Chemico-mechanical improvement of bentonite barriers for pollutant containment.

Pollution control represents one of the main problems of public interest in all industrialised countries. Since the 1970s, when the engineering of waste containment began, the overall objective of Environmental Geotechnics was to limit contaminant discharge to groundwater and subsoil. Since the 1990s design engineers and environmental agencies have shown a growing interest in the use of geosynthetic clay liners (GCLs) as an alternative to compacted clays in cover systems or in bottom lining of waste containment facilities because they have very low hydraulic conductivity to water and relatively low cost. GCLs contain a thin layer of sodium bentonite with a dry thickness between 5 and 10 mm, sandwiched between two geotextiles or glued to a geomembrane, GM. The excellent hydraulic performances of GCLs have to be attributed to bentonite characteristics and, since these last are greatly influenced by the chemical composition of the environment surrounding the barrier and by the state parameters, the performances of GCLs can be altered, and then worsened, by a simple variation of the chemical or physical boundary conditions. Aimed at solving this last issue, clay liners and GCLs have undergone great change during the last two decades, with new material being introduced (i.e. polymers) and new design methods being adopted (i.e. membrane behaviour investigation, contaminant diffusion estimation). The research project developed during the Ph.D. has been focused on bentonite barriers. The term “bentonite barriers” includes bentonite or bentonite-based barriers which find application both in urban waste landfill, hazardous or radioactive wastes final disposal and in hydrocarbon containment. The developed theoretical and experimental study has had the aim of evaluating the possible improvement of containment performance of the bentonite barriers, towards standard and non standard liquids, acting on their state parameters, chemical composition, and boundary conditions at installation. The contents of the thesis are reported below: Chapter 1 – Bentonite barriers. This chapter is an introduction to the topic of the research activity: the improvement of contaminant containment performances of the bentonite barrier. Chapter 1 gives a phenomenological and physical description of the mineralogical, chemical and physical properties of sodium and calcium bentonite. Moreover, the main features and issues concerning Geosynthetic Clay Liners, and bentonite barriers in general, are introduced. Chapter 2 – Containment performances of natural and polymer-modified bentonite barriers subjected to physical pre-treatments. The role of physical pre-treatments, such as pre-hydration, pre-consolidation and salt removal, applied to sodium and polymer modified bentonites, has been analyzed in the Paper reported in chapter 2, titled: “THE ROLE OF PHYSICAL PRETREATMENTS ON THE HYDRAULIC CONDUCTIVITY OF NATURAL AND POLYMER MODIFIED SODIUM BENTONITES”. Moreover, the effect of the presence or absence of needling across the bentonite layer has been studied. All these variables have been shown to influence the hydraulic performances of bentonite through hydraulic conductivity change in both short and long term conditions. Physical pre-treatments and polymer addiction, in fact, influence the swelling behaviour of bentonite and its response to the cation exchange phenomenon. Chapter 3 – Osmotic and swelling properties of bentonite barriers. In the Paper included in this chapter, titled “COUPLED CHEMICAL-HYDRAULIC-MECHANICAL BEHAVIOUR OF BENTONITES”, a theoretical approach has been proposed in order to derive constitutive equations which describe the coupled chemical-hydraulic-mechanical behaviour of bentonite barriers, with the aim to assess their long term performance. The phenomenological parameters that govern the transport of electrolyte solutions through bentonites, i.e. the hydraulic conductivity, the reflection coefficient, which is also called the chemico-osmotic efficiency coefficient, and the osmotic effective diffusion coefficient, have been measured through laboratory tests on a natural sodium bentonite The obtained results have been interpreted by assuming that the microscopic deviations of the pore solution state variables from their average values are negligible. In this way, it is possible to interpret the macroscopic behaviour on the basis of the physical and chemical properties of the bentonite mineralogical components. At the end of the chapter two further chemico-osmotic tests are described aimed at analysing (1) the osmotic behaviour of calcium bentonite and (2) the effects induced on osmotic behaviour by stress-strain properties. Moreover, the osmotic results are confronted with data from literature. Finally, the design of a new osmotic apparatus to measure both the swelling pressure and the reflection coefficient is proposed. Chapter 4 – Hydrocarbon containment performances of natural and polymer-modified bentonite barriers. Background information on hydrocarbon behaviour in soils is reported in the first part of this chapter. In particular, the effects of capillary forces on the distribution of immiscible fluids in porous media and the theoretical aspects, regarding the formation of tactoids induced by the low dielectric constant that characterizes the most of hydrocarbon species, are studied. An experimental study is presented in the Paper titled “HYDRAULIC PERFORMANCE OF GCLS WITH DIESEL OIL AND POLYMER TREATMENT PROPOSAL”, which is aimed at evaluating the hydraulic performance of a needle-punched GCL using both standard liquids (i.e. de-ionized water) and diesel oil in order to estimate the change in hydraulic conductivity and swelling ability upon contact or permeation with hydrocarbons. Moreover, the hydraulic conductivity to diesel oil of GCL samples saturated at different initial gravimetric water contents was investigated with the aim to analyse the effect of initial water saturation on hydrocarbon containment performances. Finally, the swelling and hydraulic performances to diesel oil of an innovative material, obtained by mixing sodium bentonite with a polymer, were measured. Chapter 5 – Containment performances of a bentonite-based barrier constituted of municipal solid waste bottom ashes. The research described in the Paper included in this chapter, titled “REUSE OF MSWI BOTTOM ASH MIXED WITH NATURAL SODIUM BENTONITE AS LANDFILL COVER MATERIAL” has the aim of evaluating the reuse of incinerator slag, mixed with sodium bentonite, for landfill capping system components. A chemical, hydraulic and mechanical characterization was performed on pure bottom ash (BA) samples from an incinerator in the North of Italy and on the BA-bentonite mixture. This study qualifies the BA-bentonite mixture as a suitable material for landfill cover in Italy. Moreover, owing to the low release of toxic compounds from BA, the proposed cover system does not affect the leachate quality in the landfill. Chapter 6 – Finite difference modelling of diffusive flux of Calcium through a bentonite barrier in in-situ conditions. The evidence of the strong degradation induced in the hydraulic performances of sodium bentonite barriers by the cation exchange phenomenon has been highlighted in the previous chapters. This experimental result underlines the need to study the transitional development of the cation exchange phenomenon with the aim to compare that to the period in which landfill barrier performances have to be guaranteed in in-situ conditions. The mathematical study developed in this chapter is focused on the evaluation of the role of the diffusive component of Calcium flux in the cation exchange phenomenon which can develop in a sodium bentonite barrier, placed in an environment inexorably rich in chemical compounds containing soluble Calcium (i.e. the natural soil, the aquifer, the drainage layer saturated with waste leachate or raining water).