Graduation Harmen van der Laan | 18 September 2009

1. Investigating subsurface iron and arsenic removal: Anoxic column experiments to explore efficiency parameters Graduation Harmen van der Laan | 18 September 2009

2. Contents Introduction Arsenic problem Subsurface iron and arsenic removal Problem description and objectives Research setup Experimental procedure Theoretical background Results and discussion Conclusions and recommendations

3. Arsenic contamination in drinking water • Arsenic problem • Naturally in ground water • Chronic exposure: higher rates of lung, bladder and skin tumors • Big social impact (ostracism) • WHO guideline: < 10 μg/L • Bangladesh • 30 million people are exposed to concentrations > 50 μg/L • Rural areas: no centralized systems (10 million tube wells)

4. Subsurface iron and arsenic removal Ground water level Ground water with Fe(II) and As

5. Subsurface iron and arsenic removal : injection phase Ground water level Injected water front O2 front Ground water with Fe(II) and As Injection water without oxygen Injection water with oxygen

6. Subsurface iron and arsenic removal: abstraction phase Ground water level iron oxide with adsorbed Fe(II) and As Ground water with Fe(II) and As Oxidation zone withfreshly formed ferric oxides Injection water without oxygen

7. Subsurface iron and arsenic removal: efficiency ratio Typically increasing over successive cycles 4 2 0 V Iron concentration [mg/L] Efficiency ratio Vi V Vinjection Volume [m3]

8. Problem description and objective Problem description There is a lack of insight in (i) the dominant mechanisms responsible for the (increasing) sorption of iron and arsenic(ii) operational factors how to optimize the removal efficiency The objective of this study To obtain reliable experimental data to investigate the parameters affecting the removal efficiency The primary goal is to gain a better understanding of the dominant sorption mechanisms and the increasing efficiency, in order to optimize the operation of this technology in the field.

9. Research setup • Four experiments, 10 injection/abstraction cycles • Experiment I: Investigation increasing capacity over successive cycles (cycle 1 – 5) • Experiment II: Influence pH: 6.5, 6.9 and 7.5 (cycle 6 – 8) • Experiment III: Influence injection volume (cycle 9) • Experiment IV: Influence increase ionic strength (0.1M NaNO3) (cycle 10) • Monitoring Fe, As, pH, Eh, Conductivity and Oxygen Anoxic column experiments to simulate several injection/abstraction cycles in Bangladesh Experimental setup • 4 columns diameter 36mm, height 308mm • 2 types of soil material Virgin Sand 0.6-1.2mm Aquifer Sand 0.12-2.5mm Fe: 2.7 and 2.5 mg/g. As: 2 and 0.5 µg/g • ‘average Bangladesh’ Synthetic Ground Water 4 mg/L Fe2+200µg/L As(III) pH 6.9 buffers: 5mM NaHCO3 1.64mM NaClIonic Strength 2·10-2

10. Experimental procedure: the story of one data point How does one data point at the graph come into existence? What is ‘the story of one ‘data point’ A short movie shows the experimental procedure

11. Met dank aan: Samuël (cameraman) en Ruben (camera én microfoon)

12. Contents Introduction Theoretical background Results and discussion Conclusions and recommendations

13. Theoretical background Iron: Fe2+ and Fe3+ Arsenic: As(III) and As(V) Arsenite Arsenate Adsorption is influenced by: • Surface charge • Chemical affinity Adsorption capacity of a material: • Number of sites (sites/nm2) • Surface area (m2/g) Furthermore, • Competing ions • Inner/outer-sphere complexationFe2+ and As(III) form inner-sphere complexes; their adsorption is fairly insensitive to ionic strength changes Example: adsorption Fe2+ M2+ H+ M2+ Fe2+ M2+ M2+ M2+ OFe+ OH OH OH OH OH Sand grain surface

14. Contents Introduction Theoretical background Results and discussion Experiment I : Influence successive cycles High adsorption capacities Increasing retardation As Stable retardation Fe2+ Experiment IV: Effect of ionic strength General discussion Conclusions and recommendations

15. Results experiment I: successive cycles Expectation, based on other experiments and literature: Retardation factor between 5 and 20, slightly increasing

16. Results experiment I: successive cycles

17. Results experiment I: successive cycles Three main findings High adsorption capacities (in absolute values) Increasing adsorption As(III) Stable adsorption Fe2+

18. High adsorption capacities How to explain this ‘miraculous sand’? • Not possible with known surface complexation characteristics improbable high site densities and/or surface areas • Laboratory artifact? Oxygen contamination and siderite (FeOH3) formation excluded as possible explanation Thus, other mechanisms … H+ Fe2+ OFe+ OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH Sand grain surface

19. High adsorption capacities Hypothesized mechanismIon exchange mechanism Ion exchange capacity determined by a.o. clay particles, in Cation Exchange Capacity (CEC). Surprisingly, a low CEC value can result in a high retardation! 2 meq/kg Retardation factor 30! (normal sandy aquifer is 10 meq/kg) Yet, ion exchange in virgin sand?! Fe2+ Na+ Na+ Na+ Na+ Fe2+ Na+ OH Sand grain surface

20. Contents Introduction Theoretical background Results and discussion Experiment I : Influence successive cycles High adsorption capacities Increasing retardation As Stable retardation Fe2+ Experiment IV: Effect of ionic strength General discussion Conclusions and recommendations

21. Retardation As(III) increasing • Note: 2.7 μM As(III) vs. 73 μM Fe2+ In other words, 3 μM sites is enough for arsenite, for ferrous iron not significant • Increasing adsorption, because of increasing amount of iron oxides. But why no increasing Fe2+ adsorption?

22. Stable Fe2+ capacity • Non-increasing capacity Fe2+ • Very remarkable! Increase iron content, thus in adsorption sites yet no increase in adsorption • In accordance with other studies and experiments • Ion exchange provides explanation: Exchange Capacity remains constant. Fe2+ Fe2+ Na+ Na+ Fe2+ Na+ Na+ Na+ Na+

23. Contents Introduction Theoretical background Results and discussion Experiment I : Influence successive cycles High adsorption capacities Increasing retardation As Stable retardation Fe2+ Experiment IV: Effect of ionic strength General discussion Conclusions and recommendations

24. Effect of the ionic strength (0.1M NaNO3)

25. Effect of the ionic strength (0.1M NaNO3)

26. Effect of the ionic strength (0.1M NaNO3) Main finding Adsorption As(III) is increasing with increasing ionic strength, while Ferrous iron adsorption is decreasing

27. Decrease Fe2+ with high ionic strength Decrease Fe2+ -70% (average) Ionic strength influenced adsorption iron? Remember: Inner-sphere complexes, thus rather insensitive for ionic strength! The ion exchange mechanism provides a clear explanation. High Na+ concentration (0.1M vs. 7 mM) results in shift exchanger composition (98% Na+ / 2% Fe2+ vs. 37% Na+ / 63% Fe2+) Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Fe2+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Fe2+ Na+ Na+ Sand grain surface

28. Increase As(III) with high ionic strength • Increase As(III) 8 – 43 in one cycle (438%) • Other studies: increasing adsorption with increasing ionic strength. But, there with negative surface charge. Here, As(III) is uncharged and positive charge. • Hypothesis : ionic strength causes surface charge of zero • Surface charge and potential becomes 0 (“point-of-zero-charge”) • thus no electrostatic repulsion • which favors adsorption of the uncharged As(III) Compare: experiment I: 10 – 50 in 5 cycles 0 As(III) ++ 0 0

29. Contents Introduction Theoretical background Results and discussion Experiment I : Influence successive cycles High adsorption capacities Increasing retardation As Stable retardation Fe2+ Experiment IV: Effect of ionic strength General discussion Conclusions and recommendations

30. General discussion • Ion Exchange mechanism • Pro’s • Not possible to describe with surface sites theory • Stable retardation Fe2+ • Decrease Fe2+ adsorption with high ionic strength • Results in adsorption capacity similar to other studies • Con’s / remaining questions • Exchange capacity (virgin) sand?! • Why no increase adsorption for ferrous iron? • Why not all Fe2+ accessible for adsorption?

31. Contents Introduction Theoretical background Results and discussion Conclusions and recommendations Iron removal mechanism Arsenic removal mechanism (Practical) implications Recommendations

32. Conclusions sorption mechanism of iron • High capacity!Much more as ‘theoretically’ possible • No increasing efficiency with increasing amount iron oxide. • Surprisingly, the ion exchange mechanism played a dominant role • Disclaimer: under laboratory circumstances Fe2+ Na+ Na+ Na+ Na+ Fe2+ Na+ OH Sand grain surface

33. Conclusions sorption mechanism of arsenic • High capacity!Much more sites accessible as expected • The efficiency is increasing (by iron oxides)1 day injection = 1 month 50% arsenic removal! • Higher ionic strength, higher efficiencyHypothesis: surface charge becomes zero • Disclaimer: under laboratory circumstances 0 As(III) ++ 0 0

34. (Practical) implications • Measure ionic strength and ‘point-of-zero-charge’ for site selectionWhere to apply subsurface arsenic removal • Honestly, more research is required for more practical implications • Biggest implication for future research if ion exchange mechanism is true, it has a large influence on interpretation results

35. Recommendations • More column experimentsvarying water quality, sand materials, experiment run times • Verify the ion exchange mechanismMeasure Cation exchange capacity, apply cation free injection water, more sampling • Focus on soil chemistrydetailed surface analyses: charge, potential, surface area (BET), X-ray diffraction

36. General conclusion Subsurface treatment has a large potential for iron and arsenic removal. Study results illustrate the theoretical possibilities under ideal circumstances More research is required to optimize the operational efficiency in the field

37. There’s Treasure Everywhere! Thank you for your attention

39. Gretha (tropen)verpleegkundigeGezondsheidstraining in communities (niet in kliniek) • HarmendrinkwateringenieurFaciliterenbijimplementatiedrinkwatersysteem in dorp • Lokaal team, Filippijnse NGO • Februari 2010 - 1 tot 1.5 jaar • Wonen in plattelandsdorp • ‘onbetaald’ – op basis van giften • Avontuur, concrete vraag, drive vanuit God • Nieuwsgierig? Harmenengretha.wordpress.com www.watervoorfilippijnen.nl DE FILIPPIJNEN