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SORPTION OF ARSENATE AND ARSENITE ON A RUTHENIUM COMPOUND: A MACROSCOPIC AND MICROSCOPIC STUDY
Citation:
Impellitteri**, C A., K G. Scheckel*, G. M. Hettiarachchi**, J A. Ryan*, AND P M. Randall*. SORPTION OF ARSENATE AND ARSENITE ON A RUTHENIUM COMPOUND: A MACROSCOPIC AND MICROSCOPIC STUDY. American Institute of Chemical Engineer 2002 Annual Meeting, Indianapolis, IN, 11/3-8/2002.
Description:
Sorption of arsenate and arsenite was examined on a ruthenium compound using macroscopic and microscopic techniques. Batch sorption experiments at pH 4,5,6, 7 and 8 were employed to construct constant solid solution ratio isotherms (CSI). After equilibration at the appropriate pH, sample supernatant was analyzed for total As by inductively coupled argon plasma spectrometry (lCAP). The oxidation state of the As in the supernatants was assessed by separation on ion exchange chromatography media followed by lCAP analysis. For spectroscopic studies, samples of the ruthenium compound were equilibrated with arsenite and arsenate at pH 7 and either dried or left in an oxygen free environment for different time periods (4 weeks to 1 day). The samples were then analyzed for solid phase oxidation state of As and molecular surface bonding characteristics of As by X-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine edge spectroscopy (EXAFS). The results show that the ruthenium compound has a high affinity for both arsenate and arsenite. Sorption capacity decreases with increasing pH. Speciation of the supernatants indicates that As initially added as arsenite is oxidized to arsenate by the ruthenium compound. This result is supported by the spectroscopic studies. XANES data indicate that the solid phase oxidation state of As is As(V) (arsenate) in all of the samples regardless of the initial oxidation state of As. The bound As remained as As(V) for all of the time periods and oxic/anoxic conditions studied. It is probablle that a two-step reaction occurs for arsenite where the sorbent oxidizes the arsenite to arsenate followed by a sorption reaction. EXAFS data suggest that the sorption reaction results in a tightly bound inner-sphere complex between As(V) and the ruthenium compound. .