Hexamethoxymethylmelamine – A precursor of persistent and mobile contaminants in municipal wastewater and the water cycle

Abstract

Hexamethoxymethylmelamine (HMMM) is a crosslinker of melamine resins and widely used in the production of coatings and plastics, e.g. for cans, coils and automobiles. Despite its widespread application little is known about the occurrence of HMMM in municipal wastewater, the extent of its removal in wastewater treatment and the fate of transformation products that may be formed from it. A lab degradation experiment with activated sludge showed that HMMM is transformed but not mineralized under aerobic conditions. By LC-UV and LC-HRMS analyses 21 transformation products (TPs) were detected and tentatively identified, with modified di(methoxymethyl)amino side chains, and a pathway for their formation was proposed. The major TP after 28 days was methylol-1,3,5-triazin-2,4,6-triamin. LC-UV and LC-HRMS response of the TPs in the lab experiment was used to quantitatively assess the concentration of TPs also in environmental samples. In municipal wastewater HMMM and its two TPs TetraMMM and DiMMM added up to median concentrations of 2.8 μg/l (7.5 nmol/L) in three wastewater treatment plants (WWTPs), and to 17.9 μg/l (66 nmol/L) in another plant with a higher contribution of industrial wastewater. Their concentration increased in the WWTPs by a factor of 2–4, likely due to ongoing formation from HMMM-related precursor compounds. Also melamine concentrations increased in the four WWTPs (to 3–8 μg/L), but this did not seem to be linked to HMMM transformation. In surface water affected by WWTP discharges HMMM and 12 TPs were determined, at median total concentrations of 5 μg/L, with an even higher proportion of the TPs relative to HMMM compared to the WWTP effluents. During bank filtration HMMM, TetraMMM and DiMMM, together with some cationic TPs, were largely removed, while three TPs, all carrying one carbonylated side chain were eventually determined in raw waters used for drinking water production, in median concentrations ranging from 0.1 to 1 μg/L. It appears that HMMM, while falling under the exemptions of REACH registration, is a precursor for widely found persistent and mobile TPs, that can reach raw waters used for drinking water production.