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).
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 HCO3- on UV/sulfite/iodide system
Effect of PO43- on UV/sulfite/iodide system
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.
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).
The degradation efficiency of HFPO-TA by UV/sulfite/iodide system in the samples from Xiaoqing river
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 . | 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.
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 |
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.