Production of sludge biochar by steam pyrolysis and acid treatment: Study of the activation mechanism and its impact on physicochemical properties

This study addresses the preparation of sludge biochar through a procedure that combines steam pyrolysis in a wide range of temperature (500–900 ºC) and acid treatment with HCl (performed before or after the pyrolysis). The acid washing after the pyrolysis maximizes BET surface area due to pore unblocking, thus making visible the pore structure generated. The drawback of the acid washing is the lower yield obtained (compared to the acid pretreatment), as well as the loss of the magnetic properties of sludge biochar. The acid pretreatment requires temperatures above 700 ºC to have a beneficial effect, due to the occurrence of additional reactions of the incorporated HCl with sludge constituents. The pretreatment results in materials with a high degree of microporosity. Among the reactions favoured, of interest is the development of magnetite in the whole temperature range. The activation protocol used in this study (especially the procedure that includes the acid pretreatment) is effective to generate biochar with magnetic properties through the transformation of the Fe introduced during the water treatment process, without adding a source of Fe. The presence of magnetite is an advantage for the reuse of sludge biochars in wastewater treatment. •Acid washing after steam activation greatly enhances porosity due to pore unblocking.•Acid pretreatment produces biochars with a high degree of microporosity.•Acid pretreatment leads to the formation of magnetite in the whole temperature range.•The acid treatment results in the generation of C=O, -OH and -NH functionalities.•The activation protocol is effective to produce magnetic biochar without adding Fe.