Technical development for in situ measurement of labile silicate in waters and soils

Abstract

Silicon is an essential component of various environmental processes and biogeochemical cycles with silicate (H4SiO4) representing the bioavailable form of silicon in waters and soils. This study aimed to develop a method for accurately measuring labile silicate levels in waters and soils using diffusive gradients in thin-films (DGT) with a hydrogel diffusive gel and a modified precipitated zirconia (mPZ) binding gel. The diffusion coefficient of silicate in the diffusive gel, measured using an independent diffusion cell, was 6.77 × 10−6 cm2 s−1 at 25 °C. The mPZ gels were found to be effective at quickly and efficiently binding silicate, and a mixed solution of 0.5 mol L−1 NaOH and 0.5 mol L−1 H2O2 was determined to effectively elute silicate from mPZ gels, with an elution factor of 1.0. Furthermore, mPZ-DGT was found to be able to measure silicate at various pH levels (4.03–7.93) and ionic strengths (0.1–1000 mmol L−1 NaNO3). Field application of mPZ-DGT devices in an urban river revealed a decrease in silicate concentration downstream compared to upstream, likely due to increased human activity. Finally, mPZ-DGT was used in four sandy or loam soils to measure silicate concentration, and the results showed relatively small variations compared to those measured by CaCl2 and ammonium oxalate extractions. The findings suggest that mPZ-DGT is a useful tool for monitoring labile silicate concentrations and assessing its bioavailability in waters and soils.