Characterization and transformation of nanche stone (Byrsonima crassifolia) in an activated hydrochar with high adsorption capacity towards metformin in aqueous solution

Abstract

Metformin (MET) is one of the main drugs to treat type 2 diabetes in humans, consequently its presence in the environment has increased in recent years. Therefore, in this work, hydrochars were synthesized from Byrsonima crassifolia stones using hydrothermal carbonization followed by cold chemical activation to enhance the removal of MET in aqueous solution. A response surface experimental design was employed to correlate the experimental adsorption capacity (qMET) with the synthesis conditions (hydrocarbonization time, activation time, and NaOH concentration) and the physicochemical properties of the adsorbents. The results showed that the hydrocarbonization and activation times are the most significant factors influencing the MET removal. According to the textural and physicochemical analyses, it was found that the combination of hydrothermal treatment and cold chemical activation increased both the presence of acid and basic active sites with the O/C ratio, which favored the removal of MET in lignin-rich hydrocarbonized materials. The adsorption equilibrium studies employing the best adsorbent evidenced that MET removal was favored at pH = 7 due to a reduction of repulsive electrostatic interactions that played a main role in the adsorption process. The maximum adsorption capacity obtained was 113.6 mg/g, which was the highest value reported in the literature. Finally, the transformation of the Byrsonima crassifolia stones suggests that the activated hydrochars synthesized by the method proposed in this work results in an effective adsorbent for MET removal in aqueous solution.