Christian M. Bako

We conducted experiments to determine whether bioaugmentation with aerobic, polychlorinated biphenyl (PCB)-degrading microorganisms can mitigate polychlorinated biphenyl (PCB) emissions from contaminated sediment to air. Paraburkholderia xenovorans strain LB400 was added to bioreactors containing PCB-contaminated site sediment. PCB mass in both the headspace and aqueous bioreactor compartments was measured using passive samplers over 35 days. Time-series measurements of all 209 PCB congeners revealed a 57% decrease in total PCB mass accumulated in the vapor phase of bioaugmented treatments relative to non-bioaugmented controls, on average. A comparative congener-specific analysis revealed preferential biodegradation of lower-chlorinated PCBs (LC-PCBs) by LB400. Release of the most abundant congener (PCB 4 [2,2'-dichlorobiphenyl]) decreased by over 90%. Simulations with a PCB reactive transport model closely aligned with experimental observations. We also evaluated the effect of the phytogenic biosurfactant, saponin, on PCB bioavailability and biodegradation by LB400. Time-series qPCR measurements of biphenyl dioxygenase (bphA) genes showed that saponin better maintained bphA abundance, compared to the saponin-free treatment. These findings indicate that an active population of bioaugmented, aerobic PCB-degrading microorganisms can effectively lower PCB emissions and may therefore contribute to minimizing PCB inhalation exposure in communities surrounding PCB-contaminated sites.

We have previously shown that nitrogen-functionalized carbon nanotubes (CNTs) represent both a precursor and source of N-nitrosodimethylamine (NDMA), a nitrogenous disinfection byproduct (DBP). Here, we explore the potential for non-functionalized, oxidized (via strong acid, ozone or chlorine), and polymer-functionalized CNTs to serve as precursors for chlorinated DBPs including regulated trihalomethanes (THMs; e.g., chloroform) and chlorinated haloacetic acids (or Cl-HAAs; e.g., mono-, di- and trichloroacetic acid). During chlorination experiments (typically 10 mg L⁻¹ CNTs, 15 mg L⁻¹ HOCl, pH 8), formation of chloroform (CF) and Cl-HAAs was greatest for CNTs functionalized with the polymers PABS and PEG, producing as much as 3.5 μg of CF per mg-CNT and 10.2 μg of Cl-HAAs per mg-CNT after 4 h of chlorination (with yields increasing over time). Further, while CF and Cl-HAAs production in PABS-CNT systems was comparable to that generated by the free polymer (i.e., PABS in the absence of CNTs), their formation was unique for PEGylated CNTs (i.e., CF and Cl-HAAs were not observed during chlorination of PEG alone, nor for physical mixtures of CNTs and PEG). While non-functionalized and acid-oxidized CNTs did not produce any measurable CF or Cl-HAAs despite exerting a chlorine demand, the ability of CNTs to function as precursors for these DBPs increased with their extent of oxidation via ozone and chlorine. For CNTs oxidized via ozonation or chlorination we observed a strong, linear correlation between formation of CF and Cl-HAAs (primarily trichloroacetic acid) and CNT surface oxygen concentration from X-ray photoelectron spectroscopy. Aside from CF and Cl-HAAs, chloropicrin (or nitrochloroform), a potent toxin used as a broad-spectrum pesticide, was also identified as a major DBP generated during chlorination of nitric acid functionalized CNTs, producing ∼1 μg per mg-CNT after 4 h. We propose its formation is the result of residual forms of oxidized nitrogen that persist on CNT surfaces after nitric acid oxidation. Although measured yields (along with low CNT levels expected in water/wastewater treatment) suggest DBP production from CNTs will be limited compared to traditional precursors (e.g., organic matter), some proposed CNT applications that utilize higher mass loadings (e.g., CNT-based filters) may produce appreciable DBP levels when in contact with residual chlorine.