ADSORPTION
Nur Istianah,ST,MT,M.Eng
08/03/2018
Definition
08/03/2018
Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation).
adsorbate: material being adsorbed
adsorbent: material doing the adsorbing. (examples are activated carbon or ion exchange resin).
d
Surface energy
The surface molecules have higher energy since they have an unbalanced force component. These surface molecules have additional energy to balance the forces.
It takes energy to put molecules on the surface, since at least one of the interior bonds must be broken to get the molecule to the surface. This excess energy is called surface tension.
Since it takes energy to create interfacial surfaces, the system will try to minimize the total interfacial surface area. Hence we see spherical droplet, meniscus etc.
Molecules were concentrated on surface
area"
surface
"
"
Volume
"
) C
C
(
initial
after adsorption
Surface excess can be defined as:
Methods
Exchange adsorption (ion exchange)– electrostatic due to charged sites on the surface. Adsorption goes up as ionic charge goes up and as hydrated radius goes down.
Physical adsorption: Van der Waals attraction between adsorbate and adsorbent. The attraction is not fixed to a specific site and the adsorbate is relatively free to move on the surface. This is relatively weak,
reversible, adsorption capable of multilayer adsorption.
Methods
Chemical adsorption: Some degree of chemical bonding between adsorbate and adsorbent characterized by strong attractiveness. Adsorbed molecules are not free to move on the surface.
There is a high degree of specificity and typically a monolayer is formed.
The process is seldom reversible.
ADSORPTION EQUILIBRIA
If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution qe = mass of material adsorbed (at equilibrium) per mass of adsorbent.
Ce = equilibrium concentration in solution when amount adsorbed equals qe.
qe/Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption, etc.
Some general isotherms are shown in the figure below.
This model assumes monolayer coverage and constant
binding energy between surface and adsorbate. The model is:
0
a e
e
e
K Q C q 1 K C
Langmuir Isotherm:
Qa
represents the maximum adsorption capacity (monolayer coverage) (g solute/g adsorbent).Ce has units of mg/L.
K has units of L/mg
0 a e 0
e a e
Q C Q
K 1 q
C
A plot of Ce/qe versus Ce should give a straight line with
intercept :
0
Q
aK 1
Langmuir model linearization
0
Q
a1
and slope:
Factors which affect adsorption
Adsorbate:
Solubility In general, as solubility of solute increases the extent of
adsorption decreases. Factors which affect solubility include molecular size (high MW- low solubility), ionization (solubility is minimum when compounds are uncharged), polarity (as polarity increases get higher solubility because water is a polar solvent).
pH pH often affects the surface charge on the adsorbent as well as the charge on the solute. Generally, for organic material as pH goes down adsorption goes up.
Temperature Adsorption reactions are typically exothermic i.e., D H rxn is generally negative. Here heat is given off by the reaction therefore as T increases extent of adsorption decreases.
Presence of other solutes
In general, get competition for a limited number of sites therefore get reduced extent of adsorption or a specific material.
Factors which affect adsorption
Adsorbent:
Activated carbon(AC)
Wood
Lignite
Coal
Nutshells
Bone
Carbon contactor
Packed bed (fixed carbon bed)
Fluidized bed
A typical packed (fixed) bed contactor looks like :
These raw materials are pyrolyzed at high temperature under low oxygen conditions (so we don’t get complete combustion). This forms a
“char”. The char is then activated by heating to 300 – 1000 oC in the presence of steam, oxygen or CO2.
Result: “Activated carbon” which is highly porous, micro-crystalline material which resembles graphite plates with some specific functional groups (e.g. COOH, OH)
Surface area of the AC is huge. Most of the surface area is interior in micro- and macropores. Typical surface area is in the range of 300- 1500 m2/gram.
Quality and hardness of the AC are a function of the starting material and the activation process.
Activated carbon(AC)
Increasing magnification
Adsorption onto AC usually is modeled as a three consecutive step process. These steps are:
film transport (through the stagnant boundary layer about the AC particle);
transport of the solute through the internal pores; and finally
adsorption to the surface site.
One or more of these steps can limit the rate of solute adsorption.
Adsorption Kinetics.
We can lump all the mass transport resistance terms into one term, k, and write:
) C
C ( a t k
C
e
k = overall mass transfer coefficient (cm/min)
a = surface area of carbon per unit volume of reactor (1/cm) Ce = concentration that would be in equilibrium with actual amount of solute adsorbed, q (g/liter).
C = actual concentration of solute in bulk solution. (g/liter).
Adsorption Kinetics.
BIOSORPTION
08/03/2018
Affinity of metal with organics
L-2-Aminopropanoic Acid (Alanine) with various metal
Log K
Ca2+ 1.30
Co2+ 4.31
Ni2+ 5.36
Cu2+ 8.11
Zn2+ 4.58
Cd2+ 3.98
Pb2+ 4.15
CHCOOH CH
| NH
3
2
ML L
M2 2
} }{L {M
{ML}
2
2
K Metal Ions
1. Immobilization of organics; 2. use of organics in natural biosolids
What is biosorption ?
• Biosorption is a property of certain types of
inactive/active organisms to bind and concentrate heavy metals from even very dilute aqueous
solutions.
• Biosorbents can be classified into:
a. Inactive organisms (mainly) include algae, fungi and bacteria
b. Their derivatives which are termed as biopolymers.
• Opposite to biosorption is metabolically driven
active bioaccumulation by living substances.
What are typical biosorbents ?
• Some of the biomass types come as a waste by- product of large-scale industrial fermentations (the mold Rhizopus, the bacterium Bacillus subtilis and waste activated sludge).
• Other metal-binding biomass types, certain abundant seaweeds (particularly brown algae e.g. Sargassum, Ecklonia ), can be readily collected from the oceans.
• Biopolymers are normally extracted from inactive organisms and processed before use (e.g. Ca- Alginate)
• These biosorbents can accumulate in excess of 25%
of their dry weight in deposited metals: Pb, Ag, Au, U, Cu.
Case presents
• Raw seaweeds – collected in Singapore
• Ca-alginate beads
• Ca-alginate based ion exchange resin
(CABIER)
Examples: Marine Algal collected in Singapore
Padina sp. Sargassum sp.
Why biosorption ?
Cu sorption
Characterization of biosorbents by instrumental analysis
• Fourier transform infrared spectroscopic (FTIR) and X- ray Photoelectron Spectroscopic (XPS) studies show that biosorbents have significant amount of COO, OH, C=O, and C-O.
• These organic functional groups would be responsible for metal uptake onto the biosorbents due to the high affinity for metal ions.
• SEM shows less pore development in bisorbents
Biosorption Equilibrium
Metal biosorptive properties: pH effect SOH + M
m+= SO-M
m++ H
+pH
1 2 3 4 5 6 7
Metal removal, %
0 20 40 60 80 100
Cu Pb
[Pb]o=[Cu]o=1x10-4 M [CABIER]=0.15 g/L
Ca-alginate
Algae as the biosorbents
B i o m a s s M e t a l i o n s qmax (mmol/g) R e f e r e n c e s
Ascophyllum spp. Ni, Pb, Cd, Cu 1.03-1.43 Volesky etal., 2000
Chlorella sp. Cd 0.99 Aksu, 2001
Cladophora sp. Pb 0.35 Jalali etal., 2002
Cyclotella sp. Cu 0.41 Schmitt etal., 2001
Cymodocea spp. Cu, Zn 0.71-0.83 Sanchez etal., 1999
Fucus sp. Pb 1.6 Volesky, 1994
Gracilaria sp. Pb 0.2-0.26 Jalali etal., 2002
Padina spp. Pb, Cu 0.31-1.05 Volesky, 1994; Jalali
etal., 2002; Kaewsarn, 2002 Phaeodactylum sp. Cu 1.67mg/g Schmitt etal., 2001
Polysiphonia sp. Pb 0.49 Jalali etal., 2002
Porphyridium sp. Cu 0.27mg/g Schmitt etal., 2001
Sargassum spp. Pb, Cu, Cd, Ni 0.71-1.99 Volesky etal., 1994,2000;
Jalali etal., 2002 Scenedesmus spp. Cu, Cd 0.06-0.21 Schmitt etal., 2001 Schizomeris spp. Pb, Cd 0.31-0.44 Ozer etal., 1999
Spirulina sp. Cd 0.87 Rangsayatorn etal., 2002
Ulva sp. Pb 0.61 Jalali etal., 2002
Conceptual model for the metal removal by ion exchange.
+ Ca2+
M = Cu and Pb
Ion exchange in biosorption (e.g. by CABIER)
1. M
2++ Ca-R M-R + Ca
2+(ion exchange) 2. M
2++ R
2- M-R (R: unreacted group)
(elementary coordination)
3. 2H
++ Ca-R H
2-R + Ca
2+(pH effect) and 4. solution and precipitation reactions……..
Chen, J.P. et al., Langmuir, Vol. 18, No. 24, pp. 9413-9421, 2002.
Bisorption Kinetics
Sorption Kinetics of Metal Ions:
Diffusion-Controlled Model
A d s o r b e n t
L i q u i d F i l m
B u l k L i q u i d C o n c e n t r a t i o n
P o r o u s
a p r , d i s t a n c e m e a s u r e d f r o m a d s o r b e n t p a r t i c l e c e n t e r
r p , e p
m
k f j C j
c j
q j q j
c j
c j ( r = a p )
D p j Model Parameters
Rate-controlling mechanism (i.e., transport-controlled or reaction-controlled cases)
Rate parameters (i.e.,
diffusion and mass transfer coefficients or rate constants)
Characterization of sorbents Sorption rate results from either mass transfer of ion species to the surface of sorbents or complexation reactions between functional groups of sorbents and ion species.
kinetics of metal biosorption
[Ca2+]o = 0, [Na+]o = 0
Time (min)
0 30 60 90 120 150 180
q (mg/g)
0 20 40 60 80 100
[Pb2+]o = 20 ppm [Pb2+]o = 36.8 ppm
pH = 4-5, m = 0.4 g/L, De = 2.95×10-11 m2/s, kf = 2.41×10-4 m/s
Advantages of Biosorption Process
• Use of naturally abundant renewable biomaterials that can be cheaply produced;
• Ability to treat large volumes of wastewater due to rapid kinetics;
• High selectivity in terms of removal and recovery of specific heavy metals;
• Ability to handle multiple heavy metals and mixed wastes;
• High affinity, reducing residual metals to below 1 ppb in many cases;
• Less need for additional expensive reagents which typically cause disposal and space problems;
• Operation over a wide range of physiochemical conditions including temperature, pH, and presence of other ions (including Ca (II) and Mg (II));
• Relatively low capital investment and low operational cost;
• Greatly improved recovery of bound heavy metals from the biomass;
• Greatly reduced volume of hazardous waste produced
THANKS FOR YOUR ATTENTION
The best person is one give something useful always
08/03/2018
