Adsorption Equilibrium Adsorption vs. Absorption Adsorption is accumulation
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 Adsorption PHASE I PHASE 2 Absorption (partitioning) PHASE I
PHASE 2 Pgas K H caq Henrys Law The Jargon of Adsorption Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+
Cu2+ Cu 2+ Cu2+ Adsorbed species present at an overall concentration of cCu(ads) Dissolved adsorbate, at concentration cCu(aq)
Cu2+ Cu2+ Cu2+ Adsorbent, in suspension at concentration csolid Adsorbed species, with adsorption density q mg Cu per g solid or per m2
Surface area per gram of solid is the specific surface area mg adsorbed mg adsorbed g solid per ci ,ads qi csolid per L of
solution per g adsorbent L of solution Causes 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) Adsorbents in Natural & Engineered Systems Natural Systems Sediments Soils Engineered Systems
Activated carbon Metal oxides (iron and aluminum as coagulants) Ion exchange resins Biosolids Engineered Systems - Removal Objectives Activated carbon (chemical functional groups) Adsorption of organics (esp. hydrophobic) Chemical reduction of oxidants Metal oxides (surface charge depends on pH) Adsorption of natural organic matter (NOM) Adsorption of inorganics (both cations & anions)
Ion exchange resins Cations and anions Hardness removal (Ca2+, Mg2+) Arsenic (various negatively charged species), NO3-, Ba2+ removal 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 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) Characteristics of Some Granular Activated Carbons Characteristics of Activated Carbons (Zimmer, 1988) F 300 H 71
C25 Bituminous Coal Lignite Coconut Shell Bed Density, F (kg/m3) 500 380
0.79 Surface Area BET (m2/g) 875 670 930 0.33 0.21
0.35 ---- 0.38 0.14 ---- 0.58 0.16 ----
1.17 0.65 Activated Carbon Raw Material Pore Volume (cm3/g) Micro- ( radius < 1nm) Meso-
(1nm < r < 25nm) MacroTotal (radius > 25nm) Oxygen-Containing Surface Groups on Activated Carbon Mattson and Mark, Activated Carbon, Dekker, 1971 Kinetics of Atrazine Sorption onto GAC 167 mg GAC/L
333 mg GAC/L Metal Oxide Surfaces Coagulants form precipitates of Fe(OH)3 and Al(OH)3 which have OH surface groups that can adsorb humics and many metals Humic substances where R is organic Sorption of NOM on Metal Oxide Sorption of Metals on Metal Oxide SOH + Me2+ SOMe+ + H+ Ion Exchange Resins
2R-Na+ + Ca2+ R2-Ca + 2Na+ R+-Cl + H2AsO4 R+- H2AsO4+ Cl Adsorptive Equilibration in a Porous Adsorbent Pore Early Later Laminar Boundary Layer GAC Particle
Adsorbed Molecule Diffusing Molecule Equilibrium Adsorption Isotherms Add Same Initial Target Chemical Concentration, Cinit, in each Control Different activated carbon dosage, Csolid, in each mg cinit c fin mg/L q fin csolid g/L
g An adsorption isotherm is a q vs. c relationship at equilibrium Assuming mineral surface started with q = 0: If mineral surface started with q >0: Commonly Reported Adsorption Isotherms Linear: q klin c Langmuir: q qmax n
q k c Freundlich: f K Lc 1 K Lc Shape of Langmuir Isotherm Shape of Freundlich Isotherm q k f c n
Shape of Freundlich Isotherm (log scale) log q log k f n log c Example. Adsorption of benzene onto activated carbon has been reported to obey the following Freundlich isotherm equation, where c is in mg/L and q is in mg/g: 0.533 qbenz 50.1 cbenz A solution at 25oC containing 0.50 mg/L benzene is to be treated in a batch process to reduce the concentration to less than 0.01 mg/L. The adsorbent is activated carbon with a specific surface area of 650 m2/g. Compute the required activated carbon dose. Solution. The adsorption density of benzene in equilibrium with ceq of 0.010 mg/L can be determined from the isotherm expression:
0.533 qbenz 50.1 cbenz 4.30 mg/g A mass balance on the contaminant can then be written and solved for the activated carbon dose: ctot ,benz cbenz qbenz c AC 0.50 0.010 4.30 mg/g c AC c AC 0.114 g/L 114 mg/L Example If the same adsorbent dose is used to treat a solution containing 0.500 mg/L toluene, what will the equilibrium concentration and adsorption density be? The adsorption isotherm for toluene is:
0.365 qtol 76.6 ctol Solution. The mass balance on toluene is: ctot ,tol ctol qtol c AC 0.50 ctol 76.6 ctol 0.365 0.114 g/L ctol 3.93x10 4 mg/L 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 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 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) v, cOUT Process operates at unsteady state, cOUT must increase over time
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