At present, heavy metal chromium pollution mainly comes from metallurgy, leather, dye, mining, electroplating and other industries^[1]. It seriously threatens the environmental safety of soil and water body. The chromium in the wastewater emitted into the environment mainly exists in two valence states, Cr(III) ion and Cr(VI) ion^[2]. Among them, Cr(III) ions in solution have almost no toxicity. However, Cr(VI) ion is carcinogenic, teratogenic and mutagenic, and highly toxic even at low concentrations^[3]. In addition, the Cr(VI) compound has strong migration ability, enrichment and strong oxidizing ability. These properties make Cr(VI) ions more dangerous and difficult to be treated^[4,5,6]. Therefore, research on pollution control of chromium wastewater becomes a hot spot in the control of environmental pollution^[7,8,9].

Adsorption technology is a simple and effective method for treatment of heavy metal pollution^[10,11]. Among the adsorbent materials, biochar is a kind of highly aromatic and difficult to capacity solid material formed by waste biomaterials under high temperature thermal cracking under conditions of oxygen deficiency or low oxygen^[12,13]. They are mostly composed of loose porous, ordered aromatic rings. They have well-developed pore structure, large specific surface area, dense surface charge and rich in carboxyl, sulfhydryl and phenolic hydroxyl groups^[14,15,16]. As a kind of agricultural and forestry waste, peanut shells are used as fuel, except for a small number of them used to make feed and building materials. This is harmful to the environment, and wastes resources. Based on above advantages, peanut shells are often used to prepare for biochar^[17].

In recent years, various natural clay materials have been used in the adsorption research of heavy metal ions due to their low cost and easy reproducibility^[18]. The experimental results showed that natural clays, such as bentonite, kaolin and diatomaceous earth, have strong adsorption capacity for heavy metal ions because of their negative charge^[19]. However, its adsorption capacity for negatively charged Cr(VI) is relatively weak. Therefore, it is generally necessary to modify the natural clay to adsorb the Cr(VI) in solution^[20,21,22].

The objective of this study is to prepare two clay- biochars and to investigate the effect of water chemical characteristics (e.g., initial solution pH, ionic strength, Cr(VI) concentration and contact time) on Cr(VI) adsorption on two clays-biochars by batch techniques. The highlight of this study is to apply biochar-based composites for decontamination of heavy metals in environmental clean up.

Kaolin and bentonite (>99.5% purity) are purchased from Sinopharm Chemical Reagent Co., Ltd. Peanut shell is obtained from vegetable market of Shaoxing city (Zhejiang province, China).

Two clay-biochars of peanut shell@Kaolin (Biochar @Kaolin) and peanut shell@Bentonite (Biochar@Bentonite) are obtained by mixing biochar derived peanut shell and two kinds of clays under magnetic stirring conditions. Briefly, the peanut shell is firstly washed and then dried at 60 ℃. The pure peanut shell is milled into powders. The peanut shell powder is filled in ceramic crucibles covered with fitted lid. Ceramic crucibles are put in a muffle furnace and burned under oxygen-limited conditions at 250 ℃ for 2 h. Then the biochar derived from peanut shell is cooled to room temperature. The obtained biochar derived from peanut shell is powdered and passed through a 50 μm(100 mesh) screen.

Then, 30 g biochar, 30 g Kaolin and Bentonite are added to 600 mL deionized water, and reacted for 2 h under vigorous stirring condition. Then, the suspensions are centrifuged at 6000 r/min for 10 min. The Biochar@Kaolin and Biochar@Bentonite are obtained by drying at 105 ℃ overnight and sieved through a 150 μm (100 mesh) screen.

Adsorption experiments are conducted in 250 mL Erlenmeyer flasks containing the clay-biochar and 100 mL Cr(VI) ion solutions with various initial concentrations. The initial pH is adjusted with HCl or NaOH solution. The flasks are placed in a shaker at a constant temperature and 200 r/min. The samples are filtered and analyzed.

The morphology of clay-biochar was observed with SEM (JEOL 6500F, Japan) and TEM (JEM-F200, Japan). XRD analysis was conducted in a D/Max-IIIA Powder X-ray Diffractomer (Rigaku Corp., Japan). NOVA 4200e Surface area and Pore size analyzer (Quantachrome, FL, USA) were used to analyze the surface area and pore size of clay-biochar at a relative pressure of 0.95. XPS (X-ray photoelectron spectrometer, Krato AXIS Ultra DLD, Japan) and the model Axis-HS (Kratos Analytical) were used to determine surface composition. FT-IR spectra of the samples were recorded on a Nexus 670 FT-IR spectrometer (Thermo Nicolet, Madison) in the wavenumber range of 400-4000 cm^-1.

The concentration of Cr(VI) ion is analyzed by atomic absorption spectrophotometry.

The q_t(mg/g) is calculated as follows:

where C₀ (mg/L) and C_t (mg/L) are the initial and equilibrium liquid-phase concentrations of Cr(VI) ion respectively. V (L) is the solution volume and m (g) is the mass of adsorbent used.

All experiments were repeated in duplicate, and the data were analyzed by the mean and standard deviation (SD). The value of the SD was calculated by Excel Software. All error estimates given in the text and error bars in figures were standard deviation of means (mean± SD). All statistical significance was noted at α=0.05 unless otherwise noted.

The surface area and average pore size of Biochar, Biochar@Kaolin and Biochar@Bentonite are listed in Table 1. As listed in Table 1, the BET specific surface area of Biochar@Kaolin and Biochar@Bentonite are 6.15 and 3.08 m²/g respectively, which is higher than that of biochar (2.79 m²/g). The total pore volume and the average pore width of Biochar@Kaolin and Biochar@Bentonite are also higher than that of original Biochar. Clay contains a proportion of mineral element, which have small surface areas and rich transitional pores, and the added clay does not block pore openings of the biochar^[23].

The surface characteristics of Biochar@Kaolin and Biochar@Bentonite are shown in Fig. 1 and Fig. 2. As shown in Fig. 1 and Fig. 2, Biochar@Kaolin and Biochar@Bentonite have rough surface with some fine particles, which indicate that the Kaolin and Bentonite successfully load on the Biochar.

EDS spectra of the Biochar@Kaolin and Biochar@Bentonite before and after the reaction with Cr(VI) are shown in Fig. 3. Biochar@Kaolin and Biochar@Bentonite contain substantial amounts of C, O Si and Al as well as a few of Ca, Na, Fe and K, which are typical of the elemental composition of clay minerals. The presence of Cr(VI) after adsorption experiment indicates that Cr(VI) is effectively removed by Biochar@Kaolin and Biochar@Bentonite^[23]. XRD patterns of Biochar@Kaolin and Biochar@Bentonite samples are shown in Fig. 4. The characteristic peaks at 2θ=20.64° and 26.40° are identified as (hkl) and (hkl) plane of expansible phyllosilicates respectively. XRD analysis suggests that the Kaolin and Bentonite successfully load on the surfaces of biochar, which are consistent with the EDS results^[24].

FT-IR spectra are used to determine the surface functional groups. FT-IR spectra of Biochar, Kaolin, Bentonite, Biochar@Kaolin and Biochar@Bentonite are depicted in Fig. 5.

As shown in Fig. 5, the peaks of Biochar@Kaolin and Biochar@Bentonite at approximately 3420, 1400, 1020- 1100 and 520-460 cm^-1 are assigned to the stretching vibration of the -OH, COO-, -C-O and -C-OH or Si-O group, respectively^[25]. The surface of compositions and chemical states of clay-biochar are further investigated by XPS. The C1s XPS spectra of Biochar@Kaolin and Biochar@Bentonite are shown in Fig. 6.

The C1s XPS spectra of Biochar@Kaolin and Biochar@Bentonite is the major component with peaks at 284.4 eV, which may be assigned to C/N-O, C-O or C-C bonds^[26]. These results suggest that Biochar@Kaolin and Biochar@Bentonite contain considerable amounts of oxygen/nitrogen groups on its surface, which is beneficial for binding Cr(VI) ions^[21].

The Cr(VI) removal efficiencies and adsorption capacities of Biochar@Kaolin and Biochar@Bentonite in solution with pH of 1-14 are shown in Fig. 7. As shown in Fig. 7, pH plays an important role in the removal of Cr(VI) in aqueous solution. At pH<7, the removal rate of Cr(VI) ions increase with pH decreasing. Under the acidic solution, the surface of Biochar@Kaolin and Biochar@Bentonite is positively charged, and is beneficial for the adsorption of Cr(VI) ion by electrostatic attraction^[25]. At pH>7, the removal rate of Cr(VI) ions increases with pH increasing due to the precipitation of Cr(VI). Previous studies have also shown that precipitation could play an important role in controlling the removal of heavy metal ions from aqueous solution by biochars at high pH^[26].

The effect of SO₄^2- ion on the removal of Cr(VI) by the Biochar@Kaolin and Biochar@Bentonite is shown in Fig. 8. No effect of SO₄^2- on the removal of Cr(VI) ions by Biochar@Kaolin, whereas the removal rates of Cr(VI) on Biochar@Bentonite decreased with the increase of SO₄^2- ions concentration. These may be attributed to the fact that there are much Na⁺ and Ca²⁺ ions exit on the surface of Biochar@Bentonite, but none of charged ions on the surface of Biochar@Kaolin. For specific adsorption sites, Cr(VI) ions are preferentially adsorbed over the SO₄^2- ions. When the specific adsorption sites are saturated, the exchange reactions of dominate and competition for these sites between Cr(VI) ions and SO₄^2- ions are important^[27,28].

Fig. 9(a) shows the adsorption data of Cr(VI) on different kinds of adsorbents. The sorption shows a rapid initial uptake followed by smooth increase. Kinetics studies are carried out and a number of well-known sorption models are applied in order to better understand the processes governing the adsorption of Cr(VI) ions to the Biochar@Kaolin and Biochar@Bentonite^[29].

(a) Adsorption data; (b) Pseudo-first order kinetic; (c) Pseudo- second order

In this study, the pseudo-first order and pseudo-second order kinetic models are used to predict the kinetic data. The pseudo-second order rate law bases on the assumption that the rate of solute sorption is directly proportional to the square of the number of vacant binding sites. The pseudo-first order and pseudo-second order^[30,31] are described as Eq. (2) and (3):

Where q_e (mg/g) is the amount of adsorbed solute at equilibrium conditions, q_t (mg/g) is the amount of adsorbed solute at the time t (min), k₂ (min^-1) and k₁ (min^-1) are the constants of pseudo-second order and pseudo-first order kinetic model respectively.

The plots of two models for Cr(VI) ions adsorbed onto the biochar, Biochar@Kaolin and Biochar@Bentonite are shown in Fig. 9. The corresponding calculated parameters are listed in Table 2.

The sorption kinetics of Cr(VI) on biochar, Biochar@Kaolin and Biochar@Bentonite are satisfactorily fitted by pseudo-second order model due to the higher correlation coefficient value (R² >0.98). It indicates that the chemisorption is involved in covalent interaction due to the sharing and exchange of electrons between Cr(VI) ions, and the adsorbent is likely the primary rate-limiting step involved in Cr(VI) ions sorption on surfaces of adsorbent^[32,33,34].

Langmuir isotherm model describes the correlation between the amounts of solute adsorbed on adsorbent (mg/g) versus the solute concentration in the solution (mg/L) at equilibrium condition. This model assumes that at equilibrium, monolayer sorption of solute occurs at fixed number of homogeneously distributed sorption sites over the adsorbent surface, and these sites also have equal affinity for the adsorbate. The non-linear equations describing the Langmuir isotherm model is given as follows^[35,36]:

Where C_e (mg/g) is the solute aqueous concentration at equilibrium, q_e (mg/g) is the amount of solute adsorbed per unit weight of adsorbent at equilibrium, Q_max is the maximum amount of solute adsorbed per unit weight of adsorbent (mg/g) to form a single layer, and K_L (L/mg) is the isotherm constant.

The Freundlich isotherm model assumes that the sorption process on a heterogeneous surface is in the form of multilayers, where adsorption sites have varied affinity for the adsorbate. The non-linear forms of the Freundlich isotherm model can be presented as^[37,38]:

Where C_e (mg/g) is the solute aqueous concentration at equilibrium, q_e (mg/g) is the amount of solute adsorbed per unit weight of adsorbent at equilibrium, K_F ((mg/g)^1/n) and 1/n are the isotherm constants.

According to experimental data, the adsorption isotherms parameters of Cr(VI) ion removal by biochar, Biochar@Kaolin and Biochar@Bentonite at different temperatures can be calculated and listed in Table 3.

According to the value of R², the simulations of the Langmuir model fits the adsorption data well and can therefore be used to describe the Cr(VI) ions adsorption behavior at different temperatures for Biochar@Bentonite. It also indicates that the adsorption process is homogeneous adsorption. The Cr(VI) ion from aqueous solution adsorption on the Biochar@Bentonite is monolayer adsorption. The maximum adsorption capacities of Biochar@Bentonite are calculated to be 20.54, 23.64 and 27.23 mg/g at 20, 30 and 40 ℃, respectively. For Biochar@Kaolin, the Freundlich adsorption model shows a better fit to the sorption data at different temperatures. It is also suggested that the sorption process on a heterogeneous surface is in the form of multilayers adsorption.

Two clay-biochars were successfully developed in this study. The batch experiments indicate that clay-biochar composite has high adsorption ability to Cr(VI) ion from aqueous solution (20.54, 23.64 and 27.23 mg/g at 20, 30 and 40 ℃, respectively). The adsorption kinetics can be satisfactorily fitted by pseudo-second order model. The adsorption isotherms of Cr(VI) on Biochar@Bentonite can be fitted by Langmuir model, whereas the Freundlich model fits better the Cr(VI) sorption on Biochar@Kaolin at different temperatures. These observations indicate that biochar-based composites can be regarded as the promising adsorbents for immobilization of heavy metals in environmental cleanup.