Effects of inorganic salts on the degradation of 2,5-dihydroxy-[1,4]-benzoquinone as a key chromophore in pulps by hydrogen peroxide under basic conditions

Abstract

Abstract 2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ) is one of the key chromophores in cellulosics. Due to its special resonance stabilization, it is a prime survivor of bleaching treatments and its degradation is essential in pulp bleaching. The clarification of the degradation mechanism of this compound is indispensable for targeting improvements of pulp bleaching processes. Previous studies revealed that DHBQ degradation by hydrogen peroxide (H2O2) is influenced by salts. This study addresses the effects of alkaline metal salts, alkaline earth metal salts, and Al2(SO4)3 on the degradation of DHBQ under simulated conditions of pulp bleaching in a peroxide stage. The degradation by excess H2O2 followed first-order kinetics in the presence of alkaline metal salts and alkaline earth metal salts, which enhanced and retarded the reaction, respectively. Kinetic studies and theoretical computations provided detailed mechanistic insights: Li+, Na+, and K+ stabilize several reaction intermediates by complex formation, causing the enhancement of the degradation, whereas Mg2+ and Ca2+ coordinate and stabilize the reactant, resulting in the observed retardation. Al2(SO4)3 exhibited a strong enhancing effect, but the degradation followed second-order kinetics with regard to DHBQ, suggesting a fundamentally different degradation mechanism.