The effect of temperature on the bioventing of soil contaminated with toluene and decane

摘要

The effect of temperature on evaporation and biodegradation during soil bioventing (SBV) was studied for a mixture of toluene (T) and decane (D) in bench-scale soil columns at a continuous air flow and consecutively at two different flow rates. Theeffect of temperature on SBV was monitored by GC headspace analysis of contaminant, CO2 and O2 concentrations in the soil gas over time. Separation of evaporation and biodegradation processes into three different phases based on their rates was used together with Q10 and E10 (the factors by which biodegradation and evaporation rates increase when the temperature is raised by 10 degrees) to compare the removal kinetics at 10 and 20 deg C. Adsorption of T and D onto soil at 20 and 10 deg C was described with linear Freundlich isotherms. A temperature decrease from 20 to 10 deg C increased soil-air partitioning coefficients by a factor of 1.8 and of 2.1 for T and D, respectively. The mean Q10 value for T biodegradation was 2.2 for a temperature rise from 10 to 20deg C. A T content of >75% of the saturation concentration in soil gas inhibited biodegradation at both temperatures. SBV efficiency was dependent on temperature with respect to remediation time. SBV at 20 deg C reduced T and D by 99.8 and 98.7%, respectively.To reach similar results at 10 deg C,～1.6 and 1.4 times as much air were required. At both temperatures the total amounts of biodegraded hydrocarbons were approximately the same. The evaporation:biodegradation ratios at 20 deg C were 82.5:17.5 for T and 16:84 for D, whereas at 10 deg C they were 71:29 and 2:98, respectively. A comparison of Q10 values showed that, except during the initial phase of SBV, only a modest decrease in biodegradation rates should be expected after a decrease in temperature from 20 to10 deg C. Flow rate reduction significantly impacted the T evaporation rate at a higher temperature, whereas for D this rate was only slightly affected by temperature. The ratio between T vapour pressures at 20 and 10 deg C may be used to predict toluene removal by evaporation.