Characteristics of biochars prepared by co-pyrolysis of sewage sludge and cotton stalk intended for use as soil amendments.

The safe disposal and utilisation of sewage sludge can be challenging because of the potential environmental risks posed by heavy metals in the sludge. Conversion of sewage sludge and agriculture biomass into biochars that can be used to improve or remediate contaminated soils is a promising solution to this problem. In this study, biochars were produced via co-pyrolysis of sewage sludge and cotton stalk (1:1, w/w) at temperatures ranging from 300°C to 600°C. Then, the potential environmental risks of heavy metals and properties of the biochars were investigated. The addition of cotton stalk promoted the migration and transformation of heavy metals from bioavailable to stable fractions, which significantly reduced the potential environmental risks of heavy metals in biochars. Moreover, compared with biochars obtained via pyrolysis of sewage sludge alone, the pH values, C contents, and adsorption capacities of biochars increased, while the yields, ash contents, specific surface areas and molar H/C ratios decreased. In summary, co-pyrolysis of sewage sludge and cotton stalk is a feasible method for alleviating the potential environmental risks of heavy metals in biochars used to treat soils.