ReviewInfluences and mechanisms of phosphate ions onto persulfate activation and organic degradation in water treatment: A review
Graphical abstract
Introduction
Recently, persulfates (PS)-based advanced oxidation processes (PS-AOPs) for organic pollutant degradation have attracted increasing attention. Commonly, PS have two types of ions, peroxymonosulfate (PMS, HSO5−) and peroxodisulfate (PDS, S2O82−) (Devi et al., 2016). According to previous reports, PS cannot directly degrade most persistent organic pollutants effectively due to low oxidation potential (E0(PDS) = 2.01 V; E0(PMS) = 1.82 V) (Peng et al., 2021b). Encouragingly, PS exhibit promising selective oxidation sometimes based on the electron-rich moieties of target pollutants (Zhou et al., 2020b). Thus, it is necessary to activate PS to generate more oxidative species such as sulfate radicals (SO4·−), hydroxyl radicals (·OH) and singlet oxygen (1O2) for contaminant degradation (Dai et al., 2021).
PS-AOPs in terms of sulfate radicals possess remarkable advantages of a high redox potential (2.5–3.1 V), wide pH range (4–9) and long half-life (30–40 μs) (Zhao et al., 2021; Zhou et al., 2018b). Many methods have been applied to activate PS, using external energy (e.g., UV, heat, ultrasound), transition metal- and carbon-based catalysts (Yan et al., 2020; Li et al., 2020a). Among them, exterior energy activation might be restricted in practical applications, as it requires more energy input (Matzek and Carter, 2016; Tan et al., 2017; Teel et al., 2009). Transition metal ions might induce secondary pollution and suffer from recycling limitation in the process of PS activation (Li et al., 2019a). Attractively, heterogeneous catalysts could address the above mentioned issues (Wu et al., 2019). Especially, carbon-based catalysts possess a high surface area and large pore volume, abundant functional groups, and high conductivity, thus receiving widespread attentions (Cheng et al., 2019). Furthermore, transition metals could be immobilized on carbon materials, exhibiting excellent stability and reusability for practical applications (Kang et al., 2020; Li et al., 2020b; Wu et al., 2018).
Phosphate ions exist widely in water environment, mainly in the forms of H3PO4, H2PO4−, HPO42− and PO43−. The dissociation balance of phosphate ions in water depends on solution pH, as shown in Eq. (1).
Where, the values are pK1 = 2.15, pK2 = 7.20 and pK3 = 12.33 (Perrin and Dempsey, 1974). Therefore, the solution pH will determine the dominant form of phosphate (Fig. 1).
The impact of phosphate ions has been reported to be either positive, negative, or negligible in different PS systems. For instance, Lou et al. (2014) prepared a phosphate buffer solution (PBS) with coexistence of H2PO4− and HPO42− at pH = 7.0. The degradation rate constant (kobs) of Acid Orange 7 (AO7) increased from 1.22 × 10−4 to 3.95 × 10−3 s−1 as the PBS concentration changed from 10.0 to 100.0 mM (Lou et al., 2014). However, the rate constant of orange II degradation decreased by 67.0% when 100.0 mM of H2PO4− was added to a MnFe2O4/(corn stems biochar)/PMS system (Fu et al., 2019). In addition, the removal rate of total organic carbon reduced slightly from 28.0% to 26.0% after adding 2.0 mM PO43− in a Co1.51Fe1.49O4/PMS system (initial pH = 6.5) (Yang et al., 2019), revealing a negligible effect of the phosphate ion on bisphenol A (BPA) mineralization. Noticeably, one phosphate form could have different effects on various PS-AOPs systems. For example, H2PO4− could inhibit tetracycline oxidation in a nanoscale zero valent iron (nZVI)/yCo3O4/PDS system (Huang et al., 2020), while promote norfloxacin degradation in a Co3O4@Fe2O3/PMS system (Chen et al., 2019). Besides, HPO42− also showed different effects on PS activation systems. Atrazine (ATZ) removal was inhibited by HPO42− in a CoFe2O4 activated PMS system (Li et al., 2018). However, decolorization of AO7 was promoted significantly in the presence of HPO42− in a HPO42−/PMS system (Peng et al., 2021a). Interestingly, varying effects were also observed after adding different forms of phosphate to one PS system. For example, H2PO4− could accelerate BPA degradation while HPO42− presented an inhibition effect in a CoMg-BO/PMS system (Dan et al., 2022).
Although the effects of phosphate on contaminant degradation have been reported in many PS-AOPs systems, the roles of phosphate ions in the processes were not well clarified. Meanwhile, no literature has summarized the mechanisms of phosphate ions in various PS systems. Herein, the main aims of this review are to (1) comprehensively assess the roles of phosphate ions in PS systems; (2) summarize the various mechanisms of phosphate ions in different PS systems; and (3) provide some references for future investigations of a PS system towards phosphate-containing organic degradation in wastewater. This review will help promote the strategic development and application of PS systems in practical wastewater treatment.
Section snippets
Influence of phosphate ions on direct oxidation by PS
PS-AOPs exhibited strong oxidation ability, reactive stability, and controllable oxidation potential in removing organic pollutants. However, the role of inactivated PS is often overlooked during contaminant oxidation (Ji et al., 2018; Yang et al., 2018; Zhou et al., 2018a). Direct oxidation by PS is a non-radical process, in which the electron-rich organics could transfer electrons to PS and then be oxidized (Peng et al., 2021b). The degradation process involves three pathways of two-electron,
Influence of phosphate ions on energy activated PS systems
The generation of oxidative species (e.g., SO4·−, ·OH and 1O2) is the key for rapid degradation of contaminants, and could be achieved by UV, visible light, thermal, and electrochemical activation of PS (Wacławek et al., 2017). UV activation could break O-O bond in PS, producing free radicals (Eqs. (4) and (5)) (Anipsitakis and Dionysiou, 2004b). Similarly, heating could also activate PS. As it is known, visible light occupies 45% of the solar spectrum and it is promising for its application in
Homogeneous catalysis
For PS activation by transition metal ions, single electron could transfer from metal ions to PDS or PMS (Ding et al., 2021), generating SO4·− and ·OH (Eqs. (28)–(30)). As it is known, Co2+ exhibited higher activity towards PMS activation than other metals (Anipsitakis and Dionysiou, 2004a). Meanwhile, iron was environmental-friendly and was considered as an effective activator for PDS (Song et al., 2020). Thus, the roles of phosphate ions (i.e., H2PO4− and the coexisting HPO42− and H2PO4−) in
Conclusions and Perspectives
As discussed above, the role of phosphate ions in a PS-AOPs system for water treatment has been investigated widely. The influences of various phosphate ions (e.g., HPO42−, and H2PO4−) on PS activation and contaminant degradation were analyzed systematically. Main conclusions are summarized as follows.
- (1)
In a PMS system without activation, HPO42− and coexisting HPO42− and H2PO4− at high concentrations could promote pollutant degradation by activating PMS to produce oxidative species, thus
CRediT authorship contribution statement
Ning Li: Conceptualization, Methodology, Formal analysis, Writing – review & editing. Yanshan Wang: Formal analysis, Writing – review & editing. Xiaoshuang Cheng: Writing – review & editing, Formal analysis, Writing – review & editing. Haoxi Dai: Writing – review & editing. Beibei Yan: Conceptualization, Writing – review & editing, Supervision. Guanyi Chen: Formal analysis, Writing – review & editing. Li'an Hou: Writing – review & editing. Shaobin Wang: Writing – review & editing, Supervision.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52100156) and Shenzhen Science and Technology Program (GJHZ20200731095801005 and JCYJ20200109150210400).
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These authors contributed equally to this work.