Enhanced Mechanism of Nano Zero-Valent Iron Activated Persulfate for Persistent Organic Pollutants in the Environment: A Critical Review

Article Type: Review Article


Introduction
With the rapid development of industry, agriculture and urbanization, organic pollutants flowing into the natural environment are becoming more and more diverse and abundant. Persistent Organic Pollutants escape into the environment, accumulate in plants and animals, and endanger life in groundwater and soil. They will enter the human body along the food chain, causing varying degrees of damage to human organs, and even threatening human's life [1]. Currently, methods of removing organic pollutants include absorption [2, 3], adsorption [4,5], membrane separation [6, 7] and advanced oxidation process [8,9]. Among these treatment methods, the advanced oxidation process (AOP) is widely used in the treatment of some difficult to remove organic pollutants due to its good treatment effect and short treatment cycle.
AOP refers to that highly reactive free radicals (i.e. hydroxyl radicals, sulfate radicals) decompose organic pollutants into small molecules, even carbon dioxide and water [10,11]. Sulfate radical (SO4 -•) can degrade most organic pollutants with a fast reaction rate and excellent degradation performance. At present, advanced oxidation technology based on persulfate (PS) has attracted extensive attention from researchers [12][13][14]. The traditional methods of activating persulfate include ultraviolet activation technology [15], metal ion activation technology [16][17][18], thermal activation technology [19], ultrasonic activation technology [20,21] and alkali activation technology [22]. However, these activation technologies require a large amount of energy consumption, strict pH conditions and cause secondary pollution. In order to overcome these shortcomings, nano-zero-valent iron (nZVI) activated PS technology (nZVI/PS) emerged as an ideal activation method. nZVI has the characteristics of large specific surface area but small particle size, high reactivity and strong reduction ability [23][24][25]. Additional, nZVI activated persulfate technology has mild reaction conditions, no extra heat and light source, environmental protection, simple equipment [26]. At present, studies have shown that nZVI activated persulfate can effectively remove organic pollutants [27][28][29][30].
In this paper, the activation pathway and degradation mechanism of organic pollutants are discussed in detail, including electron transfer, hydrogen extraction reaction and the addition reaction. The application of the nZVI/PS system to degrade organic pollutants is also investigated. And the prospect of nZVI/PS advanced oxidation system is importantly proposed combined with the above discussion. It has been proved that nano-zero-valent iron activated persulfate is very promising for the degradation of organic pollutants.

The Pathway of Activation of Persulfate by Nano-Zero-Valent Iron
Persulfate is mainly divided into peroxymonosulfate (PMS) and peroxodisulfate (PS) [31][32][33]. A comparison of PMS and PS on the degradation of organic pollutants in the past three years is shown in Table1. PMS is a compound salt, the impurities produced in the reaction process will cause a serious influence on the degradation. Therefore, compared with PMS, PS is commonly used in laboratory research due to its properties of water-soluble, stable, cheap and easy to transport [34]. However, persulfate (S2O8 2-E0=2.01V) has low activity and a slow reaction rate in water. After activation, it can generate strong oxidizing sulfate radicals, which is beneficial to the degradation of organic matter [35]. Kang et al. [36] studied the degradation effects of 1,4-dioxane (1,4-D) and As(III) in the three oxidation systems of nZVI/PS, nZVI/PMS and nZVI/hydrogen peroxide. The results showed that the removal effect of nZVI/PS system was optimal under the same conditions. The standard redox potential of SO4 -• (2.5-3.1V) is greater than that of •OH (1.8-2.7V), and the half-life of SO4 -• (4s) is thousands of times that of the hydroxyl radical (1μs). At the same time, SO4 -• can also make up for the limitation of •OH that cannot degrade some organic matter. Therefore, advanced oxidation technology based on SO4 -• has become the preferred method for pollutant degradation in recent years. nZVI activated persulfate is divided into direct and indirect activation. Direct activation refers to the direct activation of persulfate by electrons released from nZVI; indirect activation refers to the indirect activation of persulfate by reducing Fe 2+ produced by Fe 0 corrosion [37]. At present, the degradation of organic matter by SO4 -• generated by nZVI/PS system has become a research trend.

Direct Action of Persulfate Activated by nZVI
The essence of nZVI activation of persulfate is to break the O-O bond to form SO4 -•. During the activation process, the lost electrons of Fe 0 directly participate in the activation of persulfate to form SO4 -•, along with the generation of Fe 2+ and SO4 2-(Equation (1)). The generated SO4 -• reacts with water to further generate •OH and H + (Equation (2)). This lowers the pH of the reaction system and facilitates the degradation of pollutants. However, the increasing concentration of SO4 2during the reaction will occupy the active sites on the surface of nZVI particles and suppress the release of electrons and Fe 2+ , thereby hindering the activation process. 1,4-dioxane (1,4-D) and As(III) nZVI PS>PMS PS>PMS [36] toxic organic contaminants in wastewater and actual industrial wastewater microwave-ultraviolet catalyzed oxidation system (MW-UV) PS>PMS PS>PMS [44] nZVI will occur an oxidation reaction in the natural environment, losing electrons to generate ferrous ions (Equations (3), (4)) [45]. Moreover, the concentration of OHin the reaction system increases as the reaction progresses, so the pH value of the system increases. In an alkaline environment, some of SO4 -• will react with OH -(Equation (5)) to generate hydroxyl radicals [46]. The Fe 2+ and Fe 3+ formed by Fe 0 oxidation continue to react with sulfate radicals, in which Fe 3+ is reduced by Fe 0 to produce Fe 2+ and SO4 2-, and Fe 2+ occurs oxidation reaction to produce Fe 3+ and SO4 2− (Equation (6)) [47]. The above reactions realized the balance of Fe 2+ and Fe 3+ in the reaction system.
nZVI is a low-cost, low-toxic and environmentally friendly material [48]. Simultaneously, nZVI as an activator can efficiently and quickly activate persulfate to degrade organic matters. Nano-zero-valent iron is easily oxidized under natural conditions to form iron oxides, such as Fe2O3, Fe3O4 and FeO [49,50]. These oxides will be deposited on the surface of nZVI, hinder the electron transfer between nZVI core-shell, and inhibit the activation of persulfate by nZVI.
Under acidic and neutral conditions, nZVI can quickly activate persulfate to generate SO4 -•, which can promote the degradation of pollutants. However, under strong acid conditions, the SO4 -• (2.5-3.1v) will occur a selfquenching reaction, resulting in the generation of relatively weak oxidizing S2O8 2-• (E0=2.01V). Under alkaline conditions, nZVI will react with persulfate to generate hydroxyl radicals (Equation (7)), hydroxyl radicals and sulfate radicals will synergistically degrade organic pollutants. The above discussion indicates that under different conditions, the pathways of nZVI to activate PS are different.

Indirect Effect of Persulfate Activated by nZVI
Persulfate exists in the form of S2O8 2and HSO5ions in the solution. The indirect effect of nZVI activating persulfate is that the Fe 0 loses electrons during the reaction to generate ferrous ions, and the persulfate is activated by the ferrous ions to further generate sulfate radicals (Equation (8)). The sources of Fe 2+ will be discussed in the following four aspects: (1) In the process of activation of persulfate through electron transfer by nZVI, not only sulfate radical but also Fe 2+ is generated. (2) Fe 0 itself will dissolve and produce a small amount of Fe 2+ ; (3) Fe 3+ generated in the reaction process is reduced by Fe 0 to Fe 2+ [51]; (4) nZVI loses electrons to form Fe 2+ during corrosion in both aerobic and anaerobic environments. Fe 2+ mainly comes from the corrosion of Fe 0 , which can maintain the continuous generation of Fe 2+ .
Dissolved Fe 2+ plays a major role in the activation of persulfate by nZVI. Compared with the activation of persulfate by Fe 2+ alone, the merits of nZVI as an activator are as follows:(1) nZVI can slowly generate Fe 2+ , reducing the occurrence of SO4 -• quenching reaction, and improving the utilization rate of SO4 -•. (2) nZVI activated persulfate can avoid interference caused by the addition of other ions and improve the degradation efficiency of pollutants [52]. Additionally, Fe 0 can continuously reduce Fe 3+ in the reaction system to Fe 2+ (Equation (9)) [53]. Here, Fe 3+ reacts with OHto generate Fe(OH)3 deposited on the surface of nZVI particle [54]. During the process of continuous corrosion, the surface of nZVI will become uneven, and the generated iron oxides are not conducive to the release of sulfate radicals. In addition, another unfavourable factor is the excessively high persulfate concentration. Excessive persulfate will cause a self-quenching reaction of sulfate radicals before they can react with pollutants, causing the consumption of sulfate radicals. In addition, the reaction of sulfate radicals and persulfate will also cause the loss of SO4 -•. (Equation (11)) [55]. However, the reaction of SO4 -• with itself will produce S2O8 2-(Equation (10)), which realizes the regeneration of S2O8 2-.
Studies have shown that nZVI can efficiently activate persulfate. Li et al. [57]activated PS by nZVI to remove nitrobenzene, the results found that higher temperature and acidic conditions are more conducive to the removal of nitrobenzene, and the removal rate can reach 99%. Wu et al. [58] used nanoscale zero-valent iron supported on the organo-montmorillonite composite to activate PS, and the degradation rate of sulfamethazine can reach 97% within ten minutes. Zhang et al. [59] investigated the ultrasound-enhanced nano-zero-valent iron activated persulfate for degradation of chloramphenicol (CAP). The results showed that both sulfates radical and hydroxyl radical played a role in CAP degradation, but the contribution of sulfate radicals is greater.

The Mechanism of nZVI Activating Persulfate to Degrade Organic Pollutants
As mentioned above, nZVI activates persulfate through direct and indirect effects, generating reactive oxygen species (ROS) including SO4 -• and •OH are involved in the degradation of organic pollutants. SO4 -• with strong oxidizing ability has a lone-pair of electrons, the standard oxidation-reduction potential is higher than that of •OH. •OH are highly selective for organic pollutants, so there are very few organic pollutants available for degradation. Compared with •OH, SO4 -• has a strong oxidizing ability and low selectivity to organic pollutants, so most pollutants can be degraded [60]. Similar to the mechanism of •OH, SO4 -• degrades organic matter into H2O, CO2 and other small molecules through electron transfer, hydrogen extraction reaction and addition reaction.

Electronic Transfer
Lone pair electrons on sulfate radical are electrophilic groups. When reacting with organic pollutants, sulfate radicals capture electrons from organic molecules and form SO4 2− (Equation (14)). SO4 -• is more prone to electron transfer reactions than •OH. Organic compounds such as aromatic compounds, amine compounds, usually occur oxidation reaction through electron transfer. This is because these organic compounds contain multiple electrondonating groups, including hydroxyl group (-OH), amino group (-NH2), alkoxy group (-OR), etc. The presence of these electron-donating groups can promote electron transfer between sulfate radicals and target organic pollutants. Simultaneously, organic compounds contain electron-gaining groups, such as carboxyl groups (C=O) and nitro groups (-NO2) [61]. The presence of these electron-gaining groups will compete with the SO4 -• for electrons. This is not conducive to the degradation of target pollutants [62].
Nitroaromatic compounds as a kind of important chemical raw materials are widely used in dyes, medicine, pesticides and other industrial production [63,64]. Nitroaromatic compounds have the characteristics of stable structure, poor biodegradability and strong toxicity, and their existence will cause serious pollution to water bodies. An advanced oxidation system of persulfate activated by nZVI can efficiently degrade nitroaromatic compounds. Wang et al. [65] studied the green synthesis of biochar loaded with nano-zero-valent iron combined with persulfate to degrade p-nitrophenol (PNP) in water. The reduction effect of nano-zero-valent iron reduces nitrobenzene to aniline, and SO4 -• oxidizes aniline through electron transfer. Du et al. [66] studied the mechanism of simultaneous activation of hydrogen peroxide and persulfate by nZVI to degrade p-nitrophenol (PNP). •OH and SO4 -• jointly degrade p-nitrophenol. SO4 -• mainly oxidizes and degrades p-nitrophenol through electron transfer.

Hydrogen Extraction Reaction
Organic substances containing saturated bonds are prone to hydrogen extraction reactions. Such organic compounds include alkane compounds, alcohol compounds, ethers, and lipid compounds. When SO4 -• reacts with these types of pollutants, active free radicals will deprive the hydrogen atoms of the C-H bond in the organic pollutants, causing the organic matter to occur decarbonization and chain scission (Equation (15)). Anipsitakis et al. [67] found that lipid compounds are prone to hydrogen extraction reactions.
Dong et al. [68] investigated the degradation of Trichloroethylene (TCE) by sulfided nano-zero-valent iron activated persulfate, and proposed a possible degradation mechanism (Figure 1). •OH and SO4 -• attack the TCE molecules together, so that the hydrogen atoms on TCE are taken away, then SO4 -• and •OH are converted into HSO4and H2O. Ding et al. [69] prepared attapulgite-loaded nano-zero-valent iron composites to activate PMS to degrade quincloric acid. Quincloric acid contains -COOH and -Cl electrophilic groups, which is not conducive to sulfate radical attacking quincloric acid through electron transfer. Studies have found that quincloric acid mainly forms small molecular substances by capturing hydrogen atoms, oxidizing and ring-opening. Wu et al. [70] synthesized a composite material of nano-zero-valent iron and graphene, and explored the degradation mechanism of the herbicide atrazine by composite material/persulfate system. The result has found that SO4 -• degrades atrazine through hydrogen extraction. Li et al. [71] applied biochar loaded nano-zero-valent iron (BC-nZVI) to activate persulfate to remove decabromodiphenyl ether (BDE209). The results of batch experiments showed that the molar ratio of PS/BC-nZVI, the pH value of the PS solution and the temperature of the reaction system are all factors that affect the degradation efficiency of BDE209 (Figure 2). The possible degradation pathway of this system is also proposed as shown in Figure 3. In addition, the quenching experiment used in this research further proves that sulfate radicals play a dominate role under acidic and neutral conditions, while hydroxyl radicals play a dominant role under alkaline conditions.

The Addition Reaction
Addition reaction refers to that under certain conditions, the organics containing unsaturated bonds have broken the double bond (C=C, C=O) or triple bond, and other atoms or atomic groups are added to both ends of the broken bond. The strongly oxidizing sulfate radical provides electrons to unsaturated hydrocarbon organics and the addition reaction occurs. The reaction mechanism is as follows: Huang et al. [72] investigated the degradation of 59 kinds of organic compounds by persulfate oxidation process and found that persulfate oxidation process is more likely to occur addition reaction to organic compounds containing C=C double bond.
In general, nZVI activated persulfate degrades organic pollutants not by a single action, but by a combination of several actions. The specific mechanism of synergistic degradation of organic matter needs further study. In the degradation of norfloxacin (NOR), Zhu et al. [73] found that the presence of biochar-loaded nZVI/Ni bimetallic materials (BC@nZVI/Ni) can effectively activate PS to produce sulfate radicals by the electron paramagnetic resonance spectrum (EPR) test ( Figure 4). Moreover, the acidic conditions and the increase of reaction temperature are conducive to the removal of NOR. In addition, the presence of humic acid (HA), SO4 2and NO3has inhibition on the degradation of NOR, but HCO3and Clcan promote the degradation of NOR ( Figure 5).

Conclusion and Prospect
The advanced oxidation technology of persulfate activated by nZVI has been widely used in the field of environmental remediation due to its strong oxidation and low selectivity. Sulfate radicals can maintain stable activity in acidic, neutral and alkaline environments. nZVI activated persulfate advanced oxidation technology is simple, has good degradation performance, is environmentally friendly and wide range of applications. Nanozero-valent iron activated persulfate still have great potential for development in the future. Therefore, future research should focus on the following aspects :(1) Fe 2+ is the key to activating persulfate, and how to effectively control the corrosion rate of nZVI and improve the utilization rate of Fe 2+ should be considered in the future. (2) At present, the degradation effect of nano-zero-valent-iron activated persulfate on organic pollutants only focuses on the removal rate and degree of mineralization, and the degradation mechanism should be further discussed in the future.
(3) Advanced oxidation technology based on persulfate can be further studied in combination with other oxidation technologies, which can improve the degradation rate and expand the degradation range of pollutants.