Citation:

Su, C. AND Richard T. Wilkin. Removal of Arsenate and Arsenite in Equimolar Ferrous and Ferric Sulfate Solutions through Mineral Coprecipitation: Formation of Sulfate Green Rust, Goethite, and Lepidocrocite. Soil Systems. MDPI, Basel, Switzerland, 4(4):68, (2020). https://doi.org/10.3390/soilsystems4040068

Impact/Purpose:

Groundwater arsenic contamination is a global problem and there is a lack of knowledge on the degree of arsenic removal and transformation in the presence of both ferrous and ferric ions through mineral coprecipitation. This study aimed at shedding light on that question. The results showed that concentrations and species of arsenic in the parent solution influence the mineralogy of coprecipitated solid phases, which in turn affects As redox transformations. Our findings have important implications for evaluating the behavior of arsenic in chemically reducing environment.

Description:

In situ formation of mineral phases in the presence of dissolved arsenic and both ferrous and ferric iron is not well understood, yet it is an important geochemical process in the fate and transformation of arsenic and iron in groundwater systems. We conducted batch tests to precipitate ferrous (133 mM) and ferric (133 mM) ions in sulfate (533 mM) solutions spiked with As (0-100 mM As(V) or As(III)) and titrated with solid NaOH (400 mM). Goethite and lepidocrocite were formed at 0.5–5 mM As(V) or As(III). Only lepidocrocite formed at 10 mM As(III). Only goethite formed in the absence of added As(V) or As(III). Iron (II, III) hydroxysulfate green rust (sulfate green rust or SGR) was formed at 50 mM As(III) at an equilibrium pH 6.34. X-ray analysis indicated that amorphous solid products were formed at 10–100 mM As(V) or 100 mM As(III). The batch tests showed that As removal ranged from 98.65–100%. Total arsenic concentrations in the formed solid phases increased with the initial solution arsenic concentrations ranging from 1.85–20.7 g kg−1. Substantial oxidation of initially added As(III) to As(V) occurred; whereas, As(V) reduction did not occur. This study demonstrates that concentrations and species of arsenic in the parent solution influence the mineralogy of coprecipitated solid phases, which in turn affects As redox transformations.

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