Department of Earth System Science, Stanford University, Stanford, CA 94305, United States.
Department of Earth System Science, Stanford University, Stanford, CA 94305, United States.
Sci Total Environ. 2021 Sep 1;785:147096. doi: 10.1016/j.scitotenv.2021.147096. Epub 2021 Apr 14.
Adsorption of arsenate on clay minerals can control the partitioning and mobility of arsenic and subsequent contamination of groundwater. While the effect of ionic strength on arsenic adsorption to phyllosilicate minerals has been evaluated for various clay minerals, the specific ionic composition of the surrounding porewater can play a critical role in promoting adsorption (or desorption) of arsenate (HAsO). We conducted a series of adsorption isotherms to evaluate the adsorption of arsenate to various phyllosilicates in the presence of monovalent (K), divalent (Mg, Ca), and trivalent (La) cations while maintaining constant ionic strength and pH. Adsorption isotherms were combined with surface complexation modeling to examine retention processes of arsenate as a function of ionic composition in the surrounding solution. The higher charge density of greater valent cations results in stronger outer-sphere bridging complexes between negatively charged phyllosilicate mineral surfaces and negatively charged arsenate oxyanions. Higher valent cations thus enhance the propensity for arsenate adsorption on phyllosilicate minerals. We further deciphered surface complexation processes by conducting adsorption isotherms on various clay minerals including smectite, illite, and pyrophyllite to evaluate the role of interlayer, permanent charge, and terminal edge sites. We conclude that arsenate is most likely retained largely on the planar surface where structural negative charge emanates allowing cation bridging complexes to develop. Our findings illustrate that clay mineralogy of soils and sediments can combine with porewater ionic composition (and specifically the proportion of divalent cations) to describe arsenic transport, particularly in iron- or aluminum-oxide poor systems.
砷酸盐在粘土矿物上的吸附可以控制砷的分配和迁移以及随后对地下水的污染。虽然已经评估了各种粘土矿物中离子强度对层状硅酸盐矿物砷吸附的影响，但周围孔隙水中特定的离子组成在促进砷酸盐（HAsO）的吸附（或解吸）方面起着至关重要的作用。我们进行了一系列吸附等温线实验，以评估在保持离子强度和 pH 值恒定的情况下，一价（K）、二价（Mg、Ca）和三价（La）阳离子存在时，砷酸盐在各种层状硅酸盐上的吸附。吸附等温线与表面络合模型相结合，研究了周围溶液离子组成对砷酸盐保留过程的影响。更高电荷密度的高价阳离子导致带负电荷的层状硅酸盐矿物表面和带负电荷的砷酸盐含氧阴离子之间形成更强的外球桥接络合物。因此，高价阳离子增强了砷酸盐在层状硅酸盐矿物上的吸附倾向。我们通过在各种粘土矿物（包括蒙脱石、伊利石和叶蜡石）上进行吸附等温线实验，进一步解析了表面络合过程，以评估夹层、永久电荷和末端边缘位点的作用。我们得出结论，砷酸盐很可能主要被保留在结构负电荷散发的平面表面上，允许阳离子桥接络合物的形成。我们的研究结果表明，土壤和沉积物的粘土矿物学可以与孔隙水中的离子组成（特别是二价阳离子的比例）相结合，来描述特别是在铁或铝氧化物贫化系统中的砷迁移。
