC_0% sample in this study was 2.58 nm (Table 1); an analogous result has been noted in prior studies [16], [29]. Furthermore, nitrogenation introduces nitrogen-containing functional groups onto the carbon surface, such as –NH₂, pyridinic, pyrrolic, and quaternary nitrogen groups [14], which enhance the basicity of the carbon surface [36]. Phenols are more strongly removed from a solution by a basic carbon than an acidic carbon [37]; therefore, the samples functionalized with TETA always exhibited higher adsorption capacities than did those without TETA (Table 2 and Figure 7).

The adsorption capacity of MG5 onto GAC_0% (approximately 157 mg/g) was higher than that of GAC_1%−5%(approximately 27.4–101 mg/g), which might be attributed to the higher cation exchange of GAC_0% than GAC_1%−5%; this coincides with the higher Na content on the GAC_0% (5.61%) than GAC_1% (3.80%) surfaces. As Romero and others [19] reported, ion exchange plays a critical role in the adsorption of MG1 onto NaOH-AC spheres with abundant surface-grafted functional groups.

The adsorption capacity of GAC_0% (175 mg/g) for MG5 was nearly the same as that of commercial AC (178 mg/g) under the same experimental conditions; as Figure S5 illustrates, this study similarly found an excellent relationship between the maximum adsorption capacity (Q^o_max) of MG5 by GACs and non-micropore surface area (R²=0.97) and nonmicropore volume (R²=0.96). These correlated values demonstrate that the adsorption of MG5 by GACs was governed by non-micropore filling.

The maximum adsorption capacities of the TETA-modified hydrochar samples were nearly equal to those of the AC samples with TETA: GH_5% (27.5 mg/g)≈GAC_5% (27.4 mg/g), GH_4% (55.9 mg/g)≈GAC_4% (52.9 mg/g), and GH_3% (53.5 mg/g)≈GAC_2% (59.2 mg/g). This result indicates that TETA has a positive effect on the adsorption enhancement of MG5.

Clearly, the affinities of the GH samples toward AR1 were lower than those toward phenol and MG5, because the negative charge for AR1 can generate the repulsion forces of the functional groups (i.e. –COO⁻) on the hydrochar surface. If the amine groups (–NH₂) on the hydrochar samples with TETA are completely protonated to –NH₃⁺ under an acidic condition (pH=approximately 5.0), then the adsorption capacity of the hydrochar samples with TETA are enhanced because of electrostatic attraction. In this study, however, the amine groups (–NH₂) on the TETA-modified hydrochar samples were not protonated to –NH₃⁺ in the solution; therefore, the adsorption mechanisms of AR1 onto the hydrochar sample with TETA did not involve the amine groups through an anion exchange process, and the hydrogen bonding interactions between the nitrogen- and oxygen-containing functional groups on the dye molecule and the adsorbents’ surface might be responsible for the adsorption mechanisms [38], [39]. Similar to the adsorption of MG5, GAC_0% (approximately 156 mg/g) also exhibited a higher affinity toward AR1 than GAC_1%−5%(approximately 49–145 mg/g); moreover, the adsorption of AR1 by the GAC_0% (156 mg/g) and GAC_1% (145 mg/g) samples was higher than that of commercial AC (129 mg/g), indicating that GAC_1% became an excellent adsorbent.