Pyrolytic temperature evaluation of macauba biochar for uranium adsorption from aqueous solutions

This study aims to evaluate the effect of the pyrolytic temperature on the biochar derived from the macauba endocarp for the removal of uranium (VI) from aqueous solutions. The endocarp was subjected to six different pyrolytic temperatures, ranging from 250 degrees C to 750 degrees C. The biochars obtained at each temperature were evaluated for their adsorption capacities (q). The highest adsorption capacities were obtained for the biochar produced at 250 degrees C (BC250), followed by the one obtained at 350 degrees C (BC350), with removal efficiencies of 86% and 80%, respectively. The best condition was achieved when the endocarp was subjected to temperatures between 300 and 350 degrees C, at which it was possible to obtain a satisfactory balance among adsorption capacity, gravimetric yield and fixed carbon content. This characteristic, combined with the high removal efficiency, points to an ideal working temperature of 350 degrees C. Elemental analysis showed a decrease of the H/C and O/C ratios when higher pyrolytic temperatures were applied, indicating an inverse relationship between the carbonization and the surface polar functional groups, which were likely responsible for an increased adsorptive capacity in biochars produced at lower temperatures. Both FTIR and XPS analysis indicated that oxygen-containing groups such as hydroxyls and carboxylic acids were involved with the binding of uranyl ions.