Production, characterization, and application of tamarind peel-derived adsorbents for metformin removal: A comparative study with commercial activated carbon

The occurrence of pharmaceuticals such as metformin in aquatic environments is a growing concern, since conventional treatments are often ineffective for complete removal. This study produced adsorbents from tamarind peel through different treatments: raw peel, KOH activation, hydrochar by hydrothermal carbonization, and activated hydrochar (AHC), compared with commercial activated carbon (CAC). The materials were characterized by SEM, EDS, FTIR, TGA, textural analysis, and surface chemistry. Activated hydrochar showed significant modifications, with increased carbon content similar to CAC but lower surface area, possibly associated with structural rearrangements or partial pore obstruction during the activation process. Batch adsorption tests indicated activated hydrochar (AHC) as the most efficient material in the preliminary screening, removing 51% of metformin and reaching 69% under optimized conditions (pH 9.0, 50 mg). Adsorption kinetics revealed distinct mechanisms for AHC and commercial activated carbon (CAC). Although CAC exhibited a slightly higher maximum adsorption capacity under isotherm conditions, preliminary batch tests demonstrated higher removal efficiency for AHC compared to CAC. Thermodynamic analysis indicated endothermic adsorption for AHC (∆H° = +63.43 kJ mol− 1) and exothermic for CAC (∆H° = −13.07 kJ mol− 1). Results demonstrate the potential of tamarind peel-derived hydrochar as a renewable, low-cost adsorbent for pharmaceutical removal.