Abstract
BackgroundThe assessment of persistence, bioaccumulation and toxicity (PBT) is part of the regulation process of ionic organic compounds (IOCs) and a major challenge, as a commonly acknowledged approach for the estimation of the bioaccumulation potential of IOCs is still missing. The goal of the present study was, therefore, to experimentally determine the bioaccumulation of fully ionized compounds and to identify screening parameters that can indicate high bioaccumulation potential of IOCs. Three feeding studies with rainbow trout (Oncorhynchus mykiss) were carried out according to OECD TG 305. Separation of liver, gastrointestinal tract (GIT) and carcass allowed to further elucidate the tissue distribution of the individual test substances. The chemicals chosen had characteristics that made them suspect for high bioaccumulation, and included two cations (tetrabutylphosphonium bromide (TBP), trimethyloctadecyl ammonium chloride (TMOA)) and four anions (benzotriazole, tecloftalam, pentachlorophenol (PCP), MEE-phosphonate). Data on the dietary biomagnification of IOCs (strong acids) were also collected from published literature.ResultsThe highest distribution factors were found for the GIT, followed by liver. However, none of the tested IOCs showed a distinct biomagnification potential, as kinetic biomagnification factors (BMFk) ranged between 0.001 and 0.05 g/g (median 0.009 g/g). Cations showed lower assimilation efficiency (α) than anions, except for tecloftalam. In contrast, anions showed a considerably faster depuration rate (half-life less than 0.5 days) compared to cations (half-life of around 5 days). Sixteen potential screening parameters for BMF were calculated with a chemical property estimation tool (ACD/i-Lab) and correlated with the BMF data from this study and from literature. The number of hydrogen bond donors (nHBD) showed the highest correlation to measured BMF, but the prediction is only based on two values (one or two nHBD), while the other descriptors were insignificantly correlated.ConclusionThe suspected dietary bioaccumulation potential of the six IOCs could not be confirmed in the feeding studies with rainbow trout. The more than twenty screening parameters showed no particularly high correlation neither with the test results nor with the BMF values collected from literature. The results corroborate earlier findings that ionization lowers the tendency of a chemical for dietary bioaccumulation, compared to non-ionized chemicals. In addition to the lipophobicity of ionic molecule moieties, fast depuration seems to be a major reason for the observed low dietary bioaccumulation of ionic compounds, in particular anions. Fast depuration may happen due to rapid metabolism of charged compounds, and future studies should test this hypothesis.
Highlights
The assessment of persistence, bioaccumulation and toxicity (PBT) is part of the regulation process of ionic organic compounds (IOCs) and a major challenge, as a commonly acknowledged approach for the estimation of the bioaccumulation potential of Ionizable organic compound (IOC) is still missing
Reliable dietary biomagnification factor (BMF) values were selected based on quality criteria set by Organization for Economic Co-Operation and Development (OECD) TG 305 [3] and Arnot and Quinn [21], namely: (i) for BMFss, the assumption of steady state between uptake and depuration kinetics could be verified based on reported time-resolved concentration data or depuration rate; (ii) fish growth was explicitly accounted for using a growth-corrected depuration rate (k2g); and (iii) the assimilation efficiency α had been estimated to be ≤ 100%
Selection of predictors Molecular descriptors were selected to assess their capability of empirically predicting dietary BMF for ionizable substances and include: (i) acid dissociation constants (pKa); (ii) molar mass, molar volume and McGowan’s estimation of molar volume; (iii) partition coefficients Kow, Dow and K HSA; (iv) solubility at neutral pH; (v) Lipinski’s properties, namely topological polar surface area (TPSA), number of rotatable bonds, number of hydrogen bond donors and acceptors and total number of hydrogen bonds; (vi) volume of distribution
Summary
The assessment of persistence, bioaccumulation and toxicity (PBT) is part of the regulation process of ionic organic compounds (IOCs) and a major challenge, as a commonly acknowledged approach for the estimation of the bioaccumulation potential of IOCs is still missing. Within the European Union, the regulation of Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) requires testing of the bioaccumulation potential of substances during registration. Bioaccumulation is a criterion (together with persistence and toxicity) in the PBT assessment of substances, which are of major concern [2]. If the threshold value of 4.5 is exceeded, studies on aquatic bioconcentration or bioaccumulation according to OECD test guidelines 305 [3] have to be conducted [2]. Experimental determination of the bioaccumulation potential means high need for experimental animals and high costs
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