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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 7: 1030 - 1039

DOI: https://doi.org/10.7536/PC221123

Review

Mechanism of Phase Transition on Zero-Valent Aluminum Surface and Its Effect on Pollutant Removal

  • Shiying Yang , 1, 2, 3, * ,
  • Zhen Yang 3
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  • 1 The Key Laboratory of Marine Environment & Ecology, Ministry of Education,Qingdao 266100, China
  • 2 Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering (MEGE),Qingdao 266100, China
  • 3 College of Environmental Science and Engineering, Ocean University of China,Qingdao 266100, China
* Corresponding author e-mail:

Received date: 2022-11-24

  Revised date: 2023-04-05

  Online published: 2023-06-12

Supported by

Natural Science Foundation of Shandong Province(ZR2020MB093)

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Abstract

Zero-valent aluminum (ZVAl) is susceptibly oxidized in both gas and liquid media, which makes the element Al, as an “electron reservoir”, surrounded by an oxide/oxyhydroxide shell. Typically, this shell is made up of Al2O3, AlOOH, Al(OH)3 and other phases with varying structures. Furthermore, as the environment changes, the shell’s phases may transform into each other, and even the transition between different crystalline forms of the same phase may take place, finally leading to changes in the general properties of ZVAl. It is believed that the treatment of ZVAl in a variety of fields can be regarded as different regulations of its surface composition. Although it has been demonstrated that ZVAl can efficiently degrade pollutants due to its strong reducing ability, current research only focuses on the removal of the inherent oxides/oxyhydroxides on the surface of ZVAl, ignoring the transition and connection between the various phases. As a result, it is challenging to systematically clarify the impact of surface phase transformation on the reduction performance of ZVAl in the process of pollutant degradation. To provide a theoretical foundation for the investigation of the interfacial reaction processes and mechanisms between ZVAl and pollutants as well as the directional regulation of ZVAl, it is necessary to have a thorough understanding of the structure and properties of the various phases that make up the ZVAl surface, particularly the transition processes between different phases. Hence, in this review, for the first time, the reaction mechanism of the surface phase transition of ZVAl-based materials is summarized and prospectively discussed from the perspective of the type, structure, and nature of ZVAl surface phases as well as the reaction mechanism of the phase transition.

Contents

1 Introduction

2 Structure and properties of oxidized ZVAl in medias

2.1 Structure and properties of surface phases in gas media

2.2 Structure and properties of surface phases in liquid media

3 Phase transition of oxide/oxyhydroxide shells of ZVAl

3.1 To formγ-Al2O3

3.2 To formα-Al2O3

4 The influence mechanism of phase transition

4.1 Transition mechanisms in gas media

4.2 Transition mechanisms in liquid media

5 Conclusions and outlook

Key words： zero-valent aluminum; oxide/oxyhydroxide shell; phase transition; pollutant degradation; surface reaction mechanism

Cite this article

Shiying Yang , Zhen Yang . Mechanism of Phase Transition on Zero-Valent Aluminum Surface and Its Effect on Pollutant Removal[J]. Progress in Chemistry, 2023 , 35(7) : 1030 -1039 . DOI: 10.7536/PC221123

1 Introduction

As active Zero-valent metals (ZVMs), ZVAl has a core-shell structure with an outer layer of (hydroxide) oxide surrounding the inner element Al[9]. Different (hydr) oxides are produced on the surface of ZVAl with different environmental media, which mainly produce Al2O3 in dry air, and appear in the form of AlOOH, Al(OH)3, etc. In humid air or water[10,11]. In addition, even if the same phase in the surface composition, the phase transformation between different crystal forms will affect the properties and applications of ZVAl[12]. Although the covering (hydroxide) of ZVAl is very thin, its phase transformation is very important to the overall activity regulation and function[6,7,13].
In recent years, some scholars have summarized the surface action mechanism of ZVAl. In the field of energy fuels, scholars have determined the structure and location of oxides grown by ZVAl in different gaseous oxidizing media, clarified the differences of reaction mechanisms in different oxidants, and constructed ZVAl combustion models under different conditions[1]. As a hydrogen-producing material, from the perspective of ZVAl surface composition, relevant scholars summarized the progress of common additives, reaction process and mechanism, aging mode and preservation method of materials in the preparation process of hydrogen-producing aluminum matrix composites, and demonstrated the hindrance and mechanism of ZVAl surface composition on its active release[14]. In terms of pollutant removal, our research group summarized the formation process of ZVAl surface oxides, the action mechanism of ZVAl in different operations, and the effects of aqueous medium environment on its reactivity and surface action[10].
In the research on Zero-valent iron (ZVI), the surface chemical characteristics and crystal phase changes of nano-ZVI in water phase, as well as the property changes of nZVI before and after oxidation in different environmental media have been summarized, and it is clear that the dynamic changes of nZVI surface caused by the environment have an impact on the degradation of pollutants by nZVI[15,16]. However, it is only for the analysis of nanoscale ZVI.
At the same time, the study of ZVAl also lacks a systematic summary of the limiting factor of its surface phase transformation, such as the environment and other factors, and does not establish a relationship between the dynamic change of ZVAl surface and its reaction performance. Therefore, on the basis of the existing research on the surface chemistry of ZVAl, this paper summarizes the influence of external environment on the formation of surface phases of ZVAl and the conditions and processes of the transformation of various phases from the surface phase transformation process of ZVA1 with different application sizes. Based on the above contents, the mechanism of phase transformation on the surface of ZVAl with broad spectrum is discussed in order to clarify the relationship between the change of composition and structure on the surface of ZVAl and the reaction performance.Promote the mutual reference and common development of ZVAl in the field of pollutant degradation and other fields, especially promote the further research of ZVAl surface chemistry, and fully tap the application potential of ZVA1.

2 Oxidation products on the surface of zero-valent aluminum in different media.

2.1 Structure and properties of surface oxides in gaseous medium

AlOx is a general term for aluminum oxides, of which the most stable and common form is Al2O3. At normal temperature and pressure, fresh metal ZVAl exposed to dry air will be rapidly oxidized to form amorphous Al2O3, which covers the outer surface of elemental Al (Formula 1)[17,18].
4 A l + 3 O 2 → 2 A l 2 O 3
As the main component of ZVAl, the microstructure of Al2O3 affects the properties of ZVAl. The mechanism of grain growth in Al2O3 is constructed by combining the mathematical statistical model with the thermodynamics and kinetics of grain growth, which shows that the annihilation of grain boundaries depends on the number and angle between adjacent grains, and that the transport between grain boundaries can be regulated by adding dopants such as carbon nanotubes to Al2O3 through the formation of complexes at grain boundary interfaces[19].

2.2 Structure and properties of surface hydroxides in liquid medium

In humid air or liquid phase medium, H2O will undergo hydration reaction with Al2O3 on the original surface or Al in the interior, resulting in thermodynamically more stable hydroxides than Al2O3, namely AlOOH and Al(OH)3[20].

2.2.1 Aluminum oxyhydroxide (AlOOH)

AlOOH can be formed by hydration reaction only with H2O and ZVAl. When ZVAl is placed in hot water at 95 ℃ for 300 s (Formula 2), nanoscale needle-like AlOOH crystals can be gradually formed from the surface, which can enhance the mechanical bonding strength between interfaces[21]. Untreated aluminum chips are used to produce H2 and AlOOH and release heat, and the product value is 600% higher than that of the raw material itself, which can better play an economic role in industrial production[22].
2 A l + 4 H 2 O = 2 A l O O H + 3 H 2

2.2.2 Aluminum hydroxide (Al(OH)3)

When Al is gradually transformed from the surface to the Al(OH)3 process, the Al-O-Al bond in the original surface Al2O3 is first broken to form Al-OH. The results show that nano-scale Al(OH)3 with specific morphologies such as granular, spindle-like or rod-like can be prepared by ball milling ZVAl and changing the milling time during H2O[23].
The surface reaction of ZVAl and H2O at different reaction temperatures is as follows:[24]
2 A l + 6 H 2 O = 2 A l ( O H ) 3 + 3 H 2 + 427.98 k J / m o l
2 A l + 4 H 2 O = 2 A l O O H + 3 H 2 + 415.24 k J / m o l
2 A l + 3 H 2 O = A l 2 O 3 + 3 H 2 + 407.7 k J / m o l
The above reaction temperature is lower than 200 deg C, 200 to 400 deg C and higher than 400 deg C in turn. It can be seen that the reaction product and its corresponding reaction heat are related to the thermodynamic conditions. With the increase of temperature, the reaction heat decreases, and the hydroxyl content in the product decreases gradually.
The difference of hydroxyl density between AlOOH and Al(OH)3 leads to the difference of performance[25]. The surface hydroxyl groups adsorb H2O, change the dissociation degree of H2O, promote the surface passivator to crack, and stimulate the reactivity of internal Al.

3 Phase transformation between oxidation products on the surface of zero-valent aluminum

Amorphous Al2O3 exists on the surface of ZVAl at room temperature, which will transform into various crystalline Al2O3 after high temperature. At present, more than twenty crystal forms, such as α, γ, δ, τ and θ, have been found in crystalline Al2O3, and the types are still increasing[12]. The crystalline States can be transformed into each other, among which γ-Al2O3 and α-Al2O3 are the most widely studied[26⇓~28].

3.1 Active γ-Al2O3 formed by other phase transformation.

γ-Al2O3, also known as "activated alumina", has a defect spinel structure, which is a transition state crystal form of Al2O3[29]. Due to the presence of oxygen vacancies in the structure, H2O is dissociated into ions when it contacts γ-Al2O3[30]. For this reason, scholars transformed the original dense oxide layer on the surface of Al into a looser, more electron-transporting and more active γ-Al2O3 to promote reduction, adsorption and other reactions[31⇓~33].

3.1.1 The transformation of AlOx into γ-Al2O3.

The waste aluminum is dissolved, filtered, precipitated and calcined, and the original surface is reacted (formulas 6 to 11) to prepare the γ-Al2O3, which can realize the secondary utilization of the waste[34].
A l 2 O 3 ( s ) + 6 H + ( a q ) → 2 A l 3 + ( a q ) + 3 H 2 O
2 A l 3 + ( a q ) + 6 O H - ( a q ) → 2 A l ( O H ) 3 ( s )
2 A l ( O H ) 3 ( s ) + H e a t → γ - A l 2 O 3 + 3 H 2 O
A l 2 O 3 ( s ) + 2 O H - ( a q ) → 2 A l O 2 - ( a q ) + H 2 O
A l O 2 - ( a q ) + 2 H + ( a q ) + H 2 O → 2 A l ( O H ) 3 ( s )
2 A l ( O H ) 3 ( s ) + H e a t → γ - A l 2 O 3 + 3 H 2 O
Some transition state aluminum oxides can be formed on the surface of Al by artificial control, and then transformed into γ-Al2O3. Two kinds of monoclinic ordered superstructure phases (AlOx and AlOy) were prepared on the surface of ZVAl powder by chemical vapor deposition, in which Al vacancies and O vacancies were arranged in an orderly manner, and the vacancy arrangement changed from order to disorder and into γ-Al2O3 under the action of high-energy electron beam[35].

3.1.2 AlOOH transformation to form γ-Al2O3.

AlOOH on the original surface of ZVAl can be used as a precursor to prepare γ-Al2O3 by calcination (Fig. 1). Combined with mechanochemistry, mesoporous nanocrystalline AlOOH was first obtained by ball milling, followed by heat treatment at 400 ° C to produce pure mesoporous crystalline product γ-Al2O3[36]. The product has a specific surface area of 389 m2/g and a macropore volume of 1.55 cm3/g, which is very suitable for catalyst, carrier, adsorbent and other applications.
图1 Al表面AlOOH向γ-Al2O3相变转化过程[37]

Fig.1 Transformation process of AlOOH to γ-Al2 O3[37]

3.1.3 The transformation of Al(OH)3 into γ-Al2O3.

Al(OH)3, as an aluminum hydroxide, can also be used as a precursor of γ-Al2O3. Recently, an efficient and low-cost method for preparing γ-Al2O3 from industrial aluminum waste has been developed. In brief, the aluminum waste is first washed, and then NaOH and HCl are added to form a white precipitate Al(OH)3, which is filtered, dried and calcined to form a γ-Al2O3. The material has a good adsorption effect on organic dyes such as methylene blue, crystal violet and basic fuchsin[38].

3.2 Other phases transform to form stable α-Al2O3.

As the most stable crystal structure of Al2O3, α-Al2O3 can be transformed by other transition States[39,40].

3.2.1 The other crystal forms Al2O3 transform to form α-Al2O3.

In order to study the effect of phase transition of Al2O3 film on device surface, strip-shaped amorphous Al2O3 with cavity structure was fabricated on the surface of aluminum-based materials, and its solid phase epitaxy process was studied.It was found that the Al2O3 experienced a two-stage phase transition from amorphous to γ phase and then to α phase, and the whole Al2O3 film could be completely transformed into α-Al2O3 by adjusting the solid phase annealing rate[41]. In order to change the phase of Al2O3, high temperature is needed to provide energy, but the sintering temperature of α-Al2O3 powder can be reduced by the dissolution, coordination and chelation of low molecular weight organic acid and the change of Al-O bond energy in the crystal structure[42].

3.2.2 AlOOH or Al(OH)3 transformation to form α-Al2O3

Aluminum hydroxides can be converted to α-Al2O3 by releasing hydroxyl groups and crystallizing. However, most of the current studies only focus on the transformation of Al(OH)3 or AlOOH itself into α-Al2O3, and do not explore the transformation of aluminum hydroxide on the surface of ZVAl[43][44]. Moreover, there is no study on the transition between hydroxides on the surface of ZVAl, which makes it difficult to systematically reflect the influence mechanism of the transition between hydroxides on the overall activity and application.

4 Mechanism of phase transformation

The effects of phase transformation of ZVAl surface material can be divided into two types: 1) enhancing the compactness of the surface material and protecting the substrate material; 2) Weakening the restriction of surface species on active release and promoting electron transport. The two effects can regulate the release of ZVAl activity, which is helpful for the application of ZVAl in different scenarios.

4.1 Transition mechanism in gaseous medium

The Al2O3 generated on the surface of ZVAl is an insulator, which inhibits the transmission of internal electrons to the outside and affects the activity of the whole material.

4.1.1 Influence the reaction rate

Aluminum is a good energy carrier because of its high energy density, high specific energy, and low enthalpy of formation of Al2O3 produced on the surface after oxidation (-1645 kJ/mol), which will release huge heat when reacting with O2[45⇓~47]. ZVAl is also used in the field of propellants to continuously and regularly release energy as a power source based on its ability to produce violent redox reactions. In particular, the addition of nZVAl to the propellant can increase the burning rate and specific impulse of the fuel[46,48⇓-50]. The ignition and combustion characteristics of ZVAl depend on the formation and porosity of its surface oxide layer. The particle size and porosity of nZVAl are significantly smaller than those of micron-sized ZVAl (mZVAl), and the thermal insulation effect of the oxide layer will be significantly reduced with the increase of the particle size[51].
At the same time, nZVAl is easier to be oxidized by O2 than mZVAl because of its large specific surface area due to the size effect, and the passivation layer formed on the surface will limit the reaction rate. This problem can be solved by changing the surface phase type of nZVAl and reducing the activation energy of the reaction. When Ar is used as plasma gas to reduce the Al2O3 shell of ZVAl, the composition and structure of the particle surface will be induced, the thickness of the original Al2O3 passivation layer will be reduced, and the hydration will be transformed into metastable AlOOH, which can reduce the energy threshold of ignition[52].

4.1.2 Inhibition reaction proceeding

Pure aluminum is an excellent coating material with low hardness and high plastic deformation, which can inhibit the reaction with the outside world by adjusting the surface morphology, thickness and other variables of the aluminum coating to obtain stable performance[53⇓~55].
The aluminum coating can be regarded as the anode of the battery in the electrochemical reaction to protect the substrate material as the cathode. Even at high temperature, all kinds of aluminum alloys have strong oxidation resistance, which is due to the selective oxidation of Al in the alloy to the surface to form a continuous, stable and strong binding Al2O3 film, which plays a protective role[56].
The Al2O3 formed by in-situ oxidation on the surface of ZVAl can significantly improve the wear resistance of the material[57]. In addition, the corrosion potential of ZVAl can be increased, the corrosion current density can be decreased, and the corrosion resistance can be improved by preparing a uniform and dense ceramic film on the surface of ZVAl[58]. The ceramic membrane is composed of α-Al2O3 and γ-Al2O3 combined in a zigzag manner, and is divided into three parts: an innermost α-Al2O3 compact layer, an intermediate transition layer, and an outermost γ-Al2O3 loose layer.

4.2 Transition mechanism in liquid medium

In the liquid phase, not only the hydration reaction of Al and the Al2O3 and H2O on its surface occurs, but also the Al-OH produced on the surface can react with the substances in the medium.

4.2.1 Regulating the reaction of ZVAl and H2O;

4.2.1.1 Cause of corrosion

ZVAl reacts with H2O to produce gas H2, and when the H2 accumulates to a certain amount, it will break through the inherent state of the original surface of ZVAl and make the surface metastable. It not only partially converts the original oxide on the surface of ZVAl into hydroxide, but also significantly increases the concentration of electron defects in the oxide layer, resulting in a significant increase in conductivity. The sites in the oxide layer that are prone to hydration will exhibit a higher density of electronic defects and are more prone to local electrochemical breakdown, making them more permeable to water[59]. Small size and corresponding large curvature are more conducive to the formation of defects and corrosion.
ZVAl surface corrosion is a dynamic oxide layer mediated process[59]. The native oxide layer itself is a dynamic structure, and the composition and structure change with time and environment. During ZVAl surface corrosion, Al-O bonds are continuously broken and reformed by adding and removing OH- from the lattice through reversible hydrolysis reactions, in which OH- acts as a mobile phase and affects the corrosion kinetics.
Gai et Al. Used Al powder with six different particle sizes to systematically study the process of the reaction between Al and water until the H2 was produced, and found that the initial stage of the aluminum-water reaction was controlled by the surface chemical mechanism, the system temperature determined the type of surface products (Al(OH)3, AlOOH or a mixture of both), and the type of products in the later stage was determined by the diffusion of H2O molecules in the original product layer[60]. This process can also be summarized as follows: the initial Al2O3 layer on the particle surface is first transformed into small AlOOH nuclei, then aggregated and further developed to form sheet structures with a thickness of several nanometers, which are unevenly aggregated and intersected on the surface, and finally formed a layer of irregular and well-crystallized AlOOH on the outer surface[60]. By combing the particle size, the formation and fragmentation of AlOOH, and the relationship between the multi-stage reaction kinetics and the total reaction rate (Fig. 2), it is proved that the reactivity of ZVAl with water depends on the reaction temperature and particle size, and follows the Arrhenius type reaction. The higher the reaction temperature, the shorter the induction time and the faster the corrosion rate, thus increasing the reaction rate and ultimately increasing the total hydrogen production[61].
图2 铝水反应的一种收缩核模型[61]

Fig.2 A shrinkage core model for ZVAl-H2O reaction[61]

The material morphology also affects the corrosion rate. In the reaction of aluminum alloy powder with low temperature water vapor, the surface of the material is embrittled and fractured, and a popcorn-like Al(OH)3 is produced, which can promote the transport of H2O in the surface phase layer, so that the surface of the material can react with water vapor rapidly and completely to produce H2 at room temperature, which also proves that the passivation of aluminum alloy can be avoided by changing the surface morphology[62].

4.2.1.2 Corrosion Mitigation Measures

Reaction of Al-OH with chemical
In the face of the corrosion problem of aluminum-based materials, corrosion inhibitors can be added to achieve the anti-corrosion effect according to the characteristics of Al-OH on the surface. The chemical substance in the inhibitor reacts with the surface oxide to form a bond, and a new passivation layer replaces the original oxide layer on the surface.
Aluminum coatings are stabilized by surface treatment with organic or inorganic substances before use[63]. The organic inhibitor mainly combines with Al (Ⅲ) on the surface through functional groups, and forms bonds in situ on the surface to achieve the overall coating of the material. Mercapto compounds, azole derivatives, organic dyes and some polymers are the most commonly used organic corrosion inhibitors in the modification of aluminum-based materials[64]. In the past, inorganic inhibitors used heavy metal coatings such as chromate, but now they mostly use environmentally friendly alcohols and other substances as precursors to form silica coatings on the surface for protection[63].
Hydrophobic regulation
Hydrophobic manipulation of the surface also slows down the corrosion effect. The hydrophobicity of materials is determined by both surface topography and chemical composition. The surface free energy of the substrate can be reduced, the surface wettability and water adhesion can be adjusted, and the hydrophobic effect can be achieved by adjusting the microstructure and chemical composition of the ZVAl surface oxide[65]. Scholars have developed a treatment method to realize the hydrophobicity control of aluminum-based materials: due to the existence of H2O, Al(OH)3 (Formula 12) will be generated on the surface of ZVAl electrode, thereby reducing the utilization and performance of ZVAl-Air battery, so it is necessary to construct a hydrophobic surface to isolate ZVAl and H2O[66]. A hydrophobic coating with an inhibition efficiency of 81.6% can be formed by attaching PTFE to the surface of ZVAl, which prolongs the life of ZVAl electrode by about 34%. Superhydrophobic surface can also be prepared by heterogeneous nucleation and layered growth of AlOOH. Through liquid phase synthesis combined with modification of stearic acid, AlOOH with extremely low surface energy can be produced on ZVAl surface. Its structure is similar to that of lotus leaf, with a contact angle of 169 ° and a sliding angle of 4 ° to water. It remains hydrophobic after ultrasonic treatment or long-term storage (Fig. 3)[67].
2 A l + O 2 + 4 H 2 O → 2 A l ( O H ) 3 + H 2 ( g )
图3 水滴在(a)裸露的铝箔上,(b)合成的薄水铝石膜上,(c)硬脂酸改性后的薄水铝石膜上[67]

Fig.3 Water droplets on (a) exposed ZVAl, (b) synthetic AlOOH film,(c) photographs of AlOOH films modified by stearic acid[67]

It has been explored and developed in the fields of heat transfer and anticorrosive coatings to regulate the surface hydrophilic/hydrophobic properties by in-situ generated substances (such as AlOOH) on the surface of ZVAl. In the field of pollutant degradation, if the surface is hydrophobic, the hydrophobic pollutants can be enriched on the surface of the material in the degradation system, and the generated reduction electrons can be preferentially transferred to the pollutants to reduce the competition with the hydrogen side reaction, thereby improving the electron selectivity of the material.
Regulation by coexisting substances
Recently, aiming at the problem of aluminum substrate corrosion in lithium-ion batteries, electrolyte composition, thermal conditions and electrochemical parameters were used as research variables, and it was found that high concentration or local high concentration electrolyte would cover the substrate with a layer of insoluble products, which would reduce the conductivity of the electrode and hinder the ion transport[68]. By investigating the corrosion behavior of ZVAl-based materials in low concentration NaHCO3 solution at a series of high temperatures (50 ℃), it is found that the ordered charge field formed by HCO 3 - at the solid-liquid interface limits the diffusion of Al3+.Compared with deionized water, the ZVAl based material shows enhanced corrosion inhibition effect in low concentration NaHCO3 solution, which proves that environmental factors including inorganic salts also play a role in the corrosion of ZVAl surface[69].

4.2.2 Application of regulate and controlling ZVAl in pollutant treatment field

In the liquid phase, Al-OH on the surface of ZVAl will react with other substances except H2O, which can be used to realize the degradation of pollutants and further explore the performance of ZVAl in the field of pollutant treatment.

4.2.2.1 ZVAl degrades pollutants by providing electron

When ZVMs reduce and remove pollutants, the H+ of the system will be consumed, and the surface will be passivated and lose its activity[70]. The amphoteric metal ZVAl has a lower redox potential than the most commonly used ZVI, exhibits stronger reduction activity, and provides nearly four times the thermodynamic driving force for electron transfer than ZVI[71]. The reducing ability of ZVAl is gradually applied to the field of pollutant degradation[72,73].
In the liquid phase, the reaction of ZVAl with water or pollutants can be essentially regarded as the release of electrons by ZVAl as an electron donor, and the electrons are transferred to the acceptors such as water and pollutants at the interface. This process depends on the transfer of electrons from the internal ZVAl production to the external electron acceptor, so the material at the interface is required to be both loose and conductive.
Scholars have studied the oxide type, phase transformation and coverage degree on the surface of ZVAl, and found that the dense Al2O3 on the surface of ZVAl will be transformed into loose Al(OH)3, AlOOH and γ-Al2O3 after different degrees of treatment, thus promoting electron transfer to shorten the reaction induction period[74][75][76,77]. Based on this, the modified ZVAl has also been proved to be effective in removing pollutants such as bromate (Figure 4), hexavalent chromium and dyes if it is modified by a series of immersion-based modification operations and substances loaded with loose γ-Al2O3[6,7,13].
图4 基于表面氧化改性ZVAl去除溴酸盐研究模型[7]

Fig.4 Research model of bromate removal based on oxidation modification of ZVAl[7]

4.2.2.2 Activation and mechanism of reducibility of ZVAl

At present, when using ZVAl to degrade pollutants, scholars mostly adopt two kinds of ideas to eliminate the inhibition caused by the oxide layer: 1) according to the physical and chemical properties of the oxide itself, directly destroy or remove it. Therefore, acid washing, alkali washing, ultrasonic treatment, high-energy ball milling and other treatment means are needed[78⇓⇓~81]; 2) By means of ligand corrosion, construction of bimetallic system and addition of oxidant, the dissolution and modification of ZVAl surface oxide are stimulated to promote internal electron transport[82⇓~84].
Although the above methods can be regarded as the modification of the oxide on the surface of ZVAl, the phase transformation of the surface of ZVAl is regulated. However, most of the current studies focus on the overall performance changes of modified ZVAl, and the phase transformation is not studied in depth.

4.2.2.3 Anti-passivation Strategy and Mechanism of ZVAl

The prerequisite for ZVAl to degrade pollutants is to activate it. However, after being activated, ZVAl will release a large number of electrons in a short time, showing too strong reactivity to react with various oxidizing species (H2O/O2, etc.) In the system indiscriminately. The exposed Al element is oxidized again, and the product covers the surface again, resulting in secondary passivation and limiting the ZVAl reaction activity again. Therefore, it is difficult to make full use of the excellent reduction performance of ZVAl, although the effect of simply removing the oxide on the surface of ZVAl is good in a short time.
In order to enhance the anti-passivation performance of ZVAl, scholars have proposed to control the surface of ZVAl by coating, loading and other means to reduce the influence of H2O, O2 and other factors in the system. Depending on the coating material, the reactivity of ZVAl can be extended by buffering or isolation.
1) Buffering
The coating material forms a coating by reacting with the oxidizing substances on the surface of ZVAl to inhibit surface passivation. Ligand addition is one of the effective methods. The removal efficiency of BrO 3 - can be significantly increased by adding only 50 μM oxalic acid (OX) to the ZVAl-BrO 3 - system. The strong affinity of OX for Al3+ makes the coordination between Al3+ and OX, which effectively prevents the formation of a passive layer of Al(OH)3, thus keeping the reducibility of ZVAl to reduce bromate[85]. When sodium citrate (SC), as a common green non-toxic ligand, is introduced into the reaction system of mZVAl for degrading carbon tetrachloride (CT), the surface corrosion of mZVAl is enhanced at the solid-liquid interface and an Al [Cit] complex is formed, so that the internal Al0 active sites are continuously exposed, so that the mZVA1 has excellent sustainable utilization efficiency and can be continuously reused for degrading CT for five times[86]; The composite Al @ Alg-C600 was prepared by in-situ pyrolysis of sodium alginate coated with mZVAl, which not only destroyed the original passivation layer, but also produced a thin layer of Al4C3 on the surface to buffer pH, so that the material could repeatedly remove Cr (Ⅵ) and reactive black 5 in a wide pH range of 3. 00 ~ 11.0, and the material would be reactivated after re-pyrolysis[87].
2) Isolation
The porous material with low solubility can be used as a barrier to protect the ZVAl from the interference of H2O, O2 and other substances in the system, and can enhance the reduction ability by inhibiting the generation of Al2O3 on the surface of the ZVAl, and can enhance the transmission ability of pollutants in the material by means of the microporous structure. Aluminum-carbon composite AC@mZVAlbm/NaCl prepared by ball milling of activated carbon (AC) and NaCl with mZVAl[88]. Combining the adsorption of AC and the reducing ability of mZVAl can effectively solve the problem that hydrophobic hexabromocyclododecane (HBCD) can not reach the surface of hydrophilic mZVAl, and achieve the effective degradation of HBCD (Fig. 5).
图5 AC@mZVAlbm/NaCl降解HBCD的机理[88]

Fig.5 The proposed reaction mechanism of HBCD degradation by AC@mZVAlbm/NaCl[88].

5 Summary and Prospect

ZVAl is widely used in many fields because of its excellent thermal conductivity, electrical conductivity, strong reduction and other properties. The type and structure of the surface phase play a vital role in the overall performance, which can be passivated and protected by the dense oxide produced on the surface. If the inhibition of surface oxide is weakened, it can be used to produce hydrogen and degrade pollutants.
Nevertheless, the method is still in the preliminary stage of exploration, and the following problems need to be further studied:
(1) The mechanism of the effect of surface phase composition on ZVAl was discussed.
At present, the research on ZVAl surface in various fields has certain disciplinary limitations. The effect of surface phase formation and transformation on the electron release and transfer pathways of ZVAl during reaction with pollutants is very important and has not been systematically studied. In view of the above research, first of all, we can draw lessons from the relevant conclusions of aluminum-water reaction, and make a deeper and more extensive theoretical accumulation on the possible (hydroxide) oxides, properties and transformation laws on the surface of ZVAl. Next, with the help of various characterization methods, the properties of ZVAl materials can be comprehensively analyzed, and the theoretical calculation can be carried out to clarify the transformation law of ZVAl surface structure in the reaction process.
(2) Establish a ZVAl surface control method with strong applicability
At present, ZVI is the most widely studied ZVMs, and a complete set of ZVI modification system has been formed and applied to the actual environment. Because of its lower redox potential and amphoteric properties, ZVAl is more effective than ZVI in the degradation of highly refractory organic pollutants in neutral or alkaline systems[89⇓~91]. It can be used to study the phase transformation law and modification means of ZVI surface oxide layer, and provide a theoretical basis for ZVAl surface regulation. To establish a popular method, the surface morphology of ZVAl was changed from dense to stable and porous, and C, N, S and other elements were modified on the surface. By combining the electron donating ability of the ZVAl and the regulation and control effect of the surface modification substance, electrons are released from the inside and transferred in order, so that the ZVAl based material is anti-passivated and can be used for a long time until the electrons which can be released by the ZVA1 are all released and applied to the degradation of pollutants.
(3) Broaden the application of surface-regulated ZVAl in the environmental field
At present, in the field of pollutant degradation, although the modification scheme based on the idea of ZVAl surface phase transformation has been proved to be feasible from the theoretical level, the number is small and has not been applied to practical wastewater treatment. The next step is to optimize the structure of ZVAl material according to the actual treatment water quality and water treatment process characteristics, determine the correlation between them, and directionally construct materials with efficient degradation and long-term stability. At present, most of the ZVAl surface modification materials are realized in the liquid phase, and the reagent-grade mZVAl is the main one, which also requires the system conditions and cost, which is also a challenge for industrial practical application. Therefore, how to regulate the surface of ZVAl with the least cost is also the key to be solved.
When ZVAl is used as a water treatment material, it will produce Al3+ in the later stage, and aluminum salt itself is a common flocculant, which will be removed by flocculation and precipitation due to hydrolysis and polymerization of metal ions[92]. With the in-depth understanding of the properties of surface species and their mutual transformation of ZVMs, the transformation of surface species such as ZVI and ZVAl and the removal of pollutants will develop in the direction of improving the utilization rate, which will promote the wider application of ZVAl in the field of pollutant degradation[93,94].

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