Structured LDH/Bentonite Composites for Chromium Removal and Recovery from Aqueous Solutions

This study focuses on chromium removal through adsorption and ion exchange using structured calcined layered double hydroxide (LDH) (MgAl)–bentonite composites. Firstly, the powders were structured into granulates to study the effect on Cr sorption kinetics to circumvent the limitations of working with powders in real-life applications. Secondly, the regeneration of the structured composites was optimized to enable multi-cycling operation, which is the key for their applicability beyond laboratory scale. Firstly, the LDH/bentonite ratio was optimized to obtain the best performance for the removal of Cr(3+) and Cr(6+) species. In powder form, the calcined adsorbent containing 80 wt% LDH and 20 wt% bentonite performed best with an adsorption capacity of 48 and 40 mg/g for Cr(3+) and Cr(6+), respectively. The desorption was optimized by studying the effect of the NaCl concentration and pH, with a 2 M NaCl solution without pH modification being optimal. The kinetic data of the adsorption and desorption steps were modelled, revealing a pseudo-second order model for both. This was also demonstrated using XRD and Raman measurements after the Cr(3+) and Cr(6+) adsorption tests, indicating successful uptake and revealing the adsorption mechanism. Finally, five consecutive adsorption–desorption cycles were performed, each showing nearly 100% adsorption and desorption.