Amoxicillin

Phototransformation in water

Description of key information

Key study. Method similar to OECD Phototransformation of chemicals in waters; no GLP. The test item is found to be susceptible to photoinduced transformations, especially in slightly basic conditions. The test item is characterised by a maximum half-life of 9d in winter (50º latitude) and a minimum half-life of 2d in summer (20º latitude).

Key value for chemical safety assessment

Half-life in water:

9 d

Additional information

In the hydrolysis and direct photolysis experiment performed by Andreozzi et al. (2004), the abiotic degradation process of amoxicillin was described as a hydrolysis reaction through the attack of the nucleophile H2O to the β-lactam ring followed by the ring opening and the UV absorption for λ > 290 nm being the substance susceptible of photoinduced transformations. According to the obtained results, both processes would be favoured in slightly basic aqueous medium. The hydrolysis rate constants were Khydr (pH 5.5) = 0.00146 h-1 and Khydr (pH 7.5) = 0,00317 h-1. The photolysis rate constants were Kphot (pH 5.5) = 2.09*10-3 h-1 and Kphot (pH 7.5) = 3.09*10-3 h-1. In regards to the indirect photolysis process, the same authors determined that the effect of nitrate ions was negligible at both investigated pH values of pH 5.5 and pH 7.5 since the nitrate ion absorbs sunlight as amoxicillin in the middle-UV region of hydroxyl radicals production. When humic acids were presents the photodegradation rate of amoxicillin was enhanced at both pH.