Week 14 Adsorption and Ion Exchange

  Unit Process Adsorption and Ion Exchange

  1 Week 12 01/08/2019 Unit Process - Department of 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 phase, e.g., within an organic phase in contact with an air or water phase

  Adsorption Absorption (“partitioning”)

  PHASE I PHASE 2 PHASE I ‘PHASE’ 2

  gas H aq P K c

  

  Henry’s Law The Jargon of Adsorption _{Cu 2+ Cu Cu 2+ 2+ Cu 2+} Adsorbent, in _{c concentration suspension at solid Cu Cu 2+ 2+ Cu 2+ Cu} 2+ _{concentration c adsorbate, at Dissolved Cu(aq) Cu Cu 2+ 2+ Cu} 2+ _{Cu 2+ with adsorption Adsorbed species, g solid or per m} density q mg Cu per _{2 of c Adsorbed species present at an overall concentration Cu(ads)} Surface area per _{gram of solid is the specific surface area} mg adsorbed mg adsorbed g solid per       c q c _{i ads ,}  _{i solid}

        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)

  

Adsorption Phenomenon

  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.

  Physical adsorption (physisorption): Physical attractive forces (van der Waals forces)

  Adsorption e.g. Carbon ads, Activated alumina

  Phenomenon Chemical adsorption (chemisorption): the adsorbed molecules are held to the surface by covalent forces.

  (little application in ww treatment) 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 _{2+ 2+}

  

  Hardness removal (Ca , Mg )

  

  Arsenic (various negatively charged species), ₂₊ ^- NO , Ba removal ₃

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 S

  2 formation

Spent Carbon  land disposal problem, unless

regenerated Regeneration systems  Expensive +

  Air pollution problems 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

  01/08/2019 ¹¹

  Activated Carbon

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 regenerated C 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 

  Branched chains are usually more adsorbable than straight chains

  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 Mattson and Mark, Activated Carbon, Dekker, 1971

  

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
• Surface area
• Bulk density

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) Raw Material Activated Carbon Bituminous Coal Lignite Coconut Shell F 300} H 71 _{C25 Particle Density, ρ (kg/m ) 868 685 778 Bed Density, ρ (kg/m ) 500 380 500 F P}

  3 _{3 Surface Area BET (m /g) 875 670 930} Particle Radius (mm) ₂ ^{0.81 0.90 0.79} _{Micro- Pore Volume (cm /g) ( radius < 1nm)}

  3 _{0.33 0.21 0.35 Macro- (1nm < r < 25nm) (radius > 25nm)} ---- Meso- ^0.38 _{0.58 0.16 ----} ^0.14

• ---- Total
^{1.17 0.65} 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 high molecular weight organics 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
• Acid / base treatment
• Chemical Oxidation
Thermal Regeneration Drying Desorption High temperature heat treatment (650 – o

  980

  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 ↓ Generate adsorption isotherms Adsorption Isotherms 

  Technical feasibility of Activated Carbon ↓ Adsorption tests ↓ Generate adsorption isotherms

  Adsorptive Equilibration in a Porous Adsorbent

  Pore Early

  Later

  Laminar Boundary Layer

  GAC Particle Equilibrium

  Adsorbed Molecule Diffusing Molecule

  Adsorption Isotherms Add Same Initial Target Chemical Concentration, C _init , in each

  Different activated carbon dosage, C _solid , in each

  Control

      mg/L mg g g/L ^{init fin fin} _solid c c q c

       

    An adsorption ‘isotherm’ is a q vs. c relationship at equilibrium Metal Oxide Surfaces

  Coagulants form precipitates of Fe(OH) and Al(OH) _{3 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 _{2+ + +} SOH + Me + H  SOMe

  Ion Exchange Resins

• Ca + 2Na ⁺ R ⁺
• Cl ^-

  2R ^- -Na ⁺ + Ca ²⁺  R ₂

• H
₂ AsO ₄ ^-  R ⁺ - H ₂ AsO ₄ ^- + Cl ^-

  Assuming mineral surface started with q = 0: If mineral surface started with q >0: Commonly Reported Adsorption Isotherms max

  1 ^L _L K c q q

  K c   lin q k c

   n f q k c

   Linear:

  Langmuir: Freundlich: Shape of Langmuir Isotherm

  

Shape of Freundlich Isotherm

n f q k c

  

  

Shape of Freundlich Isotherm

(log scale)

log log log f q k n 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 q 50.1 c _o

_{benz benz}



  A solution at 25 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 ₂

activated carbon with a specific surface area of 650 m /g. Compute the required

activated carbon dose.

  Solution. The adsorption density of benzene in equilibrium with c of 0.010 mg/L _eq can be determined from the isotherm expression: _0.533 q 50.1 c 4.30 mg/g _{benz benz}

 

  A mass balance on the contaminant can then be written and solved for the activated carbon dose:

c c q c

_{tot benz , benz benz AC}  

  0.50 0.010 4.30 mg/g c   _AC   c 0.114 g/L 114 mg/L _AC  

  

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 q 76.6 c

   tol tol

  Solution. The mass balance on toluene is: c c q c _{tot tol , tol tol AC}   _0.365 0.50 c 76.6 c 0.114 g/L   _{tol tol}

      _{ 4} c 3.93x10 mg/L _tol  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

  

  Unsteady state

Powdered Activated Carbon (PAC)

  PAC + Coagulants

  Settled Water

  Sludge Withdrawal PAC particles may or may not be equilibrated

  PAC + Flocculated

  Coagulants Water

  Process Operates at Steady-State, c = constant in time _out

Adsorbsi: Freundlich Isoterm

   Persamaan isoterm Freundlich x

  1 / n = q = K C e f e m

  (Metcalf dan Eddy, 2002) 

  

Dimana, (x/m) atau q (mg/g) adalah massa

e adsorbat yang diadsorp per massa adsorben, K adalah faktor kapasitas adsorpsi Freundlich f

  (mg/g), C (mg/L) adalah konsentrasi e adsorbat setelah adsorpsi pada saat kesetimbangan dan 1/n adalah konstanta Freundlich.

Adsorbsi: Langmuir Isoterm

   Persamaan isoterm Langmuir q bC x maks e

  = q = e

  1 bC + m e

  

Dimana, (x/m) atau q ( mg/g) adalah massa e adsorbat yang diadsorp per massa adsorben, q (mg/g) adalah kapasitas adsorpsi maks maksimum, b (L/mg) adalah konstanta

Langmuir dan C (mg/L) adalah konsentrasi e adsorbat setelah adsorpsi pada saat kesetimbangan. 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 dan q adalah jumlah adsorbat yang diadsorp (mg/g)

_{e t} pada saat kesetimbangan dan pada waktu t. k (L/menit) _ads 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 -q ) terhadap t memberikan sebuah garis lurus _{e t} untuk kinetika adsorpsi orde satu semu. 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 (g/(mg.menit)) adalah konstanta laju orde dua semu.

₂ 

  

Persamaan di atas dapat diuubah kedalam bentuk persamaan

linier menjadi (Zhang et al., 2010) 

  Dimana h = k q dapat dianggap sebagai laju awal adsopsi pada _{2 e2} saat t mendekati 0 (nol). Plot antara t/q terhadap t memberikan _t sebuah garis lurus yang dapat digunakan untuk menentuka q _e dan k . ₂

  Massa _{adsorben (g) C e (mg/L) q e (mg/ g) Log C e Log q e C e /q e} 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

Sumber : (Hasil analisis, 2010)

  0.00 ^{0.25 0.50 0.75 1.00 1.25 1.50 0.0 0.2 0.4 0.6 0.8 1.0 f(x) = 0.37 x + 0.35 R² = 0.97} _{Log Ce} ^{L og q e}

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 1.000 2.000 3.000 4.000 f(x) = 0.12 x + 0.52 R² = 1 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.

• • Jika suatu industri elektroplating dengan debit air limbah 10 L/jam dengan konsentrasi Zn 50 mg/L

  harus mengolah air limbah sampai memenuhi baku mutu sampai 0,3 mg/L maka hitung kebutuhan adsorben jika penggantian dilakukan setiap satu minggu.

  Waktu _(menit) ^{Zn akhir} _(mg/L) ^q _t ^(mg/ _{g) (q e}

_{- q}

_{t )} ^Log _{(q e -q t ) t/q t t (0.5) 60 6,7737 3,358 0,198 -0,703 17,866 7,746}

  _{150 120} 90 _{5,8861 3,418 0,139 -0,857 26,334 9,487 3,8002 3,557 0,000 - 33,740 10,954} 4,5489 3,507 0,050 -1,302 42,775 12,247 _{210 180 4,2674 3,526 0,031 -1,507 51,056 13,416 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 Sumber : (Hasil analisis, 2010)

  40 60 80 100 120 140 160 180 200 220

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 Sumber : (Hasil analisis, 2010)

• ^{- - - - - - - f(x) = − 0 x − 0.55 R² = 0.56 waktu (menit) L og ( q}
^{e- q t) 30 60 90 120 150 180 210 240 10 20 30 40 50 60 70 f(x) = 0.28 x + 0.62 R² = 1} _{t (menit)}

^t/

^q

^t

  

  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

  

  Removal or recovery of metal 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 Environmental Engineering - ITS} Unit Process - Department of ₅₅ ION EXCHANGE (Type of Resin) a. Cationic resin - exchange positive ions

  b. Anionic resin – exchange negative ions (a) (b)

  01/08/2019 ^{Environmental Engineering - ITS 56 Unit Process - Department of}

  (Exchange Reactions)

• Cation exchange on the sodium cycle: _{+ 2+}

  Na · R + Ca  Ca · R + 2Na ₂

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: _{2+ +}

  2Na + Ca · R  Na · R + Ca ₂

  (Exchange Reactions)

• Anion exchange replaces anions with hydroxyl ions: _{- 2-}

  SO + R · (OH)  R · SO + 2OH _{4 2 4}

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: _{2- -}

  R · SO + 2OH  SO + R · (OH) _{4 4 2}

  (Basic Principles)

• ^{- +}

  H , CN H , OH Clean water

  Anion Cation Resin Resin

• ^- 3+

  Cr , CN

  (Selectivity)

• ₊ Cations: _{2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+}

  Ra > Ba > Sr > Ca > Ni > Cu > Co > Zn > Mn > Ag _{+ + + + +} >Cs > K > NH > Na > Li ₄

• Anions:
_{2- - 2- - 2- - - - -}

HCRO > CrO > ClO > SeO > SO > NO > Br > HPO >

_{4 4 4}

₄

_{4 - - - 3 - - 2- 2- 4} HA O _{s 4} > SeO > CO > CN > NO > Cl > H PO , H AsO , _{- - - 3 3 2 2 4 2 4} ^-

  HCO > OH ₃ > CH COO > F ₃ Note: The least preferred has the shortest retention time, and appears frst in the efuent and vice versa for the most preferred. 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

  

  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: ₂₊ ^-

  Cu + 2e → Cu     E° = 0.34 V _{(aq) (s)}

  

  Since both half-reactions cannot undergo reduction, we must reverse the equation of the reaction that will undergo oxidation. This will give us an electrochemical cell voltage of 0.34 V: ₂₊ E° ^-

  Cu + 2e → Cu     0.34 V _{(aq) (s) - +} H   → 2H + 2e _{2 (g) (aq) 2+}

  0.00 V ⁺ Cu + H → 2H + Cu     0.34 V _{(aq) 2 (g) (aq) (s)}

  

  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. ₂₊ ^-

   Zn + 2e → Zn     E° = -0.76 V _{(aq) (s)}

  

  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: ₂₊ E° ^- Zn   → Znu + 2e   _{(s) (aq) - +}

  0.76 V

  2H + 2e → H   _{(aq) 2 (g) 2+}

  0.00 V _{(s) (aq) (aq) 2 (g)} ⁺

   Zn + 2H → Zn + H    

  0.76 V

  

ELECTRODIALYSIS

  Basic Design of Ion Exchange Approach: 

  Scale-up approach 

  Kinetics approach 

  System operation Service 

  Backwashing 

  Regeneration 

  Rinsing 

  Operating mode

  

Let’s Have a Great Sem!

Unit Process - Department of 68 01/08/2019