The Siallon Process A Overview

The chemistry of the patented [12] Siallon process is simple in that it is a two-stage reaction involving two water-based products. The first step uses a water-based emulsifier that first desorbs the hydrocarbon from the soil and then emulsifies it within a water phase. The second stage is the addition of the Siallon reactive silicate solution to the mixture of soil and emulsified hydrocarbon. The silicate undergoes an acid-base reaction with the emulsifier micelle and is neutralized, forming a micrometer-sized particle of silica around the droplet of hydrocarbon. As the essential reaction, the neutralization of the silica, is a simple acid-base reaction, it is virtually instantaneous. The speed of this reaction leads to very high volume throughput during soil remediation; depending upon site-specific conditions, throughput ranges from 25 to 200 tons/hr.

The only limitation on the process is that the hydrocarbon or organic contaminant has to be emulsifiable. This means that a wide variety of contaminants such as gasoline, diesel fuel, jet fuel, motor oil, crude oil, greases and lube oils, coal tars, PCBs, chlorinated solvents, and many others can all be successfully treated within a soil or sludge matrix. Similarly, the physical nature of the host material plays little part in the final remediation results. All types of soils, from sand to clay, sludges, and tars, respond well to treatment, with the only modifications being to the actual processing or mixing equipment utilized. Pug mills, ribbon blenders, Bomags, and modified soil composting equipment have been used for the application and mixing of the Siallon reagents.

Applications of this process range from remediation of gasoline-contaminated soil from a UST location to in situ remediation of PCB-contaminated soil at a transformer manufacturing plant. Encapsulation efficiencies for benzene, ethylbenzene, toluene, and xylenes (BETX) and total petroleum hydrocarbons (TPH) in the gasoline-contaminated soil ranged from 99.97% to 100%. Encapsulation efficiencies for the in situ PCB remediation ranged from 99.9991% to 99.9999%.

B. Siallon Chemistry

As mentioned previously, the Siallon process of microencapsulation is a simple two-step procedure. The first step, the emulsification of the hydrocarbon, is the heart of the process. Surfactant chemistry shows that when a hydrophobic material is emulsified it forms a micelle [13] within the aqueous phase. This micelle is composed of a droplet of hydrocarbon surrounded by surfactant or emulsifier. The reason for this is the molecular structure of the surfactant. Mo-lecularly all surfactants, emulsifiers, wetting agents, and detergents have a common structure, a hydrophobic or oil-loving "tail" that is oil-soluble and a terminal hydrophilic or water-loving portion or "head" that is water-soluble. The hydrophobic tail will attach itself to the hydrocarbon particle, with the hydrophilic head conferring water solubility on the entire molecule. As emulsion proceeds, more and more surfactant molecules attach to the hydrocarbon particle in the same orientation. Such extremely close packing of the surfactant molecules occurs that the concentration of surfactant around the micelles in a given solution is much higher than the concentration within the bulk of the solution. The final result is a spherical form of hydrocarbon surrounded by close-packed surfactant molecules within the water phase. The actual amount of surfactant required to first produce desorption from soil and subsequently emulsify the hydrocarbon is extremely small in relation to the mass of soil. Consider that the clothes washing done in the average home is with an aqueous solution containing less than 0.0001 mol% surfactant for removal of body oils and grease [14]. This very high rate of activity allows the Siallon process to remediate contaminated soil with relatively small amounts of additives and no volume increase between the untreated soil and the treated soil.

The Siallon emulsifier differs from conventional surfactants or other emulsifiers in that it contains an acidic moiety as part of the hydrophilic end or head of the molecule. The acidic moiety will orient itself along with the hydrophilic end toward the aqueous phase, in essence producing a hydrocarbon microdroplet surrounded by and attached to the oil-loving portion, surrounded in turn by the water-loving portion and then by the acidic moiety. In essence there are a series of spheres or shells around the hydrocarbon, ending with a shell that has acidic reactive sites on its surface. The average diameter of these shells or micelles is less than 2 [im. The small size of the micelles results in a very stable emulsion with little or no tendency to split or revert.

All individual hydrocarbon compounds are susceptible to emulsification, and each compound responds to a very specific ratio between the size of the hydrophobic and hydrophilic portions of the emulsifier molecule. A product such as crude oil is composed of hundreds of individual compounds, each emulsifying best at a specific hydrophobic/hydrophilic ratio. To overcome this problem and produce the smallest possible micelle and most stable emulsion possible for the various hydrocarbon products requires a range of emulsifiers, each covering a range of hydrophobic/hydrophilic ratios. Each of the Siallon emulsifiers is a water-based, nontoxic product having a pH in the 4.2-5.5 range.

The second step of the Siallon process is the addition of a water-based silicate that is slightly alkaline with a pH of 9.5. The silicate undergoes an acid-base reaction with the acidic sites on the emulsifier, which neutralizes the silicate. This reaction forms silica, water, and a trace amount of salt. As the acidic sites are formed in a sphere around the emulsified micelle of hydrocarbon, the silica in turn forms a cell around the micelle. The final product is a micrometer-sized particle of silica containing both emulsifier and the drop of hydrocarbon.

Extensive characterization of the morphology and chemical nature of the silica cell was undertaken. Chemical analysis of the bulk material by both X-ray spectrometry and energy-dispersive X-ray analysis (EDXA) showed that the material formed was more than 98% Si02, with a trace of salts. These results were consistent with the neutralization of silicates in which the reaction products are normally silica, water, and simple salts.

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