Transforming contaminated biosolids into biochar for a sustainable cement replacement material

Abstract Contaminated biosolids especially with per- and polyfluoroalkyl substances (PFAS) in biosolids pose significant environmental risks, restricting their potential applications and necessitating sustainable solutions to address these challenges. In this context, pyrolysis emerges as a promising technology capable of degrading contaminants while transforming biosolids into useful products like biochar. This study demonstrates the application of pyrolysis at different temperatures of 450–750 °C to investigate its effect on contaminant removal and the properties of the resulting biochars. Subsequently, the biochars were utilized to prepare cement mortars by replacing 0.5, 1, 2, 4, and 6% of cement weight with biochar, and their compressive strengths were determined after 7 days of curing. The findings revealed that biosolids contained significant levels of PFAS, including perfluorooctanesulfonic acid (PFOS), 324 ng/g, perfluorohexanesulfonic acid (PFHxS), 9.15 ng/g, and heavy metals. Pyrolysis at 450 °C effectively degraded most contaminants, including PFAS. The biochar produced at 450 °C exhibited the highest concentrations of inorganic nutrients such as potassium (K), calcium (Ca), nitrogen (N), and phosphorus (P), though their levels decreased with increasing pyrolysis temperature. On the other hand, compressive strength tests for cement mortars with varying proportions of biochar replacement demonstrated that a 0.5% replacement was beneficial for all biochars (except 650 °C—biochar that achieved the maximum compressive strength with 2%). This resulted in a 30–45% increase in compressive strength compared to plain cement mortar. However, increasing the biochar content to 6% significantly reduced compressive strength. Overall, this study highlights the potential of biochar as a sustainable solution for enhancing cement mortar strength while mitigating biosolid contamination. Graphical abstract