Sunlight-assisted degradation of perfluorooctanoic acid (PFOA) in the presence of various silicate clay minerals = 다양한 규산염 점토광물에 따른 태양광 기반 과불화옥탄산 (PFOA) 분해 연구

다국어 초록 (Multilingual Abstract)

Per- and polyfluoroalkyl substances (PFAS) are a class of persistent organic contaminants widely detected in aquatic environments due to their extensive industrial use, high chemical stability, and resistance to conventional treatment processes. Among them, perfluorooctanoic acid (PFOA) has attracted particular concern because of its mobility, bioaccumulation potential, and documented adverse health effects. While numerous treatment technologies have been proposed for PFAS removal, most currently applied approaches rely on phase separation rather than irreversible destruction, leading to secondary waste generation and long-term management challenges. Destruction-based treatment processes, including photodegradation, have therefore received increasing attention; however, most reported photodegradation studies employ artificial high-energy UV irradiation, engineered catalysts, or aggressive reaction conditions that deviate substantially from natural environmental settings.
In this study, the phototransformation and defluorination behavior of PFOA were investigated under environmentally simulated conditions, using simulated sunlight, neutral pH, and pristine silicate clay minerals without chemical modification or external energy input. Three representative clay minerals-kaolinite, montmorillonite, and hectorite-were selected to evaluate how mineral structure influences PFOA transformation. Batch experiments were conducted under both dark and simulated solar light irradiation, and PFOA loss, formation of shorter-chain perfluorocarboxylic acids (PFCAs), and fluoride ion release were quantified. To distinguish between adsorption and true chemical transformation, extraction experiments were performed on reacted clay minerals, and mass balance analyses were conducted for total PFCAs and fluoride.
Results demonstrated that PFOA loss under dark conditions in the presence of kaolinite and montmorillonite was primarily governed by adsorption, with negligible defluorination and no detectable formation of shorter-chain PFCAs. Under simulated solar light irradiation, both clay minerals facilitated partial photodegradation of PFOA, yielding PFHpA and PFHxA as intermediates and enhanced fluoride release, indicating that adsorption, degradation, and defluorination occurred concurrently. In contrast, hectorite exhibited fundamentally different behavior. In the presence of hectorite, significant defluorination (approximately 30%) was observed under both dark and illuminated conditions, despite the absence of detectable shorter-chain PFCAs under dark conditions. Under simulated solar light irradiation, hectorite promoted successive chain-shortening of PFOA down to PFBA, accompanied by the highest defluorination ratios among the tested clay minerals.
Extraction and mass balance analyses revealed that PFOA and PFHpA were preferentially adsorbed onto clay mineral surfaces, while shorter-chain PFCAs such as PFHxA, PFPeA, and PFBA remained predominantly in the aqueous phase. Fluoride ion adsorption onto all clay minerals was negligible. These findings indicate that adsorption plays a critical role during PFOA phototransformation and that adsorption and degradation are not independent processes but rather proceed in a coupled manner under simulated solar light irradiation.
The distinct behavior observed for hectorite suggests that intrinsic structural characteristics, including its trioctahedral configuration and lithium substitution within the octahedral sheet, may facilitate defluorination pathways that are less dependent on light irradiation. Although the specific reactive species and reaction pathways were not directly identified in this study, the results provide indirect evidence that naturally abundant clay minerals can act not only as passive sorbents but also as reactive media contributing to PFOA transformation under environmentally relevant conditions.
Overall, this study demonstrates the potential of clay minerals, particularly hectorite, as components of sunlight-assisted, low-energy, and environmentally relevant destruction-based strategies for PFOA. The findings advance understanding of PFAS behavior at mineral–water interfaces and highlight the importance of considering adsorption–transformation coupling when evaluating natural attenuation and treatment processes in real environmental systems.

국문 초록 (Abstract)

과불화 및 폴리플루오로알킬물질(PFAS)은 높은 화학적 안정성과 난분해성으로 인해 수계 전반에 널리 검출되는 대표적인 잔류성 유기오염물질이다. 이 중 과불화옥탄산 (PFOA)은 높은 수용성, 이동성, 생물농축 가능성 및 다양한 독성 영향으로 인해 전 세계적으로 주요 관리 대상 물질로 인식되고 있다. 현재까지 PFAS 처리 기술의 상당수는 흡착이나 분리 기반 공정에 의존하고 있으며, 이는 오염물질을 다른 상으로 이동시킬 뿐 근본적인 분해를 달성하지 못해 2차 폐기물 발생 및 장기 관리 문제를 야기한다. 이에 따라 PFOA의 비가역적 분해와 탈불소화를 목표로 하는 파괴 기반 처리 기술에 대한 관심이 증가하고 있으나, 기존 광분해 연구의 대부분은 고에너지 자외선, 개질된 촉매, 또는 비환경적인 반응 조건을 사용하여 실제 자연환경을 충분히 반영하지 못하고 있다.
본 연구에서는 중성 pH 조건에서 태양광을 모사한 광원과 개질되지 않은 천연 규산염 점토광물을 이용하여, 환경적으로 모사된 조건에서 PFOA의 광변환 및 탈불소화 거동을 규명하고자 하였다. 대표적인 점토광물인 kaolinite, montmorillonite, hectorite를 대상으로 암조건과 태양광 조사 조건에서 배치 실험을 수행하였으며, PFOA 제거율 (PFOA loss), shorter-chain PFCAs 생성, 불소 이온 방출을 정량적으로 분석하였다. 또한 PFOA 제거가 단순 흡착에 의한 것인지 실제 화학적 분해에 의한 것인지를 구분하기 위해 반응 종료 후 점토광물에 흡착된 PFAS를 용매로 탈착 후 추출하여 분석하였다.
그 결과, kaolinite와 montmorillonite 존재 하의 암조건에서는 PFOA 제거가 주로 흡착에 의해 발생하였으며, shorter-chain PFCAs 생성 및 탈불소화는 거의 관찰되지 않았다. 반면 태양광 조사 조건에서는 두 점토광물 모두에서 PFHpA 및 PFHxA가 생성되었고, 탈불소화율 또한 암조건 대비 유의미하게 증가하여 흡착, 분해, 탈불소화가 동시에 진행됨을 확인하였다. 이에 비해 hectorite는 다른 점토광물과 뚜렷이 구별되는 거동을 보였다. Hectorite 존재 하에서는 암조건과 태양광 조사 조건 모두에서 약 30% 수준의 높은 탈불소화가 관찰되었으며, 태양광 조사 조건에서는 PFBA까지 연속적인 탄소 사슬 단축이 확인되었다. 특히 암조건에서는 탈불소화가 발생함에도 불구하고 shorter-chain PFCAs가 검출되지 않아, 암조건에서의 PFOA 변환 경로가 태양광 조사 조건과 상이할 가능성을 시사하였다.
반응 종료 후 점토광물에 흡착한 PFAS를 용매로 추출한 결과, PFOA와 PFHpA는 점토광물에 일부 흡착되었으며, PFHxA 이하의 shorter-chain PFCAs는 주로 수용액 상에 존재하였다. 불소 이온의 점토광물 흡착은 모든 조건에서 유의미하지 않은 수준이었다. 이러한 결과는 PFOA의 광변환 과정에서 흡착이 중요한 역할을 수행하며, 흡착과 분해가 서로 독립적인 과정이 아니라 상호 연계되어 함께 진행됨을 의미한다.
본 연구는 개질되지 않은 천연 점토광물이 단순 흡착체를 넘어 환경적으로 의미 있는 조건에서 PFOA의 부분적 분해 및 탈불소화를 유도할 수 있음을 보여준다. 특히 hectorite의 경우, 구조적 특성에 의해 빛에 의존하지 않는 탈불소화 경로가 작동할 가능성을 제시하였다. 이러한 결과는 자연계에서의 PFOA 분해 가능성을 확장하고, 낮은 에너지로 환경친화적인 PFOA 처리 기술 및 자연 저감(natural attenuation)의 가능성 등 중요한 시사점을 제공한다.

목차 (Table of Contents)

• Abstract i
• Table of Contents v
• List of Figures vii
• List of Tables x
• CHAPTER 1. Introduction 1
• 1.1. Background 1
• 1.2. Objectives of the study 6
• 1.3. Scope of the study 7
• CHAPTER 2. Literature review 8
• 2.1. Ongoing PFAS treatment strategies 8
• 2.1.1. PFAS treatment based on separation or destruction 8
• 2.1.2. Destruction-based treatment: Photodegradation 10
• 2.1.3. Photodegradation: Necessity of sunlight assisted approaches 11
• 2.2. Clay minerals 14
• 2.2.1. Clay minerals as a natural based green-chemical approaches 14
• 2.2.2. Clay minerals: Kaolinite 15
• 2.2.3. Clay minerals: Montmorillonite 15
• 2.2.4. Clay minerals: Hectorite 16
• 2.3. Photochemical and radical-mediated degradation of PFOA 17
• 2.3.1. Previous photodegradation approaches 17
• 2.3.2. PFOA degradation by superoxide 19
• 2.3.3. PFOA degradation by hydroxyl radicals 20
• 2.3.4. Production of superoxide and hydroxyl radical 21
• CHAPTER 3. Materials and methods 24
• 3.1. Preparation of clay minerals 24
• 3.2. PFOA phototransformation in the presence of clay minerals 25
• 3.3. Extraction of PFOA and intermediates from reacted clay minerals 28
• 3.4. Material characterization 29
• 3.5. Analysis and quantification 29
• CHAPTER 4. Results and Discussions 31
• 4.1. Characterization of the clay minerals 31
• 4.2. Photodegradation of PFOA in the presence of clay minerals 33
• 4.2.1. Kinetics and phototransformation of PFOA in the presence of kaolinite 33
• 4.2.2. Kinetics and phototransformation of PFOA in the presence of montmorillonite 36
• 4.2.3. Kinetics and phototransformation of PFOA in the presence of hectorite 39
• 4.2.4. Fluoride ion release from PFOA 43
• 4.3. PFAS and fluoride extraction from reacted clay minerals 48
• 4.3.1. Extraction from reacted kaolinite 48
• 4.3.2. Extraction from reacted montmorillonite 51
• 4.3.3. Extraction from reacted hectorite 54
• 4.3.4. Characterization by TEM-EDS on reacted clay minerals 57
• 4.4. Distinct PFOA transformation in the presence of hectorite 63
• CHAPTER 5. Conclusion 66
• References 69
• 초록 80