Experimental Determinations of Henry's Law Constants of Polybrominated Diphenyl Ethers (PBDEs) to Evaluate Exposure to Aquatic Biota
Henry’s law constants (KH) for seven polybrominated diphenyl ether (PBDE) congeners and nine polychlorinated biphenyl (PCB) congeners were determined using three variations of the gas stripping method. A modified gas stripping and an integrated gas stripping method were compared with the standard gas stripping method to identify the most accurate approach for PCB and PBDE KH determination between 5°C and 35°C. All three methods were equivalent for PCB measurements while the modified gas stripping method was more appropriate for PBDEs at or below 25°C and the integrated gas stripping method was preferred for temperatures above 25°C. Mass balance was achieved for the first time, providing a quantitative account of buffering effects and verifying the lack of degradation of the analytes during the experiment. Furthermore, the mass balance approach provided a deeper understanding of the quality of results and of the behaviour of individual compounds that cannot be achieved otherwise.
The measured Henry’s Law constants for PBDE ranged from 0.18 ± 0.34 Pa·m3·mol-1 (5°C) for 2,4,4’-tribromodiphenyl ether to 29 ± 5.7 Pa·m3·mol-1 (35°C) for 4-bromodiphenyl ether. For PCBs, Henry’s law constants ranged between 0.60 ± 0.45 Pa·m3·mol-1 (5°C) for 2,4,4’-trichlorobiphenyl to 123 ± 12 Pa·m3·mol-1 (35°C) for 2,2’,4,4’,5-pentachlorobiphenyl. KH values were generally higher for the PCBs since they were more readily volatilized from water into the air than the corresponding PBDE congeners. Although PCB KH values were not correlated to the degree of chlorine substitution, PCB KH values increased with ortho-chlorine substitution. In contrast, PBDE KH values decreased with the increasing bromine substitution and ortho-bromine substitution. The enthalpies of phase change (ΔHH) for PBDEs varied from –2.3 kJ mol-1 for 2,2’,4,4’,6-pentabromodiphenyl ether to 105 kJ mol-1 for 2,4,4’-tribromodiphenyl ether. For PCBs, ΔHH values varied from –53 kJ·mol-1 for 2,2’,4,4’-tetrachlorobiphenyl to 114 kJ·mol-1 for 2,3’,4,4’,5-pentachlorobiphenyl. According to the ΔHH values, temperature dependence of PBDE and PCB were similar.