Investigating the Effects of Long-Term Sorption of Chlorinated Volatile Organic Compounds into Sandy Soils on the Difficulty of Desorption

Soil contamination by chlorinated volatile organic compounds (CVOCs) such as tetrachloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (cDCE) has become apparent. When the contamination period increases, soil contaminated by CVOCs becomes more difficult to clean. One of the important reasons for this difficulty of remediation could be that CVOCs sorb into the deepest part of the soil particle pores over the contamination period, making desorption difficult. However, previous studies have not been able to confirm the long-term sorption behavior of CVOCs on soil because the increase in sorption was too small to measure after reaching a fast sorption equilibrium in a few hours or days. In this study, we investigated the long-term sorption behavior by measuring the difficulty of repeated solvent extraction from two types of soil that sorbed CVOCs via the gas phase over 5 months. The results showed that CVOCs become more difficult to extract from the soils as the contamination period increases. Upon comparing the two types of soils, we found that the smaller the particle size, the faster the increase in slow sorption due to the larger external surface area and shorter distance to penetrate to the deepest of pores. We also discovered that the greater the number of macropores, the easier it is for CVOCs to penetrate deeper into the soil pores. In addition, cDCE, rather than PCE and TCE, had lower absorptivity and a higher molecular diffusion coefficient, resulting in a larger diffusion coefficient for easier intragranular penetration into the deepest of pores. From the results, at the actual site, after several decades of sorption, CVOCs sorbed deep in the micropores and mesopores of the coarse particles are extremely difficult to desorption, and may be the cause of the prolonged cleanup process and rebound phenomenon.

This study confirms that the longer soil is contaminated with CVOCs, the more difficult it is to elute CVOCs from soil particles. The relationship between the difficulty of extraction and the characteristic values of soil particles and material properties was also discussed. At an actual contamination site, much of the contamination occurs over a period of several decades, so it is needed to study the long-term sorption behavior of CVOCs on soil particles. However, in previous studies, sorption behavior was examined over a short period of time as several hours or days, but long-term sorption behavior could not be measured. This study measured long-term adsorption behavior, and the results and findings could explain one of the reasons for the difficulty of remediation at actual contaminated sites. It will be useful for studying which soils and which substances are more difficult to clean up, and for considering improvements in more efficient remediation methods.