稻草生物炭对土壤中Cd2+和Pb2+的吸附特性

为了探究稻草生物炭对土壤中重金属Cd2+和Pb2+的吸附机制，采用水稻秸秆在500℃下热解制备稻草生物炭，设置不同吸附时间、Na+和Ca2+离子强度、pH等影响因素，拟合稻草生物炭对重金属的吸附动力学、吸附等温线；在此基础上，采用黄沙模拟土柱试验得出稻草生物炭固定Cd2+和Pb2+的穿透曲线，着重分析比较pH和Na+离子强度对稻草生物炭吸附固定重金属的影响.结果表明:在高pH、低离子强度下，稻草生物炭对重金属的吸附效果较好；当pH为6时，稻草生物炭对Pb2+、Cd2+的吸附效率分别为92.58%、63.36%；当离子强度为10 mmol/L时，稻草生物炭对Pb2+、Cd2+的最高吸附效率分别为97.58%、68.35%；准二级动力学模型能很好地拟合稻草生物炭对Pb2+、Cd2+的吸附规律，拟合系数（R2）均大于0.995 8，表明稻草生物炭吸附速率主要由化学吸附机制决定；此外，稻草生物炭对Pb2+的吸附规律适合采用Langmuir等温吸附模型进行描述，而对Cd2+的吸附规律采用Langmuir和Freundlich等温吸附模型均能进行很好的模拟，表明稻草生物炭对Pb2+的吸附是近似单分子层吸附，而对Cd2+的吸附存在多分子层吸附.由黄沙土柱模拟试验结果得出，稻草生物炭对Pb2+和Cd2+的滞留率随着pH的升高和离子强度的降低而增强.在Pb2+和Cd2+同时存在条件下，当pH为6、离子强度为1 mmol/L、稻草生物炭按黄沙质量的0.5%投加时，稻草生物炭对土柱中Pb2+、Cd2+的滞留效果最好.研究显示，高pH对稻草生物炭吸附固定重金属起到促进作用，而高离子强度对稻草生物炭吸附固定重金属起到抑制作用. 

Adsorptive Characteristics of Rice Straw Biochar on Composite Heavy Metals Cd2+ and Pb2+ in Soil

In order to explore the adsorptive characteristics of rice straw biochar on composite heavy metals cadmium (Cd2+) and lead (Pb2+), the rice straw was pyrolyzed at 500 ℃ for 2 h, and the adsorption kinetics and adsorption isotherms of biochar on heavy metals were investigated at different adsorption time, ionic strength of Na+ and Ca2+, pH and other influencing factors. On the basis of previous experiments, yellow sand simulated soil column filtration was carried out to obtain the breakthrough curve of heavy metals (cadmium and lead) adsorbed by rice straw biochar. The key point was to discuss the effect of pH and ionic strength on the heavy metal adsorption by rice straw biochar. The results showed that the adsorption efficiency of biochar on heavy metals was better under high pH and low ionic strength. When pH was equal to 6, the adsorption efficiency of biochar to lead was 92.58%, and the adsorption rate of cadmium was 63.36%. When the ionic strength was 10 mmol/L, the highest adsorption efficiency of biochar to lead was 97.58%, and the highest adsorption efficiency of biochar to cadmium was 68.35%. The adsorption kinetic results of rice straw biochar on Pb2+ and Cd2+ confirmed to quasi-secondary kinetics, and the correlation coefficient R2 was greater than 0.9958, indicating that the biochar adsorption rate was mainly determined by the chemical adsorption mechanism. In addition, the adsorption of Pb2+ was suitable to be described by the Langmuir isothermal equation, while the adsorption of Cd2+ was well simulated by both Langmuir and Freundlich isothermal equation, indicating that the adsorption mechanism of Pb2+ by biochar was similar to that of monolayer adsorption, but the adsorption process of Cd2+ was multi-molecular layer adsorption mechanism. From the results of the soil column simulation experiment, the retention effect of biochar on Pb2+ and Cd2+ increased with the increase of pH and the decrease of ionic strength. Under the condition of simultaneous presence of Pb2+ and Cd2+, the best retention effect on Pb2+ and Cd2+ existed when pH=6, IS=1 mmol/L and rice straw biochar was added at the weight of 0.5% of yellow sand. The Study showed that high pH promoted the adsorption of heavy metals by rice straw biochar, while high ionic strength inhibited the adsorption of heavy metals by rice straw biochar.