Enhanced and stabilized arsenic retention in microcosms through the microbial oxidation of ferrous iron by nitrate

Highlights

• Magnetite is advantageous as the host-mineral for As immobilization.

• We conduct microcosms with sediment and groundwater from an As-contaminated site.

• We trace the evolution of water composition and sediment mineralogy concurrently.

• Addition of ferrous Fe and nitrate produces mineral assemblage including magnetite.

• The study represents an initial attempt to produce relatively stable As sequesters.

Abstract

Magnetite strongly retains As, and is relatively stable under Fe(III)-reducing conditions common in aquifers that release As. Here, laboratory microcosm experiments were conducted to investigate a potential As remediation method involving magnetite formation, using groundwater and sediments from the Vineland Superfund site. The microcosms were amended with various combinations of nitrate, Fe(II) _(aq) (as ferrous sulfate) and lactate, and were incubated for more than 5 weeks. In the microcosms enriched with 10 mM nitrate and 5 mM Fe(II) _(aq), black magnetic particles were produced, and As removal from solution was observed even under sustained Fe(III) reduction stimulated by the addition of 10 mM lactate. The enhanced As retention was mainly attributed to co-precipitation within magnetite and adsorption on a mixture of magnetite and ferrihydrite. Sequential chemical extraction, X-ray absorption spectroscopy and magnetic susceptibility measurements showed that these minerals formed at pH 6–7 following nitrate-Fe(II) addition, and As-bearing magnetite was stable under reducing conditions. Scanning electron microscopy and X-ray diffraction indicated that nano-particulate magnetite was produced as coatings on fine sediments, and no aging effect was detected on morphology over the course of incubation. These results suggest that a magnetite based strategy may be a long-term remedial option for As-contaminated aquifers.

Introduction

Arsenic (As) is the 2nd most common contaminant of concern at the U.S. Superfund sites, with nearly 50% of sites having groundwater As related problems (ATSDR, 2013). Mitigating groundwater As contamination is urgently required, however, has proven difficult. Encapsulation technology, common in soil remediation, is typically impractical for diffuse contaminants. Pump-and-treat (P&T), which can control groundwater migration, is commonly used for groundwater remediation (EPA, 2003). This method is hampered for As because of slow and inefficient desorption from aquifer sediments (Wovkulich et al., 2012, Wovkulich et al., 2014). One attractive option for remediating groundwater As is in situ immobilization, which stimulates mineral formation within the aquifer to adsorb and/or precipitate As.

Iron minerals are commonly used as As sorbents (Benner et al., 2002, Dixit and Hering, 2003, Kneebone et al., 2002, Pedersen et al., 2006). The limited stability of Fe minerals under reducing conditions, however, represents a major challenge to the development of As in situ immobilization in aquifers. Reducing conditions common in aquifers can lead to reductive dissolution of many common Fe(III) (oxyhydr)oxide minerals, remobilizing the associated As (Benner et al., 2002). Arsenic can also adsorb on or co-precipitate within Fe(II) sulfide minerals (O'Day et al., 2004, Saalfield and Bostick, 2009). Sulfide-based As immobilization strategies can be effective under some conditions (Keimowitz et al., 2007, O'Day et al., 2004, Onstott et al., 2011), but these methods often are limited by sulfide mineral oxidation, and As can be solubilized as thiolated As complexes in sulfidic solutions (Wilkin et al., 2003).

Magnetite, Fe₃O₄, is a mixed-valence Fe oxide containing both tetrahedral and octahedral Fe(III) (Coker et al., 2006, Schwertmann and Cornell, 2008). Unlike many Fe minerals, magnetite is stable in a wide range of conditions including Fe(III)-reducing environments where As is typically mobilized and bioavailable (An Fe Eh-pH diagram is provided in the supplementary material (SM) Fig. S1, also showing in situ redox conditions at selected Superfund sites) (de Lemos et al., 2006, He et al., 2010, Keimowitz et al., 2005, Lipfert et al., 2006, Wovkulich et al., 2014), although biogenic and abiogenic magnetite may have different stability towards microbial Fe(III) reduction (Muehe et al., 2013b, Piepenbrock et al., 2011). Magnetite can scavenge both As(V) and As(III) from solutions through surface adsorption (Chowdhury et al., 2011, Gimenez et al., 2007, Pedersen et al., 2006, Wang et al., 2008, Yean et al., 2005). It is also capable of co-precipitating tetrahedral As(V) as well as a number of cations (e.g., Al³⁺, Ti⁴⁺, Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺) into its crystal structure (Coker et al., 2006, Cooper et al., 2000, Jang et al., 2003, Muehe et al., 2013a, Saalfield and Bostick, 2009, Schwertmann and Cornell, 2008). So far, the potential of in situ immobilization of contaminants on magnetite has seldom been applied to groundwater remediation efforts. This is in part because magnetite is typically synthesized through reductive pathways of Fe(III) oxyhydroxides at pH > 7.5 (SM Table S1), while aquifer pHs at As-contaminated sites are commonly circumneutral to acidic, i.e., conditions under which magnetite production through this pathway is inhibited (Ayala-Luis et al., 2008, Hansel et al., 2005, Jang et al., 2003, Jolivet et al., 2002, Schwertmann and Cornell, 2008, Tronc et al., 1992). Magnetite formation can also be stimulated by Fe(II)-oxidizing bacteria (Chaudhuri et al., 2001, Dippon et al., 2012), and dissolved As can be sequestered through such microbial oxidation (Hohmann et al., 2011, Hohmann et al., 2010, Senn and Hemond, 2002). Given the stability of magnetite under a relatively broad range of aquifer redox conditions, if reliably produced, magnetite could not only achieve but also maintain low groundwater As concentrations.

The objectives of the present study were to stimulate magnetite formation in amended microcosms containing natural aquifer sediments and groundwater, and to evaluate the effect on immobilizing As. The evolution of solution composition and sediment mineralogical transformations in microcosms were concurrently traced using several techniques. In all, our data suggest that magnetite formation can be achieved by the microbial oxidation of Fe(II) by nitrate, even under mildly acidic pH conditions, and that the combination of magnetite and other Fe oxides effectively sequesters dissolved As even under sustained Fe(III) reduction.