Anaerobic biodegradation of dipyrone: Determination of transformation products by LC-HRMS and ecotoxicity assessment using target organisms and QSAR model

The degradation of dipyrone (metamizole) was studied for the first time under mesophilic anaerobic conditions using a synthetic wastewater similar to that found in anaerobic digestion (AD) treatment plants. Dipyrone concentration effect (250–25,000 µg L−1) was evaluated in short term batch and long-term semi-continuous operation. After 48 days, biogas production and biodegradability were reduced in the batch AD by 17.7% and 18.8%, respectively, regardless of the dipyrone concentration. However, additions of 1000 µg L−1 to the long-term AD system stimulated biogas production and increased organic matter removal by 68% and 30%, respectively. Although removals of the main metabolite 4-MAA (N-methyl-4-aminoantipyrine) and other transformation products were observed in the batch tests after 48 days, a continuous accumulation of dipyrone by-products in the long-term AD reflected its recalcitrant nature. The occurrence of these metabolites coincided with an increased toxicity of the AD effluent to Aliivibrio fischeri of 79% bioluminescent bacteria-inhibition observed immediately after the addition of the dipyrone, while the germination of Lactuca sativa seeds and growth of Raphidocelis subcaptata were stimulated by the same conditions. The toxicity of by-products was mainly attributed to hydrazinic group according to predictions elaborated by ECOSAR software. The results showed that long term exposure to dipyrone main by-product 4-MAA, as a consequence of build up into environment over continuous discharges of effluents, can significantly affect freshwater organisms.