Bicarbonate as a hydrogen peroxide activator in the bleaching of wood pulps

In the pH range of 7--10, hydrogen peroxide (H2O 2) reacts with the bicarbonate anion (HCO3- ) to produce a low concentration of the peroxymonocarbonate monoanion (HCO4-) in an equilibrium reaction. The pK a for HCO4- dissociation to CO 42- (peroxymonocarbonate of PMC dianion) is in the range of 10.0. The PMC anions were investigated as pulp bleaching agents because low consumptions of other peracids (peracetic acid or Paa) and anionic peracids (peroxymonosulfate monoanion or HSO5-) have been shown to significantly improve the bleaching process for both chemical and mechanical pulps.;Two lignin model compounds were treated with H2O2 in 0.1M sodium bicarbonate or 0.1M sodium acetate in the pH range of 9.2--9.5. The rate of oxidation of both lignin models compounds were at least four times as high in bicarbonate as compared to acetate solutions. At pH less than 8 where HCO4- is minimally dissociated, a H 2O2/HCO3- or PB (peroxide + bicarbonate) pretreatment activated the lignin towards reaction with alkaline H2O2 and caused a significant decrease in the kappa number of an unbleached hardwood soda/AQ pulp. At pH ca. 8.5, PB treatments significantly increased the brightness of both chemical (soda/AQ and kraft) and mechanical pulps. In all cases the kappa number decrease or brightness increase per unit of H2O2 consumed was higher for PB than for conventional alkaline H2O 2 treatment.;The catalytic effects of Fe(III), Mn(II), Cu(II), V(IV), Ce(III) and Co(II) were investigated in detail because peroxide decomposition was the only potentially negative attribute of PB bleaching that was observed. Catalysis by Fe(III), Mn(II) and Co(II) was more significant than for the other metals. The sulfate salt of the transition metal was used on all occasions (VO2+ in the case of V(IV)) and citrate complex of each metal was also investigated. High decomposition rates were observed with ferric citrates, and with Mn(II) and Co(II) in the absence or presence of citrate for both metals. Free radical mechanisms were proposed to explain Fe(III) and Mn(II) catalysis while an ionic mechanism was tentatively proposed for Co(II). The investigation of Co(II) catalysis was less thorough than for Fe(III) and Mn(II) and this was due to the very low concentration of this metal in wood pulps. Rate expressions for peroxide decomposition were derived for Fe(III) and Mn(II) catalysis under various conditions and the reaction orders in terms of [H2O2], [catalyst] and [HCO 3-] were always close to the experimentally observed reaction orders.