Humic acid impacts antimony partitioning and speciation during iron(II)-induced ferrihydrite transformation

Niloofar Karimian; Edward D Burton; Scott G Johnston; Kerstin Hockmann; Girish Choppala

doi:10.1016/j.scitotenv.2019.05.305

Humic acid impacts antimony partitioning and speciation during iron(II)-induced ferrihydrite transformation

Sci Total Environ. 2019 Sep 15:683:399-410. doi: 10.1016/j.scitotenv.2019.05.305. Epub 2019 May 21.

Authors

Niloofar Karimian¹, Edward D Burton², Scott G Johnston², Kerstin Hockmann³, Girish Choppala²

Affiliations

¹ Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia. Electronic address: niloofar.karimian@scu.edu.au.
² Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia.
³ University of Bayreuth Environmental Geochemistry, Bayreuth Center for Ecology and Environmental Research (BayCEER), Universitaetsstrasse 30, 95440 Bayreuth, Germany.

PMID: 31141743
DOI: 10.1016/j.scitotenv.2019.05.305

Abstract

The Fe(II)-induced transformation of ferrihydrite, a potent scavenger for antimony (Sb), can considerably influence Sb mobility in reducing soils, sediments and groundwater systems. In these environments, humic acids (HA) are prevalent, yet their influence on Sb behaviour during ferrihydrite transformation is poorly understood. In this study, we investigated the effect of HA on (1) Sb partitioning between solid, colloidal and dissolved phases and (2) Sb redox speciation during the Fe(II)-induced transformation of Sb(V)-bearing ferrihydrite at pH 6.0 and 8.0 and Fe(II) concentrations of 0, 1 and 10 mM. The results show that, at pH 8.0 and in the presence of 10 mM Fe(II), ferrihydrite was replaced by goethite, lepidocrocite and magnetite across a wide range of HA concentrations. At pH 6.0 in the 10 mM Fe(II) treatments, ferrihydrite transformed to mainly lepidocrocite and goethite in both HA-free and low HA treatments. In contrast, high HA concentrations retarded the rate and extent of ferrihydrite transformation at both pH 6.0 and 8.0 in the 1 mM Fe(II) treatments. Antimony K-edge XANES spectroscopy revealed up to 60% reduction of solid-phase Sb(V) to Sb(III), which corresponded with an increase in the PO₄^3--extractable fraction of solid-phase Sb in HA- and Fe(II)-rich conditions at pH 8.0. In contrast to the observations at pH 8.0, minimal reduction of solid-phase Sb(V) was observed in the pH 6.0 treatments with the highest HA content, yet some reduction of Sb(V) occurred (~30-40%) at intermediate HA concentrations. Humic acid-rich conditions were also found to promote the formation of substantial amounts of colloidal Sb in the <0.45 μm to 3 kDa size range at both pH 6.0 and 8.0. Our results demonstrate that HA can exert an important control on the partitioning, mobility and speciation of Sb during Fe(II)-induced transformation of ferrihydrite in sub-surface environments.

Keywords: Antimony; Fe(II); Ferrihydrite; Humic acid; pH.