Adsorção de ciprofloxacino e norfloxacino em solução aquosa por carvão ativado produzido a partir da casca do cupuaçu (theobroma grandiflorum)

Abstract

The extensive use of antibiotics has caused several negative consequences for the environment and human health. Fluoroquinolones (FQs), present in the aquatic environment, are of particular concern. This study investigated the removal of ciprofloxacin (CIP) and norfloxacin (NOR) from aqueous solutions using activated carbon derived from cupuaçu bark (CAC), an unconventional and low-cost adsorbent. The CAC was characterized physicochemically and the adsorption experiments followed the Box-Behnken design, evaluating the effects of contact time, adsorbate concentration and adsorbent dosage on the removal and adsorption capacity of CIP and NOR. The optimal conditions of the adsorption process were determined by the desirability function, and under these conditions the kinetic, isothermal and thermodynamic adsorption experiments were carried out. CAC showed a similar yield (50.22%) to other activated carbons with H3PO4, low humidity (4.81%) and ash (4.27%). Acidic functional groups were identified in CAC in greater quantities (3.982 mg Eq/g) than basic ones (0.092 mg Eq/g), and the pHPcz was found to be 3.85. A high surface area was quantified (1335.66 m²/g), with an average pore volume and diameter of 0.753 cm³/g and 2.206 nm, respectively. Due to the zwitterionic characteristics of CIP and NOR, adsorption was more efficient at pH 5.0. In this condition, the CAC assumes a basic character and as the CIP and NOR molecules are predominantly in their cationic forms, the electrostatic interaction is facilitated. The optimal conditions for CIP adsorption were: time of 266.40 min, concentration of 192 mg/L and dosage of 0.57 g/L; for NOR, 273.60 min, 186 mg/L and 0.55 g/L. The pseudo-second order (PSO) model and the external mass transfer resistance (EMTR) model best fitted the experimental data, indicating that external mass diffusion was the controlling step of adsorption. The Langmuir model indicated that adsorption occurred in a monolayer, with CIP adsorption capacities of 6.02 mg/g and NOR of 5.70 mg/g. Thermodynamic analysis revealed that the adsorption of CIP and NOR on CAC predominantly involves physisorption. The suggested mechanism for the adsorption of CIP and NOR on CAC may involve electrostatic forces, π-π interactions and hydrogen bonds. CAC regeneration was more effective with NaOH, but after two cycles of use, the percentage of CIP and NOR removal decreased to less than 50%. Thus, CAC proved to be an effective and low-cost adsorbent for the removal of CIP and NOR, also contributing to the reuse of cupuaçu biomass.