Mild sulphuric acid pre-treatment for metals removal from biosolids and the fate of metals in the treated biosolids derived biochar

Biosolids are contaminated with heavy metals (HMs) and alkali and alkaline earth metals (AAEMs). These metals limit the suitability of biosolids for land application as well as their pyrolytic conversion to high-quality products. In this work, a mild sulphuric acid pre-treatment of biosolids was carried out at different stirring speeds (300–900 rpm), temperatures (25–100 °C), extraction time (0–180 min), and acid concentration (1–5% v/v) to reduce the metals load in biosolids and their biochar derived from pyrolysis. The metal leaching process was very rapid and reached equilibrium in less than 30 min. The optimum conditions removed about 75–95% HMs and 80–95% AAEMs except Ca due to the formation of CaSO4 hydrates. Temperature was the driving parameter for Cd and Ni extraction, whereas temperature and acid concentration played the leading roles in Cu extraction. The shrinking core product layer diffusion and surface chemical reaction models described the extraction kinetics of Ni, Cu and Cd. A leaching activation energy of 10.02 kJ/mol and 7.37 kJ/mol was estimated for Ni and Cd, respectively. FTIR, SEM and XRD characterisation of the treated biosolids jointly indicated that the leaching mechanism was dominated by acid dissolution of metal-containing components followed by ion exchange of metal species with protons from H2SO4. Treatment at 25 °C and 3% H2SO4 lowered the biosolids ash content by 50% and preserved the physicochemical attributes, which enhanced the pyrolysis upcycling of the treated biosolids. Pre-treatment influenced the migration characteristics of the metals during pyrolysis and the produced biochar had several folds lower HMs and AAEMs contents than the raw biosolids-derived biochar.