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Trimellitic anhydride (TMA) is unstable in the presence of moisture and undergoes rapid hydrolysis to trimellitic acid (TMLA). The hydrolysis of trimellitic anhydride acid chloride was virtually instantaneous at 40 degrees C (Lord and Cooper, 1992) and similarly fast hydrolysis of TMA would also be anticipated in production and manufacturing waste waters, with complete conversion to trimellitic acid occurring even before reaching waste water treatment facilities. In view of its rapid hydrolysis, it is not technically possible to determine the aqueous solubility limit of the parent anhydride, however TMLA is very soluble in water: its solubility in water is 24,400 mg/L at 20 degrees C and pH 1.8 (Villa, 2010, see Point 4.8).

None of the uses of trimellitic anhydride involve direct application to surface water. The potential for unintended exposure of the aquatic compartment is therefore confined to TMA residues contained in treatment plant effluents discharged to surface waters and, for the reasons given above, these residues will comprise derivatives of the hydrolysis product TMLA rather than the acid anhydride. Moreover, it should be noted that any production/manufacturing process effluents would be pH-balanced (neutralised) prior to treatment, to safeguard the treatment plant infrastructure from corrosion damage as well as to protect the biological purification process from adverse impacts of pH-shock. Even without this deliberate intervention, TMLA would rapidly be converted to trimellitate salts during treatment and/or in the aquatic environment. In terms of environmental exposure, trimellitate salts may therefore be more relevant than free trimellitic acid.

A number of reliable studies address the aquatic toxicity of TMA (TMLA). In these studies (Knackeret al., 1992, 1993a & b; Lebertz, 1991) TMA was first treated with aqueous NaOH solution, to convert the acid anhydride to the sodium salt(s) of TMLA, and exposure in these studies was consequently to sodium trimellitate (following neutralisation of excess alkali). No toxicity was observed under these conditions:

Acute toxicity to fish - Golden orfe (L. idus melanotus) 96 hour LC50 (static): >957 mg TMA-equiv/L, 96 hour NOEC: 957 mg TMA-equiv/L;

Acute daphnia inhibition -D. magna48 hour EC50 (static): >792 mg TMA-equiv/L, 48 hour NOEC: 792 mg TMA-equiv/L;

Algal growth inhibition -D. subspicatus72 hour ErC50: >739 mg TMA-equiv/L, 72 hour NOEC: 739 mg TMA-equiv/L.

All these endpoints are mean measured values and represent the highest concentration applied.

Toxicity to aquatic microorganisms: 3 hour EC50, based on inhibition of respiration of activated sludge, was in the range >100, <500 mg/L. Inhibition, relative to the untreated control, was observed at concentrations >/= 500 mg/L and the NOEC was 100 mg/L.

No aquatic toxicity studies with chronic exposure are available on TMA (TMLA) itself. Reliable data on fish are available for two structural analogues of the substance, methyl-tetrahydrophthalic anhydride (THPA) and phthalic anhydride, with determined NOEC values of 100 and 10 mg/L, respectively. The lower NOEC value of 10 mg/L determined for phthalic anhydride may be used as suitable read-across value for TMA considering a worst case approach. The NOEC from this study was 10 mg/L which, when corrected for differences in molecular weight, results in a calculated NOEC of 13 mg/L for TMA. Data from chronicDaphniastudies are available on three structural analogues of the substance, which after considering the differences in molecular weight, lead to comparable NOEC values. The best quality data originates from a recent study with MTHPA. The NOEC from this study was 20 mg/L which, when corrected for differences in molecular weight, results in a calculated NOEC of 23 mg/L for TMA.

In conclusion, trimellitic anhydride (TMA) and the more environmentally relevant trimellitic acid (TMLA) exhibit very low toxicity to fish, aquatic invertebrates, unicellular algae and to aquatic microorganisms including biological waste water treatment processes.