Impact of ISCO Treatment on PFAA Co-Contaminants at a Former Fire Training Area

Redox potential model for guiding moderate oxidation of polycyclic aromatic hydrocarbons in soils

In-situ chemical oxidation is an important approach to remediate soils contaminated with persistent organic pollutants, e.g., polycyclic aromatic hydrocarbons (PAHs). However, massive oxidants are added into soils without an explicit model for predicting the redox potential (Eh) during soil remediation, and overdosed oxidants would pose secondary damage by disturbing soil organic matter and acidity. Here, a soil redox potential (Eh) model was first established to quantify the relationship among oxidation parameters, crucial soil properties, and pollutant elimination. The impacts of oxidant types and doses, soil pH, and soil organic carbon contents on soil Eh were systematically clarified in four commonly used oxidation systems (i.e., KMnO4, H2O2, fenton, and persulfate). The relative error of preliminary Eh model was increased from 48-62% to 4-16% after being modified with the soil texture and dissolved organic carbon, and this high accuracy was verified by 12 actual PAHs contaminated soils. Combining the discovered critical oxidation potential (COP) of PAHs, the moderate oxidation process could be regulated by the guidance of the soil Eh model in different soil conditions. Moreover, the product analysis revealed that the hydroxylation of PAHs occurred most frequently when the soil Eh reached their COP, providing a foundation for further microorganism remediation. These results provide a feasible strategy for selecting oxidants and controlling their doses toward moderate oxidation of contaminated soils, which will reduce the consumption of soil organic matter and protect the main structure and function of soil for future utilization. ENVIRONMENTAL IMPLICATIONS: This study provides a novel insight into the moderate chemical oxidation by the Eh model and largely reduces the secondary risks of excessive oxidation and oxidant residual in ISCO. The moderate oxidation of PAHs could be a first step to decrease their toxicity and increase their bioaccessibility, favoring the microbial degradation of PAHs. Controlling the soil Eh with the established model here could be a promising approach to couple moderate oxidation of organic contaminants with microbial degradation. Such an effective and green soil remediation will largely preserve the soil's functional structure and favor the subsequent utilization of remediated soil.

Field test of thermally activated persulfate for remediation of PFASs co-contaminated with chlorinated aliphatic hydrocarbons in groundwater

The co-occurrence of per- and polyfluoroalkyl substances (PFASs) and chlorinated aliphatic hydrocarbons (CAHs) in groundwater has drawn increased attention in recent years. No studies have been conducted concerning the oxidative degradation of PFASs and/or CAHs by in situ thermally activated persulfate (TAP) in groundwater, primarily due to the difficulty in cost-effectively achieving the desired temperature in the field. In this study, the effects and mechanisms of PFASs degradation by in situ TAP at a site with PFASs and CAHs co-contaminants were investigated. The target temperature of 40.0-70.0 °C was achieved in groundwater, and persulfate was effectively distributed in the demonstration area - the combination of which ensured the degradation of PFASs and CAHs co-contaminants by in situ TAP. It was demonstrated that the reductions of perfluoroalkyl carboxylic acids (PFCAs) concentration in all monitoring wells were in the range of 43.7 %-66.0 % by in situ TAP compared to those maximum rebound values in groundwater, whereas no effective perfluoroalkane sulfonic acids (PFSAs) degradation was observed. The conversion of perfluoroalkyl acids (PFAAs) precursors was one of the main factors leading to the increase in PFCAs concentrations in groundwater during in situ TAP. CAHs were effectively degraded in most monitoring wells, and furthermore, no inhibitory effects of CAHs and Cl- on the degradation of PFASs were observed due to the presence of sufficient persulfate. Additionally, there were significant increases in SO42- concentrations and reductions of pH values in groundwater due to in situ TAP, warranting their long-term monitoring in groundwater. The integrated field and laboratory investigations demonstrated that the reductions in PFCAs and CAHs concentrations can be achieved by the oxidative degradation of in situ TAP.

Biological and Chemical Transformation of the Six-Carbon Polyfluoroalkyl Substance N-Dimethyl Ammonio Propyl Perfluorohexane Sulfonamide (AmPr-FHxSA)

Sites impacted by aqueous film-forming foam (AFFF) contain co-contaminants that can stimulate biotransformation of polyfluoroalkyl substances. Here, we compare how microbial enrichments from AFFF-impacted soil amended with diethyl glycol monobutyl ether (found in AFFF), aromatic hydrocarbons (present in co-released fuels), acetate, and methane (substrates used or formed during bioremediation) impact the aerobic biotransformation of an AFFF-derived six-carbon electrochemical fluorination (ECF) precursor N-dimethyl ammonio propyl perfluorohexane sulfonamide (AmPr-FHxSA). We found that methane- and acetate-oxidizing cultures resulted in the highest yields of identifiable products (38 and 30%, respectively), including perfluorohexane sulfonamide (FHxSA) and perfluorohexane sulfonic acid (PFHxS). Using these data, we propose and detail a transformation pathway. Additionally, we examined chemical oxidation products of AmPr-FHxSA and FHxSA to provide insights on remediation strategies for AmPr-FHxSA. We demonstrate mineralization of these compounds using the sulfate radical and test their transformation during the total oxidizable precursor (TOP) assay. While perfluorohexanoic acid accounted for over 95% of the products formed, we demonstrate here for the first time two ECF-based precursors, AmPr-FHxSA and FHxSA, that produce PFHxS during the TOP assay. These findings have implications for monitoring poly- and perfluoroalkyl substances during site remediation and application of the TOP assay at sites impacted by ECF-based precursors.

Integration of in-situ chemical oxidation and permeable reactive barrier for the removal of chlorophenols by copper oxide activated peroxydisulfate

In-situ chemical oxidation (ISCO) and permeable reactive barrier (PRB) have been used in field practices for contaminated groundwater remediation. In this lab-scale study, a novel system integrating ISCO and PRB using peroxydisulfate (PDS) as the oxidant and copper oxide (CuO) as the reactive barrier material was developed for the removal of 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP). The influences of chlorophenol concentration and flow rate on the system performance were first evaluated using synthetic solutions. The removal efficiencies of target chlorophenols were greater than 90% when sufficient PDS was supplied ([PDS]/[chlorophenol]>1). It was also found that the removal efficiencies decreased with the increasing chlorophenol concentrations (10-150 μM) and flow rates (1.8-14.4 mL/min). When three real groundwaters were employed, the removal efficiencies of 2,4-DCP and 2,4,6-TCP slightly reduced to 90% and 85%, respectively. For PCP, the removal efficiency dropped to 20% in two groundwaters with relatively high levels of alkalinity. The influences of pH and TOC were found to be insignificant for the range investigated (pH 6.5-8.7 and TOC = 0.4-1.5 mgC/L). The reduced removal efficiency could be due to the formation of weaker radicals and the stronger competition between bicarbonate ions and PDS for the activation sites on the CuO surfaces.

Monodispersed CuO nanoparticles supported on mineral substrates for groundwater remediation via a nonradical pathway

Nonradical oxidation based on singlet oxygen (¹O₂) has attracted great interest in groundwater remediation due to the selective oxidation property and good resistance to background constituents. Herein, recoverable CuO nanoparticles (NPs) supported on mineral substrates (SiO₂) were prepared by calcination of surface-coated metal-plant phenolic networks and explored for peroxymonosulfate (PMS) activation to generate ¹O₂ for degrading organic pollutants in groundwater. CuO NPs with a close particle size (40 nm) were spatially monodispersed on SiO₂ substrates, allowing highly exposure of active sites and consequently leading to outstanding catalytic performance. Efficient removal of various organic pollutants was obtained by the supported CuO NPs/PMS system under wide operation conditions, e.g., working pH, background anions and natural organic matters. Chemical scavenging experiments, electron paramagnetic resonance tests, furfuryl alcohol decay and solvent dependency experiments confirmed the formation of ¹O₂ and its dominant role in pollutants removal. In situ characterization with ATR-FTIR and Raman spectroscopy and computational calculation revealed that a redox cycle of surface Cu(II)-Cu(III)-Cu(II) was responsible for the generation of ¹O₂. The feasibility of the supported CuO NPs/PMS for actual groundwater remediation was evaluated via a flow-through test in a fixed-bed column, which manifested long-term durability, high mineralization ratio and low metal ion leaching.

Coupled photocatalytic alkaline media as a destructive technology for per- and polyfluoroalkyl substances in aqueous film-forming foam impacted stormwater

The release of aqueous film forming foam (AFFF) from fuel fire events, fire training events, and other activities has resulted in the presence of persistent and recalcitrant per- and polyfluoroalkyl substances (PFAS) in soil and water nationwide. This study describes the degradation and defluorination of PFAS in stormwater collected from an AFFF-impacted site. Silica-based granular media (SGM) containing titanium dioxide was packed into a column reactor and placed between ultraviolet (UV) lamps to excite the photocatalyst within the SGM and generate free radicals to degrade PFAS present in water that was passed through the media. The system was amended with nucleophiles (hydroxyls) to facilitate the destruction of PFAS. Results showed rapid degradation of 17 identified PFAS, including perfluoroalkyl acid (PFAA) precursors, perfluorosulfonic acids (PFSAs), and perfluorocarboxylic acids (PFCAs). Significant defluorination was observed, indicating PFAS destruction as a result of the coupled photocatalytic and nucleophilic attack. Column reactor experiment findings indicate SGM in the presence of UV light passively degraded a mixture of PFAS in a concentrated waste stream at ambient conditions.

Degradation of trichloroethylene in aqueous solution by sodium percarbonate activated with Fe(II)-citric acid complex in the presence of surfactant Tween-80

The degradation performance of trichloroethylene (TCE) by sodium percarbonate (SPC) activated with citric acid (CA) chelated Fe(II) in the presence of nonionic surfactant Tween-80 was investigated. The addition of CA successfully prevented the precipitation of iron and facilitated TCE degradation. However, Tween-80 had an inhibitory effect on TCE degradation mainly due to the competition of ^∗OH between Tween-80 and TCE. The effect of SPC and Fe(II) dosage on TCE degradation was also explored and the results displayed that 87.2% of TCE could be degraded in 15 min at the SPC/Fe(II)/CA/TCE molar ratio of 3/4/2/1. Free radical probe tests confirmed that both O2^-∗ and ^∗OH were generated in the SPC/Fe(II)/CA system. Free radical scavenging tests implied that the degradation of TCE in the SPC/Fe(II)/CA system was mainly attributed to ∗OH, while O2^-∗ was only partially involved in the degradation of TCE. In addition, TCE removal was suppressed with the raising of the initial solution pH from 3.0 to 9.0. The actual groundwater (containing Tween-80) tests confirmed that 93.2% of TCE degradation could be achieved at the SPC/Fe(II)/CA/TCE molar ratio of 30/40/10/1 and strongly demonstrated that the SPC/Fe(II)/CA process has potential for the in situ treatment of TCE contaminated groundwater in the presence of surfactant Tween-80. In conclusion, TCE degradation by Fe(II) activated SPC system in the presence of Tween-80 can be significantly enhanced with the addition of CA, and this finding offers an innovative direction for removing chlorinated organic contaminants from groundwater in contaminated site after surfactant solubilization treatment.

Use of α-cyclodextrin to Promote Clean and Environmentally Friendly Disinfection of Phenolic Substrates via Chlorine Dioxide Treatment

The use of chlorine dioxide to disinfect drinking water and ameliorate toxic components of wastewater has significant advantages in terms of providing safe water. Nonetheless, significant drawbacks toward such usage remain. These drawbacks include the fact that toxic byproducts of the disinfection agents are often formed, and the complete removal of such agents can be challenging. Reported herein is one approach to solving this problem: the use of α-cyclodextrin to affect the product distribution in chlorine dioxide-mediated decomposition of organic pollutants. The presence of α-cyclodextrin leads to markedly more oxidation and less aromatic chlorination, in a manner that is highly dependent on analyte structure and other reaction conditions. Mechanistic hypotheses are advanced to explain the cyclodextrin effect, and the potential for use of α-cyclodextrin for practical wastewater treatment is also discussed.

Discerning the inefficacy of hydroxyl radicals during perfluorooctanoic acid degradation

Perfluorooctanoic acid (PFOA) is a recalcitrant contaminant of emerging concern, and there is growing interest in advanced oxidation processes to degrade it. However, there is ambiguity in the literature about the efficacy of hydroxyl radicals (OH) for degrading PFOA. Here, we resolve this controversy by comparing PFOA degradation by UV photolysis (254 nm, 6 × 10^-6 E/L.s) versus UV + H₂O₂, which produces OH. We optimized OH production in a UV + H₂O₂ system using nitrobenzene (NB) as a OH probe, but even under optimized conditions (i.e., 5 g/L H₂O₂), no significant difference occurred in PFOA removal by UV photolysis (21.1 ± 0.4%) versus UV + H₂O₂ (19.7 ± 0.7%) after 1-day treatment. Both treatments also resulted in similar daughter by-product concentrations and defluorination efficiencies (9.5 ± 1.7% for UV photolysis and 6.8 ± 1.0% for UV + H₂O₂), which indicates that OH is ineffective towards PFOA degradation and infers that other degradation mechanisms that are independent of OH production should be explored.

Identification of hydroxyl and sulfate free radicals involved in the reaction of 1,4-dioxane with peroxone activated persulfate oxidant

This research investigates the formation of free radical intermediates in an advanced oxidation processes (AOP) capable of destroying recalcitrant contaminants. The AOP studied is marketed as OxyZone® and relies on the premise of successful persulfate activation by peroxone (hydrogen peroxide plus ozone) and the formation of free radicals. The goal of this research was to determine which radicals are involved in the treatment of the model contaminant, 1,4-dioxane, which is a ubiquitous, recalcitrant organic groundwater pollutant difficult to destroy by conventional oxidants. In a parallel study, the peroxone activation persulfate (PAP) solution investigated herein rapidly degraded 1,4-dioxane. The degradation rates of 1,4-dioxane were measured as a function the oxidant:contaminant ratio. Its degradation products or mechanism were not investigated, however. Electron paramagnetic resonance (EPR) spectroscopy spin trapping was used to identify radicals produced in the oxidant solution, its active ingredients, and their possible interplay. The data presented herein indicate that the combination of hydrogen peroxide and dissolved ozone in the presence of persulfate results in the co-occurrence hydroxyl and sulfate radicals and possibly superoxide/hydroperoxyl radicals. These findings progress our understanding of the chemical radicals formed during PAP treatment of aqueous phase contaminants, such as 1,4-dioxane.

Degradation of PFOA Substitute: GenX (HFPO-DA Ammonium Salt): Oxidation with UV/Persulfate or Reduction with UV/Sulfite?

Hexafluoropropylene oxide dimer acid (HFPO-DA, ammonium salt with trade name: GenX) has been recently detected in river water worldwide. There are significant concerns about its persistence, and potential adverse effects to the biota. In this study, the degradability of GenX by typical advanced redox technologies (UV/persulfate and UV/sulfate) is investigated. Results demonstrate that <5% GenX is oxidized after 3 h in UV/persulfate system, which is much lower than ∼27% for PFOA. In comparison, GenX can be readily degraded and defluorinated by hydrated electron (e_aq^-) generated by UV/sulfite system. Specifically, GenX is not detectable after 2 h, and >90% of fluoride ion is recovered 6 h later. This is attributed to the accumulation and subsequent degradation of CF₃CF₂COOH and CF₃COOH, which are stable intermediates of GenX degradation. Mechanistic investigations suggest that the etheric bond in the molecule is a favorable attack point for the e_aq^-. Such finding is corroborated by quantum chemical calculations. The side CF₃- at the α-carbon probably acts as an effective barrier that prevents GenX from being cleaved by SO₄^-• or OH• at its most sensible point (i.e. the carboxyl group). This study illustrates that reduction by UV/sulfite might be a promising technology to remove GenX from contaminated water.

Efficient removal of perfluorooctane sulfonate from aqueous film-forming foam solution by aeration-foam collection

Aqueous film-forming foams (AFFFs) used in fire-fighting are one of the main contamination sources of perfluorooctane sulfonate (PFOS) to the subterranean environment, requiring high costs for remediation. In this study, a method that combined aeration and foam collection was presented to remove PFOS from a commercially available AFFF solution. The method utilized the strong surfactant properties of PFOS that cause it to be highly enriched at air-water interfaces. With an aeration flow rate of 75 mL/min, PFOS removal percent reached 96% after 2 h, and the PFOS concentration in the collected foam was up to 6.5 mmol/L, beneficial for PFOS recovery and reuse. Increasing the aeration flow rate, ionic strength and concentration of co-existing surfactant, as well as decreasing the initial PFOS concentration, increased the removal percents of PFOS by increasing the foam volume, but reduced the enrichment of PFOS in the foams. With the assistance of a co-existing hydrocarbon surfactant, PFOS removal percent was above 99.9% after aeration-foam collection for 2 h and the enrichment factor exceeded 8400. Aeration-foam collection was less effective for short-chain perfluoroalkyl substances due to their relatively lower surface activity. Aeration-foam collection was found to be effective for the removal of high concentrations of PFOS from AFFF-contaminated wastewater, and the concentrated PFOS in the collected foam can be reused.