Effect of pH on degradation of acetaminophen and production of 1,4-benzoquinone in water chlorination

The concern over pharmaceuticals and their toxicity in wastewater and drinking water has grown over the past decade. In this study, the common analgesic, acetaminophen, was chlorinated with sodium hypochlorite to determine the effects of pH and chlorine-to-pharmaceutical molar ratios on the degradation of acetaminophen and the formation of the toxic byproduct 1,4-benzoquinone. Reactions were studied for pH 6.0, 7.5 and 9.0 at average molar ratios of 106±6, 1,417±285, and 9,789±1,430 over a period of 100 minutes. The degradation of acetaminophen and the formation of 1,4-benzoquinone were monitored using liquid chromatography tandem mass spectrometry (LC/MS/MS). Results indicate that acetaminophen is most reactive with free chlorine at pH 9.0 and least reactive at pH 6.0. As pH increased, degradation of acetaminophen also increased. The formation of 1,4-benzoquinone was also affected by pH and its concentration reached a maximum of 68.7% of the initial acetaminophen concentration when the pH was at 6.0, the molar ratio at 1,275, and after a contact time of 30 minutes. At all pH values the rate of degradation of acetaminophen was slowest at a molar ratio of about 100, and highest at a molar ratio of about 10,000.