Carbon Adsorbent Properties Impact Hydrated Electron Activity and Perfluorocarboxylic Acid (PFCA) Destruction.

Carbon-based adsorbents used to remove recalcitrant water contaminants, including perfluoroalkyl substances (PFAS), are often regenerated using energy-intensive treatments that can form harmful byproducts. We explore mechanisms for sorbent regeneration using hydrated electrons (eaq -) from sulfite ultraviolet photolysis (UV/sulfite) in water. We studied the UV/sulfite treatment on three carbon-based sorbents with varying material properties: granular activated carbon (GAC), carbon nanotubes (CNTs), and polyethylenimine-modified lignin (lignin). Reaction rates and defluorination of dissolved and adsorbed model perfluorocarboxylic acids (PFCAs), perfluorooctanoic acid (PFOA) and perfluorobutanoic acid (PFBA), were measured. Monochloroacetic acid (MCAA) was employed to empirically quantify eaq - formation rates in heterogeneous suspensions. Results show that dissolved PFCAs react rapidly compared to adsorbed ones. Carbon particles in solution decreased aqueous reaction rates by inducing light attenuation, eaq - scavenging, and sulfite consumption. The magnitude of these effects depended on adsorbent properties and surface chemistry. GAC lowered PFOA destruction due to strong adsorption. CNT and lignin suspensions decreased eaq - formation rates by attenuating light. Lignin showed high eaq - quenching, likely due to its oxygenated functional groups. These results indicate that desorbing PFAS and separating the adsorbent before initiating PFAS degradation reactions will be the best engineering approach for adsorbent regeneration using UV/sulfite.

Comparative Sb(V) removal efficacy of different iron oxides from textile wastewater: effects of co-existing anions and dye compounds.

Elevated Sb(V) concentration in textile wastewater is a growing environmental concern worldwide and has received wider attention in recent years. Iron oxides possess appealing characteristics as efficient and cost-effective adsorbents in large-scale applications. In the present study, Sb(V) adsorption capacity of α-Fe2O3, γ-Fe2O3, and Fe3O4 was compared under experimental conditions close to the practical textile wastewater treatment. Results demonstrated that α-Fe2O3 performed better under different pH values, reaction times, dye compounds, and co-existing ions as compared to γ-Fe2O3 and Fe3O4, and the adsorption equilibrium was achieved within 8 h. Sb(V) adsorption is found to be highly pH dependent, and higher removal was achieved in lower pH, indicating the involvement of electrostatic interactions. The pHpzc value of α-Fe2O3 was 7.15, which favored Sb(V) adsorption in practical wastewater having neutral pH value (pH ~ 7). Pseudo-first- and pseudo-second-order described the data and the simulated values of qe fitted well with the experimental values, indicating that pseudo-second-order model described the adsorption kinetics better with R2 (> 0.95) higher than of pseudo-first-order plots. The Langmuir and Freundlich models both described well the sorption data of all the adsorbents, where the R2 values were > 0.90 with a better fit in the Freundlich model for α-Fe2O3, suggesting that the adsorbent has heterogeneous surface characteristics. Similarly, characterizations revealed that the specific surface area, pore volume, and hydroxyl group content in α-Fe2O3 were higher than others, making it easier for contaminants to bind on to the active sites. Furthermore, the effect of dyes and co-existing anions on Sb(V) adsorption was negligible, except for SO42-, CO32-, and PO43- by the formation of inner-sphere complexes with iron oxides through competitive adsorption with [Sb(OH)6]-. Findings from the present study suggested that α-Fe2O3 effectively reduced Sb(V) in textile wastewater and could be a promising alternative for practical textile wastewater treatment.

Zero-valent iron-peroxydisulfate as synergistic co-milling agents for enhanced mechanochemical destruction of 2,4-dichlorophenol: Coupling reduction with oxidation.

Mechanochemical (MC) remediation with zero-valent iron (ZVI) as co-milling agent enables the non-combustion and solvent-free disposal of solid halogenated organic pollutants (HOPs) via solid-phase reaction, but suffers from incomplete dechlorination (especially for less chlorinated chemicals). Herein, a reduction-oxidation coupling strategy using ZVI and peroxydisulfate as synergistic (ZVI-PDS) co-milling agents was investigated, with 2,4-dichlorophenol (2,4-DCP) as probe contaminant. By revisiting the MC destruction process of 2,4-DCP by ZVI, the contribution of both reductive and oxidative routes is confirmed, and the inefficient •OH generation is addressed. With ball-to-material and reagent-to-pollutant mass ratios of 30:1 and 13:1, respectively, ZVI-PDS achieves higher dechlorination ratio (86.8%) for 2,4-DCP within 5 h, outcompeting sole ZVI (40.3%) or PDS (33.9%), due to the accumulation of numerous SO4•-. As suggested by a two-compartment kinetic model, the optimal ZVI/PDS molar ratio of 4:1 is determined, which balances the relative contribution of reductive/oxidative routes and leads to a maximum mineralization efficiency of 77.4%. The analysis on product distribution verifies the generation of dechlorinated, ring-opening and minor coupling products (with low acute toxicity). This work validates the necessity to couple reduction with oxidation in MC destruction for solid HOPs, and may provide information on reagent formulation.

Mechanochemical synthesis of cysteine-gum acacia intermolecular complex for multiple metal(loid) sequestration from herbal extracts.

The ubiquitous heavy metal(loid)s (HMs) contamination has triggered great concern about food safety, while sequestration and separation of trace HMs from herbal extracts still calls for appropriate sorbent materials. In this work, gum acacia was modified by cysteine to form a cysteine-acacia intermolecular complex (Cys-GA complex) via facile mechanochemical synthesis, aiming at capturing multiple HMs simultaneously. Preliminary screening confirms the superiority of Cys-CA complex for both cationic and anionic HMs, and determines an optimum Cys/GA mass ratio of 9:1 to achieve high removal capacities for Pb(II) (938 mg g-1), Cd(II) (834 mg g-1), As(V) (496 mg g-1), and Cr(VI) (647 mg g-1) in simulated aqueous solution. The analysis on HMs-exhausted Cys-GA complex indicates that Pb(II), As(V), and Cr(VI) tend to be removed through chelation, electrostatic attraction, and reduction, while Cd(II) can only be chelated or adsorbed by electrostatic interaction. The batch experiments on commercial herbal (e.g. Panax ginseng, Glycine max, Sophora flavescens, Gardenia jasminoides, Cyclocarya paliurus, and Bamboo leaf) extracts indicate that Cys-GA complex can reduce HMs concentration to attain acceptable level that comply with International Organization for Standardization, with negligible negative effect on its active ingredients. This work provides a practical and convenient strategy to purify HMs-contaminated foods without introducing secondary pollution.

Nontarget analysis and fluorine atom balances of transformation products from UV/sulfite degradation of perfluoroalkyl contaminants.

Per- and polyfluoroalkyl substances (PFAS) are a class of thousands of highly fluorinated, anthropogenic compounds that are used in a wide variety of consumer applications. Due to their widespread use and high persistence, PFAS are ubiquitous in drinking water, which is of concern due to the threats these compounds pose to human health. Reduction via the hydrated electron is a promising technology for PFAS remediation and has been well-studied. However, since previous work rarely reports fluorine atom balances and often relies on suspect screening, some transformation products are likely unaccounted for. Therefore, we performed non-target analysis using high-resolution mass spectrometry on solutions of perfluorooctanesulfonate (PFOS), perfluorobutanesulfonate (PFBS), perfluorooctanoate (PFOA), and 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)propanoate (GenX) that had been treated with UV/sulfite to produce hydrated electrons. We determined fluorine atom balances for all compounds studied, finding high fluorine atom balances for PFOS and PFBS. PFOA and GenX had lower overall fluorine atom balances, likely due to the production of volatile or very polar transformation products that were not measured by our methods. Transformation products identified by our analysis were consistent with literature, with a few exceptions. Namely, shorter-chain perfluorosulfonates (PFSA) and their H/F substituted counterparts were also detected from PFOS. This is an unexpected result based on literature, as no documented pathway exists for the formation of shorter-chain PFSA during UV/sulfite treatment. Furthermore, the nontarget approach we employed allowed for identification of novel, unsaturated products from the hydrated electron treatment of perfluorooctanesulfonate (PFOS) that warrant further investigation.

Enhancement mechanism of magnetite on the ball-milling destruction of perfluorooctane sulfonate by iron.

Zero-valent iron (Fe) is commonly employed as an additive for the mechanochemical destruction (MCD) of organic pollutants. The poly- and perfluoroalkyl substances (e.g., perfluorooctane sulfonate, PFOS) are a class of toxic environmental pollutants that are difficult to effectively degrade due to their thermodynamic and chemical stability. In this study, magnetite (Fe3O4) was applied to improve the milling performance of Fe to PFOS and its promoting mechanisms were emphatically explored. The desulfurization rate was in ahead of the defluorination rate because the C-S bond is less stable than the C-F bonds in PFOS. Fe3O4 had an excellent reinforcement effect on the milling performance of Fe, which was mainly through accelerating the electron transfer as a conductor, reacting with Fe to produce FeO, and facilitating the formation of HO●. During the MCD of PFOS with Fe/Fe3O4 as an additive, HO● played a dominant role in the defluorination process (accounting for >67%). After the elimination of sulfonate group (-SO3-), the produced radical (C7F15CF2●) continued to react through two main pathways: one was the stepwise defluorination after hydrogenation, and the other one was oxidation reaction after alcoholization to yield the corresponding aldehydes and carboxylic acids. The optimum Fe fraction (MFe) was 30%, and air atmosphere was more effective than oxygen and nitrogen conditions. This study helps to comprehensively understand the role of Fe3O4 in defluorination and fills the gap of Fe/Fe3O4 application in the MCD of PFASs.

Enhanced dechlorination of an enzyme-catalyzed electrolysis system by ionic liquids: Electron transfer, enzyme activity and dichloromethane diffusion.

Enzyme-catalyzed electrolysis system (EES) is a promising technique for the efficient dechlorination of pollutants. In this study, ionic liquids (ILs) was first introduced to enhance the dichloromethane dechlorination performance of an EES. An imidazole-based IL, 1-ethyl-3-methylimidazole tetrafluoroborate ([EMIM][BF4]), was chosen due to its excellent performance on dechlorination enhancement than other three ILs. The cyclic voltammograms with different scan rates shows that the presence of IL increased the apparent electron transfer rate constant (ks) from 0.008 to 0.013 s-1. The calculated surface electroactive species concentration (τc) also increased from 7.8 × 10-9 to 9.5 × 10-9 mol cm-2. Electrochemical impedance spectroscopy analysis illustrates that the IL mainly weakened the interfacial resistance between electrolyte and cathode to accelerate the electron communication in the EES. The introduction of IL facilitated the regeneration of reduced glutathione from oxidized glutathione, whereas inhibited the catalytic activity of dehalogenase via the disruption of secondary structure shown in circular dichroism spectra. The presence of IL was also facilitated the dichloromethane diffusion from electrolyte to cathode. The mass transfer rate constants of dichloromethane (km,d) increased by 6.9 times after the addition of IL. The optimum volume concentration, pH value, reaction temperature and applied voltage were 20%, 7, 35 °C and -0.8 V vs Ag/AgCl, respectively. The study is helpful to understand the promotion mechanism of IL on the dechlorination performance of EES when it is adopted as a treatment technique.

Pre-electrochemical treatment combined with fixed bed biofilm reactor for pyridine wastewater treatment: From performance to microbial community analysis.

To overcome the high biotoxicity and poor biodegradability of pyridine and its derivatives, a pre-electrochemical treatment combined with fixed bed biofilm reactor (EC-FBBR) was designed for multi-component stream including pyridine (Pyr), 3-cyanopyridine (3-CNPyr), and 3-chloropyridine (3-ClPyr). The EC-FBBR system could simultaneously degrade these pollutants with a mineralization efficiency of 90%, especially for the persistent 3-ClPyr. Specifically, the EC could partially degrade all pollutants, and allow them to be completely destructed in FBBR. With EC off, Rhodococcus (35.5%) became the most abundant genus in biofilm, probably due to its high tolerance to 3-ClPyr. With EC on, 3-ClPyr was reduced to an acceptable level, thus Paracoccus (21.1%) outcompeted among interspecies competition with Rhodococcus and became the dominant genus. Paracoccus was considered to participate in the subsequent degradation for the residual 3-ClPyr, and led to the complete destruction for all pollutants. This study proposed promising combination for effective treatment of multi-component pyridine wastewater.

Sulfur Loading and Speciation Control the Hydrophobicity, Electron Transfer, Reactivity, and Selectivity of Sulfidized Nanoscale Zerovalent Iron.

Sulfidized nanoscale zerovalent iron (SNZVI) is a promising material for groundwater remediation. However, the relationships between sulfur content and speciation and the properties of SNZVI materials are unknown, preventing rational design. Here, the effects of sulfur on the crystalline structure, hydrophobicity, sulfur speciation, corrosion potential, and electron transfer resistance are determined. Sulfur incorporation extended the nano-Fe0 BCC lattice parameter, reduced the Fe local vacancies, and lowered the resistance to electron transfer. Impacts of the main sulfur species (FeS and FeS2 ) on hydrophobicity (water contact angles) are consistent with density functional theory calculations for these FeSx phases. These properties well explain the reactivity and selectivity of SNZVI during the reductive dechlorination of trichloroethylene (TCE), a hydrophobic groundwater contaminant. Controlling the amount and speciation of sulfur in the SNZVI made it highly reactive (up to 0.41 L m-2 d-1 ) and selective for TCE degradation over water (up to 240 moles TCE per mole H2 O), with an electron efficiency of up to 70%, and these values are 54-fold, 98-fold, and 160-fold higher than for NZVI, respectively. These findings can guide the rational design of robust SNZVI with properties tailored for specific application scenarios.

Sulfur Dose and Sulfidation Time Affect Reactivity and Selectivity of Post-Sulfidized Nanoscale Zerovalent Iron.

Exposing nanoscale zerovalent iron (NZVI) to dissolved sulfide species improves its performance as a remediation agent. However, the impacts of sulfur dose and sulfidation time on morphology, sulfur content, reactivity, and selectivity of the resulting sulfidized NZVI (SNZVI) have not been systematically evaluated. We synthesized SNZVI using different sulfur doses and sulfidation times and measured their properties. The measured S/Fe molar ratio in the particles ([S/Fe]particle) was 10-500 times lower than [S/Fe]dosed but was predictable based on [S/Fe]dosed × tsulfidation. The low sulfur content (0.02-0.65 mol % S/Fe) inhibited the reaction of SNZVI with water (up to 13-fold) and increased its reactivity with trichloroethene (TCE) (up to 14-fold) and its electron efficiency (up to 20-fold). A higher [S/Fe]particle (0.86-1.13 mol % S/Fe) led to complex particle structures and lowered the resistance to electron transfer but did not improve the benefits realized at the lower S/Fe ratios. Adding small amounts of sulfur into NZVI led to more accumulation of acetylene, especially for low Fe/TCE conditions, suggesting that sulfur lowers the rate of hydrogenation of acetylene to ethene. These results show that [S/Fe]dosed × tsulfidation can be used to predict the measured S content in the particles and that affects reactivity, longevity, and electron selectivity, for post-SNZVI.