Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Oct 19;5(42):27323–27331. doi: 10.1021/acsomega.0c03652

Adsorption Mechanism of Hexavalent Chromium on Biochar: Kinetic, Thermodynamic, and Characterization Studies

Xiaoli Guo 1, Aiju Liu 1, Jie Lu 1, Xiaoyin Niu 1, Man Jiang 1, Yanfei Ma 1, Xinpeng Liu 1, Menghong Li 1,*
PMCID: PMC7594145  PMID: 33134695

Abstract

graphic file with name ao0c03652_0008.jpg

The adsorption mechanism of Cr6+ on biochar prepared from corn stalks (raw carbon) was studied by extracting the organic components (OC) and inorganic components (IC). Scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the properties of three kinds of carbon. Kinetic and thermodynamic experiments were performed. The results showed that the experimental data were fitted well by the Freundlich model and the pseudo-second-order kinetic model, and the adsorptions on the three kinds of carbon were all spontaneous, endothermic processes. The adsorption of Cr6+ by biochar was in accordance with a chemisorption process. The adsorption contribution rate of the OC was 97%, which was much higher than that of the IC. Electrostatic attraction and redox reaction were the main mechanisms of adsorption, and among them, the contribution rate of the redox reaction accounted for 61.49%. The reduced Cr3+ could both exchange ions with K+ and dissociate into solution by electrostatic repulsion; the amount of Cr3+ released into the solution was approximately 17.07 mg/g, and the amount of Cr3+ ions exchanged with K+ was 0.29 mg/g. These results further elucidate the adsorption mechanism of Cr6+ by biochar.

1. Introduction

In recent years, heavy metal pollution has posed a serious challenge to the ecological environment and human health because of its cumulative, latent, and nondegradable characteristics.1 Chromium, a highly toxic pollutant, is produced from electroplating, mining, tanning, and other industrial processes.2−4 Chromium is a polyvalent metal which mainly exists in the form of Cr6+ and Cr3+ and has been identified as a top-priority contaminant by the U.S. Environmental Protection Agency because of its high teratogenicity.5,6 Therefore, it is of great significance to explore a simple and effective technique for the elimination of Cr6+ pollution. There are many methods to reduce Cr6+ contamination, such as chemical precipitation, membrane separation, adsorption, and so on.7−9 Among them, adsorption has attracted increasing attention because of its simple, low-cost, and high-efficiency characteristics.

Biochar, a low-cost and environmentally friendly adsorbent,10,11 is a carbon solid produced by relatively low-temperature pyrolysis of biomasses such as wood chips and straws (<700 °C).12 It has been demonstrated that biochar can effectively remove organic pollutants and heavy metals when used as an adsorption material.13 Biochar is a highly heterogeneous material composed of inorganic components (IC, ash) and organic components (OC) containing various oxygen-containing functional groups. Both components likely play important roles in the adsorption process. For example, inorganic mineral composition greatly contributes to the binding of metal cations. However, organic compounds contain many oxygen-containing functional groups, which can interact with metal ions through electrostatic attraction and surface coordination. Lu et al. (2012) suggested that the main adsorption mechanisms of Pb2+ by sludge carbon are complexation and electrostatic adsorption with oxygen-containing functional groups in the OC, co-precipitation with inorganic mineral components, and ion exchange with Ca2+ and Mg2+ in the IC.14 Mohan et al. (2007) showed that Cr6+ could be reduced to Cr3+ by the redox reaction with oxygen-containing functional groups such as hydroxyl and carboxyl in the organic compounds and aromatic groups containing π-electrons.15 Tong et al. (2011) showed that the adsorption of Cu2+ by biochar occurred mainly through the surface complexation of heavy metals with the −COOH or −OH groups of the OC.16 However, Jiang and Xu (2013) partially attributed the adsorption of Cu2+ by biochar to precipitation with alkaline substances such as CO32– in the IC.17 It has been indicated that both the IC and OC in biochar play important roles in the adsorption of metal ions through different mechanisms. At present, research has focused on the removal of pollutants by biochar as a whole. However, studies on how the biochar components adsorb, the mechanisms by which they adsorb, and the quantification of adsorption for a particular metal ion are limited.

In this study, the OC and IC of biochar were prepared by pickling and high-temperature pyrolysis. The adsorption experiments for Cr6+ were performed, and biochar before and after adsorption was characterized. Thermodynamic and kinetic experiments were conducted simultaneously, and the experimental data were fitted with the isotherm models and kinetic equations. The adsorption mechanisms of Cr6+ on biochar were explored through experimental data analyses and characterizations, which provided empirical evidence and theoretical confirmation for the elucidation of the adsorption mechanism of Cr6+ by biochar.

2. Results and Discussion

2.1. XRD Analysis of the Different Components of Biochar before and after the Adsorption of Cr6+

Carbon in raw carbon (RC) existed mainly in the form of graphite (2θ = 26.6) and contained small amounts of KCl (2θ = 28.3), SiO2 (2θ = 22.0), and CaCO3 (2θ = 29.4) (Figure 1a). The pH of the RC was alkaline because of the presence of CaCO3 (Table 1). There was a small amount of KCl, consistent with the conclusion proposed by Guo and Thy et al. that KCl crystals in rice-based biochar are generated by pyrolysis at 500 °C.18,19 The OC were obtained by pickling with HCl and HF, which increased the content of C and eliminated detectable CaCO3 and KCl contents (Figure 1a). This showed that pickling could effectively remove metal and inorganic ions such as Ca2+, K+, and CO32–.20 The pH of the OC was lower than that of the RC (Table 1), possibly due to the replacement of cations in the functional groups such as −COOM (where M is a metal cation) with H+ during the pickling process.21 The proportion of C in the IC (the product of the RC pyrolyzed at 800 °C) was relatively decreased, whereas the quantities of KCl, CaSiO3, and CaCO3 had significantly increased (Figure 1a); these differences in the chemical compositions account for the higher pH of the IC, as compared with that of the OC. This finding is consistent with the conclusion that heat treatment could effectively remove OC from the RC. Some scholars have proposed that KCl starts to decompose at 770 °C. However, the content of KCl in the IC did not decrease but increased, compared with that in the RC. Wang et al. (2012) proposed that the decomposition temperature of KCl crystals in rice straw ranges from 692 to 1011 °C because the silica–carbon components in the IC have a significant influence on the melting and decomposition of KCl, thereby increasing the range of the decomposition temperature of KCl.22

Figure 1.

Figure 1

Open in a new tab

(a) XRD spectra before the adsorption of Cr6+ by RC, OC, and IC; (b–d) XRD comparison patterns before and after adsorption by RC, OC, and IC.

Table 1. Change of pH before and after the Adsorption of Three Kinds of Carbon.

  pH pHpzc optimum adsorption pH solution pH after adsorption mass percentage (%)
RC 9.5 7.6 2.0 2.3  
OC 2.7 6.8 2.0 2.2 77.56
IC 10.9 8.2 2.0 3.4 22.44
Open in a new tab

The C, KCl, and CaCO3 peaks were simultaneously diminished or eliminated after Cr6+ was adsorbed (Figure 1b,d). The IC accounted for 22.44% of the RC after pyrolyzing, and the remainder was mostly composed of the OC (Table 1). The oxygen-containing functional groups in the OC could reduce Cr6+ to Cr3+. Lyubchik et al. (2004) proposed that the adsorption mechanism of Cr3+ by biochar was ion exchange or a combination of ion exchange and surface complexation. Therefore, the generated Cr3+ may undergo ion exchange with K+, which resulted in a significant reduction or elimination of KCl in the IC (Figure 1d).23

2.2. SEM Analysis of Different Components of Biochar before the Adsorption of Cr6+

The RC used here is a by-product of corn stalk, produced by oxygen-limited pyrolysis at 500 °C. Many regularly arranged pores in the RC were visible at a certain magnification (Figure 2a), which indicated that the fatty components had been completely decomposed at 500 °C. These macropores are what remained after the catheter structures in the plant tissue were pyrolyzed. The fiber bundles in the OC were more obvious than those in the RC, and many spherical crystals appeared (Figure 2b). Most of the ash and mineral components had been washed away from the RC by the erosion of the strong acids used for pickling, causing the apertures to become wider and the insoluble residual silica to aggregate as spherical crystals on the surface. The IC, prepared by pyrolysis at 800 °C for 4 h, contained many bar-shaped crystals on the surface (Figure 2c), which may be KCl crystals, according to the analysis shown in Figure 1d.

Figure 2.

Figure 2

Open in a new tab

SEM images of RC, OC, and IC before adsorption.

2.3. Analysis of the Optimal pH and the Adsorption Capacity

Many experiments have shown that the solution pH can significantly affect the removal of Cr6+ by biochar.24−26 Because Cr6+ as an anion contaminant is electronegative, the gravitation or repulsion between carbonaceous materials and Cr6+ would dominate its adsorption behavior. Therefore, exploring the influence of solution pH on the adsorption of Cr6+ was of great significance. The increases in the solution pH of the three kinds of biochar after adsorption were the largest when the pH was 2–4 before adsorption (Figure 3). However, as the initial solution pH increased, the increased value of the solution pH after adsorption gradually declined because the main adsorption mechanisms of Cr6+ by biochar are adsorption–reduction–adsorption and desorption.2 During the adsorption process, the pH of the solutions always remained acidic and less than their respective pHpzc (Table 1), resulting in the three types of carbon having positive surface charges and surface functional groups that remained in a strong protonated state. Cr6+ mainly existed in the form of HCrO4– and Cr2O72– under acidic conditions. Therefore, there was a strong electrostatic adsorption between Cr6+ and the biochar surfaces, and H+ in the solutions was consumed, causing the adsorption capacities to be gradually diminished as the pH increased.

Figure 3.

Figure 3

Open in a new tab

Variation diagram of solution pH and adsorption capacity with the increase of pH before adsorption.

XPS analysis showed the characteristic peaks at 577.5, 579.5 and 587.2, 588.2 eV, corresponding to the Cr 2p3/2 orbital peak and Cr 2p1/2 orbital peak, respectively. The peak at 577.0–578.0 eV was considered to be Cr3+, and the peak at 579.0–579.5 eV was considered to be Cr6+ (Figure 4a).27,28 A portion of Cr6+ was reduced to Cr3+ after being adsorbed by biochar (Figure 4a), which confirmed that reduction was one of the removal mechanisms of Cr6+. Figure 4b shows the infrared spectra before and after the adsorption of Cr6+ by the three kinds of carbon. The vibration absorption peaks of hydroxyl (O–H) at 3440 cm–1 had decreased to varying degrees, and the absorbance of the RC and the OC decreased from 0.8996 to 0.7612 and 0.8489 to 0.7192, respectively. At the same time, the absorbance also significantly decreased at 1480 cm–1 after Cr6+ was absorbed by the RC and OC, and the vibration peak at 1480 cm–1 was attributed to a conjugated π-band (Figure 4b). Chen et al. (2020) proposed that the reduction of Cr6+ was an electron-consuming process, and some electron-donor groups such as hydroxyl and aromatic π-conjugated systems could provide electrons to Cr6+ because of the strong affinities for Cr6+ under acidic conditions.29−31 From what has been discussed above, we determined that acid conditions and oxygen-containing functional groups were favorable to the adsorption and the reduction of Cr6+ into Cr3+ according to the following formulas

2.3. 1
2.3. 2

Figure 4.

Figure 4

Open in a new tab

(a) XPS diagram after RC and OC adsorbed Cr6+, but the XPS adsorption data of Cr6+ by IC were not detected; (b) infrared spectra before and after Cr6+ was adsorbed by RC, OC, and IC.

In the adsorption and desorption stages, the reduced Cr3+ existed as a cation. Meanwhile, the solution pH values after adsorption were still less than their respective pHpzc values (Table 1), which maintained a positive surface charge on the three kinds of biochar. This resulted in electrostatic repulsion between the reduced Cr3+ and the carbon surface, and some of the Cr3+ ions were released into the solution. At the same time, OH– hindered the affinity between Cr6+ and oxygen-containing functional groups with the increase of pH, leading to the significantly lessened reduction of Cr6+.32 Therefore, the adsorption capacity of Cr6+ by biochar decreased greatly with the increase of the initial solution pH. However, the adsorption capacities of the RC and OC for Cr6+ were much greater than that of the IC (Figure 3) because the IC contained fewer functional groups than the RC and OC (Figure 4b). As a result, the electron-donor ability of IC was weaker than that of the RC and OC. In other words, the removal of Cr6+ by the IC was the result of adsorption, whereas that by the RC and OC was the result of double reactions of electrostatic adsorption and redox reaction. Therefore, the adsorption capacity of IC for Cr6+ was smaller than that of the RC and OC and was not detectable using the XPS adsorption analysis of the IC (Figure 4a).

2.4. Adsorption Isotherm

In this study, three adsorption isotherm models [Langmuir, Freundlich (F), and Dubinin–Radushkevich (D–R)] were adopted to fit the adsorption data.33,34 However, the experimental data were more consistent with the Freundlich model and the Dubinin–Radushkevich model.

The formula of the F model is as follows

2.4. 3

where qe is the adsorption capacity per unit mass adsorbent at equilibrium time (mg/g), Ce is the concentration of adsorbate in the solution at equilibrium time (mg/L), KF is the Freundlich constant, and n expresses the adsorption strength. The value of n is usually greater than 1; adsorption occurs easily if 1/n is between 0.1 and 1, and a value otherwise would indicate that adsorption is difficult.35,36 The formula of the D–R model is as follows

2.4. 4

where β is the adsorption energy constant (mol2/kJ2) and ε is the Polanyi potential energy.

2.4. 5
2.4. 6

The average free energy (E) can be calculated using β. A physical adsorption process is indicated by E < 8.0 kJ/mol, and an ion-exchange mechanism is indicated by 8.0 kJ/mol < E < 16.0 kJ/mol.37

The experimental data were applied to the above formulas, and the slopes, intercepts, adsorption constants, and correlation coefficients were obtained accordingly. The value of KF in the F model reflects the difference of adsorption performance with temperature. The KF values of the three kinds of carbon were all increased with an increase in temperature (Table 2), which indicated that the adsorptions of Cr6+ by the three types of carbon were endothermic processes. The 1/n values of the three carbons were all between 0 and 1 (Table 2), indicating that the three kinds of carbon easily adsorbed Cr6+.37 According to the D–R model, E represents the average free energy of the adsorbate transferred from the solution to a solid surface, which is independent of temperature but is dependent on the nature of the adsorbent and adsorbate.38 The E values of the three types of carbon were between 8 and 16 kJ/mol (Table 2), indicating that the adsorption processes of Cr6+ by the three types of carbon occurred by ion exchange,33 which is consistent with the conclusion that the reduction of KCl was due to ion exchange with the reduced Cr3+.

Table 2. Fitting Parameters of the F and D–R Adsorption Isotherm Models.

    F
 D–R
carbon temperature (K) KF 1/n R2 ln qm E R2
RC 293 1.67 0.31 0.997 11.08 12.66 0.995
  303 4.74 0.16 0.991 4.61 10.28 0.997
  313 5.21 0.15 0.999 5.19 11.85 0.987
OC 293 3.22 0.39 0.994 6.28 19.49 0.992
  303 4.55 0.21 0.986 5.61 12.13 0.986
  313 9.88 0.15 0.999 5.95 10.54 0.989
IC 293 1.15 0.27 0.984 6.26 14.71 0.994
  303 3.47 0.17 0.997 6.35 10.98 0.988
  313 8.42 0.09 0.995 8.24 8.84 0.999
Open in a new tab

2.5. Adsorption Thermodynamics

Through the calculation of thermodynamic parameters such as adsorption enthalpy (ΔH), adsorption free energy (ΔG), and adsorption entropy (ΔS), the adsorption mechanisms of Cr6+ on biochar could be further elucidated. The specific formulas are as follows

2.5. 7

where Kd is the distribution coefficient, T is the absolute temperature of the solution (K), and R is the universal gas constant (8.314 J/K mol); ΔH0 and ΔS0 can be obtained by calculating the slope and the intercept. Kd and ΔG0 can be obtained from the following formula

2.5. 8

where qe and Ce are the same as in eq 3. The ΔG0 value can be calculated according to the van’t Hoff equation39

2.5. 9

The ΔG values of the three carbons at the different experimental temperatures were all negative, and their absolute values increased with an increase of temperature (Table 3), which indicated that the adsorptions of Cr6+ by biochar were spontaneous, and the increases of temperature were beneficial to adsorption.40 However, the absolute values of ΔG at the same temperatures were reduced as the initial concentrations increased (Table 3), indicating that the mass transfer driving force was not the factor that controlled the adsorption process. It can be gleaned from the above discussion that the redox reaction between oxygen-containing functional groups and Cr6+ impacted the adsorption process of Cr6+.

Table 3. Thermodynamic Parameters of Different Concentrations of Cr6+ on the Three Kinds of Carbon.

        –ΔG (kJ/mol)
carbon initial concentration (mmol/L) ΔH (J/mol) ΔS (J/mol K) 293 K 303 K 313 K
RC 1 341.27 133.42 146.39 170.02 172.61
  2 139.61 57.78 128.95 137.03 140.39
  3 126.16 28.73 120.41 125.14 126.07
OC 1 387.11 147.15 142.65 161.77 171.88
  2 135.01 56.16 128.59 137.17 139.68
  3 122.83 14.54 124.29 125.11 125.33
IC 1 435.16 166.25 152.56 167.21 185.91
  2 236.59 57.14 129.86 138.55 141.14
  3 122.74 15.69 122.92 125.45 126.01
Open in a new tab

The ΔH values of the three kinds of carbon at three different concentrations were greater than 0 (Table 3), indicating that the adsorption processes were endothermic, and the increase of temperature was favorable to adsorption, consistent with the conclusion of the F model. Generally, the heat of physical adsorption (8.37–62.8 kJ/mol) was less than that of chemical adsorption (125.6–418.68 kJ/mol). The ΔH values of the three carbons were basically in the latter range (Table 3), which indicated that the adsorption mechanism of Cr6+ was chemisorption.41 In addition, if only ion exchange had occurred during the adsorption process, ΔH should be less than 8.4 kJ/mol.42 However, the ΔH values of the three types of carbon were much larger than 8.4 kJ/mol (Table 3), indicating that not only had ion exchange occurred, but other chemical reactions, such as the reduction of Cr6+ to Cr3+, had also occurred during the adsorption process. In short, the adsorption of Cr6+ was a complicated chemical process.

The ΔS value was greater than 0 and increased with an increase of the initial concentrations (Table 3), indicating that both the free movement and the freedom degree of Cr6+ in solution were increased.39 Lyubchik et al. (2004) proposed that the occurrences of ion exchange and displacement reactions lead to the formation of steric hindrance, which increases the entropy in the adsorption process, further illustrating that ion exchange is one of the chemisorption mechanisms that take place during the adsorption of Cr6+.23

2.6. Adsorption Kinetics

The pseudo-first-order (PFO) kinetic model and the pseudo-second-order (PSO) kinetic model were introduced to fit the experimental adsorption data of Cr6+, and the adsorption kinetic curves of Cr6+ on the three types of carbon were analyzed with Origin 9.

The PFO kinetic model is as follows

2.6. 10

The PSO kinetic model is as follows

2.6. 11

The correlation coefficients (R2) in the PFO kinetic model were lower than those of the PSO kinetic model, which led to a great deviation between the experimental values qe(exp) and the theoretical value qe(the) (Figure 5 and Table 4). However, the R2 values of the PSO kinetic model were all above 0.99, and the qe(exp) values were very close to qe(the) (Table 4), indicating that the adsorption of Cr6+ conforms more to the PSO kinetic model. The PSO kinetic model assumes that the adsorption rate is controlled by the chemisorption mechanism, so the rate constant (K2) is determined by the chemisorption rate of heavy metals to biochar.43 The larger the value of K2, the higher is the adsorption rate and the shorter is the time for adsorption to reach equilibrium. The order of K2 values for each carbon type was IC > OC > RC (Figure 5b), indicating that the adsorption equilibrium of the IC was the fastest, followed by the OC and the RC. This finding could be explained as follows: (I) the functional group contents of the IC were much lower than those of the RC and the OC. Therefore, the primary removal mechanism of Cr6+ by the IC was electrostatic adsorption, and the redox reaction between Cr6+ and the IC was correspondingly very weak. As a result, the qe(exp) value of the IC was 4.12 mg/g, which was much lower than that of the RC and OC (Table 4). (II) The pH of the OC (the pickling product of RC) was acidic and much lower than that of the RC, and the relative contents of oxygen-containing functional groups in the OC were higher than that of the RC. Therefore, the adsorption and reduction intensities of the OC for Cr6+ were stronger than that of the RC. Wan et al. (2019) proposed that the adsorption rate of Cr6+ was physically influenced by the porous structure, chemical functional groups, and adsorption sites. The Brunauer–Emmett–Teller (BET) value of the RC was much larger than that of the OC (Table 4), yet the K2 value of the OC was larger than that of the RC.44 This indicates that intraparticle diffusion was not the limiting step of adsorption.

Figure 5.

Figure 5

Open in a new tab

(a) PFO dynamic model diagram; (b) PSO dynamic model diagram.

Table 4. Adsorption Contribution Rates of Biochar Components for Cr6+a.

carbon experimental value qe(exp) (mg/g) theoretical value qe(the) (mg/g) experimental contribution rate (%) theoretical contribution rate (%) BET (m2 g–1)
RC 27.76 28.57     104.000
OC 34.42 35.71 96.17 96.94 13.677
IC 4.12 4.17 3.33 3.28 34.085
Open in a new tab
a

χ2 value = 0.0014 < 0.05; P = 0.999, calculated by Excel 97.

2.7. Adsorption Contribution Rate of Each Component for Cr6+

The adsorption contribution rates of the biochar components for Cr6+ were estimated by considering the mass percentage of the OC and IC (Table 1). The experimental and theoretical contribution rates were close to 1 (Table 4) because of the complexity of the biochar components; a small amount of SiO2 in the OC was not pickled off, and a small amount of C remained in the IC (Figure 1). However, the theoretical and experimental contribution rates were numerically more similar, and χ2 analysis showed that there was little deviation between them, indicating that the adsorption of Cr6+ conformed more to the PSO kinetic model. The contribution rate of the OC reached 96–97%, indicating that the OC played an important role in the removal of Cr6+.

Figure 6a illustrates the adsorption conversion process of Cr6+ by biochar. The adsorption capacity of the RC for Cr6+ was 27.76 mg/g (Table 4), and the content of Cr3+ in solution was 1.48 mg/g when equilibrium was reached (Figure 6b). Therefore, a total of 26.28 mg/g chromium (Cr6+ and Cr3+) had adsorbed on the RC surface. As can be seen in Figure 4a, Cr(VI)a accounted for 40.69% and Cr(III)r1 accounted for 59.31% (mass percentages) after Cr6+ was adsorbed by the RC, so the contents of Cr(VI)a and Cr(III)r1 were 10.69 and 15.59 mg/g, respectively. Cr(III)r was equal to the sum of Cr(III)r1 and Cr(III)r2 when the adsorption reached equilibrium. According to the above analysis, the Cr(III)r content was 17.07 mg/g, which accounted for 61.49% of the experimental adsorption value (27.76 mg/g) and 39.63% of the initial chromium content. The maximum leaching content of K+ reached 0.63 mg/g with the extension of the adsorption time, indicating that there was ion exchange between 0.29 mg/g Cr(III)r3 and K+.

Figure 6.

Figure 6

Open in a new tab

(a) Adsorption conversion diagram of Cr6+ by biochar. (I) Cr(VI)0: in the initial solution; (II) Cr(VI)e: no adsorption; (III) Cr(VI)a: adsorption on the biochar surface; (IV) Cr(III)r: reduced Cr3+; (V) Cr(III)r1: adsorbed on the biochar surface in reduced Cr3+; (VI) Cr(III)r2: released into the solution in reduced Cr3+; (VII) Cr(III)r3: ion exchange with K+ in Cr(III)r2. (b) Content variation of Cr6+, Cr3+, and K+ in solution with the adsorption time.

2.8. Adsorption Mechanism of Biochar for Cr6+

In recent years, adsorption has become the main method for the removal of Cr6+, and exploration of the adsorption mechanisms of Cr6+ through adsorption thermodynamics and adsorption kinetics has become the primary focus of research. The adsorption parameters of Cr6+ by various adsorbents have been listed in the literature (Table 5). The mechanisms of adsorption by the adsorbents were all consistent with the PSO kinetic model, indicating the adsorption of Cr6+ by these materials occurs by chemisorption. Some isotherm types conform to the Langmuir equation, and some conform to the Freundlich equation, depending on the heterogeneity of the adsorbent surface. The optimum adsorption pH of Cr6+ in all studies was 2 because the acidic conditions are conducive to the electrostatic adsorption of Cr6+ by biochar. However, acidity was the only precondition for the adsorption of Cr6+ by biochar. From the view that the adsorption capacity of Cr6+ by the OC was much larger than that of the IC, the redox reaction between the oxygen-containing functional groups on the biochar surface and Cr6+ played a major role in the adsorption process; the oxygen-containing functional groups can provide electrons to Cr6+ for reduction to Cr3+. The generated Cr3+ could be released into the solution by ion exchange with K+ in biochar.

Table 5. Comparison of Parameters among Various Adsorbents.

adsorbent adsorption capacity (mg/g) optimum pH isotherm model kinetic model adsorption form refs
thermal activated + TRB 19.00 2.0 Langmuir PSO model chemisorption (45)
HCl + thermal activated + TRB 25.20 2.0 Langmuir PSO model chemisorption (45)
HCl + TRB 136.00 2.0 Langmuir PSO model chemisorption (45)
carbon nano-onions 20.50 3.4 Langmuir PSO model chemisorption (46)
bismuth-impregnated biochar 12.23 3.0 Freundlich PSO model chemisorption (47)
acid-functionalized nanoporous carbon 35.34 2.5 Langmuir PSO model chemisorption (48)
phosphorylated tamarind nut carbon 25.00 2.0 Langmuir PSO model chemisorption (49)
Eucalyptus globulus bark biochar 21.30 2.0 Langmuir PSO model chemisorption (50)
RC 27.76 2.0 Freundlich PSO model chemisorption this work
OC 34.42 2.0 Freundlich PSO model chemisorption this work
IC 4.12 2.0 Freundlich PSO model chemisorption this work
Open in a new tab

3. Conclusions

This study explored the adsorption mechanism of Cr6+ by extracting the OC and IC of biochar. The removal contribution rate of Cr6+ by the OC was much larger than that of the IC. As the toxicity of Cr3+ is much lower than that of Cr6+, the maximum reduction efficiency of Cr6+ to Cr3+ is the essential step to eliminate Cr6+ pollution. Therefore, it is crucial that biochar with a low ash content and a high number of electron-donor groups is adopted for the removal of Cr6+.

4. Experimental Section

4.1. Preparation of Different Components of Biochar

4.1.1. Preparation of RC

The RC was purchased from Liaoning Jinhefu Technology Co., Ltd. in China. The raw materials for the RC were corn stalks, which were prepared under oxygen-limited conditions at 500 °C for 6 h.

4.1.2. Preparation of Organic Carbon

The RC was fully mixed with an acid mixture (0.3 M HF + 0.1 M HCl), and the RC–acid mixtures with a solid–liquid ratio of 1:250 (mass/volume) were undisturbed for 24 h. The residues were collected, washed thoroughly with deionized water, and dried to obtain the OC.

4.1.3. Preparation of Inorganic Carbon

The RC was pyrolyzed for 4 h at 800 °C in a muffle furnace to obtain the IC.

4.2. Determination of pH and pHpzc in Biochar

The pH of biochar was determined using a method for testing the pH of soil (NY/T 1377-2007, China). pHpzc was used to characterize the surface acidity and alkalinity of the activated carbon. Acid–base titration was adopted to determine the pHpzc value, the specific operation step referred to in ref (51).

4.3. Characterization of Biochar

The surface crystal structure of biochar was measured by XRD (Bruker AXS D8 Advance), and the data were analyzed using JADE 6 and Origin 9. The surface morphology was observed using a scanning electron microscope (FEI Sirion 200). The valence and morphology variation of Cr6+ were measured using an XPS analyzer (THS-103), and the data were analyzed by XPSPEAK41 and Origin 9.

4.4. Adsorption Experiments

4.4.1. Adsorption Thermodynamics Experiment

Different concentrations of K2CrO7 solutions were prepared, and the components of biochar were added to a solid–liquid ratio of 1:200 (mass/volume), followed by mixing in a reciprocating oscillator at 150 rpm for 12 h at 20, 30, or 40 °C. The concentrations of Cr6+ in the supernatants were determined by atomic absorption spectrophotometry after centrifugation, and the concentration of Cr3+ was obtained by subtracting the concentration of Cr6+ from the total amount of chromium.

4.4.2. Adsorption Kinetics Experiment

A 2.0 mM K2Cr2O7 solution was prepared. In accordance with a specific solid–liquid ratio of 1:200, the various components of biochar were added, and the mixture was agitated at 150 rpm in a reciprocating oscillator at 25 °C. The concentrations of Cr6+ were determined after centrifugation at regular intervals until they remained constant. The concentrations of Cr3+ and K+ in the solutions were determined simultaneously. All tests were conducted in triplicate.

4.4.3. Calculation of Adsorption Capacity

The amount of Cr6+ adsorbed on biochar was calculated by the following formula

4.4.3. 12

where qe and Ce are the same as in eq 4. C0 is the initial concentration, V is the volume of the solution (L), and m is the quality of the adsorbent.

Acknowledgments

The research was supported by the National Natural Science Foundation of China (NSFC) (nos. 41771348; 41877122; 41671322; 41703099; 51804188; and 21908134) and the Natural Science Foundation of Shandong Province, China (nos. ZR2018BEE015; ZR2019BB045).

The authors declare no competing financial interest.

References

  1. Zhao N.; Yin Z.; Liu F.; Zhang M.; Lv Y.; Hao Z.; Pan G.; Zhang J. Environmentally persistent free radicals mediated removal of Cr(VI) from highly saline water by corn straw biochars. Bioresour. Technol. 2018, 260, 294–301. 10.1016/j.biortech.2018.03.116. [DOI] [PubMed] [Google Scholar]
  2. Zhang X.; Fu W.; Yin Y.; Chen Z.; Qiu R.; Simonnot M.-O.; Wang X. Adsorption-reduction removal of Cr(VI) by tobacco petiole pyrolytic biochar: Batch experiment, kinetic and mechanism studies. Bioresour. Technol. 2018, 268, 149–157. 10.1016/j.biortech.2018.07.125. [DOI] [PubMed] [Google Scholar]
  3. Qi W.; Zhao Y.; Zheng X.; Ji M.; Zhang Z. Adsorption behavior and mechanism of Cr(VI) using Sakura waste from aqueous solution. Appl. Surf. Sci. 2016, 360, 470–476. 10.1016/j.apsusc.2015.10.088. [DOI] [Google Scholar]
  4. Zhang J.; Chen S.; Zhang H.; Wang X. Removal behaviors and mechanisms of hexavalent chromium from aqueous solution by cephalosporin residue and derived chars. Bioresour. Technol. 2017, 238, 484–491. 10.1016/j.biortech.2017.04.081. [DOI] [PubMed] [Google Scholar]
  5. Zhou L.; Liu Y.; Liu S.; Yin Y.; Zeng G.; Tan X.; Hu X.; Hu X.; Jiang L.; Ding Y.; Liu S.; Huang X. Investigation of the adsorption-reduction mechanisms of hexavalent chromium by ramie biochars of different pyrolytic temperatures. Bioresour. Technol. 2016, 218, 351–359. 10.1016/j.biortech.2016.06.102. [DOI] [PubMed] [Google Scholar]
  6. Ma Y.; Liu W.-J.; Zhang N.; Li Y.-S.; Jiang H.; Sheng G.-P. Polyethylenimine modified biochar adsorbent for hexavalent chromium removal from the aqueous solution. Bioresour. Technol. 2014, 169, 403–408. 10.1016/j.biortech.2014.07.014. [DOI] [PubMed] [Google Scholar]
  7. Zhang X.; Lv L.; Qin Y.; Xu M.; JIa X.; Chen Z. Removal of aqueous Cr(VI) by a magnetic biochar derived from Melia azedarach wood. Bioresour. Technol. 2018, 256, 1–10. 10.1016/j.biortech.2018.01.145. [DOI] [PubMed] [Google Scholar]
  8. Alvarado L.; Torres I. R.; Chen A. Integration of ion exchange and electrodeionization as a new approach for the continuous treatment of hexavalent chromium wastewater. Sep. Purif. Technol. 2013, 105, 55–62. 10.1016/j.seppur.2012.12.007. [DOI] [Google Scholar]
  9. Duan S.; Ma W.; Pan Y.; Meng F.; Yu S.; Wu L. Synthesis of magnetic biochar from iron sludge for the enhancement of Cr (VI) removal from solution. J. Taiwan Inst. Chem. Eng. 2017, 80, 835–841. 10.1016/j.jtice.2017.07.002. [DOI] [Google Scholar]
  10. Albadarin A. B.; Mangwandi C.; Al-Muhtaseb A. a. H.; Walker G. M.; Allen S. J.; Ahmad M. N. M. Kinetic and thermodynamics of chromium ions adsorption onto low-cost dolomite adsorbent. Chem. Eng. J. 2012, 179, 193–202. 10.1016/j.cej.2011.10.080. [DOI] [Google Scholar]
  11. Chen T.; Zhou Z.; Xu S.; Wang H.; Lu W. Adsorption behavior comparison of trivalent and hexavalent chromium on biochar derived from municipal sludge. Bioresour. Technol. 2015, 190, 388–394. 10.1016/j.biortech.2015.04.115. [DOI] [PubMed] [Google Scholar]
  12. Tytłak A.; Oleszczuk P.; Dobrowolski R. Sorption and desorption of Cr(VI) ions from water by biochars in different environmental conditions. Environ. Sci. Pollut. Res. 2015, 22, 5985–5994. 10.1007/s11356-014-3752-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Mohan D.; Sarswat A.; Ok Y. S.; Pittman C. U. Jr. Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent-A critical review. Bioresour. Technol. 2014, 160, 191–202. 10.1016/j.biortech.2014.01.120. [DOI] [PubMed] [Google Scholar]
  14. Lu H.; Zhang W.; Yang Y.; Huang X.; Wang S.; Qiu R. Relative distribution of Pb2+ sorption mechanisms by sludge-derived biochar. Water Res. 2012, 46, 854–862. 10.1016/j.watres.2011.11.058. [DOI] [PubMed] [Google Scholar]
  15. Mohan D.; Pittman C. U. Jr.; Bricka M.; Smith F.; Yancey B.; Mohammad J.; Steele P. H.; Alexandre-Franco M. F.; Gómez-Serrano V.; Gong H. Sorption of arsenic, cadmium, and lead by chars produced from fast pyrolysis of wood and bark during bio-oil production. J. Colloid Interface Sci. 2007, 310, 57–73. 10.1016/j.jcis.2007.01.020. [DOI] [PubMed] [Google Scholar]
  16. Tong X.-j.; Li J.-y.; Yuan J.-h.; Xu R.-k. Adsorption of Cu (II) by biochars generated from three crop straws. Chem. Eng. J. 2011, 172, 828–834. 10.1016/j.cej.2011.06.069. [DOI] [Google Scholar]
  17. Jiang J.; Xu R.-k. Application of crop straw derived biochars to Cu(II) contaminated Ultisol: Evaluating role of alkali and organic functional groups in Cu(II) immobilization. Bioresour. Technol. 2013, 133, 537–545. 10.1016/j.biortech.2013.01.161. [DOI] [PubMed] [Google Scholar]
  18. Guo J.; Chen B. Insights on the molecular mechanism for the Recalcitrance of biochars: Interactive effects of carbon and silicon components. Environ. Sci. Technol. 2014, 48, 9103–9112. 10.1021/es405647e. [DOI] [PubMed] [Google Scholar]
  19. Thy P.; Jenkins B.; Grundvig S.; Shiraki R.; Lesher C. High temperature elemental losses and mineralogical changes in common biomass ashes. Fuel 2006, 85, 783–795. 10.1016/j.fuel.2005.08.020. [DOI] [Google Scholar]
  20. Xie M.; Chen W.; Xu Z.; Zheng S.; Zhu D. Adsorption of sulfonamides to demineralized pine wood biochars prepared under different thermochemical conditions. Environ. Pollut. 2014, 186, 187–194. 10.1016/j.envpol.2013.11.022. [DOI] [PubMed] [Google Scholar]
  21. Xu X.; Cao X.; Zhao L.; Zhou H.; Luo Q. Interaction of organic and inorganic fractions of biochar with Pb(II) ion: further elucidation of mechanisms for Pb (II) removal by biochar. RSC Adv. 2014, 4, 44930–44937. 10.1039/c4ra07303g. [DOI] [Google Scholar]
  22. Wang G.; Shen L.; Sheng C. Characterization of biomass ashes from power plants firing agricultural residues. Energy Fuels 2012, 26, 102–111. 10.1021/ef201134m. [DOI] [Google Scholar]
  23. Lyubchik S. I.; Lyubchik A. I.; Galushko O. L.; Tikhonova L. P.; Vital J.; Fonseca I. M.; Lyubchik S. B. Kinetics and thermodynamics of the Cr (III) adsorption on the activated carbon from co-mingled wastes. Colloids Surf., A 2004, 242, 151–158. 10.1016/j.colsurfa.2004.04.066. [DOI] [Google Scholar]
  24. Yang P.; Guo D.; Chen Z.; Cui B.; Xiao B.; Liu S.; Hu M. Removal of Cr (VI) from aqueous solution using magnetic biochar synthesized by a single step method. J. Dispersion Sci. Technol. 2017, 38, 1665–1674. 10.1080/01932691.2016.1272058. [DOI] [Google Scholar]
  25. Xu X.; Huang H.; Zhang Y.; Xu Z.; Cao X. Biochar as both electron donor and electron shuttle for the reduction transformation of Cr(VI) during its sorption. Environ. Pollut. 2019, 244, 423–430. 10.1016/j.envpol.2018.10.068. [DOI] [PubMed] [Google Scholar]
  26. Dong H.; Deng J.; Xie Y.; Zhang C.; Jiang Z.; Cheng Y.; Hou K.; Zeng G. Stabilization of nanoscale zero-valent iron (nZVI) with modified biochar for Cr(VI) removal from aqueous solution. J. Hazard. Mater. 2017, 332, 79–86. 10.1016/j.jhazmat.2017.03.002. [DOI] [PubMed] [Google Scholar]
  27. Vajpai S.; Taylor P. E.; Adholeya A.; Ackland M. L. Chromium tolerance and accumulation in Aspergillus flavus isolated from tannery effluent. J. Basic Microbiol. 2020, 60, 58–71. 10.1002/jobm.201900389. [DOI] [PubMed] [Google Scholar]
  28. Guo N.; Lv X.; Li Q.; Ren T.; Song H.; Yang Q. Removal of hexavalent chromium from aqueous solution by mesoporous α-FeOOH nanoparticles: Performance and mechanism. Microporous Mesoporous Mater. 2020, 299, 110101. 10.1016/j.micromeso.2020.110101. [DOI] [Google Scholar]
  29. Chen M.; He F.; Hu D.; Bao C.; Huang Q. Broadened operating pH range for adsorption/reduction of aqueous Cr(VI) using biochar from directly treated jute (Corchorus capsularis L.) fibers by H3PO4. Chem. Eng. J. 2020, 381, 122739. 10.1016/j.cej.2019.122739. [DOI] [Google Scholar]
  30. Qiu Y.; Zhang Q.; Gao b.; Li M.; Fan Z.; Sang W.; Hao H.; Wei X. 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. 10.1016/j.envpol.2020.115018. [DOI] [PubMed] [Google Scholar]
  31. Sun Y.; Yu I. K. M.; Tsang D. C. W.; Cao X.; Lin D.; Wang L.; Graham N. J. D.; Alessi D. S.; Komárek M.; Ok Y. S.; Feng Y.; Li X.-D. Multifunctional iron-biochar composites for the removal of potentially toxic elements, inherent cations, and hetero-chloride from hydraulic fracturing wastewater. Environ. Int. 2019, 124, 521–532. 10.1016/j.envint.2019.01.047. [DOI] [PubMed] [Google Scholar]
  32. Usman A. R. A.; Abduljabbar A.; Vithanage M.; Ok Y. S.; Ahmad M.; Ahmad M.; Elfaki J.; Abdulazeem S. S.; Al-Wabel M. I. Biochar production from date palm waste: charring temperature induced changes in composition and surface chemistry. J. Anal. Appl. Pyrolysis 2015, 115, 392–400. 10.1016/j.jaap.2015.08.016. [DOI] [Google Scholar]
  33. Dragan E. S.; Humelnicu D.; Dinu M. V.; Olariu R. I. Kinetics, equilibrium modeling, and thermodynamics on removal of Cr(VI) ions from aqueous solution using novel composites with strong base anion exchanger microspheres embedded into chitosan/poly(vinyl amine) cryogels. Chem. Eng. J. 2017, 330, 675–691. 10.1016/j.cej.2017.08.004. [DOI] [Google Scholar]
  34. Choudhary B.; Paul D. Isotherms, kinetics and thermodynamics of hexavalent chromium removal using biochar. Chem. Eng. J. 2018, 6, 2335–2343. 10.1016/j.jece.2018.03.028. [DOI] [Google Scholar]
  35. Araújo C. S. T.; Almeida I. L. S.; Rezende H. C.; Marcionilio S. M. L. O.; Léon J. J. L.; de Matos T. N. Elucidation of mechanism involved in adsorption of Pb(II) onto lobeira fruit (Solanum lycocarpum) using Langmuir, Freundlich and Temkin isotherms. Microchem. J. 2018, 137, 348–354. 10.1016/j.microc.2017.11.009. [DOI] [Google Scholar]
  36. Zhang H.; Xiao R.; Li R.; Ali A.; Chen A.; Zhang Z. Enhanced aqueous Cr(VI) removal using chitosan-modified magnetic biochars derived from bamboo residues. Chemosphere 2020, 261, 127694. 10.1016/j.chemosphere.2020.127694. [DOI] [PubMed] [Google Scholar]
  37. Rangabhashiyam S.; Anu N.; Giri Nandagopal M. S.; Selvaraju N. Relevance of isotherm models in biosorption of pollutants by agricultural byproducts - review. J. Environ. Chem. Eng. 2014, 2, 398–414. 10.1016/j.jece.2014.01.014. [DOI] [Google Scholar]
  38. Mahanty S.; Bakshi M.; Ghosh S.; Gaine T.; Chatterjee S.; Bhattacharyya S.; Das S.; Das P.; Chaudhuri P. Mycosynthesis of iron oxide nanoparticles using manglicolous fungi isolated from Indian sundarbans and its application for the treatment of chromium containing solution: Synthesis, adsorption isotherm, kinetics and thermodynamics study. Environ. Nanotechnol. Monit. Manag. 2019, 12, 100276. 10.1016/j.enmm.2019.100276. [DOI] [Google Scholar]
  39. Sun X.; Yang L.; Xing H.; Zhao J.; Li X.; Huang Y.; Liu H. High capacity adsorption of Cr(VI) from aqueous solution using polyethylenimine-functionalized poly(glycidyl methacrylate) microspheres. Colloids Surf., A 2014, 457, 160–168. 10.1016/j.colsurfa.2014.05.061. [DOI] [Google Scholar]
  40. Zhu S.; Wang S.; Yang X.; Tufail S.; Chen C.; Wang X.; Shang J. Green sustainable and highly efficient hematite nanoparticles modified biochar-clay granular composite for Cr(VI) removal and related mechanism. J. Cleaner Prod. 2020, 276, 123009. 10.1016/j.jclepro.2020.123009. [DOI] [Google Scholar]
  41. Duranoğlu D.; Trochimczuk A. W.; Beker U. Kinetics and thermodynamics of hexavalent chromium adsorption onto activated carbon derived from acrylonitrile-divinylbenzene copolymer. Chem. Eng. J. 2012, 187, 193–202. 10.1016/j.cej.2012.01.120. [DOI] [Google Scholar]
  42. Tran H. N.; You S.-J.; Hosseini-Bandegharaei A.; Chao H.-P. Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review. Water Res. 2017, 120, 88–116. 10.1016/j.watres.2017.04.014. [DOI] [PubMed] [Google Scholar]
  43. Yang X.-L.; Jiang Q.; Song H.-L.; Gu T.-T.; Xia M.-Q. Selection and application of agricultural wastes as solid carbon sources and biofilm carriers in MBR. J. Hazard. Mater. 2015, 283, 186–192. 10.1016/j.jhazmat.2014.09.036. [DOI] [PubMed] [Google Scholar]
  44. Wan Z.; Cho D.-W.; Tsang D. C. W.; Li M.; Sun T.; Verpoort F. Concurrent adsorption and micro-electrolysis of Cr(VI) by nanoscale zerovalent iron/biochar/Ca-alginate composite. Environ. Pollut. 2019, 247, 410–420. 10.1016/j.envpol.2019.01.047. [DOI] [PubMed] [Google Scholar]
  45. Anandkumar J.; Mandal B. Adsorption of chromium (VI) and Rhodamine B by surface modified tannery waste: Kinetic, mechanistic and thermodynamic studies. J. Hazard. Mater. 2011, 186, 1088–1096. 10.1016/j.jhazmat.2010.11.104. [DOI] [PubMed] [Google Scholar]
  46. Sakulthaew C.; Chokejaroenrat C.; Poapolathep A.; Satapanajaru T.; Poapolathep S. Hexavalent chromium adsorption from aqueous solution using carbon nano-onions (CNOs). Chemosphere 2017, 184, 1168–1174. 10.1016/j.chemosphere.2017.06.094. [DOI] [PubMed] [Google Scholar]
  47. Zhu N.; Yan T.; Qiao J.; Cao H. Adsorption of arsenic, phosphorus and chromium by bismuth impregnated biochar: Adsorption mechanism and depleted adsorbent utilization. Chemosphere 2016, 164, 32–40. 10.1016/j.chemosphere.2016.08.036. [DOI] [PubMed] [Google Scholar]
  48. Pathan S. A.; Pandita N. S. Nanoporous carbon synthesized from grass for removal and recovery of hexavalent chromium. Carbon Lett. 2016, 20, 10–18. 10.5714/CL.2016.20.010. [DOI] [Google Scholar]
  49. Suganthi N. Removal of chromium (VI) from wastewater using phosphoric acid treated activated carbon. AIP Conf. Proc. 2013, 1538, 128–134. 10.1063/1.4810043. [DOI] [Google Scholar]
  50. Choudhary B.; Paul D. Isotherms, kinetics and thermodynamics of hexavalent chromium removal using biochar. Chem. Eng. J. 2018, 6, 2335–2343. 10.1016/j.jece.2018.03.028. [DOI] [Google Scholar]
  51. Ferro-García M. A.; Rivera-Utrilla J.; Bautista-Toledo I.; Moreno-Castilla C. Adsorption of Humic Substances on Activated Carbon from Aqueous Solutions and Their Effect on the Removal of Cr(III) Ions. Langmiur 1998, 14, 1880–1886. 10.1021/la970565h. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES