GEOSYNTHETIC CLAY LINERS Chemical Compatibility What is chemical compa.bility Bentonite considera.ons Tes.ng considera.ons • Prehydra.on effect • Results
What is chemical compatibility? The hydraulic conduc.vity is a func.on of both the porous medium and the proper.es of the liquid, or 𝑘=𝐾(𝛾↓𝑓 /𝜇↓𝑓 ) Where k = hydraulic conduc.vity K = intrinsic permeability 𝛾↓𝑓 = unit weight of fluid 𝜇↓𝑓 = viscosity of fluid
Changes in hydraulic conductivity can result from three mechanisms • Changes in intrinsic permeability resul.ng from applied loads; e.g., consolida.on ∆𝑘=∆𝐾(𝛾↓𝑓 /𝜇↓𝑓 ) • Permea.ng an inert porous medium with a liquid with proper.es that are different from water; e.g., oil flow through sand ∆𝑘=𝐾 ∆(𝛾↓𝑓 /𝜇↓𝑓 ) • Interac.ons between the porous medium and the liquid resul.ng in changes in the pore structure of the medium; e.g., compa.bility ∆𝑘=∆𝐾 ∆(𝛾↓𝑓 /𝜇↓𝑓 )
Bentonite considerations The main component of GCLs that is suscep.ble to compa.bility is bentonite. Therefore, some considera.on must be given to the behavior of bentonite. The main mineralogical component of bentonite is montmorillonite
Thickness of double layer T Negatively charged clay particle T Water Flow T Negatively charged clay particle T This distance controls the hydraulic conductivity
Diffuse double layer of adsorbed 5 water and cations
Double layers will contract: • If the ca.on concentra.on is increased • If the ca.ons are changed from monvalent to divalent or trivalent; e.g., from Nat+ to Ca2+ • If the dielectric constant is reduced; e.g., from 80 for water to 35 for ethanol or 2 for benzene The above factors will cause the hydrauulic conduc.vity (k) to increase
The same reactivity of bentonites that results in swelling in the presence of water and a reduction in the hydraulic conductivity (k) also results in contraction in the presence of other liquids and an increase in the hydraulic conductivity. Consider data from Ryan 1987
Testing considerations Three types of tests are used: 1. Free swell tests (ASTM D5890) & AMerberg limits to provide a preliminary indica.on of poten.al compa.bility concerns, and 2. Hydraulic conduc.vity tests using the solu.on of interest as the permeant liquid (ASTM D6766)
Free Swell Test Using 2g of Air-‐Dried Na-‐bentonite
Hydraulic conductivity testing considerations: • Equipment same as k tes.ng with water except special precau.ons for toxic and/or caus.c chemicals; e.g., membrane compa.bility issue. • Sequence of permea.on is important • Chemical equilibrium between effluent and influent solu.ons is important • k vs .me plots are virtually meaningless; always plot k vs pore volumes of flow
The factors known to potentially affect the hydraulic conductivity of bentonite dominated soils are: • increase in the valence of the ca.ons, • increase in metals concentra.on, and • decrease in pH
In addition to the termination criteria for permeation with water: • ≥ 2 pore volumes of flow should be permeated through the GCL to ensure that the residual water in the specimen has been displaced by the permeant liquid, and • Permea.on should not terminate un.l the chemical composi.on of the ouflow is similar to that of the inflow
Note: Compatibility testing should be performed in accordance with ASTM D 6766 particularly given the importance of chemical equilibrium compatibility. D6766 Standard Test Method for Evaluation of Hydraulic Properties of Geosynthetic Clay Liners Permeated with Potentially Incompatible Aqueous Solutions
Hydraulic conductivity of bentonite can be adversely affected by high cation concentrations, permeation with polyvalent cations and concentrated organic chemicals The effect of a chemical or leachate tends to be much more severe when the first wetting liquid is the chemical or leachate Bentonite is much more chemically resistant if hydrated in fresh water before exposure to chemicals or leachate
Thanks for Listening John Bowders, PE, PhD The William A. Davidson Professor of Civil Engineering University of Missouri
