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Lignin α-carbonyl groups play an important role in light-induced yellowing as initial absorber of UV light. The model compounds vanillin, verataldehyde, and acetovanillone were studied and the reaction rate coefficient  k  was calculated for the addition of bisulphite to carbonyls. UV-Vis spectra of the model compounds in bisulphite solution were measured between 200 and 400 nm and at a single wavelength of 305 nm. The reaction was fastest with vanillin, but only a minor part of acetovanillone reacted, which can be explained with different reactivity of aldehydes and ketones. Calculated reaction rate coefficients were 26×10 -3  s -1  and 18×10 -3  s -1  for vanillin and verataldehyde, respectively. Sheets from chemi-thermomechanical pulp (CTMP) were treated with bisulphite solutions (10 mM, 20 mM and 0.1 M; for 1 and 5 min) in order to study the potential of this treatment in retarding yellowing of CTMP. Initial brightness was improved with the treatments, but as the brightness loss was even more drastic, stabilization was not achieved. FT-IR spectra of bisulphite treated samples revealed that only part of the carbonyl groups reacted. The unsuccessful light stabilization of the pulp originated probably from an inadequate carbonyl addition, a problem which could not be resolved.

Addition of bisulphite to lignin α-carbonyl groups: A study on model compounds and lignin-rich pulp