重金属阳离子对可变电荷土壤吸附Cl-的诱导效应

Heavy Metal Cation-Induced Cl-Adsorption by Variable Charge Soils

选择3种代表性可变电荷土壤(红壤、砖红壤和黄壤)用一次平衡法研究了3种重金属离子(Cu(Ⅱ)、Pb(Ⅱ)和Cd(Ⅱ))对土壤吸附Cl-的影响。结果表明,与含钠离子的对照体系相比,重金属离子促进了土壤对Cl-的吸附,3种重金属离子影响Cl-吸附的大小顺序为:Pb(Ⅱ)>Cu(Ⅱ)>Cd(Ⅱ),这与土壤对这3种重金属离子吸附量的大小顺序一致。重金属离子对3种土壤中Cl-吸附促进作用的大小顺序为:砖红壤>黄壤>红壤,也与土壤对重金属吸附量的大小一致。Cl-在土壤表面主要以静电吸附为主,重金属离子可以通过3种机制促进可变电荷土壤对Cl-的吸附:(1)重金属离子在可变电荷土壤表面的专性吸附增加土壤的表面正电荷,从而增加土壤对Cl-的静电吸附量;(2)重金属阳离子降低了体系的pH,导致土壤表面正电荷增加,对Cl-的静电吸附量增加;(3)重金属阳离子与Cl-形成MCl+型络合物,当这种络合物被土壤吸附时,土壤对Cl-的吸附量增加。但3种机制中以前2种为主。

Three variable charge soils were chosen to investigate the effects of three heavy metal cations on Cl^- adsorption by the soils with batch method. The results showed that the presence of heavy metal cations induced the increase of Cl^- adsorption by the variable charge soils compared with that in Na^＋ system. The ability of heavy metal cations to enhance Cl^- adsorption followed the order： Cu（Ⅱ）、Pb（Ⅱ）和cd（Ⅱ）, consistent with the quantity of these cations adsorbed by the soils. It was found that the enhancement of Cl^- adsorption by heavy metal cations in these soils followed the order： latosol 〉 yellow soil 〉 red soil, which was also related with the amount of heavy metal cations adsorbed by the three soils. Cl^- was mainly adsorbed on the positive sites of the soils through electrostatic attraction between soil surface and cations. The enhancement of Cl^- adsorption by heavy metal cations involved in three possible mechanisms： （1） specific adsorption of heavy metal cations increased soil positive surface charge and thus increased Cl^- adsorption by the soils; （2） the presence of heavy metal cations decreased system pH and resulted in the increase of soil positive surface charge, and thus led to an increase of Cl^- adsorption; （3） the complex such as MCl^＋ was formed between heavy metal cations and Cl^-, when MCl^＋ was adsorbed by the soils, the amount of Cl^- adsorbed by the soils increased. While among the three mechanisms, the first one and the second one were the main mechanisms.