Metal Foam-Based Fenton-Like Process by Aeration.

A novel metal foam-based Fenton-like process for wastewater treatment is illustrated in this study. In the system, H2O2 was generated in situ by taking advantage of O2 in air, as metal could activate dissolved O2 to produce •O2- and then generate H2O2. Furthermore, metal foam can enhance the Fe3+/Fe2+ cycling, which eventually improved the efficiency of the Fenton process. The performance of the novel Fenton-like process was assessed by methyl blue (MB), and 94% MB removal could be achieved within 5 min in nickel (Ni) foam system. The degradation of MB in this study was based on both •OH and •O2- radicals, where •O2- radical served as the precursor to generate •OH for MB degradation through a Fenton process. The pH value of 3 with the initial Fe2+ concentration of 0.25 mM was found to be the optimum condition for the Fenton-like process. This study provides a general and new strategy for efficient wastewater treatment just using aeration and metal foams (such as Ni, Al, and Cu foams), which also offers a good alternative for rational design and application of traditional Fenton process.

Introduction

Advanced oxidation processes (AOPs), which can generate powerful chemical oxidants, have been widely used to degrade recalcitrant pollutants.[1,2] Fenton process is one of the most efficient AOPs that has been widely used in wastewater treatment because it can produce a nonselective strong oxidant, hydroxyl radical (•OH),[3,4] to oxidize contaminants. Most of the organic pollutants can be easily oxidized by •OH to be smaller organics or entirely mineralized to be carbon dioxide and water.[5] However, in traditional Fenton system, the storage and transportation of H2O2 combined with a large amount of iron sludge[6] as the byproduct may restrict its wider application.[7] To prevent the accumulation and precipitation of soluble iron, heterogeneous catalysts with low Fe dissolution, such as iron oxides,[8,9] and iron-immobilized clays[10] are widely used in heterogeneous Fenton systems to replace the homogeneous Fenton system.

In addition, electro-Fenton (E-Fenton) process has also drawn much attention due to its continuous H2O2 generation on anodal material and Fe2+ regeneration by direct cathodic reaction.[11] The E-Fenton system can continuously produce H2O2 through the oxygen reduction reaction (ORR);[12] subsequently, the generated H2O2 can react with dissolved Fe2+ to produce highly reactive •OH. Therefore, the key issues in Fenton system lie in the production of H2O2 and the reduction of Fe3+. In E-Fenton system, the efficient cathodic material should possess high electrical conductivity, large surface area with good adsorption ability, and excellent chemical stability to generate a large amount of H2O2.[13] Various electrocatalysts, such as carbon nanotubes,[14,15] graphene,[16,17] transition-metal oxides,[7,18] have been investigated to promote the ORR activity and current efficiency for H2O2 generation.[19]

To date, numerous efforts have been devoted to improving the traditional Fenton process. However, the addition of H2O2 is an inevitable part for homogeneous and heterogeneous Fenton systems, which complicates the process. Although E-Fenton can generate H2O2 in situ, the supply of current or voltage is indispensable for all E-Fenton processes. Therefore, a more convenient and efficient Fenton system is worth investigating.

In this study, a novel Fenton-like system is developed by taking advantage of O2 and metal foam under acidic condition with Fe2+ as the catalyst. This process is performed under the condition of aeration, during which metal foams could activate O2 to produce •O2–, which could then react with H+ to produce H2O2. Subsequently, a Fenton process involving Fe2+ and H2O2 can take place to produce highly reactive •OH. With the assistance of metal foams, such as Ni foam, Fe3+ can be reduced into Fe2+ to react with H2O2 again. Therefore, this cycling can effectively and continuously promote the production of •OH for pollutant removal. Methyl blue (MB) was selected as a model pollutant to evaluate the efficiency of this system with different metal foams. Four kinds of metal foams (nickel, aluminum, copper, and titanium) were studied to illustrate the reaction pathway. The results revealed that Ni foam had the best efficiency, Al and Cu foams came next, and Ti foam had little contribution to MB degradation due to its inertness to H2O2 production and Fe3+ reduction. The effects of various parameters, such as pH value and the concentration of Fe2+, were also studied and discussed. The whole process was supplied with neither current nor voltage, and the addition of H2O2 was also unnecessary in this effective system.

Results and Discussion

Ni-Foam-Based Fenton-Like System

Figure shows the degradation of MB under various reaction scenarios. The relationship among Ni, air, and Fe2+ on the degradation of MB was systematically studied. The presence of single component, such as Ni, air, or Fe2+, had little effects on MB degradation. In the presence of Ni foam, the addition of Fe2+ into the solution resulted in the efficient degradation of MB. However, the combination of Ni/air or Fe2+/air had little effect on MB degradation. Furthermore, in the presence of both Ni foam and Fe2+, the removal efficiency of MB increased dramatically as a consequence of air bubbling.

Figure 1

(a) Degradation of MB under various reaction scenarios. (b) Degradation of MB by supplying different gases (reaction conditions: initial pH = 3, [Fe2+] = 0.5 mM, [MB] = 20 mg L–1).

According to the phenomenon mentioned above, we can conclude that both Ni foam and Fe2+ were essential factors for MB degradation, and bubbling air would promote the removal efficiency of MB. To elucidate the role of air on MB degradation process, experiments were conducted by supplying different gases (air, oxygen, or nitrogen) to saturate MB solution. As shown in Figure b, faster removal of MB was observed with higher oxygen concentration (bubbling oxygen), whereas in the absence of oxygen (bubbling nitrogen), the removal of MB was strongly inhibited. Therefore, dissolved O2 was also essential for the degradation process.

It is likely that the above phenomenon could be attributed to the following Fenton-like reaction pathway. First, nickel foam played an important role to react with dissolved O2 to produce •O2–[20−23] through eq and then •O2– can react with H+ to produce H2O2[24,25] through eq . Then, Fe2+ acted as the catalyst and initiated the decomposition of H2O2 to generate the highly reactive •OH to degrade MB, which can be described as follows[26−29]In the meantime, on the basis of the standard electrode potentials of Ni2+/Ni and Fe3+/Fe2+ pairs (eqs and 5), Ni foam could promote the cycling of Fe3+/Fe2+ through eq . As a result, Fe2+ in this system can react with H2O2 continuously to generate •OH for MB degradation.On the basis of the above reaction pathway and the phenomenon observed in Figure b, it is clear that pure oxygen can accelerate the generation of •OH to attack MB. Nevertheless, employing air was effective and more economical than employing pure O2. Therefore, air bubbling was employed throughout the rest of the study.

To prove the reaction pathway proposed above, it is instrumental to measure the concentrations of H2O2 and Ni2+ in the process. Figure a shows the accumulation of H2O2 in the presence of Ni foam with continuous air bubbling. It is clear that H2O2 was generated likely through eqs and 2. In the absence of MB, the concentration of H2O2 increased dramatically to about 7.5 mg L–1 in 30 min and then reached a plateau. Ni2+ and Fe2+ were also measured during the MB degradation process. With the increase of reaction time, the concentration of Ni2+ had a similar tendency to H2O2, which was in accordance with eqs and 2. The Fe2+ concentration remained unchanged during MB degradation (Figure c), which suggests the efficient Fe3+/Fe2+ cycling promoted by Ni foam. This is a robust evidence to support the presence of eq in this system. According to eq , Ni foam could reduce Fe3+ to Fe2+. Therefore, it is expected that the introduction of Fe3+ is also beneficial for MB degradation. To investigate the effect of Fe3+, 0.25 mM Fe3+ was added into the system to degrade MB at pH 3. Under the same conditions, although Fe3+ could contribute to MB degradation, its efficiency was much lower than that of Fe2+. Figure d reveals that 56% of MB was degraded in 5 min with the addition of Fe3+, whereas Fe2+ could facilitate 94% degradation of MB. The difference between Fe2+ and Fe3+ was due to the fact that Fe2+ could be oxidized by H2O2 immediately to produce •OH for MB degradation. However, Fe3+ had to be reduced by Ni foam to generate Fe2+ first and then •OH can be produced via a Fenton reaction. Therefore, Fe2+ was a superior catalyst to Fe3+ in the metal foam-based Fenton-like system.

Figure 2

(a) Production of H2O2 in Ni foam system with air sparging at pH = 3. (b) Evolution of Ni2+ during MB degradation by Ni foam/air/Fe2+. (c) Change of Fe2+ during MB degradation. (d) Comparison of Fe2+ and Fe3+ on MB degradation by the Ni foam-based Fenton-like system (reaction conditions for (b) and (c): initial pH = 3, [Fe2+] = 0.25 mM, [MB] = 20 mg L–1. For (c), [Fe2+] = [Fe3+] = 0.25 mM).

Scanning electron microscopy (SEM) images show that Ni foam (Figure a) maintained its three-dimensional porous structure after the reaction (Figure S1), which is beneficial for the mass transfer of air.[22] However, its surface morphology changed dramatically as a result of chemical reaction. As demonstrated in Figure b,c, the original Ni foam featured smooth surface in both microscale and nanoscale. On the contrary, there was distinct corrosion on the surface of the used Ni foam (Figure d). The size of the defects on the surface of the Ni foam ranges from nanometers to micrometers (Figure e,f) as a result of the chemical reaction depicted in eqs –6. The defects on the surface of the Ni foam also indicate that nickel was leached during the reaction, which was in agreement with the increasing Ni2+ concentration during the reaction (Figure b).

Figure 3

SEM images of Ni foam before (a–c) and after (d–f) the reaction.

Quenching of •OH and •O2– in the Fenton-Like Process

To have a better understanding of the reaction pathway during the Fenton-like process, 2-propanol was used as an efficient •OH scavenger,[28,30] and p-benzoquinone as an •O2– scavenger.[31,32] As demonstrated in Figure , the addition of both p-benzoquinone and 2-propanol could inhibit the degradation of MB dramatically. When 1 mM p-benzoquinone or 2-propanol was introduced, there was no obvious change of MB concentration in the Ni foam/air/Fe2+ system for 60 min, indicating the complete inhibition of the degradation process. This phenomenon suggests that the degradation of MB in this system was based on both •OH and •O2– radicals. Here, •O2– radical likely served as the precursor to generate •OH for MB degradation rather than as a direct oxidant, as shown in eqs and 3. This is demonstrated by Figure a that the combination of Ni and air had little effect on MB removal. In such case, •O2– and H2O2 were produced; however, they did not have the capability to degrade MB. The introduction of Fe2+ to initiate Fenton reaction was crucial for the fast MB degradation. It is also evident that the quenching of •OH completely inhibited the degradation of MB (Figure b). We therefore conclude that •OH was the main radical that degraded MB in this system.

Figure 4

(a) Effect of p-benzoquinone addition on the degradation of MB. (b) Effect of 2-propanol addition on the degradation of MB (reaction conditions: initial pH = 3, [Fe2+] = 0.25 mM, [MB] = 20 mg L–1).

Effects of Fe2+ Concentration and Initial pH on MB Degradation

Solution pH and concentration of Fe2+ are known factors affecting the efficiency of Fenton process.[33]Figure a reveals that initial pH value had a great impact on the degradation of MB. The system reached the maximal MB removal efficiency at pH 3. With the increase of pH, the MB removal decreased, which was likely caused by the formation of Fe2+-based hydroxyl complexes under high pH value. Figure a also reveals that when the pH value lowered to less than 3 the MB degradation efficiency decreased. This can be attributed to the scavenging effect of •OH by H+.[34] The MB removal underwent a gradual acceleration when Fe2+ concentration increased from 0.025 to 0.25 mM (Figure b). The increased degradation of MB was ascribed to the enhanced H2O2 decomposition associated with the increase of Fe2+. However, when Fe2+ concentration continued to rise, the MB removal decreased. This is because excessive amount of Fe2+ would consume •OH,[33,35,36] which resulted in the formation of Fe3+ and OH– and consequently iron hydroxides, thus terminating the Fenton reaction (eq ). Therefore, the optimal conditions for this Fenton-like process are initial pH = 3 and Fe2+ concentration = 0.25 mM.

Figure 5

(a) Effect of initial pH on the degradation of MB in Ni foam system ([Fe2+] = 0.5 mM, [MB] = 20 mg L–1). (b) Effect of initial Fe2+ concentration on the degradation of MB in Ni foam system (initial pH = 3, [MB] = 20 mg L–1).

Comparison of Ni Foam and Ni Plate

To illustrate the role of Ni foam on MB degradation, Ni plate was also employed as another Ni source for comparison. Ni foam resulted in much faster MB degradation than Ni plate (Figure a). In Ni foam system, the MB removal reached 94% in 5 min; however, only less than 50% MB removal was achieved in the Ni plate system. The superior efficiency of Ni foam likely originated from its macroporous structure possessing more specific surface area, which can provide more reactive sites for H2O2 production. Therefore, the employment of Ni foam is another key factor to obtain high reaction efficiency. Figure b shows the degradation of three other pollutions based on our proposed Fenton-like process. It is obvious that the proposed Fenton-like process is suitable for other pollutants (such as orange II, methyl orange, and phenol). The results indicate that this process may be a general approach for wastewater treatment.

Figure 6

(a) Comparison of Ni plate and Ni foam on MB degradation. (b) Degradation of three other pollutants (orange II, methyl orange, and phenol) based on the Fenton-like process (initial pH = 3, [Fe2+] = 0.25 mM, [MB] = 20 mg L–1).

Degradation of MB Based on Other Metal Foams

In the Fenton-like system discussed above, Ni foam played an important role in degrading MB by facilitating effective •O2– production as well as Fe3+/Fe2+ cycling. Some other metals, such as Al and Cu, which have similar properties as Ni, may also have the ability to activate O2 and circulate Fe3+/Fe2+. We propose that the Ni-foam-based Fenton-like process is a general approach to produce •OH for pollutant degradation, which could be applied to other metal foams. Figure a shows the removal of MB in Fenton-like systems based on different metal foams. Three typical metals (Ni, Al, and Cu) that could reduce Fe3+ were chosen; another metal (Ti) that does not have the ability to reduce Fe3+ was also selected for comparison. It is obvious that Ni foam had the best efficiency to degrade MB. The MB removal reached 94% in 30 min. MB removal by Al foam reached 88% in 60 min, whereas Cu foam could remove 50% of MB. However, almost no MB removal was observed for Ti foam. This phenomenon confirms that metals that have the ability to reduce Fe3+ to Fe2+ can be employed in this Fenton-like system, whereas other inactive metals (such as Ti) are inefficient in this system.

Figure 7

(a) Degradation of MB based on various metal foams (initial pH = 3, [Fe2+] = 0.25 mM, [MB] = 20 mg L–1). (b) H2O2 generated by different metal foams with air sparging (initial pH = 3).

Furthermore, the differences in MB removal for the above metals were in good accordance with their H2O2 production capability. As demonstrated in Figure b, Ni foam exhibited the best H2O2 accumulation. For Al foam and Cu foam, H2O2 concentration could achieve 3.8 and 1.8 mg L–1, respectively. As a result, their corresponding MB degradation efficiency fell far behind Ni foam. As a comparison, no H2O2 was detected in Ti foam system due to its inertness to oxygen.

The performance of different metal foams with same surface area of 0.5 m2 is demonstrated in Figure S2. It is revealed that Ni foam still demonstrates the best MB removal efficiency, Cu and Al foams come next, and Ti foam has little efficiency. Furthermore, the differences in MB removal for the above metals (based on surface areas) were also in good accordance with their H2O2 production capability. It is found that Ni foam has the best H2O2 production efficiency (18.5 mg L–1 in 1 h). For Cu foam and Al foam, H2O2 concentration could achieve 3.8 and 0.93 mg L–1 in 1 h, respectively. As a result, their corresponding MB degradation efficiency fell far behind Ni foam. On the contrary, no H2O2 was detected in the Ti foam system due to its inertness to oxygen. This phenomenon is in agreement with previous comparison based on the mass.

The surface morphologies of the three metals before and after reaction vividly reveal the occurrence of the Fenton-like reaction. As demonstrated in Figure a,c, the surfaces of the original Al and Cu foam are smooth without obvious defects. However, after the Fenton-like process, their surfaces became fairly rough with numerous nanosized defects. The dramatic changes in the surface morphology were caused by the following two reasons. First and foremost, metal foams were corroded by oxygen, which produced •O2– and then H2O2. Second, metal foams reacted with Fe3+ to produce Fe2+, which activated H2O2 to produce highly oxidative •OH. On the contrary, the surface of Ti foam remained smooth and untouched after the reaction, as Ti was unreactive to either •O2– production or Fe3+ reduction.

Figure 8

Surface SEM images of Al (a, b), Cu (c, d), and Ti (e, f) foams before (left) and after (right) the reaction.

In summary, the reaction pathways for the proposed Fenton-like process are depicted in Scheme . Initially, metal foam (denoted M in Scheme ) can activate the dissolved O2 in water to produce •O2–, which then reacts with H+ in the solution to produce H2O2 in situ. The added Fe2+ reacts with H2O2 to generate •OH for MB degradation. In the meantime, metal foam plays another role in reducing Fe3+ to Fe2+. As a result of Fe3+/Fe2+ cycling, •OH will be continuously produced by the above Fenton-like process for pollutant degradation.

Scheme 1

Reaction Pathway for the Metal Foam-Based Fenton-Like Reaction

Conclusions

In this study, a novel Fenton-like system was developed by introducing aeration and metal foam under acidic condition. Metal (Ni, Al, and Cu) foam plays dual roles in this process that it can not only activate dissolved O2 to produce H2O2 but also facilitate Fe3+/Fe2+ cycling for continuous Fenton reaction. The production of both •O2– and •OH in this process was demonstrated, whereas •O2– was the precursor for H2O2 production and •OH was responsible for MB removal. Nickel foam had the best activity for H2O2 generation and MB removal, whereas Ti had little contribution to the reaction due to its inertness for O2 activation and Fe3+ reduction. The optimum conditions for this Fenton-like process were determined to be pH value = 3 and Fe2+ concentration = 0.25 mM, under which 94% of MB can be removed in 5 min by employing Ni foam. The novel Fenton-like system in this study would have good potential in wastewater treatment due to its high efficiency and low cost.

Experimental Section

Materials

All chemicals used in this study were of analytical grade. Hydrogen peroxide, FeSO4·7H2O, Fe(NO3)3·9H2O, isopropanol, and p-benzoquinone were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Potassium titanyl oxalate and methyl blue were purchased from Mclean. Nickel, aluminum, copper, and titanium foams were obtained from Suzhou Jiashide Metal Foam Co., Ltd., China. All of the chemicals were used without further purification. Deionized water was used throughout the experiments. Prior to the degradation experiment, metal foams were washed by 0.1 M HCl and deionized water to remove the oxides and grease on the surface.

MB Degradation and H2O2 Accumulation

MB solution (500 mg L–1) was kept as a stock solution. MB solution used in the degradation reaction was obtained by diluting from the stock solution. The degradation of MB was conducted in a 250 mL beaker containing metal foam (2 cm × 2 cm × 2 mm) and 200 mL of MB solution with an initial concentration of 20 mg L–1. Prior to the Fenton-like reaction, air was bubbled into the reactor for 30 min and then Fe2+ was added. H2O2 was generated in a system containing metal foam and deionized water but without MB during bubbling air. The initial pH was adjusted to the designated value using 0.1 M HCl or 0.1 M NaOH.

Characterization

The MB and H2O2 concentrations were measured by a UV–vis spectrophotometer (TU1810, Universal Analysis, Beijing, China). H2O2 was determined using the potassium titanium oxalate method at its maximum absorption wavelength of 400 nm. The MB concentration was measured at its maximum absorption wavelength of 664 nm. The standard curves of H2O2 and MB are shown in the Supporting Information (Figures S3 and S4). The Ni2+ concentration was detected by an atomic absorption spectrophotometer[37] (TAS-990F, Universal Analysis, Beijing, China). The specific surface area (Brunauer–Emmett–Teller) was measured by nitrogen adsorption/desorption (Micromeritics ASAP2020 instrument). SEM measurement was performed on a Zeiss Supra 55 scanning electron microscope.