Investigating the adsorption behavior and quantitative contribution of Pb²⁺ adsorption mechanisms on biochars by different feedstocks from a fluidized bed pyrolysis system

The aim of this study was to examine the qualitative and quantitative analysis of Pb²⁺ adsorption mechanisms performed with biochars derived from rice straw (RSBs), rice husk (RHBs) and saw dust (SDBs) at several pyrolysis temperatures (400-600 °C) in a fluidized bed system. Adsorption isotherms, kinetics, and desorption analysis were determined, and biochars were analyzed by X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope with Energy Dispersive Spectrometer (SEM-EDS) and Boehm titration method. The effect of minerals on Pb²⁺ adsorption, including precipitation and cation exchange, revealed increasing contribution of precipitation from a range of 4.13%–38.83% to a range of 34.08%–79.94% and decreasing effect of cation exchange from a range of 50.17%–69.75% to a range of 9.57%–43.47% with increasing pyrolysis temperature. However, it remained the dominant adsorption mechanism of all biochars (accounted for 69.49–89.52%). Especially, RSBs with quite high maximum adsorption capacity (qm) values (116-127.57 mg[rad]g⁻¹) were mainly due to precipitation mechanism of Pb²⁺ adsorption, which exhibited better adsorption capacities than RHBs (25.15–30.40 mg[rad]g⁻¹) and SDBs (21.81–24.05 mg[rad]g⁻¹). Only with the fluidized bed shown in this study, 2.00t RSBs could be produced and the corresponding Pb²⁺ adsorption may reach 255.50kg per year depending on its maximum adsorption capacity under 500 °C pyrolysis temperature. The results suggest that RSBs produced in a fluidized bed reactor is a promising, cost-effective, engineered biochar for application of Pb²⁺ remediation in aqueous solutions.