Page 45 - The Toxicology of Fishes
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Bioavailability of Chemical Contaminants in Aquatic Systems                  25











































                       FIGURE 2.6 Observed pH dependence of uptake rate constants (closed circles; see Saarikoski et al., 1986) and median
                       lethal concentration (open circles; see Saarikoski and Viluksela, 1981) of several substituted phenols for guppies. Dotted
                       line denotes the expected uptake rate constant based on the value of the constant at pH 3 and the assumption that uptake
                       is proportional to the fraction of the chemical in exposure water that is un-ionized.

                       pHs.  This indicates that the pH dependence of toxic water concentrations is largely attributable to
                       processes that affect accumulation; thus, toxicity as well as accumulation can be an indicator of bio-
                       availability, at least for assessing the effects of pH.
                        Because the relative amount of un-ionized phenol decreases and that of phenolate ion increases with
                       increasing pH, a likely explanation for these effects of pH on phenol bioavailability is that the ionic
                       form is less readily accumulated.  This is a reasonable expectation because fish gill epithelial cell
                       membranes should be more permeable to un-ionized phenol molecules than to their phenolate ions. This
                       explanation is further supported by the observation that significant effects of pH are only observed when
                       the pH is near or above the pK  of a particular phenol (Figure 2.6). For pH below the pK , the phenol
                                               a
                                                                                            a
                       is almost entirely in the un-ionized form, so phenol bioavailability should not depend greatly on
                       environmental pH. At higher pH, the relative amount of the un-ionized species declines appreciably with
                       increasing pH, thereby reducing the overall bioavailability of the mixture of species.
                        The effect of pH on bioavailability, however, is not simply a matter of the un-ionized phenol in the
                       exposure water being the sole basis for accumulation. If this were the case, the observed uptake rate
                       constants should decline in proportion to the fraction of the phenol that is not ionized (dotted lines in
                       Figure 2.6). Instead, once a chemical is appreciably ionized, uptake is greater than expected based on
                       the un-ionized phenol in the exposure water, sometimes by more than an order of magnitude. For uptake
                       of several chlorinated phenols by large rainbow trout, Erickson et al. (2006b) reported even greater
                       deviations of uptake rate constants from that expected based on the amount of un-ionized phenol (see
                       Figure 2.8).
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