Carbon Adsorption

Department of Civil and Environmental Engineering

CEEN 330 – Environmental Field Session

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

Absorption (“partitioning”)

PHASE I

PHASE 2

PHASE I

‘PHASE’ 2

gas H aqP K cHenry’s Law

Adsorption vs. Absorption

Adsorption Mechanisms Dislike of Water Phase – ‘Hydrophobicity’ Physical adsorption

van-der-Waals forces (adsorption energy 10-20 kJ/mol; multi-molecular adsorption)

electrostatic forces: +/-

Chemical adsorption (“chemisorption”) covalent bonding (adsorption energy 40-200 kJ/mol;

activation necessary; compound specific; maximal mono-molecular adsorption)

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) (including T&O) Chemical reduction of oxidants

Metal oxides (surface charge depends on pH) Adsorption of natural organic matter (NOM) Adsorption of inorganics (both cations & anions) Activated alumina (Al2O3) for F , As(V), POˉ 4

3 ˉ Silica-based adsorbents Ion exchange resins

Cations and anions Hardness removal (Ca2+, Mg2+) Arsenic (various negatively charged species), NO3

-, Ba2+

Contaminant Removal EffectivenessProcess Suspended

solidsDissolved organics

Dissolved inorganics

Micro-organisms

Dissolved gases

Chemical ProcessesChemical oxidation

Catalytic oxidation X X -- X X

Chlorination X X -- X --

Ozonation -- L -- X X

Wet oxidation X X -- X X

Chemical precipitation -- -- X -- --

Chemical reduction -- -- X -- --

Coagulation

inorganic chemicals X X -- X --

polyelectrolytes X X -- X --

Disinfection -- -- -- X --

Contaminant Removal EffectivenessProcess Suspended

solidsDissolved organics

Dissolved inorganics

Micro-organisms

Dissolved gases

Chemical ProcessesElectrolytic processes

Electrodialysis -- -- X -- --

Electrolysis -- -- X -- --

Extractions

Ion exchange -- -- X -- --

Liquid-liquid (solvent)

-- -- X -- --

Incineration X X -- X X

Contaminant Removal EffectivenessProcess Suspended

solidsDissolved organics

Dissolved inorganics

Micro-organisms

Dissolved gases

Physical ProcessesCarbon adsorption

Granular activated X X -- -- X

Powdered X X -- X X

Distillation X X X X --

Filtration

Diatomaceous earth filtration

X -- -- X --

Dual media X -- -- X --

Micro-screening X -- -- X --

Sand filtration X -- -- X --

Flocculation-sedimentation X -- -- X --

Foam separation X -- X -- --

Freezing -- X X -- --

Adsorption Principles Surface reaction: high specific surface

(m2/g or m2/bed volume; easily accessible) Specific affinity necessary

Applications Addition of powder/suspension (powdered activated carbon

– PAC) in batch reactors (discontinuous process) Activated carbon contactor (reactor) (granular activated

carbon - GAC) in fixed-bed adsorbers (semi-continuous process)

Activated Carbon Most active carbon is made from coconut shell or

coal materials Heated to over 2,000 °C under high pressure creates fissures that results in a large surface area

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)

Activated Carbon

Pore sizes: Micropores (pore diameter less than 20 nm) Mesopores (pore diameter 20-200 nm) Macropores (200 nm & above)

Powdered Activated carbon (PAC) Powdered activated carbon (PAC): taste, odor

Common dosage: 1 to 5 mg/L Used on a one-time basis, no recovery or reuse Typically used seasonally Can easily be added to existing conventional plants Only PAC dosing and mixing facilities required

Granular Activated Carbon (GAC) Granular activated carbon (GAC): SOC, NOM

Greater dosages and contact times required than PAC Requires carbon filters Requires reactivation or replacement Facilities need to backwash GAC beds

PAC and GAC

Parameter Unit GAC PAC

Total surface area m2/g 700 – 1,300 800 – 1,800

Bulk density kg/m3 400 – 500 360 – 740

Particle density, wetted in water kg/L 1.0 – 1.5 1.3 – 1.4

Particle size range mm (mm) 0.1 – 2.36 (5 – 50)

Effective size mm 0.6 – 0.9 na

Uniformity coefficient UC ≤1.9 na

Mean pore radius Å 16 – 30 20 – 40

Adsorption Kinetics Bulk solution transport:

Transport of adsorbate from bulk solution to boundary layer Film diffusion transport

Transport by molecular diffusion through the stationary layer of water (hydrodynamic boundary layer)

Internal pore transport Transport through the pores to available adsorption sites

Adsorption Adsorption bond is formed between adsorbate and adsorbent

The Process of Adsorption

Kinetics of Atrazine Sorption onto GAC

167 mg GAC/L

333 mg GAC/L

Readily and Poorly Adsorbed Organics

Calculating Adsorption…

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+ Cu2+

Cu2+

Cu2+

Cu2+

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

per L of solution per g adsorbent L of solutioni ads i solidc q c

Add Same Initial Target Chemical Concentration, Cinit, in each container

Different activated carbon dosage, Csolid, in each

Control

mg/Lmg

g g/Linit fin

finsolid

c cq

c

An adsorption ‘isotherm’ is a q vs. c relationship at equilibrium

Adsorption Isotherms

Isotherm

Development of Adsorption Isotherms

Adsorption experiment: Add mass of carbon (m) to defined volume (V) of sample,

measure Co and Ceq

Mass balance (batch adsorption process): V(Co – Ceq) = m * qe

qe = V/m (Co – Ceq)

Development of Adsorption Isotherms (cont.)

V/m = const.; vary Co vary V/m; Co = const.

Adsorption Equilibriumqi = f(co, eq), T = const.

Langmuir (1918)

Freundlich (1907)

Brunauer, Emmet, and Teller (1938)

Redlich-Petersen

Langmuir Isotherm Assumptions for Langmuir adsorption isotherm:

Homogeneous surface A fixed number of accessible sites, all of which have the

same energy (qe = qmax) Reversible adsorption and desorption Rate of adsorption is proportional to liquid phase

concentration

Determination of Langmuir constants: Linearization

plot 1/q vs. 1/c

Langmuir Isotherm (cont.)

Langmuir Isotherm (cont.)

Special cases: C is small: KL Ceq <<1 qe = qm KL Ceq (“linear isotherm”)

C is large: KL Ceq >>1 qe = qm (“horizontal isotherm”)

Freundlich adsorption isotherm: empirical approach (1912) applicable in specific concentrations range only

Linearization

Freundlich Isotherm

Freundlich Isotherm (cont.)

max 1L

L

K cq q

K c

linq k cn

fq k c

Linear: Langmuir:

Freundlich:

Summary of Adsorption Isotherms

Klin, KL, qmax, and n are all empirical constants

Shape of Langmuir Isotherm

nfq k c

Shape of Freundlich Isotherm

log log logfq k n c

Shape of Freundlich Isotherm (log scale)

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

PAC + Coagulants

Sludge WithdrawalPAC particles may or may not be equilibrated

Settled Water

PAC + Coagulants

Flocculated Water

Process Operates at Steady-State, cout = constant in time

Powdered Activated Carbon (PAC)

Lab Procedure Conduct jar testing with 2L river water in each jar

Use best ferric sulfate dose from Friday To 3 of the jars also add PAC-C at concentrations of 10, 20,

and 30 mg/L Measure turbidity in each of the jars at the end of

the jar testing Collect samples for TOC analysis

River water sample A composite sample of the 3 coagulant jars One sample from each of the coagulant+PAC jars

Lab Procedure Transfer 1L of clarified water from the coagulant-only

jars into new clean jars Conduct jar testing with 1 L clarified water in each jar

To the jars add PAC-C at concentrations of 10, 20, 30, and 40 mg/L

Measure turbidity in each of the jars at the end of the jar testing

Collect samples for TOC analysis