Abstract: In order to prepare calcium-iron modified biochar, optimize the removal effect of single iron-modified biochar on Pb(Ⅱ), and obtain biochar that can efficiently remove Pb(Ⅱ) from water and is easy to recycle, in this study, poplar and bamboo were used as raw materials to prepare eight kinds of biochar respectively, including poplar biochar (unmodified, FeSO₄ modified, CaCO₃+FeSO₄ modified, CaO+FeSO₄ modified) and bamboo biochar (unmodified, FeSO₄ modified, CaCO₃+FeSO₄ modified, CaO+FeSO₄ modified). First, the removal efficiency of different biochar on Pb(Ⅱ) in groundwater was evaluated through batch experiments. Then, the fixation mechanism of Pb(Ⅱ) in groundwater was explored by means of scanning electron microscopy (SEM) and X-ray absorption near-edge structure spectroscopy (XANES). The results show that the removal efficiency of Pb(Ⅱ) by calcium-iron (CaCO₃+FeSO₄, CaO+FeSO₄) modified biochars were significantly higher than those by iron (FeSO₄) modified biochar. In addition, the removal efficiency of Pb(Ⅱ) by calcium-iron modified biochar with CaO as the calcium source were all greater than 99%. While the removal efficiency of Pb(Ⅱ) by calcium-iron modified poplar biochar and calcium-iron modified bamboo biochar with CaCO₃ as the calcium source were 58% and 37% respectively. The removal efficiency of modified biochar on Pb(Ⅱ) from high to low was in the order of CaO+FeSO4 modification, CaCO₃+FeSO₄ modification, and FeSO₄ modification. The surfaces of calcium-iron modified biochar were porous and rough, which was beneficial for the attachment of Pb(Ⅱ) precipitation substances. Through the linear fitting of XANES of Pb(Ⅱ), the stable Pb(Ⅱ) components on the surface of calcium-iron modified biochar, such as basic lead carbonate and lead chlorophosphate, accounted for a higher proportion compared with other types of biochar. Among them, the proportions of basic lead carbonate and lead chlorophosphate on calcium-iron modified poplar biochar were 28% and 34% respectively, and those on calcium-iron modified bamboo biochar were 28% and 30% respectively. The formation of lead precipitates on the surface of biochar may be one of the main pathways for Pb(Ⅱ) removal.

Key words: lead, biochar, iron modification, iron-calcium modification, SEM；XANES, removal efficiency