pK_a values of the monohydroxylated polychlorinated biphenyls (OH-PCBs), polybrominated biphenyls (OH-PBBs), polychlorinated diphenyl ethers (OH-PCDEs), and polybrominated diphenyl ethers (OH-PBDEs)

Abstract

The SPARC software program aqueous pK_a prediction module was validated against corresponding experimental acidity constants for chlorinated and brominated phenols and the limited experimental aqueous pK_a data sets for monohydroxylated polychlorinated biphenyls (OH-PCBs), polychlorinated diphenyl ethers (OH-PCDEs), and polybrominated diphenyl ethers (OH-PBDEs). pK_a values were then estimated for all 837 monohydroxylated mono- through nona-halogenated congeners in each of the OH-PCB, OH-PCDE, and OH-PBDE classes, as well as for the monohydroxylated polybrominated biphenyls (OH-PBBs), giving a total of 3348 compounds. Large intrahomolog pK_a variation by up to six units is expected within each contaminant class, with pK_a values ranging from about 4 to 11 dependent on the degree and pattern of halogenation. Increasing halogenation generally decreased the average pK_a within each homolog group. Significant intrahomolog differences in pK_a values exist between OH-PCB, OH-PBB, OH-PCDE, and OH-PBDE congeners, including large acidity constant variation between isomers with equivalent halogenation patterns but varying location of the hydroxy moiety. Congener specific pH dependent investigations into the partitioning and degradation behaviors of these compounds are necessary, including greater consideration of analyte ionization effects during their extraction and analysis.

Keywords:

• Polychlorinated biphenyls
• polybrominated biphenyls
• polychlorinated diphenyl ethers
• polybrominated diphenyl ethers
• monohydroxylated derivatives
• acidity constants
• environmental fate

Acknowledgments

We thank an anonymous reviewer for valuable suggestions that improved the quality of the manuscript.