Another way to valorize by-products: Coffee Silver Skin for removing the antibiotic Norfloxacin from water

Coffee silver skin is reported as an adsorbent for treating water from Contaminants of Emerging Concern, focusing on the removal of Norfloxacin, an antibiotic. According to the sustainability principles, coffee silver skin before its use was washed with water, lowering the associated costs and the whole environmental impact. The proposed adsorbent was characterized by adopting synergistically ATR-FTIR, SEM, and TG techniques, inferring its main features before and after the pollutant removal. Specifically, the presence of the latter in water was monitored by using UV-Vis spectroscopy. Several experimental conditions were explored during the adsorption, and the roles of ionic strength, pH, adsorbent/pollutant amounts, and temperature were investigated. The thermodynamics, adsorption isotherms, and kinetics were also studied, revealing that the process was spontaneous and occurred on heterogeneous skin surfaces forming a pollutant multilayer, joining a maximum adsorption capacity of 50 mg×g-1. Interestingly, the pH values and ionic strength strongly affected the process, denoting the main presence of reversible electrostatic interaction between the pollutant and coffee silver skin. Indeed, the recycling of the proposed adsorbent was demonstrated by adopting safe and green conditions of work in the presence of diluted salt solutions or water at 50°C, also recovering the pollutant. So, this paper has the aim to present, for the first time, the kinetics and thermodynamics of an adsorption process, referred to the removal of an emerging pollutant from water, by using coffee silver skin avoding hard conditions of work to pre-treat the material before its use, proposing thus an alterative way to reuse this waste as recyclable adsorbent.