Simultaneous sampling of dissolved orthophosphate and ammonium in freshwaters using diffusive gradients in thin films with a mixed binding phase

Abstract

A new diffusive gradients in thin films (DGT) technique with a micro-sized biochar-zeolite-zirconium oxide mixed binding phase is described for simultaneous sampling of dissolved orthophosphate and ammonium in freshwaters. The performance of mixed binding layer (MBL) DGT technique was characterized by evaluation of the following parameters: (1) the adsorption efficiency of phosphate and ammonium by the mixed binding phase, (2) the elution efficiency, (3) the time-series accumulation, and (4) the influences of pH, coexisting cations and DOC on DGT uptake. The mixed binding phase had high uptake for both phosphate and ammonium. The accumulated analyte mass increased linearly with time over 48 h (Δg = 1.3 mm) and 84 h (Δg = 1.9 mm), respectively. Simultaneous sampling of phosphate and ammonium using MBL-DGT was consistent over the 10–80 mg C L−1 DOC concentration range and the 3–8 pH range for a 24 h deployment. The coexisting cations (Na+, K+, Mg2+, and Ca2+) were found to compete with the ammonium accumulation by MBL-DGT, but ammonium accumulation was consistent over concentration ranges typically found in most freshwaters (up to 0.01 mol L−1 Na+, 0.001 mol L−1 K+, 0.01 mol L−1 Mg2+, 0.03 mol L−1 Ca2+). The MBL-DGT method also performed predictably in natural freshwaters containing different concentrations of metal ions and dissolved organic carbon. The results of this project have demonstrated that this new DGT method can offer a more cost-efficient method for simultaneously determining time-weighted average concentrations of ammonium and dissolved orthophosphate in most freshwaters, but inefficiently in high salinity waters because of its extremely high concentration of competing cations.