13 adsorption
- 1. Adsorption
- 2. Adsorption Types: physical, chemical and exchange adsoprtion Physical adsoprtion – The process is quite rapid and exothermic (heat liberated is comparable to enthalpy of vapour condensation, 2-20 kJ/g.mole) – Adsorption occurs and adsorbate is held on the adsorbent surface by relaively weak intermolecular attractive forces (van der waal forces) – Adsorption is is directly proportional to the adsorbnent surface and can occur as a mono or multi layers – Reversible process – temperature increase and lowering of adsorbate concentration results in the desorption – Adsorbate is relatively free to move on the surface
- 3. Adsorption Chemical adsorption – Exothermic process and involves chemical interaction between the adsorbate and the adsorbent • Involves strong bond formation - energy liberated is 20-400 kJ/g.mole • Only monomolecular layer of adsorbate is formed on the adsorbent. – Usually irreversible – if desorbed the desorbent might have undergone chemical change and is different from the adsorbate Exchange adsorption – Charged sites on the surface are responsible Adsorption by activated carbon is a combiation of physical and chemical adsorption
- 4. Adsorbents and adsorbates • Adsorbates having low affinity for the solvent (hydrophobic molecules from water) • Applications of adsorption technologies – Contaminants in (aqueous) solutions and in exhausts/vents and emissions – Non-combustible gaseous pollutants specially present at very low concentration – Pollutants sufficiently valuable and warrant recovery – Water purification – Drying of (instrumental) air – Separation and purification of gases from air – Adsorption chillers
- 5. Adsorbents and adsorbates • Adsorbents – Activated carbon – Silica gel – Activated alumina – Synthetic zeolite (Molecular sieves) - crystalline zeolites with uniform pores to selectively separate compounds by size & shape
- 6. Properties of Molecular Sieves Anhydrous Sodium Aluminosilicate Anhydrous Calcium Aluminosilicate Anhydrous Aluminosilicate Type 4A 5A 13X Density in bulk (lb/ft3 ) 44 44 38 Specific Heat (BTU/lbo F) 0.19 0.19 - Effective diameter of pores (Å) 4 5 13 Regeneration Temp. (o C) 200-300 200-300 200-300 Max. Allowable Temp. (o C) 600 600 600 Properties of Silica Gel Bulk Density 44-56 lb/ft3 Heat Capacity 0.22-0.26 BTU/lbo F Pore Volume 0.37 cm3 /g Surface Area 750 m2 /g Average Pore Dia. 22 Å Regen Temp. 120-250 o C Max. allowable temp. 400 o C Properties of Activated Alumina Bulk density (Granules) 38-42 lb/ft3 Bulk density (Pellets) 54-58 lb/ft3 Specific Heat 0.21-0.25 BTU/lbo F Pore Volume 0.29-0.37 cm3 /g Surface Area 210-360 m2 /g Average Pore Dia. 18-48 Å Regen. temp. (steam) 200-250 o C Max. allowable temp. 500 o C Properties of Activated Carbon Bulk Density 22-34 lb/ft3 Heat Capacity 0.27-0.36 BTU/lbo F Pore Volume 0.56-1.20 cm3 /g Surface Area 600-1600 m2 /g Average Pore Dia. 15-25 Å Regen.temp.(Steam) 100-140 o C Max. allowable temp. 150 o C
- 7. Adsorption isotherms • Equilibrium adsorption isotherm – Adsorption isotherm relates the volume or mass adsorbed to the partial pressure or concentration of the adsorbate – Quantity of adsorbate adsorbed (from gas phase) per unit adsorbent is measured against partial presence of the adsorbate (in gas phase) at equilibrium at given temperature (very sensitive to T) – Incase of adsorbate in liquid/solution the measurement is against concentration of adsorbate in the solution Many equations are proposed to fit the experimental adsorption results Which model/equation to be used to describe the adsorption process is determined through experimentation
- 8. )/)(1(1)[( oom PPcPP cP V V −+− = V is the amount of adsorbed gas at a given pressure and temperature Vm is the amount adsorbed if one molecule thick layer fills the surface Po is vapor pressure of the adsorbate at the system’s temp. P is partial pressure of the adsorbate C is a parameter of adsorption process Y = C + m X BET (Brunauer,Emmett, and Teller) Adsorption Isotherm When P/P0 is <0.05 and >0.35 then the plot is not linear and this approach for estimating Vmono and C values can not be followed
- 9. Langmuir Adsorption Isotherm Langmuir adsorption isotherm assumes unimolecular layer Mass of adsorbate per unit mass of adsorbent is proportional to ‘f’ bCkbX M kC kbC M X k C k k M X C Ck Ck CCC CC C M X CCC CC fwherefC M X ee e e e e e dea ea a dea ea a 111 1 1 1 11 2 2 1' ' += + = += + = + = + == Ca and Cd are constants ‘f’ occupied fraction of the total solid surface Ce is concentration or partial pressure of the adsorbate
- 10. Fruendlich adsorption isotherm At very low and very high adsorbate partial presssure Langmuir isotherm takes fruendlich isotherm form Adsorption isotherms can also be constructed for heterogeneous mixture of compounds Parameters, like, TOC, COD, dissolved organic halogens, fluorescence, UV absorbance, are used to measure adsorbate concentration n ekC M X /1 = kC nM X e loglog 1 log += k is freundlich capacity factor n is Freundlich intensity parameter
- 11. Competetive adsorption • More than one solute competing for the same adsorption site • Multi-solute Langmuir isotherm
- 12. Factors affecting adsorption • Adsorbent – Every solid surface has the adsorption capacity • Adsorbate – Increasing solubility decreases adsorption – Solubility of adsorbate is affected by molecular size, ionization and polarity • pH – Affects surface charge of the adsorbent and the charge on the solute – Lowering pH increases adsorption for the orgnic materials • Temperature – Exothermic process – hence increasing temperature decreases adsorption • Presence of other solutes – Compete for the limited number fo adsorption sites
- 14. Adsorption Wave
- 15. Adsorption Adsorption is a surface phenomenon a film of the adsorbate is created on the surface of the adsorbent. Adsorption is increase in the concentration of a substance at the interface of a solid and a liquid or gaseous layer owing to the operation of surface forces atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates The adsorption process is classified as physisorption (weak van der Waals forces) or chemisorption (covalent bonding) Adsorption may also occur due to electrostatic attraction
- 17. Freundlich • Freundlich isotherm is a purely empirical formula for gaseous adsorbates • It was the first mathematical fit published by Freundlich and Küster (1894) • where ‘x’ is the quantity adsorbed, ‘m’ is the mass of the adsorbent, ’P’ is the pressure of adsorbate and ‘k’and ‘n’ are empirical constants for each adsorbent-adsorbate pair at a given temperature • The function is not adequate at very high pressure because in reality ’x/m’ has an asymptotic maximum as pressure increases without bound. • As the temperature increases, the constants ‘k’ and ’n’ change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface.
- 18. Langmuir • Irving Langmuir was the first to derive a scientifically based adsorption isotherm in 1918 • The model applies to gases adsorbed on solid surfaces • It is a semi-empirical isotherm with a kinetic basis and was derived based on statistical thermodynamics • It fits a variety of adsorption data • It is based on four assumptions: – All of the adsorption sites are equivalent and each site can only accommodate one molecule. – The surface is energetically homogeneous and adsorbed molecules do not interact. – There are no phase transitions. – At the maximum adsorption, only a monolayer is formed – Adsorption only occurs on localized sites on the surface, not with other adsorbates.
- 19. Langmuir • There are always imperfections on the surface • Adsorbed molecules are not necessarily inert • Mechanism is clearly not the same for the very first molecules to adsorb to a surface as for the last molecule • Frequently more molecules will adsorb to the monolayer • Langmuir isotherm is the first choice for most models of adsorption, and has many applications in surface kinetics and thermodynamics. • Langmuir suggested that adsorption takes place through this mechanism: – Here A is a gas molecule and S is an adsorption site • The direct and inverse rate constants are k and k−1. • If we define surface coverage ’ϴ’ as the fraction of the adsorption sites occupied, in the equilibrium we have:
- 20. Langmuir • Where ‘P’ is the partial pressure of the gas or the molar concentration of the solution. For very low pressures θ ~K.P and for high pressures θ~1, θ is difficult to measure experimentally; usually, the adsorbate is a gas and the quantity adsorbed is given in moles, grams, or gas volumes at standard temperature and pressure(STP) per gram of adsorbent. If we call vmon the STP volume of adsorbate required to form a monolayer on the adsorbent (per gram of adsorbent), and we obtain an expression for a straight line: • Through its slope and y-intercept we can obtain vmon and K, which are constants for each adsorbent/adsorbate pair at a given temperature. vmon is related to the number of adsorption sites through the ideal gas law. If we assume that the number of sites is just the whole area of the solid divided into the cross section of the adsorbate molecules, we can easily calculate the surface area of the adsorbent. The surface area of an adsorbent depends on its structure; the more pores it has, the greater the area, which has a big influence on reactions on surfaces. • If more than one gas adsorbs on the surface, we define as the fraction of empty sites and we have: • Also, we can define as the fraction of the sites occupied by the j-th gas:
- 21. Adsorption A surface phenomenon - a mass transfer operation • a process of accumulation of substances in solution on a suitable interface (liquid-solid, gas-solid or liquid-air) – Flotation is an example for material accumulation at the liquid-air interface • involves transfer of a constituent from a liquid phase to a solid phase (liquid solid interface process) Adsorbate: substance being transferred from liquid phase to solid phase Adsorbent: the interface (active solid phase) onto which the adsorbate accumulates Types of adsorbants: Activated carbon, Synthetic polymeric type adsorbent, Silica based adsorbent Adsorption on activated carbon is used in wastewater treatment as a polishing step (for toxicity reduction etc.)
- 22. Adsorption Refractory organics (ABS, heterocyclic organics) are difficult or impossible to remove by conventional biological treatment Adsorption can be chemical or physical: chemical adsorption results in the formation of monomolecular of adsorbate on the adsorbent Physical adsorption results in the molecular condensation in the capillaries of the adsorbent Substances of higher molecular weight are more easily adsorbed Activated carbon (powdered or granular) is used A column of granular activated carbon (GAC) very similar to pressure sand filter is used An adsorption tank and a settling tank or a filter press are used in case of adsorption with powdered activated carbon (PAC) Break point and exhaust point may be needed for the design of a GAC column Adsorption isotherms are needed for the design of PAC systems
- 23. Adsorption • Adsorption process can be considered to include – Bulk solution transport: movement adsorbate from bulk solution to the boundary film around the adsorbent by advection and dispersion – Film diffusion transport: movement of adsorbate through the film to the entrances of the pores of the adsrobent – Pore transport: molecular diffusion of adsorbate through the pore liquid and/or movement along the surface of the adsorbent – Adsorption: attachment of the adsorbate at the available adsorption site (on the outer surface and in the macro, meso, micro and submicro pores • Forces that bring about adsorption include coulombic unlike charges, point charge and dipole, dipole-dipole interactions, point charge neutral species, london or vanderwaals forces, covalent bonding with reaction and hydrogen bonding
- 24. Adsorption • Adsorption is two types: physical adsorption and chemical adsorption – Difficult to differentiate the two – Physical adsorption is rapid and one of the 3 transportation steps may limit the rates of adsorption – Chemical adsorption rate is usually limited by adsorption step itself and hence slower • With increased time of contact, the rate of adsorption equals the rate of desorption and an equilibrium concentration is achieved • Quantity adsorbate taken up by the adsorbent is function of both temperature and characterisitcs and concentration of adsorbate – Solubility, molecular structure, molecular weight, polarity and hydrocarbon saturation characterisitcs matter the most
- 25. Adsorption • Amount of adsorbate taken up is function of its concentration at a constant temperature – Adsorption isotherms are used to describe this function • Adsoprtion isotherms are developed by exposing a given amount adsorbate in a fixed volume of liquid to varying amounts of adsorbent – Duration of exposure may be 7 days for ensuring the equilibrium – GAC is powdered to reduce the duration of exposure needed prior to testing for isotherms • Depression in the adsorption capacity is experienced for any specific adsorbate if it is adsorbed from a mixture of adsorbates – Total adsorption capacity of the adsorbent is higher for a mixture of compounds than that for any of the constituent adsorbates – Size of the molecules, their adsorptive affinities and their relative concentrations apparently influence the adsoption
- 26. Adsorption Isotherms Langmuir Isotherm X is Weight of solute adsorbed on adsorbate (mg) M is weight of adsorbent (difference of initial and final amount of adsorbate in the wastewater) (grams) K is equilibrium constant b is a constant representing monolayer coverage per unit weight of adsorbent (mg/gram) Ce is equilibrium concentration of adsorbate (mg/l) Plot 1/(X/M) against 1/Ce for obtaining K and b values e e KC KbC M X + = 1 bCKb M X e 1111 + =
- 27. Freundlich Isotherm k is freundlich capacity factor n is Freundlich intensity parameter • Adsorption isotherms can also be constructed for heterogeneous mixture of compounds – Use of parameters such as TOC, COD, dissolved organic halogens, UV absorbance, fluorescence, are used to measure adsorbate concentration Adsorption Isotherms n ekC M X /1 = kC nM X e lnln 1 ln +=
- 28. Activated carbon • Activated carbon is highly porous, microcrystalline material often with specific functional groups like COOH, OH, etc. • Adsorption behavior is partially influenced by the functional groups – Activated carbon manufactured at <500C is weakly acidic and the activated carbon manufactures at >500C is weakly basic • The pores are of molecular size and hence the mass transfer rate is slower • Activated carbon is two types: PAC and GAC – PAC: powdered activated carbon – particle size is <0.074 mm (passes through 200 sieve) – GAC: granular activated carbon – particle size is >1.0 mm (about 140 seive size) • Application • Remove taste or odour producing compounds • Remove inconvenient (colour) or toxic non-polar compounds • Remove heavy metals!
- 29. Preparation of activated carbon • Carbonize or char organic materials: nut (almond, coconut and walnut) shells; wood; bone; coal, lignite; etc.) – Prepared at high temp. under low oxygen conditions - red hot heating (to 700C) under limited or insufficient (to sustain combustion) oxygen (pyrolytic) conditions – Hydrocarbons are driven out • Activation of the char • Heating (to 300-1000C) in the presence of steam, oxygen or carbon dioxide • Expose the charred material to oxidizing gases such as steam and CO2 at 800 to 900C to develop porous structure and create large internal surface – Pores are considered as micro if size is <1.0 nm, as meso if size is 1-25 nm and as macro if size is >25 nm
- 30. Regeneration and reactivation • Processes used to recover the adsorptive capacity of the spent activated carbon – Oxidation of the adsorbed materials by chemicals – Driving off the adsorbed materials by steam – Use of solvents for the removal – Biological conversion processes to remove adsorbed materials • Spent carbon is heated in a furnace to drive off the adsorbed material – some new compounds may be formed on the carbon surface - Burn off these new compounds • Methodology for the regeneration of the PAC is not well defined and not regenerated • Regeneration of activated activated carbon – Regeneration is done at 500C under low oxygen conditions in the presence of steam – Adsorbed orgaics are volatilized and/or oxidized – Regeneration usually results in the loss of 2-5% of the adsorption capacity – Attition due to mishandling contributes to 4-8% loss of the activated
- 31. Iodine number, phenol number and molasses number • Adsorption capacity • Iodine number: amount of iodine adsorbed when the iodine concentration in equilibrium is 0.02 N – One gram of iodine in one liter of iodine solution • Phenol number: concentration of phenol required in one liter of water, for the phenol concentration in equilibrium is to 0.01 ppm – One gram of pulverized activated carbon is mixed in one litre phenol solution for 4 hours • Molasses number: molasses decolourizing efficiency: ability to remove colour from a standard molasses solution by 90% efficiency
- 32. Adsorption studies with iodine solution • Weighed amount of activated carbon is treated with standard iodine solution, and iodine left in the standard solution is measured by titrating with standard sodium thiosulfate solution • Presence of volatiles, surface porosity and extractables influence the iodine adsorption number • 0.0473 N Iodine solution and 0.0394 N sodium thiosulfate solution are used • Calculations are done by ( ) ××× − = 91.126N W V B SB I I is iodine adsorption number in grams iodine per kg carbon (or mg/g) B is mL sodium thiosulfate used by blank S is mL sodium thiosulfate used by sample V is volume of iodine solution used in mL W is mass of activated carbon (in kg) N is normality of the iodine solution 126.91 is equivalent weight of iodine
- 33. Adsorption studies with iodine solution • Dry activated carbon for 1 hour at >125C in an oven (in petriplate at 10 mm depth), weigh following amounts and take in vials of 40/50 mL capacity (centrifuge tubes!) – 4.0 g, 2.0g, 1.0 g, 0.5 g, 0.25 g, 0.125 g, 0.0625 g and 0.0 g (blank) • Add 25 mL of standard iodine solution to each of the vials and cap – mix the contents on mechanical shaker for 1 minute – centrifuge for 3 minutes – decant supernatant into clean vials and immediately cap – Treat blank also similar to the sample • Take 20 ml of the decant and titrate against standard sodium thiosulfate solution – first titrate to pale yellow, add 5 drops of starch indicator and titrate to colourlessness – Carry out adsorption experiments in sequence on one vial/tube • Find iodine number and record iodine concentration in the decant in each of the cases
- 34. Adsorption studies with phenol solution • Dry the activated carbon for 1 hour at >125C in an oven (in petriplate at 10 mm depth), weigh the following quantities and take in vials of 40/50 mL capacity (centrifuge tubes!) • 4.0 g, 2.0g, 1.0 g, 0.5 g, 0.25 g, 0.125 g, 0.0625 g and 0.0 g (blank). • Add 25 mL of standard Butanol/Butyraldehyde/ Phenol solution (of 1.0 g/L strength) to each of the vials and cap – mix contents on mechanical shaker for 1 minute – centrifuge for 3 minutes – decant supernatant into clean vials and immediately cap • Treat blank also similar to the sample • Test the decanted solution for COD (requires sample digestion and titration) • Find the amount (mg) of Butanol/Butyraldehyde/Phenol adsrobed per gram of the activated carbon in each of the cases • Record the COD of the decant in each of the cases
- 36. Activated carbon contactors • Contact time: 5 to 30 minutes • Columnar fixed bed or fluidized bed type contactors – Fixed bed type cotactors • Filtration also occurs – beds need periodic backwashing or cleaning • Bed thickness can be upto 2 to 3 m • In water treatment activate carbon is used as the filter medium usually before the final chlorination step – Fluidized bed contactors • Continuous addition of fresh carbon and removal of spent carbon • Slurry type contactors – need separators
- 39. Typical isotherm solid-phase concentration (y-axis) vs liquid phase concentration (x-axis) This is a favorable isotherm: higher solid-phase concentration at low liquid concentrations
- 40. Freundlich isotherm, log-log scale
- 41. Introduction Water contains the following dissolved organic matter that can be removed by adsorption: – odor-, taste- and color-producing compounds – organic micro-pollutants (pesticides, hydrocarbon compounds) • Presence of pesticides in drinking water required extension of traditional treatment with activated carbon • THMs are toxic – activated carbon can remove the formed THMs – Chlorine can react with organic matter and tri-halo-methanes (THMs) can be formed • Under high temperature carbonaceous material becomes carbonated – the carbon is partly oxidized into CO & H2O – the carbon gets open structure – internal surface area is several times larger than the external surface area.
- 42. • Dissolved organic matter can be removed from water by filtration through a bed of activated carbon. • Organic matter diffuses from the water phase to the surface of the carbon grains • Adsorption of organic matter is not finite - there is equilibrium between the concentration of dissolved organic matter in water and the quantity of organic matter adsorbed onto the carbon • When different kinds of organic compounds are present in the water, competition will occur • Large organic molecules can block micro pores, thus preventing the smaller organic molecules from entering these micro pores. • After some time the activated carbon is saturated with adsorbed organic matter and needs to be cleaned - done by heating the carbon to 1000C • Activated carbon filters operate similar to rapid sand filters (downward flow) • When the filter is clogged with suspended matter or biomass, the filter bed is backwashed. • Activated carbon filters are usually placed after rapid sand filtration
- 43. Theory • Equilibrium is established during adsorption .�� • Maximum loading (qmax) depends on the concentration of adsorbable matter in the bulk liquid (water) • The higher this concentration, the higher the loading capacity is • The relationship between the loading capacity and the concentration of adsorbable matter in the bulk liquid is called adsorption-isotherm • Freundlich isotherm: qmax = loading capacity (g/kg); cs = equilibrium concentration (g/m3) x = adsorbed amount of compound (g) m = mass of activated carbon (kg) K = Freundlich constant ((g/kg).(m3/g)n) n = Freundlich constant (-) • The constants K and n are influenced by water temperature, pH, type of carbon and concentration of other organic compounds. • Using laboratory experiments, Freundlich constants can be determined for a single substance with a specific type of activated carbon • The higher the K-value, the better the adsorption. • non-polar substances are better adsorbed than polar substances • Substances with double bonds are better adsorbed than those with single