Relation between toxicity and accumulation of chlorophenols at various pH, and their absorption mechanism in fish

The relation between the acute toxicity and accumulation of 12 chlorophenols in goldfish exposed to media containing the chemicals at various pH, and also the transport mechanism of chlorophenols from the surrounding water in the fish were investigated. Both the acute toxicity and accumulation of chlorophenols increased with increasing number of substituted chlorine atoms, although the magnitude of their increase decreased with increasing pH. For each chlorophenol, both the toxicity and the amount accumulated in the surviving fish was almost unchanged in the range of pH < pKa, but abruptly decreased with increasing media pH in the range of pH > pKa. On the other hand, the amount found in the dead fish was little dependent on the pH of media. The distribution ratio (K) between the medium (compartment I) and fish body (compartment II) was calculated on the basis of the assumption that only the undissociated form migrates between the two compartments by diffusion and that the ratio of the concentration of the undissociated form in compartment I to that in compartment II is proportional to the partition coefficients in 1-octanol-water and n-heptane-water systems. A good correlation was observed between log bioconcentration ratio (BCR) and log K value calculated by using the partition coefficient in the 1-octanol-water system. From these results, it is concluded that the transfer of chlorophenols from media to fish body, in which the formation of intermolecular hydrogen bond between the hydroxyl group in the chemicals and the components in gill membranes plays an important role, is mainly caused by the passive diffusion of the undissociated form through the membranes, and the concentration of the chemicals in the fish body does not readily reach a lethal level with increasing pH of the medium owing to the increased conversion of the undissociated form to the dissociated form, resulting in the reduction of the toxicity of chlorophenols in the fish. However, the BCR was a little higher than that estimated from the concentration of the undissociated form in the media. The reason was presumed that the pH near the outer surface of the gill membranes is lower than that of the media.