Copper hydroxyl sulfate as a heterogeneous catalyst for the catalytic wet peroxide oxidation of phenol

Abstract

Copper hydroxyl sulfates Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B were successfully synthesized by a simple hydrothermal method and applied as heterogeneous catalysts to degrade phenol wastewater in a batch reactor in the presence of hydrogen peroxide (H₂O₂). The influence of temperature, H₂O₂ dosage, initial pH and catalyst dosage on phenol and COD removal efficiencies was investigated to get optimum conditions and to understand the degradation process more clearly. The Cu²⁺ concentrations in the solutions after three hours of reaction were also measured to prove the catalysts were stable. Excellent results, phenol removal efficiency of 99% and COD removal efficiency of 97%, were achieved when treating 100 and 500 mg L⁻¹ phenol wastewater. Even though the catalysts had low specific surface area, mesopores mainly existed to decrease diffusion control of H₂O₂ and organics.

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1. Introduction

Phenol and its derivatives are broadly applied in petrochemical, chemical, pharmaceutical, oil refinery, dye, and plastic industries as raw materials, resulting in contaminants in effluent inevitably.¹ The treatment of phenolic wastewater is necessary before discharging it directly, due to the hazard to human health and aquatic life.¹ There are many methods to deal with phenolic wastewater, but biological treatment is ineffective at a phenol concentration of 50 mg L⁻¹ or higher or needs a long time because of the biorefractory and toxic phenolic compounds.² Some physical technologies, such as adsorption,^3,4 coagulation,^5,6 and membrane separation, are only suitable for wastewater of higher concentration. The wastewater after treated is not easy to meet emission standards, still requiring biological or chemical treatment.⁷ Advanced oxidation processes (AOPs)^8–10 of chemical technologies which can eventually oxidize organic pollutants to water, carbon dioxide, and other harmless small molecules non-selectively, are the main alternatives for wastewater treatment, especially for refractory pollutants.

AOPs based on extraordinarily reactive species such as hydroxyl radicals mainly include Fenton process,^11,12 photocatalysis,^13,14 electrocatalysis¹⁵ and catalytic wet peroxide oxidation (CWPO).¹⁶ The classic Fenton process has two major disadvantages: strictly controlled pH which is around 3.5 and further treatment of much iron sludge.^11,12 A great amount of energy and special apparatuses are needed when applying photocatalysis and electrocatalysis. Whereas catalytic wet peroxide oxidation (CWPO), which almost operates under the ambient conditions (atmospheric pressure and T ≤ 323 K) has been extensively and intensively investigated as one of the most effective, environmentally friendly and economical methods for treating refractory organics.¹⁷

Copper based materials are preferred catalysts for the oxidation of phenol¹⁸ because of good catalytic effect in transition metals. However, in most cases, low phenol conversions or COD removal efficiencies are reported. In order to improve catalytic effect, copper based materials are loaded on some supports such as activated carbon¹⁹ and polymers^20,21 through complicated methods to increase the specific surface areas. In recent years, a few copper hydroxyl salts with low specific surface area were proposed as promising catalysts in azo dyes removal via catalytic wet peroxide oxidation (CWPO). In 2010, Zhan and Chen reported the degradation of azo dyes over copper hydroxyphosphate, Cu₂(OH)PO₄, and it performed well only under near-neutral pH condition.²² They also pointed out that diffusion resistance associated with microporous materials may result in low activity and, consequently, non-porous catalysts with low surface area may also exhibit satisfactory activity, but specific data of pores were not provided. Copper hydroxide nitrate, Cu₂(OH)₃NO₃, is an effective CWPO catalyst for oxidative degradation of azo dyes in a wide pH range,²³ and Cu₂(OH)₃NO₃ synthesized by solvothermal method showed higher catalytic activity, than the control sample synthesized hydrothermally by the direct reaction of Cu(NO₃)₂ with NaOH in the catalytic wet peroxide oxidation of Direct Blue15. It could be seen from the above examples that different copper hydroxyl salts and the same copper hydroxyl salts synthesized by two methods had different performances. According to Li's report,²⁴ up to now, the excellent efficiency catalyzed by 150 mg [C₁₆H₃₃(CH₃)₃N]₄H₂SiV₂W₁₀O₄₀ was obtained with the maximum 91.6% phenol (0.53 mM 50 mL) removal efficiency at pH 2.8 and 93.2% COD removal efficiency after 90 min. 300 mg H₅PV₂Mo₁₀O₄₀@SBA-15 hybrid²⁵ are need to degrade phenol (0.53 mM 100 mL), though the TOC removal could achieved 100%. However, 20 mg Cu₄(OH)₆SO₄-A can remove 99% phenol (1.06 mM 150 mL) and 97% COD in a wide pH range (4.88–7.88). The synthesis of Cu₄(OH)₆SO₄-A is simple and its dosage is fairly small compared to the above polyoxometalate. The purpose of this work was to prove the high catalytic activity of copper hydroxyl salts was because of mesopores. Copper hydroxyl sulfates which had variance in pore diameter, one of the copper hydroxyl salts, were applied to explain how they weaken diffusion resistance for organics thoroughly.

Copper hydroxyl sulfates are atmospheric corrosion products of copper surface.²⁶ Brochantite Cu₄(OH)₆SO₄, one of the most common copper hydroxyl sulfates, have been synthesized by many methods. Typical synthetic process is mixing sulfates and hydroxides^27–29 at low temperature, and hydroxides can be replaced by urea,³⁰ carbonate,³¹ ammonia and metal oxides.³² Hydrothermal treatment has been utilized for preparing large and better crystals. Different morphologies of Cu₄(OH)₆SO₄ have been synthesized by Recep Kas using copper salt and sodium peroxydisulfate under the assistance of ultrasound.³³ Ultrasound assisted method can mix the solution uniformly and decrease the diffusion control on the reaction kinetics.

In this work, Cu₄(OH)₆SO₄-A was prepared hydrothermally from CuSO₄·5H₂O and NaOH,³⁴ and copper hydroxyl sulfate (denoted as Cu₄(OH)₆SO₄-B) was synthesized based on a modfication of the hydrothermal method^32,35 from CuSO₄·5H₂O and ZnO under magnetic stirring. The catalysts were applied to degrade phenol, a model compound in wastewater, and the effects of temperature, catalyst dosage, hydrogen peroxide dosage and initial pH by Cu₄(OH)₆SO₄-A were investigated.

2. Experimental section

2.1. Materials

Copper sulfate pentahydrate (CuSO₄·5H₂O), sodium hydroxide (NaOH), tert-butanol, phenol, ZnO and H₂O₂ were purchased from Shanghai Chemical Reagent Co. Ltd. All chemicals used in this work were commercially supplied as analytical grade reagents and used without further purification. De-ionized water was used throughout the experiments.

2.2. Synthesis of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B catalysts

2.2.1 Synthesis of Cu₄(OH)₆SO₄-A. Cu₄(OH)₆SO₄-A was synthesized hydrothermally from copper sulfate pentahydrate (CuSO₄·5H₂O) and sodium hydroxide (NaOH).³⁴ The 6.6 M sodium hydroxide aqueous solution (8.7 g NaOH in 33 mL water) was dropped slowly into the 1.4 M solution of copper sulfate (5.6 g CuSO₄·5H₂O in 25 mL water) under magnetic stirring. Next, the resulting mixture was stirred for 0.5 h and sonicated at 25 °C for 0.5 h, followed by a hydrothermal treatment at 110 °C in a 100 mL Teflon-lined stainless steel autoclave for 48 h. The solid product was centrifuged, washed with de-ionized water for three times, and dried at 80 °C for 10 h. The product is named as Cu₄(OH)₆SO₄-A.

2.2.2 Synthesis of Cu₄(OH)₆SO₄-B. 7.2 g CuSO₄·5H₂O was dissolved in 80 mL water, and 0.5 g commercial ZnO powder was directly added to the above 0.6 M aqueous solution to form suspension. Next, the suspension was sonicated at 25 °C for 0.5 h and stirred for 0.5 h. The suspension was sealed in a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was heated up to 120 °C and maintained at the target temperature for 48 h. Then the autoclave was allowed to cool to ambient temperature. The product was centrifuged and washed with de-ionized water. Finally, the precipitates were dried at 80 °C for 10 h, and kept in a dry condition at room temperature. The obtained sample is hereafter referred to as Cu₄(OH)₆SO₄-B.

2.3. Characterization of the sample

The powder XRD data of as-prepared samples were characterized on a Bruker (AXS model D8 advance) powder X-ray diffractometer equipped with Cu K_α radiation, λ = 1.5419 Å, 2θ range = 10–80°. The scanning electron microscopy of catalyst samples was examined on a JEOL (JSM-6400) scanning electron microscope. The pore diameter and specific surface area were studied by using Brunauer–Emmett–Teller method (BET Model Quantachrome/Autosorb-1).

2.4. Catalytic degradation experiments

All catalytic reactions were carried out in a 250 mL three necked round-bottomed flask with a mechanical stirrer. When the temperature was constant, the designed dosage of H₂O₂ and catalyst were added to the 150 mL 100 mg L⁻¹ (1.06 mM, pH = 6.88) solution, then, the reaction was started.

Many factors had influence on the phenol and COD removal efficiencies in the degradation of phenol wastewater, such as temperature, H₂O₂ dosage, catalyst Cu₄(OH)₆SO₄-A dosage and initial pH value. To get the optimal conditions of phenol wastewater degradation and to understand degradation process more clearly, complementary experiments were performed changing the above reaction conditions. Different temperatures (40–70 °C), H₂O₂ dosages (0.35–0.92 mL), catalyst Cu₄(OH)₆SO₄-A dosages (0.02–0.05 g) and initial pH values (2.88–8.88) were tested. The desired pH values were adjusted by adding diluted ammonia water (NH₄OH) and sulfuric acid (H₂SO₄).

The phenol removal efficiency was monitored as a function of time by taking out a given amount phenol solution and measuring absorbance of the phenolic compounds using spectrophometic method^16,24,36 after catalytic treatment at given time intervals. The catalyst must be immediately centrifuged from the solution in order to avoid influence the results. The phenol removal efficiency (η_t, %) after ‘t’ min degradation was calculated by using this equation:

where A₀ was the initial concentration of phenol and A_t was the concentration of phenolic compounds (including hydroquinone and catechol) after ‘t’ min. The COD (Chemical Oxygen Demand) was measured by a standard method after the oxidation with potassium dichromate (K₂Cr₂O₇).¹⁶ The COD removal efficiency after ‘t’ min degradation was defined as:where C₀ was the initial COD of phenol aqueous solution and C_t was the COD of phenol aqueous solution after ‘t’ min.

The stability of catalysts was studied by measuring the concentration of Cu²⁺ in the solution after three hours' reaction which was determined by using an atomic absorption spectrometer. The reusability of catalysts was tested through continuous experiments for three times under the above optimal conditions.

3. Results and discussions

3.1. Characterization of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B

X-ray diffraction patterns of commercial ZnO, the as-prepared Cu₄(OH)₆SO₄-A, Cu₄(OH)₆SO₄-B, the used Cu₄(OH)₆SO₄-A and the used Cu₄(OH)₆SO₄-B were shown in Fig. 1. In this figure, the peak positions of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B are consistent with JCPDS card no. 43-1458 and no diffraction peaks for other phases or materials e.g. ZnO and CuO are observed in the XRD patterns, indicating a high purity of the final products. In this study the crystal forms of the formed products and commercial ZnO are monoclinic and hexagonal, respectively. It should be noted that the diffraction peak of ZnO in Cu₄(OH)₆SO₄-B is not observed. In previous literatures, ZnO was added into Cu(NO₃)₂ solution to produce ZnCu_1.5(NO₃)_1.13(OH)_3.88 (ref. 37) and (CuZn)₅(SO₄)₂(OH)₆ (ref. 38) was synthesized by mixing ZnO powder with CuSO₄ solution at room temperature. Zinc oxide was directly involved into the reaction and existed in the form of zinc ion in the basic salt in these examples. Thus, zinc ion was existed in the filtrate and the prepared basic salt was different based on different conditions probably. After our long and careful consideration, the residual ZnO content in the as-sample is too little to be detected due to the transformation of ZnO into ZnSO₄ according to the equation (3ZnO + 4CuSO₄ + 3H₂O = Cu₄(OH)₆SO₄ + 3ZnSO₄). In order to confirm the viewpoint, filtrate which is obtained after washing the catalyst is measured and the result proves the presence of a large amount of zinc ions.

Fig. 1 XRD patterns of the Cu₄(OH)₆SO₄-A (fresh and the used), Cu₄(OH)₆SO₄-B (fresh and the used), ZnO samples and the standard pattern of Cu₄(OH)₆SO₄.

In order to identify the structure of the synthesized catalysts, the FT-IR spectra of commercial ZnO, Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B were measured, as shown in Fig. 2.

Fig. 2 FT-IR spectra of Cu₄(OH)₆SO₄-A, Cu₄(OH)₆SO₄-B and ZnO samples.

The fundamental vibration frequency for OH is found at the range 3600–3200 cm⁻¹ depending on the degree of H-bonding. The peaks at 3564 and 3404, 3574 and 3485 cm⁻¹ indicate two different types of hydroxyl group in the Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B. Cu–O–H bending modes with different degree of H-bonding could be found in Fig. 2. The peak at 987 cm⁻¹ could be assigned to strong H-bonding in Cu–O–H. The peaks at 874 cm⁻¹ and 885 cm⁻¹ are corresponding to medium H-bonding in Cu–O–H. The IR bands at 1122(ν₃), 1088(ν₃), 1105(ν₃), 602(ν₄), 642(ν₄) and 613(ν₄) cm⁻¹ are regarded as the stretching modes of SO₄²⁻.^31,39 The typical peaks of ZnO at 532 and 503 cm⁻¹ are not obviously observed in Cu₄(OH)₆SO₄-B, agreeing with the result of XRD, and the same reason was discussed above.

The morphologies of commercial ZnO, Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B were characterized by SEM and shown in Fig. 3. From Fig. 3a, it could be seen clearly the morphology of commercial ZnO is hexagonal, and the Cu₄(OH)₆SO₄-A are short sheets observed from Fig. 3d. Whereas, Cu₄(OH)₆SO₄-B are composed of many long slender rods ranging from 2 to 10 μm in length (Fig. 3e), which are different from Cu₄(OH)₆SO₄-A in size and shape. In addition, ZnO is also not observed cleraly in the SEM images of Cu₄(OH)₆SO₄-B, and it has been explained fully above.

Fig. 3 SEM images of commercial ZnO (a and b), Cu₄(OH)₆SO₄-A (c and d) and Cu₄(OH)₆SO₄–B (e and f) in high and low resolution.

The adsorption isotherms and pore size distributions of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B were described in Fig. 4. The pore volume (V), the average pore diameter (d₁), the specific surface areas (SSA) and the most probable pore diameter (d₂) of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B were shown in Table 1. As shown in Fig. 4a, the adsorption isotherm of Cu₄(OH)₆SO₄-A is Type III adsorption isotherm, declaring less pores in Cu₄(OH)₆SO₄-A because of little change of volume from 0.05–0.8 (P/P₀), and the pore size is mainly from 2.19 to 7.75 nm (Fig. 4b). The existence of macrospores maybe results from the gap between particles. The adsorption isotherm of Cu₄(OH)₆SO₄-B depicted in Fig. 4c has type IV adsorption isotherm which is characteristic for mesoporous materials. Micropores are existed because of the upside down line from 0–0.05 (P/P₀). However, mesopores are mainly existed from the sharp increased change of volume from 0.05–0.9 (P/P₀) and pore size distribution of Cu₄(OH)₆SO₄-B. The specific surface area values and the pore volume are measured as 11.9, 39.9 m² g⁻¹ and 0.049, 0.038 cm³ g⁻¹ for Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B, which are significantly smaller than traditional catalysts'.

Fig. 4 BET results of the products: (a) adsorption isotherm of Cu₄(OH)₆SO₄-A; (b) pore size distribution of Cu₄(OH)₆SO₄-A; (c) adsorption isotherm of Cu₄(OH)₆SO₄-B; (d) pore size distribution of Cu₄(OH)₆SO₄-B.

Table 1 Calculated data of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B

Catalyst	V (cm^3 g^−1)	d1 (nm)	d2 (nm)	SSA (m^2 g^−1)
Cu4(OH)6SO4-A	0.049	1.65	2.19	11.9
Cu4(OH)6SO4–B	0.038	3.82	2.44	39.9

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3.2. The influence of factors on the degradation effect

3.2.1 Effect of temperature. It is well known that temperature was one of the important factors that influence the catalytic activity, thus temperature is selected as the first evaluated factor. The effect of temperature (40, 50, 60, 70 °C) used commonly in CWPO on the phenol and COD removal efficiency was given in Fig. 5.

Fig. 5 Effect of temperature on the catalytic degradation of phenol wastewater (catalyst Cu₄(OH)₆SO₄-A 50 mg, pH = 6.88, phenol concentration 100 mg L⁻¹, H₂O₂ 0.58 mL): (a) phenol removal efficiency; (b) COD removal efficiency.

It could be seen that the phenol was easily removed by the catalyst from Fig. 5a, and even at 40 °C the phenol removal efficiency after only 20 min could reach 98%. The excellent phenol removal efficiencies were related to the mesopores of the catalyst. The average pore diameter of Cu₄(OH)₆SO₄-A (1.65 nm) was much larger than the size of phenol (about 0.62 nm), so phenol was readily diffused into the inner pore of Cu₄(OH)₆SO₄-A to react with hydroxyl radicals.²² Although all the final phenol removal efficiencies were nearly equal to 99%, the initial phenol removal efficiency was apparently increased and the time required was obviously shortened with the increase of temperature. Furthermore, the COD removal efficiencies at 40 °C were apparently lower than those at other temperatures. Like most of the catalysts, the catalytic activity was enhanced by rising the temperature,^22,23 therefore, more hydroxyl radicals were produced gradually to oxidize organics, especially from 40 to 50 °C. However, the higher temperature, the more energy consumption was need and catalysts might be sintered. The phenol removal efficiency at 50 °C could finally approach 99% and the COD removal efficiencies at 50, 60 and 70 °C were both around 94% after around 120 min, just a little bit higher than 90% at 50 °C, thus 50 °C was chosen as the suitable reaction temperature for energy conservation. In addition, it was found that the removal of phenol and COD were dramatically different. It was illustrated that in this study phenol was easily oxidized to hydroquinone and catechol firstly³⁹ and they were easily oxidized to other intermediate products, but these intermediates were slowly mineralized.¹⁸

3.2.2 Effect of H₂O₂ dosage. Phenol and COD removal efficiency affected by hydrogen peroxide dosage directly related to hydroxyl radicals was shown in Fig. 6. The abatement of phenol was not occurred nearly with 0 mL H₂O₂ and 50 mg Cu₄(OH)₆SO₄-A, proving H₂O₂ was the oxidizer beyond doubt and the absorption of Cu₄(OH)₆SO₄-A was negligible. With 0.58 mL H₂O₂, the same amount of catalyst and tert-butanol (scavenger of hydroxyl radical)³⁵ at the same time, the phenol was not oxidized during 180 min. Whereas, the phenol removal efficiency was 91% after 5 min and COD₁₈₀ was 94% with the same H₂O₂/Cu₄(OH)₆SO₄-A. It was declared that the organics were only oxidized directly by hydroxyl radicals coming from H₂O₂ in this study, and it was known that more hydroxyl radicals were produced quickly from more H₂O₂ in the presence of catalyst. So the initial phenol and COD removal efficiencies were increased with the increase dosage of H₂O_2. However, it was noted that the final COD removal efficiency with 0.92 mL H₂O₂ was equal to that with 0.58 mL H₂O₂, though the H₂O₂ dosage was increased dramatically to four times of theoretical value. It was due to the decrease of hydroxyl radicals in generation and the increase of their elimination. Finally, the residual hydroxyl radicals to be utilized were same. The chemical equation for complete oxidation of phenol by H₂O₂ was demonstrated as eqn (3).

C₆H₅OH + 14H₂O₂ → 6CO₂ + 7H₂O (3)

2H₂O₂ → 2H₂O + O₂ (4)

HO˙ + H₂O₂ → HO₂˙ + H₂O (5)

HO₂˙ + H₂O₂ → H₂O + O₂ (6)

HO˙ + HCO₃⁻ → OH⁻ + HCO₃˙ (7)

HO˙ + HO˙ → H₂O₂ (8)

Fig. 6 Effect of H₂O₂ dosage on the catalytic degradation of phenol wastewater (catalyst Cu₄(OH)₆SO₄-A 50 mg, pH = 6.88, phenol concentration 100 mg L⁻¹, T = 50 °C): (a) phenol removal efficiency; (b) COD removal efficiency.

According to the eqn (3), 14 mol H₂O₂ was needed to completely oxidize 1 mol phenol, so 0.23 mL H₂O₂ (30%, wt%) was theoretically needed to oxidize 150 mL 100 mg L⁻¹ phenol water. However, 0.23 mL H₂O₂ was not enough to oxidize phenol because of incomplete utilization,⁴⁰ e.g. decomposition of H₂O₂ (eqn (4)), elimination of hydroxyl radicals (eqn (5), (7) and (8)) and generation of hydroperoxyl radicals (HO₂˙),¹⁸ so more H₂O₂ dosage was needed to obtain better result. The hydroperoxyl radicals (HO₂˙) were not only less reactive but also reacted with H₂O₂ to consume the resource of HO˙. The hydroxyl radicals were also consumed by bicarbonate ions from carbonic acid generated from CO₂. The lifetime of hydroxyl radicals was especially short, and they would be recombined by themselves if they were not contacted with organics in time. Consequently, more organics must be diffused into pores of catalyst rapidly to react with hydroxyl radicals before hydroxyl radicals were reacted with H₂O₂, bicarbonate ions and themselves. The diffusion of organics could be readily occurred in the mesopores of Cu₄(OH)₆SO₄-A. The good degradation performance was attributed to cumulative hydroxyl radicals after generation and consumption. But the final COD removal efficiencies with 0.58 and 0.92 mL H₂O₂ were similar, thus, 0.58 mL H₂O₂ (2.5 times of theoretical H₂O₂ dosage) was beneficial for the removal of COD.

3.2.3 Effect of initial pH. The degradation of phenol was significantly influenced by the initial pH and the experiment results were showed in Fig. 7. As the initial pH values were adjusted widely from 2.88 to 7.88, the final phenol removal efficiencies were nearly 99% after only 15 min. However, the final phenol removal efficiency was 90% after 15 min at the initial pH = 8.88. The initial phenol removal efficiencies in acid environment (pH from 2.88 to 6.88) were dropped with the decrease of initial pH because of the dissolution of catalyst.⁴¹ The less catalyst, the less active sites, hydroxyl radicals would also be less from the same H₂O₂ dosage at the same time. The initial phenol removal efficiency in alkaline environment decreased (pH from 7.88 to 8.88) as the initial pH arose. The decomposition of some H₂O₂ in alkaline environment¹ led to the reduction of hydroxyl radicals' resource. The phenol removal efficiencies in acid environment (pH = 2.88) were better than those in alkaline environment (pH = 8.88), the total amount of hydroxyl radicals was not changed as the hydroxyl radicals were generated gradually, even if the catalyst was less in acid environment. In consideration of activity of the copper ions though homogenous reaction, an additional experiment was performed using 10 mg L⁻¹ Cu²⁺. The low COD removal efficiency of 2% indicated that the excellent performance in acid environment was not resulted from the dissolved copper ions. The tendency of COD removal efficiency was similar to that of phenol removal efficiency in acid and alkaline environment, respectively. However, the removal of COD needed longer time than the removal of phenolic compounds and its reason was discussed in the above part “effect of temperature”. It was noted that COD₁₈₀ still could achieve 71% at pH = 8.88, illustrating that the decomposition of H₂O₂ was not seriously and many hydroxyl radicals were continually produced slowly within 180 min. So the COD removal efficiency was related to the cumulative amount of hydroxyl radicals during the period of intermediates were generated and degraded. In conclusion, the pH of phenol wastewater suitable for treatment was widely from 2.88 to 7.88, and it would be best at natural pH 6.88 regardless of phenol removal efficiency or COD removal efficiency which was most close to neutral, meaning less loss of catalyst and less decomposition of H₂O₂.

Fig. 7 Effect of initial pH on the catalytic degradation of phenol wastewater (T = 50 °C, catalyst Cu₄(OH)₆SO₄-A 50 mg, phenol concentration 100 mg L⁻¹, H₂O₂ 0.58 mL): (a) phenol removal efficiency; (b) COD removal efficiency.

3.2.4 Effect of catalyst dosage. Fig. 8 presented the effect of catalyst dosage on the catalytic degradation of phenol. Revealed by the above figure, the phenol removal efficiency could achieve 32% and 99% at 25 and 180 min without catalyst. Whereas, the phenol removal efficiency of 99% was obtained after 25 min with only 10 mg catalyst.

Fig. 8 Effect of catalyst Cu₄(OH)₆SO₄-A loading on the catalytic degradation of phenol (T = 50 °C, pH = 6.88, phenol concentration 100 mg L⁻¹, H₂O₂ 0.58 mL): (a) phenol removal efficiency; (b) COD removal efficiency.

Using small amount of catalyst not only could shorten the time of degradation greatly, but also improved the phenol and COD removal efficiency markedly. Without catalyst, the COD removal efficiency only reached to 2% after 180 min, illustrating phenolic compounds were only translated to intermediates which were hardly further degraded to carbon dioxide and water because few hydroxyl radicals were produced in the absence of catalyst. The η₂ arrived 35%, 40% and 44% and the COD₂₅ achieved about 5%, 11% and 44% when catalyst was added from 10 to 50 mg. It was inferred that more hydroxyl radicals were generated quickly from H₂O₂ in the presence of more catalyst which was equivalent to more active sites. Although the phenol and COD removal efficiencies were both lower with 20 mg catalyst than that with 50 mg at beginning, they could reach the similar results and even better at last. Less hydroxyl radicals were produced when adding 20 mg catalyst, resulting in a lower degradation rate. At the same time, less hydroxyl radicals were wasted compared to 50 mg catalyst as the aforementioned cause, hence the COD removal efficiency at 180 min could reach the best effect of 97%. Considering the COD removal efficiency and catalyst dosage, 20 mg (0.13 g L⁻¹) catalyst was enough to degrade 150 mL 100 mg L⁻¹ phenol wastewater.

3.3. Catalytic activity of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B

In order to compare the activity of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B, 150 mL 100 mg L⁻¹ and 500 mg L⁻¹ model phenol wastewater was degraded under the optimal conditions (50 °C, pH = 6.88, 0.13 g L⁻¹ catalyst, 2.5 times of theoretical H₂O₂ dosage) with the same amount of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B, respectively. The experiment results were showed in Fig. 9.

Fig. 9 The COD removal efficiencies of degrading 100 mg L⁻¹ and 500 mg L⁻¹ model phenol wastewater under the optimal conditions with Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B.

The COD removal efficiencies of degrading 100 mg L⁻¹ and 500 mg L⁻¹ phenol solutions by Cu₄(OH)₆SO₄-B were better than that by Cu₄(OH)₆SO₄-A. This was resulted from the bigger pore diameter (the most probable pore diameter and the average pore diameter) of Cu₄(OH)₆SO₄-B and more pores in Cu₄(OH)₆SO₄-B in Fig. 4. The more pores were equal to more active sites, resulting in the more H₂O₂ reacting with catalyst, so more hydroxyl radicals were produced. On the other hand, the diffusion resistance for organics could be reduced because of the bigger pore diameter. The performance of 500 mg L⁻¹ phenol solution was superior to that of 100 mg L⁻¹ phenol solution because of the increasing collision frequency of molecules.²³

3.4. Stability and reusability of Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B

The stability of catalyst is very essential for actual industrial application as one of the catalyst parameters. In order to illustrate the stability of catalyst, the Cu²⁺ concentration after catalytic wet peroxide oxidation for 180 min was measured. The Cu²⁺ leaching (mg L⁻¹) in treated 100 and 500 mg L⁻¹ phenol solutions by Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B were listed in Table 2. The low Cu²⁺ leaching proved that the copper hydroxyl sulfates were sable in the 100 mg L⁻¹ phenol solution. It was further seen that the Cu²⁺ leaching was increased with the increasing concentration of phenol solutions. It was the combined results of the surface-catalyzed reaction⁴² and the less acidic conditions⁴³ attributed to more organic acids. More active sites of catalyst were needed to react with H₂O₂ to produce more hydroxyl radicals as the increasing concentration of phenol solutions, and at the same time, catalyst was contaminated by more acid intermediates generated unavoidably.

Table 2 The Cu leaching (mg L⁻¹) in low and high concentration solutions for three hours treatment by Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B

Concentration of phenol solution (mg L^−1)	Cu4(OH)6SO4-A	Cu4(OH)6SO4-B
100	6	5
500	30	30

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The phenol and COD removal efficiencies of degrading 100 mg L⁻¹ phenol solution in consecutive three experiments with recycled Cu₄(OH)₆SO₄-A and Cu₄(OH)₆SO₄-B were described in Fig. 10. It was found that phenol removal efficiency in Fig. 10a and b could be maintained at 99% because of the easy conversion of phenol, and COD removal efficiency in Fig. 10a was 97%, 96% and 86% for three consecutive runs. However, in Fig. 10b the COD removal values were 97%, 97% and 94%, respectively. The decrease of COD removal efficiency was only associated with the change of catalyst under the same conditions. The XRD pattern of used catalyst was showed in Fig. 1, indicating the catalysts' structures do not change. So the corrosion from intermediate acids formed during the degradation process and the dicarboxylic acids (the typical by-product) may lead to the reduction of pore diameter. The grayish green colour of the reused catalyst (the catalysts before using were green) indicated the adsorption of acids was one of the possible reasons. The COD removal efficiency with Cu₄(OH)₆SO₄-B in the third run was higher than that with Cu₄(OH)₆SO₄-A. It was mainly because that the pore diameter of reused Cu₄(OH)₆SO₄-B still belongs to size of mesopore even it might be decreased after reusing.

Fig. 10 The phenol and COD removal efficiencies of degrading 100 mg L⁻¹ phenol solution in three recycled runs by Cu₄(OH)₆SO₄-A (a) and Cu₄(OH)₆SO₄-B (b).

4. Conclusions

Copper hydroxyl sulfates were prepared by two different hydrothermal methods and showed considerable catalytic activity in oxidizing phenol solutions of low and high concentration at mild conditions (50 °C and atmospheric pressure). Phenol removal efficiency of 99% and COD removal efficiency of 97% were obtained under the optimal conditions (pH = 6.88, 0.13 g L⁻¹ catalyst, 2.5 times of theoretical H₂O₂ dosage). The degradation efficiency was directly related to the utilized hydroxyl radicals which were equal to generation minus consumption. The generation and consumption were affected by these factors including experiment conditions and pore of catalyst, so a maximum utilization rate of hydroxyl radicals could be achieved under the optimal conditions. The mesopores of Cu₄(OH)₆SO₄-A were in favor of the diffusion of organics, and this was also confirmed by the better catalytic performance of Cu₄(OH)₆SO₄-B. Copper hydroxyl sulfates are effective and stable catalysts with low specific surface area in CWPO. Hence, the catalysts for the degradation of phenol may not be restricted to those with high specific surface area.

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