Efficient Removal of PFASs Using Photocatalysis, Membrane Separation and Photocatalytic Membrane Reactors

Abstract

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are persistent compounds characterized by stable C-F bonds giving them high thermal and chemical stability. Numerous studies have highlighted the presence of PFASs in the environment, surface waters and animals and humans. Exposure to these chemicals has been found to cause various health effects and has necessitated the need to develop methods to remove them from the environment. To date, the use of photocatalytic degradation and membrane separation to remove PFASs from water has been widely studied; however, these methods have drawbacks hindering them from being applied at full scale, including the recovery of the photocatalyst, uneven light distribution and membrane fouling. Therefore, to overcome some of these challenges, there has been research involving the coupling of photocatalysis and membrane separation to form photocatalytic membrane reactors which facilitate in the recovery of the photocatalyst, ensuring even light distribution and mitigating fouling. This review not only highlights recent advancements in the removal of PFASs using photocatalysis and membrane separation but also provides comprehensive information on the integration of photocatalysis and membrane separation to form photocatalytic membrane reactors. It emphasizes the performance of immobilized and slurry systems in PFAS removal while also addressing the associated challenges and offering recommendations for improvement. Factors influencing the performance of these methods will be comprehensively discussed, as well as the nanomaterials used for each technology. Additionally, knowledge gaps regarding the removal of PFASs using integrated photocatalytic membrane systems will be addressed, along with a comprehensive discussion on how these technologies can be applied in real-world applications.

Keywords: PFAS destructive techniques; electrostatic interactions; granular activated carbon; nanofiltration; perfluorooctane sulfonate (PFOS); perfluorooctanoic acid (PFOA); reverse osmosis.

Figure 2

Schematic representation of the (a) Effect of pH on photocatalyst’s surface charge (i) and interaction of anionic PFASs and photocatalyst’s surface at different pH values (ii) (this work). (b) Effect of pH on photocatalytic degradation of PFOA and (c) zeta potential of TiO₂ at various pH values (b and c reproduced with permission from Ref. [35]).

Figure 1

(a) Schematic representation of the efffect of light on photocatalytic degradation of PFOA (this work), (b) degradation of PFOA in pure water using UV irradiation, (c) pure water using VUV irradiation and (d) sewage water using UV and VUV irradiation (b–d reproduced with permission from Ref. [24]).

Figure 3

The effect of solute concentration on the degradation efficiency of PFOA with (a) Pb-BFO/0.5%rGO (reproduced with permission from Ref. [32]. (b) duo functional tri-metallic-oxide (f-TMO) photocatalyst (reproduced with permission from Ref. [37]) and (c) BiOI@Bi₅O₇I heterojunction photocatalyst (reproduced with permission from Ref. [38]).

Figure 4

Schematic representation (a) of the degradation efficiency of PFASs at low photocatalyst dosage(i), optimal dosage(ii) and high dosage (iii) (this work). (b) Degradation of PFOA at various photocatalyst dosages (reproduced with permission from Ref. [41]).

Figure 5

Configuration showing adsorption of PFOA on (a) In₂O₃ and (b) TiO₂.

Figure 6

(a) Degradation and (b) defluorination of PFOA using noble metal-doped TiO₂ ((a,b) reproduced with permission from Ref. [49]); (c) schematic representation of the electron trapping in metal-doped photocatalyst (this work).

Figure 7

Factors affecting the removal of PFASs via membrane separation.

Figure 8

Schematic representation of PFAS rejection efficiency in the presence of organic matter, illustrating (a) size exclusion and (b) electrostatic interactions. Black arrows indicate electrostatic exclusion between PFAS molecules and the membrane surface, as well as electrostatic shielding caused by the adsorption of cations in solution, which shields the membrane surface. (Reproduced with permission from Ref. [58]).

Figure 9

Rejection of (a) PFCAs and (b) PFSAs using NF membrane in spiked AFFF and groundwater solutions (Reproduced with permission from Ref. [8]).

Figure 10

(a) Rejection of PFOS with varying concentrations. (b) Influence of PFOS concentration on the Flux decay rate (F/F₀): F₀ is the pure water flux; F is the flux at a specific moment in time. (Reproduced with permission from Ref. [62]).

Figure 11

(a) Schematic representation of the accumulation of molecules on surface of membrane (this work). (b,c) Rejection and permeate fluxes of PFOA and PFBA at various operation pressures [57].

Figure 12

(a) Influence of pH on rejection of PFOS. (b) Zeta potentials of PMIA membrane at various solution pHs (reproduced with permission from Ref. [62]).

Figure 13

(a) Formation of CF₃(CF₂)₇SO₃Ca due to electrostatic interaction between Ca²⁺ and negatively charged sulfonate group; (b) formation of CF₃(CF₂)₇SO₃ −Ca− O₃S(CF₂)₇CF₃ through linkage of two PFOS molecules to Ca²⁺.

Figure 14

Schematic representation of the rejection proficiency of NF membrane for long- and short-chain PFASs (a), effect of organic matter and cations on rejection of long- and short-chain PFAS molecules (b) (this work) and (c) rejection proficiency of RO and NF membranes for long- and short-chain PFASs [57].

Figure 15

Schematic representation on the interaction of PFASs with membrane with (a) hydrophobic and (b) hydrophilic surface.

Figure 16

Schematic diagram showing the adsorption of PFOA onto polyamide barrier layer (a); electrostatic repulsion between polyamide barrier layer modified with carbonyl groups and PFOA (b).

Figure 17

Schematic diagram showing the removal of PFOA using NF membrane–UV hybrid system.

Figure 18

Flow diagram of NF/UV–sulphite pilot system for treatment of groundwater (reproduced with permission from Ref. [104]).

Figure 19

Schematic representation of an immobilized photocatalytic membrane reactor with crossflow filtration system.

Figure 20

A schematic diagram showing the photocatalytic degradation of PFOA adsorbed onto the photocatalytic membrane after exposure to UV.