Aqueous solubility depression for hydrophobic organic chemicals in the presence of partially miscible organic solvents

This work examines the solubility of hydrophobic organic chemicals (HOC), including naphthalene, biphenyl, 2,2'4,4'-tetrachlorobiphenyl (PCB-47), and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153), in water saturated with partially miscible organic solvents (PMOSs), including methylene chloride and chloroform. A recirculating generator column technique provided solubility measurements for the stationary-phase chemical (HOC) in the mobile phase (PMOS solution). The observed solubility of the naphthalene (log K-ow = 3.4) was not noticeably impacted by the PMOS, while that of biphenyl (log K-ow = 4.1) decreased slightly with increasing PMOS concentration. In water saturated with methylene chloride and chloroform, the observed PCB-47 (log K-ow = 6.2) aqueous concentrations were reduced to about 25% and 15%, respectively, of its aqueous solubility. HOC solubility results were further reduced for PCB-153 (log K-ow = 7.2) and the same PMOS solutions. Solventing-out, a phenomenon analogous to salting-out, is introduced as an explanation for the observed behavior. The solubility depressions increased with increasing chemical hydrophobicity (judged by K-ow) of both the HOC and the PMOS. Implications with respect to organic mixtures and contaminant transport in soils and groundwater include the following: (1) the association of the PMOS with the HOC phase will retard the transport of this relatively mobile solute through sediments contaminated with the HOC; (2) the presence of nearly saturated solutions of PMOS will reduce the apparent solubility and therefore the mobility of the HOC.