Chapter Nine - Fungal Pre-bleaching

Abstract

Multistage bleaching procedures using a combination of treatments with chlorine-based chemicals and alkaline extraction are normally used in bleaching kraft pulp, the most popular chemical pulp. There has been growing environmental concern about chlorinated organic substances, including toxic, mutagenic, and carcinogenic polychlorinated dioxins, dibenzofurans, and phenols, in the effluent from such bleaching of conventional kraft pulp. Today, general concern about the environmental impact of chlorine bleaching effluents has led to a trend toward elementary chlorine-free or totally chlorine-free bleaching methods. Biobleaching with white-rot fungi that can selectively degrade lignin in wood, has been studied by some workers to establish chlorine-free bleaching processes. Pretreatment with fungi has been shown to replace upto 70% of the chemicals needed to bleach kraft pulp. The usual specificity of biological reactions and their mild reaction conditions make biological delignification an interesting alternative to bleaching with chemicals such as pressurized oxygen or ozone. However, biobleaching with fungi is rather slow compared with chemical bleaching, and the cellulose in the pulp may be attacked by polysaccharideases. This chapter reviews the use of white-rot fungi to delignify and brighten kraft pulps. Manganese peroxidase, or laccase with a co-substrate, can demethylate and partially solubilize the lignin in pulps, mimicking the early steps of the fungal delignification.