Eco-Friendly Chemical Herders For Remediation Of Maritime Oil Spill
Crude oil spills on a sea surface can cause severe and immediate damage to marine life, as well as long term damage to the aquatic ecosystem. Current response methods for maritime oil spill remediation are twofold: The first are the use of floating barrier booms to confine and thicken the spill slick to smaller areas, where they can be subsequently removed by mechanical skimming or in-situ burning. The second are the use of dispersants which are aerially sprayed onto the slick to emulsify the slick through wave action into smaller droplets that can be dispersed through the water column for degradation. However, neither of those methods serves as a perfect solution for oil spill remediation. Booms are difficult to deploy while dispersants retain the oil in the ecosystem for extended periods of time. This thesis studies chemical herding as an alternative methodology for the remediation of oil spills on the sea surface. In chemical herding, surfactants are used as chemical herders to draw the slick upon itself to a reduced area and a large thickness so that it can be burned or skimmed. The herding surfactants are aerially sprayed onto the sea surface around the periphery of the oil slick, where they form a monolayer which reduces the air/sea tension (increases the surface pressure) at the slick edge. This reduction causes an imbalance of interfacial forces and forces the oil spill to retract to a thicker slick with smaller areas. While chemical herding has been part of the arsenal of methodologies intended to be used to remediate oil spills, the method suffers from two key drawbacks which prevent its wide spread use: The most effective surfactants used to date which are on the U.S. EPA National Contingency Plan for herding are the hydrocarbon surfactant, sorbiton monolaurate (Span 20 or Thickslick 6535) and a polydimethylsiloxane polymer surfactant (OP-40). OP-40, because of its polymeric nature, has the potential to bio-accumulate and is therefore not eco-firendly. In addition, neither of these surfactants work particularly well in the presence of seas with surface waves and this drawback is the principle reason herding has not been used extensively. The scope of this proposed research is to study two new surfactants as potential herding agents. These herding surfactants are phytanic acid, an isoprenoid fatty acid with methyl branches along its hydrocarbon chain and a carboxylic head group, and monogalactosyldiacylglycerol (MGDG), a glycolipid with unsaturated hydrocarbon chains and a galactose head group. They are chosen for three basic reasons. Both are found in the chloroplasts - the organelles in plants responsible for photosynthesis - and are therefore in abundance in aquatic life and naturally eco-friendly. Second, they are relatively insoluble and hence their ability to herd should remain steadfast as they have very limited solubility in the water column. Third, unlike other insoluble surfactants with straight hydrocarbon chains and smaller polar groups form liquid rather than solid states which may allow them to be more resilient and stable under wave action. The goals of this thesis are twofold: The first is to understand how effective phytanic acid and MGDG are at herding under calm sea conditions. Surface pressure measurements are undertaken on Langmuir troughs to study the surface pressures they can develop and these pressures are found to be large enough to herd oil. The molecular structures of the monolayers are studied using atomic force microscopy on films transferred to solid substrates. Direct measurements of the structure are undertaken by x-ray reflectivity. Second, small scale pan experiments are undertaken to record their effectiveness at compressing a crude oil spill. In each of these cases, the effect of ions in the subphase is investigated to reproduce sea water conditions. The second goal is to identify the important surface properties a surfactant herder must have to be effective at herding under wave action and to examine how effective these eco-friendly surfactants are with regard to these properties. Two surface properties are identified: The first is the intrinsic dilatational viscosity of the monolayer, as large dilatational viscosities can act to dampen wave energy so that the monolayer can remain intact. The surface dilatational viscosity of these monolayers of phytanic acid and MGDG are measured using a pendant bubble tensiometer and are shown to be relatively large. The second property is the ability of the surfactant herders to bind to the natural water soluble polysaccharides present immediately underneath the sea surface (the sea surface microlayer). The polysaccharides are in abundance in this micro layer as extrudated products of marine organisms in the layer, and upon binding to the herding monolayer can form more durable layers with potentially large dilatational viscosity. A representative negatively charge polysaccharide present in the microlayer, λ carrageenan, is used. Surface pressure measurements show that these herders bind effectively to this polysaccharide, presumably either through hydrogen bonding (MGDG) or electrostatic binding through divalent coupling of cations present in the sea surface (phytanic acid). The dilatational viscosity is found to increase. X-ray reflectivity is used to measure the binding of the carrageenan to one herder, MGDG, and significant recruitment of the carrageenan to the monolayer is found. Finally, the herding action of the MGDG monolayer, under subphases of the carrageenan polysaccharide are examined in pan-scale experiments.
City University of New York - City College
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