https://orcid.org/0000-0003-3108-8271
ABSTRACT
Hexafluoropropylene oxide trimer acid (HFPO-TA) is an emerging alternative to traditional perfluoroalkyl substances (PFASs), which is characterized by its biotoxicity and persistence. The UV/sulfite/iodide photo-induced hydrated electrons system can effectively degrade HFPO-TA under mild conditions. However, the effects of water quality on this system need to be urgently investigated. This study explored the impact of common aqueous constituents, such as Cl-, HCO3-, PO43- and humic acid (HA) on the defluorination efficiency of HFPO-TA by the UV/sulfite/iodide system. Results indicated that low concentrations of Cl- (<1.0 mM), PO43- (<0.01 mM), and HA (<1.0 mg/L) have little effect on defluorination efficiency. However, as concentrations increase, these constituents can interact with photosensitizers or reactive species within the system, leading to a decrease in defluorination efficiency. HCO3-, in their various solution states, can compete with HFPO-TA for the hydrated electron (eaq-) or engage directly with the photosensitizer, resulting in a hindrance to the defluorination capabilities of the system. Furthermore, it was identified that the components in Xiaoqing River, especially Cl- and HCO3-, could greatly inhibit the defluorination and degradation efficiency of HFPO-TA by the system. Pretreatment such as nanofiltration would effectively mitigate this problem.
HIGHLIGHTS
The study emphasizes the significance of water quality on the defluorination efficiency of the UV/sulfite/iodide system.
It examines how common aqueous constituents affect the defluorination of HFPO-TA.
The defluorination efficiency was reduced in Xiaoqing River due to Cl− and HCO3− interference.
Effective pretreatment methods can mitigate these negative impacts on defluorination.
INTRODUCTION
Hexafluoropropylene oxide trimer acid (HFPO-TA) is a novel type of perfluoroalkyl substances (PFASs) that have been introduced as an alternative to conventional perfluorinated compounds. Due to the unique physicochemical properties of HFPO-TA (such as hydrophobicity, oleophobicity, and flame retardancy), it has found extensive application across various industries over the past few decades (Dixit et al. 2021). However, studies have demonstrated that these emerging perfluorinated compounds also exhibit biotoxicity and bioaccumulation (Cui et al. 2018).
Meanwhile, HFPO-TA is now widely distributed in the aqueous environment, such as surface water, groundwater, and wastewater treatment plants around the world (Pan et al. 2018). Pan et al. (2018) reported that HFPO was detected in almost all samples from a survey of surface water in South Korea, the United States, and European countries, with detection rates of 96 and 83% for HFPO-DA and HFPO-TA, respectively. And HFPO-TA is frequently found in groundwater near fluorine chemical plants, with concentrations ranging from a few to several thousand ng/L (Feng et al. 2020; Ding et al. 2022). Even more, HFPO-TA has been detected in drinking water at low concentrations (Sun et al. 2016; Feng et al. 2021). High concentrations of HFPO-TA were detected downstream (5,200–68,500 ng/L, which is approximately 120–1,600 times greater than the background levels measured upstream) of Xiaoqing River near a fluoropolymer production plant in Shandong Province, China (Pan et al. 2017). Additionally, elevated concentrations of HFPO-TA were also detected in crucian carp (blood: 1,510 ng/mL, liver: 587 ng/g, muscle: 118 ng/g ww) caught downstream, and HFPO-TA was even detected in the serum of local residents (2.93 ng/mL) (Pan et al. 2017). In summary, the widespread presence of HFPO-TA raises concerns about its potential health risks to the environment and human health. Consequently, a comprehensive knowledge of methods to degrade HFPO-TA in natural aqueous environments is crucial for its safe application.
HFPO-TA can be effectively removed through physical methods such as adsorption, nanofiltration, and reverse osmosis (Bao et al. 2020; Wang et al. 2022). But this is not a permanent solution as it only involves phase transfer and does not degrade HFPO-TA to render them harmless. Whereas, the high thermodynamic and chemical stability of C–F bonds (∼116 kcal/mol) makes the efficient degradation of HFPO-TA very challenging, which has prompted extensive research in this field (Giesy & Kannan 2002; Bentel et al. 2019). Advanced oxidation methods such as photochemical oxidation (Shang et al. 2018) and electrochemical oxidation (Zhuo et al. 2012) have been widely applied by researchers to degrade PFASs, but studies have found that HFPOs exhibit higher resistance to oxidation than conventional PFASs. When oxidative methods are used to degrade HFPOs, the degradation products usually remain in HFPO-DA, which has strong antioxidant properties and high biological toxicity (Wang et al. 2015).
) advanced reduction method has attracted attention for its ability to degrade HFPO-TA under mild conditions. The C–F bond of HFPO-TA can be reductively converted into C–H by 
(Equation (1)), which can be generated from ultraviolet (UV) irradiation of I− (Equation (2)) or 
(Equation (3)) with a quantum yield at 0.22–0.29 and 0.11–0.14, respectively (254 nm) (Sauer et al. 2004; Li et al. 2012; Bentel et al. 2020). Due to the high molar absorption coefficient and quantum yield of I−, the UV/iodide system can efficiently degrade HFPO-TA at low concentrations of I− (O'Connor et al. 2023). However, due to the quenching effect of dissolved oxygen (Equation (4)) and the reactive iodide species (RIS) generated by I− (Equation (5)), such as I2, 
, 
, on 
, the defluorination efficiency of the UV/iodide system is often lower than expected (Park et al. 2009; Park et al. 2011; Zekun et al. 2022). The UV/sulfite system typically achieves a high level of contaminant degradation only when the concentration of 
is relatively high and the system pH is alkaline (Yu et al. 2018). The UV/sulfite/iodide composite system was used to overcome the limitations of a single system by colleagues in our group (Zhai et al. 2024). By exploiting the advantages of the high hydrated electron yield of I− and the self-cleaning ability of 
toward oxidative species (Equations (6) and (7)), efficient degradation of HFPO-TA under milder conditions was achieved.
However, the quality of different water bodies varies considerably. Some common constituents in a natural aqueous environment, such as Cl−, 
, 
, and natural organic matter (NOM), may react with active species in the system and even compete with HFPO-TA for 
. The water quality adaptability of the UV/sulfite/iodide system for degrading HFPO-TA needs to be explored. In addition, HFPO-TA is present at trace concentrations in contaminated water and usually needs to be enriched before degradation (Liu et al. 2022). Consequently, it is advisable to implement specific pretreatment procedures tailored to the coexistent constituents when utilizing this photochemical system for the degradation of HFPO-TA in natural water bodies. Nanofiltration, with pore sizes ranging from 1 to 10 nm, can effectively retain HFPO-TA when used as a pretreatment method (Bao et al. 2020).
This study aims to (1) investigate the primary coexistences influencing system performance in the aqueous environment; (2) evaluate the water quality adaptability of the UV/sulfite/iodide system by assessing its efficacy in decomposing HFPO-TA within water samples from Xiaoqing River; (3) provide guidance for the application of the UV/sulfite/KI photochemical system for the degradation of HFPO-TA in actual aqueous environment.
MATERIALS AND METHODS
Materials
HFPO-TA (C9HF17O4, >95%) was purchased from Weng Jiang Reagent Co., sodium sulfite (Na2SO3, AR), potassium iodide (KI, AR), sodium hydroxide (NaOH, >96%), hydrochloric acid (HCl, 36.0 ∼ 38.0%), sodium chloride (NaCl, AR), potassium nitrate (KNO3, AR), anhydrous sodium carbonate (Na2CO3, AR), and potassium dihydrogen phosphate (KH2PO4, AR) were purchased from Sinopharm Chemical Reagent Co., and humic acid (HA, AR) was purchased from Sigma-Aldrich. Nanofiltration membranes (polycarbonate (PCTE) 0.01 μm) purchased from GVS Filter Technology. Surface water samples were collected from Xiaoqing River, located in Huantai County, Shandong Province, at geographic coordinates 37°11′N and 118°14′E.
Photochemical reaction
All the degradation experiments were performed in a sealed photoreactor with jacketed water cooling (25 °C), which was placed on a magnetic stirrer. A 10 μM HFPO-TA solution (700 mL) containing 3 mM Na2SO3 and 0.3 mM KI with the preset concentrations of impact factors and solution pH (pH = 7.5, adjusted by 1 M H2SO4 and 0.25 M NaOH) was irradiated by a 16 W low-pressure mercury lamp (TUV 16 W T5, 254 nm beam, Philips) with a quartz sleeve.
Pretreatment experiments
Xiaoqing River water (100 L) was pre-treated by nanofiltration (PCTE, 0.01 μm) in seven passes. After completion of nanofiltration, the concentrate and retained material were collected, and the nanofiltration membrane was rinsed using 100 mL of deionized water. The concentrate and rinse solution were combined to obtain the pretreatment solution (approximately 700 mL). The concentration of HFPO-TA in the leachate and pretreatment solution was examined. A portion of the pretreatment solution was diluted to 700 mL to achieve a concentration of 10 μM HFPO-TA for subsequent photochemical experiments.
Water sample analysis
HFPO-TA and its degradation products were determined by high-performance liquid chromatography with triple quadrupole mass spectrometry (HPLC-MS/MS, Thermo Fisher Scientific TSQ Quantum). The liquid chromatography column used Agilent ZORBAX Eclipse Plus C18 (2.1 × 150 mm). The fluoride ion (F−) and other inorganic anions were measured by ion chromatography (Dionex ICS 3000, USA). The defluorination percentage (DeF %) is defined as the molar ratio between the released F− in solution and the total F in the parent HFPO-TA compounds. The concentrations of HA and the absorbance of each solution were measured by ultraviolet-visible spectrophotometer (UV-4802, UNICO).
RESULT AND DISCUSSION
Effect of co-existing water medium
Effect of Cl− on UV/sulfite/iodide system
Effect of Cl− on defluorination efficiency of HFPO-TA.
at a relatively low rate constant (k < 1.0 × 106 M−1s−1) (Buxton et al. 2009; Yu et al. 2021); (ii) according to Equation (8), Cl− could scavenge hydroxyl radicals (·OH) with a high reaction rate constant (k = 4.3 × 109 M−1s−1) which could scavenge 
(Equation (11)) (Sanchez-Polo et al. 2009). However, the degradation efficiency of the system was inhibited when the concentrations of Cl− were greater than 3.0 mM. This can be ascribed to its ability to generate some oxidative radicals, such as 
and 
(Equations (8) and (9)), which impede the breakdown of pollutants by 
(Han et al. 2021; Zhang et al. 2022). Nevertheless, Zhou et al. (2024) showed that the presence of 10 mM Cl− had no effect on As removal by UV/sulfite/iodide system at pH 9.0. Moreover, Ren et al. (2021) demonstrated that the presence of 50–150 mM chloride had no significant impact on the defluorination and degradation performance of the UV/sulfite system in treating perfluorooctanoic acid (PFOA) at pH 10.0. The systems in the references are under strong alkaline conditions (pH ≥ 9.0) or with high concentrations of sulfite (10.0 mM). Since high concentrations of sulfite would maintain the strong reductive conditions of the UV/sulfite/iodide system, the generation of chlorine radicals would be greatly inhibited (Ren et al. 2021). In addition, 
exhibits a longer lifespan in alkaline conditions (Gu et al. 2020). Therefore, the inhibitory effect of Cl− on the degradation effect of these systems is reduced compared to the system in this paper. In natural water, the concentration of Cl− is typically between 0 and 3 mM (Han et al. 2021). Consequently, it is reasonable to infer that Cl− would inhibit the defluorination and degradation of HFPO-TA in natural water.
Effect of HCO3- on UV/sulfite/iodide system
) is an essential constituent of water's alkalinity, commonly found in natural water bodies at significant concentrations that typically range from 0.4 to 4.0 mM (Ji et al. 2013). Furthermore, 
significantly impacts the degradation process of HFPO-TA within the UV/sulfite/iodide system, as illustrated in Figure 2. In the absence of 
, the HFPO-TA defluorination achieved 48.79% after 8 h of irradiation. However, with the addition of 
, the defluorination of HFPO-TA is progressively reduced to 35.33% at 0.1 mM, 26.36% at 0.5 mM, 24.92% at 1.0 mM, and notably to 10.16% at 3.0 mM 
. These data clearly demonstrate a concentration-dependent inhibitory effect of bicarbonate on the defluorination process of HFPO-TA. 
Effect of 
on defluorination efficiency of HFPO-TA.
on HFPO-TA degradation, it is essential to understand its distribution in an aqueous medium first. Given that H2CO3 is a weak dibasic acid, it will ionize in aqueous solution. The speciation of 
in an aqueous solution at 25 °C was modeled by using the Visual MINTEQ software (Figure 3). In natural aqueous environments, pH typically ranges from 6.5 to 8.5 (Kremleva & Moiseenko 2017; Raven et al. 2020; Seminskaya et al. 2020). It is reported that both 
and 
can react with 
with rates between 105 and 106 M−1·s−1 (Equations (10) and (11)) (Hart 1964; Yang et al. 2020). Moreover, the reaction rate of 
with H2CO3 (2.2 × 109 M−1S−1) is close to the diffusion-limited rates (Equation (12)) (Schwarz 1992; Amador et al. 2023). With changes in pH, the dissolved forms of [
] will shift, leading to variations in the reaction rate of 
with [
]. At pH 7.5, the proportions of [
] components are 93.44% 
, 6.41% H2CO3, 0.15% 
, and the reaction rate of [
] with 
is approximately 1 × 108 M−1S−1 (Amador et al. 2023). 
Speciation of 
as a function of pH at 25°C predicted by Visual MINTEQ.
generated by the reaction of 
and 
can compete with active iodine species for 
, affecting the cycle of I− (Equation (13)) (Neta & Huie 1985). Furthermore, 
can directly react with I− (Equation (14)) (Neta et al. 2009). Therefore, despite the relatively low reaction constants of 
with 
, it still has a significant effect on the degradation efficiency of HFPO-TA in natural aqueous environment by the UV/sulfite/iodide system.
Effect of PO43- on UV/sulfite/iodide system
) on the degradation of HFPO-TA by the UV/sulfite/iodide system is significant, as illustrated in Figure 4. Upon adding varying concentrations of 
(0.1, 0.5, 1.0, and 3.0 mM) to the system, the defluorination of HFPO-TA was observed to decrease progressively. After 8 h of UV irradiation, the defluorinations were reduced to 41.89, 35.67, 31.51, and 16.04%, respectively, from an initial of 48.79%. This indicates that 
(>0.1 mM) has a substantial inhibitory effect on the defluorination and degradation of HFPO-TA. 
Effect of 
on defluorination efficiency of HFPO-TA.
, 
, and 
. Within the pH range of 6.5 to 8.5, the solution is primarily composed of the dihydrogen phosphate ion (
) and the hydrogen phosphate ion (
), as depicted in Figure 5. Both 
and 
are capable of reacting with the hydrated electron (Yang et al. 2020). The reactivity of 
with 
is relatively low, with a rate constant of 1.4 × 105 M−1·s−1 (Equation (15)) (Buxton et al. 2009). In contrast, the reactivity of 
with 
is significantly higher, with a rate constant of 1.9 × 107 M−1·s−1 (Equation (16)) (Buxton et al. 2009). However, the concentration of 
in natural waters is relatively low, typically less than 0.01 mM (Hou et al. 2023). Consequently, 
is not likely to inhibit the degradation of HFPO-TA in natural waters by UV/sulfite/iodide system.
Speciation of 
as a function of pH at 25°C predicted by Visual MINTEQ.
Effect of HA on UV/sulfite/iodide system
NOM is prevalent across diverse aqueous environments, with humic acid (HA) being a primary constituent. HA is characterized by an array of functional groups, including carboxylic acids, hydroxyls, and aromatic hydrocarbons, endowing it with robust complexation and adsorption properties that significantly contribute to the migration and transformation processes of organic substances (Murphy et al. 2002). And the concentration of HA in natural water, soils, and sediments is typically measured in milligrams per liter (Sun et al. 2017). Consequently, investigating the effect of HA on the degradation of HFPO-TA by the UV/sulfite/iodide system is important.
Effect of HA on defluorination efficiency of HFPO-TA (pH 7.5).
Similarly, recent research suggested that low levels of HA may accelerate the degradation of PFASs (Cui et al. 2020). Sun et al. (2017) proposed that the aromatic ring in HA could react with I2 to form a π-complex, preventing the quenching of 
by oxidants like I2. Additionally, the electron-donating groups in HA are capable of transferring electrons, transforming the π-complex and reducing I2 to I−, thus expediting the iodine cycle (Sun et al. 2017). The process of generating 
in the UV/sulfite system is similar to that in the UV/iodide system, and the transformation of 
within the system is analogous to the cycle of iodide (Ren et al. 2021). In the UV/sulfite system, a low concentration of HA can also act as an electron shuttle, reducing 
to 
and 
, thereby accelerating the formation of 
(Yanghai et al. 2022). Yu et al. observed that with the rise in pH levels, the facilitative influence of HA on the reduction process intensifies, whereas its suppressive effect diminishes (Yanghai et al. 2022). And the presence of phenol and carboxyl groups within the HA could be responsible for the observed phenomenon (Ren et al. 2021). Compounds like phenol and benzoic acid are known to generate 
by UV radiation, especially in alkaline conditions (Sharpless & Blough 2014; Gu et al. 2020).
, 
, and I− for photons, leading to a diminished production of 
within the system (Cui et al. 2020). Second, HA is rich in electron-withdrawing functional groups, such as carboxyl groups (-COOH), which are known to react readily with 
. The high reactivity of HA with 
could result in a depletion of the reactive electron, leading to a less availability of 
for the reaction with HFPO-TA. Third, the decomposition and transformation of HA by UV light could produce oxidizing agents like hydroxyl radicals (·OH) and hydrogen peroxide (H2O2), which have the potential to oxidize 
, thereby diminishing its concentration within the system (Cui et al. 2020). Overall, the inhibitory effect of HA on the defluorination of HFPO-TA is relatively mild. 
UV spectra of experiments with 1.0 mg/L HA.
The degradation efficiency of HFPO-TA by UV/sulfite/iodide system in the samples from Xiaoqing river
The defluorination (a) and degradation (b) of HFPO-TA in Xiaoqing River. The concentration of HFPO-TA in Xiaoqing River was 0.12 μM, which was adjusted to 10 μM using standards.
The defluorination (a) and degradation (b) of HFPO-TA in Xiaoqing River. The concentration of HFPO-TA in Xiaoqing River was 0.12 μM, which was adjusted to 10 μM using standards.
The influence of aqueous constituents such as Cl−, 
, 
, and HA on the degradation efficiency of HFPO-TA is discussed in Section 3.1. To investigate the primary factors causing the diminished efficacy of UV/sulfite/iodide system in Xiaoqing River, key water quality parameters were analyzed. The water quality of Xiaoqing River is shown in Table 1. It was found that Cl− and 
are the predominant components in these waters, with average concentrations of 2.03 and 2.16 mM in Xiaoqing River.
Water quality parameters of water samples from Xiaoqing River
| Water | pH | DO (mM)a | Cl− (mM) |  (mM) |  (10−3 mM) | HA (mg/L) |
|---|---|---|---|---|---|---|
| Deionized water | 6.16 | 0.31 | Not detected | Not detected | Not detected | Not detected |
| Xiaoqing River | 7.39 ± 0.13 | 0.33 ± 0.05 | 2.03 ± 0.42 | 2.16 ± 0.29 | 0.74 ± 0.09 | 0.83 ± 0.11 |
| Water | pH | DO (mM)a | Cl− (mM) | (mM) | (10−3 mM) | HA (mg/L) |
|---|---|---|---|---|---|---|
| Deionized water | 6.16 | 0.31 | Not detected | Not detected | Not detected | Not detected |
| Xiaoqing River | 7.39 ± 0.13 | 0.33 ± 0.05 | 2.03 ± 0.42 | 2.16 ± 0.29 | 0.74 ± 0.09 | 0.83 ± 0.11 |
aDissolved oxygen, DO.
significantly affects the defluorination of HFPO-TA. Therefore, it can be inferred that the presence of large amounts of Cl− and 
in the Xiaoqing River would greatly reduce the defluorination and degradation of HFPO-TA by the UV/sulfite/iodide system. To test this hypothesis, deionized water was supplemented with Cl− and 
at concentrations of 2.0 and 2.16 mM, respectively, mirroring that in Xiaoqing River, to assess its impact on the defluorination of HFPO-TA. As shown in Figure 9, the degradation effect of HFPO-TA in deionized water with the added Cl− and 
is comparable to that in the samples. Given that the concentrations of constitutions like 
and HA are comparatively minimal, their impact on the defluorination of HFPO-TA is negligible. This substantiates the initial hypothesis: the high concentrations of Cl⁻ and 
in the Xiaoqing River would significantly decrease the defluorination and degradation of HFPO-TA by the system. It is noteworthy that the defluorination efficacy of HFPO-TA in Xiaoqing River is marginally lower than in deionized water containing an equivalent concentration of Cl− and 
. The observed discrepancy likely arises from the complex composition of surface water, wherein various trace elements may exert an influence on the efficacy of the system. 
Effect of Cl− (2.03 mM) 
(2.16 mM) on the system in deionized water.
The amount of substance of HFPO-TA (nHFPO-TA) in each solution during the pretreatment process
| Solution | Water sample | Leachate | Pretreatment solution |
|---|---|---|---|
| nHFPO-TA (μM) | 12.00 | 1.40 | 9.98 |
| Solution | Water sample | Leachate | Pretreatment solution |
|---|---|---|---|
| nHFPO-TA (μM) | 12.00 | 1.40 | 9.98 |
The defluorination and degradation efficiency of HFPO-TA after pretreatment.
CONCLUSIONS
This study revealed the adaptability of the UV/sulfite/iodide system to varying water quality conditions. It was observed that low concentrations of Cl− (0 ∼ 1.0 mM), 
(0 ∼ 0.1 mM), and HA (0 ∼ 1.0 mg/L) had relatively small influence on the defluorination performance of the system. The reaction rate between Cl− and 
is comparatively low, which means that only at elevated concentrations (above 3.0 mM) does it markedly impact the system. Although the reaction rate of 
with 
is relatively high, the overall concentration of [
] in the aqueous environment is typically low, which limits its impact on the effects of the system. HA can act as an electron shuttle, promoting the cycling of I− and 
to generate additional 
, thereby facilitating the defluorination of HFPO-TA at low concentrations (<0.3 mg/L). However, owing to the UV shielding property of HA, the interactions of the electron-withdrawing groups of HA and the oxidants formed during HA decomposition with 
, the addition of high concentrations of HA (>0.5 mg/L) leads to a decline in the defluorination efficiency of HFPO-TA. Given that the high reaction rate of H2CO3 with 
, and 
which is the reaction product of 
with 
, reacts with 
and I−, 
has a particularly pronounced inhibitory effect on the defluorination of HFPO-TA. The presence of high concentrations of Cl− and 
in the Xiaoqing River water samples could reduce the defluorination and degradation of HFPO-TA by the UV/sulfite/iodide system compared to deionized water. Pretreatment through adsorption/desorption or nanofiltration using highly selective materials such as COFs can selectively concentrate HFPO-TA within the water bodies, significantly reducing interference from co-existing substances during the subsequent degradation phase, which is critical for effective treatment of HFPO-TA.
ACKNOWLEDGEMENTS
This study has been supported by the National Natural Science Foundation of China (Project No. 21976135).
DATA AVAILABILITY STATEMENT
All relevant dates are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.
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