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Environmental fate & pathways

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Abiotic processes

The atmospheric oxidation half-life of dimethyl terephthalate was estimated using the AOPWIN v1.91QSAR model available from the US EPA. The estimated atmospheric oxidation DT50 of dimethyl terephthalate ranged from 18.64 days (default settings) to 27.96 days, estimated by applying the recommended northern hemisphere settings that are considered relevant in a European context.

Since dimethyl terephthalate is readily biodegradable, a formal study of the hydrolysis behaviour of DMT at three pH values is not required and has not been performed. Nevertheless, some insight is provided indirectly from other sources. Hydrolysis of dimethyl terephthalate is exploited as one of the commercial production methods of terephthalic acid, but this process requires high temperatures (260 to 280 degrees C) and pressures (4500 to 5500 kPa) and on this basis DMT may be considered unlikely to hydrolyse rapidly under normal environmental conditions.Further insight is provided by a study of the toxicity of DMT to unicellular aquatic algae (Salinas, 2010b). The concentration of dimethyl terephthalate, initially dosed to non-sterile aqueous algal growth test medium at 29.4 mg/L (measured), was reduced by 10% over the course of 72-h incubation at pH 8.1 and 23 degrees C. Although this small reduction may have been the result of biodegradation, photolysis, hydrolysis or any combination of these processes, these data (DT50 > 3 days) provide clear evidence that DMT is not prone to rapid hydrolysis in the aquatic environment.

Similarly, no studies are required or have been performed to investigate the phototransformation of dimethyl terephthalate in water, however the results of the same algal study, in which DMT remained stable following continuous bright illumination for 72 hours, suggest that dimethyl terephthalate is not prone to rapid photodegradation.

In summary dimethyl terephthalate 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 dimethyl terephthalate is readily biodegradable, with >60% mineralisation (oxidation to CO2) occuring within 14 days. Biodegradation may therefore be considered a more significant dissipation mechanism than physico-chemical processes tor DMT in the environment.

Biodegradation

Two screening tests of the ultimate "ready" biodegradability of dimethyl terephthalate are available.

In the first (Anonymous, 1993a) dimethyl terephthalate was tested for ready biodegradability according to the BODIS procedure, at a concentration of approximately 11 mg/L. Periodic measurements of biochemical oxygen demand (BOD) were compared to a theoretical oxygen uptake calculated assuming the complete mineralisation of DMT to its terminal oxidation products, and showed that 83.8% degradation occurred within 14 days. The 60% pass level was exceeded within the 10 -day window.

In the second study (CITI, 1980), dimethyl terephthalate (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 a theoretical oxygen uptake. Measured BOD expressed as %ThOD reached 84% within 14 days in this study. Confirmatory indications are provided by specific analyses for the test substance using an HPLC method - this compound-specific technique showed 100% loss of the parent test substance (primary degradation) and is consistent with the figure of 84% for ultimate biodegradation that was recorded in this study.

Both studies demonstrate that dimethyl terephthalate is readily biodegradable and this result signifies that dimethyl terephthalate 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 dimethyl terephthalate in water/sediment systems or in soil.

Based on its physico-chemical properties, dimethyl terephthalate 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 dimethyl terephthalate under anaerobic conditions have been located, however data are available for its close structural analog dimethyl phthalate (see Point 5.6). Dimethyl phthalate was biodegraded by >90% in 8 days in a series of tests designed to simulate conditions in anaerobic sludge digesters at STPs (Ziogouet al., 1989). Since dimethyl phthalate and dimethyl terephthalate are isomers, dimethyl terephthalate may be expected to undergo a similarly high degree of anaerobic biodegradation during methanogenic sludge digestion. Dimethyl terephthalate is also likley to be degraded anaerobically in water-logged soils or sediments. These findings relate to specific analytical measurements of concentrations of the parent molecule (the same method was applied to five other, related phthalate compounds in a series of similar investigations); they therefore indicate primary biodegradation (a structural transformation of the phthalate moiety) but not necessarily ultimate degradation - complete mineralisation to the terminal products CO2 and CH4.

Evidence confirming the ultimate anaerobic biodegradation potential of phthalic acid (and hence DMP and DMT following primary cleavage) is provided by Battersby and Wilson (1989) who demonstrated that 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. Since the screening method employed conservative conditions (a high test substance concentration and no other substrate feed, combined with a very low inoculum density) it may be assumed that phthalic acid will undergo complete degradation during the full-scale digestion process. This study provides evidence that the anaerobic biodegradation of dimethyl phthalate demonstrated by Ziogouet al. - and implied by read across for dimethyl terephthalate - will proceed to completion (i.e. to CO2 and CH4).

Consequently any dimethyl phthalate 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 dimethyl phthalate and dimethyl terephthalate are isomers, dimethyl terephthalate 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 DMT-laden process waste waters to anaerobic treatment. Half-lives for DMT dosed at ca. 290 mg/L to test systems inoculated from anaerobic treatment plants operated under three different regimes ranged from 39 to 58 days. These test results show that DMT is biodegradable under anaerobic, methanogenic conditions and it may be inferred that dimethyl terephthalate is also likely to be degraded in other anaerobic environments, such as water-logged soils or sediments.

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Dimethyl terephthalate 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. A study of the octanol/water partition coefficient performed according to the shake-flask procedure of OECD Guideline 107 provides a measured value for log Kow of 2.21 for dimethyl terephthalate (DMT). The US EPA's KOWWIN model predicts a log Kow of 1.66 for DMT and the database on which the model is constructed contains a published (public domain) value of 2.25 (Hansch, C.et al., 1995). All three of these log10 Kow values lie below the trigger of 3.0 and dimethyl terephthalate is therefore not expected to exhibit significant bioconcentration or bioaccumulation tendencies. Studies of bioconcentration/bioaccumulation are not triggered for DMT.

It may be concluded that dimethyl terephthalate is not bioaccumulative (not B).

Transport and distribution

(Q)SAR-modelled Koc values for dimethyl terephthalate (obtained with the KOCWIN v2.00 model of the US EPA) range from 30.96 to 109.3 L/kg. Based on these values, dimethyl terephthalate is classed as moderately mobile to mobile and is expected to have a low tendency to adsorb to soils and sediments.

The low Koc values modelled for dimethyl terephthalate 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 DMT 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.

Henry's Law constant was calculated using experimental values for vapour pressure and water solubility. The value determined is 0.870696 Pa m3/mole at 20°C (equivalent to Log H value of -0.06013).

The resulting value indicates that dimethyl terephthalate is unlikely to partition from aqueous systems to the atmosphere.