Treatability of volatile chlorinated hydrocarbon-contaminated soils of different textures along a vertical profile by mechanical soil aeration: A laboratory test

Abstract

Mechanical soil aeration is a simple, effective, and low-cost soil remediation technology that is suitable for sites contaminated with volatile chlorinated hydrocarbons (VCHs). Conventionally, this technique is used to treat the mixed soil of a site without considering the diversity and treatability of different soils within the site. A laboratory test was conducted to evaluate the effectiveness of mechanical soil aeration for remediating soils of different textures (silty, clayey, and sandy soils) along a vertical profile at an abandoned chloro-alkali chemical site in China. The collected soils were artificially contaminated with chloroform (TCM) and trichloroethylene (TCE). Mechanical soil aeration was effective for remediating VCHs (removal efficiency > 98%). The volatilization process was described by an exponential kinetic function. In the early stage of treatment (0–7 hr), rapid contaminant volatilization followed a pseudo-first order kinetic model. VCH concentrations decreased to low levels and showed a tailing phenomenon with very slow contaminant release after 8 hr. Compared with silty and sandy soils, clayey soil has high organic-matter content, a large specific surface area, a high clay fraction, and a complex pore structure. These characteristics substantially influenced the removal process, making it less efficient, more time consuming, and consequently more expensive. Our findings provide a potential basis for optimizing soil remediation strategy in a cost-effective manner.

Introduction

Volatile chlorinated hydrocarbons (VCHs), which are potential mutagens and carcinogens, are among the most commonly used industrial products. VCHs are introduced into the environment through their use as chemical intermediates. Such compounds are frequently found in contaminated soils in China and throughout the world (Wu et al., 2005).

Soil remediation technologies that are applied to sites contaminated with VCHs include soil vapor extraction (Albergaria et al., 2012), soil flushing (Kujawski et al., 2007), thermal desorption (He and Sulkes, 2011), bioremediation (Chang et al., 2002, Williamson et al., 2009), microwaving (Acierno et al., 2006, Acierno et al., 2004, Jones et al., 2002), and bioventing (Fukue et al., 2006). However, economic constraints may restrict the use of these technologies in developing countries.

Mechanical soil aeration was developed as a low-cost and highly effective remediation technology for VCH-contaminated sites and was mentioned in an annual report on remediation technologies published by the United States Environmental Protection Agency (U.S. EPA, 2007). This technology facilitates the release of volatile organic compounds (VOCs) from the soil by plowing and forced cross-ventilation. The released VOCs are then collected and treated. The advantages of mechanical soil aeration include ease of operation, significant remedial efficiency, a short remediation cycle, and cost-effectiveness. However, there are some drawbacks associated with this technology (Zhang et al., 2015). First, handling of contaminated soils is associated with an increased risk of human toxic exposure. In addition, the excavation process may pose structural risks to neighboring buildings (Yang et al., 2013).

Mechanical soil aeration removes VOCs by volatilization induced by mixing, cutting, and flipping the soils. Our previous studies on soil remediation by mechanical soil aeration in the field and laboratory revealed that agitation frequency, soil temperature, and aeration affect the removal efficiency of VCHs (Shi et al., 2012, Ma et al., 2015a, Ma et al., 2015b). Heterogeneity and non-uniformity of soil media introduce many uncertainties into the remediation process. Remediation efficiency is strongly influenced by differences in soil characteristics.

In practice, mechanical soil aeration is commonly applied after excavation of soil for ex situ remediation, without considering the effects of different soil layers along a vertical profile. It may be more cost effective to treat each soil fraction (i.e., clay, silt, and sand) separately to achieve optimal remediation efficiency, assuming that separate excavation of the soil texture types is feasible. The present study was undertaken to conduct a laboratory treatability study of three soils along a vertical profile of an abandoned chloro-alkali chemical site. Our approach aimed to investigate the response of different soil textures to mechanical soil aeration and analyze the remediation efficiency of different treatments. The results provide data to optimize decision-making for the remediation of sites contaminated with VCHs.