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

Abstract

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.

Laboratory calibration and field testing of the Chemcatcher-Metal for trace levels of rare earth elements in estuarine waters

“…The CA-membrane was chosen as the diffusionlimiting membrane because of its highly hydrophilic characteristics (Persson et al 2001). The Empore Chelating Disk is based on a backbone of polystyrene, which is cross linked with divinylbenzene.…”

Calibration and use of the Chemcatcher® passive sampler for monitoring organotin compounds in water

“…The sampler design has been described in detail elsewhere [25][26][27]. The CA diffusion membrane (47 mm diameter) was placed on the top of the conditioned C 18 Empore TM disk, and care was taken to avoid formation of air bubbles between the two layers.…”

“…The Chemcatcher ® passive sampler can be used to measure the TWA concentration of both organic [24,25] and heavy metal [26] pollutants in a range of aquatic environments. A common PTFE sampler body is used and selectivity and accumulation rates are regulated by the choice of the diffusion-limiting membrane and the receiving phase material employed in the different configurations of the device.…”

Passive Sampling Technologies

“…Sorption phases include cation-exchange resins for trace metals, iron oxides and titanium dioxide for phosphate, and silver iodide for sulfide (Zhang and Davison, 1995;Zhang et al ., 1998;Teasdale et al ., 1999;Panther et al ., 2010). The metals version of the Chemcatcher employs a 47 mm 3M Empore™ chelating disk that is covered by a 0.45 μm cellulose acetate filter (Persson et al ., 2001).…”

“…Although this device has been used mainly for nonpolar and polar organics (see previous sections), a particular configuration has been developed for measuring the TWA concentration of metals. It consists of a Teflon watertight sampler body to hold in position a cellulose acetate diffusion-limiting membrane (0.45 μm pore size and thickness of 0.135 mm) that is placed over a 3M Empore™ chelating disk (47 mm diameter) as the receiving phase (Persson et al ., 2001;Blom et al ., 2002Blom et al ., , 2003. Metal accumulation in the sampler is similar to that of the DGT and is based on the diffusion of metal species across the cellulose acetate membrane.…”