A Review of Applications of Surface-Modified Aluminum in Hydrogen Generation and Water Treatment
Yang Yang1*, Shu-Kuan Guo2 and Qin Yu3
1*Energy Materials & Physics Group, Department of Physics, Qingdao University of Technology, Qingdao 266520, China
2State Key Laboratory of Functional Crystals and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China
3School of Mechanical Engineering and Rail Transit, Changzhou University, Changzhou 213164, China
*Corresponding author: yangyang159539@163.com; Tel: +86-0539-5797912
Submitted: December 16, 2024
Revised: May 26, 2025
Accepted: June 17, 2025
Published: August 5, 2025
Abstract
Aluminum (Al) has been extensively studied for hydrogen generation and water treatment due to its abundance and low redox potential. However, the reductive capability of Al is hindered by the formation of a passive oxide layer on its surface. Besides the conventional Al surface treatment techniques such as acid/alkali corrosion, alloying and mechanical ball-milling, a novel Al surface modification technology that emerged in recent years has attracted significant attention and extensive research. This technology has been validated as an efficient method for Al activation, boasting both economic benefits and environment-friendly manufacturing processes.
In this review, the recent Al surface modification technologies and their applications in the fields of hydrogen generation and water treatment are comprehensively summarized. The advantages and disadvantages between Al surface modification technologies and common Al reactivity activation methods are compared, mainly focusing on their enhanced effect on Al performance in Al-water reactions for hydrogen generation and Al-contaminant reactions for water treatment. Finally, the obstacles that restrict Al surface modification processes in practical application in Al-water reactions for clean energy generation and Al-contaminant reactions for water treatment are examined. Several strategies to potentially address the obstacles in Al practical application are proposed.
Introduction
In recent years, the applications of metal Al in the fields of Al-water reactions for hydrogen evolution and Al-contaminant reactions for water treatment have gained increasing attention and intensive investigations. Al is the third most abundant element in the earth’s crust and the most abundant metal element (Bokare and Choi, 2009). In comparison with the metal iron (Fe, E0(Fe2+/Fe0) = -0.44V), Al has a lower redox potential [E0(Al3+/Al0) = -1.662V)] (Bokare et al., 2009; Bokare et al., 2014; Yang et al., 2016; Manilevich et al., 2024) (as listed in Table 1), which makes it a promising electron donor. In fact, Al-water reactions and Al-contaminant reactions have similar physicochemical mechanisms, both of which are based on the high reduction ability of metal Al. Distinctly, in Al-water reactions, the electrons released from inner Al are captured by water molecules to generate hydrogen molecules through reductive reaction [Eqs. (1)-(2)]. In contrast, in Al-contaminant reactions, the electrons directly attack the chemical bonds in the large contaminant compound molecules that then are decomposed into small organic fragments as shown in Eq. (3). The electrons derived from metal Al also can be captured by peroxide (hydrogen peroxide or persulfate) molecules and generate strong-oxidizing-activity radicals, and as shown in Eqs. (4)-(7), these free radicals can mineralize the organic contaminants in solution into CO2 and H2O.
Al → 3e– + Al3+ (1)
2H2O + 2e– → 2OH– + H2 ↑ (2)
large organic molecule + e– → small organic molecule fragments (3)
2e– + O2 + 2H+ → H2O2 (4)
e– + H2O2 → OH– + OH• (5)
6e– + S2O82- + 6H+ + 1.5O2 → 2SO4•– + 3H2O (6)
e– + HSO5– + H+ → SO4•– + H2O (7)
Previous investigations demonstrated that a dense oxide film would form on the Al particle surface when Al is exposed to an oxygen-containing or a humid atmosphere, which hinders the release of electrons from inner Al and subsequently leads to a decrease in Al reduction ability (Gai et al., 2012; Mercati et al., 2013; Yavor et al., 2013; Swamy et al., 2014). Hence, overcoming the obstacle of the Al surface dense oxide layer is critical for recovering the Al reduction ability and realizing its practical applications. Until now, a variety of Al activation methods have been developed, such as acid/alkali washing, alloying and mechanical ball milling, which could effectively reactivate Al and increase its electron-releasing ability. In recent years, a new method used in the treatment of the Al surface oxide layer that is named as Al surface modification technology has been developed. Hereafter springs a series of reports about its applications in the fields of Al-water reactions for hydrogen evolution and Al-contaminant reactions for water treatment.
Until now, there have been few detailed reports summarizing the applications of surface-modified Al in hydrogen energy evolution and water treatment. In this paper, different Al surface modification methods were reviewed and the underlying mechanisms responsible for the enhanced effect on Al electrons transferring were clarified. Furthermore, the performances of Al encountering different surface treatment processes in Al-water reactions for clean energy generation and Al-contaminant reactions for water treatment were compared. Finally, this paper elaborated upon the advantages of surface modification processes compared to the common Al surface treatment methods and pointed out the drawbacks of Al surface modification technology that should be overcome for its practical applications in the fields of hydrogen generation and water treatment.
1. Al Surface Treatment Methods
1.1. Acid/Alkali Corrosion
Due to their amphoteric properties, Al oxides or hydroxides can be easily corroded and dissolved in acid or alkaline conditions, recovering the reductive ability of metal Al as an electron donor (Kanakasabai et al., 2023). Belitskus et al. (1970) first proposed the method of the Al-NaOH reaction for hydrogen generation, which laid the foundation of Al-alkali aqueous solution reactions for hydrogen energy evolution. Pyun and Moon (2000) and Zhang et al. (2009) clarified the Al corrosion mechanism in alkali aqueous solutions, which proposed that OH– ions could corrode the dense oxide film on the Al surface and then react with inner Al and generate Al(OH)3. The generated Al(OH)3 further combined with an OH– ion to form Al(OH)4– ion [Eqs. (8)-(10)]. When Al(OH)4– ion concentration exceeds a critical value, it would inversely decompose into Al(OH)3 and OH– ions [Eq. (10)]. Consequently, the increasingly thickening Al(OH)3 layer would hinder the transportation of water molecules and lead to a gradual deterioration of Al reduction ability.
Al2O3 + 2OH– +3H2O → 2Al(OH)4– (8)
Al + 3OH– → Al(OH)3 + 3e– (9)
Al(OH)3 + OH– ↔ Al(OH)4– (10)
Soler et al. (2010) found that the alkaline Na2SnO3 aqueous solution also has a promoted effect on Al-water reactions for hydrogen generation. Furthermore, the performance of Na2SnO3 on activating metal Al reactivity is superior to that of NaOH and NaAlO2 at the same pH values. This could be attributed to the fact that Na2SnO3 can be reduced by metal Al and generate metal Sn covering on the surface of Al particles. The Al-Sn galvanic cell structure would further enhance Al corrosion, increase the rate of electron release and consequently increase Al-water reaction kinetics.
In 2016, Lin and Lin dissolved Al surface oxide film using hydrochloric acid. They studied the performance of the acid-corroded Al on reductively removing the aqueous BrO3– ions and the factors that influence BrO3– removal efficiency. This research laid the foundation for the application of zero-valent Al in the removal of aqueous BrO3– ions. Physicochemical analysis demonstrated that the enhanced effect brought about from acid corrosion on the performance of Al removing BrO3– ions could be attributed to two aspects. One is that as the inner fresh Al was exposed after acid washing, the direct touch between Al and BrO3– ions would greatly accelerate BrO3– ions reduction removal kinetics. The other is that the acid-corroded Al would adsorb many more BrO3– ions than the untreated Al because the acid corrosion process decreases the average Al particles sizes and subsequently increases the surface area of Al particles, as shown in Figure 1.
In 2009, Bokare and Choi studied zero-valent Al oxidatively degraded 4-chlorophenol (4-CP) in acidic aqueous solution. They found that pH value is crucial to 4-CP degradation kinetics by metal Al; a lower solution pH value would give rise to higher 4-CP degradation kinetics. Thereafter, an acidic solution is widely adopted in Al-based water treatment methods. Liu et al. (2011) adjusted the pH value of the reaction solution to the acidic range using HClO4 and studied the performance of metal Al in the removal of aqueous bisphenol A (BPA) under acidic conditions. They found that in the reaction solution with a pH value lower than 2.5, metal Al could efficiently remove the aqueous bisphenol A (BPA) at room temperature. When the initial pH value of the reaction solution exceeded 2.5, the BPA removal efficiency using metal Al would evidently decrease as the solution pH value increased. Liu et al. attributed the enhanced effect of acidic conditions on the performance of metal Al in BPA removal to two aspects. On the one hand, the acidic solution promoted the dissolution of the Al surface oxide film, which made the inner Al prone to exposure and direct touch with BPA molecules. On the other hand, the acidic solution provided a suitable condition for the generation of hydroxyl radicals (OH•) with high oxidation ability [Eqs. (11)-(12)]. Furthermore, Liu et al. found that the addition of Fe2+ ions into the reaction solution could further increase BPA degradation kinetics, which could be attributed to the synergistic effect of the Fenton process.
2Al + 3O2 + 6H+ → 2Al3+ + 3H2O2 (11)
Al + 3H2O2 → Al3+ + 3OH• + 3OH– (12)
Fu et al. (2016) and Chen et al. (2015) soaked Al powder in an acid solution with an initial pH value of ~ 2.0 for a time duration. This acid-Al system could efficiently transform high-toxic Cr(VI) ions into low-toxic Cr(III) ions through the reduction process or could mineralize the aqueous BPA in the participation of H2O2 through an advanced oxidation process.
Lin et al. (2017a) used hydrochloric acid (HCl) to wash zero-valent zinc (ZVZ); they found that the reduction ability of metal zinc in bromate removal was enhanced compared to that of untreated ZVZ. A higher ZVZ dosage and an elevated temperature had positive effects on the efficiency and kinetics of the ZVZ-bromate reduction reaction. Furthermore, pH value played a significant role in the ZVZ-bromate reduction process. Bromate removal efficiency and reaction kinetics were tremendously bolstered when the solution pH was 3 and were completely suppressed when the solution pH values were in the alkaline range. Lin et al. (2017b) also found that the addition of oxalic acid could further improve the bromate reduction efficiency by acid-washed aluminum. It is because oxalic acid could coordinate with the oxidized Al species (Al3+) and thus suppress the formation of the passivation layer of Al(OH)3.
Although acid or alkali washing is effective in activating the Al reduction ability, it poses a severe challenge to corrosion resistance of the reaction facilities, which impedes its large-scale applications in the field of clean energy generation or water treatment to some extent.
1.2. Alloying
Alloying is another widespread method for Al surface treatment. Metal Al can form alloys with other low-melting-point metals through high-temperature melting and grinding. The enhanced effect of Al alloying on activation of Al can be derived from the galvanic cell structure that effectively facilitates metal Al corrosion. Ziebarth et al. (2011) fabricated Al-Ga alloy through a high-temperature melting method and they proposed that liquid phase formation was essential for Al alloy-water reactions. Wang et al. (2021) melted the metals Ga, In, Sn, and Al, adopting the electricity arc method under an Ar atmosphere, achieving Ga-In-Sn coverage on the Al particle surface and obtaining a Ga-In-Sn-Al alloy. The alloying process not only could prevent the Al surface from passivation but could also promote the inner Al atoms’ outward diffusion and contact with water molecules (Amberchan et al., 2022; Gao et al., 2021). Chen et al. (2014) fabricated Al-Li alloy through the ball-milling process. On one hand, metal Li could increase the brittleness of the Al-Li alloy and make it prone to fracture during the reaction. On the other hand, the metal Li-water reaction is an exothermic process, which could further accelerate the breakage of the Al surface hydrated layer and the generation of hydrogen. Fu et al. (2015) and Cheng et al. (2016) deposited an Fe layer on the Al particle surface through the Fe2+ ion exchange process and obtained an Al/Fe bimetal alloy. This bimetal system could be used in the effective reduction of aqueous heavy metals such as arsenate, arsenite and Cr(VI) ions, achieving the purpose of purifying the water quality. Although the alloying process could efficiently promote metal Al corrosion, this process is usually involved in the use of noble metals, which indirectly increases the cost of the alloy materials. Apart from Fe, metal Zn also could form an alloyed system with metal Al. Fan et al. (2010) found that metal Zn could reduce the redox potential of the Al-In alloy and efficiently accelerate the diffusion of metal In around the Al particle surface. Wang et al. (2014) revealed that the enhanced corrosion ability of the Mg-Al-Fe alloy mainly derived from its galvanic cell structure.
The alloying process is an effective method in preventing the Al surface from secondary passivation and increasing Al reactivity. Nevertheless, other metals and high temperature are usually required in this process, which increases power consumption and the material-fabrication cost.
1.3. Mechanical Ball-Milling
Mechanical ball milling is a common method used for Al surface modification in hydrogen generation by the Al-water reaction. Besides Al, Al powder could also be ball-milled with carbon materials, metal oxides and soluble salts used as grinding agents. Ravavi-tuousi and Szpunar (2013) ball-milled Al powder with stearic acid as a control agent under an Ar atmosphere. The ball-milling process decreased the radius of Al particles and consequently increased the surface area of Al particles. The intrinsic Al surface oxide film was fractured during the ball-milling process, which led to the generation of lattice defects. The results showed that the ball-milling time duration is crucial to the activation effect of Al. An insufficient ball-milling process would result in large Al particle sizes and would not play a satisfactory role in increasing the Al particle surface area. However, overlong ball-milling time would conversely lead to the re-blocking of lattice defects and consequently decrease Al reduction ability.
Huang et al. (2012) fabricated an Al/C composite material using graphite as a ball-milling agent. They found that this composite material could continuously react with water to generate hydrogen at room temperature, which is attributed to the ball-milling process that could break the Al surface oxide layer and make Al particles uniformly covered by the carbon material (Xiao et al., 2021). The coverage of carbon material around the Al particle surface could prevent the Al surface from secondary passivation and have a negligible effect on the contact between the inner Al and water molecules. Furthermore, the hydrogen gas bubbles, and reaction heat released in the Al-water reaction would promote the breakage of the covered carbon material layer and ensure the direct contact of the inner Al with water molecules, as illustrated in Figure 2.
The underlying mechanism of Al activation by metal oxides used as ball-milling agents closely depends on the oxide types used. Wang et al. (2011) used nano metal oxides including TiO2, Co3O4, Cr2O3, Fe2O3, Mn2O3, NiO, CuO and ZnO as ball-milling agents. In comparison, TiO2, Co3O4 and Cr2O3 used as ball-milling agents had a greater effect on the Al-deionized water reaction for hydrogen generation at 25ºC. Furthermore, Wang et al. found that the TiO2 with average particles sizes of 14 nm were highly effective in facilitating the production of hydrogen from the reaction of Al with tap water, while other oxide nanocrystals were ineffective in promoting hydrogen generation in tap water. This is attributed to the fact that 14 nm TiO2 nanocrystals remain in intimate contact with the surface of Al particles. The anodic polarization at the contact point of the TiO2:Al2O3 layer rendered the defect species more mobile and increased reactivity.
Soluble salts, such as KCL and NaCl, could also be used as grinding agents for the treatment of the Al surface oxide layer. It is known that the crystalline salt grains have good rigidity and their sharp edges could cut and then be embedded into the Al surface oxide layer in the grinding process. Lots of micro water channels would form after the salts dissolved, which subsequently leads to the exposure of inner fresh Al. Nevertheless, some scholars attributed the promoted effect of crystalline salts on Al reactivity to that of the pit-corrosion effect by Cl– ions in preventing the Al surface from repassivation to a great extent (Chen et al., 2013; Liu et al., 2015). If other metals exist in this case, a galvanic cell system would form and Cl– ions would enhance the electric conductivity of the aqueous solution and the synergistic effect would further speed up the Al corrosion process (Fan et al., 2008).
The grinding process requires a large amount of electricity and the grinding efficiency is low. Additionally, facility abrasion is inevitable accompanying mechanical vibration and noise. These disadvantages are inconvenient for the application of the grinding process in Al surface treatment.
1.4. Surface-Modified Al Powder (SMAP)
In 2005, Deng et al. (2005a) first adopted the ceramic-fabrication technique, which involved the mixing of Al(OH)3 with Al powder, tableting, sintering, smashing and sieving, and finally obtained γ-Al2O3 modified Al powder. This modified Al powder could be used in hydrogen generation from Al-water reactions or be used in aqueous contaminant removal under mild conditions, which would provide a promising functional metal material for clean energy evolution and water treatment. In fact, the essential mechanism of Al surface modification is attributed to crystalline phase transformation (Deng et al., 2001a; Deng et al., 2001b; Deng et al., 2001c). After heat treatment for a time length, the intrinsic Al particle surface oxide or hydroxide layer would transform into γ-Al2O3 grains with fine-crystallite structures (as shown in Figure 3). The SMAP could continuously react with water and generate hydrogen at room temperature and under atmospheric pressure, demonstrating that SMAP is a kind of potential material for clean energy generation. The mechanism responsible for the promoted effect of γ-Al2O3 modification on Al-water reaction will be explained in detail in the following section.
2. Application of SMAP in Al-Water Reactions for Hydrogen Generation
SMAP was initially proposed in the study of Al-water reactions for hydrogen generation. Deng et al. (2005a) put forward a hypothesis that the enhancement effect of Al surface modification processes on Al-water reactions mainly derives from the generation of γ-Al2O3 with fine-crystallite structures, which provided a reasonable explanation for Al-water reactions or Al-contaminant reduction removal. According to the hypothesis, there is an intrinsic dense oxide film on the Al surface (Fu et al., 2024; Li et al., 2024; Yang et al., 2023; Wang et al., 2024) when Al powder is exposed to a humid environment and a hydration reaction occurs in this oxide film, which involves the breakage of the Al-O-Al bond and the generation of Al-OH pieces. Two Al-OH pieces would be generated accompanying the breakage of one Al-O-Al bond. As the hydration process proceeds, a layer of AlOOH or Al(OH)3 would consequently generate on the Al particle surface [as Eq. (13)]. When the hydrated oxide layer contacts the inner fresh Al, it will react with Al and generate molecules [as Eq. (14)] (Deng et al., 2007). Since H atoms have a limited solubility in Al particles, hydrogen molecules would immediately accumulate at the hydrated oxide layer-Al surface interface. The hydrated oxide layer will be broken when the hydrogen gas pressure exceeds the critical pressure that the hydrated layer can sustain (Kumar et al., 2023; Mutlu et al., 2024; Chen et al., 2024). From the above analysis, it can be deduced that an induction period is needed before the Al-water reaction for hydrogen generation, as shown in Figure 4.
Al2O3 + H2O → 2AlOOH (13)
6AlOOH + 2Al → 4Al2O3 + 3H2↑ (14)
After modification, the intrinsic dense oxide film on the Al surface will be transformed into a loose structure (as shown in Figure 3), and the maximum pressure that the surface oxide layer can sustain decreases. Simultaneously, as the critical hydrogen gas pressure is proportional to the tension of surface oxide film (Deng et al., 2007), it can be inferred that the SMAP surface oxide layer is more easily broken by the accumulated hydrogen gas bubbles. Then, continuous hydrogen generation at room temperature and atmospheric pressure can be achieved.
Surface modification technology attracted wide attention once it was proposed. This is mainly because the reaction of SMAP with water has no induction process, in contrast to commercial Al powder (Figure 5). SMAP is becoming a promising on-spot clean energy-generation material owing to its relatively low cost and its ability to continuously react with water at mild temperature and under atmospheric pressure.
The enhancement effect of γ-Al2O3 modification on the Al-water reaction closely relies on the percentage of γ-Al2O3 in Al/γ-Al2O3 mixed powder. As γ-Al2O3 content increased from 27% to 70%, Al-water reaction kinetics became remarkably more rapid, and the hydrogen yield increased by nearly six times in the same length of time. Besides, Al-water reaction kinetics are positively proportional to the reaction temperature (Deng et al., 2005b). Deng et al. (2010) and Liu et al. (2012) found that the enhancement effect of γ-Al2O3 modification has a positive correlation with the uniformity of γ-Al2O3 coverage on Al particles surface. Later, Gai et al. (2015) supplemented the mechanistic explanation for the enhancement effect of γ-Al2O3 modification on the Al-water reaction for hydrogen generation (Figure 6): the γ-Al2O3 grain acts like a “reactive sponge” that can store and release water molecules in a reactive way (Sohlberg et al., 1999; Yang et al., 2014). This means that water molecules could be dissociated into H+ and OH– ions when they contact γ-Al2O3 grains. When γ-Al2O3 grains contact Al particles in aqueous solution, the OH– ions dissociated from water molecules on γ-Al2O3 surfaces are easy to hydrate with the passive oxide film on the Al particle surface, which greatly accelerates the hydration process in the Al surface oxide layer. Consequently, the induction time length at the beginning of the Al-water reaction would be greatly shortened. Table 2 lists several different Al surface process methods.
Apart from the γ-Al2O3 modification, Gai et al. (2014) found that direct addition of γ-Al2O3, Al(OH)3, α-Al2O3 or TiO2 powder could also remarkably shorten the induction period time of the Al-water reaction (Figure 7), and γ-Al2O3 exhibited the most remarkable improvement. This study discovered that the phase constituents of these oxides remain nearly unchanged compared to their pristine ones, indicating that these oxides or hydroxides just acted as reaction catalysts that effectively promoted the hydration process in the Al surface oxide film. This is probably related to the structural characteristics of these oxides or hydroxides.
Compared to the common Al surface treatment method, the Al surface modification process possesses advantages such as mild fabrication conditions, no corrosion to reaction facilities, and a lower cost. However, the slow reaction kinetics limit its large-scale application to a certain degree; this could be overcome through lowering the initial reaction gas pressure and elevating the reaction temperature.
3. Application of SMAP in Water Treatment
Apart from the Al-water reaction for hydrogen generation, another successful application of SMAP is in the field of water treatment. SMAP presents excellent effects in reduction removal of aqueous contaminants such as heavy-metal ions, inorganic salts and organic dyes. Although the destination contaminants are different, their removal mechanisms by SMAP share some similarities, which are all fundamentally based on the mechanism of electron transfer, as illustrated in Figure 8.
3.1. SMAP Used for Cr(VI) Removal
Chromium (Cr)—a byproduct of industrial processes such as powder metallurgy, product manufacturing, metal surface treatment, forging and leather tanning—is one of the most concerning heavy metals in wastewater due to its high toxicity, carcinogenicity and mutagenicity. The United States Environmental Protection Agency lists chromium as one of the most poisonous of seventeen elements and set 0.1 mg L-1 of chromium as the upper limit in drinking water (Zhang et al., 2018). Chromium has diverse oxidation states, among which Cr(III) and Cr(VI) states are the most widespread and stable in the natural environment. Because Cr(VI) has a much higher toxicity and solubility than Cr(III), a common and effective way to eliminate the toxicity of chromium-containing water is the reduction from highly soluble Cr(VI) ions into Cr(III) ions with low solubility. Up to now, various methods used for reductive removal of aqueous Cr(VI) ions have been exploited, as shown in Table 3.
Yang et al. (2020) developed a simple Al surface modification method, where commercial Al powder was soaked in water for a length of time and then the Al containing suspension was centrifuged to obtain wet Al mud. Subsequently, the wet Al mud was dried at 60º and then heat-treated at 400º for a length of time to obtain the final SMAP. Compared to the common Al surface modification methods, including tableting and sintering processes (Deng et al., 2001a), Yang’s manufacturing process has a more effective enhancement on Al reactivity. That is because the Al particle surface hydrated oxide layer could be transformed into fine γ-Al2O3 grains in the heat-treatment process and these fine γ-Al2O3 grains cover the Al particle surface more uniformly and result in a more excellent Al surface modification effect.
Yang et al. (2020) further used the above SMAP in the reduction removal of Cr(VI) ions in aqueous solution, which is the first study that reported the application of SMAP in the field of water treatment. The results confirmed that the surface treatment methods through soaking and then heat treatment processes could effectively improve Al reactivity. The obtained SMAP powder exhibited high reduction ability for transformation from high-toxic Cr(VI) ions into low-toxic Cr(III) ions [as Eq. (15) and Figure 9(a)], which provided a promising water-purification material fabrication technology. Besides, the study demonstrated that as the pre-soaked time length was prolonged, the Cr(VI) removal kinetics by SMAP would be increased. This is because increasing the soaking time length would lead to the generation of a thicker Al surface hydrated layer that would be transformed into finer γ-Al2O3 grains and a uniformly surrounded Al particle surface, as shown in Figure 8. Moreover, this SMAP exhibited excellent reusability in aqueous Cr(VI) removal, since ~ 80% of aqueous Cr(VI) ions could be removed even after SMAP were reused for five cycles [Figure 9(b)]. Interestingly, the recycled SMAP exhibited a higher reactivity in the reduction from Cr(VI) ions into Cr(III) ions, as compared to the unused ones. The higher reactivity of recycled SMAP in Cr(VI) removal can be attributed to the byproducts AlOOH or Al(OH)3 generated in the Al-water reaction. The byproducts AlOOH or Al(OH)3 covered the surface of the recycled Al particles, which would further promote the hydration process in the Al surface oxide layer and greatly decrease the induction time in the Cr(VI) reduction reaction using SMAP.
Al + CrO42- + 4H2O → Al(OH)3↓+ Cr(OH)3↓+ 2OH– (15)
3.2. SMAP Used for Removal of Methyl Orange/Methyl Blue
Large amounts of organic dyes are consumed every year in textile, printing, and some other industries (Ahmad et al., 2019; Ahmad et al., 2020). Some of them enter the natural water system, becoming one of the sources of water contamination (Hao et al., 2012; Shabbir et al., 2017).
Xie et al. (2021) adopted Yang’s manufacturing processes in section 3.1 to obtain the SMAP and then studied the effect of SMAP used in the removal of aqueous methyl orange and methyl blue [Figures 10(a) and 10(b)]. It was found that the surface modification process sharply reduced the organic dye removal time length to just one third of that using the untreated Al powder. It was also found that the dye removal performance using SMAP was superior to that of the ultrasound splitting effect. Meanwhile, the SMAP demonstrated an excellent reusability in the removal of aqueous organic dyes, where more than 80% of methyl orange or methyl blue could be removed efficiently even after SMAP was used for three cycles [Figure 10(c)]. The removal of methyl orange by SMAP can be attributed to the released electrons from Al attacking -N=N- bonds in methyl orange molecules [Eqs. (16)-(19)] (Shabbir et al., 2017), which then directly decomposed into small organic fragments, achieving the purpose of decolorization or lowering wastewater toxicity. In contrast, methyl blue removal using SMAP proceeds based on the synergistic effects of the reduction mechanism and Al surface adsorption: Initially, the methyl blue molecules gained the electrons released from Al and decomposed into small organic fragments; then these fragments would be adsorbed by the surface of the Al particles. Table 4 compares the methyl orange or methyl blue removal performances using various materials.
Al → Al3+ + 3e– (16)
2H2O + 2e– → H2↑ + 2OH– (17)
Ar1-N=N-Ar2 + e– + H2 → Ar’1-NH=NH-Ar’2 (18)
Ar’1-NH-NH-Ar’2 → Ar’1-NH2 + HN2-Ar’2 (19)
3.3. SMAP Used for Bromate Ion Removal
Bromate belongs to Group 2B carcinogens to humans according to the classification of the International Agency for Research on Cancer (IARC) because even a low dosage of bromate can lead to oxidative deoxyribonucleic acid (DNA) damage (WHO, 2011). WHO has set 0.01 mg L-1 of bromate as the upper limit in drinking water (WHO, 2011).
Zhou et al. (2021) adopted four methods—soaking, soaking and then cold drying, soaking and then heat treatment, and γ-Al2O3 modification—to treat the Al particle surface oxide layer. The structure of the Al surface oxide layer became loose after using different surface treatment processes, in contrast to that of the raw Al powder (Figure 11). The researchers found that the surface treatment processes remarkably increased the bromate removal efficiency by Al under neutral aqueous conditions. The bromate ion removal kinetics by Al with different surface treatment processes ranked as γ-Al2O3 modification > soaking > soaking and then cold drying > soaking and then heat treatment > raw Al powder [Figure 12(a)]. Like the Al-water reaction for hydrogen generation, the enhanced effect of surface modification on bromate ion removal by Al powder is attributed to the fact that the surface treatment would increase the hydration process in the Al surface oxide layer and consequently increase the breakage of the Al surface hydrated oxide layer. This leads to quick contact between the aqueous bromate ions and the inner fresh Al. Consequently, the bromate removal kinetics by Al would be remarkably increased, as illustrated in Figure 8. Interestingly, the Al encountered different surface treatment processes that exhibited good reusability in bromate removal. Thus, 90% of aqueous bromate could be removed even when the SMAP was reused for four recycles [Figure 12(b)]. Table 5 summarizes the aqueous bromate ion removal performances using different metals.
(Zhou et al., 2021)
4. Conclusion and Prospects
Surface modification processes could promote the hydration process in the Al surface oxide layer and increase the breakage of the surface hydrated oxide layer, which is a promising technology used for recovering Al reactivity. In contrast to the usual Al surface treatment methods, such as acid or alkali corrosion, alloying and ball milling, the surface modification method has unique advantages: a simple fabrication process, no corrosion to reaction facilities and low cost. These merits endow SMAP with a strong application potential in the fields of clean energy generation and water treatment. The mechanisms of Al-water reactions for hydrogen generation and Al-pollutant reactions for water treatment are both essentially based on the reduction effect because Al is naturally an excellent electron donor.
Although Al surface modification has been widely studied in the fields of Al-water reaction for hydrogen generation and water treatment, several emergent problems for SMAP in the practical applications still exist:
(1) The SMAP-water reaction has slow kinetics under ordinary conditions. The question of how to further elevate Al reactivity under mild reaction conditions through improving the SMAP materials manufacturing process has guiding significance for Al practical application in the Al-water reaction for clean energy generation and water treatment;
(2) Al3+ leakage into the aqueous solution accompanying the Al-pollutant reaction process would possibly lead to secondary pollution to water bodies. The Al3+ content is a crucial standard of drinking water, with the allowed upper limit of 0.2 mg L-1 (Wu et al., 2020; US EPA, 2014). Ways to develop a method for aqueous Al3+ removal by physical adsorption using activated charcoal, zeolite, bacteria, fungi and algae, etc. should be investigated in detail in future studies;
(3) Present studies about SMAP application in water treatment are mainly based on direct electrons attaching to the chemical bonds, which is hard for mineralization of the organic contaminants. The advanced oxidation processes based on SMAP for Al application in water treatment are worth investigating because the free radicals with high oxidation ability generated in advanced oxidation processes could sufficiently mineralize the aqueous organic contaminants into CO2 and H2O;
(4) Reactivity deterioration of SMAP in the Al-water reaction for hydrogen generation or water treatment is an obstacle restricting Al reusability. As the reaction proceeds, the Al surface hydrated oxide layer would become thicker and thicker. So, studies are needed to explore a surface treatment process that can make the Al surface oxide layer more prone to be broken and prevent the Al surface from secondary passivation, both of which are helpful for enhancing Al reusability in practical applications.
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