PERFORMANCE-CHARACTERISTICS OF DIFFUSION GRADIENTS IN THIN-FILMS FOR THE IN-SITU MEASUREMENT OF TRACE-METALS IN AQUEOUS-SOLUTION

The technique of diffusive gradients in thin films (DGT) provides an in situ means of quantitatively measuring labile species in aqueous systems. By ensuring that transport of metal ions to an exchange resin is solely by free diffusion through a membrane, of known thickness, Delta g, the concentration in the bulk solution, C-b, can be calculated from the measured mass in the resin, M, after time, t, by C-b = M Delta g/DAt, where D is the molecular diffusion coefficient and A is the exposure surface area of the membrane. If a sufficiently thick (similar to 1 mm) diffusion layer is selected, the flux of metal to the resin is independent of the hydrodynamics in solution above a threshold level of convection. Deployment for 1 day results in a concentration factor of similar to 300, allowing metals to be measured at extremely low levels (4 pmol L(-1)). Only labile metal species are measured, the effective time window of typically 2 min being determined by the thickness of the diffusion layer. Because metals are quantified by their kinetics of uptake rather than the attainment of equilibrium, any deployment time can be selected from 1 h to typically 3 months when the resin becomes saturated. The measurement is independent of ionic strength (10 nM-1 M). For Chelex-100 as the resin, the measurement is independent of pH in the range of 5-8.3, but a subtheoretical response is obtained at pH <5 where binding to Chelex is diminished. The effect of temperature can be predicted from the known temperature dependence of the diffusion coefficient and viscosity. The application of DGT to the in situ measurement of Cd, Fe, Bin, and Cu in coastal and open seawater is demonstrated, and its more general applicability as a pollution monitoring tool and for measuring an in situ flux, as a surrogate for bioavailability, is discussed.