A ‘Concentrate-&-Degrade’ Technique for Cost Effective Adsorption and Degradation of Per-and Polyfluoroalkyl Substances in Water

Abstract

The overarching goal of this research is to develop and test a new class of carbon-photocatalyst composites for enhanced adsorption and destruction of per- and polyfluoroalkyl substances (PFAS) in water and field water. PFAS are ubiquitous in water, due to their widespread applications in various industrial and consumer products, and health concerns. Yet, a cost-effective technology has been lacking for the degradation of PFAS due to their resistance to conventional treatment. Thus, we developed a novel ‘Concentrate-&-Destroy’ technology, using adsorptive photocatalysts for enhanced removal of PFAS from water. Metal-doped AC-supported titanate nanotubes (M/TNTs@AC) were synthesized based on commercial activated carbon (ACs) and titanium oxide (TiO2), through a one-step facial alkaline hydrothermal process followed by proper metal doping and calcination. The material synthesis was optimized at varying AC mass contents, under different hydrothermal conditions, by doping various metals ions, and by varying calcination temperatures. Based on the experimental results, the highest photocatalytic mineralization efficiency of pre-sorbed perfluorooctane sulfonic acid (PFOS) was up to 66.2% using the 2 wt.% Ga/TNTs@Filtrosorb-400® granular activated carbon composite photocatalyst synthesized under the optimum parameters, 50 wt.% of Filtrosorb-400® granular activated carbon, 130 ℃ of hydrothermal temperature, 72 h of hydrothermal duration, and 550 ℃ of calcination temperature. The superior photoactivity of Ga/TNTs@AC is attributed to the oxygen vacancies, which not only suppressed recombination of the e-/h+ pairs, but also facilitated O2•− generation. Both h+ and O2•− played critical roles in the PFOS degradation, which starts with cleavage of the sulfonate group and converts it into perfluorooctanoate (PFOA) that is then decarboxylated and defluorinated following the stepwise defluorination mechanism. In addition, we used Ga/TNTs@AC for enhanced removal of PFAS from field water. Seven PFAS were detected in the field water, with the most predominant PFAS being PFOS. Ga/TNTs@AC (3 g/L) was able to remove ~98% of PFOS (spiked at 100 µg/L) from filed water within 10 min and offered large adsorption capacity. Subsequently, 35.5% of pre-concentrated PFOS (100 µg/L) on Ga/TNTs@AC (3 g/L) was degraded, with the defluorination rate of 25.8%. However, addition Fe3+ added in the system increased PFOS degradation to 80.4%, corresponding with the mineralization of 70.0%. The superior photoactivity is attributed to the concurrent complexes, including the Fe3+ complex with DOM and the complex between Fe3+ and PFOS. Additionally, acidic condition is favorable for not only PFOS adsorption in field water onto Ga/TNTs@AC, but also PFOS degradation in the presence of Fe3+. GenX, the ammonium salt of hexafluoropropylene oxide dimer acid, has been used as a replacement for perfluorooctanoic acid. Due to its widespread uses, GenX has been detected in waters around the world amid growing concerns about its persistence and adverse health effects. Thus, we developed an adsorptive photocatalyst by depositing a small amount (3 wt.%) of bismuth (Bi) onto TNTs@AC (Bi/TNTs@AC), and tested the material for adsorption and subsequent solid-phase photodegradation of GenX. Bi/TNTs@AC at 1 g/L was able to adsorb GenX (100 µg/L, pH 7.0) within 1 h, and then degrade 70.0% and mineralize 42.7% of pre-sorbed GenX under UV (254 nm) in 4 h. The efficient degradation also regenerated the material, allowing for repeated uses without chemical regeneration. Overall, the adsorptive photocatalysts and the ‘Concentrate-&-Destroy’ strategy represent a significant advancement in the treatment of PFAS. The new materials hold the promise to treat some of the most challenging contaminants in water in a more cost-effective manner.