Decomposition of hydrogen peroxide by the lignin-combined manganese in pulp bleaching

摘要

For hydrogen peroxide bleaching, its efficiency and selectivity are apparently affected by the accumulation of transition metal ions in the bleaching system. Lignin contains a rich number of functional groups, such as phenolic hydroxyl and carboxyl, therefore it is a very good combiner for the accumulation of transition metal ions, especially in the case of hydrogen peroxide bleaching of high yield pulp, where high amount of lignin is dissolved or dispersed in the bleaching stream. The mechanism of lignin-combined metal ion in decomposing hydrogen peroxide may be very different compared to that of the free metal ions dissolved in solution. Understanding the difference shall be important in finding good methods in improving the hydrogen peroxide bleaching efficiency. In this study, the effect of manganese ion in aqueous solution and lignin-combined manganese on decomposing hydrogen peroxide under typical pulp bleaching conditions was investigated. A variety of bleaching stabilizers, ethylenediaminetetraacetic acid (EDTA), sodium silicate (Na_2SiO_3) and magnesium sulfate (MgSO_4), were added individually or in combination, to investigate their effects in controlling the free manganese induced or lignin-combined manganese induced hydrogen peroxide decomposition. Fourier Transform Infrared Spectroscopy (FT-IR), Elemental Analyses, Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), were used to study the chemical features of the lignin-manganese composites, as well as the amount of metal ions present in the solution or adsorbed on lignin. The results showed that the hydrogen peroxide decomposition in the presence of manganese and lignin can be represented by a pseudo-first-order kinetics. The decomposition of hydrogen peroxide by free manganese ion under alkaline condition was different to that by lignin-combined manganese. EDTA added individually showed the most positive effect on reducing the decomposition of peroxide by free manganese in solution, with a pseudo-first-order rate constant (k_(obs)) of 2.57*10~(-4) min~(-1). Synergistic effect of manganese and lignin on peroxide decomposition was found to be exist, with a pseudo-first-order rate constant (k_(obs)) of 6.53* 10~(-4) min~(-1), without any common additions added. Hydrogen peroxide is the most important bleaching chemical in the production of elemental chlorine free (ECF) and totally chlorine free (TCF) pulp bleaching. Unfortunately, transition metals, especially manganese, is sensitive to catalyze decomposition of hydrogen peroxide, under certain condition of bleaching. A lot of studies involving manganese-induced peroxide decomposition have been done to control the invalid decomposition for peroxide bleaching, especially for manganese ions . As a result, pulp slurry is dealt with either acid washes or chelating washing prior to the peroxide stage, which has shown to greatly improve the removal of metal ions. Unfortunately, there is still residual metals to introduce peroxide decomposition, after washing processes. Therefore several technologies for peroxide stabilization have been developed, such as magnesium sulfate, sodium silicate and diethylenetriaminepentaacetic acid. In fact, manganese is present in many different states during the bleaching process, such as "nonequilibrium" suspended particles, oligomer dynamic equilibrium with carboxylic groups of fiber, and oligomer chelated with residual lignin by forming coordinate bonds with phenolic groups . In the case of high yield pulping, peroxide bleaching is produced together with pulping process, with certain content of dissolved lignin in flows or residual lignin from pulp fiber. Lignin is an amorphous, polyphenolic, highly cross-linked polymer consisting of certain numbers of active carboxyl and hydroxyl (phenolic or not) sites that would be readily available for complexation with metal ions. In fact, lignin have already proved to be suitable for heavy metals removal from waste water. As a result