Comparison of the adsorption capacity and mechanisms of mixed heavy metals in wastewater by sheep manure biochar and Robinia pseudoacacia biochar.

Under various pyrolysis temperatures, the characteristics and heavy metal adsorption capabilities of biochar obtained from sheep manure (SMB) and Robinia pseudoacacia (RPB) were studied. The results indicated that SMB had higher yields, pH values, and ash contents than RPB. SMB3 and RPB3 have more oxygen-containing functional groups, whereas SMB8 and RPB8 have higher aromaticity and polarity. The maximum adsorption capacities of SMB for Pb (20.2 mg·g), Cu (13.9 mg·g), Cd (3.2 mg·g), and total heavy metals (37.3 mg·g) were obtained by SMB3. However, the maximum adsorption capacities of RPB for Pb (7.4 mg·g) and Cu (10.5 mg·g) were obtained by RPB8. Furthermore, SMB and RPB had relatively higher adsorption capacities for Pb and Cu than for Cd. The pseudo-second-order model and the Freundlich Langmuir model provided a good fit to the adsorption kinetics and isotherms, indicating that chemical adsorption was dominant in the heavy metal adsorption by SMB and RPB. According to the contribution of various mechanisms, ion exchange and mineral precipitation were the primary mechanisms responsible for RPB8, while functional group complexation was the dominant mechanism for SMB3. This study provided important information on the comprehensive recycling utilization of SMB and RPB and promoted sustainable development.