A novel device for hazardous substances degradation based on double-cavitating-jets impingement: Parameters optimization and efficiency assessment

Journal of Hazardous Materials 335 (2017) 188–196

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A novel device for hazardous substances degradation based on double-cavitating-jets impingement: Parameters optimization and efficiency assessment Yuequn Tao a,b , Jun Cai a,b,∗ , Xiulan Huai a,b , Bin Liu a a b

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100080, China

h i g h l i g h t s • • • •

A novel two-cavitating-jets impinging device for hazardous substances degradation. Synergetic effect of two-cavitating-jets impingement and Fenton chemistry. Significant RB removal from aqueous solution during 2 h treatment. Advantages of reducing demand of H2 O2 and increasing treatment efficiency.

a r t i c l e

i n f o

Article history: Received 9 December 2016 Received in revised form 3 April 2017 Accepted 19 April 2017 Available online 22 April 2017 Keywords: Dye Hydrodynamic cavitation Double-cavitating-jets impingement Fenton chemistry Cavitation yield

a b s t r a c t Hydrodynamic cavitation is an effective advanced oxidation process. But sometimes it cannot obtain satisfactory treatment efficiency by using hydrodynamic cavitation individually, so it is necessary to introduce intensive methods. Based on double-cavitating-jets impingement, this paper presents a novel device that has advantages of strong heat and mass transfer and efficient chemical reactions. Based on the device, a series of experimental investigations on degradation of a basic dye, i.e. Rhodamine B were carried out. Significant Rhodamine B removal from aqueous solution was observed during 2 h treatment and the degradation reaction conformed to pseudo-first-order kinetics. The synergetic effects between double-cavitating-jets impingement and Fenton chemistry on simultaneous degradation of Rhodamine B were confirmed. Both single-variable experiments and orthogonal experiments were carried out to study the effects of initial hydrogen peroxide, ferrous sulfate and Rhodamine B concentrations and the optimum conditions were found out. Effects of jet inlet pressure in the range of 6–12 MPa and solution pH value in the range of 2–8 were also investigated. The cavitation yield was evaluated to assess the energy efficiency. The present treatment scheme showed advantages in terms of reducing the demand of hydrogen peroxide concentration and enhancing the treatment efficiency in large scale operation. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Effluents coming from dyeing and printing industry usually contain high levels of dye components. These components are photolytically stable, bio-refractory and resistant to chemical oxidation [1,2]. Generally, treatment methods as adsorption by activated carbon, coagulation by a chemical agent, or reverse osmosis are applied to such effluents [3]. Nevertheless, these are non-destructive meth-

∗ Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China. E-mail addresses: [email protected], [email protected] (J. Cai). http://dx.doi.org/10.1016/j.jhazmat.2017.04.046 0304-3894/© 2017 Elsevier B.V. All rights reserved.

ods and merely transfer contaminants from water to sludge. So there is a need to develop more environmentally friendly methods. With ability to effectively eliminate hazardous substances in wastewater, the advanced oxidation processes1 (AOPs) have increasingly aroused people’s attention [4]. The AOPs are characterized by releasing of highly oxidizing hydroxyl radicals (• OH), which can attack organic pollutants via adding to the aromatic ring or double bond, and abstracting electron or hydrogen [5]. Cavitation has been implemented as an AOP for wastewater treatment by many previous researchers [6–12]. It is a pressure-

1

Advanced oxidation processes (AOPs).

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189

Fig. 1. Schematic representation of the experimental setup.

related process and usually occurs when local pressure falls below the vapor pressure corresponding to liquid temperature. In case of acoustic cavitation, the local pressure fluctuation is caused by acoustic waves. While hydrodynamic cavitation occurs when liquid passes through constricting-diverging structures, such as throttling valve, orifice plate, and venturi. In the constriction region, the pressure decreases with the increased flow velocity and cavitation bubbles form. Subsequently, in the downstream region of constriction, the pressure is subjected to a recovery with the expansion of cross-sectional area, which leads to bubble collapse. Due to the violent compression of the internal gas during bubble collapse process, extreme environment (temperature 1000–10000 K, pressure 100–5000 bar [13,14]) is formed inside the bubble. The dissociation of water molecules and a series of chain reactions are excited in the extreme environment, giving rise to the formation of • OH [15]. Compared with acoustic cavitation, hydrodynamic cavitation has higher energy efficiency and allows for easier realization of scale-up applications [16]. Hydrodynamic cavitation has shown promising application potential for wastewater treatment, but sometimes it cannot obtain satisfactory treatment efficiency by using hydrodynamic cavitation individually [6,17]. In these situations, the intensifying methods can be introduced to enhance the efficiency and reliability of wastewater treatment systems based on hydrodynamic cavitation technique. The jets impinging technique, which was proposed by Elperin [18], is widely applied to the field of heat and mass transfer. With the capacity to decompose toxic substances and purify sewages, it has also received attention in environmental protection industry in recent years [19–21]. But to our knowledge, the combined utilization of hydrodynamic cavitation and jets impinging technique is very rare. Including hydrodynamic cavitation in the impinging jets has several important advantages in wastewater treatment: the collision, extrusion and shearing stress generated by jets impingement will divide cavitation bubbles into micro bubbles, and this will increase the bubble surface area and accelerate the reactions at the bubble surface; the severe turbulence in the impact zone will promotes cavitation inception, resulting in a secondary cavitation zone; high frequency pressure pulses caused by cavitation bubble collapse will strengthen the micro-mixing and enhance the impinging energy, leading to more efficient impingement and higher chemical reaction rate. By making use of double-cavitating-jets impingement, an innovative device was developed in this work. The device provides strong heat and mass transfer and efficient chemical reactions. To determine its applicability in wastewater treatment, a series

of experimental investigations on degradation of a basic dye, i.e. Rhodamine B2 (RB), were carried out. Effects of hydrogen peroxide, ferrous sulfate and initial RB concentrations, jet inlet pressure and solution pH were also investigated and the optimum conditions were assessed. The cavitation yield of was evaluated assess the energy efficiency. The results showed that the present treatment scheme showed advantages in terms of reducing the demand of hydrogen peroxide concentration, weakening the coalescing effects between cavitation bubbles, and enhancing the treatment efficiency in large scale operation. 2. Materials and methods 2.1. Materials RB (Analytical grade), Hydrogen peroxide (30%, chemical reagent grade), Ferrous sulfate (Analytical grade), Sulfuric acid (Analytical grade), and Sodium hydroxide (Flake, Analytical grade) were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Purified water was used throughout all experiments. 2.2. Experimental set-up Fig. 1 depicts the experimental setup. It is a closed circulation loop, which consists of a water containing vessel of 50 L volume, a plunger pump of 5.5 kW power rating and two pipelines: the main line and the bypass line. Water is drawn from the vessel by the plunger pump and flow into the main line after being pressurized to desired values by adjusting the pressure-regulating valve in the bypass line. The water in the main line then branches into two lines. Each line installs a high-pressure ball valve, an electromagnetic flow meter, a pressure gauge and a nozzle with convergent-divergent structure. Water undergoes cavitation process inside each nozzle and flow out of the nozzle in the form of high-speed cavitating jets, and then two jets impinge in the center of the main reactor chamber, where the pollutant degradation reaction mainly takes place. The schematics of the main reactor chamber and the dimensions of the nozzle are shown in Fig. 2. A fixed distance of 15 mm between the two nozzle’s outlets was adopted throughout the experiments in this paper. Water flows back to the vessel through a heat exchanger to control its temperature.

2

Rhodamine B (RB).

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Fig. 2. The schematics of the main reactor chamber and the dimensions of the nozzle.

Fig. 3. Variation of RB solution (a) color and (b) concentration with treatment time.

2.3. Experimental procedure and analytical methods Degradation of RB was carried out at different conditions using fixed solution volume of 30 L and for a constant circulation time of 120 min. Concentrations of RB, hydrogen peroxide and ferrous sulfate were varied in the range of 15.24–63.53 mg/L, 5–30 mg/L and 3.33–8.67 mg/L, respectively. The liquid temperature was set at a constant value of 26 ◦ C, while the inlet pressure was varied from 6 to 12 MPa. The absorbance of RB aqueous solution was tested at its maximum absorption wavelength of 554 nm using UVspectrophotometer (UNICO UV2100) and then the concentration was calculated according to the liner relationship between RB solution concentration and its absorbance. The pH value of the solution was controlled in the range of 2.4–8.2 by varying the amount of sulfuric acid and sodium hydroxide addition and tested using Mettler SevenExcellence Multifunctional Water Quality Tester. 3. Results and discussion 3.1. The degradation kinetics Degradation of RB in aqueous solution was investigated based on the present device with Fenton reagent addition. The variation of solution color and its concentration are shown in Fig. 3(a) and (b). It can be seen from Fig. 3(b) that the concentration of RB decreased exponentially with treatment time, which conformed to the pseudo-first-order kinetics. The relationship between RB concentration and the treatment time can be expressed as: C = C0 e−kt

(1)

where C0 (mg/L) and C(mg/L) are the concentrations of RB at initial state and at time t (min), respectively. k is the pseudo-firstorder rate constant (min−1 ). With operating conditions of jet inlet pressure 10 MPa, solution pH value 2.8, initial RB concentration 30.45 mg/L, H2 O2 concentration 20 mg/L, and FeSO4 concentrations 6.67 mg/L, the degradation rate constant k was found to be 3.07 × 10−2 min−1 with regression coefficient R = 0.9913. Though the reactions also show dependency on hydrogen peroxide concentration, it is not consistent with second-order reaction kinetics since hydrogen peroxide not only takes part into Rhodamine B degradation reaction, but also decomposes in the Fenton reaction process, as described in Refs. [22,23]. 3.2. Synergetic effects Experiments on degradation of RB with initial concentration of 33.5 mg/L were carried out using treatment methods of double-cavitating-jets impingement, Fenton chemistry and their combination at the same operating conditions. The variations of degradation rate (Eq. (2)) with treatment time are shown in Fig. 4. degradation rate =

C0 − C × 100% C0

(2)

Compared with only 33.74% reduction of RB concentration in case of only double-cavitating-jets impingement, 84.98% RB removal was achieved when initially adding 10 mg/L H2 O2 and 3.33 mg/L FeSO4 to the solution. There was only 27.44% reduction of RB concentration using the same amount of H2 O2 and FeSO4 when the solution was kept standing still. The corresponding pseudofirst-order rate constants were 0.002 min−1 , 0.004 min−1 and

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Fig. 4. Variation of RB degradation rate with treatment time using different treatment methods (conditions: solution pH, 3; jet inlet pressure, 10 MPa).

0.015 min−1 for degradation reactions via double-cavitating-jets impingement, Fenton chemistry and the combination, respectively. The synergist index (f) was calculated in Eq. (3) and was found to be 2.5, which indicates the synergetic effect of double-cavitatingjets impingement and Fenton chemistry on simultaneous removal of RB, i.e. 1 + 1 > 2. Fenton process is one of the AOPs, in which highly reactive hydroxyl radicals are produced by decomposition of hydrogen peroxide with ferrous ion (Fe2+ ) as catalyst in an acidic milieu [24]. On the one hand, both hydrodynamic cavitation and the addition of Fenton reagent provides oxidizing agents. On the other hand, hydrodynamic cavitation and jets impingement enhance the decomposing of H2 O2 and also the micro-mixing of reactants. Thus, the combination of double-cavitating-jets impingement and Fenton process has a better effect as compared to the single application of these processes.

f =

kFenton+Impingement kFenton + kImpingement

=

0.015 = 2.5 0.002 + 0.004

(3)

3.3. Effect of jet impingement By turning off one of the high-pressure ball valve, degradation of RB using one-cavitating-jet was carried out under identical conditions with that of double-cavitating-jets impingement. The main flow rate in single-cavitating-jet experiment was the same with that of double-cavitating-jets experiment, and equal to the sum of the flow rate in the two branches in double-cavitating-jets experiment. The variations of degradation rate with treatment time were shown in Fig. 5. It can be seen from Fig. 5 that the degradation rate for the combination of Fenton chemistry and single-cavitating-jet is 57.47%, while introducing jets impingement increased the degradation rate to 84.98%. The pseudo-first-order rate constants were also increased from 0.008 min−1 to 0.015 min−1 . The results show that application of double-cavitating-jets impingement has advantages of strong heat and mass transfer and efficient chemical reactions. On one hand, the collision, extrusion and shearing stress generated by jets impingement will divide cavitation bubbles into micro bubbles, and this will increase the bubble surface area and accelerate the reactions at the bubble surface. On the other hand, the severe turbulence in the impact zone will promotes cavitation inception, resulting in a secondary cavitation zone, which will strengthen the cavitation effect.

191

Fig. 5. Variation of RB degradation rate with treatment time using two different treatment methods (conditions: solution pH, 3; jet inlet pressure, 10 MPa).

Fig. 6. The degradation rate versus time at different H2 O2 concentrations (conditions: RB concentration, 30 mg/L; pH of the solution, 3; jet inlet pressure, 10 MPa; FeSO4 concentration 3.33 mg/L). Table 1 Effect of H2 O2 loading on the pseudo-first order rate constant. H2 O2 loading (mg/L)

Rate constant (min−1 )

Regression coefficient

5 10 20 30

0.014 0.019 0.022 0.045

0.971 0.979 0.972 0.941

3.4. Effect of initial solution concentration 3.4.1. Hydrogen peroxide concentration The loading of hydrogen peroxide plays a crucial role in the degradation process. The Degradation of RB was evaluated at four hydrogen peroxide concentration levels with the same set of other parameters. The degradation rate versus the experiment duration was shown in Fig. 6. As is shown in Fig. 6, increasing the initial hydrogen peroxide concentration could increase the degradation rate. 78.47% and 99.79% extent of RB removal at 120 min were achieved with the hydrogen peroxide concentrations of 5 and 30 mg/L, respectively. Kinetic studies revealed that the pseudo-first order rate constant increased from 0.014 to 0.045 min−1 with the increase in the loading of H2 O2 from 5 to 30 mg/L as shown in Table 1. The increased degradation rate at higher H2 O2 concentration can be attributed to the enhanced formation of the hydroxyl radicals due to continuous dissociation of H2 O2 under the cavitating and jets impinging conditions. The trend was consistent with those studies on the degradation of RB based on hydrodynamic cavitation technique.

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Fig. 7. The degradation rate versus time at different FeSO4 concentrations (conditions: RB concentration, 39 mg/L; pH of the solution, 3; jet inlet pressure, 10 MPa; H2 O2 concentration 10 mg/L). Table 2 Effect of FeSO4 loading on the pseudo-first order rate constant. FeSO4 loading (mg/L)

Rate constant (min−1 )

Regression coefficient

3.33 5.00 6.67 8.67

0.016 0.017 0.024 0.020

0.982 0.993 0.990 0.990

Wang et al. [25] carried out degradation of RB based on swirling jet-induced cavitation. The extent of degradation increased with higher loadings of H2 O2 . For 25 L RB solution with initial concentration of 10 mg/L, the degradation rate was 99.1% when initially adding H2 O2 concentration of 150 mg/L. Based on a venturi reactor, Mishra and Gogate [26] investigated degradation of RB using five different concentrations of H2 O2 ranging from 10 to 200 mg/L. For 4 L RB solution with initial concentration of 10 mg/L, 99.9% extent of degradation was achieved at H2 O2 concentration of 200 mg/L. It is important to note that the higher amount of RB removal was achieved in present work at relatively much lower concentration of H2 O2 . This is attributed to the simultaneously utilizing of hydrodynamic cavitation, jet impingement and Fenton chemistry, which have synergetic effects and makes the role of hydrogen peroxide given full play. The collision, extrusion and shearing stress caused by jets impingement and the severe turbulence in the impact zone will generate flow structures that promote heat and mass transfer and enhance the chemical reaction rate of hydrogen peroxide. High frequency pressure pulses generated in cavitation will strengthen micro-mixing and enhance impact energy, thus leading to a more efficient impingement and promoting the decomposition of hydrogen peroxide. Decreasing H2 O2 dose has important significance since the residual H2 O2 in water is harmful to water ecosystem. The loading of H2 O2 should be adjusted according to the pollutant concentration and make sure that almost entire amount is utilized in the process [24]. 3.4.2. Ferrous sulfate concentration The FeSO4 concentration is also an important parameter. The concentration of FeSO4 was varied from 3.33 to 8.67 mg/L to study its effect on the degradation reactions. The results are shown in Fig. 7. The removal of RB increased with increasing addition of FeSO4 and reached the maximum of 93.41% at FeSO4 concentration of 6.67 mg/L. After that, it began to drop with further increasing FeSO4 concentration. The maximum reaction rate constant 0.024 min−1 was also achieved at FeSO4 concentration of 6.67 mg/L as shown in Table 2. As a catalyst, Fe2+ can accelerate the decomposition of H2 O2 , resulting in enhanced production of hydroxyl

Fig. 8. The degradation rate versus time at different initial RB concentrations (conditions: FeSO4 concentration, 3.33 mg/L; pH of the solution, 3; jet inlet pressure, 10 MPa; H2 O2 concentration, 20 mg/L).

radicals and thus promoting RB degradation. However, due to the competition of Fe2+ and RB on hydroxyl radicals, an excess loading of Fe2+ has a negative effect on RB removal. Part of • OH produced in hydrodynamic cavitation and Fenton process are captured by the superabundant Fe2+ [27,28]. Similar result was found by Wang et al. [29] in experiments of reactive brilliant red degradation using a combination of swirling jet-induced cavitation and Fenton process. 3.4.3. Initial RB concentration The effect of initial RB concentration on the degradation rate in the range from 15.24 mg/L to 63.53 mg/L is depicted in Fig. 8. Increasing the initial RB concentration decreased the degradation rate. The pseudo-first-order rate constant increased from 0.013 to 0.040 min−1 with decreasing the initial concentration from 63.53 to 15.24 mg/L as shown in Table 3. Nevertheless, the total decomposition amount becomes higher with the increase of the initial RB concentration. As the initial concentration of RB increases, the total initial amount of RB is increased. This improves the contact between RB and hydroxyl radical, which makes more RB captured by • OH and increases the amount of RB removal. However, due to limited the oxidation capacity, the initial added RB cannot be degraded completely, which leads to the drop of the ratio of RB removal amount to the total initial amount of RB. Therefore, increasing the initial RB concentration decreased the degradation rate but increased the total removal amount of RB. The degradation experiment of imidacloprid carried out by Patil et al. [30] using the slit venturi also proved that the pseudo-first-order rate constant increased from 0.79 × 10−3 to 1.27 × 10−3 min−1 with decreasing the initial concentration from 60 to 20 ppm. Investigations by Parse et al. [31] and Wang et al. [25] on degradation of RB based on cavitation techniques show similar results. 3.4.4. Orthogonal experiment results The effects of initial H2 O2 , FeSO4 and RB concentrations on the degradation rate have been discussed by single factor experiments from Sections 3.4.1–3.4.3, and it was found that the three factors played significant role on the degradation rate. However, these studies were not enough to judge the optimum conditions and the influential order of the three factors. Therefore, an orthogonal test was carried out. Three controllable variables, initial RB concentration (A), FeSO4 concentration (B), and H2 O2 concentration(C), were set at three levels, respectively. The solution pH was 2.6 and jet inlet pressure was 10 MPa. With reference to the experimental design theory, the orthogonal array L9 (33 ) was selected to arrange the test program. The results are listed in Table 4.

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Table 3 Effect of RB initial concentration on degradation process. Initial RB concentration (mg/L)

Total amount of RB degradation (mg)

Rate constant (min−1 )

Regression coefficient

63.53 44.95 30.45 15.24

1562.27 1240.49 893.31 456.29

0.013 0.021 0.031 0.040

0.940 0.997 0.991 0.892

Table 4 Orthogonal experiment arrangements and results. NO.

A (mg/L)

1 2 3 4 5 6 7 8 9 k1 k2 k3 R

16 16 16 33 33 33 57 57 57 94.25 96.87 92.51 4.359

B (mg/L) 10 20 30 10 20 30 10 20 30 91.51 95.25 96.87 5.366

C (mg/L)

Degradation rate (%)

3.33 5.00 6.67 6.67 3.33 5.00 5.00 6.67 3.33 95.36 93.44 94.83 1.925

91.46 93.55 97.74 93.60 97.29 99.72 89.46 94.91 93.16

According to the R values in Table 4, the influential order of the three factors on RB degradation rate was B> A > C. Thus, H2 O2 concentration and initial RB concentration have a more important influence on the degradation of RB. This is because hydrodynamic cavitation and jets impingement greatly promotes the continuous decomposition of hydrogen peroxide, so the catalytic effect of Fe2+ becomes less obvious. According to the value of k, the optimum condition was A2 B3 C1 . However, the optimum condition of A2 B3 C1 did not appear in the orthogonal test. In the orthogonal test, the best condition was experiment No. 6 (A2 B3 C2 ) and its RB degradation rate was 99.72%. Thus the optimum conditions were determined as follows: Initial RB concentration 33.33 mg/L, H2 O2 concentration 30 mg/L and FeSO4 concentration 5.00 mg/L. 3.5. Effect of jet inlet pressure By adjusting the pressure-regulating overflow valve, the effect of jet inlet pressure on the degradation process was investigated over the range of 6–12 MPa and the hydrodynamic characteristics of the experimental setup is shown in Table 5. The cavitation number can be defined as: Cv =

P2 − P v

(4)

1  v0 2 2

where P2 is the pressure in the main reaction chamber and approximately equal to 0.1 MPa. Pv is saturated vapor pressure,  is the water density and v0 is the flow velocity at the nozzle hole. Fig. 9 shows the effects of inlet pressure on RB degradation rate with respect to time and Table 6 shows the pseudo-first-order rate constants at corresponding inlet pressures. It is clear that higher jet inlet pressure brings about more rapid degradation of RB. The maximum extend of RB removal (about 88.75%) was obtained at jet inlet pressure of 12 MPa whereas the degradation rates were

Fig. 9. The degradation rate versus time at different jet inlet pressures (conditions: FeSO4 concentration, 3.33 mg/L; pH of the solution, 3; H2 O2 concentration, 10 mg/L; initial RB concentration, 30 mg/L).

Table 6 Effect of RB initial concentration on the pseudo-first order rate constant. Jet inlet pressure (MPa)

Rate constant (min−1 )

Regression coefficient

6 8 10 12

0.013 0.014 0.015 0.018

0.978 0.972 0.984 0.996

78.08%, 81.70% and 83.71%% at inlet pressure of 6 MPa, 8 MPa and 10 MPa respectively under otherwise identical experimental conditions. The variation of the pseudo-first-order rate constant showed the same trend. The increasing degradation rate at higher jet inlet pressure can be attributed to the enhancement of hydroxyl radical production as a result of the intensification of cavitation activity. Increasing the jet inlet pressure also increases the jet flow velocity and this causes more violent jets impingement, stronger mixing and faster chemical reactions. Some investigations reported in literatures showed that the extent of pollutant removal firstly increased with increasing the inlet pressure to hydrodynamic cavitation device until reaching to the maximum at the optimum inlet pressure and then decreased [10,32,33]. The existing of the optimum inlet pressure in literature works is due to the coalescing effects between the cavitation bubbles. An excessive increase of inlet pressure will make the number density of cavities become so high that these cavities coalesce to form much larger cavitation bubbles. These large bubbles escape from the liquid without collapse or suffer from incomplete col-

Table 5 Hydrodynamic characteristics of the experimental system. Jet inlet pressure (MPa)

Flow rate at the nozzle upstream (L/h)

6 8 10 12

45.10 52.30 58.35 63.45

Flow velocity at the nozzle hole (m/s)

Cavitation number

Cycle number (1/min)

63.80 73.99 82.56 89.76

0.0486 0.0362 0.0291 0.0246

0.0501 0.0581 0.0648 0.0705

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Table 7 Effect of solution pH on the pseudo-first order rate constant. Solution pH

Rate constant (min−1 )

Regression coefficient

2.4 3.0 5.0 8.2

0.024 0.015 0.006 0.002

0. 990 0.984 0.976 0.941

Fig. 11. The maximum cavitation yields under the optimum conditions of different cavitation-based techniques for RB degradation.

mentioned in Section 3.5 under different jet inlet pressures are listed in Table 8, where the energy input is calculated with Eq. (7). Y1 (mg · J−1 ) =

The total amount of RB degradation The input energy

Y2 (mg · L · J−1 ) = Fig. 10. The degradation rate versus time at different solution pH values (conditions: FeSO4 concentration, 3.33 mg/L; H2 O2 concentration, 10 mg/L; initial RB concentration, 30 mg/L; Jet inlet pressure, 10 MPa).

The total amount of RB degradation The input energy per treatment volume

(5) (6)

The input energy = jet inlet pressure × flow rate × treatment time (7)

lapse, reducing the degradation rate. However, since the effects of jets impingement can divide cavitation bubbles into micro bubbles and weaken the coalescing effects, increasing the jet inlet pressure in the range of 6–12 MPa enhanced RB degradation rate in present work. 3.6. Effect of solution pH Effect of solution pH on the degradation process was investigated for pH values ranging from 2.4 to 8.2 and the results were depicted in Fig. 9 and Table 7. As shown in Fig. 10 and Table 7, the degradation rate was 94.82%, 81.70%, and 49.28% and the pseudofirst-order degradation rate constant are 0.024, 0.015 and 0.006 respectively at pH values of 2.4, 3.0 and 5.0. The degradation at natural pH value of 8.2 was only 17.60% with a degradation rate constant of 0.002. This indicates that acid condition is recommended for RB degradation. This is because the acid condition favors the generation of hydroxyl radicals due to the decomposition of hydrogen peroxide and impedes the recombination reaction among free radicals. The effect of solution pH on the degradation of diclofenac sodium was investigated by Bagal and Gogate [34] with variation of the initial pH in the range of 4–7.5. The degradation rate was raised from 14.7 to 26.8% when changing the pH from 7.5 to 4. Similarly, the degradation experiment of imidacloprid by Patil et al. [30] also demonstrated that the maximum rate of degradation was obtained at an optimum pH of 3. Under alkaline conditions, the extent of degradation was much lower than that under acidic conditions.

As is shown in Table 8, higher jet inlet pressure leads to higher degradation rate but reduces cavitation yield, since the energy input into the main reactor is higher at increased jet inlet pressure. In the practical application, the jet inlet pressure should be chosen to obtain the satisfactory degradation rate at the expense of relatively lower energy expenditure based on the water discharge standard. Besides, by increasing the hydrogen peroxide concentration, choosing the optimum value of ferrous sulfate concentration and solution pH can further enhance the cavitation yield. Many previous researchers carried out experimental studies on Rhodamine B degradation using cavitation based techniques. The experimental data and the corresponding references were listed in Table 9. It is very difficult to conclusively evaluate the energy efficiency of different cavitation reactors because the operating conditions and the hydraulic characteristics of system are diverse. To reduce the complexity, the maximum cavitation yields achieved under optimum conditions in are considered. Meanwhile, since the pressure and flow rate were not given in some references, the energy input is taken as the pump power consumption. For the degradation reaction of initial RB concentration 63.53 mg/L (Table 3), the total amount of 1562.27 mg RB removal was achieved after 120 min treatment. Therefore, Y1 and Y2 are 3.95 × 10−5 mg J−1 and 1.18 × 10−3 mg L J−1 . Compared with the maximum cavitation yield (Y1,max = 1.9 × 10−5 mg J−1 , Y2,max = 2.29 × 10−4 mg L J−1 , Fig. 11) obtained by different cavitation-based techniques [26,35–41] under optimum conditions listed in Ref. [6], the present treatment method shows certain advantages in large-scale operation and is likely to realize industrial operation.

4. Energy efficiency assessment 5. Conclusions As is suggested in Ref. [6], the cavitation yield per energy input (Y1 , Eq. (5)) and the cavitation yield per energy input into per treatment volume (Y2 , Eq. (6)) are introduced to assess the energy efficiency of present treatment method. The cavitation yields of the main double-cavitating-jet impingement rector in experiments

In this work, a novel double-cavitating-jets impinging device was introduced and the degradation of RB in aqueous solution was carried out based on combined treatment scheme of double-cavitating-jets impingement and Fenton chemistry.

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Table 8 Cavitation yield under different jet inlet pressures. H2 O2 concentration (mg/L)

FeSO4 concentration (mg/L)

Initial RB concentration (mg/L)

10 10 10 10

3.33 3.33 3.33 3.33

30.00 30.00 30.00 30.00

Jet inlet pressure (MPa)

6 8 10 12

Solution pH

Degradation Rate (%)

Y1 × 104 (mg/J)

Y2 × 102 (mg/J/L)

3 3 3 3

78.08 81.70 83.71 88.75

6.49 4.39 3.23 2.62

1.95 1.32 0.97 0.79

Table 9 The results by different cavitation based technique dealing with Rhodamine B. NO.

Cavitation apparatus

Technique

Water volume (L)

Electricity power input (W)

Time of operation (min)

Pollutant removed (mg)

Y1 (mg/J)

Y2 (mg/(J/L))

1 2 3 4 5

Swirling jet reactor [35] Orifice plate [36] Orifice plate [26] Venturi [26] 300 kHz ultrasonic generator [37] Multiple frequency (20 + 30 + 50 kHz) hexagonal cell [38] Swirling jet reactor [39] Venturi [26] Orifice plate [41] Venturi [26] Venturi [26] 24 kHz ultrasonic generator [40] 300 kHz ultrasonic generator [37] Present study

HC

20 50 4 4 0.3

3500 5500 1100 1100 60

180 60 120 120 140

187.2 43.7 9.2 10.4 1.5

4.95 × 10−6 2.21 × 10−6 1.16 × 10−6 1.31 × 10−6 2.97 × 10−6

9.90 × 10−5 1.10 × 10−4 4.64 × 10−6 5.25 × 10−6 8.93 × 10−7

7.5

900

30

2.2

1.36 × 10−6

1.04 × 10−4

HC and Fe HC and Fenton HC and CCl4 AC and H2 O2

25 4 30 4 4 0.2

2500 1100 5500 1100 1100 400

180 120 120 45 120 210

247.8 40.0 45.0 40.0 32.8 95.88

9.18 × 10−6 1.01 × 10−6 1.14 × 10−6 1.35 × 10−5 4.14 × 10−6 1.90 × 10−5

2.29 × 10−4 2.02 × 10−5 3.41 × 10−5 5.40 × 10−5 1.65 × 10−5 3.80 × 10−5

AC and CCl4

0.3

60

40

1.5

1.04 × 10−5

3.13 × 10−6

HC and Fenton

30

5500

120

1562.27

3.95 × 10−5

1.18 × 10−3

6

7 8 9 10 11 12 13 14

AC

HC and H2 O2

Note: AC represents acoustic cavitation and HC represents hydrodynamic cavitation.

The results indicated that the degradation reaction followed the pseudo-first-order kinetics. The synergistic effects between double-cavitating-jets impingement and Fenton chemistry were confirmed. The effects of solution concentrations were investigated in single variable experiments and orthogonal experiments. Jet inlet pressure and solution pH were also found to play a significant role on the degradation reactions. To optimize the degradation rate, general suggestions are as follows:

(1) Decreasing the initial pollutant concentration and increasing the hydrogen peroxide concentration will promote the degradation process. (2) There is an optimum value of ferrous sulfate concentration, which depends on the pollutant and hydrogen peroxide concentration. (3) Higher jet inlet pressure brings about more rapid degradation reactions due to the intensification of cavitation activity and jets impingement. (4) Acid condition is recommended for the degradation process since it greatly promotes the generation of hydroxyl radicals and impedes the recombination reaction among free radicals.

The present treatment method shows certain advantages in terms of cavitation yield in large-scale operation and is likely to realize industrial operation. Besides, by making use of the twocavitation-jets impinging technique, it can also reduce the demand of hydrogen peroxide concentration, weakening the coalescing effects between cavitation bubbles. Optimization of the nozzle structure and the distance between two nozzle outlet will be carried out in future work.

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