Activated persulfate by iron-based materials used for refractory organics degradation: a review

In recent decades, with the increasing manufacture and the use of synthetic chemicals, a lot of wastewater containing some refractory organics is produced. Wastewater carrying these organics is usually collected by pipelines and sent to centralized wastewater treatment plants (WWTPs) for processing. However, the removal of refractory organics in the WWTPs is not complete by the biological treatment-based secondary treatment processes. It is necessary to further treat refractory organics in a more efficient way after secondary treatment processes in order to reduce the organic pollution load of the receiving water bodies.

The advanced oxidation processes (AOPs) are currently considered to be the best processes to remove refractory organics because of their strong ability to break chemical bonds of refractory organics (Fast et al. 2017). The AOPs initially investigated were based on hydroxyl radicals (HRs, HO^•) produced in situ to degrade organic compounds. Two oxidants, hydrogen peroxide (H₂O₂) and ozone (O₃) are activated by ultraviolet (UV) or ferrous ions (Fe²⁺) to produce HRs (Deng & Zhao 2015).The main processes consisting of H₂O₂ or O₃ are H₂O₂/UV, O₃/UV, O₃/H₂O₂/UV, and Fe²⁺/H₂O₂ which often called the Fenton process (Csay et al. 2014; Ghatak 2014). Based on Fenton process, photo-Fenton (Klamerth et al. 2013; Pouran et al. 2015), electro-Fenton (Nidheesh & Gandhimathi 2012; Yahya et al. 2014), sono-Fenton (Ioan et al. 2007; Ghauch et al. 2011b), and sono-photo-Fenton (Babuponnusami & Muthukumar 2011) have also been extensively studied. However, the Fenton-based processes have several defects: Both H₂O₂ and O₃ are unstable, difficult to store and transport, and used in large quantities; the sludge generated in the reaction is difficult to treat; the optimal pH value is around 3.0, which will increase pH regulating agents (Neyens & Baeyens 2003).

In recent years, the AOPs based on sulfate radicals (SRs, SO₄^•−) are widely studied in degradation of organic pollutants because they overcome some shortcomings of AOPs-HRs. For instance, SRs have higher oxidation potential (E⁰ = 2.5–3.1 V) and longer half-life (t_1/2 = 30–40 μs) than that of HRs (E⁰ = 1.7–2.8 V, t_1/2 ≤ 1 μs), and can selectively react with unsaturated or aromatic compounds under a wide pH range of 2.0–8.0 (Olmez-Hanci & Arslan-Alaton 2013; Ike et al. 2018). PS (E⁰ = 2.01 V) can be activated to produce SRs in the presence of energy (UV, ultrasonic, and heat) (Guo et al. 2014; Darsinou et al. 2015; Ji et al. 2015), transition metals (Fe, Co, Ni, Cu, Ag, and Mn) (Liu et al. 2014; Rodriguez et al. 2014; Zhang et al. 2015a, 2015b; Kang et al. 2018), and carbon (Cheng et al. 2017). Among these activators, iron has been extensively studied in activating PS to generate SRs for refractory organics degradation due to its environmental friendliness, low price and excellent activation (Matzek & Carter 2016). Prior to PS, iron was even used as stand alone technology for the removal of refractory organics among them pesticides and pharmaceuticals(Ghauch 2008; Ghauch & Tuqan 2008, 2009; Ghauch et al. 2009, 2010a, 2010b,, 2011a). However, high concentrations of iron and iron-based materials were needed creating additional sludge formation. Therefore, researchers proposed iron use at trace level in combination with chemicals able to produce fugitive reactive oxygen species (ROS) (Ghauch 2015). Advantages of PS include chemical stability, ease of storage and transport, high solubility in water, and low cost, which broaden the application of PS in wastewater treatment (Tsitonaki et al. 2010). In addition, Sodium PS (Na₂S₂O₈) and potassium PS (K₂S₂O₈) are commonly used in research and pilot projects because they produce no secondary pollutants to water. Although high concentrations of sulfate species could be considered at risk which is not the case for PS concentration in the range of 0.1–20 mM (Liu & Wu 2018).

From the published literature available, some scholars have given an overview of the activated methods of sulfate radicals and their application in wastewater treatment (Zhang et al. 2015a, 2015b; Matzek & Carter 2016; Oh et al. 2016; Brienza & Katsoyiannis 2017; Wacławek et al. 2017; Guerra-Rodríguez et al. 2018; Xiao et al. 2018, 2019; Oh & Lim 2019). However, a detailed review about activated persulfate by iron-based materials used for refractory organics degradation has not been reported yet. Therefore, a specific and deep review on the PS activated by iron-based materials can provide state-of-the-art information for the future research of the AOPs-SRs. This paper reviews the recent advances of mechanism and application of some iron-based materials in activating PS for refractory organics degradation, and the insights towards future application of these activating methods of PS for efficient degradation of refractory organics is summarized and discussed as well.

In order to better understand the physicochemical properties of iron-based materials and analyze the morphology and structure of the materials, some methods of characterizing materials are necessary. There are several analytical methods such as SEM (scanning electron microscopy), TEM (transmission electron microscopy), EDS (energy dispersive system), XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), FTIR (Fourier transform infrared spectrometry), TGA (thermo gravimetric analysis) and BET (Brunauer-Emmett-Teller) method. The detection objects of these analysis methods are shown in Table 1.

PS and Fe²⁺ concentration in solution are critical parameters to generate SO₄^•−, and the molar ratio of Fe²⁺ to PS also determines the SO₄^•− yield. Usually, the reaction to remove refractory organics is in two stages: at initial stage of reaction, the removal rate of organics increases with the increasing Fe²⁺ concentration or PS concentration; After initial stage, even the concentration of Fe²⁺ or PS continues to increase, the removal rate decreases. This shows that an appropriate molar ratio of Fe²⁺ to PS, rather than higher concentration of Fe²⁺ or PS, can obtain a better removal rate of organics. The most used molar ratio of Fe²⁺ to PS in these studies was 1:1, and followed by 1:2 according some literature (Oh et al. 2009; Liu et al. 2012; Bu et al. 2016; Wang & Wang 2017). From the reaction of Equation (1), we can see that, 1 mole of Fe²⁺ reacts with 1 mole of PS can produce 1 mole of SO₄^•−. So, theoretically, 1:1 is the optimal ratio of Fe²⁺ to PS. However, in fact, the optimal ratio can be greater than or less than 1:1, due to the presence of some oxidized or reduced substances in the complex sewage that can consume Fe²⁺ or PS. For example, SO₄^•− can react with excess Fe²⁺ or PS (Equations (4) and (5)) without considering the influence of other ions. In addition, the ratio of oxidant (PS) or catalyst (Fe²⁺) concentration to organics concentration is also an important factor for maintaining a better organics removal rate. Some studies (Satapanajaru et al. 2015; Wu et al. 2015; Bu et al. 2016; Nie et al. 2018) stated that the concentration of oxidant or catalyst required is proportional to the concentration of contaminants due to more pollutants require more SO₄^•− generated from PS activated by Fe²⁺.

The initial pH of aqueous solution is an important factor influencing the removal efficiency of refractory organics in Fe²⁺/PS system. Some articles (Cao et al. 2008; Xu & Li 2010; Liu et al. 2012; Niu et al. 2012; Long et al. 2014; Rao et al. 2014; Epold et al. 2015; Han et al. 2015a; Satapanajaru et al. 2015; Wu et al. 2016; Nie et al. 2018; Yu et al. 2018; Shang et al. 2019; Song et al. 2019; Wang et al. 2019c) studied the effect of pH ranging from 2.0 to 11.0 on the removal of refractory organics in Fe²⁺/PS system. Based on the results of these articles (Xu & Li 2010; Liu et al. 2012; Niu et al. 2012; Long et al. 2014; Rao et al. 2014; Han et al. 2015a; Satapanajaru et al. 2015; Wu et al. 2016; Nie et al. 2018), the optimum pH range was 3.0–4.0. Within such a pH range, due to acid catalysis on PS, more SO₄^•− could be generated from decomposition of PS, which therefore enhanced the removal rate of organics (Nie et al. 2018). When the pH was less than 3.0, refractory organics removal rate decreased because pH < 3.0 promotes the formation of the ferrous complex (Fe(H₂O))²⁺, which reduces the amount of Fe²⁺ to activate PS, and thus less SO₄^•− can be formed. When the initial pH increased from 4.0 to 9.0, the removal efficiency of refractory organics gradually decreased. Four mechanisms are summarized to explain the reason why refractory organics degradation declined with increasing pH:

• (1)
  When the pH is higher than 4.0, free Fe²⁺ turns into Fe²⁺ complexes (Equation (10)), which inhibits the further reaction between Fe²⁺ and PS to form SO₄^•− (Xu & Li 2010; Rao et al. 2014; Nie et al. 2018).

  

  (10)
• (2)
  With pH increasing above 4.0, ferric hydroxide complexes, such as FeOH²⁺, Fe₂(OH)₂⁴⁺, Fe(OH)²⁺, Fe(OH)₃, and Fe(OH)₄⁻ formed in the solution have low activation capacity to generate SO₄^•− from PS, thus the efficiency of the refractory organics degradation is reduced (Nie et al. 2018). The equations of generating ferric hydroxide complexes are shown in Equations (11)–(13) (Xu & Li 2010). It can be seen from these equations that hydrogen ion is generated during the reaction and causes the decrease of pH in the solution.

  

  (11)

  

  (12)

  

  (13)
• (3)
  If the pH is higher than 8.5, the present PS in the solution may react with OH⁻ to form OH^• (Equation (14)) in alkaline solution (Satapanajaru et al. 2015). This results in the rapid decay of SO₄^•− (Liu et al. 2012; Satapanajaru et al. 2015). But hydroxyl radical is also strong in its reaction with the organic compounds.

  

  (14)
• (4)
  When the pH is greater than or equal to 8.4, SO₄^•− was found to react with HCO₃⁻ or CO₃²⁻ (Equations (15) and (16)) to form HCO₃^• or CO₃^•, any of which has poor oxidizing ability toward organics compared to SO₄^•− (Satapanajaru et al. 2015).

  

  (15)

  

  (16)

Several articles (Epold et al. 2015; Shang et al. 2019; Song et al. 2019; Wang et al. 2019c) claimed that an initial solution pH (3.0–7.0) has no significant effect on removing refractory organics. Wang et al (2019c) investigated the degradation of acetaminophen (ACT) in the Fe²⁺/PS system at different initial pH (3.0, 5.0, 7.0 and 9.0), and found that the degradation of ACT was around 70% after a 30-min reaction at each of initial solution pH. In the article by Wang et al. (2019c), regardless of how the initial pH changed (from 3.0 to 9.0), the pH of the solution after 1-min reaction dropped rapidly to 3.0, which demonstrates the degradation of ACT was not affected by the initial pH in Fe²⁺/PS system. Song et al. (2019) studied the effect of increasing pH from 4.0 to 9.0 on the degradation efficiency of triphenyl phosphate (TPhP) in Fe²⁺/PS process and found that degradation efficiency of TPhP was not impacted by pH changes. Epold et al. (2015) investigated levofloxacin (LFX) degradation at different pH (3.0, 5.0, 7.0 and 9.0) in Fe²⁺/PS system, and the degradation rates of LFX acquired were almost as same as pH from 3.0 to 7.0, while the degradation rate of LFX was decreased by about 10% at pH of 9.0. Shang et al. (2019) studied the influence of initial pH (3.0, 5.0, 7.0, 9.0 and 11.0) on diatrizoate (DTZ) degradation. It was found that DTZ degradation rate was increased to only 3% when the pH was increased from 3.0 to 9.0. However, DTZ degradation efficiency dropped significantly as the initial pH increased from 9.0 to 11.0. Shang et al. (2019) proposed that the decreased DTZ degradation at initial pH of 11.0 is due to the reduction of Fe²⁺ that tended to be hydrolyzed to ferric hydroxide and ferrous hydroxide under alkali conditions.

The findings of Cao et al. (2008) were different from the above articles. They testified that both low (pH = 1.5) and high pH (pH = 11.0) were more favorable for oxidation of Lindane in Fe²⁺/PS system. The proposed reason is that Lindane is easily catalyzed hydrolysis by acid or base.

Some inorganic anions, such as HCO₃⁻, NO₃⁻, NO₂⁻, SO₄²⁻, H₂PO₄⁻ and Cl⁻, in the water either have a positive or negative effect on sulfate radicals-based oxidation. This inorganic anion can either promote or inhibit the degradation of refractory organics in the Fe²⁺/PS system. The conclusions from some articles (Xu & Li 2010; Ghauch et al. 2013; Rao et al. 2014; Naim & Ghauch 2016; Bu et al. 2017; Amasha et al. 2018; Nie et al. 2018; Song et al. 2019; Wang et al. 2019c) indicated that NO₃⁻, SO₄²⁻, H₂PO₄⁻ and HPO₄²⁻ could inhibit degradation of refractory organics, while Cl⁻, HCO₃⁻ could play both positive and negative effects on refractory organics removal. The inhibition effect of NO₃⁻, SO₄²⁻, H₂PO₄⁻, HCO₃⁻ and Cl⁻ is attributed to the reactions occurring between inorganic anions and SO₄^•− (Equations (17)–(20)). H₂PO₄⁻, NO₃⁻, Cl⁻ and HCO₃⁻ could react with SO₄^•−, then generate HPO₄^•−, NO₃^•−, Cl^• and CO₃^•2−, respectively. Because the redox potential of HPO₄^•−, NO₃^•−, Cl^• and CO₃^•2− are lower than that of SO₄^•−, the replacement of these free radicals with SO₄^•− leads to a decrease in the rate of degradation of refractory organics in Fe²⁺/PS system. Phosphate is a special case in PS systems activated by iron species. In fact phosphate is a good complexant for Fe²⁺ species to form FeH₂PO₄²⁺ and FePO₄ (Ghauch 2008) and this is why inhibition occurs. Unless done in Fe²⁺ free solution, phosphate can generate in heat activated PS medium some phosphate radicals as mentioned here (Ghauch & Tuqan 2012).

From the above research results on chelating agents, we know that some chelating agents can promote the efficiency of Fe²⁺ activation reaction under certain conditions in the Fe²⁺/PS process, and these reaction conditions need to be further explored. In another point, the chelating agents used in the Fe²⁺/PS system should be considered not to cause secondary pollution to the solution and to be easily biodegraded.

Although Fe²⁺ appears to be a good activator for PS, Fe²⁺/PS systems still have some limitations such as scavenging effect of excess Fe²⁺ on SO₄^•−, conversion of Fe²⁺ to iron hydroxide precipitation, and acidic pH of the solution. Fe⁰ has been widely studied as an alternative source of Fe²⁺ because it can gradually release ferrous iron into the water.

Table 3 summarizes the reaction conditions and degradation efficiency of some refractory organics using Fe⁰ as an activator for PS to generate SO₄^•−. It can be seen from Table 3 that most of the activation reactions in Fe⁰/PS system can achieve high refractory organics removal rates of 70–100% in the range of pH 3.0–7.0. Some papers (Oh et al. 2009; Deng et al. 2014; Pulicharla et al. 2018; Zhang et al. 2018b) compared the effects of Fe⁰ and Fe²⁺ acting as activators for PS on refractory organics removal and all results showed that Fe⁰ has a better treatment effect on refractory organics degradation than Fe²⁺ under the same conditions of the reaction. Except, the results of Gao et al. (2018) showed that Fe⁰ and Fe²⁺ both had the same removal effect on bisphenol A removal at neutral pH. Under most conditions, the activation effect of Fe⁰ on PS is still better than that of Fe²⁺ on PS. Similar to previously discussed Fe²⁺/PS system, the molar ratio of catalyst to oxidant is an important parameter in Fe⁰/PS system. Low Fe⁰:PS molar ratio will lead to poor degradation effect on refractory organics due to lack of sufficient active sites for PS activation and SO₄^•− generation, while high Fe⁰: PS molar ratio will result in less SO₄^•− production and unnecessary Fe⁰ consumption. According to previous analysis, we know the theoretically optimal ratio of Fe²⁺: PS was 1:1. However, Fe⁰: PS molar ratio needs to be higher than 1:1 in the bulk solution in order to ensure enough Fe⁰ that can consistently liberate Fe²⁺ into the water. Moreover, the ratio of catalyst to oxidant ultimately should be determined under specific reaction conditions.

Fe⁰ shows a good activated effect on PS. However, high concentrations of Fe⁰ and PS are required, which increases treatment cost. In order to improve efficiency of Fe⁰ on PS decomposition, researchers used two thin cylindrical neodymium-iron-boron permanent magnets to make Fe⁰ magnetic, and magnetized Fe⁰ (or pre-magnetization Fe⁰) was employed to activate PS for degrading refractory organics, such as orange (OG), 2,4-dichlorophenol (2,4-DCP) (Li et al. 2017c; Pan et al. 2017). It was found that in SEM micrographs, after pre-magnetization, the Fe⁰ surface was slightly corroded and deposition of tiny particles it was noted on the surface and the BET after pre-magnetization significantly increased from 0.15 m²/g (original Fe⁰) to 1.94 m²/g, which benefited the mass transfer of Fe²⁺ released from Fe⁰ (Li et al. 2017c). For these reasons, the degradation rate of refractory organics in the pre-magnetized Fe⁰/PS is also higher than Fe⁰/PS system (Li et al. 2017c; Pan et al. 2017).

As reaction time increase, FeOOH reacted with nFe⁰ to form magnetite (Fe₃O₄) through electron transfer of bulk conduction. Simultaneously, the internal Fe⁰ volume continued to shrink, and released Fe²⁺ and Fe³⁺. Kim et al. (2018) proposed a model of core-shell structure (Figure 1) including Fe⁰(s) core, the inner layer Fe₃O₄, and the outer layer Fe³⁺. Moreover, they found that nFe⁰ can release Fe²⁺ for PS at a very fast rate, which is faster than Fe⁰ under optimal conditions.

Although nFe⁰ has a better effectiveness in activating PS for refractory organics degradation owing to its small particle size, large surface area and high reactivity, its application still faced some practical problems (Fu et al. 2014). Mueller et al. (2012), Fu et al. (2014) and Stefaniuk et al. (2016) all found that high surface energies and inherent magnetism of nFe⁰ lead to agglomerating of nFe⁰ particles, which limits the mobility of nFe⁰ and reduces electron transfer of nFe⁰. In addition, nFe⁰ is easily oxidized by the oxygen as it is exposed to the air, lack of stability and difficulty separating from the water is problematic in terms of practical application. In order to solve the weaknesses of nFe⁰ particles, some materials (such as biochar (BC), reduced graphene oxide (rGO), mesoporous carbon (MC) and montmorillonite) have been used to support nFe⁰.

Biochar (BC) is a low-cost carbonaceous adsorbent generated in an oxygen-limited or oxygen-free environment at high temperatures by pyrolysis of biomass (Yan et al. 2015; Xu et al. 2018b; Luo et al. 2019; Park et al. 2019; Wang et al. 2019a, 2019d). BC has a strong adsorption capacity due to its high surface area (SA)-to-volume ratio and porous properties (Wei et al. 2018; Wang et al. 2019b). Ahmed et al. (2016) and Ravikumar et al. (2018) found the surface of BC contains abundant oxygen-rich functional groups (such as hydroxyl (–OH) and carboxyl (–COOH)) which can quickly adsorb organic contaminants. Han et al. (2015b) testified because of these unique properties of these functional groups, BC has been used as mechanical supporting material for nFe⁰ to disperse and stabilize nFe⁰. In detail, BC has a large specific surface area and a large number of micropores, which can provide a support carbon skeleton for nFe⁰ to immobilize (Liu et al. 2018; Wang et al. 2019d). nFe⁰ is stabilized on surface of BC via adsorption, coordination, chelation and bridging (Jiang et al. 2019; Wang et al. 2019d).

Ahmad et al. (2015) had investigated different types of reactive oxygen species (ROS) generated in the nFe⁰/rGO/PS system. Because of their different reaction rate constant, tert-Butanol, NB, AN, and CT were employed as probe compounds to identify HO^•, HO^•, both HO^• and SO4^•−, and O₂^•−, respectively. The result proves the presence of three free radicals in the nFe⁰/rGO/PS system. In acidic conditions, more SO₄^•− radicals were generated compared to HO^• radicals, while O₂^•− radicals prefer a basic condition. In order to avoid the leakage of nFe⁰ and promote the tight adhesion of nFe⁰ on RGO, polydopamine (PDA), as a polymeric stabilizer, has been studied to use as nFe⁰ coating. PDA has a specialized adhesive foot protein and can adhere on nFe⁰ surface to modify nFe⁰ due to abundant functional groups possessing strong negative charge with the ability to bind positively charged iron. A study by Gu et al. (2018) has also confirmed that the nFe⁰/PDA/rGO/PS system showed better performance on pollutants removal than the nFe⁰/rGO/PS system owing to the presence of PDA. Jiang et al. (2018) investigated removal efficiency of TC in different systems, the removal rate of TC in descending order is: nFe⁰/MC/PS > MC/PS > MC > nFe⁰/MC > nFe⁰/PS >nFe0 > PS. The highest removal efficiency (92.1%) was obtained in the nFe⁰/MC/PS system. This could attribute to a synergistic effect between Fe⁰ and MC. Besides, the reaction mechanism of the nFe⁰/MC/PS system is similar to that of the Fe⁰/MC/PS system.

In addition, montmorillonite (Mt) as naturally abundant mineral clay has been investigated for support nFe⁰ to activate PS in degrading refractory organics (Wu et al. 2019) because of its high specific surface area and cation exchange capacity. Cationic surfactant-modified Mt (organo-montmorillonite, OMt) is more hydrophobic and has showed better catalytic performance than Mt (Wu et al. 2019).

Some iron-containing compounds such as hematite (Fe₂O₃), magnetite (Fe₃O₄), and ferrous sulfide (FeS), are also explored to activate PS for refractory organics degradation.

Teel et al. (2011) evaluated the effects of several soil minerals including hematite on PS activation and found that hematite is capable of activating PS for removal organic pollutants in water. This effect is attributed to the octahedral sites of hematite containing Fe²⁺ that can react with PS to produce SO₄^•−, according to Equation (1). Figure 2 illustrates the activated mechanism in which PS is activated to generate sulfate radicals for degradation of organic pollutants. The mechanism of the activation of PS by hematite is similar to that by Fe²⁺, but due to the heterogeneous catalysis of hematite, catalytic reactions will occur both on the surface of the Fe₂O₃ and in the solution. Firstly, Fe²⁺ at some sites on the surface of the hematite will adsorb PS in aqueous solution, then react with PS to form SO₄^•−, SO₄²⁻ and Fe³⁺. Fe³⁺ also reacts with PS to form Fe²⁺, which forms a cycle between Fe²⁺ and Fe³⁺ on the surface of hematite. The Fe²⁺ and Fe³⁺ leaking into the solution also undergo the same cyclic reaction as the surface. There is also the formation of SO₄^•− during this cycle. SO₄^•− formed on the surface of hematite or in the solution can capture and react contaminants to form by-products in water, which ultimately produce carbon dioxide and water. Free radical scavenging experiment in reference (Kermani et al. 2018) have shown that there is also HO^• that could be generated during oxidation of PS by hematite; HO^• also plays a role on degradation of refractory organics. The type of free radicals generated in PS/Fe₂O₃ system are determined by solution pH and the type of targeting contaminants. SO₄^•− are usually dominant under an acidic condition, while HO^• prefers a neutral and alkaline environment.

Magnetite nanoparticles (MNPs) have also been studied as a catalyst similar to hematite because they can work under mild reaction conditions and be easily separated and recovered. Magnetite is a magnetic particle, and nanometer magnetite can be synthesized by reverse co-precipitation among FeCl₃•6H₂O + FeSO₄•4H₂O + NH₃•H₂O. Literature (Leng et al. 2013, 2014; Sun et al. 2016; Zhu et al. 2018) demonstrated that treatment effects, reaction conditions and mechanisms of different types of synthetic Fe₃O₄ and its complexes with other materials (biochar, NrGO) to activate PS for effectively degradation of refractory organics. Magnetite contains Fe²⁺, which can stimulate decomposition of PS to generate free radicals. However, the Fe²⁺ may have not enough magnetite due to the slow transformation between Fe²⁺ and Fe³⁺ cycles. As a result, modifying surfaces of Fe₃O₄ MNPs and improving the redox cycle of Fe²⁺/Fe³⁺ is a very important research issue (Sun et al. 2016). Some studies investigated polyhydroquinone-coated Fe₃O₄, ascorbic acid modified Fe₃O₄, β-CD immobilized Fe₃O₄ as catalysts for the activation of PS (Leng et al. 2013, 2014; Sun et al. 2016; Zhu et al. 2018). According to Sun et al. (2016), ascorbic acid (H₂A, also called vitamin C), a redox mediator, used to modify Fe₃O₄ MNPs could enhance the ability of activating PS. Under same conditions, the effect of H₂A/Fe₃O₄ on PS activation was greater than that of Fe₃O₄ alone on PS activation. Under H₂A/Fe₃O₄ activation, a removal rate (98.5%) of 2,4-DCP can be obtained, while only 35.1% degradation of 2,4-DCP was achieved in Fe₃O₄/PS system, which was testified that the introduction of H₂A in Fe₃O₄/PS can effectively activate PS for targeting pollutants removal. Nowack (2002) and Dong et al. (2017) all testified that H₂A is an effective chelating and reducing agent that can enhance iron ions released from minerals. The primary mechanism of improved refractory organics degradation in the H₂A/Fe₃O₄/PS process is illustrated in Figure 3. First, H₂A, as a chelating agent, attached on surface of Fe₃O₄ particles combines with Fe²⁺ to form chelated iron of Fe²⁺–H₂A through the ligand exchange reaction. Then, Fe²⁺ enters into aqueous phase initiating a reaction that catalyzes PS to generate SO₄^•−. In addition, H₂A can enhance the performance of reduction of Fe³⁺ to Fe²⁺ on the Fe₃O₄ nanoparticle surface and promote more release of Fe²⁺ from Fe₃O₄ into solution.

Compared with homogenous Fe²⁺/PS system, heterogeneous FeS/PS system also shows a good performance on refractory organics removal (Pu et al. 2014; Yuan et al. 2015; Zhang et al. 2017; Fan et al. 2018). Yuan et al. (2015) found that the application ranges of pH for p-chloroaniline (PCA) fast degradation was from 3.0 to 7.0, while the optimum pH range was from 5.0 to 7.0 for complete degradation of PCA at 150 min according the research of Fan et al. (2018). The PS activation mechanism was affected by the initial solution pH. The mechanism of FeS activating PS is shown in Figure 4. As a solid particle like Fe⁰, FeS could continuously release dissolved Fe²⁺ (Yuan et al. 2015) which would activate PS to produce SO₄^•−. In addition, as an electron donor, S²⁺ was able to reduce Fe³⁺ to Fe²⁺ (Fan et al. 2018). Sustainable release of Fe²⁺ and recycling of Fe³⁺ by FeS during reaction could avoid surplus Fe²⁺ consumption and Fe³⁺ accumulation. Based on this mechanism, when the initial solution pH was 9.0–11.0, the dissolved Fe²⁺ from FeS was not enough to activate PS and relatively low PCA removal rate was acquired.

Regarding the methods for iron-based materials to activate PS to degrade organics, we are not only concerned with the efficiency, but also with the cost. Since these methods have not yet been applied in engineering, the cost of chemical agents in small experiments based on different iron materials is calculated and analyzed according to the consumption and price of chemical agents. Table 4 shows the unit price of iron-based materials found on the website of Sigma. Table 5 lists the experimental data such as oxide and catalyst concentration and removal efficiency in several representative small experiments, and also shows the cost of processing 1 L of water after calculation (in US dollars). The cost of each process is the sum of the cost of the catalyst and the oxidizing agent used to achieve an organic removal rate of more than 95%. According to the calculation results, the order of chemical costs from high to low is Fe₃O₄/PS, nFe⁰/PS, FeSO₄·7H₂O/PS, FeS/PS, Fe₂O₃/PS, and Fe⁰/PS. Since the price of the analytical reagent is used as a unit price, the calculation cost is high, and the cost will be reduced in actual engineering. In addition, the difference in the degree of difficult degradation of pollutants also determines that the same system will generate different treatment costs. These data are for reference only for small experimental chemistry costs.

In summary, a state-of-the-art review is presented on mechanisms, reaction conditions, influencing factors, and treatment effects of iron-based materials for refractory organics degradation via PS activation. The results of recently published articles affirm that activated-PS by iron-based materials has promising potentials for destruction of refractory organics in water and 100% degradation can be accomplished for some refractory organics under optimized conditions. However, challenges still remain in the improvement of refractory organics degradation rate in actual polluted water treatment using iron-based materials/PS processes, and the practical engineering application of these processes with low costs is also highly anticipated.

The activation performance of iron-based materials can be influenced by the properties and concentrations of activators, pollutants, oxidants, and water quality component along with the reaction conditions (such as pH, temperature, reaction time). Optimization of the experimental conditions is a cornerstone in achieving best removal. Deep research on environmentally friendly iron-chelators will help to increase Fe⁰ functional lifespan and improve removal rate of refractory organics in Fe⁰/PS system. Without a doubt, nFe⁰ shows the most promise for PS activation if the problems of nFe⁰ leakage and accumulation can be properly solved. BC and rGO as effective supports for nFe⁰ can reduce nFe⁰ leakage, but the increased cost should be taken into account in future engineering applications. Moreover, it is also necessary to optimize the nFe⁰/BC/PS or nFe⁰/rGO/PS process from the perspective of reaction kinetics and other reaction conditions. Other iron-containing materials (Fe₂O₃, Fe₃O₄, FeS, etc.) also are used in the catalysis activation mechanism of Fe²⁺ to activate PS. Therefore, the key issue is to identify the factors that affect the release of Fe²⁺ and maintain a dynamic balance between Fe²⁺ and Fe³⁺ when using these materials as activators. Novel reactor for the iron-based materials/PS system based on batch or continuous flow need to designed, and more pilot experiments have yet to be implemented.

In a word, because of the low cost, availability, and environmental friendliness of iron, iron-based materials activation method for PS is expected to have a better prospect with further research in this field.

The research group acknowledges the financial support provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada through the CREATE, Discovery and CRD programs and Queen's University. Pascale Champagne is thankful to the Canada Research Chairs Program. Yanjiao Gao thanks the China Scholarship Council for funding a visiting scholar (201808210024). Tiehong Song expresses gratitude for the funding (JJKH20200288KJ) of Jilin Province Education Department of China.