Water Decontamination from Cr(III)-Organic Complexes Based on Pyrite/H₂O₂: Performance, Mechanism, and Validation

Fenton reaction is a widely used pretreatment technology to degrade toxic metal-organic complexes. However, its efficiency is greatly compromised for Cr(III)-organic complexes due to accumulation of more toxic Cr(VI) and pH dependence. Herein, we proposed a combined pyrite/H₂O₂-precipitation process to efficiently remove Cr(III) (initially at 10.4 mg Cr/L) complexed by various ligands (citrate, EDTA, oxalate, and tartrate). Negligible Cr(VI) and <0.3 mg/L Cr were detected in the effluent treated by pyrite/H₂O₂-precipitation over a wide pH range of 3-9. In contrast, > 0.5 mg/L Cr(VI) and >5 mg/L Cr remained after treatment by the ZVI/H₂O₂-precipitaion process at pH₀ > 5. As for the mechanisms, pyrite/H₂O₂ produced a considerable amount of aqueous Fe(II) to initiate Fenton reaction, concurrently releasing massive H⁺ to keep the reaction pH at ∼3.0 irrespective of the initial pHs. The generated •OH radicals oxidized Cr(III) into Cr(VI) and thereby releasing the organic ligands for further mineralization. The generated Cr(VI) was in situ reduced back to Cr(III) by aqueous Fe(II) and FeS₂. Subsequently, all the free metal ions including Cr(III), Fe(III), and Fe(II) were removed via precipitation. Kinetic modeling of the pyrite/H₂O₂ process involving 17 reactions was performed to verify the proposed mechanism. Additionally, the effectiveness of the combined process was further validated by its satisfactory performance in treating authentic tannery wastewater.