Nanoscale Structural/Chemical Characterization of Manganese Oxide Surface Layers and Nanoparticles, and the Associated Implications for Drinking Wate

Water treatment facilities commonly reduce soluble contaminants, such as soluble manganese (Mn2+), in water by oxidation and subsequent filtration. Previous studies have shown that conventional porous filter system removes Mn2+ from drinking water by developing Mn-oxides (MnO x(s)) bearing coating layers on the surface of filter media. Multiple models have been developed to explain this Mn2+ removal process and the formation mechanism of MnOx(s) coatings. Both, experimental and theoretical studies to date have been largely focused on the micrometer to millimeter scale range; whereas, coating layers are composed of nanoscale particles and films. Hence, understanding the nanoscale particle and film formation mechanisms is essential to comprehend the complexity of soluble contaminant removal processes. The primary objective of this study was to understand the initial MnOx(s) coating formation mechanisms and evaluate the influence of filter media characteristics on these processes. We pursued this objective by characterizing at the micro and nanoscale MnO x(s) coatings developed on different filter media by bench-scale column tests with simulating inorganic aqueous chemistry of a typical coagulation fresh water treatment plant, where free chlorine is present across filter bed. Analytical SEM and TEM, powder and synchrotron-based XRD, XPS, and ICPMS were used for characterization of coatings, filter media and water solution elemental chemistry. A secondary objective was to model how surface coating formation occurred and its correlation with experimentally observed physical characteristics. This modeling exercise indicates that surface roughness and morphology of filtering media are the major contributing factors in surface coating formation process. Contrary to previous models that assumed a uniform distribution and growth of surface coating, the experimental results showed that greater amounts of coating were developed in rougher areas. At the very early stage of coating formation, unevenly distributed thin films and/or particle aggregates were observed, which provided active sites for further surface coating growth. The predominant MnOx(s) phase in the surface coatings was identified to be poorly crystalline birnessite having scavenging activity by intercalation and/or sorption. This would explain the enhancement of efficiency in removing soluble manganese and other contaminants during water filtration. Moreover, the increased Mn2+ removal effect of having aluminum (Al) in pre-treated water is explained. These results indicate that the surface roughness and morphology need to be incorporated into particle capture models to more precisely describe the soluble manganese removal process.