Concentrations Of Metal Pollutants Research Articles

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

Passive sampling devices accumulate chemicals continuously from water and can provide time weighted average (TWA) concentrations of pollutants over the exposure period. Hence, they offer a number 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 concentrations of labile metals, but the approaches to their calibration differ. DGT uses diffusion coefficients of metals in the hydrogel layer, whereas Chemcatcher uses metal specific uptake rates, with both sets of values obtained under controlled laboratory conditions with constant aqueous metal concentrations. However, little is known of how such samplers respond to fluctuating concentrations. We evaluated the responsiveness of these two passive sampling devices to rapidly changing concentrations of Cd, Cu, Ni, Pb and Zn in natural freshwater, over a relatively short deployment time. Maximum metal concentrations in water were varied between 70 and 140 microg L(-1). Experiments were carried out in a tank with a rotating carousel system and filled with Meuse river water, allowing a degree of control over experimental conditions while using natural river water. Fluctuating concentrations were obtained by stepwise addition of standard solutions of the metals. The reliability and accuracy of the TWA concentrations measured by the samplers were assessed by comparison with concentrations 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 microm) or ultrafiltered (5 kDa). Predictive speciation modelling using the visual MINTEQ programme was also undertaken. There was reasonable agreement between the TWA concentrations of Cd and Ni obtained with Chemcatcher and DGT and the total Cd and Ni concentrations measured in repeated unfiltered spot samples. For elements (i.e. Cu, Pb, Zn) that associate to a significant degree with suspended solids, colloids or dissolved organic carbon, or form complexes with large organic ligands, optimum agreement was with the filtered or ultrafiltered fractions and with the predicted inorganic and inorganic-fulvic acid associated fractions. While Chemcatcher-based TWA concentration 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 associated with large organic 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 concentration 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.

Adsorption of Cadmium Ions onto the Yellow River Sediment

Abstract Adsorption of cadmium ions onto Yellow River sediment was studied in a batch reactor. Equilibrium adsorption of cadmium ions onto Yellow River sediment can be well described by the Langmuir adsorption isotherm. In view of common experimental conditions in the batch reactor, a set of equations for describing variations of both the dissolved heavy metal pollutant concentration and the particulate heavy metal pollutant concentrations (the adsorption content of heavy metal pollutant on unit weight of sediment) with time was formulated with the use of an adsorptive reaction kinetic equation of heavy metal pollutant, mass balance equation and corresponding initial conditions. Furthermore, formulae for calculating the equilibrium dissolved heavy metal concentration and the equilibrium particulate heavy metal concentration (the equilibrium adsorption content of heavy metal pollutant on unit weight of sediment) in the batch reactor were obtained. These can be used as tools for further study. It was found that the saturation adsorption content, b, constant of adsorption-desorption rate, k, and coefficient of adsorption rate, k1 (and coefficient of desorption rate k2), in the adsorptive reaction kinetic equation of the Langmuir type are independent of the suspended sediment concentration. Adsorption contents of heavy metal pollutants on unit weight of sediment (the suspended particulate heavy metal concentration) decreases with increasing suspended sediment concentration and can be well explained by the law of mass conservation. Adsorption capacity, which is defined as adsorption content of heavy metal pollutant by unit volume of (muddy) water, increases with an increase in either the suspended sediment concentration or initial dissolved heavy metal concentration.

Sludge management using the biodegradable organic fraction of municipal solid waste as a primary substrate

An innovative anaerobic composting process has been used to manage wastewater treatment plant sludges using the biodegradable organic fraction (BOF) found in municipal solid waste (MSW) as a primary substrate. Three different types of wastewater treatment plant sludges (primary, thickened activated, and digested) were commingled with a simulated biodegradable organic fraction of municipal solid waste and used as a feedstock. Digester performance was evaluated for each feedstock under similar retention times, a constant total solids content, and a constant C:N ratio. Additional aspects of the process which have been analyzed include volume reduction, air emission, and humus characteristics. Results obtained indicate that stable and robust digestion was achieved using organic loading rates of 6.5‐8 g biodegradable volatile solids per kilogram active reactor mass per day. Under these organic loading rates, methane gas production rates as high as 0.36 m3/kg biodegradable volatile solids added per day were obtained, gassifying approximately half the dry mass of the feedstock. Substrate volume reduction through the process was ∼65–69% as compared with the same quantities of sludge and MSW in a well‐compacted landfill. The emissions produced from anaerobic digestion and aerobic biodrying are found to have no adverse effect on human heath and the environment. The humus produced is a fine, odorless, and pathogen‐free material with metal pollutant concentrations well below the “clean sludge” standards specified by EPA Part 503 of the U.S. Environmental Protection Agency (U.S. EPA).