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Fate of fullerene aggregate (nC60) in wastewater alum coagulation and chlorination processes

Authors Wang, Chao
Issue Date 2011
Summary C60, one of the most commonly-produced fullerene-based nanomaterials, has been widely studied as an emerging contaminant. Wastewater discharge is one likely route to release nC60 into the aqueous environment to increase environmental and human health risks. However, whether wastewater treatment can provide a good barrier to remove nC60 is unclear, primarily due to analytical difficulty. In this study, a method of LLE followed by high-performance liquid chromatography (HPLC) with UV-vis spectroscopy was selected and modified. This method provided possibility to study the removal of nC60 in wastewater. Coagulant dosages, pH, and water characteristics influenced nC60 removal in wastewater by alum coagulation. The metal precipitates generated by incorporation of hydrolyzed aluminum species onto magnesium (or calcium) carbonate or hydroxide precipitates at high pH (pH 9-10) and high level of alkalinity explained the high nC60 removal. Alkalinity in wastewater was found to be an important factor to achieve good removal of nC60 at lower alum dosages and a wider pH range by affecting the alum hydrolysis and the formation of sodium aluminum hydroxycarbonate precipitates. Suspended solids (SS) could enhance nC60 removal by acting as adsorbents and nuclei to grow flocs at low alum dosages. Sewage organic matter (OM) decreased nC60 removal by reducing the adsorption of nC60 onto SS and aluminum hydroxide precipitates through disaggregating nC60 , consuming the hydrolyzed aluminum species, and competing for the adsorption sites of SS and amorphous aluminum hydroxide precipitates. Salinity improved nC60 removal by aggregating nC60 and promoting flocculation by double-layer compression, but the effect of salinity was relatively minor. Removal of nC60 by alum coagulation was favored in the wastewater environment and it was especially true for the CEPT process treating saline wastewater in Hong Kong. The mechanisms for nC60 removal in wastewater by alum coagulation were explained by differentiating the hydrolyzed aluminum species using the ferron method. In wastewater, the nC60 removal was achieved by such ways, including adsorption of nC60 onto gravitationally settlable SS; hetero-precipitation of nC60 with SS, colloids, and Alb; sweep flocculation (enmeshment and adsorption) with Alc; adsorption to the co-precipitates of hydrolyzed aluminum species with magnesium (or calcium) carbonate or hydroxide, or coprecipitation with hydrolyzed aluminum species and magnesium (or calcium) carbonate or hydroxide at high pH (pH 9-10); adsorption to sodium aluminum hydroxycarbonate precipitates at neutral pH and low alum dosages. The enmeshment and adsorption mechanism of nC60 onto Alc was proposed as by inner-sphere complexation. Aqueous nC60 in solution could be transformed by low-level chlorine attack into products that retained absorbance at 350 nm. With the increases in chlorine dosage and reaction time, the products lost absorbance at 350 nm concurrent with the obvious decreases in size and the disappearance of the yellow color of nC60. These products were relatively more hydrophilic and more difficult to be extracted by toluene. The presence of fluorescent light could significantly accelerate the transformation of nC60. The photochlorinated products collected by centrifugation were proposed to consist of a 60-carbon cage structure with carbon-chlorine, epoxide, and hydroxyl functionalities.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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