Intensified depolymerization of aqueous polyacrylamide solution using combined processes based on hydrodynamic cavitation, ozone, ultraviolet light and hydrogen peroxide

Ultrasonics Sonochemistry 31 (2016) 371–382

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Intensified depolymerization of aqueous polyacrylamide solution using combined processes based on hydrodynamic cavitation, ozone, ultraviolet light and hydrogen peroxide Amrutlal L. Prajapat, Parag R. Gogate ⇑ Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 400 019, India

a r t i c l e

i n f o

Article history: Received 22 December 2015 Received in revised form 20 January 2016 Accepted 21 January 2016 Available online 21 January 2016 Keywords: Depolymerization Polyacrylamide Hydrodynamic cavitation Combined approaches Intrinsic viscosity reduction

a b s t r a c t The present work deals with intensification of depolymerization of polyacrylamide (PAM) solution using hydrodynamic cavitation (HC) reactors based on a combination with hydrogen peroxide (H2O2), ozone (O3) and ultraviolet (UV) irradiation. Effect of inlet pressure in hydrodynamic cavitation reactor and power dissipation in the case of UV irradiation on the extent of viscosity reduction has been investigated. The combined approaches such as HC + UV, HC + O3, HC + H2O2, UV + H2O2 and UV + O3 have been subsequently investigated and found to be more efficient as compared to individual approaches. For the approach based on HC + UV + H2O2, the extent of viscosity reduction under the optimized conditions of HC (3 bar inlet pressure) + UV (8 W power) + H2O2 (0.2% loading) was 97.27% in 180 min whereas individual operations of HC (3 bar inlet pressure) and UV (8 W power) resulted in about 35.38% and 40.83% intrinsic viscosity reduction in 180 min respectively. In the case of HC (3 bar inlet pressure) + UV (8 W power) + ozone (400 mg/h flow rate) approach, the extent of viscosity reduction was 89.06% whereas individual processes of only ozone (400 mg/h flow rate), ozone (400 mg/h flow rate) + HC (3 bar inlet pressure) and ozone (400 mg/h flow rate) + UV (8 W power) resulted in lower extent of viscosity reduction as 50.34%, 60.65% and 75.31% respectively. The chemical structure of the treated PAM by all approaches was also characterized using FTIR (Fourier transform infrared) spectra and it was established that no significant chemical structure changes were obtained during the treatment. Overall, it can be said that the combination of HC + UV + H2O2 is an efficient approach for the depolymerization of PAM solution. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction High molecular weight polyacrylamide (PAM) and its derivatives are important commercial polymers and have wide range of applications as flocculants in water treatment, as additives to enhance the oil recovery, as hydrogels in many biomedical applications and in the paper and textile industries as a thickening agent [1–4]. Similarly low molecular weight copolymers based on PAM are suitable as dispersing agents, coagulants, mud stabilizers, and crude oil thinners [5]. Due to the reduced mobility of the aqueous phase and consequently to improve the sweep efficiency, watersoluble polymers are used in many oil processing operations including oil recovery. There are many water-soluble polymers that can potentially be used in oil recovery applications. High molecular weight partially hydrolyzed polyacrylamides (PHPAM)

⇑ Corresponding author. E-mail address: [email protected] (P.R. Gogate). http://dx.doi.org/10.1016/j.ultsonch.2016.01.021 1350-4177/Ó 2016 Elsevier B.V. All rights reserved.

are most commonly used to increase the viscosity of water to improve oil displacement giving enhanced oil recovery. PHPAM is widely used in oil fields and plays a key role in tertiary oil recovery because of property of yielding modifications in the viscosity. PHPAM is also extensively used in mobility control of fluids in porous media because of its prominent performance [6]. During synthesis stage, it is not always feasible to obtain polymers with desired molecular weight (MW) and molecular weight distribution (MWD); thus the MW and MWD of commercially available samples of PAM and its copolymers are often far from the optimum required for a given application. It is important to develop reliable and fast method for obtaining the desired MW of commercial polyacrylamide. A promising way to process polyacrylamide to obtain the desired MW characteristics would be based on depolymerization where the chain scission or structure reformation can be obtained by mechanical, chemical and biological degradation methods [7]. Thermal depolymerization is the widely used conventional method for inducing polymer degradation whereas chemical, mechanical, enzymatic and acid based

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approaches are also common. Even though the enzymatic and acidic methods are convenient, these approaches offer drawback of higher treatment time and higher treatment costs especially in the case of enzymatic methods and it is difficult to achieve uniform molecular weight distribution after degradation [9]. Chemical methods are generally not considered as greener approaches due to the use of toxic chemicals. Some of the newer methods for depolymerization include methods based on the use of cavitation and irradiations such as UV or microwave. Cavitation based approach for polymer degradation is unique and fast because of the fragmentation at the center of polymer chain as well as can give uniform reduction in the intrinsic viscosity. Lu et al. [1] investigated the chemical degradation of PAM by different oxidation processes such as UV/H2O2, Fenton, UV/Fenton, visible light/Fen2
ton, visible light/Fenton/C2O2
4 (oxalate), UV/Fenton/C2O4 , visible 2
light/Fenton/C4H4O2
(tartrate) and UV/Fenton/C H and 6 4 4O6 reported higher degradation efficiency in the order of UV/Fenton/ 2
2
C 4 H 4 O 2
6 > UV/Fenton/C 2 O 4 > visible light/Fenton/C 4 H 4 O 6 > visible light/Fenton/C2O2
4 > UV/Fenton > visible light/Fenton > UV/ H2O2 > Fenton. Yen and Yang [8] studied the effect of ultrasound on the polyacrylamide solution at different irradiation time (0–400 min), reaction temperature (20 °C, 30 °C, 40 °C and 50 °C) and reaction concentration (0.50, 0.75 and 1 g/dl). It was reported that the ultrasound induced degradation of PAM solution increased with an increase in the irradiation time and reaction temperature as well as with a decrease in the solution concentration. Though ultrasound induced PAM depolymerization has been reported to be efficient, literature analysis revealed that hydrodynamic cavitation, which also generates similar conditions in an more efficient manner, has not been applied for the degradation of polyacrylamide solution. The present work deals with application of hydrodynamic cavitation for polyacrylamide depolymerization. Hydrodynamic cavitation (HC) has been proposed as an alternative method of generating cavitating conditions for polymer degradation, also giving superior scale-up potential with lower investment and the operating costs [10,11]. Hydrodynamic cavitation is generated when a liquid is passed through a constriction like throttling valve, venturi, orifice etc. [11]. The conditions of collapsing cavity downstream of the constriction generates effects like intense turbulence, liquid streaming at micro level as well as hot spots similar to the ultrasound assisted cavitation. There have been some reports related to the use of hydrodynamic cavitation reactors for the depolymerization dealing with understanding the effect of operating parameters for different type of cavitating devices as orifice plate, throttling valve and venturi [10,12]. Prajapat and Gogate [10] studied the depolymerization of guar gum solution using hydrodynamic cavitation and investigated the effect of initial concentration of polymer, inlet pressure, geometry of cavitating device, operating temperature and addition of potassium persulfate (KPS) on the extent of viscosity reduction. It was reported that the use of hydrodynamic cavitation alone under optimized conditions (slit venturi at 3 bar pressure) resulted in about 74% reduction in intrinsic viscosity whereas addition of 1 g/L KPS enhanced the extent of intrinsic viscosity reduction to 98%. It was also reported that the depolymerization of guar gum solution depends on the geometry of the cavitating devices, upstream pressure and KPS loadings. Huang et al. [12] studied the degradation of chitosan using hydrodynamic cavitation and reported that lower initial concentration, lower pH, higher upstream pressure and higher treatment time were favorable for the degradation of chitosan solution. It was also reported that the degradation of chitosan depends on the geometry of the orifice plate with larger number of holes and smaller hole diameter favoring the extent of chitosan degradation. During the treatment using hydrodynamic cavitation reactors, the molecular chains get shorter due to the breakage resulting from the physical effects of cavitation

and the number of bonds also decreases due to the shear induced bond breakage as well as due to the OH radical attack. A detailed analysis of the literature reveals that there has been no work on the application of hydrodynamic cavitation for polyacrylamide depolymerization and in general also, limited studies are available for polymer depolymerization. In the present work, depolymerization of aqueous solution of polyacrylamide has been investigated using hydrodynamic cavitation based on the use of slit venturi as a cavitating device. Use of hydrogen peroxide, ultraviolet light and ozone can intensify the depolymerization rate due to the production of oxidizing radicals (OH) and hence were used as an intensifying approach to enhance the depolymerization quantified in terms of the change in the viscosity of polymer solution. Viscometry is a reliable and practical method for monitoring the degree of depolymerization in aqueous solutions though more sophisticated methods like Gel Permeation Chromatography can also be used to monitor the changes in the molecular weight distribution. Further, to check whether the chemical identity of the polyacrylamide solution is retained after treatment with hydrodynamic cavitation, FTIR analysis for the untreated and treated polyacrylamide solution was also performed. The novelty of the present work lies in developing the combined process based on the use of hydrodynamic cavitation, ultraviolet light, hydrogen peroxide and ozone for efficient depolymerization.

2. Materials and methods 2.1. Materials Polyacrylamide was obtained from Himedia labs Pvt. Ltd. Mumbai. Hydrogen peroxide (H2O2) (30%, w/v) was procured from S.D. Fine Chemicals Ltd., Mumbai, India. All the chemicals were of analytical grade quality and used as received from the supplier. Distilled water has been used as a solvent to prepare the required solutions freshly in the laboratory.

2.2. Experimental setup 2.2.1. Hydrodynamic cavitation reactor The schematic representation of the hydrodynamic cavitation set up and cavitating device (slit venturi) used in the present work has been shown in Fig. 1(a) and (b). A closed loop system with holding tank, a reciprocating pump of power rating 1.1 kW, control valves, flanges to accommodate the cavitating device (slit venturi having a total length of 87 mm with convergent and divergent section length as 20 mm and 65 mm respectively), a bypass line to control the flow through the main line and a glass rotameter constitutes the entire assembly. Half angle of convergent section was 23.5° whereas for the divergent section, half angle was 5.5°. The dimensions of throat are W = 6 mm, H = 1.9 mm and L = 1.9 mm. Detailed description of the flow loop has been described in our earlier work [10].

2.2.2. Ultraviolet (UV) irradiation assembly A single UV tube (Philips TUV 8 W/G8T5), having power rating of 8 W and dominant emitting wavelength of 254 nm was used as a source of ultraviolet irradiation. Different UV tubes with a power rating of 8 W each have been used with an objective of investigating the dependency of the intrinsic viscosity reduction on the power dissipation. The tubes were placed inside the holding tank and the top of the holding tank was covered using black cover to avoid the diffusion of the irradiations outside the holding tank.

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373

Fig. 1. Schematic representation of (a) hydrodynamic cavitation reactor setup (b) front view of slit venturi.

2.2.3. Ozone (O3) generator A laboratory scale ozone generator (Model-DOZ400) was procured from Eltech Engineering, Mumbai, with a rated maximum output flow of 400 mg/h. Ozone was generated from air using a laboratory-scale oxygen generator and continuously bubbled into the polyacrylamide solution holding tank using a ceramic diffuser. The ozone injection point was decided based on the results of our earlier investigation [13] where it was confirmed that lower cavitational effects are obtained when ozone is injected at the cavitating device as compared to the holding tank. Different flow rates as 100, 200, 300, and 400 mg/h have been used in the present work for investigating the effect of ozone loading. 2.3. Experimental methodology All experiments were performed using fixed volume (5 L) with constant treatment time of 3 h. The concentration of

polyacrylamide solution was kept constant at 0.2% (w/v) for all the studies related to understanding the effect of inlet pressure and combined operation with UV irradiations, H2O2 and ozone. The effect of inlet pressure was studied over the range of 1–4 bar whereas constant reaction temperature of 32 °C was used in the present work. Constant temperature has been selected during all the experiments as our previous studies have shown marginal effect of temperature in the case of hydrodynamic cavitation [14]. Experiments were performed for different treatment strategies such as only hydrodynamic cavitation, only ultraviolet irradiation, only ozone, combination of HC + UV, HC + O3, HC + H2O2, UV + O3, UV + H2O2, HC + UV + O3 and HC + UV + H2O2. The loading of H2O2 was varied over the range of 0.01–0.2% to investigate the degree of intensification. Similarly the effect of ozone (O3) loading was also investigated over a range of 100–400 mg/h. At regular intervals of 30 min, samples were collected to monitor the progress of depolymerization. The withdrawn samples

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70

Intrinsic Viscosity (ml/g)

60

1 Bar

50 40

2 Bar

30

3 Bar

20

4 Bar

10 0 0

30

60

90

120

150

180

210

Time (min) Fig. 2. Effect of inlet pressure on the extent of intrinsic viscosity reduction.

were cooled to 30C in cold water bath to quench the reaction and intrinsic viscosity reduction was quantified using glass capillary Ubbelohde viscometer. All the details related to the calculation of intrinsic viscosity as well as kinetic model for the estimation of depolymerization rate constant have been described in our earlier work [10]. The rate constant was calculated based on the graph of ln(A) against time t (min) where ðA ¼ g0
g1 =gt
g1 Þ prepared for all the experimental runs knowing the initial intrinsic viscosity ðg0 Þ, intrinsic viscosity at time t ðgt Þ and the limiting intrinsic viscosity ðg1 Þ of the polymer solution. The chemical structural identity of native polyacrylamide and processed polyacrylamide based on different approaches (after 3 h treatment) was analyzed using Fourier transform infrared spectra (FTIR) (MIRacle-10, Shimadzu). 3. Results and discussion 3.1. Effect of inlet pressure The obtained results for the effect of inlet pressure have been depicted in the Fig. 2 in terms of the extent of intrinsic viscosity reduction. The kinetic data of the first order degradation rate constant has been given in Table 1. The extent of intrinsic viscosity reduction increased with an increase in the inlet pressure from 1 bar till an optimum of 3 bar, beyond which the rate of depolymerization decreased at pressure of 4 bar. The results of kinetic studies (Table 1) showed that order rate constant increased from 0.014 min
1 to 0.02 min
1 with an increase in pressure from 1 bar to 3 bar beyond which it decreased to 0.017 min
1 at operating pressure of 4 bar. Maximum extent and rate of depolymerization of polyacrylamide solution obtained at optimum pressure was 35.38% and 0.02 min
1 respectively. Initially, with the use of higher operating pressures (below the optimum), the intense collapse of cavities intensifies the extent of dissociation of water molecules as well as produces intense shear effects. Enhanced dissociation rates generate higher quantum of free hydroxyl radicals (OH) which increases the extent of viscosity reduction of polyacrylamide solution. Based on these results, inlet pressure of 3 bar has been chosen as the optimum pressure for studying the effect of other parameters and combination approaches. In hydrodynamic cavitation, the enhanced depolymerization with an increase in inlet pressure can also be explained on the basis of cavitation number, defined as follows

CV ¼

P2
PV 1 2

qv 20

where, P2 is the fully recovered downstream pressure, PV is the vapor pressure of the liquid, q is the density of solvent, v 0 is the velocity at the throat of the constriction which can be measured by knowing the area of the opening and main line flow rate. An increase in the flow through the main line with an increase in the pressure results in increase in the velocity at the throat, which gives lower cavitation number. In the present study, cavitation number has been observed to decrease from 0.45 to 0.21 with an increase in pressure from 1 bar to 3 bar confirming higher cavitational intensity and hence higher extent of viscosity reduction is obtained. Similar results have been reported in the literature [10,12]. Prajapat and Gogate [10] investigated the effect of inlet pressure for the depolymerization of guar gum and reported the existence of optimum inlet pressure as 3 bar beyond which reduction in the extent of depolymerization has been observed. Huang et al. [12] investigated the effect of upstream pressure over the operating range of 0.1–0.5 MPa using orifice plate for the degradation of chitosan with solution concentration of 0.5 g/L and treatment time of 3 h at pH 4.4. It has been reported that with an increase in the upstream pressure from 0.1 MPa to 0.5 MPa, the intrinsic viscosity reduction also increased from 27% to 48%. It can be said that the existence of optimum pressure depends on the type of the cavitating device as well as the range of operating pressures investigated and hence the importance of the present work to establish the optimum pressure is confirmed. 3.2. Effect of ultraviolet (UV) irradiation The obtained results for the effect of power dissipation (8 W and 16 W) in the case of only UV on the extent of viscosity reduction have been presented in Fig. 3. It has been observed that the extent of the intrinsic viscosity reduction obtained for different UV power as 8 W and 16 W were 40.83% and 42.31% respectively. During the treatment with ultraviolet irradiation, water soluble polymer chain can undergo rupture to smaller chains due to the formation of hydroxyl (OH) radicals which induce the breakage and results in the viscosity reduction of the polymer solution [15–17]. UV irradiation can also rupture a carbonAcarbon bond of polymer backbone chain and form carbon radicals [3] and the subsequent radical termination reactions will involve disproportionation with a consequent lowering of the molecular weight and viscosity. The covalently unsaturated chromophore groups present in the polymer structure are responsible for the molecular and electronic absorption in the UV–visible region of the spectrum

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A.L. Prajapat, P.R. Gogate / Ultrasonics Sonochemistry 31 (2016) 371–382 Table 1 Kinetic rate constant, extent of viscosity reduction and final intrinsic viscosity values for various treatment approaches. Approach

Intrinsic viscosity g (ml/g)

Extent of viscosity reduction (%)

Kinetic rate constant (min
1)

R2

Only HC Inlet pressure (bar) 1 2 3 4

44.03 41.05 37.46 40.6

24.05 29.19 35.38 29.96

0.014 0.017 0.02 0.017

0.961 0.941 0.977 0.934

Only UV 8W 16 W

34.3 33.44

40.83 42.31

0.024 0.025

0.988 0.945

Only ozone 100 mg/h 200 mg/h 300 mg/h 400 mg/h

40.54 36.15 32.72 28.79

30.07 37.64 43.56 50.34

0.017 0.022 0.025 0.029

0.936 0.999 0.943 0.932

Combined approaches 8 W UV + 3 bar HC 3 bar HC + 0.01% H2O2 8 W UV + 0.01% H2O2 3 bar HC + 0.1% H2O2 8 W UV + 0.1% H2O2 3 bar HC + 0.2% H2O2 8 W UV + 0.2% H2O2 3 bar HC + 8 W UV + 0.01% H2O2 3 bar HC + 8 W UV + 0.1% H2O2 3 bar HC + 8 W UV + 0.2% H2O2 100 mg/h ozone + 3 bar HC 200 mg/h ozone + 3 bar HC 300 mg/h ozone + 3 bar HC 400 mg/h ozone + 3 bar HC 100 mg/h ozone + 8 W UV 200 mg/h ozone + 8 W UV 300 mg/h ozone + 8 W UV 400 mg/h ozone + 8 W UV 100 mg/h ozone + 8 W UV + 3 bar 200 mg/h ozone + 8 W UV + 3 bar 300 mg/h ozone + 8 W UV + 3 bar 400 mg/h ozone + 8 W UV + 3 bar

24.12 32.64 17.8 23.81 7.32 10.28 6.08 12.5 4.69 1.58 35.4 31.14 28.33 22.81 26.29 21.24 19.73 14.31 18.85 15.75 12.5 6.34

58.39 43.70 69.29 58.93 87.37 82.27 89.51 78.44 91.91 97.27 38.93 46.28 51.13 60.65 54.65 63.36 65.97 75.31 67.48 72.83 78.44 89.06

0.029 0.025 0.04 0.034 0.051 0.048 0.052 0.046 0.054 0.057 0.022 0.027 0.03 0.035 0.032 0.037 0.038 0.044 0.039 0.042 0.046 0.052

0.988 0.942 0.983 0.923 0.973 0.934 0.933 0.987 0.989 0.995 0.944 0.981 0.952 0.972 0.983 0.978 0.979 0.973 0.922 0.911 0.975 0.961

HC HC HC HC

[18]. In the case of UV irradiation, only 2% difference in the extent of viscosity reduction of PAM in aqueous solution was observed with an increase in power from 8 W to 16 W, indicating that the extent of increase in the depolymerization of PAM caused by enhanced power of direct photolysis is very limited. The kinetic rate constant (Fig. 4) was found to be 0.024 min
1 and 0.025 min
1 for power dissipation of 8 W and 16 W respectively. It appears that the effect of power dissipation is dependent on the type of polymer mainly in terms of water soluble natural polymer or the synthetic polymers. In our earlier work [17], effect of power on the ultraviolet induced degradation of aqueous solution of guar gum was investigated over the power dissipation range of 8–32 W and it was established that the extent of viscosity reduction increased from 33.38% to 77.94% with an increase in the UV power dissipation from 8 W to 32 W, which is quite significant as compared to that observed in the present work. The observed trends clearly establish the need of detailed understanding into effect of power dissipation and the methodology presented here would be very useful. When the polyacrylamide solution was treated with hydrodynamic cavitation (3 bar) combined with ultraviolet radiation, the intrinsic viscosity of PAM solution decreased from 57.97 ml/g to 24.12 ml/g in 180 min of treatment. In the combined HC + UV irradiation treatment, the extent of viscosity reduction (58.4%) was higher than the values obtained by the UV irradiation alone (40.8%) and the individual HC treatment under optimum conditions (35.38%). The obtained results confirmed that HC is the primary contributor for the depolymerization of the polymer and

the effect of UV irradiation is only to accelerate the depolymerization in the initial time period. 3.3. Combined treatment involving H2O2, HC and UV To investigate the effect of combination approach on depolymerization of polyacrylamide, 5 L PAM solution (0.2% w/v) was treated with different concentrations of hydrogen peroxide combined with ultraviolet radiation (8 W) and hydrodynamic cavitation (3 bar) for 180 min at optimized condition of inlet pressure. The intrinsic viscosity reduction was plotted as a function of the treatment time and the results are depicted in Fig. 5(a) –(c). Table 1 represents the kinetic rate constant obtained for the depolymerization of polyacrylamide. It was observed that combination of ultraviolet irradiation and hydrodynamic cavitation in the presence of 0.01% H2O2 resulted in a decrease in the intrinsic viscosity of polyacrylamide from 57.97 ml/g to 12.5 ml/g (Fig. 5 a). The extent of viscosity reduction was 78.44%. For 0.1% and 0.2% loading of H2O2, the extent of intrinsic viscosity reduction was 91.91% and 97.27% respectively. When PAM solution was treated with 0.01%, 0.1% and 0.2% H2O2 along with only HC, the intrinsic viscosity decreased from 57.97 ml/g to 32.64 ml/g, 23.81 ml/g and 10.28 ml/g respectively (Fig. 5b). The corresponding extents of viscosity reduction were 43.70%, 58.93% and 82.27% respectively. From Fig. 5(b), it can be also observed that the initial viscosity reduced to only 47.45 ml/g due to the addition of 0.2% H2O2 with circulation only without HC and the extent of viscosity reduction was 18.15%.

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70

Intrinsic Viscosity (ml/g)

60

8W UV

50

16W UV

40 30

8W UV + 3 Bar

20 10 0 0

30

60

90

120

150

180

210

Time (min) Fig. 3. Effect of UV power and HC + UV combination on the extent of intrinsic viscosity reduction.

5.00 4.50 4.00

Ln (A)

3.50

y = 0.024x R² = 0.988

8W UV

y = 0.025x R² = 0.945

16W UV

y = 0.029x R² = 0.988

8W UV + 3 Bar

3.00 2.50 2.00 1.50 1.00 0.50 0.00 0

50

100

150

200

Time (min) Fig. 4. Plot of ln(A) against irradiation time for the effect of UV power on extent of viscosity reduction.

When PAM solution was treated with ultraviolet irradiation combined with 0.01%, 0.1% and 0.2% H2O2 (Fig. 5c), the reduction in the intrinsic viscosity of PAM was from 57.97 ml/g to 17.8 ml/g, 7.32 ml/g and 6.08 ml/g with an extent of viscosity reduction as 69.29%, 87.37% and 89.51% respectively. It has been observed that due to the addition of H2O2, a significant enhancement in the PAM depolymerization rate is obtained extent of which increases with an increase in the H2O2 loading. This is due to the fact that in the presence of UV radiation, higher quantum of radicals is generated by UV induced dissociation of hydrogen peroxide giving faster polymer degradation [16]. During irradiation of short-wave ultraviolet wavelengths (200–280 nm), the highest hydroxyl radical yields are obtained because of the stronger absorption by the peroxide at lower wavelengths [1]. Similar results were reported by Prajapat and Gogate [17] for the degradation of guar gum solution using 8 W UV irradiations in the presence of H2O2 over the range of 0.01–0.1% loading. It was reported that with an increase in the H2O2 concentration from 0.01% to 0.1%, the extent of intrinsic viscosity reduction increased from 93.94% to 99.08%. It is expected that dissociation of hydrogen peroxide in the presence of UV irradiations is significantly higher as compared to the hydrodynamic cavitation based dissociation. From Table 1 it can been seen that in the presence of 0.01% H2O2

loading, extent of increase in the viscosity reduction for UV (8 W power) was 69.29% whereas for HC (3 bar) it was 43.70%. It has been established that the ultraviolet radiation-induced accelerated depolymerization of polyacrylamide is dominant as compared to HC induced dissociation. Also, the UV + HC + H2O2 treatment was more effective than HC + H2O2 and UV + H2O2 treatment. 3.4. Degradation of PAM using ozone For performing experiments related to the use of only ozone for polymer degradation, the main line valve was completely closed such that the flow was not allowed to pass through the cavitation chamber and the efficacy of only ozone induced in the liquid holding tank for the intrinsic viscosity reduction of PAM solution was investigated. The outlet of ozone generator was placed in the solution feed tank so that ozone was introduced directly in the solution well below the liquid level. Four different loadings of ozone as 100, 200, 300 and 400 mg/h were used and the obtained results for the effect of varying ozone flow rate on the extent of viscosity reduction of PAM in 180 min treatment has been depicted in Fig. 6. It was observed that about 30–50% depolymerization of PAM was obtained in 180 min for the varying ozone flow over the range of

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377

70

3 Bar + 8W UV + 0.01% H2O2

Intrinsic Viscosity (ml/g)

60 50

3 Bar + 8W UV + 0.1% H2O2

40 30

3 Bar + 8W UV + 0.2% H2O2

20 10 0 0

30

60

90

120

150

180

210

Time (min)

(a) 70

3 Bar + 0.01% H2O2

Intrinsic Viscosity (ml/g)

60

3 Bar + 0.1% H2O2

50 40

3 Bar + 0.2% H2O2

30

0.2% H2O2 only

20 10 0 0

30

60

90

120

150

180

210

Time (min)

(b) 70

8W UV + 0.01% H2O2

Intrinsic Viscosity (ml/g)

60

8W UV + 0.1% H2O2

50

8W UV + 0.2% H2O2 40 30 20 10 0 0

30

60

90

120

150

180

210

Time (min)

(c) Fig. 5. Effect of different concentrations of hydrogen peroxide combined with (a) HC + UV (b) hydrodynamic cavitation (3 bar) (c) ultraviolet radiation (8 W) for 180 min treatment at optimized conditions.

100–400 mg/h respectively. The intrinsic viscosity of PAM decreased from initial 57.97 ml/g to 40.54 ml/g and from 57.97 ml/g to 28.79 ml/g in 180 min of treatment time for ozone

loading as 100 and 400 mg/h respectively. The first order rate constant obtained at different ozone flow rate of 100, 200, 300 and 400 mg/h were 0.017 min
1, 0.022 min
1, 0.025 min
1 and

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70

100 mg/h + circulaon

Intrinsic Viscosity (ml/g)

60

200 mg/h + circulaon

50

300 mg/h + circulaon

40

400 mg/h + circulaon

30 20 10 0 0

30

60

90

120

150

180

210

Time (min) Fig. 6. Effect of ozone on PAM depolymerization.

0.29 min
1 respectively. Similar results for degradation of chitosan were reported by Yue et al. [19] for the varying ozone dosage of 35 ± 5 mg/min, 50 ± 5 mg/min and 65 ± 5 mg/min for 30 min treatment. It was reported that the viscosity-average molecular weight of chitosan decreased from 667 to 459, 420 and 391 kDa in 30 min, respectively. The extent of decrease in viscosity-average molecular weight increased from 31.18% to 41.38% for an increase in the flow rate from 35 ± 5 mg/min to 65 ± 5 mg/min. Prajapat and Gogate [17] and Yue et al. [20] have also studied the effect of ozone on guar gum and chitosan polymers respectively and reported similar results. It is also important to understand that the extent of increase in the intrinsic viscosity reduction obtained at varying ozone flow rates is indeed dependent on the type of polymer and hence the importance of the present work is established. 3.5. Combined approaches based on ozone (O3), HC and UV for degradation of PAM In order to investigate whether the synergetic effect is obtained for the depolymerization of polyacrylamide solution using combined approaches, treatment was performed with different flow rates of ozone (100, 200, 300 and 400 mg/h) combined with ultraviolet irradiation (8 W power) and hydrodynamic cavitation (3 bar) for 180 min of total treatment time. The corresponding intrinsic viscosity reduction as a function of the time has been illustrated in Fig. 7(a)–(c). Table 1 represents the kinetic rate constant of degradation of polyacrylamide for these treatment approaches. For the combined operation of ozone and hydrodynamic cavitation, four different flow rates of ozone (O3) as 100, 200, 300 and 400 mg/h were used in the study. It was observed that combined approach of O3 + HC resulted in an enhanced reduction in the intrinsic viscosity of PAM from 57.97 ml/g to 35.5, 31.14, 28.33 and 22.81 ml/g for 180 min of treatment time at different flow rates as 100, 200, 300 and 400 mg/h, respectively (Fig. 7a). The corresponding extent of depolymerization of PAM was observed to be 38.93%, 46.28%, 51.13% and 60.65%, respectively. When the polyacrylamide solution was treated with ozone combined with ultraviolet radiation, the intrinsic viscosity of PAM decreased from 57.97 to 26.29, 21.24, 19.73 and 14.31 ml/g respectively with extent of depolymerization being 54.65%, 63.36%, 65.97% and 75.31% respectively (Fig. 7b). The depolymerization of PAM by combination of ozone, hydrodynamic cavitation and ultraviolet radiation was also investigated and results have been depicted in Fig. 7(c). When the polyacry-

lamide solution was treated with O3 + UV + HC, the intrinsic viscosity of PAM decreased from 57.97 to 18.85, 15.75, 12.50 and 6.34 ml/g respectively with extent of depolymerization as 67.48%, 72.83%, 78.44% and 89.06% respectively. The enhanced degradation of PAM in the combined approach as compared to individual approach is attributed to the dual oxidation mechanism based on molecular ozone and hydroxyl radicals formed due to decomposition of ozone in the presence of UV light. The higher extent of viscosity reduction for the combination with UV irradiation can be attributed to the mechanism of ozone decomposition. UV irradiation was reported to significantly enhance the dissociation of ozone [20–23] as compared to hydrodynamic cavitation. The enhanced dissociation of ozone in the PAM solution can result in the formation of enhanced OH radicals. Generally it can be established that the enhanced depolymerization of the polymer obtained due to the combined effects depends on the reactivity of hydroxyl radicals with the polymer. The enhanced generation of hydroxyl radicals, which are powerful oxidizing agents and can attack the chain of polyacrylamide polymer, may play a significant role for the acceleration of degradation of polyacrylamide treated with ozone combined with ultraviolet radiation. The degree of intensification would be dependent on the type of polymer and reactivity with hydroxyl radicals and need to be established with studies similar to that reported in present work. Overall, the combined approach of O3 + UV + HC, resulted in higher extent of depolymerization as compared to the HC + O3 and UV + O3 approaches. 3.6. Mechanisms of intensification for combination approaches of H2O2 + UV, O3 + UV and O3 + HC 3.6.1. Case-I: hydrogen peroxide and ultraviolet irradiation The direct photolysis of H2O2 under UV irradiation leads to the formation of OH radicals [24]:

H2 O2 þ hm ! 2 OH Furthermore HO
2 , which is in an acid–base equilibrium with H2O2, absorbs the UV irradiation of the wavelength 254 nm leading to formation of OH radicals

H2 O2 $ HO
2 þ Hþ HO
2 þ hm !  OH þ O


A.L. Prajapat, P.R. Gogate / Ultrasonics Sonochemistry 31 (2016) 371–382

70

100 mg/h + 3 Bar

60

Intrinsic Viscosity (ml/g)

379

200 mg/h + 3 Bar

50

300 mg/h + 3 Bar

40

400 mg/h + 3 Bar

30 20 10 0 0

30

60

90

120

150

180

210

Time (min)

(a) 70

100 mg/h + 8 W UV

Intrinsic Viscosity (ml/g)

60

200 mg/h + 8 W UV

50 40

300 mg/h + 8 W UV

30

400 mg/h + 8 W UV

20 10 0 0

30

60

90

120

150

180

210

Time (min)

(b) 70

100 mg/h + 8 W UV+ 3 Bar

Intrinsic Viscosity (ml/g)

60

200 mg/h + 8 W UV+ 3 Bar

50 40

300 mg/h + 8 W UV+ 3 Bar

30

400 mg/h + 8 W UV+ 3 Bar

20 10 0 0

30

60

90

120

150

180

210

Time (min)

(c) Fig. 7. Effect of different flow rates of ozone combined with (a) HC (b) UV (8 W) (c) HC + UV for 180 min at optimized conditions.

UV + H2O2 approach has been successfully used for the degradation of polyacrylamide [1] and polyvinyl alcohol [16,25]. Ghafoori et al. [25] successfully employed the degradation of aqueous poly-

vinyl alcohol using UV/H2O2 in a photoreactor. Lu et al. [1] also reported the beneficial effects for the chemical degradation of polyacrylamide based on combined advanced oxidation processes.

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3.6.2. Case-II: ozone and ultraviolet irradiation Dissociation of the molecular ozone in water under UV irradiation is accompanied by the formation of hydrogen peroxide and free radicals [22,26–27] based on following two mechanisms (a) UV irradiation of ozone initially results in formation of hydrogen peroxide and later free radicals

O3 þ H2 O ! O2 þ H2 O2 H2 O2 ! 2 OH (b) direct generation of free radicals as a result of interaction of UV and ozone

O3 þ H2 O ! O2 þ 2 OH It has been reported that the possibility of generation of free radicals directly from ozone and hydrogen peroxide depends on the UV irradiation intensity [22,26,27]. 3.6.3. Case-III: ozone and Hydrodynamic cavitation The free hydroxyl radicals are generated from cavitation of H2O in the case of hydrodynamic cavitation [19,28]:

H2 OþÞÞÞ !  OH þ H When polyacrylamide solution was treated with ozone in combination with hydrodynamic cavitation, the hydroxyl radicals can be generated based on following sequence of reactions

H2 OþÞÞÞ !  OH þ H O3 ! O2 þ O O þ H2 O ! 2 OH When polyacrylamide solution was treated with ozone in the presence of hydrodynamic cavitation, oxygen is produced in the collapsing bubble. Additional OH radicals are generated under an oxygen atmosphere, by the dissociation of molecular oxygen in the bubble as per reaction given above. In combined approach of HC + O3, the turbulence generated by hydrodynamic cavitation enhances the oxidizing action of molecular ozone leading to better utilization. The reactive species formed due to decomposition of ozone i.e. emerging oxygen and hydroxyl radicals in the vapor phase of a cavitation bubble significantly

enhances the production of hydroxyl radical from dissociation of ozone. Hence, due to higher availability of the hydroxyl radical for the attack on the polymer chain, the viscosity of PAM decreases at a faster rate. 3.7. FTIR analysis of PAM solution before and after treatment using different approaches In order to investigate the changes in the molecular structure of PAM induced by different treatments, FTIR spectra of PAM samples subjected to different treatments were analyzed. Fig. 8 showed the spectra of native PAM (a), HC treated PAM (b), PAM after H2O2 treatment (c) and PAM subjected to ozone treatment (d) respectively. It can be observed from figure that the FTIR spectrum of the treated PAM is mostly superimposable over that of the native PAM. The characteristic bands for acrylamide were observed at 3400– 3100, 1631, 1550 and 1470 cm
1 ascribed to the symmetric and anti-symmetric vibrations of NH2 (3330–3100 cm
1), the symmetric stretching vibrations of the C@O on the amide group (1631 cm
1), symmetric stretching vibrations of the C@O on the carboxyl group (1500 cm
1) [29] and stretching vibration of CAN (1470 cm
1) [30]. NAH wagging vibrations were represented at 557, 597 and 696 cm
1 respectively [31] and band region between the 500 cm
1 and 700 cm
1 represented the crystallinity of the polymer [32]. The FTIR results have clearly established that there were no significant changes in the chemical structure and functional groups during the treatment. Hence, the free radical induced viscosity reduction was only based on the breakage of the polymer structure without any changes in the activity or the functional properties. 3.8. Comparison of different approaches The comparison of the different depolymerization processes of PAM have been now discussed in terms of the extent of depolymerization. Fig. 9 shows the comparison of extent of depolymerization obtained using different techniques. It can be observed that combination of HC (3 bar) + UV (8 W) + H2O2 (0.2%) is the most effective combination process resulting in higher depolymerization (97.27%) in a treatment time of 180 min. In the case of UV (8 W) + HC (3 bar), the depolymerization extent was 58.39% in

Fig. 8. FTIR analysis of native PAM as well as treated PAM (a = native PAM, b = HC treated PAM, c = PAM after H2O2 treatment and d = ozone treated PAM).

381

Extent of Degradaon (%)

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100 90 80 70 60 50 40 30 20 10 0

91.91 97.27

89.06

78.44

75.31 60.65

58.39 35.38

50.34

40.83

18.15

Fig. 9. Comparison of percentage degradation obtained using different combination approaches.

treatment time of 180 min whereas approaches using only HC (3 bar) and UV (8 W) gave extent of viscosity reduction as 35.38% and 40.83%. In the case of approach involving ozone, it has been observed that higher extent of viscosity reduction was obtained by using combination of 400 mg/h (ozone) + UV (8 W) + HC (3 bar) i.e. 89.06% as compared to only ozone (400 mg/h), ozone (400 mg/h) + HC (3 bar) and ozone (400 mg/h) + UV (8 W) approach where the extent of viscosity reduction was 50.34%, 60.65% and 75.31% respectively. Overall, it can be said that combination of different approaches using HC, ozone and UV irradiations for polymer depolymerization is beneficial and significant intensification can be obtained as compared to individual processes. 4. Conclusions The detailed investigation of depolymerization of polyacrylamide polymer using different approaches revealed that hydrodynamic cavitation in combination with UV, ozone and H2O2 is more effective, which can be attributed to the enhanced formation of free radicals in the polymer solution. Following guidelines can be established based on the obtained results:  Hydrodynamic cavitation is effective for the depolymerization of polyacrylamide solution at optimum operating pressure of 3 bar at large operating capacity of 5 L as used in the present work.  Degradation is favored under UV irradiation and not significantly dependent on the power of UV light.  Combination of hydrodynamic cavitation (3 bar) with 0.01%, 0.1% and 0.2% H2O2 concentrations gave enhanced depolymerization of 43.70%, 58.93% and 82.27% respectively whereas 0.01%, 0.1% and 0.2% H2O2 concentrations combined with UV (8 W) approach gave higher extents of degradation as 69.29%, 87.37% and 89.51% respectively in a treatment time of 180 min.  Extent of viscosity reduction observed in the case of HC + UV + H2O2 was higher than that obtained for HC + H2O2 and UV + H2O2.  Degradation was also favored under higher ozone loadings for the approaches based on ozone.

 Combination of O3 + UV + HC was found to be efficient technique for depolymerization of PAM giving 89.06% depolymerization under optimized conditions.  The extent of depolymerization using HC (3 bar) + UV (8 W) + H2O2 (0.2%) combination was maximum (97.27%) in 180 min of treatment time as compared to all the other combination approaches.  FTIR spectra confirmed that the chemical structure of PAM was preserved even after treatment with hydrodynamic cavitation and using various combination approaches and no significant modification was observed in degraded PAM. Overall, it has been established that combination of hydrodynamic cavitation using slit venturi as cavitating device with UV/ Ozone/H2O2 treatments can be effectively used for depolymerization of polyacrylamide and hybrid treatment approaches give higher extents of viscosity reduction as compared to individual treatment processes.

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