Aspects of ionic diffusion through thick matrices of charged particles

Self-generation of concentration gradients of divalent cations in commercial bentonite membrane during ultrafiltration

“…In this evaluation, characteristics of pure Na-bentonite-NaCl solution system, calculated on the basis of [Cl − ]'s reported in [8,14], will be compared to those calculated for a mixture of pure Na-bentonite + 0.68% divalent carbonate-NaCl solution system. At this low level, divalent carbonates do not change the expansion characteristics of Na-bentonite [8]. Unfortunately, however, CaCO 3 /MgCO 3 is unknown.…”

“…At the end of the experiment, they determined molarity of [Cl − ] within the column at different distances from the upstream end of the diffusion bed. These data have been collected in [8,14]. These [Cl − ] values could be fitted with R 2 = 0.9998 to the equation The interface charge, σ , when both monovalent and divalent ions are present, is related to the interface potential, ψ , and interface salt concentrations by (4) σ 2 = 2εε 0 kT Na + e −eψ/kT + e eψ/kT − 2…”

“…In the course of a literature survey it has been noted that a number of important aspects of ionic diffusion through thick barriers of charged matrices (commercial bentonite) are not getting their due consideration [8]. In the clay field a number of workers have reported different apparent diffusivity of cations and anions [9][10][11][12].…”

“…Crooks and Quigley also reported that at a distance from the landfill a ring of carbonate bloom formed. In a recent review of the relevant literature it has been suggested that during diffusion of (neutral) alkali halides through commercial bentonite, Ca 2+ and/or Mg 2+ ions, from CaCO 3 or MgCO 3 , co-diffuse with alkali and chloride ions to the downstream and at the same time a corresponding amount of CO 2− 3 ions diffuse to the upstream solution [8]. This double diffusion keeps both the upstream and the downstream solutions electroneutral.…”

“…Equation(2)gives [Cl − ] at the upstream and downstream solution interfaces as 0.0662 and 0.0076 M. At these interfaces [NaCl] is the same as [Cl − ]. From these concentrations, interface potentials are calculated, following the method described in[8]. This calculation requires an evaluation of the interface charge density (σ ).…”