Week 14 Adsorption and Ion Exchange

Unit Process

Adsorption and Ion

Exchange

1

Week 12

Adsorption Equilibrium



_{Adsorption vs. Absorption}



Adsorption is accumulation / adhesion of

molecules at the surface of a solid material

(usually activated carbon) in contact with an

air or water phase



Absorption is dissolution of molecules within a

Adsorption

Absorption (“partitioning”)

PHASE I

PHASE 2

PHASE I

‘PHASE’ 2

gas

H aq

P

_

K c

Henry’s Law

The Jargon of Adsorption

Adsorbent, in suspension at concentration

csolid

Dissolved adsorbate, at concentration cCu(aq)

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

Cu2+ Cu2+

Adsorbed species present at an overall concentration of cCu(ads)

mg adsorbed

mg adsorbed

g solid per

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)

The surface of a solid shows a strong affinity for

molecules that come into contact with it.

Certain solid materials concentrate specific substances

from a solution onto their surfaces.

Adsorption Phenomenon

Physical adsorption (physisorption):

Physical attractive forces

(van der Waals forces)

e.g. Carbon ads, Activated alumina

Adsorption

Phenomenon

Chemical adsorption (chemisorption):

the adsorbed molecules are held to the

surface by covalent forces.

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 (Ca

2+

, Mg

2+

)



_{Arsenic (various negatively charged species),}

Activated Carbon Systems

Carbon systems generally consist of vessels in

which granular carbon is placed, forming a

flter bed through which ww passes.

Activated Carbon Systems

Area requirement: less

If anaerobic conditions occur



Biological activity in carbon beds



H

₂

S

formation

Spent Carbon



land disposal problem, unless

regenerated

Regeneration systems



Expensive +

11

Adsorption Mechanism



2) Chemical adsorption



Results from a chemical interaction between the

adsorbate and adsorbent. Therefore formed bond is

much stronger than that for physical adsorption



Heat liberated during chemisorption is in the range

of 20-400 kj/g mole

Activated Carbon Systems

Pretreatment is important to reduce solids

loading to granular C systems.

Powdered Activated Carbon (PAC) can be fed to

ww using chemical feed equipment.

Activated Carbon Systems

Mostly used for organic matter removal. AC remove

variety of organics from water (not selective)

Metal removal:

Recent applications in metal removal

Few in full scale

Pretreatment by sedimentation / fltration to

remove precipitated metals

Remaining dissolved metals adhere to the

carbon until all available sites are exhausted.

Spent carbon



Replaced with new or

Factors efecting Carbon Adsorption



_{Physical and chemical characteristics of}

carbon (surface area, pore size)



_{Physical and chemical characteristics of}

adsorbate ?

(molecular size, molecular polarity, chemical

composition)

Higher molecular weight



more easily

adsorbed

Molecular weight



Size

Factors efecting Carbon Adsorption



_{Concentration of adsorbate in the liquid phase}

(solution)



_{Characteristics of the liquid phase ?}

(pH, temperature)



_{Contact time}



_{Increasing solubility of the solute in the liquid}

carrier decreases adsorbability

Factors efecting Carbon Adsorption



_{Substituent groups (hydroxyl, amino, carbonyl}

groups, double bonds)



_{Molecules with low polarity are more sorbable}

than highly polar ones.

Oxygen-Containing Surface Groups on Activated Carbon

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

Properties of of Ativated Carbon

Made from: (?)

- Wood

- Lignin

- Bituminous coal

- Lignite

- Petroleum residues

Standards for specifc applications:

- Pore size

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)

Activated Carbon F 300 H 71 C25

Raw Material Bituminous Coal Lignite Coconut Shell

Bed Density, ρ_F (kg/m3_{) 500 380 500}

Particle Density, ρ_P (kg/m3_{) 868 685 778}

Particle Radius (mm) 0.81 0.90 0.79

Surface Area BET (m2_{/g) 875 670 930}

Other parameters used for AC

characterization



_{Phenol Number:}

_{Index of carbon’s ability to}

remove taste and odor compouns



_{Iodine Number:}

_{Adsorption of low-molecular}

weight substances

Micropores, radius <2 µm



_{Molasses Number:}

_{Carbon’s ability to adsorb}

high molecular weight substances

Pores 1 – 50 µm

Other parameters used for AC

characterization

High iodine number



Efective for ww with

low molecular weight organics

High molases number



Efective for ww with

Kinetics of Atrazine Sorption onto

GAC

167 mg GAC/L

333 mg GAC/L

Carbon Regeneration

Objective:

Remove the previously adsorbed

materials from the carbon pore structure

Methods:

- Thermal

- Steam

- Solvent extraction

Thermal Regeneration

Drying

Desorption

High temperature heat treatment (650 –

980

o

C) in the presence of water vapor, fue

gas, oxygen

- Multiple heat furnaces

- Fluidized bed furnaces are used.

Adsorption Isotherms



_{Technical feasibility of Activated Carbon}

↓

Adsorption tests

↓

Adsorption Isotherms



_{Technical feasibility of Activated Carbon}

↓

Adsorption tests

↓

Generate adsorption isotherms

Adsorptive Equilibration in a

Porous Adsorbent

Adsorbed Molecule

Diffusing Molecule

Equilibrium

Pore

GAC Particle

Early

Later

Adsorption Isotherms

Add Same Initial Target Chemical Concentration, C

_init

, in each

Different activated carbon dosage, C

_solid

, in each

Control

relationship at equilibrium

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

Sorption of NOM on Metal Oxide

Sorption of Metals on Metal Oxide

Ion Exchange Resins

2R

-

-Na

+

+ Ca

2+



R

2

-Ca + 2Na

+

R

+

_-Cl

-

+ H

2

AsO

4-



R

+

- H

2

AsO

4-

+ Cl

If mineral surface started with

q

>0:

Assuming mineral surface started with

q

= 0:

Shape of Langmuir Isotherm

Shape of Freundlich Isotherm

n

f

Shape of Freundlich Isotherm

(log scale)

log

q

_

log

k

_f

_

n

log

c

0.533

tot benz benz benz AC

c



c



q

c





0.50 0.010





4.30 mg/g

c

_AC

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:

A solution at 25

o

C 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 m

2

_{/g. Compute the required}

activated carbon dose.

Solution.

The adsorption density of benzene in equilibrium with

c

_eq

of 0.010 mg/L

can be determined from the isotherm expression:

0.365

3.93x10 mg/L

tol

c





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:

Solution.

The mass balance on toluene is:

General Process Design

Features



_{Contactors provide large surface area}



_{Types of contactors}



Continuous fow, slurry reactors



Batch slurry reactors (infrequently)



Continuous fow, packed bed reactors



_{Product water concentration may be}



Steady state or

PAC +

Coagulants

Sludge Withdrawal

PAC particles may or

may not be equilibrated

Settled

Water

PAC +

Coagulants

Flocculated

_Water

Powdered Activated Carbon (PAC)

Process Operates at Steady-State,

c

_out

= constant in time

Adsorbsi: Freundlich

Isoterm



_{Persamaan isoterm Freundlich}

(Metcalf dan Eddy, 2002)



Dimana, (x/m) atau q

_e

(mg/g) adalah massa

adsorbat yang diadsorp per massa adsorben,

K

_f

adalah

faktor kapasitas adsorpsi Freundlich

(mg/g), C

_e

(mg/L)

adalah konsentrasi

adsorbat setelah adsorpsi pada saat

Adsorbsi: Langmuir Isoterm



_{Persamaan isoterm Langmuir}



_{Dimana, (x/m) atau q}

_e

_{( mg/g) adalah massa}

adsorbat yang diadsorp per massa adsorben,

q

maks

(mg/g) adalah kapasitas adsorpsi

maksimum, b (L/mg) adalah konstanta

Langmuir dan C

e

(mg/L) adalah konsentrasi

adsorbat setelah adsorpsi pada saat

kesetimbangan.

01/08/2019 Unit Process - Department of Environmental Engineering - ITS 47

Kinematika adsorbsi

Orde satu semu



_{Kinetika orde satu semu disebut juga dengan persamaan}

Lagergren yang menunjukkan laju adsorpsi adsorbat pada

permukaan adsorben:

(Zhang

et al.

, 2010)



_{Dimana q}

_e

_{dan q}

_t

_{adalah jumlah adsorbat yang diadsorp (mg/g)}

pada saat kesetimbangan dan pada waktu t. k

ads

(L/menit)

adalah konstanta laju kinetika adsorpsi orde satu semu.



_{Persamaan Zhang et al. dapat diubah kedalam bentuk}

persamaan linier:

(Zhang

et al.

, 2010)



_{Plot dari log (q}

_e

_-q

_t

_{) terhadap t memberikan sebuah garis lurus}

Kinematika adsorbsi

Orde dua semu



_{Kinetika orde dua semu dikembangkan oleh Ho. Model ini}

diaplikasikan secara luas untuk beberapa sistem adsorpsi

logam. Persamaan kinetika orde dua semu:

(Zhang

et al.

, 2010)



_{Dimana k}

₂

_{(g/(mg.menit)) adalah konstanta laju orde dua semu.}



_{Persamaan di atas dapat diuubah kedalam bentuk persamaan}

linier menjadi

(Zhang

et al.

, 2010)



_{Dimana h = k}

₂

_q

_e2

_{dapat dianggap sebagai laju awal adsopsi pada}

saat t mendekati 0 (nol). Plot antara t/q

t

terhadap t memberikan

sebuah garis lurus yang dapat digunakan untuk menentuka q

e

dan k

2

.

Massa

0,5

_23,128

_6,804

_1,364

_{0,833 3,399}

1,0

6,789

5,036

0,832

0,702 1,348

1,5

3,800

3,557

0,580

0,551 1,068

2,0

1,925

2,761

0,284

0,441 0,697

Tabel 4.14 Perhitungan Isoterm Lumpur Alum

Treated

dengan Waktu Kontak 120 Menit pada pH 4 dengan

Konsentrasi Awal Zn

2+

_{57,150 mg/L; Volume 100 mL}

01/08/2019 Unit Process - Department of Environmental Engineering - ITS 51

0.00

0.25

0.50

0.75

1.00

1.25

1.50

0.0

Gambar 4.14 Isoterm Freundlich Lumpur Alum

Treated

dengan Waktu Kontak 120 Menit pada pH 4 dengan

Konsentrasi Awal Zn

2+

_57,150mg/L

Sumber : (Hasil analisis, 2010)

0.000 5.000 10.000 15.000 20.000 25.000 0.000

Isoterm Langmuir 120 menit pH 4

Ce

C

e/

q

e

Gambar 4.15 Isoterm Langmuir Lumpur Alum

Treated

dengan Waktu Kontak 120 Menit pada

pH 4 dengan Konsentrasi Awal Zn

2+

_57,150mg/

L

Sumber : (Hasil analisis, 2010)

•

Hitunglah kapasitas adsorbsi masing-masing dan konstanta reaksi masing-masing.

Waktu

60

_6,7737

_3,358

_0,198

_-0,703

_17,866

_7,746

90

_5,8861

_3,418

_0,139

_-0,857

_26,334

_9,487

120

_3,8002

_3,557

_0,000

_-

_{33,740 10,954}

150

_4,5489

_3,507

_0,050

_-1,302

_{42,775 12,247}

180

_4,2674

_3,526

_0,031

_-1,507

_{51,056 13,416}

210

_5,0523

_3,473

_0,083

_-1,078

_{60,463 14,491}

Tabel 4.17 Perhitungan Kinetika Lumpur Alum

Treated

pada Dosis 15 g/L Menit pH 4 dengan

Konsentrasi Awal Zn

2+

_57,150mg/L

01/08/2019 Unit Process - Department of Environmental Engineering - ITS 53

Gambar 4.18 Kinetika Orde Satu Semu Lumpur

Alum

Treated

pada Dosis 15 g/L pH 4 dengan

Konsentrasi Awal Zn

2+

_57,150mg/L

Sumber : (Hasil analisis, 2010)

Gambar 4.19 Kinetika Orde Dua Semu Lumpur Alum

Treated

pada Dosis 15 g/L pH 4 dengan Konsentrasi

Awal Zn

2+

_57,150mg/L

ION EXCHANGE



_Defnition

Ion exchange is basically a reversible chemical

process wherein an ion from solution is

exchanged for a similarly charged ion attached

to an immobile solid particle.

Removal of undesirable anions and cations from

solution through the use of ion exchange resin



_Applications



Water softening



Removal of non-metal inorganic

ION EXCHANGE

(Medium - resin)



_{Consists of an organic or}

inorganic network

structure with attached

functional group



_{Synthetic resin made by}

the polymerisation of

organic compounds into a

porous three dimensional

structure



_{Exchange capacity is}

determined by the number

of functional groups per

unit mass of resin

01/08/2019 55

ION EXCHANGE

(Type of Resin)

a.

Cationic resin - exchange positive ions

b.

Anionic resin – exchange negative ions

ION EXCHANGE

(Exchange Reactions)

•

_{Cation exchange on the sodium cycle:}

Na

₂

· R + Ca

2+



_{Ca · R + 2Na}

+

where R represents the exchange resin. When all exchange sites are

substantially replaced with calcium, resin is regenerated by passing a

concentrated solution of sodium ions (5-10%) through the bed:

2Na

+

_{+ Ca · R}



_Na

2

· R + Ca

2+

ION EXCHANGE

(Exchange Reactions)

•

_{Anion exchange replaces anions with hydroxyl ions:}

SO

₄

2-

_{+ R · (OH)}

2



R · SO

4

+ 2OH

-where R represents the exchange resin. When all exchange sites are

substantially replaced with sulphate, resin is regenerated by passing a

concentrated solution of hydroxide ions (5-10%) through the bed:

R · SO

₄

+ 2OH

-



_SO

ION EXCHANGE

(Basic Principles)

Cation

Resin

Cr

3+

_{, CN}

-H

+

, CN

-Anion

Resin

H

+

, OH

-Clean

water

ION EXCHANGE

Note: The least preferred has the shortest retention time, and

appears frst in the efuent and vice versa for the most

Ion exchange-electrochemistry



_{During redox reactions, electrons pass from one}

substance to another.

Electrochemistry

is the branch

of chemistry that deals with the conversion between

chemical and electrical energy.



_{The fact that diferent substances are oxidized more}

readily than others is the driving force behind

electrochemical cells, and it is this force that forces

electrons through the external circuit from the anode

(site of oxidation) to the cathode (site of reduction).

This force is known as the

potential diference

or

electromotive force

(

emf

or

E

). Potential diference

is measured in

volts

(V), and thus is also referred to

as the

voltage

of the cell. Voltage is a measure of the

tendency of electrons to fow. The higher the voltage,

the greater the tendency for electrons to fow from the

anode to the cathode.



_{For example, if copper and hydrogen half-cells are}

joined together we fnd that the copper half-cell will

gain electrons from the hydrogen half-cell. Thus the

copper half-cell is given a positive voltage and given

a relative value of +0.34 V:

Cu

2+

(aq)

+ 2e

-

→ Cu

(s)

    E° = 0.34 V



_{Since both half-reactions cannot undergo reduction,}

we must reverse the equation of the reaction that

will undergo oxidation. This will give us an



_{We see in the Table of Standard Reduction Potentials}

that zinc has a negative E° indicating that it is not as

good at competing for electrons as hydrogen.

 Zn

2+

(aq)

+ 2e

-

→ Zn

(s)

    E° = -0.76 V



_{Therefore if zinc and hydrogen are paired together in}

an electrochemical cell, the hydrogen would be

reduced (gain the electrons) and zinc would be

oxidized (losing electrons). To determine the net

redox reaction as well as the voltage of the

electrochemical cell we reverse the zinc equation,

and also reverse it's sign

before adding the

equations and E° together:

E°

Basic Design of Ion Exchange

Approach:



_{Scale-up approach}



_{Kinetics approach}



_{System operation}

Service



_Backwashing



_Regeneration



_Rinsing



_{Operating mode}