Makers of OilSorb™ and Other State-of-the-Art Filtration Media
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Makers of OilSorb™ and Other State-of-the-Art Filtration Media
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REMOVING OILS FROM WATER WITH ORGANOCLAYS George R. Alther, 2002 AWWA Journal Organically modified clays have long been used by industry to remove oil from water. A study was undertaken to verify results from real-world applications and develop a better understanding of the chemical conditions under which organoclays are most effective. Systematic testing was conducted on some 50 oils, including mineral oils, vegetable oils, animal fats, fish oils, and synthetic oils. In addition, the adsorption capacity of activated carbon was tested in order to compare the effectiveness of the two media. Tests also investigated the removal capacity of organoclays for various surfactants as well as the influence such surfactants exert on the removal efficiency of the organoclays. Results showed organoclays are more effective than activated carbon in removing oil from water and cationic organoclays are superior to nonionic organoclays for some removal applications. Granular organoclays have been used for a number of years as a prepolisher to activated carbon and for removal of small amounts of oil from industrial processing water and groundwater. However, neither industry nor academia has conducted studies to determine whether organoclays work with all oils, including mineral oils, plant oils, animal oils, crude oils, and refined oils. In addition, data are sparse as to which chemical conditions are most conducive to organoclay efficiency or whether one particular type of organoclay fits all circumstances. This article provides the results of a systematic laboratory study conducted to investigate various organoclay applications. ALL ABOUT OIL An understanding of the removal mechanism of organoclays requires familiarity with oil and its classifications. The sidebar details the various kinds of oils, including crude oil, mineral oil, petroleum, animal oils, vegetable oils, and essential oils. Table 1 lists oils by type and function. Oil contaminants in water. Oil and grease are commonly found in many process waters and groundwater. Oil found in contaminated water can be classified into five areas: • Free oil is oil that rises rapidly to the surface under calm conditions. • Mechanically emulsified oil consists of fine droplets ranging in size from microns to a few millimeters. These droplets are electrostatically stabilized without influence of surfactants. • Chemically stabilized emulsions have surface active agents that provide enhanced stability because of interaction at the oil-water interface. • Chemically emulsified or dissolved oil includes finely divided oil droplets (0.5-p diameter), benzene, and phenols. • Oil-wet solids are oils that adhere to sediments, metal shavings, or other particulate matter in wastewater (Braden, 1991). Types of oil found in wastewater can include fats, lubricants, cutting fluids, and heavy hydrocarbons such as tar, grease, crude oil, and diesel oil. Other oils are classified as light hydrocarbons (e.g., kerosene, jet fuel, and gasoline) and can also be found in wastewater. The major components of oils found in contaminated groundwater are benzene, toluene, xylene, naphthalene, benzo(a)anthracene, benzo(a)pyrene, polychlorinated biphenyl (PCB), trichloroethylene, 1,1,1-trichloroethane, tetrachloroethylene, methyl tertiary butyl ether, polynuclear aromatic hydrocarbons (PAHs), and phenols. Petroleum products tend to float on top of water as a sheen, but a small fraction is water-soluble. Some compounds may be adsorbed to solids or sink. Low-molecular-weight alkanes such as pentane and hexane are slightly water-soluble. Alkenes are slightly more water-soluble then alkanes, and aromatics are even more water-soluble, e.g., the benzene-toluene-ethyl benzene-xylenes (BTEX). Even when crude oil comes into contact with water, a small portion dissolves. All petroleum products have a water-soluble fraction, with the light end having a higher fraction than the heavy end. The heavier the hydrocarbon, the higher its boiling point, and the less soluble it is in water (The Nalco Water Handbook, 1979). Emulsions. An emulsion can be defined as a heterogeneous system that consists of at least one immiscible liquid intimately dispersed in another liquid in the form of droplets, whose diameter generally exceeds 0.1 μ. Emulsified oil is oil that has been broken up into droplets that disperse in the water. The smaller the droplets, the more stable the emulsion. When the droplets contact each other, they tend to coalesce and rise to the surface. If oil is not emulsified, it floats on top of the water as a sheen. When oil is emulsified, it is emulsified either mechanically or chemically. In chemical emulsions, an emulsifier usually a surfactant, detergent, or soap is present. Surfactants consist of a hydrophilic-oleophobic end and a hydrophobic-oleophilic-yophylic end. They act as a coupling agent between the oil-water phase. Because the emulsifier is polar on one end (i.e., it has a charge) and non-polar at the other end, it prevents the oil droplets from approaching each other and coalescing (Lissant, 1983). Surfactants and finely divided solids increase the stability of the emulsion because they impart a like charge on the oil droplets, causing them to repel each other and to disperse (Becher, 1965). ORGANOCLAYS' Manufacture of organoclays. Organoclays are manufactured by modifying bentonite with quaternary amines, a type of surfactant that contains a nitrogen ion. The nitrogen end of the quaternary amine (i.e., the hydrophilic end) is positively charged and ion exchanges onto the clay platelet for sodium or calcium (Lagaly, 1984). The most commonly used quaternary amine is of the dimethyl dyhydrogenated tallow ammonium type, which may include-A.-benzyl molecule if the application requires it. Bentonite is chemically-altered volcanic ash that consists primarily of the clay mineral montmorillonite. The bentonite has a charge of 70-90 meq/g. After it is treated with the quaternary amine, some 30-40 meq/g remain, which enables the organoclay to remove small amounts of the common heavy metals, e.g., lead, copper, cadmium, and nickel. How organoclays work. When the organoclay is introduced into water, the quaternary amine is activated and extends perpendicularly off the clay platelets into the water. A chlorine or bromine ion is loosely attached to the carbon chain. Because the sodium ions that were replaced by the nitrogen are positively charged, they bond with the chlorine ion, resulting in sodium salt that is washed away during backwashing when the clay is a granular material in an adsorber tank. What remains is a neutral surfactant with a solid base, i.e., the organoclay. Because "like dissolves like," the oleophilic end of the amine dissolves into the oil droplet, thus removing that droplet from water. The partition reaction takes place "outside" of the clay particle, in contrast to adsorption of oil by carbon, which takes place inside its pores; because partition takes place outside the particle, the organoclay does not get fouled quickly. Partition is discussed in detail in a subsequent section. Granular organoclay is blended with anthracite, which also has oil removal capability and has about the same bulk density (56 Ib/cu ft) as the organoclay. The blending helps prevent the interstitial pores from being immediately filled with oil as the wastewater containing oil is passed through the adsorber. Organoclays effectively remove chlorinated phenols and other hydrophobic, sparingly chlorinated, soluble compounds, e.g., PAHs and PCBs (Mortland et al, 1986). In combination with activated carbon, organoclays also provide cost-effective removal of BTEX; the organoclay removes the compounds with lower solubility, i.e., xylene and toluene, leaving the carbon able to remove benzene exclusively. Partitioning. The simplest definition of partitioning is that like liquid compounds dissolve into other liquid compounds of a similar nature when both are present in a third liquid of a different nature. An example would be two non-polar liquids dissolving into each other when present in a polar liquid. Oil and water do not mix. The water molecule is small and highly polar, whereas oil is a combination of neutral, non-polar organic molecules. Organic compounds with low solubility and non-polar characteristics (e.g., hydrophobicity) stick together. Because liquids like to dissolve into each other, it becomes a question of compatibility. When granular organoclay is placed into a filter vessel filled with water, the non-polar amine chains become activated, extending out from the clay surface into the water phase. Low-soluble, non-polar organics are attracted to the activated amine chains and attach themselves by dissolving or partitioning into these chains where they are held by coulombic forces.
Jar contents indicate the progress of oil removal treatment with organoclays through various stages. From left to right, the jars show (1) raw wastewater with 3% oil, (2) 200 mg/L oil after coalescer, (3) 3 mg/L oil after organoclay, and (4) no oil detectable after activated carbon. STUDY OBJECTIVES AND METHODS Objective. Because limited data are available about organoclays, the most effective chemical conditions for their use, and the type of organoclay needed for various circumstances, a systematic laboratory study was conducted to investigate organoclay applications. Isotherms cannot be run effectively with oils because many oils-particularly the heavy ones of near-zero solubility, such as bunker C oil or number 4 oil-tend to coat the mixing devices. The testing method used in this study was designed to circumvent some of these problems. Tests were conducted on some 50 oils, including mineral oils, vegetable oils, animal fats, fish oils, and synthetic oils. Occasional testing of the adsorption capacity of activated carbon allowed for comparison of the effectiveness of the two media. In addition, tests were performed to determine the removal capacity of organoclays for various surfactants, followed by another test series in which detergents were added simultaneously with the oil to determine the influence such surfactants exert on the removal efficiency of the organoclays. All tests were performed with powdered organoclays and activated carbon. In this research, organoclays of both the nonionic form and cationic form were used, because some of the oils were too polar for removal by the nonionic organoclay. Laboratory methods. Preparation of emulsions. Softened tap water (499.50 g) was weighed into a clean glass blender container. Next, 0.50 grams of the oil to be tested was added to the water, and the mixture was blended in the blender at high speed for 5 min. Eight 500-g batches were subsequently combined together to create a composite batch, which was continuously stirred with an overhead mixer to ensure uniformity of the samples. Organoclay testing. The composite oil-water emulsion (349.72 g) was weighed out into a clean 600-mL beaker; and a magnetic stir bar was inserted. The beaker was then tared on the same scale on which the emulsion was measured. The beaker was placed on a stir plate, and a stirring motion was set to establish a vortex that extended halfway down the emulsion. After the emulsion was allowed to stir for 1 min, 0.28 g of organoclay was added to the agitating composite for 4 min. (The 0.28-g figure was established as the ideal amount of organoclay to be added to the emulsion for maximum, but still economical, removal capacity.) As the organoclay powder was dispersed, the speed of the mixer was increased by extending the vortex down to the bottom of the beaker. The mixture was then placed into the blender and sheared at high speed with cutting blades for another 10 min. After it was discovered that filtering the sample through filter paper removed additional oil, a pipette method was substituted. A 2-mL disposable pipette was used to pipette out a portion of the treated emulsion, with care taken to avoid any organoclay particles or clumps. (It was observed that after the mixing, the organoclay powder would coagulateinto a clump and float to the surface. Thus, pipetting without sucking up powder was not an issue.) The composite was tested for oil content with a portable oil detection instrument (sensitivity – 0.50 mg/L). In a few tests, the author replaced the organoclay with a sample of powdered activated carbon to establish some comparative values with a known filtration medium. The carbon was flocculated out of the water by the addition of two drops of 0.1% aqueous solution of anionic polymer to 100 ML of the emulsion. This mixture was stirred on a stir plate for 1 min and then passed through a metal tea strainer before it was analyzed for oil content. Data presentation and calculation. Data are presented in terms of amount of oil removed. The oil-removed data were calculated based on the initial oil content of the nontreated emulsion. The amount of oil content removed was obtained by subtracting the amount of oil content remaining from the initial oil content of the non-treated batch emulsion. The percent of oil content removed was calculated by dividing the oil removed value by the initial nontreated oil content and multiplying by 100. The grams of oil removed were obtained by expressing the oil content removed as a percentage (e.g., 715 mg/L = 0.0715%) and then obtaining that percentage from 350 mL. The percent of oil removed on a clay-weight basis was calculated by dividing the grams of oil removed by 0.28 g and multiplying by 100. The assumption was that the oil was truly removed from the emulsion and that the organoclay adsorbed all of the removed oil. Occasionally samples were replicated. Because this study was designed to aid the wastewater engineer in the design of wastewater treatment systems, the idea was to discern trends, which is why not every sample was replicated. Mixing times were established by increasing mixing times until the amount of oil removed leveled out. RESULTS Figure 1 compares the relative capacity of organoclay and activated carbon to remove oil from water. (Results for this figure were not generated in this study but were included for comoparison of organoclay and activated carbon.) As the figure shows, granular organoclay (in this case blended with anthracite and the wastewater being passed through a column) was far superior to activated carbon at removing oil – some seven times more effective, depending on the kind of oil that was being removed from the wastewater. Figure 2 shows the capacity of nonionic organoclay to remove various mineral oils from water. Some of the results from tests with heavy oils (e.g., bunker C, number 4 lubricating oil) may not be completely reliable because the oil sticks to the side of the wall of the jar and the spindle. Figure 3 shows the same kind of data generated with plant oils. The important conclusion from this figure is that nonionic organoclay is just as efficient at removing plant oil from water as it is at removing mineral oils. Figure 4 shows test results of miscellaneous oils removed from water with organoclay. It is evident that some oils, particularly crude oils, were not well removed. The reason is that crude plant oils contain phospholipids and fatty acids. These chemicals act as emulsifiers, reducing the ability of the organoclay to remove the crude oil from water. The test for removing miscellaneous oils was repeated, this time with a cationic organoclay, i.e., one that is positively charged, to determine whether this substitution would improve removal levels. Figure 5 shows that the cationic organoclay increased oil removal. Compared with the nonionic organoclay, the cationic organoclay was nearly twice as successful at removing synthetic watch oil. CONCLUSIONS Test results yielded the following conclusions: • Organoclays are more effective than activated carbon at removing all oils from water, because they do not experience the problem of blinding of pores. • One standard, nonionic organoclay does not fit all applications effectively; for example, a cationic organoclay is more effective for removal of the more-polar crude oils. • Organoclays remove all surfactants as effectively as activated carbon: • The cationic and anionic surfactants quickly retard the ability of the organoclay to remove oils from water. In addition to these findings, another set of data showed that nonionic organoclays are effective over the entire pH range, proving the veracity of the partitioning removal mechanism. Organoclays have been shown to be effective at removing any oil-nonpolar or semipolar from water. In addition, organoclays remove other organics of medium to low solubility from water with the same efficiency. In the author's experience, the effectiveness of organoclays has been proven by many real-world applications over the past 15 years. In one case history, a manufacturer had 3% semisynthetic lubricating oil in its wastewater collection system. The wastewater was treated with conventional coalescing equipment, followed by a bag filter for dirt removal, followed by organoclay and activated carbon as a final polish. When the company began reusing its wastewater, it saved approximately $30,000 per year in disposal costs as well as additional savings from reduced water needs and associated sewer discharge costs. The system paid for itself in less than two years. Although effective for the removal of both oils and detergents, organoclays remove oils preferentially over detergents, because the oil droplets are heavier and primarily nonionic, except in the case of the slightly polar crude oil. In real-world applications, therefore, organoclay-carbon systems should be used. This results in potable water when the organoclay-carbon system is used in series, i.e., the organoclay is placed into the first adsorber, followed by a tank filled with activated carbon |
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