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Environmental fate & pathways

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

In the first (Anonymous, 1993a) dimetyl 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 (Ziogou et 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 Ziogou et 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).