Degradation of lignins by wet oxidation: model water solutions

Abstract

Nowadays many paper industries are faced with the problem of wastewaters being badly biodegradable because of tannins, lignins, etc. Wet oxidation (WO) is an appropriate met hod for the destruction of very complex, multi-dimensional, and permanent compounds. This paper studies the WO of lignin water as a model case with the aim of degrading lignin and enhancing water biodegradability. The experiments were performed at various temperatures (110-190 °C), partial oxygen pressures (0.5-1.5 MPa), and pH values (5, 9, and 12) in a batch stainless steel high pressure reactor. The experiments showed that increasing the temperature improved the processes efficiency. At the lowest temperature tested 75% lignin reduction was detected and a temperature increase improved the lignin removal to 100% at 190 °C. The effect of temperature on the COD removal rate was lower, but detectable: 20% of organics was oxidized at 110 °C, but 53% at 190 °C. Oxygen partial pressure changes affected the process results modestly. Lignin removal increased by about 10% and COD removal by 4% with an oxygen partial pressure increase from 0.5 to 1.5 MPa. The effect of pH occurred mostly on lignin removal. Increasing the pH enhanced the lignin removal efficiency from 60% to nearly 100%. In all cases, a good biodegradability (BOD/COD ratio over 0.5) was achieved starting at a temperature of 150 °C, pH 9, irrespective of the experimented pressures. The biodegradability (BOD/COD) of the remaining organic matter increased during WO and approached almost 1 at the highest temperature of 190 °C.