Degradation of chitosan with self-resonating cavitation

4.1.1 Effect of chitosan initial concentration

The effect of initial concentration on chitosan degradation was investigated by self-resonating cavitation. The experiments were performed under the conditions of initial concentrations of 1–10 g·L⁻¹, an inlet pressure of 0.4 MPa, pH of 4.4, temperature of 60 °C, and treatment time of 180 min. The degraded samples were withdrawn every 30 min. Then, the intrinsic viscosity of degraded chitosan solutions was measured, and the intrinsic viscosity reduction rate was calculated. The results are shown in Figs. 3 and 4. The kinetic rate constants k″ and the determination coefficient R² (Eq. (6)) are shown in Table 2.

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Fig. 3

Effect of initial concentration on the degradation of chitosan. Note: The symbols (a–c) stand for the significance difference values. Curves with the same symbols are not different significantly (P > 0.05), the same as Figs. 5, 7, 9 and 11.

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Fig. 4

The plots of ${{(1/\eta }_t)}^{1/\alpha }-({{1/\eta }_0)}^{1/\alpha }$ versus degradation time under the conditions of different initial chitosan concentrations.

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As shown in Fig. 3, the initial concentration of chitosan had a great effect on the degradation efficiency. With the initial concentration increasing from 1 to 10 g·L⁻¹, the intrinsic viscosity reduction rate was decreased from 94% to 56%. This phenomenon suggested that the degradation efficiency decreased with the increase of initial concentration of chitosan and a lower initial concentration promoted the degradation of chitosan through self-resonating cavitation. According to the results of Table 2 and Fig. 4, the R² values in all the fitting results under the condition of different initial chitosan concentrations were greater than 0.9. The great linearity indicated that the degradation reaction caused by self-resonating cavitation was a random first-order reaction. The results of kinetic studies (Table 2) showed that the first-order rate constant decreased from 3.27 × 10⁻⁵ to 0.33 × 10⁻⁵ with the increase of initial concentration from 1 to 10 g·L⁻¹, which is consistent with the tendencies in the cavitation degradation of chitosan with a turbine, orifice plate and Venturi tube (Huang et al., 2013; Wu et al., 2014; Xu et al., 2016). This is because the hydroxyl radicals of system provided was certain under the same cavitation conditions, so the degradation efficiency decreased with the increase of concentration. The stretching degrees of chitosan molecular chains decreased with the increase of solution concentration, and intramolecular and intermolecular hydrogen bonds were further strengthened. The probability of hydroxyl radicals attacking chitosan and the number of glycosidic bonds cleavaged were decreased (Huang et al., 2004). In addition, the viscosity and surface tension of the solutions increased with the increase of solution concentration, and cavitation bubbles had to overcome greater intermolecular forces for expansion, so the formation and transportation of cavitation bubbles were impeded. Thus, for improving the effect of chitosan degradation, the solution concentration should be lower. Considering the economy of the process, the initial concentration of 3 g·L⁻¹ could be better.

4.1.2 Effect of solution temperature

Temperature is an important factor for chitosan degradation caused by self-resonating cavitation. The effect of temperature on chitosan degradation was investigated under the conditions of an initial solution concentration of 3 g·L⁻¹, inlet pressure of 0.4 MPa, pH of 4.4, and treatment time of 180 min. Then, the intrinsic viscosity values of degraded chitosan solutions at 30, 40, 50, 60 and 70 °C were measured, and the intrinsic viscosity reduction rates were calculated. The results are shown in Figs. 5 and 6. The kinetic rate constants k″ and the determination coefficient R² following Eq. (6) are shown in Table 3.

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Fig. 5

Effect of solution temperature on the degradation of chitosan.

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Fig. 6

The plots of ${{(1/\eta }_t)}^{1/\alpha }-({{1/\eta }_0)}^{1/\alpha }$ versus degradation time at different temperatures.

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Fig. 5 showed that the temperature had a great influence on chitosan degradation. Under the same conditions, as the temperature increased from 30 to 70 °C, the intrinsic viscosity reduction rate was increased from 52% to 96%. Fig. 6 and Table 3 showed the results of kinetic studies. The first-order rate constant increased from 0.25 × 10⁻⁵ to 7.09 × 10⁻⁵ with the increase of temperature from 30 to 70 °C. The results indicated that a higher temperature facilitated the degradation process, which is consistent with the trends in the cavitation degradation of chitosan with a turbine and Venturi tube (Wu et al., 2014; Xu et al., 2016). This may be due to the fact that the solution viscosity and surface tension decreased and molecules moved faster with the increase of temperature, which led to an increase of intermolecular collision probability, promoting the degradation reaction. On the other hand, the hydrogen bonds and salt linkages were gradually destroyed and steric hindrance disappeared with the increase of temperature, and the glycosidic bonds in chitosan became fragile and subject to degradation. In addition, water vapor pressure increased with increase of temperature, thus reducing the difference between vapor pressure and atmospheric pressure, which was conducive to the formation of cavitation bubbles and enhanced the cavitation effect. Fig. 5 showed that the intrinsic viscosity reduction rate leveled off at above 50 °C. This is probably because most salt linkages had been destroyed and the fracture of glycosidic bonds in molecular chains was not obvious at above a certain temperature (Chen and Han, 2006). Another reason may be that the vapor pressure increased faster than the solution temperature with the increase of temperature. Therefore, the transient high temperature and high pressure generated by cavitation collapse decreased, and the cavitation intensity decreased, thus lowering the degradation rate (Huang et al., 2015). It should be noted that the effect cannot be generalized because the increase of temperature also led to an increase in the vapor pressure of medium. This result implies that vaporous cavities would be generated in the system, leading to lower intensity of cavitation (Dhanke and Wagh, 2020; Mishra and Gogate, 2010). Similar results regarding the effect of operating temperature were reported in the literatures. Wang et al. (Wang and Zhang, 2009) investigated the effect of temperature on the extent of degradation of alachlor using hydrodynamic cavitation and reported that the temperature increased from 30 to 40 °C resulted in an increase in the extent of degradation, but the rates of degradation decreased when the temperature increased to 60 °C. Dhanke et al. (Dhanke and Wagh, 2020) studied the degradation of Acid RED-18 by hydrodynamic cavitation and found similar results.

4.1.3 Effect of pH on the degradation of chitosan

The effect of initial pH on chitosan degradation was investigated by using the self-resonating cavitatior. The experiments were performed at different pH values (3.2, 3.6, 4.0, 4.4 and 4.8) under the conditions of a solution concentration of 3 g·L⁻¹, inlet pressure of 0.4 MPa, temperature of 60 °C, and treatment time of 180 min. Then, the intrinsic viscosity values of degraded chitosan solutions were measured, and the corresponding intrinsic viscosity reduction rates were calculated. The results are shown in Figs. 7 and 8. The kinetic rate constants $k^{\text{'}\text{'}}$ and R² values are shown in Table 4.

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Fig. 7

Effect of pH on the degradation of chitosan.

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Fig. 8

The plots of ${{(1/\eta }_t)}^{1/\alpha }-({{1/\eta }_0)}^{1/\alpha }$ versus degradation time at different pH values.

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As shown in Figs. 7, 8 and Table 4, the intrinsic viscosity reduction rate was increased and first-order rate constant was increased from 0.25 × 10⁻⁵ to 2.88 × 10⁻⁵ with the increase of pH from 3.2 to 4.4. This may be because the concentration of protons increased with the decrease of pH, and the concentration of the anions in the acidic solution would be also increased, forming more salt linkages in chitosan molecules and thus increasing the steric hindrance of the glycosidic bonds to be cleavaged and reducing the rate of chitosan degradation (Zhang et al., 2006). The intrinsic viscosity reduction rate decreased when the pH was greater than 4.4. This is due to the degradation of chitosan is an acid-catalyzed reaction in the first place, and the reaction should be carried out under acidic conditions. The optimal pH obtained in this study should be chosen to be 4.4.

4.1.4 Effect of inlet pressure

The effect of inlet pressure, in the range of 0.1–0.5 MPa, on the degree of degradation was investigated. The experiment was performed with the initial solution concentration of 3 g·L⁻⁻¹, pH of 4.4, 60 °C, and treatment time of 180 min. Then, the intrinsic viscosity values of degraded chitosan solutions were measured, and the corresponding intrinsic viscosity reduction rate was calculated. The results are shown in Figs. 9 and 10. The kinetic rate constants $k^{\text{'}\text{'}}$ and R² values are shown in Table 5.

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Fig. 9

Effect of the inlet pressure on the degradation of chitosan.

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Fig. 10

The plot of ${{(1/\eta }_t)}^{1/\alpha }-({{1/\eta }_0)}^{1/\alpha }$ versus the time at different inlet pressures.

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It was found that the intrinsic viscosity reduction rate increased first and then decreased with the increase of inlet pressure (Fig. 9). The intrinsic viscosity reduction rate was increased from 82.5% to 92.2% with the increase in pressure from 0.1 to 0.4 MPa. Fig. 10 and Table 5 showed the results of kinetic studies. The first-order rate constant was increased from 1.19 × 10⁻⁵ to 2.88 × 10⁻⁵ with the increase in pressure from 0.1 to 0.4 MPa. This could be explained by the fact that the flowing velocity of liquid increased with the increase of inlet pressure, thereby (i) decreasing the number of cavities which were beneficial for the formation of cavitation; (ii) increasing the frequency of cavitation in per volume of solution; and (iii) enhancing the resonant frequency and the resonant amplitude of jet leading to the increase of the velocity gradient of liquid in the resonant cavity, which could strengthen the mechanical shearing force of fluid (Ma et al., 2016). Thus, the effect of degradation was improved. However, the reduction rate of intrinsic viscosity was decreased when the pressure was higher than 0.4 MPa, which was caused by a super cavitation phenomenon at high pressures. Moreover, the decrease in the intrinsic viscosity reduction rate at a higher inlet pressure (0.5 MPa) can be attributed to choked cavitation, and the entire downstream area of the cavitation device was completely filled with a large number of cavities. These cavities would be integrated into larger bubbles, thus reducing the cavitational intensity because of the incomplete or cushioned collapse of these cavities (Saharan et al., 2011). Therefore, the parameter 0.4 MPa was chosen as the optimal inlet pressure in this study. When chitosan was degraded by cavitation with a turbine and orifice plate, the degradation efficiency increased with the increase of pressure in a certain range. These results are different from the trend in self-resonationg cavitation, which should be attributed to the different cavitation mechanisms (Huang et al., 2013; Wu et al., 2014). Cavitation with a turbine is the cavitation effect caused by cavitation bubbles generated during the rotation process of turbine. Higher pressures led to a faster injection speed in the swirl chamber, resulting in a higher pressure drop in the center of vortex, so more cavitation bubbles could be formed and the influence of cavitation intensity was stronger. For the cavitation with an orifice plate, the increase of pressure could increase the velocity of jets and the pressure difference around the center of orifice plate, thus promoting the degradation. The self-resonating cavitation process is a type of cavitation mode to strengthen the hydrodynamic cavitation through the self-excited oscillation produced by the resonant cavity, in which the influence of pressure change on the fluids in the resonant cavity (such as the influence on the frequency and amplitude of the oscillation) is complex (Ma et al., 2016). Therefore, the trend is different and the specific reasons need to be further studied.

4.1.5 Effect of treatment time

The effect of treatment time on chitosan degradation was studied. The experiment was performed under the conditions of initial solution concentration of 3 g·L⁻¹, pH of 4.4, 60 °C, and treatment time of 180 min. Then, the intrinsic viscosity of degraded chitosan solutions was measured, and the intrinsic viscosity reduction rate was calculated. The results are shown in Fig. 11.

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Fig. 11

Effect of time on the degradation of chitosan.

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The intrinsic viscosity reduction rate increased with the increase of treatment time and reached 92.2% at 180 min in Fig. 11. It was also observed that the intrinsic viscosity reduction rate increased rapidly in the range of 0–90 min. 90 min later, the intrinsic viscosity reduction rate leveled off. This was probably because the breakage of the glycosidic bonds took place during the degradation of chitosan. At the beginning of the treatment, the molecular chains of chitosan were long and contained glycosidic bonds, and the degradation rate was high. With the continuous treatment, the molecular chains became shorter, and the number of glycosidic bonds became smaller, which slowed the degradation rate. In addition, in the early stage of cavitation, more oxygen was dissolved in the solution, which made cavitation more obvious. After a period of cavitation treatment, the oxygen content in the solution was gradually decreased, weakening the effect of cavitation. Thus, the treatment time should not be too long.

4.2.1 Measurement of viscosity-average molecular weight

The experiment was performed under the conditions as follows: a solution concentration of 3 g·L⁻¹, pH 4.4, at pressure 0.4 MPa and temperature 60 °C, for cavitation time 60 min. Based on a viscometric method (Fan et al., 2002), the specific viscosity of initial and degraded chitosan was measured under the conditions of different solution concentrations, and the intrinsic viscosity values of initial and degraded chitosan were calculated to be 649.19 and 116.94 cm³·g⁻¹, respectively, according to Eq. (5). On the other hand, based on an acid–base titration method, the DD values of initial and degraded chitosan were measured to be 89.34% and 88.05%, respectively. Then, the viscosity-average molecular weights of initial and degraded chitosan were calculated to be 651 and 104 kD, respectively, according to Eq. (4). The data indicated that after degradation, the DD value of chitosan was slightly changed and the degradation of chitosan by self-resonating cavitation was highly efficient.

4.2.2 FT-IR spectral analysis

FT-IR is a powerful tool for the study of physicochemical properties of polysaccharides. The FT-IR spectra of original and degraded chitosan are shown in Fig. 12. The main bands in the spectrum of original chitosan could be assigned as follows: the bands in the range of 3600–3200 cm⁻¹ were attributed to the stretching vibration of O-H and N-H (Huang et al., 2008); the signals at 1646 and 1346 cm⁻¹ were assigned to amide I and III bands, respectively; the band at 1570 cm⁻¹ could be attributed to amide II band (Wang et al., 2008); the bands at 1151 and 1093 cm⁻¹ were assigned to C-O-C and C-O groups (Gu et al., 2010); the band at 929 cm⁻¹ was assigned to the β-(1,4)-glycosidic bonds in chitosan (Yue et al., 2009). No bands between 1660 and 1900 cm⁻¹ were observed, indicating that carboxyl or carbonyl groups (Xu et al., 2016) were absent in the degradation products. Compared with the spectrum of original chitosan, the intensity of bands in the spectrum of degradation chitosan was changed, but the wavenumber values did not change, which showed that no side reactions occurred in the degradation process of chitosan by self-resonating cavitation, and the functional groups and main-chain structures of the products were not damaged except for the breaking of glycosidic bonds on the molecular chains. This may be because the free radicals and mechanical effects, produced by self-resonating cavitation, did not cause the changes in the backbone structures and only broke the β-(1,4) glycosidic bonds of chitosan molecules (Zhou, 2008). These results are also consistent with the findings of degradation of chitosan with a turbine, orifice plate and Venturi tube (Huang et al., 2013; Wu et al., 2014; Xu et al., 2016).

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Fig. 12

FT-IR spectra of original (A) and degraded chitosan (B).

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4.2.3 XRD spectral analysis

The X-ray diffraction patterns of initial and degraded chitosan are shown in Fig. 13. The XRD pattern of the former exhibited two characteristic peaks at 2θ of 9.59° and 20.00°, assigned to the crystal forms I and II (Jiang et al., 2011; Wang et al., 2007). In contrast, the intensity of peak at 2θ of 20.24° was decreased after the degradation and the other peak disappeared, indicating that the number of intermolecular hydrogen bonds in chitosan was reduced after the degradation. Thus, the degradation of chitosan by self-resonating cavitation destroyed a proportion of crystals in the chitosan matrix, and the destruction could occur in both the amorphous and crystalline regions. In other words, the chitosan in the amorphous region was firstly degraded into water-soluble molecules, and was then dissolved in the water. With the proceeding of degradation, the crystalline structure was destroyed and the crystallinity decreased (Qin et al., 2002). In contrast, the chitosan degraded with an orifice plate and Venturi tube cavitator only showed weakened characteristic XRD signals, while after the degradation of chitosan by self-resonating cavitation, one of the two characteristic peaks disappeared and the weakened extent of the other was greater than the former cavitator, which indicated that the degradation effect of self-resonating cavitation on chitosan was stronger (Huang et al., 2013; Xu et al., 2016).

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Fig. 13

X-ray diffraction spectrum of original (A) and degraded chitosan (B).

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4.2.4 ¹H NMR spectral analysis

The ¹H NMR spectra of original and degraded chitosan samples are shown in Fig. 14. Assignments of initial chitosan are shown in Fig. 14A. The signals can be identified as follows: the small signal at 4.82 ppm is attributed to H¹ (Qin et al., 2012). Multiplets at 3.85–3.70 ppm correspond to H³, H⁴, H⁵ and H⁶ of the glucosamine molecules (Kumar et al., 2012). The singlet at 3.11 ppm is assigned to H² and the signal at 2.01 ppm is assigned to —CH₃ (Ma et al., 2009). Assignments of degraded chitosan are shown in Fig. 14B. Signals at 3.69–3.95 ppm attributed to H³, H⁴, H⁵ and H⁶ protons of the polysaccharide are significantly enhanced. Other signals at 4.30 and 4.63 ppm are assigned to H¹, and these signals are overlapped with those of the solvent. The signal at about 2.06 ppm is assigned to —NH(CO)—CH₃. The ¹H NMR results indicated that degradation process did not affect the main-chain structure of chitosan, which is consistent with the FT-IR results.

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Fig. 14

¹H NMR spectra of original (A) and degraded chitosan (B).

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4.2.5 Polydispersity analysis

As shown in Fig. 15, the polydispersity indices of products became lower and decreased from 3.17 at 0 min to 2.75 at 180 min, which indicated that the molecular-weight distribution of products was more concentrated after degradation by self-resonating cavitation. This may be because of the strong mechanical effect in the process of self-resonating cavitation. Chitosan is a linear polymer and the mechanical degradation effect usually occurs in the center of the backbone (Bose and Git, 2004). And, the greater the molecular weight is, the higher the degradation degree by mechanical effect is. Therefore, in the system containing polydisperse chitosan materials, macromolecules were degraded into molecules with medium molecular weights through the mechanical effect, and the molecular weights of products would be more concentrated, thus reducing the polydispersity index of products.

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Fig. 15

Polydispersity indices of products at different degradation times.

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