An additional observation made within this report is the need to differentiate between acids and bases, particularly when considering the relative importance of trigger values. This also relates to likely differences in sorption behaviour of the two different classes of chemicals, which will likely result in significant differences in bioavailability. Based on the data presented in Figures 14 and 15 (Section 4.1), the anionics, even with relatively large DOW may have a greater PEC than cationics with similar DOW. Consequently, the trigger value for the acid and base should be represented accordingly.
It is assumed that the trigger values, based on KOW and summarised in Table 5 of Section 2.2.3, for bioconcentration testing are meant to assess the bioconcentration potential of chemicals based on their potential for enhanced hydrophobic interactions. There has been an historic reliance on the use of octanol as a surrogate for biological lipids (Leo et al, 1971), and therefore exposure to chemicals with KOW values > 1,000 can be used as a means for screening chemicals that may have the potential to accumulate in biological lipids. The assumption appears to be generally applicable to neutral organic compounds, but there are concerns regarding the applicability of using KOW as a trigger value for bioconcentration testing for ionisable organic compounds. The main concern regards the relative importance of hydrophobic interactions for ionisable organic compounds, versus other potential mechanisms of interaction. As a preliminary step towards addressing this concern, it is suggested that DOW, reported at pH 7, represents a better metric as a trigger value, than does KOW, since it should account for the ionisation state of the chemical. Unfortunately, there is a paucity of data to definitively demonstrate the utility of using DOW as opposed to KOW. It is acknowledged, however, that the current trigger values listed in Table 5, Section 2.2.3, appear to be responsible for a relatively significant increase in the number of bioconcentration tests being commissioned by pharmaceutical companies. Thus, analogous to data illustrated in Figure 16, which attempts to provide a low-tiered approach for estimating the sorption of monoprotic bases to sludge solids, based on a relationship with DOW, it may be possible to acquire data that could be used to demonstrate the role that DOW might play in screening ionisable organic compounds for bioconcentration. The Task Force thus recommends that data being generated regarding the bioconcentration of ionisable organic compounds are made publicly available to allow greater transparency which would help to improve the mechanistic understanding with respect to BCF, and strengthen the argument of using DOW as opposed to KOW as a more appropriate trigger value. The recent CEFIC-LRI request for proposal (LRI-ECO21) pertaining to the development of a mechanistic model for the bioaccumulation of ionisable organic compounds in fish, may lead to timely insight towards an improved understanding.