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Complexation and precipitation reactions in the ternary As(V)-Fe(III)-OM (organic matter) system

Author:
  • Anneli Sundman
  • Torbjorn Karlsson
  • Staffan Sjoberg
  • Per Persson
Publishing year: 2014
Language: English
Pages: 297-314
Publication/Series: Geochimica et Cosmochimica Acta
Volume: 145
Document type: Journal article
Publisher: Elsevier

Abstract english

The predominant forms of arsenic (As) in anoxic and oxic environments are As(III) and As(V), respectively, and the fate of these forms is influenced by interactions with mineral surfaces and organic matter (OM). Interactions between As(V) and OM are believed to occur mainly via iron(Fe)-bridges in ternary Fe-arsenate complexes, but direct evidence for these interactions are scarce. Furthermore, since the speciation of Fe in the presence of organic matter varies as a function of pH and Fe concentration, a central question is how different chemical conditions will affect the As-Fe-OM interactions. In order to answer this, the As(V)-Fe(III)-OM system have been studied under a large range of experimental conditions (6485-67,243 ppm Fe(III) and Fe(III): As(V) ratios of 0.5-20 at pH 3-7), with Suwannee River natural organic matter and Suwannee River fulvic acid as sources of OM, using Fe and As K-edge X-ray absorption spectroscopy (XAS), infrared (IR) spectroscopy and chemical equilibrium modeling. Our collective results showed that interactions in the ternary As(V)-Fe(III)-OM system were strongly influenced by pH, total concentrations and ratios among the reactive species. In particular, the high stability of the Fe(III)-OM complexes exerted a strong control on the speciation. The predominant species identified were mononuclear Fe(III)-OM complexes, Fe(III) (hydr) oxides and FeAsO4 solids. The experimental results also showed that at low concentrations the Fe(III)-OM complexes were sufficiently stable to prevent reaction with arsenate. The chemical equilibrium models developed corroborated the spectroscopic results and indicated that As(V) was distributed over two solid phases, namely FeAsO4(s) and Fe(OH)(1.5)(AsO4)(0.5)(s). Thus, neither ternary As(V)-Fe(III)-OM complexes nor As(V) surface complexes on Fe(III) (hydr) oxides were necessary to explain the collective results presented in this study. (C) 2014 Elsevier Ltd. All rights reserved.

Keywords

  • Biological Sciences
  • Earth and Related Environmental Sciences

Other

Published
  • MICCS - Molecular Interactions Controlling soil Carbon Sequestration
  • ISSN: 0016-7037
Per Persson
E-mail: per [dot] persson [at] biol [dot] lu [dot] se

Professor

MEMEG

+46 46 222 17 96

+46 70 266 38 79

E-C350

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Professor

Centre for Environmental and Climate Research (CEC)

+46 46 222 17 96

+46 70 266 38 79

D350

Ekologihuset, Sölvegatan 37, Lund

50