Environmental Pollution 210 (2016) 371e379
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Impact of soil pH and organic matter on the chemical bioavailability of vanadium species: The underlying basis for risk assessment* € Hartikainen Inka Reijonen*, Martina Metzler, Helina Soil and Environmental Chemistry, Department of Food and Environmental Sciences, Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 27, 00014 Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2015 Received in revised form 17 December 2015 Accepted 21 December 2015 Available online xxx
The main objective of this study was to unravel the chemical reactions and processes dictating the potential bioavailability of vanadium (V). In environmental solutions V exists in two stable oxidation states, þIV and þV, of which þ V is considered to be more toxic. In this study, the effect of speciation and soil pH on the chemical accessibility of V was investigated with two soils: 1) field soil rather rich in soil organic matter (SOM) and 2) coarse mineral soil low in SOM. Fresh soil samples treated with V(þV) (added as NaVO3) or V(þIV) (added as VOSO4) (pH adjusted to the range 4.0e6.9) were incubated for 3 months at 22 C. The adsorption tendency of V species was explored by water extraction (Milli-Q water, 1:50 dw/V) and by sequential extraction (0.25 M KCl; 0.1 M KH2/K2HPO4; 0.1 M NaOH; 0.25 M H2SO4, 1:10 dw/V). The potential bioavailability of V was found to be dictated by soil properties. SOM reduced V(þV) to V(þIV) and acted as a sorbent for both species, which lowered the bioaccessibility of V. A high pH, in turn, favored the predominance of the V(þV) species and thus increased the chemical accessibility of V. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bioavailability Vanadium Soil Speciation Chemistry
1. Introduction Vanadium (V) is a transition metal that exists in soils within a range of 20e110 mg kg1 (Kabata-Pendias, 2004). However, during the past decades industrial use and combustion of fossil fuels have considerably increased anthropogenic V emissions. Additionally, V is released to the soil from V bearing fertilizers, leachates from mine tailings, municipal sewage sludge and slags from steel manufacturing (ATSDR, 2012). Despite the elevating V loading, the research on the environmental chemistry of V is relatively scarce. However, knowledge of the chemical behavior of contaminants and pollutants in the environment creates a scientifically sound basis for risk assessment, remediation strategies and legislation. In soil, the mobility and potential bioavailability (availability to organisms) of elements relies on their chemical reactions and processes. In the case of V, this is particularly challenging due to its redox-sensitivity rending its chemistry complicated. In environmental conditions, it exists in two stable oxidation states, þIV and þV. Vanadium is also found in
*
This paper has been recommended for acceptance by Jay Gan. * Corresponding author. E-mail address: inka.reijonen@helsinki.fi (I. Reijonen).
http://dx.doi.org/10.1016/j.envpol.2015.12.046 0269-7491/© 2016 Elsevier Ltd. All rights reserved.
the oxidation state þ III in some primary and mafic minerals (Essington, 2015) but not in environmental solutions. Because bioaccessible species are considered to be readily or potentially soluble, V(þIII) is not relevant in the terms of chemical bioavailability. Nonetheless, research on the environmental risks related to potential bioavailability of various V species in waste deposits is urgently needed. The biological activity and toxicity of V is affected by its speciation, which varies considerably depending on the oxidation state. Vanadium has been identified as a micronutrient for several organisms, although its essentiality for humans has not been completely established (French and Jones, 1993; Domingo, 1996). Controversially, some V species are classified as teratogens and potential carcinogens (Rehder, 1991). Generally, the toxicity of V(þV) is considered greater than that of V(þIV) (Evangelou, 2002), which may be attributable to the analogy of V(þV) species with phosphate and consequently to inhibition (or in some case activation) of various phosphatases, ATPases and other important enzymes (Leonard and Gerber, 1998). Acute toxicity values (LD50) on mammals, e.g. rats, are considerably higher for V(þV) than V(þIV) (Thompson et al., 1998). However, information on the chronic toxicity of different V species is limited. At low concentrations and within pH ~3 to 7, inorganic V(þV)
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I. Reijonen et al. / Environmental Pollution 210 (2016) 371e379
exists mainly as monomeric H2VO-4 (Tracey et al., 2007). According to pKa values, further deprotonated oxyanionic vanadate species are prevalent only in alkaline conditions (Heath and Howarth, 1981): 2 þ H2 VO 4 ¼ HVO4 þ H 3 þ HVO2 4 ¼ VO4 þ H
pKa ¼ 7:1 pKa ¼ 12
However, in millimolar solutions the self-condensation of vanadate becomes critical. As a consequence, in mM concentrations (>51 mg V L1) and at pH 3e6 the predominant species of V(þV) is decavanadate (HVO10O5 28 ) (Tracey et al., 2007). This means that polymeric species can be formed at rather modest V concentrations. In soil solution, a mM concentration of V(þV) can be obtained, for instance, when more than 10% of the total V(þV) 100 mg kg1 is in the soluble fraction (at the water content of 20%). Therefore, the polymeric species are presumably present in notable amounts in V-contaminated soils. Furthermore, vanadate is able to form polymeric species with oxyanions such as phosphates and arsenates (Gresser and Tracey, 1990; Mikkonen and Tummavuori, 1994). In contrast to the anionic vanadate species, inorganic V(þIV) occurs as cationic species of vanadyl (VO2þ) (Crans et al., 1998). Above pH 4, it is hydrolyzed to form VO(OH)þ and upon increasing pH, further to the weakly soluble hydroxide, VO(OH)2 (Baes and Mesmer, 1976):
VO2þ þ H2 O ¼ VOðOHÞþ þ Hþ VOðOHÞþ þ H2 O ¼ VOðOHÞ2
ðsÞ
þ Hþ
The dissimilar chemical accessibility of V(þIV) and V(þV) in soil relies on the speciation chemistry. Generally, the mobility of V(þIV) in the biosphere is low due to the formation of hydrous oxides and organic complexes of low reactivity (Tracey et al., 2007). The covalent bonding of VO2þ with organic matter is very strong and for example the vanadium porphyrins in fossil fuels belong to the class I highest stability category in the Buhler classification system (Buchler and Smith, 1975). The solubility of inorganic hydrolyzed VO2þ species is limited also by specific adsorption e.g. on the surfaces of aluminum (hydr)oxides and smectite clays (McBride, 1979; Wehrli et al., 1990). In contrast to the reduced species, the mobility of V(þV) species is considered to be high (Tracey et al., 2007). However, in soil solution its concentration may be reduced by specific reversible adsorption onto iron (Fe)- and aluminum (Al)(hydr)oxides (Wehrli and Stumm, 1989; Peacock and Sherman, 2004). Conversion in the oxidation state affects the chemical bioavailability of V. On the one hand, the hydrolyzed species of V(þIV) can easily be oxidized to V(þV) by molecular oxygen (Wehrli and Stumm, 1989), which increases the mobility. On the other hand, in oxic conditions V(þV) can be reduced by soil organic components via abiotic and biological pathways. The reduction rate by soil organic matter (SOM) depends on its quality as well as on soil pH, temperature and microbiological activity. For instance, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Shewanella oneidensis are reported to mediate reduction of V(þV) (Carpentier et al., 2003; Bredberg et al., 2004). Generally, the microbial processes enhance the overall reduction by SOM. This reaction pattern is demonstrated, for instance, in the case of ferric iron Fe(III) and hexavalent chromium (Cr) (Chen et al., 2003; Chiu et al., 2009). Easily degradable organic matter is considered a better reducing material than humic substances: fulvic acids (FAs), humic acids (HA) and humin. However, the role of FAs and HAs in V reduction is acknowledged (Wilson and Weber, 1979).
Furthermore, the reduction of V(þV) is found to be followed by V(þIV) retention as an organic inner sphere complex. Interestingly, the reduction and adsorption reactions of V are depending on each other. Reduction of V(þV) to V(þIV) enhances the retention due to the greater affinity of vanadyl species for organic matter. However, complexation of V(þV) with organic substances, e.g. FAs, has also been demonstrated (Lu et al., 1998). Overall, the negative impact of anthropogenic V deposits on soil systems is alleviated by organic matter (OM) via two pathways. Firstly, the ecotoxicity of V is lowered by reduction of V(þV) to the less toxic V(þIV) species. Secondly, chemical bioavailability of V is limited by sorption onto SOM. Thus, FAs and other V(þV) binding humic substances are likely diminish toxicity beyond their reducing capacities by acting as adsorbents. The solubility of organic matter is dictated by pH. At higher pHs, the deprotonation of organic functional groups (e.g. phenolic and carboxylic groups) increases the hydrophilicity and, thus, the solubility of SOM. Consequently, also the solubility of organic V complexes increases. As for humus, this holds true for HAs. However, the humin fraction is not soluble either in alkali or acid whereas FAs are soluble regardless of the pH. In addition to SOM solubility, the redox reactions in soils are regulated by pH. At elevated pHs, reduction of V(þV) to (V þ IV) by SOM is declined whereas oxidation of V(þIV) to V(þV) by O2 is enhanced. This oxidation is particularly prevalent in circumneutral and alkaline conditions (Tracey et al., 2007). The prerequisite to assess the potential bioavailability of V is to know the pH-dependent reaction patterns of its various species in soil. This systematic laboratory study was undertaken to unravel the chemical reactions of V species at different pHs in two contrasting soils differing in their SOM. Scheme 1 outlines our hypotheses for reactions of V species (at pH 4e7) and it serves as the basis for this research. Our aim was to discover the impact of soil pH on the reactions regulating the chemical accessibility of V(þV) and V(þIV). Our hypotheses were that 1) the potential bioavailability of oxyanionic species of V(þV) exceeds that of V(þIV) which has a higher tendency to be adsorbed or precipitated and 2) a high pH enhances the solubility of V, because V(þV) species become predominant. Furthermore, the role of SOM in controlling the accessibility through reduction reactions of V was investigated. Soil organic matter was expected to bind V but also to act as a reductant of V(þV). These reactions were hypothesized to result in a high extractability of the organic V fraction as well as in elevated concentrations of dissolved organic carbon (DOC) due to V(þV)induced oxidation of SOM. This study focuses on the abiotic V processes, although the potential influence of microbes on the chemical reactions related to V bioavailability in soils is acknowledged. To determine the potentially bioavailable and mobile V fractions (i.e. soluble or potentially released from the solid phase) relevant to the assessment of environmental risks, we introduced a sequential extraction procedure. The extractants used were selected to distinguish between various V fractions on the basis of their increasing binding strength: readily and easily soluble < V bound by ligand exchange < V bound to solid organic matter < strongly bound V. The chemical accessibility of V was considered to decline according to this order. 2. Materials and methods 2.1. Soil incubation The effect of soil pH on the chemical accessibility of V species was investigated with two test soils, classified as fine sand (according to the U.S. Department of Agriculture, U.S.D.A.) but differing markedly in their SOM content: 1) surface soil (3.5% SOM)
I. Reijonen et al. / Environmental Pollution 210 (2016) 371e379
373
Scheme 1. Summary of the working hypotheses on the chemical processes and factors (pH and soil organic matter) dictating the chemical bioavailability of V species. The illustrated V(þIV) (middle left side) and V(þV) (middle right side) species demonstrate the predominant species in dilute (
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Impact of soil pH and organic matter on the chemical bioavailability of vanadium species: The underlying basis for risk assessment* € Hartikainen Inka Reijonen*, Martina Metzler, Helina Soil and Environmental Chemistry, Department of Food and Environmental Sciences, Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 27, 00014 Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2015 Received in revised form 17 December 2015 Accepted 21 December 2015 Available online xxx
The main objective of this study was to unravel the chemical reactions and processes dictating the potential bioavailability of vanadium (V). In environmental solutions V exists in two stable oxidation states, þIV and þV, of which þ V is considered to be more toxic. In this study, the effect of speciation and soil pH on the chemical accessibility of V was investigated with two soils: 1) field soil rather rich in soil organic matter (SOM) and 2) coarse mineral soil low in SOM. Fresh soil samples treated with V(þV) (added as NaVO3) or V(þIV) (added as VOSO4) (pH adjusted to the range 4.0e6.9) were incubated for 3 months at 22 C. The adsorption tendency of V species was explored by water extraction (Milli-Q water, 1:50 dw/V) and by sequential extraction (0.25 M KCl; 0.1 M KH2/K2HPO4; 0.1 M NaOH; 0.25 M H2SO4, 1:10 dw/V). The potential bioavailability of V was found to be dictated by soil properties. SOM reduced V(þV) to V(þIV) and acted as a sorbent for both species, which lowered the bioaccessibility of V. A high pH, in turn, favored the predominance of the V(þV) species and thus increased the chemical accessibility of V. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bioavailability Vanadium Soil Speciation Chemistry
1. Introduction Vanadium (V) is a transition metal that exists in soils within a range of 20e110 mg kg1 (Kabata-Pendias, 2004). However, during the past decades industrial use and combustion of fossil fuels have considerably increased anthropogenic V emissions. Additionally, V is released to the soil from V bearing fertilizers, leachates from mine tailings, municipal sewage sludge and slags from steel manufacturing (ATSDR, 2012). Despite the elevating V loading, the research on the environmental chemistry of V is relatively scarce. However, knowledge of the chemical behavior of contaminants and pollutants in the environment creates a scientifically sound basis for risk assessment, remediation strategies and legislation. In soil, the mobility and potential bioavailability (availability to organisms) of elements relies on their chemical reactions and processes. In the case of V, this is particularly challenging due to its redox-sensitivity rending its chemistry complicated. In environmental conditions, it exists in two stable oxidation states, þIV and þV. Vanadium is also found in
*
This paper has been recommended for acceptance by Jay Gan. * Corresponding author. E-mail address: inka.reijonen@helsinki.fi (I. Reijonen).
http://dx.doi.org/10.1016/j.envpol.2015.12.046 0269-7491/© 2016 Elsevier Ltd. All rights reserved.
the oxidation state þ III in some primary and mafic minerals (Essington, 2015) but not in environmental solutions. Because bioaccessible species are considered to be readily or potentially soluble, V(þIII) is not relevant in the terms of chemical bioavailability. Nonetheless, research on the environmental risks related to potential bioavailability of various V species in waste deposits is urgently needed. The biological activity and toxicity of V is affected by its speciation, which varies considerably depending on the oxidation state. Vanadium has been identified as a micronutrient for several organisms, although its essentiality for humans has not been completely established (French and Jones, 1993; Domingo, 1996). Controversially, some V species are classified as teratogens and potential carcinogens (Rehder, 1991). Generally, the toxicity of V(þV) is considered greater than that of V(þIV) (Evangelou, 2002), which may be attributable to the analogy of V(þV) species with phosphate and consequently to inhibition (or in some case activation) of various phosphatases, ATPases and other important enzymes (Leonard and Gerber, 1998). Acute toxicity values (LD50) on mammals, e.g. rats, are considerably higher for V(þV) than V(þIV) (Thompson et al., 1998). However, information on the chronic toxicity of different V species is limited. At low concentrations and within pH ~3 to 7, inorganic V(þV)
372
I. Reijonen et al. / Environmental Pollution 210 (2016) 371e379
exists mainly as monomeric H2VO-4 (Tracey et al., 2007). According to pKa values, further deprotonated oxyanionic vanadate species are prevalent only in alkaline conditions (Heath and Howarth, 1981): 2 þ H2 VO 4 ¼ HVO4 þ H 3 þ HVO2 4 ¼ VO4 þ H
pKa ¼ 7:1 pKa ¼ 12
However, in millimolar solutions the self-condensation of vanadate becomes critical. As a consequence, in mM concentrations (>51 mg V L1) and at pH 3e6 the predominant species of V(þV) is decavanadate (HVO10O5 28 ) (Tracey et al., 2007). This means that polymeric species can be formed at rather modest V concentrations. In soil solution, a mM concentration of V(þV) can be obtained, for instance, when more than 10% of the total V(þV) 100 mg kg1 is in the soluble fraction (at the water content of 20%). Therefore, the polymeric species are presumably present in notable amounts in V-contaminated soils. Furthermore, vanadate is able to form polymeric species with oxyanions such as phosphates and arsenates (Gresser and Tracey, 1990; Mikkonen and Tummavuori, 1994). In contrast to the anionic vanadate species, inorganic V(þIV) occurs as cationic species of vanadyl (VO2þ) (Crans et al., 1998). Above pH 4, it is hydrolyzed to form VO(OH)þ and upon increasing pH, further to the weakly soluble hydroxide, VO(OH)2 (Baes and Mesmer, 1976):
VO2þ þ H2 O ¼ VOðOHÞþ þ Hþ VOðOHÞþ þ H2 O ¼ VOðOHÞ2
ðsÞ
þ Hþ
The dissimilar chemical accessibility of V(þIV) and V(þV) in soil relies on the speciation chemistry. Generally, the mobility of V(þIV) in the biosphere is low due to the formation of hydrous oxides and organic complexes of low reactivity (Tracey et al., 2007). The covalent bonding of VO2þ with organic matter is very strong and for example the vanadium porphyrins in fossil fuels belong to the class I highest stability category in the Buhler classification system (Buchler and Smith, 1975). The solubility of inorganic hydrolyzed VO2þ species is limited also by specific adsorption e.g. on the surfaces of aluminum (hydr)oxides and smectite clays (McBride, 1979; Wehrli et al., 1990). In contrast to the reduced species, the mobility of V(þV) species is considered to be high (Tracey et al., 2007). However, in soil solution its concentration may be reduced by specific reversible adsorption onto iron (Fe)- and aluminum (Al)(hydr)oxides (Wehrli and Stumm, 1989; Peacock and Sherman, 2004). Conversion in the oxidation state affects the chemical bioavailability of V. On the one hand, the hydrolyzed species of V(þIV) can easily be oxidized to V(þV) by molecular oxygen (Wehrli and Stumm, 1989), which increases the mobility. On the other hand, in oxic conditions V(þV) can be reduced by soil organic components via abiotic and biological pathways. The reduction rate by soil organic matter (SOM) depends on its quality as well as on soil pH, temperature and microbiological activity. For instance, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Shewanella oneidensis are reported to mediate reduction of V(þV) (Carpentier et al., 2003; Bredberg et al., 2004). Generally, the microbial processes enhance the overall reduction by SOM. This reaction pattern is demonstrated, for instance, in the case of ferric iron Fe(III) and hexavalent chromium (Cr) (Chen et al., 2003; Chiu et al., 2009). Easily degradable organic matter is considered a better reducing material than humic substances: fulvic acids (FAs), humic acids (HA) and humin. However, the role of FAs and HAs in V reduction is acknowledged (Wilson and Weber, 1979).
Furthermore, the reduction of V(þV) is found to be followed by V(þIV) retention as an organic inner sphere complex. Interestingly, the reduction and adsorption reactions of V are depending on each other. Reduction of V(þV) to V(þIV) enhances the retention due to the greater affinity of vanadyl species for organic matter. However, complexation of V(þV) with organic substances, e.g. FAs, has also been demonstrated (Lu et al., 1998). Overall, the negative impact of anthropogenic V deposits on soil systems is alleviated by organic matter (OM) via two pathways. Firstly, the ecotoxicity of V is lowered by reduction of V(þV) to the less toxic V(þIV) species. Secondly, chemical bioavailability of V is limited by sorption onto SOM. Thus, FAs and other V(þV) binding humic substances are likely diminish toxicity beyond their reducing capacities by acting as adsorbents. The solubility of organic matter is dictated by pH. At higher pHs, the deprotonation of organic functional groups (e.g. phenolic and carboxylic groups) increases the hydrophilicity and, thus, the solubility of SOM. Consequently, also the solubility of organic V complexes increases. As for humus, this holds true for HAs. However, the humin fraction is not soluble either in alkali or acid whereas FAs are soluble regardless of the pH. In addition to SOM solubility, the redox reactions in soils are regulated by pH. At elevated pHs, reduction of V(þV) to (V þ IV) by SOM is declined whereas oxidation of V(þIV) to V(þV) by O2 is enhanced. This oxidation is particularly prevalent in circumneutral and alkaline conditions (Tracey et al., 2007). The prerequisite to assess the potential bioavailability of V is to know the pH-dependent reaction patterns of its various species in soil. This systematic laboratory study was undertaken to unravel the chemical reactions of V species at different pHs in two contrasting soils differing in their SOM. Scheme 1 outlines our hypotheses for reactions of V species (at pH 4e7) and it serves as the basis for this research. Our aim was to discover the impact of soil pH on the reactions regulating the chemical accessibility of V(þV) and V(þIV). Our hypotheses were that 1) the potential bioavailability of oxyanionic species of V(þV) exceeds that of V(þIV) which has a higher tendency to be adsorbed or precipitated and 2) a high pH enhances the solubility of V, because V(þV) species become predominant. Furthermore, the role of SOM in controlling the accessibility through reduction reactions of V was investigated. Soil organic matter was expected to bind V but also to act as a reductant of V(þV). These reactions were hypothesized to result in a high extractability of the organic V fraction as well as in elevated concentrations of dissolved organic carbon (DOC) due to V(þV)induced oxidation of SOM. This study focuses on the abiotic V processes, although the potential influence of microbes on the chemical reactions related to V bioavailability in soils is acknowledged. To determine the potentially bioavailable and mobile V fractions (i.e. soluble or potentially released from the solid phase) relevant to the assessment of environmental risks, we introduced a sequential extraction procedure. The extractants used were selected to distinguish between various V fractions on the basis of their increasing binding strength: readily and easily soluble < V bound by ligand exchange < V bound to solid organic matter < strongly bound V. The chemical accessibility of V was considered to decline according to this order. 2. Materials and methods 2.1. Soil incubation The effect of soil pH on the chemical accessibility of V species was investigated with two test soils, classified as fine sand (according to the U.S. Department of Agriculture, U.S.D.A.) but differing markedly in their SOM content: 1) surface soil (3.5% SOM)
I. Reijonen et al. / Environmental Pollution 210 (2016) 371e379
373
Scheme 1. Summary of the working hypotheses on the chemical processes and factors (pH and soil organic matter) dictating the chemical bioavailability of V species. The illustrated V(þIV) (middle left side) and V(þV) (middle right side) species demonstrate the predominant species in dilute (
