UTILIZZO DI SURFATTANTI PER IL RISANAMENTO DI SUOLI CONTAMINATI DA IDROCARBURI

Abstract

The present study aimed to evaluate the effect of a non-ionic (polyoxyethylene-(20)-sorbitan monooleate - Tween 80) and an anionic (Sodium Dodecyl Benzensulphonate - SDBS) surfactant, in the remediation of soils contaminated by petroleum hydrocarbons by means of Soil Flushing, in terms of hydrocarbons removal efficiency and soil residual toxicity. Soil pollution by fuel hydrocarbons is a serious and widespread environmental threat since they represent the 50-60% of the main contaminants found in soils. Among the remediation interventions for the removal of hydrocarbons from the soil, one of the most effective is represented by washing techniques with surfactant solutions, both in-situ (Soil Flushing) and ex-situ (Soil Washing). The use of surfactants, in general, improves the applicability of the treatment as they facilitate the mechanisms of mobilization, solubilization and desorption of organic contaminants from the solid matrix, lowering the surface and the interface tension in a soil-water-NAPL system. In detail, once the concentration of a surfactant in aquatic media exceeds its critical micelle concentration (CMC), generation of micelles takes place. These micelles can act as a hydrocarbon-like segment thus promoting the mobilization of pollutants from the media. In this context, to study the effectiveness of two different surfactants, an experimental laboratory-scale apparatus was set up. During the experimental campaign, several tests were conducted to simulate a soil flushing remediation on a sandy soil artificially contaminated by diesel fuel. At the end of experiments, the removal efficiency obtained during each test was evaluated and phytotoxicity tests were performed by means of germination index (GI). In particular, the hydrocarbon removal efficiency was evaluated by washing the soil with only water and with a solution of water and surfactant at different flow rates and concentrations. The assessment of TPH concentrations on the solid matrix was carried out according to “Procedura per l’analisi degli idrocarburi > C12 in suoli contaminati - Manuali e Linee Guida 75/11” proposed by ISPRA (2011), which provides, after the extraction and purification on Florisil, the TPH analysis by GC-FID. TPH concentration was determined by headspace gas-chromatographic analysis using a gas-chromatograph (Agilent 6890N Network GC System) equipped with a Flame Ionization Detector (FID) and the column Agilent 7683 Series; helium was used as gas carrier, oven temperature was set at 170°C and injection temperature was 250°C. Phytotoxicity tests employing seed germination and root elongation were used following the APAT procedure (APAT, 2004). Lepidium sativum seeds were used for germination and growth assays on sand aqueous solutions. Results showed that the use of both surfactants allows to reach high removal efficiency (~50% Tween 80 and ~70% SDBS) of hydrocarbons and that both the surfactant concentration and the contact time between surfactant and contaminant affect the removal efficiency. On the other hand, the results on the germination index showed that the two surfactants have different effects on the phytotoxic characteristics of the treated soil. The behaviour of Tween 80, contrarily to SDBS (toxic and poorly biodegradable), seems to favour seed germination; indeed, since it contains carbon potentially bioavailable, it would lead to a more efficient absorption of nutrients by the root system. In particular, while the soil treated with SDBS led to a lowering of the GI (10.88%), the soil samples washed with Tween 80 were characterized by higher values (146.61%). The use of both surfactants in the washing solution allowed to obtain significantly higher removal rates compared to only water, confirming their important role in the remediation of hydrocarbon-contaminated soils. In addition, although the use of Tween 80 resulted in lower removal efficiencies than that of SDBS in l