Tailored carbon adsorbents developed from coconut shells and coconut palm fibers for the removal of contaminants from anodizing wastewater

Abstract

In the present work, new efficient carbon adsorbents derived from coconut shells and coconut palm fibers (Cocos nucifera) were prepared with the purpose of using them in the removal of anions and cations present in anodizing wastewater, including PO43-, SO42-, F-, SiO4-4, Na+, K+ and Al3+. The methodology of preparation comprises the oxidation, impregnation and carbonization of biomass using different heating systems and two impregnation salts (magnesium and calcium). The materials obtained were characterized using FT-IR spectroscopy, potentiometric titration, nitrogen adsorption isotherms at 77 K and SEM/EDX analysis. The adsorbents were used for the removal of anions and cations from monocomponent solutions and a real anodizing wastewater employing batch systems under constant agitation. When working with monocomponent solutions, the best performing adsorbent for the removal of PO43- was the sample obtained from coconut palm fibers, oxidated with HNO3, modified with calcium chloride, and carbonized in a tubular furnace at 1073.15 K (CF-HNO3-I-1Ca-800), rendering an adsorption capacity of 1.76 mol/kg. However, the most efficient carbons for the removal of the rest of the ionic pollutants (also in monocomponent solutions) were obtained from coconut shells modified with calcium or magnesium chloride with the adsorption capacities as follows: 0.22 mol SO42-/kg, 0.25 mol SiO44-/kg, 0.33 mol F-/kg, 2.01 mol Na+⁄kg, 0.68 mol K+/kg and 1.33 mol Al3+/kg. Finally, when the real anodizing wastewater was used in the adsorption studies, a lower removal efficiency was observed for all the pollutants in comparison with the monocomponent solutions study due to complexity of the wastewater. However, for some species such as SO42- and F-, the decrease in the removal was lower than 20%, thus opening the possibility of using some of the materials prepared in this work as a realistic alternative for the tertiary treatment of anodizing wastewater.