Superfund Research Program
Role of Mineral Genesis, Dissolution, and Sorption on Arsenic Fate in Contained Waste Sites
The focus of this project is to understand the stability of arsenic under various environmental conditions and to engineer processes that will stabilize arsenic wastes so as to minimize human and ecological risk. Researchers are investigating both biological and abiotic aspects of arsenic fate in the environment. On the biological side, it is well known that microbes can cycle arsenic between the As(V) and As(III) states and that the mobility of arsenic is strongly related to its valence state. In a groundbreaking discovery, researchers have demonstrated that there are some microorganisms that by themselves can perform the entire cycle (both oxidation and reduction of arsenic). Further research is necessary to characterize the mechanisms responsible for this ability and to better understand the ecological implications associated with this novel discovery.
A second research area relates to the fate of As(III) or As(V) in the presence of the plethora of surfaces that are available for adsorption in water treatment or environmental systems. A key control over adsorption of arsenic is the presence or absence of other materials that can compete for sorption sites. Among the most important competing solutes is dissolved silica. Using molecular spectroscopy to study the competitive sorption of As(V) and silica on iron oxides, researchers have found that dissolved silica effectively diminishes As(V) sorption, which has important implications for As fate because silica is ubiquitous in natural waters. Molecular modeling is underway to help researchers resolve these competitive effects. A second key control over arsenic sorption is reduction-oxidation reactions that control As mobility. When As(V) sorbed to iron oxides is subjected to conditions intended to model landfill leachate conditions, researchers find that As(V) is reduced to As(III), Fe(III) is reduced to Fe(II), and a large fraction of As is mobilized into the solution phase. The remaining fraction of As is primarily associated with residual Fe(III) iron solids. Understanding why such residual solids persist under reducing conditions is a key goal for current research.