Emerging organic contaminants are widely present in the waters and have potential threat to the ecosystem and human health. How to remove the emerging organic contaminants has become a growing concern for researchers. High valent manganese and iron, i.e. potassium permanganate (Mn(Ⅶ), KMnO4) and potassium ferrate (Fe(Ⅵ), K2FeO4), are two effective inorganic chemicals for decomposing emerging organic contaminants in water treatment. Permanganate and ferrate are environmentally friendly, and have been drawing more and more attention due to their high efficiency. Permanganate and ferrate have similar chemical properties and close behavior in the removal of emerging organic contaminants. Recent studies on the oxidative degradation of emerging organic contaminants by permanganate and ferrate mainly focus on the kinetic models, the role of manganese- and iron-intermediates, the role of free radicals generated when permanganate and ferrate are combined with other chemicals or processes, and their application in real waters. Therefore, this paper summarizes the reaction kinetics of permanganate and ferrate, and compares the roles of permanganate- and ferrate-intermediates and free radicals, and their performance in real waters.
Contents
1 Introduction
2 Mechanism of permanganate and ferrate oxidation
3 Oxidation kinetics of organic pollutants by permanganate and ferrate
4 Formation and functions of permanganate-and ferrate-intermediates
Jing Zhang, Dingxiang Wang, Honglong Zhang.
Oxidative Degradation of Emerging Organic Contaminants in Aqueous Solution by High Valent Manganese and Iron.
Progress in Chemistry, 2021, 33(7): 1201-1211 DOI:10.7536/PC200756
emerging organic pollutants (emerging organic contaminants, EOCs) are a class of organic pollutants that have no environmental monitoring standards or emission standards at present, and have a negative impact on the ecology and human health[1,2]. Common emerging organic pollutants include endocrine disrupting compounds (EDCs), pharmaceutical and personal care products (PPCPs), persistent organic pollutants (POPs) and disinfection by-products (DBPs). The concentration of emerging organic pollutants in surface water and groundwater can reach ng/L ~ μg/L, so how to effectively remove them has attracted the attention of researchers at home and abroad[3⇓⇓~6]. The traditional drinking water treatment technology based on coagulation-sedimentation-filtration-disinfection process and the traditional wastewater treatment process based on activated sludge process can not completely remove the emerging organic pollutants, so new water treatment technologies such as ozone oxidation, Fenton technology, adsorption technology and so on are gradually used to remove organic pollutants in water[7][8][9][10,11][12]. Potassium permanganate and potassium ferrate are two kinds of inorganic high-efficiency water treatment agents with similar chemical properties and "green oxidants", which can quickly remove most of the emerging organic pollutants[13,14]. Recent studies on the oxidative degradation of emerging organic pollutants by potassium permanganate and potassium ferrate mainly focus on the following four aspects: (1) kinetic models of the degradation of emerging organic pollutants by potassium permanganate and potassium ferrate; (2) the role of manganese and iron in the intermediate valence state; (3) The effect of free radicals that may be produced when combined with other substances or processes; (4) Application in actual water body. In this paper, based on the recent research on the oxidative degradation of emerging organic pollutants by potassium permanganate and potassium ferrate, the characteristics of potassium permanganate and potassium ferrate in the above four aspects are discussed and summarized, and the similarities and differences are analyzed and discussed.
2 Oxidation mechanism of high-valence manganese and iron
High-valent manganese, mainly referred to as potassium permanganate (potassium permanganate,KMnO4), is a kind of highly efficient inorganic water treatment agent. It is a deep purple crystalline solid at room temperature, easily soluble in water, and its aqueous solution is rose red. In the process of water treatment, potassium permanganate can be used in pre-oxidation, disinfection and other processes, which can effectively remove chroma and odor substances, inhibit the growth of algae, and reduce the formation of disinfection by-products. The reduction product of potassium permanganate in the common water treatment process (under neutral conditions), MnO2, will not pollute the water quality, and can further improve the water treatment effect through adsorption, flocculation and other processes. Therefore, potassium permanganate is called "green oxidant"[15,16].
The reduction products of potassium permanganate are different at different pH values. Under acidic conditions, the reduction products of potassium permanganate are divalent manganese ions:
Under neutral conditions, the reduction product of potassium permanganate is manganese dioxide:
Under alkaline conditions, the reduction product of potassium permanganate is potassium hexavalent manganate:
High-valent iron, referred to herein as potassium ferrate (potassium ferrate,K2FeO4), has received increasing attention over the past few decades. Potassium ferrate is a dark purple solid at room temperature and can be preserved for a long time in an air-isolated and dry environment; Unlike potassium permanganate, it is highly susceptible to oxidative decomposition when exposed to air. In the process of water treatment, the final product Fe (Ⅲ) of potassium ferrate will not pollute the water quality, and it exists in the form of ferric hydroxide precipitation in water, which has the function of flocculation. Therefore, ferrate is also known as "green oxidant" and "environment-friendly oxidant"[17].
Potassium ferrate is easily soluble in water, and its color is purplish red after it is dissolved in water. It exists in different forms in aqueous solution under different pH values, as shown in Formulae 4 to 6 and Figure 1:[18]
Similar to potassium permanganate, the form of Fe (Ⅲ) is different at different pH. Fe (Ⅲ) exists in a dissolved state under acidic conditions:
Under neutral or alkaline conditions, Fe (Ⅲ) exists in the form of Fe(OH)3 precipitate:
Potassium ferrate is strongly oxidizing at all pH values and is more oxidizing than potassium permanganate[17]. Under strong acidic conditions, the redox potential of potassium ferrate (+ 2.20 V) is higher than that of ozone (+ 2.08 V) and potassium permanganate (+ 1.67 V), but lower than that of hydroxyl radical (+ 2.80 V), which indicates that potassium ferrate is one of the strong oxidants in water treatment.
In contrast to potassium permanganate, potassium ferrate is unstable in aqueous solution and spontaneously decomposes to produce iron oxyhydroxide (FeOOH), hydroxide ions (OH-), and molecular oxygen (O2)[19]. This low stability makes the actual oxidation capacity of potassium ferrate lower than the theoretical value, so the negative effects of self-decomposition of potassium ferrate must be considered in practical applications.
3 Kinetics of oxidation of organic compounds by potassium permanganate and potassium ferrate
In the process of oxidation and degradation of organic matter by potassium permanganate, the total reaction is a second-order reaction, and the consumption of potassium permanganate or the degradation of organic matter is a first-order reaction. The kinetic model is shown in Equation 10:[20]
Where the kapp is the apparent second-order rate constant; The [Mn(Ⅶ)]tot is the total concentration of Mn (Ⅶ); The [X]tot is the total organic concentration.
Similar to the kinetic model of organic matter oxidation and degradation by potassium permanganate, the total reaction in the process of organic matter oxidation and degradation by potassium ferrate is second-order, and the consumption of potassium ferrate or the degradation of organic matter is first-order reaction. The kinetic model is shown in Equation 11:
Where the kapp is the apparent second-order rate constant; The [Fe(Ⅵ)]tot is the total concentration of Fe (Ⅵ). The [X]tot is the total organic concentration. Integrate Equation 11 to obtain Equation 12:
Wherein the [X]0 and the [X]t are the concentrations of the organic matter at time 0 and time t; The is the integral of the ferrate concentration over time. If the pseudo-first-order reaction kinetic model is used, that is, the concentration of potassium ferrate is set to be much higher than the concentration of organic matter, the consumption of potassium ferrate can be ignored relative to its initial concentration. However, the stability of potassium ferrate in acidic and near neutral aqueous solution is poor, for example, in borate buffer solution with pH = 7. 5, the self-decomposition of potassium ferrate is more than 50% within 5 min[21]. Therefore, the self-decomposition of potassium ferrate must be taken into account in the kinetic study of the oxidation of organic compounds by potassium ferrate under acidic and near-neutral conditions.
Fe (Ⅵ) exists in different forms in aqueous solutions with different pH values. In addition, organic substances can also be dissociated in aqueous solutions. Therefore, according to the acid-base protonation theory, the second-order reaction kinetic model of the oxidation and degradation of organic substances by potassium ferrate can be further transformed into Formula 13:
In which that kij is the rate constant of the reaction between the species; αi and βj are the speciation distribution coefficients of Fe (VI) and organics (Fig. 1); I and J represent the species of Fe (VI) and organics in aqueous solution. Potassium ferrate may exist in the form of H3Fe, , H2FeO4, HFe and Fe in water.At neutral pH, HFe is the dominant species.
Figure 2 summarizes the second-order reaction rates for the oxidative degradation of emerging organic pollutants in water by potassium permanganate and potassium ferrate. The oxidation of potassium permanganate and potassium ferrate has selectivity, which can effectively remove some emerging organic pollutants in the water environment.Generally, the structure of these emerging organic pollutants contains unsaturated functional groups (such as carbon-carbon double bonds, etc.) Or electron-rich groups (such as phenolic hydroxyl, amino, etc.) Which are easily oxidized by potassium permanganate and potassium ferrate, while the oxidation ability of unsaturated functional groups such as benzene rings (such as toluene, nitrobenzene, etc[7]. It can be seen from Figure 2 that the reaction rate of potassium ferrate in oxidizing organic pollutants such as phenol, bisphenol A, 4-chlorophenol, 2,4-dichlorophenol, carbamazepine and triclosan is 2 to 10 times faster than that of potassium permanganate, which is also one of the advantages of potassium ferrate.
The redox potential of potassium permanganate increases with the decrease of pH value, while the organic matter exists in the form of molecular state or deprotonated state at different pH values. Different forms of organic matter have different reaction properties with potassium permanganate, so the reaction rate is also different. Stewart et al. Found in 1971 that the second-order reaction rate constant of the oxidative degradation of hydroxycyclohexane carboxylic acids by potassium permanganate increases with the increase of pH value and reaches a maximum when the pH value is close to the pKa of the organic matter.Then the second-order reaction rate constant decreased with the further increase of pH value, so the second-order reaction rate constant showed a parabola-like shape in the whole pH range, which was reported for the first time[34]. Du et al used the second-order reaction kinetic model to fit the degradation of phenolic organic compounds by potassium permanganate oxidation. The second-order reaction rate constants of some organic compounds also showed a parabola-like shape, and the second-order reaction rate reached its maximum when the pH value was close to pKa[35]. They also suggest that the parabola-like model is not universally applicable, for example, the effect of pH on the reaction rate of potassium permanganate oxidation of phenol is monotonically increasing, while the effect on potassium permanganate oxidation of pentachlorophenol is monotonically decreasing. Du et al. Proposed a proton transfer mechanism to explain this phenomenon, that is, the phenol-permanganate complex is formed between the deprotonated phenolic organic matter and permanganate ion, and the complex is combined with the H+ and then decomposed into the final product[35]. The experimental results of Jiang et al and Pang et al also support this theory[36][25]. When investigating the oxidation of bisphenol by potassium permanganate (A(pKa 9.2), Zhang et al found that the second-order reaction rate constant varied with pH in a parabola-like manner and reached a maximum at pH = 9 ~ 10[22]. The second-order reaction rate constant of the oxidation of organic compounds by potassium permanganate shows a parabolic model with pH value, which can not be simply explained by the oxidation characteristics of potassium permanganate or the acid-base protonation theory, and may be related to the pKa of the target organic compounds, that is, the form of organic compounds. Taking phenols as an example, protic phenols can be directly oxidized with potassium permanganate to form oxidation products, while deprotic phenols will first form intermediate products with potassium permanganate, and then combine with protons to form final products. With the increase of pH value, the contribution of protonic phenols decreases, while the contribution of ionic phenols is parabolic, so the overall trend is parabola-like (Formula 14)[25,35,37].
Where, the kapp is the second-order apparent rate constant; Alpha is a constant; [H+] is the variable of hydrogen ion concentration in the reaction process, which can be calculated from pH; The Ka is the ionization constant of the target organic substance and can be calculated by the pKa; The k1 and k2 are the reaction rate constants of oxidation of protonic and ionic phenols by potassium permanganate, respectively.
The redox potential of potassium ferrate increases with the decrease of pH, and the main species of potassium ferrate at different pH are different, and its oxidation performance is also different. It is generally believed that the conjugated acids of , and H2FeO4, are more oxidizing, that is, the oxidizing ability of potassium ferrate increases with the decrease of pH value. On the other hand, the stability of potassium ferrate increases with the increase of pH value, for example, the self-decomposition rate constant decreases by four orders of magnitude when pH value increases from 2 to 8[38]. The oxidation-reduction potential and stability of potassium ferrate will affect its oxidation performance. With the increase of pH value,
The stability of potassium ferrate increases while the redox potential decreases, so the oxidation characteristics of potassium ferrate are determined by both its stability and redox potential. Manoli et al. Found that acid-activated potassium ferrate can oxidize caffeine in water within seconds to minutes, so its oxidation performance is higher than that of non-acid-activated potassium ferrate[39]. Chen et al found that the reaction rate of potassium ferrate degradation of polychlorinated diphenyl sulfides decreased with the increase of pH value[40]. Yang Tao used the second-order reaction kinetic model to fit the kinetics of the oxidative degradation of bisphenol S and bisphenol AF by potassium ferrate (as shown in Figure 3)[24].
It was found that the second-order reaction rate constant decreased with the increase of pH value. And the main ferrate species reacting with bisphenol S and bisphenol AF at pH = 6.5-10 is . Kim et al. Investigated the kinetics of the degradation of sulfonamides by potassium ferrate at pH = 2 ~ 10, and found that the second-order reaction rate constant under acidic conditions was several orders of magnitude higher than that under alkaline conditions[41]. It is believed that the reaction ability of protonated Fe (Ⅵ) with sulfonamide is stronger under acidic conditions, and shows a trend of H3 >H2FeO4> . Kamachi et al., using density functional theory (DFT) calculations, also confirmed that the oxidation ability of Fe (Ⅵ) follows the trend of density functional theory > [42].
Fig. 4 shows the variation of the second-order kinetic rate constants of the oxidative degradation of typical emerging organic pollutants bisphenol A by potassium permanganate and potassium ferrate with pH value. The rate constants of potassium permanganate and bisphenol A show a typical parabola-like curve with pH value, while the second-order rate constants of potassium ferrate and bisphenol A increase with the decrease of pH value[22][23]. Comparing the oxidation of organic compounds by potassium permanganate and potassium ferrate, it can be seen that potassium ferrate has stronger oxidation performance under acidic conditions, because the content of , H2FeO4 and in H3Fe is higher under acidic conditions. However, in the permanganate system, only the acid-base proton theory can not explain the second-order reaction rate constant of permanganate oxidation degradation of organic compounds with the increase of pH value showing a parabolic law, which may be related to the pKa of target organic compounds, and more research is needed.
4 Formation and action of intermediate valence iron or manganese
In the process of potassium permanganate oxidation, in addition to Mn (Ⅶ), there are also Mn (Ⅵ), Mn (Ⅴ), Mn (Ⅳ), manganese (Ⅲ) and manganese (Ⅱ). However, some intermediate valences of Mn are unstable, and only Mn (Ⅱ) and Mn (Ⅳ) (in the form of MnO2) can exist stably in acidic and neutral/alkaline aqueous solutions, respectively. Mn (Ⅲ) and MnO2;Mn(Ⅴ) are easily disproportionated to Mn (Ⅱ) and Mn (Ⅶ) in aqueous solution, and MnO2;Mn(Ⅵ) is easily disproportionated to Mn (Ⅶ) and MnO2 in aqueous solution. Mn (Ⅵ) is very unstable at neutral pH, and can effectively oxidize mandelic acid and cyclobutanol in strong alkaline environment[43]. Mn (Ⅴ) is also very unstable. Sim Simándi et al. Used As (Ⅲ) to reduce Mn (Ⅶ) under acidic conditions to produce Mn (Ⅴ), which accelerated the degradation of phenolic organic compounds[44]. Gao et al speculated that there might be an intermediate valence state of manganese in the system of Mn (Ⅲ) and complexing agent enhancing the oxidative degradation of phenols and triclosan by KMnO4[45,46]. Jiang et al considered that the oxidative degradation of methyl-p-tolyl sulfoxide by Mn (Ⅶ) was a series of two-electron transfer processes (Mn (Ⅶ) → Mn (Ⅴ) → Mn (Ⅲ)), and speculated that Mn (Ⅴ) participated in the reaction[47]. Gao et al., using methylphenyl sulfoxide as a probe compound, confirmed that there was a two-electron transfer process in Mn (Ⅶ)/sulfite system, that is, the active species in this system was manganese in the intermediate valence state[48]. Jiang Jin and Pang Suyan have found that there is an obvious autocatalytic phenomenon in the oxidative degradation of phenolic compounds by KMnO4, which is due to the catalytic effect of in-situ generated MnO2 on the system[7][49]. However, Sun Bo believes that the promotion effect of MnO2 on Mn (Ⅶ) is due to its own oxidation properties, rather than catalytic effect[50]. Therefore, the mechanism of Mn (Ⅶ) oxidation promoted by MnO2 is still controversial and contradictory, and further research is needed. Recently, Mn (Ⅲ) has been found to be involved in the process of rapid degradation of pollutants in experimental studies. Gao et al and Sun et al also proposed that Mn (Ⅲ) may be the active species of Mn(Ⅶ)/NaHSO3. Li et al also proposed recently that Mn (III) generated in situ in the Mn(Ⅶ)/Na2SO3 system is the active species of rapid decomposition of activated sludge[51][52][53,54][55]. However, Mn (Ⅲ) is not stable in aqueous solution. Therefore, Gao et al. Suggested that besides inducing the formation of Mn (Ⅲ) in the system, another function of bisulfite is to complexe Mn (III) and enhance its stability in aqueous solution[52].
Fe (Ⅴ) and Fe (Ⅳ) are the intermediate ions that may be produced in the reaction of potassium ferrate, and they also have high oxidizing properties[56]. Machla et al considered that Fe (Ⅴ) and Fe (Ⅳ) were formed during the reduction of Fe (Ⅵ) to Fe (Ⅲ)[57]. Sharma pointed out that the reaction of Fe (Ⅴ) with organic compounds followed the first-order kinetic model, and the oxidation performance of Fe (Ⅴ) was 3 ~ 5 orders of magnitude higher than that of Fe (Ⅵ)[58]. Fe (Ⅴ) and Fe (Ⅳ) can even oxidize some organic compounds which are difficult to be oxidized by Fe (Ⅵ)[59]. Sharma et al. Also studied the reaction of Fe (Ⅵ), Fe (Ⅴ) and Fe (Ⅳ) with cyanide, and found that the oxidation of Fe (Ⅴ) and Fe (Ⅳ) was 4 and 2 orders of magnitude higher than that of Fe (Ⅵ), respectively[60]. Terryn et al confirmed that Fe (Ⅴ) has the strongest oxidation ability, Fe (Ⅳ) is in the middle, and Fe (Ⅵ) is the weakest, which is consistent with the experimental results of Sharma et al[61]. This indicates that the reaction of potassium ferrate with organic compounds can be enhanced in the presence of intermediate valence iron. Manoli et al. Found that potassium ferrate can be activated by simple acids (such as hydrochloric acid, nitric acid and acetic acid) under weak alkaline conditions to produce intermediate Fe (Ⅴ)/Fe (Ⅳ), which enhances the oxidation ability of the system[59]. Sun et al. Used methylphenyl sulfoxide as a probe compound to demonstrate that Fe (Ⅴ)/Fe (Ⅳ) formed in situ between carbon nanotubes and Fe (Ⅵ) during the degradation of bromophenol by potassium ferrate activated by carbon nanotubes could enhance the removal of bromophenol[62]. Tian et al. Enhanced the formation of intermediate valence iron by the interaction between biochar and ferrate, which accelerated the degradation of various organic compounds[63].
The oxidative degradation of organic compounds by potassium ferrate can be achieved by either single electron transfer (Fe (Ⅵ) → Fe (Ⅴ) → Fe (Ⅳ) → iron (Ⅲ)) or double electron transfer (Fe (Ⅵ) → iron (Ⅳ) → ferric (Ⅱ)) mechanisms, depending on the nature of the target oxide and the coexisting species[64]. 2,2 '-Azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) can be used as an electron carrier to induce Fe (Ⅵ) to produce Fe (Ⅴ) by single electron transfer, which can promote the oxidation rate of diclofenac by Fe (Ⅵ)[65]. Zhao et al speculated that Fe (Ⅲ)/Fe (Ⅱ) may induce Fe (Ⅵ) to produce Fe (Ⅴ)/Fe (Ⅳ) with stronger oxidizing ability[66].
In the oxidation system of potassium permanganate and potassium ferrate, the intermediate valences of manganese and iron are produced. In the potassium permanganate oxidation system, Mn (Ⅶ) can generate Mn (Ⅵ) and Mn (Ⅴ) through single electron transfer or double electron transfer, and some experiments show that these intermediate valences of manganese may be active species in the potassium permanganate oxidation system. Mn (Ⅲ), as one of the reduction products in the system, usually needs a stabilizer or a complexing agent. At the same time, Mn (Ⅲ) has a certain oxidation ability, which can participate in the oxidative degradation of organic matter. MnO2 is the main reduction product of potassium permanganate in conventional water treatment, which can be used to remove pollutants through adsorption, coagulation and other ways.Some researchers believe that MnO2 itself has a certain oxidation, and some researchers believe that the contribution of MnO2 to potassium permanganate oxidation system comes from its catalytic performance. In the oxidation system of potassium ferrate, Fe (Ⅵ) can generate Fe (Ⅴ) and Fe (Ⅳ) with stronger oxidizability through single electron transfer or double electron transfer, and this kind of intermediate valence iron enhances the oxidation ability of the system. As a reduction product in the system, Fe (Ⅲ) itself has no oxidation, but some researchers believe that Fe (Ⅲ) can induce Fe (Ⅵ) to produce Fe (Ⅴ) and Fe (Ⅳ), thus enhancing the oxidation of the system.
Therefore, there are intermediate valences of manganese and iron in the oxidation system of potassium permanganate and potassium ferrate, and their contribution to the degradation of organic matter can not be ignored. Although some studies have tried to put forward the production path of intermediate valence manganese and iron, there is still a lack of direct evidence, and the kinetics and oxidation mechanism of intermediate valence manganese and iron oxidation of organic matter deserve further discussion, and finally how to efficiently use these intermediate valence manganese, iron also deserves further study.
5 Generation and action of free radicals
Potassium ferrate and potassium permanganate can not only oxidize organic pollutants by themselves, but also induce other oxidants or reducing agents to produce free radicals, or produce free radicals by combining with other technologies. The generated free radicals with stronger oxidation performance can further enhance the degradation of target organic compounds, so the generation and action mechanism of free radicals in the oxidation process of potassium ferrate and potassium permanganate have become one of the hot spots of current research.
When potassium permanganate is combined with other suitable processes or substances, it can not only give full play to the advantages of potassium permanganate as a "green oxidant", but also improve its oxidation performance. Song et al. Found that ABTS as an electron shuttle carrier can promote the rate of Mn (Ⅶ) oxidation of phenol, because ABTS accelerates the oxidation of phenolic organic compounds through the ABTS·+ produced by single electron transfer[67]. Song Jian investigated the effectiveness of the combination of Mn(Ⅶ)/O3 in degrading bisphenol A in water, and found that hydroxyl radical was the main active species in degrading bisphenol A[68]. A series of active species, such as , , NH4 + and · OH, can be produced by the combined system of Mn(Ⅶ)/NaHSO3[69⇓~71]. Chen et al. Found that the intermediate valence Mn (Ⅵ), Mn (Ⅴ) and free radicals ( and hydroxyl radicals) in the combined system of Mn(Ⅶ)/NaHSO3 may cause rapid degradation of organic pollutants[72]. According to the above literature reports, the possible electron transfer pathways of the combined system of Mn(Ⅶ)/Na2SO3 are summarized in this paper, as shown in Fig. 5 (a ~ C).
Potassium ferrate itself has a strong oxidizing ability, which can be further enhanced when combined with other appropriate processes or substances. Han et al. Investigated the generation of hydroxyl radical in the system of Fe(Ⅵ)/O3 combined with Tetrabromobisphenol A. Compared with the single oxidation of Fe (Ⅵ) or O3, the system of Fe(Ⅵ)/O3 combined with Tetrabromobisphenol a has more advantages in the degradation, mineralization and control of by-products[76]. Feng et al. Found that peroxymonosulfate (PMS) can enhance the removal of fluoroquinolones by Fe (Ⅵ), and they speculated that it may be the role of intermediate valence iron or [77]. Wang Yicheng inspected the degradation efficiency of the combined system of Fe(Ⅵ)/Na2SO3 on diclofenac sodium, and believed that in the low concentration area of Na2SO3, the Na2SO3 mainly reduced Fe (VI) to produce intermediate valence iron to degrade the target.However, in the region of high concentration of Na2SO3, Fe (Ⅵ) promotes the formation of S from Na2SO3[78]. Sun et al confirmed that was the main oxidation species in the degradation of N, N-diethyl-3-methylaniline by Fe(Ⅵ)/Na2SO3 system, and bisulfite and pyrosulfite could also accelerate the degradation of target pollutants by Fe (Ⅵ)[79]. However, Shao et al. Investigated the active species in the combined system of Fe(Ⅵ)/CaSO3 and found that it was Fe (Ⅴ)/Fe (Ⅳ) rather than free radicals generated in the system that accelerated the oxidation of organic compounds[75]. Feng et al. And Zhang et al. Also considered that Fe (Ⅴ)/Fe (Ⅳ) and hydroxyl radical may be the active substances in the combined system of Fe(Ⅵ)/S2 and [73,80][81]. According to the above literature reports, it is speculated that the electron transfer pathway possibly involved in the combined system of Fe(Ⅵ)/Na2SO3 is as shown in Equations 15 to 20 and Fig. 5 (d to f):
Hydroxyl radical was the main active species in both Mn(Ⅶ)/O3 and Fe(Ⅵ)/O3 systems. The active species in the combined system of Mn (Ⅶ) or Fe (Ⅵ) and bisulfite are still controversial. Some researchers believe that the active species may include , , and hydroxyl, but some researchers believe that the active species are intermediate valences of manganese and iron. Different researchers have different views on the path of free radical production, but the production of is also beyond doubt. The mechanism and contribution of free radical generation in the combined system of Mn (Ⅶ) or Fe (Ⅵ) and bisulfite still need to be further explored.
6 Application in actual water body
The background substances in the actual water body are very complex, and there are many anions and cations.Uch as Mg2+, Ca2+ and ,, , Cl-, etc.There is also a large amount of natural organic matter (NOM), especially humic acid. The concentration of these substances varies greatly with time and region, and may also affect the degradation of organic matter by potassium permanganate and potassium ferrate.
Yang Tao pointed out that the presence of humic acid reduced the degradation of bisphenol S and bisphenol AF by ferrate, and with the increase of humic acid concentration, the degradation efficiency gradually decreased[24]. Jiang et al. Suggested that natural organic matter as a complexing agent could increase the stability of Fe (Ⅵ)[21]. Zhang et al. Found that humic acid can enhance the rate of oxidation and degradation of bisphenol A by potassium permanganate, but too high concentration of humic acid can also compete with bisphenol A for oxidants, thus reducing the oxidation ability of potassium permanganate[22]. Xu et al. Proposed that the reason why humic acid enhances the oxidation of organic matter by potassium permanganate is that humic acid and Mn (Ⅲ) form a complex, and further produce and hydroxyl radicals[82]. Pang et al. Suggested that humic acid would compete to consume potassium permanganate on the one hand, and on the other hand, it would complex intermediate valence manganese (such as Mn (Ⅲ)) to stabilize it[31]. Although there are different explanations for the enhanced oxidation of humic acids, the dual effects of humic acids do exist, that is, humic acids can accelerate the oxidation of organic matter by potassium permanganate or potassium ferrate, and may competitively consume oxidants to inhibit the degradation of organic matter. The magnitude of this dual effect is affected by the pH value of the system and the concentration of humic acid.
The common inorganic ions in water environment include Ca2+, Mg2+, , , and so on. It was found that Ca2+(0~1.5 mM) and Mg2+(0~1.6 mM) had little effect on the oxidative degradation of BPA by potassium permanganate at pH = 7.0 ( [Mn (Ⅵ)] = 10 μM; [BPA] = 5 μM)[83]. The results of Ding Hao's experiment showed that at pH = 4 ~ 5, Ca2+(1 mM) and Mg2+(1 mM) could inhibit the oxidative degradation of diclofenac (DCF) by potassium permanganate ( [Mn (Ⅵ)] = 60 ~ 180 μM;[DCF] = 5.92 μM), the reason is that the coagulation aid of Ca2+ and Mg2+ inhibits the surface adsorption of MnOx in the system and catalyzes KMnO4 to oxidize and degrade organic matter[84]. When pH = 6 ~ 10, Ca2+ and Mg2+ have no effect on the system, because MnOx has little effect on the system when pH > 6. Therefore, the effect of Ca2+ and Mg2+ on the oxidation of potassium permanganate may be related to the pH value and the intermediate valence of manganese. Our experimental results showed that the presence of could promote the oxidative degradation of BPA by potassium permanganate, but the mechanism still needs to be further explored[83].
For the oxidation of organic compounds by potassium ferrate, the effect of inorganic ions is not significant. Zhao Junfeng's experimental results show that Ca2+ and Mg2+ have no effect on the oxidation of diclofenac sodium by potassium ferrate[85]. The experimental results of Fan et al show that Cl-, and have no effect on the oxidation of arsenic acid by potassium ferrate[86]. Zhao Junfeng's experimental results also showed that Cl- and had no effect on the oxidation of diclofenac sodium by potassium ferrate, but had a strong inhibitory effect on the reaction, which might be due to the hydrolysis of affecting the pH value of the system and the quenching of hydroxyl radicals by [85]. The reason for the different effects of and in the two systems may be that the former uses a buffer system and the latter does not use a buffer system, and not all systems produce hydroxyl radicals, so the reaction between and hydroxyl radicals should be further discussed according to the specific system.
In the actual water, the oxidation rate of potassium ferrate is slower than that in pure water, which may be due to the background substances in the actual water reducing the oxidation performance of potassium ferrate[86][25]. Anquandah et al. Found that the amount of potassium ferrate needed to remove the same trimethoprim in the secondary effluent of sewage treatment plants was more than that in pure water, and speculated that the coexisting substances in the actual water might consume potassium ferrate, resulting in an increase in its dosage[87]. Lee et al. Also confirmed that other organic pollutants compete with target pollutants for oxidants (such as chlorine, chlorine dioxide, ozone, and potassium ferrate) during wastewater treatment[88]. However, Jiang et al found that potassium permanganate was more efficient in oxidizing and degrading phenolic organic compounds and triclosan in actual water, which may be due to the fact that background organic compounds or inorganic salts in actual water can act as complexes to stabilize intermediate Mn (Ⅲ), thus enhancing the oxidation capacity of the system, and the specific mechanism of these background organic compounds still needs further study[14,89].
In the actual water, the reduction products of permanganate and ferrate, MnO2 and Fe (Ⅲ), can further remove the target organic matter through flocculation. At present, the reports of potassium permanganate and potassium ferrate in the actual water are still relatively limited, most of the literature reports are completed in pure water or artificial water conditions, so the oxidation characteristics of potassium permanganate and potassium ferrate in the actual water treatment process still need further study.
7 Conclusion and prospect
Potassium permanganate and potassium ferrate, two highly effective inorganic oxidants, have many similarities in removing emerging organic pollutants, but there are also differences:
(1) The kinetic models of the oxidation of emerging organic pollutants by potassium permanganate and potassium ferrate are all second-order reaction: the second-order reaction rate constant of the oxidation of organic pollutants by potassium permanganate usually varies with pH in a parabola-like manner and reaches a maximum near the pKa of organic substances, which may be related to the speciation distribution of organic substances in water; The second-order reaction rate constant of the oxidation of organic compounds by potassium ferrate usually increases with decreasing pH, which is related to the dissociation of Fe (Ⅵ) in water. It is generally believed that the oxidation of and H2FeO4 with higher species proportion is higher at acidic pH.
(2) In the system of oxidation and degradation of emerging organic pollutants by potassium permanganate and potassium ferrate, active species such as intermediate valence manganese and iron will be produced, which will enhance the oxidation ability of the system. Mn (Ⅲ), MnO2 and Mn (V), which are the reduction products of potassium permanganate, can participate in the oxidative degradation of organic compounds. Fe (Ⅲ), the reduction product of ferrate, can induce Fe (Ⅵ) to produce Fe (Ⅴ) and Fe (Ⅳ), thus enhancing the oxidation of the system. In addition, MnO2 and Fe (Ⅲ) can also remove the target organic matter through adsorption and coagulation.
(3) Potassium permanganate and potassium ferrate can be combined with O3 to generate hydroxyl radicals to enhance the oxidation performance of the system, and can also be combined with sulfite/bisulfite to generate free radicals such as , , , hydroxyl radicals and the like to accelerate the degradation of organic pollutants,However, there may be intermediate valences of manganese and iron in these systems, so the mechanism and contribution of free radical generation in the combined system of Mn (Ⅶ) or Fe (Ⅵ) and sulfite still need to be further explored.
(4) In the actual water, the oxidation ability of potassium permanganate and potassium ferrate is affected by the background organic matter and anions and cations in the water: on the one hand, natural organic matter will compete with the target organic pollutants for oxidants, resulting in the reduction of the removal rate of the target organic pollutants; On the other hand, it may also induce the production of highly active species such as intermediate iron and manganese to promote the degradation of target organic matter. The influence of anions and cations in water may be related to the pH value of the experimental system.
In terms of environmental pollution control, potassium permanganate and potassium ferrate can quickly remove emerging organic pollutants, which are "green oxidants" and "environmentally friendly oxidants". Potassium permanganate has been widely used in drinking water treatment, which can not only remove organic pollutants quickly, but also control odor, microbial growth, inhibit algae growth and reduce the production of disinfection by-products. Compared with potassium permanganate, potassium ferrate has a higher redox potential, which can not only quickly remove organic pollutants, but also sterilize and remove algae, and has a wide range of applications in drinking water treatment and wastewater treatment.
However, there are still some problems to be solved in the oxidation of potassium permanganate and potassium ferrate, mainly including the following aspects: (1) the formation mechanism of intermediate valence manganese and iron in different oxidation processes and their specific action mechanism are still to be clarified; (2) The mechanism of enhancing the oxidation of high-valent manganese and iron by redox mediators still needs further investigation; (3) The reduction products of potassium permanganate and potassium ferrate can be used as flocculants, and how to combine the oxidation of high-valent manganese and iron with the subsequent flocculation process is a problem worth exploring. In addition, there are still some limitations in the application of potassium ferrate in actual water treatment, such as the high production cost of potassium ferrate, its large-scale application is rarely reported, and related research is still concentrated in the laboratory. How to reduce the production cost of potassium ferrate and combine it with the existing technology to achieve more efficient purposes remains to be solved.
KamachiT, MiyanishiM, YoshizawaK. DFT Study on the pH Dependence of the Reactivity of Ferrate(Ⅵ). Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation, Washington D C:ACS Symposium Series, 2016.1238.
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