Innovative nZVI/chlorine system for efficient micropollutants degradation: mechanistic insights and process optimization toward sustainable water treatment applications

Abstract

In this work, a novel oxidation process combining nanoscale zero-valent iron (nZVI) and chlorine is reported for the efficient degradation of a model azo dye (CG-HG) in aqueous solutions. The originality of the process lies in the in situ generation of high-valent iron species (Fe(IV)) as the dominant selective oxidants, rather than classical hydroxyl or chlorine radicals. This provides enhanced selectivity and reduced susceptibility to common radical scavengers. Under optimized conditions (100 mg L⁻¹ nZVI, 250 μM chlorine), the system achieved >95% dye removal within 5 minutes, with a synergy index up to 14.43. Radical quenching experiments and mechanistic investigations confirmed Fe(IV) as the primary reactive species. The process remained effective across varying pollutant concentrations and demonstrated long-term durability with over 80% efficiency retained after 10 reuse cycles. The robustness of the system was further evaluated under realistic conditions, showing variable sensitivity to common inorganic ions (Cl⁻, SO₄²⁻, NO₃⁻, NO₂⁻, Br⁻), surfactants, and humic acids. Notably, Fe(IV)'s high reactivity with nitrite and bromide led to complete inhibition, while chloride and nitrate had minor effects. Unexpectedly, sulfate significantly suppressed performance at higher concentrations, likely due to oxygen salting-out, which reduced Fe(II) release. Finally, tests in natural mineral water, river water, treated wastewater effluent, and seawater demonstrated the system's practical relevance. While moderate salt content in mineral water enhanced dye removal, seawater imposed severe inhibition. Despite the strong primary degradation performance, the process achieved a moderate TOC removal of 38%, indicating the persistence of some by-products and the potential need for complementary post-treatments (e.g., biological processes) for full mineralization. These findings underline the importance of matrix composition and support the feasibility of the nZVI/chlorine process as a selective, rapid, and durable oxidation process for pollutant degradation in real water systems, especially, natural mineral water.