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 pH(0) > 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 similar to 3.0 irrespective of the initial pHs. The generated center dot 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.