Facet-dependent surface charge and Pb²⁺ adsorption characteristics of hematite nanoparticles: CD-MUSIC-eSGC modeling

Accurate prediction of the environmental fate of Pb depends on the understanding of Pb coordination to mineral surfaces. Here, the proton and Pb adsorption and speciation on hematite nanocrystals with different exposed crystallographic facets were investigated. High-resolution transmission electron microscopy images revealed that hematite nanoplates (HNP) were of 75.3 ± 9.5% (001) facets and 24.6 ± 9.3% (012) facets, while hematite nanocubes (HNC) were of 76.0 ± 11.1% (012) facets and 24.0 ± 3.2% (110) facets. Our modeling results revealed that the proton affinity constant (log K_H) of ≡FeOH^-0.5 and ≡Fe₃O^-0.5 was 7.8 and 10.8 on hematite (012) facets, and changed to 7.7 and 11.7 on (110) facets, respectively. Owing to the different atomic arrangements, (012) facets not only have higher adsorption performance for Pb, but also present a greater dependence on pH than (110) facets. Additionally, our modeling further indicated that (012) facets bind Pb via both bidentate and tridentate complexes, while (110) facets bind Pb only through bidentate complexes at pH 3.0-6.5. These results facilitate a more detailed understanding of the complex species of Pb on hematite surface while also provide new insight into the reactivity mechanism of individual hematite facets.