Adding value to gasification and co-pyrolysis chars as removal agents of Cr ³⁺

The present work aims to assess the efficiency of chars, obtained from the gasification and co-pyrolysis of rice wastes, as adsorbents of Cr ³⁺ from aqueous solution. GC and PC chars, produced in the gasification and co-pyrolysis, respectively, of rice husk and polyethylene were studied. Cr ³⁺ removal assays were optimised for the initial pH value, adsorbent mass, contact time and Cr ³⁺ initial concentration. GC showed a better performance than PC with about 100% Cr ³⁺ removal, due to the pH increase that caused Cr precipitation. Under pH conditions in which the adsorption prevailed (pH < 5.5), GC presented the highest uptake capacity (21.1 mg Cr ³⁺ g ⁻¹ char) for the following initial conditions: 50 mg Cr ³⁺ L ⁻¹ ; pH 5; contact time: 24 h; L/S ratio: 1000 mL g ⁻¹ . The pseudo-second order kinetic model showed the best adjustment to GC experimental data. Both the first and second order kinetic models fitted well to PC experimental data. The ion exchange was the dominant phenomenon on the Cr ³⁺ adsorption by GC sample. Also, this char significantly reduced the ecotoxicity of Cr ³⁺ solutions for the bacterium Vibrio fischeri. GC char proved to be an efficient material to remove Cr ³⁺ from aqueous solution, without the need for further activation.