New insights into stoichiometric efficiency and synergistic mechanism of persulfate activation by zero-valent bimetal (Iron/Copper) for organic pollutant degradation

Highlights

• DCP oxidation can be notably enhanced by Fe/Cu compared to nZVI.

• The rate constant of DCP oxidation is linearly correlated to the Cu ratio in Fe/Cu.

• A synergistic effect exists between Cu and Fe in activating persulfate.

• The Fe and Cu species in Fe/Cu determine their synergistic effects in DCP oxidation.

• Cu atoms on Cu₂O layers are the sites dominantly responsible for PS activation.

Abstract

Extensive studies have been devoting to investigating the catalytic efficiency of zero-valent iron (Fe⁰)-based bimetals with persulfate (PS), while little is known in the stoichiometric efficiency, underlying mechanisms and reaction center of zero-valent bimetallic catalysts in activating PS. Herein, nanoscale zero-valent Fe/Cu catalysts in decomposing 2,4-dichlorophenol (DCP) have been investigated. The results show that the increase of Cu ratio from 0 to 0.75 significantly enhances the DCP degradation with a rate constant of 0.025 min⁻¹ for Fe⁰ to 0.097 min⁻¹ for Fe/Cu(0.75) at pH ∼3.3, indicating Cu is likely the predominate reaction centers over Fe. The PS decomposition is reduced with the increase of Cu ratios, suggesting the stoichiometric efficiency of Fe/Cu in activating PS is notably enhanced from 0.024 for Fe⁰ to 0.11 for Fe/Cu(0.75). Analyses indicate Cu atoms are likely the predominant reaction site for DCP decomposition, and Fe atoms synergistically enhance the activity of Cu as indicated by DFT calculations. Both ${\text{SO}}_4^{\text{⦁}-}$ and ⦁OH radicals are responsible for reactions, and the contribution of ${\text{SO}}_4^{\text{⦁}-}$ is decreased at higher pH conditions. The findings of this work provide insight into the stoichiometric efficiency and the reaction center of Fe/Cu catalysts to activate PS for pollutant removals.

Introduction

Sulfate radical (${\text{SO}}_4^{\text{⦁}-}$)-based catalytic oxidation for the destruction of refractory organic pollutants has received great interests in treating contaminated groundwater and soils (Zhong et al., 2015; Liu et al., 2014), because of its relatively higher redox potential of 2.5–3.1 V compared to the hydroxyl radical (⦁OH; 1.8–2.7 V) (Huang et al., 2017). Both peroxymonosulfate (PMS) and persulfate (PS) have been used as precursors for ${\text{SO}}_4^{\text{⦁}-}$ generation (Zhong et al., 2015; Zhang et al., 2018). To promote ${\text{SO}}_4^{\text{⦁}-}$ yield, great progress has been made in applying transition-metal oxide (e.g. Co, Fe, Cu, and Mn)-based catalysts for enhancing the PS and PMS activations (Liang and Guo, 2010; Lei et al., 2015; Anipsitakis and Dionysiou, 2003). Compared with PMS, PS is relatively low-cost and more stable for practical use but more difficult to be activated, restricting its extensive applications (Yu et al., 2016).

Recent studies show that a combination of transition metals such as Co/Fe, Mn/Fe, and Cu/Fe to form binary metallic oxides can significantly enhance PS/PMS activation (Deng et al., 2013; Bao et al., 2019). Within these element combinations, Cu and Fe are particularly considered to be favorable for the heterogeneous PS activation due to their benign nature compared to other metals like Co (Lei et al., 2015). For instance, two parallel studies developed a novel Fe(II)/CuO system, and a similar synergistic effect of Fe(II) with CuO was observed for activating PS (Zhang et al., 2018, 2017). Besides, a binary Fe/Cu oxide catalyst, CuFeO₂ has also shown a great capability for organic pollutant degradation with PS as the oxidant (i.e. about one magnitude greater than that with Fe₂O₃) (Feng et al., 2016). A common explanation for such synergistic effects is that the acceleration of electron cycling within the redox couples between the two metals in composites [e.g. Cu(II)/(I) and Fe(III)/Fe(II)] (Deng et al., 2013; Feng et al., 2016; Zhang et al., 2013). Interestingly, evidence also suggests that the presence of Cu(I) is of particular importance on enhancing PS activation and therefore the ${\text{SO}}_4^{\text{⦁}-}$/⦁OH radical generation (Zhang et al., 2018; Feng et al., 2016; Xia et al., 2017). Recently, Huang and coworkers discovered that the synergistic efficiency of a Fe/Mn oxide catalyst (Mn_1.8Fe_1.2O₄) was highly related to the stoichiometric ratio of the two metals, which is likely due to the different roles of the two metals with Mn primarily acting as active site and Fe(III) as adsorption site (Huang et al., 2017). A very recent study conducted by Bao and coworkers has also reported that the stoichiometric efficiency of the Co/Fe oxides plays a crucial role in PMS activation and radical generation for oxidizing sulfamethoxazole (Bao et al., 2019). However, the stoichiometry of Fe/Cu on its capability for PS activation and radical generation remains unclear, and the corresponding radical generation performance can certainly vary accordingly.

In addition to metallic oxides, their zero-valent forms particularly Fe° nanoparticles (nZVI) have also been studied to activate PS/PMS for removing organic pollutants (Liang and Lai, 2008; Xiong et al., 2014). More strikingly, better performances of pollutant removal by nZVI with PS have been frequently observed compared to its oxides (Xiong et al., 2014; Guan et al., 2015). This is likely due to their low reduction potentials [e.g. E°(Fe²⁺/Fe°) = -0.44 V] (Liu et al., 2020), benefiting for PS activation and pollutant removal. Because of the important role of nZVI in groundwater remediation, different nZVI-based binary nanoparticles (e.g. Fe/Pd, Fe/Cu) have been developed and shown their superiority in promoting electron transfer and pollutant removal (He and Zhao, 2008; Fang et al., 2018a). Importantly, our previous studies have also shown that the reductive transformation of organic pollutants by zero-valent binary metals (e.g. Fe/Cu) is highly related to the stoichiometric ratio of Fe/Cu under anaerobic conditions (Fang et al., 2018a; Xu et al., 2019). This unique feature probably also determines their capability in activating PS. Nevertheless, knowledge of the performance of nanoscale zero-valent Fe/Cu and underlying mechanisms in consideration of its stoichiometric efficiency is still unknown.

In the present study, binary zero-valent Fe/Cu nanoparticles with different stoichiometric ratios were prepared and characterized, and the prepared nanoparticles were systematically examined for their performance in PS activation. 2,4-dichlorphenol (DCP) was selected as the model compound, to investigate the catalytic oxidation efficiency of Fe/Cu catalysts with PS under the pH-controlled conditions, because it is a representative for phenolic compounds with a well-known structure and a high solubility. Accordingly, the stoichiometric efficiency of Fe/Cu for PS activation and DCP degradation has been systematically analyzed by combining the findings of batch experiments with the structural evolution and elemental states of Fe/Cu nanoparticles during the activation processes and theoretical calculations. In final, the underlying mechanisms of synergistic effects of Fe and Cu determining the stoichiometric efficiency of the catalysts were systematically addressed.