Natural organic matter (NOM) adsorption onto and coprecipitation with solids formed during softening
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Softening, which reduces the hardness in source waters by precipitating the divalent hardness ions, was evaluated as a mechanism to also remove natural organic matter (NOM) and subsequently reduce disinfection by-product (DBP) formation. The research hypotheses were that NOM removal in softening occurs by adsorption onto or coprecipitation with CaCO3, Mg-CaCO3, and Mg(OH)2, and that the chemical composition and surface characteristics of those precipitates affect the degree of NOM removal. Three phases of experimental research were conducted to investigate these hypotheses. In Phase I, softening experiments using synthetic water were conducted under a range of pH values and Ca:Mg ratios. The precipitates were analyzed by environmental scanning electron microscopy and electrophoresis to determine the morphology and surface charge. Adsorption studies in Phase II evaluated how efficiently pure solids, similar in composition to softening precipitates identified in Phase I, adsorb NOM. In Phase III, the extent of NOM removal through coprecipitation was evaluated; softening experiments were performed in which the NOM was added prior to precipitation of the solids. NOM extracts from two different hard waters (Lake Austin and Missouri River) were used in the research. Phase I results revealed precipitate surface charge increased to more positive values with softening, presenting favorable conditions for adsorbing NOM. CaCO3 exhibited the calcite morphology and the Mg(OH)2 precipitates were amorphous. Also, some magnesium was incorporated into CaCO3, resulting in increased surface charge and a less-structured morphology, favorable characteristics for NOM removal. The Phase II research showed that Mg(OH)2 adsorbed NOM to the greatest extent, followed by freshly-precipitated CaCO3, and then MgCO3. The aged CaCO3 and Mg-CaCO3 (echinoids) did not remove NOM. The Phase III research revealed both magnesium and calcium were able to coprecipitate NOM. Coprecipitation with CaCO3 occurred through NOM adsorption onto the flocs as they formed, whereas NOM removal with Mg may have occurred by both direct precipitation and adsorption as Mg(OH)2 precipitated. The maximum degree of removal was equivalent for coprecipitation with both Ca and Mg; however significantly less Mg precipitation was required to achieve the same removal. Finally, NOM removal by the softening precipitates was limited by the NOM characteristics themselves.