Color reduction of sulfonated eucalyptus kraft lignin

- The less-degradative decoloration method of lignin, different from conventional bleaching, was investigated.
- The sulfonating and color reducing were processed simultaneously under mild conditions, which saved time and energy.
- The decoloration was evaluated by a quantifiable brightness method and the brightness value was improved by 76.1%.

Several eucalyptus lignins named as HSL, SML and BSL were prepared by high temperature sulfonation, sulfomethylation, butane sultone sulfonation respectively. The color properties of samples were investigated. Under optimized conditions the sulfonic group (SO3H) content of HSL, SML and BSL reached 1.52, 1.60 and 1.58 mmol/g, respectively. Samples were characterized by UV-vis spectroscopy, FTIR spectroscopy, 1H NMR spectroscopy, GPC and brightness test, respectively. The results revealed that BSL performed a higher molecular weight and lighter color due to the phenolic hydroxyl blocking by 1,4-butane sultone (1,4-BS). The color reduction of sodium borohydride treated BSL (labeled as SBSL) was further enhanced and the brightness value was improved by 76.1% compared with the darkest HSL. SBSL process was much better than HSL and SML process. Hydroxyl blocking effect of 1,4-BS and reducibility of sodium borohydride played important roles in the color reduction of sulfonated eucalyptus kraft lignin.