Production of magnetic sewage sludge biochar: investigation of the activation mechanism and effect of the activating agent and temperature

Abstract Sewage sludge-based porous materials were prepared through an integrated and energy-saving procedure that combines chemical activation with an alkali (NaOH or K 2 CO 3 ) and physical activation with CO 2 in a single step. The study was conducted in a wide temperature range (600–1000 °C), using higher temperatures than those commonly used. From the standpoint of textural properties, the impregnation results in a decrease in the optimum activation temperature: 800 °C for untreated samples, and 600–700 °C for impregnated samples. The impregnation increases the amount of –OH and –NH functional groups, and it also leads to the generation of new oxygen- and/or nitrogen-containing functionalities (above 800 °C for the impregnation with K 2 CO 3 and in the whole temperature range for the impregnation with NaOH). The chemical treatment favors the development of magnetic properties on biochars, which is an advantage for its reuse in wastewater treatment, since it favors the removal of anionic surfactants and heavy metals such as lead, copper, zinc, and manganese. The uptake of phenol and methylene blue (MB) is maximized with the combination of the impregnation (with NaOH for phenol and K 2 CO 3 for MB) and the use of a moderate temperature (600–700 °C). The optimum uptake values represent an increase of 87% and 152% for MB and phenol, respectively, compared to the highest value of non-impregnated samples (activated at 800 °C), with the advantage of the lower temperature required. Graphical abstract