Soil/Air Partitioning of Semivolatile Organic Compounds. 2. Influence of Temperature and Relative Humidity

The influence of temperature and relative humidity (RH) on the equilibrium partitioning of polychlorinated biphenyls (PCBs), pentachlorobenzene, and hexachlorobenzene between a sandy loam soil and air was investigated. The measurements were conducted at 6 temperatures from 5 to 60 °C at each of 5 different relative humidities from 29 to 93% in a solid-phase fugacity meter. The soil/air equilibrium partition coefficient ( K SA ) was very sensitive to temperature and humidity. For instance, K SA for PCB 52 was 46,000 times higher at T = 5 °C, RH = 29% than at T = 60 °C, RH = 93%. For a given RH, a linear relationship between ln K SA and 1/ T was observed. The heats of phase change were similar for the di- through octachlorinated PCB congeners but varied with RH, increasing from 103 kJ/mol at RH = 29% to 116 kJ/mol at RH = 66% and then decreasing to 98 kJ/mol at RH = 93%. At a given temperature, an inverse linear relationship between ln K SA and RH was observed. The slopes of these lines were similar for all compounds studied at all temperatures. The increase in K SA with diminishing RH was much greater than could be explained by adsorption to soil minerals. This suggests that sorption in organic material also increases with diminishing RH. A simple predictive equation was developed to calculate K SA as a function of soil properties, physical chemical properties, soil temperature and soil relative humidity. The correlation coefficient for the 1131 data points predicted and the measured K SA was 0.97, with a root-mean-square residual of 0.24 log units. This equation provides an initial basis for exploring the influence of the high sensitivity of K SA to temperature and relative humidity on the environmental fate of SOCs.