Toward the Development and Application of an Environmental Risk Assessment Framework for Microplastic.

Emissions of plastic waste to the environment and the subsequent degradation into microplastic particles that have the potential to interact with biological organisms represent a concern for global society. Current understanding of the potential impacts on aquatic and terrestrial population stability and ecosystem structure and function associated with emissions of microplastic particles is limited and insufficient to fully assess environmental risks. Multistakeholder discussions can provide an important element in helping to identify and prioritize key knowledge gaps in assessing potential risks. In the present review, we summarize multistakeholder discussions from a 1-d International Council of Chemical Associations-sponsored symposium, which involved 39 scientists from 8 countries with representatives from academia, industry, and government. Participants were asked to consider the following: discuss the scientific merits and limitations of applying a proposed conceptual environmental risk assessment (ERA) framework for microplastic particles and identify and prioritize major research needs in applying ERA tools for microplastic particles. Multistakeholder consensus was obtained with respect to the interpretation of the current state of the science related to effects and exposure to microplastic particles, which implies that it is unlikely that the presence of microplastic in the environment currently represents a risk. However, the quality and quantity of existing data require substantial improvement before conclusions regarding the potential risks and impacts of microplastic particles can be fully assessed. Research that directly addresses the development and application of methods that strengthen the quality of data should thus be given the highest priority. Activities aimed at supporting the development of and access to standardized reference material were identified as a key research need. Environ Toxicol Chem 2019;38:2087-2100. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Binding catalysis and inhibition by metal ions and protons in the enolization of phenylacetylpyrazine.

A study of the enolization of phenylacetylpyrazine (PzCOCH(2)Ph) catalyzed by acid, base and metal ions in aqueous solution shows, unusually, that metal ions are more effective catalysts than protons, e.g., for zinc k(Zn)/k(H) = 600. Such behavior contrasts with that of the structurally related phenacylpyridine (PyCH(2)COPh) for which k(Zn)/k(H) = 0.0065. To interpret this difference, equilibrium constants for the tautomerization of phenylacetylpyrazine and for binding of protons and metal ions to its keto tautomer and enolate anion have been measured or estimated and are compared with existing measurements for phenacylpyridine. A tautomeric constant, K(E) = 1.2 x 10(-3) (pK(E) = 2.9), is derived by combining forward and reverse rate constants for enolization measured, respectively, by iodination or bromination of the keto tautomer and relaxation of the less stable enol. For the keto tautomer, NMR measurements yield a pK(a) = -0.90 for N-protonation, and spectrophotometric measurements give pK(a) = 11.90 for ionization to an enolate anion. For the enol, pK(a) values of 0.44 and -4.80 for mono- and diprotonation are obtained from the pH profile for ketonization and absorbance measurements for the transient enol reactant. Binding constants for metal ions (Cu(2+), Ni(2+), Zn(2+), Co(2+), and Cd(2+)) are derived from the saturation of their catalysis of the ketonization reaction. It is found that ketonization is efficiently catalyzed by metal ions but inhibited by acid. These findings, and the striking difference from phenacylpyridine, are ascribed to differences in thermodynamic driving force arising from stronger binding of the proton to the more basic pyridine than pyrazine nitrogen atom in both the reactant keto tautomer and in the enaminone or zwitterion product of the rate-determining (proton transfer) step of the enolization.