Implications of Selected Models Used for SPMDs and BMOs

IMPLICATIONS OF SELECTED MODELS USED FOR SPMDs AND BMOs 

Herein, we examine several eqnations which are nsed to determine BCF% and BAFs, SPMD-water partition coefficients (Ksv,s), and the more complex exchange processes of some BMOs. Hopeffilly, the formulation and the assumptions behind these equations further clarify some of commonalities and differences between residue accumulation in BMOs and SPMDs. 

Phillips (1980) andPhillips and Rainbow (1993) have stated that each species of aquatic BMO exhibits unique uptake and elimination kinetics for a particular HOC. The ramification of this statement is revealed in the following simple one-compartment model, which is often used for the determination of steady-state BCFs and Ks s. 

Inspection of Eqs. 7.2 and 7.3 shows that k is inversely proportional to the lipid content of the biota (Vt/A/wb), whereas ku is independent of the lipid content. Thus, the generally much lower neutral lipid contents of BMOs result in much higher values of BMO keS. Also, the BMO BCFs are generally much smaller than the SPMD ATswS. 

Equation 7.1 utilizes exchange coefficients to predict steady-state BCFs and ATswS, and the model assumptions include a uniform lipid phase enclosed in a non-interactive membrane. The model shows that the magnitude of a BMO s BCF or an SPMD s Ksvj is affected by variations in ku and/or ke, unless both constants rise or fall proportionally. In the case of SPMDs, Huckins et al. (1993,2002a) have shown that the uptake and release process is essentially isotropic for HOCs. When residue exchange is isotropic, AfswS will remain relatively constant even when exposure conditions affect SPMD ku and ke values. This is not always the case for BMOs, yet isotropic exchange is a fundamental assumption of EP theory. 

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