Removal of Arsenate and Arsenite in Equimolar Ferrous and Ferric Sulfate Solutions through Mineral Coprecipitation: Formation of Sulfate Green Rust, Goethite, and Lepidocrocite

Abstract

An improved understanding of in situ mineralization in the presence of dissolved arsenic and both ferrous and ferric iron is necessary because it is an important geochemical process in the fate and transformation of arsenic and iron in groundwater systems. This work aimed at evaluating mineral phases that could form and the related transformation of arsenic species during coprecipitation. 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 of 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.

Keywords: Raman spectroscopy; X-ray absorption spectroscopy; arsenic coprecipitation; iron oxides; redox transformation.

Figure 1.

(A): X-ray diffractogram of coprecipitate minerals formed at varying dissolved arsenate concentrations of 0, 0.5, 1.0, 5.0, 10.0, 50.0, and 100 mM (L = lepidocrocite, G = goethite); (B) X-ray diffractogram of coprecipitate minerals formed at varying dissolved arsenite concentrations of 0, 0.5, 1.0, 5.0, 10.0, 50.0, and 100 mM (SGR = sulfate green rust, L = lepidocrocite, G = goethite).

Figure 2.

Raman spectra of coprecipitates formed at varying initial concentrations of dissolved As(V) (A) and As(III) (B).

Figure 3.

Normalized As XANES spectra and quantitative fit deconvolutions with reference As(V) (11876 eV; white line) and As(III) (11873 eV; white line) and spectra for coprecipitates at various initial As(III) concentrations (0.5–100 mM).