In situ Fe oxide synthesis for stabilization of Cd in soil: Repetitive application for enhanced sequestration and field demonstration = 토양 내 카드뮴 안정화를 위한 원위치 철산화물 합성: 반복적용을 통한 격리 향상 및 현장 실증

다국어 초록 (Multilingual Abstract)

Among various remediation approaches, in situ iron (Fe) oxide synthesis offers the advantage of applicability in areas where aggressive remediation methods such as excavation are not feasible. This method enables rapid cadmium (Cd) stabilization through surface adsorption and sequestration utilizing naturally abundant materials. However, since surface-adsorbed species exhibit higher mobility than sequestered Cd under leaching conditions, research has been conducted to enhance sequestered Cd forms. Nevertheless, existing studies have focused primarily on mechanistic investigations in homogeneous systems or soil application efficiency, with limited understanding of Cd sequestration mechanisms in heterogeneous soil environments.
Chapter 3 demonstrated that repeated synthesis in aqueous solutions enhanced Cd stabilization through encapsulation of surface-adsorbed Cd on previously synthesized Fe oxide by newly synthesized Fe oxides. Sequential extraction, X-ray absorption spectroscopy (XAS), and aberration-corrected transmission electron microscopy (Cs-TEM) confirmed Cd migration from Fe oxide surfaces to interior sites without causing structural deformation.
Chapter 4 evaluated repetitive synthesis applicability using artificially contaminated sand and field-contaminated soils. Sand matrix experiments revealed that increasing Fe concentrations enhanced discrete Fe oxide formation in soil solution, which provided primary sequestration sites through structural incorporation. Repetitive synthesis using descending Fe injection concentrations successfully enhanced Cd stabilization through encapsulation mechanisms while avoiding destabilization of previously formed Fe oxides. Field-soil samples demonstrated that stabilization efficiency varies significantly with Fe/Cd molar ratios in soil solution. In soils which available soil Fe participated in stabilization mechanisms, single synthesis achieved substantial leachate reduction. Soils with lower ratios showed surface adsorption as the predominant stabilization mechanism, where repetitive synthesis enhanced stabilization through encapsulation effects. These results emphasize that Fe/Cd ratios must be considered prior to field applications.
Field demonstration was conducted at an active smelting facility using a radial multi-horizontal injection well system. Site characterization revealed significant Cd contamination predominantly bound to Fe and Mn oxide fractions, with 30-year leaching predictions indicating substantial long-term metal release without intervention. Two repeated synthesis cycles were applied using 0.5 M FeCl3 and 1.7 M NaOH solutions. Electron probe microanalysis coupled with energy-dispersive spectroscopy (EPMA-EDS) analysis confirmed successful Fe oxide formation in treated soils. Synthetic Precipitation Leaching Procedure (SPLP) results demonstrated that most treated soils achieved non-detectable Cd leaching levels after repetitive synthesis applications, demonstrating that repetitive in situ Fe oxide synthesis provides effective and scalable remediation for contaminated sites.
This study demonstrates that repetitive in situ Fe oxide synthesis offers a practical and effective remediation approach for Cd-contaminated sites. The enhanced method is expected to serve as an effective remediation approach for contaminated sites where excavation is restricted.

국문 초록 (Abstract)

다양한 중금속 안정화 기술 중 원위치 철산화물 합성법(in situ Fe oxide synthesis)은 굴착과 같은 적극적 정화가 불가능한 지역에서도 적용 가능하다는 장점이 있다. 이 방법은 자연 기원 물질을 활용하여 표면흡착과 격리를 통해 단시간 내에 카드뮴 안정화가 가능하다. 하지만 표면흡착 카드뮴은 용출환경에서 격리된 카드뮴 보다 이동성이 더 높기 때문에 격리형태의 카드뮴을 향상시키기 위한 연구가 수행되어 왔다. 그러나 기존 연구는 주로 균질 매질에서의 기작 연구나 토양 적용 후 효율성 평가에 국한되어, 실제 복잡한 토양 환경에서 카드뮴 격리 기작이 어떻게 작용하는지에 대한 연구는 미흡한 실정이다. 따라서 본 연구는 3가 철 기반의 원위치 철산화물 반복합성 기법을 통해, 기존의 철산화물 표면에 흡착된 중금속을 격리 형태로 전환하는 기술을 제안하고 기작을 검증하고 안정화 효과를 평가하였다.
3장에서 수용액 카드뮴에 원위치 철산화물 반복합성 적용한 결과, 격리 형태의 카드뮴이 25%에서 76%로 증가했다. X-선 흡수 분광법(XAS)과 구면수차보정이 가능한 투과전자현미경(Cs-TEM) 분석을 통해 반복적으로 합성된 철산화물이 물리적으로 기존 형성된 철산화물을 코팅하여 표면에 흡착된 중금속을 encapsulation하는 기작임을 확인하였다.
논문 4장에서는 인공오염 및 현장 오염 토양에 반복합성을 적용하고 격리 효율을 확인하였다. 토양 매질에서는 주입하는 철의 분배를 고려해 토양수 내 최소 60 Fe/Cd 몰 비율일 때 격리된 카드뮴의 비율이 80% 였다. 하지만 반복합성 시에는 저농도 철을 주입 방식이 격리형태의 카드뮴 비율을 증가시킬 수 있었다. 현장 오염토양에서 토양수 내 Fe/Cd 몰 비율에 따라 안정화 효율이 민감하게 달라짐을 확인하였다. 토양 내 이용가능한 철로 인해 최소 Fe/Cd 몰 비율을 상회(400-2,800)하거나 이를 만족했을 때 원토양의 강우모사용출시험(SPLP)을 통한 카드뮴 용출량이 64%-99% 저감했다.
5장에서는 연구대상지역인 운영중인 제련소에 수평관정을 이용하여 원위치 철산화물 반복 합성법을 총 2회 적용하였다. 에너지 분산형 분광법이 결합된 전자탐침 미세분석(EPMA-EDS) 분석을 통해 토양에서 관측된 철산화물의 빈도수가 증가하여 성공적인 철산화물의 형성을 확인했다. 또한 토양 내 카드뮴의 용출량이 강우모사용출시험에서 불검출 수준까지 저감되는 것을 확인하였다. 이를 통해 원위치 철산화물 반복합성법이 카드뮴 안정화를 위한 효과적이고 현장 적용 가능한 정화기술임을 확인하였다.
본 연구에서는 원위치 철산화물의 반복 합성법을 활용하여 카드뮴의 격리와 이에 따른 안정화 효율을 향상시켰다. 본 연구를 통해 양이온 중금속의 격리 기작에 대한 이해를 확장하였으며, 개선된 기술은 굴착이 제한된 오염부지의 정화방안으로 활용될 수 있을 것으로 판단된다.

목차 (Table of Contents)

• Chapter 1 Introduction 1
• 1.1 Background 1
• 1.1.1 Environmental significance of inaccessible sites 1
• 1.1.2 Environmental significance of Cd contamination 3
• 1.2 In situ stabilization of heavy metals 5
• 1.3 Objectives 9
• 1.4 Dissertation structure 10
• References 12
• Chapter 2 Literature review 15
• 2.1 In situ Fe oxide synthesis 15
• 2.2 Fe oxide transformation and metal sequestration 16
• 2.3 Fe oxide precipitation at soil-water interface 19
• 2.4 Characterization of heavy metal sequestration 21
• 2.4.1 Cs-TEM 22
• 2.4.2 XAS: XANES and EXAFS 23
• 2.4.3 EPMA 24
• 2.5 In situ synthesis for heavy metals stabilization 25
• References 28
• Chapter 3 Spectroscopic evidence for the sequestration of Cd via repetitive in situ Fe oxide synthesis 32
• 3.1 Introduction 32
• 3.2 Materials and methods 35
• 3.2.1 Procedure for repetitive in situ Fe oxide synthesis 35
• 3.2.2 Measurement of surface-adsorbed and sequestered Cd 38
• 3.2.3 Instrumental analysis 38
• 3.2.3.1 SEM-EDS 39
• 3.2.3.2 XRD 39
• 3.2.3.3 Cs-TEM 39
• 3.2.3.4 EMPA 40
• 3.2.3.4 Particle size distribution 40
• 3.2.3.4 XAS: XANES and EXAFS 40
• 3.3 Results and discussion 41
• 3.3.1 Behavior of the surface-adsorbed Cd during repetitive synthesis 41
• 3.3.2 Local structures of the Cd sequestered by repetitive Fe oxide synthesis 46
• 3.3.3 Spatial distribution of the encapsulated Cd by repetitive Fe oxide synthesis 51
• 3.4 Summary 53
• References 55
• Chapter 4 Stabilization of Cd in soil using repetitive in situ Fe oxide synthesis 59
• 4.1 Introduction 59
• 4.2 Materials and methods 61
• 4.2.1 In situ Fe oxide synthesis in sand 61
• 4.2.2 Repetitive synthesis for sequestration of Cd in soil 63
• 4.2.3 Application of repetitive synthesis on field-soil samples 64
• 4.3 Results and discussion 65
• 4.3.1 Fe concentration-dependent Cd sequestration 65
• 4.3.2 Enhanced Cd sequestration using repetitive Fe oxide synthesis 67
• 4.3.3 Applicability of in situ Fe oxide on field-soil 72
• 4.4 Summary 78
• References 79
• Chapter 5 Field demonstration of repetitive in situ Fe oxide synthesis 83
• 5.1 Introduction 83
• 5.2 Materials and methods 85
• 5.2.1 Site description 85
• 5.2.2 Characterization of site contamination 86
• 5.2.2.1 Long-term leaching prediction 89
• 5.2.3 Radial multi-horizontal injection well installation 92
• 5.2.4 Field repetitive in situ Fe oxide synthesis 92
• 5.3 Results and discussion 96
• 5.3.1 Physicochemical properties of soil samples 96
• 5.3.2 Thirty-year prediction of Cd leachate mass from study site 98
• 5.3.3 Effectiveness of repetitive in situ Fe oxide synthesis 101
• 5.4 Summary 106
• References 107
• Chapter 6 Conclusion 111
• 국문초록 113