Spectroscopic characteristics of ultrafiltration fractions of fulvic and humic acids isolated from an eucalyptus bleached Kraft pulp mill effluent

Abstract

In order to investigate the chemical heterogeneity of fulvic and humic acids previously isolated from a bleached Kraft pulp mill effluent, a sequential ultrafiltration (UF) scheme through four polyethersulphone membranes was applied. The unfractionated fulvic and humic acids and their fractions were characterized by UV-VIS, synchronous fluorescence (with Δ λ=60 nm) and FTIR spectroscopies. The FTIR spectra were compared with those of lignin isolated from Eucalyptus globulus wood and from the black liquor of a Kraft pulping process. The results highlighted that fulvic acids fractions of low molecular sizes contain more lignin derived phenolic units, while those of higher molecular size exhibit a higher content of carbohydrate structures. However, the shift observed in the UV-VIS absorbance and fluorescence intensity towards higher wavelength, suggests a higher degree of conjugation of π-bonds in the fractions of highest molecular sizes. In what concerns the humic acids size fractions, the FTIR spectra did not exhibit major differences but, as observed for the fulvic acids’ fractions, UV-VIS and synchronous fluorescence spectra also suggest a higher degree of conjugation of π-bonds in the fractions with the highest molecular sizes. It was also observed that the fulvic and humic acids fractions of the same molecular size, operationally defined by the UF process, exhibit major differences in their spectroscopic features.