A new insight on the core-shell structure of zerovalent iron nanoparticles and its application for Pb(Ⅱ) sequestration

摘要

Nanoscale zerovalent iron (nZVI) has shown a high efficacy for removing heavy metals from liquid solution. However, its removal capacity has not been fully explored due to its common shell composition (FeOOH). In this study, a much higher removal capacity of Pb(Ⅱ) is observed (1667 mgPb(Ⅱ)/g Fe), which is over 100% higher than the highest removal capacity of nZVI reported before. High-resolution X-ray photoelectron spectroscopy (HR-XPS) reveals that through restricting the dehydration process of Fe(OH)_3, nZVI can acquire a unique shell, which is composed of 45.5% Fe(OH)_3 and 54.5% FeOOH. The presence of Fe(OH)_3 suppresses the reduction of Pb(Ⅱ), but greatly promotes the co-precipitation and adsorption of Pb(Ⅱ). Combining the ratio of Fe-released to Pb-immobilized and the result of HR-XPS, a reaction between Fe° core, Fe(OH)_3, and Pb(Ⅱ) is proposed. The Fe released from the Fe~0 core leads to the core depletion, observed by transmission electron microscopy (TEM) under high Pb(Ⅱ) loading. While temperature has little influence on the removal capacity, pH affects the removal capacity greatly. pH ＜4.5 favors Fe dissolution, while pH ＞ 4.5 promotes Pb(Ⅱ) adsorption. Given the high Pb removal capacity via the Fe(OH)_3 shell, nZVI can be used to remedy Pb(Ⅱ) contamination.