Peroxymonosulfate (SO, PMS) and peroxydisulfate (S, PDS) are two common persulfates that are utilized in AOPs, both of which are chemically stable under mild conditions when they are not activated [ 53 54 ]. Taking advantage of easy separation, heterogeneous activation has attracted increasing attention compared with homogeneous systems [ 55 ], especially for nZVI, which can be conveniently recovered by a magnet and recycled. When activated with nZVI, the hydro-peroxide bond (O&#;O) of persulfate can be broken via homolytic or heterolytic cleavage and can generate various ROS for the degradation of organic pollutants [ 34 ].
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In addition, some recent investigations indicated that nZVI-based AOP treatment has the potential to destroy antibiotic resistance genes that are released from damaged antibiotic-resistant bacteria in sewage and sludge, which may be beneficial to the enhancement of the performance of disinfection and the alleviation of bacterial resistance risks [ 98 99 ]. Specifically, Duan et al. adoptedL. leaf extract-modified nZVI to activate PS to produce ROS for the removal of antibiotic resistance genes, and achieved satisfactory removal efficiencies towards1,1, and the bacterial 16S rRNA gene [ 99 100 ].
Organic pollutants not only exist in water systems, they can also be adsorbed and accumulated in soil systems, causing soil pollution and adverse effects on the growth of crops [ 93 ]. Therefore, there is an important theoretical and practical significance for the development of efficacious remediation technologies for the degradation of organic pollutants in soil [ 95 96 ]. Compared with the methods of biological degradation, physical adsorption, and reduction for the remediation of polluted soil, AOPs have more advantages concerning efficiency and operational costs [ 93 97 ]. Significantly, persulfate-based AOPs conducted using nZVI has been applied for the remediation of TBBPA-polluted soil (concentration of TBBPA was 5 mg/kg soil, nZVI dosage was 3 g/kg soil, and reaction temperature was 25 °C), petroleum-polluted soil (concentration of total petroleum hydrocarbons was ± 115 mg/kg soil, nZVI dosage was 2 g/kg soil, and reaction temperature was 25 °C), and anthracene-polluted soil (concentration of anthracene was 100 mg/kg soil, nZVI dosage was 1.77 g/kg soil, and reaction temperature was 20 °C), yielding a removing efficiency of 78.32% in 12 h, 96% in 10 h, and 76.4% in 12 h, respectively [ 56 97 ].
Notably, these research studies revealed that nZVI materials can be conveniently recovered by a magnet and reused 3&#;5 times in AOPs ( Figure 5 ), which reinforces their potential recyclable application in practice [ 58 87 ]. After storage for 6 months, nZVI/CF still had a favorable levofloxacin degradation efficiency, which confirms that nZVI materials can possess excellent long-term stability ( Figure 5 B) [ 69 ], while some recycling experiments in the reported research suffered an obvious decrease in degradation efficiency [ 60 88 ], for example, the degradation rate of atrazine was only 40.1% in the seconds run, which is a significant reduction of 53.7% below the initial rate [ 70 ].
The degradation efficiency of the organic pollutants depends on multiple factors, such as temperature, pH of the reaction solution, catalyst dosage, persulfate concentration, and impurities in water [ 92 ]. In general, an increased temperature and a low pH were classified as favorable conditions for the removal of pollutants ( Figure 2 ); however, they are inhibited when there is an excess of nZVI or persulfate ( Figure 3 ), considering the rapid self-scavenging ability of un-reacted ROS [ 68 81 ]. The presence of natural organic compounds and inorganic anions in the environment was an unfavorable condition for the removal of pollutants ( Figure 4 ) [ 62 78 ].
The catalytic application of nZVI in environmental remediation can be seriously hindered with agglomeration and passivation that arises from the high surface energy and high surface activity of NPs [ 89 ]; more and more studies have loaded nZVI on/in a porous support, such as carbon, reduced graphene oxide (rGO), reduced graphene oxide aerogel (rGOA), biochar (BC), cotton carbon fiber (CF), graphene-like carbon sheet (CS), MoSnanosheets, and so on [ 90 ]. Supported and polymer-modified nZVI was usually spherical [ 71 84 ]; however, special geometric forms of nZVI, such chains or nanocracked spheres ( Figure 1 ), may also be formed due to magnetic interactions, van der Waals forces, or the regulation of soluble functional species (i.e., phosphorus species, polyphenols) [ 45 91 ].
The applications of persulfate-based AOPs are summarized in Table 1 . nZVI and the nZVI supported by porous materials were adopted for the activation of persulfate (Na, K, or KHSO) for the degradation of organic pollutants, such as chlorophenol (CP), phenols, dichlorophenol (DCP), sulfamethazine (SMZ), sulfamethoxazole (SMX), atrazine, oxytetracycline (OTC), trichloroethylene (TCE), tetracycline (TC), tetrabromobisphenol A (TBBPA), methyl orange (MO), bisphenol A (BPA), organophosphorus pesticides (OPPs), pyrene, rhodamine B (RhB), gamma-hexachlorocyclohexane (γ-HCH), and various antibiotics ( Table 1 ).
Fe0 + 2H2O &#; Fe2+ + H2 &#; + 2OH&#;
(1)
2Fe0 + O2 + 2H2O &#; 2Fe2+ + 4OH&#;
(2)
Fe0 + O2 + 2H+ &#; Fe2+ + H2O2
(3)
Fe0 + S2O82&#; &#; Fe2+ + 2SO42&#;
(4)
Fe2+ + S2O82&#; &#; Fe3+ + SO42&#; + &#;SO4&#;
(5)
&#;SO4&#; + OH&#; &#; SO42&#; + &#;OH
(6)
&#;SO4&#; + H2O &#; SO42&#; + &#;OH + H+
(7)
Fe0 + 2S2O82&#; &#; Fe2+ + SO42&#; + &#;SO4&#;
(8)
Fe0 + 2S2O82&#; + 2H2O &#; Fe2+ + 4SO42&#; + 2&#;OH + 2H+
(9)
&#;SO4&#; + Fe2+ &#; SO42&#; + Fe3+
(10)
&#;SO4&#; + &#;SO4&#; &#; 2SO42&#; or S2O82&#;
(11)
2Fe3+ + Fe0 &#; 3Fe2+
(12)
The activation mechanism of persulfate using nZVI relies on the corrosion of the core of nZVI and the resultant release of ferrous ions [ 101 ]. The possible generation pathway of Fein persulfate-based AOPs conducted using nZVI includes the oxidation of nZVI with water, oxygen, and persulfate, which is described as Equations (1)&#;(4). After the formation of Fe, &#;SOis subsequently produced via Equation (5). The presence &#;SOradicals can also cause a reaction with HO and OHthat generates &#;OH (Equations (6) and (7)) [ 72 ]. Moreover, persulfate (besides being able to be activated with Fe) can also be directly activated with Fevia electron transfer to generate &#;SOand &#;OH (Equations (8) and (9)), both of which are the major ROS generating in persulfate-based AOPs and which played a predominant role in the oxidized degradation of organic pollutants [ 93 ]. However, excessive &#;SOor Fecan result in a rapid elimination of &#;SO(Equations (10) and (11)), diminishing the removal efficiency of the pollutants [ 87 93 ]. The byproduced Fecan be further reduced with nZVI and regenerate Fe(Equation (12)), yielding an Fe/Fecycle, and thus providing nZVI with persistent reactivity for the activation of persulfate [ 40 87 ].
2+ catalyzes HSO5&#; to produce &#;OH or &#;SO4&#;, as shown in Equations (13) and (14) [69,83,0 via electron transfer to generate &#;SO4&#; (Equation (15)), similarly to the activation mechanism of PDS [Fe2+ + HSO5&#; &#; Fe3+ + OH&#; + &#;SO4&#;
(13)
Fe2+ + HSO5&#; &#; Fe3+ + &#;OH + SO42&#;
(14)
Fe0 + 2HSO5&#; &#; Fe2+ + 2OH&#; + 2&#;SO4&#;
(15)
Furthermore, supposing that the persulfate is PMS, the formation pathway of &#;OH has a slight difference; that is, Fecatalyzes HSOto produce &#;OH or &#;SO, as shown in Equations (13) and (14) [ 40 88 ]. PMS can also directly react with Fevia electron transfer to generate &#;SO(Equation (15)), similarly to the activation mechanism of PDS [ 84 ].
4&#; and &#;OH are considered as the major ROS that are responsible for the degradation of organic contaminants (64,65,67,71,72,73,74,75,76,81,4&#;. This may be attributed to the hydrolysis of SO4&#; (Equations (6) and (7)) during the reaction, which can be promoted by the modification of nZVI, and thus generate more &#;OH and incidentally cause the system to be acidic [4&#; plays a more dominant role in the reaction compared to &#;OH [59,66,68,69,80,83,84,4&#;, while neutral and alkaline conditions were more conducive to the formation of &#;OH (4&#; in the degradation process [+ and scarce in OH&#;, both of which are adverse to the formation of &#;OH via Equations (6) and (7). Instead, the alkaline condition is rich in OH&#; and scarce in H+, which will facilitate the reaction of Equations (6) and (7) and transform more &#;SO4&#; ions into &#;OH.According to the reported studies on persulfate-based AOPs, both &#;SOand &#;OH are considered as the major ROS that are responsible for the degradation of organic contaminants ( Table 1 ) [ 15 82 ]. With regard to the dominant ROS, a few research studies revealed that &#;OH played a dominant role in the degradation compared to &#;SO. This may be attributed to the hydrolysis of SO(Equations (6) and (7)) during the reaction, which can be promoted by the modification of nZVI, and thus generate more &#;OH and incidentally cause the system to be acidic [ 57 63 ]. Additionally, most of the research stated that &#;SOplays a more dominant role in the reaction compared to &#;OH [ 40 88 ]. Investigations suggested that an acidic environment is favorable for the generation of &#;SO, while neutral and alkaline conditions were more conducive to the formation of &#;OH ( Figure 6 A); hence, this could have affected the domination of &#;OH and &#;SOin the degradation process [ 60 76 ]. The acidic condition is rich in Hand scarce in OH, both of which are adverse to the formation of &#;OH via Equations (6) and (7). Instead, the alkaline condition is rich in OHand scarce in H, which will facilitate the reaction of Equations (6) and (7) and transform more &#;SOions into &#;OH.
2+ via the reaction of Fe0 and H+ (5&#; or S2O82&#;) and further inhibit the activation of persulfate [Overall, acid condition is suitable for the activation of persulfate [ 57 ]. In fact, an acid condition is a benefit for the formation of Fevia the reaction of Feand H Figure 2 F); furthermore, the surface of a catalyst will be negative charged in the alkaline condition, which will cause the catalyst to repel the persulfate anion (HSOor S) and further inhibit the activation of persulfate [ 57 ]. Thus, the optimal pH of the reaction solution for the degradation of contaminants is usually acidic ( Figure 2 ), for instance, 4-CP can degrade more effectively in a pH of 6 than in a pH of 11 [ 57 ].
2&#;) and singlet oxygen (1O2) may also be the dominant ROS for the degradation of contaminants (85,86,1O2 and &#;OH were verified as the dominant ROS (2&#; and 1O2 played dominant roles in their AOP system (2&#; and 1O2 is shown as Equations (16)&#;(21). The formation of &#;O2&#; (E0 = 1.56 V) is originated from the hydrolysis of persulfate and the reaction between Fe2+ and O2 (Equations (16)&#;(18)) [1O2 (E0 = 0.81 V) is usually generated from the reactions among &#;O2&#;, &#;OH, and &#;SO4&#; via Equations (19)&#;(21) [1O2 [S2O82&#; + 2H2O &#; 2SO42&#; + HO2&#; + 3H+
(16)
S2O82&#; + HO2&#; &#; SO42&#; + &#;SO4&#; + &#;O2&#; + H+
(17)
Fe2+ + O2 &#; &#;O2&#; + Fe3+
(18)
2&#;O2&#; + 2H+ &#; 1O2 + H2O2
(19)
&#;O2&#; + &#;OH &#; 1O2 + OH&#;
(20)
&#;O2&#; + &#;SO4&#; &#; SO42&#; + 1O2
(21)
Aside from the above-mentioned dominant ROS modes in persulfate-based AOPs conducted with nZVI, some researchers declared that the superoxide radical (&#;O) and singlet oxygen () may also be the dominant ROS for the degradation of contaminants ( Table 1 ) [ 77 87 ]. Huang et al. prepared supported nZVI with P-doped biochar and employed it in persulfate-based AOPs, in whichand &#;OH were verified as the dominant ROS ( Figure 6 B) [ 85 ]. A study conducted by Cao et al. demonstrated that &#;Oandplayed dominant roles in their AOP system ( Figure 6 C) [ 87 ]. The probable generation pathway of &#;Oandis shown as Equations (16)&#;(21). The formation of &#;O(E= 1.56 V) is originated from the hydrolysis of persulfate and the reaction between Feand O(Equations (16)&#;(18)) [ 102 ].(E= 0.81 V) is usually generated from the reactions among &#;O, &#;OH, and &#;SOvia Equations (19)&#;(21) [ 79 80 ]. And, the introduction of N species on the nZVI material is commonly considered to be a profit for the generation of 77 ].
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1O2) dominated the pattern in acidic conditions and radicals (&#;SO4&#;) dominated the pattern in alkaline conditions [+/OH&#;-dependent reaction pathway determines the formation of ROS (Equations (19) and (20)), the electron donating capacity of nZVI at different pH and the M(n+1)+/Mn+ redox cycles between Fe species or doping metals (i.e., the Co metal) also matters [60,62,71,72,73,78,81,Co2+ + S2O82&#; &#; Co3+ + SO42&#; + &#;SO4&#;
(22)
Fe2+ + Co3+ &#; Fe3+ + Co2+
(23)
Cu+ + S2O82&#; &#; Cu2+ + &#;SO4&#; + SO42&#;
(24)
2Cu2+ + Fe0 &#; 2Cu+ + Fe2+
(25)
Ni+ + S2O82&#; &#; Ni2+ + &#;SO4&#; + SO42&#;
(26)
2Ni2+ + Fe0 &#; 2Ni+ + Fe2+
(27)
Mo4+ + HSO5&#; &#; Mo5+ + &#;SO4&#; + OH&#;
(28)
Mo5+ + HSO5&#; &#; Mo6+ + &#;SO4&#; + OH&#;
(29)
Mo4+ + 2Fe3+ &#; Mo6+ + 2Fe2+
(30)
Mn2+ + HSO5&#; &#; Mn3+ + SO42&#; + &#;OH
(31)
In the research conducted by Huang et al., the activation mechanism of persulfate was proposed, as nonradicals () dominated the pattern in acidic conditions and radicals (&#;SO) dominated the pattern in alkaline conditions [ 60 ]. Besides the H/OH-dependent reaction pathway determines the formation of ROS (Equations (19) and (20)), the electron donating capacity of nZVI at different pH and the M/Mredox cycles between Fe species or doping metals (i.e., the Co metal) also matters [ 60 ]. Additionally, results from the electron paramagnetic resonance (EPR), X-ray photoelectron spectrum (XPS), and a series of screening experiments revealed the synergistic effect between Fe and Co/Cu/Ni/Mo/Mn ( Figure 6 D) in redox cycling (Equations (22)&#;(31)) [ 40 84 ].
2+, imposing an impact on the generation of ROS and further affecting the degradation efficiency of contaminants [&#;, Br&#;, NO3&#;, CO32&#;, HCO3&#;, PO43&#;, H2PO4&#;) and natural organic matter (i.e., humic acid (HA)) existing ubiquitously in natural water can often bring varying diverse effects on the degradation efficiency of contaminants (70,73,81,4&#;, and generating less oxidative capacity free radicals (Equations (32)&#;(37)), but to HA as well [73,81,3&#; > PO43&#; > NO3&#; > Cl&#; [3&#; > SO42&#; > Cl&#; > NO3&#; [3&#; and H2PO4&#; are believed to impose a more significant inhibiting effect on contaminant degradation compared with other inorganic anions (2+, the latter of which will make Fe species unavailable for persulfate activation [71,
Cl&#; + &#;SO4&#; &#; SO42&#; + &#;Cl
Cl&#; + &#;OH &#; OH&#; + &#;Cl
(32)
Br&#; + &#;SO4&#; &#; SO42&#; + &#;Br
Br&#; + &#;OH &#; OH&#; + &#;Br
(33)
NO3&#; + &#;SO4&#; &#; SO42&#; + &#;NO3
NO3&#; + &#;OH &#; OH&#; + &#;NO3
(34)
CO32&#; + &#;SO4&#; &#; SO42&#; + &#;CO3&#;
CO32&#; + &#;OH &#; OH&#; + &#;CO3&#;
(35)
HCO3&#; + &#;SO4&#; &#; SO42&#; + &#;HCO3
HCO3&#; + &#;SO4&#; &#; SO42&#; + &#;CO3&#; + H+
(36)
HCO3&#; + &#;OH &#; OH&#; + &#;HCO3
HCO3&#; + &#;OH &#; H2O + &#;CO3&#;
(37)
Additionally, regulating the reactivity of nZVI alters the production rate of Fe, imposing an impact on the generation of ROS and further affecting the degradation efficiency of contaminants [ 75 ]. Furthermore, inorganic anions (Cl, Br, NO, CO, HCO, PO, HPO) and natural organic matter (i.e., humic acid (HA)) existing ubiquitously in natural water can often bring varying diverse effects on the degradation efficiency of contaminants ( Figure 4 ) [ 60 103 ]. This may attribute not merely to inorganic anions having the ability to quench ROS, such as &#;OH and &#;SO, and generating less oxidative capacity free radicals (Equations (32)&#;(37)), but to HA as well [ 104 ]. HA can not only competitively react with radicals, but can also block the active sites of catalyst, resulting in an increased inhibiting effect on the removal efficiency of contaminants with the increase of their concentration [ 71 87 ]. As for the intensity of the inhibitory effects, the results in the research conducted by Diao et al. indicated that the effect occurred as follows: HA > HCO> PO> NO> Cl 67 ]. Alternatively, results in the research conducted by Rao et al. indicated that it occurred as follows: HCO> SO> Cl> NO 73 ]. Furthermore, HCOand HPOare believed to impose a more significant inhibiting effect on contaminant degradation compared with other inorganic anions ( Figure 4 ), and owe this to the low oxidative capacity of their corresponding free radicals and their complex with Fe, the latter of which will make Fe species unavailable for persulfate activation [ 70 105 ].
2 and H2O with ROS. In the proposed degradation process, these macromolecular compounds firstly suffer from the attack of ROS and form a variety of intermediates, which are subsequently oxidized into small intermediates and gradually decomposed to much smaller intermediates, and are ultimately mineralized to CO2, H2O, or/and inorganic salts. The suggested degradation mechanisms of RhB and BPA are shown in 77,The organic contaminants, especially macromolecular compounds, cannot be directly oxidized to COand HO with ROS. In the proposed degradation process, these macromolecular compounds firstly suffer from the attack of ROS and form a variety of intermediates, which are subsequently oxidized into small intermediates and gradually decomposed to much smaller intermediates, and are ultimately mineralized to CO, HO, or/and inorganic salts. The suggested degradation mechanisms of RhB and BPA are shown in Figure 7 84 ].
To enhance the efficiency of ZVI, nanoscale zero-valent iron (nZVI) was invented. Nanomaterials are defined as those materials whose key physical characteristics are defined by the nano-objects they contain (Kharisov et al. ). nZVI ranges from 10 to 100 nm (Li et al. ).
For laboratory-sized applications, the generally used method for nZVI manufacturing is the boron hydride reduction of ferrous salts (Crane and Scott ; Chekli et al. ; Sun et al. ). But for the production of a bigger amount, there are many different methods, e.g. chemical vapour deposition, pulsed laser ablation, sputtering gas aggregates, thermal decomposition, thermal reduction of oxide compounds and many more (Crane and Scott ).
An enormous advantage of nZVI is that it can be directly injected into the contaminated groundwater plumes or source zones without the need for ex situ methods or intensive digging and without clogging of pores (Cundy et al. ). nZVI is generally injected into the subsurface as a slurry or as an emulsion with a hydrophobic fluid, this should prevent the agglomeration of particles and enhance the reactivity and mobility (Cundy et al. ). Due to this method, the injection wells can be installed at almost any location and depth with minimal disturbance to the area above (Chekli et al. ; Hashim et al. ). Even areas that are unreachable with most other conventional technologies, including those underneath surface barriers, where only in situ methods are feasible, can be treated (He et al. ). The injected nanoparticles should be virtually immobile once the external injection pressure is released and they should pose no adverse environmental effect (Zhao et al. ).
The application of nZVI for environmental remediation (groundwater, wastewater, soil) has extensively been studied in the last 15&#;20 years (Calderon and Fullana ; Guan et al. ; Vanzetto and Thomé ; Li et al. ). Contaminated sites often contain a complex and varied mixture of contaminants from heavy metals to organic pollutants (Cundy et al. ; Li et al. ). Because of such complex contaminations, it is highly important that nZVI can perform fast and concurrent removal of different heavy metal ions (Li et al. ). There are already some field applications of nZVI, most sites are located in the USA, and only a small number of pilot studies have been conducted in Europe (Karn et al. ).
The zero-valent iron nanoparticles age rapidly in water due to interaction with dissolved compounds and corrosion, and this results in structural and chemical changes of the nZVI depending on the environment (Calderon and Fullana ). Calderon and Fullana () studied the ageing of nZVI with Zn, Cd, Ni, Cu and Cr as typical contaminants found in groundwater and wastewater. The removal efficiency of all elements in the first 3 h was &#;&#;&#;99.9%, but only Cr showed no remobilization after an extended period, because it was the only element that was incorporated into the nZVI core. Cd and Ni showed the maximum remobilization with 65% and 27% after 21 days. Crane et al. () and Dickinson and Scott () showed similar results in the removal of uranium with nZVI, with a partial release of 20% of the retained uranium after one week and 60% after 3 weeks. At a Portuguese metal contamination site, Gonçalves () applied nZVI and observed that at first the concentration was reduced to 60% below initial concentration, but within 4 weeks the elevation of the contaminants was close to the initial values. In contrast, Gonçalves () attributed this effect to groundwater recharge and not to some kind of ageing effect of the nZVI. Calderon and Fullana () also proved that mainly the pH changes of the solution were responsible for the release of metal ions into the water; this also shows that some metals are only in equilibrium with the shell of the nZVI and not entrapped in the core.
The different factors (e.g. mobility, reactivity, tendency to aggregate) determining the feasibility of nZVI depend heavily on properties such as particle size, surface charge, surface chemistry and bulk composition (Chekli et al. ). The particle size of nZVI has a major influence on the reactivity and mobility of the nanoparticles (Hassellöv et al. ; Christian et al. ). In comparison with highly dispersed nanoparticles, highly aggregated particles have reduced available reactive surface sites (Nurmi et al. ). The composition of the oxide shell and the metallic iron content of the implemented nZVI has a substantial influence on the efficacy of the nZVI treatment (Grieger et al. ; O&#;Carroll et al. ). The composition of the outer shell is dependent on the method of synthesis and the nanoparticle environment (Nurmi et al. ).
Factors affecting the reactivity of nZVI are diverse and include surface area, nZVI stabilizer, pH, soil organic matter, soil type, temperature, dissolved oxygen, soil moisture and aged soil (Jiang et al. ; Xue et al. ; Zhao et al. ). The reactivity of some pollutants with ZVI depends also on the age of the pollutants in the soil, and El-Temsah and Joner () reported that the degradation of dichlorodipenyltrichloroethane (DDT) in spiked soil was 50%, whereas the removal efficiency in aged DDT polluted soil was only at 24%. This could be due to desorption, solubilization and dissolution of DDT in the soil. A weakly acidic soil increases the dechlorination rate of polychlorinated biphenyls (Wang et al. ). The pH also has an impact on the reactive lifetime due to corrosion effects and the reactivity towards the target contaminant (Zhao et al. ; Dickinson and Scott ). The pH is also a key parameter for metal transfer in soil, e.g. the As availability is lower in acidic soils than in calcareous ones (Xue et al. ; Gil-Díaz et al. ). The soil organic matter has many different contrary effects; for example, the organic matter can coat the nZVI and form a barrier for electron transfer which inhibits the reactivity towards the contaminants, but the same effect leads to a prolonged reactive lifetime of the nZVI (Zhang et al. ). Zhao et al. () also concluded that the soil organic matter can form complexes with heavy metals and thus change the adsorption and immobilization of metals. Stabilizers for nZVI lead to a better soil mobility due to less particle agglomeration (Jiang et al. ). It was also proved that the efficiency of nZVI in soil is related to the leachability and bioavailability of contaminants, higher leachability and bioavailability are beneficial, but those factors depend heavily on the soil type (Xue et al. ; Chen et al. ). For example, a higher sand content in comparison with a high clay content was shown to be preferable for a high removal rate (Chen et al. ).
Even though the chemistry behind nZVI and ZVI or mZVI (macroscale ZVI) is much the same, nZVI, due to the large surface area, is more reactive than ZVI, shows higher reductive efficiency and is also able to migrate in the underground to some extent, which allows active remediation not only of the plume but also of the source of contaminants (Mueller et al. ). nZVI holds even some more advantages, as summarized by Guan et al. (): (1) it can successfully remove some aqueous contaminant species due to its large surface area that mZVI cannot remove, (2) it degrades some contaminants more rapidly and (3) it avoids the formation of some undesirable by-products due to a more complete degradation.
nZVI has been proved to be a feasible method to transform a wide range of contaminants to low toxicity or inert compounds (Crane and Scott ; Xue et al. ), including all kinds of metals and metalloids like chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), nickel (Ni), zinc (Zn), cadmium (Cd), arsenic (As), selenium (Se) and actinides such as uranium (U) and plutonium (Pu) (e.g. Table 4). Also some organic compounds like chlorinated pesticides, organophosphates, nitroamines, nitroaromatics, p-chlorphenol and some inorganic anions such as nitrate could be removed (e.g. Table 4), whereas nitrate was reduced to ammonia (Sohn et al. ).
Table 4 Compilation of different studies regarding the application of nZVIFull size table
Even though nZVI seems to be already more feasible than ZVI for environmental remediation, the enhancement of nZVI has been thoroughly studied (Jiang et al. ; Zhao et al. ), especially since bare nZVI has a limited soil mobility and deliverability (Crane and Scott ). Thus, in order to enhance the feasibility of nZVI and decrease the toxic effects on soil micro-organisms and the entire biosphere, chemical and physical methods were developed to improve the performance (Jiang et al. ). Physical enhanced technologies include dispersion and acoustic cavitation of nZVI using ultrasonic technology for nanoparticle, UV light or electric forces (Jiang et al. ). The nanoparticles can also be combined with another metal, bound onto a support or the surface modified (Xue et al. ). For example, to improve the reduction reactions nZVI has been coated with a catalyst, such as platinum (Pt), nickel (Ni), gold (Ag), palladium (Pd) or copper (Cu), and those products are then called bimetallic nanoparticles (Guan et al. ). nZVI could also be deployed in a combination with biotechnology, such as phytoremediation or microbial remediation, which would be an environmental-friendly and cost-effective remediation strategy (Jiang et al. ).
Stabilized nZVI has three significant advantages: (1) the stabilizers prevent the aggregation through electrostatic and/or steric repulsion, (2) they may offer a greater reactivity and (3) due to different physical and chemical properties the dispersibility, chemical reactivity, reactive longevity and toxicity of nZVI may be better controlled (Zhao et al. ; Xue et al. ). The coating of stabilizers onto nanoparticles may also passivate the reactive particle surface from reaction with the media (e.g. H2O and dissolved oxygen) (Zhao et al. ).
Zhao et al. () categorized possible stabilizers for nZVIs into the following five groups: (I) surfactants, (II) synthetic or natural macromolecules or polyelectrolytes, (III) viscosity modifiers, (IV) oil emulsifiers and (V) micro-scale solid supports or coatings. Stabilized nZVI shows promising results for the degradation of redox-active organics and reductive immobilization of many metals, metalloids and radionuclides (Zhao et al. ). Cao et al. () tested the feasibility of nZVI in combination with organic acids. The removal efficiency of heavy metals was improved when nZVI was combined with organic acids, such as citric acid, tartaric acid and oxalic acids. The removal efficiency increased with increasing concentration of nZVI and solid-to-liquid ratio and decreased with increasing solution pH. Gopal et al. () enhanced nZVI-Cu particles with bentonite support to overcome colloidal attraction and enhance total carbon removal. Li et al. () anchored nZVI on Zn-MOF-74, thus significantly enhancing the removal of U(VI).
nZVI is extremely challenging to characterize and analyse due to their high reactivity, but it is crucial for the understanding and reproducibility of each project to report appropriate and detailed characterization of the nZVI (Chekli et al. ). There is also a definite lack of appropriate analytical methods for on-site detection and measurement of nZVI (Chekli et al. ). Natural environments and contaminations sites can vary widely from studies conducted in a closed system in the laboratory, and this can lead to a drastic overestimation of nZVI performance (Calderon and Fullana ; Guan et al. ; Li et al. ; Ye et al. ).
One of the biggest disadvantages of nZVI is its tendency for agglomeration during migration probably due to weak surface charges, and this leads to a reduced surface area and mobility as well as a deterioration of reactivity and an overall reduced efficiency (Chekli et al. ; Sun et al. ; Ye et al. ; Guan et al. ). However, sometimes lower reactivity is preferred, especially when treating slow moving groundwater, as in this case high reactivity requires more frequent injection of nZVI, which also leads to undesired cost and maintenance issues. Another one is the high reactivity of dry, bare nZVI with oxygen in the atmosphere; this leads to a passivation of the surface with a thin oxide layer (Chekli et al. ; Sun et al. ; Ye et al. ; Guan et al. ). This exothermic reaction entails also a logistic and safety problem, and there are two main approaches to make nZVI transportable: firstly as a slurry and secondly to deliberately passivate the surface (Ribas et al. ). To mitigate the decrease in reactivity due to the passivation, Ribas et al. () invented an activation process, which degrades the oxide shell and enhances the electron transfer and increases the specific surface area of the nZVI.
Cundy et al. () reasoned that nZVI could not be employed in a permeable reactive barrier (PRB) system or a similar flow-through application due to their lack of durability and extremely high pressure drops. Seven years later, Zhang et al. () confirmed the feasibility of nZVI as the reactive medium in PRB on the basis of in situ remediation of uranium-contaminated red soils, even though there are still major influences (poor stability, mobility of nZVI and tendency for aggregation), limiting the reduction reactivity in the soil. This technology may not be suited for sites with a slow contaminant desorption rate, as a result of the relative short reactive lifetime (Zhao et al. ).
nZVI that was injected during the in situ treatment of a contaminated site was left in the underground (Crane and Scott ). The contaminants that were not destroyed by chemical reactions were trapped in an immobile state and were not extracted from the groundwater or soil (Calderon and Fullana ). If those metastable systems gradually reverse towards a pre-injection state, a significant remobilization can occur (Calderon and Fullana ; Crane et al. ; Crane and Scott ). nZVI also has the disadvantage of relatively high costs in contrast to macro-ZVI and granular iron (Table 5).
Table 5 Short overview of the costs of nZVI, ZVI and granular ironFull size table
Many toxicological concerns derive from the beforehand desired properties of nZVI like the small size, high reactivity and mobility (Cundy et al. ). Most research works regarding the toxicity of nZVI have shown a negative impact on many micro-organisms, especially for gram-negative bacteria (Fajardo et al. ). According to Vanzetto and Thomé (), USA and China are the leading nations in research regarding the toxicological impact of nZVI. The cytotoxicity includes disruption to the cell membrane integrity, interference with respiration and oxidative damage of DNA or enzymatic proteins by the generation of reactive oxygen species (ROS) (Vanzetto and Thomé ; Xie et al. ; Xue et al. ). The general trend was that with increasing nZVI concentration the toxicity to bacterial cells also increases (Fajardo et al. ), even though some researchers found the inverse trend that high concentrations were less toxic (Fajardo et al. ). Some surface stabilizers like CMC (carboxymethyl cellulose) are able to decrease the damages to cell membrane integrity, whereas the modification with other metals increases the toxicity (Xue et al. ). The treatments of aquifer sediments with nZVI showed a long-term adverse impact on the micro-organism (Vanzetto and Thomé ). It was found that fungal cells have a higher tolerance for nZVI than bacteria cells (Xue et al. ). It was observed that the organic matter decreases the toxicity of nZVI (Navarro et al. ; Pawlett et al. ). Aged nZVI also had a reduced toxic effect, probably due to the formation of the iron oxide layer (El-Temsah and Joner ). Baragaño et al. () found the best results of removal rate of 89.5% at a dose of 2% nZVI (also tested 0.5%, 2%, 5% and 10%), with no negative effects on soil parameter and the soil phytotoxicity was also reduced. Dong et al. () summarized the effect of nZVI on three types of functional anaerobic bacteria in the remediation of contaminated groundwater. The study proved that nZVI could create suitable environmental conditions for cell growth, but that it was highly dependent on the species and dosage of nZVI.
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