Reductive dissolution of As(V)-bearing Fe(III)-precipitates formed by Fe(II) oxidation in aqueous solutions

Abstract

Iron(III)-precipitates formed by the oxidation of dissolved Fe(II) are important sorbents for major and trace elements in aquatic and terrestrial systems. Their reductive dissolution in turn may result in the release of associated elements. We examined the reductive dissolution kinetics of an environmentally relevant set of Fe(II)-derived arsenate-containing Fe(III)-precipitates whose structure as function of phosphate (P) and silicate (Si) content varied between poorly-crystalline lepidocrocite, amorphous Fe(III)-phosphate, and Si-containing ferrihydrite. The experiments were performed with 0.2–0.5 mM precipitate-Fe(III) using 10 mM Na-ascorbate as reductant, 5 mM bipyridine as Fe(II)-complexing ligand, and 10 mM MOPS/5 mM NaOH as pH 7.0 buffer. Times required for the dissolution of half of the precipitate (t50%) ranged from 1.5 to 39 h; spanning a factor 25 range. At loadings up to ~ 0.2 P/Fe (molar ratio), phosphate decreased the t50% of Si-free precipitates, probably by reducing the crystallinity of lepidocrocite. The reductive dissolution of Fe(III)-phosphates formed at higher P/Fe ratios was again slower, possibly due to P-inhibited ascorbate binding to precipitate-Fe(III). The slowest reductive dissolution was observed for P-free Si-ferrihydrite with ~ 0.1 Si/Fe, suggesting that silicate binding and polymerization may reduce surface accessibility. The inhibiting effect of Si was reduced by phosphate. Dried-resuspended precipitates dissolved 1.0 to 1.8-times more slowly than precipitates that were kept wet after synthesis, most probably because drying enhanced nanoparticle aggregation. Variations in the reductive dissolution kinetics of Fe(II) oxidation products as reported from this study should be taken into account when addressing the impact of such precipitates on the environmental cycling of co-transformed nutrients and contaminants.Electronic supplementary materialThe online version of this article (10.1186/s12932-019-0062-2) contains supplementary material, which is available to authorized users.

Groundwater As Removal by As(III), Fe(II), and Mn(II) Co-Oxidation: Contrasting As Removal Pathways with O2, NaOCl, and KMnO4

“…Although the twice as high residual As in the NaOCl system (Si700+Ca1 = 4.0 μg/L) compared to the KMnO 4 system (Si700+Ca1 = 1.8 μg/L) is only several μg/L, this disparity in As removal can easily result in noncompliance with stringent As drinking water targets (e.g., 5 μg/L in New Hampshire and New Jersey, USA; 1 μg/L in The Netherlands). − While we show that KMnO 4 can benefit As­(III) treatment, there are some potential drawbacks. For example, As can be released from the surface of both Fe­(III) precipitates , and MnO 2 during aging and transformation, , but the mobilization of As from MnO 2 is a greater concern because MnO 2 minerals are more susceptible to reductive dissolution (e.g., catalyzed by light or organic carbon) than Fe­(III) precipitates. , In addition, colloidally stable suspensions formed when KMnO 4 was dosed to Ca-free solutions, consistent with the role of bivalent cations to aggregate MnO 2 , , though we note groundwater free of bivalent cations is uncommon . Therefore, the trade-off between enhanced As­(III) removal and decreased particle separation efficiency and the potential for As release from the solid phase must be considered before implementing KMnO 4 in treatment plants.…”

“…5−7 While we show that KMnO 4 can benefit As(III) treatment, there are some potential drawbacks. For example, As can be released from the surface of both Fe(III) precipitates 71,81 and MnO 2 during aging and transformation, 54,82 but the mobilization of As from MnO 2 is a greater concern because MnO 2 minerals are more susceptible to reductive dissolution (e.g., catalyzed by light or organic carbon) than Fe(III) precipitates. 54,82 In addition, colloidally stable suspensions formed when KMnO 4 was dosed to Ca-free solutions, consistent with the role of bivalent cations to aggregate MnO 2 , 83,84 though we note groundwater free of bivalent cations is uncommon.…”

Effect of Sb on precipitation of biogenic minerals during the reduction of Sb-bearing ferrihydrites

“…2). The observed decrease in the rate of reduction may be related to the decline of a significant number of reactive sites on the Fh surface due to Sb(V) loading (Fredrickson et al, 2001;Ekstrom et al, 2010;Voegelin et al, 2019).…”

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

“…The effect of phosphate competition on As retention has been well documented (Dixit and Hering, 2003;Hug et al, 2008;Meng et al, 2002;Voegelin et al, 2019Voegelin et al, , 2010, with phosphate commonly outcompeting As for sorption sites. Increased As release was concurrent with rising bicarbonate concentrations, resulting from lactate mineralisation, which may have further enhanced the adverse influence of phosphate.…”