Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6e. Surfactant-Enhanced Remediation

Formatted Title:

Surfactant Enhanced Mobilization of Polycyclic Aromatic Hydrocarbons from Marine Sediment by Washing and Flushing Processes

Background/Objectives:

Polycyclic aromatic hydrocarbons (PAHs) are one of the most impactful pollutant classes of soil, sediments, and groundwaters quality due to their strong toxicity (many of them are carcinogens and/or mutagens) and the high persistence of their secondary sources. Due to their chemical-physical properties, they are strongly sorbed onto soils and sediments, especially in historical contamination. Thus, development of new methods to mobilize and make them more available are necessary for remediation purposes. In recent years, great scientific efforts have been concentrated on the possibility of employing surfactant solutions in flushing or washing processes of the contaminated matrix to increase the mobility and solubility of hydrophobic organic compounds (such as PAHs) and their mass transfer in aqueous phase.In this context, a lab-scale study was carried out to evaluate the ability of eight biodegradable non-ionic surfactants, such as synthetic alkylpolyglycides (APGs), biological sophorolipids (SLs) and biological rhamnolipids (RLs) to solubilize and mobilize PAHs from a historically heavy contaminated marine sediment.The investigated sediment comes from the Italian National Priority Site of Bagnoli-Coroglio (Naples), which suffers from historical hydrocarbon contamination attributed to local steel industries that began operations in the early 20th century and were decommissioned in the early 1990s. Consequently, this represents an aged contamination scenario where the remaining pollutants are highly recalcitrant, constituting the most persistent fractions, and exhibit markedly reduced mobility under standard environmental conditions.

Approach/Activities:

To evaluate the effectiveness of surfactants in solubilizing PAHs from contaminated sediment, a systematic study was conducted. First, the sediment was characterized to measure the initial PAH contamination level. Then, a preliminary batch washing test was performed at a surfactant concentration of 5 times the critical micelle concentration (CMC) to evaluate the ability of the surfactants to solubilize PAHs in the aqueous phase. Next, additional batch tests were performed to investigate the effects of multiple sediment washes and surfactant concentration on PAHs mobilization efficiency. The results showed that multiple sediment washes significantly increased the mobilization of PAHs, and that the optimal surfactant concentration varied depending on the specific used surfactant. Finally, a continuous configuration column experiment was carried out by selecting the best surfactants in terms of mobilization efficiency and their optimal concentration. Column experiments allowed to investigate PAHs mobilization under conditions representative of a real flushing process.

Results/Lessons Learned:

The present work provides a systematic guideline for the preliminary selection of surfactants for flushing/washing operation, the optimal operative conditions, and the technical approach for PAHs extraction from the sediment, especially in the context of a real aged contamination scenario. Experimental results showed that all investigated surfactants contributed to enhance the mass transfer of pollutants in the aqueous phase, with respect to the water. The process conditions study highlighted a great improvement in PAHs removal from the sediment with multiple washes test, especially for the heavy PAHs fraction (more than 3 aromatic rings). In this case, almost quadrupled PAHs removal efficiency was observed after the third wash of the same sediment sample. Also, the increase in surfactant concentration, led to an important increase in PAHs removal from the sediment, reaching almost 30% of efficiency using a 5% wt of surfactant concentration. Finally, column test showed a great efficiency of these surfactants in PAHs mobilization in continuous flushing of the contaminated matrix. Surfactants’ performances resulted 40 times greater than water mobilization ability. Moreover, it will be shown almost the totality of the mobilization in the flushing process with the selected surfactants occurred in just two pore volumes of the feeding flushing solution. These data clearly underline and emphasize the incredible potential of surfactants in the treatment of matrices impacted by hydrophobic organic contaminants even in situations of historical contamination and particularly aged sources.

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Gabriele Moscatelli