Mechanism And Remediation Effect Of Biochar And Soil Conditioner On Cadmium Contaminated Soil

The process of Cadmium（Cd） pollution in soil system has the characteristics of concealment, latent, accumulating and long-term. Cadmium goes into the organism through the food chain, and endangers human health after a long-term accumulation. Along with the large-scale industrial urbanization process of our country, cadmium is introduced into contaminated soils through industrial emission, atmospheric deposition, sewage irrigation, application of sludge and fertilizer contaminated with Cd, which results in the severe Cd pollution of the fields throughout China. Cadmium contaminated vegetable soils are also widespread in China, especially in suburban areas. Most of the non-point source polluted farmland soils in our country are mild or moderate pollution. Remediation technologies of soil Cd pollution mainly include engineering method, chemical remediation, phytoremediation and agronomic regulation. In numerous remediation methods, in situ remediation technology of passivation is low cost, easy to operate and has quick effect, which is widely used in contaminated soil, especially in mild or moderate polluted soils. Biochar has large specific surface area, surface energy and the role of binding heavy metal ions, so it is able to passivate heavy metals in soil. Soil conditioner made of inorganic-organic blend of passivating agents, can reduce the effectiveness of heavy metals in soil. In this research, through the adsorption experiment, pot and field experiments, the effects of biochar and soil conditioner on growth, physiological characteristics, Cd accumulation and chemical forms in pakchoi, as well as Cd chemical form in soil. We investigate the mechanism of biochar and soil conditioner on cadmium uptake by pakchoi, and find out the optimum adding amount of biochar and soil conditioner repairing soil, in order to provide theoretical foundation on Cd remediation. The main results are as follows:（1） In the scanning electron microscopy（SEM） and energy spectrum analysis, biochar was observed to have a smooth surface and dense carbon layer structure, with many voids and holes unevenly distributed; soil conditioner surface was found to be rough, highly irregular shape and have more porous structure. Both before and after the adsorption, the percentage of Cd element increased significantly, the percentages of the other elements have a corresponding change. Biochar and soil conditioners on Cd2 + adsorption tests showed that the Cd²⁺ adsorption capacity of biochar and soil conditioners increased with increasing Cd²⁺ concentration in the equilibrium solution, and the Cd²⁺ adsorption curve can be fitting well by Langmuir isotherm. Biochar and soil conditioner has large adsorption capacity, high adsorption efficiency, and they can achieve balance in a short time; the adsorption of Cd²⁺ is in accordance with the secondary dynamic model. Great influence of solution p H on Cd²⁺ adsorption by biochar and soil conditioners was found in our study. In the range of solution p H 2-9, Cd²⁺ adsorption capacity of biochar and soil conditioners increased with increasing solution p H first, and then tended to the balance. Higher Cd²⁺ adsorption capacities were observed at solution p H from 5 to 7.（2） In the pot experiment, the biochar and soil conditioners and their complex treatments increased the dry weight of shoot and root, and total plant dry mass in pakchoi. The dry weight of shoot and root, and total plant dry mass increase with increasing applied amount of the biochar and soil conditioners and their complex. In most cases, chlorophyll A, chlorophyll B, total chlorophyll and carotenoid content of pakchoi significantly increased by the soil conditioner and biochar and their complex. Applying biochar and soil conditioner into the contaminated soil can improve the nutritional quality of pakchoi, which not only significantly decrease the activities of CAT, SOD and POD, and MDA content in leaf of pakchoi, but also increase the content of reducing sugar, free amino acids and vitamin C content in pakchoi, and reduce the nitrate content.（3） The sodium chloride extraction（FNa Cl） and acetic acid extraction Cd（FHAc） were predominant forms of Cd in pakchoi leaves. Applying different concentrations of biochar and soil conditioner into the cadmium polluted soil significantly reduced the concentrations of FE, FW, FNa Cl, FHAc, FHCl, FR and the total extraction amount of Cd in the leaves by 12.9%- 58.9%, 13.0%-58.2%, 6.6%- 50.7%, 16.3%- 41.4%, 13.3%-47.5%, 7.7%-39.1% and 11.9%- 48.0%, respectively. Cadmium mainly accumulated in the roots of pakchoi. After the application of biochar and soil conditioner and their complex, Cd concentration of each tissue and plant total Cd accumulation of pakchoi decreased. Compared with two materials, the reducing effect on the concentration of Cd and the accumulation of Cd in shoot at biochar treatments were higher than those at soil conditioner treatments.（4） The order of concentration of Cd chemical forms in the pot soil was EXC-Cd>CAB-Cd > RES-Cd > Fe Mn-Cd > OM-Cd. The concentration of exchangeable Cd（EXC-Cd） in the soil decreased with increasing applied amount of biochar and soil conditioner and their complex. On the contrary, the concentrations of CAB-Cd, Fe Mn-Cd and OM-Cd significantly increased by applying biochar and soil conditioner. Biochar and soil conditioner and complex significantly reduced the total extraction amount of soil Cd by 5%. The reducing effect of the combined treatments of biochar and soil conditioneron the total extraction amount of soil Cd was better than that ofthe single treatments at low applied amount（from 0.5% to 2%）.（5） In the field experiment, applying different concentrations of biochar and soil conditioner and their complex in cadmium polluted soil variably reduced the concentration of Cd in shoot and root of pakchoi by 24.3%-52.2% and15.5%-34.3%, 18.5%-40.9% and18.1%-28.5%, 31.2%-41.5% and 26.9%-33.2%, respectively. Expect T1 and T/C2: 1 treatments increasing Cd accumulation, soil conditioner and biochar reduced Cd accumulation in shoot of pakchoi, and decreased with increasing applied amount of soil conditioner and biochar. Compared with two materials, the decreasing effect of biochar on Cd accumulation in each tissue and the total Cd of plant in pakchoi is higher than that of soil conditioner.（6） The order of Cd concentration of various chemical forms in the mining area was EXC-Cd>RES-Cd>CAB-Cd>Fe Mn-Cd>OM-Cd, and the concentrations of EXC-Cd and RES-Cd were 1.565-2.953 mg·kg-1 and 1.118-1.425 mg·kg-1, accounting for 29.0%-49.4% and 18.7%-26.4% of the total extraction amount of Cd, respectively. Concentration of EXC-Cd and the total extraction amount of soil Cd decreased by biochar and soil conditioner, and reducing effect of biochar was significantly higher than that of soil conditioner; reducing effect of the combined treatment（T/C1:2） was better than that in the single treatments at low applied amount（from 0.5% to 2%）.（7） In correlation analysis, Cd concentration in shoot of pakchoi was significantly positively correlated with root Cd concentration（P<0.01）. Significant positive correlations were found in Cd concentrations of shoot and root with concentration of EXC-Cd in soil（P<0.01）, while significant negative correlations were also observed in Cd concentrations of shoot and root with the concentrations of CAB-Cd, Fe Mn-Cd, OM-Cd and RES-Cd in soil（P<0.01）. The concentration of EXC-Cd was significantly negatively correlated with concentrations of CAB-Cd, Fe Mn-Cd, OM-Cd and RES-Cd in soil.