Response of tungsten (W) solubility and chemical fractionation to changes in soil pH and soil aging

Abstract

A thorough understanding of the geochemical behavior of W in soils is crucial for environmental risk assessment. Soil pH is known as master variable of element solubility and bioavailability in soils. Here we report on effects of soil pH (modified by liming and acid – base additions) and soil aging on the environmental availability of W in soil using W solubility and chemical fractionation as indicators. Experimental soils included two naturally acidic soils with contrasting soil texture (SAND, CLAY), at native pH or limed with 2.5% CaCO3, and spiked with increasing concentration of W.Our results showed that W was significantly more labile in alkaline compared to acidic soils, confirming the validity of results of pure-mineral studies for more heterogeneously composed soils. While labile W was generally greater in the SAND compared to the CLAY soil, the reverse trend was observed in the limed soils at the highest W addition (5000 mg kg−1). Combining our results with previous mechanistic reports suggests that clay edge sorption sites significantly contributed to W retention in treatments with low to medium W additions, resulting in lower environmental availability for W in the CLAY soil. At high W concentrations and high pH, the stronger W retention in the SAND was attributed to continuous formation of W surface polymers on the more abundant metal (oxyhydr)oxides, a process that has been previously reported to occur even under alkaline conditions. A first comparison of various soil chemical methods (Bray & AB-DTPA extractions, soil solution centrifugation CL, diffusion-based DGT) to predict W phytoavailability in soil also revealed a strong pH dependency challenging the identification of a suitable method. This study is one of the first demonstrating the pH dependence of W in natural soils and delivers evidence for increased risk of W mobilization in W polluted, alkaline soil environments.