1 Adsorption and Absorption l Adsorption »Process by which a solute accumulates at a solid-liquid interface l Absorption »partitioning of solute into a solid material (at molecular level) Sorption = Adsorption + Absorption

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

3 Adsorption to a pore

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

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

6 Physical (cont.) l Hydrogen Bonding »special case of dipole-dipole interaction, involves hydrogen atom with slightly positive charge l 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 Reactive (Active) surface site l A location on the surface of the adsorbent where the physical/chemical attraction is favorable Reactive surface site

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

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

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

11 Adsorbates of Interest (cont.) l Humic substances »large natural organics, often color forming, with molecular weights ranging from few hundred to hundred thousands. Adsorption properties vary widely. C H HH 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.

12 Adsorbates of Interest (cont.) l Some metals »antimony, arsenic, silver, mercury,... l Viruses l Other inorganics »Chlorine, Bromine

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

14 What is Activated Carbon? l 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 ü m 2 /g »Creates active adsorption sites ücarbon is non-polar, good for adsorbing non- polar compounds

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

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

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

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

19 GAC Use (cont.) l Clean contaminated groundwater »Counter flow üdirtiest GAC contacts dirtiest water ücontinuous or batch addition of fresh GAC & removal of dirty GAC

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

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

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

23 Single Tank Dirty Water Clean Water Dirty GAC Clean GAC

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

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

26 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m

27 Breakthrough Curve Time (days) Column 1 Column 2 Column 3 C out /C o C out = concentration exiting a column

28 Bed-Depth Service Time Service Time (days) Bed Depth (m) % Feed Concentration 10 % Feed Concentration AZ UW ac = Unit weight of act. carbon, mass/volume M ac = Activated carbon needed, mass/time a = slope of service time lines, time/length 1/a = velocity of AZ, length/time A = Column cross-sectional area, area M ac = A(1/a)UW ac AZ = Adsorption Zone length l = length of column # = No. of columns required # = (AZ / l) + 1 then round UP

29 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 0

30 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 1

31 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 2

32 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 3

33 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 4

34 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 5

35 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 6

36 SP2 SP3 SP1 SP4 Experiment Column 1 Column 2 Column m AZ = 2.5 m < to 0.9 >0.9 Time Step 7

37 Breakthrough Curve Time (days) Column 1 Column 2 Column 3 C out /C o C out = concentration exiting a column

38 Bed-Depth Service Time Service Time (days) Bed Depth (m) % Feed Concentration 10 % Feed Concentration AZ UW ac = Unit weight of act. carbon, mass/volume M ac = Activated carbon needed, mass/time a = slope of service time lines, time/length 1/a = velocity of AZ, length/time A = Column cross-sectional area, area M ac = A(1/a)UW ac AZ = Adsorption Zone length l = length of column # = No. of columns required # = (AZ / l) + 1 then round UP