High-resolution two-dimensional quantitative analysis of phosphorus, vanadium and arsenic, and qualitative analysis of sulfide, in a freshwater sediment

Recently introduced techniques that can provide two-dimensional images of solution concentrations in sediments for multiple analytes have revealed discrete sites of geochemical behaviour different from the average for that depth (microniches). We have developed a new preparation method for a binding phase, incorporated in a hydrogel, for the diffusive gradients in thin films (DGT) technique. It allows co-analysis of sulfide and the reactive forms of phosphorus, vanadium and arsenic in the porewaters at the surface of the device. This gel, when dried and analysed using laser ablation mass spectrometry, allows the acquisition of high-resolution sub-millimetre-scale data. The binding phase was deployed within a DGT device in a sediment core collected from a productive lake, Esthwaite Water (UK). Localised removal of phosphate and vanadium from the porewaters has been demonstrated at a microniche of local sulfide production. The possible removal processes, including bacterial uptake and reduction of vanadate to insoluble V-III by sulfide, are discussed. Understanding processes occurring at this scale may allow improved prediction of pollutant fate and better prediction of past climates where trace metals are used as paleoredox proxies.