Extractants for the Removal of Mixed Contaminants from Soils

Abstract

The development of improved and effective remediation technologies for soils contaminated by heavy metals as well as polycyclic aromatic hydrocarbons (PAHs) has emerged as a significant environmental priority. One technology that appears to have considerable promise is the ex-situ soil washing that involves the use of extracting agents such as surfactants, cosolvents, cyclodextrins, chelants, and weak acids. This study investigates the use of various single and sequential flushing schemes using different extracting agents (two surfactants, two cosolvents, two cyclodextrins, two chelants, and phosphoric acid and six organic acids) for the simultaneous removal of PAHs and metals from three different soils. Specifically, this study examines the effectiveness of different extractants on the removal of phenanthrene (representative PAH) and nickel, lead, and zinc (representative heavy metals) from the soils and evaluates the optimum composition of extractants in sequential flushing schemes. Soils used in this study were artificially spiked kaolin soil (a representative clayey soil with negligible organic content), Soil A collected from a former manufactured gas plant (MGP) site dominated by sand and high organic content, and Soil D collected from a different MGP site having high clay and low organic contents. Batch experiments with a single extraction step using surfactants showed that the removal of phenanthrene varied from 4.9% to 52% for kaolin, 40% to 100% for Soil A, and 21% to 72.6% for Soil D. 1M phosphoric acid and 1M citric acid were found to be effective for the removal of the three heavy metals from all soils. Sequential extraction schemes were designed and investigated for the simultaneous removal of phenanthrene and metals from all the soils. Results showed that 5% Tween 80 followed by 1M citric acid or 1M citric acid followed by 5% Igepal CA-720 schemes have potential for the removal of both phenanthrene and nickel from kaolin soil. However, 5% Tween 80 followed by 1M citric acid scheme or 0.2M EDTA followed by 5%Tween 80 scheme showed maximum removal of phenanthrene, lead, and zinc from Soil A and Soil D. Overall this study demonstrates that the removal of both heavy metals and PAHs from soils can be accomplished by developing a sequential flushing scheme based on specific site soil composition.