The application of heterogeneous catalysts for destruction of per- and polyfluoroalkyl substances (PFAS) in subcritical hydrothermal water (350 degrees C, 16.5 MPa) is a promising remediation strategy. Among catalysts screened, carbon-supported noble metals (Ru/C, Rh/C, Pt/C, and Pd/C) were more effective than metal oxides (Al2O3, FeOOH, TiO2, and ZrO2) at promoting degradation and defluorination of perfluorobutane sulfonic acid (PFBS), a representative perfluoroalkyl sulfonic acid (PFSA). After 3 h, PFBS and perfluorooctane sulfonic acid (PFOS) were degraded > 98% and defluorinated > 10% by a 5 wt% ruthenium-on-carbon (Ru/C) catalyst. Defluorination increased to > 20% when reaction was extended to 5 h, but a large fraction of the fluorine mass balance remained unaccounted for. Mono-substituted PFSA intermediates were identified in solution and unextractable organic fluorine species on the Ru/C surface were confirmed by spectroscopic analysis. The bound species were released to solution as F- when exposing the Ru/C to hydrothermal alkaline treatment (HALT) conditions (1 M NaOH, 350 degrees C), closing the fluorine mass balance. A tentative mechanism initiated by homolytic cleavage of carbon-carbon and carbon-sulfur bonds along the perfluoroalkyl chain is proposed, with chemisorbed fluorine resulting from coupling of C-centered radicals to the carbon support. These findings introduce an innovative strategy for more sustainable remediation of PFAS contamination.

Increases in perfluorinated sulfonic acid (PFSA) porewater concentrations over a 35 month duration following in situ flushing were monitored at an aqueous film-forming foam (AFFF) site using porous cup suction lysimeters within a highly instrumented test cell. Results provided evidence that perfluorooctane sulfonate (PFOS) slow desorption kinetics contributed to slow contaminant rebound in measured porewater concentrations. PFSAs in the shallow (0.23 m depth) highly PFSA-impacted soils migrated downward during the monitored post-flushing period, with short-chained PFSAs migrating more rapidly in porewater than long-chained PFSAs. Following flushing, apparent equilibrium porewater concentrations at a depth of 0.61 m below ground surface were attained within two months for perfluoropentane sulfonate (PFPeS), between 2 and 20 months for perfluorohexane sulfonate (PFHxS), and 25 months for PFOS. For PFPeS and PFHxS, apparent steady-state rebound concentrations (to 38% of their pre-flushing baseline levels, with no increasing or decreasing trend over time subsequently observed) were reasonably predicted based on an equilibrium model. PFOS rebound and ultimately vertical migration were highly impacted by non-equilibrium soil desorption. Excavation of elevated PFSAs in surface soils had no impact on PFSA porewater concentrations 0.38 m below the excavation over a 1.2 year post-excavation monitoring period. Together, these long-term rebound data highlight the potential importance of mass transfer-controlled processes for PFOS leaching, and suggest that removal of elevated PFSAs in surface soils may take years until PFSA discharges to groundwater are diminished.

Hydrothermal alkaline treatment (HALT) is an innovative approach that was developed for the destruction of per- and polyfluoroalkyl substances (PFAS). While HALT has been shown to effectively destroy a wide range of PFAS detected in various sample matrices, a comprehensive understanding of the controlling reaction mechanisms and transformation pathways remains limited. Herein, we selected trifluoromethanesulfonate (TFMS), the shortest-chain and likely one of the most recalcitrant PFAS reported to date, to probe degradation mechanisms and identify transformation products. The results indicate that HALT of TFMS proceeds via general nucleophilic substitution and base-promoted pathways, leading to 65.7% mineralization of the parent compound after a 180 min reaction of 0.1 M TFMS at 350 °C in 1 M NaOH. For the degraded TFMS, approximately 100% of fluorine and sulfur were converted to fluoride and sulfate, respectively, while carbon was distributed mainly as carbonate (95%) and formate (5%), along with the production of hydrogen. These findings are supported by both experimental and computational evidence. Hydroxide plays dual roles by initiating the reaction as the nucleophile and promoting subsequent steps by maintaining strongly basic conditions. The initial degradation step is rate-determining, with an estimated energy barrier of 27.8 kcal/mol. Similar mechanisms are proposed for reactions of longer-chain perfluoroalkyl sulfonic acids (PFSAs). Finally, the key factors governing PFSA reactivity across chain lengths from C1 to C8 were identified as reaction temperature, nucleophile type and concentration, and reaction time. This study addresses a critical knowledge gap in PFAS hydrothermal reactions and further establishes HALT as an effective technology for PFAS destruction and defluorination.

Per- and polyfluoroalkyl substances (PFASs) are synthetic chemicals used across numerous industrial and consumer applications. Their persistence and toxicological impacts necessitate their removal from the environment and their complete destruction; however, many PFAS destruction technologies release gas-phase and aerosol-phase fluorinated products of incomplete destruction (PIDs). In this Review, we discuss the PIDs released by PFAS destruction methods and approaches to categorize and measure them. Existing and emerging technologies use thermal, chemical, electrical or biological approaches to degrade PFASs, with varying degrees of success. Although many technologies achieve destruction and removal efficiencies of more than 99.99

Per- and polyfluoroalkyl substances (PFAS) pose a significant challenge for water treatment facilities facing strict regulatory standards. Granular activated carbon (GAC) adsorption is effective for PFAS removal, but media exhaustion and replacement can be costly, highlighting the need for innovative GAC regeneration methods. While thermal reactivation of GAC can eliminate adsorbed PFAS, it requires high temperatures and is mainly feasible for large-scale media users. This study investigates spent GAC regeneration by hydrothermal alkaline treatment (HALT), which applies subcritical water (e.g., 350 °C, 16.5 MPa) amended with strong base (e.g., NaOH) to destroy PFAS. Previous research indicates that HALT successfully degraded and defluorinated PFAS while maintaining GAC surface area and equilibrium adsorption capacity. This study presents data from rapid small-scale column tests (RSSCTs) demonstrating effective removal of long-chain PFAS by a HALT-treated spent GAC sample collected from a long-term PFAS treatment field pilot study (BV50 > 50,000 for PFOS, PFHxS, and PFNA). HALT-treated virgin GAC and untreated virgin GAC evaluated using RSSCTs exhibited similar PFAS breakthrough behavior, with comparable overall PFAS removal to the HALT-treated spent GAC. Physisorption measurements revealed that HALT recovers GAC pore surface area lost during field-use. Surface chemical characterization techniques indicated mostly similar surface composition and functional groups in virgin and HALT-treated GAC, with limited change in the carbon structure following HALT and differences between virgin and field-spent samples. Analyses of reactor liquid products, media mass loss, and NaOH neutralization by GAC also provided evidence for removal of adsorbed non-target organic matter and possible GAC surface renewal by carbon gasification reactions occurring in parallel with PFAS destruction, analogous to surface carbon burn-off that occurs during high-temperature thermal reactivation. Retention of adsorbed metal ions that accumulated on the spent GAC during field testing may be responsible for enhanced adsorption behavior observed for some PFAS following HALT regeneration. Results indicate that HALT can enable reuse of spent GAC, potentially alleviating the high demand for virgin media in PFAS treatment processes.

This study provides a global review of per- and polyfluoroalkyl substances (PFAS) occurrence in industrial wastewater from six key industrial sectors and critically evaluates the performance of currently employed treatment processes for removing PFAS from wastewater. The analysis incorporates publicly available data (2006.11-2025.02) from 205 industrial sites across Asia, Europe, and North America. The dataset includes 1635 concentration records from targeted analysis (77 PFAS) and 137 records from non-targeted analysis (31 PFAS). The results revealed pronounced sectoral clustering in terms of data availability: fluorochemical, electronics, textile, and electroplating wastewater data accounted for over 85 % of the dataset, while PFAS data remaining were limited for pharmaceuticals and food processing. PFAS concentrations spanned ∼12 orders of magnitude in industrial wastewater (2.1 ×10-3 to 1.7 ×109 ng/L). Fluorochemical wastewater exhibited the highest diversity (73 PFAS), dominated by short-chain and emerging PFAS. Electronics industry wastewater showed a shift toward short- and ultrashort-chain PFAS, while textile wastewater featured overall lower PFAS concentrations but was enriched in long-chain PFAS and ether-based alternatives. Electroplating effluents contained elevated levels of perfluorooctane sulfonate (PFOS) and its replacement (perfluoro (2-(6-chlorohexyl) oxy) ethanesulfonic acid and 6:2 fluorotelomer sulfonic acid). Analysis of 734 PFAS data records from 21 full-scale industrial wastewater treatment plants (WWTPs) showed that advanced processes such as adsorption, membrane technology, and the Fenton process achieved removal rates exceeding 90 % for long-chain PFAS (e.g., PFOS), which is substantially higher than the < 50 % removal typically observed for traditional processes. This study highlights the complexity and persistence of industrial PFAS pollution, calling for enhanced monitoring of PFAS and their precursors, development of effective and sustainable treatment technologies, and implementation of life-cycle-based regulatory frameworks to reduce environmental and health risks.

Non-targeted analysis of complex per- and polyfluoroalkyl substances (PFAS) via Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) promises unprecedented insights into the "fluorinome", i.e., the complete set of organofluorine compounds in a given sample. In this study, we present a Python-based workflow developed in parallel with a carefully constructed PFAS formula database comprising ∼ 20 million entries to facilitate the assignment of chemical formulas to ultrahigh-resolution mass spectra. PFAS assignments based on the analysis of complex aqueous film-forming foam (AFFF) on the world's highest-resolving 21 tesla FT-ICR MS were compared to quadrupole time-of-flight (QTOF) MS data, validating 22 common and discovering 19 previously undetected PFAS classes. We then explored the viability of ultrahigh-resolution FT-ICR MS analysis for forensic profiling purposes on PFAS-impacted groundwater samples, identifying bis-perfluoroalkyl sulfonimides (bis-FASIs) as long-suspected ingredients of electrochemical fluorination AFFFs. While our newly developed workflow may also be used for other types of high-resolution mass spectrometers such as QTOF and Orbitrap, this work leverages the unique ultrahigh-resolving power, sub-ppm mass measurement accuracy, and high dynamic range of 21 tesla FT-ICR mass spectrometry to maximize information from complex contaminant mixtures in environmental samples.

Per- and polyfluoroalkyl substances (PFASs) are a large group of anthropogenic fluorinated chemicals. Ultrashort-chain perfluoroalkyl acids (PFAAs) have recently gained attention due to their prevalence in the environment and increasing environmental concerns. In this review, we established a literature database from 1990 to 2024, encompassing environmental and biological concentrations (>3,500 concentration records) of five historically overlooked ultrashort-chain PFAAs (perfluoroalkyl carboxylic and sulfonic acids with less than 4 carbons): trifluoroacetic acid (TFA), perfluoropropanoic acid (PFPrA), trifluoromethanesulfonic acid (TFMS), perfluoroethanesulfonate (PFEtS), and perfluoropropanesulfonate (PFPrS). Our data mining and analysis reveal that (1) ultrashort-chain PFAAs are globally distributed in various environments including water bodies, solid matrices, and air, with concentrations usually higher than those of longer-chain compounds; (2) TFA, the most extensively studied ultrashort-chain PFAA, shows a consistent upward trend in concentrations in surface water, rainwater, and air over the past three decades; and (3) ultrashort-chain PFAAs are present in various organisms, including plants, wildlife, and human blood, serum, and urine, with concentrations sometimes similar to those of longer-chain compounds. The current state of knowledge regarding the sources and fate of TFA and other ultrashort-chain PFAAs is also reviewed. Amid the global urgency to regulate PFASs, particularly as countries worldwide have intensified such efforts, this critical review will inform scientific research and regulatory policies.

Some Per- and polyfluoroalkyl substances (PFAS) are strongly retained in the vadose zone due to their sorption to both soils and air-water interfaces. While significant research has been dedicated to understanding equilibrium behavior for these multi-phase retention processes, leaching and desorption from aqueous film-forming foam (AFFF) impacted soils under field relevant conditions can exhibit significant deviations from equilibrium. Herein, laboratory column studies using field collected AFFF-impacted soils were employed to examine the leaching of perfluoroalkyl acids (PFAAs) under simulated rainfall conditions. The HYDRUS 1-D model was calibrated to estimate the unsaturated hydraulic properties of the soil in a layered system using multiple boundary condtions. Forward simulations of equilibrium PFAS partitioning using the HYDRUS model and simplified mass balance calculations showed good agreement with the net PFAS mass flux out of the column. However, neither were able to predict the PFAS concentrations in the leached porewater. To better understand the mechanisms controlling the leaching behavior, the HYDRUS 1-D two-site leaching model incorporating solid phase rate limitation and equilibrium air-water interfacial partitioning was employed. Three variations of the novel model incorporating different forms of equilibrium air-water interfacial partitioning were considered using built-in numerical inversion. Results of numerical inversion show that a combination of air-water interfacial collapse and rate-limited desorption from soils can better predict the unique leaching behavior exhibited by PFAAs in AFFF-impacted soils. A sensitivity analysis of the initial conditions and rate-limited desorption terms was conducted to assess the agreement of the model with measured data. The models demonstrated herein show that, under some circumstances, laboratory equilibrium partitioning data can provide a reasonable estimation of total mass leaching, but fail to account for the significant rate-limited, non-Fickian transport which affect PFAA leaching to groundwater in unsaturated soils.

Complete defluorination of per- and polyfluoroalkyl substances (PFAS) by a non-thermal technology is not easy to achieve. A rationally designed photochemical–electrochemical treatment train realizes complete defluorination of PFAS mixtures in complex water matrices.

The widespread use of aqueous film-forming foam (AFFF) for firefighting and firefighter training has led to extensive per- and polyfluoroalkyl substance (PFAS) contamination in the environment. Challenges remain in the analytical determination of PFASs via liquid chromatography-mass spectrometry (LC-MS), particularly when attempting to include ultrashort-chain perfluoroalkyl acids (PFAAs) and longer-chain anionic and zwitterionic PFASs in a single direct injection. In this study, we assessed the performance of three analytical LC columns (C18, JJ, and Acclaim columns) to separate targeted and suspect PFASs in AFFF-impacted water samples collected from five sites. The C18 column failed to retain ultrashort-chain PFAAs while the JJ and Acclaim columns were not suitable for hydrophobic PFASs. Ultrashort-chain PFAAs were detected at three sites and comprised 1.6-18% of the total perfluoroalkyl carboxylic and sulfonic acids. Semi-quantified concentrations of suspect PFASs comprised 0.70-13% of the total PFASs. When attempting to capture the entirety of the PFAS mass in a water sample, the C18 column captured the broadest suite of suspect PFASs, while the JJ column quantified the most total PFAS mass. Results of this study highlight the importance and tradeoffs of LC column choice to comprehensively determine the composition of PFASs and their concentrations in AFFF-impacted water samples.

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.

Granular activated carbon (GAC) adsorption is the most common technology applied to treat water contaminated with per-and polyfluoroalkyl substances (PFASs), but rapid exhaustion of the media necessitates frequent replacement and costly off-site thermal regeneration. Here, we extend the application of hydrothermal alkaline treatment (HALT), which uses strong alkali and near-critical temperatures and pressures (e.g., 350 degrees C, 16.5 MPa, and 1 M NaOH) to degrade and mineralize PFASs, to the regeneration of spent GAC. Mass balance experiments wherein a known mass of perfluorooctanesulfonate (PFOS) was adsorbed onto GAC prior to treatment showed that HALT achieved >99% destruction of PFOS and 96 +/- 4% defluorination with no observed fluoro-organic intermediates [167 g L-1 GAC, 350 degrees C, 1 M NaOH, t(rxn)= 400 min, and 10-15 mg of PFOS (g of GAC)(-1)]. Treatment of GAC collected from a field pilot study also showed effective destruction of the range of adsorbed PFASs. Moreover, repeated HALT cycles did not significantly affect the GAC specific surface area, and similar adsorption isotherms for perfluoropentanoic acid and PFOS were recorded for virgin and HALT-treated GAC. These findings suggest a promising strategy for on-site regeneration of PFAS-contaminated GAC and other adsorbent media that may be an alternative to off-site thermal regeneration practices.

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.

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.

The critical challenge of hydrothermal liquefaction (HTL) for bio-oil production from biomass is the production of large amounts of aqueous products (HTL-AP) with high organic contents. The present study investigated the anaerobic digestion (AD) performances of HTL-AP under both thermophilic and mesophilic conditions, and molecular and metabolic analysis were conducted to provide insights into the different performances. The results showed that thermophilic AD had lower COD removal efficiency compared to mesophilic AD (45.0% vs. 61.6%). Liquid chromatography coupled with organic carbon detection and organic nitrogen (LC-OCD-OND) analysis showed that both high molecular weight (HMW) and low molecular weight (LMW) compounds were degraded to some extent and more LMW acids (LMWA) and recalcitrant aromatic compounds were degraded in the mesophilic reactor, which was the main reason of higher COD removal efficiency. Phenyl compounds (e.g. phenol and 2 methoxyphenol), furans and pyrazines were the recalcitrant chemicals detected through GC-MS analysis. Fourier transform ion cyclone resonance mass spectrometry (FT-ICR-MS) analysis demonstrated the complexity of HTL-AP and the proportions of phenolic or condensed aromatic compounds increased especially in the thermophilic effluents. Metabolites analysis showed that the reasons contributing to the differences of mesophilic and thermophilic AD were not only related to the degradation of organic compounds (e.g. benzoate degradation via CoA ligation) in HTL-AP but also related to the microbial autogenesis (e.g. fatty acid biosynthesis) as well as the environmental information processing. In addition, the enrichment of Mesotoga, responsible for the high degradation efficiency of LMWA, and Pelolinea, involved in the degradation of phenyl compounds, were found in mesophilic reactor, which was consistent with higher removal of corresponding organics.

Here, we report a promising new strategy for achieving rapid and complete destruction of perfluorooctane-sulfonate (PFOS) through the application of hydrothermal conditions (condensed water, 200-350 degrees C, 2-16.5 MPa) to solutions amended with NaOH. Initial screening experiments with a wide range of solution amendments (e.g., acids, alkalis, oxidants, reductants) revealed highly variable extents of PFOS defluorination, ranging from 0% to 80% after 90 min of reaction at 350 degrees C. The most reactive amendments, regardless of type, shifted solution pH to highly alkaline conditions (pH >= 9), suggesting a base-promoted mechanism. For NaOH-amended solutions, rates of PFOS degradation increased with temperature and followed a second-order rate law, -d-[PFOS]/dt = k(2)[OH-][PFOS], with k(2) = 0.052 +/- 0.004 M-1 min(-1) at 350 degrees C, and F-19-NMR measurements show complete conversion of C-F bonds to F- (2.5 g/L PFOS) within 40 min for the reaction with 1 M NaOH. Small quantities of short-chain perfluorocarboxylic acids (<= 1.5% [PFOS](init)) were detected as transient intermediates, indicating that an initial OH--catalyzed cleavage of the sulfonate headgroup is followed by rapid sequential decarboxylation reactions, eventually leading to complete mineralization. These findings suggest a promising technology for destruction of PFOS-containing wet concentrates (e.g., aqueous film-forming foam (AFFF) stockpiles, industrial waste, sorbent regenerate, and membrane reject waste streams).

Hydrothermal liquefaction of sewage sludge to produce bio-oil and hydro-char unavoidably results in the production of high-strength organic wastewater (HTLWW). However, anaerobic digestion (AD) of HTLWW generally has low conversion efficiency due to the presence of complex and refractory organics. The present study showed that granular activated carbon (GAC) promoted the AD of HTLWW in continuous experiments, resulting in the higher methane yield (259 mL/g COD) compared to control experiment (202 mL/g COD). It was found that GAC increased the activities of both aceticlastic and hydrogenotrophic methanogens. The molecular transformation of organics in HTLWW was further analyzed. It was shown GAC promoted the degradation of soluble microbial by-products, fulvic- and humic-like substances as revealed by 3-dimensional fluorescence excitation-emission matrix (3D-EEM) analysis. Gas chromatography mass spectrometry (GC-MS) analysis showed that GAC resulted in the higher degradation of N-heterocyclic compounds, acids and aromatic compounds and less production of new organic species. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis also showed that GAC promoted the degradation of nitrogenous organics. In addition, it was shown that GAC improved the removal of less oxidized, higher nitrogen content, and higher double bond equivalent (DBE) organic compounds. Microbial analysis showed that GAC not only increased the microbial concentration, but also enriched more syntrophic bacteria (e.g., Syntrophorhabdus and Synergistes), which were capable of degrading a wide range of different organics including nitrogenous and aromatic organics. Furthermore, profound effects on the methanogens and the enrichment of Methanothrix instead of Methanosarcina were observed. Overall, the present study revealed the molecular transformation and microbial mechanism in the AD of HTLWW with the presence of GAC.

Anaerobic digestion (AD) has shown potential to convert hydrothermal liquefaction wastewater (HTLWW) into biogas in previous studies. However, the identification of refractory components and further insights into the molecular transformations of organics in HTLWW are essential for developing more efficient AD processes. In this study, two HTLWWs were obtained from the temperature-derived hydrothermal liquefaction of sewage sludge at 170 ℃ and 320 ℃. Their molecular compositions, as well as their modifications in the subsequent AD process, were characterized using a suite of advanced molecular tools. The dissolved organic matter (DOM) in the high temperature-derived HTLWW was lower in molecular weight, less saturated, less oxidized, and enhanced in nitrogenous substances. During the AD process, most of the volatile compounds and low molecular weight (LMW) neutrals were removed, while biopolymers were the most refractory. Carboxylic-rich alicyclic molecules (CRAM), particularly those containing 3 to 5 N for low temperature-derived DOM and 1 to 3 N for high temperature-derived DOM, were resistant to anaerobic biodegradation. Meanwhile, compounds with fewer nitrogens and more carboxyl groups were preferentially produced. This molecular characterization of HTLWW-derived DOM and examination of its transformation during AD will contribute to the development of efficient methods for HTLWW treatment in the future.