Correspondence Comment on “Bioconcentration Factor Hydrophobicity Cutoff: An Artificial Phenomenon Reconstructed” Recently Jonker et al. (1) determined the bioconcentration factors (BCFs) for selected polycyclic aromatic hydrocarbons using six different chemical/biological approaches and evaluated the relationship between the log BCFs and the logarithm of octanol-water partition coefficient Kow of the PAHs, and concluded that there is no hydrophobicity cutoff for the measured BCFs, at least not for PAHs with log Kow of up to 7.0 and polychlorinated biphenyls with log Kow of up to 7.41. They ascribed the previously observed cutoff phenomenon to experimental artifacts. We believe that Jonker et al.’s results do not provide any new insight that has not been obtained by other researchers. In addition, their conclusion is misleading and may compromise ongoing efforts to investigate the mechanics behind the BCFhydrophobicity cutoff phenomenon. Herein we will discuss the deficits in Jonker et al.’s experiments and then provide alternative explanations for the cutoff phenomenon. To aid readers in better understanding the discussions below, a brief description of the six different approaches used by Jonker et al. is presented in the Supporting Information.
Deficits in Jonker et al.’s Experimental Designs Use of Inadequate Target Analytes. If the uncertainties in measured log Kow values from the literature are considered (Supporting Information), the linear relationship between the log BCFs or the logarithmic lipid-normalized necroconcentration factors (log NCFs) and log Kow of the selected PAHs and PCB congeners (Figure S3 in ref 1) with log Kow < 7-7.5 is consistent with what have been observed in aquatic species (2, 3) and biomimic probes (4, 5). Obviously, Jonker et al. were unable to examine the relationship between log BCF and log Kow for very hydrophobic organic chemicals (VHOCs) such as octa-, nona-, and deca-chlorinated biphenyl congeners with log Kow > 7.5-8, which is the centerpiece of a more significant scientific question raised by other researchers (2, 6) and critical for bioaccumulation modeling efforts (7). Contribution of Dissolved Organic Carbon (DOC) to the Hydrophobicity Cutoff. Jonker et al. (1) attributed the leveloff of log BCF obtained with approach 2 to insufficient timeto-equilibrium and overestimated Cw obtained with approach 1 to the different BCF values obtained with approaches 1 and 2 (because total analyte concentration instead of freely dissolved fraction in the dissolved phase was measured with solvent extraction in approach 1). Herein we will focus on the effects of DOC based on Jonker et al.’s experimental results, because time-to-equilibrium is obviously a factor but its influence can be largely eliminated by using a prolonged extraction time. Assuming that Jonker et al.’s attribution was correct, DOC concentrations estimated from measured BCFapp (approach 1) and BCF (approach 2) using eq S1 should be similar for all target PAHs because the procedures of approaches 1 and 2 were identical except for the methods to determine Cw. Figure S1 of the Supporting Information clearly shows that the estimated DOC values vary in a wide range (-62 to 712 mg/L). In particular, the DOC values for two compounds, benzo[k]fluoranthene and benzo[a]pyrene (both with log Kow ) 6.2), are 712 and 464 ppm, respectively, substantially higher than those for other PAH analytes. The negative DOC values 10.1021/es801084g CCC: $40.75
Published on Web 11/14/2008
2008 American Chemical Society
are likely to stem from experimental uncertainties because of negligible DOC effects for relatively less hydrophobic chemicals. Nevertheless, the large variability in the estimated DOC values indicates that the measured BCF values intrinsically had large uncertainties, especially for PAHs with log Kow > 5.5. Apparently, Jonker et al.’s attribution was not supported by their own data. Overestimated Cb Obtained with Approaches 5 and 6. As discussed in the Supporting Information, the concentration of an HOC in dead worm or liposome obtained with approaches 5 and 6 was calculated by subtracting the sum of the chemical masses in water and POM or SPME fiber from the spiked (total) mass, and the values of NCF and Klip were subsequently derived. Clearly, Jonker et al. did not consider losses of VHOCs due to association with DOC, as well as adsorption to glassware, which have been well characterized (8). These artifacts unintentionally led to overestimation of NCF and Klip, and also better linear relationships between bioconcentration and hydrophobicity obtained with approaches 5 and 6 (Figure 2e and f in ref 1) than with approaches 3 and 4 (Figure 2c and d in ref 1).
Importance of BCF Cutoff in Risk Assessment Jonker et al. questioned whether the cutoff of BCFs exists within the hydrophobicity/size range applicable to anthropogenic HOCs that merit risk assessment (1). This statement may be misleading because, as an example, hexa- or higher brominated congeners of polyborminated diphenyl ethers largely have log Kow values greater than 8 (9). In addition, the log Kows of penta- or higher chlorodibenzo-p-dioxins are also higher than 7.5 (10). Before more data become available to clarify whether BCF-hydrophobicity cutoff is also applicable to polybrominated diphenyl ethers and polychlorodibenzop-dioxins, it is at least premature to question the necessity to conduct risk assessment for these known toxins.
Alternative Explanations for Curvilinear BCF Alternative explanations for the curvilinear shape for VHOCs include but are not limited to: (1) octanol is a poor surrogate for lipids (11); (2) elimination of VHOCs into feces etc. (6); (3) occurrence of metabolism, e.g. for PCB congeners with large Kow (12); and (4) restricted membrane permeability (13) and/or the difficulty to associate with internal lipid constituents for VHOCs (14). Our previous study (5) suggested that octanol is a poor surrogate of polydimethylsiloxane (PDMS) coating because a larger magnitude of chain rotation is needed to create sufficient volume to accommodate large solute molecules in PDMS compared to that in octanol. Consequently, the cutoff or leveloff of logarithm of PDMS-water partition coefficient (log Kfw) for PCBs with log Kow > 7-7.5 was resulted. Based on the similarity between log Kfw and log BCF and assuming that all experimental artifacts can be eliminated, the cutoff or leveloff of BCF may still occur because octanol is a poor surrogate for lipids (11). Organism membranes made of lipid bilayers that molecules pass through are structurally different from octanol.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (20677061, 40532013 and 40821003) and the K.C. Wong Education Foundation, Hong VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Kong Special Administrative Region, China. This is contribution NO. IS-1010 from GIGCAS.
Supporting Information Available Additional text, equations, references, and a figure. This material is available free of charge via the Internet at http:// pubs.acs.org.
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(9) Braekevelt, E.; Tittlemier, S. A.; Tomy, G. T. Direct measurement of octanol-water partition coefficients of some environmentally relevant brominated diphenyl ether congeners. Chemosphere 2003, 51563–567. (10) Govers, H. A. J.; Krop, H. B. Partition constants of polychlorinated dibenzofurans and dibenzo-p-dioxins. Chemosphere 1998, 37, 2139–2152. (11) Chessells, M.; Hawker, C. W.; Connell, D. W. Influence of solubility in lipid on bioconcentration of hydrophobic compounds. Ecotoxicol. Environ. Saf. 1992, 23, 260–273. (12) Zhao, R.-b.; Sun, D.-y.; Fu, S.; Wang, X.-f.; Zhao, R.-s. Bioconcentration kinetics of PCBs in various parts of the lifecycle of the tadpoles Xenopus lavis. J. Environ. Sci. 2007, 19, 374–384. (13) Shaw, G. R.; Connell, D. W. Physicochemical properties controlling polychlorinated biphenyl (PCB) concentrations in aquatic organisms. Environ. Sci. Technol. 1984, 18, 18–23. (14) Stange, K.; Swackhamer, D. L. Factors affecting phytoplankton species-specific differences in accumulation of 40 polychlorinated biphenyls (PCBs). Environ. Toxicol. Chem. 1994, 13, 1849–1860.
Ze-Yu Yang State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, and Graduate School, Chinese Academy of Sciences, Beijing 100039, China
Eddy Y. Zeng State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ES801084G
