Dow and Kaw,eff vs. Kow and K°aw: Acid/base ionization effects on partitioning properties and screening commercial chemicals for long-range transport and bioaccumulation potential

Abstract

A set of 543 ionizable commercial organic compounds with various acid/base functionalities and experimental octanol-water partitioning coefficients (log Kow) were obtained from the Canadian Domestic Substances List. Corresponding pH-dependent octanol-water distribution coefficients (log Dow) and air-water partitioning coeffients (log Kaw,eff) were estimated using the SPARC software program, as were log Kow and log K°aw values for the neutral forms of each chemical. Significant ionization dependent effects on chemical screening results at various pH values were obtained using established criteria for bioaccumulation potential (BAP) in aquatic organisms, terrestrial animals, and humans, as well as for atmospheric long range transport potential (LRTP). Future modelling efforts for environmental and toxicological screening of commercial chemicals should therefore explicitly include the influence of ionization for both weak and strong organic acids and bases on bioavailability and air-water mobility within the respective regulatory frameworks. Functional group specific sorption of both ionizable and neutral compounds to particulate and dissolved inorganic and organic matter will also affect chemical screening results for BAP and LRTP. More complex sorption related modelling in various types of representative aquatic systems also appears necessary to achieve reliable chemical screening results for commercial organic compounds.