Stable Mo isotopic fractionation during complexation onto hematite surface: Molecular insights from Ab-initio MD and X-ray absorption spectroscopy

Abstract

Iron (hydr)oxides are key sorbents of molybdenum (Mo) in a wide range of environmental settings. Among them, hematite (α-Fe₂O₃) is the most stable iron oxide under surficial conditions; however, the extent and molecular-scale mechanisms of Mo isotopic fractionation on hematite across variable pH conditions have remained poorly constrained. In this study, we conducted a series of Mo adsorption experiments at pH 5.0–9.0 to investigate isotopic fractionation and identified two distinct fractionation regimes under acidic and alkaline conditions. Mo isotopic fractionation remained relatively constant between pH 4.0 and 7.0, with Δ⁹⁸Mo_{[Mo(aq)-Mo(hematite)]} ≈ 2.10‰, indicating minimal pH sensitivity. Above pH 7, however, fractionation decreased markedly, with values declining to 1.80‰ at pH 8 and 1.37‰ at pH 9.

Ab initio molecular dynamics (AIMD) simulations indicate that Mo is predominantly present as a five-fold coordinated complex at the water–hematite interface. This structural motif is supported by Mo K-edge X-ray absorption near-edge structure (XANES) spectra, which show that the local coordination environment of sorbed Mo differs from that of MoO₃ (octahedral geometry) and cannot be explained by a simple mixture of tetrahedral and octahedral species. Extended X-ray absorption fine structure (EXAFS) analysis yields an average Mo–O coordination number of 4.3 ± 0.9 at pH = 5.0 and 4.7 ± 0.9 at pH = 7.0, consistent with prior observations (5.3 ± 0.6), thereby corroborating the predominance of a five-fold Mo species.

Theoretical isotopic fractionation for this five-fold complex, calculated as Δ⁹⁸Mo_{[Mo(aq)-Mo(five-fold complex)]} = 2.01 ‰ at 20 °C, agrees well with our experimental results (Δ⁹⁸Mo_{[Mo(aq)-Mo(hematite)]} = 1.899 ± 0.322 ‰) and previous data (2.19 ± 0.54 ‰). A two-component model incorporating both five-fold and tetrahedral Mo complexes successfully reproduces the observed pH-dependent fractionation trend. Collectively, these findings demonstrate that Mo isotope fractionation on hematite is strongly pH-dependent and governed by changes in surface complexation geometry, with implications for the application of Mo isotopes as tracers in both natural and anthropogenically influenced systems, from continental weathering profiles to aqueous transport pathways.