Effects of Binding of Metals to the Hydrogel and Filter Membrane on the Accuracy of the Diffusive Gradients in Thin Films Technique

Abstract

The effect of metal binding to the diffusion layer of diffusive gradients in thin films (DGT) on measurements of time-integrated concentrations (CDGT) was studied experimentally and using a numerical model. Cationic divalent Cu and Cu-fulvic complexes bind to sites in the filter membrane and polyacrylamide (APA) gel, together comprising the diffusion layer of DGT. Less Cu was found in Chelex gels retrieved immediately after DGT deployment than in Chelex gels retrieved after storage of deployed DGTs. The difference was attributed to postdeployment transport of Cu from the diffusion layer to the Chelex gel. Binding of Cu in the diffusion layer delays the formation of a steady concentration gradient between solution and Chelex gel and can, depending on diffusion layer thickness, speciation, and concentration of metal, significantly bias the measurement of CDGT for short deployment times. However, C(DGT) will converge with the concentration in solution provided that the deployment time is sufficiently long (for a 1.0 mm diffusion layer, a diffusion coefficient of 5.5 x 10(-6) cm2 s(-1) and a concentration of Cu in solution of 0.1 nM, the difference is less than 11% and 5% for deployment times longer than 32 and 64 h, respectively). Minimal bias is encountered when measuring Cu contaminated waters (> or = 100 nM) using DGTs with thinner (< or = 0.5 mm) diffusion layer thicknesses. Diffusion layer binding of cationic Cd in the presence of cationic Cu was insignificant, and C(DGT) of Cd was found to converge with the concentration in solution after deployment times of 4 h and less. These findings have implications for the maximum temporal resolution that can be achieved with DGT.