The application of the zerovalent iron (ZVI) for water treatment of metal(loid) (oxyan)ions has been a research hotspot in recent years. In practical applications, how to simultaneously improve the reactivity and electron efficiency of contaminants sequestration by ZVI are highly critical for the progress in ZVI-based technology. This review summarizes the improvements of ZVI technologies proposed in the past 10 years (2011—2021), including the sulfidation, weak magnetic field (WMF), dosing of Fe2+ and oxidants, along with other novel technologies. In addition, the performances (e.g., reactivity, removal capacity and electronic efficiency) and mechanisms of these technologies for contaminants removal are summarized and compared. The enhanced performance of ZVI technology should be complementarily analyzed not only from the broad-spectrum studies of different systems but also from the specific study of a single system. Finally, in order to promote the further improvement and development of ZVI technology, the future research direction of ZVI technology is outlooked. This review is expected to provide a new research direction and a complete theoretical basis for improving the performances of ZVI technology in real environmental application.
Contents
1 Introduction
2 Methods for enhancing reactivity of ZVI
2.1 Sulfidation
2.2 Addition of divalent metal cation
2.3 Weak magnetic field
2.4 Premagnetization
2.5 Addition of oxidants
2.6 Novel enhanced methods
2.7 Summary
3 Methods for enhancing electron efficiency of ZVI
Jinhui Zhang, Jinhua Zhang, Jiwei Liang, Kaili Gu, Wenjing Yao, Jinxiang Li.
Progress in Zerovalent Iron Technology for Water Treatment of Metal(loid) (oxyan) Ions: A Golden Decade from 2011 to 2021.
Progress in Chemistry, 2022, 34(5): 1218-1228 DOI:10.7536/PC210622
In 1994, Gillham and O 'Hannesin and Matheson and Tratnyek published papers on the removal of halogenated organics from water by ZVI respectively, and ZVI technology began to attract attention in the field of water treatment because of its low cost and convenient operation[1][2][3,4]. Researchers have applied ZVI technology to remove various pollutants in water, such as halogenated organics, nitroaromatics, dyes, phenolic compounds, heavy metals and oxyanions[1,2,5][6,7][8][9][10][11⇓⇓⇓~15].
The mechanism of ZVI removing pollutants in water is mainly reduction, oxidation and adsorption coprecipitation[16,17]. ZVI undergoes a series of corrosion reactions in the H2O/O2 system, and the transformation of the released intermediate species (such as Fe2+) is accompanied by the release of hydrogen and (or) the reduction of oxygen (Fig. 1)[18,19]. However, in the long-term practical application, the defects of ZVI, such as low reactivity, low selectivity and easy passivation, and limited treatment effect on some refractory pollutants under aerobic conditions, are gradually exposed[20⇓~22]. Guan et al. Summarized the countermeasures proposed from 1994 to 2014 to overcome or improve the defects of ZVI technology, which can be mainly divided into seven categories: (1) pretreatment of ZVI to remove the original passivation layer, such as pickling, H2 and ultrasonic pretreatment[20]; (2) synthesize nanoscale zero-valent iron (nZVI) to increase that surface area of ZVI; (3) synthesis of ZVI based bimetals, such as Ni-Fe0 and Pd-Fe0;(4), using the catalytic ability of noble metals to enhance the performance of ZVI by physical methods, such as external ultrasound, ultraviolet-visible light, microwave and weak magnetic field; (5) loading ZVI on other adsorptive materials, such as ZVI/activated carbon, ZVI/zeolite, ZVI/pillared bentonite, and ZVI/iron oxide; (6) chemical methods are used to enhance the performance of ZVI, such as the addition of organic ligands and ions; (7) activate and aging ZVI, such as electrochemical method, addition of Fe2+ or Mg2+, external weak magnetic field and bacteria-mediated regeneration. Although these technologies can improve the reactivity of ZVI to remove pollutants to a certain extent, it is still difficult to be used in practical wastewater treatment. For example, the cost of pickling pretreatment is high, and a large amount of waste liquid containing metal ions is difficult to treat; The operation of H2 pretreatment is complex; The synthesized nZVI particles are easy to agglomerate and have potential biological toxicity; Ultrasound, ultraviolet-visible light, microwave, electrochemical and other methods need to consume additional energy; Loss of precious metals and secondary passivation in bimetallic system.
In the past, most of the methods to improve the pollutant removal efficiency of ZVI were based on breaking the passive film on the surface of ZVI or improving its surface activity. In the long-term reaction process, the regeneration of the passive film will still affect the ability of ZVI to remove and degrade pollutants. In the past, the research usually focused on the reaction rate of ZVI decontamination, and there were few evaluation indexes for various lifting technologies, and there was a lack of evaluation for the electronic efficiency of ZVI decontamination.
Liu et al. Studied the corrosion of ZVI and its Electron transfer, proposed the index of electron selectivity of ZVI, and first proposed the concept of electron efficiency (EE), which is the ratio of the amount of electrons used by ZVI to reduce pollutants (Ne) to the total amount of electrons provided by ZVI (Nt) in the process of removing pollutants, to quantitatively evaluate the electron selectivity of ZVI[23⇓~25]. The specific calculation formula is shown in Formula 1.
EE=
It can be seen that how to simultaneously improve the reactivity and electronic efficiency of ZVI reduction decontamination is very important to promote the further development of this technology. In the past groundwater remediation and wastewater treatment, the rapid reduction reaction of ZVI and target pollutants was generally pursued, ignoring the importance of ZVI utilization efficiency[24]. As an electron donor, the electrons given by ZVI are not only transferred to the target pollutant, but also other oxidizing substances (such as O2 and H+) in the water will compete for electrons and reduce the electron efficiency of ZVI, which is usually an undesirable side reaction[26]. Although some lifting technologies promote the corrosion and electron release of ZVI, most of the released electrons are consumed by oxidizing substances such as O2 and H+, and less electrons are really transferred to the target pollutants, resulting in excessive dosing, excessive consumption and waste of electrons of ZVI, which increases the cost of ZVI technology and the difficulty of subsequent treatment and disposal[27,28].
Dissolved oxygen (DO) is ubiquitous in shallow groundwater or industrial wastewater. As a highly reactive oxidizing species, DO has a significant impact on the corrosion of ZVI and its subsequent removal of pollutants[29][30]. No uniform conclusion has been reached to date regarding the potential impact of DO on ZVI performance. Some studies suggest that a certain concentration of DO can promote the removal of pollutants by ZVI, but excessive DO has a more obvious inhibitory effect on the removal of pollutants by ZVI[31]. Sun et al. Proposed that different reaction conditions lead to different effects of DO in different studies, and whether DO plays a promoting role depends largely on the removal mechanism of the target pollutant[32]. For pollutants removed by adsorption co-precipitation or oxidation process, DO promotes the formation of ZVI corrosion products or participates in the oxidation reaction itself, which usually plays a promoting role[18]. However, for the removal of pollutants involving the reduction process, the presence of DO will compete for the electrons given by ZVI, and the corrosion products will form and deposit on the surface of ZVI, which will hinder the conduction of electrons from the internal ZVI and inhibit the reduction of pollutants by ZVI[33].
In recent years, there have been more and more studies on the electronic efficiency of ZVI decontamination, which has filled the gap in the research field that only focused on the reactivity of ZVI in the past, and the proposed technical means for improving ZVI have become more and more mature[34⇓⇓~37]. Therefore, on the basis of the existing research at home and abroad, this paper reviews the ZVI upgrading technology proposed in the past decade (2011-2021) and its improvement factor for the reactivity of ZVI to remove (similar) metal (oxygen-containing) ions in water.Sulfuration, addition of Fe2+, addition of H2O2 and external weak magnetic field, which have good application prospects, are used to summarize the improvement times and mechanism of ZVI removal capacity and ZVI electronic efficiency, so as to promote the further improvement and development of ZVI technology.
2 Technical Means to Improve ZVI Reactivity
In view of the defects of ZVI technology, the upgrading technology proposed in the past decade is no longer confined to breaking the passive film on the surface of the original ZVI, but pays more attention to the improvement of the reactivity of ZVI in the long-term application process. In this section, the research progress of sulfurization, addition of divalent metal cations, external weak magnetic field (WMF), pre-magnetization and addition of oxidants to enhance the reactivity of ZVI are reviewed, and the enhancement factors are summarized.
Taking sulfidation as an example, the ratio of the reaction rate constant K+S for pollutant removal by S-ZVI to the reaction rate constant K-S for pollutant removal by ZVI, , is calculated according to Equation (2), and the value of is the multiple of the improvement of ZVI reaction activity by sulfidation. In the same way, the enhancement factor of ZVI reaction activity by pre-magnetization can be calculated.Addition of Fe2+, addition of H2O2, enhancement factor of WMF/Fe2+ system and enhancement factor of WMF RWMFK.
RSK=
2.1 Vulcanization
As early as 1994, it was shown that sulfide could promote the removal of pollutants by ZVI, but it was not until Kim et al. First synthesized zero-valent iron sulfide nanoparticles in 2011, which significantly improved the removal rate of TCE by ZVI, that the research on zero-valent iron sulfide began to attract more and more attention[38][39]. Trichloroethylene (TCE) is an organic pollutant that has been widely studied. Fan et al. Found that sulfurization can improve the dechlorination rate of ZVI and inhibit the production of H2 in the reaction[40]. Gu et al. Showed that ZVI almost completely lost its reactivity when the pH value was greater than 9, while S-ZVI did not show any decrease in activity during the whole reaction stage when the pH value was between 6 and 10. Sulfidation expanded the effective working pH range of ZVI[41]. Wang et al. Applied S-ZVI to the decolorization of dyes, and the decolorization rate of dyes was increased from 52% ~ 79.4% to 69.5% ~ 100% by sulfurization, and the ratio of decolorization rate of S-ZVI to that of ZVI was 1.3 ~ 1.4[8].
In view of the possible coexistence of chromium and antimony in industrial wastewater, Wang et al. Showed that sulfidation could significantly promote the simultaneous removal of Cr (Ⅵ) and Sb (Ⅲ)[42]. Li et al. Used two different sulfidizing agents, elemental sulfur and Na2S, to study the reaction activity of S-ZVI for removing Cr (Ⅵ) under aerobic conditions. Compared with the original ZVI, the reaction activity of the two S-ZVIs increased by a factor of 1.4 – 29.9, as shown in Fig. 2 (a)[43]. Industrial wastewater contains not only the target pollutants, but also a large number of coexisting ions, which have a significant impact on the reactivity and long-term effectiveness of ZVI. Qiao et al. Systematically studied the performance of unmodified ZVI and S-ZVI for the reduction of Se (Ⅵ) by ZVI under aerobic conditions in the presence of different coexisting ions such as Cl-, , , Ca2+ and Mg2+[44]. The results showed that vulcanization enhanced the reactivity of ZVI in the presence of X and , and the enhancement effect was more significant with the increase of and concentration. Sulfidation increased the Se (Ⅵ) removal capacity of ZVI from 15.6 mg/G to 30.4 mg/G at a background Cl- concentration of 1.0 mmol·L-1; Sulfidation increased the Se (Ⅵ) removal capacity of ZVI from 29.6 mg/G to 44.8 mg/G at a background Mg2+ concentration of 2.0 mmol·L-1.
S-ZVI is usually synthesized by liquid phase synthesis. Gu et al. Proposed a new method for solid-phase synthesis of S-ZVI, that is, ZVI was directly reacted with elemental sulfur powder in solid phase by ball milling, so that sulfur-iron compounds were formed on the surface of ZVI[41,53]. The distribution of Fe and S in the particles is relatively uniform, and the agglomeration of the obtained material is improved.
2.2 Adding divalent metal cation
Various cations, especially divalent metal cations, present in industrial wastewater,Such as Fe2+, Co2+, Mn2+,Cu2+, Pb2+, Zn2+ and Ni2+, etc.Considered to have the ability to improve the performance of ZVI[54]. Huang et al. Found that Fe2+ could promote the reduction of nitrate in aerobic system[29]. Molybdate is a substance that inhibits the corrosion of ZVI, and it is difficult to be effectively removed in the ZVI system. Huang et al added Fe2+ to the ZVI system, and found that Mo (Ⅵ) was rapidly reduced to a lower valence state. The consumption ratio of molybdate to Fe2+ was 1 ∶ 1.1 during the reaction, and it was considered that Fe2+ had the ability to passivate ZVI[55]. Selenate, one of the most stable and mobile species, is difficult to remove by ZVI alone[56]. Tang et al. Showed that Fe2+ could significantly improve the reaction activity of ZVI for Se (Ⅵ) removal, and the removal efficiency of Se (Ⅵ) increased with the increase of Fe2+ concentration[57]. Mn2+ and Co2+ have similar chemical properties to Fe2+. Tang et al. Studied the promotion of Mn2+ and Co2+ on the reduction of selenate by ZVI[58]. The results show that Co2+ promotes the release of Fe2+ and the formation of Co0-Fe0 bimetal through redox reaction, and Mn2+ and Co2+ provide about 30% and 69% of the electrons for the reduction of selenate, respectively. Doong and Lai also pointed out that the added Cu2+, Co2+ and Ni2+ can be reduced to zero valence by ZVI, thus forming bimetals and accelerating the dechlorination rate of PCE[59].
In addition to improving the reactivity of ZVI, it has been shown that divalent metal cations can restore the reactivity of aged ZVI. Liu et al. Studied the effect of different divalent metal cations on the dechlorination TCE of aged ZVI, and the results showed that the removal rate constant of TCE was positively correlated with the log K value of the affinity of different cations for hydrated iron oxide on the surface of ZVI in the presence of specific cations[60⇓~62]. The cations with larger log K value can better complex with the passive layer on the surface of ZVI to dissolve it and expose the lower layer of magnetite, and ZVI can transfer electrons to TCE through the conductive magnetite to complete dechlorination. For some cations (such as Co2+, Ni2+, Cu2+, and Pb2+), they will be reduced to the zero or + 1 valence state with strong reducibility during the depassivation process, which can promote the reduction of TCE.
2.3 External weak magnetic field
Guan's research group accidentally found in the experiment that the reaction activity of ZVI to remove Se (Ⅳ) was significantly improved after the stirring mode was changed from mechanical stirring to magnetic stirring[13]. Due to the presence of magnets in the magnetic stirrer and rotor, the weak magnetic field (WMF) provided by the magnets affects the activity of the ZVI. Liang et al. Studied the effect of WMF on the removal of Se (Ⅳ) by ZVI, and the results showed that the presence of WMF significantly accelerated the removal of Se (Ⅳ) and expanded the effective working pH range of ZVI from 4.0 ~ 6.0 to 4.0 ~ 7.2, and the enhancement effect of WMF was more obvious at a lower initial Se (Ⅵ) concentration[13]. Sun et al. Used WMF to enhance the removal of As (V) and As (Ⅲ) by ZVI at pH 3. 0-9. 0. The results showed that WMF could significantly improve the reactivity of ZVI for arsenic removal even in the presence of different background ions, and WMF improved the reactivity of ZVI by promoting the corrosion of ZVI[63]. Jiang et al. Found that WMF could enhance the removal of Cu (Ⅱ) by ZVI at pH 3.0 ~ 6.0, and the reaction activity was increased by 10.8 ~ 383.7 times[64]. Feng et al. studied the promotion effect of WMF on the removal of Cr (Ⅵ) by ZVI at pH 4.0 ~ 5.5, and WMF could improve the removal rate of Cr (Ⅵ) by ZVI by about 1.1 ~ 5.89 times, as shown in Fig. 2 (B)[12]. Xu et al. Also found that WMF could improve the removal rate of Sb (Ⅲ) by ZVI by 6 ~ 8 times[45].
WMF was also able to significantly improve the reactivity of aged ZVI. Liang et al. Found that the reaction activity of WMF for removing Se (Ⅳ) by ZVI aged for 6 ~ 60 H at pH 6.0 was increased by 10 ~ 100 times[11]. Xu et al. Further studied the promotion effect of WMF on the removal of Se (Ⅳ) by aged ZVI, and found that the magnetic field intensity required to activate aged ZVI was related to the properties of the outer passivation layer of ZVI[49]. A weak magnetic field with a field strength of 1 mT is sufficient to restore the reactivity of aged ZVI samples with a passive film with Fe3O4 as the main component or a thin layer of α-Fe2O3 and γ-FeOOH in the outer passive film. If the aged ZVI sample is covered by a thicker and denser passive film and contains more α-Fe2O3 and γ-FeOOH, the magnetic field strength for activating the aged ZVI sample needs to be increased to 2 mT or even 5 mT.
2.4 Premagnetization
ZVI is a ferromagnetic substance with ferromagnetic memory, so it can be magnetized in an external magnetic field. Even if the external magnetic field is removed, there is residual magnetism inside ZVI, which can still maintain the magnetized state[65]. Under the condition of different magnetic field intensity, the remanence intensity inside ZVI is different, so the magnetic field intensity will affect the reactivity of ZVI. Li et al. First used pre-magnetization to enhance the reactivity of ZVI toward As (Ⅲ)[52]. With the increase of the magnetic field intensity from 0 to 500 mT, the reaction rate of As (Ⅲ) removal by ZVI increases from 0.0182 min-1 to 0.0544 min-1, and the increase factor is 1.6 ~ 3.0 times. The reaction rate of total As removal gradually increases from 0.0159 min-1 to 0.0667 min-1, with an enhancement factor of 1.9 – 4.2 times, As shown in Fig. 2 (C). The enhancement of ZVI reactivity is related to the presence of remanence inside the ZVI. The residual magnetism retained in the ZVI particle after pre-magnetization makes the ZVI particle become a small magnet, so that an uneven magnetic field is generated on the surface of the magnetized ZVI particle, and the paramagnetic ions move under the action of the magnetic field gradient force, thereby promoting the corrosion of the ZVI. Li et al. Further used ZVI from different sources to study the broad spectrum of different pollutants in water (such as AR27, Pb (II), Cu (II), Se (IV), Ag (I) and Cr (VI)), and found that pre-magnetization all showed a promotion effect, and the rate constant of pollutant removal of pre-magnetized ZVI samples was 1.2 ~ 12.2 times that of the original ZVI samples[14]. Li et al. Pointed out that pre-magnetization promoted the release of Fe2+ and the generation of hydroxyl radicals through SEM and XRD characterization[66].
Pre-magnetization solves the problem of ZVI particle agglomeration caused by high magnetic field intensity, and has the advantages of simple operation, high efficiency, no additional energy, low cost and no secondary pollution, so it is a ZVI lifting technology with broad application prospects.
2.5 Dosing of oxidizer
In 2015, Guo et al. Found that oxidants could enhance the reactivity of ZVI, thus significantly improving the reduction of nitrate in water by ZVI[67]. When the pH value is 5.0, the addition of KMnO4 and H2O2 can increase the reduction rate of nitrate to 94% and 97%, respectively. The addition of KMnO4 and H2O2 increased the nitrate reduction to 74% and 98%, respectively, at pH 7.0. Guo et al. Further found that the combination of oxidant and ZVI could also efficiently remove heavy metals from water[10]. As (V) with a liquid-solid ratio of 200 ∶ 1 and a concentration of 1 mg/L can be completely removed within 10 min by adding 0.5 mmol·L-1NaClO, KMnO4 or H2O2. Cd (Ⅱ) and Hg (Ⅱ) can be simultaneously removed from an initial concentration of 0. 2 mg/L to 2. 4-4. 4 μg/L and 4. 0-5. 0 μg/L within 30 min. They believe that the addition of a proper amount of oxidant accelerates the surface corrosion of ZVI, thus continuously activating the surface of ZVI and continuously generating a large number of active iron oxides, which is conducive to the adsorption and co-precipitation of metal ions. The conclusion of this study also challenges the view that traditional oxidants will affect the electron transfer of ZVI and reduce the reactivity of ZVI by promoting the formation of ZVI passivation layer.
2.6 New Hoisting Technology
In addition to the above several popular lifting technologies in recent years, Hu et al. Proposed that liquid nitrogen treatment is also a promising physical method to activate ZVI[68]. It was found that the apparent removal rate constant of Cr (Ⅵ) by ZVI was increased by 4 ~ 120 times by liquid nitrogen treatment, and the mechanism was due to the different thermal expansion coefficients of iron oxide and iron.The low temperature of liquid nitrogen at 77 K causes a large number of cracks in the iron oxide shell of ZVI, which provide a suitable mass transfer channel for water/oxygen molecules to transfer inward to the surface of ZVI core, and then the Fe2+ generated in situ reduces Cr (Ⅵ) to Cr (Ⅲ).
Li et al. Proposed that phosphate modification could enhance the reactivity and electronic efficiency of nanoscale zero-valent iron (nZVI) for heavy metal ion removal[69]. The results show that the removal rate of Cr (Ⅵ) by phosphorylated nanoscale zero-valent iron (p-nZVI) is 4 times higher than that of the original ZVI, and the electron efficiency is increased from 6. 1% to 31. 3%. Phosphate group adsorbs on the surface of p-nZVI in a monodentate mononuclear configuration, which can inhibit the side reaction of ZVI with oxygen and (or) water, and the adsorption configuration of Cr (Ⅵ) on the surface of p-nZVI changes from monodentate mononuclear to bidentate binuclear, which is beneficial to the adsorption and reduction of Cr (Ⅵ). In a further study, Li et al. Found that the Kirkendall effect promoted the removal of heavy metal ions by p-nZVI[70]. Phosphorylation limits the circumferential stress of nZVI particles and "breaks" the compact spherical nZVI to produce a large number of radial nanocracks, which facilitate the inward diffusion of heavy metal ions, the outward transfer of electrons and Fe2+, and the exchange of electrons at the boundary. Whether the application of phosphate modification to micron zero-valent iron can also achieve better lifting effect still needs further study.
2.7 Brief summary
As shown in Fig. 2 and Fig. 3, the enhancement factor of sulfidation on the removal of Cr (Ⅵ) by ZVI is concentrated at 1.4 ~ 53 times, the enhancement factor of sulfidation on Sb (Ⅲ) is concentrated at 5.0 ~ 15.3 times, and the enhancement effect on Se (Ⅵ) is the best, which is concentrated at 27 ~ 92.7 times. Sulfidation shows a good enhancement effect on the reaction activity of different pollutants. The reaction activity of ZVI for removing various pollutants is improved by 1.2 to 12.2 times by premagnetization; The enhancement ratio of adding Fe2+ to ZVI reaction activity is 1.9 ~ 18.6 times, the enhancement ratio of adding H2O2 to ZVI reaction activity is 1.9 ~ 9.0 times, and the enhancement ratio of WMF/Fe2+ system to ZVI activity is 6.5 ~ 15.5 times, which is lower than that of vulcanization. The enhancement factor of WMF for ZVI reaction activity is in the range of 1. 1 ~ 147.4, and two values show strong enhancement effect, that is, the enhancement factor for Se (Ⅳ) is up to 330. 4 times, and the enhancement factor for paramagnetic ion Cu (Ⅱ) is up to 384. 7 times. The enhancement effect of external weak magnetic field on ZVI reactivity is not only the most significant in the overall enhancement factor, but also superior to other technologies in the maximum enhancement factor.
As mentioned above, each lifting technology improves the pollutant removal rate by promoting the corrosion of zero-valent iron, but this does not guarantee that the activity of ZVI will fully and effectively act on pollutants under the interference of other oxidizing substances in the water environment. Based on this, the effects of various lifting technologies on the removal capacity of pollutants were systematically investigated in this paper.
The numerical value of the ratio of the enhanced ZVI to pollutant removal capacity RC+X to the original ZVI to pollutant removal capacity RC-X, calculated according to formula (3), RX RC,RX RC, is the enhancement factor of the ZVI decontamination removal capacity by each lifting technology.
RXRC=
As shown in fig. 4, the removal capacity of Se (Ⅵ) by ZVI is increased by 4.6 to 10.3 times by sulfidation; The Se (Ⅵ) removal capacity was increased by 1.4 ~ 17.9 times by adding Fe2+, by 1.1 ~ 1.2 times by adding Ca2+, and by 1.1 ~ 1.5 times by adding Mg2+. The effect of adding Fe2+ was better. The removal capacity of Se (Ⅵ) can be improved by 1.9 ~ 10.4 times by adding H2O2. The overall enhancement factor of WMF for Se (Ⅵ) removal is in the range of 2. 0-6. 9, and only one value reaches 12. 5. The overall enhancement factor of WMF for other different pollutants is in the range of 2. 7-5. 5. It can be seen that WMF shows an absolute advantage in improving the reactivity of ZVI, but in improving the pollutant removal capacity, the effect of adding Fe2+ is the most significant. In order to comprehensively compare the lifting technologies, it is necessary to introduce the most critical electronic efficiency evaluation index.
3 Technical means to improve the electronic efficiency of ZVI
The means of improving the reactivity of ZVI proposed in the early studies also objectively improved the electronic efficiency of ZVI, but the improvement factor of electronic efficiency of ZVI was not quantitatively described and studied. Since the concept of ZVI electronic efficiency was proposed and attracted attention, the technical means proposed in the past decade have gradually focused on the improvement of ZVI electronic efficiency. This section summarizes the research progress of four representative technologies for improving the electron efficiency of ZVI pollution removal in recent years, including vulcanization, addition of Fe2+, addition of H2O2 and external weak magnetic field, and summarizes the improvement times of electron efficiency of ZVI pollution removal.
In order to better analyze the enhancement effect of each lifting technology on the ZVI decontamination electron efficiency, the value of the ratio of the enhanced ZVI decontamination electron efficiency EE+X to the original ZVI abatement electron efficiency EE-X, RX EE,RX EE, is calculated according to Equation (4), which is the enhancement factor of each lifting technology on the ZVI abatement electron efficiency.
RX EE=
3.1 Vulcanization
In 2015, Rajajayavel and Ghoshal found in their research that sulfurization can inhibit the hydrogen evolution rate of ZVI in water and improve the reactivity of ZVI. The reactive sites on the FeS layer formed on the surface of S-ZVI have a stronger binding effect with TCE and can transfer electrons to TCE more preferentially[72]. In 2016, Fan et al. Proposed that sulfurization could significantly improve the electronic efficiency of nZVI for pollutant removal[73]. In 2018, Li et al. First studied the enhancement effect of sulfidation on the electronic efficiency of mZVI for chromate removal under aerobic conditions, and the results showed that the electronic efficiency of the two S-ZVI for Cr (Ⅵ) removal was 14.6% and 13.3%, respectively, which was 10.7 and 7.5 times that of the original ZVI[43]. Sulfidation increases the surface area of ZVI, and the ion selectivity of FeS promotes the enrichment of Cr (Ⅵ) on the surface of ZVI, which is beneficial to the electron transfer from ZVI nucleus to Cr (Ⅵ). Gu et al. Prepared S-ZVI by ball milling, and the electronic efficiency of TCE degradation by ZVI increased from 8% to 87% under the condition of insufficient ZVI[41]. Under the condition of sufficient ZVI, the electronic efficiency of TCE degradation by ZVI increased from 0. 19% to 9. 8%. Sulfurization has a good application prospect in improving the electronic efficiency of ZVI.
3.2 Adding divalent metal cation
In 2017, Qin et al. Focused on the effect of adding Fe2+ on the electronic efficiency of Se (Ⅵ) removal by ZVI and the utilization rate of ZVI under aerobic conditions[34]. The results show that the electron efficiency of ZVI increases from 3.2% – 3.6% to 6.2% – 6.8% after adding 1.0 mmol·L-1Fe2+, and the utilization of ZVI increases by 5.0% – 19.4%. Qiao et al. Further found that although Ca2+ and Mg2+ plasma had no significant effect on the removal rate of Se (Ⅵ), they could improve the electron efficiency by 3.0% – 50.3%[44]. XAFS characterization showed that divalent metal cations affected the electron transfer from ZVI to Se species by affecting the transformation of corrosion products on the surface of ZVI. The addition of divalent metal cations improves the electronic efficiency of ZVI and is also beneficial to the long-term sustainability of ZVI technology.
3.3 Dosing of oxidizer
At present, there are few studies on the improvement of ZVI electron efficiency by oxidants, and the electron transfer mechanism among ZVI, oxidants and pollutants is complex, and the effect of oxidants on ZVI electron transfer mechanism in water environment is not clear[37]. In 2019, Fan et al. Mentioned the enhancement effect of H2O2 on the electronic efficiency of ZVI pollutant removal when comprehensively comparing the enhancement performance of several enhancement technologies on ZVI selenate removal, and the addition of H2O2 increased the electronic efficiency from about 0.53% to 1.21% – 4.63%, with an enhancement factor of 2.28 – 8.74[48].
Although some studies have shown that oxidants can improve the electronic efficiency of ZVI in removing pollutants, no study has clearly pointed out the reasons for the improvement of its electronic efficiency. Oxidants accelerate the corrosion of ZVI and promote the release of electrons, while a large number of electrons produced are inevitably consumed by other oxides in the reaction system, so the mechanism of oxidants improving the electron removal efficiency of ZVI needs further study.
3.4 External weak magnetic field
The study of external weak magnetic field in improving the electron efficiency of ZVI is late. As early as 2011, Kim et al. Found that WMF could not only increase the degradation rate of 4-chlorophenol by ZVI from 22. 5% to 51%, but also reduce the H2 production by 59. 29%, but it was not until 2020 that Li et al. Proposed in detail that WMF could improve the electronic efficiency of ZVI[74][51]. Li et al. Studied the effect of WMF on the electronic efficiency of metalloid oxyanions (such As As (Ⅲ), As (V), Sb (Ⅲ), Sb (V), Se (Ⅳ) and Se (Ⅵ)) under aerobic conditions[51]. The results show that the specific removal capacity of ZVI for metalloid oxyanions is increased from 1. 8-19. 0 mg/G Fe to 12. 6-85. 3 mg/G Fe, and the electronic efficiency of ZVI for Se (Ⅳ)/Se (Ⅵ) removal is increased by 3. 7-14. 1 times. Because the magnetic field gradient force generated by WMF can drive the movement of paramagnetic Fe2+ and metalloid oxyanions, the subsequent adsorption of metalloid oxyanions and the in situ generation of iron oxides can occur synchronously, so that more electrons are transferred to metalloid oxoanions, and the electron efficiency of ZVI is improved. WMF can improve the electronic efficiency of ZVI decontamination under aerobic conditions, which is an improved method with good application prospects.
3.5 Brief summary
As shown in fig. 5, the electron efficiency improvement of sulfurization on the removal of Se (Ⅵ) by ZVI is concentrated at 6.1 to 9.5 times; The electronic efficiency of Se (Ⅵ) removal by adding Fe2+ is increased by 1.4 ~ 19.2 times, the electronic efficiency of Se (Ⅵ) removal by adding Ca2+ is increased by 1.1 ~ 1.3 times, and the electronic efficiency of selenium removal by adding Mg2+ is increased by 0.8 ~ 1.5 times. Although Mg2+ improves the reaction activity of ZVI, it has little effect on the improvement of electronic efficiency, and even inhibits it. The Se (Ⅵ) removal capacity of the H2O2 is increased by 2.2 ~ 9.8 times. The removal capacity of WMF for Se (Ⅵ) is increased by 5.4 ~ 14.0 times, and the removal capacity for Se (Ⅳ) is increased by 3.6 ~ 4.2 times. It can be seen that Fe2+ has the most significant effect on improving the electron efficiency of ZVI, which is the same as the conclusion of removal capacity. The improvement of electron efficiency makes more electrons given by ZVI transfer to the target pollutant, reducing the occurrence of side reactions, thus improving the removal capacity of pollutants.
Figs. 2 to 5 show the broad-spectrum analysis of ZVI pollutant removal efficiency by various lifting technologies under different pollutants and different reaction conditions. Fan et al. Used Se (Ⅵ) as the target pollutant, and compared the effects of four lifting technologies, including sulfurization, addition of Fe2+, H2O2 and external weak magnetic field, on ZVI pollutant removal efficiency in a single and systematic way[48]. The results show that the performance of Se (Ⅵ) removal by ZVI can be significantly improved by the four methods. Sulfurization has a great advantage in improving the reactivity of ZVI, which is contrary to the conclusion that the effect of external weak magnetic field is the best in the broad-spectrum analysis. The addition of Fe2+ has the best effect on improving the pollutant removal capacity and electronic efficiency of ZVI, which is the same as the conclusion in the broad-spectrum analysis. Fan et al. Proposed that the feasibility verification of ZVI reactivity enhancement by Fe2+ should be based on broad-spectrum analysis of ZVI from different sources, different target pollutants, and different background matrices[48]. Therefore, the evaluation based on broad-spectrum analysis or single system analysis is one-sided, and the evaluation of the promotion effect of ZVI upgrading technology needs to combine broad-spectrum analysis with single analysis, and to explore a new and more scientific evaluation method.
4 Action Mechanism of Hoisting Technology
The mechanism of action of the four lifting techniques is shown in Figure 6. In essence, for (quasi) metal (oxygen-containing) ions removed by reduction, such as Cr (VI), Cu (II), Pb (II), U (VI) and so on, each promotion technology promotes ZVI corrosion, releases more Fe2+ and electrons for reducing pollutants, and improves the reactivity of ZVI pollutant removal. For (quasi) metal (oxygen-containing) ions removed by oxidation, such As Sb (Ⅲ) and As (Ⅲ), each lifting technology promotes the corrosion of ZVI to produce Fe2+ and H2O2 under aerobic conditions, and produces hydroxyl radicals and Fe (Ⅳ) with strong oxidizing properties under low and high pH conditions, respectively, to oxidize highly toxic low-valence ions to low-toxic high-valence ions[75,76]. The Fe2+ and Fe3+ produced by ZVI corrosion can be adsorbed and coprecipitated with metal ions in the process of hydrolysis and transformation into iron (hydroxide) oxides, thus removing pollutants. In the process of ZVI decontamination, the removal mechanism is usually not only one, for example, Se (Ⅳ) and Se (Ⅵ) are first adsorbed and then reduced. Promoting the corrosion of ZVI is the key to enhance the reactivity of ZVI by various promotion technologies.
Sulfidation makes the surface of ZVI covered with a hydrophobic FeS film, which improves the conductivity of ZVI, so that electrons can be transferred from the core Fe0 and preferentially transferred to the target pollutants. At the same time, sulfidation accelerated the corrosion of ZVI and the release of electrons. Qiao et al. Found that the Fe0 in S-ZVI was completely corroded within 24 H, and the main corrosion products were lepidocrocite and magnetite[44]. Qin et al. Also found a similar mechanism when dosing Fe2+ to promote ZVI decontamination. Under aerobic conditions, with the increase of Fe2+ concentration, the proportion of Fe0 and Fe2O3 in the process of Se (Ⅵ) removal by ZVI decreased, while the proportion of lepidocrocite and magnetite increased, indicating that Fe2+ can also promote the transformation of corrosion products, which is conducive to the adsorption and reduction of Se (Ⅵ)[34]. During the reaction, the Fe2+ can also maintain the solution pH at a low level, which is beneficial to the corrosion of ZVI, while the Fe2+ itself also participates in the reaction as part of the electron donor. Fan et al. Further confirmed that under aerobic conditions, sulfidation plus Fe2+ can synergistically improve the reactivity and electron selectivity of ZVI for Se (Ⅵ)[47]. The promotion effect is mainly manifested in the following aspects: (1) during the removal of Se (Ⅵ) by S-ZVI, the Fe2+ can maintain a low pH value; The (2)S-ZVI/Fe2+ can inhibit the consumption of Fe0 by the non-target pollutant O2/H+, thereby improving the electronic efficiency of S-ZVI; The (3)Fe2+ can realize electron transfer by forming a semiconductor Fe3O4.
The technical basis of the ZVI/oxidizer system is the half-reaction based on the cathodic reduction of the oxidizer[77]. Oxidants react with the surface of ZVI rapidly and stably, driving ZVI to corrode rapidly, thus significantly improving the removal capacity of ZVI for a broad spectrum of pollutants. Heavy metals, metalloids and nitrates in water are removed through various mechanisms such as precipitation, co-precipitation, adsorption and reduction[78,79]. In aqueous solution, the pollutant removal mechanism of ZVI is mainly to transfer an electron from ZVI to DO or water, and then indirectly to the target pollutant, converting the target pollutant into a less toxic substance. In the ZVI/oxidizer system, the oxidizer will compete with the target pollutant for electrons, resulting in a decrease in electron efficiency, and a small part of the electrons will be used to corrode rather than attack the target pollutant, which is contrary to the promotion effect of H2O2 on the electron efficiency of ZVI for Se (Ⅵ) removal mentioned in Fan et al. The effect of oxidizer on the electron efficiency of ZVI for Se (Ⅵ) removal and its mechanism still need further study[77][48].
Under aerobic conditions, Liang et al. And Sun et al. Revealed that WMF could accelerate the corrosion of ZVI and the transformation of corrosion products to lepidocrocite, which was beneficial to the removal of pollutants, for the reasons of the enhanced reactivity and selectivity of ZVI/WMF system[13][63]. In addition, Li et al. Further confirmed that the enhancement effect of WMF was mainly attributed to the magnetic field gradient force (FΔB), which can drive paramagnetic ions such as Fe2+ to move to the position with higher induced magnetic field[80]. On this basis, Sun et al. Demonstrated the coupling effect of coexisting negative ions and WMF on ZVI reactivity enhancement, which is related to the simultaneous movement of negative ions and paramagnetic Fe2+ in solution to maintain local electrical neutrality[81]. The magnetic field gradient force can selectively drive the metalloid oxyanion to move to the in-situ generated iron oxide, which is beneficial to the electron transfer to the target pollutant, thus improving the reactivity and electron efficiency of ZVI.
5 Conclusion and prospect
With the in-depth study of ZVI enhancement technology, the improvement of ZVI reactivity is no longer the only goal of research, and the improvement of electronic efficiency has gradually become the focus of ZVI enhancement technology research. Sulfidation, addition of divalent metal cations, addition of WMF and addition of oxidant can significantly improve the reactivity and electronic efficiency of ZVI for pollutant removal, but the degree of improvement is different. To sum up, in order to scientifically and effectively evaluate the performance of ZVI lifting technology, broad-spectrum research in different systems should be combined with specific research in a single system.
Although the use and development of ZVI technology have been on the rise in the past decade, there are still limitations in the application of ZVI technology. In order to overcome or alleviate these limitations and expand the application of ZVI technology, research and development in this field are necessary. On this basis, the focus of future research is proposed as follows.
(1) Further investigate the influence of various technical means on the corrosion process of ZVI, clarify the response relationship between the corrosion process of ZVI to improve the reactivity and electronic efficiency of ZVI, so as to explore more scientific and effective means to improve the technical evaluation.
(2) To further investigate the improvement of the reactivity and electronic efficiency of ZVI in real water by the combination of different lifting technologies, and to explore the coupling mechanism between them.
(3) Phosphate modification can significantly enhance the decontamination performance of nZVI, which has broad application prospects in the actual groundwater remediation and industrial wastewater treatment. Therefore, it is necessary to further explore the effect of phosphate modification on the reduction and decontamination efficiency of ZVI, and to reveal how it changes the surface structure of ZVI at the molecular level to improve the reactivity and electronic efficiency of ZVI.
(4) Considering that there are a large number of metal ions in saline wastewater and seawater, the technology of adding metal cations has unique advantages for (aging) zero-valent iron in the treatment of high-salinity wastewater and seawater. Subsequent studies should further clarify the effects of metal ions such as Ca2+, Mg2+, Fe2+, and Co2+ in actual water bodies (saline wastewater and seawater) on the reactivity and electronic efficiency of ZVI reduction and decontamination, and evaluate the operation cost of the technology.
(5) It is expected to reduce the cost of ZVI technology by improving the electronic efficiency of ZVI decontamination, and subsequent studies should further evaluate the safe disposal pressure of different technologies on iron-containing sludge and solid waste produced in the process of ZVI decontamination.
PDF
