Dissolved lignin components in spent liquor of simple inorganic complex bleaching eucalyptus pulp

Abstract

Promising results from the catalytic bleaching (Cat-bleaching) of oxygen-delignified hardwood kraft pulp (Eucalyptus urophylla × Eucalyptus grandis) with simple inorganic complexes prompted interest in discovering the soluble lignin components. The dissolved substances can be harmful to the environment and some dissolved substances may also have toxic effects on the environment. For these reasons, study of the the composition and structure of dissolved lignin components are of great importance. This investigation is part of a project on Cat-bleaching to obtain more information regarding the lignin components of bleaching effluents. The Cat-treatments of the pulp were performed at bench scale using simple inorganic complexes composed of (NH 4 ) 2 S 2 O 8 plus Cu(CH 3 COO) 2 (Cu) or Co(CH 3 COO) 2 (Co). The spent liquors obtained after Cat-bleaching were extracted using chloroform, in portions, and the compounds of interest from both the chloroform extracts were isolated using gravity column chromatography with the aid of the UV technique. The samples were analyzed by spectrometric (FT-IR, HSQC 2D-NMR, GC-MS) methods. The dissolved components extracted from the spent liquors from the Cat-bleaching were confirmed to be structurally similar to lignin. The major constituents extracted by organic solvents were identified as phenols, benzaldehydes, phenylethanones, benzoic acid and benzoic acid ethyl ester, which indicated extensive oxidation reactions on the pulp lignin including C α –C β and alkyl-aryl cleavage in the lignin upon catalytical attack. The 2-methoxy phenol predominated, and its proportion was higher from the Cu-bleaching. But the carbonyl compounds (benzaldehyde + phenylethanone) released from the pulp after Co-bleaching had a higher ratio than those after Cu-bleaching. Identification of these carbonyl compounds from among the dissolved compounds contributed to the relation between lignin oxidation and lignin removal.