Biochar from malt residue: Toward a circular economy for sustainable fluoroquinolone removal in aqueous systems

Malt bagasse holds potential as a high-value material, contributing to a circular economy. Biochars were synthesized via pyrolysis (PC-500) and hydrothermal carbonization (HC-150), with one-pot activation using phosphoric acid. Additionally, PC-500 activated with H3PO4 was synthesized for comparison. Various analytical methods were used to characterize the materials. The PC-500 and PCA-500 samples exhibited higher degrees of carbonization, whereas the HC-150 sample retained its functional groups more effectively. Biochars showed higher porosity than biomass, with microspheres present in HC-150. Evaluations were conducted on the removal of fluoroquinolones from water using biochars. Maximum removal efficiency was obtained at pH 8, favoring neutral fluoroquinolone species with a Brønsted acid-base nature. The experimental outcomes were further rationalized by the computational calculation of the physicochemical properties of the adsorbates, in analogy with the structural activity relationship approach in medicinal chemistry. The pseudo-second-order kinetic model proved to be the most suitable for fitting the experimental data, while the Langmuir model effectively described the adsorption isotherms. Ciprofloxacin, enrofloxacin, and norfloxacin showed maximum adsorption capacities (qmax) of 95, 164, and 99 mg g−1, respectively, when HC-150 was used. On the other hand, with PC-500, qmax of 57, 12, and 20 mg g−1 were obtained for ciprofloxacin, enrofloxacin, and norfloxacin, respectively. Although PCA-500 enhanced adsorption capacity compared to PC-500, the qmax remained lower than the results obtained with HC-500, measuring at 51, 102, and 95 mg g−1. These findings emphasize malt bagasse biochar as a viable adsorbent. [Display omitted] •Malt bagasse was reused as an adsorbent based on a circular economy.•The physicochemical properties of the fluoroquinolones were calculated.•Adsorption kinetics and adsorption isotherms were evaluated for the three drugs.•Elucidation of adsorption phenomena in analogy with the structure-activity