Qualitative and relative distribution of Pb²⁺ adsorption mechanisms by biochars produced from a fluidized bed pyrolysis system under mild air oxidization conditions

Qualitative and relative distribution of Pb²⁺ adsorption mechanisms was investigated using corn stalk biochars (CSBs) and rice husk biochars (RHBs) produced from a pilot-scale fluidized bed pyrolysis system under mild air oxidization conditions (oxygen content 0–6%) at low temperature (450 °C). Boehm titration, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscope with Energy Dispersive Spectrometer (SEM-EDS) were used to analyze the characteristics of biochars, and adsorption isotherms and kinetics analysis were studied. The Pb²⁺ adsorption capacities of CSBs (about 40 mg/g) presented about twice the amount of RHBs (about 22 mg/g), which were mainly caused by cation exchange and precipitation mechanisms, with the contribution content to the total sorption up to 90%. With oxygen content increasing from 0% to 6%, the absorbed lead capacity due to cation exchange of CSBs increased from 19.10 to 30.40 mg/g, and the corresponding contributions to total Pb²⁺ sorption increased from 49.00% to 73.80%. When the oxygen content of pyrolysis atmosphere increased, the contribution of cation exchange in Pb²⁺ adsorption exhibited extremely different tendencies in CSBs and RHBs under the combined action of metallic accumulation in ash and non-active silica embedding in the retract process, which plays a dominant role in Pb²⁺ adsorption. This study suggests that CSBs produced by fluidized bed under mild air oxidation improved cation exchange capacity, and were promising, low-cost biochar for Pb²⁺ remediation in the aqueous environment.