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Solid Phase Speciation and Solubility of Vanadium in Highly Weathered Soils Worachart Wisawapipat, and Ruben Kretzschmar Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01005 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017
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Environmental Science & Technology
Solid Phase Speciation and Solubility of Vanadium in Highly Weathered Soils Worachart Wisawapipat *,1,2 and Ruben Kretzschmar*,2
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Department of Soil Science, Faculty of Agriculture, Kasetsart University,
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Bangkok 10900, Thailand 2
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Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich,
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CHN, 8092 Zürich, Switzerland
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TOC–ART
*
Corresponding Authors
W.W. E-mail:
[email protected]; Phone: + 66–2–9428104 ext. 208; Fax: + 66–2–4415910 R.K. E-mail:
[email protected]; Phone: +41–44–6336003; Fax: +41–44–6331118
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Environmental Science & Technology
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ABSTRACT
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Vanadium (V) is increasingly recognized both as a medical trace element with essential
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biological functions and as a potentially toxic environmental pollutant, yet the current
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knowledge on V speciation in soils is limited. Here, we investigated the chemical speciation
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and extractability of V in highly weathered tropical soils, which are often rich in V compared
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to soils of temperate climatic regions. Vanadium K-edge X-ray absorption near edge structure
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(XANES) spectra of soil samples, along with a range of reference compounds differing in V-
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oxidation state and coordination chemistry, revealed the predominance of V4+/5+ in a primarily
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octahedral or tetrahedral coordination. The soil spectra were best fitted with linear
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combinations of reference spectra of V4+ in the structure of kaolinite, V5+ adsorbed to kaolinite,
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and other V5+-sorbed solids. Vanadate adsorbed to goethite, ferrihydrite, gibbsite, and/or
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Fe(III)–natural organic matter complexes and V4+ in the structure of goethite may be present,
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but cannot unequivocally be distinguished from each other by XANES spectroscopy.
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Sequential and single chemical extractions provided complementary information on the
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solubility of V under various conditions. The most labile V fractions, interpreted as weakly and
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strongly adsorbed V5+, are the most relevant to V mobility and bioavailability in the
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environment, and accounted for only ~2% of total soil V. Our results demonstrate that kaolinite
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and Fe oxides can effectively sequester V in highly weathered soils by mechanisms of
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adsorption and structural incorporation, and are relevant to other Fe-oxide-rich environments
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under acidic and oxic conditions.
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Environmental Science & Technology
INTRODUCTION
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Vanadium (V) has been recently recognized as a medical element for humans, since it
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plays indispensable roles in biological systems particularly assisting the alleviation of
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diabetes,1, 2 hyperlipidemia and hypertension,3 and certain types of cancer.4 However, V has
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also been notorious as a toxic element at equivalent levels to arsenic when taken up at elevated
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concentrations.5-7 The increasing demand for V in industry, e.g., for steel alloys, sulfuric acid
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production and mass energy storage batteries, along with V leaching from petroleum coke and V
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emission from fossil combustion bears the risk of increasing environmental contamination.8-11
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Bioavailability and toxicity of V to organisms are greatly influenced by its oxidation state
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and coordination chemistry.12 In aqueous systems, V can exist in multiple oxidation states (V2+,
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V3+, V4+, and V5+) depending on solution pH, redox conditions, and the type of ligands.13, 14
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Vanadyl (V4+) and vanadate (V5+) species are the most stable forms under oxic conditions and
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in most biological systems. Solubility of V4+ and V5+ is higher than for lower oxidation states
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(V2+ and V3+), resulting in more profound impacts on biological activity. V4+ occurs as cationic
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VO2 + or VO(OH)+ species, whereas V5+ occurs mainly as oxyanions (VO43-, HVO42- and
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H2VO4-) that are thought to affect biota most strongly, especially being mutagenic and
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carcinogenic.10 Cationic V5+ species (VO2+), however, occur under extremely acidic conditions
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(pH
