Mechanism and thermodynamics of scorodite formation by oxidative precipitation from arsenic-bearing solution

Zanlang Tang; Xincun Tang; Zeyu Xiao; Haonan Liu

doi:10.1016/j.envres.2024.118500

Mechanism and thermodynamics of scorodite formation by oxidative precipitation from arsenic-bearing solution

Environ Res. 2024 Feb 22:250:118500. doi: 10.1016/j.envres.2024.118500. Online ahead of print.

Authors

Zanlang Tang¹, Xincun Tang², Zeyu Xiao³, Haonan Liu³

Affiliations

¹ College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China. Electronic address: tangzanlang@163.com.
² College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China. Electronic address: tangxincun@csu.edu.cn.
³ College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China.

PMID: 38387492
DOI: 10.1016/j.envres.2024.118500

Abstract

Arsenic pollution is a challenging environmental issue caused by arsenic-bearing wastes from nonferrous metallurgy. Oxidative precipitation via introducing O₂ into an ionic Fe(II)-As(V) solution is an advanced method for arsenic immobilization. However, the underlying mechanism is still not well understood. This study proposed a mechanism for scorodite formation by oxidative precipitation, and its thermodynamics were calculated using Gaussian software. Scorodite formation was divided into three stages: precursor formation (3-90 min), oxidative conversion (90-270 min) and crystallization (270-720 min) from the variation in precipitates and solution characterization and parameters such as initial pH, arsenic concentration, and ferrous dosage. In the scorodite formation mechanism, the precursors originate from the coordination polymerization of aqueous Fe(H₂O)₆²⁺ and H₂AsO₄^-, which contributes to the oxidative conversion of coordinated polymers ([Fe(H₂O)₄(H₂O)]_nⁿ⁺) to basic Fe(H₂O)₂AsO₄ until regular octahedral crystals are formed via nucleation and growth during crystallization. The Δ_rG_m^θ for polymerization varied from -491.96 kJ mol^-1 to -33.30 kJ mol^-1, and the Δ_rG_m^θ of oxidative conversion changed from -982.16 kJ mol^-1 to -224.82 kJ mol^-1, demonstrating the feasibility in scorodite formation. This research is significant for understanding scorodite formation in As(V) solutions. It can provide schemes for controlling and modifying the conditions of arsenic-bearing waste immobilization in the laboratories and industries.

Keywords: Arsenic pollution; Coordination polymerization; Oxidative conversion; Scorodite formation.