Controls on Arsenic Mobility in Contaminated Wetland and Riverbed Sediments

Arsenic mobility and transport in the environment are strongly influenced by associations with solid phases. This dissertation investigates the mechanisms affecting arsenic retention in contaminated wetland and riverbed sediments. A sequential extraction procedure was designed and tested to differentiate solid phase arsenic speciation, including adsorbed As and As coprecipitated with amorphous and crystalline oxides and sulfides. The sequential extraction was performed on Wells G & H wetland (Woburn, MA) sediments, and the inferred As associations were compared to XANES analyses. Geochemical modeling was used to predict redox conditions and As associations. We found that most As in the wetland was adsorbed onto amorphous Fe (hydr)oxide phases. Riverbed sediments differed from wetland sediments in that redox conditions were more reducing, and As was associated with more reducing and crystalline phases, including sulfides. As(lll) and As(V) oxidation states coexist in the wetland and riverbed sediments, with more oxidized As in the wetland. We tested the hypothesis that As associations with more oxidized phases in the wetland may result from wetland plant activities, including root oxygenation in anoxic sediments. We investigated the extent of Fe plaque formation on Typha latifolia roots (cattail), and the mechanism of As sequestration in plaques and (near-root) rhizosphere sediment. The plaque was approximately 30 gm thick, with a strong correlation between As and Fe, as determined by XRF microtomography. Most As was adsorbed, likely to Fe hydroxides in the plaque. Root plaque oxidation state maps showed that both As(Ill) and As(V) were retained in the plaque, suggesting that the plaque sequesters even the more mobile As(Ill) species. Mass balance analysis of As revealed that the plaque was a significant sink of aqueous As. We concluded that the As retained in the Wells G & H wetland is susceptible to redox changes (particularly reduction of Fe hydroxides onto which most As is adsorbed) and displacement by competitive anions. Iron plaque formation on T. latifolia roots was shown to affect As cycling within root zone contaminated sediments by providing a substrate onto which aqueous As can adsorb. Thesis Supervisor: Harold F. Hemond Title: Professor of Civil and Environmental Engineering