Preparation of low molecular weight chitosan through hydroxyl radicals generated via hydrodynamic cavitation

Low molecular weight chitosan is highly desired for its improved solubility and biological activity compared to high molecular weight chitosan; however, effective, mild and environmentally friendly degradation methods remain a challenge. In this study, we used hydrodynamic cavitation generated by orifice plate constrictions in combination with H₂O₂ as an oxidizing agent to degrade chitosan to low molecular weight species under mild reaction conditions (room temperature, short reaction time). We systematically investigated parameters such as cavitation intensity, H₂O₂ concentration, sample preparation, and medium pH that influence the efficiency of chitosan degradation. At 20 °C and after 48 min, corresponding to 800 cavitation passes, the weight-average molecular weight of chitosan decreased from 710 to 20 kDa, with the molecular weight distribution remaining largely unchanged. The degradation performance of cavitation combined with H₂O₂ clearly exceeded that observed when chitosan was treated with the oxidizing agent alone or with cavitation alone at the same temperature and treatment time (resulting in 390 and 510 kDa, respectively). The degraded chitosan retains the backbone of the original chitosan, while oxidative cleavage of the glycosidic bonds by ^•OH leads to the formation of chromophore groups, probably located at the chain ends, but their concentration is below the detection limit of FTIR and NMR. The crystallinity and thermal properties of degraded chitosan are largely comparable to those of the original chitosan. The disclosed method provides an environmentally friendly and sustainable alternative to other degradation processes for producing low molecular weight chitosan.