Charles D. Shackelford

Laboratory diffusion testing for waste disposal: a review

This paper reviews the state-of-the-art for the measurement in the laboratory of diffusion coefficients of chemical waste constituents in fine-grained soils. The purpose of the review is to present the experimental and analytical methods for determining liquid-phase diffusion coefficients which can be used in practice for the design and evaluation of waste containment barriers. After the appropriate equations describing mass transport in soil are presented, the practical significance of diffusion coefficients in soil (known as “effective diffusion coefficients”) are described. Appropriate analytical solutions required to calculate the effective diffusion coefficient (D∗) from the measured laboratory data also are presented for several different initial and boundary conditions. The advantages and disadvantages of each method are noted. A summary of effective diffusion coefficients from the literature suggests that the major physical factor affecting the value of the measured diffusion coefficient is the degree of saturation of the soil, with D∗-values for nonreactive and reactive solutes in saturated soils being as much as 10–20 times higher than the corresponding values in unsaturated soils. Most of the other physical factors only become important in soils which are highly unsaturated. In addition, the diffusive transport rates of reactive solutes subject to reversible sorption reactions can be as much as 5000 times lower than those of nonreactive solutes in saturated soils and from 20 to 630,000 times lower in unsaturated soils.

Flow and transport through clay membrane barriers

Flux equations for liquid and solute migration through clay barriers that behave as semi-permeable membranes used in waste containment and remediation applications, known as clay membrane barriers (CMBs), are discussed. The results of a simplified analysis of flow through a geosynthetic clay liner (GCL) using measured values for the chemico-osmotic efficiency coefficient (ω) of the GCL indicate a total liquid flux that counters the outward Darcy (hydraulic) flux due to chemico-osmosis associated with clay membrane behavior of the GCL. Also, the solute (contaminant) flux through the GCL is reduced relative to the solute flux that would occur in the absence of membrane behavior due to chemico-osmotic counter advection and solute restriction. Since diffusion commonly controls solute transport through GCLs and other low-permeability clay barriers, the implicit (empirical) correlation between ω and the effective salt-diffusion coefficient of the migrating contaminant is an important consideration with respect to contaminant restriction in CMBs.

A laboratory apparatus to measure chemico-osmotic efficiency coefficients for clay soils

A laboratory apparatus for measuring the chemico-osmotic efficiency coefficient, ω, for clay soils in the presence of electrolyte solutions is described. A chemico-osmotic experiment is conducted by establishing and maintaining a constant difference in electrolyte concentration across a soil specimen while preventing the flow of solution through the specimen. The chemico-osmotic efficiency coefficient is derived from a measured pressure difference induced across the specimen in response to the applied concentration difference. The effective diffusion coefficient (D*) and retardation factor (Rd) of the electrolytes (solutes) also can be determined simultaneously by measuring the diffusive solute mass flux through the specimen until steady-state diffusion is achieved. Experimental results using specimens of a geosynthetic clay liner subjected to potassium chloride solutions indicate that the measurement of ω may be affected by soil-solution interactions, as well as by changes in the induced chemico-osmotic pressure difference due to solute diffusion. As a result, ω should be evaluated using the induced pressure difference at steady state. The time required to achieve a steady-state response in induced pressure difference is related to the time required to achieve steady-state diffusion of all solutes, and may be affected by the circulation rate at the specimen boundaries. The circulation rate should be sufficiently rapid to minimize changes in the boundary concentrations due to diffusion, but sufficiently slow to allow measurement of solute mass flux at the lower concentration boundary for evaluating D* and Rd.

THE DESTRUCTIVE ROLE OF DIFFUSION ON CLAY MEMBRANE BEHAVIOR

The results of a combined chemico-osmotic/diffusion experiment conducted on a geosynthetic clay liner (GCL) containing Na-bentonite illustrate the destructive role of diffusion on the ability of the GCL to act as a semipermeable membrane. The experiment is conducted by maintaining a concentration difference of 5 mM CaCl2 across the GCL specimen while preventing the flow of solution through the specimen. A time-dependent membrane efficiency is derived from measured pressure differences induced across the specimen in response to the applied concentration difference. The diffusive mass fluxes of the solutes (Cl− and Ca2+) through the specimen are also measured simultaneously. An initial increase in induced pressure difference across the specimen to a peak value of 19.3 kPa is observed, followed by a gradual decrease to zero. The decrease in induced pressure difference is consistent with compression of diffuse double layers between clay particles and particle clusters due to diffusion of Ca2+, resulting in a concomitant increase in pore sizes and decrease in the observed membrane behavior. The time required for effective destruction of the initially observed semipermeable membrane behavior correlates well with the time required to achieve steady-state Ca2+ diffusion. The results have important implications for the ability of clays to sustain membrane behavior.

Transit-time design of earthen barriers

Abstract Transit-time analyses are used to evaluate the design thickness of earth-lined waste containment barriers. Solute transit times are determined from dimensionless charts based on an existing analytical solution for transient solute transport in saturated porous media. Both solute concentration and solute flux are considered in the transit-time analyses. With respect to solute flux, a dimensionless parameter termed the “flux number” is introduced. The results of an example design problem using measured parameters indicate that the effects of diffusion and retardation on solute transit times can be significant in low-permeability (i.e.,≤ 1.0 · 10−9m/s), fine-grained barrier materials. The method is relatively simple and can be used for preliminary design of earthen waste containment barriers, evaluation of remedial measures, and/or verification of more sophisticated numerical models.

Long-Term Hydraulic Conductivity of a Bentonite-Polymer Composite Permeated with Aggressive Inorganic Solutions

AbstractBentonite was modified to prevent alterations in hydraulic conductivity when permeated with aggressive inorganic solutions. Acrylic acid within bentonite slurry was polymerized to create a bentonite-polymer composite (BPC). Tests indicate that BPC generally swells more and retains low hydraulic conductivity compared with natural sodium bentonite (Na-bentonite) when contacted with aggressive inorganic solutions. BPC in deionized water swelled greater than 3.8 times the swell of the Na-bentonite used to create BPC (73 versus 19 mL/2 g). In 500 mM CaCl2, however, swell of BPC was similar to swell of calcium bentonite ( 2 years). In contrast, Na-bentonite and superabsorbent polymer (similar...

Consolidation and Hydraulic Conductivity of Zeolite-Amended Soil-Bentonite Backfills

The effect of zeolite amendment for enhanced sorption capacity on the consolidation behavior and hydraulic conductivity, k ,o f a representative soil-bentonite (SB) backfill for vertical cutoff walls was evaluated via laboratory testing. The consolidation behavior and k of test specimens containing fine sand, 5.8% (dry weight) sodium bentonite, and 0, 2, 5, or 10% (dry weight) of one of three types of zeolite (clinoptilolite, chabazite-lower bed, or chabazite-upper bed) were measured using fixed-ring oedometers, and k also was measured on sep- arate specimens using a flexible-wall permeameter. The results indicated that addition of a zeolite had little impact on either the consolidation behavior or the k of the backfill, regardless of the amount or type of zeolite. For example, the compression index, Cc, for the unamended backfill specimen was 0.24, whereas values of Cc for the zeolite-amended specimens were in the range 0:19 ≤ Cc ≤ 0:23. Similarly, the k for the unamended specimen based on flexible-wall tests was 2: 4×1 0 � 10 m=s, whereas values of k for zeolite-amended specimens were in the range 1: 2×1 0 � 10 ≤ k ≤ 3: 9×1 0 � 10 m=s. The results of the study suggest that enhancing the sorption capacity of typical SB backfills via zeolite amendment is not likely to have a significant effect on the consolidation behavior or k of the backfill, provided that the amount of zeolite added is small (≤ 10%). DOI: 10.1061/(ASCE)GT.1943-5606.0000566.

Membrane Behavior of Compacted Clay Liners

The containment function of clay barriers used for waste containment applications (e.g., landfills) can be enhanced if such clays exhibit membrane behavior or the ability to restrict the migration of solutes (e.g., contaminants). In this regard, compacted specimens of a locally available natural clay known as Nelson Farm Clay (NFC), as well as NFC amended with 5% (dry weight) sodium bentonite, were evaluated for hydraulic conductivity, k , and the potential for membrane behavior. The membrane efficiencies of specimens of both soils compacted such that k was less than 10−7  cm/s were measured by establishing steady salt (KCl) concentration differences, −Δ Co , ranging from 3.9 to 47 mM across the specimens in a flexible-wall cell under closed-system boundary conditions. The measured membrane efficiency for the unamended NFC was negligible (i.e., ≤1.4% ), even though the k was suitably low (i.e., k< 10−7  cm/s ). In contrast, compacted specimens of the bentonite amended NFC exhibited not only lower k but al...

Diffusion of inorganic chemical species in compacted clay soil

This research was conducted to study the diffusion of inorganic chemicals m compacted clay soil for the design of waste containment barriers The effective diffusion coefficients (D*) of amonlc (C1 , Br , and I ) and cationic (K ~, Cd 2+, and Zn 2÷) species in a synthetic leachate were measured Two clay soils were used in the study. The soils were compacted and pre-soaked to mlmmlze mass transport due to suction in the soil The results of the diffusion tests were analyzed using two analytical solutions to Flcks second law and a commercially available seml-analyhcal solution, POLLUTE 3.3 Mass balance calculations were performed to indicate possible sinks/sources in the diffusion system Errors in mass balance were attributed to problems with the chemical analysis (I), the inefficiency of the extraction procedure (K ÷), precipitation (Cd 2. and Zn 2÷ ), and chemical complexatlon (C1- and Br-) The D* values for C1 reported in this study are in excellent agreement with previous findings for other types of soil The D* values for the metals (K + , Cd 2÷ , and Zn 2~ ) are thought to be high (conservative) due to (1) Ca 2÷ saturation of the exchange complex of the clays, (2) preclpitatmn of Cd 2÷ and Zn 2÷ , and (3) nonlinear adsorptmn behavior In general, high D* values and conservative designs of waste containment barriers will result if the procedures described in this study are used to determine D* and the adsorption behavior of the solutes is similar to that described in this study

Compaction of Sand-Processed Clay Soil Mixtures

The effects of type of processed clay soil, curing period, and mixing procedure on laboratory compaction of sand-attapulgite clay (S-AC), sand-granular bentonite (S-GB), sand-powdery bentonite (S-PB), and sand-attapulgite clay-granular bentonite (S-AC-GB) mixtures are evaluated. Compaction is evaluated for total clay soil contents of 10, 15, and 20%. Different trends in optimum water content, wopt, and maximum dry unit weight, γdmax, versus clay soil content among the S-AC, S-GB, and S-PB mixtures are attributed, in part, to (1) the greater water sorptivity and lower swelling potential of attapulgite clay relative to the bentonites, (2) the larger particle sizes of the granular bentonite in the air-dried condition relative to the powdery bentonite, and (3) the possible correlation between the wopt and the plasticity index of the sand-bentonite mixtures. The Δwopt values and Δγdmax values resulting from one-day versus seven-day curing periods before compaction of the S-GB and S-PB mixtures are ∼0.5 percentage points and ≤0.08 kN/m3 (≤0.5 pcf), respectively, and result in different trends in γdmax versus bentonite content for the two types of sand-bentonite mixtures. Also, mixing the sand and bentonite in a dry condition before adding water consistently results in greater wopt and γdmax values than mixing the sand with the appropriate amount of water before adding the bentonite regardless of the type of bentonite. Finally, mixing the attapulgite clay and granular bentonite together in small amounts for each individual compaction point for the S-AC-GB mixtures consistently results in higher γdmax and wopt values relative to mixing the attapulgite clay and granular bentonite together in large amounts sufficient to cover all compaction points.