Bioconcentration Model for Non-Ionic, Polar, and Ionizable Organic Compounds in Amphipod

The present study presents a bioconcentration model for non-ionic, polar, and ionizable organic compounds in amphipod based on first-order kinetics. Uptake rate constant k(1) is modeled as logk(1) = 1/0:81/logK(OW)+0.15(root mean square error [RMSE] = 0.52). Biotransformation rate constant k(M) is estimated using an existing polyparameter linear free energy relationship model. Respiratory elimination k(2) is calculated as modeled k1 over theoretical biota-water partition coefficient K-blow considering the contributions of lipid, protein, carbohydrate, and water. With negligible contributions of growth and egestion over a typical amphipod bioconcentration experiment, the bioconcentration factor (BCF) is modeled as k(1)/(k(M) (+) k(2)) (RMSE = 0.68). The proposed model performs well for non-ionic organic compounds (log K-OW range = 3.3-7.62) within 1 log-unit error margin. Approximately 12% of the BCFs are underpredicted for polar and ionizable compounds. However, >50% of the estimated k(2) values are found to exceed the total depuration rate constants. Analyses suggest that these excessive k2 values and underpredicted BCFs reflect underestimation in Kbiow, which may be improved by incorporating exoskeleton as a relevant partitioning component and refining the membrane-water partitioning model. The immediate needs to build up high-quality experimental kM values, explore the sorptive role of exoskeleton, and investigate the prevalence of k(2) overestimation in other bioconcentration models are also identified. The resulting BCF model can support, within its limitations, the ecotoxicological and risk assessment of emerging polar and ionizable organic contaminants in aquatic environments and advance the science of invertebrate bioaccumulation. (C) 2018 SETAC.