Valorization of orange peels through the production of eco-friendly, highly effective adsorbents for herbicides based on one-step CO₂-activation under conventional/microwave heating

Abstract

Orange peels were used to produce new eco-friendly activated carbons with high adsorption performance towards diuron and glyphosate, even in aqueous systems with complex composition. Activated carbons were obtained through a one-step process involving biomass pyrolysis and CO₂-consuming physical activation under conventional or microwave-assisted heating. Surprisingly, despite worse textural parameters, materials produced in a conventional manner showed a higher ability to immobilize herbicides of different nature than analogous solids obtained in a microwave furnace. With the initial diuron concentration of 10 mg/L, 97 % (48.68 mg/g) and 73 % (35.57 mg/g) of the herbicide were adsorbed on the materials produced through conventional and microwave heating, respectively. In the case of glyphosate, the adsorption efficiency was 58 % (2.91 mg/g) for the carbonaceous material obtained in a conventional furnace and 25 % (1.26 mg/g) for that treated with microwaves. The adsorption of diuron and glyphosate was also studied in complex systems containing metals (Cu, Cd), metalloids (As, Se) or polymers (synthetic cationic and anionic polyacrylamide, bacterial exopolysaccharide). It was noted that cationic polyacrylamide favored glyphosate binding, but its impact on the herbicide adsorption was rather small. The desorption degree of diuron from the produced materials was rather at low level (4.36 % in total after 3 desorption cycles conducted with water), but the addition of polymers further decreased this value. Cationic polyacrylamide significantly decreased the total desorption of glyphosate, i.e., from 40 % to even 0 %. The activated carbon of the highest adsorption capacity appeared to be non-phytotoxic and able to enhance plant growth in the glyphosate presence.