Droplet Size
For optimal oil distribution in the subsurface, the emulsion employed should have a uniform, small droplet size and surface characteristics that reduce its potential to flocculate.
EOS Remediation products are engineered to have droplet sizes significantly smaller than most pore spaces, with an effective diameter of approximately 1.0 micron. (See photo below.)
It is generally reported that pore diameters of silty and clayey sands typically vary between 20 and 100 microns. Both field and laboratory studies show EOS Remediation oil droplets can easily pass through these pores with negligible clogging (Borden et al., 2004; Coulibaly and Borden, 2004). However, using an emulsion with smaller oil droplets would be wasteful, since most bacteria are larger than 1 micron and could not access oil in the smallest pores.
The image on the left shows the emulsified oil substrate manufactured by EOS Remediation with uniformly sized droplets (size bar = 10 micron). The image on the right shows an emulsion at similar magnification that was field processed using a Silverson high shear mixer.
A second important consideration in evaluating an amendment is the transport, dispersion and retention of edible oil emulsions in a wide variety of soil types. This process can be described by colloid transport theory. When the oil droplets are significantly smaller than pore spaces (as with EOS®), oil retention is controlled by the chemical properties of the droplet and soil surfaces.
The interaction between oil droplets also plays an important role in colloidal stability. Recent work by Jain and Demond (2002) indicates that droplet capture and associated permeability loss may also be strongly related to the surface charge characteristics of the oil droplets. Depending on the type of surfactant used in preparing the emulsion and the ionic strength of the groundwater, oil droplets may repel each other or they may stick together (flocculate). If they stick together, they can coat the pore walls forming mats of droplets many layers thick.
The left photomicrograph below shows a pore with many tiny emulsion droplets, much smaller than the pore throat. However, when they clump together forming mats, they can clog very large pores (30-70 µm). The right photomicrograph shows how these mats can break off, migrate downgradient and clog other pores. As a consequence, it is very important to use emulsions that do not clump together.
The Secret of Good Oil Distribution “Emulsions should NOT Flocculate”
The surfactants used in EOS® 598B42 have been tested and chosen so sorption to silts and clay soils in the aquifer sediment is relatively low. Our oil droplets are engineered to have a slight negative surface charge. This negative charge allows the oil droplets to repel each other, while sorbing onto the slightly positively charged aquifer soils. Since most aqueous colloidal systems are stabilized by electrostatic repulsion, the larger the repulsive forces between particles, the less likely they are to flocculate.
Independent test results indicate that:
• Our EOS® 598B42 blend has a zeta potential (the measure of the repulsive forces between particles) of –35 mV.
• The electro-negative charge of EOS® 598B42 will allow the emulsion to remain stable in suspension (i.e. the droplets charge repels one another).
Other emulsion blenders make their emulsions, in part, with lecithin. Lecithin contains functional groups with both positive charges (ammonia groups) and negative charges (phosphate groups). As a consequence, lecithin-based emulsions adsorb very strongly to clays and more substrate is needed to treat the same volume of aquifer. EOS® 598B42 requires less product to do the same job.
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