Francesco Mazzieri

DIFFUSION OF CALCIUM CHLORIDE IN A MODIFIED BENTONITE: IMPACT ON OSMOTIC EFFICIENCY AND HYDRAULIC CONDUCTIVITY

Chemically modified bentonites are being developed with the aim of preserving low hydraulic conductivity in the presence of potentially aggressive permeants in pollutant-containment applications. ‘Multiswellable’ bentonite (MSB) has been obtained by treating standard sodium bentonite with propylene carbonate. Research on the engineering properties of MSB has focused mainly on permeability and chemical compatibility. Solute diffusion and membrane behavior in MSB have not yet been investigated. A combined chemico-osmotic/diffusion test was performed on a MSB specimen using a 5 mM CaCl2 solution. Permeability with distilled water and with the 5 mM CaCl2 solution was measured prior to and after the chemico-osmotic/diffusion tests. The material exhibited time-dependent membrane behavior with a peak osmotic efficiency value (ω) of 0.172 that gradually shifted to zero upon breakthrough of calcium ions. Effective diffusion coefficients of calcium and chloride ions were in the range commonly described for untreated bentonite at similar porosities. After the chemico-osmotic/diffusion stage and permeation with 5 mM CaCl2, the hydraulic conductivity of MSB increased from 1.1 × 10−11 m/s to 7.0 × 10−11 m/s. The MSB was apparently converted into a calcium-exchanged bentonite at the end of the test. Prehydration and subsequent permeation might have contributed to elution of the organic additive from the clay. Further investigation is recommended to clarify the effect of prehydration on the hydraulic performance of MSB in the presence of potentially aggressive permeants.

Retention of heavy metals in conventional and factory-prehydrated GCLs materials

A conventional Geosynthetic Clay Liner (C-GCL) and a dense prehydrated (DPH) GCL were permeated with an acidic solution (pH=2) containing relatively high concentrations (0.025 M) of Pb, Zn, and Cu as nitrate salts. Both GCLs were permeated with distilled water prior to exposure to the metal solution. The DPH-GCL preserved hydraulic conductivity about one order of magnitude lower than the C-GCL. Breakthrough of metals occurred within three months of permeation for the C-GCL whereas it was not complete after more than one year of permeation for the DPH- GCL material. Cation exchange and precipitation of metals were the likely retention mechanisms.

Modified clays for barriers: a review

The aim of this Specialized Lecture is to present the recent advances and issues, as well as original research, on Modified Clays for Barriers. Topics of interest include: (1) long-term hydraulic performance of modified clays for GCLs, (2) chemico-osmotic and diffusion efficiency of modified clays, (3) modeling coupled chemical-hydraulic-mechanical behavior of modified clays, (4) wet and dry ageing of modified clays, (5) use of novel bentonites for vertical barrier applications, and (6) organoclays for various barrier applications. In addition, the possible reuse of dredged sediments after polymer treatment will also be discussed. Environmental management and handling of dredged sediments are important worldwide because enormous amounts of dredged material emerge from maintenance, construction and remedial works within water systems. Usually these materials after temporary upland disposal in lagoons are disposed in landfills. The aim of this study is to analyse the possible reuse of these sediments as a low-cost alternative material for landfill covers. The mechanisms through which polymers can improve the efficiency of dredged sediments for waste containment low permeable barriers are discussed.

Discussion of “Chemical Compatibility of Model Soil-Bentonite Backfill Containing Multiswellable Bentonite” by Michael A. Malusis and Matthew D. McKeehan

The paper discussed presents a comprehensive and very interesting investigation regarding the chemical compatibility of soil-bentonite backfills to CaCl2 solutions. Standard (NG) as well as modified bentonites (MSB and SW101) were used as amendments. The discussion paper commented on some aspects that dealt with the paper in light of their past experience with MSB.

Hydraulic Conductivity and Sorption Capacity of Special Barrier Materials in Inorganic Solutions

The hydraulic performance and migration parameters of cementitious mixtures are currently under investigation by an experimental study. The main aim of the research is to investigate the diffusion coefficient and sorption capacity of the mixtures and their interaction with inorganic sulphate solutions at different concentrations, as a function of their composition and curing time. The paper shows and discusses the results related to the hydraulic performance and the sorption capacity as a function of the mixtures composition. Simulations of solute migration through cut-offs made by the different mixtures investigated show the importance of taking into account their sorption capacity to optimize the design.

Modeling Solute Transport Through Geosynthetic Clay Liners Permeated with Inorganic Solutions

GCLs are employed in containment applications, where both advection and diffusion need to be evaluated as possible migration mechanisms of contaminants. Permeation column tests were carried out on a GCL using two synthetic multispecies inorganic solutions of different ionic strength and containing equimolar concentration of heavy metals (Pb, Zn, Cu). The hydraulic conductivity (k) of the GCL to inorganic solutions increased by one order of magnitude relative to permeation with water. The solute breakthrough curves were interpreted using the software Pollute®, which allows modeling solute transport in case of variable transport parameters and nonlinear equilibrium sorption. The experimental data were best fitted by assuming effective diffusion coefficients of solute species increasing with time.

About the state parameters of active clays

The common use of active clays, such as bentonites, as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical models that take into account the physical, chemical and mechanical coupled phenomena. The elementary particle (platelet) of a bentonite is characterized by a high total specific surface (S) and a permanent or fixed negative electric surface charge (sigma) that interacts with the ions in solution at different concentration (cs) in the pore water. This interaction is the primary factor that determines the final fabric of the solid skeleton and, consequently, most of the bentonite properties in terms of both mechanical behavior and solute transport. The tactoid formation or flocculation is the primary phenomenon that influences bentonite behavior and can be quantified with appropriate state parameters, such as the effective specific surface, Seff, the average number of platelets per tactoid, Nl,AV, and the electric fixed-charge concentration of the solid skeleton, csk,0. On the basis of the aforementioned state parameters, a theoretical hydro-chemico-mechanical framework has been developed. The validity of this framework has been tested, within the present paper, by comparison of its predictions with some of the available experimental results on bentonites with more than 70% montmorillonite content, in terms of hydraulic conductivity, swelling pressure and osmotic efficiency versus void ratio and ion concentrations of the pore solution.

Modeling contaminant leaching and transport to groundwater in Tier 2 risk assessment procedures of contaminated sites

Abstract The current approach to the management of contaminated sites includes a risk assessment procedure as a crucial step. In some jurisdictions (e.g., Italy), groundwater must be considered as a target of contamination. In other cases (e.g., USA), risks from exposure to contaminated groundwater (e.g., through the ingestion of contaminated water) must be taken into account. If the contaminant source is located in the vadose zone, the risk assessment requires modeling of contaminants leaching and subsequent transport to groundwater. In Tier 2 risk assessments, analytical transport models are used for this purpose. This paper compares steady state and a transient approach for modeling the leaching and migration of contaminants to groundwater. The steady-state model that was used considers soil–water partitioning, soil attenuation during transport and dilution with groundwater in a defined mixing zone. The transient model that was used considers source depletion due to leaching and volatilization, one-dimensional solute transport and a novel approach to the modeling of dilution with groundwater. Comparative simulations were performed with regard to a given site geometry and to index substances representing different contaminant classes: aromatic solvents, chlorinated solvents, and heavy metals. The results of the study indicate that, although steady-state models are simple and easy-to-use, taking into account the time variable is essential for a reasonable simulation of contaminant transport to groundwater, particularly for contaminants with high partition coefficients migrating through fine-grained soils.

Stabilisation/solidification of soils contaminated by chlorinated pesticides

This article describes laboratory tests carried out to identify an effective stabilisation/solidification method for soils contaminated by chlorinated pesticides at a disused industrial site. The effectiveness of the treatment was evaluated using leach tests. Powdered activated carbon (PAC) was used as the primary amending agent, thanks to the strong affinity for organic molecules and high sorption capacity. For some areas of the site, test results showed that treatment with PAC was decisive in reducing leaching of the pollutants below the prescribed limits. For areas with higher concentrations, treatment with PAC alone was not sufficient, although the reduction was significant. According to a detailed programme, further tests were performed: different percentages of cement were added to the soil–PAC mixture as a binding/solidifying agent. For all the agents, the optimum dosage that led to dissolved phase concentration standards for pesticides was identified. The test results are evaluated in view of possible field applications. Factors to be further addressed for an overall evaluation of the technology are discussed.