A Passive Sampling System for Monitoring Metals in the Aquatic Environment
Water monitoring is undertaken routinely as a requirement of water quality legislation. It is usual that water monitoring is undertaken through spot sampling. A passive sampling system has been developed for the aquatic environment that provides time-averaged (integrated) concentrations for metals. Transport of metal into the passive sampler system is based on Fickian diffusion. The sampler design includes a Nafion-coated diffusion limiting membrane and a receiving membrane where metals are retained. The passive sampler is suitable for a wide range of aquatic environments and can be adapted for both inorganic and organic pollutants.
Environmental factors that affect the diffusion rate across the diffusion limiting membrane include turbulence, biological growth and temperature. The effect of turbulence was studied through experiments with; a rotating sampler to change the boundary layer thickness, magnetic stirring to simulate totally mixed flow conditions and double diffusion limiting membranes to eliminate exterior flow conditions. Under turbulent conditions the effect on the diffusion rate is negligible, but under quiescent conditions the diffusion rate is slower. The Nafion coating was found to reduce biofouling under low growth conditions. A Nafion-coated passive sampler was found to provide a speciation procedure due to permselectivity. It is possible to exclude large colloids (natural humic substances) from the Nafion-coated membrane, while small synthetic ligands pass through. Membrane material can be chosen to target metal species or to permit the collection of all metals. The passive sampler was found to give the same concentration as the electroavailable concentration for time-dependent composite samplers in stormwater.
passive sampling system