(217d) Fate of Inorganic Oxide Nanoparticles in Semiconductor Manufacturing Effluents During Activated Sludge Treatment

The rapid increase in the application of engineered nanomaterials has created concerns whether these materials have potential environmental, safety and health (ESH) implications. Nano-sized silica (SiO2), ceria (CeO2), and alumina (Al2O3) are used in many industrial processes, including semiconductor manufacturing. Slurries utilized in chemical mechanical planarization (CMP) contain abrasive nanoparticles (NPs) of these inorganic oxides to facilitate the polishing of wafers. These inorganic oxides also have rapidly growing applications in catalysis, polymers, coatings, etc. CMP slurries undergo dilution during CMP rinsing operations, and the concentration of abrasive particles in the CMP waste effluents is typically in the range of 50 to 500 mg/L. Inspite of the ESH concerns, there is very limited information available on the fate of these NPs during municipal wastewater treatment. In CMP slurries, abrasive NPs are chemically stabilized to form highly stable aqueous dispersions. The stability of these nanosized oxide dispersions in sewage is likely to be altered due to the significant changes in the water chemistry (e.g., circumneutral pH values, high concentrations of organic matter, reduced levels of surfactants, etc.), which might result in particle agglomeration and eventually even sedimentation. In addition to particle agglomeration, interactions of NPs with microorganisms involved in biological wastewater treatment might be an additional mechanism contributing to the removal of NPs. The objective of this research is to investigate the removal of CMP NPs during municipal wastewater treatment and elucidate the main mechanisms contributing to their removal. The study was conducted in a laboratory-scale activated sludge system (A/S) fed with municipal wastewater collected from a local treatment facility and a dispersion nanosized CeO2 (average particle size = 50 nm). Additional studies are being conducted with alumina. To garner additional mechanistic information, experiments on the stability of the NPs dispersions were conducted at a range of pH values in the presence of model compounds representative of wastewater constituents such as proteins, amino acids, surfactants, humic acids and phosphate ions. As far as we know, this is the first study that attempts to perform a systematic evaluation of the aggregation behavior of these nano-sized inorganic oxides in municipal wastewater. The aggregation behavior of NPs was evaluated by determining the concentration of total and dispersed inorganic oxides in the influent and in the clarified effluent using sequential membrane filtration (1 um, 200 nm, and 20 nm) and ICP-OES analysis of the digested samples. In some experiments, NP stability was evaluated by determining the particle size distribution and zeta potential of the dispersions. The organic matter removal efficiency was determined by monitoring the total as well as the soluble chemical oxygen demand (COD) in the influent and effluent. The reactor operation was divided into two periods, phase I when the influent consisted of only wastewater, and phase II when the reactor was fed simultaneously with wastewater and the CeO2 dispersion (to a final concentration of 50 mg/L). The COD removal efficiency of the reactor system was high (>65%). Results from toxicity batch bioassays confirmed that the nano-sized ceria did not inhibit respiration of the activated sludge at concentrations as high as 1000 mg/L. Our results indicated that a significant fraction of the CeO2 NPs was removed during treatment in the A/S system. The average removal of total ceria was 97%. The ceria removal efficiency based on influent and effluent samples filtered through the 200 nm and 20 nm filters was 97 and 92%, respectively. While it's true the NPs were largely removed, nonetheless the results indicate that some unagglomerated CeO2 NPs was detected in the treated water. This was unexpected considering that the pH of the wastewater (7.6-7.7) was close to the isoelectric point (pI) of CeO2 in pure water (approximately 8). The stability of the NP dispersion is expected to be lowest at a pH value very close to the pI as there are no interparticle repulsive forces due to absence of particle surface charges. Additional experiments confirmed that certain organic constituents in the wastewater, such as proteins, surfactants and humic acids, had a strong impact on improving the stability of CeO2 NPs dispersion in aqueous solution at neutral pH conditions. Isotherms conducted the NP dispersions with washed activated sludge indicated that CeO2 NPs had a strong tendency to partition onto the sludge biomass, with a concentration factor of approximately 1000 when comparing the initial aqueous concentration mg/L with the final mg Ce/kg sludge dry weight. SEM-EDS analysis of the sludge indicated the presence of Ce in discernable agglomerates as well as an association with the surfaces of bacteria. Taken as a whole the results indicate a large fraction of CeO2 NP will be removed by A/S due to two mechanisms. One mechanism is the destabilization of the NP dispersion and subsequent sedimentation of agglomerated particles. The second mechanism is association of NPs with the surfaces of microorganisms in the sludge. A small fraction of the CeO2 NPs was observed to pass through A/S even though the pH conditions were expected to destabilize the NP dispersion. This phenomenon is most likely due to dispersing effect of certain organic polymers in the wastewater