Elsevier

Chemosphere

Volume 270, May 2021, 128639
Chemosphere

Solid peroxides in Fenton-like reactions at near neutral pHs: Superior performance of MgO2 on the accelerated reduction of ferric species

https://doi.org/10.1016/j.chemosphere.2020.128639Get rights and content

Highlights

  • The performance of four solid peroxides in Fenton-like reactions were compared.

  • Image 1
    generated from metallic peroxides is critical in promoting initial reactions.

  • MgO2 has the highest

    Image 11
    production rate and reactivity for Fe(III)/Fe(II) recycles.

  • A mixture of MgO2 and H2O2 enhances the utilization efficiency of active oxygen.

Abstract

Fenton-like reactions at near neutral pHs are limited by the slow reduction of ferric species. Enhancing generation of

Image 2
from solid peroxides is a promising strategy to accelerate the rate-limiting step. Herein, the H2O2 release and Fenton-like reactions of four solid peroxides, MgO2, CaO2, ZnO2 and urea hydrogen peroxide (UHP), were investigated. Results indicated that UHP can release H2O2 instantly and show a similar behavior as H2O2 in the Fenton-like reactions. MgO2 released H2O2 quickly in phosphate buffered solutions, which was comparable to CaO2 but faster than ZnO2. Metal peroxides induced higher initial phenol degradation rates than UHP and H2O2 when the same theoretic H2O2 dosages and Fe(III)-EDTA were used. MgO2 displayed a superior performance for phenol degradation at pH 5, resulting in more than 93% phenol reduction at 1.5 h. According to kinetic analyses, the generation rate of
Image 2
in the MgO2 system was 18 and 3.4 times higher than those in ZnO2 and CaO2 systems, respectively. The addition of MgO2 significantly promoted H2O2 based Fenton-like reactions by increasing production of
Image 2
, and the mixture of MgO2 and H2O2 had an improved utilization efficiency of active oxygen than the MgO2 system. The findings suggested the critical roles of metal peroxides in favoring Fenton-like reactions and inspired strategies to simultaneously accelerate Fenton-like reactions and improve utilization efficiency of active oxygen.

Introduction

Fenton based advanced oxidation technologies (AOTs) have been widely applied for the removal of recalcitrant organic contaminants in wastewater (Yang et al., 2019; Zhang et al., 2019; Zhu et al., 2019; Zhou et al., 2020). In these reactions, highly active hydroxyl radicals can be produced by using iron containing substance and H2O2, and the mechanism can be described by Haber-Weiss reactions (R1 and R2) (Lin and Gurol, 1998; Xing et al., 2018). However, these processes are limited by the narrow applicable pH range and the sluggish reduction of ferric to ferrous ions (R2, k = 0.0010.022M1s1) (Lin and Gurol, 1998). Chelating agents (EDTA, citric acid, etc.) or immobilized iron containing materials were employed to widen the pH range by avoiding the precipitation of iron hydroxides at near neutral pHs (Ling et al., 2014; He et al., 2015; Zhou et al., 2017). Several strategies were also proposed to target the kinetic problems, including the introduction of external energy like UV or visible light (Zou et al., 2018; Liu et al., 2018; Serna-Galvis et al., 2018), electricity (Ganiyu et al., 2018) and ultrasound (Hassani et al., 2018; Wei et al., 2019a), the addition of organic reductants (e.g., ascorbic acid, hydroxylamine) (Qin et al., 2015; Hou et al., 2016), and employing nanomaterials like molybdenum disulfide (Zhu et al., 2020) and crystal boron (Zhou et al., 2020). However, it is still a challenge to develop facile and efficient strategies to accelerate the slow redox cycles of Fe(III)/Fe(II) and to make the reaction kinetics meet the applications’ requirements.

Generally, radical dotOH and

are dominant radicals in Fenton or Fenton-like systems (Fu et al., 2016; Hayyan et al., 2016; Vione and Scozzaro, 2019; Zhu et al., 2019). Superoxide radical (
) is one typical reactive oxygen species (ROS), while the chemical generation of
by the reaction between ferric ion and H2O2 (R2) is slow. In fact, the reaction rate constant of Fe(III) reduction by
is very high (R3, ∼107108M1s1) (Li et al., 2016). Thus, the Fe(III)/Fe(II) cycles can be efficiently accelerated by enhancing
production. The increased generation of
can be realized by several methods, such as increasing H2O2 concentrations (Hayyan et al., 2016), using alkali metal superoxide (e.g. KO2) as oxidants (Smith et al., 2004), or catalyzing H2O2 decomposition by birnessite (γ-MnO2) (Furman et al., 2009) or Mn2+ ions (Li et al., 2016). Recently, we found that when using CaO2 as the oxidant, the generation rate of
in the Fenton-like system of CaO2/Fe(III)-EDTA was about four orders of magnitude higher than that in the H2O2 system (Pan et al., 2018). On the other hand,
is a relatively unreactive radical, and its reactivity depends greatly on the generation method, cosolvent polarity, contaminant type, and surface area of materials in the heterogeneous systems (Smith et al., 2004; Hayyan et al., 2016; Zhao et al., 2020). Therefore, simultaneous increasing of the reactivity and production rate of
would be a feasible strategy to improve the removal of organic contaminates in Fenton or Fenton-like systems.

Solid peroxides, like CaO2, were frequently used as the alternative sources of H2O2 in Fenton based AOTs (Northup and Cassidy, 2008; Goi et al., 2011; Lu et al., 2017; Mosmeri et al., 2018; Xue et al., 2019; Wu et al., 2019; Yuan et al., 2019). The advantages of solid peroxides over liquid H2O2 are the more convenience and safety in storage and transportation, and to initiate Fenton-like oxidation within a wider pH range by utilizing the slowly released H2O2 in water. Apart from the most frequently studied CaO2, other solid peroxides, such as ZnO2, MgO2 and urea hydrogen peroxide (UHP) were also investigated in the Fenton-like reactions. A nano-ZnO2/Fe2+ system showed high efficiency in removal of Rhodamine B under faintly acidic conditions (Prasanna and Vijayaraghavan, 2017). High-purity MgO2 nanoparticles were synthesized and successfully applied for methylene blue degradation by using ferric ions as catalysts (Wu et al., 2019). Both MgO2 and CaO2 performed well in the remediation of oil-contaminated soil by using natural minerals as catalysts (Goi et al., 2011). A microwave assisted UHP/Fe2+ system was used for decontamination of halogenated aromatics polluted soil (Cravotto et al., 2007), and a couple of UHP and Fe impregnated biochar was used to reduce fumigant emission in soil (Qin et al., 2020). However, the distinctions among these solid peroxides as Fenton-like oxidants have not been studied yet, especially the reactivity and generation rate of

from the solid peroxides.

Herein, four solid peroxides (MgO2, CaO2, ZnO2 and UHP) including both metallic and non-metallic peroxides were selected. They are common oxidants that have been frequently employed in Fenton and Fenton-like reactions. Their performance on Fenton-like reactions for phenol degradation at near neutral pHs were investigated. Based on mechanistic analyses, the intrinsic differences on the generation rates and reactivity of

were discussed, and a strategy to accelerate the Fenton-like reactions and to improve utilization efficiency of active oxygen was proposed.

Section snippets

Materials

MgO2 was synthesized by a modified method based on previous reports (Navik et al., 2017). Other materials were commercially purchased, and details can be found in Text S1 and S2. All other chemicals were of the highest grade and used without purification.

Release of H2O2

The release of H2O2 from the solid peroxides were measured at the pHs of 5 or 7 by using phosphate buffers (0.05 M). Specifically, a certain amount of solid peroxide was added into the buffered aqueous solution, which was magnetically stirred

Active oxygen contents of peroxides and H2O2 release

The active oxygen contents (wt%) and purities of the four solid peroxides, MgO2, CaO2, ZnO2 and UHP, were determined. Theoretically, MgO2 has the highest active oxygen content (28.4 wt%), which was about twice of that in the commercial 30 wt% H2O2 solution. As shown in Table S1, the measured active oxygen contents were 13.0 ± 0.34%, 12.8 ± 0.19%, 8.22 ± 0.16% and 16.4 ± 0.08% for MgO2, CaO2, ZnO2 and UHP, respectively. According to the ratio of the measured active oxygen to the theoretical

Conclusions

In summary, according to a comparison of four solid peroxides, MgO2 displayed a superior performance to CaO2 and ZnO2 in the generation rate and reactivity of

, which resulted in accelerated Fe(III) reduction and organic degradation in the Fenton-like reactions. UHP behaved like H2O2 due to the instant release of H2O2 and the absence of precipitates. Although currently the cost of MgO2 is higher than H2O2, MgO2 has advantages over H2O2 in the convenience and safety of transportation and

Credit author statement

Yitong Zhu: Data curation, Methodology, Investigation, Visualization, Writing-original draft. Jiaolong Qin: Resources, Investigation, Methodology. Shuqi Zhang: Investigation, Methodology. Adi Radian: Validation, Methodology, Writing - review & editing. Mingce Long: Conceptualization, Supervision, Funding acquisition, Resources, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2017YFE0195800), the National Natural Science Foundation of China (21876108) and Shanghai Municipal International Cooperation Foundation (19230713800). The authors also acknowledge the experimental support of the BET-BJH analysis from Ms. Jie Zhang of Instrumental Analysis Center of Shanghai Jiao Tong University.

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