Fluorescence spectroscopy for chromophore studies on bleached kraft pulps

Fluorescence techniques are highly sensitive and, since aromatic lignin is the most likely source of natural pulp fluorescence, they are suitable for detailed investigation of residual lignin in bleached pulps. Such investigations are important to our understanding of the bleachability and brightness reversion of pulps. In this study, the effect of bleaching on pulps was evaluated using fluorescence spectra of two softwood kraft pulps bleached in six elemental chlorine-free and totally chlorine-free sequences. Fluorescence spectra were recorded in each bleaching stage using four different instrument settings. In general, pulp fluorescence at excitation wavelengths of 270 and 350 nm increased as bleaching proceeded. Below a certain lignin content, however, pulp fluorescence measured at an excitation wavelength of 430 nm decreased with the residual lignin content. Carbonyl groups have a quenching effect on fluorescence, and the formation of carbonyl groups after oxygen and ozone delignification induced a decrease in pulp fluorescence. The increase in pulp fluorescence after peroxide bleaching stages was due to the removal of carbonyl groups. Reactions of chromophores during brightness reversion also had an effect on pulp fluorescence. These findings demonstrate that fluorescence spectroscopy provides an interesting tool for pulp bleaching studies.