Iron-containing palygorskite clay as Fenton reagent for the catalytic degradation of phenol in water

Abstract

Heterogeneous Fenton systems have great application prospects in the catalytic degradation of organic wastewater; however, they are still not widely used in operation due to the difficulty of preparing catalysts in low yields and the high manufacturing cost. Herein, we report that a pristine iron-containing palygorskite clay can be used as a Fenton catalyst reagent without any retreatment. The composition, morphology, and structure of palygorskite clay, as well as the distribution and content of Fe element in palygorskite, were characterized via several physicochemical techniques. The degradation reaction of phenol in water was carried out as a probe reaction for the palygorskite Fenton reagent. The effects of the palygorskite content, pH value, and hydrogen peroxide concentration on the degradation efficiency of phenol were studied. Under optimum operating conditions, the chemical oxygen demand (COD) degradation efficiency of phenol reaches 94% with a reaction temperature of 20 °C and a reaction time of 15 min.

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1. Introduction

Water pollution is a critical issue affecting and constraining people's life and society's development.^1,2 The increased phenol pollution has become a major concern in environmental protection because they are highly toxic, carcinogenic and difficult to degrade.^3,4 Traditional research methods for phenol removal include biological treatment,⁵ extraction,⁶ and wet oxidation.⁷ It was impossible to go into operation as all of these methods have limitations of high cost, low efficiency, secondary pollution and other factors. In recent years, advanced oxidation processes (AOPs) have been widely studied by scholars,⁴ which generally includes Fenton oxidation,^8,9 ozone oxidation,¹⁰ sonolysis oxidation,¹¹ super critical oxidation,¹² and photochemical oxidation.¹³

Compared with other methods of AOPs, the Fenton oxidation is easy to operate, and the process is relatively environmentally friendly. The mechanism of the Fenton oxidation is described as following:^14,15

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH (1)

Fe³⁺ + H₂O₂ → Fe²⁺ + HO₂˙ + H⁺ (2)

Fe²⁺ + HO₂˙ → Fe³⁺ + HO₂⁻ (3)

Fe³⁺ + HO₂˙ → Fe²⁺ + O₂ + H⁺ (4)

H₂O₂ + ˙OH → HO₂˙ + H₂O (5)

As mentioned above, in the presence of hydrogen peroxide (H₂O₂), (OH) was generated by the reaction between Fe²⁺ and H₂O₂, which can degrade the macromolecules into micromolecules or mineralize the organic molecules into inorganic matters such as CO₂ and H₂O. The clear disadvantage of the traditional homogeneous Fenton system was that it needed a higher concentration of Fe²⁺ in the solution. Some heterogeneous Fenton systems have been studied for improving the Fenton oxidation property. For example, Guo et al. synthesized a sulfur-doped α-Fe₂O₃ (α-Fe₂O₃/S) from ferrous sulfate and Na₂S₂O₃ through a mixed hydrothermal calcination treatment for the degradation of acid orange 7 and phenol.¹⁶ Xu et al. prepared a magnetic nano-scale Fe₃O₄/CeO₂ composite via the impregnation method as a heterogeneous catalyst for the degradation of 4-chlorophenol.¹⁷ In addition, some scholars found that the introduction of electricity, light and ultrasound can improve the Fenton oxidation efficiency, which led to the research on electron-Fenton,^18,19 photo-Fenton,^20,21 sono-Fenton,^22,23 sono-photo-Fenton,²⁴ sono-electron-Fenton,²⁵ and photo-electron-Fenton.²⁶ Although numerous methods were used for Fenton reagents to degrade organic wastewater, they are still difficult to be used in actual application because the methods are limited by the difficulty of preparing catalysts in low yields and high cost of catalyst preparation. Therefore, it is necessary to develop a simple heterogeneous Fenton system for the degradation of organics in water.

Palygorskite clay, also known as attapulgite, is a kind of hydrous magnesium-rich silicate clay mineral with a chain-layered structure.^27–30 Its theoretical chemical formula is Mg₅Si₈O₂₀(HO)₂(OH₂)₄·4H₂O. Palygorskite is a fibrous porous crystal. The crystal structure unit layer is arranged by 8 Si–O tetrahedrons in a 2 : 1 type layer, where Si⁴⁺ can be replaced by Fe³⁺ or Al³⁺, and Mg²⁺ by Fe²⁺, Fe³⁺ or Al³⁺. The basic crystal unit forms a bundle with a chain structure, and the crystal is acicular, fibrous, or fibrous aggregate.²⁹ Palygorskite clay has been widely used in the fields of chemical industry, light industry, textile, building materials, environmental protection and pharmaceutical due to its specific fiber structure and excellent adsorption and decolorization properties.³⁰ In this study, we reported that a pristine iron-containing palygorskite clay can be used as a Fenton heterogeneous reagent without any retreatment. The composition, morphology, and structure of palygorskite clay, as well as the distribution and content of Fe element within it were studied via several physicochemical techniques. The effects of palygorskite content, pH value, and hydrogen peroxide concentration on the phenol degradation efficiency were also studied and the optimum operating conditions concluded were catalyst dosage of 0.5 g L⁻¹, initial pH = 3 and hydrogen peroxide dosage of 30 mmol L⁻¹. This study provides a catalyst material for improving the heterogeneous Fenton oxidation property.

2. Materials and methods

2.1 Materials

All the chemical reagents (phenol, hydrogen peroxide, sulfuric acid, sodium hydroxide, and hydrochloric acid) were of A.R. grade and used without further purification. The raw palygorskite clay, provided by Gansu Cuihua Technology Co., Ltd China, was passed through an 80-mesh sieve before use.

2.2 Characterization

X-ray diffraction (XRD) pattern was collected on a Shimadzu XRD-6000 X-ray powder diffractometer with Cu Kα radiation (λ = 1.5406 Å), and the scan rate was 10° min⁻¹. Scanning electron microscopy (SEM) was carried out on Zeiss Supra 55. High-resolution transmission electron microscopy (HRTEM) uses Tecnai G2 produced by FEI. The determination of COD was based on a 5B-3C COD rapid analyzer produced by Lianhua Technology Co., Ltd China. The content of Fe³⁺ in the solution after the reaction was determined via ion chromatography (ICS-90A). The composition of palygorskite clay was tested on an X-ray fluorescence spectrometer (XRF-1800).

2.3 Catalytic experiment

Unless otherwise specified, all experiments were performed using 100 mL phenol (100 mg L⁻¹) with an appropriate pH value adjusted using 0.5 mol L⁻¹ H₂SO₄ and 1.0 mol L⁻¹ NaOH. During the experiment, 0.5 g L⁻¹ palygorskite was added to the phenol solution and magnetically stirred for 30 min (500 rpm) to establish an adsorption/desorption equilibrium at 25 °C. Subsequently, different volumes of H₂O₂ (30%) were added at the required temperature. Finally, 5 mL of the mixed solution was taken out after a reaction time of 15 min and centrifuged at high speed (10 000 rpm, 3 min) to measure the COD and phenol concentration. The catalytic degradation efficiency is described as: [1 − (C_c/C₀)] × 100, where C₀ and C_c are the initial phenol COD value and the phenol COD value after the reaction, respectively. Effects of the palygorskite content, pH value, and hydrogen peroxide concentration on phenol degradation efficiency were also studied.

3. Results and discussion

3.1 Characterization of palygorskite clay

Fig. 1 shows the XRD pattern of raw palygorskite clay provided by Gansu Cuihua Technology Co., Ltd, China. There are numerous peaks stacked together. The highest peak was located at 2θ 26.6°, which is characteristic of the quartz (101) crystal plane. In order to analyze the specific composition of palygorskite clay, we compared the impurity crystals with the JCPDS card. We can see that besides the palygorskite (JCPDS no. 21-0958), the raw attapulgite powder includes dolomite (JCPDS no. 36-0426), clinochlore (JCPDS no. 29-0701), muscovite-3T (JCPDS no. 07-0042), feldspar (JCPDS no. 70-1862), quartz (JCPDS no. 46-1045), calciye (JCPDS no. 47-1743), gypsum (JCPDS no. 21-0816),³¹ and Fe₂O₃ (JCPDS no. 40-1139).

Fig. 1 XRD pattern of raw palygorskite clay.

SEM image of palygorskite with and without ultrasonic treatment of ethanol is shown in Fig. 2. It can be seen that various crystals interacted closely and superimposed on each other without ethanol ultrasound (Fig. 2a). However, the rod-shaped palygorskite crystals could not be distinguished from the other crystals. After palygorskite was ultrasonically dispersed in ethanol, we can clearly see the rod-shaped palygorskite (Fig. 2b). Palygorskite has a length of about 0.4–3 μm. In addition, we can see numerous flake impurity crystals after ultrasonic treatment of ethanol.

Fig. 2 SEM images of palygorskite without (a) and with ethanol ultrasound (b).

HRTEM was used to study the fine structure of the palygorskite sample (Fig. 3). The plane spacing between adjacent stripes on the edge of the palygorskite was measured to be 0.249 nm in HRTEM (Fig. 3a), exactly matching with the (110) (JCPDS no. 33-0664) crystal plane of α-Fe₂O₃.³² Linear scanning maps of Mg, Al, Si and Fe were obtained along the line shown in the HRTEM of the palygorskite sample (Fig. 3b and c), which shows that α-Fe₂O₃ is located at the edge of the palygorskite clay. An energy dispersive spectrometer was used to test the content of the elements of palygorskite clay (Fig. 3d). It was confirmed that palygorskite clay contained Mg, Al, Si and Fe, and their contents were determined to be 8.17%, 23.38%, 43.32% and 24.11%, respectively. In addition, the XRF test shows the content of Fe₂O₃ to be 9.9%. We suspected that Fe exists in the form of Fe₂O₃ on the surface of palygorskite clay.

Fig. 3 (a) Enlarged HRTEM image of palygorskite clay. (b) HRTEM image of palygorskite clay. (c) Linear scanning maps of Mg, Al, Si and Fe along the line shown in the HRTEM of palygorskite clay. (d) EDS of palygorskite clay.

3.2 Catalytic activity of palygorskite clay

3.2.1 Effect of palygorskite clay content. The effect of the amount of palygorskite clay on the phenol degradation efficiency was investigated in the range of 0.01–2 g L⁻¹ of palygorskite (Fig. 4). As the content of palygorskite increases from 0.01 to 0.5 g L⁻¹, the degradation efficiency of COD increases from 62% to 94%. With further increase in the quality of the palygorskite clay further increased, the COD degradation rate showed a decreasing trend, reaching a minimum of 60% at 2 g L⁻¹. According to the mechanism of the Fenton system as described above,^14,15 the palygorskite clay content was smaller, and the ˙OH radicals generated by the action between H₂O₂ and Fe²⁺ were less, leading to low phenol degradation efficiency. If we added excessive palygorskite clay, a large amount of ˙OH groups may rapidly be generated in the system. The excessive ˙OH may accumulate and react with each other to offset the utilization of ˙OH and reduce the degradation rate, thereby reducing the phenol degradation efficiency. We selected 0.5 g L⁻¹ as the appropriate dose for subsequent experiments. Furthermore, the concentration of Fe³⁺ in the solution was measured after the reaction was carried out to evaluate the practicability of palygorskite in the degradation of organic wastewater. We can see that the Fe³⁺ concentration is lower than 5 ppm in the dosage range of 0.01–2 g L⁻¹ of palygorskite.

Fig. 4 Effect of palygorskite content on phenol degradation efficiency (other reaction conditions: H₂O₂ concentration 30 mmol L⁻¹, initial pH value = 3, reaction temperature 20 °C, and reaction time 15 min). The ordinate on the right represents the dissolution rate of Fe³⁺ in an aqueous solution. The results of the COD degradation efficiency presented in the graph are the average value calculated from three parallel experiments.

In order to evaluate the effect of the Fe content of palygorskite on the phenol degradation efficiency, raw palygorskite was acidified using 6 mol L⁻¹ hydrochloric acid (with a liquid-to-solid ratio of 10 : 1). The XRF test shows the content of Fe₂O₃ was 3.5% for the acidified palygorskite. With the addition of 0.5 g L⁻¹ palygorskite, 30 mmol L⁻¹ H₂O₂ and with an initial pH value of 3, the phenol degradation efficiency was 34% for the above-acidified sample. The content of Fe₂O₃ in the palygorskite clay was an important factor for degradation efficiency.

3.2.2 Effect of H₂O₂ concentration. The effect of H₂O₂ concentration on the degradation rate of phenol is shown in Fig. 5. When the concentration of H₂O₂ increased from 10 to 30 mmol L⁻¹, the phenol degradation efficiency improved. However, when the H₂O₂ increased to 130 mmol L⁻¹, the phenol degradation efficiency slowly decreased. When the concentration of H₂O₂ is low, increasing H₂O₂ will increase ˙OH, resulting in an increase in the phenol degradation efficiency. Overdose of H₂O₂ will lead to the rapid oxidation of Fe²⁺ to Fe³⁺, thus consuming H₂O₂ and inhibiting the generation of ˙OH. 30 mmol L⁻¹ of H₂O₂ will be used for the phenol degradation experiment. In addition, the Fe³⁺ content increases with the increase in the H₂O₂ concentration, which may due to the reaction between H₂O₂ and Fe²⁺ to form Fe³⁺.

Fig. 5 Effect of H₂O₂ concentration on phenol degradation efficiency (other reaction conditions: palygorskite content 0.5 g L⁻¹, initial pH value of 3, reaction temperature 20 °C, and reaction time 15 min). The ordinate on the right represents the dissolution rate of Fe³⁺ in an aqueous solution. The results of the COD degradation efficiency presented in the graph are the average value calculated from three parallel experiments.

3.2.3 Effect of pH value. A relatively wide range of pH 1–9 was selected for testing the effect of pH on the Fenton reaction (Fig. 6). It was found that the phenol degradation efficiency gradually increased with the increase in pH from 1 to 3, reaching the maximum value of 94% when the pH was 3. As pH exceeds 3, the degradation efficiency gradually decreased. According to literature, the optimum pH of heterogeneous Fenton was 2.8.^33,34 If the pH value is lower than the optimum value, H⁺ in the system would be higher, which will break the conversion between different valence states of iron. If the pH value is higher, there would be too little H⁺ in the system, which will inhibit the formation of ˙OH and influence degradation efficiency. Therefore, pH = 3 is selected as the optimum pH. In the experiment, the phenol degradation efficiency could be kept above 70% at a relatively broad pH of 1–9. In addition, we can see that Fe³⁺ decreases with the increase in pH and disappears in the solution after pH = 7. Based on the above experimental results, we found that the optimum operating conditions are the following: catalyst dosage of 0.5 g L⁻¹, initial pH value of 3, and hydrogen peroxide dosage of 30 mmol L⁻¹.

Fig. 6 Effect of initial pH value on phenol degradation efficiency (other reaction conditions: palygorskite content 0.5 g L⁻¹, H₂O₂ concentration 30 mmol L⁻¹, reaction temperature 20 °C, and reaction time 15 min). The ordinate on the right represents the dissolution rate of Fe³⁺ in an aqueous solution. The results of the COD degradation efficiency presented in the graph are the average value calculated from three parallel experiments.

4. Conclusion

In summary, we reported that a pristine iron-containing palygorskite clay can be used as a heterogeneous Fenton reagent without any retreatment. Besides, palygorskite pristine powder sample includes impurity crystals such as gypsum, quartz, fluorphlogopite, and dolomite. We can clearly see the rod-shaped palygorskite after the powder sample was ultrasonically dispersed in ethanol. HRTEM and linear scanning map indicate that Fe exists in the form of Fe₂O₃ on the surface of the palygorskite clay. The degradation of phenol in water was carried out as a probe reaction for the palygorskite Fenton reagent. The impact of several roles, such as palygorskite content, H₂O₂ concentration, and pH value, on the phenol degradation efficiency has been studied. The optimum operating conditions utilized were palygorskite content of 0.5 g L⁻¹, initial pH = 3, and hydrogen peroxide dosage of 30 mmol L⁻¹. Under the optimum operating conditions, the COD degradation efficiency of phenol reached 94% within a reaction time of 15 min. The degradation ability of the palygorskite clay may be attributed to Fe₂O₃ on the surface of palygorskite.

Conflicts of interest

All authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21376019, 21676013) and Beijing Engineering Center for Hierarchical Catalysts.

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Footnote

† These authors contributed equally to this work.

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