Jinxiang Li

Although numerous strategies have been developed to enhance the performance of zero-valent iron (ZVI) for wastewater treatment, a systematic application-oriented evaluation of their reactivity, durability, and practical suitability remains lacking. In this study, four representative enhancement strategies—ZVI/S, ZVI/Fe²⁺, ZVI/WMF (weak magnetic field), and ZVI/H₂O₂—were systematically evaluated through batch and multi-cycle experiments using commercially sourced ZVI materials. All enhancement methods improved Cr(VI) removal relative to pristine ZVI, while their overall performances followed the order of ZVI/Fe²⁺ > ZVI/S > ZVI/WMF > ZVI/H₂O₂ > pristine ZVI in terms of reaction kinetics, removal capacity, selective reduction efficiency (SRE), durability, and cost-effectiveness. Among them, ZVI/Fe²⁺ exhibited the highest Cr(VI) removal capacity (6.73 mg g⁻¹), sustained reactivity over 6 consecutive cycles, and superior adaptability toward elevated Cr(VI) concentrations, aged ZVI, and complex wastewater matrices containing EDTA–Cu(II). Mechanistic investigations combining SEM, FIB-SEM, XPS, Raman spectroscopy, and electrochemical analyses revealed that Fe²⁺ induced the formation of cracked and conductive corrosion layers enriched with magnetite and lepidocrocite, thereby facilitating Cr(VI) transport and subsequent electron transfer for Cr(VI) reduction. Finally, this study elucidates the application-specific suitability of ZVI enhancement strategies, identifying ZVI/Fe²⁺ as particularly effective for Cr(VI) wastewater treatment, while outlining the key challenges and future perspectives of ZVI-based technologies. Overall, this study provides a performance-based analysis of ZVI enhancement technologies, guiding the selection of ZVI with enhancement methods for wastewater treatment and supporting their environmental application.

A comprehensive comparative evaluation of zero-valent iron (ZVI) systems is essential for optimizing ZVI-based technologies in practical application. In this study, four representative enhancement methods—ZVI/S, ZVI/Fe²⁺, ZVI/WMF, and ZVI/H₂O₂—were systematically investigated through single-batch and multi-cycle experiments, with emphasis on Cr(VI) removal kinetics, reduction capacity, durability, and application feasibility. All enhanced systems significantly outperformed pristine ZVI, with overall performance following the order: ZVI/Fe²⁺ > ZVI/S > ZVI/H₂O₂ ≈ ZVI/WMF > pristine ZVI, as evaluated by reaction rate constants, sustained reactive time, specific reduction capacity (SRC), cost, and removal capacity. Notably, ZVI/Fe²⁺ exhibited the most outstanding performance, far exceeding other systems. Mechanistic analyses using FIB-SEM, XPS, electrochemical impedance spectroscopy, and Raman spectroscopy revealed that ZVI with Fe²⁺ dosing promoted Cr(VI) reduction by inducing cracked and porous corrosion layers and facilitating the formation of conductive iron (hydr)oxide phases, thereby enhancing mass transport and interfacial electron transfer. Moreover, ZVI/Fe²⁺ enabled the simultaneous removal of multiple contaminants, highlighting its adaptability in complex industrial wastewater matrices, particularly under aging and high Cr(VI) concentration effects. This study further provides comparative insights into application-specific suitability of ZVI with enhancement methods, demonstrating that ZVI/Fe²⁺ is particularly well-suited for industrial wastewater treatment under oxic conditions. Overall, this work establishes a mechanism-informed performance hierarchy and offers a rational framework for tailoring ZVI enhancement strategies to specific environmental settings, thereby supporting the targeted design and scalable implementation of ZVI-based technologies for Cr(VI) remediation in wastewater systems.

The resource recovery treatment of uranium-containing wastewater presents a significant challenge for both nuclear energy and environmental sustainability. This study develops the system of coupling microbial fuel cells with sulfidated zero-valent iron (MFC@S-ZVI) to enhance U(VI) removal and facilitate resource recovery from low C/N ratio wastewater. Sulfidation significantly improves electron transfer, thereby enhancing the removal of U(VI) by ZVI under various reaction conditions, including different pH levels and coexisting ions, with the removal rate increasing by a factor of 1.29 to 20.90 depending on the specific conditions. In the MFC@S-ZVI system, a U(VI) removal rate of 98.8% is achieved within 36 hours, surpassing other configurations such as MFC@ZVI (82.0%) and MFC (57.6%). The synergistic effect of S-ZVI and microbial activity optimizes electron flow, resulting in a 2.7-fold improvement in U(VI) removal efficiency compared to single MFC systems. Additionally, XPS and SEM-EDS analyses confirm that U(VI) is predominantly reduced to U(IV) and adsorbed on the S-ZVI surface as high-purity crystalline UO₂, offering a promising method for uranium recovery. Moreover, the MFC@ZVI system maintains enhanced reactivity toward U(VI) across multiple cycles, ensuring long-term sustainability for wastewater treatment. In general, this study underscores the potential of the MFC@S-ZVI system for treating uranium-contaminated wastewater and highlights its broader applicability in resource recovery, particularly in wastewater treatment systems with low C/N ratios.

A comprehensive comparative evaluation of zero-valent iron (ZVI) systems is essential for optimizing ZVI-based technologies in practical application. In this study, four representative enhancement approaches—ZVI/S, ZVI/Fe²⁺, ZVI/WMF, and ZVI/H₂O₂—were systematically investigated through single-batch and multi-cycle experiments, with emphasis on Cr(VI) removal kinetics, reduction capacity, durability, and application feasibility. All enhanced systems significantly outperformed pristine ZVI, with overall performance following the order: ZVI/Fe²⁺ > ZVI/S > ZVI/H₂O₂ ≈ ZVI/WMF > pristine ZVI, as evaluated by reaction rate constants, sustained reactive time, specific reduction capacity (SRC), cost, and removal capacity. Notably, ZVI/Fe²⁺ exhibited the most outstanding performance, far exceeding other systems. Mechanistic analyses using FIB-SEM, XPS, electrochemical impedance spectroscopy, and Raman spectroscopy revealed that ZVI with Fe²⁺ dosing promoted Cr(VI) reduction by inducing cracked and porous corrosion layers and facilitating the formation of conductive iron (hydr)oxide phases, thereby enhancing mass transport and interfacial electron transfer. Moreover, ZVI/Fe²⁺ enabled the simultaneous removal of multiple contaminants, highlighting its adaptability in complex industrial wastewater matrices, particularly under aging and high Cr(VI) concentration effects. This study further provides comparative insights into application-specific suitability of ZVI with enhancement methods, demonstrating that ZVI/Fe²⁺ is particularly well-suited for industrial wastewater treatment under oxic conditions. Overall, this work establishes a mechanism-informed performance hierarchy and offers a rational framework for tailoring ZVI enhancement strategies to specific environmental settings, thereby supporting the targeted design and scalable implementation of ZVI-based technologies for Cr(VI) remediation in wastewater systems.

Sulfidation has gained increasing attention due to its merits to improve the structural-activity of nanoscale zerovalent iron (nZVI) and thus enhance its reactivity toward contaminants. Few studies have been conducted to elucidate the correlation between the structural-activity and reactivity of nZVI, which is important for up-scaling such a decontamination strategy. Taking chromate (Cr(VI)) as the targeted contaminant, this study found that sulfidation enhanced the reactivity of nZVI toward Cr(VI) to varying extents, which was closely related to the degree and order of sulfidation. Particularly, the optimal rate constants of S-nZVI for Cr(VI) removal were 9.79 and 1.48 times higher than that of nZVI in the batch and column systems, respectively. In addition, this study suggested that sulfidation enhanced the electrical conductivity of nZVI by forming conductive iron sulfides (FeSx), while simultaneously reducing the particle aggregation and thus attenuating the settling rate of nZVI in water. More importantly, the reactivity of S-nZVI toward Cr(VI) exhibited negative correlations with its sedimentation activity and electrical conductivity. These relationships can be potentially used to predict the decontamination reactivity of S-nZVI if its sedimentation or conductivity activity was known in advance. Finally, this study clarified the sulfidation-induced improvement in reactivity of nZVI toward Cr(VI), which should be primarily associated with the improved reactive site of S-nZVI due to excellent dispersion and excellent conductivity due to FeSx introduction, ultimately facilitating the reduction of Cr(VI) by nZVI.