镉污染水稻秸秆生物炭对土壤中镉稳定性的影响

中国农田土壤镉等重金属污染问题突出，对其生产过程中产生的镉污染水稻秸秆进行无害化和资源化利用研究具有重要意义。该研究通过连续提取试验、风险评价指数法、吸附动力学/热力学、土柱试验，以及X射线衍射分析、傅里叶变换红外光谱分析等手段，探究了不同热解温度下制备的镉污染水稻秸秆生物炭对土壤中Cd的稳定特性。研究结果表明，镉污染水稻秸秆热解制备的生物炭可有效吸附土壤镉。热解温度显著影响生物炭对Cd的吸附能力（P<0.05），高温生物炭对Cd吸附容量大，700 ℃下制备的生物炭对Cd的吸附容量可达72.57 mg/g。生物炭对Cd的吸附主要通过含氧官能团表面络合和碳酸盐共沉淀吸附，其吸附过程符合Langmuir方程和准二级动力学模型，吸附过程受化学速率控制。土柱试验表明，镉污染水稻秸秆生物炭能有效降低土壤Cd的下渗迁移能力，其作用机制主要是将土壤Cd从酸可提取态转化为残渣态，施入高温生物炭的土壤中Cd的残渣态比例最高。上述结果表明，热解可有效处理镉污染水稻秸秆，制备的生物炭可用于Cd等重金属污染土壤的稳定修复，有效解决镉污染水稻秸秆的潜在二次污染问题并实现其安全利用。

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.