Isophthalic acid

Endpoint summary

Additional information

Abiotic processes

The atmospheric oxidation half-life of isophthalic acid (IPA) was estimated using the AOPWIN v1.91model available from the US EPA. The estimated atmospheric oxidation DT50 of isophthalic acid ranged from 8.18 days (default settings) to 12.27 days, estimated by applying the recommended northern hemisphere settings that are considered relevant in a European context.

 

Since isophthalic acid is readily biodegradable, a formal study of the hydrolysis behaviour of IPA at three pH values is not required and has not been performed. Nevertheless, some insight is provided by a study of the toxicity of IPA (converted to its sodium salt(s) prior to test initiation) to unicellular aquatic algae (Knacker et al., 1993c). The IPA-equivalent concentrations, initially dosed to non-sterile aqueous algal growth test medium at 67.9, 298 and 1023 mg/L (measured), were reduced by - on average - ca. 16% over the course of 96-h incubation at pH ca 7and 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 > 4 days) provide clear evidence that IPA is not prone to rapid hydrolysis in the aquatic environment.

 

Similarly, no studies are required or have been performed to investigate the phototransformation of isophthalic acid in water, however the results of the same algal study, in which IPA remained stable following continuous bright illumination for 96 hours, suggests that isophthalic acid is not prone to rapid photodegradation. No data are available for the phototransformation of IPA in soil, however rapid photolysis may be considered unlikely, based on its apparent resistance to rapid phototransformation in water.

 

In summary isophthalic acid is generally resistant to physico-chemical degradation processes under the range of conditions likely to be encountered in the aquatic and terrestrial environment. Other data (see Point 5.2.1) show that isophthalic acid is readily biodegradable, with >60% mineralisation (oxidation to CO2) occurring within 5 to 7 days. Biodegradation may therefore be considered a more significant dissipation mechanism than physico-chemical processes for IPA in the environment.

 

Biodegradation

Two screening tests of the "ready" biodegradability of isophthalic acid are available.

 

In the first (Lebertz, 1991a) isophthalic 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 IPA 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, 1976), isophthalic 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 IPA, calculated assuming its complete mineralisation to terminal oxidation products. This comparison provides a measure of ultimate biodegradation. Measured BOD expressed as %ThOD reached 77.7% within 14 days in this study and further measurements indicated 85.3% ultimate degradation based on TOC removal (the "pass" criterion that applies to this parameter is 70%). Confirmatory indications are provided by specific analyses for the test substance using GC and UV-VIS spectrometric methods - these compound-specific techniques showed 100% and 96% loss of the parent test substance (primary degradation) and are consistent with the indications of ultimate biodegradation recorded in this study.

 

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

 

Based on its physico-chemical properties, isophthalic 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 quantity may become associated with sludge solids during primary settlement or with waste activated sludge during secondary treatment 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.

 

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 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 isophthalic acid and phthalic acid are structural analogues, the same conclusions may be drawn for the behaviour of isophthalic acid during methanogenic sludge digestion.

 

Confirmation is provided by tests performed by Kleerebezem et al. (1999) to assess the amenability of IPA-laden process waste waters to anaerobic treatment. Half-lives for IPA dosed at ca. 310 mg/L to test systems inoculated from anaerobic treatment plants operated under three different regimes ranged from 74 to 156 days. These test results show that IPA is biodegradable under anaerobic, methanogenic conditions and it may be inferred that isophthalic acid is also likely to be degraded in other anaerobic environments, such as water-logged soils or sediments.

 

Isophthalic acid is not persistent (not P).

 

Bioaccumulation

The threshold at which it becomes necessary 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 isophthalic acid (IPA) and the database on which the model is constructed contains a published (public domain) value of 1.66 for IPA (Hansch, C. et al., 1995). Both these log10 Kow values lie below the trigger of 3.0 and isophthalic 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 IPA.

 

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

 

Isophthalic acid is not expected to remain stable in the form of the free acid under environmental conditions. Aquatic ecotoxicology studies have been conducted with IPA 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 IPA in the environment. The increased aqueous solubility of isophthalate salts relative to that of the free acid implies a corresponding decrease in log10 Kow and hence bioconcentration/bioaccumulation potential.

 

Transport and distribution

(Q)SAR-modelled Koc values for isophthalic acid (obtained with the KOCWIN v2.00 model of the US EPA) range from 11.86 to 79.24 L/kg. Based on these values, isophthalic acid is classed as moderately to very 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, isophthalic 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 isophthalic 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 IPA 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 neutralised to protect both the plant hardware (concrete and metalwork) from corrosion and the biological treatment process from pH-shock effects, IPA 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 isophthalic acid to bind to sludge solids.

 

Henry's Law constants for isophthalic 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 isophthalic 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 isophthalic acid is unlikely to partition from aqueous systems to the atmosphere.