Adsorption and interactions of methyl green with montmorillonite and sepiolite

G. Rytwo*, S. Nir, M. Crespin, L. Margulies

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

88 Scopus citations

Abstract

The divalent organic cation, methyl green (MG), undergoes a slow transformation (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 MG in suspension were lower than the cation-exchange capacity (CEC) of the clay (0.8 mol(c)/kg clay), no measurable amount of MG remained in solution. The maximal amounts of MGOH+ adsorbed were larger than those of MG2+, being 1.15 and 0.75 mol MG/kg day, respectively, corresponding to 140% of the CEC in the first case. On a charge basis the adsorption of added MG2+ amounts to 185% of the CEC, which raises the possibility that a certain fraction of MG2+ transformed into the monovalent form during the incubation period, since other divalent organic cations previously studied only adsorbed up to the CEC (paraquat), or slightly above it (diquat). Adsorption of MG on sepiolite (CEC = 0.15 mol(c)/kg) further emphasizes the two patterns of its adsorption. The maximal adsorbed amounts of MG2+ and MGOH+ were 0.09 and 0.30 mol/kg day, respectively. X-ray diffraction measurements gave lower values for the basal spacings for montmorillonite-MG+ than for MGOH+, suggesting that MG2+ binds two clay platelets together, as in the case of other divalent cations. A competition for adsorption between MG and the monovalent 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 into the interlammelar space. Spectra of montmorillonite-MG particles in the visible range exhibited significant differences between clay-MG and clay- carbinol. (C) 2000 Academic Press.

Original languageEnglish
Pages (from-to)12-19
Number of pages8
JournalJournal of Colloid and Interface Science
Volume222
Issue number1
DOIs
StatePublished - 1 Feb 2000

Keywords

  • Adsorption
  • Clay minerals
  • Methyl green
  • Montmorillonite
  • Organic cations
  • Sepiolite

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