Author: Ma Yi-yang Corresponding author: Zhang Zhenghua Communications Unit: Tsinghua University, Shenzhen Graduate School of International

Paper DOI: 10.1016/j.apcatb.2020.119720
Graphic abstract

Introduction
Recently, Zhang Zhenghua’s team from the Shenzhen International Graduate School of Tsinghua University published a titled "Catalytic degradation of ranitidine using novel magnetic Ti3C2-based MXene nanosheets modified with nanoscalezero-valent iron" in the well-known journal Applied Catalysis B: Environmental (IF=16.683) in the environmental field. particles" research paper (DOI: https://doi.org/10.1016/j.apcatb.2020.119720).
Full text at a glance

This study introduces a magnetic composite material (nZVI@Ti3C2) that uses Ti3C2 phase MXene nanosheets as the catalytic material structure frame and modified nano-zero-valent iron particles on its surface (nZVI@Ti3C2), and constructs a heterogeneous class by catalyzing hydrogen peroxide- Fenton oxidation system to achieve the removal of ranitidine in water. The Ti3C2 phase MXene nanosheet not only solves the problem of easy agglomeration of nano zero-valent iron particles, but also forms a positive synergistic effect between the two through the special shape design. Hydroxyl radicals generated by the activation of hydrogen peroxide are the main reactive oxide species (ROS) for the removal of ranitidine, and the free radical quenching experiment is used to evaluate the catalytic performance of its different forms. The nano-zero-valent iron particles in the composite material have been proved to be the reactive center of the heterogeneous oxidation system and directly affect the catalytic stability of the entire composite material. This article provides a profound discussion on this.
Introduction

Ranitidine is usually used as a powerful H2 receptor antagonist to inhibit gastric acid secretion and reduce the activity of gastric acid and gastric enzymes. However, after being ingested into the human body, most of ranitidine is excreted through the kidneys in the form of the original drug. It has been confirmed that the conversion rate of it into dimethylnitrosamine (NDMA) with high carcinogenicity is as high as 89.9-94.2%. Traditional Fenton advanced oxidation is suitable for the treatment of difficult-to-degrade organic pollution, but shortcomings such as large chemical sludge output, narrow applicable pH range and low hydrogen peroxide utilization rate strictly limit the wide application of this technology. In recent years, with the gradual development of heterogeneous-Fenton catalytic oxidation technology, many defects of traditional processes have been overcome.
It can be seen that the development of solid-phase catalytic materials with the advantages of cheap and easy-to-obtain,

strong reaction activity and no secondary pollution has become the core of heterogeneous-Fenton oxidation technology.

Among them, nano-zero-valent iron exhibits high reactivity in water,

Figure 1. (a) SEM image of monolithic Ti3C2 phase MXene; (bc) SEM image of multi-layer structure/monolithic structure nZVI@Ti3C2 nanosheets (illustration is conceptual model); (dg) monolithic structure EDS element distribution map of nZVI@Ti3C2 nanosheets; (h) AFM image of monolithic nZVI@Ti3C2 nanosheets; (i) high-resolution TEM image of monolithic nZVI@Ti3C2 nanosheets (inset is low-resolution TEM Figure); (j) Size distribution of nano-zero-valent iron particles on the surface of Ti3C2 phase MXene.

The Ti3C2 phase MXene nanosheets are prepared by a two-step method of etching and layer peeling using Ti3AlC2 as the raw material. nZVI@Ti3C2 nanosheets realize special structure design through reduction and co-precipitation. In Figure 1a, it can be observed that the Ti3C2 phase MXene nanosheets exhibit obvious lamellar structure characteristics. Nano-zero-valent iron particles were modified on the surface of Ti3C2 phase MXene nanosheets by liquid-phase reduction and co-precipitation, and two nZVI@Ti3C2 nanosheet composites with morphological characteristics were obtained (Figure 1b-c). In Fig. 1d-g, the elements on the surface of the monolithic nZVI@Ti3C2 nanosheets are uniformly distributed, and the weak signal of Al element is caused by the etching of the aluminum layer in the primary phase of Ti3AlC2. Figure h shows the sheet size of the monolithic nZVI@Ti3C2 nanosheet, and in Figure i, the lattice fringes of zero-valent iron and Ti3C2 phase MXene can be observed. In Figure j, it is calculated that the particle size of the nano-zero-valent iron particles on the surface of the Ti3C2 phase MXene nanosheets is mainly concentrated in the range of 10-40 nanometers.

Figure 2. (a) Adsorption-desorption isotherms of nZVI, Ti3C2 phase MXene and nZVI@Ti3C2 nanosheets; (b) XRD of Ti3AlC2, Ti3C2 phase MXene and nZVI@Ti3C2 nanosheets; (c) Ti3C2 phase MXene and FTIR spectra of nZVI@Ti3C2 nanosheets; (d) Raman spectra of single Ti3AlC2, Ti3C2 phase MXene and nZVI@Ti3C2 nanosheets.

In Figure 2a, the typical IV isotherm and H3 hysteresis loop show that nZVI@Ti3C2 nanosheets have an obvious mesoporous structure, with a pore diameter of 36.4 nm and a specific surface area of 14.56 m2/g compared to nano-zerovalent iron. Raised to 28.25. In Figure 2b, it can be observed that the diffraction peaks of (002) and (004) of Ti3AlC2 broaden and move to a lower angle, and the intensity of the diffraction peak at 39° is significantly reduced, indicating that the aluminum layer in Ti3AlC2 is etched and Form a two-dimensional layered structure. In Figure 2c, nZVI@Ti3C2 nanosheets have a large number of hydroxyl groups attached to the surface to make them exhibit good hydrophilicity. In Figure 2d, the Ti3C2 phase MXene nanosheets are shown at 158, 202, 375, 575, 620 and 720 cm-1. Vibration response of functional groups on the surface of the sheet.

Figure 3. nZVI@Ti3C2 nanosheets repeated experiments five times in a row (reaction conditions: reaction temperature is 25℃, [solution pH]0 = 4.5, [H2O2]0 = 0.5mM, [ranitidine concentration]0 = 5 mg/L and [nZVI@Ti3C2nanosheets dosage]0 = 0.5 g/L): (a) Different processing methods; (b) XRD of nZVI@Ti3C2 nanosheets after five reactions; (c) Hysteresis loop of nZVI@Ti3C2 nanosheets before and after the reaction ; (D) High-resolution TEM image of nZVI@Ti3C2 nanosheets after reaction.
The nano-zero-valent iron in the composite nZVI@Ti3C2 nanosheet component is used as the reactive center, and its particle surface passivation is a direct factor affecting the catalytic activity. In Figure 3a, the dropwise addition of hydrogen peroxide, dilute acid washing, and nitrogen protection all effectively improved the recyclability of nZVI@Ti3C2 nanosheets, and Figure 3b shows that nZVI@Ti3C2 nm after different treatment methods XRD pattern of the film. In Figure 3c, the magnetic enhancement of the nZVI@Ti3C2 nanosheets after the reaction is due to the oxidation of the surface of the nano-zero-valent iron particles in the composite material to generate iron-based oxides. In Figure 3d, it can be observed that the zero-valent iron particles present an obvious core-shell structure, and there is no particle agglomeration after the reaction.

Figure 4. (a) EH-pH and Bhuby diagram of Fe-H2O system at 25℃; (b) Zeta (ζ) potential diagram of nZVI@Ti3C2 nanosheets; (c) nZVI@Ti3C2 nanosheets in [DMPO ]= EPR of DMPO-OH after five minutes in 0.1M; (d) Hydroxyl radical quenching experiment (reaction conditions: reaction temperature is 25℃, [solution pH]0 = 4.5, [H2O2]0 = 0.5mM, [ ranitidine concentration]0 = 5 mg/L and [nZVI@Ti3C2nanosheets dosage]0 = 0.5 g/L).

In Fig. 4a, combining EH-pH and Bhuby diagram, we explored the change rule of the passivation layer on the surface of nano-zero-valent iron particles in nZVI@Ti3C2 nanosheets under different pH conditions. It is fully proved that under acidic conditions, the passivation layer on the surface of zero-valent iron particles can be corroded to expose more active sites. On the contrary, under alkaline conditions, the growth rate of the passivation layer can be accelerated. In Figure 4b, it can be observed that the isoelectric point of the nZVI@Ti3C2 nanosheet is about 6.37. The EPR spectrum (Figure 4c) confirmed that hydroxyl radicals are the main reactive oxide species for ranitidine removal, and the zero-valent iron in the composite material can be further determined as the reactive center based on the response signal intensity. In Figure 4d, it can be observed that n-butanol and potassium iodide as active free radical quenchers have different degrees of inhibition on the removal of ranitidine. The experimental results confirm that the surface-bound hydroxyl radicals have a more significant effect on the degradation of ranitidine.

Figure 5. The conceptual diagram of ranitidine degradation mode in the heterogeneous Fenton oxidation system constructed by nZVI@Ti3C2 nanosheets catalyzed by hydrogen peroxide.

In this study, nZVI@Ti3C2 nanosheets have two morphological structures. According to the distribution characteristics of nano-zero-valent iron particles, the activation mechanism of hydrogen peroxide is also different. Among them, as shown in Type II, the nano-zero-valent iron particles existing between the lamellae can exert a confined catalytic effect, which can achieve a faster degradation reaction rate compared to the traditional heterogeneous oxidation process. Therefore, by optimizing the preparation method, screening out nZVI@Ti3C2 nanosheets with type II structural characteristics will be the focus of subsequent research.
to sum up
The nZVI@Ti3C2 nanosheet magnetic material prepared in this research has abundant hydrophilic functional groups on the surface. Under mild reaction conditions, hydrogen peroxide is activated to generate hydroxyl radicals to achieve ranitidine degradation. Cyclic experiment results show that the surface passivation of zero-valent iron particles directly affects the reusability of nZVI@Ti3C2 nanosheets, and dilute HCl treatment is the best treatment method that can make the catalyst easy to regenerate without destroying the structural characteristics of the catalyst. The quenching experiment proved that reducing [H] can degrade a small amount of ranitidine molecules and accelerate the valence cycle of Fe(III)/Fe(II), but ranitidine molecules are mainly degraded by the attack of hydroxyl radicals , Especially the surface-bound hydroxyl radicals.
About the Author
Author: Ma Yi-yang, graduated from China University of Geosciences (Beijing), now a postdoctoral Tsinghua University, Shenzhen Graduate School of International. The main research directions include advanced oxidation and membrane separation technology for water treatment.

Corresponding author: Professor T. David Waite Zhang Zhenghua, Tsinghua University, Shenzhen Graduate School of International, special researcher, doctoral tutor, doctoral and post-doctoral period under the tutelage of the US Academy of Engineering from the University of New South Wales, Australia (The University of NewSouth Wales), access to environmental Doctor of Engineering and Australian Postgraduate Award. He is also a member of the Youth Expert Committee of the China Seawater Desalination and Water Reuse Society. He is also a high-level overseas talent in Shenzhen-Peacock Project, and a high-level domestic talent.

Main research directions: 1) Membrane water treatment: preparation of functional membranes, membrane water/sewage treatment processes, membrane pollution control and cleaning strategies; 2) Advanced oxidation water treatment: electrochemistry, restricted catalysis, Fenton-like, Photocatalysis, etc.; 3) Preparation of functional materials and water treatment applications: electrospinning functional materials, carbon quantum dots, functional polymer materials, etc.

So far, more than 60 journals and conference papers have been published, of which the first author/corresponding author has published more than 40 professional TOP journal papers (JCR District 1); co-authored an English monograph by Elsevier; applied for 13 patents, including 1 PCT international patents, 4 patents have been authorized, including 1 Australian patent; 17 scientific research projects have been presided over and participated in, 12 of which have been presided over, including the National Natural Science Foundation (in research), Shenzhen Overseas High-level Innovation and Entrepreneurship Project (in A total of 12 projects including research) and Shenzhen basic research (in research); key participating projects include 5 major basic research projects of Australia Linkage and the National Ministry of Science and Technology 973 project.

Email:zhenghua.zhang@sz.tsinghua.edu.cn

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Past recommendations Paper Recommendation ES&T Editor-in-Chief/Associate Editor-in-Chief: Why is my paper rejected without being submitted for review? Angew, Academician Qu Jiuhui of the School of Environment, Tsinghua University: Restricted and enhanced fast Fenton-like reaction led by free radicals
The team of Hongying Zhao and Guohua Zhao from Tongji University ES&T: Electric Fenton Cathodic Oxidation-Reduction Synergy Advanced Treatment of Halogen-containing Pollutants

Past recommendations Paper RecommendationES&T Editor-in-Chief/Associate Editor-in-Chief: Why is my paper rejected without being submitted for review? Angew, Academician Qu Jiuhui of the School of Environment,Tsinghua University: Restricted and enhanced fast Fenton-like reaction led by free radicals
The team of Hongying Zhao and Guohua Zhao from Tongji University ES&T: Electric Fenton Cathodic Oxidation-Reduction Synergy Advanced Treatment of Halogen-containing Pollutants

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