Sodium alginate-modified activated binder derived from TiO2 nanoparticles and carbon-rich limestone: A new bio-based composite for methylene blue adsorption

Herein, a mixture of naturally carbonized limestone (BL) and rutile nanoparticles (TiO₂) was modified with sodium alginate (SA) to produce a novel biosorbent (SA-AATiO₂/BL) for methylene blue (MB) uptake. The physicochemical characterization data suggested a unique SA-AATiO₂/BL properties rich in surface functionalities with improved adsorptive properties. The adsorption experiments suggested that pseudo-first-order (PFO) and Langmuir models offered the best fit for the kinetic and equilibrium studies, respectively. The maximum adsorption capacity was 263.40 mg/g at 25 °C, and it enhanced to 283.42 mg/g when the temperature increased to 55 °C. The statistical monolayer model's physicochemical parameters were used to understand MB adsorption per functional group, active site density of SA-AATiO₂/BL interaction energies, and sorption capacities at saturation. Theoretically, the adsorption process across all temperatures involves a multi-molecular adsorption mechanism and vertical positioning. The thermodynamic studies indicated an endothermic and spontaneous adsorption, in which the MB molecules adsorption was governed by physical forces such as hydrogen bonds and electrostatic interactions, with energies ranging from 16.80 to 17.53 kJ/mol. According to the regeneration results of up to six rounds, the SA-AATiO₂/BL demonstrated stability and suitability for reusing in actual wastewater treatment.