Natural Mineral Supported Zerovalent Iron Composites for the Remediation of Hexavalent Chromium Contaminated Groundwater = 천연광물 지지 영가철(ZVI) 복합소재를 이용한 6가 크롬 오염 지하수 정화 연구

다국어 초록 (Multilingual Abstract)

The contamination of groundwater with hexavalent chromium (Cr(VI)) poses serious environmental and human health risks due to its high toxicity, mobility, and persistence in the subsurface environments. The widespread presence of Cr(VI) in industrially impacted aquifers highlights the urgent need for in situ remediation technologies capable of achieving rapid removal and long-term effectiveness. This dissertation investigates the development, characterization, and performance evaluation of zero-valent iron (ZVI) composites supported on natural and modified zeolites (NZ, PZ) and bentonite (Bt) for effective in situ Cr(VI) remediation. The objective was to enhance the stability, dispersibility, and reactivity of nZVI as an in situ remedial agent, ensuring sustained contaminant removal under realistic subsurface conditions.
The effects of alkali treatment on natural zeolites (NZ) were systematically assessed to optimize nZVI incorporation and stabilization. Alkali-modified zeolites (PZ) exhibited increased Fe uptake and uniform nZVI distribution on both external surfaces and internal channels, effectively mitigating nanoparticle aggregation commonly observed in bare nZVI systems. Mechanistic investigations revealed that structural defects, including pore and cage openings, along with Na⁺-mediated ion exchange, were key factors controlling nZVI encapsulation. Three distinct incorporation pathways were identified: (i) surface loading onto NZ, (ii) framework integration within PZ, and (iii) channel encapsulation in PZ. Higher NaOH concentrations favored complete encapsulation, slowing electron transfer and extending nZVI reactive lifespan. These findings indicate that controlled structural modification of zeolites can substantially improve nZVI stability and long-term reactivity.
The transport behavior and Cr(VI) removal efficiency of nZVI@zeolite composites were evaluated through batch experiments, column studies, and three-dimensional sandbox simulations replicating realistic aquifer conditions. Among the tested composites, nZVI@PZ(0.1) displayed superior mobility, reduced aggregation, and deeper penetration into porous media. Electrochemical analyses confirmed enhanced corrosion resistance and sustained electron transfer capacity during sustained exposure to Cr(VI). Although nZVI@NZ exhibited higher removal efficiency in batch experiments, nZVI@PZ(0.1) achieved approximately 3.4 times higher in situ removal in flow-through sandbox tests. This difference was attributed to secondary reactions induced by physical interactions with soil particles, which exposed internal Fe(0) and maintained prolonged Cr(VI) reduction, highlighting the importance of considering hydrodynamic and transport effects in nZVI-based remediation design.
Bentonite-supported nZVI (bmZVI@Bt) synthesized via mechanochemical ball milling demonstrated enhanced structural and chemical activation compared to pristine and ball-milled ZVI. Characterization revealed uniform nZVI dispersion on bentonite surfaces, increased Fe(0) exposure, and higher Fe²⁺/Fe³⁺ ratios. Batch experiments showed rapid and nearly complete Cr(VI) reduction under both deionized water and groundwater conditions, and continuous-flow column tests confirmed stable long-term removal with a capacity approximately 4.5 times higher than bmZVI alone. Mechanistic analyses indicated that Cr(VI) removal involved adsorption onto reactive Fe sites, reduction to Cr(III), and immobilization via precipitation and coprecipitation. Bentonite served as a stabilizing support, mitigating premature nZVI passivation while enhancing electron transfer and mass transport.
This dissertation demonstrates that zeolite- and bentonite-supported nZVI composites are highly promising materials for in situ Cr(VI) remediation. By improving nanoparticle dispersibility, corrosion resistance, and the longevity of reactive Fe sites, these composites offer a sustainable and effective approach for long-term groundwater remediation. The results highlight the critical interplay between material design, structural modification, and subsurface transport behavior, providing a solid scientific basis for scaling up supported nZVI technologies and for developing next-generation in situ remediation strategies that optimize reactivity, durability, and mobility

국문 초록 (Abstract)

지하수 내 육가크롬(Cr(VI)) 오염은 높은 독성, 이동성 및 지하 환경에서의 지속성으로 인해 심각한 환경적·보건적 문제를 야기한다. 산업 활동의 영향을 받은 대수층에서 Cr(VI)이 광범위하게 존재함에 따라, 신속한 제거와 장기적인 지속 가능성을 동시에 달성할 수 있는 원위치(in situ) 정화 기술의 개발이 절실히 필요하다. 본 논문에서는 나노영가철(nZVI)을 천연 및 알칼리 전처리 제올라이트(NZ, PZ)와 벤토나이트(Bt)를 지지체로 활용하여 복합체를 개발하고, 이를 활용한 Cr(VI) 원위치 정화 효율과 특성을 종합적으로 평가하였다. 연구의 주요 목표는 실제 지중 환경을 고려한 환경에서 nZVI의 안정성, 분산성 및 반응성을 향상시키고, 장기적인 오염물질 제거를 실현하는 것이었다.
천연 제올라이트에(NZ) 대한 알칼리 처리 효과를 체계적으로 평가하여 nZVI 지지 및 안정화 메커니즘을 최적화하였다. 알칼리 처리된 제올라이트는 철 흡착량을 증가시키고, nZVI가 제올라이트 외부 표면과 내부 채널에 균일하게 분포하도록 하여 일반적인 nZVI 응집 문제를 효과적으로 완화하였다. 분석 결과, 기공 및 케이지 구조의 결함과 Na⁺ 매개 이온교환이 nZVI 캡슐화에 중요한 역할을 하는 것으로 나타났다. 세 가지 주요 Fe 도입 메커니즘은 1) NZ 표면 적재, 2) PZ 구조 내 통합, 3)PZ 채널 캡슐화로 확인되었으며, 높은 NaOH 농도에서는 완전한 캡슐화가 촉진되어 전자 전달 속도가 늦춰지고 nZVI의 반응 수명이 연장되었다. 이러한 결과는 제올라이트 구조의 조절을 통해 nZVI 안정성과 장기적인 반응성을 크게 향상시킬 수 있음을 시사한다.
nZVI@제올라이트 복합소재의 이동성과 Cr(VI) 제거 효율은 배치 실험, 컬럼 실험, 3차원 샌드박스 모사 실험을 통해 쳬계적으로 평가하였다. 알칼리 전처리 천연 제올라이트(PZ)에 지지된 nZVI 복합소재(nZVI@PZ(0.1))는 다공성 매질 내에서 뛰어난 이동성을 보였으며, 입자 응집을 억제하면서 오염원이 존재하는 깊은 다공성 매질 내부까지 효과적으로 침투할 수 있었다. 전기화학 분석에서는 Cr(VI) 존재 하에서도 보다 음의 값의 부식 전위(Ecorr)를 유지하여 장기적인 노출에도 전자 전달 능력이 지속됨을 확인하였다. 배치 실험에서는 nZVI@NZ가 더 높은 제거 효율을 보였지만, 연속 흐름 샌드박스 실험에서는 nZVI@PZ(0.1)이 약 3.4배 높은 현장 내 제거 효율을 나타냈다. 이는 모래 입자와의 물리적 상호작용을 통해 내부 Fe(0)가 노출되고 장기적인 Cr(VI) 환원이 지속된 결과로 해석되며, 지하수 내 이동 및 물리적 상호작용이 사질 대수층 내 nZVI 기반 정화 효율에 중요한 영향을 미친다는 것을 보여준다.
또한, 벤토나이트 지지 nZVI(bmZVI@Bt)는 기계화학적 볼밀링을 통해 합성되었으며, 마이크론 크기의 ZVI (mZVI) 및 나노 크기의 볼밀 ZVI (bmZVI) 대비 구조적·화학적 활성화가 향상되었다. 특성 분석 결과, nZVI가 벤토나이트 표면에 균일하게 분산되고 Fe(0) 노출이 증가했으며 Fe²⁺/Fe³⁺ 비율이 높아 반응성이 강화되었다. 배치 실험에서는 탈이온수 및 지하수 조건 모두에서 Cr(VI)의 빠르고 완전한 제거를 나타내고, 연속 유속 컬럼 실험에서도 안정적인 장기 제거 효율이 확인되었다. 제거 용량은 bmZVI 단독 대비 약 4.5배로 나타났으며, Cr(VI) 제거 과정은 반응성 Fe 부위에 의한 흡착, 전자 매개 환원, 그리고 침전 및 공동침전을 통한 고정화 과정을 포함하였다. 벤토나이트는 nZVI의 조기 수동화를 방지하고 전자 전달을 유지하며, 물질 전달을 향상시키는 안정화 지지체로 작용하였다.
본 논문은 제올라이트 및 벤토나이트 지지 nZVI 복합체가 Cr(VI) 오염 지하수와 사질 대수층에서 장기적·지속 가능한 정화 소재로 매우 유망함을 보여준다. 나노입자의 분산성, 부식 저항성, 반응성 Fe 사이트의 장기적 유지를 향상시킴으로써 실제 지중 환경에서도 효과적인 장기 오염물 제거가 가능함을 입증하였다. 연구 결과는 재료 설계, 구조적 조절, 지하수 내 이동성 간 상호작용이 nZVI 성능 최적화에 결정적임을 강조하며, 천연광물 지지체 활용한 nZVI 기술의 현장 적용 확대와 차세대 원위치 정화 전략 개발을 위한 과학적 근거를 제공한다.

목차 (Table of Contents)

• Ⅰ. Introduction ·············································································· 1
• 1.1. Introduction ······································································· 1
• 1.2. Research Objectives and Scopes ················································ 4
• Ⅱ. Research Backgrounds ······························································ 6
• 2.1. Overview of Chromium (VI) Contamination in Groundwater ················ 6
• 2.2. Strategies for Cr (VI) remediation in Groundwater ·························· 7
• 2.3. In Situ Reactive Materials ······················································· 8
• 2.3.1. Nanoscale Zerovalent Iron (nZVI) ········································· 8
• 2.3.2. Synthesis of nZVI ··························································· 9
• 2.3.2.1. Bottom-Up Synthesis Approach ·········································· 9
• 2.3.2.2. Top-Down Synthesis Approach ········································· 10
• 2.3.2.3. Modification of nZVI ···················································· 10
• 2.4. nZVI-Based Composites on Supports ········································· 11
• 2.4.1. Inorganic Supporting Materials ············································· 11
• 2.4.2. Organic Supporting Materials ··············································· 11
• 2.4.3. Zeolite and Bentonite-Based nZVI composites ··························· 12
• Ⅲ. Methodology ······································································· 13
• 3.1. Chemicals and Materials ························································ 13
• 3.1.1. Natural mineral ····························································· 13
• 3.1.2. Chemicals ····································································· 13
• 3.2. Synthesis of nZVI@Natural Mineral Composites ···························· 14
• 3.2.1. Alkaline Treatment of Natural Zeolites ···································· 14
• 3.2.2. Synthesis of Zeolite-Supported nZVI (nZVI@Zeolite) via Liquid-Phase Reduction ·············································································· 14
• 3.2.3. Synthesis of Bentonite-Supported nZVI (bm-ZVI@Bt) via Ball Milling Process ················································································· 15
• 3.3. Reactor Design for In Situ Remediation Simulation ························· 15
• 3.3.1. Column Reactor System ····················································· 15
• 3.3.2. Sandbox Reactor System ···················································· 16
• 3.4. Sedimentation Experiment ······················································ 17
• 3.5. Transportability Tests Using Column and Sandbox Reactors ················ 18
• 3.6. Cr(VI) Removal Experiments ·················································· 20
• 3.6.1. Batch Experiments for Cr (VI) Removal ··································· 20
• 3.6.2. Column Experiments for Cr (VI) Removal ································ 22
• 3.7. In Situ Flow-Through Cr (VI) Remediation Using Sandbox ················ 22
• 3.8. Continuous In Situ Cr (VI) Remediation Using a Diffusion-Inhibiting Barrier System in a Sandbox ····························································· 24
• 3.9. Analytical Method ······························································· 25
• 3.9.1. Surface Analyses ····························································· 25
• 3.9.2. Electrochemical Corrosion Measurements ································· 26
• 3.9.3. Analytical Methods Used for Experiments ································ 27
• Ⅳ. Nano Fe (0)-functionalized Zeolites: Boosting Oxidation Resistance and Longevity of Cr (VI) Removal Through Alkaline Treatment ······················· 29
• 4.1. Introduction ······································································ 29
• 4.2. Results and Discussion ·························································· 32
• 4.2.1. Characterization of Various NZs and PZs ·································· 32
• 4.2.2. General Effects of Alkali-Treatment ······································· 38
• 4.2.3. Effect of NaOH Concentration on nZVI formation ······················· 40
• 4.2.4. Cr (VI) Removal by Air-aged nZVI on NZ(2), PZ(2)0.1 and PZ(2)1 ··· 47
• 4.2.5. Encapsulation Mechanism of nZVI on PZ surface ························ 52
• 4.3. Summary and Conclusion ····················································· 58
• Ⅴ. Unveiling the Effect of Reactive Surface Exposure during In Situ Sandy Aquifer Remediation of Cr (VI) by Nanoscale Zerovalent Iron@Zeolite Composites: Contrary Results in Batch and Sandbox Experiments ····························· 59
• 5.1. Introduction ······································································ 59
• 5.2. Results and Discussion ·························································· 62
• 5.2.1. Characterization of NZ and PZ ············································· 62
• 5.2.2. Batch Experiments and Removal Mechanism of Cr(VI) by nZVI@NZ and nZVI@PZ ············································································· 76
• 5.2.3. In Situ Flow-Through Cr (VI) Remediation in Sandbox ················· 85
• 5.2.4. Collision-Driven Enhanced Cr (VI) Removal by nZVI@PZ(0.1) in Sandbox Experiment ············································································ 94
• 5.2.5. Remediation of Cr (VI)-contaminated Real Groundwater in Sandbox Experiment ··········································································· 100
• 5.3. Summary and Conclusion ···················································· 106
• Ⅵ. Dual Functionality of Co-milled Bentonite@Zerovalent Iron Composite as reactive cut-off barriers: Mechanochemical Activation for Enhanced Cr(VI) Reduction and Prolonged ZVI Stability in Groundwater ························· 107
• 6.1. Introduction ····································································· 107
• 6.2. Results and Discussion ························································· 109
• 6.2.1. Characterization of bm-ZVI and bm-ZVI@Bt ··························· 109
• 6.2.2. Activation Mechanism by Ball-Milling Process ·························· 114
• 6.2.3. Cr (VI) Removal Batch Test by mZVI, bm-ZVI and bm-ZVI@Bt ····· 117
• 6.2.3.1. Isotherm Test ····························································· 117
• 6.2.3.2. Batch Kinetic Test ······················································· 118
• 6.2.4. Cr (VI) Removal Mechanism by bm-ZVI and bm-ZVI@Bt in DIW and GW ························································································ 123
• 6.2.5. Application of bm-ZVI@Bt for In Situ Cr (VI) Remediation using GW 132
• 6.3. Summary and Conclusion ···················································· 137
• Ⅶ. Conclusion and Recommendations ············································ 138
• 7.1. Conclusions ····································································· 138
• 7.2. Recommendations ······························································ 141
• References ·············································································· 143
• Abstract (in Korean) ································································· 152