Assessing PAH removal from clayey soil by means of electro-osmosis and electrodialysis

https://doi.org/10.1016/j.scitotenv.2012.07.010Get rights and content

Abstract

Polycyclic aromatic hydrocarbons (PAH) are persistent and toxic contaminants which are difficult to remove from fine porous material like clayey soils. The present work aims at studying two electroremediation techniques for the removal of PAHs from a spiked natural silt soil from Saudi Arabia and a silty loam soil from The Netherlands which has been exposed to tar contamination for over 100 years. The two techniques at focus are electro-osmosis and electrodialysis. The latter is applied for the first time for the removal of PAH. The efficiency of the techniques is studied using these two soils, having been subjected to different PAH contact times.

Two surfactants were used: the non-ionic surfactant Tween 80 and anionic surfactant sodium dodecyl sulphate (SDS) to aid desorption of PAHs from the soil. Results show a large discrepancy in the removal rates between spiked soil and long-term field contaminated soil, as expected. In spiked soil, electro-osmosis achieves up to 85% while electrodialysis accomplishes 68% PAH removal. In field contaminated soil, electro-osmosis results in 35% PAH removal whereas electrodialysis results in 79%. Short recommendations are derived for the up-scale of the two techniques.

Highlights

► PAHs are persistent and ubiquitous contaminants with difficult removal from clays. ► Electroremediation techniques (electro-osmosis and electrodialysis) may be a solution. ► Electrodialysis is used here for the first time to study the removal of PAH from soil. ► Removal efficiencies varied between 35 and 85%. ► Results show high discrepancies in PAH removal on spiked and field contaminated soils.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are organic non-polar contaminants which are very difficult to remove from soils, especially at high organic matter content, since PAHs tend to strongly bind to this fraction (Sposito, 2008). Biodegradation is often the selected soil remediation strategy, but faces limitations such as oxygen and substrate accessibility (Sobisch et al., 2000).

Electrokinetics has been applied effectively to soils since the 1930s (Reuss, 1809). Initially, the intent was the dewatering of clayey soil and stabilization of quick clays through electro-osmosis (Casagrande, 1952). In the 1990s, the process became quite popular for soil remediation of heavy metals, giving rise to different patents (e.g. Lageman, 1993, Ottosen and Hansen, 1992). After this first challenge, the same processes were also applied in the remediation of a range of waste porous media, from soil (Ottosen et al., 1997) to fly ash (Lima et al., 2009), some achieving quite good results. At the same time, the removal of organic contaminants was attempted, where some promoted temperature rise and evaporation of the organics (Lageman and Godschalk, 2006), others promoted mobilization of dissolved charged organic species by convection under an electric field (Ribeiro et al., 2005). Removal of non-polar and non-charged organic contaminants, such as PAHs, from soils has been attempted by water transport by electro-osmosis as driving force in both spiked soil (Saichek and Reddy, 2003) and field contaminated soil (Lima et al., 2011). These studies use surfactants, which are amphiphilic polysorbents that aid desorption of non-polar substances.

Electrodialysis is based on electrokinetics with the introduction of selective ion-exchange membranes (Ottosen et al., 1997). These membranes allow only the transfer of cations to the cathode, where a cation-exchange membrane is located, and anions to an anion-exchange membrane at the anode (Hansen, 1995). Our recently modified electrodialytic cell focuses on remediation of soil through electromigration and electrophoresis, i.e. the movement of particles in an electric field, since it uses a soil suspension rather than consolidated soil (Pedersen, 2002). This cell has never been used to remove non-polar substances from soil or other porous materials. In order to enable the removal of PAHs by electromigration, PAHs need to become electrically charged through the addition of an ionic surfactant.

In this study, removal of PAHs by electro-osmosis and by electrodialysis is being compared. Two soils are used: a silt soil from Saudi Arabia, spiked with phenanthrene, and a silty loam from The Netherlands, field contaminated at a former tar-production site. These soils were selected because they represent two extremes with respect to the genesis of the clayey material they contain. The soil from The Netherlands is formed by fluvial deposition in a moderate Holocene climate, whereas the soil from Saudi Arabia is formed by in situ weathering in an arid climate. Field contaminated soil from Saudi Arabia was not available. Thus, the available pristine soil from Saudi Arabia could only be studied by spiking it with phenanthrene. We used phenanthrene as a model substance of PAHs due to its intermediate size and behavior. Our interest in studying fine grained Saudi soil is based on the country's abundance of petrochemical industry and crude oil exploitation. In addition to comparing the two processes, we aim to get an insight in the effect of residence time of PAHs – aged vs. spiked – in electroremediation of these soils.

Section snippets

Soil sampling and characterization

Two soils were used in this study. One is from a date farm in the Al-Hasa oasis near Hofuf, Saudi Arabia, and the other one from a tar contaminated site in the IJssel river flood plain near Olst, The Netherlands.

The Olst soil is contaminated peaty silty loam, collected from a depth of 12 to 13 m on a former asphalt factory site, active from 1903 to 1983. The soil was sampled in 2009. This deep layer (12–13 m) is named “eemklei” and is characterized by the uncommonly high organic matter content

Phenanthrene removal

Fig. 2 shows the voltage oscillations during all experiments. Fig. 2a depicts electro-osmosis experimental results, where Exp. 2 shows a negative voltage across the Olst clay. This is a result of using reference electrodes to monitor voltage oscillations. The sign of the voltage difference indicates the electro-osmotic flux direction, as explained elsewhere (Lima et al., 2011). As Fig. 3b shows, the electro-osmotic flux in Exp. 2 (Olst soil) occurred in the direction of the anode, in agreement

Final considerations and future applications

Laboratory experiments of electro-osmosis and electrodialysis were carried out to test PAH removal from both a spiked silt soil and a field tar-contaminated silty loam soil.

Contrary to the field contaminated soil, the electro-osmosis experiment in a spiked soil was highly successful, indicating that contaminant behavior is quite different under field conditions with long-term and aged contamination than under spiked conditions in the laboratory. Using spiked soil for studying the effect of

Acknowledgments

We thank Bärbel Agres (Technische Universität München) for the PAH analysis. Frank Volkering from TAUW B.V., Deventer, The Netherlands is kindly acknowledged for providing the sample material and information about the contaminated site. This publication was based on work supported by award no KUK-C1-017-12, made by King Abdullah University of Science and Technology (KAUST).

References (31)

  • M. Alexander

    Aging, bioavailability, and overestimation of risk from environmental pollutants

    Environ Sci Technol

    (2000)
  • L. Casagrande

    Electro-osmotic stabilization of soils

    J Boston Soc Civ. Eng.

    (1952)
  • E. Chladek et al.

    Use of bonded phase silica sorbents for the sampling of priority pollutants in wastewaters

    J Chromatogr Sci

    (1984)
  • Hansen HK., 1995. Practical and theoretical aspects concerning the use of ion exchange membranes and resins in...
  • R. Hartmann

    Polycyclic aromatic hydrocarbons (PAHs) in forest soils: critical evaluation of a new analytical procedure

    Int J Environ Anal Chem

    (1996)
  • Cited by (0)

    View full text