Issue 9, 2019

Emerging investigator series: interdependency of green rust transformation and the partitioning and binding mode of arsenic

Abstract

We investigated the impact of aging-induced structural modifications of carbonate green rust (GR), a mixed valent Fe(II,III) (hydr)oxide with a high oxyanion sorption affinity, on the partitioning and binding mode of arsenic (As). Suspensions of carbonate GR were produced in the presence of As(V) or As(III) (i.e. co-precipitated with As(III) or As(V)) and aged in anoxic and oxic conditions for up to a year. We tracked aqueous As over time and characterized the solid phase by X-ray absorption spectroscopy (XAS). In experiments with initial As(V) (4500 μg L−1, As/Fe = 2 mol%), the fresh GR suspension sorbed >99% of the initial As, resulting in approximately 14 ± 8 μg L−1 residual dissolved As. Anoxic aging of the As(V)-laden GR for a month increased aqueous As to >60 μg L−1, which was coupled to an increase in GR structural order revealed by Fe K-edge XAS. Further anoxic aging up to a year transformed As(V)-laden GR into magnetite and decreased significantly the aqueous As to <2 μg L−1. The As binding mode was also modified during GR transformation to magnetite from sorption to GR particle edges to As substitution for tetrahedral Fe in the magnetite structure. These GR structural modifications altered the ratio of As partitioning to the solid (μg As/mg Fe) and liquid (μg As per L) phase from 2.0 to 0.4 to 14 L mg−1 for the fresh, month, and year aged suspensions, respectively. Similar trends in GR transformation and As partitioning during anoxic aging were observed for As(III)-laden suspensions, but occurred on more rapid timescales: As(III)-laden GR transformed to magnetite after a day of anoxic aging. In oxic aging experiments, rapid GR oxidation by dissolved oxygen to Fe(III) precipitates required only an hour for both As(V) and As(III) experiments, with lepidocrocite favored in As(V) experiments and hydrous ferric oxide favored in As(III) experiments. Aqueous As during GR oxidation decreased to <10 μg L−1 for both As(V) and As(III) series. Knowledge of this interdependence between GR aging products and oxyanion fate improves biogeochemical models of contaminant and nutrient dynamics during Fe cycling and can be used to design more effective arsenic remediation strategies that rely on arsenic sorption to GR.

Graphical abstract: Emerging investigator series: interdependency of green rust transformation and the partitioning and binding mode of arsenic

Supplementary files

Article information

Article type
Paper
Submitted
31 may 2019
Accepted
19 iyl 2019
First published
22 iyl 2019
This article is Open Access
Creative Commons BY license

Environ. Sci.: Processes Impacts, 2019,21, 1459-1476

Emerging investigator series: interdependency of green rust transformation and the partitioning and binding mode of arsenic

C. M. van Genuchten, T. Behrends and K. Dideriksen, Environ. Sci.: Processes Impacts, 2019, 21, 1459 DOI: 10.1039/C9EM00267G

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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