Influence of the Addition of Cotton Stalk during Co-pyrolysis with Sewage Sludge on the Properties, Surface Characteristics, and Ecological Risks of Biochars

Sewage sludge produced by municipal sewage treatment plants can potentially be used as a biomass energy source because of its high organic content. Presently, the conversion and utilization of rapidly growing amounts of sewage sludge represent an urgent challenge in China. Thermal conversion of sewage sludge to biochar through pyrolysis is a promising solution to this problem. However, biochar produced by pyrolysis of sewage sludge alone has a poor pore structure as a result of its low C content and high ash content. Furthermore, it is enriched in heavy metals that may pose high ecological risks. In this study, we addressed these issues through co-pyrolysis of sewage sludge and cotton stalks (1:1, wt./wt.) at different pyrolysis temperatures ranging from 350°C to 750°C. The properties and surface characteristics of the biochars were investigated. Meanwhile, the transformation behavior of heavy metals during the co-pyrolysis process was studied, and the potential ecological risks of heavy metals in biochars were assessed. The results showed that elevated pyrolysis temperatures reduced the biochar yield and C content of the biochars, whereas such temperatures increased the pH value and ash content of the biochars. The biochars prepared at different pyrolysis temperatures were all mesoporous materials. The elevated temperatures promoted the transformation of heavy metals from mobile fractions to stable ones, thus resulting in a significant decrease in the ecological risks. In summary, co-pyrolysis of sewage sludge with cotton stalks proved to be a feasible method for the conversion and utilization of sewage sludge.