Phenanthrene removal in unsaturated soils treated by electrokinetics with different surfactants—Triton X-100 and rhamnolipid

Abstract

In this study, the remediation performance of electrokinetic (EK) technology integrated with different surfactants for removing phenanthrene from unsaturated soils was investigated. A synthetic surfactant (Triton X-100) and a biosurfactant (rhamnolipid) were used to enhance phenanthrene solubility and removal efficiency during EK process. Results indicate that the electro-osmotic flow (EOF) rate in the rhamnolipid system is higher than that in Triton X-100. Using the EK technology for the removal of phenanthrene in the presence of rhamnolipid was more efficient than in the presence of Triton X-100. In addition to the transport mechanism of phenanthrene in EK system, the presence of rhamnolipid may promote microbial growth in the soil–water system and bring about biodegradation of phenanthrene. A diffusion–advection–sorption (DAS) model was solved by MATLAB, based on the linear sorption isotherm at the non-equilibrium condition, which is feasible to simulate the movement of phenanthrene during the EK + Triton X-100 treatment.

Introduction

The removal of polyaromatic hydrocarbons compounds (PAHs) has been considered as a challenge of soil remediation due to their high hydrophobicity and low water solubility. In order to clean sites contaminated with PAHs there are many treatment methods, such as thermal desorption and pump-and-treat techniques. A promising technique called electrokinetic (EK) remediation has been applied to treat many different types of polluted soils [1]. The EK process is feasible to remove contaminants from soils with low hydraulic conductivity, to control the transport direction of contaminants, and to treat various contaminants simultaneously [2], [3], [4], [5]. In addition, EK can be integrated into various other remediation techniques such as bioremediation and zero-valent iron which can be used to treat very specific pollutants [6], [7]. In order to increase the solubility of PAHs for enhancing removal efficiency, surfactants have been integrated into many soil remediation processes. In this research, EK and different species of surfactants were combined to enhance the phenanthrene removal efficiency from contaminated soils. A mathematic model was used to simulate the phenanthrene transport (removal) behavior in unsaturated soil under EK + different surfactant processes.

The EK process applies a direct-current (DC) electric field to soils to invoke some electrokinetic transport mechanisms such as electro-osmosis, electrophoresis, and electrolytic migration [8]. For the application to soil remediation, EOF is the movement of groundwater induced by the excessive surface charge of soil particles (zeta potential represents its magnitude); electrophoresis is the motion of charged colloids in the soil–groundwater mixture, and electrolytic migration is the migration of ionic species (based on size and charge of ions) in the groundwater. According to specific physical–chemical properties of both contaminants and soils, target contaminants can be removed from soils by one or a couple of these mechanisms. In the real contaminated sites, the PAHs have been always uptaken in the vadoze zone (unsaturated soils) [9], [10]. Because the water content of soils influence the EOF and the sorption/desorption of target compounds dramatically [11], [12], the removal phenomena of pollutants in the EK system will alter under saturated/unsaturated conditions. In other research, the mobility of inorganic and organic contaminants in saturated soils has been extensively studied [13], [14], [15], [16]; however, there is little information on the transport of PAHs in unsaturated soils under EK conditions. Accordingly, it is essential to understand the EK performance as served for treating the unsaturated soils.

In an aqueous solution, surfactant molecules form micelles when the surfactant concentration exceeds a certain value. This concentration is called the critical micelle concentration (CMC). The CMC value of a surfactant is a function of surfactant type (nonionic surfactants generally have a lower CMC than ionic surfactants) and system conditions, e.g., temperature and hardness [17]. Micelles consist of two portions: the hydrophilic exterior, which is oriented towards the water phase, and the hydrophobic interior, which is orientated away from the water phase and toward the center of the micelle. Using this mechanism of micellation hydrophobic organic compounds are solubilized within the hydrophobic interior of the micelles, thus increasing their solubility in water. The degree of organic solubilization depends on the class and the concentration of the surfactant. Although surfactants have the potential to increase the aqueous solubility of hydrophobic contaminants, which in turn enhances organic removal efficiency, the accumulation of contaminants at the soil–water interface has many disadvantages in engineering applications. In this case surfactant sorption on the soil is undesirable.

More frequently chemical–synthetic surfactants are used in environmental engineering such as sodium dedecyl sulfate (SDS) and Triton X-100. Among many different chemical–synthetic surfactants, Triton X-100 has been used to enhance desorption of phenanthrene effectively [18], [19], hence, it was utilized in this study. However, more recently various biosurfactants produced by different bacterial species are being used in water remediation. Biosurfactants are classified into several broad groups: glycolipids, lipopeptides, lipopolysaccharides, phospholipids, and fatty acids/neutral lipids [20]. The largest and most studied groups of biosurfactants are glycolipids, which include the sophorose-, rhamnose-, trehalose-, sucrose-, and fructose-lipids. Many factors including growth conditions, culture medium nutrients, temperature, pH, and agitation control the production and purification of biosurfactants [21]. The biosurfactant has a typical molecular weight range from approximately 500 to 1500 MW and CMC values of 1 to 200 mg L⁻¹ [22]. Like many chemical–synthetic surfactants, biosurfactants can improve the removal efficiency of the organic contaminants. Compared to the chemical–synthetic surfactants, and in terms of remediation, biosurfactants have several advantages over synthetic ones. They are biodegradable, cost-effective, and can be produced in situ at contaminated sites. A widely used biosurfactant, rhamnolipid, is able to effectively facilitate the transport and biodegradation of PAHs in the porous media. The retardation factor could be up to eightfold lower than that without rhamnolipid [23] and a 15 times higher microbial population as compared to the situation without rhamnolipid [24]. Thus, rhamnolipid was selected in this work. To show a comparison between the effectiveness of chemical–synthetic and bio-produced surfactants on water remediation, an integrated EK treatment technique was employed to remove phenanthrene in unsaturated soils.

In our previous research, a quantitative electrokinetic transport model of the chlorinated organic solvents by EK was established [25]. The mathematical model was based on the diffusion–advection–sorption (DAS) equation [26]. For the sorption term in the DAS equation, a linear sorption isotherm is commonly employed to describe the partitioning of organic contaminants in soils and is frequently applied to transport models [27]. Another important consideration is whether the sorption–desorption of non-ionic organic compounds reaches equilibrium or not during the EK process. To date, the equilibrium status is widely adopted to describe the pollutants transport under EK conditions [28]. On the basis of the above, the DAS equation with equilibrium and nonequilibrium assumptions was applied to simulate the transport kinetics of phenanthrene in unsaturated soils by EK treatment. During the simulation, the parameters of diffusion, advection and sorption varied with experimental conditions and treatment time. The results are discussed in terms of the current understanding of the transport mechanisms involved.