Elsevier

Chemical Engineering Journal

Volume 337, 1 April 2018, Pages 372-384
Chemical Engineering Journal

Synthesis of magnetically recoverable Fe0/graphene-TiO2 nanowires composite for both reduction and photocatalytic oxidation of metronidazole

https://doi.org/10.1016/j.cej.2017.12.090Get rights and content

Highlights

  • nZVI assembled on graphene-TiO2 nanowires (Fe@GNW) composite was synthesized.

  • Fe@GNW exhibited excellent adsorption, reduction and oxidation abilities.

  • Metronidazole (MNZ) removal experienced two stages of reduction and oxidation.

  • Micro-graphene-Fe0 batteries and photo-Fenton system are formed during MNZ removal.

  • The magnetic Fe@GNW can be easily recovered from residual solution using a magnet.

Abstract

Novel ternary nanocomposite based on nano zero-valent iron (nZVI) and graphene-TiO2 nanowires (Fe@GNW) was successfully synthesized for both reduction and photocatalytic oxidation of metronidazole (MNZ). Fe@GNW exhibits synergetic effects and significant properties, such as facilitating the photogenerated charges separation, enhancing the surface activity of nZVI, improving the adsorbent ability of catalyst, possessing the magnetic property for facile recycling. Due to the formation of micro-graphene-nZVI batteries and heterogeneous photo-Fenton system, Fe@GNW showed a superior activity in removal of MNZ (99.3%) compared with TiO2 nanowires (43.0%) and graphene-TiO2 nanowires (67.6%). Moreover, Fe@GNW retained excellent stability without apparent loss in catalytic activity after five cycles. MNZ removal by Fe@GNW experienced a two-stage process of reduction-adsorption and photocatalytic oxidation, which could be well-described by a revised pseudo-first order kinetics. The decomposition pathways of MNZ were proposed based on the analysis of intermediates. The quenching tests demonstrated that h+ and radical dotOH are responsible for MNZ decomposition. New insights into the mechanism of the enhanced reduction and photodegradation of MNZ were proposed.

Introduction

Pharmaceutical and personal care products (PPCPs) have recently aroused particular concern as emerging contaminants. Some PPCPs are listed as priority pollutants by the European Union Water Framework Directive [1]. Among PPCPs, antibiotics have been increasingly detected in surface water, groundwater, sewage effluent, and drinking water. Metronidazole (MNZ), a common antibacterial and antiprotozoal drug. It is widely used for treating infectious diseases for both human and veterinary [2]. MNZ may pose a serious, long-term risk for human health and aquatic ecosystems due to its non-degradability, potential mutagenicity, and carcinogenicity. As MNZ is continuously released into the environment, it was prone to accumulating in animals, fish farm water and effluents from meat industries and hospitals [3], [4], and then pass through wastewater treatment plants (WWTPs) [5]. To develop promising strategies for effective degradation of MNZ is hence of great environmental interest.

So far, considerable efforts have been made to applications of nanomaterials, especially by using zero-valent iron (nZVI)) to environmental remediation [6], [7], [8], [9]. However, the activity of nZVI declined over time due to the formation of a passivation layer on its surface and aggregation of nZVI [10], [11], [12]. Moreover, nZVI technology is sometimes considered as limited chemical reduction process because of the generation of recalcitrant intermediates during the decomposition of parent compound. Although MNZ could almost be completely eliminated by nZVI at the acidic and neutral conditions, extremely low removal efficiency of total organic carbon (TOC) has been described [2], [13], [14]. To avoid toxic refractory by-products or intermediates, removal of MNZ for thorough mineralization/oxidation by using photocatalysis has been pursued.

Anatase TiO2 has been widely studied because of its unique characteristics, such as cost-effective, environmentally-benign, excellent stability and photocatalytic activity [15]. Nevertheless, TiO2-based catalysts still have several remaining challenges (e.g. rapid recombination of photogenerated charge carriers, and low quantum efficiency) [16], [17], [18], [19]. The activities of photocatalysts are dependent on its morphology and structure, and morphology-controlled method [20]. Particularly, one dimensional (1D) structures of TiO2, including nanofibers, nanorods and nanotubes, have sparked immense interest [21], [22], [23]. Unfortunately, the performances of these photocatalysts fail to meet the requirements of applications because they are usually used in the form of nanopowders that is inconvenient to recycle, resulting in loss of catalyst and secondary pollution. However, pilot-scale studies indicate that antibiotics can be effectively degradated and mineralized by using solar fixed-bed reactor. Bansal et al. reported that 70% cephalexin was removed after 10 h of solar irradiation in the pilot-scale fixed-bed baffled solar reactor [24]. Pereira et al. carried out solar photocatalytic degradation experiment of oxytetracycline (OTC) in a solar pilot plant. Results showed 100% OTC removal and 80% TOC removal were achieved with 1.8 kJ/L and 11.3 kJ/L of photo treatment energy, respectively [25]. Therefore, photocatalytic degradation of antibiotics has good application perspectives from both engineering and commercial point of view.

The electrons generated from band gap of TiO2 can be theoretically trapped by the half reaction of Fe3+/Fe2+ [26]. TiO2 and nZVI have been combined to eliminate phenol [27], nitrate [28], various chlorinated solvents [26], [29] and dye effluents [30]. Although the coexistence of nZVI and TiO2 is able to remediate each other, the blocking of TiO2 reactive sites by nZVI and its corrosion products might weaken the photocatalytic activity of TiO2. The recent advent of graphene revolution has opened up tremendous possibilities for exploring and constructing new carbon-based multifunctional composites. Immobilization of nZVI onto graphene matrix can restrain aggregation of iron particles, contributing to enhance its performance toward pollutants removal [19]. Herein, we design and construct a ternary nanocomposite of magnetic nZVI/graphene-TiO2 nanowires (Fe@GNW), with the first-time usage of the novel functional material for synergistic degradation of antibiotic MNZ. To the best of our knowledge, there currently is no available literature on evaluating the potential of the composites composed of photocatalytic TiO2, highly reductive nZVI and graphene for complete decomposition of antibiotics in water environment. The main objective of this study is to provide a novel composite with magnetic property and higher photocatalytic activity for antibiotic wastewater treatment. Prior to illumination, nZVI as a strong reductant can reduce most of MNZ to intermediates and simultaneously establish the adsorption equilibrium with magnetic stirring. While TiO2 shows powerful photocatalytic ability under ultraviolet (UV) irradiation, and thus the intermediates and residual MNZ can be completely mineralized. The other objectives of this research were to: (1) examine the MNZ removal efficiency achieved by Fe@GNW under different initial MNZ concentrations, pH values, reaction temperatures, and loaded nZVI content because excessive iron load amount may serve as recombination centers for the photogenerated carrier, (2) characterize the newly prepared catalysts using various techniques to study their morphology, crystalline phase, specific surface area, elemental composition, optical and luminescent properties, (3) study the degradation kinetics of MNZ in two stages of reduction-adsorption and photodegradation, respectively, as well as (4) propose a possible reaction mechanism in the Fe@GNW redox system and reveal the promotional role of nZVI, graphene and nanowires (NWs) structure in the degradation process of MNZ.

Section snippets

Materials

MNZ was obtained from Fuchen Chemical Regents Factory (Tianjin, China). TiO2 Degussa P25 was supplied by Germany Degussa. Graphite powder were acquired from Kelong Chemical Reagent Co., Ltd. (Chengdu, China). Ferrous sulphate heptahydrate (FeSO4·7H2O), potassium borohydride (KBH4) and other chemicals were purchased from Aladdin Reagent Database Inc., China. All the chemicals were of analytical grade and used as received without further purification.

Synthesis of G-TiO2NWs composites

Graphene oxide (GO) was synthesized with an

Morphology and microstructure

The detailed morphology and structure of the as-prepared samples were clearly demonstrated by TEM. From Fig. 2A, it can be seen that the synthesized TiO2 NWs are elongated with length from hundreds of nanometers to several micrometers, although occasionally, some of the NWs tend to be aggregated into bundles. The selected-area electron diffraction (SAED) pattern (inset in Fig. 2B) exhibits single crystal feature across the entire length of anatase TiO2 NWs, which can be confirmed by the results

Conclusion

In this study, magnetic high-efficiency Fe@GNW nanocomposites have been fabricated, characterized, and tested for removal of MNZ from aqueous solution. The as-prepared Fe@GNW exhibits excellent activity over the reaction course, which mainly arises from the formation of graphene-nZVI micro-batteries and heterogeneous photo-Fenton system (TiO2/Fe0/H2O/O2/UV). The outstanding advantage of Fe@GNW lies in that the photocatalytic activity of TiO2 and reductive performance of nZVI can be completely

Acknowledgment

The authors appreciate the support provided by Program for National Natural Science Foundation of China (NSFC, Grant Nos. 51368025 and 51068011).

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