Developing Adsorption Filters for Fluoride and Arsenic Removal from Water By Simona Dossi April 2018 Academic Supervisor: Dr. Enzo Mangano Industrial Supervisor: Mr. Aaron Krupp Abstract This report presents the experimental work conducted during a six months Engineers without Borders UK placement in effort to progress Caminos de Agua’s design of an adsorption filter in San Miguel de Allende, Mexico. The filter design’s objective is to reduce local groundwater concentrations below the World Health Organisation’s potable water limits of 10 µg/L and 1.5 mg/L, arsenic and fluoride respectively. Through batch experiments, Bayoxide ® E33 was concluded to be the most effective commercial arsenic adsorbent for the local groundwater over Bayoxide ® E33 HC and TiO particles 2 suspended in sand. Fixed bed and batch experiments, using Caminos de Agua’s internally produced bone char fluoride adsorbent, investigated adsorption capacity and bone char production optimisation. The bone char’s adsorption capacity was found to be independent of pyrolysis time, in a fixed bed system the adsorption capacity tested as 5.5 ± 0.3 mg/g for fluoride, acetic acid treated bone char performed with a 12 % higher adsorption capacity than untreated bone char, and an investigation of fixed bed system performance under varying solute flowrates was completed. Recommendations regarding improvement possibilities for future experiments conclude the report. 2 Caminos de Agua Simona Dossi Table of Contents ABSTRACT 2 TABLE OF CONTENTS 3 I. NOMENCLATURE 5 NOTATION ............................................................................................................................................. 5 GREEK LETTERS ....................................................................................................................................... 5 SUBSCRIPTS ............................................................................................................................................ 6 ACRONYMS ............................................................................................................................................ 6 II. LIST OF FIGURES 7 III. LIST OF TABLES 7 INTRODUCTION 9 1.1 ENGINEERS WITHOUT BORDERS UK AND CAMINOS DE AGUA ..................................................................... 9 1.2 PROJECT MISSION ............................................................................................................................ 10 1.3 CONTEXT OF PROJECT WITHIN CAMINOS DE AGUA ................................................................................. 11 1.3.1 Fluoride Adsorbent Production ........................................................................................................ 11 1.3.2 Arsenic Adsorbent Production .......................................................................................................... 12 1.3.3 Lead Lag Design ................................................................................................................................. 12 1.4 STRUCTURE OF REPORT ..................................................................................................................... 13 2. BACKGROUND AND THEORY 13 2.1 SEPARATION PROCESS SELECTION........................................................................................................ 14 2.2 ADSORPTION THEORY ....................................................................................................................... 14 2.1.1 Forces and Energetics ....................................................................................................................... 15 2.1.2 Sorption Equilibrium ......................................................................................................................... 16 2.1.4 Transport ........................................................................................................................................... 18 2.2 ADSORPTION SYSTEMS ...................................................................................................................... 21 2.2.1 Batch .................................................................................................................................................. 21 2.2.2 Fixed Bed Columns ............................................................................................................................ 22 2.3 AQUEOUS ARSENIC AND FLUORIDE ADSORPTION LITERATURE REVIEW ....................................................... 23 2.3.1 Aqueous Fluoride Adsorption on BC ................................................................................................ 23 2.3.2 Aqueous Arsenic Adsorption ............................................................................................................ 24 3. EXPERIMENTAL 25 3.1 EXPERIMENTS INTRODUCTION AND OBJECTIVES ..................................................................................... 25 3.1.1 Overall Project Quantitative Objective ............................................................................................ 25 3.1.2 Experiments Outline .......................................................................................................................... 26 3.2 APPARATUS AND ERROR .................................................................................................................... 27 3.2.1 Water Testing and Sampling ............................................................................................................. 27 3.2.2 Batch Experiments ............................................................................................................................ 28 3.2.3 Column Experiments ......................................................................................................................... 28 3.2.4 Error Calculation ................................................................................................................................ 29 3.3 ADSORBENT MATERIALS TESTED ......................................................................................................... 29 3.4 METHODOLOGY ............................................................................................................................... 30 3.4.1: Feed Water Preparation .................................................................................................................. 30 3.4.2: 24 hour Kinetic Tests Methodology ................................................................................................ 30 3.4.3: 1 Week Batch Test Methodology .................................................................................................... 31 Simona Dossi Engineers without Borders UK 3 3.4.4: Column Experiments Methodology ................................................................................................ 31 3.5 EXPERIMENTAL DESIGN ..................................................................................................................... 31 3.5.1 Batch Experimental Design ............................................................................................................... 31 3.5.2 Fixed Bed Experimental Designs ....................................................................................................... 32 4. RESULTS AND ANALYSIS 34 4.1 KINETIC BATCH EXPERIMENTS ............................................................................................................ 35 4.1.1 BC Charring - Varying Duration Comparison .................................................................................... 35 4.1.2 Commercial Arsenic Adsorbents Comparison ................................................................................. 36 4.1.3 Commercial Arsenic Adsorbents Kinetic Test .................................................................................. 38 4.2 FIXED BED EXPERIMENTS ................................................................................................................... 40 4.2.1 Acid Treated and Untreated BC Fluoride Adsorption: Fixed Bed ................................................... 41 4.2.2 Desorption Experiment Acid Treated and Untreated BC: Packed Bed ........................................... 44 4.2.3 Adsorption Experiment Varying Flowrates: Packed Bed ................................................................. 46 4.3 MASS BALANCE ............................................................................................................................... 49 5. CRITICAL ANALYSIS 50 5.1 ISSUES ENCOUNTERED AND PROPOSED SOLUTIONS ................................................................................. 50 5.1.1 Flowrate Control .......................................................................................................................... 50 5.1.2 Arsenic Test Kit Error and Experimental Noise ................................................................................ 51 5.1.3 Variance in Experiments .............................................................................................................. 52 5.2 SUGGESTIONS FOR FUTURE EXPERIMENTS ............................................................................................. 52 5.1.4 Temperature and pH Dependence ............................................................................................. 52 5.1.5 General next steps for filter goal ................................................................................................ 53 6. CONCLUSION 53 6.1 SUMMARY OF PROJECT AND RESULT ............................................................................................ 53 6.2 EXPERIMENTAL RECOMMENDATIONS SUMMARY ............................................................................ 54 ACKNOWLEDGMENTS 55 REFERENCES 55 APPENDIX A: DETAILED METHODOLOGIES 58 APPENDIX A.1: ADSORBENT PRODUCTION .................................................................................................. 58 A.1.1 Bone Char Production ....................................................................................................................... 58 A.1.2 Reverse Osmosis Flushing Bone Char – Phosphate Leaching ......................................................... 58 A.1.3 Acid Treatment of Bone Char ........................................................................................................... 59 APPENDIX A.2 BATCH EXPERIMENTS ......................................................................................................... 59 A.2.1 24 hour kinetic experiments ............................................................................................................. 59 A.2.1 1 week batch experiments ............................................................................................................... 60 APPENDIX A.3 COLUMN EXPERIMENTS ...................................................................................................... 61 APPENDIX B: FEED WATER CHEMISTRY INFORMATION 65 APPENDIX C: ROLES AND RESPONSIBILITIES 65 APPENDIX D: HACH AND ITS ARSENIC FIELD TEST KITS ANALYSIS 66 4 Caminos de Agua Simona Dossi i. Nomenclature Notations Symbol Name Units a Empirical constant [kg/m3)/(kg/m)]-n A Area [m2] c Concentration of adsorbate solute [mol/m3] i cs adsorbate concentration at adsorbent surface [mol/m3] i d Average particle diameter [m] p J Diffusion flux [mol/m2s] i J Pore diffusion flux [mol/m2s] pi k mass transfer coefficient [m/s] f L Bed length [m] m Adsorbent solid mass [g] ad n Number of moles [mol] i N Mass transfer flux between adsorbent and bulk fluid [mol/sm2] i p Partial pressure [Pa] i Pref Standard state reference pressure [Pa] i q Specific adsorption capacity [mol/kg] t Q Volumetric flowrate [L/hr] t Distance between the centres of the two isolated [m] r and interactive molecules R Ideal gas constant [J/Kmol] Re Reynolds number / t Time passed [hr] T Temperature [K] u Superficial fluid velocity [g/s] s V volume [m3] x mole fraction / i Greek Letters 𝝐 Potential well depth [m] 𝝈 Distance where the potential is equal to zero [m] ∅ Overall potential [J/mol] 𝝀 Isosteric heat of adsorption for component i [Jk/mol] 𝒊 𝝁 Chemical potential of component i [J/mol] 𝒊 𝓓 Diffusivity for component i [m2/s] 𝒊 𝓓 Pore diffusivity of component I [m2/s] 𝒑𝒊 𝜺 Adsorbent porosity / 𝒑 𝝉 Tortuosity / 𝒑 ∆𝒑 Pressure drop [Pa] 𝝆 T fluid density [kg/m3] 𝝁 Fluid viscosity [kg/ms2], 𝝅 Spreading pressure [Pa] Simona Dossi Engineers without Borders UK 5 𝜸 The adsorbed-phase activity coefficient / 𝒊 𝜺 Bed void fraction / 𝒃 Subscripts ad Adsorbent b Fixed bed D Attractive i Component i LJ Lennard jones p Pore P Polarisation Q Gradient-quadrupole R Repulsive S Sorbate-sorbate s Superficial t At time t 0 At time 0 Acronyms BC Bone Char DI Deionised EWB-UK Engineers with Borders UK HAP Hydroxyapatite IAST Ideal Adsorbed Solution Theory LES Length Equivalent Equilibrium Section LUB Length of Unused Bed MTZ Mass Transfer Zone NGO Non-Governmental Organisation TDS Total Dissolved Solids TO Treatment Objective TLUD Top-Lit-Updraft WASH Water, Sanitation, and Hygiene WHO World Health Organisation 6 Caminos de Agua Simona Dossi ii. List of Figures Figure 1: Independence Aquifer well general geographic location and quality ............... 10 Figure 2: Finalised BC gasifier design with pyrolysis process illustration ......................... 12 Figure 3: Lead lag design schematic illustration ............................................................... 13 Figure 4: Lennard Jones Potential Mathematical Illustration........................................... 15 Figure 5: IUPAC classification of adsorption isotherm for gas-solid equilibria ................ 17 Figure 6: General illustration of transfer locations in porous packed bed ....................... 18 Figure 7: Schematic of pore diffusion resistances combination....................................... 19 Figure 8: Schematic illustration - concentration profile in an adsorption fixed bed........ 22 Figure 9: BC and HAP point of zero charge ....................................................................... 23 Figure 10: Hach fluoride test colorimeter (Hach, 2017) ................................................... 27 Figure 11: ITS arsenic field test kit .................................................................................... 27 Figure 12: tumbler used for batch experiments ............................................................... 28 Figure 13: Fixed bed experimental set up ........................................................................ 28 Figure 14: Batch and dose experiments schematic design............................................... 32 Figure 15: Schematic diagram of experiment 4.2.1 .......................................................... 33 Figure 16: Experiment 4.2.2 schematic diagram .............................................................. 33 Figure 17: Schematic Diagram Experiment 4.2.3 .............................................................. 34 Figure 18: Experiment 4.1.2 average adsorption capacities ............................................ 37 Figure 19: Bayoxide ® E33 and TiO2 adsorption capacity - 1 week batch experiment .... 39 Figure 20: TiO2 adsorption capacity over 1 week batch experiment ............................... 39 Figure 21: Experiment 4.2.1 column 1 breakthrough curves ........................................... 41 Figure 22: Experiment 4.2.1 column 2 breakthrough curves ........................................... 42 Figure 23: Experiment 4.2.1 flowrate control................................................................... 42 Figure 24: Outlet fluoride concentration – desorption experiment 4.2.2 ........................ 44 Figure 25: Experiment 4.2.2 flowrate monitoring ............................................................ 45 Figure 26: Experiment 4.2.3 column 1 breakthroughs ..................................................... 46 Figure 27: Experiment 4.2.3 column 2 breakthroughs ..................................................... 47 Figure 28: Experiment 4.2.3 flowrate control data .......................................................... 47 Figure 29: Column Set Up Diagram ................................................................................... 61 iii. List of Tables Table 1: Design requirements for adsorption filter design............................................... 10 Table 2: Fluoride and arsenic water separation processes .............................................. 14 Table 3: Physical adsorption and chemisorption typical characteristics .......................... 15 Table 4: ITS arsenic field test kit errors ............................................................................. 27 Table 5: Adsorbent materials tested ................................................................................ 30 Table 6: Fixed bed systems parameters ........................................................................... 32 Table 7: Operating conditions experiment 4.2.3 .............................................................. 34 Table 8: Experiment 4.1.1 feed water information .......................................................... 35 Table 9: Experiment 4.1.1 calculated adsorption capacities – average of doubles ......... 35 Table 10: Inclusive range of adsorption capacities for different burn times ................... 36 Simona Dossi Engineers without Borders UK 7 Table 11: Experiment 4.1.2 feed water information ........................................................ 37 Table 12: Summary of calculated parameters experiment 4.1.3 ..................................... 37 Table 13: Experiment 4.1.3 feed water information ........................................................ 38 Table 14: Saturation adsorption capacity ......................................................................... 40 Table 15: Feed water information experiments 4.2.1 – 4.2.3 .......................................... 40 Table 16: Summary of calculated parameters – experiment 4.2.1 .................................. 43 Table 17: Summary of calculated parameters – Experiment 4.2.2 .................................. 45 Table 18: Summary of calculated parameters experiment 4.2.3 ..................................... 48 Table 19: Summary of EBCT variation ............................................................................... 48 Table 20: Friction factors estimation ................................................................................ 48 Table 21: Mass balances for experiments 4.1.1 – 4.1.3 ................................................... 50 Table 22: Information on test water - Ex-Hacienda de Jesus community ........................ 65 8 Caminos de Agua Simona Dossi Introduction This technical report presents and analyses the experimental work conducted working within Caminos de Agua as an Engineers without Borders UK (EWB-UK) volunteer. The project work consists of experimental investigations regarding an aqueous arsenic and fluoride adsorption filter design. This introductory section will introduce the two organisations involved in the project, the project aims, and the context of the work within Caminos de Agua’s previous contributions to the project. 1.1 Engineers without Borders UK and Caminos de Agua EWB-UK is a nongovernmental organisation (NGO), which provides engineering expertise to projects in the developing world and inspires global responsibility within the engineering sector. EWB-UK believe that engineering should provide solutions for people in need of safe and healthy living environments regardless of income, nationality, or gender (Engineers without Borders UK, 2017). As well as running national educational projects, EWB-UK provides qualified engineer volunteers to non-profit organisations around the world. Volunteers assist projects which range from cook stove design to wind turbine implementation; all projects are divided within three engineering sectors: i. clean energy, ii. built environment, and iii. water, sanitation and hygiene (WASH) (Engineers without Borders UK, 2017). The work presented in this report summarises a EWB-UK WASH placement completed within Caminos de Agua between June and December 2017. Caminos de Agua is an accredited NGO founded and based in San Miguel de Allende, Mexico. Caminos de Agua has collaborated with five EWB-UK volunteers including myself on the filter project; the project design decisions and work completed prior to the scope covered in this report are summarised in Section 1.3. Caminos de Agua’s mission is to aid the water quality and scarcity crisis “for at-risk communities on [the] Independence Aquifer in Central Mexico” (Caminos de Agua, 2018); the aquifer is an underground water reservoir and the source of 99% of water usage for over half a million local residents (Caminos de Agua, 2018). The water level of the aquifer has been rapidly decreasing since the 1950s, thus endangering the availability and quality of its water (Caminos de Agua, 2018). Arsenic and fluoride concentrations above the World Health Organisation (WHO) potable limits have been recorded in water wells in the region; Figure 1, indicates the wells’ general geographical locations and the well water’s arsenic and fluoride concentrations relative to the WHO limits (Caminos de Agua, 2018; Targetmap, 2018). Part of Caminos de Agua’s efforts to combat this issue consist in investigating functionalised bio-chars to develop low cost, sustainable, adsorption filters to treat the contaminated groundwater; this report presents contributions to this project. Simona Dossi Engineers without Borders UK 9 Well Water Arsenic and Fluoride Concentrations  Both contaminants concentrations below WHO potable limits  At least one contaminant concentration over ≤ 60% of WHO limit  At least one contaminant concentration over > 60% of WHO limit  At least one contaminant concentration over > 100% of WHO limit Figure 1: Independence Aquifer well general geographic location and quality 1.2 Project Mission The final objective of the project is to design and implement adsorption filters in communities throughout the San Miguel municipality to reduce well water concentrations of arsenic and fluoride to below the WHO potable limits: 10 g/L and 1.5 mg/L respectively (World Helath Organisation, 2008). The feed water concentrations used for experiments are 40 g/L of arsenic and 8.5 mg/L of fluoride because these values match or exceed, up to the 90th percentile of monitored wells’ contaminant concentrations (Caminos de Agua, 2018). Caminos de Agua calculated the necessary volume of potable water for an average family of five for cooking and drinking as 25.2 litres per day, thus defining the specific filter capacity. Lastly, the filter needs to be accessible and practical for implementation in isolated rural communities. Adsorption was the separation process chosen due to the local and inexpensive fluoride adsorbent production; more information on process selection is presented in Section 3.1. All filter design objectives are summarised in Table 1. Table 1: Design requirements for adsorption filter design Adsorbent/s ▪ Locally sourced/produced ▪ Low cost Capacity ▪ Flowrate: 10 Caminos de Agua Simona Dossi