Comparative adsorption of levofloxacin by Cu–Fe layered double hydroxides and mixed oxides: kinetics, isotherms, mechanisms, energy efficiency, and green valorization of waste adsorbents

Abstract

The increasing prevalence of levofloxacin (LV) and related antibiotics in aquatic systems constitutes a critical environmental and public health concern, driving the need for advanced water treatment solutions. This investigation focuses on the synthesis and characterization of a copper-iron layered double hydroxide (Cu–Fe LDH) and its calcined mixed oxide derivative (Cu–Fe MO), assessing their performance as adsorbents for aqueous LV. Comparative analysis elucidates their respective removal efficiencies and underlying sequestration mechanisms. The maximum adsorption capacity for LV was quantified as 45.474 mg g⁻¹ on Cu–Fe LDH at 25 °C, increasing to 59.216 mg g⁻¹ at 55 °C. In contrast, the Cu–Fe mixed oxide (MO) exhibited lower capacities of 31.386 mg g⁻¹ and 38.68 mg g⁻¹ at 25 °C and 55 °C, respectively. These Langmuir-derived q_max values represent the definitive adsorption capacities for performance evaluation. Analysis of the statistical monolayer model's parameters provided insights into the adsorption energy, active site density, and saturation capacity per functional group. Theoretically, the data suggest a multi-layer adsorption mechanism with a vertical molecular orientation on the LDH surface, whereas LV adopts both vertical and horizontal alignments at the 96.4 kg⁻¹ interface. Thermodynamic parameters revealed spontaneous, endothermic adsorption behavior. The observed energy range of 18.29–21.4 kJ mol⁻¹ for Cu–Fe LDH and 18.05–20.9 kJ mol⁻¹ for Cu–Fe MO implies that the binding mechanism evolves from physisorption at ambient temperature toward chemisorption at elevated temperatures. The practical viability of both composites was demonstrated through four regeneration cycles, highlighting their robust reusability for complex wastewater matrices. An evaluation of the procedural environmental impact, conducted via the AGREEprep, ESA, and AMVI green metrics, substantiated the method's alignment with sustainable principles. The electrocatalytic performance of the synthesized materials for methanol oxidation was notably enhanced following their application in adsorption. The Cu–Fe LDH composite exhibited an increase in current density from 107 mA cm⁻² to 189.12 mA cm⁻² post-adsorption. A similar trend was observed for the Cu–Fe MO, which showed a rise from an initial ∼96 mA cm⁻² to 146.03 mA cm⁻² after the adsorption cycle. From an economic perspective, the manufacturing cost was estimated at $94.2 per kilogram of the Cu–Fe LDH composite.