Removal of oxide nanoparticles in a model wastewater treatment plant: influence of agglomeration and surfactants on clearing efficiency.

The rapidly increasing production of engineered nanoparticles has created a demand for particle removal from industrial and communal wastewater streams. Efficient removal is particularly important in view of increasing long-term persistence and evidence for considerable ecotoxicity of specific nanoparticles. The present work investigates the use of a model wastewater treatment plant for removal of oxide nanoparticles. While a majority of the nanoparticles could be captured through adhesion to clearing sludge, a significant fraction of the engineered nanoparticles escaped the wastewater plant's clearing system, and up to 6 wt % of the model compound cerium oxide was found in the exit stream of the model plant. Our study demonstrates a significant influence of surface charge and the addition of dispersion stabilizing surfactants as routinely used in the preparation of nanoparticle derived products. A detailed investigation on the agglomeration of oxide nanoparticles in wastewater streams revealed a high stabilization of the particles against clearance (adsorption on the bacteria from the sludge). This unexpected finding suggests a need to investigate nanoparticle clearance in more detail and demonstrates the complex interactions between dissolved species and the nanoparticles within the continuously changing environment of the clearing sludge.

Dynamic equilibrium dissolution of complex nonaqueous phase liquid mixtures into the aqueous phase.

Human health risks posed by hazardous substances seeping from a pool of nonaqueous phase liquids (NAPLs) into groundwater change over time because the more soluble compounds such as benzene, toluene, ethylbenzene, and xylene (BTEX) dissolve faster into the aqueous phase than less soluble compounds such as polycyclic aromatic hydrocarbons (PAH). Long-term dissolution from diesel fuel into the aqueous phase was determined experimentally in a continuous flow-through system using the slow-stirring method. The data obtained are interpreted using a dynamic equilibrium dissolution model based on Raoult's law. The predicted temporal development of aqueous concentrations are in good agreement with the experimental results. When a compound in the NAPL approaches complete depletion, a tailing behavior is observed, which is assigned to nonequilibrium effects, such as mass transfer limitations in the NAPL phase. The model predicted an increase of the mean molar mass of the diesel fuel of 1.5% over the entire experimental period. It should be noted that, if the dissolution process were to proceed further, the change in the mean molar mass could become significant and render the simple model inaccurate. Yet the simple model supports the assessment of initial action after a contamination event as well as the planning of long-term remedial strategies.

Movement and fate of residual mineral oil contaminants in bioremediated soil.

After the bioremediation of soils contaminated with mineral oil products, residual contaminants (designated as total solvent extractable material [TSEM]) remain therein. Since limited information is available about the movement and fate of these residual contaminants in the environment, the potential environmental hazards are difficult to assess. The aim of this study was to identify and quantify the relevant transport and transformation processes of the residual contaminants in bioremediated soil such as volatilization, leaching, and further biodegradation and to conduct a mass flow analysis for the TSEM in bioremediated soil for the first year after application as top soil. The results indicate that after 1 year, the major portion (93%) of the TSEM can be recovered in the top soil and 7% of the TSEM was lost. The majority of the total losses (>98%) was due to transformation processes (biodegradation and aging effects), while small amounts escaped into the atmosphere (0.08%), plant uptake was negligible, (<0.001%) and leaching, identified as the major transport process, accounted for only small effects (1.7%).