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Environmental fate & pathways

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(Q)SAR-modelled Koc values for terephthalic acid (obtained with the KOCWIN v2.00 model of the US EPA) range from 18.28 to 79.24 L/kg. Based on these values, terephthalic acid is classed as moderately mobile to mobile and is 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, terephthalic acid 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 values modelled for terephthalic acid also imply a low tendency to associate with sludge solids during the primary settlement and secondary biological stages of waste water treatment. The majority of the TPA 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 neutalised to protect both the plant hardware (concrete and metalwork) from corrosion and the biological treatment process from pH-shock effects, TPA 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 terephthalic acid to bind to sludge solids.

Henry's Law constants for terephthalic acid were estimated using the HENRYWIN v3.20 QSAR model available from the US EPA. Estimated Henry's Law constants of 2.21E-7 and 3.93E-8 Pa m3/mole at 25 degrees C were obtained by the bond estimation and group estimation methods, respectively.

After conversion to log10, the estimated Henry's Law constants (HLc) for terephthalic acid range from ca. -7 to -8. According to the tables presented in Appendix II of Part II of the Technical Guidance Document on Risk Assessment (European Commission, 2003), the threshold value of log10 HLc at which significant (=/>10%) air-stripping occurs under conditions of forced aeration during waste-water treatment is +1.0, and no air-stripping occurs at log10 HLc values =/<1.0. Both HLc estimates provided by HENRYWIN v3.20 indicate that terephthalic acid is unlikely to partition from aqueous systems to the atmosphere. Moreover, TPA will occur in the environment in ionised form, with an even lower tendency to volatilise to the atmosphere than the parent monomer.