Utilization of encapsulating, anti-oxidizing, and green capping features of metakaolin based geopolymers for Mn, Cr and Co salts in catalytic degradation of dyes

Abstract

Green and sustainable alkali-activated aluminosilicates (geopolymers, GPs), with zeolite-like structure were used previously to prepare metal oxides/GP composites by ion-exchange of metal ions with Na⁺ balancing [AlO₄]⁻ in GP. The present work attempts preparing metal oxides embedded in GP, for the first time, by direct mixing of 30 wt% KMnO₄, K₂CrO₄ or CoCl₂ with alkali activated Aldrich metakaolin (MK) and local Jordanian metakaolin (JMK) followed by calcination at 650 °C. FTIR, XRD, SEM and TEM of the products showed dispersion of nanostructures of metal oxides (Mn₃O₄ (II, III), Cr₂O₃ (III) and Co₃O₄ (II, III)) suggesting reducing/anti-oxidizing and capping power of aluminosilicate medium. The degradation behavior of acid blue (AB) and Congo red (CR) was studied in the presence of GP samples, H₂O₂ and absence of illumination. Using 5.0 g/L catalyst dose, 50 mg/L dye concentration, 88 mM H₂O₂, 30 °C and pH 10; MK-Mn and JMK-Mn catalysts induced degradation of 90 % AB within 65 min. MK-Co and JMK-Co resulted in degradation of 50–74 % of CR within 210 min at 50 °C. Although CR (26.9, 11.0 and 53.3 %) was found to have higher affinity than AB (1.2, 7.7 and 26.4 %) for adsorption onto JMK-Cr, JMK-Mn and JMK-Co, the second order rate constants (k₂) of CR (8.14 × 10², 0.0 and 67.93 min⁻¹ M⁻¹) were lower than those of AB (7.29 × 10³, 1.81 × 10³ and 5.23 × 10² min⁻¹ M⁻¹). In comparison with photocatalytic degradation, the present work exclude electricity and UV lamps cost with relatively lower k₂ and longer treatment.