Study on the treatment of simulated coking wastewater by O₃ and O₃/Fenton processes in a rotating packed bed

Abstract

In this study, simulated coking wastewater was treated by the O₃/Fenton process in a rotating packed bed (RPB) and the results were compared with those by the O₃ process. Contrast experiments indicated that the degradation rates of phenol, aniline, quinoline and NH₃–N in the wastewater reached 100%, 100%, 95.68% and 100% respectively under the optimum operating conditions in the O₃/Fenton process and were much higher than those in the O₃ process. The BOD₅/COD value of the simulated coking wastewater treated in the O₃/Fenton process reached 0.46 and was 135% higher than that in the O₃ process. The degradation pathways of phenol, aniline, quinoline and NH₃–N in the simulated coking wastewater were also discussed. The results indicated that a combination of the advanced oxidation processes and the RPB can enhance the treatment efficiency of coking wastewater.

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1. Introduction

Coking wastewater is one of the largest industrial wastewaters with many poorly biodegradable organics including phenol, aniline, quinoline, as well as inorganic ammonia, which are refractory and cause difficulties in biochemical treatment.^1,2 Therefore, coking wastewater treatment processes are complex, and combined treatment processes with various chemical and physical methods are usually adopted in order to achieve a certain effect, resulting in high operation costs.

In recent years, a variety of new technologies have emerged for the treatment of wastewater with complex components, among which advanced oxidation processes (AOPs), based on chain reactions of hydroxyl radicals with pollutants, have attracted wide attention due to their high efficiency and non-selectivity.³ The most commonly used AOPs include O₃, Fenton, O₃/Fenton, O₃/H₂O₂, and so on.^4–9 It was reported that AOPs are efficient methods for coking wastewater treatment.^10–12 In O₃-related AOPs, pollutants are degraded by reacting with O₃ molecules (direct oxidation) or with hydroxyl radicals (indirect oxidation), which are stronger oxidants produced through chain reactions of ozone.¹³

High gravity technology is carried out in a rotation packed bed (RPB), which creates a simulated high gravity environment using centrifugal force generated by the rotation of a rotor in the RPB. Liquid flowing through the porous packing in the rotor is split into micro- or nano-droplets, threads and thin films, and there is a huge and violently renewed gas–liquid interface, leading to a significant intensification of mass transfer and micromixing.^14,15 It is known that O₃-related AOPs are limited by O₃ absorption into water.¹⁶ Thus high gravity technology can be used to intensify these processes to improve the treatment effect of wastewater.

This study employed the O₃ and O₃/Fenton AOPs and an RPB as the reactor to treat a simulated coking wastewater containing phenol, aniline, quinoline and ammonia. The effect of different operating conditions on the degradation of phenol, aniline, quinoline and ammonia was investigated, in an attempt to provide a new process for the treatment of coking wastewater.

2. Experimental section

2.1 Materials

The simulated coking wastewater was prepared in the laboratory and its composition and properties are shown in Table 1.

Table 1 Composition and properties of the simulated coking wastewater

Parameter	Value
Phenol (mg L^−1)	100
Aniline (mg L^−1)	100
Quinoline (mg L^−1)	50
NH3–N (mg L^−1)	83
pH	6.58
Chemical oxygen demand (COD) (mg L^−1)	588
Biochemical oxygen demand after 5 days (BOD5) (mg L^−1)	147
BOD5/COD	0.25

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The chemical reagents used in the experiments include ferrous sulfate heptahydrate, hydrogen peroxide (30%, w/w), sodium hydroxide, sulfuric acid (98%), phenol (≥99.0%), aniline (≥99.5%), quinolone (≥99.5%) and ammonia (≥99.0%). They are of analytical grade and were purchased from Beijing Chemical Works, China. One mM L⁻¹ of sodium hydroxide and 1 mol L⁻¹ of sulfuric acid were used to adjust pH value of the solution.

The RPB consists mainly of a stationary casing and a packed rotor. The rotor has an inner diameter of 40 mm, an outer diameter of 120 mm, and an axial length of 15 mm. The diameter of the casing is 170 mm. Stainless steel wire mesh (Beijing Hongyahong Mesh Sale Center, Beijing, China) was used as the packing material. For further information on the RPB, please refer to our previous paper.¹⁷

2.2 Experimental procedures

The experimental setup is shown in Fig. 1. The simulated coking wastewater was adjusted to a certain pH value and divided into two portions with equal volume, one of which was added with a certain amount of H₂O₂, and the other with a certain amount of FeSO₄·7H₂O. The two liquid streams were simultaneously pumped into the RPB via two respective liquid inlets with the same flow rate, while the ozone-containing gas was produced by an ozone generator and introduced into the RPB via a gas inlet. The liquid streams and gas stream contacted countercurrently in the RPB to achieve the mixing of liquid streams and the absorption of O₃ into the liquid as well as the degradation of the pollutants. The remaining O₃ in the gas stream exiting the gas outlet was absorbed by the KI solution. Samples were taken at the liquid outlet to analyze the pollutant concentration, COD and BOD₅.

Fig. 1 Experimental setup for the treatment of simulated coking wastewater in an RPB with the O₃/Fenton process.

2.3 Analytical methods

Ozone concentration was analyzed by a dual ultraviolet ozone concentration detector (LM S-150, Guangzhou Li Mei Co., China).

COD was measured in terms of the Chinese Standard GB HJ/T 399-2007 (water quality – determination of the chemical oxygen demand – fast digestion – spectrophotometric method, which is similar to the method by APHA¹⁸), and COD removal rate was calculated according to the following eqn (1):

where COD₀ is the initial COD value of the wastewater, mg L⁻¹; COD_i is the COD value of the liquid samples taken at the liquid outlet of the RPB, mg L⁻¹.

BOD₅ was measured in terms of the Chinese Standard GB 7488-87 (water quality – determination of biochemical oxygen demand after 5 days – dilution and seeding method, which is based on ISO 5815:1983 (ref. 19)), and BOD₅ removal rate was calculated according to the following eqn (2):

where C₁, C₂: dissolved oxygen concentration measured immediately after the test specimen was prepared, and measured after the test specimen was cultivated for 5 days, respectively, mg L⁻¹.

C₃, C₄: dissolved oxygen concentration in a blank water specimen measured immediately after the specimen was prepared, and measured after the specimen was cultivated for 5 days, respectively, mg L⁻¹.

V_e, V_t: sample volume in the test specimen and total volume of the test specimen, respectively, mL.

BOD₅/COD (B/C) is the ratio of BOD₅ to COD. B/C is a biodegradable index, and a high B/C means high biodegradability of organic matters in wastewater. In general, wastewater is suitable for biological treatment when B/C is higher than 0.3.

Phenol, aniline and quinoline concentrations were measured with a high performance liquid chromatograph (Waters 2695, USA), while NH₃–N concentration was determined by a portable ammonia meter (GDYS-101SA, Xiaotiane Instrument Co. Ltd., Changchun, China).

3. Results and discussion

3.1 Effect of rotation speed

The effect of rotation speed on COD removal in the O₃ and O₃/Fenton processes is presented in Fig. 2. It can be seen that the highest COD removal rates of 39.36% and 17.5% were reached at the rotation speed of 1000 rpm in the O₃ and O₃/Fenton processes, respectively. When the rotation speed increased initially, liquid was split into smaller droplets and thinner films, leading to a larger gas–liquid interface and better absorption of O₃ and thus higher COD removal rate. Nevertheless, a further increase in the rotation speed resulted in a reduction in liquid residence time in the RPB and thus a decrease in the COD removal. It is also found from Fig. 2 that the O₃/Fenton process has better effect on the COD removal than the O₃ process due to the presence of H₂O₂ and Fe²⁺ to enhance the generation of hydroxyl radicals.

Fig. 2 Effect of rotation speed on the COD removal rate. (C_O3 = 30 mg L⁻¹, pH = 6.58, C_Fe(II) = 0.4 mmol L⁻¹, C_H2O2 = 6.5 mmol L⁻¹, G = 150 L h⁻¹, L = 30 L h⁻¹).

The B/C values were 0.24 and 0.38 at the rotation speed of 1000 rpm in the O₃ and O₃/Fenton processes respectively, indicating that the wastewater treated by the O₃/Fenton process is suitable for subsequent bio-treatment.

A large amount of hydroxyl radicals is generated in the O₃/Fenton process through the reaction of O₃ with Fenton. The synergic mechanism of O₃ and Fenton is shown by the following eqn (3)–(12).^20–23

Fe²⁺ + O₃ → Fe³⁺ + ˙O₃⁻ (3)

˙O₃⁻ + H⁺ → O₂ + ˙OH (4)

Fe²⁺ + O₃ → FeO²⁺ + O₂ (5)

FeO²⁺ + O₃ + H₂O₂ → Fe³⁺ + ˙OH + OH⁻ (6)

H₂O₂ → H⁺ + HO₂⁻ (7)

O₃ + HO₂⁻ → O₂ + ˙OH + O₂⁻ (8)

O₃ + O₂⁻ → O₃⁻ + O₂ (9)

O₃⁻ + H⁺ → ˙HO₃ (10)

˙HO₃ → O₂ + ˙OH (11)

Fe²⁺ + H₂O₂ → Fe³⁺ + ˙OH + OH⁻ (12)

3.2 Effect of gas flow rate

The effect of gas flow rate on the phenol, aniline, quinoline, NH₃–N removal in the O₃ and O₃/Fenton processes is presented in Fig. 3. It can be seen from Fig. 3 that the degradation of the pollutants increased with an increasing gas flow rate and tended to stability at the gas flow rate of 300 L h⁻¹ in both the O₃ and O₃/Fenton processes. The phenol, aniline, quinoline, NH₃–N degradation rates reached 61.97%, 98.78%, 60.0% and 100% respectively at the gas flow rate of 300 L h⁻¹ in the O₃ process, while their degradation rates reached 91.51%, 100%, 88.78% and 100% at the gas flow rate of 300 L h⁻¹ in the O₃/Fenton process, indicating that the O₃/Fenton process has better effect than the O₃ process. As the gas flow rate increased, the pollutant degradation rates increased because higher gas flow rate provided more ozone and higher mass transfer driving force, promoting the mass transfer between ozone and liquid and accelerating the degradation of the pollutants.

Fig. 3 Effect of gas flow rate on pollutants degradation in the O₃ process (a) and O₃/Fenton process (b) (C_O3 = 30 mg L⁻¹, R = 1000 rpm, pH = 6.58, L = 20 L h⁻¹, C_Fe(II) = 0.4 mM L⁻¹, C_H2O2 = 6.5 mM L⁻¹).

The effect of the gas flow rate on the B/C value in the O₃/Fenton process is given in Table 2, which shows that the B/C value at the gas flow rate of 300 L h⁻¹ reached 0.43 and was higher than that at the gas flow rate of 150 L h⁻¹, indicating that the treated water is suitable for subsequent bio-treatment.

Table 2 Effect of gas flow rate on the B/C value in the O₃/Fenton process

Gas flow rate	150 (L h^−1)	300 (L h^−1)
COD (mg L^−1)	418.7	366.8
BOD (mg L^−1)	159.1	157.7
B/C	0.38	0.43

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3.3 Effect of liquid flow rate

The effect of liquid flow rate on the phenol, aniline, quinoline, NH₃–N removal in the O₃ and O₃/Fenton processes is presented in Fig. 4. It can be seen from Fig. 4(a) that the pollutants degradation rate decreased as the liquid flow rate increased. An increase in the liquid flow rate led to lower gas–liquid ratio, resulting in reduced ozone mass transfer into per volume liquid. In addition, higher liquid flow rate caused larger liquid droplet size and shorter residence time of the liquid in the reactor.¹⁴ All these were unfavorable for mass transfer between ozone and the liquid and consequently resulted in decrease in the removal rates of the pollutants.

Fig. 4 Effect of liquid flow rate on pollutants degradation in the O₃ process (a) and O₃/Fenton process (b) (C_O3 = 30 mg L⁻¹, R = 1000 rpm, pH = 6.58, G = 300 L h⁻¹, C_Fe(II) = 0.4 mM L⁻¹, C_H2O2 = 6.5 mM L⁻¹).

In order to maintain a reasonable throughput capacity, the suitable liquid flow rate was determined as 20 L h⁻¹ in this study. Fig. 4 also demonstrates that the O₃/Fenton process has better effect than the O₃ process with the degradation rates of phenol and quinoline reaching 76.4% and 78.63% respectively in the O₃/Fenton process but 55.69% and 50.96% respectively in the O₃ process at the liquid flow rate of 20 L h⁻¹.

Table 3 gives the effect of the liquid flow rate on the B/C value and shows that the B/C value reached 0.43 at the liquid flow rate of 20 L h⁻¹ in the O₃/Fenton process, indicating a high biodegradability.

Table 3 Effect of liquid flow rate on the B/C value in the O₃/Fenton process

Liquid flow rate	20 (L h^−1)	50 (L h^−1)
COD (mg L^−1)	373.4	449
BOD (mg L^−1)	160.6	152.7
B/C	0.43	0.34

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3.4 Effect of initial pH

The solution pH plays an important role in the reaction of organic compounds with ozone. The effect of initial pH on the phenol, aniline, quinoline, NH₃–N removal in the O₃ and O₃/Fenton processes is presented in Fig. 5, which indicates that the highest aniline, quinoline, NH₃–N degradation rates were attained at the initial pH 7 in both processes. At a lower pH, the scavenging effect of hydroxyl radicals by H⁺ becomes obvious and H₂O₂ reacted with H⁺ to form H₃O₂⁺, leading to an improved stability of H₂O₂ and weakened degradation effect.²⁴ The pollutants degradation increased with a rising pH until 7 because the increase of hydroxyl ion concentration enhanced the formation of hydroxyl radical, a more active oxidant than ozone, through hydroxyl ion reaction with O₃.¹⁵ In addition, the increase of pH to 7 in the O₃/Fenton process led to coagulation whereby the pollutants were also removed by complexation reactions due to the conversion of Fe²⁺ and Fe³⁺ to Fe(OH)_n type structures.²⁵

Fig. 5 Effect of initial pH on pollutants degradation in the O₃ process (a) and O₃/Fenton process (b) (C_O3 = 47 mg L⁻¹, R = 1000 rpm, L = 20 L h⁻¹, G = 300 L h⁻¹, C_Fe(II) = 0.4 mM L⁻¹, C_H2O2 = 6.5 mM L⁻¹).

When the pH was higher than 7, the degradation of phenol constantly increased, while the degradation of aniline, quinoline, NH₃–N decreased with an increasing pH. The increasing phenol degradation rate can be explained by the reaction of phenol with more hydroxyl radicals induced by the increasing hydroxyl ion concentration. It is deduced that aniline and quinoline exist as the emprotid that is active and easy to react with hydroxyl ions in acidic condition, but they exist as stable molecules in basic condition, leading to a decreasing degradation rate with increasing pH.²⁶

It can be seen from Fig. 5 that the phenol, aniline, quinoline, NH₃–N degradation rates reached 70.78%, 87.41%, 58.25% and 95.0% respectively at the initial pH of 7 in the O₃ process, while they increased to 88.39%, 94.97%, 76.31% and 100% respectively at the initial pH of 7 in the O₃/Fenton process. These results confirmed that the O₃/Fenton process has a higher efficiency for coking wastewater treatment.

The B/C values of the wastewater treated at different initial pH in the O₃/Fenton process are given in Table 4, which indicates that the highest B/C value of 0.49 was attained at the pH 7. The natural pH of the simulated coking wastewater is 6.58, which is close to 7 and was chosen as a suitable pH because there was no need to adjust the pH of the wastewater with reagents.

Table 4 Effect of initial pH on the B/C value in the O₃/Fenton process

pH	3	5	6	7	9	12
COD (mg L^−1)	424.3	419.5	400.1	386.6	422.4	426.5
BOD (mg L^−1)	148.5	155.2	176.2	190.2	167.2	153.5
B/C	0.35	0.37	0.44	0.49	0.40	0.36

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3.5 Effect of Fe(II) concentration

The effect of Fe(II) concentration on the phenol, aniline, quinoline, NH₃–N removal in the O₃/Fenton process is presented in Fig. 6. It can be seen from Fig. 6 that the phenol, aniline, quinolone and NH₃–N degradation rates reached 100%, 100%, 95.68% and 100% respectively at the Fe(II) concentration of 0.4 mM L⁻¹ in the O₃/Fenton process. The combination of Fe(II) and hydrogen peroxide can promote the generation of hydroxyl radicals and improve the oxidation efficiency (eqn (3)–(12)). Therefore, the increase in Fe(II) concentration enhanced the formation of hydroxyl radicals, thus boosting coking wastewater degradation. However, when Fe(II) concentration exceeded 0.4 mM L⁻¹, hydroxyl radicals were consumed by the excess Fe(II) (eqn (13) and (14)), leading to a decrease in coking wastewater degradation efficiency. The optimum Fe(II) concentration was thus determined as 0.4 mM L⁻¹.

Fe²⁺ + ˙OH → Fe³⁺ + OH⁻ (13)

Fe²⁺ + FeO²⁺ + 2H⁺ → 2Fe³⁺ + H₂O (14)

Fig. 6 Effect of Fe(II) concentration on pollutants degradation in the O₃/Fenton process (C_O3 = 47 mg L⁻¹, R = 1000 rpm, pH = 6.58, L = 20 L h⁻¹, G = 300 L h⁻¹, C_H2O2 = 6.5 mM L⁻¹).

Higher degradation rate and B/C value can be attained in the O₃/Fenton process due to the synergic effect of O₃ and Fenton reagent. Table 5 indicates that the B/C value reached 0.46 at the Fe(II) concentration of 0.4 mM L⁻¹, compared to 0.35 and 0.38 at the Fe(II) concentration of zero and 0.8 mM L⁻¹ respectively, confirming that 0.4 mM L⁻¹ was a suitable Fe(II) concentration.

Table 5 Effect of Fe(II) concentration on the B/C value in the O₃/Fenton process

Fe(II) concentration	0	0.4 (mM L^−1)	0.8 (mM L^−1)
COD (mg L^−1)	440.6	392.6	418.8
BOD (mg L^−1)	154.2	180.6	159.1
B/C	0.35	0.46	0.38

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3.6 Degradation pathways

The degradation pathways of the coking wastewater are complex and involve mainly the oxidative ring-opening reaction, nitration reaction, and so on. In the O₃/Fenton process, the Fenton reagent with Fe²⁺ and H₂O₂ can catalyze O₃ decomposition to produce hydroxyl radicals and thus improve the degradation efficiency of the coking wastewater significantly.

1) Phenol degradation pathway. The mechanism of oxidative degradation of phenol is shown in Scheme 1. Intermediates are generated by the attack of hydroxyl radicals on phenol, and the main intermediate is hydroquinone. Then the generated quinone substances are further oxidized to form small molecular substances through ring-opening reaction and eventually converted into carbon dioxide and water.^27,28

Scheme 1

2) Aniline degradation pathway. Aniline mineralization is initiated by the attack of O₃ and/or ˙OH on aniline to yield mainly benzoquinonimine and some nitrobenzene. p-Benzoquinone is subsequently produced by hydrolytic decomposition of benzoquinonimine. Further degradation of p-benzoquinone, as well as of nitrobenzene with release of NO₃⁻, leads to the formation of maleic acid which is mineralized to CO₂.^29,30 The mechanism of oxidative degradation of aniline is shown in Scheme 2.

Scheme 2

3) Quinoline degradation pathway. The mechanism of oxidative degradation of quinoline is shown in Scheme 3. Hydroxyl radicals attack preferentially C3 and C6 sites at the benzene ring to activate quinoline, which are quickly oxidized to form 5,8-dicarbonyl quinolone. Aldehyde and acid are released through the ring-opening reaction of 5,8-dicarbonyl quinolone, and by-products, mainly pyridine, are produced. With the attack of hydroxyl radicals and O₂ on pyridine, NH₃, N₂, CO₂, CO, H₂O and other small molecule compounds are generated.³¹

Scheme 3

4) NH₃–N degradation pathway. As shown in the following eqn (15) and (16), NH₃ is removed by the reaction with O₃ and hydroxyl radicals, and N₂ and NO₃⁻ are produced. It is also found that hydroxyl radical concentration in the solution has a great influence on the degradation rate of NH₃–N.³²

NH₃ + O₃ → N₂ + H₂O (15)

NH₃ + ˙OH → N₂ + H₂O + NO₃⁻ + H⁺ (16)

4. Conclusions

The degradation of simulated coking wastewater by the O₃/Fenton process in the RPB was investigated in this study. The O₃/Fenton process was compared with the O₃ process, and it is found that the O₃/Fenton process was more effective due to the synergistic effect of O₃ and Fenton. The optimum operating conditions for the degradation of phenol, aniline, quinoline and NH₃–N in the coking wastewater were determined as rotation speed of 1000 rpm, gas flow rate of 300 L h⁻¹, liquid flow rate of 20 L h⁻¹, C_Fe(II) of 0.4 mM L⁻¹, C_H2O2 of 6.5 mM L⁻¹ and pH of 6.58. Under these conditions, phenol, aniline, quinoline and NH₃–N removal rates in the O₃/Fenton process reached 100%, 100%, 95.68% and 100%, respectively, which are much higher than those in the O₃ process. The BOD₅/COD value of the simulated coking wastewater treated by the O₃/Fenton process reached 0.46 and was 135% higher than that treated by the O₃ process. It can be deduced that the O₃/Fenton process in an RPB is a feasible way to increase biodegradability in the treatment of coking wastewater.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21276013, 20990221).

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