Impact of humic acid on the photoreductive degradation of perfluorooctane sulfonate (PFOS) by UV/Iodide process

Photochemical decomposition of perfluorochemicals in contaminated water

Perfluorochemicals (PFCs) are a set of chemicals containing C-F bonds, which are concerned due to their bioaccumulation property, persistent and toxicological properties. Photocatalytic approaches have been widely studied for the effective removal of PFCs due to the mild operation conditions. This review aims to provide a comprehensive and up-to-date summary on the homogenous and heterogeneous photocatalytic processes for PFCs removal. Specifically, the homogenous photocatalytic methods for remediating PFCs are firstly discussed, including generation of hydrated electrons (e_aq^‒) and its performance and mechanisms for photo-reductive destruction of PFCs, the active species responsible for photo-oxidative degradation of PFCs and the corresponding mechanisms, and metal-ion-mediated (Fe(III) mainly used) processes for the remediation of PFCs. The influences of molecular structures of PFCs and water matrix, such as dissolved oxygen, humic acid, nitrate, chloride on the homogenous photocatalytic degradation of PFCs are also discussed. For heterogeneous photocatalytic processes, various semiconductor photocatalysts used for the decomposition of perfluorooctanoic acid (PFOA) are then discussed in terms of their specific properties benefiting photocatalytic performances. The preparation methods for optimizing the performance of photocatalysts are also overviewed. Moreover, the photo-oxidative and photo-reductive pathways are summarized for remediating PFOA in the presences of different semiconductor photocatalysts, including active species responsible for the degradation. We finally put forward several key perspectives for the photocatalytic removal of PFCs to promote its practical application in PFCs-containing wastewater treatment, including the treatment of PFCs degradation products such as fluoride ion, and the development of noble-metal free photocatalysts that could efficiently remove PFCs under solar light irradiation.

Structural Dependence of Reductive Defluorination of Linear PFAS Compounds in a UV/Electrochemical System

Per and polyfluoroalkyl substances (PFAS), legacy chemicals used in firefighting and the manufacturing of many industrial and consumer goods, are widely found in groundwater resources, along with other regulated compounds, such as chlorinated solvents. Due to their strong C-F bonds, these molecules are extremely recalcitrant, requiring advanced treatment methods for effective remediation, with hydrated electrons shown to be able to defluorinated these compounds. A combined photo/electrochemical method has been demonstrated to dramatically increase defluorination rates, where PFAS molecules sorbed onto appropriately functionalized cathodes charged to low cell potentials (-0.58 V vs Ag/AgCl) undergo a transient electron transfer event from the electrode, which "primes" the molecule by reducing the C-F bond strength and enables the bond's dissociation upon the absorption of a hydrated electron. In this work, we explore the impact of headgroup and chain length on the performance of this two-electron process and extend this technique to chlorinated solvents. We use isotopically labeled PFAS molecules to take advantage of the kinetic isotope effect and demonstrate that indeed PFAS defluorination is likely driven by a two-electron process. We also present density functional theory calculations to illustrate that the externally applied potential resulted in an increased rate of electron transfer, which ultimately increased the measured defluorination rate.

An analysis of the versatility and effectiveness of composts for sequestering heavy metal ions, dyes and xenobiotics from soils and aqueous milieus

The persistence and bioaccumulation of environmental pollutants in water bodies, soils and living tissues remain alarmingly related to environmental protection and ecosystem restoration. Adsorption-based techniques appear highly competent in sequestering several environmental pollutants. In this review, the recent research findings reported on the assessments of composts and compost-amended soils as adsorbents of heavy metal ions, dye molecules and xenobiotics have been appraised. This review demonstrates clearly the high adsorption capacities of composts for umpteen environmental pollutants at the lab-scale. The main inferences from this review are that utilization of composts for the removal of heavy metal ions, dye molecules and xenobiotics from aqueous environments and soils is particularly worthwhile and efficient at the laboratory scale, and the adsorption behaviors and effectiveness of compost-type adsorbents for agrochemicals (e.g. herbicides and insecticides) vary considerably because of variabilities in structure, topology, bond connectivity, distribution of functional groups and interactions of xenobiotics with the active humic substances in composts. Compost-based field-scale remediation of environmental pollutants is still sparse and arguably much challenging to implement if, furthermore, real-world soil and water contamination issues are to be addressed effectively. Hence, significant research and process development efforts should be promptly geared and intensified in this direction by extrapolating the lab-scale findings in a cost-effective manner.

Destruction of Per- and Polyfluoroalkyl Substances (PFAS) with Advanced Reduction Processes (ARPs): A Critical Review

Advanced reduction processes (ARPs) have emerged as a promising method for destruction of persistent per- and polyfluoroalkyl substances (PFAS) in water due to the generation of short-lived and highly reductive hydrated electrons (e_aq^-). This study provides a critical review on the mechanisms and performance of reductive destruction of PFAS with e_aq^-. Unique properties of e_aq^- and its generation in different ARP systems, particularly UV/sulfite and UV/iodide, are overviewed. Different degradation mechanisms of PFAS chemicals, such as perfluorooctanoic acid (PFOA), perfluorooctanesulfonate (PFOS), and others (e.g., short chain perfluorocarboxylic acids (PFCAs) and perfluorosulfonic acids (PFSAs), per- and polyfluoro dicarboxylic acids, and fluorotelomer carboxylic acids), are reviewed, discussed, and compared. The degradation pathways of these PFAS chemicals rely heavily upon their head groups. For specific PFAS types, fluoroalkyl chain lengths may also affect their reductive degradation patterns. Degradation and defluorination efficiencies of PFAS are considerably influenced by solution chemistry parameters and operating factors, such as pH, dose of chemical solute (i.e., sulfite or iodide) for e_aq^- photoproduction, dissolved oxygen, humic acid, nitrate, and temperature. Furthermore, implications of the state-of-the-art knowledge on practical PFAS control actions in water industries are discussed and the priority research needs are identified.

Degradation of Perfluorooctanesulfonate by Reactive Electrochemical Membrane Composed of Magnéli Phase Titanium Suboxide

This study investigated the degradation of perfluorooctanesulfonate (PFOS) in a reactive electrochemical membrane (REM) system in which a porous Magnéli phase titanium suboxide ceramic membrane served simultaneously as the anode and the membrane. Near complete removal (98.30 ± 0.51%) of PFOS was achieved under a cross-flow filtration mode at the anodic potential of 3.15 V vs standard hydrogen electrode (SHE). PFOS removal efficiency during the REM operation is much greater than that of the batch operation mode under the same anodic potential. A systematic reaction rate analysis in combination with electrochemical characterizations quantitatively elucidated the enhancement of PFOS removal in REM operation in relation to the increased electroactive surface area and improved interphase mass transfer. PFOS appeared to undergo rapid mineralization to CO₂ and F^-, with only trace levels of short-chain perfluorocarboxylic acids (PFCAs, C₄-C₈) identified as intermediate products. Density functional theory (DFT) simulations and experiments involving free radical scavengers indicated that PFOS degradation was initiated by direct electron transfer (DET) on anode to yield PFOS free radicals (PFOS^•), which further react with hydroxyl radicals that were generated by water oxidation and adsorbed on the anode surface (^•OH_ads). The attack of ^•OH_ads is essential to PFOS degradation, because, otherwise, PFOS^• may react with water and revert to PFOS.

Photo-degradation dynamics of five neonicotinoids: Bamboo vinegar as a synergistic agent for improved functional duration

The effects of photo-degradation on the utilization of pesticides in agricultural production has been investigated. Various influencing factors were compared, with results showing that the initial pesticide concentration, light source, water quality and pH possessed different effects on neonicotinoids photo-degradation. The initial concentration and pH were found to be most critical effects. The photo-degradation rate decreased by a factor of 2-4 when the initial concentration increased from 5 mg L-1 to 20 mg L-1, particularly for acetamiprid and imidacloprid. The photo-degradation pathways and products of the five neonicotinoids were also investigated, with similar pathways found for each pesticide, except for acetamiprid. Degradation pathways mainly involved photo-oxidation processes, with products identified using liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS) found to be consistent with literature reported results. Bamboo vinegar exerted a photo-quenching effect on the neonicotinoids, with an improved efficiency at higher vinegar concentrations. The photo-quenching rates of thiamethoxam and dinotefuran were 381.58% and 310.62%, respectively, when a 30-fold dilution of vinegar was employed. The photo-degradation products in bamboo vinegar were identical to those observed in methanol, with acetic acid being the main factor influencing the observed quenching effects.

Removal of iodide from water using silver nanoparticles-impregnated synthetic zeolites

Synthetic zeolite-based Ag-nanocomposites were synthesized, characterized and used to remove iodide from aqueous solutions. The results showed high removal efficiency (up to 94.85%) and the formation silver iodide which is stable into the material. The maximum achieved adsorption capacity of the nanocomposites was between 19.54 and 20.44mg/g. The removal mechanism was meticulously studied by taking into account both water chemistry and surface interactions backed by multiple characterization techniques, such as XRD, XRF, SEM/EDX, TEM and BET. The qualitative and quantitative examination of pre- and post-adsorption of nanocomposite samples proved that the anchored silver iodide was formed via oxidation of initial silver nanoparticles followed by reaction with iodide to form a stable crystalline precipitate on the surface of the materials. A diffusion-based adsorption model indicated that the controlling mechanism is a slow intraparticle surface diffusion with diffusion coefficients in the range of 0.37-1.72×10^-13cm²/s. The investigation of competing and co-existing anions (Cl^-, Br^-, CO₃^2-, and CrO₄^2-) on the removal efficiency of iodide demonstrated a negligible effect showing a kinetically favorable precipitation reaction of iodide over other anions.

Enhancing photo-degradation of ciprofloxacin using simultaneous usage of eaq- and OH over UV/ZnO/I- process: Efficiency, kinetics, pathways, and mechanisms

The aim of this study is to develop the process relies on the UV irradiation of ZnO and I^-, i.e. UV/ZnO /I^- (UZI), to create both oxidizer and reducer agents simultaneously for photo-degradation of the Ciprofloxacin (CIP). This paper shows that while applying UV irradiation, UV/ZnO and UV/I^- for 20 min can lead to achieve 37.5%, 58.12%, and 61.4% photo-degradation of 100 mg L^-1 CIP at pH 7, respectively. Moreover, the UZI treatment can provide 91.54% photo-degradation efficiency. The LC-MS analysis of the UZI effluent indicates that 10 min process was adequate to degrade CIP into simple ring-shaped metabolites while 15 min treatment, mostly of CIP intermediates were linear and biodegradable organic compounds. Furthermore, fourteen little fragments were identified in the CIP photo-degradation via UZI, during the photoreaction time of 2.5 to 20 min. Then, a pseudo first-order kinetics equation was utilized to model the observed photo-degradation process. Finally, the computational results show that the increased concentration of the CIP solution from 100 to 400 mg L^-1 decreases the observed rate constant (k_obs) from 0.4125 to 0.2189 min^-1 while increases the photoreaction rate (r_obs) from 41.25 to 87.56 mg L^-1 min^-1.

Effects of fluorescent dissolved organic matters (FDOMs) on perfluoroalkyl acids (PFAAs) in lake and river water

This study presents the effects of fluorescent dissolved organic matters (FDOM) on perfluoroalkyl acids (PFAAs) in western Lake Chaohu and its inflow rivers. The surface water samples from the 27 sites in western Lake Chaohu and its inflow rivers were collected in March and September 2013. The contents of PFAAs and the FDOM in the water samples were measured by a high-performance liquid chromatograph - mass spectrometer (HPLC-MS) and by a fluorescence spectrophotometer, respectively. The temporal-spatial distributions of PFAAs and FDOM, as well as their interrelationships, were investigated. Eleven PFAA components were detected, and the mean concentration of total PFAAs (TPFAAs) in western Lake Chaohu and its inflow rivers were 12.93 ± 5.19 ng/L and 11.84 ± 9.50 ng/L, respectively. PFOA was the predominant contaminant in two regions (7.13 ± 3.07 ng/L and 4.30 ± 2.14 ng/L) followed by PFHxA (1.72 ± 0.80 ng/L and 1.42 ± 1.41 ng/L) and PFBA (1.44 ± 0.78 ng/L and 1.37 ± 0.78 ng/L). The mean concentration of total FDOM in western Lake Chaohu and its inflow rivers were 220.0 ± 40.30 μg quinine sulfate units (Q.S.)/L and 406.3 ± 213.1 μg Q.S./L, respectively. The significant, positive correlations were observed between the PFAAs and FDOMs in both the lake area and the inflow rivers. However, no significant correlation was observed between PFAAs and the dissolved organic carbon (DOC) in the lake area. This finding indicated that the residues and distributions of PFAAs were significantly dependent on the compositions of dissolved organic matters (DOM) and not on the total content of DOM.

Graphene based ZnO nanoparticles to depolymerize lignin-rich residues via UV/iodide process

In this work, potassium iodide (KI) and graphene oxide (GO) were utilized to promote the selectivity of photocatalytic process for alkali lignin oxidation over ZnO. Different concertation of GO was added during the microwave synthesis procedure of ZnO, and the characterization results revealed that graphene can shift the conduction band to more reducing potential, resulting to higher production of superoxide anion radicals (O₂^-) compared to OH. Response Surface Methodology revealed the most suitable interaction among loading of GO, KI and irradiation time on lignin and total phenolic compound (TPC) degradation. Specifically, the optimal conditions (i.e. maximum lignin (52%) and minimum TPC (55%) degradation) were at [KI] = 0.64 mM; GO content into ZnO 1.2 mg/mL; 240 min of irradiation time. The results showed that higher addition of graphene into structure of ZnO could preserve more phenolics from degradation due to less production of OH. Furthermore, the addition of KI at optimized conditions could enhance the selectivity of degradation of lignin and phenolics via producing I radicals and quenching the excess amount of generated OH, respectively. The lower generation of OH at optimized conditions was quantitatively confirmed by a photoluminescence simplified technique. In addition, the effect of the photocatalytic process on substrate's anaerobic degradability was examined in order to evaluate the suitability of the pretreated solution for energy recovery. Indeed, besides the higher TPC concentration, the biogas production of treated straw at optimized conditions was increased by 35% compared to the untreated sample.

Formation of perfluorocarboxylic acids from photodegradation of tetrahydroperfluorocarboxylic acids in water

Tetrahydroperfluorocarboxylic acids (2H,2H,3H,3H-PFCAs) have aroused the interest of scholars worldwide due to their potential to generate perfluorinated compounds. In this work, we systematically examined the photodegradation kinetics and mechanisms of typical 2H,2H,3H,3H-PFCAs (C_nF_2n+1C₂H₄COOH, n = 6, 7, 8) in aqueous solution by a 500 W Hg lamp. The photodecomposition of 2H,2H,3H,3H-PFCAs all followed pseudo-first-order kinetics, and the photolysis rate coefficients increased with the increasing carbon chain length. Under the same reaction condition, 2H,2H,3H,3H-PFCAs degraded much faster than the corresponding PFCAs. The photodecomposition rate coefficient of C₈F₁₇CH₂CH₂COOH was accelerated by low pH and Fe³⁺ addition, but decreased by the existence of humic acid, carbonate and bicarbonate. Compared with ultrapure water, a decreased removal of 2H,2H,3H,3H-PFCAs was observed in four types of natural waters, i.e., tap water, Jiuxiang river water, primary effluent and secondary effluent. According to mass analysis, C₈F₁₇CH₂CH₂COOH was mainly decomposed into 8:2 fluorotelomer acid (C₈F₁₇CH₂COOH), shorter-chain perfluorocarboxylic acids (PFCAs), perfluoro-1-enes (C_nF_2n) and perfluoroketenes (C_nF_2n+1CF = C = O). Thus, α-oxidation, decarboxylation and elimination reaction were proposed as reaction pathways. ECOSAR predictions showed that photolysis generally decreased the aquatic toxicity of C₈F₁₇CH₂CH₂COOH.

Cultivar-Dependent Accumulation and Translocation of Perfluorooctanesulfonate among Lettuce ( Lactuca sativa L.) Cultivars Grown on Perfluorooctanesulfonate-Contaminated Soil

Perfluorooctanesulfonate (PFOS) is a toxic and persistent organic pollutant that can be widely detected in agricultural soils. In this study, two lettuce cultivars with low PFOS accumulation were screened out to reduce the exposure of PFOS to the human body via vegetable consumption. The screened low-PFOS cultivars may help to ensure food safety, despite planting in highly PFOS-polluted soils (1.0 mg/kg), due to their high tolerance to PFOS and 4.4-5.7 times lower shoot PFOS concentration than the high-PFOS cultivars. Protein content and protein-mediated transpiration played key roles in regulating PFOS accumulation in the lettuce cultivars tested. Lower protein content, lower stomatal conductance, and lower transpiration rate resulted in low PFOS accumulation. This study reveals the mechanism of forming low-PFOS accumulation of lettuce cultivars at physiological and biochemical levels and lays a foundation for developing a cost-effective and safe approach to grow vegetables in PFOS-polluted soils.

Perfluorooctanesulfonate Degrades in a Laccase-Mediator System

Perfluorooctanesulfonate (PFOS) is a compound that has wide applications with extreme persistence in the environment and the potential to bioaccumulate, and could induce adverse effects to ecosystems. We investigated the degradation of PFOS by laccase-induced enzyme catalyzed oxidative humification reactions (ECOHRs) using 1-hydroxybenzotriazole (HBT) as a mediator. Approximately 59% of PFOS was transformed over 162 days of incubation, and the reaction appeared to follow a pseudo-first-order model with reaction rate constant of 0.0066/ d ( r² = 0.87) under one condition tested. Using differential absorption spectra and theoretical simulation, we elucidated the interaction between Cu²⁺/Mg²⁺ and PFOS, and proposed that Cu²⁺ and Mg²⁺ could serve as a bridge to bring the negatively charged PFOS and laccase to proximity, thus increasing the chance of radicals that are released from laccase to reach and react with PFOS. In addition, density functional theory modeling showed that PFOS complexation to the metal ions could unlock its helical configuration and decrease the C-C bond energy of PFOS. These changes allow the attack of PFOS C-C backbone by radicals to become easier. On the basis of products identification, we proposed that direct attack of PFOS by the HBT radical initiated the free radical chain reaction processes and led to the formation of fluoride and partially fluorinated compounds. These results suggest that ECOHR is a potential pathway by which PFOS could be degraded in the environment, and it may make a viable approach to remediate PFOS contamination via amendment of appropriate enzymes and mediators.

Distribution of perfluorinated compounds in drinking water treatment plant and reductive degradation by UV/SO32- process

Perfluorinated compounds (PFCs), which are widely used in industrial and residential areas, have a large negative impact on the environment. This study investigated the removal efficiency of five PFCs in a drinking water treatment plant. The results indicate that the total PFC concentration in raw water is 261.51 ng L^-1 and that perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are the predominant pollutants. Among all of the treatment processes, coagulation sedimentation process had the highest removal ratio of PFCs (36.12%), and removal ratio was the least in the sand filtration process. The ozonation/activated carbon and disinfection processes increased the concentration of PFCs. Therefore, developing an effective treatment to degrade PFCs is feasible. In this study, we proposed a method using UV irradiation of SO₃^2- at 365 nm to degrade PFCs. The SO₃^2- concentration, pH, and initial concentration had profound impacts on the degradation of PFCs. When the PFC initial concentration was 20 mg L^-1, the SO₃^2- concentration was 2.4 g L^-1, and in the presence of buffer, the degradation of PFCs was the most efficient, with the degradation ratio close to 100% after 60 min of reaction. During the degradation of PFCs, short-chain PFCs and hydrofluorinated carboxylic acid were generated. From the above, we proposed a detailed mechanism of degradation and its possible pathways.