Abstract

Modified walnut shell (MWS) was obtained using diethylenetriamine through a grafting reaction and its adsorption capacity toward Cr(VI) was enhanced. The adsorbent was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and elemental analysis and the results showed that the modification was effective. To optimize experimental conditions, the effect of temperature, solution pH, salinity, contact time, and Cr(VI) concentration on adsorption quantity were performed in batch mode. It showed that the adsorption ability for Cr(VI) onto MWH can reach 50.1 mg·g−1 at 303 K with solution pH 3. Both the solution pH and salinity had a great impact on the adsorption capacity. The Langmuir model can predict the equilibrium process while the pseudo-second-order model can describe the kinetic process. The Yan model can be used to predict the column process. Additionally, there was also some regeneration ability for Cr-loaded MWH. Consequently, MWS is effective for removing Cr(VI) from solution.

INTRODUCTION

Water is the source of life. With the rapid development of mining, electroplating, metallurgy and other industries, large amounts of industrial wastewater containing heavy metal ions such as lead, zinc, mercury and chromium have been discharged into various water bodies. The accumulation of heavy metal ions in the environment and the human body is possibly irreversible even at a trace level (Xu et al. 2020), and the pollution of water, air and soil has been a severe issue worldwide. Unlike organic contaminants, heavy metals are non-biodegradable, toxic and easily accumulated in the body through water (Fu & Wang 2011). The heavy metals commonly feature the atomic weights between 63.5 and 200.6 and specific gravities greater than 5.0, and there are about 60 kinds of metallic element (Srivastava & Majumder 2008). The World Health Organization (WHO) and Environmental Protection Agency (EPA) stipulate that the value of the maximum allowable emission volume of heavy metals into the environment is 2.0 mg·L−1 (Kurniawan et al. 2006; Hashim et al. 2011). However, large amounts of water with higher heavy metals concentrations, which surpass the allowable level, are still discharged into the environment and create environmental pollution and serious disorders of human health (Carolin et al. 2017). Thus it is urgent to purify the water environment.

Because of corrosion resistance and aesthetic properties (Carolin et al. 2017), chromium and its compounds have many valence states, and the trivalent and hexavalent states have been widely used in industrial production. In these valence states, Cr(III) is a necessary trace element in mammals (Mohan & Pittman 2006), and the trivalent form is unstable and easily oxidized to a higher valence state in the oxic environment (Qi et al. 2016). But Cr(VI) is the most toxic valence state and easily absorbed by people, and shows strong carcinogenicity and mutagenicity. Therefore, it is necessary to find an effective method to remove the hexavalent chromium ion from water. Many methods have been developed to remove Cr(VI) from wastewater, including physical, chemical and biological techniques, such as adsorption (Lesmana et al. 2009), chemical precipitation (Boamah et al. 2015), reverse osmosis (Carolin et al. 2017), membrane filtration (Salehi et al. 2016), electrochemical treatment (Fu & Wang 2011), bioleaching process (Dinker & Kulkarni 2015) and so on.

Adsorption is one of the most commonly used methods of water treatment, with high efficiency, short cycle, good adaptability, easy operation and other advantages (Hua et al. 2012; Zhou et al. 2019). Currently, the adsorption of Cr(VI) in wastewater has been researched to some extent. For example, natural dolomite and wheat bran modified by tartaric acid have been used to adsorb Cr(VI) in wastewater (Albadarin et al. 2012; Kaya et al. 2014).

Compared with other types of adsorbents such as activated carbons, zeolites, fibers, organic resins and so on, biomass adsorbents from a variety of agro-forestry wastes have been used in water treatment in quantity because of the advantages of low costs, easy operations, being economically acceptable (Zhang et al. 2014) and having good availability (Wartelle & Marshall 2005). Some biomass materials naturally have an adsorption ability for contaminants, while others can be enhanced through a simple modification of surface functional groups to enhance their adsorption ability (Islam et al. 2019). For example, amine groups, quaternary ammonium and metal ions can improve sorption specificity and efficiency (Zhang et al. 2013; Xu et al. 2016).

Among a variety of biomass adsorbents, walnut shell (WS) could be used to remove dyes and heavy metal ions from water directly, due to its rich lignin, cellulose and hemicellulose (Yang et al. 2016; Liu et al. 2019). It also includes several functional groups like the carboxyl group and carbonyl and phenolic hydroxyl groups (Lu & Guo 2019). So, it can be widely used in the removal of heavy metal ions and dyes from solutions. For instance, Aguayo-Villarrel et al. (2013) used walnut shell to adsorb anionic dyes in water. Altun & Pehlivan (2012) used citric acid to modify walnut shell to improve the adsorption capacity for Cr(VI). Ding et al. (2014) used walnut shell modified by nickel chloride and potassium ferricyanide to prepare a new type of adsorbent, which showed satisfying adsorption performance on Cs+, and the adsorption rate was fast, up to adsorption equilibrium within 2 hours.

Amino modification may be an effective method to promote the adsorption ability of walnut shell. The amino groups existing in the surface of the adsorbent can bind heavy metal positive ions through complexation (Deng & Ting 2005). It can also adsorb anionic ions from solution through static-electric attraction with protonated amino groups (Wang et al. 2013; Wang et al. 2016; Xu et al. 2016; Dong et al. 2019). The aim of this study was to prepare modified walnut shell (WS) by diethylenetriamine through grafting, and modified walnut shell (MWS) was obtained. The adsorption of Cr(VI) by MWS was performed. The breakthrough curve is also discussed. Finally, the regeneration performance of the adsorbent is explored.

MATERIALS AND METHODS

Materials

Walnut shell: the raw material was taken locally. After crushing and sieving, walnut shell was taken with particle size between 40 and 60 mesh, rinsed with deionized water, and dried to obtain the original walnut shell (WS).

Potassium chromate was used to prepare the adsorbate solution in the experiment and other common chemical reagents such as absolute ethanol (Fuchen Chemical Reagent, Tianjin, China), sodium hydroxide (Fuchen Chemical Reagent, Tianjin, China), sodium carbonate (Fuchen Chemical Reagent, Tianjin, China), epichlorohydrin (Kermel Chemical Reagent, Tianjin, China) and diethylenetriamine (Kermel Chemical Reagent, Tianjin, China) were all analytical grade.

Preparation of MWS

The walnut shell was modified by a grafting reaction; 5 g WS, 50 mL 2 mol·L−1 sodium hydroxide solution and 50 mL epichlorohydrin were added into a 250 mL Erlenmeyer flask, and the flask was placed in a thermostat oscillator and vibrated for 12 h at room temperature. Then, the mixtures were filtrated, and the solids were put in a new Erlenmeyer flask that containing 50 mL absolute ethanol, 50 mL diethylenetriamine, and a small amount of sodium carbonate. Next, the mixtures were vibrated for 12 h at room temperature to react completely. After that, the mixtures were filtrated and the obtained solids were washed several times by deionized water until neutral. Finally, the desired product, MWS, was dried at 40 °C in an oven. The process is shown as below:

Characterization of WS and MWS

Several analytical techniques were used to characterize the adsorbents. The pH at point zero charge of MWS was evaluated by the 0.01 mol·L−1 NaCl solid addition method. Fourier transform infrared spectroscopy (FT-IR Spectrometer, Nicolet iS50, USA) was used to identify the characteristic functional groups of WS and MWS. The content of nitrogen, carbon and hydrogen elements in WS and MWS were determined by automatic element analyzer (Flash EA 1122, Thermo Fisher Scientific). The crystal textures of WS and MWS were imaged by X-ray powder diffractometer (D/MAX-RA, Japan).

Adsorption experiments

Taken in series, a conical flask and 10 mL Cr(VI) solution (mass/solution volume, 50 mg·L−1 or other concentration) and 0.010 g adsorbent were added in the flask, the conical flask was placed in the thermostatic oscillator (Guohua Enterprise SHZ-82, China) and vibrated for 6 h, filtered, and the residual Cr(VI) concentration of the solution was calculated by spectrophotometer (752, Shanghai Shun Yu Hengping Science Instrument Co., Ltd, China).

Influencing factors include: (1) effect of pH – the pH of the initial adsorbate solutions was controlled by 0.1 mol·L−1 NaOH and HCl solution, (2) effect of salinity, (3) effect of adsorbate concentration and temperature – the initial concentrations of Cr(VI) were adjusted to 10–200 mg·L−1, (4) effect of contact time and adsorbate concentration.

The adsorption quantity of Cr(VI) loaded onto unit weight of MWS was calculated using Equation (1):  
formula
(1)
where V is the volume (L) of Cr(VI) solution, C0 is the initial concentration (mg·L−1) of Cr(VI), Ce is the concentration of Cr(VI) at any time t or equilibrium (mg·L−1), and m is the mass of MWS (g).

Desorption study

A given mass of MWS was added to Cr(VI) solution, of which the concentration was 150 mg·L−1, and which was shaken at a constant temperature of 303 K until the adsorption was saturated, filtered and dried, and then the desorption experiment was performed with 0.01 mol·L−1 NaOH, and the desorption efficiency was calculated. After that, Cr(VI) was re-adsorbed by the treated adsorbent, and the regeneration efficiency was calculated.

The desorption efficiency (d) and regeneration efficiency (η) were calculated using the following equations:  
formula
(2)
 
formula
(3)
where m is the mass of Cr(VI) (mg) which is desorbed from the adsorbent and mc is the remaining Cr(VI) mass on MWS before desorption (mg); qn and qe are the adsorption quantities of regenerative MWS for recycle times and the primitive MWS in the same experimental conditions, respectively.

Column adsorption

The adsorption experiment in a fixed-bed column was performed in one glass column (1 cm ID and 25 cm height), packed with 1.03 g MWS (height 2 cm). A solution with 30.0 mg·L−1 Cr(VI) was pumped in downflow mode at flow rates of 8.0 mL·min−1 using a peristaltic pump. Samples from the effluent were collected at regular intervals to measure the concentration of Cr(VI). Then the breakthrough curve (Ct/C0t) could be obtained.

RESULTS AND DISCUSSION

Characterization of WS and MWS

The pH of point zero charge of WS and MWS in the solution is presented in Figure 1, which suggests that the pH of point zero charge was basically unchanged, at pH 7. At pH < 7, the surface was positively charged in the solution while it was negatively charged at pH > 7.

Figure 1

The zero point charge of WS and MWS.

Figure 1

The zero point charge of WS and MWS.

Elemental analysis showed that the content of common elements was 48.4% for carbon and 6.02% for hydrogen in terms of WS, while it was 47.26% for carbon, 6.59% for hydrogen and the content of nitrogen increased from 0% to 1.69% regarding MWS, which confirmed that the amino groups were introduced onto the surface of WS successfully.

The X-ray diffraction (XRD) spectra of WS and MWS are shown in Figure 2. It was noticed that the positions of the diffraction peak were similar before and after the modification, indicating that the modification might have no great influence on the crystal structures. There were two characteristic diffraction peaks at about 22° and 16°, which represented highly ordered cellulose crystal structure and less-ordered polysaccharide structure respectively.

Figure 2

XRD spectra of WS and MWS.

Figure 2

XRD spectra of WS and MWS.

In order to determine the type of functional groups on the WS and MWS, FT-IR spectra were carried out and are shown in Figure 3. The wide absorption peak at 3,423 cm–1 was due to the stretching vibration of –OH or –NH bonds. The strong peak of WS at 1,741 cm–1 represented the stretching vibration of C = O in acetyl groups or carbonyl groups and the weakening of this peak from MWS was probably caused by a large amount of PEI which was on the surface of WS (Shang et al. 2016). The evident peak at 1,150 cm−1 from the C–N stretching vibration indicated that PEI was introduced on the surface. The absorption peak at 1,244 cm–1 was the characteristic absorption peak of the C–O–C stretching vibration in the alkyl aryl ether bond of WS. After modification, this peak was divided into absorption peaks of 1,228 cm–1 and 1,267 cm–1, and the absorption intensity decreased to some extent. Relatively, the wide absorption peak near 1,038 cm−1 represented the stretching vibration of the C–O bond in alcohol and the increase of the adsorption peak proved that the modification process changed some functional groups on the surface of the walnut shell.

Figure 3

FT-IR spectra of WS and MWS.

Figure 3

FT-IR spectra of WS and MWS.

The results from FT-IR and elemental analysis showed that MWS had some advantages in the adsorption of some pollutants.

Batch adsorption

The effect of pH on adsorption

The pH of the solution often plays an important role in the adsorption process and the effect is shown in Figure 4. When the pH value was in the range of 1–2, the unit adsorption capacity of Cr(VI) by MWS was about 22 mg·g–1; then, qe increased with the increasing pH, and the adsorption capacity reached 28 mg·g–1 when the solution pH value was 3. With the continuous increase of the pH value, the adsorption capacity of Cr(VI) by MWS decreased gradually. At last, qe was as low as 0.91 mg·g–1 at pH 8. The chemical properties of Cr(VI) depend on pH value and its concentration. It is well known that the main form of Cr(VI) is HCrO4 when the pH value is less than 5. With the increasing of pH value, HCrO4 might shift to other forms, CrO42− and Cr2O72− (Cimino et al. 2000). In aqueous solution, Cr(VI) mainly exists in the oxyanion form. There was positive charge on the surface functional groups of the adsorbent like hydroxyl and amine groups due to protonation at lower pH. Thus, the main force between MWS and the Cr(VI) was electrostatic attraction. With the increase of the solution pH value, the degree of the protonation of functional groups in the adsorbent was decreased, which resulted in being less positively charged, and the HCrO4 might shift to CrO42− and Cr2O72−. So the electrostatic attraction was weakened and the adsorption capacity decreased. In order to facilitate the study of the adsorption behavior of Cr(VI) by MWS, the pH value of Cr(VI) solution was adjusted to 3 in subsequent experiments.

Figure 4

The effect of solution pH on adsorption (T = 303 K, C0 = 50 mg·L−1, MWS dosage = 0.01 g, t = 360 min).

Figure 4

The effect of solution pH on adsorption (T = 303 K, C0 = 50 mg·L−1, MWS dosage = 0.01 g, t = 360 min).

The effect of salinity on adsorption

The salinity also has a great impact on the adsorption quantity. The results are shown in Figure 5. The presence of salt had a negative influence on the adsorption quantity of MWS, and qe gradually decreased with the increase of salt concentration. The addition of inorganic salts increased the ionic strength of the solution, reduced the activity coefficient, and the effective concentration of Cr(VI) was decreased, which might weaken the electrostatic attraction between MWS and Cr(VI). The anions in the salt solution might compete with Cr(VI) for the adsorption sites on the surface of MWS, which result in the decrease of adsorption capacity.

Figure 5

The effect of salt concentration on adsorption (T = 303 K, C0 = 50 mg·L−1, MWS dosage = 0.01 g, t = 360 min).

Figure 5

The effect of salt concentration on adsorption (T = 303 K, C0 = 50 mg·L−1, MWS dosage = 0.01 g, t = 360 min).

The effect of contact time on adsorption

Kinetic process is also important for adsorption. The effect of contact time on adsorption quantity is depicted in Figure 6. It is obviously shown in Figure 6 that a three-stage kinetic behavior was evidently observed for MWS: a rapid initial adsorption over 40 min (adsorption quantity: 19.9 mg·g–1), followed by a period of much slower uptake of less than 150 min (27.5 mg·g–1) and gradual equilibrium time to 180 min (28.0 mg·g–1). At the initial stage of the reaction, the adsorption rate increased rapidly, which was probably because a large amount of Cr(VI) occupied the reaction sites on the surface of WS quickly. At the second stage, the adsorption sites became fewer, and the concentration of Cr(VI) also decreased. In addition, the absorbed Cr(VI) had a blocking effect on the absorption process, and therefore, the reaction rate became slow and reached equilibrium at the end.

Figure 6

Effect of contact time on adsorption quantity and kinetic fitted curves (T = 303 K, C0 = 50 mg·L−1).

Figure 6

Effect of contact time on adsorption quantity and kinetic fitted curves (T = 303 K, C0 = 50 mg·L−1).

For WS, a two-stage kinetic process was observed: fast at initial adsorption, then near equilibrium the adsorption quantity was only 5.20 mg·g–1. So the adsorption capacity of MS toward Cr(VI) was significantly enhanced after amino introduction.

Kinetic analysis was performed and the pseudo-second-order kinetic model was used to fit the kinetic data:  
formula
(4)
where qt is adsorption quantity (mg·g–1) at time (t), qe is adsorption quantity at equilibrium (mg·g–1), and k2 (mg·g–1·min–1) is the kinetic rate constant (Ho et al. 2000).

Nonlinear regressive analysis was applied to fit the data and the fitted results are presented in Table 1. The fitted curves are also presented in Figure 6.

Table 1

Parameters of pseudo-second-order kinetic model

qe(exp) (mg·g−1)qe(cal) (mg·g−1)k2 (g·mg−1 ·min−1)R2SSE
WS 5.20 5.44 ± 0.12 0.0261 ± 0.0036 0.973 0.0381 
MWS 28.0 28.7 ± 0.9 0.00266 ± 0.00044 0.960 1.69 
qe(exp) (mg·g−1)qe(cal) (mg·g−1)k2 (g·mg−1 ·min−1)R2SSE
WS 5.20 5.44 ± 0.12 0.0261 ± 0.0036 0.973 0.0381 
MWS 28.0 28.7 ± 0.9 0.00266 ± 0.00044 0.960 1.69 

It is noticed from Figure 6 and Table 1 that there are a higher determination coefficient (R2) and lower values of SSE, and the values of qe from the model are closer to the values of qe from the experiments. Furthermore, the fitted curves are also closer to the experimental curves. So it could be concluded that the pseudo-second-order kinetic model can be used to predict the kinetic process of Cr(VI) adsorption onto WS and MWS. This model consists of all three steps of adsorption: (1) external film diffusion – transport of solute from bulk aqueous phase to the film of adsorbent molecules, (2) surface diffusion – diffusion of solute from the film to the pores of the adsorbent, and (3) intra-particle diffusion – adsorption of solutes onto the interior surface of the pores (Nethaji & Sivasamy 2014; Wu et al. 2015). It also showed that chemisorption may be the main rate-limiting step (Jung et al. 2013).

The effects of adsorbate concentration and reaction temperature on adsorption

The effects of Cr(VI) concentration and solution temperature on values of qe were investigated, and the results are presented in Figure 7.

Figure 7

Adsorption isotherms and Langmuir model fitted curves.

Figure 7

Adsorption isotherms and Langmuir model fitted curves.

It is clearly noticed from Figure 7 that the value of qe gradually increased with the increase of Cr(VI) equilibrium concentration, and the extent of increase was from fast to slow. Finally, the equilibrium adsorption capacity tended to a stable value, and the adsorption was basically at saturation. At different temperatures, three isotherms showed the same trend and the adsorption quantity was to 50.1, 51.1 and 52.8 mg·g–1 from experiments at 303, 313, 323 K, respectively. So the higher temperature was in favor of Cr(VI) adsorption. However, the effect of temperature on the adsorption capacity was not significant.

The Langmuir model was used for the fitting analysis of the equilibrium data (Liu & Liu 2008; Zhao et al. 2017):  
formula
(5)
where qe is the equilibrium adsorption capacity (mg·g–1), qm is the maximum adsorption capacity (mg·g–1), KL is a constant related to the affinity of the binding sites and energy of adsorption (L·mg–1), and Ce is equilibrium concentration (mg·L–1).

The Langmuir model was used to fit the equilibrium data using nonlinear regressive analysis and the results are shown in Table 2. The fitted curves are also shown in Figure 7.

Table 2

Parameters of Langmuir model of Cr(VI) adsorption onto MWS

T/KKL/(L·mg−1)qm(exp)/(mg·g−1)qm(theo)/ (mg·g−1)R2SSE
303 0.0346 ± 0.0016 50.1 58.5 ± 0.9 0.998 0.444 
313 0.0389 ± 0.0033 51.1 60.0 ± 1.7 0.994 1.86 
323 0.0516 ± 0.0046 52.8 60.4 ± 1.6 0.993 2.22 
T/KKL/(L·mg−1)qm(exp)/(mg·g−1)qm(theo)/ (mg·g−1)R2SSE
303 0.0346 ± 0.0016 50.1 58.5 ± 0.9 0.998 0.444 
313 0.0389 ± 0.0033 51.1 60.0 ± 1.7 0.994 1.86 
323 0.0516 ± 0.0046 52.8 60.4 ± 1.6 0.993 2.22 

From the fitting results, the Langmuir model could well describe the adsorption process of Cr(VI) by MWS. The R2 of the fitting were all over 0.99, and the errors were relatively small, and the maximum unit adsorption amounts calculated by the Langmuir model were also close to the experimental results. It was indicated that the adsorption of Cr(VI) could occur on the homogeneous surface by MWS through monolayer adsorption (Wang et al. 2018). Most of the adsorptions in accordance with the Langmuir model were chemisorption.

There have been many studies now on the adsorption capacity of Cr(VI) around the world. Table 3 lists the comparative data of adsorption capacity of Cr(VI) by various adsorbents in other studies. It can be seen that MWS has a good adsorption capacity toward Cr(VI) and there is some competence in application.

Table 3

The comparison of adsorption capacity of Cr(VI) with different adsorbents

Adsorbentqe (mg·g−1)Reference
Dolomite 10.01 Albadarin et al. (2012)  
Modified wheat bran 5.28 Kaya et al. (2014)  
Green coconut shell 22.96 Kumar & Meikap (2014)  
CA-WNS 30.99 Altun & Pehlivan (2012)  
Activated carbon 6.01 Nethaji & Sivasamy (2014)  
Surface modified nano-zeolite 14.16 Tashauoei et al. (2010)  
MWS 50.1 This work 
Adsorbentqe (mg·g−1)Reference
Dolomite 10.01 Albadarin et al. (2012)  
Modified wheat bran 5.28 Kaya et al. (2014)  
Green coconut shell 22.96 Kumar & Meikap (2014)  
CA-WNS 30.99 Altun & Pehlivan (2012)  
Activated carbon 6.01 Nethaji & Sivasamy (2014)  
Surface modified nano-zeolite 14.16 Tashauoei et al. (2010)  
MWS 50.1 This work 

Calculation of thermodynamic parameters

The adsorption of Cr(VI) by MWS was an adsorption equilibrium process. In practice, it could be used to obtain the values of ΔG0, ΔH0, as well as ΔS0. The equilibrium constant (Kc) could be obtained according to the following Equation (6):  
formula
(6)
Ce and Cad,e are the concentrations of Cr(VI) in the solution as well as on MWS.
Calculating Kc, Equations (7) and (8) are used to get values of ΔG0, ΔH0, and ΔS0. Since the calculation of Kc requires the solution to be a very dilute ideal solution, which is difficult to achieve under experimental conditions, this paper used the several initial points on the isotherm to calculate lnKc, and then drew the line of Ce with lnKc. The intercept on the y-axis of the obtained line can be approximately considered as lnKc under ideal conditions.  
formula
(7)
 
formula
(8)
where ΔG0 means the Gibbs free energy (J), T denotes the absolute temperature (K), and R represents the gas constant (8.314 J·mol–1·K–1).

The thermodynamic parameters of adsorption were calculated and the results are shown in Table 4. It can be seen that the adsorption reaction was a spontaneous and entropy-increasing endothermic reaction. The change of enthalpy change was small during the adsorption process, indicating that the adsorption of Cr(VI) by MWS may be physical adsorption. In the adsorption kinetics analysis, the adsorption of Cr(VI) by MWS was consistent with the pseudo-second-order kinetic model, which indicated that there might be chemical reactions in the adsorption process. In the adsorption process, physical adsorption is often accompanied by chemical adsorption, so it was concluded that the adsorption process may have both chemical adsorption and physical adsorption.

Table 4

Thermodynamic parameters of adsorption

ΔH/(kJ·mol–1)ΔS/(J·mol–1·K–1)ΔG/(kJ·mol–1)
303 K313 K323 K
15.7 56.4 –1.47 –1.76 –2.60 
ΔH/(kJ·mol–1)ΔS/(J·mol–1·K–1)ΔG/(kJ·mol–1)
303 K313 K323 K
15.7 56.4 –1.47 –1.76 –2.60 

Desorption and regeneration experiments

Recycling of adsorbent and recovery of adsorbate will make the treatment process economical (Han et al. 2010). Multiple regeneration experiments were conducted on MWS with 0.01 mol·L–1 NaOH solution. The desorption efficiency of three regenerations was not high (19.5%, 20.9%, 18.0%), and the regeneration effect was also not good (71.8%, 60.7%, 51.2%). With the increase of regeneration time, the regeneration efficiency gradually decreased, which indicated that Cr(VI)-loaded MWS was not easily reused through regeneration. This implied the adsorption between Cr(VI) and MWS was probably dominated by chemical adsorption.

Column study

Continuous adsorption can be further performed in column mode. The breakthrough curve of Cr(VI) binding onto MWS is illustrated in Figure 8. It can be seen that the shape of the breakthrough curve was ‘S’ type. The breakthrough time (t0.05) and half breakthrough time (t0.5) (at time of Ct/C0 = 0.5) were 18 min and 130 min, respectively.

Figure 8

Breakthrough curve and fitted curve of Cr(VI) adsorption on MWS (MWS 1.027 g, column height 2 cm, C0 = 30 mg·L−1, v = 8 mL·min−1).

Figure 8

Breakthrough curve and fitted curve of Cr(VI) adsorption on MWS (MWS 1.027 g, column height 2 cm, C0 = 30 mg·L−1, v = 8 mL·min−1).

The adsorption uptake (qe(exp)) was obtained using the following expression:  
formula
(9)
where v, ttotal and A are volumetric flow rate (mL·min–1), total flow time (min), and the area under the breakthrough curve, respectively; m is the dry weight of MWS (g).

According to Equation (9), the value of qe(exp) was 31.2 mg·g–1.

The Yan model was applied as the following expression (Yan et al. 2001; Song et al. 2011; Su et al. 2013):  
formula
(10)
where both a and b are parameters of the Yan model.

Using the nonlinear regressive analysis method, the values of a, b, determination coefficient (R2), SSE and fitted curve are also shown in Figure 8. There was a higher value of R2 and lower value of SSE, and the value of q0 (bC0/m) from the Yan model was 28.1 mg·g–1, smaller than qe(exp). Furthermore, it can be observed from Figure 8 that the fitted curve was closer to the experimental curve. So the Yan model can be selected to predict the column process and adsorption quantity.

CONCLUSION

Compared with the WS, the adsorption performance of Cr(VI) by MWS was obviously improved. Solution pH value had a great influence and the presence of salt was not conducive to Cr(VI) adsorption. The adsorption isotherm was in accordance with the Langmuir model while the kinetic process was well fitted by the pseudo-second-order kinetic model. The Yan model could describe the column process well. As a consequence, MWS can be used to remove Cr(VI) from solution as a promising material.

ACKNOWLEDGEMENTS

This work was financially supported by the Henan province basic and advanced technology research project (142300410224).

REFERENCES

REFERENCES
Aguayo-Villarreal
I. A.
Ramírez-Montoya
L. A.
Hernández-Montoya
V.
Bonilla-Petriciolet
A.
Montes-Morán
M. A.
Ramírez-López
E. M.
2013
Sorption mechanism of anionic dyes on pecan nut shells (Carya illinoinensis) using batch and continuous systems
.
Ind. Crop. Prod.
48
,
89
97
.
Albadarin
A. B.
Mangwandi
C.
Al-Muhtaseb
A. H.
Walker
G. M.
Allen
S. J.
Ahmad
M. N. M.
2012
Kinetic and thermodynamics of chromium ions adsorption onto low-cost dolomite adsorbent
.
Chem. Eng. J.
179
,
193
202
.
Boamah
P. O.
Huang
Y.
Hua
M. Q.
Zhang
Q.
Wu
J. B.
Onumah
J.
Sam-Amoah
L. K.
Boamah
P. O.
2015
Sorption of heavy metal ions onto carboxylate chitosan derivatives – a mini-review
.
Ecotox. Environ. Safe.
116
,
113
120
.
Carolin
C. F.
Kumar
P. S.
Saravanan
A.
Joshiba
G. J.
Naushad
M.
2017
Efficient techniques for the removal of toxic heavy metals from aquatic environment: a review
.
J. Environ. Chem. Eng.
5
,
2782
2799
.
Dong
J. J.
Du
Y. Y.
Duyu
R. S.
Shang
Y.
Zhang
S. S.
Han
R. P.
2019
Adsorption of copper ion from solution by polyethylenimine modified wheat straw
.
Bioresour. Technol. R.
6
,
96
102
.
Han
R. P.
Wang
Y.
Sun
Q.
Wang
L. L.
Song
J. Y.
He
X. T.
Dou
C. C.
2010
Malachite green adsorption onto natural zeolite and reuse by microwave irradiation
.
J. Hazard. Mater.
175
,
1056
1061
.
Hashim
M. A.
Mukhopadhyay
S.
Sahu
J. N.
Sengupta
B.
2011
Remediation technologies for heavy metal contaminated groundwater
.
J. Environ. Manage.
92
,
2355
2388
.
Ho
Y. S.
Ng
J. C. Y.
McKay
G.
2000
Kinetics of pollutant sorption by biosorbents: review
.
Separ. Purif. Method.
29
,
189
232
.
Hua
M.
Zhang
S. J.
Pan
B. C.
Zhang
W. M.
Lv
L.
Zhang
Q. X.
2012
Heavy metal removal from water/wastewater by nanosized metal oxides: a review
.
J. Hazard. Mater
211–212
,
317
331
.
Islam
M. A.
Angove
M. J.
Morton
D. W.
2019
Recent innovative research on chromium (VI) adsorption mechanism
.
Environ. Nanotechno. Monit. Manage.
12
,
100267
.
Jung
C.
Park
J.
Lim
K. H.
Park
S.
Heo
J.
Her
N.
Oh
J.
Yun
S.
Yoon
Y.
2013
Adsorption of selected endocrine disrupting compounds and pharmaceuticals on activated biochars
.
J. Hazard. Mater.
263
,
702
710
.
Kurniawan
T. A.
Chan
G. Y. S.
Lo
W.
Babel
S.
2006
Physico–chemical treatment techniques for wastewater laden with heavy metals
.
Chem. Eng. J.
118
,
83
98
.
Lesmana
S. O.
Febriana
N.
Soetaredjo
F. E.
Sunarso
J.
Ismadji
S.
2009
Studies on potential applications of biomass for the separation of heavy metals from water and wastewater
.
Biochem. Eng. J.
44
,
19
41
.
Liu
Y.
Liu
Y. J.
2008
Biosorption isotherms, kinetics and thermodynamics
.
Separ. Purif. Techno.
61
,
229
242
.
Lu
X. G.
Guo
Y. T.
2019
Removal of Pb (II) from aqueous solution by sulfur-functionalized walnut shell
.
Environ. Sci. Pollut. Res.
26
,
12776
12787
.
Qi
W. F.
Zhao
Y. X.
Zheng
X. Y.
Ji
M.
Zhang
Z. Y.
2016
Adsorption behavior and mechanism of Cr(VI) using Sakura waste from aqueous solution
.
Appl. Surf. Sci.
360
,
470
476
.
Salehi
E.
Daraei
P.
Arabi Shamsabadi
A.
2016
A review on chitosan-based adsorptive membranes
.
Carbohydr. Polym.
152
,
419
432
.
Shang
Y.
Zhang
J. H.
Wang
X.
Zhang
R. D.
Xiao
W.
Zhang
S. S.
Han
R. P.
2016
Use of polyethylenemine-modified wheat straw for adsorption of Congo red from solution in batch mode
.
Desalin. Water Treat.
57
,
8872
8883
.
Song
J. Y.
Zou
W. H.
Bian
Y. Y.
Su
F. Y.
Han
R. P.
2011
Adsorption characteristics of methylene blue by peanut husk in batch and column modes
.
Desalination
265
,
119
125
.
Tashauoei
H. R.
Attar
H. M.
Kamali
M.
Amin
M. M.
Nikaeen
M.
2010
Removal of hexavalent chromium (VI) from aqueous solutions using surface modified nanozeolite A
.
Int. J. Environ. Res.
4
,
491
500
.
Wang
X. G.
Zhang
Y. L.
Li
J.
Zhang
G. Z.
Li
X. M.
2016
Enhance Cr(VI) removal by quaternary amine-anchoring activated carbons
.
J. Taiwan Inst. Chem. Eng.
58
,
434
440
.
Xu
X.
Gao
B. Y.
Jin
B.
Yue
Q. Y.
2016
Removal of anionic pollutants from liquids by biomass materials: a review
.
J. Mol. Liq.
215
,
565
595
.
Xu
C. H.
Shi
S. Y.
Wang
X. Q.
Zhou
H. F.
Wang
L.
Zhu
L. Y.
Zhang
G. H.
Xu
D.
2020
Electrospun SiO2-MgO hybrid fibers for heavy metal removal: characterization and adsorption study of Pb(II) and Cu(II)
.
J. Hazard. Mater.
381
,
120974
.
Yan
G.
Viraraghavan
T.
Chen
M.
2001
A new model for heavy metal removal in a biosorption column
.
Adsorpt. Sci. Technol.
19
,
25
43
.
Yang
F. C.
He
Y. Y.
Sun
S. Q.
Chang
Y.
Zha
F.
Lei
Z. Q.
2016
Walnut shell supported nanoscale Fe0 for the removal of Cu(II) and Ni(II) ions from water
.
J. Appl. Polym. Sci.
133
,
43304
.
Zhang
S. L.
Zhang
R. D.
Xiao
W.
Han
R. P.
2013
Adsorption of chloro-anilines from solution by modified peanut husk in fixed-bed column
.
Water Sci. Technol.
68
,
2158
2163
.
Zhao
B. L.
Xiao
W.
Shang
Y.
Zhu
H. M.
Han
R. P.
2017
Adsorption of light green anionic dye using cationic surfactant-modified peanut husk in batch mode
.
Arab. J. Chem.
10
(
S2
),
S3595
S3602
.
Zhou
Y. B.
Lu
J.
Zhou
Y.
Liu
Y. D.
2019
Recent advances for dyes removal using novel adsorbents: a review
.
Environ. Pollut.
252
,
352
365
.