The effect of metformin and drinking water quality variation on haloacetamide formation during chlor(am)ination of acetaminophen.

Acetaminophen (Apap) is widely used and is known to form toxic haloacetamides (HAcAms) during chlorination. Metformin (Met) is a typical medication with usage much higher than that of Apap and its ubiquitous presence in the environment is known. The objective of this study was to investigate the effects of Met which contains multiple amino groups potentially joining reactions and different chlorination methods on HAcAm formation from Apap. In addition, a major drinking water treatment plant (DWTP) using the largest river in southern Taiwan was sampled to study the influence of Apap in a DWTP on the HAcAm formation. Results showed increasing dichloroacetamide (DCAcAm) molar yields of Apap at a Cl/Apap molar ratio of 5 during chlorination (0.15%) and two-step chlorination (0.03%). HAcAms were formed by the chlorine substitution of hydrogen on the methyl group in Apap followed by the cleavage of the bonding between nitrogen and aromatic. While a high Cl/Apap ratio during chlorination led to reactions between chlorine and HAcAms formed decreasing the HAcAm yields, the two-step chlorination further reduced the HAcAm formation during chlorination by a factor of 1.8-8.2. However, Met which limitedly formed HAcAms increased the DCAcAm yields of Apap by 228% at high chlorine dosages during chlorination and by 244% during two-step chlorination. In the DWTP, trichloroacetamide (TCAcAm) formation was important. The formation was positively correlated with NH4+, dissolved organic carbon (DOC), and specific ultraviolet absorbance (SUVA). DCAcAm dominated in the presence of Apap. The DCAcAm molar yields were 0.17%-0.27% and 0.08%-0.21% in the wet and dry seasons, respectively. The HAcAm yields of Apap in the DWTP were limitedly changed between different locations and seasons. Apap could be one important cause for HAcAm formation in a DWTP, as the presence of other pharmaceuticals such as Met possibly worsens the situation in chlorine applications.