Mechanisms of Organic Fouling and Chemical Cleaning of RO

1. Mechanisms of Organic Fouling and Chemical Cleaning of RO/NF Membranes Menachem Elimelech Department of Chemical Engineering Environmental Engineering Program Yale University

2. Mention that it is most common and ubiquitous.Mention that it is most common and ubiquitous.

3. Mention that it is most common and ubiquitous.Mention that it is most common and ubiquitous.

4. Fouling Happens! Fouling is inevitable in membrane separations (and other filtration devices) We can minimize the rate of fouling, but we cannot eliminate fouling (unless very extensive pre-treatment is applied)

5. Can We Live with Fouling?

6. Chemical Cleaning Commercial products: combination of alkaline, metal chelating agents, and surfactants Cleaning methods are based on trial and error

7. Objectives To elucidate the mechanisms of organic fouling and chemical cleaning of organic-fouled RO/NF membranes To relate fouling and cleaning mechanisms to intermolecular adhesion forces Emphasize this is the first time to study the mechanisms on the molecular level.Emphasize this is the first time to study the mechanisms on the molecular level.

8. Overview of Interfacial Force Measurement

9. Force Measurement by Atomic Force Microscopy (AFM) Explain how to convert deflection vs. displacement curve to force vs. distance curve.Explain how to convert deflection vs. displacement curve to force vs. distance curve.

10. Functionalized Colloidal Probe

12. Interfacial Force Measurement

13. Organic Fouling of RO and NF Membranes

14. Mention that it is most common and ubiquitous.Mention that it is most common and ubiquitous.

15. Model Polysaccharide: Alginate Represents hydrophilic fraction of EfOM (polysaccharides) Also found in surface waters (originating from algae) Molecular weight ranging from 12 to 80 kDa Contains carboxylic functional groups

16. RO and NF Membranes RO Membrane LFC1, “low fouling” (Hydranautics) Fully aromatic; rough surface Commonly used for wastewater reclamation NF Membrane NF-270, “loose” NF (FilmTec) Relatively smooth surface

17. Effect of Divalent Cations on NF Humic Acid Fouling: Mg2+ vs. Ca2+ Change axis ticks Change axis ticks

18. Interaction Force (Approach Curve) between Humic and Fouling Layer

19. Bridging Effect of Ca2+

20. Influence of Divalent Ions (Ca2+ vs Mg2+) on Alginate Fouling of RO Membrane

21. Gel Formation of Alginate by Ca2+: “Egg-box” Structure In the presence of Ca cations, the gelation and crosslinking of the polymers are mainly achieved by the exchange of sodium ions from the guluronate groups with the Ca cations, and the stacking of these guluronate groups to form the ‘egg-box’ model. The divalent cations bind to the guluronic blocks in a highly cooperative manner. Each alginate chain can dimerize to form junctions with many other chains, and as a result, gel networks are formed. The gel network adds to the hydraulic resistance in RO membrane process. During chemical cleaning, the efficiency of a cleaning agent depends on the ability of the cleaning agent to break down the alginate gel network in the fouling layer formed in the presence of calcium through chemical reaction. In the presence of Ca cations, the gelation and crosslinking of the polymers are mainly achieved by the exchange of sodium ions from the guluronate groups with the Ca cations, and the stacking of these guluronate groups to form the ‘egg-box’ model. The divalent cations bind to the guluronic blocks in a highly cooperative manner. Each alginate chain can dimerize to form junctions with many other chains, and as a result, gel networks are formed. The gel network adds to the hydraulic resistance in RO membrane process. During chemical cleaning, the efficiency of a cleaning agent depends on the ability of the cleaning agent to break down the alginate gel network in the fouling layer formed in the presence of calcium through chemical reaction.

22. Gel Formation Leads to Severe Fouling

25. Relating Fouling to Interfacial Adhesion Forces

26. AFM Force Measurement

27. Effect of Divalent Cations: Humic Acid Fouling of NF Membrane

28. Effect of Ionic Strength: Alginate Fouling of RO Membrane

29. Effect of Divalent Ions (Ca2+ vs Mg2+): Alginate Fouling of RO Membrane

30. Relating Fouling to Adhesion Force: Bridging by Ca2+ This is a schematic of the foulant-foulant interactions which was mentioned in the earlier slide. The CML particle contains a highly charge layer of carboxylic groups, which at the ambient pH of 5.8 – 6.0 , would be deprotonated and would have a strong tendency to adsorb Ca2+ which would act as bridging agents for the SA molecules to be attached on the particle surface. The membrane, at the ambient pH of 5.8 – 6.0, would also have the carboxylic functional groups on the surface deprotonated and adsorbed the divalent cations which would bridge the SA molecules. Thus, after equilibration, it is assumed that a layer of SA molecules could be adsorbed to the clean membrane, so as to simulate foulant-foulant interactions. The presence of Ca ions would bridge the SA adsorbed on the CML and the SA on the membrane surface.This is a schematic of the foulant-foulant interactions which was mentioned in the earlier slide. The CML particle contains a highly charge layer of carboxylic groups, which at the ambient pH of 5.8 – 6.0 , would be deprotonated and would have a strong tendency to adsorb Ca2+ which would act as bridging agents for the SA molecules to be attached on the particle surface. The membrane, at the ambient pH of 5.8 – 6.0, would also have the carboxylic functional groups on the surface deprotonated and adsorbed the divalent cations which would bridge the SA molecules. Thus, after equilibration, it is assumed that a layer of SA molecules could be adsorbed to the clean membrane, so as to simulate foulant-foulant interactions. The presence of Ca ions would bridge the SA adsorbed on the CML and the SA on the membrane surface.

31. Relating Fouling to Adhesion Force:Influence of Divalent Ions (Ca2+)

32. Remarkable Correlation between Fouling Rate and Adhesion Force

33. Cleaning of Organic Fouled RO and NF Membranes

34. Cleaning Chemicals Deionized water (as a baseline) Alkaline: NaOH, pH 11 Metal chelating agent: EDTA Anionic surfactant: SDS

35. Fouling/Cleaning Experimental Protocol

36. Chemical Aspects of Cleaning:SDS and EDTA as a Function of pH

37. Chemical Aspects of Cleaning:SDS and EDTA as a Function of pH

38. Chemical Aspects of Cleaning:Influence of Cleaning Agent (EDTA) Dose

39. EDTA Cleaning: A Ligand-Exchange Mechanism

40. Chemical Aspects of Cleaning:Influence of Cleaning Agent (SDS) Dose

41. Physical Aspects of Cleaning

42. Physical Aspects of Cleaning

43. Can AFM Data Explain Chemical Cleaning Behavior?

44. Interfacial Force Measurement

46. Effect of Cleaning Chemicals on Humic Acid Adhesion Force

47. Relating Chemical Cleaning Efficiency to Adhesion (Humic-NF System)

48. Conceptual Model for Chemical Cleaning Chemical cleaning of organic fouled membranes involves two steps: Chemical step: Reaction between the chemical cleaning agent and the foulant ? need favorable “chemistry” and stoichiometry Physical step: Mass transfer of cleaning agent into the fouling layer and of foulants away from the surface

49. [32] Based on the experimental results, the cleaning mechanisms of a alginate-fouled membrane is proposed. In presence of calcium ions, the cross-linked fouling layer is formed on the membrane surface. This binds organic foulants and form bridges between adjacent foulant molecules. [32] Based on the experimental results, the cleaning mechanisms of a alginate-fouled membrane is proposed. In presence of calcium ions, the cross-linked fouling layer is formed on the membrane surface. This binds organic foulants and form bridges between adjacent foulant molecules.

50. [33] During cleaning, the cleaning agent is transferred into the fouling layer.[33] During cleaning, the cleaning agent is transferred into the fouling layer.

51. [34] The cleaning agent reacts with the foulants in the fouling layer.[34] The cleaning agent reacts with the foulants in the fouling layer.

52. [35] When chemical reaction is favorable, these reaction products and the foulant are removed from the fouling layer to the bulk solution through the hydrodynamics/mass transfer. [35] When chemical reaction is favorable, these reaction products and the foulant are removed from the fouling layer to the bulk solution through the hydrodynamics/mass transfer.

53. [36] The process is then repeated, when there is a transfer of the cleaning agents into the fouling layer.[36] The process is then repeated, when there is a transfer of the cleaning agents into the fouling layer.

54. [37] Chemical reaction takes place between cleaning agent and foulants, and this loosens structural integrity of subsequent fouling layer. Therefore, efficient cleaning can be achieved through the coupling between the chemical reaction and mass transfer by optimizing cleaning conditions.[37] Chemical reaction takes place between cleaning agent and foulants, and this loosens structural integrity of subsequent fouling layer. Therefore, efficient cleaning can be achieved through the coupling between the chemical reaction and mass transfer by optimizing cleaning conditions.

55. Concluding Remarks Adhesion force measurements provide valuable information on organic fouling potential and chemical cleaning efficiency Ca2+ greatly enhances fouling by bridging organic foulant macromolecules and forming a cross-linked matrix Efficient chemical cleaning is achieved by breaking Ca2+ binding/bridging

56. Acknowledgments Graduate students and post-docs USBR, NSF