Degradation of Pulp-Mill Effluent by Oxygen and Na₅[PV₂Mo₁₀O₄₀], a Multipurpose Delignification and Wet Air Oxidation Catalyst

Water-soluble salts of polyoxometalate (POM) anions can be used as (net) catalysts for selective, effluent-free oxygen delignification (bleaching) of unbleached kraft wood-pulps. Essential to this process is the use of the POM anions as catalysts for the wet oxidation of the lignin removed from the pulp fibers. In anaerobic bleaching, a 0.5 M solution of α-Na₅[PV₂Mo₁₀O₄₀] (POM_ox) is combined with wood pulp and the mixture heated to 125 °C. This treatment dissolves certain organic compounds (some lignin and polysaccharide fragments) with a combined chemical oxygen demand (COD) of 1500-2000 mg of O₂/L. Before the solution can be used again for bleaching, the POM anions (POM_red) must be reoxidized and the dissolved organic compounds removed. Here, a second function of the POM anions is realized. They are used under mild conditions (150 °C, 0.7 MPa of O₂) to catalyze the wet air oxidation of the dissolved organic compounds. After several cycles of anaerobic bleaching and wet oxidation, a steady-state condition in COD (at ca. 550 mg of O₂/L) is achieved.