Valorization of agroindustrial orange peel waste during the optimization of activated carbon–multiwalled carbon nanotubes–zinc oxide composites used in the removal of methylene blue in wastewater

Activated carbons (AC) produced from agroindustrial orange peel (OP) wastes and/or commercial multi–walled carbon nanotubes (MWCNTs) were modified with ZnO nanoparticles in different amounts (15 and 30 wt–%) for the removal of methylene blue in aqueous solutions. The MWCNTs functionalization was carried out by typical procedure in acid medium using a ratio of 3:1 (HNO3/H2SO4). A batch adsorption analysis was carried out evaluating different parameters: MB concentration from 40 to 100 mg L–1; pH values between 4 and 10 and adsorption time that varied from 1 to 48 h. The modified materials were characterized through the techniques XRD, FTIR, BET, TGA, SEM and UV–vis. The results showed an adequate dispersion of ZnO NPs on the carbon surfaces with a typical flower–like morphology this type of shape developed micropores that facilitate adsorption phenomena. Under the evaluation conditions, 20 mg of carbon–based materials can remove up to 89.5 ± 4.47 % of MB (80 ppm) after 24 h of exposure at a regulated pH of 8. The equilibrium adsorption data were better fitted by a Langmuir isotherm (R2 = 0.9999) than the Freundlich model (R2 = 0.9366); showing in the best case a maximum adsorption capacity of 1250 mg g−1 for both materials with 30 wt–% of ZnO NPs (AC–MWCTs or MWCNTs). Experimental data also suggest that the most likely mechanism is chemisorption followed by physical adsorption. Then, the compounds with 30 wt–% metal loading (ZnO NPs), AC–MWCNTs–ZnO or MWCNTs–ZnO, displayed the most interesting properties for MB dye removal. AC–ZnO compounds can also remove about 88.75 ± 4.43 % of MB, demonstrating that it can also be a facile and low–cost method to improve the surface area in carbonaceous structures by promoting a strong interaction with the ZnO NPs, which in turn helps to remove contaminants in water efficiently.