New insight into PFOS transformation pathways and the associated competitive inhibition with other perfluoroalkyl acids via photoelectrochemical processes using GOTiO2 film photoelectrodes.

The global distribution and environmental persistence of perfluoroalkyl acids (PFAAs) has been considered a critical environmental concern. In this work, we successfully fabricated a graphene oxide-titanium dioxide (GOTiO2) photoelectrode for perfluorooctane sulfonate (PFOS) degradation in a photoelectrochemical (PEC) system. The results reveal that a 5 wt.% GOTiO2 anode possesses the optimal PEC performance, with a band gap (Eg) of 2.42 eV, specific surface area (SBET) of 72.6 m2 g-1 and specific capacitance (Cs) of 4.63 mF cm-2. In the PEC system, PFOS can be efficiently removed within 4 h of reaction time, with a pseudo-first-order rate constant of 0.0124 min-1, under the optimized conditions of current density = 20 mA cm-2, electrode distance = 5 mm, solution pH = 5.64, [PFOS]0= 0.5 µM and NaClO4 electrolyte concentration = 50 mM. The electron transfer pathway, hydroxyl radicals and superoxide radicals are all responsible for PFOS decomposition/transformation. New degradation pathways were identified; a total of 25 PFOS byproducts are reported in this work; and perfluoroalkane sulfonates (PFSAs), perfluorinated aldehydes (PFALs) and hydrofluorocarbons (HFCs) were identified for the first time. PFOS degradation involves the desulfonation pathway as the first step, followed by oxidation and subsequent defluorination, decarboxylation, decarbonylation, sulfonation, defluorination and hydroxylation. The results from this work also show that the reactivity of PFAAs is related to their carbon chain length, with shorter-chain PFAAs exhibiting a lower degradation rate. In a PFAA mixture, a decline in the degradation rate was observed for the shorter-chain-length PFAAs, suggesting stronger competitive inhibition and indicating stronger environmental recalcitrance during the treatment process. Novelty statement: Although many efforts have been made to identify perfluorooctane sulfonate (PFOS) degradation byproducts, previous studies were only able to identify byproducts that are related to perfluorinated carboxylic acids (PFCAs). This is the first study to elucidate the new PFOS degradation pathway; furthermore, this is the first report to identify byproducts containing sulfonate groups (perfluoroalkane sulfonates, PFSAs), aldehyde groups (perfluorinated aldehydes, PFALs), and hydrofluorocarbons (HFCs). This study further systematically explores how perfluoroalkyl acid (PFAA) degradation may be affected in the mixture system: shorter-chain-length PFAAs suffer stronger competitive inhibition in the photoelectrochemical (PEC) system. By utilizing the graphene oxide-titanium dioxide (GOTiO2) photoelectrode fabricated in this work, PFOS can be successfully decomposed during the PEC process for the first time.

Kinetics and mechanism of 4-methylbenzylidene camphor degradation by UV-activated persulfate oxidation.

4-Methylbenzylidene camphor (4-MBC), a widely used ultraviolet (UV) filter detected in various aquatic environments, has been shown to evoke estrogenic activity. In this study, the use of UV light-activated persulfate for 4-MBC degradation is evaluated for the first time. Our results showed that the combination of UV and persulfate (UV/persulfate) can significantly remove 4-MBC, with a pseudo-first-order rate constant (kobs) of 0.1349 min-1 under the conditions of [4-MBC]0 = 0.4 µM, [persulfate]0 = 12.6 µM, and initial pH = 7. The kobs and persulfate dose exhibited a linear proportional relationship in the persulfate dose range of 4.2-42 µM. The kobs remained similar at pH 5 and pH 7 but significantly decreased at pH 9. A radical scavenging test indicated that SO4-• was the dominant species in 4-MBC degradation; the second-order rate constant of SO4-• with 4-MBC was calculated to be (2.82 ± 0.05) × 109 M-1 s-1. During the UV/persulfate reaction, 4-MBC was continuously degraded, while SO4-• was gradually converted to SO42-. 4-MBC degradation involved the hydroxylation and demethylation pathways, resulting in the generation of transformation byproducts P1 (m/z 271) and P2 (m/z 243), respectively. The Microtox® acute toxicity test (Vibrio fischeri) showed increasing toxicity during the UV/persulfate degradation of 4-MBC. The 4-MBC degradation rate was markedly lower in outdoor swimming pool water than in deionized water. Graphical abstract.

Clean water generation through a multifunctional activated carbon-TiO2 interfacial solar distillation system.

Solar distillation is emerging as an environmentally friendly and energy-effective technology for clean water generation. However, bulk water heating and the possibly complex composition of water matrices of source water could undermine the system efficacy. In this study, an interfacial evaporation device consisting of activated carbon combined with P25 TiO2 as the top layer and polyethylene foam as the bottom layer (AC-P25/foam device) was established. With the excellent optical absorbance of AC and the heat localization effect contributed by the PE foam, the evaporation rate (r evp) of the device (r evp = 2.1 kg m-2 h-1) was improved by 209% and 71% compared with that of the water-only (r evp = 0.68 kg m-2 h-1) and conventional evaporation (i.e., submerged AC-P25) systems (r evp = 1.23 kg m-2 h-1), respectively. The reusability test showed the stable evaporation performance of AC-P25/foam within 7 cycles; this interfacial evaporation was also found to be less affected by suspended solids in water due to a reduction in the influence of light scattering. The AC-P25/foam device not only possessed photothermal ability for water distillation but was also able to prevent enrichment of volatile organic compounds (i.e., phenol) with ∼95% removal efficiency through adsorption and photocatalytic reactions under illumination. Additionally, an outdoor solar distillation test performed with synthetic saline water demonstrated the desalination ability of the AC-P25/foam device, with the concentrations of all ions in the distilled water ≤3.5 mg L-1, far below the drinking water guideline value provided by the World Health Organization. The materials of the AC-P25/foam photothermal device are readily available and easily fabricated, showing the practical feasibility of this device for clean water generation.

Solar photodegradation of the UV filter 4-methylbenzylidene camphor in the presence of free chlorine.

UV filters are essential ingredients in sunscreens and many personal care products. The coexposure of UV filters to solar photolysis and free chlorine (solar/free chlorine) is inevitable in outdoor swimming pools and many other aquatic matrices, and this study aims to investigate the degradation mechanism of one specific UV filter, 4-methylbenzylidene camphor (4MBC), under solar/free chlorine system. Under solar irradiation alone, 4MBC only undergoes isomerization from (E)- to (Z)-4MBC; however, in the solar/free chlorine system, 4MBC was significantly degraded, with a pseudo-first-order rate constant of 0.0137 s-1 (pH = 7). The effects of the initial free chlorine concentration, solution pH and water matrix (presence of dissolved organic matter, HCO3- and Cl-) were studied. The results revealed that reactive chlorine species (RCS) are the dominant species influencing 4MBC degradation via solar/free chlorine, while OH and O3 played minor roles. These species would likely react with the 4-methylstyrene moiety of 4MBC and subsequently lead to 4MBC degradation through hydroxylation, chlorine substitution, oxidation and demethylation. Nevertheless, the dramatic increase in acute toxicity (Microtox®) during solar/free chlorine degradation of 4MBC highlights the need to further explore the transformation byproducts as well as their associated risks to humans and the environment.

Photoelectrochemical degradation of perfluorooctanoic acid (PFOA) with GOP25/FTO anodes: Intermediates and reaction pathways.

Perfluorooctanoic acid (PFOA) have been widely studied due to their persistence, bioaccumulation and possible toxic effects. In this work, we investigated a photoelectrochemical (PEC) system consisting of a graphene oxide-titanium dioxide (GOP25) anode coated on fluorine-doped tin oxide (FTO) glass for removal of PFOA in an aquatic environment. The GOP25/FTO anode was fabricated and well characterized. Nearly complete decomposition of 0.5 mg/L PFOA was achieved after 4 h of PEC treatment with an initial pH of 5.3 and a current density of 16.7 mA cm-2. The presence of graphene oxide (GO) on the TiO2 anode could enhance its electrochemical performance, thereby leading to increased decomposition efficiency. A total of 18 PFOA transformation products, including short-chain perfluoroalkyl acids, are reported in this work, and 13 products were observed for the first time. Four possible routes of PFOA decomposition, namely, decarboxylation followed by oxidation, defluorination, hydroxylation and Cl atom substitution, were determined. The presence of chlorinated byproducts in the system indicated that reactive chlorine species contributed to PFOA degradation.

Pharmaceutical and anticorrosive substance removal by woodchip column reactor: removal process and effects of operational parameters.

Urban stormwater has recently been considered a potential water resource to augment urban water supplies; however, the existence of emerging contaminants limits urban stormwater utilization. This study aims to use woodchip bioreactors, which are natural and inexpensive, to remove emerging contaminants from artificial stormwater, with a focus on the contaminant removal processes in the woodchip bioreactor and on the effects of operational parameters on the system performance. Seven commonly detected emerging contaminants - acetaminophen (ACE), caffeine (CAFF), carbamazepine (CBZ), ibuprofen (IBU), sulfathiazole (SFZ), benzotriazole (BT) and 5-methyl-1H-benzotriazole (5-MeBT) - were studied. The results showed that the removal efficiency and removal processes are heavily dependent on the compound. ACE and CAFF have the highest removal efficiencies (≥80%), and sorption and biodegradation are both crucial for their removal. However, IBU exhibits very limited sorption and biodegradation and hence has the worst removal (≤15%). The removal efficiencies of the other compounds (SFZ, CBZ, BT and 5-MeBT) range from ∼30 to 60%, and sorption is likely the main removal process. The effects of several operational parameters, including woodchip type, operation time, season and flow rate, on the removal rate of emerging contaminants were also explored. The results of this study showed that the woodchip column system, which is capable of sorption and biodegradation, represents a promising treatment process for removing emerging contaminants from urban stormwater.

Degradation of methadone by the sunlight/FC process: Kinetics, radical species participation and influence of the water matrix.

Free chlorine sunlight photolysis (sunlight/FC) markedly enhances the degradation rate of methadone, a synthetic opioid used medically, over that obtained using sunlight alone. The pseudo-first-order rate constants of methadone degradation under acidic conditions ([methadone] = 0.2 µM, [free chlorine] = 4 µM, and pH = 4) for sunlight/FC and sunlight photolysis are 7.0 ±â€¯1.1 × 10-2 min-1 and 1.4 ±â€¯0.2 × 10-2 min-1, respectively. The improved methadone degradation can be attributed to the production of HO and reactive chlorine species (RCS) during sunlight/FC photolysis. HO and RCS predominantly accounted for degradation during sunlight/FC photolysis under acidic and neutral conditions, while direct photolysis was the major contributor towards methadone degradation in alkaline conditions. The initial pH (pH 4-11) and free chlorine concentration (1-6 µM) significantly influenced the overall degradation efficiency of methadone. The presence of HCO3-, Cl- and dissolved organic matters, which may competitively react with HO and RCS, retard the degradation of methadone in synthetic wastewater. Consequently, a 50% lower methadone degradation rate was observed when deionized (DI) water was replaced with tap water. These results emphasize the need to consider different water matrices when applying sunlight/FC photolysis for water treatment.

The effects and the toxicity increases caused by bicarbonate, chloride, and other water components during the UV/TiO2 degradation of oxazaphosphorine drugs.

The influences of HCO3-, Cl-, and other components on the UV/TiO2 degradation of the antineoplastic agents ifosfamide (IFO) and cyclophosphamide (CP) were studied in this work. The results indicated that the presence of HCO3-, Cl-, NO3-, and SO42- in water bodies resulted in lower degradation efficiencies. The half-lives of IFO and CP were 1.2 and 1.1 min and increased 2.3-7.3 and 3.2-6.3 times, respectively, in the presence of the four anions (initial compound concentration = 100 µg/L, TiO2 loading =100 mg/L, anion concentration = 1000 mg/L, and pH = 8). Although the presence of HCO3- in the UV/TiO2/HCO3- system resulted in a lower degradation rate and less byproduct formation for IFO and CP, two newly identified byproducts, P11 (M.W. = 197) and P12 (M.W. = 101), were formed and detected, suggesting that additional pathways occurred during the reaction of •CO3- in the system. The results also showed that •CO3- likely induces a preferred ketonization pathway. Besides the inorganic anions HCO3-, Cl-, NO3-, and SO42-, the existence of dissolved organic matter in the water has a significant effect and inhibits CP degradation. Toxicity tests showed that higher toxicity occurred in the presence of HCO3- or Cl- during UV/TiO2 treatment and within 6 h of reaction time, implying that the effects of these two anions should not be ignored when photocatalytic treatment is applied to treat real wastewater.

The role of bicarbonate anions in methotrexate degradation via UV/TiO2: Mechanisms, reactivity and increased toxicity.

Bicarbonate anion (HCO3-) is a major constituent in wastewater and natural water matrices, and the aim of this study was to investigate its roles in the degradation of the antineoplastic agent methotrexate via UV/TiO2. A comprehensive investigation of reaction mechanisms was performed by conducting scavenger experiments and substructure reactivity and Microtox® toxicity tests. In the presence of HCO3-, the methotrexate degradation rate substantially increased, indicating the involvement of CO3-. The estimated second-order rate constants of methotrexate with CO3- and OH were 1.4 × 107 M-1 s-1 and 8.7 × 109 M-1 s-1, respectively. Both the valence hole (hvb+) and OH resulted in the generation of CO3-. Initial transformation pathways of methotrexate were proposed, including the addition of atomic oxygen, hydroxylation, deamination, CC cleavage and CN cleavage. CN cleavage at the aniline moiety (the N(13) position) is the primary decomposition pathway, leading to an aminopterin yield of 43%. CO3- preferentially reacted with the 4-aminobenzamide (ABZ) moiety and generated toxic byproducts during the later stages of decomposition, which was not observed in the UV/TiO2 system. The reactivity of the three methotrexate substructures decreased in the following order in the presence of HCO3-: ABZ â‰« DHP â‰« LG∼0; however, without HCO3-, the following order was observed: ABZ âˆ¼ DHP > LG. The results of this work suggest that the increase in toxicity induced by the presence of HCO3- likely occurs in many other OH-based advanced oxidation processes in wastewater containing pharmaceutical cocktails with ABZ moieties.

Oxidative Transformation of Controlled Substances by Manganese Dioxide.

This study investigated the oxidative transformation of four controlled substances (ketamine, methamphetamine, morphine, and codeine) by synthesized MnO2 (δ-MnO2) in aqueous environments. The results indicated that ketamine and methamphetamine were negligibly oxidized by MnO2 and, thus, may be persistent in the aqueous environment. However, morphine and codeine were able to be oxidized by MnO2, which indicated that they are likely naturally attenuated in aqueous environments. Overall, lower solution pH values, lower initial compound concentrations, and higher MnO2 loading resulted in a faster reaction rate. The oxidation of morphine was inhibited in the presence of metal ions (Mn(2+), Fe(3+), Ca(2+), and Mg(2+)) and fulvic acid. However, the addition of Fe(3+) and fulvic acid enhanced codeine oxidation. A second-order kinetics model described the oxidation of morphine and codeine by MnO2; it suggested that the formation of a surface precursor complex between the target compound and the MnO2 surface was the rate-limiting step. Although the target compounds were degraded, the slow TOC removal indicated that several byproducts were formed and persist against further MnO2 oxidation.

Occurrence of pharmaceuticals, hormones, and perfluorinated compounds in groundwater in Taiwan.

In this work, we investigated the emerging pollutants in Taiwanese groundwater for the first time and correlated their presence with possible contamination sources. Fifty target pharmaceuticals and perfluorinated chemicals in groundwater were mostly present at ng L(-1) concentrations, except for 17α-ethynylestradiol, sulfamethoxazole, and acetaminophen (maximums of 1822, 1820, and 1036 ng L(-1), respectively). Perfluorinated compounds were detected with the highest frequencies in groundwater at almost all of the sample sites, especially short-chained perfluorinated carboxylates, which were easily transferred to the groundwater. The results indicate that the compounds found to have high detection frequencies and concentrations in groundwater are similar to those found in other countries around the world. Most common pharmaceuticals that contain hydrophilic groups, such as sulfonamide antibiotics and caffeine, are easily transported through surface waters to groundwater. The results also indicated that the persistent natures of emerging contaminants with high detection frequencies in surface water and groundwater, such as perfluorooctanesulfonate (risk quotient >1), caffeine, and carbamazepine, should be further studied and evaluated.

TiO2 photocatalytic degradation and transformation of oxazaphosphorine drugs in an aqueous environment.

This study investigated the TiO2 photocatalytic degradation and transformation of the oxazaphosphorines ifosfamide (IFO), cyclophosphamide (CP) and trofosfamide (TRO). Under the optimum conditions of TiO2=100mg/L, IFO=100µg/L and solution pH=5.5, IFO was completely removed within 10min (k=0.433min(-1)). The results indicated that OHfree radicals generated by valence holes in the bulk solution were the predominant species for the degradation of IFO. At higher initial concentrations of oxazaphosphorines (20mg/L), >50% of TOC remained after 6h of reaction time, indicating that parent compounds were transformed to byproducts, which exhibit higher Microtox acute toxicities; chlorinated byproducts were likely the source of toxicity. Photocatalytic degradation pathways of the three oxazaphosphorines were proposed. IFO, CP and TRO follow very similar pathways and bond-breaking processes: ketonization and breaking of the CCl bond, the PN bond and the CN bond (N-dechloroethylation). Chloride (Cl(-)) release is likely the first and primary step in the decomposition process. Several of the identified byproducts were also metabolites, which implies that photocatalytic oxidation proceeds through pathways that are similar to metabolic pathways.

Removal of pharmaceuticals and organic matter from municipal wastewater using two-stage anaerobic fluidized membrane bioreactor.

The aim of present study was to treat municipal wastewater in two-stage anaerobic fluidized membrane bioreactor (AFMBR) (anaerobic fluidized bed reactor (AFBR) followed by AFMBR) using granular activated carbon (GAC) as carrier medium in both stages. Approximately 95% COD removal efficiency could be obtained when the two-stage AFMBR was operated at total HRT of 5h (2h for AFBR and 3h for AFMBR) and influent COD concentration of 250mg/L. About 67% COD and 99% TSS removal efficiency could be achieved by the system treating the effluent from primary clarifier of municipal wastewater treatment plant, at HRT of 1.28h and OLR of 5.65kg COD/m(3)d. The system could also effectively remove twenty detected pharmaceuticals in raw wastewaters with removal efficiency in the range of 86-100% except for diclofenac (78%). No other membrane fouling control was required except scouring effect of GAC for flux of 16LMH.