Abstract
Clay mineral has excellent adsorption capability for heavy metal ions in general. However, recent studies showed that humic substance could affect the retardation capability of clay mineral for heavy metals. This study divided the main influence into three parts: organic content (Aldrich humic acid), clay minerals (Jishin bentonite, montmorillonite, kaolinite, and vermiculite), and the environmental parameter, such as the soil pH influence for further investigating the influence of humic acid on Cd and Pb adsorbed onto clay minerals.The adsorption characteristics of humic acid onto clay minerals are related with the basic properties of clay minerals. In addition, clay H+- solution Mn+ ion exchange reaction was discussed to be one of the possible mechanisms. Moreover, the order of Kd values is consistent with average precipitated percentages of the humic acid on clay minerals, indicating that precipitation caused an organic film formed on the surface of clay, and substantially enhanced the adsorption capability of clay minerals by the organic partitioning.The revised 1:2 complexation model, which was based on the traditional 1:1 complexation model, was evaluated to give the suitable description of the participating humic acid concentration and make connection to the evaluation of equilibrium constants of complexation (at pH 6.5, 0.01 N ionic strength, and 25℃). The binding capability of the Aldrich humic acid for Pb2+ was higher than Cd2+. Pb2+ generally has higher adsorption capability onto clay minerals than Cd2+ even without considering the precipitation.In Cd2+ and Pb2+ co-existed condition, stability constants and total concentrations of deprotonated humic acid were influenced by the competitive effects. Less competitive effect of Cd2+ and Pb2+ occurred at higher amounts of aggregated deprotonated humic acid. Meanwhile, the influence extent of Aldrich humic acid was also depressed in the competitive system. By applying the concept of ion exchange equilibrium, heavy metal not only complexes with humic acid but also with other adsorption processes, such as partitioning, and the influence of Aldrich humic acid can only up to 10-1 meq/L degree.The results of this study discussed the possible mechanisms and soil pH influencing the heavy metal sorption behaviors. The competitive effect and influenced extent of different concentrations of humic acid offer the information of estimating the trend of complicated environmental pollution, and more significantly, this perspective has significance for the use of biogeochemical safety assessment and the pollution retardation especially on the influence of organic content.