Efficient degradation of organic pollutants by low-level Co2+ catalyzed homogeneous activation of peroxymonosulfate
Graphical abstract
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 (OH) 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 (SO4−) based AOPs are developed as a promising alternative, which can work in a wide pH range [6,7].
Typically, SO4− 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+ + SO4− + H2OCoO+ + 2H+ → Co3+ + H2OCo3+ + HSO5− → Co2+ + SO5− + H+
The formation of CoOH+ (Eq. (1)), which was reported to facilitate the PMS activation [28,29], is important for the generation of SO4− 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 SO5− 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 SO4− as main reactive species, and the generation of SO4− is enhanced by
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grants Nos. 21777194 and 21477043).
References (40)
- et al.
Performance of magnetic activated carbon composite as peroxymonosulfate activator and regenerable adsorbent via sulfate radical-mediated oxidation processes
J. Hazard. Mater.
(2015) - et al.
Bromate formation in bromide-containing water through the cobalt-mediated activation of peroxymonosulfate
Water Res.
(2015) - et al.
Degradation of sulfamonomethoxine with Fe3O4 magnetic nanoparticles as heterogeneous activator of persulfate
J. Hazard. Mater.
(2011) - et al.
Degradation of atrazine by cobalt-mediated activation of peroxymonosulfate: different cobalt counteranions in homogenous process and cobalt oxide catalysts in photolytic heterogeneous process
Water Res.
(2009) - et al.
Application of a peroxymonosulfate/cobalt (PMS/Co(II)) system to treat diesel-contaminated soil
Chemosphere
(2009) - et al.
Oxidative degradation of dyes in water using Co2+/H2O2 and Co2+/peroxymonosulfate
J. Hazard. Mater.
(2010) - et al.
Effects of chloride ions on bleaching of azo dyes by Co2+/oxone reagent: kinetic analysis
J. Hazard. Mater.
(2011) - et al.
Magnetic CoFe2O4 nanoparticles supported on titanate nanotubes (CoFe2O4/TNTs) as a novel heterogeneous catalyst for peroxymonosulfate activation and degradation of organic pollutants
J. Hazard. Mater.
(2016) - et al.
Enhancing the catalytic activity of g-C3N4 through Me doping (Me = Cu, Co and Fe) for selective sulfathiazole degradation via redox-based advanced oxidation process
Chem. Eng. J.
(2017) - et al.
Co3O4 nanocrystals/3D nitrogen-doped graphene aerogel: a synergistic hybrid for peroxymonosulfate activation toward the degradation of organic pollutants
Chemosphere
(2018)