Release of Vanadium from soils by conventional leaching procedures and extractions

Abstract

1. Introduction Despite the fact that vanadium (V) is among the 20 most abundant elements in the earth's crust5, with average concentrations between 50 and 150 µg g-1, this element receives relatively few attention in the scientific literature on soil and sediment geochemistry. However, the last few years, the attention for this potentially harmful element is growing. Some countries adopted threshold values for vanadium in soils, sediments, ground- or surface water, but in general vanadium is of little importance in environmental legislation. In the present study, the release of vanadium from soils and from certified reference materials was investigated by means of several types of leaching tests and extractions that are frequently use for soil and sediment characterization. The data of the physico-chemical soil characterization, extractions and leaching tests can be used estimate the actual and potential risk for vanadium release into the environment. 2. Methodology Dredged sediment derived soils and alluvial soils from the northern part of Belgium were subjected to a physico-chemical (grainsize, pH, CEC, and total element concentrations) and mineralogical (XRD) characterization. Single extractions with CaCl2 (0.01 M), acetic acid (0.43 M) and ammonium-EDTA (0.05 M) were performed to estimate the actual and potential mobilization of V from the sediments. The SMT sequential extraction scheme was applied on a selection of samples. pHstat leaching tests were applied according to the method described by Van Herreweghe et al.8 3. Results and discussion Ammonium-EDTA was used to estimate the mobilizable element concentrations. The average EDTA-extractable V concentration in the dredged sediment derived soils was 2.46 mg/kg which is 2 comparable with average EDTA-extractable V-concentrations (3.40 mg/kg) reported by Gäbler et al.4 In the alluvial soil samples, the average ammonium-EDTA-extractable V concentration was 11.2 mg/kg. These soils are contaminated with V (with V-concentrations up to 301 mg kg-1), which can also explain for the higher potentially mobile V-content in these soils. Acetic acid only released very low V-concentrations, (between 0.5 and 6 mg/kg), accounting for less than 5% of the total V-content in the samples. The maximum CaCl2-extractable V concentration, which can be considered representative for porewater composition was 12 mg kg-1 with an average concentration of 4 mg kg-1, which is far below the value of 3 mg mg kg-1, which is proposed by Edwards et al. and would cause significant toxic effects to plants. In the sequential extractions, the residual fraction of V was dominant for all samples, which together with the reducible fraction accounted for 90% of the total V concentration in the samples. The acid extractable fraction of V was very low (below 5%), whereas between 5 and 10 % of the total V content was recovered in the oxidisable fraction. Leaching of V as a function of pH was generally characterized by a typical V-shaped pattern, with maximum release at very low (pH 2) and very high pH (pH 10). At pH 2, The release of V as a function of time followed the similar pattern as described before for heavy metals. Geochemical speciation modeling indicates that V occurs as VO2+ species at pH 2, whereas at more neutral and alkaline pH values (pH 6 tot 10), VO2(OH)2- and VO3OH2-are the prevailing V-species1. 4. Conclusion The leaching tests and extractions applied in this study show that vanadium generally displays a very limited actual and potential mobility in soil. Mobile V concentrations, as estimated by the amount of V released by a single extraction with CaCl2 0.01 mol L-1, were low, even in the most contaminated soil samples. Reducing conditions may initially cause a release of V into the environment, but under reducing, saturated conditions, V is immobile9.status: publishe