carbon adsorption department of civil and environmental engineering ceen 330 – environmental field...
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Carbon Adsorption
Department of Civil and Environmental Engineering
CEEN 330 – Environmental Field Session
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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
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Adsorption
Absorption (“partitioning”)
PHASE I
PHASE 2
PHASE I
‘PHASE’ 2
gas H aqP K cHenry’s Law
Adsorption vs. Absorption
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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)
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Adsorbents in Natural & Engineered Systems Natural Systems
Sediments Soils
Engineered Systems Activated carbon Metal oxides (iron and aluminum as coagulants) Ion exchange resins Biosolids
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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+
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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 --
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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
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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 -- --
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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)
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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
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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
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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)
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Activated Carbon
Pore sizes: Micropores (pore diameter less than 20 nm) Mesopores (pore diameter 20-200 nm) Macropores (200 nm & above)
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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
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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
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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
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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
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The Process of Adsorption
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Kinetics of Atrazine Sorption onto GAC
167 mg GAC/L
333 mg GAC/L
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Readily and Poorly Adsorbed Organics
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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
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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
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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)
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Development of Adsorption Isotherms (cont.)
V/m = const.; vary Co vary V/m; Co = const.
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Adsorption Equilibriumqi = f(co, eq), T = const.
Langmuir (1918)
Freundlich (1907)
Brunauer, Emmet, and Teller (1938)
Redlich-Petersen
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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
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Determination of Langmuir constants: Linearization
plot 1/q vs. 1/c
Langmuir Isotherm (cont.)
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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”)
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Freundlich adsorption isotherm: empirical approach (1912) applicable in specific concentrations range only
Linearization
Freundlich Isotherm
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Freundlich Isotherm (cont.)
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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
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Shape of Langmuir Isotherm
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nfq k c
Shape of Freundlich Isotherm
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log log logfq k n c
Shape of Freundlich Isotherm (log scale)
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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
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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)
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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
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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