Adsorption

1. Adsorption • Adsorption • Process by which a solute accumulates at a solid-liquid interface Acknowledgement: Some of these slides were prepared by Dr. J. W. Everett Professor of Civil and Environmental Engineering Rowan University

2. Adsorption Aqueous Phase Napthalene dissolved in aqueous phase Solid Surface Reactive surface site Adapted from Fundamentals of Environmental Engineering, Mihelcic

3. Adsorption to a pore

4. Terms • Adsorbate • Substance removed from liquid phase • Adsorbent • Solid phase on which accumulation occurs • Example • color can be removed from water using activated carbon. Color is the adsorbate, activated carbon is the adsorbent

5. Physical Adsorption • Electrostatic attraction • oppositely charged particles • Dipole-Dipole Interaction • Attraction of two Polar Compounds • Polar compounds have an unequal distribution of charge (e.g., one end of molecule has slight + charge, the other a - charge)

6. Physical (cont.) • Hydrogen Bonding • special case of dipole-dipole interaction, involves hydrogen atom with slightly positive charge • Vander Waals Force • Weak attraction caused when close proximity of two non-polar molecules causes change in distribution of charges, setting up a slight dipole-dipole attraction

7. Another way to look at adsorption • Molecules prefer to be in lower energy state • If molecule can attain lower energy state by “sticking” to a solid surface, it will. • E.g., hydrophobic compounds

8. Equilibrium • At equilibrium, the chemical of concern will be found… • Dissolved in aqueous phase AND • Adsorbed to solid phase adsorbent • Adsorption is Reversable • add more to aqueous phase - get more adsorption • reduce concentration in aqueous phase, get desorption

9. Adsorbates of Interest • Taste and Odor (major interest) • Synthetic Organic Compounds (SOC) • Aromatic solvents (benzene, toluene) • Chlorinated aromatics • Pesticides, herbicides • Many more

10. H H C H H • Halomethanes can be formed when water containing humic substances is chlorinated. • Methane molecule with halogens (Cl, Br,...) substituted for H’s. Some are carcinogens. Adsorbates of Interest (cont.) • Humic substances • large natural organics, often color forming, with molecular weights ranging from few hundred to hundred thousands. Adsorption properties vary widely.

11. Adsorbates of Interest (cont.) • Some metals • antimony, arsenic, silver, mercury,... • Viruses • Other inorganics • Chlorine, Bromine

12. Adsorbents • Activated Carbon • Will remove all of the adsorbates mentioned above (to varying degrees) • by far most popular adsorbent • Synthetic resins • Zeolites • Clays with adsorptive properties

13. History of Activated Carbon Adsorption on porous carbons was described as early as 1550 B.C. in an ancient Egyptian papyrus and later by Hippocrates and Pliny the Elder, mainly for medicinal purposes. In the 18th century, carbons made from blood, wood and animals were used for the purification of liquids. What about the role of Rosalind Franklin? Her work focused on a wartime problem: the nature of coal and charcoal and how to use them most efficiently. She published five papers on the subject before she was 26 years old. Her work is still quoted today, and helped launch the field of high-strength carbon fibers. At 26, Franklin had her PhD and began working in x-ray diffraction.

14. What is Activated Carbon? • Carbon that has been pyrolyzed (heated in a low oxygen environment) • Burns off tar, volatizes off gases • Creates material with lots of pores, thus lots of surface area • 500 - 1000 m2/g • Creates active adsorption sites • carbon is non-polar, good for adsorbing non-polar compounds

15. Activated Carbon Picture Source: solomon.bond.okstate.edu/thinkchem97/experiments/lab7.html

16. Types • PAC: Powdered activated carbon • A fine powder, < 0.05 mm dia. • As much as 100 acres of surface area / lb • Pore sizes down to 10 x10-7 m. • GAC: Granular activated carbon • 0.3 - 3 mm • Not as much surface area as PAC

17. How do we use PAC? • Water Treatment • Add it to rapid mix unit, remove in filter • do not regenerate • Typical dose ~ 5 mg/L • Used to remove taste and color

18. How do we use GAC? • Water Treatment • As filter media to assist in taste and odor removal Water Head Activated Carbon Bed Sand Bed Under drain

19. GAC Use (cont.) • Clean contaminated groundwater • Counter flow • dirtiest GAC contacts dirtiest water • continuous or batch addition of fresh GAC & removal of dirty GAC

20. Gas Station Site Clean Water Dirty Water Drums of Activated Carbon Groundwater Pumping Well Contaminated Aquifer

21. Gas Station Site First Drum gets dirty fastest Drums of Activated Carbon Groundwater Pumping Well Contaminated Aquifer

22. Gas Station Site Add clean drum at end Pull first drum Drums of Activated Carbon Groundwater Pumping Well Contaminated Aquifer

23. Dirty Water Dirty GAC Clean GAC Clean Water Single Tank

24. Design • Pass contaminated water through single or series of columns • Use constant Loading Rate (flow/area) and Influent Concentration • Record concentration at difference points along column(s) over time • Plot Breakthrough & Bed Service Time Curves • Determine width & velocity of Adsorption Zone • determine # of columns needed, amount of adsorbant needed

25. Terms • Loading Rate • Flux through column, Flow / Area • Co • Concentration in influent to 1st column • Adsorption Zone • Zone where majority of adsorption is occurring • Defined as zone where concentration is between 10 and 90 % of Co.

26. Experiment SP1 Column 1 Column 2 Column 3 2.3 m SP3 SP4 SP2