Partitioning-Enhanced Photochemical Reductive Dehalogenation of Organohalogen Compounds in Wastewater-Associated Dissolved Organic Matter and Foams

Abstract

Reactive reducing species (RRS) – including hydrated electron (eaq–), hydrogen atom (H•), and other reducing species – may form during ultraviolet irradiation of chromophoric dissolved organic matter (CDOM) in natural or engineered aquatic systems. Traditionally, RRS have been assumed as insignificant in photochemical processes due to their rapid scavenging by aqueous molecular oxygen and protons. However, within CDOM microenvironments, RRS may be shielded from scavengers, resulting in higher levels than in the bulk aqueous phase. CDOM may also sorb hydrophobic and amphiphilic trace contaminants (TCs), including those susceptible to RRS-mediated degradation, such as organohalogens. In such contexts, the extended RRS lifetimes, enriched organohalogen concentrations, and enhanced spatial proximity between RRS and organohalogens within CDOM may facilitate elevated rates of reductive dehalogenation relative to the bulk aqueous phase. To quantify the potential for organohalogen TCs to be transformed within DOM microenvironments, experimental work investigated partitioning of select organohalogen TCs into natural and wastewater-associated DOM, applying solid-phase microextraction (SPME) batch sorption isotherms to derive the organic carbon–water partitioning coefficient (KDOM). Method development addressed analytical artifacts and kinetics through targeted assessments such as SPME fiber fouling by DOM and kinetic screening tests. A key finding was that DOM sorption isotherms were linear, indicating that sorption sites were not limiting and generally contained similar free energies for association of the organohalogen TC with DOM. KDOM values derived from SPME tests revealed differences of up to three orders of magnitude in equilibrium partitioning, indicating intermolecular interactions of the organohalogen TC with DOM were substantially different. In particular, higher molecular weight organohalogen TCs with larger molar volumes and those with a greater degree of halogen substitution tended to have greater KDOM values, likely due to greater hydrophobic van der Waals interactions. Further, organohalogen TCs capable of hydrogen-accepting and - interactions had a greater association with DOM. Finally, acid/base speciation significantly impacted partitioning, where KDOM values were lower for anionic species, likely due to electrostatic repulsion due to the negative surface charge associated with DOM. Thus, key intermolecular interactions between organohalogen TCs and DOM were van der Waals hydrophobic interactions, acid/base speciation and associated electrostatic interactions, and - interactions. This research further characterized and compared the degrees to which representative CDOM macromolecules (i.e., whole natural organic matter, humic and fulvic acids, model biomolecules, and an authentic effluent organic matter (EfOM) isolate) generated RRS during exposure to (1) broadband simulated sunlight generated by a Xe arc lamp solar simulator, and (2) 254 nm UV light generated by a low-pressure Hg lamp. The sorption-enhanced effects of RRS-mediated organohalogen degradation within CDOM microenvironments were assessed relative to the bulk aqueous phase using hexachloroethane (HCE) and chloroacetate (CA) as probe compounds, respectively. HCE served as an intra-CDOM RRS probe due to its CDOM partitioning (logKDOM = 4.14), high second-order rate constant for reaction with eaq– ( = 3.9×1010 M–1 s–1), and resistance to direct photolysis or degradation by co-occurring reactive species (e.g., hydrogen atom, H● and hydroxyl radical, HO●). CA served as a bulk aqueous phase RRS probe due to its high water solubility, high second-order rate constant for reaction with eaq– ( = 1×109 M–1 s–1), and resistance to direct photolysis. A key finding was that biomolecule components, particularly those with phenolic or indole moieties, were confirmed to be capable of generating eaq–. Further, the eaq– quantum yield under low pressure irradiation was estimated as 126±48× higher within DOM microenvironments compared to the bulk aqueous phase. The eaq– quantum yield under simulated sunlight was moderately lower within DOM microenvironments but still significant at 73±41× compared to bulk aqueous measurements. Thus, this research highlighted substantial increases in eaq– quantum yields and steady state concentrations within DOM microenvironments relative to bulk water. These data provide further context that may be used to estimate the consequent extent(s) to which organohalogen TC degradation by RRS with DOM microenvironments is/are likely to occur in engineered and natural systems. Building upon the investigation of DOM partitioning, this work further explored whether foam microenvironments can enrich organohalogen TCs relative to bulk water, using a bench-scale synthetic-foam matrix representative of surface water and a foam reactor coupled with SPME to quantify sorption/partitioning metrics (e.g., logKDOM and foam/water partitioning expressed via enrichment factors). Consistent with earlier DOM partitioning behavior, the organohalogen TCs exhibited substantial association with DOM components (logKDOM values in the ~4–6 range for SRHA and the plant-based surfactant saponin), with compound- and DOM-specific behavior such as 4,4′-DDT sorbing more strongly to SRHA than saponin (associated with π–π interactions in SRHA’s aromatic fractions). However, foam fractionation produced only modest contaminant enrichment overall (EFs generally ~1 to 1.6). Mirex had the highest enrichment factor (~1.6) consistent with its high hydrophobicity. FOSA paradoxically had slightly higher foam enrichment at pH 8 despite lower DOM sorption—supporting the conclusion that air–water interfacial partitioning, rather than DOM-driven hydrophobic association, can dominate foam association for some compounds. Importantly, DOM itself was not enriched into the generated foam, contrasting with much larger DOM enrichment factors reported for natural marine/freshwater/wastewater foams. The simplified synthetic matrix tested in this work lacked suspended biomass and partially degraded biomolecules that are known to promote foaming, which may have limited the foam enrichment factors observed. Future work is recommended with real waters or amended matrices to better emulate environmental conditions conducive to foam formation and TC partitioning. This work is novel in that it explored the partitioning behavior of organohalogen TCs to DOM, the potential role of RRS-mediated organohalogen degradation within CDOM microenvironments in both natural and engineered systems (i.e., municipal wastewaters and wastewater-impacted receiving streams), and investigated the potential for enrichment within foams as an additional multiphase heterogeneous microenvironment that can influence contaminant partitioning and has potential to influence photochemical transformation. This research further highlights underrecognized but potentially important pathways for organohalogen contaminant fate and attenuation.