At present, the production of SO₄^●− mainly comes from the activation of PMS and PS, which are the monosubstituted or symmetrically substituted derivatives of hydrogen peroxide by sulfonic acid group (-SO₃), respectively. PMS has been widely used in organic compound synthesis and as a chlorine-free additive for disinfecting swimming pools at a rate of about 1–2 pounds per 10,000 gallons of pool water ^[1][11]. Under the presence of 25 mg/L PMS and 0.1 mg/L Co²⁺, the removal rate of E. coli reached 99.99% after 1 h of reaction ^[2][12]. PMS is white solid powder. It is stable when pH is less than 6 or pH is 12. When pH is 9, it showed the poorest stability where half of HSO₅⁻ decomposes to SO₅^{2− [3]}[4]. At present, the widely used potassium bisulfate complex salt (2KHSO₅·KHSO₄·K₂SO₄) is composed of three components, potassium peroxymonosulfate, potassium hydrogen sulfate, and potassium sulphate, and its main active substance is potassium peroxymonosulfate (KHSO₅). The salt is marketed under the trade names Caroat and Oxone registered by Evonik and DuPont, respectively. Oxone is a white granular powder crystal salt, which is stable, non-toxic, inexpensive, and soluble in water. The peroxide bond (O-O) distance is 1.453 Å, and the bond energy is 140–213.3 kJ/mol. PMS is most stable when the solution pH is less than 6 and equal to 12. When pH is 9, the stability is worst, and the concentration of HSO₅⁻ and SO₅²⁻ in the solution is almost equal. When pH is less than 1, PMS will undergo hydrolysis reaction to produce H₂O₂ ^[4][13]. PS, an oxidant with symmetrical structure, was first known as the initiator of polymerization reaction. The O-O distance is 1.497 Å, and the bond energy is 140 kJ/mol ^[5][14]. PS, often in the form of potassium persulfate or sodium persulfate, has been widely used as bleaching agents, oxidants, emulsion polymerization promoters, and water or soil remediation agents. The related properties of PMS and PS are shown in Table 1. Both PMS and PS are strong oxidants, but their direct reaction rates with most pollutants are very low. Therefore, it is necessary to activate them through appropriate ways to destroy the O-O bond and generate strong oxidizing free radicals, ¹O₂ and other ROS to degrade organic pollutants quickly and efficiently.

Because PS and PMS are solid powders, they can be transported and stored more easily. Compared with H₂O₂, the anions of PS and PMS remain stable in water for a much longer time until they are properly activated. In addition, PMS and PS-based AOPs can proceed smoothly in a wide solution pH range from acidic to alkaline (pH = 2–10), while H₂O₂-based Fenton process requires strict acidic conditions (pH = 2.7–3). Generally, PS and PMS can be activated with the assistance of ultraviolet light, heat, alkali, or metal catalysts, etc. Different types of oxidants and activation methods will produce different ROS. The advanced oxidation processes dominated by SO₄^●− and ¹O₂ were discussed in this rpapesearchr.

advanced oxidation process (AOPs) based on sulfate radical (SO₄^●−) and singlet oxygen (¹O₂) has attracted a lot of attention because of its characteristics of rapid reaction, efficient treatment, safety and stability, and easy operation. SO₄^●− and ¹O₂ mainly comes from the activation reaction of peroxymonosulfate (PMS) or persulfate (PS), which represent the oxidation reactions involving radicals and non-radicals, respectively. The degradation effects of target pollutants will be different due to the type of oxidant, reaction system, activation methods, operating conditions, and other factors.

2. Sulfate Radicals-Based Advanced Oxidation

Studies have shown that AOPs based on SO₄^●− and ^●OH is an effective method for the degradation of refractory organic pollutants, such as pharmaceuticals, pesticides, personal care products, steroids, endocrine disruptors, etc. ^[6][15]. However, AOPs based on ^●OH degrades organic pollutants through a non-selective, multi-step approach that typically requires an acidic environment. In addition, the oxidation process is severely limited by the large amount of dissolved organic matter and anions in complex environments, which are the main scavengers of ^●OH. SO₄^●− is inherently more oxidizing than ^●OH and lasts longer in aqueous solutions, and in some cases SO₄^●− can oxidize contaminants that ^●OH cannot. In recent years, SO₄^●−-based AOPs have replaced ^●OH-based AOPs to some extent. The essence of the advanced oxidation process based on SO₄^●− is to activate PMS or PS to form SO₄^●− to achieve the removal of organic matter. Besides SO₄^●−, there are also associated or indirect generation of ^●OH, superoxide radical (O₂^●−), or other radicals, but SO₄^●− plays a leading role in the degradation of pollutants. These free radicals (SO₄^●−, ^●OH, and O₂^●−) could be detected by using 5,5-dimethyl-1-pyrroline-1- oxide (DMPO) as a spin trapping agent in an electron paramagnetic resonance spectroscopy (EPR) ^[7][16]. Both ethanol (Et) and tert-butanol (TBA) could quench ^●OH rapidly (k_Et = 1.2–2.8 × 10⁹ M⁻¹s⁻¹, k_TBA = 3.8–7.6 × 10⁸ M⁻¹s⁻¹), and the reaction rate between Et and SO₄^●− is much faster than that of TBA (k_Et = 1.6–7.7 × 10⁷ M⁻¹s⁻¹, k_TBA = 4.0–9.1 × 10⁵ M⁻¹s⁻¹). Therefore, Et and TBA can be used as capture agents to identify who contributes more to the degradation of pollutants ^[8][17]. PMS and PS can be activated to produce SO₄^●− through energy input, transition metal ions and their oxides, non-metallic materials, etc. Studies on the degradation of pollutants by activating PMS and PS in different ways to generate SO₄^●− are summarized in Table 2.

3. Singlet Oxygen-Based Advanced Oxidation

With the report of PMS and PS non-radical activation, people gradually have a strong interest in the process of non-radical oxidation. Non-radical reactive species are generally thought to be resistant to common free radical scavengers (e.g., methanol, ethanol, and tert-butanol), selective to electron-rich organic compounds, and particularly sensitive to organic substrates with mild redox potentials ^[37][8]. At present, non-radical oxidation processes have been found in a variety of reaction systems, especially in PMS and PS activation. However, the mechanism remains controversial. Based on current reports, carbon or metal catalysts mainly realize non-radical activation of oxidants (PMS, PS, H₂O₂, or O₃) through three ways: electron transfer process, generation of activated complexes (or surface-bound radicals) and singlet oxygen participating in pollutant degradation, respectively, as shown in Figure 14. Among them, ¹O₂ shows significant advantages in the selective removal of organic pollutants, which has become the focus of research.