ADSORPTION OF MOLYBDATE ON NANO-BALL ALLOPHANE

Abstract

Adsorption behavior of Mo (Na₂MoO₄) on three nano-ball shaped allophane samples was different between lower (up to 0.1mM) and higher (0.2 to 1.6mM) Mo concentrations. The difference may be due to chemical species of Mo in solution: at lower concentration, Mo exists as monomeric H₂MoO₄, HMoO₄^- and MoO₄^2-, and at higher concentrations as polymeric forms. The monomeric molybdate species, especially the anionic species, had very strong affinity for the allophane at lower pH where the allophane had much surface positive charge and essentially no negative charge. All of the molybdate added at equilibrium pH 3.7 to 3.9 and initial concentration up to 0.1mM, 200 mmol kg^-1, was adsorbed on allophane samples with lower Si/Al ratio. With increasing pH, the affinity or amount of adsorption decreased markedly. At equilibrium pH near 5, adsorption isotherm was Langmuir type, whereas at higher pH such as 6 or 8 the isotherm was linear or Freundlich type. The strong interaction between molybdate anion and positively charged allophane was concluded as electrostatic one followed by ligand exchange reaction between Mo-O^- and aluminol groups. At higher Mo concentrations, more than 0.2 mM, shape of the isotherms were linear indicating further polymerization reaction on the allophane surfaces, and partial destruction of allophane structure was observed as Si and Al release with the adsorption.