Equilibrium and kinetic solid-phase microextraction determination of the partition coefficients between polychlorinated biphenyl congeners and dissolved humic acid.

A conventional solid-phase microextraction (SPME) method combined with liquid-liquid extraction was applied under equilibrium and nonequilibrium conditions to determine the partition coefficients (K(doc)) of 25 polychlorinated biphenyl congeners (PCBs) between Sigma-Aldrich humic acid (HA) and water. The values of log K(doc) determined with equilibrium SPME were linearly correlated with the logarithm of the octanol-water partition coefficients (K(ow)) for PCB congeners at logK(ow)< approximately 7.2, but the trends were disrupted for logK(ow) from approximately 7.2 to 8.18. In addition, short-term (5 min to 4 days) and long-term (5-44 days) uptake profiles of PCBs were established, from which a pseudo-equilibrium for sorption of PCBs was revealed at approximately 4 days of extraction. To understand this phenomenon, the uptake profiles were fitted with two equations (one equation is often used for pure water samples and the other one is applicable for samples containing complex matrices) derived from a first-order kinetics model. Subsequently, K(doc) values obtained through kinetic approaches were compared with those acquired from equilibrium SPME. The comparison of K(doc) values indicated that the pseudo-equilibrium was caused by the slow desorption of PCBs from HA rather than the biphasic desorption mechanism.