Enhanced adsorptive removal of methylene blue by low-temperature biochar derived from municipal activated sludge

Low-temperature biochar was prepared by pyrolyzing municipal waste activated sludge and used for the adsorptive removal of a cationic dye methylene blue (MB). It was observed that both the as-prepared and demineralized sludge biochars pyrolyzed at 200 degrees C (BC200) outperformed other biochars for MB uptake. The sophisticated pore structure and suitable surface properties of sludge biochar BC200 might facilitate the uptake of MB, in which the abundant surface functional groups played the key role. The uptake of MB on both as-prepared and demineralized BC200 increased dramatically from pH 5, while the demineralized BC200 performed better than the as-prepared BC200 at pH >= 7. The experimental data was better fitted by the pseudo-second-order model, indicating a possible chemisorption process. The Langmuir model worked better to describe the isotherm data than Freundlich model for both the as-prepared and demineralized BC200, suggesting a monolayer coverage of MB on both biochars. At 298 K, the q(m) values for the as-prepared and demineralized BC200 were 177.6 and 184.9 mg/g, respectively, much higher than those of other biochars reported in literature. The demineralized BC200 had an almost equivalent adsorption capability to the as-prepared BC200.