Multicomponent sorption of heavy metals and ion exchange on montmorillonite.

摘要

Due to their toxicity and persistence, heavy metals such as zinc (Zn), nickel (Ni), and cadmium (Cd) pose a continuing threat to the environment and human health when released to surface or subsurface waters as constituents of waste. The fate and transport of heavy metals such as Zn, Ni, and Cd in the subsurface are governed in large part by ion exchange and sorption interactions with the surrounding soil and aquifer material. In addition, sorption reactions influence the speciation and thus the bioavailability of trace metals. The available research on metal sorption has largely been limited to the overall sorption behavior of individual metals despite the presence of numerous metal constituents at many contaminated sites and the ubiquitous occurrence of potentially competing major cations like Ca{dollar}sp{lcub}2+{rcub}{dollar} and Mg{dollar}sp{lcub}2+{rcub}{dollar} in natural porewaters and ground water. In addition, ion exchange models that can incorporate the influence of lateral interactions with neighboring ions and that can be applied to naturally-occurring oxides such as montmorillonite have not been available.; Laboratory and modeling techniques were developed and employed to examine the interaction of suspensions of montmorillonite clay with various ions and heavy metals in solution under widely ranging conditions to elucidate the fundamental mechanisms of sorption and ion exchange. Batch tests were conducted for a wide range of conditions to determine the sorption of Zn, Ni, and Cd by montmorillonite in single- and multi-solute systems. A wide range of concentrations was examined here to encompass the range of concentrations encountered in natural, undisturbed environments, incidents of groundwater contamination, and industrial wastes. Titrations on montmorillonite suspensions were performed to determine its surface charge characteristics under various conditions and to determine the applicability of an ion exchange model developed for "pure" oxides for use in describing ion exchange on a natural metal oxide such as montmorillonite.; Results from the titrations and subsequent modeling confirm that the lateral interaction/ion exchange model can be successfully applied to montmorillonite. Indeed, the montmorillonite is seen to have similar ion exchange behavior as TiO{dollar}sb2{dollar} and {dollar}rm Fesb3Osb4{dollar} prepared in the laboratory. Batch tests reveal that metal sorption to montmorillonite is virtually instantaneous, the significance of which is that sorbed metals may be expected to rapidly remobilize in the natural subsurface under the influence of changing porewater chemistry. Sorption was observed to be strongly influenced by the solids concentration {dollar}(Csb{lcub}p{rcub}),{dollar} presumably due to particle aggregation at high {dollar}Csb{lcub}p{rcub}{dollar} and high pH. Metal sorption was not observed to be influenced by the presence of any other trace metal ions as potentially competing cations, suggesting the possible existence of a variety of reactive surface sites on the montmorillonite, each with characteristic geometric and electrical/chemical properties that induce ion-specific selectivity. Ion exchange was seen to play an important role in determining the extent of desorption as evidenced by the order of magnitude increases in the fraction desorbed under Ca-saturating solutions relative to deionized water.