Fate, Exposure and Effects

The direct release of MBMBP to surface water could occur in sewage treatment plant (STP) effluent, at STPs that receive influent from the processing facilities. Releases to air are expected to be negligible, because MBMBP’s vapour pressure is very low. Releases to soil could occur from the application of STP sludge that contains MBMBP. MBMBP could partition to sediment from surface water that contains MBMBP.

The Level III fugacity model (CEMC, 2002) has been used to predict the environmental fate of MBMBP. According to Level III fugacity modelling, MBMBP is expected to partition mainly to soil (65.3%) and sediment (25.7%), assuming equal release to all media. If released to water, most (74.3%) of the substance would tend to partition to the sediment phase, while release only to soil or only to air would result in partitioning mostly (>90%) into the soil.

Experimental results indicate that MBMBP is persistent in water (CITI, 1992). Predicted results indicate that MBMBP is not persistent in air (SRC, 2001). No experimental data were found for half-lives in soil or sediment.

Data concerning measured levels of this substance in air, water, soil and sediment in Canada were not found. The Screening Information Data Set (SIDS) Initial Assessment Report (SIAR) (OECD, 2001) reports that no quantitative monitoring data are available globally, including Canada. No data could be found for concentrations of MBMBP in wildlife in Canada or worldwide.

Environmental concentrations were calculated given potential losses during plastic processing. Plant effluents were assumed to be treated by municipal STPs before discharge to the environment. The local concentration in STP effluent is calculated using ChemSim, which is a modelling program developed for Environment Canada that predicts aquatic concentrations downstream from point sources of a substance’s release (Canadian Hydraulics Centre, 2003). In order to run ChemSim, inputs, including the loading rate, are required. In this case, the loading rate is the mass of MBMBP in STP effluent released in one day. To calculate the loading rate, the following conservative assumptions were made:

• The upper range of the quantity of MBMBP imported in 2000 is 100 tonnes (100 000 kg). For the conservative exposure scenario, we have assumed that this quantity is imported by one distributor who sells to one customer who:
  
  
  • uses the total quantity of MBMBP as a plastic additive,
    
  • uses the total quantity at one processing facility in one calendar year, and
    
  • discharges plant effluent into a municipal sewage treatment system.
    
• For calculating the amount of MBMBP released from the processing facility into the municipal sewage system, the Organisation for Economic Co-operation and Development (OECD) emission release scenario for plastic additives was used, with a release percentage of 0.65% (OECD, 2003). For determining the number of operating days (300 per year), the European Union Technical Guidance Document was used (European Chemicals Bureau, 2003).
  
• The 92.2% removal rate for MBMBP at the STP was calculated using the STP fugacity model within the EPI Suite of models, version 3.10 (SRC, 2001).

The ChemSim model run assumed a standard river in Southern Ontario with a flow rate of 5 m³/s and predicted a maximum concentration of MBMBP of 7.46 × 10⁻³ mg/L, 50 m from the point of impingement (release). Further assumptions required to run ChemSim (pertaining to river flow and channel geometry) are listed in the ChemSim report for this substance (Environment Canada, 2004). The estimated exposure value for the aquatic medium (EEVaq) is therefore 7.46 × 10⁻³ mg/L.

Most of the MBMBP in STP influent is removed (92.2%) and ends up in the sewage sludge. Since application of sewage sludge to agricultural land is a possibility, we have considered an exposure scenario involving sewage sludge–amended soil. No data on MBMBP concentrations in Canadian sewage sludge or soil were found. For the conservative soil exposure scenario, we calculate a concentration of MBMBP in sewage sludge to be 257 mg/kg dry weight, based on standard calculations, adapted to MBMBP, for estimating the concentration of a substance in sewage sludge (Droste, 1997). Using this MBMBP concentration in sewage sludge and assuming that MBMBP-containing sludge is applied to the land for 10 years (OMOE, 1996) and that no or little biodegradation of the MBMBP occurs, this would result in a soil concentration of 1.64 mg/kg dry weight. The estimated exposure value for soil (EEV_soil) is therefore 1.64 mg/kg dry weight.

MBMBP is not likely to be bioaccumulative, according to experimental results (OECD, 2001). The highest reported bioconcentration factor in this study was 125, significantly below the criterion for bioaccumulation of 5000 in the Persistence and Bioaccumulation Regulations of CEPA 1999 (Government of Canada, 2000). Caution should be exercised in the interpretation of these results, however, because the study used a solubilizer to achieve MBMBP concentrations above its aqueous solubility limit, so the actual bioconcentration factor might be higher than reported.

Experimental toxicity data exist for effects in aquatic organisms (green algae, water flea and fish; OECD, 2001). There is some uncertainty connected to these data, because the toxicity values are all above the water solubility limit. The critical toxicity value (CTV) selected is the lowest acceptable chronic value of 0.89 mg/L, the lowest-observed-effect concentration (LOEC) for immobility in Daphnia magna (water flea). No toxicity data were found for effects on soil or sediment-dwelling organisms, terrestrial plants or wildlife. A study found that MBMBP has a predicted relative binding affinity of 0.0034, which indicates that MBMBP is a low priority for further studies to determine its endocrine modulating activity (Klopman and Chakravarti, 2003).