Complexation mechanism of Pb²⁺ on Al-substituted hematite: A modeling study and theoretical calculation

Hematite nanoparticles commonly undergoes isomorphic substitution of Al³⁺ in nature, while how the Al-substitution-induced morphological change, defective structure and newly generated Al-OH sites affect the adsorption behavior of hematite for contaminants remains poorly understood. Herein, the interfacial reactions between Al-substituted hematite and Pb²⁺ was investigated via CD-MUSIC modeling and DFT calculations. As the Al content increased from 0% to 9.4%, Al-substitution promoted the proportion of (001) facets and caused Fe vacancies on hematite, which increased the total active site density of hematite from 5.60 to 17.60 sites/nm². The surface positive charge of hematite significantly increased from 0.096 to 0.418 C/m² at pH 5.0 due to the increases in site density and proton affinity (logK_H) of hematite under Al-substitution. The adsorption amount of hematite for Pb²⁺ increased from 3.92 to 9.74 mmol/kg at pH 5.0 and 20 μmol/L initial Pb²⁺ concentration with increasing Al content. More Fe vacancies may lead to a weaker adsorption energy (E_ad) of hematite for Pb²⁺, while the E_ad was enhanced at higher Al content. The adsorption affinity (logK_Pb) of bidentate Pb complexes slightly increased while that of tridentate Pb complexes decreased with increasing Al content due to the presence of ≡ AlOH^-0.5 and ≡ Fe₂AlO^-0.5 sites. Tridentate Pb complexes were dominant species on the surface of pure hematite, while bidentate ones became more dominant with increasing Al content. The obtained model parameters and molecular scale information are of great importance for better describing and predicting the environmental fate of toxic heavy metals in terrestrial and aquatic environments.