Synthesis of a halloysite/MnFe2O4 heterogeneous Fenton catalyst for the efficient degradation of organic pollutants

Abstract

To address the limitations associated with conventional Fenton processes, which often exhibit a restricted pH range and present challenges in terms of catalyst recovery and second pollutant, magnetic heterogeneous halloysite (HNT)/MnFe2O4 catalysts were optimally synthesized, which could achieve 90% removal efficiency for 50 mg/L methylene blue (MB) at pH 4–10 and have high hydrogen peroxide (H2O2) utilization efficiencies. In addition, the catalysts could be easily separated from a solution through magnetic separation. The degradation efficiency of MB exhibited remarkable resilience against common aqueous interferents with anions (NO3−, Cl−, SO4^2−, CO3^2−, HCO3−) and humic acid, demonstrating negligible inhibitory effects. Notably, carbonate species (CO3^2− and HCO3−) even elicited a promotional effect on the catalytic process. Furthermore, the removal efficiency of MB only decreased by less than 10% in the fifth cycle compared with that of a fresh catalyst. Furthermore, the HNT/MnFe2O4 catalyst effectively degraded various organic pollutants, such as benzohydroxamic acid, xanthate, and eosin Y. The excellent catalytic performance of the catalysts was attributed to the synergistic effects between HNT and MnFe2O4. The electron paramagnetic resonance spectra and quenching experiments indicated that the main reactive oxygen species that participated in the degradation process were ·OH and ·O2−. ·OH directly attacked MB molecules, and ·O2− accelerated the reduction of metal ions. Therefore, the catalysts showed considerable potential for organic pollutant degradation. This study provides valuable insights into the synthesis of novel catalysts and their practical applications in organic wastewater purification.

1. Introduction

Advanced oxidation processes (AOPs) have been considered promising strategies for wastewater treatment because they can effectively break down organic pollutants into nontoxic by-products or improve the biodegradability of recalcitrant pollutants without secondary pollution [1–4]. Among AOPs, Fenton oxidation stands as one of the most effective because of its high efficiency and simple operation [5–6]. In a conventional Fenton reaction, typical reactive oxygen species (ROS), such as ·OH, will be generated through the soluble ferrous iron (Fe2+) salt-mediated decomposition of hydrogen peroxide (H2O2), and the formed ROS attack and decompose target organic pollutants [7]. However, several obstacles, such as the acidic pH requirement and generation of massive iron mud, limit its large-scale application [8–9].

To offset these limitations, the traditional Fenton reaction system has been continuously improved, and heterogeneous Fenton oxidation has been proposed [10–11]. In a typical heterogeneous Fenton reaction system, soluble Fe2+ salts are replaced by iron-based solid catalysts; this approach suppresses iron leaching and considerably broadens the adapted pH range [5,12]. Many studies are focused on Fe3O4 as a heterogeneous Fenton catalyst [13–14]. It has a cubic inverse spinel structure in which Fe(II) occupies the octahedral position, and Fe(III) occupies the tetrahedral position and half of the octahedral position [15]. The high electron mobility between Fe(II) and Fe(III) in octahedral structure facilitates the reversible inversion of oxidation and reduction state [16]. Besides, Fe3O4 can be easily separated and recycled in the presence of an external magnetic field [17].

Recently, some studies have provided evidence that the incorporation of transition metals into Fe3O4 could effectively improve the catalytic efficiency and promote synergy among adjacent active substances [15,18–19]. For example, Carvalho et al. [20] synthesized Mn-substituted Fe3O4 to degrade a dye and found that the insertion of Mn considerably enhanced the catalytic efficiency of Fe3O4. Du et al. [18] prepared magnetic porous Mn–Fe bimetallic oxide, which exhibited better Fenton catalytic performance than monometallic oxide because of the synergism between Mn and Fe species. Notably, manganese ferrate (MnFe2O4) with an inverse spinel structure similar to that of Fe3O4 but with Mn(II) replacing Fe(II) in its octahedral position [20] displays high catalytic activity in a neutral environment [19,21]. Consequently, MnFe2O4 shows great potential as a high-performance heterogeneous Fenton catalyst because of its improved catalytic activity.