1. Challenge the future Delft University of Technology Investigating subsurface iron and arsenic removal: Anoxic column experiments to explore efficiency parameters Graduation Harmen van der Laan | 18 September 2009

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

3. 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. 4 Subsurface iron and arsenic removal Ground water level Ground water with Fe(II) and As

5. 5 Subsurface iron and arsenic removal : injection phase Ground water level O 2 front Injected water front Ground water with Fe(II) and As Injection water without oxygen Injection water with oxygen

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

7. 7 Subsurface iron and arsenic removal: efficiency ratio Volume [m 3 ] Iron concentration [mg/L] 420420 V injection V V ViVi Efficiency ratio Typically increasing over successive cycles

8. 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. 9 Research setup 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 Fe 2+ 200µg/L As(III) pH 6.9 buffers: 5mM NaHCO 3 1.64mM NaCl Ionic Strength 2·10 -2 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 NaNO 3 ) (cycle 10) Monitoring Fe, As, pH, Eh, Conductivity and Oxygen

10. 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. 11 Met dank aan: Samuël (cameraman) en Ruben (camera én microfoon)

12. 12 Contents i. Introduction ii. Theoretical background iii. Results and discussion iv. Conclusions and recommendations

13. 13 Adsorption is influenced by:  Surface charge  Chemical affinity Adsorption capacity of a material:  Number of sites (sites/nm 2 )  Surface area (m 2 /g) Furthermore,  Competing ions  Inner/outer-sphere complexation Fe 2+ and As(III) form inner-sphere complexes; their adsorption is fairly insensitive to ionic strength changes Theoretical background Fe 2+ OH OFe + H+H+ OH Sand grain surface M 2+ Example: adsorption Fe 2+ Iron: Fe 2+ and Fe 3+ Arsenic:As(III) and As(V) Arsenite Arsenate

14. 14 Contents i. Introduction ii. Theoretical background iii. Results and discussion i. Experiment I : Influence successive cycles a. High adsorption capacities b. Increasing retardation As c. Stable retardation Fe 2+ ii. Experiment IV:Effect of ionic strength iii. General discussion iv. Conclusions and recommendations

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

16. 16 Results experiment I: successive cycles

17. 17 Results experiment I: successive cycles Three main findings a.High adsorption capacities (in absolute values) b.Increasing adsorption As(III) c.Stable adsorption Fe 2+

18. 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 (FeOH 3 ) formation excluded as possible explanation Thus, other mechanisms … Sand grain surface Fe 2+ OH OFe + H+H+ OH

19. 19 High adsorption capacities Hypothesized mechanism Ion 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/kgRetardation factor 30! (normal sandy aquifer is 10 meq/kg) Yet, ion exchange in virgin sand?! Fe 2+ Na + OH Na + Sand grain surface Na + Fe 2+

20. 20 Contents i. Introduction ii. Theoretical background iii. Results and discussion i. Experiment I : Influence successive cycles a. High adsorption capacities b. Increasing retardation As c. Stable retardation Fe 2+ ii. Experiment IV:Effect of ionic strength iii. General discussion iv. Conclusions and recommendations

21. 21 Retardation As(III) increasing  Note: 2.7 μM As(III) vs. 73 μM Fe 2+ 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 Fe 2+ adsorption?

22. 22 Stable Fe 2+ capacity Non-increasing capacity Fe 2+  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. Fe 2+ Na + Fe 2+ Na +

23. 23 Contents i. Introduction ii. Theoretical background iii. Results and discussion i. Experiment I : Influence successive cycles a. High adsorption capacities b. Increasing retardation As c. Stable retardation Fe 2+ ii. Experiment IV:Effect of ionic strength iii. General discussion iv. Conclusions and recommendations

24. 24 Effect of the ionic strength (0.1M NaNO 3 )

25. 25 Effect of the ionic strength (0.1M NaNO 3 )

26. 26 Effect of the ionic strength (0.1M NaNO 3 ) Main finding Adsorption As(III) is increasing with increasing ionic strength, while Ferrous iron adsorption is decreasing

27. 27 Decrease Fe 2+ with high ionic strength Decrease Fe 2+ -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% Fe 2+ vs. 37% Na + / 63% Fe 2+) Fe 2+ Na + Sand grain surface Na + Fe 2+ Na +

28. 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 As(III) 0 ++ 0

29. 29 Contents i. Introduction ii. Theoretical background iii. Results and discussion i. Experiment I : Influence successive cycles a. High adsorption capacities b. Increasing retardation As c. Stable retardation Fe 2+ ii. Experiment IV:Effect of ionic strength iii. General discussion iv. Conclusions and recommendations

30. 30 General discussion Ion Exchange mechanism Pro’s  Not possible to describe with surface sites theory  Stable retardation Fe 2+  Decrease Fe 2+ 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 Fe 2+ accessible for adsorption?

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

32. 32 Conclusions sorption mechanism of iron I.High capacity! Much more as ‘theoretically’ possible II.No increasing efficiency with increasing amount iron oxide. III.Surprisingly, the ion exchange mechanism played a dominant role Disclaimer: under laboratory circumstances Fe 2+ Na + OH Na + Sand grain surface Na + Fe 2+

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

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

35. 35 Recommendations I.More column experiments varying water quality, sand materials, experiment run times II.Verify the ion exchange mechanism Measure Cation exchange capacity, apply cation free injection water, more sampling III.Focus on soil chemistry detailed surface analyses: charge, potential, surface area (BET), X-ray diffraction

36. 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. 37 Thank you for your attention

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39. 39 DE FILIPPIJNEN Gretha (tropen)verpleegkundige Gezondsheidstraining in communities (niet in kliniek) Harmen drinkwater ingenieur Faciliteren bij implementatie drinkwatersysteem 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 www.watervoorfilippijnen.nl