In recent years, zero⁃valent Aluminum (ZVAl) has been widely used for contaminants removal in water due to its extremely low redox potential and being an excellent electron donor. However, the dense oxide film on its surface restrains the activity of ZVAl, and it is easy to form Al⁃(hydr)oxide on its surface during the reaction, resulting in a secondary passivation and a short reacting life for ZVAl. The research shows that ZVAl could overcome its own disadvantages after compounded with iron⁃based materials such as zero⁃valent iron, iron ore and iron⁃containing clay minerals. Besides changing the physical properties such as hardness and magnetism of ZVAl, it is more meaningful that its efficiency for pollutants removal from water through chemical mechanisms is improved, such as 1) speeding up the reaction rate, 2) broadening pH ranges, 3) promoting durability, and 4) enhancing reaction selectivity. Therefore, on the basis of systematically summarizing the combining mechanisms between iron⁃based materials and ZVAl (i.e., redox reaction, intermetallic reaction or self⁃propagating reaction) under different synthetic methods, this review mainly focuses on the enhancing mechanisms of ZVAl composites modified by iron⁃based materials for pollutants removal in water, and further on the optimization strategies such as introducing a third metal, adding non⁃metallic elements, polymer or ligand, and constructing a load in detail. Finally, the research direction of ZVAl composites modified by iron⁃based materials is prospected in order to promote further research and wider application in environmental field for the novel ZVAl based water or wastewater technology.
Shiying Yang, Qianfeng Li, Sui Wu, Weiyin Zhang.
Mechanisms and Applications of Zero⁃Valent Aluminum Modified by Iron⁃Based Materials.
Progress in Chemistry, 2022, 34(9): 2081-2093 DOI:10.7536/PC211102
2 Combining mechanism between iron⁃based materials and ZVAl
2.1 Redox reaction
2.2 Intermetallic reaction
2.3 Self⁃propagating reaction
3 Strengthening mechanism of ZVAl composites modified by iron⁃based materials in water treatment application
3.1 Speeding up the reaction rate
3.2 Widening pH ranges
3.3 Promoting durability
3.4 Enhancing reaction selectivity
4 Optimization strategies of ZVAl composites modified by iron⁃based materials for removing contaminants from water
4.1 Introducing a third metal
4.2 Adding non⁃metallic elements
4.3 Adding polymer
4.4 Adding ligand
4.5 Constructing a load
5 Conclusions and outlook
1 Introduction
Aluminum is the most abundant metallic element in the earth's crust, with light weight (ρ=2.70 g/cm3), amphoteric nature, extremely low redox potential (E0=-1.662 V), higher thermal driving force of electron transfer, and excellent electron donor (one atom can produce three electrons). These chemical characteristics have made it more and more used in environmental research in recent years[1]. Existing studies have shown that zero-valent aluminum (ZVAl) has a strong ability to remove a variety of pollutants, such as heavy metals (Cr (VI)), nitrobenzene, trichloroethylene, phenol, etc., and can also remove the persistent organic pollutant hexabromocyclododecane (HBCD), and the debromination rate can be close to 100% under optimized conditions[2][3][4,5][6][7,8].
However, although ZVAl has a strong potential to remove pollutants and is a sufficient "electron reservoir", there is a dense oxide film on the surface of ZVAl, which will hinder the release of electrons on the surface of ZVAl, thereby inhibiting the removal of pollutants by ZVAI, and the activated ZVAI is easily passivated again by oxygen in the environmental medium[9⇓~11]. In order to solve the above problems and stimulate the strong potential of ZVAl to remove pollutants, the introduction of other materials to modify it is one of the main means[12,13].
Iron-based materials, such as zero-valent iron (ZVI), some natural ores (such as pyrite (FeS2), magnetite (Fe3O4), hematite (Fe2O3), etc.) and iron-bearing clay minerals have attracted wide attention because of their good reactivity and environmental harmlessness, and more and more scholars have focused on the research of ZVI, iron ore, iron-bearing clay minerals and other iron-based materials in the field of wastewater treatment[14⇓⇓~17][18⇓~20][21].
At the same time, iron-based materials can transfer electrons better than alumina, the insulator on the surface of ZVAl[22]. Therefore, ZVAl is used as an electron donor, and iron-based materials can be used as electron transporters to build favorable channels for electrons to transfer to target pollutants, thereby improving the removal rate of pollutants. At the same time, the iron-based material can also overcome the passivation of the ZVAl surface caused by the (hydroxide) formed on the surface in the reaction process[23]; The two can also synthesize an integrated solid-phase Fenton reagent[24]; At the same time, the differences between ZVAl and iron-based materials, such as the redox potential difference between aluminum and iron, as well as the differences in hardness, magnetism and other physical properties, will lead to new chemical and physical properties in the composite system of ZVAl modified by iron-based materials[25⇓~27]. Therefore, the characteristics of ZVAl and iron-based materials and their respective pollutant removal performance and potential can make the iron-based modified ZVAl composite produce synergistic effect in water treatment.
Based on this, the composite system composed of iron-based materials modified by ZVAl has gradually become a hot research issue in the fields of environment, materials and so on. However, in the field of water treatment, the chemical synthesis of Fe/Al bimetal and its application are the most widely used in the modification of iron-based materials ZVAl, but there is a lack of systematic summary of different modification methods and applications[28]. At the same time, for composite materials, the current research mostly focuses on the modification strategy of Al-modified ZVI composite materials for the disadvantages of the original ZVI, for example, the reduction potential of Al is lower than that of Fe, so using the principle of cathodic protection of sacrificial anode, the introduction of Al can keep Fe in zero-valent state to prevent Fe deactivation; On the contrary, the strengthening mechanism of ZVAl in the composites modified by iron-based materials has not been paid enough attention[29]. Moreover, the modified ZVAl of iron-based materials has also been developed in the field of hydrogen production and materials, and its modification mechanism will have important reference significance for the development of new environmental functional materials[30⇓~32][33⇓~35].
To sum up, it is very necessary and valuable to summarize the mechanism in the process and application of ZVAl modification of iron-based materials. Therefore, based on the existing research progress, this paper systematically summarizes: (1) the reaction mechanism of ZVAl modified by iron-based materials under different methods; (2) The enhancement mechanism of iron-based material modified ZVAl system compared with ZVAl system alone when it is used in the field of water treatment; (3) To further optimize the series of strategies adopted in the water treatment application of the composite system constructed by the iron-based material modified ZVAl. On this basis, the future development direction of composite materials constructed by iron-based materials modified by ZVAl in the field of water treatment was prospected.
2 Modification mechanism of iron-based materials on ZVAl
Different chemical reactions will occur in the modification process of iron-based materials on ZVAl, and then there will be different mechanisms of action on ZVAl, resulting in different excellent properties, which can be used in the fields of environment, materials and energy.
This section summarizes the typical modification methods under different reaction mechanisms and the potential advantages and disadvantages of various methods (Table 1).
2.1 Redox reaction
Oxidation-reduction reaction occurs in the process of modification of ZVAl by some iron-based materials, which changes the original surface oxide film of ZVAl. These include the dissolution of the original oxide film of ZVAl in the liquid phase environment for wet redox, or the physical cutting of the dry reduction to destroy the dense oxide film of ZVAl for dry redox, and then the iron-based materials are loaded on it.
2.1.1 Change ZVAl oxide film
(1) Chemical deposition
Wet redox reaction can be used to modify ZVAl, a typical iron-based material, to synthesize Fe/Al bimetal. In the synthesis process, concentrated hydrochloric acid is first added to dissolve the dense oxide film on the surface of ZVAl to expose the fresh surface[36]. Subsequently, Fe was deposited on the surface of Al by the redox reaction of Al powder with Fe (II) salt or Fe (III) salt, taking advantage of the chemical property that the redox potential of Al is lower than that of Fe[37]. This common method of in situ synthesis of bimetals from reactive metal powders and slightly less reactive metal salts is called "chemical deposition"[38].
In addition, the metal salt can also be reduced by NaBH4 to achieve the effect of depositing ZVI nanoparticles on the surface of ZVAl to synthesize Fe/Al bimetallic particles (formula (1))[39]. As shown in Fig. 1, compared with the ZVAl particles, the final synthesized Fe/Al bimetal can see obvious nano-zero-valent iron (NZVI) particles on the surface of ZVAl, and the energy spectrum analysis shows that the Fe/Al bimetal is mainly composed of Al and Fe[39]. It should be noted that when the chemical deposition method is used to prepare bimetals, Al powder will cause surface corrosion and gas evolution under acidic conditions. In order to solve this problem, the coordination ion F- of Al (Ⅲ) can be introduced into the reaction system to form a stable [AlF6]3- complex, so as to realize the rapid dissolution of alumina film on the surface of Al powder in weak acid medium[40].
(2) electroless plate
The surface of micron-sized Al powder can be coated with a uniform Fe layer by electroless plating, and a core-shell coating with Al as the core and Fe as the shell can be obtained.
Mixing powder; Sodium hypophosphite is used as a reducing agent, and Al powder is used as a substrate for electroless iron plating in an alkaline solution, specifically, sodium hypophosphite and iron complex ions are adsorbed on the surface of the Al powder which is washed by alkali and has catalytic activity, and the iron complex ions are deoxidized into iron atoms by free electrons released by catalytic dehydrogenation of sodium hypophosphate. Due to the heterogeneous nucleation theory, iron atoms nucleate and grow on the surface of Al powder, and finally the Al/Fe composite powder with good coating effect is obtained. Iron on the surface of Al powder is granular, and the coating is more complete, but the coating is less dense[41]. At the same time, as shown in fig. 2, the surface of the Al particle can also be covered with a thick and dense layer of iron oxide by this method. The iron oxide coated on the surface of the Al particle has a flower cluster structure, and each flower cluster is composed of nano-flake iron oxide[42].
(3) Co-reduction method
In addition to electroless deposition and electroless plating, there is also a co-reduction process in which two different metal salts and a NaBH4 are oxidized and reduced together. In theory, in the co-reduction process, the metal with a high redox potential is first reduced and formed, while the metal with a lower reduction potential is deposited on the core in the form of a shell[56]. However, in practice, the Al-coated Fe structure may also occur when the Fe/Al bimetal is synthesized by the co-reduction method. When the NaBH4 solution is slowly dropped into the FeCl3 solution as a reducing agent, the FeCl3 is first reduced to nano-zero-valent iron (nZVI), and then the AlCl3 solution and the NaBH4 solution are added to reduce the deposition on the surface of Fe particles, so that the synthesized Al/Fe bimetal has an Al-coated Fe structure (Fig. 3)[23].
2.1.2 Destroy ZVAl oxide film
The solvent-free mechanical ball milling method of dry redox reaction mechanism has the advantages of mild reaction conditions and simple process, and has been used to activate metal materials and prepare composite materials in recent years[57][58,59]. It was found that when ZVA1 was modified by ball milling brittle FeSO4·7H2O crystals, it could be observed that the particle surface of mZVAl(Fe⁃mZVAlbm) after mechanochemical modification not only broke the original dense oxide layer to produce a rough fresh reactive surface, but also attached a rough Fe oxide layer uniformly distributed on the mZVAl (Fig. 4)[46].
In addition to the above brittle crystal ferrite, when four clay minerals with different compositions were mixed with ZVAl for mechanical ball milling, the electrons in the mechanochemical reaction during ball milling may be transferred from the ZVAl to the structural Fe (Ⅲ) of the clay through the basal plane or edge of the clay.Then Fe (II) will be partially produced on the Clay surface, and the composite of ZVAl and clay (ZVAl Clay) can be obtained, thus realizing the modification of ZVAl by iron clay minerals[45]. At the same time, after mechanical ball milling, the structural Fe (Ⅲ) in iron clay minerals is reduced to Fe (II), and ZVAl with strong reducing ability acts as an electron pool in the Fe (II)/Fe (Ⅲ) cycle to continuously regenerate the dissolved and structural Fe (Ⅱ)[60]. Therefore, it can be seen that the mechanical ball milling synthesis process can induce the compaction between particles, and can also promote the internal electron transfer process to achieve dry redox reaction[45].
2.2 Reaction between metals
In addition to redox reaction, the modification of ZVAl by iron-based materials can also be completed by intermetallic reaction. This reaction mechanism can enhance the interfacial bonding between iron-based materials and ZVAl, or change some of the original physical properties of ZVAl, for example, through the modification of iron-based materials, the original soft and non-magnetic Al can enhance the mechanical properties and magnetic properties.
2.2.1 Enhanced interfacial bonding
Using iron-based materials to modify ZVAl to alloy aluminum can enhance the interface bonding between them. The metal components in the aluminum alloy are uniformly distributed, and the bonding between metals is much stronger than that of bimetal[61].
The alloying process mainly uses the reaction mechanism between metals, which can be completed by high temperature melting. High-purity Al and Fe metals with a certain mass ratio are put into a vacuum induction melting furnace together, and the Al powder and the Fe powder are subjected to intermetallic reaction by high-temperature treatment to obtain the Al-Fe alloy containing Al, Fe and Al-Fe intermetallic compounds[32]. The intermetallic compounds present include the AlFe compounds Al13Fe4, Fe3Al, FeAl2 (Fig. 5)[62].
In addition to the above melting method, the mechanical ball milling method can not only achieve the crushing effect on the material, but also promote the interfacial interaction between the grinding components[63]. Al, Fe and Mg powders were used as starting materials, and stearic acid was used as milling aid. The highly activated Mg _ Al _ Fe materials were prepared by mechanical milling in argon atmosphere. By increasing the milling time and iron content, the grain size of Al can be reduced, and the hydrolytic reactivity of the composites can be significantly improved[30].
At the same time, the high temperature melting method and the mechanical ball milling method can also be combined to prepare the Al-Fe composite material by intermetallic reaction, firstly, Fe2Al5 intermetallic alloy is prepared by melting Al powder and Fe powder in an induction furnace,Then the intermetallic alloy was mechanically milled. The results show that the formation rate and induction time of hydrogen do not change much with the increase of mechanical milling time after the formation of intermetallic reaction powder[64].
2.2.2 Changing the physical properties of ZVAl
The recombination mechanism between iron-based materials and ZVAl can not only change the chemical properties of ZVAl by changing or destroying the surface oxide film and the interfacial bonding between them, but also change some of the original physical properties of ZVAl, for example, the hardness and magnetism of the original soft and non-magnetic aluminum can be enhanced by the modification of iron-based materials.
(1) enhanced hardness
As a light metal, aluminum has ductility, which is beneficial to the morphology control of materials, but its mechanical properties will also affect its thermal stability. Studies have shown that Al/Fe alloys prepared by mechanical alloying have extremely high hardness.This extremely high hardness can be attributed to the synergistic effect of nanocrystalline Al13Fe4 intermetallic compound, the absence of soft aluminum phase, and a large number of defects induced during mechanical milling[33]. In addition, it is found that Al/Fe bimetal castings need to be heat treated before practical use to further improve their mechanical properties. The nano-hardness of the reaction layer of Al/Fe bimetal castings with and without heat treatment is significantly higher than that of the base metal, and the nano-hardness of the reaction layer after heat treatment is 10% higher than that of the as-cast sample[65]. At the same time, some studies have proposed that the microhardness of Al-Fe alloy synthesized by high temperature melting will increase after atomization, and the composite is composed of α-Al, lamellar eutectic (Al⁃Al13Fe4) and rod-like eutectic (Al⁃Al6Fe)[66].
(2) enhanced magnetic
Iron is a magnetic material with strong ferromagnetism, so when ZVAl is modified by iron-based materials, the obtained composite material is rich in magnetism, and the reaction mechanism is mostly intermetallic reaction[35]. For example, microwave melting is a technology suitable for rapid and uniform heating of metal-containing components, and magnetic refrigerant AlFe2B2 can be prepared by microwave melting using Al powder, Fe powder and B crystal as raw materials, and the initial Al/Fe loading ratio will strongly affect the magnetic properties of the sample, and the increase of Al content will lead to the decrease of magnetocaloric properties[34]. A plasma gas condensation cluster deposition system is used, and a single glow discharge power source is adopted to sputter Fe and Al atoms to form a cluster, so that an Al-Fe alloy cluster can be obtained; Fe/Al core-shell cluster composite was sputtered by double glow discharge[51]. In the former, there are ferromagnetic Al-rich Fe alloy clusters, and Al and (or) Al oxide clusters cover the iron core, while in the latter, the iron core oxidation and magnetic coupling are inhibited[51].
2.3 Self-propagating reaction
During the modification of ZVAl by iron-based materials (iron oxides, iron sulfides), a mechanochemical reaction called Mechanically Induced Self sustaining Reactions (MSR) can also occur. In the SHS reaction, the ratio of the standard reaction enthalpy to the product heat capacity (△H/△Cp) needs to be higher than about 2000 K[67,68].
2.3.1 Synthetic NZVI
The self-propagating reaction can be realized by using iron sulfide or oxide minerals and active metal Al. The ZVI aluminum sulfide compound (formula (2)) is prepared by reducing pyrite through a mechanochemical reaction between the pyrite (FeS2) and Al through ball milling in an inert atmosphere[69,70]. However, there are two possible ways for the mechanochemical reaction between ZVAl and iron-containing oxides during mechanical ball milling: one is the direct reaction of Fe3O4 to form Fe (formula (3)); The second is the reduction of Fe3O4 to form FeO by an intermediate reaction, followed by the formation of Fe (formulas (4 – 9))[55].
3FeS2+4Al→3Fe+2Al2S3
3Fe3O4+8Al→4Al2O3+9Fe
Fe3O4→3FeO+O2
2Al+O2→Al2O3
2Al+3FeO→Al2O3+3Fe
2FeO+Al→Fe+FeAl2O4
3FeAl2O4+2Al→3Fe+4Al2O3
3Al+Fe→Al3Fe
2.3.2 Specific surface area increase
In the preparation of mixed aluminum matrix nanocomposites, the grain size is also significantly refined, so the specific surface area is increased, and the nanocomposites can be obtained from micron-sized precursor materials by controlling the process variables and under appropriate mechanochemistry[55]. Due to ball milling, the dislocation density and crystal defects will increase. In addition, the increase of specific surface area increases the energy stored in the powder particles, which will lead to an increase in the reaction rate. By reducing the particle size of the powder and activating it through the ball milling process, the generated aluminum sulfide or alumina plays a role in protecting the ZVI phase from agglomeration and oxidation[70].
3 Reinforcement mechanism of iron-based material modified ZVAl composite for water treatment application
ZVAl alone has limitations in water treatment. First, with the extension of reaction time, its activity will be limited by the surface passivation layer, which will lead to its rapid deactivation. Second, under neutral pH conditions, the oxide film on the metal surface will be more easily formed, which will affect the removal and degradation of pollutants, resulting in a narrow pH range for its application[28]. In the field of water treatment, the current research on the modification of ZVAl by iron-based materials mainly focuses on the Fe/Al bimetal and the activated Al-Fe alloy, and the research shows that the ZVAl modified by iron-based materials has enhanced reactivity and applicability compared with the ZVAl or ZVI system alone[52,71].
3.1 Accelerated reaction rate
3.1.1 Primed galvanic effect
Compared with the ZVAl monometallic system, the ZVI on the surface of Fe/Al bimetallic particles not only provides electrons for the reduction process of heavy metals, but also forms a large number of galvanic cells on the bimetallic particles and internal particles.Al acts as the anode and Fe acts as the cathode, so it can transfer the electrons of ZVAl through the galvanic reaction. The existence of ZVAl prolongs the reduction lifetime of ZVI and accelerates the transfer of electrons to heavy metal ions[39]. The removal efficiency of Fe/Al bimetallic particles for heavy metal ions is much higher than that of single NZVI and ZVAl particles or the sum of two particles, showing a significant synergistic effect between Fe and Al (Fig. 6)[37,71]. In addition, the aluminum-iron alloy can simultaneously remove , Cu (II), Cr (VI) and Cr (III), and can reduce nitrate to ammonium, reduce Cu (II) to Cu (0) and reduce Cr (VI) to Cr (III), and then precipitate in a Cr(OH)3 or a compound of Cr (Ⅲ) -Al (Ⅲ) -Fe (Ⅲ) hydroxide/oxyhydroxide[52]. At the same time, compared with Al alone, Al-Fe alloy can also effectively produce high-purity hydrogen under alkaline etching due to the existence of galvanic corrosion, and the concentration of Fe and intermetallic compounds is the main factor affecting the galvanic corrosion and intergranular corrosion, thereby affecting the hydrogen production rate[32].
At the same time, even if Fe/Al bimetallic particles or Al-Fe alloy are not used, only the mixture of pickled ZVI and ZVAl is physically mixed to form a packed chromatographic column, when the mixture is used as a reaction medium in a permeable reaction barrier PRB, the galvanic effect caused by the coexistence of ZVI and ZVAl,Its performance is much better than that of the packed chromatographic column using ZVI or ZVAl alone, because the formed Al-Fe galvanic cell is conducive to the removal of heavy metals, in which Al as an electron source can provide electrons, and Fe can transfer electrons to reduce heavy metal ions[72]. The results show that the performance of the chromatographic column can be improved by using acid-washed Fe and Al as reaction media to treat heavy metal wastewater containing cadmium, nickel and copper due to the enhanced reducing power of the galvanic effect[73].
3.1.2 Promoted Fenton-like reaction
Zero-valent metals are introduced as effective reductants for the activation of molecular oxygen in the AOPs[6]. Compared with single ZVAl, Fe/Al bimetal can not only generate H2O2 in situ through electron transfer from ZVAl to O2 (formulas (10 ~ 13)), but also generate · OH in situ through Fenton reaction (formulas (14 ~ 16)), so the oxidation ability of bimetallic Al⁃Fe/O2 will be better than that of ZVI/O2 and ZVAl/O2, and it will also show higher reactivity in the degradation of pollutants[24,74].
Al0→Al3++3e-
O2+H++e-→HO2·
2HO2·→H2O2+O2
Al0+3H2O2→Al3++3·OH+3OH-
Fe0+O2+2H+→Fe2++H2O2
Fe0+H2O2→Fe2++2OH-
Fe2++H2O2→·OH+OH-+Fe3+
At the same time, compared with the original ZVA1, the maximum removal rate of pollutants and the corresponding TOC removal of the composite modified by iron-containing clay minerals increased by 23 times and 38. 7%, respectively, indicating that the structural iron in the ZVA1 clay material has a positive effect on the decomposition of H2O2 generated in situ during the reaction[45].
3.2 Widening the application range of pH
Because both oxides and hydroxides of ZVAl are amphoteric, scholars have focused on the application of ZVAl in acidic or alkaline conditions, and confirmed the reduction ability of ZVA1 in acidic and alkaline conditions[10]. Extensive pH applications of ZVAl have only been realized in nanoscale ZVAl systems[3]. However, Fe/Al bimetal is not only easy to remove Cr (VI) under acidic pH conditions, but also shows high removal efficiency of Cr (VI) in neutral and alkaline solutions, and under neutral conditions, the performance of Fe/Al bimetal is much better than that of ZVA1 and ZVA1 after acid washing[37].
It is obvious that the reduction potential of ZVI usually decreases significantly from acidic pH to alkaline pH, and the oxidation of ZVI by water inevitably increases the pH of the solution, but when combined with ZVAl, ZVAl can maintain the zero-valent state of ZVI and make it difficult to be oxidized by water. At the same time, if Al and Fe are synthesized into Al-Fe alloy, the strong interfacial bonding between them can further make the deposited ZVI not easy to fall off from the surface of ZVAl, so it shows relatively constant reactivity to the reduction of pollutants in a wide range of pH = 2 ~ 12[52]. At the same time, the mesoporous ferromagnetic Fe/Al bimetallic particles synthesized by Cheng et Al. Can remove As (Ⅲ) from water at pH values of 3 ~ 11, and it is found that arsenite is mainly removed by bimetallic surface adsorption and free radical oxidation and intraparticle reduction in the presence of ZVAl and ZVI (Fig. 7)[44].
3.3 Prolonged response persistence
Fe/Al bimetallic or Al and Fe alloying can improve the performance of metal deposits, thereby enhancing their persistence in removing heavy metals from water or degrading chlorinated organic matter, while inhibiting the leaching of iron ions. The ZVI on the surface of bimetallic particles can not only directly provide electrons for the reduction of pollutants, but also transfer the electrons of ZVAl. ZVAl is considered to be the electron bank of ZVI, which prolongs the reduction life of ZVI. Therefore, Fe/Al bimetallic or Al-Fe alloy particles have high reactivity and persistence[23,53]. Reductive dechlorination and oxidation of Fe occur on the Fe surface. However, the presence of ZVAl reduced the dissolved Fe to ZVI, while the Fe corrosion products did not precipitate on the surface of bimetallic particles. The dissolved Al (Ⅲ) produced by the oxidation of ZVAl precipitates rapidly from the solution, and the residual amount of dissolved Al is relatively low[75].
3.4 Enhanced reaction selectivity
Generally, in the reduction of nitrate by single metal, Fe and Al will reduce nitrate to nitrite first, and ammonium is the final product, which will not be reduced to N2. But ammonium is even more toxic than nitrate and is therefore an unwanted toxic product. The researchers observed that the N2 selectivity of the Al-Fe alloy was increased from 0 to 60% compared with Fe and Al, and the nitrogen selectivity of the Al-Fe alloy for nitrate reduction was most likely increased due to the catalytic effect of the intermetallic Al-Fe compound (Al13Fe4) in the alloy, which can convert nitrate into N2[62].
4 Optimization Strategy of Fe-based Material Modified ZVAl Composite for Pollutant Removal in Water
In addition to ZVAl and iron-based materials, ZVAl modified by iron-based materials can remove pollutants in water more effectively by introducing a third metal, adding a non-metallic element, adding a polymer, adding a ligand and building a load to optimize the system.
4.1 Introducing a third metal
4.1.1 Enhanced catalytic activity
The reduction of nitrate in water by 90% Al-10% Fe alloy particles can be significantly promoted by introducing CuCl2 for Cu loading. The Al-Fe alloy shows the highest efficiency and the fastest nitrate reduction kinetics, which may be due to the synergistic effect between Cu and the intermetallic compounds of Al and Fe. Cu significantly enhances the reactivity of Al-Fe alloy particles[76]. Meanwhile, deposition of a small amount of Pd-modified trimetallic Pd/Fe/Al particles on the bimetallic Fe/Al surface was also proved to improve the reactivity of bimetallic Fe/Al and enhance the dechlorination efficiency of CCl4[77].
In the field of hydrogen production, the addition of Fe-Co-B catalyst in the aluminum-water reaction will greatly shorten the induction time and increase the amount of hydrogen production (from 850 to 1000 mL·g-1)[31]. The study showed that in the aqueous phase hydrogenolysis performance of the Fe/Al based composites for glycerol (a by-product of biodiesel production), all the catalysts had low carbon yields for gases and high carbon yields for liquid products (mainly 1,2-propanediol, acetol, and ethylene glycol). It is obvious that the addition of metal Ni has higher catalytic activity, and the specific order is Ni/Al3Fe>Ni/AlFe>Ni/AlFe3>Ni/Al>Ni/Fe[78].
4.1.2 Induced reductive atomic hydrogen formation
Adsorbed hydrogen ions can accept electrons on the surface of some metals, thereby reducing to atomic hydrogen (Hads). It has been shown that Hads reduces various contaminants faster than direct electron transfer from metals[79,80]. When the system does not contain pollutants, hydrogen is formed and saturated. Compared with this, the hydrogen evolution rate decreases significantly in the reaction system with repeated addition of pollutants, which indicates that hydrogen is consumed in the degradation process of pollutants. The author believes that the formed atomic hydrogen participates in the indirect reduction of pollutants[81]. In this mechanism, the charge transfer reaction occurs not between the metal and the organic pollutant, but between the metal and water molecules, which generates adsorbed Hads at the local cathode, and these Hads in turn participate in the catalytic hydrogenation reaction, which in turn leads to pollutant reduction[82]. And according to the difference between the theoretical exchange current density between metals and the experimental value, it is judged that the initial dechlorination rate of pollutants depends on the concentration of Hads adsorbed on the surface of the third metal and is controlled by the cathode[83].
The H2 bubbles generated by the Al _ Fe alloy particles are faster than those generated from the Al or Fe particles. Al and (or) Hads rapidly reduce nitrate to nitrite, and then the resulting nitrite is further reduced to ammonium under the catalysis of Cu (fig. 8)[76].
Meanwhile, when a noble metal such as Pd is used as a third metal to introduce an Fe/Al bimetal as its shell metal, its indirect reduction in the dechlorination reaction involves the formation of a Hads,H2 was evolved and consumed during the degradation of CCl4 by Pd/Fe/Al, which indicates that the indirect reduction involves the formation of Hads on the metal surface, and the main reason for the increased degradation rate is their outstanding catalytic ability through indirect reductive dechlorination[77]. The hydrogen adsorption of the third metal on the metal substrate can be represented by the following formulas (17, 18), and the noble metal Pd can also easily adsorb the H2 as an adsorbed surface Hads (formula (19))[84].
H+(Cu)+e-→Hads(Cu)
H+(Pd)+e-→Hads(Pd)
H2+Pd→2Hads(Pd)
4.2 Additional non-metallic simple substance
Different types of metal (non-metal) particles have different electrode potentials, so under the condition of micro-electrolysis, after adding carbon materials in the bimetallic system, the Fe/Al/C internal micro-electrolysis system is used to treat the actual produced water in the oilfield, and the removal efficiency of chemical oxygen demand (COD) reaches 52.6%, which creates favorable conditions for the subsequent biological treatment[85].
In order to improve the selectivity of the zero-valent metal activated O2 for generating H2O2 in situ, a metal-carbon composite with a welding interface is prepared by using a metal and a conductive carbon material,By forming a corrosion cell in water containing O2, in the electrode reaction, the electrons lost by the zero-valent metal as the anode are transferred to the surface of the cathode (conductive carbon material), and the O2 is reduced by gaining electrons at the surface of the cathode[86]. High energy ball milling and high temperature melting method was used to prepare ZVAl carbon graphite (Gr) which can produce H2O2 in situ, and liquid phase reduction method was used to attach nano-ZVI to ZVAl carbon graphite (Gr) to prepare a new type of functional material (ZVI) which can produce H2O2/·OH in situ. Because of ZVI, it has good magnetic separation characteristics.At the same time, ZVAl in ZVAl Gr in ZVI activated the O2 to produce a large amount of H2O2 in situ efficiently by forming a corrosion cell, while ZVI in ZVAl Gr in ZVI catalyzed the decomposition of the H2O2 produced in situ into strong oxidizing OH, which realized the efficient degradation of antibiotics[87]. At the same time, the Al⁃CNTs⁃Fe3O4 composite particles of Al, carbon nanotubes (CNTs) and Fe3O4 were formed by high energy ball milling, which had abundant pore structure and large specific surface area due to the existence of CNTs.While increasing the mass transfer rate, it can adsorb and enrich the organic pollutants in the sewage, thus increasing the concentration of reactants in the degradation process, which is very beneficial to improving the degradation rate[88].
4.3 Additive polymer
The introduction of a third metal can improve the activity of the bimetal, but its activity is not maintained. Chitosan has excellent biocompatibility and can be used as a polymer to protect the reactivity of the bimetal[89]. In recent years, chitosan-modified bentonite has been used to remove pollutants from water[90]. After coating with chitosan and bentonite, the increase of isoelectric point will make Fe/Al bimetallic have the ability to adsorb nitrate in a wide range of pH values, and it is unrealistic to recover Fe/Al bimetallic particles from water.Chitosan can enhance the separation of bentonite from water, increase the reaction rate, reduce secondary pollution and total nitrogen (TN) in the removal of nitrate, and has a higher selectivity for N2 than Fe/Al bimetallic[91].
4.4 Dosing Ligand
The efficiency of the Fe/Al/O2 system as well as the reaction durability can be improved by the addition of iron complexing ligands. For example, ethylenediaminetetraacetic acid (EDTA) enhances the in situ generation of H2O2 and · OH, and can activate dissolved oxygen and continuously produce H2O2 through a series of complex reactions (Fig. 9), thus increasing the activation rate of Fe/Al[92]. At the same time, EDTA can make the Fe/Al/O2 degrade simultaneously with 4 BP, EDTA and TOC in a long time range[92].
At the same time, the results show that the yield of oxidant can be significantly improved by adding oxalate into Fe/Al oxygen saturated solution due to the solubility, pH buffer and complexation characteristics of oxalate[74].
4.5 Build load
When single metal or bimetal is used alone, it will have microbial inhibition, and Al (Ⅲ), Fe (Ⅱ), Fe (Ⅲ) will be released into water in the reaction process, which will pose the threat of secondary pollution[28,93]. Bentonite is an efficient adsorbent with cation exchangeability, and some toxic reduction products produced during the reaction and iron ions released from ZVI can be effectively removed[94]. When the Fe/Al bimetal loaded on bentonite was used to treat nitrate, it could significantly reduce the total nitrogen in water, effectively remove Fe (Ⅱ), Fe (Ⅲ) and Al (Ⅲ) released in the reaction process, and avoid secondary pollution[91]. The columnar clay calcined by bentonite and bimetallic ions at high temperature can be used as a stable catalyst for catalytic wet peroxide oxidation, and has different structural sensitivity at different calcination temperatures[95].
In the above strategy of modifying ZVAl with iron-based materials, there are additional substances whose respective characteristics produce different optimization results, such as the simultaneous introduction of a third metal and a ligand, or the higher reactivity for the degradation of a ligand as a pollutant while enhancing the oxidation efficiency. The combination of two may produce obvious synergistic effect, so whether this can be used to make the composite material have two optimization results at the same time is worth further discussion.
5 Conclusion and prospect
ZVAl modified by iron-based materials can complement each other's advantages and is developing into a new type of environmental functional materials in the field of water treatment.
In the modification and application of the above iron-based materials to ZVAl, the main action mechanisms are as follows: the oxidation and reduction reactions existing in the chemical deposition method, the chemical plating method and the co-reduction method are used to change or destroy the ZVAl oxide film; An intermetallic reaction mechanism occurs to enhance the interfacial bonding between the ZVAl and the iron-based material or to change the physical properties of the hardness and magnetism of the ZVAl; The modification of ZVAl by iron-based materials can also be realized by mechanochemical self-propagating reaction. The composite material modified by the iron-based material ZVAl can improve the application of the ZVAl, especially in the environmental field, the reaction rate is accelerated, the pH application range is widened, the reaction durability is prolonged, and the reaction selectivity is enhanced compared with the composite material without the modification. In addition, the system of ZVAl modified by iron-based materials for removing pollutants in water was optimized by introducing a third metal, adding a non-metallic element, adding a polymer, adding a ligand, and constructing a load. Therefore, the development of new materials and new technologies for ZVAl water treatment has been strongly promoted. In recent years, although there are more and more studies on the methods and applications of ZVAl modified by iron-based materials, there are still some problems to be further studied.
(1) Find a green and simple new method to modify ZVAl for iron-based materials
Chemical deposition is the most commonly used method to prepare ZVI composite ZVAl (Fe/Al bimetallic), but the existence of excessive secondary metal on the bimetallic system prepared by this method can completely cover the surface of the primary metal, resulting in low pollutant degradation efficiency. In addition, the secondary metal covering the primary metal will be released into the water during the corrosion and dissolution of the primary metal, resulting in a significant reduction in the activity of the bimetal, and a large number of chemical reagents need to be consumed in the preparation process, which not only does harm to the environment, but also brings risks to the experimental operation[92]. In comparison, the mechanochemical method does not require pickling pretreatment, and the controllable amount of ball milling method can avoid the surface over-deposition of the replacement method, and the full fusion of bimetals can reduce the stripping between metals in the reaction process. In addition, ball milling can produce a large number of internal strains and lattice defects, which play a very important catalytic role[2]. However, due to the good ductility of ZVAl itself, it is difficult to prepare Fe/Al core-shell bimetal by mechanochemical method, and the appropriate iron source and specific milling conditions need to be further studied.
(2) In-depth study of the mechanochemical mechanism in the process of ZVAl modification of iron-based materials.
Current studies have found that the oxide film on the surface of ZVAl can be mechanically activated and modified by ferrous salts or iron-containing minerals, but the focus of the study is generally focused on the subsequent removal mechanism of pollutants and the interaction mechanism between pollutants and the surface of materials.In the process of mechanical activation modification, the mechanochemical process is a "black box", and how to effectively control the formation of new passive films on the surface of ZVAl is very important for water treatment applications, so the mechanochemical process between ZVAl and iron-based materials is worth further study.
(3) To further explore the application potential of the composite materials constructed by the modification of iron-based materials with ZVAl in the environmental field.
The composite material modified by the iron-based material ZVAl can be applied to the reduction of pollutants under a wide range of pH conditions. However, in the oxidation system, the excellent degradation ability of pollutants is mostly reflected under acidic conditions, and when the initial pH value is greater than 7, the degradation of pollutants will be strongly inhibited. The application of iron-based modified ZVAl materials in advanced oxidation is also a direction worthy of breakthrough. Moreover, aluminum salts have been widely used as coagulants in water treatment to remove organic pollutants by coagulation[96]. The concentration of residual Al (Ⅲ) can be controlled by adjusting the pH value. At the same time, aluminum treatment is an economical and commonly used method for phosphate treatment. Al (Ⅲ) can precipitate phosphate, so it can be designed to remove pollutants and phosphate simultaneously and reduce the dissolution of Al (Ⅲ) in application[97]. At the same time, due to the coagulation of aluminum ions and aluminum substances produced by metal corrosion, the resulting aluminum-iron mud can be treated harmlessly such as cement-based solidification, and can also be recycled by appropriate filter AIDS, which provides a new idea for the treatment of harmful impurities[98].
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