15.10.2018 Lecture No. 21 Lecture Name: Geomaterial Characterization Sub-topics • Chemical characterization Sorption-Desorption Characteristics Determination of k d (The Distribution coefficient) • Thermal Characterization • Electrical Characterization
Sorption Adsorption Absorption Incorporation, Assimilation, Accumulation of substance on surface Inclusion Atoms or molecules move from the bulk phase (that is, solid, atoms or molecules move liquid, or gas) onto a solid or into the bulk of a porous liquid surface. material, e.g. the absorption of water The adhesion of a thin layer of by a sponge molecules of some substance to the surface of a solid or liquid e.g. purification by adsorption where impurities are filtered from liquids or gases by their adsorption onto the surface of a high-surface-area solid such as activated charcoal
Terms related to Sorption • Adsorbates - molecules that have been adsorbed onto solid surfaces • Substrate or Adsorbent - the surface to which adsorbates are adsorbed • e.g. in case of adsorbed cations tightly held on surfaces of negatively charged dry clay particles, clay particle is substrate and cations are adorbates. cations Clay particle
Desorption • a phenomenon whereby a substance is released from or through a surface • process is the opposite of sorption • occurs in a system being in the state of sorption equilibrium between bulk phase (fluid, i.e. gas or liquid solution) and an adsorbing surface (solid or boundary separating two fluids) • When the concentration (or pressure) of substance in the bulk phase is lowered, some of the sorbed substance moves to the bulk state
SORPTION & DESORPTION CHARACTERISTICS of Geomaterials Excellent philosophy to simulate geomaterial-contaminant interaction Sorption and Desorption are opposite interaction mechanisms Strategies for Remediation of Contaminated Soils/Geomaterials (Site cleanup/Site remediation) Soil washing Soil flushing Vitrification Solidification Immobilization Important to determine Sorption and Desorption Characteristics of Geomaterial and Immobilizing Agents
Importance of Sorption and Desorption Characteristics Fate and Transport of Reactive Contaminant(s) Efficiency of Environmental Cleanup Strategies Selection of Suitable Geological Formations and Backfill Materials Design of Barrier Layers of Waste Containment System Accumulation of Heavy Metals and Pesticides in Subsoil Challenge Precise Determination of these Characteristics (k d & k dl ) in a short duration k d : the coefficient describing sorption process k dl : the coefficient describing desorption process (subscript l corresponds to leaching process)
The coefficient k d Also known as the partition(ing) or distribution coefficient Is a measure of sorption of contaminants to soils/rocks/admixtures (geomaterials). Defined as the ratio of the quantity of the sorbate sorbed per unit mass of solids (C s ) to the amount of the sorbate remaining in solution (C w ), at equilibrium. The reverse is true for desorption (leaching) process K d measurement, some issues Experimental conditions Measurement methodology Contaminant chemical characteristics Sorbents (particle size, geochemistry) Type (active/passive) and concentration of the sorbate
Partitioning Relationships C mg / Kg Solid S • Solid ↔ water K d C mg / L Water W C s : Sorbate sorbed per unit mass of solids C w : Sorbate remaining in the solution, at equilibrium. 3 • Water ↔ vapor C mol / m air g Henry' s law constant (H) 3 C mg / m water w Contaminant Concentration in geomaterials Total mass in unit volume of geomaterial θ w : Volumetric water content .C s + θ w . C W + θ g . C g C T = θ g : Volumetric vapour content b θ = Saturation Porosity ( ) If soil is saturated, θ g = 0 and θ w = = bulk density of porous medium C s + C w b C T b
Determination of Sorption and Desoprtion Characteristics of Geomaterials Fail to simulate “ Geomaterial-Contaminant- 1. Batch Tests immobilizing agent Interaction ” in a realistic manner Fail to come up with recommendations regarding Generalized Isotherm Quite Time and Cost Intensive 2. Column Tests “ Low Hydraulic Conductivity ” Accelerated Physical Modeling Using a Geotechnical Centrifuge seems to be a viable option In situ field batch tests Field modelling tests k oc method k oc = k d .(100/OC) k oc = organic carbon normalized adsorption coefficient OC = percentage of organic carbon in the sample (g/g)
Available Methodologies ASTM (American Society of Testing and Materials). 1987. “ 24-hour Batch- Type Measurement of Contaminant Sorption by Soils and Sediments. ” In Annual Book of ASTM Standards, Water and Environmental Technology, Volume 11.04, pp. 163-167, Philadelphia, Pennsylvania. ASTM 1988. “Determining a Sorption Constant (k oc ) for an Organic Chemical in Soil and Sediments. ” In Annual Book of ASTM Standards, Water and Environmental Technology. Volume 11.04, pp. 731-737, Philadelphia, Pennsylvania. OCED TG 106, 2000, Determination of Soil Adsorption/Desorption Using a Batch Equilibrium Method. OECD: Organization for Economic Co-operation and Development
Factors Influencing Sorption and Desorption Characteristics Specific-surface Area Pozzolanic Activity (Lime Reactivity) CEC Value Liquid to Solid Ratio pH of the Soil Solution Buffer Capacity of the Sorbent Temperature Grain Size Presence of other Ions Ionic Strength Organic Content & Fe- Mn Oxides Carbonate Content
Batch Sorption Tests Geomaterial (in powder form) + Contaminant (in solution form) is allowed to interact for certain duration (with continuous stirring) Factors Influencing Liquid to solid ratio (L/S = 10, 20, 50, 100, 200) Interaction time Variation of C e with interaction time 100 C i Initial concentration of contaminant L/S 10 20 50 100 200 80 C e Concentration of contaminant in solution after time interaction time is to be 60 recorded by sampling the solution C e (mg/l) frequently 40 C s Concentration of the contaminant sorbed 20 C C C (L/S) s i e 0 24 h 0.01 0.1 1 10 100 t s (h)
Sorption Characteristics (Isotherms) L/S 10 20 50 100 200 2.8 LR 4 mg/kg) Linear Isotherm (LR) 2.1 C s = K d · C e 1.4 C s ( 10 0.7 0.0 0 1000 2000 3000 4000 5000 1.00 Langmuir Isotherm (LM) LM 0.75 C e /C s (kg/l) C 1 C e e 0.50 C K b b d 0.25 s 0.00 0 1000 2000 3000 4000 5000 FH Freundlich Isotherm(FH) 5 10 C s (mg/kg) -1 log(C ) log(K ) n log(C ) 3 s d e 10 1 10 -2 0 2 4 10 10 10 10 C e (mg/l)
Desorption Characteristics (Isotherms) C sl : the amount of contaminants present in the sorbate after desorption phenomena C el : the equilibrium concentration of contaminants C C C (L/S) s s el l present in the solution after equilibration time C s : Concentration of the contaminant sorbed Variation of C el with leaching time 100 L/S Linear Isotherm (LR) 10 20 50 100 200 C K C sl dl el 10 Langmuir Isotherm (LM) C el (mg/l) C 1 C el el C K b b d sl l l l 1 Freundlich Isotherm (FH) -1 log(C ) log(K ) n log(C ) 2 h sl dl l el 0.1 0.01 0.1 1 10 100 t l (h)
Relationship between CEC and K d CEC K d Principal minerals (meq/100 g) (ml/g) Quartz, Montmorillonite 49-57 3347-3580 Quartz, Orthoclase 6.4-6.6 1497-1530 10000 K d (ml/g) 1000 100 0 20 40 60 80 100 L/S
Some Important Relationships 3600 kd (ml/g) 3200 2800 12000 2400 0 5 10 15 20 25 10000 t (h) 8000 Kd (ml/g) 6000 4000 2000 0 3 4 5 6 7 8 9 pH
5 10 Material “Accelerated Physical Modeling of CS Sorption and Desorption Characteristics WC 4 10 of Geomaterials and Immobilizing Agents” IC EC (mS/cm) RSS Dali Naidu (2006) BSS FA-I 3 10 FA-II Material 2 CS WC IC RSS BSS FA-I FA-II 10 5 10 1 10 4 10 0 1 2 3 10 10 10 10 L/S EC (mS/cm) 1000 3 10 2 10 89 9.8 1 10 -1 0 1 2 3 4 -2 -1 0 1 2 3 10 10 10 10 10 10 10 10 10 10 10 10 Kd (l/kg) Kdl (l/kg)
Column Tests 55 The BTC Inner cylinder 1.0 Desorption starts @ 62 h Sorption 0.8 140 C t/ C 0 0.6 h 1 0.4 Middle cylinder sample L 0.2 Outer cylinder Porous disc 0.0 Base plate 10 0 20 40 60 80 100 120 140 160 t (h) 100 PV= V sol [ L ( d 2 /4)] -1 V sol is the volume of the solute passing through the sample L is the length of the sample, is the porosity
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