Complexation mechanism of Pb 2+ on Al-substituted hematite: A modeling study and theoretical calculation.

Hematite nanoparticles commonly undergoes isomorphic substitution of Al ³⁺ 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 Pb ²⁺ 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/nm ² . The surface positive charge of hematite significantly increased from 0.096 to 0.418 C/m ² 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 Pb ²⁺ increased from 3.92 to 9.74 mmol/kg at pH 5.0 and 20 μmol/L initial Pb ²⁺ concentration with increasing Al content. More Fe vacancies may lead to a weaker adsorption energy (E _ad ) of hematite for Pb ²⁺ , 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 ≡ Fe ₂ AlO ^-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.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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