In situ generation of H₂O₂ using MWCNT-Al/O₂ system and possible application for glyphosate degradation

Highlights

• Al-CNTs composite was prepared by high-energy ball milling and sintering process.

• The accumulative concentration of H₂O₂ reached 947 mg/L in Al-CNTs/O₂ system.

• In situ generated H₂O₂ could be used for the degradation of glyphosate.

Abstract

Hydrogen peroxide (H₂O₂), as a green oxidant, has been widely applied into advanced oxidation processes (AOPs) for the degradation of toxic organic pollutants. The in situ generation of H₂O₂ can not only improve the storage and transportation safety of H₂O₂ but also reduce the capital and operation costs. In the present work, a novel system, i.e., multi-walled carbon nanotube‑aluminum (MWCNT-Al) composite was used to in situ generate H₂O₂ through micro-electrolysis. The MWCNT-Al composite was characterized and optimized. The accumulation concentration of H₂O₂ reached 947 mg/L at the initial pH of 9.0, the MWCNT-Al composite dosage of 8 g/L and oxygen gas flow rate of 400 mL/min after 60 min. The in situ generation of H₂O₂ was achieved by MWCNT-Al/O₂ system, mainly owing to the direct contact between Al⁰ and MWCNT in MWCNT-Al composite, which accelerated the transfer of electrons from Al⁰ to O₂, as well as the excellent electrocatalytic activity of MWCNT toward the two-electron reduction of oxygen. When H₂O₂ in situ generation technology was used in peroxone process (O₃/H₂O₂ process) to degrade glyphosate in aqueous solution, the removal efficiency of TOC and total phosphorus was 68.35% and 73.27%, respectively. Finally, the possible mechanism of in situ generation of H₂O₂ in MWCNT-Al/O₂ system was temporarily proposed.

Introduction

Advanced oxidation processes (AOPs) that use hydroxyl radical (OH) with redox potential of 2.80 eV as principal active species for the degradation and mineralization of refractory pollutants are considered as promising methods of wastewater treatment (Wang and Xu, 2012; Wang and Bai, 2017). Hydrogen peroxide (H₂O₂) is used for OH generation in many AOPs, such as H₂O₂/UV, H₂O₂/Fe²⁺, H₂O₂/O₃, etc. (Neyens and Baeyens, 2003; Wan and Wang, 2016; Tang and Wang, 2018). In these H₂O₂-based AOPs, the potential safety risks from the transportation, and storage process of commercially available and concentrated H₂O₂ have received increasing attention due to its chemical instability (Asghar et al., 2015; Zhang et al., 2018). Therefore, a technology that can generate H₂O₂ in situ would be highly desirable.

Among various alternative approaches for the in situ generation of H₂O₂, the reduction of O₂ to H₂O₂ is the most attractive because O₂ can be obtained conveniently from air. Many reducing agents have been used for the in situ generation of H₂O₂, such as hydrogen, hydrogen substituted organic compounds, photoelectron, cathode of electrochemical or microbial fuel cells, active metals, etc. (Asghar et al., 2015; Edwards et al., 2015; Fu et al., 2010; Annabi et al., 2016; Trovó et al., 2009; Luo et al., 2015; Yalfani et al., 2011; Liu et al., 2017). The direct reaction of hydrogen and oxygen to H₂O₂ is conceptually the most straightforward, however, there is potential problem of operational safety due to the wide explosive range of H₂/O₂ mixtures (Liu et al., 2018a). Hydrogen substituted organic compound such as hydrazine, formic acid and hydroxylamine has been suggested to overcome the shortcomings of direct method (Yalfani et al., 2011). However, incomplete conversion of organic compound may increase the toxicity of the treated wastewater and the decomposition of H₂O₂ over catalyst surface may decrease the process efficiency (Liu et al., 2018b). The photochemical reactions of semiconductors (ZnO, TiO₂ etc) can produce electrons and holes. The electron as a reducing agent, helps to the production of H₂O₂ and the holes helps to the degradation of organic contaminant (Hu et al., 2018; Clarizia et al., 2017). However, it cannot be considered as an alternative for in situ H₂O₂ generation because of its intensive energy demand (Asghar et al., 2015). The electrochemical synthesis via a 2-electron oxygen reduction reaction (ORR) on suitable cathode was confirmed to be more effective in H₂O₂ generation rate and O₂ utilization efficiency. The accumulation of H₂O₂ could reach 566 mg/L in 0.05 M Na₂SO₄ at a current density of 7.1 mA/cm² and air flow rate of 0.5 L/min after 180 min (Yu et al., 2015). The in situ generation of H₂O₂ in microbial fuel cells can reduce the intense energy requirement by the production of electricity (Zhuang et al., 2010; Chen et al., 2014). However, a lot of inorganic electrolyte was required in electrochemical system and many nutrient substances must be provided for microbial fuel cells. Active metals such as zinc, aluminum and iron have been known as an attractive reducers for reducing oxygen into H₂O₂ because of its simple steps and no extra cost on material and energy (Keenan and Sedlak, 2008; Fan et al., 2015; Wen et al., 2014). However, the maximal concentration of H₂O₂ was only 180 μM in Al/O₂ system at 250 min (Fan et al., 2015), which limited its application in the wastewater treatment.

The electrochemical corrosion theory implies that when the metals contact directly with other conducting material, with higher electrode potential, the galvanic-type corrosion cell will be formed, which will accelerate the corrosion rate of metal and most of the depolarizing agent reduction on the surface of metal will be transferred to cathode. Based on this theory, our group has successfully prepared zinc carbon nanotubes (Zn-CNTs) composite and used it to generate H₂O₂ in situ. The results showed that the maximum cumulative concentration of H₂O₂ in Zn-CNTs/O₂ system was 293.51 mg/L under the initial pH 3.0, Zn-CNTs dosage 2 g/L and O₂ flow rate 400 mL/min, which was 15 times of zinc alone (Gong et al., 2018). Aluminum has lower electrode potential and more abundant storage in the earth than zinc. The fresh aluminum compound produced by the corrosion of aluminum has strong flocculation properties and the ability of precipitation to remove phosphorus. Therefore, multi-walled carbon nanotubes-Al (MWCNT-Al) composite may have greater potential than Zn-CNTs composite for the in situ generation of H₂O₂ and for the removal of toxic contaminants from the wastewater.

Glyphosate (N‑(phosphonomethyl) glycine) is an organophosphorus compound, which has been extensively used as herbicide (Lan et al., 2016; Wang et al., 2016). Glyphosate has been classified as “probably carcinogenic in humans”, which can cause a variety of symptoms including eye and skin irritation, contact dermatitis, eczema, cardiac and respiratory diseases and allergic reactions (Hu et al., 2011).

In this paper, we synthetized MWCNT-Al composite for the first time by high-energy ball milling and sintering the mixture of aluminum and MWCNT directly. XRD, SEM, EDS and Raman were used to characterize the as-prepared MWCNT-Al composite. The catalytic performance of MWCNT-Al composite for the in situ generation of H₂O₂ under oxygen aeration was investigated. Meanwhile, the effects of operational factors, such as the initial pH in solution, operating temperature and MWCNT-Al composites dosage, on the in situ generation of H₂O₂ were also discussed. The in situ generated H₂O₂ was used to peroxone process (O₃/H₂O₂ process) for the degradation of glyphosate in aqueous solution Glyphosate was selected as the model pollutant due to its high toxicity and extensive application (Koskinen et al., 2016; Ibáñez et al., 2005).