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Environmental fate & pathways

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The most significant values in terms of environmental assessment are relative to the acid form of the anhydride, because trimellitic anhydride (TMA) hydrolyses almost instantaneously on contact with water to give trimellitic acid. (TMLA).

Adsorption / desorption

(Q)SAR-modelled adsorption coefficient (Koc) values for trimellitic anhydride (TMA) and trimellitic acid (TMLA) obtained with the KOCWIN v2.00 model of the US EPA range from ca. 0 to 387 L/kg. Based on these values, TMA and TMLA are classed as moderately to very mobile and are expected to have a low tendency to adsorb to soils and sediments. Koc may be influenced by and vary significantly in response to pH. Under environmental conditions where TMA is expected to have undergone complete hydrolysis, TMLA will rapidly be converted to salts whose Koc may be expected to be lower (mobility higher) than that of the free acid.

The low Koc value modelled for TMLA also implies a low tendency to associate with sludge solids during the primary settlement and secondary biological stages of waste water treatment. The majority of the TMLA load contained in a treatment plant influent load may therefore be expected partition to the aqueous phase and to be routed toward aerobic biological treatment. Since process effluents discharged to treatment facilities are typically neutralised to protect both the plant hardware (concrete and metalwork) from corrosion and the biological treatment process from pH-shock effects, TMLA is likely to be discharged in the form of salts that are more highly water soluble and have a correspondingly lower Koc than the parent acid. Salts formed by the pre-treatment neutralisation step are likely to have an even lower tendency than that of free TMLA to bind to sludge solids.

Henry's Law constants

The HLC value of 1.29E-05 Pa m³/mol at 25 deg C indicates that TMA is essentially not volatile from surface water

The HLC limit value of 1.01E-07 Pa m³/mol at 25 deg C indicates that TMLA is essentially not volatile from surface water.

Distribution modelling

Distribution in environmental compartments has been calculated using a Fugacity model according to Mackay, Level III, giving the following results:

- Trimellitic anhydride (TMA) - Emission scenario: 1000 kg/h into each of air and soil compartments.

Air= ca. 0%; Water= 15.5%; Soil= 84.4% and Sediment= 0.04%.

- Trimellitic acid (TMLA) - Emission scenario: 1000 kg/h into each of air, water and soil compartments

Air= ca. 0%; Water= 39; Soil= 60.9% and Sediment= 0.07%.

The soil, and so less water, were then found to be the major target compartments.