Removal of norfloxacin in deionized, municipal water and urine using rice (Oryza sativa) and coffee (Coffea arabica) husk wastes as natural adsorbents.

The removal of the widely used antibiotic norfloxacin (NOR), the presence of which has been reported in natural water, was evaluated using rice (RH) and coffee (CH) husk wastes as adsorbents. Low particle sizes and natural pH in distilled water favored NOR elimination in both materials. In order to investigate the type of adsorption, the data was adjusted to the Langmuir, Freundlich and Redlich-Peterson isotherms. The best fit for the Langmuir and Redlich-Peterson isotherms suggested a monolayer-type adsorption model. Kinetic models of pseudo first and second order were also evaluated, the latter being the most suitable to represent the NOR adsorption phenomenon. Meanwhile, the intraparticle diffusion model indicated that the adsorption of NOR occurs both at the surface and within the pores of the material. Studies performed on thermodynamic aspects such as activation energy (Ea), enthalpy change (ΔH˚) and Gibbs free energy change (ΔG˚) suggest that the physisorption of the pollutant takes place through a spontaneous endothermic process. Additionally, PZC determination, Boehm method, chemical composition, thermodynamic analysis, and FTIR spectra before and after the adsorption of the antibiotic suggest that in CH adsorbents this occurred mainly through electrostatic interactions, while in RH hydrogen bonds also contributed significantly. Finally, the efficiency of natural adsorbents for the removal of NOR was evaluated in synthetic matrices of municipal wastewater and urine, and promising results were obtained despite the complexity of these matrices. The results presented in this work show the potential application of RH and CH residues as a low-cost alternative for the removal of NOR even in complex matrices. However, despite the similarities between the materials, CH waste showed better properties for the removal of the tested NOR due to its higher surface area, lower PZC and higher number of acid groups.