Application of a cellulose phosphate ion exchange membrane as a binding phase in the diffusive gradients in thin films technique for measurement of trace metals

The technique of diffusive gradients in thin films (DGT) is a newly developed analytical technique capable of measuring in situ concentrations of trace metals in the environment. The technique employs a thin film diffusive hydrogel (with well-defined diffusion properties) in contact with a binding phase capable of binding metal ions of interest. In this work, we demonstrate, for the first time, the use of a commercially available solid ion exchange membrane (Whatman P81) as the binding phase in DGT analysis. The cellulose phosphate-based Whatman P81 membrane is a strong cation exchange membrane. Its performance characteristics as a new binding phase in DGT measurement of Cu²⁺ and Cd²⁺ were systematically investigated. Several advantages over the conventional ion exchange resin-embedded hydrogel binding phases used in DGT were observed including simple preparation, ease of handling, and reusability. The binding capacities of the material to various metal ions were examined both collectively and individually. The binding phase preferentially binds to transition metal ions rather than matrix ions such as potassium, sodium, calcium and magnesium, which are competitive species in natural waters. Within the optimum pH range (pH 4.0-9.0), the maximum non-competitive binding capacities of the membrane for Cu²⁺ and Cd²⁺ were 3.22 and 3.07 μmol cm⁻², respectively. The suitability of the new membrane-based binding phase for DGT applications was validated experimentally. The experimental results demonstrated excellent agreement with theoretically predicted trends. The measurement was not degraded after four consecutive reuses of the cellulose phosphate binding phase.