Tuning crystallization for controlled morphology of Zeolite A by an eco-friendly sonochemical precursor-less method

Abstract

This study describes the sustainable synthesis of zeolite 4A from industrial solid residues without the addition of chemical precursors, as well as the optimization of its structural and morphological control using a central composite design. High-modulus silicate and aluminate are extracted from fly ash and red mud by low-temperature alkali fusion (350 °C) followed by ultrasonication. The transformation of colloidal aluminosilicate sol into a stable crystal of zeolite 4A by the hydrothermal method was studied to observe morphological changes and hypothesize a crystal growth mechanism. A total of 20 experiments were conducted to optimize high-quality truncated edge cubic zeolite 4A formation. A pure zeolite 4A (85% of crystallinity, 79.825 m2/g of specific surface area, uniform pore size distribution of 3.69 nm) is obtained under the optimized conditions of FA/RM extract = 1.02, crystallization temperature = 90 °C, crystallization time = 10.25hrs. The surface morphology of zeolite 4A appears in three distinct forms: sharp edge cube when FA/RM extract is greater than 1.02 (Run 19), truncated edge cube when FA/RM extract is equal to 1.02 (Run 2), and rounded edge cube when FA/RM extract is less than 1.02 (Run 18). Based on the ANOVA and desirability function results, we can infer that our anticipated model is a fit and the desired goal of maximum crystallinity was achieved with 95% confidence. The optimized zeolite 4A shows a higher hardness removal capacity of Ca2+(184.6 Ca2+mg/g in just 15 min) and Mg2+ (253.94 mg Mg2+/g in 5 h) for moderately hard pond water than commercial one. The kinetic and thermodynamic study reveals a better fit for the pseudo 2nd order kinetic model with a faster exchange rate of Ca2+(1.5 × 10−2 g.mg−1. min−1) than Mg2+ (7.4 × 10−5 g mg−1. min−1) and cation exchange is a spontaneous endothermic process.