The effect of thiourea activation on physicochemical and catalytic properties of biochars and hydrochars from sugarcane bagasse: Application to metal recovery from wastes

Two biochars were prepared from pyrolysis of sugarcane bagasse (SC) at 550 and 750 °C for 3 h and named as BC550 and BC750, respectively. One hydrochar was prepared from Hydrothermal Carbonization (HTC) of SC solution at 200 °C for 2 h (HC200). Chars (BC550, BC750 and HC200) were mixed with thiourea (10 % in weight) and heated at 350 °C for 1 h leading to BC550-TU, BC750-TU, and HC200-TU, respectively. Pristine and thiourea-activated biochars were characterized by FTIR, N₂ isotherms, XRD, XRF and ultimate analysis. Thiourea activation increased the H/C of biochar, indicating that the presence of N and S and their reaction with carbon structures of biochar hinders the formation of aromatic carbon. Thiourea activation increased the surface area of biochar prepared at 550 °C, while decreasing the surface area of biochar prepared at 750 °C or hydrochar. Finally, both pristine and activated biochars were used as catalysts in the oxidation of sulfide minerals for metal recovery. The addition of thiourea-activated biochars significantly enhanced the extraction of Cu and Zn from mining waste, from 47.4 % of Cu and 88.1 % of Zn to values of 99.4–100 %, while reducing As extraction. However, in leaching processes involving minerals with higher Cu and Zn content, the extraction percentages did not reach such high values (<50 % for Cu and < 90 % for Zn). It can be concluded that the use of thiourea-activated biochars enable the development of low cost and sustainable processes for the recovery of metals from wastes.