Synthesis of zeolite-based Cu/Fe-X from coal gangue for Fenton-like catalytic degradation of Rhodamine B

In this paper, coal gangue, a solid waste is used as raw material to provide silicon and aluminum sources for synthesis of zeolite. A coal gangue zeolite based Cu/Fe-X catalyst is successfully prepared by immersion-calcination method, which is used to remove Rhodamine B from aqueous solution. The characterization results show that Cu and Fe have been successfully loaded on zeolite, not damaged its structure, and these nanoparticles are highly dispersed and low crystallinity. This special structure will enhance its catalytic ability to activate H 2 O 2 . The experiment showed that hydroxyl radical (•OH) was the main active species for catalytic degradation of Rhodamine B, and the circulation of Cu 2+ to Cu 1+ and Fe 3+ to Fe 2+ could synergistically produce •OH. Furthermore, the redox potential of Cu 1+ and Cu 2+ is lower than Fe 2+ and Fe 3+ , which can promote Fe 3+ circulate to Fe 2+ , further promote Fe 2+ to activate H 2 O 2 to decompose into •OH, and overcome the speed limiting step of Fenton-Like catalytic reaction. The Cu/Fe-X catalyst can activate H 2 O 2 in wide pH range (1–10). Under optimal conditions the catalytic degradation rate can reach 99.9%, and the TOC removal rate is as high as 98.5%. Therefore, Cu/Fe-X/H 2 O 2 system can effectively remove organic dyes, and has a high industrial application prospect.


Introduction
Inner Mongolia is rich in coal resources, which will produce a large number of coal gangue in the process of coal mining and washing.A large number of coal gangue piles not only occupy a large amount of land in China [1] , but also cause air, soil and water pollution, and also cause greater economic burden to enterprises.Therefore, the treatment and comprehensive utilization of coal gangue have attracted much attention.
Zeolite molecular sieve is a kind of porous aluminum silicate crystal, which has good adsorption performance and ion exchange capacity.It has been widely used in wastewater treatment, chemical separation catalytic reaction, material industry and other elds.The synthesis of zeolite molecular sieve from coal gangue is a way to e ciently utilize the effective components in coal gangue.Generally speaking, the main chemical components of coal gangue are SiO 2 and Al 2 O 3 [2] , with a total amount of more than 60%, which can provide the required silicon and aluminum sources for the synthesis of zeolite molecular sieves.It is the theoretical basis for the synthesis of zeolite molecular sieves using coal gangue as raw materials [3] .
With the development of industry, the use of organic dyes has caused a series of environmental problems.It is reported that the global annual production exceeds 7×10 5 tons of colored dyes, with more than 1000 kinds [4] .In fact, the reason why organic dye compounds are particularly stable and insoluble is that they have special aromatic structures and are not easy to be degraded by biophysical methods.Therefore, it is urgent to develop an appropriate and effective method to treat dye wastewater.Now, catalytic degradation has been widely used in wastewater treatment.
Fenton process is one of the advanced oxidation technologies (AOPs) used to degrade organic pollutants.However, many defects of classical Fenton system limit its application.Therefore, nonhomogeneous Fenton catalysis has been widely used.Its main goal is to develop effective and stable solid catalysts.Zeolite molecular sieve is a good choice for catalyst carrier because of its low cost and environmental protection.In addition, zeolite has the advantages of large speci c surface area and rich pore structure of non-homogeneous Fenton catalyst.
Iron based Fenton process has been widely used in wastewater treatment [5][6][7] , due to its advantages of high oxidation e ciency, wide pH range, low negative impact, etc. [8][9][10] .However, the catalytic activity still needs to be further strengthened to obtain better oxidation e ciency.In the iron based Fenton-Like process, the generation of hydroxyl radicals requires the reaction of Fe (II) and H 2 O 2 , and the recycling of Fe (II) is generally the rate control step of the fenton reaction [11,12] .In recent years, people have developed a variety of methods, such as photo/electrochemical assistance, catalyst modi cation, addition of reducing agent, to improve the rate of rate limiting step in Fenton-Like reaction [13][14][15][16][17] .Adding another metal to heterogeneous catalysts (such as copper, cobalt or nickel) is a method with low negative effects and high e ciency [18][19][20] .Among these metals, copper is most commonly used to synthesize iron based bimetallic catalysts due to its low cost, low toxicity and high e ciency.In addition, copper can also catalyze H 2 O 2 to generate hydroxyl radicals [21,22] .More importantly, Fe (III) reduced by Cu (I) can promote the regeneration of Fe (II) in iron-copper system [23] .Therefore, in this paper, X-type zeolite prepared from coal gangue, which is used as the carrier for loading copper and iron, then the copper-iron bimetallic reaction active centers non-homogeneous Fenton catalyst is prepared, and used it to degrade dye wastewater.The implementation of this project can provide a theoretical basis for the treatment of dye wastewater, and also provide a solution for the resource utilization of coal gangue.
Experimental Section In this experiment, we will optimize the conditions of loading amount of copper and iron, dosage, dye concentration, pH value of solution, reaction temperature, H 2 O 2 dosage.Determine the optimal catalytic conditions and explore its catalytic mechanism.

Characterizations
UItima Results And Discussion

X-ray diffraction analysis
XRD was used to characterize the crystal structures of the catalysts.As shown in Figure .1, the obvious characteristic diffraction peak of X-type zeolite [24] is shown, and the XRD analysis of X-type zeolite is at 2θ = 6.38°, 15.76°, 23.64°, 26.99° and 31.30°respectively correspond to planes (101), (302), ( 404), ( 112) and (406) [25] , indicating that the high crystallinity and structural form of X-type zeolite are good.The XRD patterns of Cu/Fe-X is basically consistent with X-type zeolite, indicating that Cu and Fe metal doping has little effect on the structure of X-type zeolite.The observation shows that the diffraction peak of X-type zeolite slightly decreases with the increase of copper and iron content, but no new diffraction peak is found and there is no obvious shift, indicating that the Cu/Fe-X catalyst structure is well maintained.The surface properties of X-type zeolite and Cu/Fe-0.5 catalyst were measured by N 2 adsorptiondesorption technique.The results are shown in Figure .2.The isotherm of X-type zeolite and Cu/Fe-0.5 catalyst is type IV, with H4 hysteresis loop [26] .The pore size distribution of BJH reveals the mesoporous structural characteristics of X-type zeolite and Cu/Fe-0.5 catalyst [27] , and the most probable pore size is about 3.6 nm.Table 2 shows that speci c surface area and pore volume of X-type zeolite are 511.2m 2 /g and 0.31 cm 3 /g respectively at relative pressure (P/P 0 ) = 0.4 < 1.The speci c surface area and pore volume of Cu/Fe-0.5 catalyst were 438.6m 2 /g and 0.17cm 3 /g respectively.This phenomenon can be explained as Fe 3+ and Cu 2+ are introduced into the microporous structure and form small nanoparticles during the calcination process, thus reducing the speci c surface area of Cu/Fe-0.5 catalyst.

SEM analysis
The appearance and morphology of X-type zeolite and Cu/Fe-X catalyst were observed by SEM.It can be seen from Fig. 3 (a) that X-type zeolite is in blocky structure, with an average diameter of 2µm to 5µm of Na-X-type zeolite and there are quite a lot of pores, and the dye is likely to be adsorbed and trapped in these pores [28] .And there are irregular lumps on the surface, which is caused by the residue of mesoporous template agent.After metal loading, the morphology of Cu/Fe-X catalyst is roughly the same as that of X-type zeolite (Fig. 3 (b) and Fig. 3 (c)).A small amount of metal oxide particles can be observed on the surface of Cu/Fe-X catalyst, and they are uniformly distributed on the surface of Cu/Fe-X catalyst without any aggregation.

TEM analysis
Figure 4 shows TEM and HRTEM images of Cu/Fe-0.5 catalyst.It can be seen from the TEM image that the grain boundary edge of Cu/Fe-0.5 is clear, the lattice is relatively complete and the crystallinity is high.
The microstructure and the existence of Fe and Cu species in Cu/Fe-0.5 were further studied by HRTEM.
The lattice spacing between two adjacent stripes is 0.232 nm and 0.154 nm, which match the Bragg re ection from the (10-2-2) and (11-11-1) planes, and well matched with the diamond hematite [29] .
However, the oxide particles formed do not show obvious peaks in the XRD spectrum (Fig. 1), indicating that these nanoparticles are highly dispersed and have low crystallinity.This special structure will enhance its catalytic ability to activate H 2 O 2 .

XPS analysis
As shown in Fig. 5, XPS spectroscopy is used to verify the surface element composition and electronic state of X-type zeolite and Cu/Fe-X catalyst.Different peaks of elements C, O, Fe and Cu can be observed in the wide scan XPS spectrum (Fig. 5 (a)).In the C1s spectrogram of Cu/Fe-X catalyst (Fig. 5 (b)), three peaks at 284.2eV, 286.4eV and 288.5eV can be observed, which are respectively attributed to the vibrations of C-C, C-O and C = O [30,31] .In Fig. 5 (c), the peaks at 932.8eV and 955eV represent Cu 0 or Cu 2 O, proving the existence of reducing copper [32] , while the satellite peaks at 941.1eV, 943.6eV and 962.3eV are attributed to Cu 2+ [33,34] , indicating that Cu 2+ is reduced during the reaction, Then H 2 O 2 is decomposed into •OH [35] .For Fe2p (Fig. 5  .The satellite peaks at 712.6eV and 730.2eV indicate the presence of Fe 3+ [37] .Fe 2+ can lose electrons to H 2 O 2 during the surface reaction, which further decomposes H 2 O 2 into •OH [35] , The change of Fe2p surface atoms before and after the reaction proves that it participates in the Fenton -Like reaction.High resolution XPS analysis results show that the multivalent state of Fe and Cu in Cu/Fe-X catalyst is proved to be very bene cial to the activation of H 2 O 2 [37,38]   , and the redox potential of Cu 0 or Cu 1+ and Cu 2+ is lower than that of Fe 2+ and Fe 3+ , which can promote the circulation of Fe 3+ to Fe 2+ , further promote the decomposition of Fe 2+ activated H 2 O 2 to produce •OH, and overcome the speed limiting step of Fenton-Like catalytic reaction [35] .

FT-IR analysis
FT-IR studies the functional groups existing in X-type zeolite and Cu/Fe-0.5 catalyst before and after use, ranging from 4000 − 500 cm − 1 .The energy spectrum is plotted in Fig. 6.The sharpest peak observed at 982 cm − 1 is related to the asymmetric stretching vibration of Al-Al-OH and Al-Si-O [39] .The peaks at 676 cm − 1 and 758 cm − 1 can be attributed to the symmetric stretching vibration mode of Al-O and Si-O tetrahedrons.Because the zeolites are hydrophilic, the peaks at 3480 cm − 1 correspond to the stretching vibration of OH groups in water or hydroxyl groups, and the peaks at 1633 cm − 1 can be attributed to the bending vibration of physically adsorbed water [40] .After Fe and Cu are loaded, the absorption peak of Cu/Fe-X basically keeps the characteristic peak of X-type zeolite, and moves slightly to a lower wave number, mainly because of the stable interaction between metal oxides and X-type zeolite.The main functional groups of Cu/Fe-0.5 catalyst did not change before and after Fenton-Like reaction, indicating that its structure was stable.
Catalytic Performance And Mechanism

Effect of copper and iron loading
Different metal loading catalysts Cu/Fe-X were used to evaluate the RhB degradation e ciency under neutral conditions.As shown in Fig. 7 (a), the degradation e ciency increases with the increase of metal loading.The catalysts Cu/Fe-0.5 and Cu/Fe-0.7 can achieve almost complete decolorization.It is because the electrons generated by a large number of bimetallic ions in the redox reaction process promote the rate of mutual oxidation reaction, thus promoting the H 2 O 2 convert into active free radicals [41] .However, the degradation e ciency of catalyst Cu/Fe-0.7 is slightly lower than Cu/Fe-0.5, because excessive metal oxide particles will rst react with free radicals generated by H 2 O 2 to form hydroxide [42] .
Therefore, the catalyst Cu/Fe-0.5 is selected for subsequent experiments.

Effect of reaction temperature
Temperature is also a key factor affecting the removal e ciency in Fenton reaction, so the effect of reaction temperature on the performance of Cu/Fe-0.5 catalyst for the degradation of RhB.
As shown in Fig. 7 (b), the removal e ciency increases signi cantly with the temperature increasing from 20 ℃ to 80 ℃.The reaction rate is obviously accelerated, and the degradation e ciency can reach 99.0% after 45 minutes of reaction at 80 ℃.This is because the increase of temperature can provide energy to overcome the activation energy of Fenton reaction, accelerate the decomposition of H 2 O 2 , and produce a large number of free radicals.Based on factors such as removal rate and high temperature energy consumption, the optimal temperature for RhB degradation with Cu/Fe-0.5 catalyst is 60 ℃.

Effect of dosage
Figure .7 (c) shows the effect of catalyst dosage on RhB degradation.With the increase of catalyst dosage from 0.7 g/L to 1.5 g/L, the degradation rate of RhB is signi cantly improved, because the increase of catalyst can provide more active sites for RhB to stimulate H 2 O 2 to produce free radicals [43] .
When the amount of catalyst Cu/Fe-0.5 reaches 1.0 g/L, the removal e ciency can reach more than 99.0% within 75 minutes.However, further increasing the amount of catalyst to 1.5 g/L did not improve the reaction rate and decolorization e ciency.The reason may be that excessive Fe 3+ and Cu 2+ will consume the free radicals, cause•OH to be consumed [44] .•OH generated by the reaction is constant, and the degradable is also constant, leading to the degradation e ciency of RhB decreases with the increase of its concentration [45] .In the actual treatment process of dye wastewater, economic and e ciency factors are considered, so the RhB concentration is selected as 100 mg/L.

Effect of H 2 O 2 dosage
In the whole Fenton catalytic system, the amount of H 2 O 2 affects the catalytic activity.In the reaction process, the production of active free radicals is related to the H 2 O 2 .Therefore, it is necessary to discuss the in uence of H 2 O 2 dosage on RhB removal e ciency.As shown in Fig. 7 (e), with the increase of H 2 O 2 dosage from 1.0mL to 2.0mL, the degradation rate of RhB changes from 58.4-99.9%.However, with the further increase of H 2 O 2 dosage from 2.0 mL to 2.5 mL, the removal rate of RhB even decreased slightly (98.3%).These results show that the amount of H 2 O 2 from 1.0 mL to 2.0 mL can most effectively react with the active sites on the catalyst surface to produce more •OH radicals, and excessive H 2 O 2 concentration will inhibit the catalytic degradation of RhB.This is because H 2 O 2 is not only related to the generation of •OH, but also related to the elimination of •OH [46] .Excessive H 2 O 2 can convert the highly active •OH into the less active HO 2 • [47] .Therefore, when the concentration of H 2 O 2 is high enough, the degradation e ciency of RhB is basically stable or even slightly decreased.

4.6Effect of initial pH of solution
The initial pH of the solution has an important in uence on the decolorization of dyes in the nonhomogeneous Fenton reaction.Therefore, the effect of initial solution pH on the removal of RhB by Cu/Fe-0.5 catalyst was studied.As shown in Fig. 7 (f), when pH = 1.0, 4.0, 7.0 and 10.0, RhB basically realizes complete decolorization, and when pH = 1.0 and 4.0, the degradation rate can reach 99.99% in 30 minutes and 45 minutes, similar to typical Fenton reaction.Generally, under acidic conditions has more H + , which is more conducive to generae H 2 O 2 , H 2 O 2 can be rapidly decomposed to produce •OH to enhance the degradation of RhB [48] .
However, unlike the classical Fenton process, the degradation e ciency of RhB can still reach 98.9% and 97.6% at pH = 7.0 and 10.0.This means good catalytic degradation effect can still be obtained under neutral and alkaline conditions.In conclusion, the decolorization of RhB by Cu/Fe-0.5 is almost complete under acidic, neutral or alkaline conditions, indicating that the prepared Cu/Fe-0.5 catalyst has excellent catalytic activity in a wide pH range.This is particularly important in practical applications.

Recycling of catalyst
The stability of catalyst is an important index to evaluate its performance.As shown in Figure .8,the degradation rate of RhB decreased to 98.9% after the catalyst was recycled for 5 times.This is because, with the increase of the number of repetitions, copper and iron in the catalyst are continuously consumed and reduced, which not only weakens the oxidative decomposition process of H 2 O 2 , but also increases the leaching amount of copper and iron, resulting in adverse effects on Fenton reaction.However, it still shows a high degradation e ciency, indicating that Fe and Cu can rmly adhere to the X-type zeolite.These results show that the Cu/Fe-0.5 catalyst has excellent stability and recycling performance.
Compared with the clay based catalyst previously reported, the Cu/Fe-0.5 catalyst shows excellent decolorization e ciency and reusability, realizing complete mineralization of organic dyes in water.

Catalytic reaction mechanism
In order to determine the active substances generated in the non-homogeneous Fenton catalytic process of Cu/Fe-0.5 catalyst, common free radical inhibitors such as isopropyl alcohol (IPA), p-Benzoquinone (BQ), NaN3 and KI are used to capture hydroxyl radical (•OH), peroxide radical (•O 2 − ) [50] , singlet oxygen ( 1 O 2 ) and hole (h + ) [51] generated in the oxidation process.The results in Fig. 9 (a) show that the Fenton-Like degradation of RhB is the result of the synergistic effect of copper iron and H 2 O 2 in Cu/Fe-0.5 catalyst.Figure 9 (b) shows that the addition of IPA signi cantly inhibits the degradation of RhB.In the presence of IPA, the removal rate of RhB decreased from 99.9-37.8%.The difference is that the removal e ciency of RhB does not change signi cantly with the addition of other free radical inhibitors.These results show that the active substances are mainly •OH in this non-homogeneous fenton system [52] .In order to further verify the type of free radicals, the Cu/Fe-0.Figure 10 shows the possible mechanism of RhB degradation by Cu/Fe-X as non-homogeneous Fenton catalyst.First, RhB is adsorbed on the surface of Cu/Fe-X catalyst.Due to the large speci c surface area and complex pore structure of Cu/Fe-X, as well as the metal cations on the catalyst surface, dye RhB can be effectively adsorbed.Physical and chemical adsorption between the catalyst and dye is the premise of non-homogeneous Fenton catalysis [53] .H 2 O 2 reacts with Fe 3+ , Cu 2+ and metal oxides on the catalyst surface to form •OH. In addition, the redox cycle of Fe 3+ /Fe 2+ and Cu 2+ /Cu + can promote the interface electron transfer of Cu/Fe-X, so that H 2 O 2 can produce more •OH, which is the real rate control step of the whole non-homogeneous Fenton reaction.The •OH can also be used as reactants to enter the bulk solution and generate other active free radicals again through chain reaction.The reaction process [54] as follows (Equations ( 1)-( 7)): Fe 3+ + Cu + → Fe 2+ + Cu 2+ (5) To sum up, •OH is the main free radical.The redox cycle of Fe 3+ /Fe 2+ and Cu 2+ /Cu + controls the generation rate of reactive free radicals in non-homogeneous Fenton systems and promotes degradation to produce CO 2 and H 2 O of RhB.

Conclusion
Non-homogeneous Fenton catalyst (Cu/Fe-X) was successfully prepared by impregnation and calcination of X-type zeolite supported on coal gangue to remove RhB from aqueous solution.The following conclusions are drawn.
SEM, BET and XRD spectra show that Fe and Cu have been successfully loaded on X-type zeolite.
Compared with X-type zeolite supported by single metal, Cu/Fe-X shows more excellent catalytic activity in Fenton process, because electrons generated in the redox cycle of bimetallic ions promote the rate of mutual oxidation reaction, thus improving the conversion of H 2 O 2 .
The free radical quenching experiment and the change of copper and iron multivalent states before and after the reaction showed that hydroxyl radical (•OH) was the main active species for catalytic degradation of rhodamine B, and the circulation of Cu 2+ to Cu 1+ and Fe 3+ to Fe 2+ could synergistically produce •OH, thereby improving the catalytic e ciency; At the same time, the redox potential of Cu 1+ and Cu 2+ is lower than that of Fe 2+ and Fe 3+ , which can promote Fe 3+ to circulate to Fe 2+ , further promote Under the conditions of dye concentration of 100 mg/L, 60 ℃, copper iron loading of 0.5g/L, catalyst dosage of 1.0 g/L and 2.0 mL H 2 O 2 , the catalytic degradation rate is as high as 99.9%, and the TOC removal rate is as high as 98.5%.The catalyst has good recyclability.Therefore, Cu/Fe-X/H 2 O 2 system can effectively remove organic dyes, and has a high industrial application prospect.

Figures
Figure 1 XRD patterns of CG-X and CG-X with different copper iron content IV powder X-ray diffractometer (XRD, Nihon Nihon Co., Ltd) was used to analyze the existing forms of each component in the sample.ASAP2020 pore structure speci c surface area analyzer (BET, American Micromeritic Company) was used to analyze and determine the speci c surface area and pore size distribution of samples.The microstructure and morphology of the prepared catalyst were analyzed by FEI Tecnai F30 transmission electron microscope (TEM, FEI Company, USA); The component species and surface functional groups of the prepared catalyst were analyzed by 6700 Fourier infrared spectrometer (FT-IR, Nicolet Company, USA); The surface morphology of the samples was analyzed by SU4800 scanning electron microscope (SEM, Hitachi); ESCAlab250xi X-ray photoelectron spectrometer (XPS, Thermo Fisher Scienti c, USA) was used to analyze the samples; Multi N/C 3100 TOC analyzer (TOC, Jena Analytical Instruments, Germany) was used to measure the total organic carbon content before and after catalytic degradation; JESFA200 electron paramagnetic resonance spectrometer (ESR, Japan Electronics JES-FA Company) was used to analyze the •OH and superoxide radical (•O 2 − ) captured by DMPO; UV-5100 ultraviolet visible spectrophotometer (Shanghai Yuanan Instrument Co., Ltd.) was used to measure the concentration change of the degraded dye.

Figure. 7
Figure.7 (d) shows the in uence of RhB concentration on its degradation e ciency.The initial concentration of RhB increased from 50mg/L to 100mg/L, and the degradation effect was almost the same.When the concentration of RhB reaches 125mg/L, the degradation e ciency decreases.This is mainly because when the amount of Cu/Fe-0.5 catalyst and the amount of H 2 O 2 are xed, the amount of

5 + H 2 O 2 +
RhB system was analyzed by ESR using DMPO as the spin trapping agent.The results are shown in Figure.9(c).It can be seen that there are obvious DMPO-•OH signals (four characteristic peaks, 1:2:2:1), proving that •OH is generated in the system of Cu/Fe-0.5 + H 2 O 2 + RhB.The ESR results were consistent with the free radical capture experiment, indicating that the presence of •OH led to the degradation of RhB in the Cu/Fe-0.5 + H 2 O 2 + RhB system.Figure.9(d) shows that •OH plays a leading role in the degradation of rhodamine B and the removal rate of TOC in Fenton-Like reaction.

Fe
2+  to activate H 2 O 2 to decompose into •OH, and overcome the speed limiting step of Fenton-Like catalytic reaction.The obtained Cu/Fe-X catalyst can e ciently degrade RhB by activating H 2 O 2 in wide pH range(1-10).

Figure 7 The
Figure 7

Table 1 ,
the major elements of the raw coal gangue are Al 2 O 3 and SiO 2 , and the content of Al 2 O 3 and SiO 2 is higher than 60%, which can be used to prepare zeolite molecular sieve.In addition, the content of Fe 2 O 3 , MgO and Na 2 O is small, and the impact on the test is negligible.
2 solution to start non-homogeneous Fenton catalytic reaction.HCl and NaOH solutions are respectively used to adjust the pH value of the solution.The effects of different parameters on the degradation e ciency of RhB were investigated.Test the degradation rate ( ) of RhB by ultraviolet spectrophotometer, = (C 0 -C t )/C 0 , where C t is the RhB concentration at a certain time, and C 0 is the initial concentration of RhB.And the mineralization degree can be obtained through TOC test.

Table 2
The parameters of the surface structure of X-type zeolite and Cu/Fe-0.5 (d)), the binding energies are 710.5eV(Fe 2p3/2) and 724.7eV (Fe 2p1/2) respectively.There is a swing satellite peak at 718.3eV, which proves that the iron oxide of the medium Cu/Fe-X catalyst is Fe 2 O 3