Conversion of waste biomass to designed and tailored activated chars with valuable properties for adsorption and electrochemical applications

Abstract

Waste biomass, a renewable energy source, is inexpensive material that has great potential in sorption and electrochemical application. The selected waste materials (corncobs, coconut shells, walnuts, and pistachio husks) allow to close the production cycle and enable material recycling, which are important aspects in the hierarchy of waste management. The proposed methodology for production and activation of biochars can be used industrially due to highly porous structure, developed surface area, and sorption ability of the obtained activated carbons (AC). A significant increase (from 4 up to more than 10 times) in specific surface area (SSA) is observed for all samples after the CO2 activation process (0.5 h at 800 °C) up to 725 m2 g−1 for corncobs, 534.9 m2 g−1 for pistachio husks, 523 m2 g−1 for coconut shells, and 393 m2 g−1 for walnut husks. The highest value of SSA is achieved for the AC derived from corncobs. This material is evaluated for use as an adsorbent, revealing 99% removal of Rhodamine B (dye/AC ratio of 0.0017) and 69% removal of chromium (dye/AC ratio of 0.0028). Based on the adsorption kinetics analysis, it is demonstrated that the Cr(VI) undergoes physical adsorption, while RhB undergoes chemisorption. In addition, corncob-derived AC exhibits superior electrochemical performance in 6 M KOH compared to the nonactivated biochar. A specific capacitance of 70 F g−1 at 5 A g−1 is achieved, along with outstanding rate capability (45 F g−1 at 50 A g−1) and cycling stability (94% at 10 A g−1 after 10,000 cycles). In contrast, the nonactivated sample shows only 34 F g−1 at 5 A g−1 and 13 F g−1 at 50 A g−1, with a stability of 91.4%.