Biological and chemical transformation of hexamethoxymethyl melamine

Hexamethoxymethyl melamine (HMMM) is an emerging micropollutant. It has recently gained attention due to its widespread occurrence across the world, in surface water, seas, lake, road runoff, stormwater, soil and even crops like lettuce. The primary point source of this pollutant is industrial wastewater, while another significant contributor to its release into the environment is tire wear leachate, constituting a diffuse source. This compound is also abundant in Ergene River, Türkiye. Its structural similarity to currently banned triazine pesticides such as atrazine and its world-wide presence in various ecological systems prompts studies that can provide further insight into its source, fate in the environment and its transformation mechanisms via biotic and abiotic degradation. Although HMMM was detected in various matrices, literature regarding its biotransformation, chemical oxidation, and the mechanism by which these transformations occur remains limited. In this study, persistence of HMMM was confirmed along Ergene River. Further, its biotransformation was studied both with a single strain of bacterium, Pseudomonas sp. BIOMIG1BAC, and activated sludge. Both inocula achieved over 80% removal of HMMM within 45 to 100 days. However, complete mineralization of HMMM was not observed, rather, transformation products accumulated. Melamine was found to be a possible dead-end product. A biotransformation pathway was proposed using the detected metabolites. Additionally, advanced oxidation of HMMM via UV- TiO2 photocatalysis was studied and rapid removals ranging between 50-100% were achieved. Similar transformation products were observed during advanced oxidation and biological treatments, suggesting that degradation likely proceeds through similar mechanisms dominated by oxidation and hydrolysis reactions leading to sequential loss of methoxy- methyl groups. This study contributes to a better understanding of biological and chemical degradation mechanisms of HMMM. Furthermore, it is the first ever report in the literature to assess its degradation via UV-TiO2 photocatalysis and to identify its oxidation byproducts.