A Winning Combination

• Oct 16, 2007

Creosote and other non-aqueous phase liquids (NAPLs) are responsible for high pump-and-treat costs in groundwater remediation because, due to their molecular size, they tend to clog activated-carbon pores quickly. The result is frequent change outs, which renders the pump-and-treat method too expensive.

NAPLs are a class of hydrophobic-phase organic liquids that can form discrete layers in the aquifer. NAPLs are divided into two classes depending on their specific gravity.

• Dense, non-aqueous phase liquids
. Those compounds with a specific gravity greater than fresh water are dense, non-aqueous phase liquids (DNAPLs). Wood preservation waste, called creosote, is classified as a DNAPL. Creosote includes the phenols, coal tar wastes from manufactured gas processing sites, organic solvents perchloroethylene (PCE), trichloroethylene (TCE), 1,1,1, tributyl acetate (TCA), and others. They are present as one or several compounds and tend to sink into the aquifer.

• Light, non-aqueous phase liquids
. Compounds with a specific gravity lower than that of water are light, non-aqueous phase liquids (LNAPLs). Petroleum-based fuels including gasoline, jet fuel, and even crude oils are LNAPLs.

LNAPLs have an affinity to partition into organoclays, which are organically modified clays consisting of bentonite modified with quaternary amines. More on this topic can be found in an article by George Alther that was published in Environmental Protection magazine's February 2000 issue. The article, "Maximize Water Cleanup Performance," can be accessed free of charge on Environmental Protection's Web site, www.eponline.com, by searching in Archives.

DNAPLs, particularly creosotes, i.e. phenolic compounds, also have a great affinity for organoclays. DNAPLs may form pools within the aquifer, or they may be residual, meaning they are held under negative pressure by surface tension as coating on soil particles and within fractures and pore spaces. DNAPLs are mobile -- they can migrate vertically through an aquifer under positive pressure and into zones of lower permeability such as fractured or otherwise permeable rock, sediments (glacial till, moraines), and soils. Migration of such plumes can occur quickly, i.e. several feet per day. If the water table is close to the surface, this migration can have an adverse effect on the environment because soils will adsorb the creosote compounds. Migration behavior is thus very complex and has been discussed thoroughly by industry experts. More information about migration behavior can be found on The National Groundwater Association's Web site, www.ngwa.org.

Because of the migration complexity of DNAPLs, in-situ remediation methods are sometimes not appropriate for cleaning up aquifers, such as old wood-treating sites or manufactured gas-storage sites. If pump and treat is used, it may be necessary to inject co-solvents such as surfactants, soaps, or alcohols into the aquifer in order to desorb the DNAPLs from the soils or shales. Ultimately, it may be advisable to use the pump-and-treat method for quick removal of the largest concentration of DNAPLs in a plume, followed by an in-situ and passive method to deal with smaller contamination levels. In a pump-and-treat situation, if the chemical oxygen demand (COD) is 50 parts per million (ppm) or higher, Fenton chemistry may be applied in the equalization tank, followed by an organoclay/activated carbon adsorption train. Fenton chemistry is the chemistry that occurs when metal ions, such as ferrous or ferric ions in water, interact with peroxide. This chemistry is characterized by the liberation of oxygen, accompanied by the production of a flux of hydroxyl radicals. If surfactants were added for desorption, demulsification may be required, which is achieved by Fenton reactants. The organoclay/carbon method is then used to remove intermediaries. Creosote removal has been performed successfully with organoclays.

What is Creosote?
Like oil, creosote does not have a single chemical formula; it is a mixture of various chemicals. Creosote can be derived from several sources: the resin of the creosote bush (creosote bush resin), beech and other woods (wood creosote), and coal tar and coal tar pitch (coal tar creosote).

Creosote bush resin comes from the creosote bush, officially named the Larrea Tridentata, which thrives in the Southwestern deserts of the United States. It is found in all five Southwestern deserts, which are located in southern Nevada, southwestern Utah, southeastern California, the southern third of Arizona, southern New Mexico, and into the southern part of Texas and northern Mexico. This tough plant can tolerate the arid conditions in these deserts, often pushing out other plants. Creosote bushes have several flexible stems and grow on average to 4 feet height; but with abundant water they can grow up to 12 feet in height. The bushes bloom from February to August and produce flowers that are approximately an inch wide and have yellow petals. As it blooms, the creosote flowers turn into small, white, fuzzy bloom capsules containing five seeds each. But it is the resin from the leaves of the creosote bush that are used to make creosote oil.

Wood creosotes Chemical Abstracts Service Number (CAS#) 8021-39-4 are created by the high-temperature treatment of beech and other woods. They create a colorless to yellowish gray, greasy liquid with a smoky odor and sharp burned taste. In wood creosote, the major chemicals are phenol, cresol, and guaiacol. Although now rarely used in the United States, wood creosote was formerly used as a disinfectant, a laxative, and a cough treatment.

Coal tar (CAS#8007-45-2) and coal tar pitch (CAS67996-93-2) are the byproducts of the high-temperature treatment of coal to manufacture coke or natural gas. By heating coal in the absence of air, a thick, black or brown liquid called coal tar is generated, which is then distilled to form coal tar creosote. This creosote is a thick, oily liquid typically amber to black in color. Coal tar creosote (CAS#8001-58-9) is commonly used as a wood preservative in the United States. It is used for the impregnation of railroad ties, telephone poles, pilings, fence posts, log homes, and other outdoor wooden structures. It is also a restricted-use pesticide. The major chemicals that are dangerous in coal tar pitches are polyciclic aromatic hydrocarbons, phenols, and cresols. Even dioxin is found in creosote.

Chemically, creosote is a blend of several hundred compounds, the most prevailing ones being polycyclic aromatic hydrocarbons and phenolic compounds, particularly pentachlorophenol (PCP). PCP is a major constituent of wood preservatives. Trichlorophenol is used as a preservative for wood and leather and as a biocide. Cresols have a methyl group attached to the benzene ring of toluene. Before 1950, creosote was the primary preservative chemical; afterward, isolated PCP became the preferred preservative.

Health Problems
Of the three types of creosote, it is the variety derived from coal tar and coal pitch that is a known health threat. Several governmental agencies have concluded that exposure to coal tar creosote has carcinogenic effects. After assessing that coal tar creosote is probably a human carcinogen, and that coal tar pitch is definitely carcinogenic, the U.S. Environmental Protection Agency included coal tar creosote on its 1992 Toxic Release Inventory. The International Agency for Research on Cancer also has decided that coal tar creosote is most likely carcinogenic to humans.

According to the National Safety Council (NSC), long exposure to coal tar creosote chemicals can lead to skin cancer and cancer of the scrotum, particularly in situations where the chemicals directly contact skin. These health problems have been reported in occupations that use coal tar creosote, such as in wood treatment or in manufacturing creosote-treated products. Even brief exposure of large amounts of coal tar creosote can cause significant health problems such as skin irritations, chemical burns to the eye, convulsions, mental confusion or liver problems, unconsciousness, or even death.

The NSC (www.nsc.org) also warns against eating food or drinking water contaminated with high levels of creosote, which can lead to a burning in the mouth and throat and stomach pains. Of course, given its wide-spread use, creosote has managed to contaminate several locations and is present at various Superfund sites.

Cleaning Up Creosote-Contaminated Sites
Removal of creosote from water is a task prevalent at a number of Superfund sites. Usually, the water is passed through an adsorber filled with activated carbon, which turns out to be an expensive proposition. The cause of the expense involves the fact that the major compounds contained in creosote are large organic hydrocarbons of low solubility in water, resulting in frequent carbon change outs.

A number of contractors that service these creosote sites are now using organically modified clay (organoclay) as a way to lower operations costs, using carbon as a post polisher. The reason? Organoclay removes large organic hydrocarbons of low solubility, including chlorinated phenols, polynuclear aromatics (PNAs), polychlorinated biphenyls (PCB), and oil and grease, at seven times the rate of activated carbon, lowering operations costs by 50 percent.

What are Organoclays?
Organically modified clays, or organoclays, consist of bentonite, which has been chemically modified with cationic quaternary amines. The result of this process is that the bentonite now becomes a non-ionic surfactant with a solid base. The organoclay is usually blended with anthracite to reduce the quick filling of interstitial pore space with the contaminant that is being removed. Organoclay removes creosote compounds, such as phenolic compounds, by a chemical process called partition (versus adsorption taking place inside carbon pores). This process is very successful in the removal of non-polar compounds. As phenolic compounds increasingly become chlorinated, they become more neutral, and thus become more prone to partition by organoclays. As the chlorine content increases, the solubility of these compounds decreases. Activated carbon is also an excellent adsorber for phenols, but as the phenols increase in size, the efficiency of activated carbon decreases due to blinding of the pores by the compounds. It also is known that activated carbon works extremely well at lower concentrations, i.e. less than 5 ppm, while organoclay works much more efficiently at higher concentrations.

Successful removal rates combined with the economic benefits of removing creosote with organoclay make this method an attractive option for creosote removal and a viable solution for preventing creosote-related health problems.

Case Histories
The three case histories below show how effective the pump-and-treat method became when organoclay and activated carbon where combined.

1. A creosote Superfund site on the East Coast installed a pump-and-treat system consisting of two filter vessels, each containing 20,000 pounds of activated carbon. The flow rate is 170 gallons per minute (gpm). The COD consisted of 40 ppm to 60 ppm, including benzene, volatile organic compounds (VOCs), and phenols. The activated carbon lasted about two weeks, with a breakthrough of 7-ppm to 12-ppm COD, then it had to be changed out. After another vessel containing 19,000 pounds of organoclay was installed, the effluent after the activated carbon was non detect. Furthermore, there was a total suspended solids content of 32 ppm to 35 ppm (discharge limit is 40 ppm), primarily due to the presence of ferric iron. Once the organoclay was installed, the total suspended solids (TSS) content in the effluent was 3 ppm. This is because the organoclay, being a bentonite, also removes heavy metals by ion exchange.
2. An old wood-treating site in Colorado is situated above an aquifer that had a concentration of 30 ppm of an oil and 25 ppm of PCP. The discharge limit for PCP is 50 parts per billion (ppb). When an activated carbon system was installed, change out was required within one month. After 20,000 pounds of organoclay was installed prior to the activated carbon, discharge limits where met, and change out was required after 12 months to 15 months.
3. An old railroad site in the southeastern United States, where railroad ties were once treated with creosote, requires excavating the soil and thermally treating it to destroy the creosote. A condensate builds up that contains PCP. Rather than accepting the high cost of incinerating the condensate water, it is passed through an organoclay/carbon system and discharged locally.
4. Two sets of field tests where run with water from a pond closure near an old wood treating site in Florida:

In these tests, some 300,000 gallons of water was passed through the organoclay and carbon vessel. At another site, not only was PCP removed, the treatment helped to remove dioxin as well.

These case histories show that using organoclays for pre-polishing, followed by activated carbon, is an effective method of increasing the economics of using such media as activated carbon and Mycelx, a technology that uses a chemical compound to prevent the formation of micelles (oil droplets) in water and causes oil products to affix themselves to the Mycelx compound for removal from the water.

This article originally appeared in the July/August 2004 issue Water and Wastewater Products, Vol. 4, No. 4.