Treatment of electrochemical plating wastewater by heterogeneous photocatalysis: the simultaneous removal of 6:2 fluorotelomer sulfonate and hexavalent chromium

Abstract

6:2 fluorotelomer sulfonate (6:2 FtS) is being widely used as a mist suppressant in the chromate (Cr(VI)) plating process. As a result, it is often present alongside Cr(VI) in the chromate plating wastewater (CPW). While the removal of Cr(VI) from CPW has been studied for decades, little attention has been paid to the treatment of 6:2 FtS. In this study, the removal of Cr(VI) and 6:2 FtS by Ga₂O₃, In₂O₃, and TiO₂ photocatalysts was investigated. In the Ga₂O₃/UVC system, over 95% of Cr(VI) was reduced into Cr(III) after only 5 min. Simultaneously, 6:2 FtS was degraded into F⁻ and several perfluorocarboxylates. The predominant reactive species responsible for the degradation of 6:2 FtS in the Ga₂O₃ system were identified to be h_VB⁺ and O₂˙⁻. In addition, it was observed that the presence of Cr(VI) helped accelerate the degradation of 6:2 FtS. This synergy between Cr(VI) and 6:2 FtS was attributable to the scavenging of e_CB⁻ by Cr(VI), which retarded the recombination of e_CB⁻ and h_VB⁺. The In₂O₃/UVC system was also capable of removing Cr(VI) and 6:2 FtS, although at significantly slower rates. In contrast, poor removal of 6:2 FtS was achieved with the TiO₂/UVC system, because Cr(III) adsorbed on TiO₂ and inhibited its reactivity. Based on the results of this study, it is proposed that CPW can be treated by a treatment train that consists of an oxidation–reduction step driven by Ga₂O₃/UVC, followed by a neutralization step that converts dissolved Cr(III) into Cr(OH)_3(S).

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1 Introduction

Owing to their simultaneous lipophobicity and hydrophobicity, high chemical and thermal stability, and surfactant property, per- and polyfluorinated alkyl substances (PFAS) are being used in a wide variety of industrial and commercial products, including in fire-fighting foams, flame retardants, mist suppressants, adhesives, and surface coatings, among others.¹ The extensive use of PFAS and the release of PFAS-containing wastes into the environment have led to their detection in air,^2,3 surface water,^4,5 groundwater,^6–9 drinking water,^10,11 soils^6,8 and biosolids.^12–14 Exposure to PFAS has been linked to various health problems, such as increased blood cholesterol, liver damage, kidney and testicular cancer, and thyroid dysfunction.^15–18 Therefore, guidelines and cleanup criteria in the low part-per-trillion levels are being proposed by jurisdictions around the world. For example, the US Environmental Protection Agency has recently established a health advisory level of 70 ng L⁻¹ for perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS).¹⁹ Health Canada has proposed drinking water screening values of 200–600 ng L⁻¹ for several PFAS compounds.^20,21 As our knowledge about the occurrence, fate, and toxicity of PFAS continues to grow, the guidelines for these compounds may become increasingly more stringent. Therefore, there is an urgent need to develop effective technologies for the treatment of PFAS-containing wastes to meet these future guidelines and to protect ecosystem and human health.

This study concerns the treatment of PFAS in chromate plating wastewater (CPW). During electrochemical plating, the evolution and burst of gas bubbles produced by electrolysis generates chromic acid mists, which pose significant health risks to workers on site. Therefore, a surfactant is usually added to the plating bath to suppress mist formation as well as to decrease surface wettability. Certain PFAS compounds are uniquely suited as mist suppressants in electrochemical plating because of their high aqueous solubility and resistance to electrochemical degradation. PFOS was traditionally the most popular mist suppressant in the electroplating industry, but this compound has been gradually replaced by 6:2 fluorotelomer sulfonate (6:2 FtS),²² a less recalcitrant and bioaccumulative compound.^23,24 The concentration of 6:2 FtS in electrochemical plating baths is typically tens of mg L⁻¹, resulting in a concentration of 6:2 FtS in CPW ranging up to hundreds of μg L⁻¹.²²

While the treatment of CPW has been investigated for decades, previous efforts focused primarily on removing hexavalent chromium (Cr(VI)) and other metals,²⁵ with little/no attention being paid to the removal of 6:2 FtS. However, it can be expected that conventional CPW treatment processes such as chemical reduction and coprecipitation are unlikely effective at removing 6:2 FtS. This is because like most PFAS, 6:2 FtS does not adsorb to metal hydroxides to an appreciable extent. Also, 6:2 FtS is generally resistant to chemical degradation; recent studies have shown that PFAS can only be degraded by harsh treatment processes such as thermal treatment,^26–28 plasma-based oxidation and reduction,²⁹ and reduction by hydrated electrons.^30–32

Several studies have reported that PFAS can be degraded by heterogeneous photocatalysis. For example, it has been shown that PFOA was destroyed by photogenerated valence band holes (h_VB⁺) produced when wide-bandgap semiconductors such as In₂O₃,^33,34 Ga₂O₃,³⁵ and In modified-Ga₂O₃ ³⁶ were irradiated with ultraviolet (UV) light. It has also been shown that PFOA could be degraded by the widely used TiO₂ photocatalyst,^33,34 although the degradation rate was relatively slow since PFOA did not react appreciably with hydroxyl radical (˙OH) (i.e., the predominant reactive species in the TiO₂/UV system). In a recent study, an excellent removal of PFOA was achieved with a commercial boron nitride (BN) photocatalyst.³⁷ The degradation of PFOA in this system was attributable to multiple reactive species, including h_VB⁺, ˙OH, and superoxide (O₂˙⁻). In addition to oxidative degradation, it appears that PFOA can also be reductively destroyed via reactions with photogenerated conduction band electrons (e_CB⁻)³⁸ or by surface hydrogen.³⁹ While the generation of a suite of different reactive species capable of destroying PFAS makes heterogeneous photocatalysis appealing, a major drawback of this approach is that h_VB⁺ and e_CB⁻ usually recombine rapidly, resulting in inefficient production of reactive species. This limitation can be overcome by the addition of a scavenger (e.g., H₂O₂, IO₄⁻, HS⁻, I⁻) that react with either h_VB⁺ or e_CB⁻, thereby extending the lifetime of the other species.⁴⁰

We hypothesize that heterogeneous photocatalysis is a promising approach for the treatment of CPW, owing to its potential to remove both Cr(VI) and 6:2 FtS in a single treatment step. Specifically, we envision in that in heterogeneous photocatalysis Cr(VI) will be reduced by e_CB⁻ into Cr(III), which then can be separated as Cr(OH)_3(s) by precipitation. Simultaneously, 6:2 FtS will be oxidized by h_VB⁺ and/or ˙OH. Moreover, we hypothesize that there could be a synergistic effect between the removals of Cr(VI) and 6:2 FtS—that is the removal of each contaminant will be enhanced in the presence of the other. This is because the scavenging of e_CB⁻ by Cr(VI), and of h_VB⁺ by 6:2 FtS could potentially mitigate the recombination of h_VB⁺ and e_CB⁻ as discussed above.

Photocatalyst + hν → h_VB⁺ + e_CB⁻ (1)

e_CB⁻ + Cr(VI) →→→ Cr(III) (2)

h_VB⁺ + 6:2 FtS → byproducts (3)

h_VB⁺ + H₂O → ˙OH + H⁺ (4)

˙OH + 6:2 FtS → byproducts (5)

To test the hypotheses above, the removals of Cr(VI) and 6:2 FtS were investigated using three commercially available photocatalysts, namely Ga₂O₃, In₂O₃, and TiO₂. A series of experiment with probe compounds were conducted to gain insights into the reactive species responsible for degradation of 6:2 FtS. In addition, the 6:2 FtS degradation byproducts were measured throughout the experiment to close mass balance and elucidate the degradation pathway. Finally, the reusability of Ga₂O₃ (which turned out to be the most promising among the studied materials) was investigated through a repeated contaminant removal experiment.

2 Materials and methods

2.1 Materials

The photocatalysts Ga₂O₃ (≥99.99%), In₂O₃ (99.9%), and TiO₂ (Degussa P25, 99.5%) were purchased from Sigma Aldrich and were used as received. The surface morphology and elemental composition of these materials were characterized by scanning electron microscopy (SEM)/energy dispersive X-ray spectrometry (EDS), while their surface area was determined using the Brunauer–Emmett–Teller (BET) N₂ physisorption method. The results of these characterizations are available in the ESI (Table S1 and Fig. S1†). 6:2 FtS (2-(perfluorohexyl)ethane-1-sulfonic acid sodium salt, >98%) was purchased from Toronto Research Chemicals (Toronto, ON), while PFAS analytical native and mass-labelled standards were purchased from Wellington Laboratories (Guelph, ON). All other chemicals were purchased at the highest purity available from Sigma Aldrich or Fisher Scientific. A stock solution of 5 mg L⁻¹ 6:2 FtS was prepared by dissolving 6:2 FtS in an aqueous solution consisting of 15 mM methanol. Other stock solutions, namely Cr(VI) (100 mg L⁻¹ as Cr), Cr(III) (100 mg L⁻¹ as Cr), p-benzoquinone (BQ, 100 mM) and bromate (BrO₃⁻, 500 mM), were prepared by dissolving K₂Cr₂O₇, CrCl₃·6H₂O, BQ, and KBrO₃ in 18.2 MΩ cm MilliQ water (Millipore).

2.2 Photolysis experiments

All experiments were carried out in a reactor chamber that was equipped with 6 × 4 W UVC lamps (Fig. S2 in the ESI†). A quartz container, which held the reaction suspension, was placed at the center of the chamber. In most experiments, the reaction suspension was prepared by adding a photocatalyst and aliquots containing 6:2 FtS and Cr(VI) from their respective stock solutions to MilliQ water such that [catalyst] = 0.5 g L⁻¹, [6:2 FtS]₀ = 100 or 500 μg L⁻¹, and [Cr(VI)]₀ = 0 or 1 mg L⁻¹. Although the concentration of Cr(VI) in chromium plating wastewaters can be in the order of tens of mg L⁻¹, the concentration of Cr(VI) employed in most experiments was kept low to avoid damaging the liquid chromatograph mass spectrometer (LC/MS/MS). However, in one experiment, the initial concentrations of Cr(VI) and 6:2 FtS were 20 mg L⁻¹ and 2 mg L⁻¹, respectively. The samples collected from this experiment were diluted 400 times prior to LC/MS/MS analysis. In some experiments, the reaction suspension also contained formic acid (HCOOH, a h_VB⁺ scavenger),⁴⁰ tert-butyl alcohol (t-BuOH, an ˙OH scavenger),⁴¹ BQ (a O₂˙⁻ scavenger),⁴² or BrO₃⁻ (a e_CB⁻ scavenger).⁴³ The initial pH of the reaction suspension was adjusted to pH = 3 ± 0.1 using either 1 M H₂SO₄ or 1 M NaOH. The pH of the suspension varied by less than 0.1 unit throughout the entire course of each experiment.

Preliminary experiments showed that approximately 10% of the initial 6:2 FtS was lost by adsorption on the photocatalysts, and that adsorption equilibrium was attained within 30 min. As such, the suspension was vigorously mixed in the dark for 1 hour. Subsequently, photochemical reactions were initiated by turning the UVC lamps on. At predetermined time intervals, aliquots of suspension were withdrawn and subjected to analyses for dissolved Cr, fluoride (F⁻), as well as 6:2 FtS and its degradation products.

As will be presented in the Results and discussion sections, Ga₂O₃ was observed to be the photocatalyst most efficient at removing 6:2 FtS and Cr(VI). Therefore, the reusability of Ga₂O₃ was evaluated over 3 experimental cycles wherein at the end of each cycle Ga₂O₃ particles were separated and added to a fresh reaction solution containing 100 μg L⁻¹ 6:2 FtS and 1 mg L⁻¹ Cr(VI). Subsequently, the photolysis experiment was repeated.

All experiments were carried out at least in triplicate, and the average values are presented along with one standard deviation. The evolution of the concentrations of different species in the solution is expressed against energy flux (J cm⁻²) received by the reaction suspension, which was determined based on ferrioxalate actinometry.

2.3 Analytical methods

For the analysis of F⁻, Ga₂O₃ particles were separated from the suspension by centrifugation, and F⁻ in the supernatant was measured spectrophotometrically using the modified SPADNS method (limit of quantitation of 50 μg L⁻¹).⁴⁴ For the analysis of dissolved Cr, the suspension was first acidified to pH < 2 to dissolve particulate Cr(III) (which was formed due to the photoreduction of Cr(VI) to Cr(III)). Subsequently, Ga₂O₃ particles were separated by centrifugation, and Cr(VI) and total Cr (i.e., Cr(VI) + Cr(III)) in the acidified supernatant were measured spectrophotometrically using the 1,5-diphenylcarbazide method.⁴⁵ The limits of quantitation by this method are 30 μg L⁻¹ for Cr(VI), and 50 μg L⁻¹ for total Cr. The concentration of Cr(III) in the sample was calculated based on the difference between the concentrations of total Cr and Cr(VI).

For the analysis of PFAS by LC/MS/MS, each sub-aliquot was diluted 9 times in a solution consisting of 50 v% methanol and 50 v% 5 mM ammonium acetate in water. The purpose of this dilution was threefold: (a) to desorb PFAS from the catalyst in order to enable closing mass balance, (b) to precipitate Cr(III) from the sample in order to minimize the amount of dissolved Cr introduced into the LC/MS/MS, and (c) to obtain a sample consisting of ca. 50/50 v/v methanol/water for analysis by LC/MS/MS. Following dilution, the suspension was centrifuged to separate Ga₂O₃ and Cr(OH)₃, and the supernatant was added to an HPLC vial. Prior to LC/MS/MS analysis, mass-labelled PFAS, which served as internal standards, were spiked into each vial. Details about the LC/MS/MS operating condition and the list of PFAS compounds analysed in this study can be found in the ESI (Tables S2 and S3†).

3 Results

Photolysis without a catalyst did not result in 6:2 FtS or Cr(VI) removal (data not shown). Also, both contaminants were not removed appreciably by adsorption in the absence of light (Fig. S3†). In contrast, both contaminants were removed when Ga₂O₃, In₂O₃, or TiO₂ was present, although the reaction rates varied significantly (Fig. 1). In the Ga₂O₃ system, the concentration of Cr(VI) decreased to around the method quantitation limit after only 5 min of irradiation (Fig. 1a). Simultaneously, the concentration of 6:2 FtS gradually decreased throughout the course of the experiment, with 90% 6:2 FtS removed after 2 hours (equivalent to a fluence of 19 J cm⁻², Fig. 1d). Interestingly, the 6:2 FtS degradation rate was over twice slower when Cr(VI) was absent (k_observed = 0.34 h⁻¹, versus k_observed = 0.73 h⁻¹ in the presence of Cr(VI)). Excellent removals of Cr(VI) and 6:2 FtS were also observed in the experiment wherein the initial concentrations of the contaminants were increased by 20 times (i.e., [Cr(VI)]₀ = 20 mg L⁻¹, [6:2 FtS] = 2 mg L⁻¹; Fig. S4†).

Fig. 1 Removals of Cr(VI) and 6:2 FtS by Ga₂O₃ (a) and (d), In₂O₃ (b) and (e) and TiO₂ (c) and (f) photocatalysts. [Catalyst] = 0.5 g L⁻¹, [6:2 FtS]₀ = 100 μg L⁻¹, [Cr(VI)]₀ = 1 mg L⁻¹, pH = 3.0 ± 1. The solution was vigorously mixed in the dark for 1 hour. Subsequently, photolysis was initiated by turning UV-C lamps on at t = 0 h.

Cr(VI) and 6:2 FtS were also simultaneously removed in the In₂O₃ system, although at significantly slower rates. After 8 hours, the concentrations of Cr(VI) and 6:2 FtS decreased by only 50% and 35%, respectively (Fig. 1b and e). As in the Ga₂O₃ system, the presence of Cr(VI) enhanced the rate of 6:2 FtS degradation (k_observed = 0.12 h⁻¹ and 0.10 h⁻¹ in the presence and absence of Cr(VI), respectively). On the contrary, the presence of Cr(VI) inhibited the degradation of 6:2 FtS in the TiO₂ system: whereas 80% of 6:2 FtS was removed within the first 20 min in the absence of Cr(VI), only 30% removal was achieved in the presence of Cr(VI) even after 8 hours (k_observed = 0.04 h⁻¹) (Fig. 1f). If the 6:2 FtS degradation was very slow, the reduction of Cr(VI) in the TiO₂ system was slightly faster than that in the In₂O₃ system (Fig. 1c).

The varying reactivity among the materials discussed above is attributable to their inherent photocatalytic reactivity rather than to the difference in their surface area. In the absence of Cr(VI), the 6:2 FtS degradation rate with TiO₂ (k_observed = 2.39 h⁻¹) was over seven times faster than that with Ga₂O₃ (k_observed = 0.34 h⁻¹), although the surface area of TiO₂ (51.9 m² g⁻¹) is only three times greater than that of Ga₂O₃ (19.5 m² g⁻¹). In the presence of Cr(VI), the 6:2 FtS degradation rate with Ga₂O₃ (k_observed = 0.73 h⁻¹) was over six times greater than that of In₂O₃ (k_observed = 0.12 h⁻¹), even though the surface area of the latter (7.1 m² g⁻¹) is only 2.7 times less than that of the former.

Not only did the contaminant removal rates vary but also the reactive species responsible for the degradation of 6:2 FtS appeared to be significantly different in the three photocatalyst systems. With In₂O₃, 6:2 FtS did not degrade appreciably in the presence of either 10 mM HCOOH or 10 mM t-BuOH (Fig. S5a†). With TiO₂, the degradation of 6:2 FtS was also significantly inhibited by HCOOH, but was only partially inhibited by t-BuOH (Fig. S5b†). Somewhat surprisingly, neither of these two scavengers had any effect on the degradation of 6:2 FtS by Ga₂O₃ (Fig. 2a). However, the degradation was partially inhibited when BrO₃⁻ was present, and was completely inhibited in the presence of HCOOH and BrO₃⁻, or HCOOH and BQ (Fig. 2a).

Fig. 2 (a) Effect of various scavengers on the degradation of 6:2 FtS; (b) evolution of byproducts; (c) F mass balance. [Ga₂O₃]₀ = 0.5 g L⁻¹, [6:2 FtS]₀ = 100 μg L⁻¹ in (a) 500 μg L⁻¹ in (b) and (c), [Cr(VI)] = 1 mg L⁻¹, [t-BuOH]₀ = [HCOOH]₀ = [BrO₃⁻]₀ = 10 mM, [BQ]₀ = 1 mM, pH = 3.0 ± 1. It is noted that Cr(VI) was not added in the experiments wherein other scavengers (i.e., HCOOH, BQ, BrO₃⁻, and t-BuOH) were present.

Because Ga₂O₃/UVC was effective at removing both Cr(VI) and 6:2 FtS, the evolution of the 6:2 FtS degradation byproducts in this system was further investigated to gain insight into the reaction pathway. Besides F⁻, a total of 5 byproducts, i.e., 6:2 fluorotelomer carboxylate (6:2 FtCA), perfluoroheptanoate (PFHpA), perfluorohexanoate (PFHxA), perfluoropentanoate (PFPeA), perfluorobutanoate (PFBA), were observed as 6:2 FtS was degraded (Fig. 2b). The fluorine mass balance indicates that the sum of F⁻, the F in the 5 byproducts, and the F in the remaining 6:2 FtS accounted for 80% of the total F initially added as 6:2 FtS (Fig. 2c). This result suggests that there must be other byproducts that were not detected by the LCMS analytical method used in this study. Among the measured byproducts, 6:2 FtCA exhibited a two-phase concentration–time profile wherein its concentration first increased and subsequently decreased (Fig. 2b). For the other 4 byproducts, their concentrations increased gradually (Fig. 2b). By the end of the experiment, the concentration of F⁻ was 135 μg L⁻¹ (equivalent to approximately 50% of the total F added).

In the Ga₂O₃ reusability experiment, excellent removals of both Cr(VI) and 6:2 FtS were observed over three cycles (Fig. 3a). In each cycle, more than 95% of Cr(VI) was reduced into Cr(III), 80% of which can be separated by Cr(OH)_3(s) from the treated stream by raising the solution pH to 7.5 (Fig. 3b). The concentration of dissolved Cr after pH adjustment and Cr(OH)_3(s) separation (by centrifugation) was 0.2 mg L⁻¹, well below the US EPA's discharge limit of 2–4 mg L⁻¹.⁴⁶ Meanwhile, the fraction of 6:2 FtS degraded in each cycle ranged between 86 and 92%. The slightly lower 6:2 FtS removal in cycle 3 could be due to the adsorption of Cr species on Ga₂O₃, thereby diminishing its photocatalytic reactivity.

Fig. 3 (a) Degradation of 6:2 FtS by Ga₂O₃/UV-C over three cycles. The concentration of Ga₂O₃ was 0.5 g L⁻¹. At the beginning of each cycle, the spent solution was replaced with a fresh solution containing [6:2 FtS]₀ = 100 μg L⁻¹ and [Cr(VI)]₀ = 1 mg L⁻¹; (b) the concentrations of dissolved Cr(VI) and Cr(III) at the beginning of each cycle (labelled as “initial”), after photolysis (labelled as “final”), and after adjusting pH to 7.5 and separating Cr(OH)_3(s) (labelled as “pH adjust”).

4 Discussion

4.1 Reactive species responsible for contaminant removal

The Cr mass balance and the stoichiometric production of Cr(III) in all three photocatalytic systems suggest that Cr(VI) was removed via reactions with e_CB⁻ (reaction (2)). (The reduction of Cr(VI) via direct reactions with 6:2 FtS can be ruled out as Cr(III) was not produced in the absence of photocatalysts. The production of Cr(III) from direct reactions between Cr(VI) and the byproducts of 6:2 FtS can also be ruled out, since 6:2 FtS was not degraded appreciably in the first 5 min in the Ga₂O₃ system (Fig. 1a)). The much faster Cr(VI) reduction in the Ga₂O₃ compared to that in the other two systems is attributable to Ga₂O₃'s higher conduction band position (E_CB,Ga2O3 = −2.95 eV versus E_CB,TiO2 = −4.21 eV and E_CB,In2O3 = −4.30 eV), resulting in e_CB⁻ that possesses a greater reduction power.^47–49 Despite a rapid removal at the beginning, the concentration of Cr(VI) in the Ga₂O₃ system never decreased to zero. Instead, a steady state concentration of Cr(VI) of ca. 30 μg L⁻¹ was maintained in the solution throughout the course of the experiment. As such, it is hypothesized that there was a redox cycle between Cr(VI) and Cr(III) in this system, driven by reaction (2) and the oxidation of Cr(III) back to Cr(VI):

h_VB⁺ + Cr(III) →→→ Cr(VI) (6)

The role of the Cr(VI)–Cr(III) redox cycling toward sustaining the enhanced 6:2 FtS degradation (compared to when Cr was absent) in the Ga₂O₃ system will be discussed further in Section 4.2.

Whereas e_CB⁻ was the only reactive species responsible for Cr(VI) removal, a few different reactive species appears to be involved in the degradation of 6:2 FtS. In the In₂O₃ system, the predominant oxidant likely was ˙OH, although h_VB⁺ also could have had a partial contribution, since the 6:2 FtS degradation was somewhat further inhibited by HCOOH (a h_VB⁺ scavenger, which is also expected to inhibit reaction (4)) (Fig. S5a†). In the TiO₂ system, both h_VB⁺ and ˙OH played an important role because HCOOH and t-BuOH appreciably retarded the degradation of 6:2 FtS (Fig. S5b†). Because h_VB⁺ and ˙OH can oxidize substrates via hydrogen abstraction,⁴⁰ it is proposed that the oxidation of 6:2 FtS proceeded as follows:

C₆F₁₃C₂H₄SO₃⁻ + h_VB⁺ → C₆F₁₃CH₂˙CHSO₃⁻ + H⁺ (7)

C₆F₁₃C₂H₄SO₃⁻ + h_VB⁺ → C₆F₁₃˙CHCH₂SO₃⁻ + H⁺ (8)

C₆F₁₃C₂H₄SO₃⁻ + ˙OH → C₆F₁₃CH₂˙CHSO₃⁻ + H₂O (9)

C₆F₁₃C₂H₄SO₃⁻ + ˙OH → C₆F₁₃˙CHCH₂SO₃⁻ + H₂O (10)

Previous studies have reported that ˙OH played a minor role in the degradation of PFOA by TiO₂- and In₂O₃-based photocatalysts.^33,34 This is because PFOA, a fully fluorinated compound, does not react with ˙OH to an appreciable extent. The second-order rate constant for the reaction between PFOA and ˙OH is thought to be k_˙OH < 10⁶ M⁻¹ s⁻¹, although the precise value is not available in the literature.⁵⁰ For 6:2 FtS, the second-order rate constant with ˙OH was estimated by density functional theory to be 1.0 × 10⁶ (reaction (9)) and 3.0 × 10⁶ M⁻¹ s⁻¹ (reaction (10)).⁵¹ It is generally agreed that contaminants with a k_˙OH < 10⁸ M⁻¹ s⁻¹ will not be removed appreciably by ˙OH-based processes, due to the ˙OH scavenging by HCO₃⁻ and other solutes. However, since CPW is acidic (pH < 3), the concentration of HCO₃⁻ in CPW should be very small (H₂CO₃ has a pK_a1 = 6.5). In case where the scavenging of ˙OH by other background solutes is significant, 6:2 FtS could still be degraded by h_VB⁺ (reactions (7) and (8)).

It has been well established that the irradiation of Ga₂O₃ by UVC generates h_VB⁺ and ˙OH. As mentioned previously, however, neither HCOOH nor t-BuOH had any appreciable effect on the oxidation of 6:2 FtS (Fig. 2a). The rate of 6:2 FtS removal remained the same even when the concentration of HCOOH was increased to 50 mM (Fig. S6†). At a first glance, these results seem to suggest that h_VB⁺ and ˙OH played a minimal role in the degradation of 6:2 FtS in the Ga₂O₃ system. This hypothesis appears to be further corroborated by the experiment in the presence of BrO₃⁻, which indicates that 6:2 FtS was degraded by reductants such as e_CB⁻ and/or superoxide radical:^42,52

e_CB⁻ + O₂ → O₂˙⁻ (11)

e_CB⁻ + 6:2 FtS → byproducts (12)

O₂˙⁻ + 6:2 FtS → byproducts (13)

e_CB⁻ + BrO₃⁻ → BrO₃˙⁻ (14)

However, if e_CB⁻ and O₂˙⁻ were the only reactive species involved in the degradation of 6:2 FtS, it would not have been possible to explain why the presence of HCOOH together with BrO₃⁻ further inhibited the degradation (Fig. 2a). In fact, the inhibitory effect of HCOOH + BrO₃⁻ suggests that the involvement of h_VB⁺ cannot be ruled out. To reconcile these seemingly conflicting experimental observations, the degradation of 6:2 FtS by the Ga₂O₃/UVC system is proposed to have occurred as follows. First, both h_VB⁺ and e_CB⁻ should be important to the degradation of 6:2 FtS. It is noted that the degradation of 6:2 FtS is determined not only by the rate at which h_VB⁺ and e_CB⁻ were produced, but also by the rate at which these species recombined. Thus, although the presence of HCOOH would on the one hand suppress the oxidation of 6:2 FtS, the scavenging of h_VB⁺ by HCOOH would on the other hand mitigate the recombination of h_VB⁺ and e_CB⁻, thereby extending the lifetime of the latter. As a result, while the removal of 6:2 FtS by reactions (7)–(10) would be retarded, the removal by reactions (12) and (13) would be enhanced, resulting in no net change in the removal rate. On the contrary, h_VB⁺ and e_CB⁻ would be scavenged when both HCOOH and BrO₃⁻ were present, resulting in the inhibition of reactions (7)–(13). It is further hypothesized that the involvement of ˙OH (i.e., reactions (9) and (10)) is likely minimal, since t-BuOH (which is not expected to affect the production or recombination of h_VB⁺ and e_CB⁻) did not influence the rate of 6:2 FtS degradation. Finally, it appears that O₂˙⁻ played a greater role than e_CB⁻ in the reductive degradation 6:2 FtS. This hypothesis is supported by two lines of evidence. Firstly, the 6:2 FtS degradation profile in the presence of HCOOH + BQ was similar to that in the presence of HCOOH + BrO₃⁻. (BQ only scavenges O₂˙⁻ but not e_CB⁻). Secondly, removing O₂ from the solution (by sparging N₂) also retarded the degradation to the same extent as BrO₃⁻ did (Fig. S7†).

4.2 The effect of Cr(VI) on 6:2 FtS removal

In agreement with the hypothesis that a synergistic effect may exist between Cr(VI) and 6:2 FtS, the 6:2 FtS degradation rate was enhanced in the Ga₂O₃ and In₂O₃ systems when Cr(VI) was present. In the In₂O₃ system, reaction (2) would mitigate the h_VB⁺/e_CB⁻ recombination, thereby making h_VB⁺ more available to H₂O—as discussed above, ˙OH, which is generated from the reaction between h_VB⁺ and H₂O (reaction (4)), is likely the primary species involved in the degradation of 6:2 FtS in the In₂O₃ system. In the Ga₂O₃ system, the presence of Cr(VI) can on the one hand enhance reactions (7) and (8) (by retarding the h_VB⁺/e_CB⁻ recombination), but on the other hand suppress reaction (11) (and thereby suppressing reaction (13)). However, it is hypothesized that the production of O₂˙⁻ by reaction (11) was not appreciably affected by Cr(VI), since the concentration of dissolved O₂ (0.25 mM) in the solution were over 400 times greater than that of Cr(VI) (30 μg L⁻¹, or 0.57 × 10⁻³ mM). (As the rate constants for reactions (2) and (11) are not known, the exact proportions of e_CB⁻ that react with O₂ and Cr(VI) cannot be estimated).

Unlike the other two photocatalysts, TiO₂ was not effective at removing 6:2 FtS when Cr(VI) was present (Fig. 1c). As has been established above, e_CB⁻ and HO₂˙ are not involved in the degradation of 6:2 FtS in the TiO₂ system. Therefore, the inhibitory effect of Cr(VI) cannot be ascribed to the scavenging of e_CB⁻ by reaction (2). Instead, the most plausible explanation could be that the reactivity of TiO₂ was severely reduced by the deposition of Cr(III) on the surface of TiO₂. Whereas the color of In₂O₃ and Ga₂O₃ did not change throughout the experiment, TiO₂ rapidly turned grey/black when photolysis started (Fig. S8†). Surface elemental analysis by EDS revealed that the surface of the spent TiO₂ catalyst consisted of 3.7 wt% of Cr (this number was 0.22 wt% and 0.44 wt% for the spent Ga₂O₃ and In₂O₃, respectively) (Fig. S9†). Therefore, it is hypothesized that the Cr(III) adsorbed on TiO₂ suppressed the degradation of 6:2 FtS by inhibiting the reactive sites on TiO₂ and/or competing with 6:2 FtS for h_VB⁺ (reaction (6)). A similar hypothesis was invoked in several studies to explain for the reactivity decrease of TiO₂.^53,54 In the Ga₂O₃ system, essentially all Cr remained in the solution by the end of the experiment (Fig. 3b), suggesting that the adsorption of Cr on Ga₂O₃ was minimal, which is consistent with SEM/EDS results (Fig. S9a†).

4.3 The transformation pathway of 6:2 FtS in the Ga₂O₃/UV system

According to the discussion above about the reactive species involved and the concentration–time profiles of the byproducts, the transformation of 6:2 FtS in the Ga₂O₃/UV system is proposed to take place as follows (Scheme 1). Upon reacting with h_VB⁺ (reactions (7) and (8)), 6:2 FtS is oxidized into intermediate radicals IM1a and IM1b. 6:2 FtS can also be reduced by O₂˙⁻ into IM2 (reaction (13))—this nucleophilic substitution reaction is thought to occur in a way similar to the dehalogenation of alkyl halides by O₂˙⁻.^42,55 Next, IM1a, IM1b and IM2 are oxidized into 6:2 FtCA. It is noted that the exact mechanism through which 6:2 FtCA is produced is unclear at this point. It is also noted that IM1a, IM1b could also have been converted into other intermediates that were not detected by the LCMS analytical method used in this study. For example, by employing suspected screening-high resolution mass spectrometry, Zhang et al.⁵¹ observed that sulfonate byproducts such as C₆F₁₃C₂H₂OSO₃⁻, C₆F₁₃C₂H₂OSO₃⁻, and C₆F₁₃C₂H₄O₂SO₃⁻ were produced when ˙OH reacted with 6:2 FtS (˙OH was generated using Co²⁺/peroxymonosulfate). Whether these sulfonate byproducts can be produced via reactions with h_VB⁺ remains to be explored. However, as was discussed earlier, approximately 20% of F was unaccounted for in the fluorine mass balance. This missing F fraction could be that associated with sulfonate intermediate products, as well as other short-chain fluorinated compounds such as trifluoroacetic acid.

Scheme 1 Proposed 6:2 FtS degradation pathway.

Most previous studies have proposed that the degradation of PFOA by h_VB⁺ and ˙OH takes place via a stepwise decarboxylation mechanism PFOA → PFHpA → PFHxA → PFPeA → PFBA.^56,57 This stepwise mechanism implies that the concentration of the preceding byproduct will rise and fall before the subsequent byproduct is generated. However, it was observed that PFHpA, PFHxA, PFPeA and PFBA were generated simultaneously, and that none of these byproduct exhibited a two-phase concentration–time profile (Fig. 2b). Therefore, we argue that PFHpA, PFHxA, PFPeA and PFBA were generated directly from 6:2 FtCA. Additional research is needed to investigate the mechanism through which this transformation occurs.

4.4 Proposed treatment train for the removal of 6:2 FtS and Cr(VI) in CPW

Based on the results of this study, it is proposed that Cr(VI) and 6:2 FtS in CPW can be removed using a treatment train that consists of an oxidation–reduction step driven by Ga₂O₃/UVC, followed by a neutralization step (Scheme 2). The neutralization step not only will convert dissolved Cr(III) into Cr(OH)_3(S), which can be separated (e.g., by filtration, centrifugation, or sedimentation), but also will raise the pH to the range that would comply with discharge regulations (typically pH = 6–8). If required, the effluent from this treatment system can be further polished using ion exchange or activated carbon to enhance the removal of Cr species as well as the fluorinated byproducts. Because conventional CPW treatment systems usually already consist of a neutralization step and a polishing step, existing systems can be readily upgraded by placing the Ga₂O₃/UVC unit in the front. Although existing systems with ion exchange or activated carbon could also remove 6:2 FtS, the upfront degradation by Ga₂O₃/UVC will significantly reduce the loading of fluorinated compounds on the polishing step. Recall that under the condition employed in this study, 50% of the initial organic fluorine was converted into F⁻. While F⁻ could be toxic to aquatic life,⁵⁸ its concentration in the treated CPW is not expected to exceed discharge limits (which is typically above 1 mg L⁻¹),^59,60 since raw CPW only contains up to a few hundreds μg L⁻¹ of 6:2 FtS.²²

Scheme 2 Process flow chart for CPW treatment including the oxidation–reduction step using Ga₂O₃/UVC system.

5 Conclusions

Heterogeneous photocatalysis is a promising approach for the treatment of complex waste streams, owing to its ability to generate several types of oxidative and reductive species, namely h_VB⁺, ˙OH, e_CB⁻, and O₂˙⁻. This study showed that the two commercial wide-band gap photocatalysts In₂O₃ and Ga₂O₃ were capable of simultaneously removing Cr(VI) and 6:2 FtS in chromate plating wastewater. With a significantly higher reactivity, Ga₂O₃ appeared to be superior to In₂O₃. Through a detailed investigation of how different probe compounds influence 6:2 FtS removal, h_VB⁺ and O₂˙⁻ were identified to be the primary reactive species involved in the 6:2 FtS degradation in the Ga₂O₃/UVC system. It was also found that there was a synergistic effect between Cr(VI) and 6:2 FtS. The faster degradation of 6:2 FtS in the presence of Cr(VI) was attributable to the scavenging of e_CB⁻ by Cr(VI), which prevented the recombination h_VB⁺ and e_CB⁻.

While this study has successfully demonstrated how Cr(VI) and 6:2 FtS can be simultaneously removed using photocatalysis, additional research is needed to improve the efficiency of this approach. For example, a recent study reported that In-doped Ga₂O₃ is significantly more reactive than Ga₂O₃. Future research should explore if In-doped Ga₂O₃ as well as other highly reactive catalysts (e.g., BN) can be employed for the treatment of CPW. Future studies should also employ real CPW samples to explore how factors such as the catalyst dosage, the solution pH, the concentration of Cr(VI) and 6:2 FtS, and the presence of other contaminants typically present in CPW (e.g., heavy metals) may affect the treatment efficacy. Additionally, future research should investigate if photocatalysis can be used to treat other types of PFAS used in electrochemical plating, such as PFOS and F-53B (i.e., ClC₆F₁₂OCF₂CF₂SO₃⁻).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Funding for this research was provided by Natural Sciences and Engineering Research Council of Canada (Discovery Grant #2015-04850), and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF-2021R1A6A3A03044333).

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Footnote

† Electronic supplementary information (ESI) available: Text S1, Tables S1–S3 and Fig. S1–S9. See DOI: 10.1039/d1ra06235b

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