Pilot Scale Feasibility Test for In-situ Chemical Oxidation Coupled with Bioremediation to Remediate the Diesel Contaminated Military Site, Korea

Abstract

Objectives of this study are to test the feasibility of the combination process of in-situ flushing by the chemical oxidation using hydrogen peroxide (H2O2) solution with the bioremediation to remove TPH from the contaminated site in a pilot scale test. The test site (A area), located within a military base in Busan, was contaminated with TPH because of fuel leakage from underground pipeline. TPH concentrations of soils in a depth of 0-1 meter (Sandy loam) and in a depth of 2-3 meter (Silty loam) were 3,399 mg/kg and 2,632 mg/kg, respectively. The study was divided into three steps to determine an appropriate remediation process for a clean-up of contaminated site. The first step was the batch and continuous column experiment for in-situ flushing to remove TPH. The second step was the bioremediation batch test with indigenous microorganisms to remove TPH from the contaminated soil. And the third step was the pilot scale feasibility test for the chemical oxidation combined with the bioremediation. To determine the optimal operation conditions having the effective removal efficiency of TPH, batch experiments using alcohols, surfactants and H2O2 solution were performed. The diesel solubility using 1 % of ethanol and methanol solutions increased up to 1.5 times and 3 times of only water solubility, and by using 2 % of TW-80 (Tween-80) solution and 1 % of TW-60 (Tween-60) solution, the diesel solubility increased 9 times. Results suggest that diesel solubility of surfactant solution was higher than that of alcohol solution. For soil flushing experiments using surfactants for TPH contaminated soil, when the mass ratio of flushing solution to soil was 1:1, TPH concentrations of soil flushed by 2 % of TW-80 solution and 1 % of TW-60 solution decreased by 52 % and 56 %, respectively. When using 5 % of H2O2 solution for the contaminated soil, TPH removal efficiencies were represented as 55 % (the mass ratio of soil vs. H2O2 solution = 1:1) and 63 % (the mass ratio of soil vs. H2O2 solution = 1:2), respectively. TPH removal efficiencies ranged from 94 % to 99 % with the addition of 0.3-1 % ferrous iron with H2O2 solution. Results suggested that TPH removal efficiency by chemical oxidation using H2O2 solution was higher than that by those using alcohols and surfactants. In continuous column experiments, for using 20 % of H2O2 solution without the ferrous iron, TPH removal efficiency maintained at more than 87 %, suggesting that TPH in soil was effectively removed by using only H2O2 solution with the autochthonous iron (Fe concentration in the soil: 29,507 mg/kg). Indigenous microorganisms isolated from the test site and identified by the 16S rRNA gene sequence were presented as P. frederiksbergensis JAJ 28 (M-1), M. arborescens DSM 20754 (M-2), P. argentinensis LMG 22563 (M-3) and V. Ginsengisoli KCTC 12583 (M-4). For injecting 0.75 ml of microorganism in TPH contaminated soil, TPH removal efficiencies were 69 % (M-1), 73 % (M-2), 67 % (M-3), 63 % (M-4) and 66 % (M-5: mixed microorganisms). The first order constant of TPH degradation for each microorganism was calculated and ranged from 1.28 × 10-2 to 2.02 × 10-2. TPH half-life of M-2 (34.3 days) was shorter 19.8 days than that of M-4 having the longest half-life (54.1 days). From the calculation, TPH removal efficiency will reach about 90 % within 180 days of bioremediation with indigenous microorganism. To investigate the activity of the bioremediation by indigenous microorganisms after the chemical oxidation by using H2O2 solution, batch experiments for the bioremediation with pre-treated soil were duplicated and TPH removal efficiency was 84 %, suggesting that the combination of the chemical oxidation with bioremediation for in-situ flushing process would be successfully applied to remove TPH from TPH contaminated site. For the pilot scale feasibility test, A area contaminated with TPH was selected and five injection wells and one extraction well were installed in the site (2.5 m × 2.7 m × 1 m). The average injection rate of 17.5 % of H2O2 solution was fixed at 140 ml/min. Total volume of H2O2 solution injected and extracted were 718 L and 400 L, respectively. After injecting H2O2 solution, TPH concentration in the effluent solution somewhat decreased from the initial TPH concentration (4.62 mg/L), and maintained at lower than 1.59 mg/L during H2O2 solution flushing. Results suggested that TPH mass in soils was removed from the decomposition process by H2O2 solution rather than the increase in solubility and mobility of TPH in the flushed solution. After 0.3 pore volumes of H2O2 solution flushing, the average TPH concentration in soils was 1,192 mg/kg and TPH removal efficiency was about 50 %. After H2O2 solution flushing for 17 days, mixed microorganism culture solution (43 L) was injected into injection and extraction wells at two times. After 14 days of the injection of culture solution, the average concentration of TPH remained in soil was 740 mg/kg, showing that 19 % of TPH was additionally removed by the bioremediation (total TPH removal efficiency: 69 %). From the study, it was investigated that the usage of the in-situ flushing using H2O2 solution inducing the chemical oxidation conjugated with the bioremediation is an effective process to remove TPH from the contaminated site.