Evaluation of the Chemcatcher and DGT passive samplers for monitoring metals with highly fluctuating water concentrations

Passive sampling devices accumulate chems. continuously from water and can provide time weighted av. (TWA) concns. of pollutants over the exposure period. Hence, they offer a no. of advantages over other conventional monitoring techniques such as spot or grab sampling. The diffusive gradient in thin film (DGT) and the Chemcatcher passive samplers can be used to provide TWA concns. of labile metals, but the approaches to their calibration differ. DGT uses diffusion coeffs. of metals in the hydrogel layer, whereas Chemcatcher uses metal specific uptake rates, with both sets of values obtained under controlled lab. conditions with const. aq. metal concns. However, little is known of how such samplers respond to fluctuating concns. We evaluated the responsiveness of these two passive sampling devices to rapidly changing concns. of Cd, Cu, Ni, Pb and Zn in natural freshwater, over a relatively short deployment time. Maximum metal concns. in water were varied between 70 and 140 mg L-1. Expts. were carried out in a tank with a rotating carousel system and filled with Meuse River water, allowing a degree of control over exptl. conditions while using natural river water. Fluctuating concns. were obtained by stepwise addn. of std. solns. of the metals. The reliability and accuracy of the TWA concns. measured by the samplers were assessed by comparison with concns. of the metals in spot samples of water taken regularly over the deployment period. The spot samples of water were either unfiltered (total), filtered (0.45 mm) or ultrafiltered (5 kDa). Predictive speciation modeling using the visual MINTEQ program was also undertaken. There was reasonable agreement between the TWA concns. of Cd and Ni obtained with Chemcatcher and DGT and the total Cd and Ni concns. measured in repeated unfiltered spot samples. For elements (i.e. Cu, Pb, Zn) that assoc. to a significant degree with suspended solids, colloids or dissolved org. carbon, or form complexes with large org. ligands, optimum agreement was with the filtered or ultrafiltered fractions and with the predicted inorg. and inorg.-fulvic acid assocd. fractions. While Chemcatcher-based TWA concn. ranges for Cu and Zn were in best agreement with the total filtered fraction, there was lack of agreement for Pb. The combined use of DGT devices with open pore (OP) and restricted pore (RP) gels allowed the labile fraction of metal assocd. with large org. ligands or DOC to be differentiated and quantified, since this is available to DGT OP but unable to diffuse into the DGT RP. This evaluation of the two sampling devices clearly demonstrated their ability to react reliably to transient peaks in concn. of metal pollutants in water and indicated where future efforts are needed to improve calibration data. Such samplers may prove valuable in responding to the monitoring requirements of the European Union's Water Framework Directive. [on SciFinder (R)