Field, Experimental, and Modeling Study of Arsenic Partitioning across a Redox Transition in a Bangladesh Aquifer
Hun Bok Jung; Benjamin C. Bostick; Yan Zheng
- Field, Experimental, and Modeling Study of Arsenic Partitioning across a Redox Transition in a Bangladesh Aquifer
Jung, Hun Bok
Bostick, Benjamin C.
- Lamont-Doherty Earth Observatory
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- Environmental Science & Technology
- To understand redox-dependent arsenic partitioning, we performed batch sorption and desorption experiments using aquifer sands subjected to chemical and mineralogical characterization. Sands collected from the redox transition zone between reducing groundwater and oxic river water at the Meghna riverbank with HCl extractable Fe(III)/Fe ratio ranging from 0.32 to 0.74 are representative of the redox conditions of aquifers common in nature. One brown suboxic sediment displayed a partitioning coefficient (Kd) of 7â€“8 L kgâ€“1 at equilibrium with 100 Î¼g Lâ€“1 As(III), while two gray reducing sediments showed Kd of 1â€“2 L kgâ€“1. Lactate amendment to aquifer sands containing 91 mg kgâ€“1 P-extractable As resulted in the reduction of As and Fe with sediment Fe(III)/Fe decreasing from 0.54 to 0.44, and mobilized an equivalent of 64 mg kgâ€“1 As over a month. Desorption of As from nonlactate-amended sediment was negligible with little change in sediment Fe(III)/Fe. This release of As is consistent with microbial reduction of Fe(III) oxyhydroxides and the resulting decrease in the number of surface sites on Fe(III) oxyhydroxides. Arsenic partitioning (Kd) in iron-rich, sulfur-poor aquifers with circumneutral pH is redox-dependent and can be estimated by HCl leachable sediment Fe(III)/Fe ratio with typical Fe concentrations.
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