Elsevier

Journal of Hazardous Materials

Volume 371, 5 June 2019, Pages 456-462
Journal of Hazardous Materials

Efficient degradation of organic pollutants by low-level Co2+ catalyzed homogeneous activation of peroxymonosulfate

https://doi.org/10.1016/j.jhazmat.2019.03.002Get rights and content

Highlights

  • Efficient catalyzed activation of peroxymonosulfate with low-level Co2+ was realized.

  • Acetate enhanced the catalytic activity of Co2+ and increased the formation of SO4radical dot.

  • The enhancement was attributed to the coordinating and electron donating abilities.

Abstract

Dissolved cobalt ions (Co2+) are an excellent catalyst for activating peroxymonosulfate (PMS) to generate SO4radical dot for the degradation of organic pollutants, but fairly high level of Co2+ is generally required, which may cause secondary pollution due to its toxicity. Herein, we demonstrate a novel strategy for this catalytic oxidation treatment in which the required Co2+ addition is very small, being much less than the emission limit of 17 μmol L−1. This new strategy is based on the much enhanced catalytic effect by the addition of small organic acids (SOAs). In a typical case, all the added diclofenac (30 μmol L−1) was degraded in 20 min by using 2 μmol L−1 Co2+, 0.15 mmol L−1 PMS and 0.5 mmol L−1 acetate with a degradation rate constant of 0.482 min−1, which was about 10 times higher than that (0.048 min−1) of equivalent Co2+-PMS system without acetate. The formation of SO4radical dot was greatly enhanced by introducing acetate, and this novel system is universal for enhanced degradation of various organic pollutants. Similarly, formate, propionate, and butyrate also exhibited enhancing effects on the catalytic ability of Co2+. The enhancement mechanism of SOAs on catalytic activation of PMS by low-level Co2+ was also proposed.

Introduction

Over-growing organic pollutants in surface and underground water have attracted extensive attention due to their adverse effects to the quality of human life through food chains or/and environmental cycles [1]. Most of the organic pollutants come from the disposal of industrial and civil wastewater. To control organic pollution from the wastewater, advanced oxidation processes (AOPs), such as Fenton and Fenton-like processes, have attracted increasing attentions due to their potential capability of generating strong oxidizing species like hydroxyl radical (radical dotOH) to degrade organic pollutants into innocuous or lowly toxic small compounds, and even harmless carbon dioxide and water [2,3]. As the Fenton reagents, the combination use of Fe2+ and H2O2 requires a narrow acidic operation pH (2.5–3.5), and conventional Fenton processes often produce a significant accumulation of iron-containing sludge, which limits their practical applications [4,5]. Therefore, sulfate radical (SO4radical dot) based AOPs are developed as a promising alternative, which can work in a wide pH range [6,7].

Typically, SO4radical dot is generated from the activation of peroxymonosulfate (PMS) and persulfate. PMS is possibly activated by various transition-metal ions, and Co2+ is found to be the best non-noble metal catalyst for activating PMS [8]. In the reported homogenous Co2+-PMS systems, the Co2+ addition is usually in a range from 100 to 300 μmol L−1 [[9], [10], [11], [12], [13]], and it may be as high as 1244 and 7675 μmol L−1 in some cases [8,14]. Such an addition of high-level Co2+ will induce a risk of secondary pollution, because the added level of Co2+ is much higher than its emission limit in water specified by many countries. For example, this is tens to hundreds times higher than the emission limit in water (17 μmol L−1) specified in China according to Chinese standard of GB 25467-2010. Heterogeneous cobalt-based catalysts have been developed to decrease the addition of dissolved Co2+ [[15], [16], [17], [18], [19], [20], [21]]. Recently, we reported a catalytic and adsorptive bifunctional material of cobalt particles encapsulated and nitrogen-doped bamboo-like carbon nanotubes (Co/N-CNTs) for efficient removal of organic pollutants from wastewater in the presence of PMS [22]. In general, the reported heterogeneous cobalt-based catalysts permitted a wide working pH range, and limited the leaching of Co2+ at fairly low concentrations below 20 μmol L−1 under optimal conditions [16,[23], [24], [25], [26]]. However, such heterogeneous catalysts would decrease the utilization efficiency of Co active sites, because only a small amount of cobalt on the surface involved in the catalytic reaction [27]. In view of the maximum atomic utilization of Co2+, it is significant to develop a promising strategy with only a very small Co2+ addition for the activation of PMS and the degradation of organic pollutants.

A generally accepted mechanism for the PMS activation by Co2+ can be described as follows,Co2+ + H2O → CoOH+ + H+CoOH+ + HSO5 → CoO+ + SO4radical dot + H2OCoO+ + 2H+ → Co3+ + H2OCo3+ + HSO5 → Co2+ + SO5radical dot + H+

The formation of CoOH+ (Eq. (1)), which was reported to facilitate the PMS activation [28,29], is important for the generation of SO4radical dot radicals in Eq. (2) accompanying with the oxidation of Co2+ to Co3+. The regeneration of Co2+ is achieved by Eq. (4) along with the generation of SO5radical dot radicals. Unfortunately, the reaction rate of Co3+ and PMS is much slower than that of Co2+ and PMS (Eqs. (1) and (2)), which was proved to be the rate-limiting step, especially at a low level of Co2+ [30]. Therefore, it is of great importance to realize efficient Co3+/Co2+ cycle in a homogenous Co2+-PMS system with low-level Co2+ for the degradation of organic pollutants.

Small organic acids (SOAs) as electron donors have been observed to enhance the photocatalytic removal of Cr(VI) [31] and the photocatalytic degradation of ronidazole [32]. We speculated that SOAs could improve the cycle of Co3+/Co2+ due to their electron donating ability. Therefore, the main objectives of this study were (1) to develop a low-level Co2+ catalytic system for the degradation of organic pollutants with the aid of SOAs, (2) to investigate the generality for the enhancement effect of SOAs, and (3) to propose the enhancement mechanism of SOAs for the catalytic activation of PMS by low-level Co2+.

Section snippets

Chemicals and materials

Co(NO3)2⋅6H2O, sodium formate, sodium acetate (NaAc), sodium propionate, rhodamine B (RhB), methylene blue (MB), methyl orange (MO), methanol (MeOH), and tert-butyl alcohol (TBA) were provided by Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Sodium butyrate, diclofenac (DCF), bisphenol A (BPA), sulfadiazine and tartrazine were bought from Aladdin Chemistry Co., Ltd (Shanghai, China). Oxone (2KHSO5·KHSO4·K2SO4), a favorable source of PMS, was purchased from Energy Chemical (Shanghai,

Catalytic degradation of diclofenac by low-level Co2+ with the assistance of acetate

DCF, a typical pharmaceutical, was selected as a model organic pollutant to investigate the catalytic oxidative degradation of organic po llutants by low-level Co2+, due to its risk of unpredictable long-term effects [33,34]. Fig. 1a showed the degradation kinetics of DCF in different reaction systems. No degradation of DCF occurred in the Co2+-Ac system, and the degradation of DCF was also negligible in the PMS-Ac system. It was reported that organic pollutants, such as atrazine and azo dyes

Conclusions

In this work, a novel method for the efficient degradation of organic pollutants was successfully developed by using low-level Co2+ catalyzed homogeneous activation of PMS with the assistant of SOAs. The addition of SOAs can greatly reduce the required Co2+ addition for activating PMS effectively through their enhancing effect on the catalytic performance of low-level Co2+. The catalytic activation of PMS by Co2+ produces SO4radical dot as main reactive species, and the generation of SO4radical dot is enhanced by

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants Nos. 21777194 and 21477043).

References (40)

  • X. Li et al.

    FexCo3-xO4 nanocages derived from nanoscale metal-organic frameworks for removal of bisphenol A by activation of peroxymonosulfate

    Appl. Catal. B

    (2016)
  • X.K. Tian et al.

    Controlled synthesis of dandelion-like NiCo2O4 microspheres and their catalytic performance for peroxymonosulfate activation in humic acid degradation

    Chem. Eng. J.

    (2018)
  • W. Tian et al.

    Bread-making synthesis of hierarchically Co@C nanoarchitecture in heteroatom doped porous carbons for oxidative degradation of emerging contaminants

    Appl. Catal. B

    (2018)
  • M. Chen et al.

    Cobalt particles encapsulated and nitrogen-doped bamboo-like carbon nanotubes as a catalytic and adsorptive bifunctional material for efficient removal of organic pollutants from wastewater

    J. Environ. Chem. Eng.

    (2017)
  • Y. Hardjono et al.

    Synthesis of Co oxide doped carbon aerogel catalyst and catalytic performance in heterogeneous oxidation of phenol in water

    Chem. Eng. J.

    (2011)
  • X. Lin et al.

    LiCoPO4 (LCP) as an effective peroxymonosulfate activator for degradation of diethyl phthalate in aqueous solution without controlling pH: efficiency, stability and mechanism

    Chem. Eng. J.

    (2017)
  • W. Zhang et al.

    Supported cobalt oxide on MgO: highly efficient catalysts for degradation of organic dyes in dilute solutions

    Appl. Catal. B

    (2010)
  • P. Shi et al.

    Synergistic catalysis of Co3O4 and graphene oxide on Co3O4/GO catalysts for degradation of Orange II in water by advanced oxidation technology based on sulfate radicals

    Chem. Eng. J.

    (2014)
  • Q. Yang et al.

    Nanocrystalline cobalt oxide immobilized on titanium dioxide nanoparticles for the heterogeneous activation of peroxymonosulfate

    Appl. Catal. B

    (2007)
  • F.J. Rivas et al.

    Aqueous pharmaceutical compounds removal by potassium monopersulfate. Uncatalyzed and catalyzed semicontinuous experiments

    Chem. Eng. J.

    (2012)
  • Cited by (0)

    View full text