Waste chars from wood gasification and wastewater sludge pyrolysis compared to commercial activated carbon for the removal of cationic and anionic dyes from aqueous solution

This study compared the properties of waste chars from gasification of wood chips and pyrolysis of wastewater sludge with a commercial activated carbon for the adsorption of dyes. Before the adsorption tests, sludge char was steam-activated at various temperatures between 650 and 900 °C. The activated sample with the largest porosity was chosen for further testing alongside the original sludge char. Results from adsorption tests with methylene blue (cationic dye) and amaranth (anionic dye) were analysed with equilibrium and kinetic models. Wood char showed the highest dye removal efficiency, followed by commercial activated carbon. Monolayer completion was not achieved for these materials in the tested conditions. Sludge char showed maximum adsorption capacities of 13.4 and 8.4 mg/g for the anionic and cationic dye, respectively. Steam activation improved these values to 19.6 and 12.3 mg/g. The cationic dye uptake was in all cases higher compared with the anionic dye.