Assessing the solubility, chemical stability and ecotoxicology of an emerging non-halogenated flame retardant, melamine cyanurate, against a prevalent halogenated congener, tetrabromophthalic anhydride

Concerns over plastic-associated chemical toxicity are increasing amid the plastic pollution crisis. Halogenated flame retardants, though effective in plastic development, are being phased out due to toxicity, while nitrogen-based alternatives, such as melamine cyanurate (MC), are considered more chemically stable and less toxic. Here, we assess the solubility and chemical stability of MC in freshwater using various solvents and evaluate degradation after UV exposure. Additionally, we compare the acute and chronic aquatic toxicity of MC to the more widespread halogenated flame retardant tetrabromophthalic anhydride (TBA) using the Daphnia magna invertebrate model. Toxicity of a common solvent, dimethyl sulfoxide (DMSO), was also assessed. MC was insoluble in 16 of 18 tested solvents, with solubility only seen in a strong acid and base. UV exposure for 72 h within freshwater media indicated minimal degradation, classifying MC as a highly stable compound. Acute toxicity tests at 1-20 mg/L showed no significant difference in EC₅₀ values between TBA (0.16 - 11.46 mg/L) and MC (5.91 - 13.23 mg/L). Chronic toxicity tests at 0.5, 5, and 15 mg/L yielded NOEC values of ≤5 mg/L for TBA and <5 mg/L for MC. At 15 mg/L, chronic exposure to TBA, MC, and DMSO resulted in 100% mortality in D. magna. These findings challenge the assumption that DMSO is a low-toxicity solvent in aquatic testing. Overall, the study highlights the difficulty in assessing the toxicity of highly stable flame retardants like MC, while indicating that they may exhibit similar aquatic toxicity as halogenated congeners.