Goethite Enhances the Immobilization of Pb2+ by Biochar: Formation of Biochar-Goethite-Pb Ternary Complexes

Biochar is a promising remediation agent for heavy metal contaminated soils, and its immobilization effect on heavy metals depends not only on its structure composition but also on its interaction with soil components such as soil minerals. In this study, we investigated the effect of two typical soil iron minerals, hematite and goethite, on Pb2+ immobilization by barley grass-derived biochar through batch adsorption and model fitting, meanwhile elucidated the underlying mechanisms through XRD, FTIR, and SEM-EDS by dividing biochar into undissolvable biochar and dissolvable biochar. Dissolvable biochar had more excellent Pb(2+ )removal capacity than undissolvable biochar (Gmax in Langmuire model were 0.356 mmol L-1 and 0.288 mmol L-1, respectively). The presence of goethite increased Pb2+ removal rates by 70.8% and 72.2% for undissolvable and dissolvable biochar, respectively, while hematite slightly reduced their Pb2+ removal rates. The main mechanism was the formation of a ternary complex of biochar-goethite-Pb through -OH groups on the surface of goethite and biochar. Above novel findings emphasized that the dissolvable organic fraction in biochar played an important role in immobilizing Pb2+, and the interaction between biochar and soil iron minerals significantly affected final remediation effect.