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Novel chitosan and Laponite based nanocomposite for fast removal of Cd(II), methylene blue and Congo red from aqueous solution

  • Guobin Xu , Yuejun Zhu , Xiujun Wang , Shanshan Wang , Tianxiang Cheng , Rang Ping , Jie Cao EMAIL logo and Kaihe Lv EMAIL logo
Published/Copyright: May 29, 2019
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e-Polymers
From the journal e-Polymers Volume 19 Issue 1

Abstract

A series of chitosan and Laponite based nano-composite adsorbents, which showed an excellent performance for fast and efficient removal of Cd(II), methylene blue (MB) and Congo red (CR) from aqueous solution, were prepared. In the adsorbent, with the increase of Laponite component, the surface area increased from 44.69 m2 g-1 to 64.58 m2 g-1. As a result, the adsorption rates were enhanced by increasing Laponite component. The adsorption capacities for Cd(II) and MB increased with increasing Laponite component due the cationic characteristic of two pollutants, and the opposite result was found for the removal of CR. The impacts of some factors, e.g. solution pH, temperature, pollutant concentration and salt, on the adsorption capacity were investigated. Additionally, this adsorbent could be effectively regenerated by dilute HCl solution after the adsorption of Cd(II), and the mixture of methanol and acetic acid was a suitable eluent after the adsorption of two dyes.

Graphical Abstract

Keywords: wastewater treatment; fast adsorption; inorganic-organic hybrid; Laponite; nanocomposite

1 Introduction

As a result of rapid urbanization and industrialization, water pollution, which is caused by the continuous release of toxic pollutants from heavy metals, dyes, synthetic manure and organic compounds into the encompassed water bodies, has attracted worldwide attention (1, 2, 3). Cd(II) is considered as one of the most toxic heavy metal ions. Even at very low concentration, the presence of Cd(II) is tremendously harmful to the water environment and human health in terms of renal disturbances, muscular cramps, high blood pressure, lung insufficiency, proteinuria, bone lesions, destruction of red blood cells and testicular tissues (4). Pollution by Cd(II) usually comes from several industries such as electroplating, metal production, alloy manufacturing, plastic, the combustion of fossil fuels, the manufacturing of batteries, pigments, and screens (5,6). Around 100,000 types of dyes are produced with a yearly production rate of over 7 × 105 to 1 × 106 tons and utilized as a part of a few businesses, for example, material, textile, leather, paper, printing, paint, pigments, rubber and plastic (7). Methylene blue (MB), a cationic dye, is widely used for colouring paper, printing cotton, dyeing leather, and indicating oxidation-reduction in analytical chemistry and also used as an antiseptic (8). Congo red (CR), an anionic dye, is used for dyeing plastic and textile and also used as a biological stain (9). From short time and prolonged contact with MB and CR dyes, many diseases can be caused, such as tissue necrosis, cyanosis, and heart beat increase in humans and also threats to marine life (10).

From wastewater treatment, heavy metal ions and dyes have been removed by various technologies, such as chemical precipitation, ultrafiltration, ion exchange, oxidation/reduction, membrane separation and electrochemical technologies (11, 12, 13). However, each method has its inherent limitations, such as high energy consumption, generation of toxic sludge, incomplete treatment, and high reagent requirement (14). The adsorption method has been developed as a suitable process for the treatment of heavy metal ions from contaminated water or soil because of its low cost, great removal efficiency and simple regeneration (15). For preparing effective adsorbent, efficient adsorption group is crucial for adsorbent. The introduction of different adsorption groups into adsorbent is one of the basic methods to improve the adsorption performance of adsorbent. A lot of adsorbents with novel molecular structures and excellent properties have been prepared and studied (16, 17, 18). Additionally, surface area and microstructure are also important for fast adsorption and high removal efficiency (19). High surface area and large amount of micropores in adsorbent is convenient for the penetration of water and heavy metal ions into the interior and thus enhances the adsorption rate and removal efficiency. Additionally, the surface area and microstructure of adsorbent could also affect the utilization of functional groups for adsorption (20,21).

Laponite could be exfoliated into individual particles with a thickness of about 1 nm, and a diameter of about 25 nm in aqueous solution (3). Due to its significant surface area and charged surface, Laponite is suitable for the adsorption of several chemicals (22,23). However, Laponite dispersion cannot be directly mixed with many chemicals to prepare Laponite based nanocomposite because Laponite particles flocculate in salt or polymer solution. It has been reported that nanometer Laponite particles exhibit much better dispersion performance of salt resistance after being pre-adsorbed by 2-acrylamido-2-methylpropanesulfonic acid (AMPS) (24). Chitosan is the most abundant biopolymer after cellulose. Chitosan is regarded as an efficient component in the adsorption of pollutants due to its high content of -NH2 groups and -OH groups, which are conducive to chelating and surface contact with pollutant molecules (17,25). Up to now, chitosan based materials have been prepared and applied in the removal of Cd(II), Pb(II), Cu(II), Hg(II), Congo red, methylene blue, methyl orange, malachite green and tartrazine from aqueous solution (26).

In our previous experiment, we prepared a novel chitosan and Laponite based hybrid nanocomposite adsorbent (3). The introduction of AMPS into the nanocomposite could obviously improve the dispersion stability of Laponite, which increased the surface area and total pore volume of this adsorbent. As a result, this adsorbent exhibited an excellent adsorption capacity and high adsorption rate for Cu(II). To further understand this kind of adsorbent, the removal of Cd(II), MB and CR from aqueous solution was studied in detail in this paper.

2 Experimental

2.1 Materials

Chitosan with a deacetylation degree of 85% was purchased from Jinan Haidebei Marine Bioengineering Co. (China), and purified twice as follows: it was dissolved in HCl solution with a concentration of 0.1 mol L-1, then the solution was filtered and precipitated with ethanol, and the product was dried in vacuo at 50°C for 48 h. The average molecular weight of chitosan was measured by intrinsicviscosity method and the result was 2.4 × 105 g mol-1. Laponite XLG (Mg5.34Li0.66Si8O20(OH)4Na0.66) was obtained from Rockwood (Wesel, Germany). Monomers, including acrylic acid (AA, 98%), acrylamide (AM, 98%), 2-acrylamido-2-methylpropanesulfonic acid (AMPS, 98%), initiator potassium persulfate (KPS, 99%) and crosslinker N,N’-methylenebisacrylamide (MBA, 98%) were obtained from Shanghai Sinopharm Chemical Reagent Co. Ltd. (China). Other chemicals were of analytical grade and used as received.

2.2 Adsorbent preparation

An AA aqueous solution with a concentration of 2 wt% was prepared, and chitosan with a concentration of 5 wt% was dissolved in the solution under agitation for 48 h at room temperature. A transparent Laponite XLG aqueous dispersion was obtained under ultrasonication for 30 min, and the concentration of Laponite was 2 wt%. AMPS was added into the dispersion, and after stirring for 2 h, other chemicals including AM, AA, MBA, and chitosan were added. After stirring for 30 min under a nitrogen atmosphere, 10 g of KPS solution (0.5 wt%) was added and polymerization was performed at 70°C for 4 h. The product was dried under vacuum at 65°C for 48 h and sieved to give 40-20 mesh particles.

2.3 Characterization

The molecular structure of adsorbent was confirmed by Fourier transform infrared spectroscopy (FTIR) on a Tensor 27 spectrometer (Bruker, Switzerland). The sample was prepared as KBr pellet and the result was obtained at a resolution of 4 cm-1 in the frequency range of 4000-400 cm-1 for a total of 16 scans. Thermogravimetric analysis (TGA) of the adsorbent was performed using a thermogravimetric analyzer (Mettler TGA / SDTA851) under a nitrogen atmosphere at a heating rate of 10°C min-1. N2 adsorption-desorption experiment was performed by a specific surface area and pore analyzer (NOVA 3200e, Quantachrome), and the specific surface area, average pore size, and pore size distribution of adsorbent was obtained. Before the experiment, each sample was treated with organic solvents to remove moisture. The specific surface area was calculated according to Brunauer-Emmett-Teller (BET) equation. The volume of adsorbed N2 at a relative pressure of 0.99 was used to calculate the total pore volume. The desorption isotherm was used to study the pore size distribution via the Barrett-Joyner-Halenda (BJH) method. The average pore size D (nm) was calculated by Eq. 1 (27):

(1)D=4Vtotal/ABET

where Vtotal (cm3 g-1) is the total pore volume, and ABET (m2 g-1) is the specific surface area.

2.4 Adsorption experiments

The effects of adsorption time, initial solution pH, temperature, salt concentration and equilibrium pollutant concentration on the adsorption capacity of different adsorbents were analyzed by batch adsorption experiments. For each experiment, 0.050 g of adsorbent was added into 50 mL of pollutant solution. The solution was stirred by a magnetic stirrer through the whole adsorption process. After adsorption, the solution was analyzed using an atomic absorption spectrophotometer (Varian Spectra HP 3510) to determine the residual Cd(II) concentration, whereas the equilibrium concentration of dyes was determined with a UV-1780 (Shimadzu, Japan) spectrophotometer. The adsorption capacity q was calculated by Eq. 2:

(2)q=(c0−ce)×vm

where C0 (mmol L-1) is the initial concentration of pollutant, Ce (mmol L-1) is the concentration of pollutant at equilibrium, V (L) is the volume of solution, and m (g) is the mass of adsorbent used.

2.5 Desorption experiments

After the 90-min adsorption of Cd(II) (50 mL, 500 mg L-1) experiment, the adsorbent (0.05 g) was filtered out from solution. The adsorbed pollutant was removed from the adsorbent being washed in HCl solution (50 mL) for 30 min. And then, the adsorbent was used in the same Cd(II) solution again. For the removal of MB and CR, the regeneration of adsorbent was performed in the same process except that 50 mL of methanol/acetic acid (v/v, 9/1) was used as eluent. The reutilization experiment of adsorbent was performed for 5 consecutive times.

2.6 Model to experimental data

For adsorption kinetic analysis, two models were used to evaluate the time required to remove pollutants.

The pseudo-first-order kinetic model, described as:

(3)qt=qe−qee−k1t

where qt and qe are adsorption capacity (mmol g-1) at time t (min) and at equilibrium respectively, k1 (min-1) is the kinetic rate constant.

The pseudo-second-order kinetic model, defined as:

(4)qt=k2qe2t1+k2tqe

where k2 (g mmol-1 min-1) is the pseudo-second-order rate constant.

For adsorption isotherm analysis, two common isotherm models were employed to fit the experimental data.

The first adsorption isotherm model is the Langmuir isotherm model expressed by Eq. 5:

(5)q=kLqmce1+kLce

where qe (mg g-1) is the equilibrium adsorption capacity of the adsorbent, Ce is the equilibrium pollutant concentration in solution (mg L-1), qm (mg g-1) is the maximum adsorption capacity of the adsorbent, and KL (L mg-1) is the Langmuir adsorption constant.

The second adsorption isotherm model is the Freundlich isotherm model, which is expressed as:

(6)qe=KFCe1/n

where qe and Ce are defined as above, KF (L mg-1) is the Freundlich constant, and n is the heterogeneity factor.

3 Results and discussion

3.1 Characterization of adsorbents

The molecular structures of four samples (Table 1) were analyzed by FTIR analysis, and the results are shown in Figure 1. The FTIR spectra exhibit O-H stretches near 3551 cm-1, the C-H stretching vibrations around 2932 cm-1, and the C-H bending vibrations around 1450 cm-1. The characteristic peaks around 1034 cm-1 and 1184 cm-1 are attributed to the absorption band of sulfonic group. The peak at 1109 cm-1 is classically assigned to the absorption band of polysaccharide molecule backbone, which indicates the introduction of chitosan component in four samples (3). In general, the broad peak at around 1800-1600 cm-1 corresponds to the absorption band of carbonyl group in ketone, carboxylic acid, carboxylate and amide, furthermore, the carbonyl group in carboxylic acid usually exhibits a high absorption peak at around 1750 cm-1. As a result, the peak at 1728 cm-1 could be attributed to the absorption of carboxylic acid, and this is consistent with the adsorbent preparation process (acidic condition). The peaks at 519 and 455 cm-1 are attributed to the absorption band of Si-O from Laponite component, and the absorption intensity of these peaks continuously increases from S-1 to S-4, which is consistent with the gradual increase of the feeding ratio of Laponite (28).

Table 1

The feeding composition of four samples.

SampleAM (g)AA (g)AMPS (g)MBA (g)H2 O (g)Laponite (g)Chitosan (g)KPS (g)Yield (%)
S-15.02.52.50.0390.00.50.50.0598.6
S-25.02.52.50.0390.01.00.50.0598.8
S-35.02.52.50.0390.01.50.50.0598.2
S-45.02.52.50.0390.02.00.50.0597.9

It has been reported that the incorporation of inorganic nanoparticles into polymer matrix is expected to improve its surface properties, such as its surface area and pore diameter (29). The microstructure difference in the four adsorbents was investigated by BET analysis as shown in Figure 2 and Table 2. For S-4, the BET surface area and pore volume are measured to be 64.58 m2 g-1 and 0.1713 cm3 g-1. However, the BET surface area and pore volume for S-1 are only 44.69 m2 g-1 and 0.0772 cm3 g-1, which are much smaller than S-4. This reflects that the increase of Laponite component could enhance the surface area and total pore volume of the adsorbent. For adsorbents with similar molecular structures, higher surface area might be more beneficial for rapid and effective adsorption of pollutant from wastewater. The BJH analysis method was further used for BET results to investigate the pore-size distribution for each sample, and the results are shown in Figure 3. By comparing S-1 and S-3, it is found that S-1 has smaller total pore volume and lower porosity than S-3, and obviously, S-1 has narrower pore-size distribution. Furthermore, it is worth noting that the average pore size also gradually increases with the increase of Laponite component. Thus, for this adsorbent, greater surface area, pore volume, and average pore size are pronounced with more Laponite addition.

Table 2

Structural characteristics of four samples.

SampleABET (m2 g-1)Vtotal (cm3 g-1)D (nm)
S-144.690.07726.90
S-254.450.12168.93
S-361.630.14879.65
S-464.580.171310.61

The results for TGA measurements and differential thermogravimetric analysis (DTGA) for four samples are shown in Figure 4. For any sample, a mass-loss peak could be observed from 200°C to 280°C, which is generally attributed to the thermal decomposition of chitosan

Figure 1 FTIR analysis of four samples.
Figure 1

FTIR analysis of four samples.

Figure 2 N2 adsorption-desorption isotherms of four adsorbents (inset, adsorbed N2 volume at low pressure)
Figure 2

N2 adsorption-desorption isotherms of four adsorbents (inset, adsorbed N2 volume at low pressure)

Figure 3 Pore size distributions of four adsorbents.
Figure 3

Pore size distributions of four adsorbents.

Figure 4 TGA and DTGA curves of four adsorbents.
Figure 4

TGA and DTGA curves of four adsorbents.

component and the imidization reaction between amide groups on polymer backbone (30, 31). The maximum mass-loss peak during the whole thermal degradation process is found at around 330°C, and it is interpreted as the breakdown of the polymer backbone and the imides formed in the above decomposition region. However, as shown in DTGA inset, from S-1 to S-4, the temperature for this mass-loss process is 324°C, 330°C, 337°C and 345°C, respectively, which could indicate that the thermal stability of polymer backbone is reinforced due to the increase of Laponite. Additionally, the residual weight percentage of thermal decomposition at 700°C is in the same order of Laponite concentration because inorganic component is more stable at high temperature.

3.2 Adsorption studies

3.2.1 Adsorption kinetics

From adsorption kinetics investigation, key information about the adsorption rate and the pathway for adsorption process can be obtained, which could mostly determine the potential applications of adsorbent. The relationship between adsorption capacity and contact time by four samples is illustrated in Figure 5. It can be seen that four adsorbents have rapid adsorption rate compared to conventional polymeric adsorbent. The increase of Laponite component in adsorbent will improve the adsorption rate. For example, for the removal of Cd(II), the adsorption equilibrium time for S-3 and S-4 is 20 min, and that for S-2 and S-1 is 25 min and 35 min, respectively. This can be attributed to the increase of surface area and pore volume by the formation of nanocomposite in the adsorbent.

For the removal of Cd(II), the adsorption capacity happens in the order of S-3 (0.47 mmol g-1) > S-4 (0.43 mmol g-1) > S-2 (0.4 mmol g-1) > S-1 (0.36 mmol g-1). For an adsorbent, the adsorption is induced by the combination interaction from its different functional groups. As a result, an optimal ratio between different groups exists in this process (32). For the removal of MB, four adsorbents exhibit excellent adsorption capacities, and the removal efficiency for each sample is more than 95%. A slight increase of adsorption capacity with the increase of the Laponite concentration is found, and S-4 presents the best adsorption capacity (0.31 mmol g-1). Different from the above two cases, for the removal of CR, the adsorption capacity continuously reduces with the increase of the Laponite concentration. CR is an anionic compound and Laponite is a high negatively charged component. The electrostatic repulsion between them could be an unfavorable factor for the adsorption (10). S-1 shows the best adsorption capacity (0.091 mmol g-1), and its removal efficiency is 63.4%.

The adsorption kinetics curve fittings by the pseudo-first-order kinetic model and the pseudo-second-order kinetic model have been carried out and the corresponding parameters were calculated as shown in Table 3. For each adsorbent, the higher coefficient of determination (R2) isobtained for the pseudo-first-order kinetic model. Moreover, the closer agreement of the calculated equilibrium adsorption capacity (qe) from the pseudo-first-order kinetics with the experimentally determined value further proves that the adsorption processes for four adsorbents follow the pseudo-first-order kinetic model. Taking the removal of Cd(II) as an example, the numerical result of k1 (pseudo-first-order model) is in the order of S-4 (0.152) > S-3 (0.148) > S-2 (0.138) > S-1 (0.092). As a result, the increase of Laponite component obviously improves the adsorption rate.

Table 3

Adsorption kinetics constants for Cd(l I), MB and CR adsorbed by four adsorbents.

SampleCd(ll)MBCR
Pseudo-firstPseudo-secondPseudo-firstPseudo-secondPseudo-firstPseudo-second
k1qeR2K2qeR2k1qeR2K2qeR2k1qeR2K2qeR2
S-l0.0920.360.99450.3480.400.96060.0840.290.98520.3780.320.94290.0790.0940.98351.0390.1050.9348
S-20.1380.400.99870.5500.430.97230.0960.300.99090.4260.340.95800.0820.0920.98081.1150.1020.9288
S-30.1480.470.99460.5170.500.95980.1170.310.98810.5740.330.94550.1060.0870.98231.7020.0960.9291
S-40.1520.440.99330.5870.470.95400.1200.310.99190.5620.340.95430.1130.0810.98452 .0140.0880.9345
Figure 5 Adsorption kinetic on the adsorption of (a) Cd(II), (b) MB and (c) CR. (Conditions: initial pollutant concentration 100 mg L−1; pH 6.0; 30°C; adsorbent dosage 1 g L−1).
Figure 5

Adsorption kinetic on the adsorption of (a) Cd(II), (b) MB and (c) CR. (Conditions: initial pollutant concentration 100 mg L−1; pH 6.0; 30°C; adsorbent dosage 1 g L−1).

3.2.2 Adsorption isotherm

The amount of adsorbed pollutant versus equilibrium concentrations, which is also called as adsorption isotherm, are shown in Figure 6, and the corresponding parameters from the Langmuir and the Freundlich models are summarized in Table 4. As the pollutant concentration increases, all adsorbent samples have a similar trend in equilibrium adsorption capacity for the removal of Cd(II), MB or CR. For example, for the removal of Cd(II), the adsorption capacity of S-1 increases from 11.0 to 93.7 mg g-1 with increasing the initial concentration from 20 to 400 mg L-1, and the adsorption capacity shows a slight increase with further increasing the concentration. With the increase of pollutant concentration, the driving force for the adsorption is enhanced. At lower pollutant concentration, almost all pollutant molecules can contact with the active sites on the surface of adsorbent, as a result, a significant enhancement in the adsorption capacity of adsorbent can be observed with the increase of pollutant concentration. However, the adsorption sites will reach saturation at high pollutant concentration, and a balanced adsorption capacity is found (33).

Table 4

Adsorption isotherm constants for Cd(ll), MB and CR adsorbed by four adsorbents.

SampleCd(ll)MBCR
LangmuirFreundlichLangmuirFreundlichLangmuirFreundlich
qeKLR2KfnR2qmKLR2KfnR2qmKLR2KfnR2
S-l139.60.00620.99517.482.380.9507576.60.0230.991265.672.790.9452399.40.00570.999513.682.030.9642
S-2170.10.00700.99649.862.420.9436566.90.0380.991083.603.050.9488394.10.00490.999211.681.960.9738
S-3236.30.00620.993211.282.270.9514563.60.0680.9862110.093.440.9385390.30.00430.997810.211.900.9766
S-4267.80.00440.99508.042.000.9575569.40.0940.9840121.313.530.9306368.30.00380.99518.531.870.9784

The Langmuir model assumes that adsorption occurs at specific homogenous sites of adsorbent and all adsorption sites are energetically equivalent (34). However, the Freundlich model is suitable for describing multilayer adsorption on heterogeneous adsorbent surface. The Freundlich model assumes that the distribution ofadsorption sites is heterogeneous on adsorbent surface (35). In comparison to the Freundlich model, the Langmuir model describes the isotherm data noticeably better for each pollutant. For the removal of Cd(II), the maximum adsorption capacity (qm) is in the order of S-4 (267.8 mg g-1) > S-3 (236.3 mg g-1) > S-2 (170.1 mg g-1) > S-1 (139.6 mg g-1). However, an opposite result is found for the removal of CR, and qm is in the order of S-1 (399.4 mg g-1) > S-2 (394.1 mg g-1) > S-3 (390.3 mg g-1) > S-4 (368.3 mg g-1). For the removal of MB, the greatest qm is obtained by S-1 with a value of 576.6 mg g-1, and lowest qm is obtained by S-3 with a value of 563.6 mg g-1. The interaction between adsorbent and dye molecules generally contains van der Waals forces, hydrogen bond and electrostatic interaction (36). The significant difference in the adsorption capacities between anionic and cationic dyes means the electrostatic interaction might play an important role in the adsorption.

3.2.3 Effect of pH

Since pH is one of the most important factors that may affect the adsorption capacity of adsorbent, the adsorption capacities of S-3 for Cd(II), MB and CR over a pH range of 2.0-6.0, 2.0-10.0 and 2.0-10.0, respectively, were investigated (Figure 7). For the removal of Cd(II), at lowest pH, the adsorption capacity is insignificant and it sharply increases with the increase of pH value. The change of pH value alters the surface charge on the hydrated Cd(II) ion, because pH affects the protonation or deprotonation of surface functional groups (37). pH also affects the charge of functional groups in polymer and Laponite. Increased H+ ion restricts the approach of Cd(II) ion to the functional groups because of the repulsive force (38,39). For the removal of MB, an increase in solution pH apparently favors the adsorption capacity. This could also be due to the cationic characteristic of MB in aqueous solution. In contrast to Cd(II) and MB, CR removal seems to diminish with expanding the pH value. This may be attributed to the effect of acidic media on the surface of the adsorbent. The acidic conditions may cause surface protonation, and the adsorbent surface becomes positively charged (1).

3.2.4 Effect of temperature

Generally, two major effects on the adsorption process could be found from temperature. The temperature dependence of the adsorption system determines the adsorption to be endothermic or exothermic. Due to the decrease in viscosity of the solution, it is believed that increasing the temperature increases the rate of diffusion of the adsorbate. For the removal of Cd(II), there is an

Figure 6 Adsorption isotherm on the adsorption of (a) Cd(II), (b) MB and (c) CR. (Conditions: time 90 min; pH 6.0; 30°C; adsorbent dosage 1 g L−1).
Figure 6

Adsorption isotherm on the adsorption of (a) Cd(II), (b) MB and (c) CR. (Conditions: time 90 min; pH 6.0; 30°C; adsorbent dosage 1 g L−1).

obvious increase with temperature increasing from 20°C to 40°C, and a noticeable decrease with temperature increasing from 40°C to 60°C (Figure 8). The increase of temperature could facilitate the transfer of Cd(II) from aqueous solution into adsorbent, however, higher temperature may also induce the desorption process of pollutant, especially for the pollutant adsorbed by multilayer adsorption (40). For the removal of MB, the adsorption capacity increases from 426.5 mg g-1 to 509.1 mg g-1 when the temperature increases from 20°C to 60°C, and a continuous increase in the adsorption capacity can also be found for the adsorption of CR.

3.2.5 Effect of salt

Because almost all wastewater contains a certain amount of salt, the effect of salt (NaCl and CaCl2) on the adsorption capacities of Cd(II), MB and CR onto S-3 was studied, and the results are shown in Figure 9. For the removal of Cd(II), a continuous decrease of adsorption capacity is observed in each salt solution, which could be attributed to the competitive binding of the cations for the surface functional groups of the adsorbent (41). When the concentration is 0.5 mol L-1, the adsorption capacity is 133.2 mg g-1 and 124.8 mg g-1 for NaCl and CaCl2, respectively. Compared to the monovalent cation, the divalent cation presents a more significant suppressive effect on Cd(II) adsorption, likely due to the stronger complexing ability of functional groups with Ca2+ ion (27). For the removal of MB, it is obvious that two salts play negative role on the performance. When the salt concentration increases from 0 to 0.5 mol L-1, the adsorption capacity decreases from 433.7 mg g-1 to 377.2 and 355.6 mg g-1 for NaCl and CaCl2, respectively. This negative effect might suggest that the ion-exchange interaction plays an important role during the adsorption process because the adsorbent is an anionic substance and MB is a cationic compound (42). With the increase of salt concentration, the active sites on the surface of the adsorbent and the active concentration of MB will decrease, causing the decrease of the interaction between positive charge and negative charge (43). For the removal of CR, the existence of salt is found to be favor for the adsorption property. The adsorption capacity in distilled water, NaCl solution (0.5 mol L-1) and CaCl2 solution

Figure 7 Effect of initial pH on the adsorption capacity of S-3. (Conditions: time 90 min; initial pollutant concentration 500 mg L−1; 30°C; adsorbent dosage 1 g L−1).
Figure 7

Effect of initial pH on the adsorption capacity of S-3. (Conditions: time 90 min; initial pollutant concentration 500 mg L−1; 30°C; adsorbent dosage 1 g L−1).

Figure 8 Effect of temperature on the adsorption capacity of S-3. (Conditions: time 90 min; initial pollutant concentration 500 mg L−1; pH 6.0; adsorbent dosage 1 g L−1).
Figure 8

Effect of temperature on the adsorption capacity of S-3. (Conditions: time 90 min; initial pollutant concentration 500 mg L−1; pH 6.0; adsorbent dosage 1 g L−1).

(0.5 mol L-1) is 218.7, 247.9 and 241.0 mg g-1, respectively. The performance difference between MB and CR is mainly due to their different molecular structures. The introduction of salt may screen the electrostatic interaction between like charges, as a result, the electrostatic repulsion between the negative charged CR and the anionic adsorbent could be reduced (44,45).

3.3 Desorption studies

For practical point of view, reusability is an important feature of an advanced adsorbent because this reduces the material-cost significantly. According to our previous study, different solutions, such as HCl, EDTA, KCl and CaCl2 solutions, were used as eluents for the regeneration

Figure 9 Effects of (a) NaCl and (b) CaCl2 on the adsorption capacity of S-3. (Conditions: time 90 min; initial pollutant concentration 500 mg L−1; pH 6.0; adsorbent dosage 1 g L−1).
Figure 9

Effects of (a) NaCl and (b) CaCl2 on the adsorption capacity of S-3. (Conditions: time 90 min; initial pollutant concentration 500 mg L−1; pH 6.0; adsorbent dosage 1 g L−1).

of this adsorbent after the adsorption of heavy metal ions, and dilute HCl solution was proved to be the most efficient one (3). During the removal process of Cd(II), the regeneration of S-3 with HCl solution (0.1 mol L-1) was investigated as shown in Figure 10. After the first reuse cycle, a slight loss of adsorption capacity is observed, and the adsorption capacity remains broadly stable from the second reuse cycle to the fifth reuse cycle. After the fifth reuse cycle, the equilibrium adsorption capacity decreases from 165.2 mg g-1 to 151.8 mg g-1. The dilute HCl or NaOH solution is not a proper eluent for the desorption of MB or CR Other methods, such as thermal desorption, and eluting with different kinds of organic solution (ethanol, methanol, acetic acid and a mixture of methanol and acetic acid) were tested, and the mixture of methanol and acetic acid (v/v, 9/1) was found to be the best eluent (46,47). After the fifth reuse cycle, the adsorption capacity decreases from 433.7 mg g-1 to 348.6 mg g-1, and from 218.7 mg g-1 to 173.5 mg g-1 for the removal of MB and CR, respectively.

Figure 10 The regeneration and recycle of S-3.
Figure 10

The regeneration and recycle of S-3.

3.4 Comparison of various adsorbents

The removal of heavy metal ions and dyes from aqueous solution has been extensively investigated by many adsorbents. Table 5 compares the adsorption capacities of the nanocomposite obtained in this work with different adsorbents previously used for the removal of pollutants. It can be seen that there are not many adsorbents capable of well adsorbing heavy metal ions, anionic dyes and cationic dyes simultaneously. Therefore, this nanocomposite adsorbent might be used as a universal adsorbent for different pollutants. In addition, the adsorption capacities of the nanocomposite adsorbent are much greater than that of many other previously reported adsorbents indicating that the as-prepared adsorbent has great potential application in pollutant removal from aqueous solution.

Table 5

Comparison of the maximum adsorption capacity of heavy metals, MB or CR onto different adsorbents.

Adsorbentsqm (mg g-1)References
Heavy metalsMBCR
Laponite nanocomposite236.3 (Cd)563.6390.3This work
Serpentine76.3 (Cr)58.593.4(1)
Katira gum based hydrogel–62.573.4(10)
Polymer nanocomposite158.8 (Ni)––(15)
Modified guar gum––226.2(29)
Pyrolysis bio-char–16.9–(42)
Rice husk ash–18.17.0(44)
Chelating resin140.5 (Cd)––(48)
Activated carbon–934.6–(49)
Chitosan nanoparticle12.2 (Cu)––(50)
Magnetic nanoparticle56 (Cd)––(51)
Biogenic wastes43.8 (Cd)251.2157.2(52)

4 Conclusions

We successfully synthesized and characterized a chitosan and Laponite-based nanoparticle-polymer hybrid material. This nanocomposite adsorbent shows an excellent capacity for removing Cd(II), MB and CR from aqueous solution rapidly and efficiently due to its large surface area and total pore volume, which is enhanced by the Laponite component. With the increase of Laponite component in the adsorbent, the adsorption equilibrium time decreases from 35 min to 20 min. Additionally, this adsorbent exhibits a good reusability. In conclusion, this novel nanocomposite adsorbent is a promising material for wastewater treatment.

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Acknowledgements

We appreciate the financial support from the National Natural Science Foundation of China (51874347, 21607174 and U1762212), the Fundamental Research Funds for the Central Universities (17CX02053), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R58).

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Received: 2018-09-21
Accepted: 2018-12-11
Published Online: 2019-05-29

© 2019 Xu et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.

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https://doi.org/10.1515/epoly-2019-0025
Keywords for this article
wastewater treatment; fast adsorption; inorganic-organic hybrid; Laponite; nanocomposite
Creative Commons
BY 4.0

Articles in the same Issue

  1. Special Issue: Polymers and Composite Materials / Guest Editor: Esteban Broitman
  2. A novel chemical-consolidation sand control composition: Foam amino resin system
  3. Bottom fire behaviour of thermally thick natural rubber latex foam
  4. Preparation of polymer–rare earth complexes based on Schiff-base-containing salicylic aldehyde groups attached to the polymer and their fluorescence emission properties
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  6. Effect of different nucleating agent on crystallization kinetics and morphology of polypropylene
  7. Effect of surface modifications on the properties of UHMWPE fibres and their composites
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  16. Polycation-globular protein complex: Ionic strength and chain length effects on the structure and properties
  17. Improving the flame retardancy of ethylene vinyl acetate composites by incorporating layered double hydroxides based on Bayer red mud
  18. N, N’-sebacic bis(hydrocinnamic acid) dihydrazide: A crystallization accelerator for poly(L-lactic acid)
  19. The fabrication and characterization of casein/PEO nanofibrous yarn via electrospinning
  20. Waterborne poly(urethane-urea)s films as a sustained release system for ketoconazole
  21. Polyimide/mica hybrid films with low coefficient of thermal expansion and low dielectric constant
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