Atomistic insights into the As(V) and trace divalent metal co-stabilization in scorodite via precipitation and...

This study systematically investigated how trace divalent metal cations (M(II): Cu^2 +, Zn²⁺, Cd²⁺, and Ni²⁺) affect the precipitation, stability, and phase transformation of hydrous ferric arsenate (HFA) as well as the mechanisms governing As(V) and M(II) mobilization in acidic systems. Natural crystalline ferric arsenate (scorodite) samples from Guangxi, China, validated the environmental relevance of the findings. The results showed that coexisting M(II) slightly inhibited the precipitation of Fe(III)-As(V), and the addition of M(II) promoted the partial dissolution of pre-formed HFA. Density functional theory (DFT) calculations attributed these effects to the site-selective incorporation of M(II) into the HFA chain (Fe1-As1-Fe2-As2) via substitution at the Fe1 site. Aging experiment results (70 °C) revealed that the doping level of M(II) in HFA and secondary scorodite is controlled by a combination of thermodynamics and kinetics. DFT and extended X-ray absorption spectroscopy (EXAFS) results confirmed that the scorodite lattice exerts a thermodynamic screening effect on incorporated M(II). Both Zn²⁺ and Cd²⁺ displayed favorable formation energies, whereas Cu²⁺ and Ni²⁺ tended toward remobilization. The resulting M(II) incorporation capacities in scorodite follow the order: Zn²⁺ (15.2 g/kg) > Cu²⁺ (12.6 g/kg) > Ni²⁺ (8.3 g/kg) > Cd²⁺ (3.4 g/kg). This study proposes a kinetic entrapment-thermodynamic screening mechanism for M(II)-As(V) co-immobilization that provides atomic-scale insights into As and M(II) in multi-M(II) acidic waste streams.