Elsevier

Applied Geochemistry

Volume 16, Issues 11–12, August–September 2001, Pages 1419-1428
Applied Geochemistry

Enhanced biodegradation of polycyclic aromatic hydrocarbons using nonionic surfactants in soil slurry

https://doi.org/10.1016/S0883-2927(01)00043-9Get rights and content

Abstract

The effect of nonionic surfactants on the solubility and biodegradation of polycyclic aromatic hydrocarbons (PAHs) in the aqueous phase and in the soil slurry phase, as well as the fate of these surfactants, were investigated. The PAH solubility was linearly proportional to the surfactant concentration when above the critical micelle concentration (CMC), and increased as the hydrophile–lipophile balance (HLB) value decreased. Substantial amounts of the sorbed phenanthrene in the soil particles were desorbed by non-ionic surfactants into the liquid phase when the ratio of soil to water was 1:10 (g/ml). Brij 30 was the most biodegradable surfactant tested, showed no substrate inhibition up to a concentration of 1.5 g/l, and was definitely used as a C source by the bacteria. Naphthalene and phenanthrene were completely degraded by phenanthrene-acclimatised cultures within 60 h, but a substantial amount of naphthalene was lost due to volatilization. The limiting step in the soil slurry bioremediation was bioavailability by the micro-organisms for the sand slurry and mass transfer from a solid to aqueous phase in the clay slurry.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are major recalcitrant components in oil contaminants and are known to be carcinogenic to humans and other living organisms. These compounds are produced by industrial activities such as oil processing and storage, and are often found in contaminated soil. Physical, chemical and biological methods can all be used for the remediation of such contaminated sites, but biological treatment is known to be advantageous for both environmental and economic reasons. Due to the low aqueous solubility and strong sorption properties of PAHs; however, soils contaminated with PAHs are not easily treated by biological means.

Surfactants may be useful for the bioremediation of sites contaminated with PAHs because they enhance the desorption and solubility of hydrophobic compounds. Many studies have been conducted to enhance the biodegradation of PAHs using surfactants to increase the solubility and desorption rates of the PAHs from soil particles (Bury and Miller, 1993, Volkering et al., 1995, Doong et al., 1996). However, little information is available concerning the effects of surfactants on the solubility and the biodegradation of PAHs. Furthermore, the studies that do exist show contradictory results, where some authors reported enhanced effects (Aronstein and Alesxander, 1993, Boonchan et al., 1998), and others reported inhibitory effects (Laha and Luthy, 1991). Also, the biodegradation and toxicity of surfactants by and on microorganisms has not been researched in depth.

The objectives of this study were to determine the capacity of nonionic surfactants to affect the bioremediation of PAHs and to define the fate and effects of the surfactants used in either an aqueous or soil slurry phase. To achieve these objectives, 3 experimental sets were conducted. In the first, the physico-chemical effects of 3 nonionic surfactants (Triton X-100, Tween 80, and Brij 30) were examined with respect to their solubilities and desorption rates of two PAHs (naphthalene and phenanthrene) in the aqueous phase and the soil slurry phase. In the second, the biodegradation and toxicity of the surfactants were tested using a respirometer to measure the O2 uptake. Finally, batch experiments for the biodegradation of PAHs with nonionic surfactants were conducted in a soil slurry reactor.

Section snippets

Soil and chemicals

Sand (0.3% organic matter) and clay (0.9% organic matter) were obtained from Kwangju Science Co. (Kwangju, Korea). Reagent-grade phenanthrene, naphthalene and Brij 30 (a nonionic surfactant) were purchased from Aldrich (Milwaukee, Wisconsin, USA), and HPLC-grade acetonitrile was purchased from Duksan Pure Chemicals (Ansan, Kyungkido, Korea). The non-ionic surfactants (Tween 80 and Triton X-100) were obtained from Sigma Chemical Co. (St. Louis, MO, USA). The hydrophile–lipophile balance (HLB;

CMC

The CMC values were determined by plotting the surface tension versus the surfactant solution for various concentrations (Fig. 1). The CMC values for Brij 30, Tween 80, and Triton X-100 were 3.50×10−5 M (9.7 mg/l), 2.50×10−5 M (13.4 mg/l), and 1.85×10−4 M (13.6 mg/l), respectively.

Solubility of PAHs

In Fig. 2, the PAH solubility was plotted as a function of surfactant solution concentration. The naphthalene and phenanthrene solubilities were linearly proportional to the surfactant concentration in the range of

Conclusions

This experiment showed that the solubility of the PAHs was proportional to the concentrations of the surfactants when above the CMC. However, when below or near the CMC, additional surfactant did not enhance the solubility of the PAHs. Thus, the formation of the micelle resulted in an increase of the apparent solubility, which was confirmed using the aqueous–micelle partition coefficient values. Brij 30, which has an HLB value of 8, showed a stable milky dispersion in the aqueous phase, while

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