As(Ⅴ)在粉质黏土中的吸附特性研究

Adsorption characteristics of As(Ⅴ) on silty clay

目前对土中重金属吸附特性的研究多集中在金属阳离子,对As(Ⅴ)等以阴离子基团形式存在的重金属关注较少。研究了As(Ⅴ)在粉质黏土中的吸附,并与常见阳离子Pb(Ⅱ)进行了对比。考虑了土水比、反应时间、溶液浓度、pH和温度对吸附的影响,通过吸附动力学、吸附等温线、吸附热力学模型和微细观测试分析了吸附机制,另外研究了吸附As(Ⅴ)和Pb(Ⅱ)后土的渗透特性。试验结果表明,As(Ⅴ)在粉质黏土上的最大吸附量远低于Pb(Ⅱ);As(Ⅴ)吸附量随溶液浓度增加线性增加;Pb(Ⅱ)吸附量随溶液p H增加而增加,而As(Ⅴ)吸附量在碱性条件下稍有减小。As(Ⅴ)和Pb(Ⅱ)的吸附都符合准二阶动力学模型,都是以化学吸附为主,Pb(Ⅱ)吸附与颗粒内扩散和液膜扩散有关,而颗粒内扩散是As(Ⅴ)吸附的主要控速因素;Langmuir模型对As(Ⅴ)拟合较差,As(Ⅴ)属于中等难吸附;Pb(Ⅱ)的吸附是吸热过程,温度高有利于Pb(Ⅱ)吸附,而As(Ⅴ)的吸附过程放热,吸附量随温度升高而降低。微观测试表明,粉质黏土对As(Ⅴ)和Pb(Ⅱ)的吸附基本发生在晶格以外;吸附Pb(Ⅱ)后土颗粒团聚导致孔隙变大,而吸附As(Ⅴ)后土颗粒趋于分散;土中有官能团参与了吸附过程。随着土中重金属含量增加,Pb(Ⅱ)污染土渗透系数增大,而As(Ⅴ)污染土渗透系数减小,酸性条件下含重金属的土渗透系数均增大。得到的吸附机理和参数对研究砷污染具有重要意义。

Adsorption of metal cations has been extensively studied. However, the researches on adsorption of anions are still lacking. Adsorption of As(Ⅴ) on a silty clay is investigated and compared with that of a common cation, Pb(Ⅱ). The effects of solid-solution ratio, reaction time, concentration, pH and temperature on the adsorption are studied. The adsorption kinetics,isotherms and thermodynamics model sand meso/micro tests are used to investigate the adsorption mechanism involved. The hydraulic conductivities of the contaminated soil are studied. The test results show that the adsorption capacity of As(Ⅴ) is much lower than that of Pb(Ⅱ). The adsorption of As(Ⅴ) increases linearly with the increasing concentration. The adsorption of Pb(Ⅱ) increases with pH, whereas that of As(Ⅴ) decreases slightly under alkaline condition. The adsorptions of As(Ⅴ) and Pb(Ⅱ)can both be described by the pseudo-second-order kinetic model, and are both dominated by chemical adsorption. Both intraparticle and film diffusions are involved in the adsorption of Pb(Ⅱ), whereas the intraparticle diffusion is the rate-limiting factor for the adsorption of As(Ⅴ). The Langmuir model gives poor fit for the adsorption of As(Ⅴ), which can be rated as moderately difficult. The adsorption of Pb(Ⅱ) is endothermic and favored by high temperature, whereas that of As(Ⅴ) is exothermic and decreases with temperature. The meso/micro analysis shows that the adsorptions of As(Ⅴ) and Pb(Ⅱ) happen mainly out of the crystal structure. The soil pores become larger after the adsorption of Pb(Ⅱ) due to particle aggregation,whereas the soil particles become more scattered after that of As(Ⅴ). Some functional groups participate in the adsorption. As heavy metal content increases, the hydraulic conductivity of Pb(Ⅱ)-contaminated soil increases whereas that of As(Ⅴ) decreases.The alkaline condition increases the conductivity. The obtained adsorption mechanisms and parameters are important for the researches on arsenic pollution.