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Sulfate Radical-Based Advanced Oxidation Treatment for Groundwater Water Treatment and Potable Water Reuse

  • Author(s): Li, Wei
  • Advisor(s): Liu, Haizhou
  • et al.
Abstract

Climate change and population growth pose increasing challenges to the availability of fresh water resources. Remediation of contaminated groundwater and recycling of wastewater are two promising solutions to increase fresh water supplies. Persulfate (S2O82-)-based advanced oxidation processes (AOPs) have gained increasing attention in recent years due to the generation of highly reactive and selective sulfate radicals (SO4•-). S2O82--based AOPs have been widely used in in situ chemical oxidation (ISCO) for groundwater remediation. UV/S2O82- is also an alternative to UV/H2O2 that can be used to treat recycled wastewater into high quality drinking water.

In this dissertation work, we examined the application of S2O82--based AOPs in ISCO and UV/AOPs. First, the effects of important groundwater chemical parameters, i.e., alkalinity, pH and chloride on benzene degradation via heterogeneous S2O82- activation by three Fe(III)- and Mn(IV)-containing aquifer minerals (e.g. ferrihydrite, goethite and pyrolusite) were examined. A comprehensive kinetic model was established to elucidate the mechanisms of radical generation and mineral surface complexation. Results showed

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that an increase of alkalinity up to 10 meq/L decreased the rates of persulfate decomposition and benzene degradation, which was associated with the formation of unreactive surface carbonato complexes. An increase in pH generally accelerated persulfate decomposition due to enhanced formation of reactive surface hydroxo complexation. A change in the chloride level up to 5 mM had a negligibly effect on the reaction kinetics. Kinetics modeling also suggested that SO4•- was transformed into hydroxyl radicals (HO•) and carbonate radicals (CO3•-) at higher pHs.

Secondly, we investigated 1,4-dioxane degradation by UV/S2O82- in the presence of monochloramine (NH2Cl), a membrane anti-fouling reagent. The mixing effects of NH2Cl and S2O82- on 1,4-dioxane degradation were examined at various oxidant doses, chloride concentrations, solution pHs and dissolved O2 levels. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond which the scavenging effects of NH2Cl on HO•, SO4•-, and Cl2•- radicals decreased 1,4-dioxane degradation rate. At the optimal ratio, the degradation rate of 1,4-dioxane increased linearly with the total oxidant dose up to 6 mM. The simultaneous photolysis of NH2Cl and S2O82- was sensitive to the solution’s pH, due to a disproportionation of NH2Cl into less photo-reactive dichloramine (NHCl2) and radical scavenger NH4+ at pH lower than 6. An increase of chloride concentration transformed reactive HO• and SO4•- to less reactive Cl2•-, while the presence of dissolved O2 promoted 1,4-dioxane degradation and gaseous nitrogen production by participating in the 1,4-dioxane and NH4+ oxidation steps.

Third, based on the experimental investigations, a kinetic model was developed and

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the fundamental mechanisms of contaminant degradation by three UV/AOPs, i.e., UV/hydrogen peroxide (H2O2), UV/S2O82- and UV/chlorine (HOCl) were simulated. Results showed that the treatment efficiency of contaminants generally followed the order of UV/S2O82- > UV/H2O2 > UV/HOCl under chemical conditions relevant to reverse osmosis (RO) permeate. The generation of HO� was important in UV/H2O2, whereas both SO4•- and HO� were important in UV/S2O82-. CO3•- predominated in UV/HOCl. Among the three UV/AOPs, the treatment efficiency of UV/S2O82- was most sensitive to pH, chloride and inorganic carbon. The results provided guidance on the design and optimization of UV/AOP systems for water reuse under diverse chemical conditions.

Finally, the oxidation byproducts (OBPs) of benzene and 1,4-dioxane by S2O82- based AOPs were investigated, and the cytotoxicity and genotoxicity levels of the parent compound and oxidation byproducts were compared. Results showed that phenol and aldehyde are the major products from benzene degradation. 1,4-dioxane was transformed in to six major intermediates after UV/AOP treatments including ethylene glycol diformate, formaldehyde, glycolaldehyde, glycolic acid, formic acid, and methoxyacetic acid. Toxicity assays showed that the aldehyde and phenol were extremely genotoxic and cytotoxic compare to benzene and 1,4-dioxane. Results suggested that evaluation of the efficiency of any AOPs applied to water treatment must not only include a study of the disappearance of the parent compound, but also needs a study of the toxicity of the subsequential oxidation byproducts.

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