Novel (Advanced) Oxidation and Disinfection Processes based on Reactions of Free Available Chlorine or Hydrogen Peroxide with Unconventional Activating Agents: Superoxide Radical and Non-Ferrous/Non-Cuprous Metal(loid) Oxide Catalysts

Abstract

Advanced oxidation processes (AOPs) are a suite of physical/chemical treatment processes that are increasingly investigated and applied to enhance attenuation of trace organic contaminants in water and wastewater treatment and reuse, through in situ generation of hydroxyl radical (HO•) and various other radical or reactive oxidant species. The most prevalent AOPs are typically driven by using UV light and/or ozone (O3) to convert (or “activate”) hydrogen peroxide (H2O2) or free available chlorine (FAC) to HO• and other more reactive oxidants, and often require specialized capital- and energy-intensive infrastructure (e.g., arrays of low- or medium-pressure Hg vapor lamps, or ozone generators), or are constrained by mass-transfer, hydrodynamics, and/or spatial heterogeneity in light transmission. As potential alternatives to such “conventional” AOP approaches, several novel reaction systems were investigated in this work, in which FAC or H2O2 were “activated” using several unconventional agents, including the superoxide radical (O2•–) and a suite of non-ferrous/non-cuprous metal(loid) oxides (MeOx). MeOx minerals studied in this work included amorphous zirconium oxide (a-ZrO2), monoclinic zirconium oxide (m-ZrO2), cerium oxide (CeO2), titanium oxide (TiO2), aluminum oxide (Al2O3), and silicon oxide (SiO2). The reaction of O2•– with FAC (O2•–/FAC) was found capable of effectively generating HO• as well as various reactive chlorine species (RCS), including chlorine monoxide radical (ClO•), chlorine radical (Cl•), and dichloride radical (Cl2•–) with exposure levels comparable to many commonly applied AOPs such as UV/H2O2, O3/H2O2, and UV/FAC under representative water treatment and reuse conditions. Direct formation of halogen-containing disinfection byproducts (DBPs) during O2•–/FAC treatment was found to be minimal. The reactions of MeOx species with H2O2 (MeOx/H2O2) or FAC (MeOx/FAC) were (tentatively) found to be capable of (catalytically) generating the reactive oxidant species singlet oxygen (1O2) and/or superoxide (O2•–) in aqueous phase and/or on the MeOx mineral surfaces, and limited amounts of HO• radical, as identified by multiple radical probes and radical scavengers. FAC-containing systems also appear to generate reactive chlorine species (potentially including Cl• and ClO•). Due to their tendency to generate more selective oxidants (1O2, ClO•) in favor of less selective/stronger oxidants (HO•, Cl•), the MeOx/H2O2 and MeOx/FAC reactions appear less useful as advanced oxidation processes, but could be quite effective as targeted oxidation processes (consistent with observations obtained using the 1O2 probe furfuryl alcohol). Several MeOx/H2O2 and MeOx/FAC reactions were also investigated as the basis for potential (advanced) disinfection processes, using two commonly-employed surrogates of water-borne pathogens, Bacillus subtilis endospores and MS2 bacteriophage. Limited effectiveness was observed toward the highly disinfection-resistant B. subtilis endospores, whereas the CeO2/H2O2 and m-ZrO2/H2O2 reactions were each found to be quite effective toward MS2, which is known to be sensitive to 1O2 (where MS2 was stable toward MeOx or H2O2 alone). In addition to contaminant oxidation and disinfection, several of the MeOx minerals could provide substantial ancillary benefits as (i) adsorbents (e.g., a-ZrO2 is highly effective for adsorption of phosphate, as well as of MS2), or (ii) oxidant quenchers, where a-ZrO2 and CeO2 in particular were found to be especially capable of rapidly and catalytically decomposing H2O2 and FAC. This latter benefit could prove especially advantageous in post-processing of waters treated using conventional AOPs (which frequently require removal of H2O2) or of chlorinated (waste)waters from which removal of FAC is desired prior to use or discharge (e.g., point-of-use consumption or discharge of wastewater to natural receiving waters). Findings reported in this study provide a promising foundation for further examination of a wide array of potential applications of O2•–/FAC, MeOx/H2O2, and MeOx/FAC treatment for mitigating organic contaminants and microbial pathogens (via inactivation and/or adsorption), as well as for rapid, sustained removal of residual H2O2 or FAC during or following (waste)water treatment when desirable.