Diffusional behaviour of metals in a passive sampling system for monitoring aquatic pollution

Passive sampling systems are an emerging technology for detecting pollutants in the aquatic environment. A passive sampling system has been developed based on diffusion through a porous surface to a receiving phase, where the analyte is removed by chelation at a solid phase. The diffusion process can be described by Fickian diffusion through the sampler. The sampler has a well-defined surface area, which permits calibration in terms of concentration. Passive sampling systems can be used to determine pollutant concentrations if the diffusion process can be described and understood within environmental limits. In natural water systems, diffusion coefficients for metal transport across the porous membrane will be affected by external conditions, including biofouling and variation in turbulence and temperature. Uptake rates for the analytes Cu, Cd and Pb have been determined for the complete passive sampling system. Two different cases have been investigated, a batch case, where the bulk concentration decreases with time, and a flow-through case, where the bulk concentration remains constant. Diffusion coefficients were determined for the two conditions and compared with the calculated value obtained for the Stokes–Einstein equation in pure water. Diffusion coefficients for metals were found to be lower than for diffusion in pure water, a difference attributed to the effect of the porous membrane. The effect of the hydraulic conditions on the metal diffusion was studied for both a conventional magnetic stirrer creating turbulence in the system and for a rotated sampler, the latter providing a well-defined boundary layer system. The boundary layer was found to be negligible compared with the diffusion limiting membrane in the presence of sufficient turbulence or if the rotation of the sampler was high.