Determination of the partition coefficient between dissolved organic carbon and seawater using differential equilibrium kinetics

Because the freely dissolved fraction of highly hydrophobic organic chemicals is bioavailable, knowing the partition coefficient between dissolved organic carbon and water (K_DOCw) is crucial to estimate the freely dissolved fraction from the total concentration. A kinetic method was developed to obtain K_DOCw that required a shorter experimental time than equilibrium methods. The equilibrium partition coefficients of four polychlorinated biphenyls (PCBs) (2,4,4′-trichlorobiphenyl (PCB 28), 2,2′,3,5′-tetrachlorobiphenyl (PCB 44), 2,2′,4,5,5′-pentachlorobiphenyl (PCB 101), and 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB 153)) between dissolved organic carbon and seawater (K_DOCsw) were determined using seawater samples from the Korean coast. The log K_DOCsw values of PCB 28 were measured by equilibrating PCB 28, the least hydrophobic congener, with seawater samples, and the values ranged from 6.60 to 7.20. For the more hydrophobic PCBs (PCB 44, PCB 101, and PCB 153), kinetic experiments were conducted to determine the sorption rate constants (k₂) and their log K_DOCsw values were obtained by comparing their k₂ with that of PCB 28. The calculated log K_DOCsw values were 6.57-7.35 for PCB 44, 6.23-7.44 for PCB 101, and 6.35-7.73 for PCB 153. The validity of the proposed method was further confirmed using three less hydrophobic polycyclic aromatic hydrocarbons. This kinetic method shortened the experimental time to obtain the K_DOCsw values of the more hydrophobic PCBs, which did not reach phase equilibrium. A new kinetic method was proposed to determine K_DOCw for more hydrophobic organic chemicals by comparing their desorption rate constants with that for less hydrophobic organic chemical of which K_DOCw is measured at equilibrium.