Adsorption of Formaldehyde from Aqueous Solutions Using Metakaolin-Based Geopolymer Sorbents

Abstract

Geopolymer samples prepared through alkali activation of metakaolin under varying conditions of synthesis (100°C (14 h); 130°C (3 h); clinoptilolite filler) are employed as formaldehyde sorbents from aqueous solutions. The structure and physicochemical properties of geopolymer sorbents are studied using XRD, XRF, SEM, N₂ adsorption–desorption, and chemical methods. Zeolite A is the main component among the structural components in the synthesized specimens (zeolite А, hydroxysodalite, and quartz). Addition of the clinoptilolite filler in the synthesis of geopolymers has resulted in a slight increase in their specific surface and pore volume. Geopolymer sorbent specimens display strong adsorption ability in relation to sorbate molecules in aqueous solutions of formaldehyde (0.064–0.366 M). The cation-exchange capacity of the geopolymers, which is 2–3 times as large as that of the natural clinoptilolite specimen, is the probable reason of their strong adsorption capacity to formaldehyde. The nature of the exchanged cation (Na⁺, \({\text{NH}}_{4}^{ + },\) Cu²⁺) determines the magnitude and the predominant mechanism of sorption uptake of formaldehyde (physical adsorption, chemisorption, and complexation). Higher adsorption values of formaldehyde are obtained in the case of \({\text{NH}}_{4}^{ + }\)- and Cu²⁺-forms of geopolymer sorbents compared to the Na⁺-form.