Wet oxidation of process water from hydrothermal carbonization of biomass with nitrate as oxidant

Wet oxidation of a wastewater highly contaminated with biogenic organic compounds was investigated with nitrates as oxidant. Iron(III) nitrate proved to be a powerful oxidant at a relatively low treatment temperature of 120 °C. 50% of the chemical oxygen demand (COD) and 30% of the dissolved organic carbon (DOC) could be eliminated within the first hour of reaction. Iron(III) ions significantly support the nitrate based wet oxidation under mild reaction conditions. It may function as catalyst and/or co-oxidant. Iron oxides/hydroxides precipitate after cooling the reaction medium. However, the resulting iron sludge can be re-dissolved in HNO3 and recycled as oxidant. Oxidation efficiency with this mixture was enhanced significantly compared to oxidation with HNO3 only. At 200 °C similar DOC and COD reductions are achieved with Fe(NO3)3 and with HNO3. At 120 °C Fe(NO3)3 outperforms HNO3 clearly. This points to a nitrate activation by iron(III) which is effective already at low temperatures. 3-Chlorophenol as a representative of chlorinated aromatic pollutants could be completely eliminated by the nitrate-based hydrothermal oxidation under mild reaction conditions (2 h at 120 °C). Thus, wet oxidation using nitrate as oxidant can be feasible for elimination of recalcitrant pollutants at relatively low temperatures of 120 °C.