Assessment of the semipermeable membrane effect of illite and montmorillonite on dissolved thallium diffusion

The natural barrier properties of low-permeability clay layers have led to widespread interest in their application in the environmental field. Research on the protective and retardant effects of clay layers on low concentrations of highly toxic thallium is limited. A comparison revealed that the proportion of the active fraction of thallium in illite and montmorillonite was relatively low, indicating that these clays play an important role in thallium fixation. The focus of this work was an experimental investigation of the semipermeable membrane behaviour of illite and montmorillonite. Chemical osmotic differential pressure tests were performed at five thallium concentrations (0.5 mg/L to 50 mg/L), and the chemico-osmotic efficiency coefficients (ω) were calculated. Montmorillonite (ω, 0.048–0.651) has greater chemico-osmotic efficiency coefficients than illite (ω, 0.045–0.437). ω increased as the concentration of dissolved thallium decreased, indicating that the semipermeable membrane effects of the illite and montmorillonite layers were stronger at low thallium concentrations. The effective diffusion coefficients and retardation factors for thallium in illite and montmorillonite were calculated. The findings of this study must be considered in the context of ion competition effects within complex chemical environments. Studying the semipermeable membrane effect of clays on low concentrations of dissolved Tl may aid in clarifying the natural cleaning and barrier properties of clay, as well as support its potential use as an artificial remediation material.