Sorption of Perfluorochemicals to Granular Activated Carbon in the Presence of Ultrasound

Perfluorochemicals (PFCs) are emerging pollutants of increasing public health and environmental concern due to their worldwide distribution, environmental persistence, and bioaccumulation potential. Activated carbon adsorption is an effective method to remove PFCs from water. Herein, we report on the sorption of four PFCs: perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA), perfluorobutane sulfonate (PFBS), and perfluorobutanoate (PFBA), from deionized water (MQ) and landfill groundwater (GW) by granular activated carbon (GAC) in the absence and presence of 20 kHz ultrasound. In all cases, the adsorption kinetics were found to be well-represented by a pseudosecond-order model, with maximum monolayer sorption capacity and initial sorption rate values following the orders q_(e)^(PFOS) > q_(e)^(PFOA) > q_(e)^(PFBS) > q_(e)^(PFBA) and v_(0)^(PFOS) > v_(0)^(PFBS) > v_(0)^(PFOA) > v_(0)^(PFBA), respectively. The equilibrium adsorption was quantified by the BET multilayer absorption isotherm, and the monolayer sorption capacity increased with increasing PFC chain length: q_(m)^(PFOS) > q_(m)^(PFOA) > q_(m)^(PFBS) > q_(m)^(PFBA). The equilibrium PFC sorption constants, q_e and q_m, and the sorption kinetic constants, v_0 and k_2, were greater in Milli-Q water than in landfill groundwater with or without pretreatment, indicating competition for sorption sites by natural and cocontaminant groundwater organics. Ultrasonic irradiation significantly increased the PFC−GAC sorption kinetics, 250−900%, and slightly increased the extent of PFC equilibrium adsorption, 5−50%. The ultrasonic PFC−GAC sorption kinetics enhancement increased with increasing PFC chain length, suggesting ultrasound acts to increase the PFC diffusion rate into GAC nanopores

Sonolytic Decomposition of Aqueous Bioxalate in the Presence of Ozone

Ultrasonic irradiation in the presence of ozone is demonstrated to be effective for the rapid oxidation of oxalic acid, bioxalate, and oxalate (H_(2)C_(2)O_(4)/HC_(2)O_(4)−/C_(2)O_(4)^2−) in aqueous solution to CO_2 and H_(2)O. The degradation rate of bioxalate exposed to “sonozone” (i.e., simultaneous ultrasonication and ozonolysis) was found to be 16-times faster than predicted by the linear addition of ozonolysis and ultrasonic irradiation rates. The hydroxyl radical (•OH) is the only oxy-radical produced that can oxidize oxalate on a relevant time-scale. Thus, plausible •OH production mechanisms are evaluated to explain the observed kinetic synergism of ultrasonication and ozonolysis toward bioxalate decomposition. •OH production via decomposition of O_3 in the cavitating bubble vapor and via the reaction of O_3 and H_(2)O_2 are considered, but kinetic estimations and experimental evidence indicate neither to be a sufficient source of •OH. A free-radical chain mechanism is proposed in which the HC_(2)O_(4)− + •OH reaction functions as a primary propagation step, while the termination occurs through the O_3 + CO_(2)•− reaction via an O-atom transfer mechanism. Kinetic simulations confirm that ozone reacts efficiently with the superoxide (O_(2)•−) ion that is produced by the reaction of O_2 and CO_(2)•− to form •OH radical, and that the reaction of O_3 + CO_(2)•− must be chain terminating. Oxalate is also readily oxidized by “peroxone” treatment (i.e., H_(2)O_2 and O_3). However, the addition of H_(2)O_2 during the course of the sonolytic ozonation of oxalic acid does not appear to increase the observed degradation rate and decreases rates at millimolar levels

Sonochemical Degradation of Perfluorooctane Sulfonate (PFOS) and Perfluorooctanoate (PFOA) in Landfill Groundwater: Environmental Matrix Effects

Perfluorinated chemicals such as perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are environmentally persistent and recalcitrant to most conventional chemical and microbial treatment technologies. In this paper, we show that sonolysis is able to decompose PFOS and PFOA present in groundwater beneath a landfill. However, the pseudo first-order rate constant for the sonochemical degradation in the landfill groundwater is reduced by 61 and 56% relative to MilliQ water for PFOS and PFOA, respectively, primarily due to the presence of other organic constituents. In this study, we evaluate the effect of various organic compounds on the sonochemical decomposition rates of PFOS and PFOA. Organic components in environmental matrices may reduce the sonochemical degradation rates of PFOS and PFOA by competitive adsorption onto the bubble−water interface or by lowering the average interfacial temperatures during transient bubble collapse events. The effect of individual organic compounds depends on the Langmuir adsorption constant, the Henry’s law constant, the specific heat capacity, and the overall endothermic heat of dissociation. Volatile organic compounds (VOCs) are identified as the primary cause of the sonochemical rate reduction for PFOS and PFOA in landfill groundwater, whereas the effect of dissolved natural organic matter (DOM) is not significant. Finally, a combined process of ozonation and sonolysis is shown to substantially recover the rate loss for PFOS and PFOA in landfill groundwater

Reductive defluorination of aqueous perfluorinated alkyl surfactants : effects of ionic headgroup and chain length

Perfluorinated chemicals (PFCs) are distributed throughout the environment. In the case of perfluorinated alkyl carboxylates and sulfonates, they can be classified as persistent organic pollutants since they are resistant to environmentally relevant reduction, oxidation, and hydrolytic processes. With this in mind, we report on the reductive defluorination of perfluorobutanoate, PFBA (C_3F_7CO_2−), perfluorohexanoate, PFHA (C_5F_(11)CO_2−), perfluorooctanoate, PFOA (C_7F_(15)CO_2−), perfluorobutane sulfonate, PFBS (C_4F_9SO_3−), perfluorohexane sulfonate, PFHS (C_6F_(13)SO_3−), and perfluorooctane sulfonate, PFOS (C_8F_(17)SO_3−) by aquated electrons, eaq−, that are generated from the UV photolysis (λ = 254 nm) of iodide. The ionic headgroup (-SO_3− vs -CO_2−) has a significant effect on the reduction kinetics and extent of defluorination (F index = −[F−]_(produced)/[PFC]_(degraded)). Perfluoroalkylsulfonate reduction kinetics and the F index increase linearly with increasing chain length. In contrast, perfluoroalkylcarboxylate chain length appears to have a negligible effect on the observed kinetics and the F index. H/F ratios in the gaseous fluoro-organic products are consistent with measured F indexes. Incomplete defluorination of the gaseous products suggests a reductive cleavage of the ionic headgroup occurs before complete defluorination. Detailed mechanisms involving initiation by aquated electrons are proposed

Electrochemical Water Splitting Coupled with Organic Compound Oxidation: The Role of Active Chlorine Species

The need for alternative energy sources with minimal to no carbon footprint is growing. A solar-powered electrochemical system that produces hydrogen via water splitting using organic pollutants as sacrificial electron donors is a possible solution. The hybridization of a BiO_x−TiO_2/Ti anode with a stainless steel cathode powered by a photovoltaic (PV) array has been shown to achieve this process. The electrochemical degradation kinetics of a variety of organic substrates is investigated as a function of a background electrolyte, NaCl versus Na_2SO_4. The observed substrate (S) degradation kinetics (k_(obs)^S) are found to correlate well with the cell current (I_(cell)) and the H_2 production energy efficiency (EE) in the presence of NaCl as the background electrolyte. In the case of Na_2SO_4, no correlation is observed and the degradation rates are greatly reduced in comparison to NaCl. This suggests that the primary chemical oxidant is electrolyte-dependent. The k_(obs)^S’s are found to be proportional to the bimolecular rate constants of Cl_2^(•−) with the substrate (k_(Cl_2^(•−) + S)) and to substrate-induced ΔEEs (EE with substrate − EE without substrate) in the presence of NaCl. The ΔEE correlation arises from the active chlorine species acting as an electron shuttle, which compete with H_2 production for cathodic electrons. In the presence of the organic substrates, the active chlorine species are quenched, increasing the fraction of electrons utilized for the H_2 production

Sonochemical Degradation of Perfluorooctane Sulfonate (PFOS) and Perfluorooctanoate (PFOA) in Groundwater: Kinetic Effects of Matrix Inorganics

Ultrasonic irradiation has been shown to effectively degrade perfluorinated chemicals (PFCs) such as perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) in aqueous solution. Reduced PFC sonochemical degradation rates in organic-rich groundwater taken from beneath a landfill, however, testify to the negative kinetic effects of the organic groundwater constituents. In this study, the PFOX (X = S or A) sonochemical degradation rates in a groundwater sample with organic concentrations about 10 times lower than those in the groundwater taken from beneath a landfill are found to be 29.7% and 20.5% lower, respectively, than the rates in Milli-Q water, suggesting that inorganic groundwater constituents also negatively affect PFC sonochemical kinetics. To determine the source of the groundwater matrix effects, we evaluate the effects of various inorganic species on PFOX sonochemical kinetics. Anions over the range of 1−10 mM show Hofmeister effects on the sonochemical degradation rates of PFOX, k_(ClO_4)^(−PFOX) > k_(NO_3)^(−PFOX) ~ k_(Cl^−)^(−PFOX) ≥ k_(MQ)^(−PFOX) > k_(HCO_3)^(−PFOX) ~ k_(SO_(4)^(2−)^(−PFOX). In contrast, common cations at 5 mM have negligible effects. Initial solution pH enhances the degradation rates of PFOX at 3, but has negligible effects over the range of 4 to 11. The observed inorganic effects on sonochemical kinetics are hypothesized to be due to ions’ partitioning to and interaction with the bubble−water interface. Finally, it is shown that the rate reduction in the groundwater in this study is primarily due to the presence of bicarbonate and thus can be fully rectified by pH adjustment prior to sonolysis

Sonochemical Degradation of Perfluorooctanesulfonate in Aqueous Film-Forming Foams

Aqueous film-forming foams (AFFFs) are fire extinguishing agents developed by the Navy to quickly and effectively combat fires occurring close to explosive materials and are utilized today at car races, airports, oil refineries, and military locations. Fluorochemical (FC) surfactants represent 1−5% of the AFFF composition, which impart properties such as high spreadability, negligible fuel diffusion, and thermal stability to the foam. FC’s are oxidatively recalcitrant, persistent in the environment, and have been detected in groundwater at AFFF training sites. Ultrasonic irradiation of aqueous FCs has been reported to degrade and subsequently mineralize the FC surfactants perfluorooctanoate (PFOA) and perfluorooctanesulfonate (PFOS). Here we present results of the sonochemical degradation of aqueous dilutions of FC-600, a mixture of hydrocarbon (HC) and fluorochemical components including cosolvents, anionic hydrocarbon surfactants, fluorinated amphiphilic surfactants, anionic fluorinated surfactants, and thickeners such as starch. The primary FC surfactant in FC-600, PFOS, was sonolytically degraded over a range of FC-600 aqueous dilutions, 65 ppb < [PFOS]_i < 13100 ppb. Sonochemical PFOS−AFFF decomposition rates, R_(AFFF)^(−PFOS), are similar to PFOS−Milli-Q rates, R_(MQ)^(−PFOS), indicating that the AFFF matrix only had a minor effect on the sonochemical degradation rate, 0.5 < R_(AFFF)^(−PFOS)/R_(MQ)^(−PFOS) < 2.0, even though the total organic concentration was 50 times the PFOS concentration, [Org]_(tot)/[PFOS] ~50, consistent with the superior FC surfactant properties. Sonochemical sulfate production is quantitative, Δ[SO_4^(2−)]/Δ[PFOS] ≥ 1, indicating that bubble-water interfacial pyrolytic cleavage of the C−S bond in PFOS is the initial degradation step, in agreement with previous studies done in Milli-Q water. Sonochemical fluoride production is significantly below quantitative expectations, Δ[F^−]/Δ[PFOS] ~4 vs 17, suggesting that in the AFFF matrix, PFOS’ fluorochemical tail is not completely degraded, whereas Milli-Q studies yielded quantitative F− production. Measurements of time-dependent methylene blue active substances and total organic carbon indicate that the other FC-600 components were also sonolytically decomposed

Reductive degradation of perfluoroalkyl compounds with aquated electrons generated from iodide photolysis at 254 nm

The perfluoroalkyl compounds (PFCs), perfluoroalkyl sulfonates (PFXS) and perfluoroalkyl carboxylates (PFXA) are environmentally persistent and recalcitrant towards most conventional water treatment technologies. Here, we complete an in depth examination of the UV-254 nm production of aquated electrons during iodide photolysis for the reductive defluorination of six aquated perfluoroalkyl compounds (PFCs) of various headgroup and perfluorocarbon tail length. Cyclic voltammograms (CV) show that a potential of +2.0 V (vs. NHE) is required to induce PFC oxidation and −1.0 V is required to induce PFC reduction indicating that PFC reduction is the thermodynamically preferred process. However, PFCs are observed to degrade faster during UV(254 nm)/persulfate (S2O82−) photolysis yielding sulfate radicals (E° = +2.4 V) as compared to UV(254 nm)/iodide (I−) photolysis yielding aquated electrons (E° = −2.9 V). Aquated electron scavenging by photoproduced triiodide (I3−), which achieved a steady-state concentration proportional to [PFOS]0, reduces the efficacy of the UV/iodide system towards PFC degradation. PFC photoreduction kinetics are observed to be dependent on PFC headgroup, perfluorocarbon chain length, initial PFC concentration, and iodide concentration. From 2 to 12, pH had no observable effect on PFC photoreduction kinetics, suggesting that the aquated electron was the predominant reductant with negligible contribution from the H-atom. A large number of gaseous fluorocarbon intermediates were semi-quantitatively identified and determined to account for [similar]25% of the initial PFOS carbon and fluorine. Reaction mechanisms that are consistent with kinetic observations are discussed

Experimental Anion Affinities for the Air/Water Interface

Anion affinities, γ_(X^−), for the aerial interface of aqueous (Br^- + NO_3^- + I^- + SCN^- + BF_4^- + ClO_4^-) solutions are determined by electrospray ionization mass spectrometry. The composition of the ions ejected from the surface of fissioning nanodroplets shows that γ_X(−^ )increase (decrease) exponentially with anionic radii, a_(X−(dehydration free energies, ^dG_(X^−)), and selectively respond to the presence of surfactants. BF_4^-, the least hydrated and polarizable anion of the set, has one of the largest γ_(X^−) values. Non-ionic surfactants decrease γ_(I^−) and γ_(SCN^−) but increase γ_(BF_4^−). Cetyltrimethyl ammonium markedly enhances the γ_(X^−) of smaller anions. A similar but weaker effect is observed upon lowering the pH of the bulk solutions from 8.2 to 3.0. Dodecyl sulfate has a negligible effect on γ_(X^−). Considering that (i) universal many-body electrodynamic interactions will progressively stabilize the interfacial layer as its dielectric permittivity falls relative to that of the bulk solution and (ii) water permittivity is uniformly depressed by increasing concentrations of these anions, we infer that the observed Hofmeister correlation, ln γ_(X^−) ∝ - ^dG_(X^−), is consistent with the optimal depression of the permittivity of the drier interfacial layer by the least hydrated ions. Interfacial ion−ion interactions can significantly influence γ_(X^−) in environmental aqueous media