Importance of pyrolysis temperature and particle size on physicochemical and adsorptive properties of urban wood-derived biochar

Physicochemical properties of biochars made from urban wood waste at different pyrolysis temperatures (400–700 °C) and particle sizes (1–2 mm and 63–75 μm) were compared to their ability for heavy metals adsorption at various contact times (30–300 min) and adsorbent dosages (0.1–5 g L−1) in real landfill leachate. The yield, volatile matter, hydrogen and oxygen contents, and functional groups of biochars decreased with increasing pyrolysis temperature; while pH, electrical conductivity, ash, fixed carbon, and specific surface area (SSA) increased. FE-SEM images showed that the creation of pores on the surface of biochars increased significantly with increasing pyrolysis temperature. Moreover, particle size exhibited no significant effect on the properties of biochars (p > 0.05), except for the SSA. The SSA of biochars with a particle size of 63–75 μm was significantly higher than that measured in 1–2 mm biochars. The adsorption efficiency of heavy metals from landfill leachate increases with increasing pyrolysis temperature, contact time and adsorbent dosage. Conversely, reducing the particle size increases the adsorption efficiency. The highest adsorption efficiency for Pb, Zn, Ni, Cd, and Mn was obtained as a result of using biochar pyrolyzed at 700 °C with a particle size of 63–75 μm (i.e., BC700/63–75) by 60.39%, 75.01, 64.36, 58.87%, and 71.15%, respectively. Pseudo second-order kinetic and the Langmuir isotherm were recognized as the most appropriate models to describe the adsorption process. The chemical adsorption was identified as the main mechanism for the adsorption of metal ions onto BC700/63–75. identified as the main factors for the adsorption of metal ions onto BC700/63–75.