Prussian-blue-modified iron oxide magnetic nanoparticles as effective peroxidase-like catalysts to degrade methylene blue with H₂O₂

Abstract

Prussian-blue (PB)-modified γ-Fe₂O₃ magnetic nanoparticles (PBMNPs) were successfully synthesized based on electric interactions between negatively charged [Fe(CN)₆]⁴⁻ and positively charged γ-Fe₂O₃ nanoparticles. The in situ PB coating was generated by the coordinating reaction between the adsorbed [Fe(CN)₆]⁴⁻ and the ferric ions on the surface of γ-Fe₂O₃ NPs. The as-prepared PBMNPs were characterized by FT-IR, XRD, TEM, and used to remove organic pollutants from aqueous solution, namely, using methylene blue (MB) as model compound. The experimental results showed that the target compound could be removed efficiently from solution over a wide pH range from 3 to 10 in the presence of PBMNPs as peroxidase-like catalyst and H₂O₂ as oxidant. Under optimal conditions, MB could be removed completely after 120 min of reaction at 298 K; the chemical oxygen demand (COD) removal efficiency and the total organic carbon (TOC) abatement efficiency were 53.6% and 35%, respectively. Furthermore, the PBMNPs catalysts showed high magnetization, temperature tolerance, long-term storage and operational stability, and they could be readily separated from solution by applying an external magnetic field. Finally, a possible reaction mechanism for MB degradation was also discussed.

Introduction

The widely used dyes in various industries such as textile, printing, rubber, food plants and cosmetics are the main source of environmental pollution and are considered as a serious threat to ecosystem. Therefore, the treatment of effluent containing dyes is of interest due to their harmful impacts on receiving waters. During the past ten years, the various conventional treatment methods, such as physical adsorption [1], [2], [3], chemical oxidation [4], [5], [6], biological degradation [7], [8], have been developed for the removal of such contaminants in wastewater. However, they suffer from disadvantages including incomplete removal, high cost, disposal problem of sludge, and secondary metal ion pollution, etc. There is a growing demand for improved methods of treatment. In recent years, enzyme as biocatalyst has been widely used in wastewater treatment. For example, it has been demonstrated that horseradish peroxidase (HRP) can catalyze hydrogen peroxide to oxidize various organic compounds including phenol [9], [10], chlorophenols [11], and remazol blue [12], etc. There were also attempts trying to immobilize enzymes on various matrices such as cellulose filter paper, nylon balls, and nylon tubing [13]. However, the application of natural peroxidase enzymes in the treatment of wastewater is limited due to their instability and high cost [14]. Therefore, new catalytic materials are needed for the degradation of organic pollutants such as dyes in wastewater.

Recently, Gao et al. [15] reported for the first time that Fe₃O₄ magnetite nanoparticles (FMNPs) possess intrinsic enzyme mimetic activity similar to natural peroxidases. They catalyzed the oxidation of different peroxidase substrates such as TMB (tetramethylbenzidine), DAB (di-azo-aminobenzene) and OPD (o-phenylenediamine) to give the same color variation as that of HRP [15]. On this basis, other studies [16], [17] investigated the use of FMNPs to degrade other organic pollutants such as phenolic and aniline compounds. However, a long reaction time (6 h, at 308 K over 5 g/L FMNPs in the presence of 1.2 mol/L H₂O₂) was needed to complete the removal of two compounds due to the relatively low H₂O₂-activating ability of FMNPs [17]. In order to increase the H₂O₂-activating ability of FMNPs, ultrasonic irradiation was introduced to the preparation of FMNPs with much improved peroxidase-like catalytic ability to activate H₂O₂ [18]. In this case, only 1 h was needed for 90% removal of rhodamine B as a model compound [18]. It was reported [15] that the peroxidase-like activity of FMNPs was derived from superficial ferrous atoms of nanoparticles. Hence, there are some challenges to enhancing the peroxidase-like activity and long-term stability of FMNPs due to ease oxidation of ferrous atoms. Based on Zhang et al. [19], in this research, Prussian-blue (PB)-modified γ-Fe₂O₃ (PBMNPs) as peroxidase-like catalysts were prepared, in which PB plays a role like superficial ferrous ions in FMNPs and γ-Fe₂O₃ keeps their magnetic property.

The peroxidase-like activity, high stability and convenient separation from solution by external magnetic field makes PBMNPs a potential catalyst for the degradation of organic pollutants. Here, we demonstrated that the PBMNPs could effectively degrade MB in the presence of H₂O₂. MB was selected as a model target because it is one of the most commonly used dyes and has been widely used for coloring paper, temporary hair colorant, dyeing cottons, wools, and coating for paper stock [20]. Also, it can cause burning sensation, nausea, and vomiting, mental confusion and methemoglobinemia upon ingestion [21]. Various parameters affecting the degradation, including potassium ferrocyanide, PBMNPs and H₂O₂ concentrations, pH and temperature, were investigated in detail. Furthermore, the TOC removal and the stability of PBMNPs were also studied. Finally, we proposed that the MB might be degraded by reactive oxygen species (ROS) including OH and O₂⁻ generated during the reaction. Our results demonstrated that the PBMNPs possess excellent catalytic activity toward the oxidation of MB in the presence of H₂O₂, with advantages such as wide working pH range, easy separation, high-temperature tolerance and stability, and they may become a promising catalyst in environmental application.