Adsorption of cadmium by magnesium-modified biochar at different pyrolysis temperatures

Abstract

Metal pollution in soil is an increasing concern. Cadmium poses significant risks to ecosystems, and methodologies for its removal, including adsorption, have been researched. There are several environmentally friendly adsorbing materials (such as Biochar) for Cd removal. In this study, to improve the adsorptive capacity of Cd, coconut and peanut shells were used as raw materials to prepare Biochar at 300 °C and 600 °C. The effects of the pyrolysis temperature and material type on the physicochemical properties of the adsorbents were investigated by elemental analysis, scanning electron microscopy, and Fourier transform infrared spectroscopy. Magnesium-loaded BC was synthesized to determine its Cd2+ absorptivity. The adsorption characteristics and mechanisms of Cd2+ in an aqueous phase were studied through batch adsorption experiments. The results demonstrated that the pseudo-second-order kinetics model accurately described the adsorption kinetics of adsorbents of Cd2+. The adsorption behavior of the Cd2+ adsorbent conforms to the single layer adsorption described by the Langmuir model. Adsorption of Cd2+ involves a spontaneous endothermic process. The initial pH of the solution greatly influenced the adsorption of Cd2+ and showed a trend of rapid growth and then slow growth. Thus, magnesium-modified biomass carbon has good potential for applications in pollutant remediation.

Full Article

Adsorption of Cadmium by Magnesium-modified Biochar at Different Pyrolysis Temperatures

Ya Chen, Ruifeng Shan,* and Xiaoyin Sun

Metal pollution in soil is an increasing concern. Cadmium poses significant risks to ecosystems, and methodologies for its removal, including adsorption, have been researched. There are several environmentally friendly adsorbing materials (such as Biochar) for Cd removal. In this study, to improve the adsorptive capacity of Cd, coconut and peanut shells were used as raw materials to prepare Biochar at 300 °C and 600 °C. The effects of the pyrolysis temperature and material type on the physicochemical properties of the adsorbents were investigated by elemental analysis, scanning electron microscopy, and Fourier transform infrared spectroscopy. Magnesium-loaded BC was synthesized to determine its Cd²⁺ absorptivity. The adsorption characteristics and mechanisms of Cd²⁺ in an aqueous phase were studied through batch adsorption experiments. The results demonstrated that the pseudo-second-order kinetics model accurately described the adsorption kinetics of adsorbents of Cd²⁺. The adsorption behavior of the Cd²⁺ adsorbent conforms to the single layer adsorption described by the Langmuir model. Adsorption of Cd²⁺ involves a spontaneous endothermic process. The initial pH of the solution greatly influenced the adsorption of Cd²⁺ and showed a trend of rapid growth and then slow growth. Thus, magnesium-modified biomass carbon has good potential for applications in pollutant remediation.

Keywords: Cd adsorption; Modified BC; Mg doping; Adsorption mechanism

Contact information: College of Geography and Tourism, Qufu Normal University, Rizhao, 276826, P. R. China; *Corresponding author: ruifengshan@sina.com

INTRODUCTION

Soil heavy metal pollution is a common environmental issue. Cadmium （Cd²⁺）is one of the most toxic heavy metals, causes great harm to human health, and is a serious threat to the surrounding environment. The concentration of Cd²⁺ in mining, smelting, and sewage irrigation areas in China is significantly higher than in remote areas. In spatial distribution, the content of Cd²⁺ in southern areas is higher than in northern areas. The content of Cd²⁺ in soils is 0.27 mg·kg^-1 in southern areas, which exceeds the national first-level soil environmental quality standard (Luo. 2018). In April 2014, the Ministry of Environmental Protection and the Ministry of Land and Resources of China announced the state of soil pollution in the entire country. The results demonstrated that cadmium was the main soil pollutant in soil of China.

Biochar (BC) is one of the many soil amendments. Because of its special structure and property, such as huge specific surface area, its richness in oxygen-containing functional groups, and a large number of surface charges, BC could be an alternative absorbent. In recent years, BC has been widely used in the repair of soils contaminated by heavy metals.

Recent studies have encouraged the production and optimization of adsorbent materials that have characteristics similar to that of activated carbon (C) materials but are cost efficient and environmentally friendly (Inyang et al. 2010). For example, BC is a pyrogenic carbon material produced by thermal conversion of lignocellulose biomass under oxygen-free or limited conditions (Lehmann et al. 2006; Song et al. 2014). Because of its unique properties such as high surface area and cation exchange capacity, BC can be used for soil improvement, fertility enhancement, and carbon sequestration applications (Zimmerman et al. 2011; Mohana et al. 2014). However, the ability of BC to remove contaminants from aqueous solutions is limited (Yao et al. 2013). Improvements are reflected in the large number of engineered BCs with novel structures and surface properties (Zhang et al. 2013; Song et al. 2014). Studies have focused on the synthesis of carbon-based binary metal oxide composites for removing heavy metals from aqueous solutions (Tang et al. 2014).

Copper (Cu²⁺) and Cd²⁺ have been successfully removed from aqueous solution by adsorption by ferromanganese binary oxide–BC composites prepared through impregnation/sintering methods. The process is accurately represented by the Freundlich and Langmuir models and is driven by spontaneous endothermic entropy reduction (Zhou et al. 2018; Wang et al. 2015). Coconut shell BC, is mixed and wrapped with impurities during its production process, which reduces its original microstructure characteristics and remediation efficiency (Jia et al. 2016; Paranavithana et al. 2016). Acid pickling and ultrasonic treatment methods improve the physicochemical properties of BCs, the immobilization effect of heavy metals, and the soil micro-environment. The acid-soluble Cd, Nickel (Ni), and Zinc (Zn) elements were decreased by 30.1%, 57.2%, and 12.7%, respectively (Liu et al. 2018a). Therefore, it is necessary to modify the microstructure and surface properties of BC to enhance its remediation ability (Rajapaksha et al. 2016; Fang et al. 2018). Magnesium-modified biochar (MBC) is a kind of new material that can load magnesium oxide on the surface of BC (Zhou et al. 2012). It can be made from agricultural straw, garden waste, municipal sludge, and other biomass wastes (Fang et al. 2015). Moreover, the pyrolysis conditions, such as the solubility of the impregnated solution and the pyrolysis temperature, affect the surface properties of the BC, thus affecting its adsorption properties for inorganic and organic compounds (Novais et al. 2018). The adsorbed MgO modified BC can be used in agricultural slow-released fertilizer to supplement phosphorus and potassium and improve soil nutrition. Meanwhile, due to its rich fixed carbon, it can improve the carbon content in the soil and improve the physical and chemical properties of the soil (Beesley et al. 2011). Therefore, the use of biomass materials such as waste to prepare MBC to repair heavy metals in the soil can improve soil conditions, and can also realize waste resource treatment, providing a more ecological and green solution for waste treatment. However, the effectiveness of magnesium-modified and ultrasonic-modified BC and their implications for contaminated soil remediation processes have not been systematically studied. Therefore, magnesium modification and ultrasonic treatment were chosen to enhance the physical and chemical properties of BC, as well as the immobilization effect of heavy metals and water micro-environment.

The objectives of this study were (1) to prepare and characterize BC and MBC composites; (2) to analyze the adsorption properties of MBC in an aqueous solution; and (3) to define the mechanisms involved in the adsorption of Cd²⁺ by MBC.

EXPERIMENTAL

BC Preparation

The coconut shell was produced in Hainan, and the peanut shell was produced in Shandong, China. The coconut and peanut shells were washed with deionized water, dried and pulverized, passed through a 10-mesh sieve. The coconut shell biomass and peanut shell biomass were then placed into a porcelain crucible and capped, were pyrolyzed at desired temperature (300 or 600 °C) under N₂ environment at the rate of 10 °C min⁻¹, and then kept for 2 h at that temperature (Sun et al. 2017). After cooling to room temperature (25 °C), the BC was ground and sieved through a 0.6 mm sieve and washed with deionized water for a few times, then dried at 70 °C for 6 h, and finally put in a closed container before use.

0.1 M MgCl₂ solution was prepared to modify the BC. A certain amount of the coconut shell BC and peanut shell BC were immersed in the MgCl₂ solution (solid/liquid ratio was fixed at 1:10), soaked in an ultrasonic oscillator for 6 h, filtered, cleaned, placed in a crucible, and oven-dried at 105 ℃. The samples were placed in a resistance furnace control box and carbonized at 300 ℃ for 2 h (without protecting gas), then cooled to room temperature to obtain a modified coconut shell charcoal and modified peanut shell, respectively. The original BC samples were designated “BC” and the modified BC samples were designated “MBC” with a suffix indicating the peak temperature of pyrolysis: coconut shell pyrolyzed at 300 ℃ (300 YBC); coconut shell pyrolyzed at 600 ℃ (600 YBC); peanut shell pyrolyzed at 300 ℃ (300 HBC); poultry manure pyrolyzed at 600 ℃ (600 HBC); coconut shell pyrolyzed at 300 ℃ and doped with MgCl₂ (300 MYBC); coconut shell pyrolyzed at 600 ℃ and doped with MgCl₂ (600 MYBC); peanut shell pyrolyzed at 300 ℃ and doped with MgCl₂ (300 MHBC); and peanut shell pyrolyzed at 600 ℃ and doped with MgCl₂ (600 MHBC).

Sorbent Characterization

The surface morphology of the samples was characterized using by scanning electron microscopy (SEM) (Hitachi S-4800, Tokyo, Japan). The surface functional groups were determined via Fourier transform infrared spectroscopy (FTIR) (Bruker TENSOR 27, Karlsruhe, Germany). Brunauer–Emmett–Teller (BET) specific area, porosity and pore size distribution were characterized by a porosimetry analyzer (AUTO-SORBiQ2, Quantachrome, USA).The elemental composition of BC was determined with an elemental analyzer (Vario EL III, Elementar, Berlin, Germany). The crystallographic structure of the samples was examined through X-ray diffraction (XRD) analysis (Ultima IV, Japan).

Adsorption Experiments

All adsorption experiments were conducted in 100 mL conical glass bottles. The bottles were placed in a thermostatic rotary shaker with a speed of 200 rpm for 24 h at 25 ± 1 °C. All sorption studies were conducted in triplicates, and the average values were used for the data analysis.

For the adsorption kinetic studies, 0.05 g of the prepared samples were added to a Cd²⁺ solution (25 mL, 100 mg·L^-1) and stirred at 200 rpm. Aliquots (0.5 mL) were sampled at different time intervals (5, 10, 15, 30, 60, 120, 300, 600, 720, and 1440 min), filtered through a 0.45-μm filter, and analyzed by atomic absorption spectrophotometer (AAS). The following pseudo-first-order, pseudo-second-order kinetic, and intra-particle diffusion equations were used to fit the data (Chen et al. 2018a),

pseudo-first-order equation:  (1)

pseudo-second-order equation:  (2)

Intra-particle diffusion equation:  (3)

where q_t is the adsorption amount at the time, (mg·g^-1); K₁ is the pseudo-first- order kinetic constant, (min^-1); K₂ is the pseudo-second-order adsorption rate constant, (g·mg^-1·min^-1); K_p is the intra-particle diffusion constant, (mg·g^-1·min^-1/2); t is the adsorption time, (min); and C is the intercept, which indicates the size of the boundary reaction, assuming that q_t and t^1/2 are linear. At the coordinates (0,0) of the coordinate curve, the adsorption reaction belongs to the intra-particle diffusion.

The isotherm was recorded at the initial Cd²⁺ concentrations of 20.0, 40.0, 80.0, 120.0, 160.0, and 200.0 mg·L^-1 at 25/35 and 45 °C. 0.05 g of the prepared samples were added to a Cd²⁺ solution and stirred at 200 rpm then filtered through a 0.45-μm filter, and analyzed by AAS. The dosage of the samples was 25 mL, and the solution pH was 6.03. The following Langmuir and Freundlich equations were used to fit the data and study the equilibrium adsorption behavior of MgBC,

Langmuir equation:  (4)

Freundlich equation:  (5)

where K_L represents the interaction energy (mg·L^-1), K_F represents coefficient of affinity; q_m is the theoretical adsorption saturation, (mg·g^-1); q_e is the maximum adsorption capacity (mg·g^-1); C_e is the concentration of Cd²⁺ in the equilibrium solution (mg·L^-1), and n is linearity constant.

To test the pH effect, the solution was adjusted to a constant pH of 3, 4, 5, 6, 7, and 8. 0.05 g·L^-1 of all adsorbents was mixed with 25 mL of the 100 mg·L^-1 Cd²⁺ solution and stirred at 200 rpm then filtered through a 0.45-μm filter, and analyzed by AAS.

Statistical Analysis

The experimental data were analyzed and plotted with Excel 2010 and Origin 8.0.

RESULTS AND DISCUSSION

Sorbent Properties

The pore structure of BC has a great influence on the physical adsorption process. Different pore structures engender an inconsistent transport and diffusion rate of Cd²⁺ in BC. The stability of Cd²⁺ adsorbed by BC is low. Thus, Cd²⁺ is easily rereleased when the conditions change (Ahmad et al. 2013). The structure of the BC material is changed by modifications. The analysis of the parameters of the specific surface area, total pore volume, and average pore size are shown in Table 1. The order of the specific surface area of BC was 300 MYBC > 600 MYBC > 300 YBC > 600 YBC; 300 MHBC > 600 MHBC > 300 HBC > 600 HBC. After Mg modification, the specific surface area and pore volume of raw carbon increased, but the average void width decreased compared with the raw carbon. These results demonstrate that oxidation of MgCl₂ expands the specific surface area and pore volume of raw carbon.

Table 1. Pore Structure of the Carbon Materials

The main factors affecting the adsorption performance of BC are the specific surface area and pore volume (Cui et al. 2010). Generally, the larger the specific surface area and pore volume, the stronger the adsorption performance of BC. Figure 1 shows the nitrogen adsorption-desorption curves of coconut shell and peanut shell BC samples.

Fig. 1. The nitrogen adsorption-desorption curves of BC and MBC at different temperatures: (a) 300YBC and 300MYBC (b) 600YBC and 600MYBC (c) 300HBC and 300MHBC (d) 600HBC and 600MHBC

It can be seen from Fig. 1 that when P / P₀ = 1, the nitrogen adsorption capacity increased at 300 ~ 600 ℃, indicating that the pore structure of the material developed gradually and the total pore volume increased with the increase of pyrolysis temperature in this stage. The adsorption isotherms of coconut shell at 300 ℃ and 600 ℃ showed obvious adsorption platforms. According to the six types of adsorption isotherms defined by IUPAC (Xu et al. 2015), the isotherms are typical T-type, namely microporous type. The adsorption isotherms of peanuts shell BC are all in inverse S shape. When the relative adsorption pressure was low (P / P₀ < 0.2), the adsorption capacity of the sample at 300 ℃ declined. At this time, there is an inflection point, indicating that the pore completes the monolayer adsorption, and then with the increase of the relative adsorption pressure, the adsorption capacity of nitrogen increased slightly, but the increased amplitude of capacity was small, and so the curve tended to be horizontal. When the relative adsorption pressure is low, the adsorption capacity of nitrogen slightly increases. When the pressure was high (P / P₀ > 0.8), the nitrogen adsorption capacity of the samples increased, which indicated that the pore structure of BC was mainly mesopores and macropores (Liu et al. 2005), and the adsorption capacity of magnesium modified BC and 600 ℃ peanut shell samples in the low pressure zone increased rapidly. It reflects the existence of micropores in the BC material at this time. When the pressure was close to saturation, the adsorption capacity of nitrogen increased. It is also obvious that the reason may be the capillary condensation phenomenon similar to the macropore. This is similar to the conclusion following from pore size analysis.

The functional groups of BC affect its chemisorption. There are multiple functional groups such as carboxyl, hydroxyl, and amino groups on the surface of carbon materials. A higher number of oxygen-functional groups results in stronger cation exchange ability and stronger adsorptive capacity of Cd²⁺ (Ahmad et al. 2013). Figure 2 (a) shows that the functional groups of magnesium-modified BC (300 MYBC, 600 MYBC) were similar to those of coconut shell BC (300 YBC, 600 YBC). The broad peak of coconut shell BC was observed at 3423 cm^-1, which corresponds to the stretching vibration of hydroxyl (-OH). The peak at 2922 cm^-1 corresponds to the stretching of the C-H bonds of methyl and methylene. The peak at 1567 cm^-1 represents the antisymmetric stretching of -COO (Yuan et al. 2011). The peak at 1014 cm^-1 is attributed to the stretching vibration of the alcohol group and C-OH of the carboxylic acid. In addition, the polarization vibration and symmetrical stretching of carboxylate (-COO) are observed at 776 cm^-1. The absorption peak of coconut shell BC decreased when the temperature rose to 600 ℃.

As observed in Fig. 2 (b), peanut shell BC showed obvious absorption peaks at 3462, 2944, 1633, 1450, 1009, and 833 cm^-1, indicating that it contained abundant functional groups, but the surface functional groups of pyrolysis BC at different temperatures were different. When the pyrolysis temperature was increased from 300 ℃ to 600 ℃, the stretching vibration of -OH near 3462 cm^-1 almost disappeared, indicating that the increase in pyrolysis temperature decreased the number of hydroxyl radicals, which may be due to the separation of bound water and the breakage of hydrogen bonded hydroxyl radicals (Jian et al. 2016). With the increased pyrolysis temperature, the aliphatic C-H (2944 cm^-1) decreased gradually, while the aromatic C-H (833 cm^-1) increased. This result indicates that the alkyl group of the peanut shell disappears gradually during the pyrolysis and the aromatization degree of BC increases gradually (Marco et al. 2010).

The infrared spectrum analysis showed that the characteristic peaks of MgO modified BC were similar to those of unmodified BC, indicating that MgO particles have little effect on the types of organic functional groups in BC (Fang et al. 2014; Takaya et al. 2016).

Fig. 2. Infrared spectra of BC and modified BC at different temperatures: (a) YBC and MYBC and (b) HBC and MHBC

As shown in Fig. S1 (a, b) of the Supplementary material, the surface of coconut shell BC presented multiple pores with different-sizes. On the pore walls of large holes, smaller holes were observed, and the pore walls show a network structure. In Fig. S2 (a, b) of the Supplementary material, the cross-section of the peanut shell BC presented an irregular columnar or massive surface, and a large number of pores, resulting in a large specific surface area, facilitating the adsorption of heavy metal pollutants. As shown in Supplementary material Fig. S1 (c, d) and Fig. 3 (c, d), a large number of cubic crystal particles appeared on the surface of modified BC. From the Energy Dispersive Spectrometer (EDS) test results in Fig. 3, the modified BC presented a Mg peak and the O peak increased. The white particles on the surface of the modified BC are magnesium oxides or hydroxyl compounds. These particles are MgO crystals (Jung et al. 2015b). The medium crystalline particles of 600 MYBC and 600 MHBC were evenly distributed compared to 300 MYBC and 300 MHBC.

Figure 4 shows the XRD wide angle diffraction patterns of YBC, MgO-YBC, HBC and MgO-HBC. The comparisons of the two samples modified by magnesium to the unmodified samples are shown in Fig. 4a and 4b, respectively. There is an obvious diffraction peak at 2 θ = 42.7 °. It has been shown that the peak should be the characteristic peak of MgO, which indicates that magnesium ions have been loaded on BC (Meng et al. 2017; Yang et al. 2018). Therefore, MgO was the most important crystal phase in the modified BC particles. Meanwhile, the related research also shows that when MgCl₂ is used as the modifier, the existing form of magnesium in the modified BC is MgO.

Fig. 3. Energy spectrum analysis charts of different types of BC

Fig. 4. XRD analysis charts of different types of BC

BC Adsorption Behavior for Cd²⁺

Adsorption kinetics

Figure 5 shows the adsorption curve of BC and modified BC for Cd²⁺ at different adsorption times (the reaction contact time was controlled for 0 to 24 h). The adsorption capacity increased rapidly at the initial stage of adsorption, and then it gradually reached equilibrium. This is due to the abundant adsorption sites on the surface of BC in the initial stage and the high Cd²⁺ content in the solution, which leads to a higher adsorption rate. In the later stage, the adsorption sites on the surface of BC tend to be saturated. The Cd²⁺ content in the solution decreases, the Cd²⁺ diffuses into the void of BC, and the adsorption rate decreases.

To deeply analyze the adsorption mechanism of coconut shell BC, peanut shell BC, and magnesium-modified BC for Cd²⁺ and the possible control steps, pseudo-first-order, pseudo-second-order kinetic, and intra-particle diffusion models were used to fit the parameters and curves, respectively, as shown in Tables 2, 3 and Fig. 5 and 6. By comparing the fitting coefficients (R²) of the three kinetic equations in Tables 2 and 3, it was observed that the pseudo-second-order kinetic equation presents the most suitable fit. This result reflects more accurately the adsorption process of Cd²⁺ for all carbon materials. The pseudo-first- order kinetic model can adapt to the initial stage of the adsorption process, and then deviate from the adsorption process. The kinetic adsorption process of the unmodified and magnesium-modified materials was in good agreement with the pseudo-second-order kinetic model. The pseudo-first-order equation and the intra-particle diffusion model of the MgBC adsorption of Cd²⁺ presented smaller coefficient of determination values.

Fig. 5. Fitting parameters of the kinetic equation of Cd²⁺ with different adsorbents: (a) pseudo-first-order kinetics YBC and MYBC; (b) pseudo-second-order kinetics of YBC and MYBC; (c) pseudo-first-order kinetics of HBC and MBC and (d) pseudo-second-order kinetics of HBC and MBC

As shown in Fig. 6, the adsorption process is divided into two linear stages. The first stage represents the instantaneous adsorption, mainly surface adsorption, and the second stage represents the gentle adsorption, which indicates that the intra-particle diffusion is the main speed-limiting step. There was no linear relationship between the amount of Cd²⁺ adsorbed by MgBC and t^1/2. Because the curve does not go through the origin, the adsorption of Cd²⁺ by coconut shell BC and peanut shell BC was controlled by intergranular diffusion and multi-step control. In addition, the adsorptive capacity of Cd²⁺ by peanut shell BC and modified peanut shell BC was greater than that of coconut shell BC and coconut shell modified BC.

Table 2. Fitting Parameters of the Kinetic Equation of Cd²⁺ with Different Adsorbents

Table 3. Fitting Parameters of the Intra-Particle Diffusion of Cd²⁺ with Different Adsorbents

Fig. 6. Fitting curves of the intra-particle diffusion model for Cd²⁺ with different adsorbents

Adsorption isotherm

The adsorption isotherms of different adsorbents for Cd²⁺ at different temperatures are shown in Figs. 7 and 8.

Fig. 7. Isothermal fitting curves of YBC and MYBC for Cd²⁺ at different temperatures

Fig. 8. Isothermal fitting curves of Cd²⁺ between HBC and MHBC at different temperatures

The Langmuir and Freundlich isotherm adsorption models were used to fit the experimental data. The fitting parameters of the isotherm adsorption model are shown in Table 4. When the initial concentration of Cd²⁺ was below 80 mg·L^-1, the adsorption capacity of Cd²⁺ by BC increased with the increase in Cd²⁺ concentration and reached equilibrium when the concentration was greater than 100 mg·L^-1. This result indicates that increasing the initial concentration of Cd²⁺ is beneficial to the adsorption of Cd²⁺ by BC. Moreover, a higher pyrolysis temperature resulted in a larger adsorption capacity. The same pyrolysis temperature promotes the adsorption of Cd²⁺ by BCs. The maximum adsorption capacity of the modified carbon material was greater than that of original carbon.

The MgO crystal formed on the modified carbon surface, which enhances the adsorption capacity of the modified carbon material. It was stronger than the original carbon material, especially 600 MHBC. The maximum adsorption capacity of the modified carbon material reached 75.35 mg·g^-1, which indicates that the MgO crystal formed on the modified carbon surface enhanced the adsorption capacity of the modified carbon material. MHBC had a strong adsorption capacity for cadmium-contaminated solution. Comparing the fitting coefficients R² of the two models, the fitting parameters R² of the Langmuir model were larger than that of the Freundlich model, which indicates that the Langmuir model can better describe the adsorption behavior of adsorbents of Cd²⁺, and that they adsorb Cd²⁺ on a single layer. In addition, isothermal adsorption experiments were performed at 298 K, 308 K, and 318 K. Table 4 shows that the K_F values of the adsorbents increased with increasing ambient temperature. In particular, the adsorptive strength of adsorbents increased with the increasing ambient temperature. The Langmuir model parameters, q_m, of the adsorbents also showed an increasing trend with the increase in ambient temperature. Thus, the increase in temperature improved the adsorptive capacity of the adsorbents for Cd²⁺.

The environmental temperature also affects adsorption. Thermodynamic adsorption experiments were performed at 298 K, 308 K, and 318 K. The experiment was recorded at the initial Cd²⁺ concentrations of 20.0, 40.0, 80.0, 120.0, 160.0, and 200.0 mg·L^-1 at 25, 35, and 45 °C. 0.05 g of the prepared samples were added to a Cd²⁺ solution and stirred at 200 rpm then filtered through a 0.45-μm filter, and analyzed by AAS. The dosage of the samples was 25 mL, and the solution pH was 6.03. To examine the adsorption thermodynamics of Cd²⁺ by MgBC, the ΔG⁰ of the adsorption process was calculated by Eq. 6. The slope and intercept of T were obtained by fitting the relationship between ΔG⁰ and ΔH⁰ and ΔS⁰, expressed in Eq. 7,

ΔG⁰ = –RTlnK₀ (6)

ΔG⁰ = ΔH⁰ – TΔS⁰ (7)

where ΔG⁰ is the free energy change, (kJ·mol^-1); ΔS⁰ is the entropy change, (kJ·mol^-1·K^-1); ΔH⁰ is the enthalpy change, (kJ·mol^-1); R is the gas constant, (8.314 J·mol^-1.K^-1); T is the thermodynamic temperature, (K); and K₀ is the Langmuir isothermal equation constant K_L multiplied by 1000; and transformed into a dimensionless number (Zhang et al. 2012b). The values of ΔH⁰ and ΔS⁰ are shown in Table 5.

Table 4. Isothermal Adsorption Model Parameters of Cd²⁺ by Different Adsorbents Adsorption Thermodynamics

Based on the experimental data of the adsorbent adsorbing Cd²⁺ at 298 K, 308 K, and 318 K, a thermodynamic analysis of adsorption was performed. The thermodynamic analysis diagrams are shown in Figs. 5 and 6, and the thermodynamic parameters are shown in Table 5. Table 5 demonstrates that the Gibbs free energy of different adsorbents was negative, indicating that adsorbents adsorbed Cd²⁺ spontaneously. Moreover, ΔG⁰ decreased with the increase of temperature, indicating that the increase of ambient temperature plays a positive role in spontaneous adsorption. The enthalpy change ΔH⁰ was positive, indicating that the adsorbent adsorbing Cd²⁺ is an endothermic process at 298 to 318 K. The entropy change ΔS⁰ was positive, indicating that the adsorbent adsorbs Cd²⁺ spontaneously. The structure of the adsorbents and heavy metals changes during the adsorption process. The entropy changed to a positive value of ΔS⁰, which suggests that the order of the adsorbate molecules decreases, and the degree of chaos increased during the adsorption process (Lang et al. 2015).

Table 5. Thermodynamic Model Parameters of Adsorption of Cd²⁺ by Different Adsorbents

Effect of pH

The adsorption of Cd²⁺ in aqueous solution in different initial pH conditions is shown in Fig. 9. The initial pH of the solution had a great influence on the adsorption of Cd²⁺ and showed a trend of rapid growth and then slow growth. The adsorption of Cd²⁺ by BC in Figs. 9a and 9b increased with the increased solution pH. In Fig. 9a, when the pH was 3 to 7, the adsorption capacity increased rapidly, and when the pH was greater than or equal to 7, the adsorption capacity increased slowly. In Fig. 9b, when the pH was 3 to 6, the adsorption capacity increased rapidly, and when the pH was greater than or equal to 6, the adsorption capacity increased slowly.

The adsorption amount varied with the initial value of the solution because, at low pH, the adsorption sites on the surface of the carbon particles were occupied by a large number of H⁺, which prevented the approach of Cd²⁺. Therefore, the removal rate of Cd²⁺ was relatively low. With increasing pH, the negative charge on the surface of carbon increases (Houben et al. 2013), and the electrostatic attraction of Cd²⁺ increases. With the increase in pH, the surface of biomass carbon is negatively charged.

Biomass carbon mainly depends on the electrostatic adsorption of heavy metal ions in solution (Kadirvelu and Namasivayam 2004). In addition, because of the electronic layer structure of Cd²⁺, the increase in pH stimulates hydrolysis, and the adsorption affinity of BC by metal hydroxyl ions produced by the hydrolysis is greater than that of free ions (Fristak et al. 2015).

Fig. 9. Adsorption of Cd²⁺ in an aqueous solution under different initial pH conditions: (a) YBC and MYBC and (b) HBC and MHBC

CONCLUSIONS

1. Coconut and peanut shells were used as raw materials. Magnesium oxide was formed on the surface of coconut and peanut shell BC by using a chemical impregnation method. Magnesium-loaded BC materials were prepared, which had good adsorption effect on Cd²⁺.
2. The results of SEM-EDS, BET, and FTIR analyses showed that MgO was successfully loaded on BC and played an important role in the adsorption of Cd²⁺.
3. The pseudo-second-order kinetics and Langmuir isothermal adsorption equation accurately described the adsorption process of BC and MBC for Cd²⁺. At a higher temperature, the adsorptive performance of adsorbents was better. At high pH, the adsorptive effect of the modified material for Cd²⁺ was stronger. At the same time, under the same adsorption conditions, the amount of Cd²⁺ adsorbed by peanut shell BC and modified peanut shell BC was larger than that by coconut shell BC and modified coconut shell BC.
4. The MgO particles on the surface of the modified material play an important role in the adsorption of Cd²⁺, and the adsorption process mainly consists of chemical adsorption. MBC is a low-cost adsorbent that has strong adsorptive properties on Cd²⁺ and a great potential for environmental remediation.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (No. 41501542).

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Article submitted: July 26, 2019; Peer review completed: October 10, 2019; Revised version received: November 19, 2019; Accepted: November 22, 2019; Published: December 11, 2019.

DOI: 10.15376/biores.15.1.767-786

APPENDIX

Supplementary Material

Fig. S1. SEM diagrams of YBC and MYBC at different temperatures: (a) 300 YBC, (b) 600 YBC, (c) 300 MYBC; and (d) 600 MYBC

Fig. S2. SEM diagrams of HBC and MHBC at different temperatures: (a) 300 HBC, (b) 600 HBC, (c) 300 MHBC; and (d) 600 MHBC