Improving alternatives assessment of plastic additives : exploring in silico tools to identify less hazardous flame retardants
Abstract: Alternatives assessment is applied for replacing hazardous chemicals with viable, safer substitutes. High quality experimental hazard data, however, are usually unavailable for this purpose, and obtaining in silico data is the only approach to fill in data gaps. In silico tools also have the advantage of providing a large amount of data with much lower cost and time requirements.The aim of this PhD project was to explore the use of in silico tools for alternatives assessment, and develop practical tools for alternatives assessment of organic plastic additives. For this purpose, flame retardants were used as case chemicals. The major results were:1. Quantitative structure-activity relationship (QSAR) models for endocrine disruption were developed and explored (Paper I and II). These developed models were able to identify chemical properties that impact the binding affinities of brominated organic chemicals with estrogen-related receptor γ (Paper I), and to predict the androgen receptor activity of several organic chemicals, including flame retardants (Paper II);2. A hazard ranking tool was developed for alternatives assessment based on the hazard properties of persistence (P), bioaccumulation (B), mobility in the aquatic environment (M) and toxicity (T). The flame retardant decabromodiphenyl ether (decaBDE) and 16 of its alternatives were taken as case chemicals to develop the tool. From a comparison of experimental and in silico data for these case chemicals, hazard data predicted by in silico tools were identified as the more suitable data source for the hazard ranking tool as the experimental data were confounded by large data gaps (Paper III);3. The inclusion of chemical transformation products for the hazard ranking tool were studied with the case of decaBDE and its alternatives. Several in silico tools were used to predict transformation products, and a strategy for prioritizing chemical transformations with high occurrence potential in the environment was developed (Paper IV);4. Multicriteria decision analysis (MCDA) tools were used to evaluate diverse P, B, M and T endpoints of parent compounds (Paper III) and their transformation products (Paper IV) simultaneously based on in silico data. Three different MCDA methods were explored, and one of them was developed to include the consideration of uncertainties of in silico data;5. In the studied case of decaBDE alternatives, the three different MCDA methods generally agreed on the most and least hazardous alternatives. With the consideration of hazard for the studied flame retardants and their in silico predicted transformation products, two alternatives, melamine and bis(2-ethylhexyl) tetrabromophthalate, were identified as the least hazardous of considered alternatives for decaBDE (Paper III and IV);6. It is critical for the exposure aspect of alternatives assessment to identify the key properties that influence the emission process. For this, a fast measuring method for the emission of polymer additives was developed based on a Quartz Crystal Microbalance (QCM). Empirical linear models were applied to describe the emission patterns (Weibull model) to better understand the chemical mechanism behind the emissions of organophosphate flame retardants from various polymers. The results showed that the octanol-water partitioning coefficient and molecular size are key parameters for the emission process, but also showed that the emission process is complex and is likely driven by a combination of both polymer and additive properties, as well as their interactions (Paper V).This research shows how alternatives assessment can make more effective use of in silico tools. and it also highlights current challenges in the use of these in silico tools that require further development. The next steps to make a holistic alternatives assessment would include an exposure assessment procedure based on the work in Paper V, and combining this with the hazard ranking tools (developed in Paper III and IV), including information on technical feasibilities, economic feasibilities, and also life cycle impacts. The MCDA methods for hazard ranking in Paper III and IV can be further adapted for the decision component of such a more complete alternatives assessment for specific uses of chemicals.
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