CEEN 572 Environmental Engineering Pilot Plant Laboratory Introduction to Adsorption

1. CEEN 572Environmental Engineering Pilot Plant Laboratory Introductionto Adsorption

2. Adsorption Equilibrium • Adsorption vs. Absorption • Adsorption is accumulation of molecules on a surface (a surface layer of molecules) in contact with an air or water phase • Absorption is dissolution of molecules within a phase, e.g., within an organic phase in contact with an air or water phase

3. Adsorption vs. Absorption Adsorption PHASE I ‘PHASE’ 2 Absorption (“partitioning”) PHASE I Henry’s Law PHASE 2

4. Mechanisms of Adsorption • Dislike of Water Phase – ‘Hydrophobicity’ • Attraction to the Sorbent Surface • van der Waals forces: physical attraction • electrostatic forces (surface charge interaction) • chemical forces (e.g., - and hydrogen bonding)

5. Adsorbents in Natural & Engineered Systems • Natural Systems • Sediments • Soils • Engineered Systems • Activated carbon • Metal oxides (iron and aluminum as coagulants) • Ion exchange resins • Biosolids

6. Engineered Systems - Removal Objectives • Activated carbon (chemical functional groups) • Adsorption of organics (esp. hydrophobic) (including T&O) • Chemical reduction of oxidants • Metal oxides (surface charge depends on pH) • Adsorption of natural organic matter (NOM) • Adsorption of inorganics (both cations & anions) • Activated alumina (Al2O3) for Fˉ, As(V), PO43 ˉ • Silica-based adsorbents • Ion exchange resins • Cations and anions • Hardness removal (Ca2+, Mg2+) • Arsenic (various negatively charged species), NO3-, Ba2+

7. Contaminant Removal Effectiveness

8. Contaminant Removal Effectiveness

9. Contaminant Removal Effectiveness

10. Adsorption Principles • Surface reaction: high specific surface (m2/g or m2/bed volume; easily accessible) • Specific affinity necessary • Applications • Addition of powder/suspension (powdered activated carbon – PAC) in batch reactors (discontinuous process) • Activated carbon contactor (reactor) (granular activated carbon - GAC) in fixed-bed adsorbers (semi-continuous process)

11. Activated Carbon • Most active carbon is made from coconut shell or coal materials • Heated to over 2,000 °C under high pressure • creates fissures that results in a large surface area

12. Steps in Preparation of Activated Carbon • Pyrolysis – heat in absence of oxygen to form graphitic char • Activation – expose to air or steam; partial oxidation forms oxygen-containing surface groups and lots of tiny pores

13. Factors Affecting Activated Carbon Properties • Starting materials (e.g., coal vs. wood based) and activation • Pores and pore size distributions • Internal surface area • Surface chemistry (esp. polarity) • Apparent density • Particle Size: Granular vs. Powdered (GAC vs. PAC)

14. Activated Carbon • Pore sizes: • Micropores (pore diameter less than 20 nm) • Mesopores (pore diameter 20-200 nm) • Macropores (200 nm & above)

15. Activated Carbon • Removes a broad spectrum of organic substances • Pesticides • Taste and odor causing compounds • NOM

16. Powdered Activated carbon (PAC) • Powdered activated carbon (PAC): taste, odor • Common dosage: 1 to 5 mg/L • Used on a one-time basis, no recovery or reuse • Typically used seasonally • Can easily be added to existing conventional plants • Only PAC dosing and mixing facilities required

17. Granular Activated Carbon (GAC) • Granular activated carbon (GAC): SOC, NOM • Greater dosages and contact times required than PAC • Requires carbon filters • Requires reactivation or replacement • Facilities need to backwash GAC beds

20. PAC and GAC

21. Properties of GAC Produced in the US

22. Adsorption Mechanisms • Physical adsorption • van-der-Waals forces (adsorption energy 10-20 kJ/mol; multi-molecular adsorption) • electrostatic forces: +/- • Chemical adsorption (“chemisorption”) • covalent bonding (adsorption energy 40-200 kJ/mol; activation necessary; compound specific; maximal mono-molecular adsorption)

23. Adsorption Kinetics • Bulk solution transport: • Transport of adsorbate from bulk solution to boundary layer • Film diffusion transport • Transport by molecular diffusion through the stationary layer of water (hydrodynamic boundary layer) • Internal pore transport • Transport through the pores to available adsorption sites • Adsorption • Adsorption bond is formed between adsorbate and adsorbent

24. The Process of Adsorption

25. Adsorptive Equilibration in a Porous Adsorbent Pore Early Later Laminar Boundary Layer GAC Particle Equilibrium Adsorbed Molecule Diffusing Molecule

26. Kinetics of Atrazine Sorption onto GAC

27. Readily and Poorly Adsorbed Organics

28. The Jargon of Adsorption

29. Adsorption Isotherms Add Same Initial Target Chemical Concentration, Cinit, in each container Control Different activated carbon dosage, Csolid, in each An adsorption ‘isotherm’ is a q vs. c relationship at equilibrium

30. Development of Adsorption Isotherms • Adsorption experiment: • Add mass of carbon (m) to defined volume (V) of sample, measure Co and Ceq • Mass balance (batch adsorption process): • V(Co – Ceq) = m * qe • qe = V/m (Co – Ceq) Isotherm

31. Development of Adsorption Isotherms (cont.) V/m = const.; vary Co vary V/m; Co = const.

32. Adsorption Equilibrium qi = f(co, eq), T = const. • Langmuir (1918) • Freundlich (1907) • Brunauer, Emmet, and Teller (1938) • Redlich-Petersen

33. Langmuir Isotherm • Assumptions for Langmuir adsorption isotherm: • Homogeneous surface • A fixed number of accessible sites, all of which have the same energy (qe = qmax) • Reversible adsorption and desorption • Rate of adsorption is proportional to liquid phase concentration

34. Langmuir Isotherm (cont.)

35. Langmuir Isotherm (cont.) If c(t) = const. = Ceq, q(t) = qe with KL = k/k’

36. Langmuir Isotherm (cont.) • Determination of Langmuir constants: • Linearization • plot 1/q vs. 1/c

37. Langmuir Isotherm (cont.) • Special cases: • C is small: • KLCeq <<1 qe = qm KLCeq (“linear isotherm”) • C is large: • KLCeq >>1  qe = qm (“horizontal isotherm”)

38. Freundlich Isotherm • Freundlich adsorption isotherm: • empirical approach (1912) • applicable in specific concentrations range only • Linearization

39. Freundlich Isotherm (cont.)

40. Competitive Adsorption Atrazine

41. Summary of Adsorption Isotherms Linear: Langmuir: Freundlich: Klin, KL, qmax, and n are all empirical constants

42. Shape of Langmuir Isotherm

43. Shape of Freundlich Isotherm

44. Shape of Freundlich Isotherm (log scale)

45. General Process Design Features • Contactors provide large surface area • Types of contactors • Continuous flow, slurry reactors • Batch slurry reactors (infrequently) • Continuous flow, packed bed reactors • Product water concentration may be • Steady state or • Unsteady state

46. Powdered Activated Carbon (PAC) PAC + Coagulants Settled Water Sludge Withdrawal PAC particles may or may not be equilibrated PAC + Coagulants Flocculated Water Process Operates at Steady-State, cout = constant in time

47. Packed Bed Adsorption v, cIN Natural Packed Bed – subsurface with groundwater flow Engineered Packed Bed- granular activated carbon EBCT = empty-bed contact time (Vbed/Q) Adsorptive capacity is finite (fixed amount of adsorbent in bed) Process operates at unsteady state, cOUT must increase over time v, cOUT