Adsorption and reaction of Pb(2+) and Cu(2+) on pristine and modified oxides in aqueous suspension.

摘要

The surfaces of high surface area oxides and clays are accepted as holding a controlling influence over the fate and transport of metal cations in the environment. Environmental systems have abundant supplies of potential reactants yet previous studies of metal sorption on minerals have rarely considered the potential reactivity of adsorbed metals at the oxide/water interface. The work described in this dissertation combines classical and modern techniques to characterize the behavior of lead and copper in suspensions of the iron and aluminum oxyhydroxides, goethite ({dollar}alpha{dollar}-FeOOH), and boehmite ({dollar}gamma{dollar}-ALOOH). These minerals are models of adsorbents one might find in soils. The simplest form of cation reaction is a reaction with itself yielding metal-hydroxide surface precipitates. X-ray absorption spectroscopy (XAS) indicates lead participates in this kind of reaction at near-neutral pH on both materials but to a greater extent on boehmite. Copper resists precipitation on boehmite at neutral pH despite high surface loadings. EPR measurements indicate copper ions on the surface are in close enough to experience dipolar relaxation from one another. XAS rules out any significant precipitation.; Reaction of adsorbed metals with a co-adsorbate such as phosphate or sulfate is a second type of potential reaction at the oxide/water interface. The pH dependent adsorption of lead on anion modified particles compared to the pristine case was measured. The lead adsorption isotherm for the goethite system was little changed by the presence of anions, however, XAS and infrared spectroscopy indicate lead reacts with phosphate and sulfate to form a new surface phase. The pH dependent adsorption of lead on boehmite is sharply effected by anions. The same phosphate reaction was observed but there was no direct reaction with sulfate although sulfate appears to mediate lead adsorption on the basis of electrophoretic mobility and solution concentration data. Copper adsorption is enhanced on boehmite by the presence of anions but no reaction between the species is detected on the surface. The implication of surface phase formation to surface complex modeling is discussed.