Fixing of electrophoretically deposited natural zeolite particles by geopolymer reaction for heavy metal ion adsorbents

Abstract

The fixing of electrophoretically deposited natural zeolite particles on a metal substrate by “geopolymer reaction,” i.e. the polymerization of silicate monomers in a solution to form a polymer phase that can bind particles onto solid materials, was investigated. Finely ground natural zeolite (clinoptilolite) particles were dispersed in ethanol, and deposited on a stainless-steel electrode by electrophoretic deposition (EPD) with silica sol particles and polyvinylpyrrolidone (PVP) as binder materials. The deposit was then fixed to the electrode by curing in an alkali silicate aqueous solution containing Na₂SiO₃ and NaOH at 40°C for 24 h. The addition of the mixed binder of silica sol and PVP to the suspension used for the EPD was effective for the fixing treatment. After the fixing treatment, the durability of the zeolite deposit in stirred water was largely enhanced. It was also found that fixed deposits treated in solutions containing a smaller amount of Na₂SiO₃ showed better durability. Fixed deposits, containing the starting zeolite phase, moreover, showed the adsorption property of Pb²⁺ ions in the aqueous solution.