Precipitative iron coagulation : the impacts of organic matter and inorganic species, and their reactions on the formation and growth of amorphous iron hydroxide

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2021-05

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The incoming water at treatment plants contains various constituents: particles, inorganic contaminants (e.g., arsenic and/or chromium), and natural organic matter (NOM). Due to their detrimental health effects and the absence of federal regulation (e.g., hexavalent chromium), public concerns arose and still remain. Consequently, a proper treatment of those constituents has been an intensive research subject for many years. Coagulation/flocculation is considered economically feasible because of its versatile applications. This treatment technology has been historically applied for turbidity control, but research showed its effectiveness in NOM and inorganic species control as well. Precipitative iron coagulation was extensively investigated in this research along with the influencing parameters: turbidity, coagulant dose, pH, the presence of organic matter (OM) and inorganic ions, their types (e.g., functional groups or oxidation states), and their reactions in the formation and growth of amorphous iron hydroxide. Different types of organic matter were selected based on their acidic ligand properties, particularly their carboxylic and phenolic charge density. Multiple influencing factors (e.g., turbidity, coagulant dose, operating pHs) were tested to investigate the iron hydroxide precipitation under more relevant condition at treatment plants. Turbidity changed the impact of NOM on enhancing nucleation and flocculation because of the competition between particles and NOM in the use of limited amount of Fe ions. However, under the condition allowed sufficient Fe ions, the competition receded. NOM enhanced the precipitation of CaCO₃ (s) at high pH just as it did for Fe(OH)₃ (s) at low pH. The removal trend of each inorganic species under various pH conditions was compared in the absence and presence of NOM that showed either synergistic or competitive reaction, and this reaction mechanism was further investigated through the binding strength between NOM and inorganic species. The results also showed the different reaction mechanisms depending on the density of each functional group; high phenolic content resulted in rapid nucleation and flocculation while high carboxylic content showed the interference in those reactions. A mathematical modeling data of flocculation was compared to experimental results in number distribution and fraction remaining. The comparison showed the excellent agreement between the model and experimental results that the model development from basic principles was valid, and it successfully predict the flocculation of nano size particles. Arsenic and chromium oxyanions were chosen to examine their impacts on the formation and growth of iron hydroxide, and the effect of NOM in their interaction was also tested. By the adsorption and coprecipitation, the formation of iron hydroxide was affected by the oxyanions. In dual ligand condition when NOM was co-presented, the formation and growth of iron hydroxide was more dictated by NOM. The findings in this research indicated that the type of organic matter played an important role in the formation and growth of iron hydroxide. Furthermore, the NOM-altered iron precipitation and the presence of NOM affected the interaction between inorganic species and the precipitates. The research outcome in this study is critical that it provides meaningful insights regarding the impacts of the inorganic and organic substances on iron hydroxide precipitation in a relatively short term which ultimately affect their control at water treatment plants.

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