Enhanced Dechlorination of o-Chlorophenol by Sodium Alkalis in Supercritical Water

摘要

Effects of sodium alkalis on the dechlorination of o-chlorophenol (o-CP) in supercritical water (SCW) have been studied. Experiments were carried out under the conditions of 450℃ and 25 MPa. Results indicated that sodium alkalis could enhance the dechlorination of o-CP significantly and sodium hydroxide and sodium carbonate had similar activities. Dechlorination selectivities at residence time of 19s increased from 59.4% without alkali to 94.5% and 94.4% in the presence of sodium hydroxide and sodium carbonate, respectively. The dechlorination proceeded mainly via two pathways, hydrodechlorination and hydrolysis. Sodium alkalis could promote both of the two pathways. Sodium ion accelerated the overall dechlorination of o-CP via promoting the hydrodechlorination pathway, while hydroxide ion accelerated the overall dechlorination of o-CP via promoting the hydrolysis pathway. The ratio of hydrodechlorination reaction to hydrolysis reaction was about 1:3 regardless of the addition of sodium alkalis. This revealed that hydrolysis was the dominant pathway and the addition of sodium alkalis could promote both pathways similarly. Dechlorination of o-CP prior to SCWO process with addition of sodium alkalis could not only avoid the reactor corrosion caused by the generated hydrochloric acid in the followed SCWO of o-CP, but also reduce the formation of toxic chlorinated byproducts.