The effect of soil organic matter and aging on the bioavailability of total petroleum hydrocarbons

Biodegradation of hydrocarbon compounds was defined as a process involving interactions among soil particles, pollutants, water, and microorganisms. In particular, soil-pollution compound interactions have been found influenced by soil organic matters (SOM), both in amount and in the nature. Early studies demonstrated that aging problem is more marked in soils with high SOM than with low SOM. It has been found that several organic chemicals become sequestered as they age or persist in soil, and the resultant sequestration is evident by the decline in bioavailability to bacteria. When most of the existing studies focused on bioavailability of polyaromatic hydrocarbons, the present paper presented two lab-scale biopile case studies to demonstrate the effects of SOM and aging on TPH bioavailability to indigenous bacteria in soils. Duplicated bioplies were operated for over about 200 days. Case I study was conducted with P batch soil containing SOM 70.5%, ST batch soil containing SOM 9.0%, and ST control containing SOM 2.3%. The 1^st-order kinetic rate constants during the 1^st-stage degradation indicated the highest rate resulting from ST batch and ST control (0.0188 day^-1 and 0.0133 day^-1, respectively) were about two folds of that resulted from P batch (0.0106 day^-1). The final TPH removal percentage also indicated that ST batch and ST_C achieved higher removal rate (∼85%) than that of P batch (72.7%). Relative high SOM could reduce the initial biodegradation rate and the final TPH removal amount. Additionally, in the final stage, introduction of rhamnolipid to P batch tended to enhance the associated bioavailability. Case II study was designed to distinguish the different bioavailability after being sequestrated in sealed and dark glass jars at room temperature for 0 day (ST0), 38 days (ST38), and 101 days (ST101). With the increase of the sequestration time, 1^st-order kinetic rates were decreased (ST0: 0.0245 day-1, ST38: 0.0128 day^-1, ST101:0.0090 day^-1). Also, the TPH removal rates decreased with the increase of the aging time: ST0: 87%, ST38: 71%, and ST: 58%.