Competitive adsorption of anionic and cationic molecules on three activated carbons derived from agroindustrial waste

This study examined the feasibility of sugarcane bagasse (sb), spent coffee-grounds (sg), and orange peel (op) activated carbons for the removal of methyl orange (MO) and methylene blue (MB). For such purpose, individual dye solutions and binary mixtures under different pH and initial concentrations were used. The study was complemented with Density Functional Theory (DFT) calculations to assess the carbon surface preference to-wards a specific molecule. The DFT method, using molecular descriptors, showed a potential application for predicting the order by which the carbons sorb a given set of contaminants. For the three carbon materials studied, the mono-dye experiments adsorption capacities ranged from 60 to 140 mg/g for MB and from 146 to 65 mg/g for MO at pH values of 5, 7 and 9. While the best methylene blue adsorbent was the sugarcane bagasse carbon, the orange peel carbon showed the best methyl orange adsorption capacity. Interestingly, the spent coffee-ground carbon exhibited a balanced behavior when both molecules were added separately. When the dyes were added as a mixture, the three carbons adsorbed methylene blue preferentially over methyl orange. While sb adsorbed up to 75 mg/g of MB and 10 mg/g of MO, op adsorbed 60 mg/g of MB and up to 25 mg/g of MO. The trend displayed in mono-dye systems was kept in binary systems. This is important since real wastewater systems contain mixtures with two or more pollutants. In the case of the binary mixtures, the three activated carbons can adsorb both type of dyes at concentrations up to 300 mg/L, the pH did not affect the MB adsorption, whereas MO uptake was greatly affected at pH > 7. Thus, it is important to study versatile materials obtained from organic wastes capable of removing more than one species from wastewater.

Automated summary

This study examined the feasibility of using sugarcane bagasse, spent coffee-grounds, and orange peel activated carbons for the removal of methyl orange and methylene blue. The results showed that these three carbon materials can adsorb both dyes simultaneously and exhibit different adsorption capacities under different pH and initial concentrations. This is important for the treatment of wastewater containing mixed pollutants.