Hard-core/zeolitized-shell beads obtained by surface zeolitization of activated clay particles

Abstract

This work presents a novel and cost-effective procedure that improves the reactivity of kaolinitic clay particles toward the generation of millimetric hard-core/zeolitized-shell units. The obtained particulate material is ready to use as ion exchanger beads in a fixed adsorption bed. Starting from properly activated, millimeter-sized clay units, the transition of their external surface towards a new crystalline ordering could be induced. The Na₂CO₃ alkaline activation treatment applied to previously calcined clay particles allowed obtaining different levels of NaAlSiO₄ polymorphs on the external surface of the particles, which are easily transformed to zeolitic products. By varying the synthesis conditions, FAU and LTA zeolite structures were successfully obtained on the surface of hardened particles, reaching conversions close to 90%. The starting clay, pretreated clays, and products obtained after hydrothermal synthesis were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray microanalysis (EDX). The versatility of the method is exemplified in obtaining adsorbents specifically designed for the retention of Cd²⁺ and Ni²⁺ contaminating ions. A total removal of Cd²⁺ and Ni²⁺ was achieved at 150 min from initial 40 ppm solutions, being 98% and 94% from initial 120 ppm solutions for cadmium and nickel, respectively.