Kinetic Model for Predicting Perfluoroalkyl Acid Degradation During UV-Sulfite Treatment.

Hydrated electron (eaq-) treatment processes show great potential in remediating recalcitrant water contaminants, including perfluoroalkyl and polyfluoroalkyl substances (PFAS). However, treatment efficacy depends upon many factors relating to source water composition, UV light source characteristics, and contaminant reactivity. Here, we provide critical insights into the complex roles of solution parameters on contaminant abatement through application of a UV-sulfite kinetic model that incorporates first-principles information on eaq- photogeneration and reactivity. The model accurately predicts decay profiles of short-chain perfluoroalkyl acids (PFAAs) during UV-sulfite treatment and facilitates quantitative interpretation of the effects of changing solution composition on PFAS degradation rates. Model results also confirm that the enhanced degradation of PFAAs observed under highly alkaline pH conditions results from changes in speciation of nontarget eaq- scavengers. Reverse application of the model to UV-sulfite data collected for longer chain PFAAs enabled estimation of bimolecular rate constants (k2, M-1 s-1), providing an alternative to laser flash photolysis (LFP) measurements that are not feasible due to the water solubility limitations of these compounds. The proposed model links the disparate means of investigating eaq- processes, namely, UV photolysis and LFP, and provides a framework to estimate UV-sulfite treatment efficacy of PFAS in diverse water sources.

Characterizing the Opportunity Space for Sustainable Hydrothermal Valorization of Wet Organic Wastes.

Resource recovery from wet organic wastes can support circular economies by creating financial incentives to produce renewable energy and return nutrients to agriculture. In this study, we characterize the potential for hydrothermal liquefaction (HTL)-based resource recovery systems to advance the economic and environmental sustainability of wastewater sludge, FOG (fats, oils, and grease), food waste, green waste, and animal manure management through the production of liquid biofuels (naphtha, diesel), fertilizers (struvite, ammonium sulfate), and power (heat, electricity). From the waste management perspective, median costs range from -193 $·tonne-1 (FOG) to 251 $·tonne-1 (green waste), and median carbon intensities range from 367 kg CO2 eq·tonne-1 (wastewater sludge) to 769 kg CO2 eq·tonne-1 (green waste). From the fuel production perspective, the minimum selling price of renewable diesel blendstocks are within the commercial diesel price range (2.37 to 5.81 $·gal-1) and have a lower carbon intensity than petroleum diesel (101 kg CO2 eq·MMBTU-1). Finally, through uncertainty analysis and Monte Carlo filtering, we set specific targets (i.e., achieve wastewater sludge-to-biocrude yield >0.440) for the future development of hydrothermal waste management system components. Overall, our work demonstrates the potential of HTL-based resource recovery systems to reduce the costs and carbon intensity of resource-rich organic wastes.

Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA).

Per- and polyfluoroalkyl substances (PFAS) have received increased attention due to their environmental prevalence and threat to public health. Trifluoroacetic acid (TFA) is an ultrashort-chain PFAS and the simplest perfluorocarboxylic acid (PFCA). While the US EPA does not currently regulate TFA, its chemical similarity to other PFCAs and its simple molecular structure make it a suitable model compound for studying the transformation of PFAS. We show that hydrothermal processing in compressed liquid water transforms TFA at relatively mild conditions (T = 150-250 °C, P < 30 MPa), initially yielding gaseous products, such as CHF3 and CO2, that naturally aspirate from the solution. Alkali amendment (e.g., NaOH) promotes the mineralization of CHF3, yielding dissolved fluoride, formate, and carbonate species as final products. Fluorine and carbon balances are closed using Raman spectroscopy and fluoride ion selective electrode measurements for experiments performed at alkaline conditions, where gas yields are negligible. Qualitative FTIR gas analysis allows for establishing the transformation pathways; however, the F-balance could not be quantitatively closed for experiments without NaOH amendment. The kinetics of TFA transformation under hydrothermal conditions are measured, showing little to no dependency on NaOH concentration, indicating that the thermal decarboxylation is a rate-limiting step. A proposed TFA transformation mechanism motivates additional work to generalize the hydrothermal reaction pathways to other PFCAs.

Carbocatalysts for Enhancing Permanganate Oxidation of Sulfisoxazole.

Permanganate (Mn(VII)) is extensively applied in water purification due to its stability and ease of handling, but it is a mild oxidant for trace organic contaminants (TrOCs). Hence, there is significant interest in strategies for enhancing reaction kinetics, especially in combination with efficient and economical carbocatalysts. This study compared the performance of four carbocatalysts (graphite, graphene oxide (GO), reduced-GO (rGO), and nitrogen-doped rGO (N-rGO)) in accelerating sulfisoxazole (SSX) oxidation by Mn(VII) and found that GO exhibited the greatest catalytic performance. Besides, the Mn(VII)/GO system shows desirable capacities to remove a broad spectrum of TrOCs. We proposed that the degradation of SSX in Mn(VII)-GO suspensions follows two routes: (i) direct oxidation of SSX by Mn species [both Mn(VII) and in situ formed MnO2(s)] and (ii) a carbocatalyst route, where GO acts as an electron mediator, accepting electrons from SSX and transferring them to Mn(VII). We developed a mathematical model to show the contribution of each parallel pathway and found one-electron transfer is primarily responsible for accelerating SSX removal in the Mn(VII)/GO system. Findings in this study showed that GO provides a simple and effective strategy for enhancing the reactivity of Mn(VII) and provided mechanistic insights into the GO-catalyzed redox reaction between SSX and Mn(VII).

ABNO-Functionalized Silica as an Efficient Catalyst for Enhancing Permanganate Oxidation of Emerging Contaminants.

Recent reports indicate that some soluble electron-shuttling compounds (or organic mediators) can accelerate reactions between permanganate (Mn(VII)) and contaminants of emerging concern. However, practical application is limited to homogeneous electron-shuttling compounds. This study reports on the development and application of a heterogeneous electron-shuttling catalyst for Mn(VII) reactions with bisphenol A (BPA). First, we screened a series of poly/monocyclic nitroxides, finding that 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) provides the most significant enhancement of Mn(VII)/BPA reaction kinetics, where Mn(VII) oxidizes ABNO to BPA-reactive ABNO+. Next, we immobilized ABNO onto silica (SiO2) by covalent bonding of 9-azabicyclo[3,3,1]nonan-3-one-9-oxyl (keto-ABNO) via a 3-aminopropyltriethoxysilane bridge to yield an ABNO@SiO2 heterogeneous catalyst. The performance of ABNO@SiO2 in catalyzing Mn(VII)/BPA reactions is demonstrated, with BPA reaction kinetics being highly dependent on catalyst dosage and pH conditions. The stability of ABNO@SiO2 was retained at pH 5.0 and decreased slightly at pH 7.0 over five successive Mn(VII)/BPA reaction cycles. Kinetics modeling shows that BPA reacts with immobilized ABNO+, Mn(VII), and in situ formed MnO2. Moreover, ABNO+ can form via ABNO reactions with both Mn(VII) and the in situ formed MnO2. These results indicate a promising strategy for developing practical heterogeneous catalysts for enhancing Mn(VII) reactivity and treatment applications.

Influence of Carbonate Speciation on Hydrated Electron Treatment Processes.

Advanced reduction processes (ARPs) that generate hydrated electrons (eaq-; e.g., UV-sulfite) have emerged as a promising remediation technology for recalcitrant water contaminants, including per- and polyfluoroalkyl substances (PFASs). The effectiveness of ARPs in different natural water matrices is determined, in large part, by the presence of non-target water constituents that act to quench eaq- or shield incoming UV photons from the applied photosensitizer. This study examined the pH-dependent quenching of eaq- by ubiquitous dissolved carbonate species (H2CO3*, HCO3-, and CO32-) and quantified the relative importance of carbonate species to other abundant quenching agents (e.g., H2O, H+, HSO3-, and O2(aq)) during ARP applications. Analysis of laser flash photolysis kinetic data in relation to pH-dependent carbonate acid-base speciation yields species-specific bimolecular rate constants for eaq- quenching by H2CO3*, HCO3-, and CO32- (kH2CO3* = 2.23 ± 0.42 × 109 M-1 s-1, kHCO3- = 2.18 ± 0.73 × 106 M-1 s-1, and kCO32- = 1.05 ± 0.61 × 105 M-1 s-1), with quenching dominated by H2CO3* (which includes both CO2(aq) and H2CO3) at moderately alkaline pH conditions despite it being the minor species. Attempts to apply previously reported rate constants for eaq- quenching by CO2(aq), measured in acidic solutions equilibrated with CO2(g), overpredict quenching observed in this study at higher pH conditions typical of ARP applications. Moreover, kinetic simulations reveal that pH-dependent trends reported for UV-sulfite ARPs that have often been attributed to eaq- quenching by varying [H+] can instead be ascribed to variable acid-base speciation of dissolved carbonate and the sulfite sensitizer.

Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater.

While foam fractionation (FF) process has emerged as a promising technology for removal of per- and polyfluoroalkyl substances (PFASs) from contaminated groundwater, management of the resulting foam concentrates with elevated concentrations of PFASs (e.g., >1 g/L) remains a challenge. Here, we applied hydrothermal alkaline treatment (HALT) to two foam concentrates derived from FF field demonstration projects that treated aqueous film-forming foam (AFFF)-impacted groundwater. Results showed >90% degradation and defluorination within 90 min of treatment (350 °C, 1 M NaOH) of all 62 PFASs (including cations, anions, and zwitterions) identified in foam concentrates. Observed rate constants for degradation of individual perfluoroalkyl sulfonates (PFSAs, CnF2n+1-SO3-), the most recalcitrant class of PFASs, in both foam concentrates were similar to values measured previously in other aqueous matrices, indicating that elevated initial PFAS concentrations (e.g., PFHxSinit = 0.55 g/L), dissolved organic carbon (DOC; up to 4.5 g/L), and salt levels (e.g., up to 325 mg/L chloride) do not significantly affect PFAS reaction kinetics. DOC was partially mineralized by treatment, but a fraction (∼15%) was recalcitrant. Spectroscopic characterization revealed molecular features of the HALT-recalcitrant DOC fraction, and nontarget high-resolution mass spectrometry tentatively identified 129 nonfluorinated HALT-recalcitrant molecules. Analysis of process energy requirements shows that treating PFAS-contaminated foam concentrates with HALT would add minimally (<5%) to the overall energy requirements of an integrated FF-HALT treatment train.

Abatement of Organic Contaminants by Mn(VII)/TEMPOs: Effects of TEMPOs Structure, Organic Contaminant Speciation, and Active Oxidizing Species.

In this study, a representative redox mediator, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), and its para-substituted derivatives (TEMPOs: 4-hydroxyl-TEMPO, 4-acetylamino-TEMPO, and 4-amino-TEMPO) significantly accelerated the abatement of trace organic contaminants (TrOCs, i.e., bisphenol-A (BPA), phenol, amines, and phenylbutazone) by Mn(VII) over a wide pH range of 4.0-9.0. The addition of substituents at para to the > N-O• moiety significantly influenced the degradation kinetics of TrOCs by changing the reduction potentials of TEMPOs and the corresponding oxoammonium cations (TEMPOs+); a linear relationship was observed between the substituents' para Hammett sigma constants and the reduction potentials of TEMPOs and TEMPOs+. Pseudo-first-order reaction rate constants (kobs, min-1) of TrOC degradation by Mn(VII)/TEMPOs were also affected by the pKa of the TrOCs. Generally, the highest kobs values for individual TrOCs were observed at pH near the pKa even for TEMPOs+ with relatively pH-invariant reduction potentials. Overall, TrOC abatement kinetics were related to a combination of reactive species (Mn(VII), in situ formed MnO2, and TEMPOs+). For BPA, the relative contributions (R) of reactive species ranked as R(TEMPOs+) > R(Mn(VII)) > R(in situ formed MnO2) at pH 4.0-8.0, whereas R(Mn(VII)) > R(TEMPOs+) at pH 9.0 mainly owing to a change in BPA speciation as the pH approached the pKa1 value for BPA. The results of this study are useful for the development of heterogeneous TEMPO-based redox mediators and future applications of TEMPO-mediated oxidation systems for accelerated abatement of TrOCs in water.

Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil.

Hydrothermal alkaline treatment (HALT) can effectively degrade per- and polyfluoroalkyl substances (PFASs) present in aqueous film-forming foam (AFFF). However, information is lacking regarding the treatment of PFASs in actual groundwater and soil from AFFF-impacted sites, especially for complex soil matrices. Given the lack of studies on direct soil treatment for PFAS destruction, we herein applied HALT to two groundwater samples and three soil samples from AFFF-impacted sites and characterized the destruction of PFASs using high-resolution mass spectrometry. Results showed that the 148 PFASs identified in all collected field samples, including 10 cationic, 98 anionic, and 40 zwitterionic PFASs, were mostly degraded to nondetectable levels within 90 min when treated with 5 M NaOH at 350 °C. The near-complete defluorination, as evidenced by fluoride release measurements, confirmed the complete destruction of PFASs. While many structures, including perfluoroalkyl carboxylic acids and polyfluorinated substances, were readily degraded, perfluoroalkyl sulfonates (PFSAs, CnF2n+1-SO3-), most notably with short chain lengths (n = 3-5), were more recalcitrant. Rates of PFSA destruction in groundwater samples were similar to those measured in laboratory water solutions, but reactions in soil were slow, presumably due to base-neutralizing properties of the soil. Further, the degradation of PFASs in groundwaters and soils was found to be a function of reaction temperature, NaOH concentration, and reaction time. These findings have important implications for the remediation of AFFF-impacted sites.

Application of High-Resolution Mass Spectrometry to Evaluate UV-Sulfite-Induced Transformations of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF).

UV-sulfite has been shown to effectively degrade per- and polyfluoroalkyl substances (PFASs) in single-solute experiments. We recently reported treatment of 15 PFASs, including perfluoroalkyl sulfonic acids (PFSAs), perfluoroalkyl carboxylic acids (PFCAs), and fluorotelomer sulfonic acids (FTSs), detected in aqueous film-forming foam (AFFF) using high-resolution liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) targeted analysis. Here, we extend the analysis within those original reaction solutions to include the wider set of PFASs in AFFF for which reactivity is largely unknown by applying recently established LC-QTOF-MS suspect screening and semiquantitative analysis protocols. Sixty-eight additional PFASs were detected (15 targeted + 68 suspect screening = 83 PFASs) with semiquantitative analysis, and their behavior was binned on the basis of (1) detection in untreated AFFF, (2) PFAS photogeneration, and (3) reactivity. These 68 structures account for an additional 20% of the total fluorine content in the AFFF (targeted + suspect screening = 57% of total fluorine content). Structure-reactivity trends were also revealed. During treatment, transformations of highly reactive structures containing sulfonamide (-SO2N-) and reduced sulfur groups (e.g., -S- and -SO-) adjacent to the perfluoroalkyl [F(CF2)n-] or fluorotelomer [F(CF2)n(CH2)2-] chain are likely sources of PFCA, PFSA, and FTS generation previously reported during the early stages of reactions. The results also show the character of headgroup moieties adjacent to the F(CF2)n-/F(CF2)n(CH2)2- chain (e.g., sulfur oxidation state, sulfonamide type, and carboxylic acids) and substitution along the F(CF2)n- chain (e.g., H-, ketone, and ether) together may determine chain length-dependent reactivity trends. The results highlight the importance of monitoring PFASs outside conventional targeted analytical methodologies.

Role of TEMPO in Enhancing Permanganate Oxidation toward Organic Contaminants.

Permanganate (Mn(VII)) has been widely applied as an oxidant in water treatment plants. However, compared with ozone, Fenton, and other advanced oxidation processes, the reaction rates of some trace organic contaminants (TrOCs) with Mn(VII) are relatively low. Therefore, further studies on the strategies for enhancing the oxidation of organic contaminants by Mn(VII) are valuable. In this work, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), as an electron shuttle, enhanced Mn(VII) oxidation toward various TrOCs (i.e., bisphenol A (BPA), phenol, estrone, sulfisoxazole, etc.). TEMPO sped up the oxidative kinetics of BPA by Mn(VII) greatly, and this enhancement was observed at a wide pH range of 4.0-11.0. The exact mechanism of TEMPO in Mn(VII) oxidation was described briefly as follows: (i) TEMPO was oxidized by Mn(VII) to its oxoammonium cation (TEMPO+) by electron transfer, which was the reactive species responsible for the accelerated degradation of TrOCs and (ii) TEMPO+ could decompose TrOCs rapidly with itself back to TEMPO or TEMPOH (TEMPO hydroxylamine). To further illustrate the interaction between TEMPO and target TrOCs, we explored the transformation pathways of BPA in Mn(VII)/TEMPO oxidation. Compared to Mn(VII) alone, adding TEMPO into the Mn(VII) solution significantly suppressed BPA's self-coupling and promoted hydroxylation, ring-opening, and decarboxylation. Moreover, the Mn(VII)/TEMPO system was promising for the abatement of TrOCs in real waters for humic acid, and ubiquitous cations/anions had no adverse or even beneficial impact on the Mn(VII)/TEMPO system.

Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Aqueous Film-Forming Foam.

The widespread use of aqueous film-forming foam (AFFF) for firefighting activities (e.g., fire training to extinguish fuel-based fires at aircraft facilities) has led to extensive groundwater and soil contamination by per- and polyfluoroalkyl substances (PFASs) that are highly recalcitrant to destruction using conventional treatment technologies. This study reports on the hydrothermal alkaline treatment of diverse PFASs present in AFFFs. Quantitative and semiquantitative high-resolution mass spectrometry analyses of PFASs demonstrate a rapid degradation of all 109 PFASs identified in two AFFFs (sulfonate- and fluorotelomer-based formulations) in water amended with an alkali (e.g., 1-5 M NaOH) at near-critical temperature and pressure (350 °C, 16.5 MPa). This includes per- and polyfluoroalkyl acids and a range of acid precursors. Most PFASs were degraded to nondetectable levels within 15 min, and the most recalcitrant perfluoroalkyl sulfonates were degraded within 30 min when treated with 5 M NaOH. 19F NMR spectroscopic analysis and fluoride ion analysis confirm the near-complete defluorination of PFASs in both dilute and concentrated AFFF mixtures, and no stable volatile organofluorine species were detected in reactor headspace gases by the gas chromatography-mass spectrometry analysis. These findings indicate a significant potential for application of hydrothermal treatment technologies to manage PFAS waste streams, including on-site treatment of unused AFFF chemical stockpiles, investigation-derived wastes, and concentrated source zone materials.

Removal of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF) Using Ion-Exchange and Nonionic Resins.

Despite benefits to the firefighting industry, the release of per- and polyfluoroalkyl substances (PFASs) from aqueous film-forming foam (AFFF) into aquatic systems poses significant risks to human health and other organisms. While anion-exchange technologies have proven to be effective for removing perfluoroalkyl acids (PFAAs) from water, their effectiveness for removing the diverse PFAS structures discovered in AFFF remains unknown. Here, we report on the adsorption of 75 PFASs, including 63 polyfluorinated substances, in a diluted AFFF mixture using 14 commercially available ion-exchange (IX)/nonionic resins and granular activated carbon (GAC). Results showed that anion-exchange resins (AERs) exhibited significant adsorption of PFASs compared to cation-exchange resins (CERs), nonionic resins (NIRs), and GAC regardless of the PFAS's predicted charge. Isotherm data showed that macroporous AERs have a higher PFAS adsorption capacity compared to gel-type AERs. Cross-correlation comparison of PFAS/Cl- selectivity coefficients (Kex) for each PFAS-AER combination showed that the hydrophobicity of the AER functional group, and polymer matrix played a dominant role in determining resin affinity for PFASs. PFAS structural characteristics also significantly affected adsorption, with increasing chain length and a net negative charge increasing the extent of adsorption. Results from this study provide guidelines for the selection of resins to adsorb a wider range of PFASs and meaningful insights for the development of quantitative models for IX treatment of AFFF-impacted water.

Insights into the Oxidation of Organic Cocontaminants during Cr(VI) Reduction by Sulfite: The Overlooked Significance of Cr(V).

Literature works reported that organic cocontaminants could be degraded during Cr(VI), a contaminant, reduction by sulfite (Cr(VI)/sulfite process). However, the role of Cr(V) and Cr(IV) intermediates in the Cr(VI)/sulfite process has been overlooked. In this study, we confirmed the generation of Cr(V) and proposed a new mechanism for the decomposition of coexisting organic contaminants during Cr(VI)/sulfite reactions occurring in oxygenated solutions at pHini 4.0 with the molar ratio of sulfite to Cr(VI) of 10.0. UV-visible and electron paramagnetic resonance (EPR) spectra indicate that Cr(V) was the predominant Cr intermediates in oxygenated solutions, while Cr(IV) accumulated in deoxygenated solutions. The contribution of Cr(V) to the degradation of organic contaminants was verified by the EPR spectra collected at 2 K and using methyl phenyl sulfoxide as a probe compound. Both Cr(V) and SO4•- contributed to the decomposition of organic contaminants in oxygenated solutions, with the relative contributions from each species being strongly dependent on properties of the target organic cocontaminants. The key mechanisms responsible for Cr(V) accumulation were supported by DFT calculations, and the degradation kinetics of organic cocontaminants was simulated with the program Kintecus 6.51. This work advances the fundamental understanding of the oxidative transformation of coexisting organic contaminants in this process.

Destruction of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF) with UV-Sulfite Photoreductive Treatment.

Ultraviolet photochemical reaction of sulfite (SO32-) photosensitizer generates strongly reducing hydrated electrons (eaq-; NHE = -2.9 V) that have been shown to effectively degrade individual per- and polyfluoroalkyl substances (PFASs), including perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). However, treatment of complex PFAS mixtures in aqueous film-forming foam (AFFF) remains largely unknown. Here, UV-sulfite was applied to a diluted AFFF to characterize eaq- reactions with 15 PFASs identified by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) targeted analysis. Results show that reactivity varies widely among PFASs, but reaction rates observed for individual PFASs in AFFF are similar to rates observed in single-solute experiments. While some structures, including long-chain perfluoroalkyl sulfonic acids (PFSAs) and perfluoroalkyl carboxylic acids (PFCAs) were readily degraded, other structures, most notably short-chain PFSAs and fluorotelomer sulfonic acids (FTSs), were more recalcitrant. This finding is consistent with results showing incomplete fluoride ion release (up to 53% of the F content in AFFF) during reactions. Furthermore, results show that selected PFSAs, PFCAs, and FTSs can form as transient intermediates or unreactive end-products via eaq- reactions with precursor structures in AFFF. These results indicate that while UV-sulfite treatment can be effective for treating PFOS and PFOA to meet health advisory levels, remediation of the wider range of PFASs in AFFF will prove more challenging.

Enhanced Transformation of Emerging Contaminants by Permanganate in the Presence of Redox Mediators.

In this study, a permanganate/redox mediator system for enhanced transformation of a series of emerging contaminants was evaluated. The presence of various redox mediators (i.e., 1-hydroxybenzotriazole, N-hydroxyphthalimide, violuric acid, syringaldehyde, vanillin, 4-hydroxycoumarin, and p-coumaric acid) accelerated the degradation of bisphenol A (BPA) by Mn(VII). Since 1-hydroxybenzotriazole (HBT) exhibited the highest reactive ability, it was selected to further investigate the reaction mechanisms and quantify the effects of important reaction parameters on Mn(VII)/redox-mediator reactions with BPA and bisphenol AF (BPAF). Interestingly, not only HBT accelerated the degradation of BPA, but also BPA enhanced the decay of HBT. Evidence for the in situ formation of HBT· radicals as the active oxidant responsible for accelerated BPA and BPAF degradation was obtained by radical scavenging experiments and 31P NMR spin trapping techniques. The routes for HBT· radical formation involving Mn(VII) and the electron-transfer pathway from BPA/BPAF to HBT· radicals demonstrate that the Mn(VII)/HBT system was driven by the electron-transfer mechanism. Compared to Mn(VII) alone, the presence of HBT totally inhibited self-coupling of BPA and BPAF and promoted ß-scission, hydroxylation, ring opening, and decarboxylation reactions. Moreover, Mn(VII)/HBT is also effective in real waters with the order of river water > wastewater treatment plant (WWTP) effluent > deionized water.

Ruthenium Catalysts for the Reduction of N-Nitrosamine Water Contaminants.

N-Nitrosamines have raised extensive concern due to their high toxicity and detection in treated wastewater and drinking water. Catalytic reduction is a promising alternative technology to treat N-nitrosamines, but to advance this technology pathway, there is a need to develop more-efficient and cost-effective catalysts. We have previously discovered that commercial catalysts containing ruthenium (Ru) are unexpectedly active in reducing nitrate. This study evaluated supported Ru activity for catalyzing reduction of N-nitrosamines. Experiments with N-nitrosodimethylamine (NDMA) show that contaminant is rapidly reduced on both commercial and in-house prepared Ru/Al2O3 catalysts, with the commercial material yielding an initial metal weight-normalized pseudo-first-order rate constant ( k0) of 1103 ± 133 L·gRu-1·h-1 and an initial turnover frequency (TOF0) of 58.0 ± 7.0 h-1. NDMA is reduced to dimethylamine (DMA) and ammonia end-products, and a small amount of 1,1-dimethylhydrazine (UDMH) was detected as a transient intermediate. Experiment with a mixture of five N-nitrosamines spiked into tap water (1 µg L-1 each) demonstrates that Ru catalysts are very effective in reducing a range of N-nitrosamine structures at environmentally relevant concentrations. Cost competitiveness and high catalytic activities with a range of contaminants provide a strong argument for developing Ru catalysts as part of the water purification and remediation toolbox.

Mineral- and Base-Catalyzed Hydrolysis of Organophosphate Flame Retardants: Potential Major Fate-Controlling Sink in Soil and Aquatic Environments.

The ubiquitous occurrence of organophosphate flame retardants (OPFRs) in aquatic and soil environments poses significant risks to human health and ecosystems. Here, we report on the hydrolysis of six OPFRs and three structural analogues in the absence and presence of metal (hydr)oxide minerals. Eight of the target compounds showed marked degradation in alkaline solutions (pH 9-12) with half-lives ranging from 0.02-170 days. Kinetics follow a second-order rate law with apparent rate constants for base-catalyzed hydrolysis (kB) ranging from 0.69-42 000 M-1 d-1. Although hydrolysis in homogeneous solution at circumneutral pH is exceedingly slow (t1/2 > 2 years, except for tris(2,2,2-trichloroethy) phosphate), rapid degradation is observed in the presence of metal (hydr)oxide minerals, with half-lives reduced to <10 days for most of the target OPFRs in mineral suspensions (15 m2/L mineral surface area loading). LC-qToF-MS analysis of transformation products confirmed ester hydrolysis as the active degradation pathway. Values of kB for individual OPFRs are highly variable and correlate with acid dissociation constants (pKa) of the corresponding alcohol leaving groups. In contrast, kinetic parameters for mineral-catalyzed reactions are much less sensitive to OPFR structure, indicating that other factors like mineral-OPFR interactions are rate controlling. Given the documented recalcitrance of OPFRs to biodegradation and photodegradation, these results suggest that mineral-catalyzed hydrolysis may be a major fate-controlling sink in natural environments.

Quantitative Evaluation of an Integrated System for Valorization of Wastewater Algae as Bio-oil, Fuel Gas, and Fertilizer Products.

Algal systems have emerged as a promising strategy for simultaneous treatment and valorization of wastewater. However, further advancement and real-world implementation are hindered by the limited knowledge on the full energetic and nutrient product potentials of such systems and the corresponding value of these products. In this work, an aqueous-based system for the conversion of wastewater-derived algae and upgrading of crude products was designed and demonstrated. Bio-oil, fuel gas, and fertilizer products were generated from algal biomass harvested from a municipal wastewater treatment facility. Experiments showed that 68% of chemical energy contained in the algal biomass could be recovered with 44% in upgraded bio-oil and 23% in fuel gas (calculated as higher heating values), and 44% and 91% of nitrogen and phosphorus element contents in the original feedstock could be recovered as fertilizer products (ammonium sulfate and struvite), respectively. For 1,000 kg of such dry algal biomass, these products had an estimated total value of $427 (in 2014 U.S. dollars). For the first time, experiment-based energy and nutrient recovery potentials of wastewater-derived algae were presented in an integrated manner. Findings also revealed critical research needs and suggested strategies to further improve resource recovery and waste valorization in these systems.

Electrochemical Transformations of Perfluoroalkyl Acid (PFAA) Precursors and PFAAs in Groundwater Impacted with Aqueous Film Forming Foams.

While oxidative technologies have been proposed for treatment of waters impacted by aqueous film forming foams (AFFFs), information is lacking regarding the transformation pathways for the chemical precursors to the perfluoroalkyl acids (PFAAs) typically present in such waters. This study examined the oxidative electrochemical treatment of poly- and perfluoroalkyl substances (PFASs) for two AFFF-impacted groundwaters. The bulk pseudo first order rate constant for PFOA removal was 0.23 L h-1 A-1; for PFOS, this value ranged from 0.084 to 0.23 L h-1 A-1. Results from the first groundwater studied suggested a transformation pathway where sulfonamide-based PFASs transformed to primarily perfluorinated sulfonamides and perfluorinated carboxylic acids (PFCAs), with subsequent defluorination of the PFCAs. Transient increases in the perfluorinated sulfonamides and PFCAs were observed. For the second groundwater studied, no transient increases in PFAAs were measured, despite the presence of similarly structured suspected PFAA precursors and substantial defluorination. For both waters, suspected precursors were the primary sources of the generated fluoride. Assessment of precursor compound transformation noted the formation of keto-perfluoroalkanesulfonates only in the second groundwater. These results confirm that oxidation and defluorination of suspected PFAA precursors in the second groundwater underwent transformation via a pathway different than that of the first groundwater, which was not captured by total oxidizable precursor assay.