Influence of fluoride on the compressibility of montmorillonites

Abstract

The influence of fluoride contamination on the plasticity, compressibility and permeability of a bentonite clay and a montmorillonitic soil and the physico-chemical mechanisms involved are investigated. Treatment with fluoride degraded the mineral structure and decreased the cation exchange capacity (CEC) of beotonite clay and the montmorillonitic Chitoor soil by 60% and 71% respectively. This decrease in the CEC is attributed to aluminium as Al(OH)₃ extracted from the clay lattice, displacing the naturally adsorbed cations and blocking the pathway to these sites. X-ray diffraction patterns of the fluoride-treated specimens on potassium saturation, glycerol solvation and heating to 500°C did not show any reflections around 10 Å suggesting that the precipitated Al(OH)₃ does not enter the interlayer space but is possibly located on clay domains or in the pores between clay domains and effects their aggregation by electrostatic bonding. The liquid limit of bentonite clay decreased from 495% to 256% and that of Chitoor soil from 124% to 81% on fluoride treatment. The decrease in CEC and in surface area (due to particle aggregation) on fluoride treatment leads to a decrease in the diffuse double-layer thickness and in the liquid limit values. Oedometer test results showed that fluoride-treated bentonite clay and Chitoor soil exhibit appreciably lower equilibrium voids ratios at any pressure increment. Treatment of the montmorillonitic clay/soil specimen with fluoride causes a reduction in the repulsion forces due to a decrease in the diffuse double-layer thickness, leading to lower equilibrium voids ratios at any applied pressure. Treatment with fluoride increases the permeability of the clay/soil specimen appreciably. On fluoride treatment, the precipitated Al(OH)₃ binds adjacent silicate surfaces, facilitating the formation of larger effective pores. Further, as these pores are relatively less constricted by the reduction in diffuse double layers, they are more open for water flow, which yields higher permeability coefficients.