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

Silicon is an essential component of various environmental processes and biogeochemical cycles with silicate (H 4 SiO 4 ) 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 cm 2 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 H 2 O 2 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 NaNO 3 ). 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 CaCl 2 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.