Sequential Soil Flushing for Remediation of Complex Contaminated Soils: A Pilot-Scale Test

Abstract

In this study, an efficient soil flushing process was developed for the remediation of soil complex contaminated with heavy petroleum oils (HPOs) and heavy metals. In most cases, remediation of contaminated soil is carried out after all industrial activity is suspended and removal of facilities. Therefore, in-situ remediation becomes more favored over ex-situ technologies albeit relatively long remediation periods are needed. In particular, soil flushing has been employed as an efficient in-situ technology most frequently in many railroad and industrial sites still in business. The objective of this study was to develop an in-situ soil flushing method using horizontal injection/suction channels. A pilot-scale box reactor (1 m × 0.6 m × 0.7 m) was employed to evaluate desorption of complex contaminants from complex contaminated soils by flushing agents. Since HPOs and heavy metals can be removed by different mechanisms, various flushing agents were required for the treatment of HPOs and heavy metals. Hydrogen peroxide (H2O2) and citric acid were selected and injected sequentially as flushing agents for HPOs and heavy metals, respectively. Soils complex contaminated with HPOs, Zn, and Pb were collected from a railroad site, Seoul, Korea, and they were packed into the pilot-scaled reactor. Two horizontal channels were installed: injection channel was placed 10 cm below the top of soil surface and suction channel was placed 10 cm above the bottom of the reactor. Flushing agents were injected at a flow rate of 3.86 mL/min for 1 month. After flushing, soil samples were collected separately from various points of the reactor (divided into 5 vertical layers and 15 horizontal sections), and then each soil sample was analyzed for the soil flushing efficiency. The initial concentrations of HPOs, Zn and Pb were 4685.5±374.4 mg/kg, 204.9±60 mg/kg, and 139.8 mg/kg (n = 3). After soil flushing, the concentrations were decreased to 1448.4±166.7 mg/kg, 143.4 mg/kg, and 99.5 mg/kg (total removal rates = 69%, 30% and 28.9%, for HPOs, Zn, and Pb, respectively). Hence, it was confirmed in this pilot-scale study that sequential soil flushing by combination of flushing agents was effective for soils complex contaminated with HPOs and heavy metals. These results must be useful for field-scale application of soil flushing remediation for the complex contaminated soils.