Effects of biochar from Cd-containing rice straw on stabilization of Cd in soils

Heavy metal pollution (such as cadmium, Cd) in farmland soil has posed a great threat to crop production in China. It is highly urgent to produce the harmless and resource utilization of cadmium contaminated rice straw. In this study, the stability of cadmium contaminated rice straw was evaluated to prepare the biochar at different pyrolysis temperatures on Cd in soil, using the sequential extraction test, Risk Assessment Code (RAC), adsorption kinetics/thermodynamics, soil column test, X-ray diffraction analysis, and Fourier transform infrared spectrometry. The results showed that the biochar prepared by pyrolysis of cadmium contaminated rice straw had effectively adsorbed the cadmium in soil. The pyrolysis promoted the volatilization of Cd in the cadmium contaminated rice straw, indicating the low Cd content of high-temperature biochar for a less risk to ecological environment. The total content of Cd in BC700 (BC represented the biochar and 700 represented the pyrolysis temperature) decreased by 26%, and the proportion of stable state increased by 41%, compared with the rice straw powder (RS). There was the lowest RAC value of BC500 (12.58%). The pyrolysis temperature was also dominated the physicochemical properties of cadmium contaminated rice straw biochar. The yield of biochar decreased from 39.26% to 25.13%, when the pyrolysis temperature increased from 300°C to 700°C, whereas, the pH value, specific surface area and pore volume increased, indicating more adsorption sites for the cadmium adsorption. The Fourier Transform Infrared Spectrometry (FTIR) analysis showed that the pyrolysis temperature posed a great effect on the type and strength of functional groups of biochar from the cadmium contaminated rice straw. The content of oxygen-containing functional groups of biochar decreased, whereas, the degree of aromatization increased gradually, with the increase in pyrolysis temperature. The pyrolysis temperature was significantly determined the adsorption capacity of biochar for the Cd (P<0.05). The adsorption capacity of biochar at 300, 500, and 700°C increased from 25.31 to 72.57 mg/g, indicating the larger adsorption capacity of biochar at high-temperature pyrolysis. The adsorption of Cd by the biochar was conformed to the Langmuir equation and pseudo-second-order kinetic model. The adsorption process was mainly controlled by the chemical rate. The X-Ray Diffraction (XRD) patterns showed that the Cd removal mechanism in the cadmium contaminated rice straw biochar was the surface complexation of oxygen-containing functional groups and carbonate coprecipitation, where -COOH, CO32-, -OH were important in the adsorption, and the cadmium oxalate and cadmium carbonate were the main precipitates in BC300 and BC700, respectively. The soil column test showed that the cadmium contaminated rice straw biochar was effectively reduced the infiltration and migration of soil Cd, where the soil Cd was transformed from acid leaching state to residue state. The proportion of residual Cd in BC700 was the highest after adding biochar, which was 26.78%, compared with the control. Once the pyrolysis temperature increased from 300°C to 700°C, the acid extractable state of soil Cd was further transformed into the residual state. In conclusion, pyrolysis can be widely expected to effectively treat the cadmium contaminated rice straw. The prepared biochar can be used for the stable remediation of Cd and other heavy metal contaminated soil. The finding can provide a strong reference for the potential secondary pollution cadmium contaminated rice straw for safe utilization. © 2022, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.