Pyrithione (PT) is used as an active agent in some shampoos and other household products, and consequently may enter freshwaters following use. Dissolved metal (copper, zinc) salts of pyrithione are toxic to freshwater organisms, with toxicity dependent upon the metal. For this reason, the chemical speciation of pyrithione may be important in controlling its environmental toxicity. The objective of this work was to gather and assess data on the binding equilibria of pyrithione that could be used to assess its environmental speciation. Binding constants for the proton and a small number of metals were found in the literature.\ud They were supplemented by binding constants for other metals estimated from constants for a ligand with an analogous structure to pyrithione. Using these tentative binding constants demonstration calculations of pyrithione speciation in a circumneutral freshwater were done.\ud Pyrithione speciation was dominated by the binding of iron(III) with smaller contributions from aluminium, copper and zinc. Speciation was highly dependent upon water pH and the\ud concentration of natural organic matter. These calculations demonstrate the feasibility of modelling to elucidate the speciation of pyrithione in freshwaters; for more robust\ud calculations, experimental determination of more binding constants is needed. Knowledge of pyrithione speciation may be useful in other aspects of its risk assessment, specifically in understanding and predicting the rate at which it degrades in the natural environment
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