Water treatment techniques for destructive removal of perfluoroalkyl substances (PFAS) have only recently begun to emerge in the research literature, comprising unconventional advanced oxidation and reduction methods. Photocatalytic degradation of PFAS has not been widely pursued, which is a result of the limited ability of common semiconductor materials to induce C−F bond cleavage in aqueous systems. Herein, degradation of perfluorooctanoic acid (PFOA) by bismuth phosphate photocatalysts under ultraviolet irradiation has been investigated for the first time, including the relatively well-known monoclinic BiPO 4 wide band gap semiconductor, as well as a novel Bi 3 O(OH)(PO 4 ) 2 (BOHP) composition. Compared to BiPO 4 and a β-Ga 2 O 3 nanomaterial reference catalyst, BOHP microparticles achieved dramatically faster PFOA degradation and mineralization, despite both a smaller surface area and a lower band gap energy. The rate constant for degradation of PFOA by BOHP in a pure water solution was ∼15 times greater than those of both BiPO 4 and β-Ga 2 O 3 (∼20−30 times greater when normalized for surface area) and was on the same order of magnitude as that of phenol degradation by P25 TiO 2 in the same photoreactor. The superior performance of BOHP was primarily related to the surface charge and adsorption behavior of PFOA, in combination with the favorable redox potentials of BOHP charge carriers. The catalyst was further tested at low PFOA concentrations (i.e., microgram per liter range) in the presence of natural organic matter, and rapid degradation of PFOA was also observed, indicating the potential of BOHP to enable practical ex situ destructive treatment of PFAS-contaminated groundwater.

Semiconductor photocatalysis is currently being explored as a treatment tool for wastewaters contaminated with poly-/ perfluoroalkyl substances (PFAS), such as groundwater impacted by aqueous film-forming foams. While numerous catalysts have been shown to degrade perfluorocarboxylic acids (PFCAs) such as PFOA, research thus far has been confined to bench-scale evaluations that offer little insight into the practical aspects and potential energy efficiency expected during full-scale application. Herein, we advanced such understanding using the recently discovered Bi 3 O(OH)(PO 4 ) 2 catalyst system (UV/ BOHP) by first elucidating the basic PFCA degradation mechanisms and behavior, followed by comparisons among different photoreactor designs. The BOHP suspension degraded PFCAs primarily through direct heterogeneous oxidation by valence band holes, and kinetics correlated positively with chain length. Degradation of PFCAs was further compared between stirred immersion photoreactors, bench-scale confined-flow high-intensity slurry photocatalytic reactors (CHISPRs), and a larger commercial CHISPR system. Complete degradation (>99%) of long-chain PFCAs was observed in the immersion reactors within 60 min, while the CHISPRs degraded all PFAS tested within 20 min; however, control tests revealed that direct photolysis by vacuum UV was the main driver in the CHISPRs. Despite their faster kinetics, the energy consumption (per order removal) of PFOA photolysis in the unmodified CHISPRs was significantly higher (51−124 kWh/m 3 ) compared to PFOA photocatalysis in the immersion reactors (25 ± 4 kWh/ m 3 ). Based on these findings, practical photoreactor design criteria were proposed which incorporate both photolysis and photocatalysis, and which have implications beyond just the UV/BOHP process.

In this study, for the first time a statistical analysis based on the response surface methodology (RSM) was employed to investigate individual and interaction effects of key operating parameters of the photocatalytic degradation under visible-light irradiation using Ag-S/PEG/TiO 2 . Ag-S/PEG/ TiO 2 is a visible-light-driven photocatalyst and was synthesized (based on earlier research) by co-doping of TiO 2 with silver and sulphur and addition of polyethylene glycol (as a reagent template). In addition, the model pollutant was methylene orange (MO) and the studied operating parameters included the photocatalyst loading, initial concentration of the pollutant, and pH of the solution. The statistics-based experimental design and RSM was utilized to find a quadratic model as a functional relationship between the degradation efficiency and the three operating parameters. The regression analysis with R 2 value of 0.9678 showed a close fit between the model prediction and experimental data of the degradation efficiency. The analysis of variance based on the model indicated that pH of the solution was the most influential factor, while the two other operating parameters were also significant. The efficiency of MO degradation reached 94.0 % under the optimum conditions (i.e. photocatalyst loading of 1.20 g/L, MO concentration of 5 mg/L, and pH of 2).

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