Catalytic wet air oxidation of Kraft bleaching plant effluents in the presence of titania and zirconia supported ruthenium

The wet air oxidation (WAO) of two acidic and alkaline Kraft bleaching plant effluents (total organic carbon (TOC) content 665 and 1380 mg l−1, respectively) has been investigated in a batch slurry reactor in the presence of titanium or zirconium oxides, or ruthenium catalysts supported on these oxides. At 463 K and 5.5 MPa total air pressure, a drastic extent of TOC removal was observed with addition of the oxides (up to 88 and 79%, respectively). The rate of TOC decrease was not influenced by the nature and structure of the oxides nor by the acidic/basic character of the surface, but it was found to increase with the specific surface area of the oxides. The rate of TOC removal was further enhanced by deposition of metallic ruthenium on the oxides. In that case, the treatment produced decolorised liquid effluents, with very low TOC values (TOC removal >99.7%), constituted of traces of acetic acid. No leaching of Ru, Ti or Zr was detected by ICP–AES analysis to the detection limits of 0.2, 0.1 and 0.1 mg l−1, respectively. The quality of water obtained may allow its recycling in the process.