Further insights into the activation process of sewage sludge-based precursors by alkaline hydroxides

The present work extends previous activation results obtained with a sewage sludge to others and deepens into the study and characterisation of sewage sludge-based sorbents prepared by alkaline hydroxide activation. Results obtained show that different sewage sludges, whose compositions and treatments vary from each other, can be successfully activated by KOH. A wide range of porosities are achieved, being remarkable that the activation of a biologic sewage sludge leads to surface areas in the range of 1900 m2 g−1 using a very low KOH/precursor ratio. Some of the prepared sorbents exhibit an important mineral matter contribution, which has shown to be porous as well. The chemical changes occurring during the activation of these precursors were followed by different techniques. The combination of conventional and in situ XRD allowed us to conclude that KOH acts not only as an activating agent of the carbonaceous matter present in the sludge-based precursors, but also produces a reaction (alkaline fusion) with the inorganic matter present in the precursors. Regarding the activation of the carbonaceous part, the sludge-based precursors show specific features when compared to other conventional carbon precursors. A considerable proportion of the nitrogen from the microorganisms present in the sludges is maintained after the pyrolysis, leading to an interesting nitrogen enriched carbonaceous precursor. These findings could be of relevance considering that these sorbents are promising for odorous compounds removal, such as those derived from wastewater treatment plants, and for other environmental applications requiring porous sorbents.