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TECHNICAL REPORTS
Heavy Metals in the Environment

Adsorption of Arsenate and Arsenite on Ferrihydrite in the Presence and Absence of Dissolved Organic Carbon

^a Dep. of Plant and Soil Sciences, Univ. of Delaware, 152 Townsend Hall, Newark, DE 19717
^b Dep. of Crop and Soil, Environmental Science, 236 Smyth Hall, Virginia Polytech. Inst. & State Univ., Blacksburg, VA 24061
^c Dep. of Plants, Soils, and Biometeorology, College of Agriculture, Utah State Univ., Logan, UT 84322

* Corresponding author (eick{at}vt.edu)

Received for publication January 19, 2001. The adsorption of As(V) and As(III) on synthetic two-line ferrihydrite in the presence and absence of a peat humic acid (HA_p), Suwannee River fulvic acid (FA), or citric acid (CA) was investigated. Previous work with goethite has demonstrated the ability of dissolved organic carbon (DOC) to decrease As(V) and As(III) adsorption. The results obtained demonstrate that As(V) adsorption on ferrihydrite was decreased only in the presence of CA. Arsenate decreased the adsorption of all organic acids except HA_p. Both FA and CA reduced As(III) adsorption on ferrihydrite, while HA_p had no effect. Fulvic and citric acid adsorption on ferrihydrite was decreased in the presence of As(III); however, FA and CA adsorption increased at lower pH, which was consistent with decreased As(III) adsorption. Peat humic acid did not decrease As(III) adsorption, and we believe that the adsorption process of HA_p and As(III) and As(V) on ferrihydrite are independent of each other. Previously, we observed that As(V) adsorption on goethite decreased in the presence of HA_p > FA > CA, while As(III) adsorption on goethite was decreased similarly to that on ferrihydrite in the presence of CA > FA HA_p, yet As(III) adsorption on ferrihydrite was greater than on goethite. The observed differences between this study and the earlier study on goethite are believed to be an intricate function of ferrihydrite's surface characteristics, which affect the mechanisms of adsorption and hence the affinity of organic acids such as HA_p, FA, and CA for the ferrihydrite surface. As such, the adsorption of DOCs to ferrihydrite are assumed to be less favorable and to occur with a fewer number of ligands, resulting in lower surface coverage of weaker bond strength.

Abbreviations: AHS, acidified humic substance • CA, citric acid • DOC, dissolved organic carbon • DOM, dissolved organic matter • FA, Suwannee River fulvic acid • HA_p, peat humic acid

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