Effects of cyclodextrins and rhamnolipids on the extraction of PAHs and metals from an aged creosote polluted soil.

Abstract

Creosote has been used as wood preservatives worldwide. Former wood preservation facilities are ofien highly eontaminated due to practices such as the use of unlined wood treatment celIs, disposal of wastes in unlined lagoons and long-term dripping of preservatives from treated timber onto site soils. Creosote, produced from the high temperature carbonization of bituminous coal, is a eomplex mixture of over 200 eompounds, predominantly PAHs (about 85% ofthe creosote eomposition) as welI as phenolic compounds (10%) and nitrogen-, sulphuror oxygen-containing heterocycles (5%). Polycyclic aromatic hydrocarbons (PAHs) are of human health concern due to their known genotoxic, mutagenic or carcinogenic effeets. On the other hand, other compounds used for wood protection include sorne potentialIy toxic elements (EPT). This is the reason why soil EPT total and available contents of sorne facilities are high, especialIy in the ease of Cr, Zn, Cu and As. This study is focussed on a creosote contaminated soil where railway sleepers were impregnated with creosote to preserve and waterproof these wooden structures. Besides P AHs content, EPTs were also present. This results in a mixed organie-inorganic eontamination that makes difficult the selection of an adequate remediation treatment. Even more, another peculiarity shown by the soil under study is the low percentage of low molecular weight PAHs, which is an added difficulty for selecting an appropriate decontamination method. Biological treatment is the preferred technology for remediation of wood-treatment contaminated soil due to its low cost and represents an environmentalIy friendly alternative since the organic components may be completely mineralized to C02 and H20. However, due to their low aqueous solubility, high hydrophobicity and strong sorption to soil, biological remediation can be limited by the bioavailability of soil-bound P AHs, which is increased by the long ageing of the contamination in soils. Sorne extractants such as cyclodextrins (CDs) and rhamnolipids have been proposed as alternative non-toxic agents to enhance the water solubility of hydrophobic compounds, thus increasing their availability for biodegradation. In the most recent reviews about technologies for soil PAH remediation using extraetants no references to studies on the extraction of the 16 EPA P AHs and EPTs from real aged contaminated soils are included. Therefore, the objeetives of the present study were (i) to thoroughly analyse a real creosote contaminated soil to determine its P AH content and the presence of potentialIy toxic elements; Oi) to evaluate the abilities of a rhamnolipid (JBR 425) and several eyclodextrins: a natural CD (p-cyelodextrin), and three chemicalIy modified CDs (HP-P-CD, RAMEB and HP-y-CD), to extract the 16 EPA-PAHs; Oii) to evaluate the effect of these treatments on the availability of EPTs, which could inerease their mobility and endanger groundwater quality. The study concludes the high potential of rhamnolipids to extract PAHs from soil.