Review on the degradation of chlorinated hydrocarbons by persulfate activated with zero-valent iron-based materials Open Access

Chlorinated hydrocarbons (CHCs) are an important organic solvent and chemical intermediates widely used in industrial production and human activities (Kueper et al. 1989). Most CHCs, such as trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and carbon tetrachloride (CT), have stable chemical properties and easily dissolve into non-polar substances. Therefore, in recent decades, CHCs have often been used as solvents, cleaners, or extractants in the pharmaceutical, chemical, and electrical engineering industries (Huang et al. 2014a). However, they often enter into groundwater through unreasonable discharge of industrial wastewater, accidental leakage of storage tanks, infiltration of leachate from waste storage sites, or improper leakage of toxic and hazardous chemical wastes, etc., leading to groundwater pollution and threatening human health (Aranzabal et al. 2014). Besides, CHCs can remain in the environment for a long time and they are not easily degraded naturally, making them to be a focus of attention in the field of environmental remediation.

Advanced oxidation processes (AOPs) have been widely studied in the field of wastewater treatment and used to degrade organic pollutants that are not easily biodegraded in industrial water (Yang et al. 2009). Traditional AOPs refer to oxidation technologies using hydroxyl radical (⁠⁠) as the active species, including Fenton, ozone, photocatalysis, electrochemical oxidation, microwave and ultrasonic irradiation, supercritical water and wet air oxidation, and others (Pera-Titus et al. 2004; Choi et al. 2010; Capodaglio 2020). is one of the strongest oxidants and can unselectively oxidize almost any organic compound by the following basic pathways: radical addition, hydrogen abstraction, electron transfer, and radical combination (Afzal et al. 2012; Capodaglio 2020; Ma et al. 2022). The produced reaction chains will further lead to the generation of reactive species such as H₂O₂ and super oxides (⁠⁠). Thus, the coexistence of multiple reactive species often occurs in the process of advanced oxidation reaction. These active species have a high redox potential, meaning that they can easily oxidize other substances (Moreira et al. 2017). Therefore, AOPs based on have been widely used in wastewater treatment. However, the application of this technology is limited to some extent by factors such as the narrow range of pH and complex operation (Gao et al. 2020). The disadvantages are particularly significant in the in-situ chemical remediation process of groundwater, causing researchers to increasingly shift their focus to persulfate (PS) systems (Ikea et al. 2018).

Persulfate, including peroxymonosulfate (PMS) and peroxydisulfate (PDS), generates free radicals with strong oxidation capacity through certain technical means (Oturan & Aaron 2014; Oh et al. 2016; Ren et al. 2023). PS oxidation is a promising technology based on sulfate radicals (⁠⁠) for degradation of organic pollutants. In fact, PS itself is relatively chemically stable, and barely reacts with organic compounds, but it can be activated by a variety of media to realize its oxidation function, such as heat (Zhang et al. 2015; Zhu et al. 2018a), alkali (Furman et al. 2011), ultraviolet light (Ghauch et al. 2017), microwaves (Qi et al. 2014), and transition metals (Anipsitakis & Dionysiou 2004). ⁠, with strong oxidation ability, is created during the activation process. In contrast to the non-selective oxidation of ⁠, is more inclined to oxidize unsaturated hydrocarbons, and its redox potential (E₀ = 2.5 − 3.1 V) is higher than that of (E₀ = 1.7 − 2.8 V) (Wei et al. 2016; Yan et al. 2016). Meanwhile, the half-life of (t_1/2 = 30 − 40 μs) is much longer than (t_1/2 < 1 μs) (Olmez-Hanci & Arslan-Alaton 2013; Hu & Long 2016; Matzek & Carter 2016; Shokoohi et al. 2019), ensuring that it has enough time to contact with contaminants and mineralize them. Besides, it is an environmentally friendly oxidation active free radical that can degrade organic pollutants, converting itself into non-toxic during the reaction. In addition, compared with other common oxidants, PS exhibits outstanding advantages, such as aqueous solubility, high activity, and stability, which make it be a promising oxidant for environmental remediation (Lei et al. 2015).

Although various studies on the degradation of organic pollutants in nano zero-valent iron (nZVI)–PS system have been reported, there are still some controversies about the mechanism of oxidation process. Besides, there is no conclusive study on the degradation of CHCs by zero-valent iron-based materials and PS. The main purpose of this paper is to summarize the PS activation pathways by nZVI and modified nZVI to analyze the degradation mechanism of CHCs. Meanwhile, several common factors influencing the activation process are also summarized. Finally, the application prospects of nZVI and modified nZVI to degrade CHCs are analyzed, in order to provide reference for related engineering applications.

Common transition metal ions used to activate PS include Fe²⁺, Ag⁺, Co²⁺, Mn²⁺, and Cu²⁺. Different metal ions show different activation properties under the same reaction conditions. The optimal combination of metal ions and oxidants depends on the target pollutant and reaction conditions (Wacławek et al. 2017). It seems that Ag⁺ is more effective than other ions in activating PDS to degrade organic pollutants (Gong & Lin 2011; Parenky et al. 2020), whereas Co²⁺ is more suitable for activating PMS (Anipsitakis & Dionysiou 2004). However, rare metals such as Ag and Co can easily cause secondary pollution to human body and environment, which greatly limits their practical application in groundwater treatment. By contrast, iron is considered as one of the most feasible PS activators because of its non-toxicity, environmentally friendliness and low-cost (Rastogi et al. 2009; Bae et al. 2018).

Although Fe²⁺ can be used alone to effectively activate PS, there are still some issues that need to be addressed. Firstly, acidic conditions are required for this process (Boczkaj & Fernandes 2017), and Fe²⁺ reacts very quickly with PS, usually within a few minutes. Secondly, Fe²⁺ is highly reductive, it will participate in a variety of redox reactions during the PS activation process, so it is easily oxidized to Fe³⁺, losing the ability to activate PS. To overcome these deficiencies, ZVI is used as a source of Fe²⁺ to activate PS for degradation of contaminants. The feasibility of degrading CHCs by activated PS using ZVI as the Fe²⁺ source has been demonstrated (Rajajayavel & Ghoshal 2015; Han & Yan 2016). Fe⁰ is more effective than Fe²⁺ in activating PS for mineralization of contaminants (Oh et al. 2009; Kusic et al. 2011; Barzegar et al. 2018). On the one hand, it releases Fe²⁺ slowly in case of excessive presence of Fe²⁺ in the system (Fang et al. 2013a). On the other hand, it is superior in terms of subsurface migration (Kim et al. 2018; Wu et al. 2019). It is worth noting that the ZVI particle size affects the release rate of Fe²⁺ (Rodriguez et al. 2014). Smaller particles of ZVI usually needs less time to release Fe²⁺, the higher PS activation efficiency can be achieved.

Both ZVI and nanometer ZVI (nZVI) can serve as stable sources of Fe²⁺. Although they have the same valence state, their surface structures are different. nZVI has a larger specific surface area than ZVI, which gives it stronger adsorbability, reactivity, and subsurface mobility (Lefevre et al. 2016). The catalytic performance of nZVI, ZVI, and Fe²⁺ for PS degradation of TCE has been studied (Al-Shamsi & Thomson 2013). The results showed that TCE degradation rate was the fastest in the nZVI/PS system, its degradation rate in the first 3 min was 21 times higher than that in the ZVI/PS system.

Also, some researchers proposed that Fe²⁺ was not the sole activator in the nZVI/PS system, they believed that Fe⁰ could activate PS directly to produce oxidizing radicals (Oh et al. 2010). Oh et al. (2010) indicated that the optimal molar ratio of PS to nZVI was 1 during the degradation of 2,4-dinitrotoluene (DNT), in which no was generated theoretically via reaction Equation (9), and more PS was required to produce ⁠, which was bound to increase the molar ratio of PS to nZVI to above 1. Thus, a separate mechanism was proposed that electrons could directly transfer from the nZVI surface to PS. However, the optimal molar ratio of PS to nZVI should be related to the target pollutant and not fixed (Hou et al. 2021). Therefore, the theoretical view that electrons transfer directly from nZVI to PS is unreliable.

The concentrations of PS and Fe²⁺ in solution are crucial parameters for the formation of ⁠, and the molar ratio of Fe²⁺ to PS also determines the yield of ⁠. Generally, the CHCs removal reaction is divided into two stages: in the initial stage of the reaction, the CHCs removal rate increases with the increase of Fe²⁺ or PS concentration. After the initial phase, the removal rate will decrease even if the concentration of Fe²⁺ or PS continues to increase (Gu et al. 2015). This suggests that an appropriate molar ratio of Fe²⁺ to PS, rather than a higher concentration of Fe²⁺ or PS, can obtain a better CHCs removal rate.

It has been mentioned above that transition metal ions can be oxidized to higher valence states during activation of PS (Wang et al. 2018). The formation of hypervalent metals is caused by the oxidation of transition metals by PS, resulting in formation of two oxidizing species – free radicals and hypervalent metals, both of which can degrade organic pollutants. However, the ability of high-valence metals removing organic pollutants is not absolute. The current consensus is that the reaction mechanism of hypervalent metals can transform organic pollutants, but cannot further completely mineralize them. Recent studies have confirmed that ferryl iron (Fe(IV)) is produced during degradation of sulfone organics in the nZVI/PS system. Fe(IV) converts the target pollutants to other forms without mineralizing the contaminants (Wang et al. 2020).

Besides, nZVI particles tend to aggregate due to strong interparticle attraction and inherent magnetic interaction (Li et al. 2011; Rayaroth et al. 2017), which usually results in low mobility and weak reactivity. In addition, nZVI is susceptible to be oxidized in aqueous solution, resulting in its stability and utilizability being reduced (Fan et al. 2016a; Bae et al. 2018). These defects limit its application in soil and groundwater remediation.

Given the above drawbacks of nZVI during the PS activation process, a novel material with good slow-release Fe²⁺ function, strong sustained performance, and suitability for migration in soil voids has become a focus of research. The aim of modified nZVI is to improve the PS activation ability and to enhance its mobility and stability in groundwater. Up to now, a variety of modified nZVI materials have been developed. The four main types of modification methods for nZVI that have been widely studied are supported nZVI, bimetallic modification, sulfide-modified nZVI, and surface-coated nZVI.

Heterogeneous support refers to the combination of nZVI with a solid carrier such as silica gel, carbon, or polymeric resin. In order to effectively hinder the aggregation of nZVI particles and enhance the reactivity, the above materials can be used as a supporter, so that nZVI particles can be evenly and stably dispersed on the surface of the material to form supported nZVI (Yu et al. 2019). With large specific surface area, large pore volume, low cost, and wide sources, carbon has become the most widely used supported material (Liu et al. 2016b; Fan et al. 2019). The carbon-supported nZVI composite can improve the adsorption capacity of the catalyst and it has been shown to be effective in activating PS to degrade contaminants (Wang et al. 2015). Commonly used carbon-based materials include activated carbon (AC), biochar (BC), reduced graphene oxide (rGO), and carbon nanotubes (CNTs) (Huang et al. 2014b; Wu et al. 2018a).

Carbon-based adsorbent, with high specific surface area and porosity, owns extremely strong adsorption capacity (Wei et al. 2018). The adsorption of the carrier can increase the contact chance between nZVI and pollutants so as to accelerate the reaction rate (Fan et al. 2019). When nZVI is uniformly supported on the surface of carbon-based materials, the aggregation phenomenon between particles can be eliminated (Mandal et al. 2020). Meanwhile, the mobility of nZVI is significantly enhanced, thus the reactivity can be improved (Yang et al. 2018). The degradation effect of nZVI/BC/PS system on TCE has been studied (Yan et al. 2015). In the absence of activator, PS itself cannot effectively degrade TCE. When nZVI was used as an activator, the degradation rate of TCE was significantly improved. By contrast, nZVI/BC composite catalyst was selected to activate PS for removing TCE, the degradation rate was much higher than that of pure nZVI (Shan et al. 2021a).

rGO also provides rich adsorption sites for nZVI immobilization due to its unique two-dimensional layered structure and large specific surface area (Hao et al. 2018). The activation mechanism of PS by rGO-nZVI is similar to that by BC-nZVI. In addition to supporting nZVI, oxygen-rich functional groups such as –COOH and –OH on the surface of rGO also catalyze PS to release and (Wu et al. 2018a). In addition, is also found in the rGO- nZVI system as one of the active free radicals (Ahmad et al. 2015), and tends to be produced under acidic conditions, while prefers alkaline conditions.

It is worth noting that carbon materials are easily deactivated by oxidation of free radicals, and the small molecular organic matter produced in the reaction process tends to block the internal pores of the carbon, and mask the active sites, thus reducing the removal capacity of pollutants (Wu et al. 2018b; Wang et al. 2019a).

Sulfide-modified nZVI (S-nZVI) is the chemical modification of nZVI by adding sulfide agent to form iron pyrite on the surface of nZVI particles. Research on sulfidation of nZVI has received increasing attention recently due to its ability to improve the reactivity of nZVI. S-nZVI has lower hydrophilicity and less magnetism than nZVI (Xu et al. 2019a; Mangayayam et al. 2020; Li et al. 2021a), which is mainly attributed to the formation of an iron sulfide (FeS_x) protective shell on the surface of nZVI. FeS_x can increase the surface roughness and specific surface area of the particles, which inhibits nZVI aggregation, facilitates the adsorption of pollutants, and improves the reactivity (Cao et al. 2017; Song et al. 2017). FeS_x on the surface of nZVI also prevents the corrosion reaction between nZVI and H₂O, and inhibits hydrogen precipitation (Han & Yan 2016), driving more nZVI to participate in the activation reaction.

Two main types of iron sulfide are formed in the nZVI sulfidation process, namely FeS and FeS₂. Both of them have been proven to be effective for PS activation (Fan et al. 2018; Lian et al. 2019). However, FeS minerals are generally considered to be more hydrophobic and more conductive than FeS₂ (Zhang et al. 2016; Sun et al. 2018). Therefore, S-nZVI with a FeS shell is selected for most laboratory research and engineering applications.

Meanwhile, sulfidation can also enhance the conductivity of nZVI. Delocalized electrons in the FeS interlayer can be used as an electronic conductor, promoting transfer of electrons from nZVI to pollutants, and improving the degradation rate (Kim et al. 2011; Li et al. 2016). Kim et al. (2011) studied the influence of sulfidation on the surface potential and conductivity of nZVI and found that the electronic mobility was significantly enhanced on the surface of S-nZVI.

Although the formation of FeS_x on the surface of nZVI increases the distance between particles and inhibits the aggregation of nZVI, there is no significant difference in the aggregation behavior between S-nZVI and nZVI when divalent cations exist in the system (Song et al. 2017). This is mainly attributed to that the aggregation between S-nZVI particles is affected by the inhibition of electrostatic repulsion, while divalent cations can shield the electrostatic charge on the surface of iron nanoparticles (Adeleye et al. 2013). Thus, the aggregation is not eliminated.

In recent years, S-nZVI has been successfully applied to activate PS to remove CHCs. Recent research confirmed that it was difficult to degrade TCE using PS or S-nZVI alone. The activation of PS by nZVI could reduce the concentration of TCE to a certain extent, and the removal rate of TCE by using S-nZVI/PS was higher than that of nZVI/PS at various pH conditions (Dong et al. 2019). Thus, S-nZVI was superior to nZVI in activating PS to degrade TCE. However, the dechlorination efficiency of the two did not show a significant difference. The dechlorination efficiency of TCE in acidic, neutral, and alkaline conditions was 45.83, 38.10, and 35.28% in the S-nZVI/PS system, and that was 41.71, 35.90, and 33.49% in the nZVI/PS system, respectively. This result indicated that only a portion of the TCE was completely degraded in the S-nZVI/PS system.

It should be noted that the activation effect is not proportional to the sulfide content. The reactivity of S-nZVI is affected by many factors, particularly the S/Fe molar ratio (Li et al. 2017). With the molar ratio increasing, more iron sulfide is generated on the surface of the S-nZVI, improving its conductivity and facilitating removal of organics. However, hollow structures will be formed as the molar ratio is excessive, reducing the reactivity of the S-nZVI (Xu et al. 2019b). Besides, a high molar ratio will lead to FeS₂, with lower reactivity, accumulated on the surface (Xie & Cwiertny 2010; Nunez et al. 2016), which decreases the dosage of nZVI and the surface area of composite materials, and further reduces the pollutant removal ability (Song et al. 2017; Dong et al. 2018).

Heterogeneous materials based on metal catalysts are widely used for PS oxidation due to the easy recovery and wide sources. In recent years, metal PS catalysts have been developed from single metal form to bimetal or even polymetallic forms.

Bimetallic nanoparticles formed by adding less reactive metals to nZVI can increase the reactivity of the particles and provide good protection against passivation (Yan et al. 2013). nZVI composites with other metals are one of the most widely studied bimetal systems because of their high reactivity and large specific surface area, and they have been applied in treatment of halogenated organically polluted wastewater (Su et al. 2016; Zhu et al. 2016). The catalytic activity of metal catalysts to PS is enhanced by their synergistic effect, such as electron transfer and their ilka catalytic effect on PS. Metals commonly doped to nZVI to form bimetallic system include Co, Cu, Ni, Pt, Ag, and Pd (Li et al. 2021b). Ag and Ni are toxic, Pt and Pd are rare and expensive, which restricts their environmental applications. Cu is relatively inexpensive and less toxic than other several precious metals, therefore it has been increasingly combined with nZVI. Researchers prefer to choose Cu doped onto nZVI to activate PS for removing organic pollutants.

Besides, acidic conditions can accelerate corrosion of nZVI–Cu, releasing more Fe²⁺ and producing extra (Lai et al. 2013; Hussain et al. 2017), thus achieving higher removal rate of pollutants than that in neutral or alkaline solutions. The synthesized bimetallic particles form galvanic cells on nZVI surface, promoting electron transfer and accelerating the reaction degree (Shi et al. 2016; Zeng et al. 2017). However, studies have confirmed that excessive Cu might inhibit nZVI corrosion, and the amount of doped copper required to achieve the highest removal rate differs for different pollutants (Qu et al. 2020).

In order to evaluate the CHCs degradation ability of bimetallic materials activating PS, relevant studies were also conducted. Polyvinylpyrrolidone functionalized Fe/Cu bimetallic nanoparticles (PVP-nZVI/Cu) were synthesized and used to activate PS for degradation of TCE (Idrees et al. 2021). It was found that about 5.3 and 7.1% of TCE was lost by PVP-nZVI/Cu and PS alone, respectively. PVP-nZVI activated PS obtained 68.8% of TCE removal rate. By contrast, PVP-nZVI/Cu nanoparticles could almost completely remove TCE.

Bimetallic-modified nZVI materials improve the reactivity and pollutant removal rate to some extent. However, up to now they are mostly used in remediation of heavy metal contaminated soil and groundwater in practical applications (Zhu et al. 2017, 2018b, 2018c), but rarely in combination with PS for degradation of CHCs.

Due to the van der Waals force and magnetic properties of iron, as well as the surface energy of nanometer materials, nZVI particles tend to agglomerate, affecting their migration and reactivity in soil and groundwater (Jiemvarangkul et al. 2011). Therefore, improving the dispersion of nZVI has become a research hotspot in recent years.

The purpose of nZVI surface modification is to increase its dispersion and liquidity in water medium, and the ideal modifier is characterized by strong adherence to the particle surface, strong stability, and no secondary pollution. The surface modification mechanism mainly changes the surface charge distribution of nZVI through adsorption of the modifier, and prevents electrostatic attraction and aggregation of nZVI by means of electrostatic stability, steric hindrance, or the combination of the two (Tesh & Scott 2014; Zhao et al. 2016). One method to maintain the nZVI particles in stable state is to increase the electrostatic repulsion between them. Several studies have demonstrated that certain polymeric materials can stabilize nanoparticles in aqueous solutions, including polyacrylic acid (PAA), polyaspartic acid (PAP), Tween 80, and biopolymers such as soy protein, starch, and carboxymethyl cellulose (CMC) (Liu et al. 2016a; Saha et al. 2019; Li et al. 2020). Different types of surface modifiers have been used to coat nZVI particles, ranging from long-chain polyelectrolytes such as polyethylene vinyl pyrrolidone (PVP) and CMC, to short-chain surfactants such as rhamnol (RL) (Bhattacharjee et al. 2016). High-concentration nZVI aqueous dispersion has been obtained after modification by Tween 80 (Soukupova et al. 2015). Compared with unstabilized nZVI, nZVI modified by Tween 80 has been shown strong oxidation resistance, time stability, as well as high reactivity to target pollutants. It is possible that the surface layer formed by Tween 80 on the particles plays a key role. Biosurfactants such as rhamnolipid have also been shown to be excellent surface modifiers, significantly reducing accumulation of nZVI and improving its transportation (Liang et al. 2014).

CMC is currently one of the most widely used polysaccharide derivatives for nZVI modification. Complexation between the carboxylic acid base group and iron ions, as well as the intermolecular hydrogen bonds between CMC and the iron particle surface, are considered to be the main mechanisms of stabilization. Fatisson et al. (2010) synthesized coated nZVI by CMC, and studied the effect of organic matter in natural water on the aggregation and surface charge of nanoparticles before and after coating. The results showed that CMC was bonded to the surface of the nanoparticles by covalent bonds, which effectively inhibited their aggregation in solution. The study also found that electrostatic repulsion was the main reason why CMC inhibited aggregation of nZVI particles. Meanwhile, CMC surface modification could also reduce the average size of nZVI nanoparticles and improve their specific surface area (Eljamal et al. 2020), which was conducive to the removal of pollutants. Dong et al. (2016) found that CMC-coated nZVI not only showed good dispersity and stability, but also had low toxicity to cells, alleviating the long-standing concerns about nZVI practical application.

Although surface modification has been used for field testing, the main disadvantage is that coating can occupy the reactive sites of the nZVI, which affects its ability to react with contaminants and decreases the pollutant removal efficiency. Meanwhile, there is still little research to date on the oxidation degradation of CHCs by CMC-modified nZVI activating PS, most of which focuses on the reduction effect of CMC-nZVI on CHCs. In order to facilitate the comparison of the advantages and disadvantages of the four modification methods, the above contents are simply summarized (as shown in Table 1).

In order to more comprehensively understand the degradation mechanism of CHCs by the nZVI/PS system, researchers conducted a number of exploratory experiments, and the debate about which of the two is dominant has not been settled.

The non-free radical oxidation pathways found in current PS systems are mainly oxidation and electron transfer (Hu et al. 2017; Li et al. 2019). Non-free radicals have much higher selectivity than free radicals and they can resist interference from environmental factors more effectively (Ma et al. 2018). has a selective oxidation tendency, and it can oxidize pollutants containing electron-rich functional groups (e.g. –OH), but has difficulty in degrading pollutants with electron donating groups (e.g. –Cl, –COOH, –NO₂) (Hu et al. 2021). Therefore, degradation of CHCs mainly depends on free radicals in the system.

The development of materials science is changing rapidly, the factors influencing the applications of iron-based materials activating PS are not the same. Therefore, the influencing factors in the process of removing pollutants by nZVI activating PS will be discussed from a broad perspective, rather than a specific modification technology.

The concentration of free radicals is closely related to the concentration of PS, because PS must be activated to produce highly reactive species (⁠⁠) (Miserli et al. 2022). Studies have shown that activator concentration (Zeng et al. 2022), activator type (Huang et al. 2021; Liang et al. 2021; Zheng et al. 2021, 2022; Zhu et al. 2022), and activation method (Ismail et al. 2017; Khajeh et al. 2021; Satizabal-Gómez et al. 2021) all affect the activation of PS. These studies generally show that efficiency of pollutant removal exhibits a positive relationship with concentration within a reasonable range, but excessive amounts affect the degradation rate. Wang et al. (2022) synthesized a novel porous sodalite (SOD) through reactive oxidation species from industrial waste lithium silicon fume (LSF) to stabilize nZVI. SOD@nZVI was used as an outstanding PS activator for degradation of organics. At the beginning of the reaction, with the increase of catalyst usage, more free radicals were produced and the removal rate of target pollutants increased. However, further increase of the catalyst dosage (0.4–-2.0 g/L) decreased removal rate of pollutants, which was mainly caused by the self-quenching reaction of excess free radicals. Overall, the core element of pollutant degradation is free radicals, so free radicals with high activity, effective concentration and durable stability are the focus of research.

pH is a key factor used to assess the feasibility of nZVI-based material activating PS in the in-situ remediation process (Yan et al. 2019; Rayaroth et al. 2020; Gao et al. 2021). Many studies have shown that the removal effect of organic pollutants under acidic conditions is better than that under alkaline conditions, whether in ordinary nZVI/PS systems or modified nZVI/PS systems (Liu et al. 2020; Zhu et al. 2020). The type of iron ion is significantly affected by pH value (Gao et al. 2021). As shown in Figure 2, H⁺ can inhibit the precipitation of Fe³⁺ under acidic condition, resulting in more Fe²⁺ existing in solution and promoting the utilization efficiency of iron. In contrast, ferrous hydroxide and iron hydroxide will be formed on the surface of nZVI under alkaline or neutral condition than under acidic condition (Li et al. 2014b), the released Fe²⁺ will decrease and the catalytic effect will go down significantly.

Although the degradation rate of pollutants by the nZVI/PS system and the modified nZVI/PS system increased with the increase of pH, the removal effect of target pollutants in the modified nZVI/PS system was higher than that of the pure nZVI/PS system under the same pH condition. In addition, as mentioned above, pH affects the dominant active species in the system, i.e. is more dominant in acidic conditions and is preferred in neutral and alkaline conditions. Therefore, pH is an important factor needed to be considered in practical application.

In recent years, PS has been successively applied for in-situ chemical oxidation (ISCO) to repair the polluted soil and groundwater (Tsitonaki et al. 2010). Rayaroth et al. (2020) also confirmed that the proposed ISCO system was effective for the removal of mixture of pollutants such as arsenite and 1,4-dioxane. Besides CHCs, it has been shown that PS can effectively degrade multiple pollutants, such as polychlorinated biphenyls (PCBs; Fang et al. 2013b; Fan et al. 2014, 2016b), diesel fuel (Do et al. 2010), polycyclic aromatic hydrocarbons (PAHs; Ranc et al. 2016; Song et al. 2019; Wang et al. 2021), and total petroleum hydrocarbons (TPHs; Sra et al. 2013; Lominchar et al. 2018; Ossai et al. 2020; Zhang et al. 2020; Liu et al. 2021). A large number of previous studies have only focused on the degradation of a single pollutant, ignoring the consumption of oxidants by non-target pollutants. Recently, Xu et al. (2021b) conducted comprehensive remediation of organic-contaminated sites through a modified alkaline heat/persulfate (MAH/PS) system, finding that MAH/PS were more effective in degrading benzene and 1,2-dichloroethane with simple molecular configurations. The degradation efficiency of the complex pollutants such as benz[a]anthracene, benzo[a]pyrene, and TPHs was much lower. Therefore, the structural type of non-target pollutants has a significant impact on the degradation efficiency of the system.

Different activation processes involve different mechanisms for the production of free radicals. Therefore, four factors influencing the application of iron-based materials activating PS are summarized. As mentioned above, the degradation efficiency of the iron-based materials activating PS is mainly limited by the concentration of free radicals, pH, inorganic anions, and non-target pollutants. These factors require more rigorous experimental design to achieve an economical, efficient, and environmentally friendly degradation of CHCs in engineering applications.

This paper has systematically reviewed the activation mechanism, influential factors, and treatment effects of CHCs degradation by PS activated by nZVI and its modifications. PS activated by nZVI-based materials oxidize CHCs by two types of activation: free radical oxidation (⁠⁠, ⁠, and ⁠) and non-radical oxidation (⁠⁠). The free radical oxidation pathway mainly relies on production of active species (especially and ⁠) to attack the pollutants, while non-free radical oxidation is mainly completed by electrons transfer between the pollutants and the PS. Although non-free radical oxidation cannot be ignored, free radicals play a dominant role in degradation of CHCs by nZVI activating PS.

In order to overcome the weakness of nZVI, the research on nZVI modification by different ways was discussed, including supported nZVI, sulfidation, bimetallic modification, and surface-coated modification. These modification methods improve the dispersity, stability, and migration ability of nZVI in groundwater to some extent, providing theoretical supports for overcoming some drawbacks in degradation of pollutants by nZVI/PS systems. Besides, the activation properties of nZVI-based materials are affected by the concentration of free radicals, initial pH of solution, inorganic anions, and non-target pollutants in the system. A full understanding of these adverse factors will be beneficial to the practical application of modified nZVI/PS systems. Through the above analysis, it can be concluded that the application of the modified nZVI/PS system in CHCs polluted groundwater still faces some challenges.

• i.

  Most of the advantages of modified nZVI are only exhibited under laboratory conditions. The practical application of the nZVI-based material/PS system in degradation of CHCs polluted groundwater still needs to be further verified. Besides, the high temperature, high pressure, and other harsh conditions required in the above four modification methods tend to greatly increase the use cost. Even if the modified materials can achieve better results, the acceptability of the cost needs to be reassessed.

• ii.

  In the process of in-situ groundwater restoration, over-acid and over-alkali conditions will cause irreversible damage to the soil and groundwater ecosystem. In addition, it is difficult to adjust pH in the process of in-situ restoration. Therefore, a modified nZVI/PS system that can adapt to a wider pH range is particularly important.

• iii.

  The composition of groundwater is complex, and various inorganic anions will consume free radicals, which greatly reduces the removal rate of CHCs. It is hard to undo its effects. Therefore, in the practical application process, the type and content of inorganic anions need to be tested in detail, although most of them do not belong to pollutants. In addition, if possible, different types of anion shielding techniques can be tried to block or delay the contact with free radicals, so that CHCs can be degraded preferentially in the system. By solving the above problems, a low-cost, high availability, and environmentally friendly modified nZVI/PS system for CHCs degradation is highly anticipated in practical engineering applications.

• iv.

  As for the applications of the nZVI/PS system in dealing with CHCs contamination, surface pollution is relatively easy to remove because of the controllable operating conditions. By contrast, the remediation of groundwater pollution is not easy to achieve. One of the most important limiting factors is the acidic environment. The actual pH of groundwater is close to neutral in most parts of the globe, including CHC contaminated areas. Surprisingly, rapid reactions between PS and nZVI can induce acidic conditions immediately in an aquifer with high buffering capacity, making it possible for the reaction to persist. Therefore, we believe that the nZVI/PS system has a promising future in in-situ remediation of CHCs polluted groundwater.

The authors gratefully acknowledge financial support for this work from the Scientific research project of Tianjin North China Geological Exploration Bureau (HK2022-B1).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.