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

Abstract

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 {alpha}-Na{sub 5}[PV{sub 2}Mo{sub 10}O{sub 40}] (POM{sub 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 1,500--2,000 mg of O{sub 2}L. Before the solution can be used again for bleaching, the POM anions (POM{sub 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 O{sub 2}) 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 O{sub 2}/L) is achieved.