A novel Mg(OH)2 binding layer-based DGT technique for measuring phosphorus in water and sediment

Abstract

Diffusive gradients in thin films (DGT) have gained wide attention for in situ measurement of reactive phosphorus species (PO₄) in natural water, sediments and potentially soils. In this study, a novel Mg(OH)₂ binding gel was formed using magnesium hydroxide obtained by in situ hydration of calcined magnesium oxide. Laboratory scale experiments showed that the novel Mg(OH)₂ gel had a homogeneous dispersion of fine particles of Mg(OH)₂ with a particle size of 2–5 μm. With 10 mL of 2.0 mol L⁻¹ NaOH as the eluting agent, the optimal elution efficiency of PO₄ on the Mg(OH)₂ gel was 72 ± 5%. There were linear relationships between the accumulated PO₄ mass and the applied PO₄ concentration (0.1 to 20 mg P per L), time (0 to 24 h) and temperature (22 to 40 °C). The capacity of the Mg(OH)₂ binding layer was determined to be 99.5 μg P per disc. Tests in synthetic seawater, Chaohu Lake and Yihai Pond confirmed that Mg(OH)₂-DGT was able to accurately measure phosphorus up to 10 days. This was indicated by the good agreements between the concentrations measured by DGT (C_DGT) technology and by an ex situ chemical method in solution (C_soln), with a C_DGT/C_soln ratio between 0.91 and 1.09.