Performance evaluation of 3D-printed geopolymer monoliths for cadmium adsorption

This study presents a novel strategy for cadmium removal from industrial wastewater using metakaolin-based geopolymer monoliths fabricated via Direct Ink Writing (DIW). The performance of the monoliths was systematically compared with compositionally identical geopolymer beads produced by line injection method. Batch adsorption experiments with powdered geopolymer established the optimal adsorption conditions, with equilibrium data fitting well to the Langmuir isotherm (R² > 0.98), indicating monolayer adsorption and a maximum capacity of 87.1 mg/g. Dynamic column experiments were conducted for both monolith and bead beds under varying bed heights (0.5–1.5 cm) and flow rates (5–15 mL/min). The highest cadmium uptake was achieved at 0.5 cm bed height and 5 mL/min flow rate. Maximum dynamic adsorption capacities were 37.5 mg/g for beads and 35.9 mg/g for monoliths. Breakthrough curve analysis revealed that, although beads exhibited slightly higher adsorption capacity, monoliths demonstrated superior mass transfer characteristics, evidenced by a shorter mass transfer zone and enhanced intraparticle diffusion. Notably, the monolithic bed retained both structural integrity and adsorption efficiency over eight adsorption–desorption cycles, whereas the bead bed sustained only three. This work highlights 3D-printed geopolymer monoliths as durable and scalable bulk adsorbents, eliminating the need for costly downstream separation and offering a promising pathway for industrial-scale wastewater treatment.