Removal of Perfluorooctane Sulfonate (PFOS) and Related Compounds From Industrial Effluents

Abstract

Perfluorooctane sulfonate (PFOS) and related perfluoroalkyl surfactants (PFAS) are ubiquitous contaminants of increasing public concern due to their environmental persistence, toxicity, and bioaccumulation. These perfluorinated compounds have been used for more than half a century in a wide variety of industrial and consumer products ranging from stain repellents such as Teflon® to aqueous fire-fighting foams and to grease-proof food packing. The public health and environmental risks posed by PFAS have driven environmental agencies and the industry to restrict their use to specific applications where they cannot be replaced by other chemicals. The sources and pathways of PFOS and its derivatives in the environment are not well understood. Analysis of environmental samples is critical to understand the fate, transport and persistence of these emerging contaminants. Techniques based on fluorine nuclear magnetic resonance (¹⁹F NMR) spectroscopy and high performance liquid chromatography (HPLC) with suppressed conductivity detection were successfully developed to monitor the presence of PFAS in water samples. Chromatographic separation of C₄ to C₈ PFAS surfactants was achieved using a C₁₈ reversed-phase column and a mobile phase consisting of a mixture of boric acid and acetonitrile at mixing ratios ranging from 75:25 to 45:55 (v/v). The combination of these two techniques was very effective for characterization and routine quantification of PFOS and related chemicals. Analytical methods based on ¹⁹F NMR, HPLC-suppressed conductivity detection, and liquid chromatography with tandem mass spectrometry (LC-MS/MS) were employed to characterize commercial PFOS samples. Linear and branched PFOS isomers in a percentage ratio of 75:25 were identified. Municipal wastewater treatment systems are one of the major sources of PFAS emissions into the environment. The presence of PFAS in sewage sludge from two wastewater treatment plants in Tucson, Arizona, was investigated. Sludge samples were washed with acetic acid and extracted with a mixture of acetic acid and methanol. The extract was cleaned and concentrated by means of solid phase extraction. LC-MS/MS operating in the selective ion monitoring (SIM) mode was employed to assess the presence of perfluorosulfonates, perfluorosulfonamides, and perfluorocarboxylates in sewage sludge samples. PFOS was the only perfluoroalkyl chemical detected in municipal sludge samples at a concentration of 77 ± 5 g kg⁻¹ sludge dry weight. Cost-effective treatment techniques for removing PFAS from industrial effluents are needed to minimize discharges of these pollutants. Reductive dehalogenation is widely applied to the degradation of highly chlorinated compounds. Hence, the susceptibility of PFOS and related compounds to biological and chemical reductive dehalogenation was evaluated in batch assays. PFAS were not reductively dehalogenated by different microbial consortia even after periods of incubation exceeding 2 y, confirming the high resistance of these compounds to microbial degradation. The anaerobic biodegradability of PFOS and perfluorobutane sulfonate (PFBS) samples exposed to electrochemical pretreatment with boron-doped diamond film electrodes was also investigated. The oxidation decreased the concentration of PFAS and dissolved organic carbon in solution, confirming the destruction of these compounds. However, the oxidative treatment did not enhance the susceptibility of PFAS to microbial degradation even after extended periods of incubation (> 1 y). In contrast, PFOS was reductively dehalogenated with a biomimetic system based on vitamin B12 as the catalyst and Ti(III) citrate as the reducing agent. The optimal treatments conditions of the reaction were 260 μM vitamin B₁₂, 36 mM Ti(III) citrate, 70°C and solution pH 9.0. Interestingly, branched PFOS isomers were more prone to degradation by vitamin B₁₂ catalysis compared to the linear isomer. Removal of 3 mol Fper mol of technical PFOS and 12 mol F- per mol of branched PFOS isomers was achieved. Defluorination of PFOS was also observed at environmental relevant conditions of 30°C and pH 7.0, albeit at lower degradation rates. Fluoride and carbon dioxide were identified as the major products of the chemical defluorination. Traces of partially fluorinated volatile compounds were also detected in the headspace. The feasibility of removing PFAS compounds from aqueous streams by sorption onto granular activated carbon (GAC), zeolite, and wastewater treatment sludge was examined in batch isotherm experiments. The fluorocarbon chain and the functional group influenced sorption of the anionic surfactants, PFOS adsorbed more strongly to GAC than perfluorooctanoic acid (PFOA) and PFBS. Activated carbon showed the highest affinity for PFOS (Freundlich K(F) values of 36.7 to 60.9) followed by the hydrophobic, high-silica zeolite NaY (Si/Al 80, K(F) of 31.8) and lastly anaerobic sludge (K(F) of 0.95 to 1.85). GAC sorption is a suitable treatment for the removal of anionic perfluoroalkyl surfactants when present at low concentrations. Fluoride has been identified as the major product of the reductive dehalogenation of PFOS and derivatives. Thus, the toxicity of inorganic fluoride towards the main microbial populations responsible for the removal of organic constituents and nutrients in wastewater treatment processes was also studied. Fluoride concentrations ranging from 18 to 43 mg L⁻¹ caused 50% inhibition (IC₅₀) of the activity of propionate- and butyratedegrading microorganisms and of acetate-utilization by methanogens evaluated under mesophilic and thermophilic conditions. All other microbial populations evaluated in this study, i.e., glucose fermenters, aerobic glucose-degrading heterotrophs, denitrifying bacteria, and H₂-utilizing methanogens tolerated fluoride at very high concentrations (> 500 mg L⁻¹). In the same manner, H₂-utilizing methanogens also tolerated PFOS and PFBS at concentrations as high as 200 and 500 mg L⁻¹, respectively.