Formation of Regenerative Reducing Zone with Structural Fe-bearing Clay Minerals for Electron-mediated Hexavalent Chromium Immobilization in Groundwater = 구조철 함유 점토광물을 이용한 지하수 내 재생가능한 환원 구역 생성과 전자매개 기반 6가 크롬의 고정화

국문 초록 (Abstract)

다양한 산업활동에서 배출되는 6가 크롬은 높은 용해도와 이동성으로 인해 넓은 범위로 확산되어 지하수 오염을 유발하며, 발암성을 포함한 독성으로 인체 건강에 심각한 위해를 초래한다. 6가 크롬은 환원 과정을 거쳐 상대적으로 안정한 3가 크롬으로 전환될 수 있으며, 다양한 환원처리 기술이 지하수 정화에 적용되고 있다. 그중 ISRM(in situ redox manipulation)은 대수층 내 환원대를 조성하여 산화환원성 오염물질을 제거하는 대표적 방법으로, 형성된 환원대의 매질은 반응성 매체로 기능한다. 특히 구조철을 포함한 점토광물은 환원제로부터 전자를 전달하여 전자 공급원으로 작용할 수 있다. 본 연구는 점토광물의 구조철 환원 거동과 안정성, Cr(VI) 환원 및 고정화 특성, 그리고 흐름 조건에서의 적용 가능성을 규명하기 위해 수행되었다.
세 가지 점토광물인 카올리나이트(0.01 wt.%), 몬모릴로나이트(2.3 wt.%), 논트로나이트(22.3 wt.%)를 대상으로 pH 3–11 범위에서 50 mM 디티오나이트를 이용하여 화학적 환원을 수행하였다. 알칼리 조건에서 디티오나이트 안정성이 높고 산화환원전위가 낮았기 때문에 구조철의 환원율(Fe(II)/Total Fe)은 pH 증가에 따라 상승하였으며 pH 9에서 가장 높은 값을 보였다. pH 9에서 환원율은 카올리나이트 1.00, 몬모릴로나이트 0.67, 논트로나이트 0.49로 나타났고, 철 함량이 낮은 점토에서 더 높은 환원율이 확인되었다. 환원속도상수도 같은 경향을 보여 각각 5.8, 3.8, 1.3 M⁻¹h⁻¹로 관측되었다. 주목할 점은 pH 11에서 디티오나이트 안정성이 가장 높았음에도 불구하고 몬모릴로나이트와 논트로나이트의 환원율이 감소한 것이다. 이는 구조철 자체의 환원성이 고알칼리 조건에서 낮아졌음을 보여준다. 점토광물의 겉보기 표준 산화환원전위를 계산하여 각 점토광물의 환원성을 정량적으로 평가할 수 있었다. 또한, 각 점토광물의 겉보기 표준 산화환원전위는 pH 증가에 따라 값이 낮아지는 경향을 통해, pH 11에서 환원율이 감소한 원인을 설명할 수 있었다. 환원 이후 Fe2+ 용출은 카올리나이트에서 28–77%로 크게 나타났고, 몬모릴로나이트와 논트로나이트는 각각 0.47–2.05%, 0.11–0.53% 수준에 머물렀다. 안정성 측면에서 카올리나이트의 높은 Fe 용출은 1:1 층구조와 관련되며 전자셔틀 활용에는 불리하다. 반복 환원–산화 실험에서는 몬모릴로나이트와 논트로나이트 모두 구조철 재생이 가능했으나, 디티오나이트 농도가 과도하게 높을 경우(≥100 mM) 구조철 용해와 Cr 환원 성능 저하가 발생하였다.
pH 2.5–11 범위에서 두 점토광물에 의한 Cr(VI) 환원이 모두 관찰되었으며, 구조 Fe(II)는 1:3 화학양론비로 Cr(VI)과 반응하였다. 산성 조건에서는 환원이 더 빠르고 완전하게 진행되었으나, pH 9와 11에서는 Cr(VI) 산화환원전위가 낮아 13–32%가 잔류하였다. 생성된 Cr(III)은 산성 조건에서는 부분적으로, 중성 및 알칼리 조건에서는 모두 고정되어 수용액상에서 제거되었다. 고체를 회수하여 SEM-EDS, XPS, DTPA 추출 분석을 수행한 결과, 낮은 pH에서는 흡착이, pH 7 이상에서는 침전이 주요 제거 기작으로 확인되었다. Fe(II) 함량이 높을수록 환원 속도는 증가하였으며, 몬모릴로나이트가 논트로나이트보다 일관되게 더 빠른 반응성을 보였다. 또한, 구조철 환원 및 크롬 환원 반복실험을 진행하였는데 두 점토 모두 전자셔틀로 재사용이 가능했으나, 몬모릴로나이트는 Fe 손실이 거의 없이 안정적인 성능을 유지한 반면 논트로나이트는 구조철 용해로 인해 효율이 저하되었다. 따라서 몬모릴로나이트가 장기적인 Cr(VI) 환원과 고정화에 더 적합한 점토광물로 판단된다.
1차원 모래–몬모릴로나이트 혼합 컬럼(내경 8 cm × 높이 8 cm)을 이용하여 환원대 형성과 Cr(VI) 제거 성능을 평가하였다. 중량대비 1%와 5%의 몬모릴로나이트가 포함된 컬럼에 디티오나이트를 주입하여 구조 Fe(III)를 Fe(II)로 환원시킨 뒤 Cr(VI) 용액(50 mg/L)을 주입하였다. 디티오나이트 펄스 주입(1–2 공극부피)으로 구조 Fe(III)가 효과적으로 환원되었으며, Fe(II)/Total Fe 비율은 각각 0.67과 0.64에 도달하였다. 생성된 Fe(II)를 함유한 컬럼에 Cr(VI)을 주입하자 Cr(VI) 이동은 대조 컬럼에 비해 크게 지연되었고, 중량대비 1%와 5% 컬럼에서 각각 4와 19 공극부피 이후에 파과가 나타났다. 고체상을 회수하여 SEM-EDS, EPMA, XPS 분석을 수행한 결과 Cr(OH)3 침전물이 점토광물과 결합된 형태로 존재하면서 Cr(III)이 고정화된 사실이 확인되었다. 흡착과 환원 속도를 반영한 이중 반응매질 수송모델은 실험 결과를 잘 재현하였으며, Cr(VI) 제거가 구조 Fe(II) 환원에 의해 주도됨을 보여주었다. 디티오나이트 재주입을 통한 환원대 재생 실험에서는 4회 사이클까지 높은 제거 효율이 유지되었으나, 구조 Fe 손실, 우선 유동 발달, 2차 Fe–Cr 수산화물의 불안정성이 누적되면서 성능 저하가 발생하였다. 따라서 장기적 적용을 위해서는 지속적인 모니터링과 관리가 필요하다.
본 연구는 Fe 함유 점토광물을 단순한 대수층 매질이 아니라 Cr(VI) 정화를 위한 능동적 산화환원 매개체로 활용할 수 있음을 제시하였다. 나아가, 구조철의 환원 특성, Cr(VI) 환원 및 고정화 반응, 그리고 흐름 조건에서의 성능을 종합적으로 분석하여 현장 적용에 필요한 과학적 근거와 실용적 매개변수를 제공하였다.

다국어 초록 (Multilingual Abstract)

Hexavalent chromium (Cr(VI)) from industrial activities represents a persistent groundwater contaminant, with its high solubility and mobility enabling extensive migration through subsurface environments. Its severe toxicity, particularly its carcinogenic property, poses serious risks to human health, making effective treatment necessary. Reductive treatment is widely recognized as an effective strategy for Cr(VI) remediation as it converts Cr(VI) to the less toxic and immobile Cr(III). Among available methods, in situ redox manipulation (ISRM) has been widely applied to establish reductive zones within aquifers, enabling sustained contaminant removal. Structural iron in clay mineral is key material which can act as an electron mediator when reduced to Fe(II), providing continuous reductive capacity in subsurface environments. Accordingly, this study focuses on understanding the reduction behavior and stability of structural iron, the performance of Cr(VI) reduction and subsequent immobilization mechanisms induced by structural Fe(II), and the feasibility of these processes under flow conditions relevant to groundwater systems. To achieve these objectives, both batch and column experiments were conducted to investigate the processes.
Three types of clay minerals with varying iron contents, including kaolinite (0.01 wt.%), montmorillonite (2.3 wt.%), and nontronite (22.3 wt.%), were subjected to abiotic reduction using 50 mM dithionite at pH 3–11. The reduction ratio (Fe(II)/Total Fe) of structural iron increased up to pH 9, which can be attributed to the higher stability and lower redox potential of dithionite under alkaline conditions. Clay minerals with lower iron contents exhibited higher reduction ratios, reaching 1.00, 0.67, and 0.49 and higher reduction rate constants (kobs) of 5.8, 3.8, and 1.3 M⁻¹h⁻¹ for kaolinite, montmorillonite, and nontronite, respectively, at pH 9. At pH 11, despite dithionite having the highest stability and lowest redox potential, the reduction ratios decreased for montmorillonite and nontronite. This behavior indicates that the structural iron in these clay minerals inherently became more difficult to reduce at pH 11. The calculated apparent standard redox potential of structural iron (apparent Eh0structural iron) showed the highest values in kaolinite, followed by montmorillonite and nontronite at all pH conditions. Notably, a decrease in apparent Eh0structural iron with increasing pH was observed, which explains the diminished extent of reduction at pH 11. Upon reduction, partial Fe2+ release was observed, with values of 0.11–0.53% for nontronite, 0.47–2.05% for montmorillonite, and as high as 28–77% for kaolinite. The pronounced Fe leaching from kaolinite can be attributed to its 1:1 layer structure, indicating its unsuitability as an electron mediator. Repeated reduction–oxidation cycles further confirmed that structural iron in montmorillonite and nontronite can be regenerated, While higher dithionite concentrations enhanced Fe(II) yields, excessive treatment (≥100 mM) caused substantial structural Fe dissolution and impaired chromium reduction capacity.
Montmorillonite and nontronite reduced with 50 mM dithionite at pH 9, yielding structural Fe(II) ratios of 0.67 and 0.49, were employed in hexavalent chromium (Cr(VI)) reduction experiments. The Fe(II)-bearing clay minerals successfully reduced Cr(VI) across a wide pH range (2.5–11), with greater extents and rates observed at lower pH. At pH 7 and below, nearly all structural Fe(II) participated in the reduction following a 1:3 stoichiometric ratio ([Cr(VI)] to [Fe(II)]), resulting in complete removal. In contrast, 13–32% of Cr(VI) remained unreacted at pH 9 and 11 due to the relatively lower redox potential of Cr(VI). The reduced Cr(III) was only partially immobilized at low pH, while at neutral and alkaline pH it precipitated as Cr(OH)3 and deposited on mineral surfaces, as confirmed by SEM-EDS, XPS, and DTPA extraction. Kinetic analysis showed that the observed rate constants of reduction increased with Fe(II) content and were consistently higher for montmorillonite than for nontronite. Across three reaction cycles, both clays acted as renewable electron mediators, but montmorillonite maintained stable performance with minimal Fe loss, whereas nontronite exhibited declining efficiency due to structural Fe dissolution. These results indicate that montmorillonite possesses greater reaction rates and long-term stability, making it the more suitable clay mineral for practical Cr(VI) immobilization applications.
A one-dimensional sand–montmorillonite column study (8 cm inner diameter x 8 cm height) was conducted to evaluate the applicability of generating reductive zones and their Cr(VI) reduction performance using montmorillonite as a reactive medium. Columns containing 1 and 5 wt.% montmorillonite were treated with dithionite to reduce structural Fe(III) to Fe(II), after which a Cr(VI) solution (50 mg/L) was introduced. Pulse injections of dithionite (1–2 pore volumes) effectively reduced structural Fe(III) in montmorillonite, achieving maximum Fe(II)/total Fe ratios of 0.67 and 0.65 in the 1 and 5 wt.% columns, respectively. Cr(VI) transport was substantially delayed in the presence of Fe(II)-bearing montmorillonite, with breakthrough at 4 and 19 pore volumes for 1 wt.% and 5 wt.% columns, respectively, due to reduction by structural Fe(II). Solid analyses (SEM–EDS, EPMA, XPS) confirmed immobilized Cr(III) as Cr(OH)3 precipitates on montmorillonite surfaces. A dual-site reactive transport model incorporating adsorption and reduction kinetics successfully simulated the experimental results, indicating that Cr(VI) removal was primarily governed by structural Fe(II)-mediated reduction. Repeated regeneration cycles further showed that reductive capacity could be restored by dithionite reinjection, maintaining high removal efficiency for up to four cycles. However, excessive regeneration led to performance decline caused by structural Fe loss, preferential flow development, and decreased stability of secondary Fe–Cr hydroxides, indicating the need for monitoring and management to ensure long-term effectiveness.
This work establishes Fe-bearing clay minerals as active redox mediators for chromium remediation rather than conventional passive adsorbents. By evaluating reduction extents, kinetics, and underlying mechanisms, the study provides laboratory-scale evidence and practical parameters for in situ groundwater treatment.

목차 (Table of Contents)

• Abstract
• Table of Contents
• List of Tables
• List of Figures
• CHAPTER 1. INTRODUCTION 1
• 1.1. Background 1
• 1.2. Objective and scope 4
• 1.3 References 6
• CHAPTER 2. LITERATURE REVIEW 9
• 2.1. Environmental issues related to hexavalent chromium (Cr(VI)) 10
• 2.2. Remediation approaches for Cr(VI)-contaminated groundwater 19
• 2.3. Potential of iron-bearing clay minerals as redox-active materials 29
• 2.4. Transport and reaction processes of Cr(VI) in porous media 38
• 2.5. References 47
• CHAPTER 3. Redox behavior of structural iron by dithionite in Fe(III)-bearing clay minerals and its utilization as an electron mediator for redox-sensitive contaminants 59
• 3.1. Introduction 59
• 3.2. Materials and Methods 63
• 3.2.1. Determination of stability and redox potential of dithionite 63
• 3.2.2. Clay mineral purification, size fractionation 65
• 3.2.3. Reduction of structural iron in clay minerals by dithionite 66
• 3.2.4. Mssbauer spectrometry 67
• 3.2.5. Surface area of nontronite 68
• 3.2.6. Chromium reduction experiments 69
• 3.2.7. Redox potential of dithionite 70
• 3.2.8. Reduction rate and apparent standard redox potential of structural iron 71
• 3.3. Results and Discussions 73
• 3.3.1. Determination of structural iron reduction in clay minerals by dithionite 73
• 3.3.2. Reduction of structural iron by dithionite in clay minerals at varying pH 76
• 3.3.3. Redox behavior with different structural iron contents 79
• 3.3.4. Apparent standard redox potential of structural iron in clay minerals 81
• 3.3.5. Role of surface on electron transfer from dithionite to structural iron in nontronite 85
• 3.3.6. The stability of Fe(II) for potential utilization as an electron mediators for chromium reduction 91
• 3.4. Summary 96
• 3.5 References 97
• CHAPTER 4. Reduction of hexavalent chromium by Fe(II)-bearing clay minerals as electron mediators and pH-dependent immobilization mechanism involved 109
• 4.1. Introduction 109
• 4.2. Materials and Methods 113
• 4.2.1. Preparation of Fe(II)-bearing clay mineral 113
• 4.2.2. Surface charge characterization 115
• 4.2.3. Cr(VI) reduction by Fe(II)-bearing clay minerals 116
• 4.2.4. Characterizing Cr-associated clay minerals 119
• 4.2.5. Diethylenetriaminepentaacetic acid extraction test 120
• 4.3. Results and discussions 121
• 4.3.1. Cr(VI) reduction in presence of clay minerals at different pH 121
• 4.3.2. Distribution of reduced chromium species in clay mineral suspensions 129
• 4.3.3. XPS analysis 132
• 4.3.4. SEM-EDS analysis 134
• 4.3.5. Immobilization mechanism of the reduced chromium by Fe(II)-bearing clay minerals 136
• 4.3.6. Effect of clay mineral content on chromium reduction 140
• 4.3.7. Effect of background ions on reduction rate 144
• 4.3.8. Regeneration of Fe(II)-bearing clay minerals for Cr(VI) immobilization 146
• 4.4. Summary 151
• 4.5. References 152
• CHAPTER 5. In situ generation of reductive zone by dithionite for hexavalent chromium immobilization mediated Fe-bearing clay mineral 163
• 5.1. Introduction 163
• 5.2. Materials and methods 167
• 5.2.1. Preparation of porous media 167
• 5.2.2. Columns design and preparation 168
• 5.2.3. Generation of reductive zones 171
• 5.2.4. Chromium removal test 172
• 5.2.5. Solid-phase analysis 173
• 5.2.6. Model development 175
• 5.3. Results and Discussions 179
• 5.3.1. Generation of the reductive zone by dithionite 179
• 5.3.2. Removal of Cr(VI) by the reductive zone 185
• 5.3.3. Evidence of immobilized Cr(III) in the column 190
• 5.3.4. Reactive transport modeling of Cr(VI) removal 192
• 5.3.5. Regeneration of the reductive zone 201
• 5.3.6. Physical and chemical changes in the porous media 207
• 5.4. Summary 212
• 5.5. References 213
• CHAPTER 6. CONCLUSION 219
• 6.1. Summary of the dissertation 219
• 6.2. Procedure for field application 224
• 6.3. Field case study applying the decision flowchart 232
• 6.4. Suggested further study 241
• 6.4.1. Effect of the background ions in groundwater 241
• 6.4.2. Coupled effects of pH and flow on Cr(VI) reduction 243
• 6.4.3. Potential microbial influence under field conditions 245
• 6.4.4. Influence of heterogeneity and development of improved modeling approaches 246
• 6.5. References 247
• 국문초록 251