Elsevier

Water Research

Volume 222, 15 August 2022, 118896
Water Research

Review
Influences and mechanisms of phosphate ions onto persulfate activation and organic degradation in water treatment: A review

https://doi.org/10.1016/j.watres.2022.118896Get rights and content

Highlights

  • The effects and mechanisms of phosphate ions on persulfate (PS) activation are discussed.

  • Phosphate ions favor to attack asymmetric PMS compared to PDS.

  • The research directions of phosphate ions in PS-AOPs are proposed.

Abstract

Currently, various strategies have been applied to activate persulfate (PS) for contaminant removal from water. However, the background phosphate ions in water affect PS activation and organic degradation, and the mechanism of their influence on the processes is still controversial. In this review, the possible effects of different phosphate forms (HPO42−, H2PO4, and PO43−) on PS activation and contaminant degradation were systematically evaluated and summarized. Specifically, HPO42− promotes contaminant degradation in direct peroxymonosulfate (PMS) oxidation and thermal/PMS systems, while it exhibits inhibition to thermal/peroxodisulfate (PDS) and ultraviolet (UV)/PDS systems. Meanwhile, H2PO4 inhibits most oxidation processes based on PMS and PDS, except for non-metal dominated and metal assisted PMS systems. Coexisting HPO42− and H2PO4 could present beneficial effects in thermal, Co2+ and non-metal activated and metal assisted PMS systems. Nevertheless, their inhibitory effects were found in direct PMS oxidation, UV/PMS (or PDS) and metal dominated PMS systems. Generally, phosphate ions inhibit PMS/PDS activation through competing adsorption with PMS or PDS on the solid surface, forming a complex with metal ions, as well as occupying active sites on solid catalysts. In addition, phosphate ions can quench radicals for reduced degradation of contaminants. However, phosphate ions could weaken the bond dissociation energy via combining with PMS and contaminants or form a complex with Co2+, thus displaying a facilitative effect. This review further discusses major challenges and opportunities of PS activation with co-existing phosphates and will provide guidance for better PS utilization in real water treatment practice.

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).H3PO4K1H2PO4K2HPO42K3PO43

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).

References (96)

  • Y. Ding et al.

    Nonradicals induced degradation of organic pollutants by peroxydisulfate (PDS) and peroxymonosulfate (PMS): recent advances and perspective

    Sci. Total Environ.

    (2021)
  • P. Duan et al.

    Effect of phosphate on peroxymonosulfate activation: accelerating generation of sulfate radical and underlying mechanism

    Appl. Catal. B Environ.

    (2021)
  • P. Duan et al.

    Enhanced degradation of clothianidin in peroxymonosulfate/catalyst system via core-shell FeMn@N-C and phosphate surrounding

    Appl. Catal. B Environ.

    (2020)
  • H. Fu et al.

    Activation of peroxymonosulfate by graphitized hierarchical porous biochar and MnFe2O4 magnetic nanoarchitecture for organic pollutants degradation: structure dependence and mechanism

    Chem. Eng. J.

    (2019)
  • J. Fu et al.

    Electrochemical activation of peroxymonosulfate (PMS) by carbon cloth anode for sulfamethoxazole degradation

    Chemosphere

    (2022)
  • M.A. Fulazzaky et al.

    Precipitation of iron-hydroxy-phosphate of added ferric iron from domestic wastewater by an alternating aerobic–anoxic process

    Chem. Eng. J.

    (2014)
  • Y. Gao et al.

    Pyrene degradation in an aqueous system using ferrous citrate complex activated persulfate over a wide pH range

    J. Environ. Chem. Eng.

    (2021)
  • A. Ghauch et al.

    Oxidation of bisoprolol in heated persulfate/H2O systems: kinetics and products

    Chem. Eng. J.

    (2012)
  • Y. Hong et al.

    Heterogeneous activation of peroxymonosulfate by CoMgFe-LDO for degradation of carbamazepine: efficiency, mechanism and degradation pathways

    Chem. Eng. J.

    (2020)
  • L. Hu et al.

    Enhanced degradation of Bisphenol A (BPA) by peroxymonosulfate with Co3O4-Bi2O3 catalyst activation: effects of pH, inorganic anions, and water matrix

    Chem. Eng. J.

    (2018)
  • B. Huang et al.

    Ultrafast degradation of contaminants in a trace cobalt(II) activated peroxymonosulfate process triggered through borate: indispensable role of intermediate complex

    J. Hazard. Mater.

    (2022)
  • W. Huang et al.

    Activation of persulfates by carbonaceous materials: a review

    Chem. Eng. J.

    (2021)
  • X. Huang et al.

    Mechanism insight into efficient peroxydisulfate activation by novel nano zero-valent iron anchored yCo3O4(nZVI/yCo3O4) composites

    J. Hazard. Mater.

    (2020)
  • Y.H. Huang et al.

    Efficient decolorization of azo dye reactive black B involving aromatic fragment degradation in buffered Co2+/PMS oxidative processes with a ppb level dosage of Co2+-catalyst

    J. Hazard. Mater.

    (2009)
  • Z. Huang et al.

    Novel green activation processes and mechanism of peroxymonosulfate based on supported cobalt phthalocyanine catalyst

    Appl. Catal. B Environ

    (2014)
  • A.A. Isari et al.

    Peroxymonosulfate catalyzed by core/shell magnetic ZnO photocatalyst towards malathion degradation: Enhancing synergy, catalytic performance and mechanism

    Sep. Purif. Technol.

    (2021)
  • Y. Ji et al.

    Non-activated peroxymonosulfate oxidation of sulfonamide antibiotics in water: kinetics, mechanisms, and implications for water treatment

    Water Res.

    (2018)
  • J. Jia et al.

    Visible-light-induced activation of peroxymonosulfate by TiO2 nano-tubes arrays for enhanced degradation of bisphenol A

    Sep. Purif. Technol.

    (2020)
  • D. Kanakaraju et al.

    TiO2 photocatalysis of naproxen: effect of the water matrix, anions and diclofenac on degradation rates

    Chemosphere

    (2015)
  • J. Kang et al.

    The enhanced peroxymonosulfate-assisted photocatalytic degradation of tetracycline under visible light by g-C3N4/Na-BiVO4 heterojunction catalyst and its mechanism

    J. Environ. Chem. Eng.

    (2021)
  • Y.G. Kang et al.

    Fe(III) adsorption on graphene oxide: a low-cost and simple modification method for persulfate activation

    Chem. Eng. J.

    (2020)
  • C. Li et al.

    Highly efficient activation of peroxymonosulfate by natural negatively-charged kaolinite with abundant hydroxyl groups for the degradation of atrazine

    Appl. Catal. B Environ.

    (2019)
  • C.X. Li et al.

    Interactions between chlorophenols and peroxymonosulfate: pH dependency and reaction pathways

    Sci. Total. Environ.

    (2019)
  • J. Li et al.

    Enhancement of the degradation of atrazine through CoFe2O4 activated peroxymonosulfate (PMS) process: kinetic, degradation intermediates, and toxicity evaluation

    Chem. Eng. J.

    (2018)
  • M. Li et al.

    Iron-tannic modified cotton derived Fe0/graphitized carbon with enhanced catalytic activity for bisphenol A degradation

    Chem. Eng. J.

    (2019)
  • X. Li et al.

    High-efficiency degradation of organic pollutants with Fe, N co-doped biochar catalysts via persulfate activation

    J. Hazard. Mater.

    (2020)
  • Y. Li et al.

    Visible light induced efficient activation of persulfate by a carbon quantum dots (CQDs) modified γ-Fe2O3 catalyst

    Chin. Chem. Lett.

    (2020)
  • F. Liu et al.

    Heterogeneous degradation of organic contaminant by peroxydisulfate catalyzed by activated carbon cloth

    Chemosphere

    (2020)
  • J. Liu et al.

    Phosphate adsorption on hydroxyl–iron–lanthanum doped activated carbon fiber

    Chem. Eng. J.

    (2013)
  • X. Lou et al.

    Peroxymonosulfate activation by phosphate anion for organics degradation in water

    Chemosphere

    (2014)
  • J. Ma et al.

    Impacts of inorganic anions and natural organic matter on thermally activated persulfate oxidation of BTEX in water

    Chemosphere

    (2018)
  • G. Mamba et al.

    Graphitic carbon nitride (g-C3N4) nanocomposites: a new and exciting generation of visible light driven photocatalysts for environmental pollution remediation

    Appl. Catal. B Environ.

    (2016)
  • L.W. Matzek et al.

    Activated persulfate for organic chemical degradation: a review

    Chemosphere

    (2016)
  • M. Nie et al.

    Degradation of chloramphenicol by persulfate activated by Fe2+ and zerovalent iron

    Chem. Eng. J.

    (2015)
  • G. Peng et al.

    Activation of peroxymonosulfate by phosphite: kinetics and mechanism for the removal of organic pollutants

    Chemosphere

    (2021)
  • J. Peng et al.

    Degradation of atrazine by persulfate activation with copper sulfide (CuS): kinetics study, degradation pathways and mechanism

    Chem. Eng. J.

    (2018)
  • W.Y. Peng et al.

    Non-radical reactions in persulfate-based homogeneous degradation processes: a review

    Chem. Eng. J.

    (2021)
  • Y.F. Rao et al.

    Degradation of carbamazepine by Fe(II)-activated persulfate process

    J. Hazard. Mater.

    (2014)
  • Cited by (97)

    View all citing articles on Scopus
    1

    These authors contributed equally to this work.

    View full text