Sequential oxidative transformation of the chalcopyrite (001)-S surface by H 2 O and O 2 : electronic structure...

Chalcopyrite (CuFeS $_2^{}$ ) is the primary copper mineral worldwide, yet its hydrometallurgical extraction remains severely hindered by slow leaching kinetics, commonly attributed to surface passivation. Understanding the atomistic events governing the early stages of chalcopyrite surface oxidation is essential for developing more efficient and sustainable copper extraction technologies. In this work, a four-step oxidative mechanism on the CuFeS $_2^{}$ $\left(001\right)$ -S surface was investigated and characterized electronically. O $_2^{}$ co-adsorption with H $_2^{}$ O on Fe sites triggers charge transfer into antibonding $\pi _{}^{\ast }$ orbitals, activating O $_2^{}$ into a superoxide-like intermediate. Subsequent proton transfer yields Fe-OOH and Fe-OH species, followed by the dissociative adsorption of a second H $_2^{}$ O molecule onto the S-S dimer, forming S $_2^{}$ OH intermediates accompanied by S-Cu bond elongation from $2.35$ to $3.62$  Å. In the final step, H $_2^{}$ O adsorption on copper drives significant interfacial charge redistribution, with electronic states near the Fermi level progressively localized on oxygenated intermediates. Overall, iron, copper, and sulfur play distinct but electronically coupled roles in the progressive oxidative transformation of the chalcopyrite surface, providing atomistic insight into the early stages of passivation under hydrometallurgical conditions.

All calculations were performed within the DFT+U framework using the PBE functional and a Hubbard correction ( $U=2$  eV for Fe $_{}^{3+}$ ), as implemented in QUANTUM ESPRESSO (v7.3). The CuFeS $_2^{}$ (001)-S surface was modeled as a periodic slab with a $15$  Å vacuum layer, employing ultrasoft pseudopotentials and a plane-wave kinetic energy cutoff of $60$  Ry. Brillouin zone sampling used $1\times 1\times 1$ and $4\times 4\times 1$ $k$ -point grids for geometry optimizations and electronic property calculations, respectively. Antiferromagnetic ordering along the crystallographic c-axis was adopted throughout. Electronic structure characterization was performed through Bader charge analysis, projected density of states (PDOS), charge density difference maps, and integrated local density of states (ILDOS).