In situ arsenic immobilisation for coastal aquifers using stimulated iron cycling: Lab-based viability assessment

Abstract

Arsenic (As) is one of the most harmful and widespread groundwater contaminants globally. Besides the occurrence of geogenic As pollution, there is also a large number of sites that have been polluted by anthropogenic activities, with many of those requiring active remediation to reduce their environmental impact. Cost-effective remedial strategies are however still sorely needed. At the laboratory-scale in situ formation of magnetite through the joint addition of nitrate and Fe(II) has shown to be a promising new technique. However, its applicability under a wider range of environmental conditions still needs to be assessed. Here we use sediment and groundwater from a severely polluted coastal aquifer and explore the efficiency of nitrate-Fe(II) treatments in mitigating dissolved As concentrations. In selected experiments >99% of dissolved As was removed, compared to unamended controls, and maintained upon addition of lactate, a labile organic carbon source. Pre- and post experimental characterisation of iron (Fe) mineral phases suggested a >90% loss of amorphous Fe oxides in favour of increased crystalline, recalcitrant oxide and sulfide phases. Magnetite formation did not occur via the nitrate-dependent oxidation of the amended Fe(II) as originally expected. Instead, magnetite is thought to have formed by the Fe(II)-catalysed transformation of pre-existing amorphous and crystalline Fe oxides. The extent of amorphous and crystalline Fe oxide transformation was then limited by the exhaustion of dissolved Fe(II). Elevated phosphate concentrations lowered the treatment efficacy indicating joint removal of phosphate is necessary for maximum impact. The remedial efficiency was not impacted by varying salinities, thus rendering the tested approach a viable remediation method for coastal aquifers.

Keywords: arsenic remediation; bioremediation; coastal aquifer; in situ mineral precipitation.

Figure 1

Total sediment Fe (top) and As (bottom) concentrations as determined by acid digestion for three site sediments, E1.1, E1.2 and E1.3 collected 11 to 12 m below surface. The reactive fraction is the sum of sequential extraction analysis results subtracted from acid digest totals (orange). The remainder is referred to as unreactive Fe and As (grey).

Figure 2

Sequential extraction Fe mineralogy fractions (top) and As mineralogical associations (bottom) averaged from three site sediments (E1.1, E1.2 and E1.3). Initial fractions (far left) compared to final fractionations in microcosms (from left to right): artificial water treated with 10 mM FeSO₄ and 10 mM NaNO₃ (AWT), groundwater treated with 10 mM FeSO₄ and 10 mM NaNO₃ (GWT), the respective artificial water control (AWC) and the respective groundwater control (GWC).

Figure 3

Dissolved As (top) and Fe (bottom) concentrations in artificial water microcosms (left, circles) and site groundwater microcosms (right, triangles). Data presented is the average of the three E1 scenarios (Table S1). FeSO₄ (10 mM) and NaNO₃ (10 mM) treatment results are shown in solid blue and comparative controls (no FeSO₄, NaNO₃ or lactate) in hollow red. Concentrations are reported in mM. Grey shading indicates 1 mM lactate addition in treatments only.

Figure 4

A: Day 3 FeSO₄ (10 mM) and NaNO₃ (10 mM) treatment (first, third and fifth pairs as duplicates) and respective control microcosms (second, fourth and sixth pairs as duplicates, no FeSO₄ or NaNO₃). B: Day 9 FeSO₄ (10 mM) and NaNO₃ (10 mM) treatments showing progressive colour change to red brown (first, third and fifth pairs as duplicates) and respective control microcosms (second, fourth and sixth pairs as duplicates, no FeSO₄ or NaNO₃) showing no colour change C: Day 9 (left), Day 12 (centre) and Day 34 (right) SO₄-lactate (10 mM) microcosm duplicate pairs showing progressive colour change to black.

Figure 5

Solution pH (top), alkalinity (middle) and calcium (bottom) concentrations in artificial water microcosms (left, circles) and site groundwater microcosms (right, triangles). Data presented is the average of the three E1 scenarios (Table S1). FeSO₄ (10 mM) and NaNO₃ (10mM) treatment results are shown in solid blue and comparative controls (no FeSO₄, NaNO₃ or lactate) in hollow red. Concentrations are reported in pH units, meq CaCO₃ L⁻¹ and mM, respectively. Grey shading indicates 1 mM lactate addition from Day 12 in the treatment microcosms only.

Figure 6

Dissolved As (left) and Fe (right) concentrations in E.2 (Table S2) SO₄-lactate (10 mM added from the start) treated microcosms of low salinity (5.3 mS cm⁻¹) (solid stars) and high salinity (15.5 mS cm⁻¹) (hollow triangles). Reported in mM.

Figure 7

Dissolved As (left) and Fe (right) concentrations from three E2 microcosms (Table S2) amended with FeSO₄ (5 mM) and NaNO₃ (10 mM). (a) E2.2 being low (1.5 mM) phosphate (1.5 mM) and low salinity (5.3 mS cm⁻¹) (solid stars), (b) E2.3 being low phosphate (1.5 mM) and high salinity (15.5 mS cm⁻¹) (hollow circles) and (c) E2.5 being high phosphate (4.5 mM) and low salinity (5.3 mS cm⁻¹) (hollow triangles). Reported in mM. Grey shading indicates 10 mM lactate addition from Day 12.

Figure 8

Solution Eh in artificial water microcosms (left, circles) and site groundwater microcosms (right, triangles). Data presented is the average of the three E1 scenarios (Table S1). FeSO₄ (10 mM) and NaNO₃ (10mM) treatment results are shown in solid blue and comparative controls (no FeSO₄, NaNO₃ or lactate) in hollow red. Concentrations are reported in mV. Grey shading indicates 1 mM lactate addition from Day 12.