Adsorption and interactions of methyl green with montmorillonite and sepiolite

The divalent organic cation, methyl green (MG), undergoes a slow transforma tion (6 h) to a monovalent cation, carbinol (MGOH(+)) upon dilution of its solution (10 mM), or in a buffer at neutral pH, Adsorption isotherms of MG on montmorillonite were determined by two procedures, both of which yield a final pH of suspensions between 7 to 7.4. When the amounts of NG in suspen sion were lower than the cation-exchange capacity (CEC) of the clay (0.8 mo l(c)/kg clay), no measurable amount of MG remained in solution. The maximal amounts of MGOH(+) adsorbed were larger than those of MG(2+), being 1.15 a nd 0.75 mol MG/kg clay, respectively, corresponding to 140% of the CEC in t he first case. On a charge basis the adsorption of added MG(2+) amounts to 185% of the CEC, which raises the possibility that a certain fraction of MG (2+) transformed into the monovalent form during the incubation period, sin ce other divalent organic cations previously studied only adsorbed up to th e CEC (paraquat), or slightly above it (diquat). Adsorption of MG on sepiol ite (CEC = 0.15 mol(c)/kg) further emphasizes the two patterns of its adsor ption. The maximal adsorbed amounts of MG(2+) and MGOH(+) were 0.09 and 0.3 0 mol/kg clay, respectively. X-ray diffraction measurements gave lower valu es for the basal spacings for montmorillonite-MG(+) than for MGOH(+), sugge sting that MG(2+) binds two clay platelets together, as in the case of othe r divalent cations, A competition for adsorption between MG and the monoval ent organic cation, acriflavin (AF), gave lower adsorbed amounts of AF when competing with MG(+), which is interpreted to be due to the smaller basal spacing in this case, which partially inhibits the entry of AF molecules in to the interlammelar space, Spectra of montmorillonite-MG particles in the visible range exhibited significant differences between clay-MG and clay-ca rbinol. (C) 2000 Academic Press.