Efficient cadmium removal from phosphoric acid by adsorption onto functionalized bentonite and activated carbon derived from rosemary root

Abstract

Cadmium content in wet phosphoric acid remains a major challenge in the industrial production of phosphate fertilizers. This study investigates the removal of cadmium using two novel adsorbent materials: modified bentonite clay and activated carbon derived from rosemary root. The bentonite clay was functionalized by intercalating silver hydroxide (Ag(OH)) between its layers, followed by grafting diethyl O,O-dithiophosphoric ester (-SH group) onto its surface. Activated carbon was prepared via chemical carbonization of rosemary root using phosphoric acid. The adsorbents were characterized using XRD, SEM, EDX/EDS, and BET techniques, and their cadmium adsorption performance was evaluated in a batch system. A full factorial design was employed to study the effects of adsorbent specific mass, phosphoric acid concentration (P₂O₅ %), and temperature on cadmium removal efficiency. The results showed that modified bentonite achieved up to 89 % cadmium removal at a specific mass of 6 g per 100 g of wet phosphoric acid, particularly at P₂O₅ concentrations between 6.8 % and 12.6 %, with higher pH favoring improved performance. Activated carbon achieved a maximum removal efficiency of approximately 70 % at a specific mass of 1 g per 100 g of wet phosphoric acid for P₂O₅ concentrations below 10 %. The enhanced performance of modified bentonite is attributed to chemisorption through covalent bonding between cadmium ions and surface sulfur groups. These findings highlight the potential of functionalized bentonite as an effective and economical adsorbent for cadmium removal in industrial phosphoric acid processing.