Investigation of the fluoride removal in groundwater by bone-char in Turkana County (Kenya)

다국어 초록 (Multilingual Abstract)

In Turkana County- Kenya, the scarcity of perennial surface water forces residents to depend exclusively on groundwater sources. The aquifers in these arid and semi- arid areas, however; are often enriched with geogenic fluoride, with concentrations exceeding the World Health Organization’s drinking water guideline of 1.5 mg/L, thereby posing significant risks to public health. This study evaluated the effectiveness of bone char, a carbon-apatite composite derived from bovine bones, as a locally available and low-cost adsorbent for defluoridation. Bone char was produced under controlled pyrolysis conditions of 550 °C and assessed through batch adsorption experiments using synthetic groundwater representative of field fluoride levels (2-10 mg/L). The influence of operational factors-pH, contact time, dosage, initial concentration, and temperature-was systematically assessed. Adsorption equilibrium data were fitted to Langmuir and Freundlich isotherms, with the Freundlich model (R² = 0.98) providing the best fit, indicating adsorption on heterogeneous surfaces. Kinetic studies revealed rapid uptake within 10 minutes, with the pseudo-second-order model confirming chemisorption as the dominant mechanism. Under near-neutral pH (6-8), ambient temperature (18-30 °C), and a dosage of 4 g/L; removal efficiencies reached up to 98%, with Langmuir-estimated capacities of 8-11 mg/g. The findings highlight the potential of bone char as a sustainable, community-level treatment medium, aligning with local resource availability and advancing progress towards Sustainable Development Goal 6 on safe and affordable drinking water Keywords: Bone char adsorption, Freundlich isotherm, Geogenic fluoride, Groundwater defluoridation, Hydroxyapatite-based adsorbent, Low-cost water treatment, SDG 6, Turkana County (Kenya), Fluoride contamination

국문 초록 (Abstract)

케냐 투르카나(Turkana) 카운티에서는 지속적인 지표수의 부족으로 인해 주민들이 전적으로 지하수에 의존하고 있다. 그러나 이와 같은 건조 및 반건조 지역의 대수층은 종종 지질 기원(geogenic) 불소가 풍부하게 포함되어 있으며, 그 농도가 세계보건기구(WHO)의 음용수 불소 기준치인 1.5 mg/L를 초과하여 심각한 공중보건 위험을 초래한다. 본 연구는 지역에서 쉽게 구할 수 있고 비용이 저렴한 흡착제로서 탄화골(bone char)—소 뼈를 이용해 제조한 탄소-아파타이트 복합체—의 제불소화(defluoridation) 효과를 평가하였다. 본 연구에서 탄화골은 550 °C의 제어된 열분해 조건에서 제조되었으며, 현장 불소 농도(2–10 mg/L)를 반영한 합성 지하수를 이용한 배치 흡착 실험을 통해 평가되었다. pH, 접촉시간, 흡착제 투여량, 초기 농도, 온도 등 운전 인자의 영향을 체계적으로 분석하였다. 흡착 평형자료는 Langmuir 및 Freundlich 등온식에 적용되었으며, Freundlich 모델(R² = 0.98) 이 가장 높은 적합도를 보였는데, 이는 탄화골 표면의 비균질적 흡착 특성을 반영한다. 흡착 속도 연구에서는 10분 이내에 빠른 불소 제거가 이루어졌으며, 유사 2차 속도식(pseudo-second-order model) 이 화학적 흡착(chemisorption)이 지배적임을 확인하였다. pH 6–8의 중성 부근, 18–30 °C의 상온 조건, 4 g/L의 흡착제 투입량에서 최대 98%의 제거 효율을 나타냈으며, Langmuir 모델 기반 최대 흡착용량은 8–11 mg/g으로 산정되었다. 이러한 결과는 탄화골이 지역 자원 활용성과 경제성을 동시에 갖춘 지속가능한 지역사회 기반 제불소화 매체로서의 잠재력을 보여준다. 또한 안전하고 경제적인 식수 공급을 목표로 하는 지속가능발전목표(SDG) 6 달성에 기여할 수 있음을 시사한다.

키워드: 탄화골 흡착, 프렌들리히 등온식, 지질 기원 불소, 지하수 제불소화, 하이드록시아파타이트 기반 흡착제, 저비용 수처리 기술, 지속가능발전목표 6, 투르카나 카운티(케냐), 불소 오염

목차 (Table of Contents)

• Chapter One 1
• 1.0 Introduction 1
• 1.1 Background 1
• 1.2 Problem Statement 3
• 1.3 Objectives and Scope 4
• 1.3.1 Study’s main objective 4
• 1.3.2 Specific objectives. 4
• 1.4 Research questions 5
• 1.4.1 Main research question 5
• 1.4.2 Specific research questions 5
• 1.5 Justification & significance of the study 5
• 1.6 Scope & limitations of study 7
• 1.6.1 Scope of the Study 7
• 1.6.2 Limitations of the Study 8
• 1.7 Organization of the thesis 9
• 1.8 Study area 9
• 1.8.1 Location and extent of study area 9
• 1.8.2 Climate and Hydrology 9
• 1.8.3 Geology and Soils 10
• Chapter 2 12
• 2 Literature review 12
• 2.1 Overview of fluoride contamination in Water 12
• 2.2 Health and Environmental impacts of Fluoride 13
• 2.3 Global & local hotspots for fluoride prevalence in groundwater 14
• 2.4 Chemical Composition and properties of Fluorides 16
• 2.4.1 Fluoride in Water 16
• 2.4.2 Key Properties of Fluorine 18
• 2.4.3 Health Considerations of fluoride and other uses 18
• 2.5 Fluoride Mitigation Measures 19
• 2.6 General defluoridation system set up and water feed 19
• 2.7 Defluoridation Methods 20
• 2.8 Bone Char Method as a green adsorbent 22
• 2.8.1 Bone char composition and fluoride removal mechanism 23
• 2.11 Technical configurations for use in rural setup adsorption filters 27
• Chapter 3 29
• 3 Methodology 29
• 3.1 Study Area and Water Sample Simulation 29
• 3.2 Research Design 29
• 3.3 Materials and Equipment 30
• 3.3.1 Instrumental set up 32
• 3.3.2 Bone Char Preparation 32
• 3.3.3 Zirconyl-acid reagent preparation 32
• 3.3.4 SPADNS Solution preparation 33
• 3.3.5 Preparation of Acid zirconyl-SPADNS reagent 33
• 3.3.6 Reference solution 33
• 3.3.7 Stock fluoride solution 33
• 3.3.8 Standard fluoride solution for calibration curve: 33
• 3.4 Measurement of fluoride adsorption at different pH 34
• 3.5 Setting up batch adsorption experiments 34
• I. Establishing key parameters 34
• II. Preparations of calibration Curves 34
• III. Kinetics Equilibrium 36
• IV. Isotherm Equilibrium 36
• V. Fitting kinetic models (using non-linear or linearized forms): 37
• Chapter 4 38
• 4.0 Results and Discussions 38
• 4.1 Calibration of Fluoride Measurement 38
• 4.2 Adsorption Isotherm Studies 39
• 4.2.1 Langmuir Isotherm 40
• 4.2.2 Freundlich Isotherm (ln qe vs ln Ce) 44
• 4.2.3 Experimental versus Model-Predicted Isotherms (Fluoride-Bone char relationship) 45
• Practical implications 50
• 4.3 Kinetic Studies 50
• 4.3.1 Investigation on Contact time 51
• 4.3.2 Pseudo First Order Kinetics 54
• 4.3.3 Pseudo-Second Oder (PSO) 55
• 4.4 Temperature 58
• 4.5 Bulk Density 59
• 4.6 pH Conditions 60
• 4.7 Adsorbent dosage effect of bone-char 62
• 4.8 Estimation of Surface Area and Mesopore Volume from Methylene Blue Adsorption 65
• 4.8.1 Langmuir Isotherm Fitting for Methylene Blue Solution 65
• 4.9 Socio-Cultural and Environmental Feasibility 68
• 4.10 Comparison with other recent studies 69
• Chapter 5: Conclusion & recommendations 73
• 5.1 Conclusion 73
• 5.2 Recommendations 74