Organoclay Keeps Activated Carbon Working
Bituminous activated carbon has an upper-limit pore diameter for effective absorption of 100 angstroms (0.01 micron). The pores rapidly lose their absorption energy as they become larger. Therefore, when attempting to remove organics using activated carbon, even the smallest oil droplets (0.05 micron) will block the pores of the activated carbon, quickly exhausting its capacity for removing any other organics.
Fortunately a means to circumvent this dilemma exists.
A "facilitator" in the form of organoclays has been identified and applied successfully during the last few years. Organoclays comprise suitably modified bentonites (95% clay mineral montomorillonite, 5% gangue minerals).
Bentonite essentially consists of sheets of aluminosilicates separated by an interlayer containing both inorganic cations, such as calcium and potassium, and water molecules. The cations in native clays, however, can be easily replaced by other cations through aqueous ion-exchange processes. For the remediation of organic materials, clays are traditionally treated with quaternary ammonium cations that contain one long-chain hydrocarbon tail. These cations render the surface of the clay hydrophobic. The resulting materials have great affinity for non-polar and modestly polar organic compounds.
Organoclays' ability to remove oil from water is seven or more times that of activated carbon. Also, organoclays can remove the full range of petroleum contaminants, from polynuclear aromatics to gasoline, diesel fuel, kerosene, to Bunker C oil—that is, from the light end to the heavy-molecular weight-end.
When used jointly with organoclays, activated carbon will be totally dedicated to the removal of more soluble compounds, such as phenol and BTEX.
The organoclay is blended with anthracite to prevent early plugging of interstitial pore spaces. This blend is capable of removing 50 to 60% of its weight in oil, depending on the molecular weight and solubility of the oil. The higher the oil's molecular weight and the lower the solubility of the organic compound to be removed by the activated carbon, the more effective is its removal rate.
In this fashion, organoclays are used as post polishers for oil/water separators and dissolved-air flotation units, frequently in a stand-alone mode (depending on the discharge limits). More often the organoclay is used as a pre-polisher for activated carbon and to prevent fouling of ion-exchange resins and membranes.
The combination of organoclay and activated carbon allows for effective in situ treatment of oil-contaminated water that may be stored in ponds or large storage tanks, eliminating the need for hauling and disposing of the water.
The ultimate advantage of an organoclay/activated carbon water-treatment system is that the level of contaminants can be economically reduced to non-detectable concentrations, allowing for zero discharge and recycling of the wastewater.
This is important in vehicle wash operations, laundries, rolling steel plants, and other types of facilities that use large amounts of water. It is not unusual for such facilities to pay as much as U.S.$40,000 per month for water use and discharge.
Organoclays also may be used in place of sand as pre-polishers.
A word of caution: If the oil is chemically emulsified, it must be de-emulsified first—that is, the surfactant must be dislodged from the oil and water interface. This is done by adjusting the pH and by other methods.
About the author: George R. Alther is a principal with Biomin, Inc., Ferndale, MI. Tel: 248-544-2552; Fax: 248-544-3733, E-mail: biomin@aol.com.