Study on mechanism of chitosan degradation with hydrodynamic cavitation

Highlights

• The Monte Carlo algorithm can analyze the mechanism of chitosan degradation.

• Chitosan degradation is caused by chemical and mechanical effects.

• The relative proportion of the effects is affected by degradation conditions.

Abstract

Hydrodynamic cavitation is an effective method for chitosan degradation, of which the mechanism directly determines the molecular weight distribution of degradation products. In this study, based on the Monte Carlo simulation and experimental results, the mechanism of chitosan degradation with hydrodynamic cavitation and molecular weight distribution of products were analyzed. The results showed that the algorithm established in the simulation could effectively analyze degradation mechanism and the factors that influenced degradation mechanism and molecular weight distribution of products. The degradation with hydrodynamic cavitation was caused by chemical and mechanical effects, of which the former dominated the degradation process. The outlet and inlet angles and throat length of the cavitator had major and minor influences on the degradation pattern, respectively. The chemical effect led to random cuts resulting in wide distribution of the products, while the mechanical effect led to central cuts resulting in narrow distribution of the products. With more central cuts, the slide-shaped molecular weight distribution curve of degradation products was gradually transferred into a bell-shaped curve. These results provide instructions for researches on the molecular weight distribution of chitosan products degraded with hydrodynamic cavitation.

Introduction

Chitosan, a product of N-deacetylated chitin, is a natural polymer composed of β-(1,4)-2-amino-2-deoxy-β-D-glucose and β-(1,4)-2-acetamido-2-deoxy-β-D-glucose. Because chitosan oligosaccharides with a specific molecular weight have unique properties [1], the production of these oligochitosans obtained through chitosan degradation has drawn widespread attention.

Compared with ultrasonic cavitation, hydrodynamic cavitation exhibits many advantages such as convenient operations, a high efficiency, uniform cavitation fields and so on [2], [3], [4], [5], [6], [7], [8], and hence has been applied in the degradation of organic materials [9], [10], [11], [12], [13], [14] including chitosan [15], [16]. The cavitation degradation of polymers is mainly caused by mechanical, chemical, thermal and supercritical effects [3], [17], [18]. At present, the research on the mechanism of organic polymers degradation by cavitation mainly focuses on chemical and mechanical effects, and mostly concentrates on the field of ultrasonic cavitation. Yasui et al. [19] found that when the temperature in collapsing bubble was higher than 2500 K, all the water molecules inside the bubble were dissociated, thus producing hydroxyl radicals and other chemicals. Didenko and Cravotto et al. [20], [21] founded that the local high temperature (4500–5000 K) and high pressure (>1000 atm) generated from the collapse of cavitation bubbles broke water molecules to produce free radicals, including hydroxyl radicals, peroxide radicals and hydrogen peroxide [22]. Li et al. [23] reported that hydrodynamic cavitation improved the degradation of Rhodamine B assisted by Fe³⁺ doped TiO₂, and analyzed the reaction mechanism, suggesting that the highly oxidative hydroxyl radical generated by cavitation, H₂O₂ formed by hydroxyl radical combining and peroxide radicals oxidized Rhodamine B. However, few experiments directly determined the concentration of free radicals in the process of cavitation degradation of polymers due to the high reactivity and extremely short life of free radicals, and competition between free radical quenching agent and degradation substances. Koda et al. [24] degraded methyl cellulose, pullulan, dextran and poly(ethylene oxide) in aqueous solutions with ultrasound, proving that the degradation rate was faster at high ultrasonic frequencies. The addition of free radical scavenger (t-BuOH) inhibited the degradation of water-soluble polymer and the chemical effect of cavitation degradation polymers was proven. Daraboina [25] found that the ultrasonic degradation rate of polymer increased with the increase of side-chain length of polymer, which increased the mechanical tensile force of molecule in the process of cavitation, so the degradation may be caused by the mechanical effect in the process of cavitation. Taghizadeh [26] studied the degradation of poly(vinyl-pyrrolidone) with different initial molecular weights by ultrasonic cavitation and the degradation rate increased with the increase of molecular weight. In addition, they also confirmed that the shear force produced by the rapid movement of solvent after cavitation collapsed led to the fracture of the polymer chemical bonds. Madras [27] studied the dynamics of molecular weight distribution of polymers in ultrasonic degradation. The pattern of ultrasonic cavitation degradation of polymer was central cut, and the polymer chains with low molecular weights would not fracture in the middle. In order to explore the impact of degradation on the molecular weight distribution of products, a numerical simulation of ultrasonic degradation of polymers was established by the authors. Portenlänger [28] studied the influences of frequency and other irradiation parameters on ultrasonic cavitation mechanics and free radical effect. They found that at a low ultrasonic frequency (35 kHz), the degradation was caused by mechanical effect, and the degradation rate was proportional to the molecular weight of polymer. When the molecular weight of polymer was lower than 40000, the mechanical effect would disappear; while at a high ultrasonic frequency (>500 kHz), the degradation was caused by the free radical effect. In conclusion, according to the similar mechanisms of hydrodynamic cavitation and ultrasonic cavitation, this paper assumed that the degradation of chitosan caused by hydrodynamic cavitation should be ascribed to the combination of chemical and mechanical effects. These different effects could probably affect the molecular weight distribution of chitosan degradation products with hydrodynamic cavitation, and thus it is of great significance to study the mechanism of degradation with hydrodynamic cavitation for regulating the molecular weight distribution and improving the degradation efficiency.

The main cleavage patterns of linear polymers include random cut, central cut, percent cut, unzip cut, ends cut and so on, and these patterns have varied influences on the molecular weight distribution of products. It was reported that the degradation of polymers in cavitation mainly depended on chemical and mechanical effects [29], [30], [31], [32]. Huang et al. [33] found that the oxidative degradation of chitosan in a homogeneous system followed a random degradation manner. Bose et al. [34], [35], [36], [37], [38], [39] discovered that the mechanical degradation often occurred in the central link of chain backbone under the action of shear force or ultrasound. Therefore, the simulation was carried out based on the assumption of chitosan degradation with hydrodynamic cavitation in manners of random and central cuts, and the correctness of the hypothesis was deduced through the simulation results.

In this paper, the molecular weight distribution of degradation products was simulated with Monte Carlo method [40], [41] based on the assumption of chitosan degradation with hydrodynamic cavitation in manners of random and central cuts. The simulation results were compared with the experimental results under the same conditions to testify the hypothesis of mechanism of chitosan degradation with hydrodynamic cavitation and the proportion of chemical and mechanical effects in the degradation process. The effects of different cleavage patterns on the molecular weight distribution were analyzed. This paper is theoretical basis for the production of oligochitosan with homogeneous molecular weights through hydrodynamic cavitation.