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Terephthalic acid (TPA) is only slightly soluble in water (ca. 17 mg/L at 25 degrees C, according to Park & Sheehan (1996)) and this property limits the exposure concentrations of TPA that are achievable under the conditions of aquatic toxicity tests.

None of the uses of terephthalic acid involve direct application to surface water. The potential for unintended exposure of the aquatic compartment is therefore confined to TPA residues contained in treatment plant effluents discharged to surface waters. Moreover, it should be noted that production/manufacturing process effluents will 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. Even without this deliberate intervention, TPA would rapidly be converted to terephthalate salts during treatment and/or in the aquatic environment. In terms of environmental exposure, the much more highly water-soluble terephthalate salts are therefore more relevant than the parent terephthalic acid.

Two sets of reliable (Klimisch 1, guideline- and GLP-compliant) studies address the aquatic toxicity of TPA.

In the first set (Government of Japan, Ministry of the Environment, 2003a-d), aquatic organisms were exposed to high purity TPA (free acid), dosed from stock solutions prepared with DMSO, at a concentration intended to approximate to TPA's aqueous solubility limit. No toxicity was observed under these test conditions:

Himedaka (O. latipes) 96 -h LC50 (semi-static): >18.6 mg TPA/L, 96 -h NOEC: 18.6 mg TPA/L;

D. magna 48 -h EC50 (semi-static): >20.1 mg TPA/L, 48 -h NOEC: 20.1 mg TPA/L;

P. kirchneriella: 72 -h ErC50 (static): >19.0 mg TPA/L, 72 -h NOErC: 19.0 mg TPA/L;

D. magna: 21 -d NOEC (semi-static): 19.5 mg TPA/L.

All these endpoints are mean measured values and all represent the highest or the single limit concentration achieved under the test conditions.

In the second set (Knacker et al., 1993a-c) TPA was first treated with NaOH solution, to convert the acid to its much more soluble sodium salt(s), and exposure in these studies was consequently to sodium terephthalate (following neutralisation of excess alkali). No toxicity was observed under these conditions:

Golden orfe (L. idus melanotus) 96 -h LC50 (static): >961 mg TPA-equiv/L, 96 -h NOEC: 961 mg TPA-equiv/L;

D. magna 48 -h EC50 (static): >967 mg TPA-equiv/L, 48 -h NOEC: 967 mg TPA-equiv/L;

D. subspicatus 72 -h ErC50: >668 mg TPA-equiv/L, 72 -h NOEC: 668 mg TPA-equiv/L.

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

Additional acute aquatic toxicity data, deemed not reliable (Klimisch 3) on the basis of the scant detail provided in the report (Lockhart, 1977) are available for other fish and invertebrate species and, deficiencies notwithstanding, add to the weight of evidence of the low toxicity of terephthalic acid to aquatic organisms. In these studies, TPA was first treated with NaOH solution and neutralised, and exposure was therefore to sodium terephthalate:

Fathead minnow (P. promelas) 96 -h LC50: >100 mg TPA-equiv/L;

D. magna, Dugesia tigrina (flatworm) and Helisoma trivolvis (gastropod mollusc): 96 -h LC50 values all >100 mg TPA-equiv/L.

In conclusion, terephthalic acid and its more environmentally relevant sodium terephthalate salt exhibit very low toxicity to fish, aquatic invertebrates and unicellular algae.

In a reliable (Klimisch 1, guideline- and GLP-compliant) study of the toxicity of terephthalic acid to aquatic microorganisms (Lebertz, 1991), the 3 -h EC50, based on inhibition of respiration of activated sludge, was 1392.8 mg/L. Inhibition, relative to the untreated control, was observed at concentrations >/= 1000 mg/L and the NOEC was 500 mg/L. All the nominal concentrations applied exceeded the water-solubility of the test substance, although it is likely that some more highly soluble terephthalate salts were formed by interaction between the acid and the mineral constituents of the synthetic sewage present in the test system. No pH measurements were reported; consequently it is uncertain to what degree a lowering of pH may have caused or contributed to the observed effect.

Nevertheless, it may be concluded that terephthalic acid presents low toxicity to aquatic microorganisms and to biological waste water treatment processes.