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Environmental fate & pathways

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Abiotic processes

The atmospheric oxidation half-life of terephthalic acid was estimated using the AOPWIN v1.91QSAR model available from the US EPA. The estimated atmospheric oxidation DT50 of terephthalic acid ranged from 8.65 days (default settings) to 12.97 days, estimated by applying the recommended northern hemisphere settings that are considered relevant in a European context.

Since terephthalic acid is readily biodegradable, a study of the hydrolysis behaviour of TPA is not required and has not been performed. Nevertheless, some insight is provided indirectly by a study of the toxicity of TPA to unicellular aquatic algae (Government of Japan, Ministry of the Environment 2003d). Terephthalic acid, dosed to aqueous algal growth test medium at a measured initial concentration of 19.0 mg/L, remained intact over the course of 72-h incubation at pH 7.8 +/- 0.3 and 22.3 +/- 0.1 degrees C. These data (DT50 > 3 days) provide evidence that TPA is not prone to rapid hydrolysis in the aquatic environment.

Similarly, no studies have been performed to investigate the phototransformation of terephthalic acid in water, however the results of the same algal study, in which TPA remained stable following continuous bright illumination for 72 hours, suggests that terephthalic is not prone to rapid photodegradation.

In summary terephthalic acid is generally resistant to physico-chemical degradation processes under the range of conditions likely to be encountered in the aquatc and terrestrial environment. Other data (see Point 5.2.1) show that terephthalic acid is readily biodegradable, with >60% mineralisation (oxidation to CO2) occuring within 5 days. Biodegradation may therefore be considered a more significant dissipation mechanism than physico-chemical processes for TPA in the environment.

Biodegradation

Two screening tests of the "ready" biodegradability of terephthalic acid (TPA) are available.

In the first (Lebertz, 1991a) terephthalic acid was tested for ready biodegradability according to the 1984 OECD 301B (Sturm Test) procedure, at concentrations of approximately 10 and 20 mg/L. The measured CO2 yield from TPA exceeded 60% of theoretical at both concentrations and the 60% threshold was crossed within the "10-day window",i.e. within 10 days of CO2 production reaching 10% of theoretical.

In the second (CITI, 1975), terephthalic acid (100 mg/L) was tested for biodegradability by the Chemicals Inspection and Testing Institute of Japan to fulfil the requirements of the Japanese Chemical Substances Control Law. A composite inoculum (applied at 30 mg suspended solids/L) originating from ten specified locations around Japan, not deliberately adapted to the test substance, fed with peptone and glucose prior to use and renewed at regular intervals (see OECD Guideline 301C 1984 and 1992 for details) was employed as standard practice at CITI for these investigations. An automated respirometer was used to make continuous measurements of biochemical oxygen demand (BOD) and recorded BOD was compared to the theoretical oxygen demand (ThOD) for TPA, calculated assuming its complete mineralisation to terminal oxidation products. This comparison provides a measure of ultimate biodegradation. Measured BOD expressed as %ThOD reached 74.7% within 14 days in this study. Confirmatory indications are provided by specific analyses for the test substance using UV-VIS and HPLC methods - these compound-specific techniques respectively show 99.3% and 100% loss of the parent test substance (primary degradation) and are consistent with the figure of 74.7% for ultimate biodegradation that was recorded in this study.

Both studies demonstrate that terephthalic acid is readily biodegradable and this result signifies that terephthalic acid will degrade rapidly and completely, without the formation of stable metabolites, under aerobic conditions in a variety environmental compartments (aquatic and terrestrial) and that extensive biodegradation may be anticipated in aerobic biological wastewater treatment processes. This (in addition to exposure considerations) obviates the need for studies of the degradation of terephthalic acid in water/sediment systems or in soil.

Based on its physico-chemical properties, terephthalic acid is expected to partition mainly toward the aqueous compartment during wastewater treatment and to be channelled predominantly toward aerobic biological (e.g. activated sludge) treatment. Nevertheless, a significant (albeit minor) proportion may become associated with sludge solids during primary settlement or with waste activated sludge and be directed toward thermophilic anaerobic digestion, which typically precedes the disposal of wastewater treatment sludges to land or alternatively by land-filling or incineration.

No guideline studies of the degradation of terephthalic acid under anaerobic conditions have been located, however data are available for its close structural analog phthalic acid (1,2 -dicarboxylic acid). Phthalic acid was completely mineralised (converted to CH4 and CO2) within 4 weeks in a screening test designed to assess the potential of organic compounds to undergo biodegradation under methanogenic conditions in digesting sludge (Battersby & Wilson, 1989, see Point 5.6). Since the screening method employed conservative conditions (a relatively high test substance concentration and no other substrate feed, combined with a very low inoculum density) it may be assumed that phthalic acid will also undergo complete degradation during the full-scale digestion process. Consequently any phthalic acid that partitions to wastewater treatment sludge solids (either primary sludge and/or surplus activated sludge) may be expected to be completely degraded before the digested product becomes available for application to soil. Since terephthalic acid and phthalic acid are structural analogues, terephthalic acid may be expected to undergo a similarly high degree of anaerobic biodegradation during methanogenic sludge digestion.

Confirmation is provided by tests performed by Kleerebezem et al. (1999) to assess the amenability of TPA-laden process waste waters to anaerobic treatment. Half-lives for TPA dosed at ca. 310 mg/L to test systems inoculated from anaerobic treatment plants operated under three different regimes ranged from 44 to 61 days. These test results show that TPA is biodegradable under anaerobic, methanogenic conditions and it may be inferred that terephthalic acid is also likely to be degraded in other anaerobic environments, such as water-logged soils or sediments.

Terephthalic acid is not persistent (not P).

Bioaccumulation

The threshold that triggers the need to investigate a potential bioconcentration/bioaccumulation tendency experimentally is a log10 Kow value greater than or equal to 3.0. The US EPA's KOWWIN model predicts a log Kow of 1.76 for terephthalic acid (TPA) and the database on which the model is constructed contains a published (public domain) value of 2.00 for TPA (Hansch, C.et al., 1995 - data attributed to Chan, T. & Hansch, C., Pomona College, unpublished results). Both these log10 Kow values lie below the trigger of 3.0 and terephthalic acid is therefore not expected to exhibit significant bioconcentration or bioaccumulation tendencies. The US EPA's model BCFBAF v3.00 predicts a bioconcentration factor in fish of 3.16 L/kg wet weight, derived from the measured log Kow value. Studies of bioconcentration/bioaccumulation are not triggered for TPA.

It may be concluded that terephthalic acid is not bioaccumulative (not B).

Terephthalic acid is not expected to remain stable in the form of the free acid under environmental conditions. Aquatic ecotoxicology studies have been conducted with TPA after converting it to its disodium salt to increase its solubility and the range of achievable exposure concentrations. This is considered representative of the likely behaviour of TPA in the environment. The increased aqueous solubility of terephthalate salts relative to that of the free acid implies a corresponding decrease in log10 Kow and hence an even lower bioconcentration/bioaccumulation potential.

This is confirmed by measured data available for isopthalic acid. Isophthalic acid is a structural analog of TPA that has an identical KOWWIN log Kow estimate of 1.76. The octanol/water partition coefficient of isophthalic acid has been determined according to the shake-flask procedure, in a system buffered to pH 7 (see Point 4.7, Hatoum & Garthwaite, 1992). The mean log Kow obtained for IPA under these conditions was -2.34. This value implies a much higher relative solubility of IPA in the aqueous phase than the log Kow indicated by KOWWIN QSAR. This is likely to have been caused by the presence of the buffer used to maintain the test system at pH 7: IPA would have been converted under these conditions to salts with higher aqueous solubility than that of the free acid. Similar behaviour, with a log Kow similarly lower than that of the free acid, may be expected for TPA under comparable conditions, following conversion of the parent monomer to its more soluble salts.

Transport and distribution

(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 alkaline 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 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.