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

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Complexation and precipitation reactions in the ternary As(V)-Fe(III)-OM (organic matter) system. / Sundman, Anneli; Karlsson, Torbjorn; Sjoberg, Staffan; Persson, Per.

In: Geochimica et Cosmochimica Acta, Vol. 145, 2014, p. 297-314.

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Sundman, Anneli ; Karlsson, Torbjorn ; Sjoberg, Staffan ; Persson, Per. / Complexation and precipitation reactions in the ternary As(V)-Fe(III)-OM (organic matter) system. In: Geochimica et Cosmochimica Acta. 2014 ; Vol. 145. pp. 297-314.

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TY - JOUR

T1 - Complexation and precipitation reactions in the ternary As(V)-Fe(III)-OM (organic matter) system

AU - Sundman, Anneli

AU - Karlsson, Torbjorn

AU - Sjoberg, Staffan

AU - Persson, Per

PY - 2014

Y1 - 2014

N2 - 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.

AB - 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.

U2 - 10.1016/j.gca.2014.09.036

DO - 10.1016/j.gca.2014.09.036

M3 - Article

VL - 145

SP - 297

EP - 314

JO - Geochmica et Cosmochimica Acta

JF - Geochmica et Cosmochimica Acta

SN - 0016-7037

ER -