Deeper Defluorination and Mineralization of a Novel PFECA (C7 HFPO-TA) in Vacuum UV/Sulfite: Unique Mechanism of H/OCF3 Exchange.

C7 HFPO-TA is a newly identified alternative to PFOA, which possesses a unique structure fragment (CF3O-CF(CF3)-). In this study, we evaluated the chemical reactivity of C7 HFPO-TA in advanced oxidation and reduction processes for the first time, which revealed a series of unexpected transformation mechanisms. The results showed that reductive degradation based on hydrated electrons (eaq-) was more feasible for the degradation of C7 HFPO-TA. For oxidative degradation, the branched -CF3 at the α-position carbon posed as the spatial hindrance, shielding the attack of SO4•- to -COO-. The synergistic effects of HO•/eaq- and direct photolysis led to deeper defluorination and mineralization of C7 HFPO-TA in the vacuum UV/sulfite (VUV/SF) process. We identified a unique H/OCF3 exchange that converted the CF3O-CF(CF3)- into H-CF(CF3)- directly, and the SO3•- involved mechanism of C7 HFPO-TA for the first time. We revealed the branched -CF3 connected to the same carbon next to the CF3O- group affected the C-O bond cleavage site, preferring the H/OCF3 exchange pathway. The defluorination of C7 HFPO-TA was compared with PFOA and three PFECAs in the VUV/SF process, which was highly dependent on structures. Degradation kinetics, theoretical calculations, and products' analysis provided an in-depth perspective on the degradation mechanisms and pathways of C7 HFPO-TA.

Degradation of trichloroethylene in aqueous solution by nanoscale calcium peroxide in the Fe(II)-based catalytic environments.

In this study, nCaO2 was synthesized successfully and applied in the Fe(II)-based catalytic environments in investigating trichloroethylene (TCE) removal performance. nCaO2 with the particle sizes in the range of 50-200 nm was prepared, and it performed better for TCE removal when compared to the conventional CaO2. Further experimental results showed that 70.4% of TCE could be removed in 180 min at the nCaO2/Fe(II)/TCE molar ratio of 1/2/1, while this data was elevated to 86.1% in the presence of citric acid (CA) at the nCaO2/Fe(II)/CA/TCE molar ratio of 1/2/2/1 in the same test period. Probe compound tests, specifically designed for free radicals confirmation, demonstrated the presence of HO• and O2 -•. Moreover, scavenging tests indicated that HO• was the major radical responsible for TCE degradation but O2 -• promoted the removal of TCE in both nCaO2/Fe(II) and nCaO2/Fe(II)-CA system. In addition, the effects of initial solution pH and anions (Cl-, HCO3 -) were also evaluated. The performance of TCE degradation in actual groundwater demonstrated that both nCaO2/Fe(II) and nCaO2/Fe(II)-CA systems can be applicable for TCE removal in ISCO practice and the nCaO2/Fe(II)-CA system is much promising technique. These fundamental data strongly confirmed the feasibility and potential of nCaO2 based technique in the remediation of TCE contaminated groundwater.

Insight into the key role of oxygen dopants over ball-milled boron nitride for efficient degradation of PFOS alternative 6:2 fluorotelomer sulfonic acid.

6:2 Fluorotelomer sulfonic acid (6:2 FTS) has been identified as an alternative to perfluorooctane sulfonic acid but has been proven to cause potential threats to humans and the environment. In this study, boron nitride (BN) photocatalysis was explored for 6:2 FTS degradation with 100% removal (kobs=1.8 h-1) and desulfurization rate of 100% as well as the defluorination rate of 57.3%. The superior performance of BN was primarily related to oxygen dopants defects (O-dopants). In addition, O-dopants contribution was confirmed by ball-milled BN (B-BN), which introduced more O-dopants and exhibited an increased 6:2 FTS degradation rate of 2.88 h-1. The decomposition of 6:2 FTS was attributed to holes (h+), hydroxyl radicals (•OH), and superoxide (•O2-) and proceeded via two pathways, the hydrogen abstraction from ethyl carbons by •OH and the C-S bond activation by h+ and •OH. To the best of our knowledge, this is the first study demonstrating that h+, •OH, and •O2- played significant roles in the heterogeneous photocatalytic degradation of 6:2 FTS.

Effective Breaking of the Fluorocarbon Chain by the Interface Bi2O2X···PFOA Complex Strategy via Coordinated Se on Construction of the Internal Photogenerated Carrier Pathway.

The destruction of perfluorooctanoic acid (PFOA) from outside was inhibited by the "barrel spiral" barrier, but the construction of the photocatalyst-PFOA complex provided a direct attack on photogenerated reactive species (RSs). Here, we investigated the bridging ability of bismuth oxychalcogenide (Bi2O2X) for constructing an effective photocarrier pathway to PFOA. The experimental results and DFT calculations showed that a more intense internal access of Bi2O2Se was built via the terminal carboxylate tail, and the weaker electrostatic interaction of Bi-Se bonds helped realize the smaller band gap and slower recombination of photocarriers, thereby inhibiting the invalid annihilation of holes with H2O and facilitating the transformation of electrons to O2-•. The pseudo-first-order rate coefficient (kobs) was 2 and 4 times higher than Bi2O2S and TiO2, respectively, showing the outstanding photocatalytic activity of Bi2O2Se. A broad pH (4-8) adaptability of Bi2O2Se was observed for defluorination, especially in alkali condition. This new understanding may inspire the development of Se-coordinated catalysts.

Determination of total reducible organofluorine in PFAS-impacted aqueous samples based on hydrated electron defluorination.

Per- and polyfluoroalkyl substances (PFASs) is a large group of thousands of anthropogenic chemicals. Recently, measurement of total organic fluorine (TOF) to reflect the total PFASs has been recommended in limits and advisories. In this study, a total reducible organofluorine (TROF) assay is developed based on hydrated electron (eaq-) conversion of PFASs into inorganic fluorine combined with ion chromatograph, which is a common and widespread instrument. The eaq- is generated in UV/sulfite system with alkaline condition, and the concentration of TROF (CF_TROF) is the difference of fluoride concentration before and after assay. Method validation uses perfluorooctanesulfonic acid, perfluorooctanoic acid and their main alternatives, and F- recoveries are 76.6%-101%, except for perfluorobutanesulfonic acid (48.5%). Method application of TROF assay uses industrial surfactant products and fluorochemical industry-contaminated water, meanwhile, target PFAS analysis and total oxidizable precursors (TOP) assay are concurrently conducted. Concentrations of PFASs detected in target analysis and TOP assay were converted to fluorine equivalents concentrations (CF_Target and CF_TOP). ∑CF_Target and ∑CF_TOP account for 0.80%-36% of CF_TROF in industrial samples, 0.12%-54% in environmental water and 9.7%-14% in wastewater. The TROF assay can be used to initially judge whether PFASs contamination occurred near a hotspot with known sources. The CF_TROF could infer the extent of PFAS contamination in PFAS-impacted samples and estimate the fraction of uncharacterized PFAS.

Efficient removal of trichloroethene in oxidative environment by anchoring nano FeS on reduced graphene oxide supported nZVI catalyst: The role of FeS on oxidant decomposition and iron leakage.

The performance of trichloroethene (TCE) removal was initially investigated in sodium persulfate (SPS) or potassium monopersulfate triple salt (PMS) oxidative environment by reduced graphene oxide (rGO) supported nZVI (nZVI-rGO) catalyst and further the role of sulphur by anchoring nano FeS on nZVI-rGO (FeS@nZVI-rGO) was evaluated. The high usage of oxidants and stability of FeS@nZVI-rGO catalyst were significantly improved due to the insoluble nature of this innovative catalyst by involvement of nano FeS which limited the rapid iron loss caused by the corrosion of active sites and mitigated rapid oxidants decomposition in FeS@nZVI-rGO/SPS and FeS@nZVI-rGO/PMS systems. The tests for target contaminant removal showed that over 95 % TCE could be removed at 100 mg L-1 FeS@nZVI-rGO and 1.2 mM SPS or 0.3 mM PMS dosages, in which over 85 % TCE could be dechlorinated. The reactive oxygen radicals (ROSs) generation mechanisms and their contribution to TCE removal were investigated through radical scavenge tests in both systems, indicating that both HO and SO4- were the major ROSs rather than O2-. In conclusion, this study revealed the well function and fundamental mechanism of this innovative catalyst by anchoring nano FeS and worth of further demonstration of this technique in TCE contaminated groundwater remediation application.

The impact of surface properties and dominant ions on the effectiveness of G-nZVI heterogeneous catalyst for environmental remediation.

The surface properties of nanocomposites are influenced by the existence of inorganic species that may affect its performance for specific catalytic applications. The impact of different ionic species on particular catalytic activity had not been investigated to date. In this study, the surface charge (zeta potential) of graphene-oxide-supported nano zero valent iron (G-nZVI) was tested in definitive cationic (Na+, K+, Ca2+ and Mg2+) and anionic (Br-, Cl-, NO3-, SO42-, and HCO3-) environments. The efficiency of G-nZVI catalyst was inspected by measuring the generation of reactive oxygen species (ROS) for the degradation of 1,1,1-trichloroethane (TCA) in sodium percarbonate (SPC) system. Tests conducted using probe compounds confirmed the generation of OH and O2- radicals in the system. In addition, the experiments performed using scavenging agents certified that O2- were primary radicals responsible for TCA removal, along with prominent contribution from OH radicals. The study confirmed that G-nZVI catalytic capability for TCA degradation is notably affected by various cationic species. The presence of Ni2+ and Cu2+ significantly enhanced (94%), whereas Na+ and K+ had minor effects on TCA removal. Overall, the results indicated that groundwater ionic composition may have low impact on the effectiveness of G-nZVI-catalyzed peroxide TCA treatment.

Enhanced degradation of trichloroethylene in oxidative environment by nZVI/PDA functionalized rGO catalyst.

Nano zero-valent iron (nZVI) particles with higher reactivity have been recognized as more efficient catalysts than Fe(II) for the groundwater remediation. The rapid emergence of novel catalyst supports efficiently prevent the rapid aggregation of nZVI and further improve catalytic reactivity. However, the lack of ability to avoid the potential oxidation of bare nZVI-support structure in air environment hinders its wider application in the actual contaminated sites. In this study, nZVI on reduced graphene oxide (rGO) functionalized by polydopamine (PDA) (nZVI-PDA@rGO) was synthesized successfully and applied into sodium persulfate (SPS), potassium monopersulfate (PMS) and H2O2 oxidative environments to remove trichloroethylene (TCE). For comparison, nZVI supported on solely rGO was prepared. The XRD test displayed the stronger stability of α-Fe(0) in nZVI-PDA@rGO catalyst against oxidation exposed to air. Compared with nZVI-rGO, a core shell structure of nZVI-PDA@rGO was observed in TEM image obviously. The dosage tests showed nZVI-PDA@rGO had a better catalytic reactivity than nZVI-rGO for TCE removal at lower catalyst and oxidant dosages, i.e. PMS dosage: 0.3 mM, catalyst dosage: 50 mg L-1, TCE removal: 45.0% (nZVI-rGO) up to 99.6% (nZVI-PDA@rGO). TCE removal mechanisms were revealed through radical scavenger tests, demonstrating sulfate radicals played more important role in nZVI-PDA@rGO catalyzed-oxidant systems.

Degradation of trichloroethylene in aqueous solution by rGO supported nZVI catalyst under several oxic environments.

The reduced graphene oxide (rGO) supported nano zero-valent iron (nZVI) (nZVI-rGO) was synthesized successfully and applied in the several oxic environments to remove trichloroethylene (TCE). The nZVI-rGO had a better catalytic performance than bare nZVI for the TCE removal. Both aggregation of nZVI and agglomeration of rGO were in part prevented by loading the nZVI nanoparticles on the rGO sheet. Among all the oxic environments, the better removal of TCE was followed as the order of PMS > SPS > H2O2. Chemical scavenger tests were carried out to identify the reactive oxygen species (ROSs) generated in the removal of TCE, showing that in PMS and SPS systems, SO4- and HO were main radicals responsible for TCE removal, while HO and O2- were main radicals in H2O2 system. The possible mechanisms were proposed with nZVI-rGO under several oxic environments. The recyclability of nZVI-rGO, dechlorination and mineralization of TCE were investigated. These fundamental data confirmed the effectiveness of nZVI-rGO to remove TCE and could help selecting the suitable oxidants to use with nZVI-rGO in the actual field groundwater remediation.