土壤pH、有机质和含水氧化物对镉、铅竞争吸附的影响

采用一次平衡法（振荡16h）研究了土壤pH值、有机质和含水氧化物（以晶质氧化铁和非晶质氧化铁为代表）对Cd^2＋、Pb^2＋在水稻土上竞争吸附的影响。结果表明，pH值是影响土壤吸附Cd^2＋、Pb^2＋的主要因素，土壤对金属离子Cd^2＋、Pb^2＋的吸附量随pH值的增大而增加。土壤pH值为5．0和6．5时，对Cd^2＋、Pb^2＋的吸附量分别达到最大。当土壤去除有机质后，土壤对Cd^2＋、Pb^2＋的吸附均降低，与对照相比，分配系数Kd,cd、Kd,pb西分别下降54％～64％和36％-52％；土壤去除非晶质氧化铁后，Kd,cd、Kd,pb与对照相比分别下降32％～45％和15％～33％。有机质和非晶质氧化铁对Cd^2＋、Pb^2＋的选择性为：Pb^2＋〉Cd^2＋.当土壤晶质氧化铁和非晶质氧化铁均去除后．土壤吸附Cd^2＋、Pb^2＋不但未降低，反而有不同程度的增加，Kd,cd、Kd,pb西明显高于对照。

Effects of pH, Organic Matter and Hydrous Oxides on Competitive Adsorption of Cd2+ and Pb2+ by Soil

The preferential retention of heavy metals by soils is critical to their availability to plant and mobility through the soils down to the groundwater. We examined the competitive adsorption of Cd2+ and Pb2+ by a paddy soil as affected by pH, organic matter and hydrous oxides. Cadmium and lead adsorption from Cd-Pb binary solution produced two different adsorption isotherms. The adsorption isotherm of the metals showed that maximum adsorption occurred at pH 5.0 for Cd2+ and pH 6.5 for Pb2+, probably due to the difference in the first hydrolysis constants of the metals. The distribution coefficient Kd, which defines the affinity of the metals for the soil adsorbents, was much greater for Pb2+ than Cd2+. The removal of organic matter from the soil reduced 54%~64% for Kd,Cd and 36%~52% for Kd,Pb as compared to the natural soil. Similarly, removing amorphous hydrous oxide by oxalate extraction prior to the introduction of Cd and Pb reduced Kd,Cd 32%~45% and Kd,Pb 15%~33% as compared to the natural soil, but when both amorphous and crystalline hydrous oxides were removed before Cd2+ and Pb2+ addition, the Kd, Cd increased 100%, whereas Kd, Pb increased by more than twice as compared to the natural soil. It seemed that reactions with organic matter and amorphous oxides were the major influencing factors on Cd2+, whereas Pb2+ might be largely sorbed by cation exchange reaction when in competition with Cd2+ for binding sites in soils. The metal binding sites in the amorphous hydrous oxides and organic matter were more selective for Pb2+ than Cd2+.