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Fate of Transition Metals in Acellular Phosphate -Based Assays for PM Toxicity: Equilibrium Modelling and Macroscopic Experimental Study
JAYASHREE YALAMANCHILI, Christopher Hennigan, Brian Reed, University of Maryland, Baltimore County
Abstract Number: 58
Working Group: Health-Related Aerosols
Abstract
Among the diverse chemical constituents that make up atmospheric aerosols, transition metals are hypothesized to possess an enhanced toxicity based on their ability to support electron exchange, and to catalyze and generate reactive oxygen species (ROS) in biological tissues. The toxicity of metals is related to the oxidative stress induced by the ROS that are generated. The most widely used acellular assays for ROS measurements in PM employ phosphate (PO4) buffers (pH = 7.4 and T = 37 °C) to simulate conditions in the human body. However, the high PO4 concentrations (typically 0.1 – 0.5 M) and pH can induce physical and chemical transformations of soluble transition metals. In this study, we show that precipitation of the transition metals Fe(II), Fe(III), Cu(II), and Mn(II) are thermodynamically favored in phosphate-based assays used to measure the oxidative potential (OP) of PM. Fe and Mn precipitation is likely to occur at extremely low metal concentrations (< 0.5 µM), levels that are imperceptible to the naked eye. Cu is the most soluble of the four metals, which may partly explain its heightened sensitivity in OP assays. Macroscopic experimental results at higher metal concentrations (> 100 µM) with visible precipitates provide quasi-validation of the thermodynamic modeling. At lower metal concentrations (< 10 µM) chemical analysis of dissolved metals and laser particle light scattering measurements confirm reductions in transition metal concentrations, precipitate formation and significant oxidation of Fe(II) to Fe(III) and Mn(II) to Mn(II). The current results suggest an artifact in acellular assays of aerosol OP that is likely widespread; the implications for studies of PM toxicity are discussed.