From agricultural wastes to a resource: Kiwi Peels, as long-lasting, recyclable adsorbent, to remove emerging pollutants from water. The case of Ciprofloxacin removal

This work accounts for the first example of dry Kiwi Peels to remove emerging pollutants from water. Both the inner and outer sides of Kiwi Peels were characterized by using in synergy FTIR-ATR, TG, and SEM analyses before and after its use and re-use, proposing it as long-lasting, recyclable adsorbent material. Among the tested 14 emerging pollutants, 6 of them were successfully removed by Kiwi Peels, also if present in mixtures, calculating the Kiwi Peels maximum adsorption capacities, occurring in the range of 1–4 mg/g. To infer information about the behavior of Kiwi Peels during water treatments, Ciprofloxacin, a well-known and largely used antibiotic, was selected as a dangerous model contaminant. The roles of ionic strength, pH values, adsorbent/pollutant amounts, and temperature values during the adsorption process were assessed, giving physical and chemical information about the whole adsorption process. The thermodynamics, the adsorption isotherms, and kinetics were studied. The Freundlich model, with a good correlation, well described the obtained results, indicating the heterogenous character of the Ciprofloxacin adsorption with the formation of a multilayer of pollutant molecules onto the adsorbent surface. If, on the one hand, in the range of temperatures 283–303 K, the Ciprofloxacin adsorption was favored by increasing the temperature, on the other hand, the adsorption was hindered by further incrementing of temperature due to the pollutant desorption. Until the occurrence of desorption, the process occurred with ΔH° and ΔS° > 0 with a ΔG° < 0. Both the pseudo-first and pseudo-second-order kinetic equations seemed to describe the process with the applicability of the Weber-Morris model. The results suggested the main presence of electrostatic interaction between the pollutant and adsorbent by changing the pH values and ionic strength of Ciprofloxacin solutions. As a whole, from the obtained results, the best condition to remove Ciprofloxacin is neutral pH in the absence of salt at 303 K. The Kiwi Peels and Ciprofloxacin recycling was also investigated by performing desorption experiments using 0.1 M MgCl2 solution, increasing the Kiwi Peels maximum adsorption capacity at least from 4 to 40 mg/g for Ciprofloxacin. At least 10 cycles of adsorption/desorption were performed, desorbing almost 75% of adsorbed pollutants during each run. Preliminary information about the possibility of inducing the solid-state pollutant photodegradation by using Advanced Oxidation Processes was also explored, giving a possible alternative for pollutant desorption and adsorbent recycling.