Initial delignification and cellulose degradation of conventional and ethanol-assisted ozonation

It has been proposed that the initial regime of pulp ozonation is a diffusion controlled process, due to the low solubility of ozone and its very fast reaction with lignin. However, whether the process is governed by diffusion on a fibre wall scale (macro diffusion control), on a (micro)fibril scale (micro diffusion control), or by reaction with lignin located on the fibre surface followed by diffusion control is an open question. In order to address this, a 31 kappa Hemlock kraft pulp impregnated with acidified (pH = 2.0) water or acidified (pH = 2.0) ethanol-water (70% w/w) was ozonated at high consistency in a differentially operated packed bed reactor. The cellulose degradation and pulp delignification rates were determined from the change in pulp viscosity and kappa number with ozonation time. Transmission electron microscopy (TEM) was used to visualize the lignin distribution of KMnO(4) stained fibre walls. The TEM pictures clearly show reacted lighter regions at exposed fibre surfaces, which are sharply separated from unreacted darker areas inside the fibre wall. Therefore, these pictures support the macro diffusion theory. The macro diffusion theory was used to describe the progress of delignification and cellulose degradation as a sharp reaction front moving through the fibre wall from the external fibre surface towards the lumen. In agreement with this so called diffusion controlled shrinking core model, the present experimental results show that the delignification and cellulose degradation increase proportionally with the square root of the ozonation time. The effects of ozone partial pressure and ethanol-water impregnation on the delignification and cellulose degradation rates are also described.