Bromide Sequestration Using a Low-Cost Hybrid Composite of Calcium, Activated Carbon, Kaolin, and Starch

Bromide in source waters promotes the formation of toxic brominated disinfection byproducts during chlorination and ozonation, necessitating efficient, affordable pretreatment materials. This study developed a low-cost hybrid composite, denoted Calcium-Activated Carbon-Kaolin-Starch (CAKS) composite, from kaolin, activated carbon, a calcium precursor and starch through starch-assisted consolidation followed by calcination under nitrogen. The synthesized material was characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, nitrogen adsorption-desorption analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. CAKS exhibited a heterogeneous mesoporous structure with a Brunauer-Emmett-Teller surface area of 212.5 m² g^-1, a pore volume of 0.384 cm³ g^-1, and an average pore diameter of 7.2 nm. Batch adsorption experiments showed that bromide uptake was strongly dependent on solution pH, adsorbent dosage, contact time, initial concentration, agitation speed, temperature, and the presence of competing ions. Maximum removal of approximately 97.3% was achieved at pH 5, close to the apparent point of zero charge of 5.5. Adsorption was rapid during the first 60 min and approached equilibrium after 90-120 min. The experimental adsorption capacity increased with increasing bromide concentration, reaching approximately 40.0 mg g^-1 at an initial bromide concentration of 200 mg L^-1. Equilibrium data were best described by the Langmuir model, which yielded a model-estimated maximum adsorption capacity of 81.30 mg g^-1. Kinetic data were described best by the pseudo-second-order model, although film diffusion and intraparticle diffusion also contributed to the overall uptake process. Equilibrium data indicated heterogeneous adsorption behavior, while thermodynamic analysis suggested that bromide adsorption was thermodynamically favorable under the investigated conditions. Regeneration with 0.1 M Sodium hydroxide and reuse over five adsorption-desorption cycles was also evaluated. Results were obtained under controlled laboratory conditions.