Final Report: Surface Chemistry of Oil/Soil/Water Systems for Improved Oil Removal from Contaminated Soil by Air-Sparged Hydrocyclone Flotation

EPA Grant Number: R825396
Title: Surface Chemistry of Oil/Soil/Water Systems for Improved Oil Removal from Contaminated Soil by Air-Sparged Hydrocyclone Flotation
Investigators: Miller, Jan D.
Institution: University of Utah
EPA Project Officer: Packard, Benjamin H
Project Period: January 20, 1997 through January 19, 2000 (Extended to January 19, 2001)
Project Amount: $315,706
RFA: Exploratory Research - Water Chemistry and Physics (1996) RFA Text |  Recipients Lists
Research Category: Water , Land and Waste Management , Engineering and Environmental Chemistry


Unrestricted discharge of various oil-bearing wastes and accidental oil spills on lands and in water streams has created significant environmental problems in the United States. Spilled petroleum products cause ecological and esthetic degradation, economic losses, and pose a threat to human health. Although much research has been conducted in the removal/separation of oils from contaminated soil, technological issues as well as economic factors have limited the successful cleanup of oil-contaminated sites on a large scale. Froth flotation is one of the least costly technologies that can be used to remove oil from contaminated soil. Air-sparged hydrocyclone (ASH) flotation technology is a new high specific capacity flotation technology being developed at the University of Utah during the past 20 years. Initial research and development efforts were made for the fast and efficient flotation separation of fine particles in the mineral industries. During the most recent 10 years, the ASH technology has been successfully evaluated for environmental remediation; cleanup of wastewater, wastepaper recycle, and reclamation of fine coal refuse. Also, the ASH technology has been demonstrated to be applicable for dispersed oil flotation.

However, the lack of development of a more efficient deoiling flotation process has limited our ability to do large-scale cleanup of oil-contaminated soil. In essence, the process must consist of two consecutive steps: release/displacement of oil from the soil, and separation of dispersed oil from the water phase. In this regard, comprehensive fundamental studies on the surface chemistry of oil/soil/water systems have been completed, including examination of physicochemical properties, surface hydrophobicity, oil/soil interactions, stability of oil-in-water emulsions, release of oil from mineral surfaces, coalescence of oil droplets with air bubbles, oil spreading on air bubble surfaces, the effect of oil on foam stability, and the treatment of oil contaminated soils by ASH flotation.

The overall research objective was to establish a surface chemistry and engineering basis for the treatment of oil contaminated soil using air sparged hydrocyclone (ASH) flotation technology. In this program, three major areas associated with oil/soil/water systems have been identified in order to undertake a comprehensive study and realize the overall research objective. These three research areas include: (1) selection of surface and solution chemistry conditions for oil flotation, (2) understanding and control of froth stability in relation to oil flotation, and (3) specification of operating variables for cleanup of oily soil using ASH flotation technology.

To complete the first task and then successfully apply it in practice, understanding of the surface chemistry at a detailed level was necessary. Significant effort has been devoted to develop methodologies for study of interaction forces in model systems related to the oil/soil/water/air system, and to characterize the influence of surface and solution chemistry on these forces. These results obtained using the atomic force microscopy (AFM) colloidal probe technique, together with wetting and spreading experiments, have allowed for fundamental insight into the physicochemistry at the molecular level and to apply these findings in subsequent tasks.

Appropriate froth stability is a necessary condition for successful separation using flotation techniques. Because of the existence of a strong centrifugal forcefield, combined with the pronounced shearing forces, froth stability during ASH flotation required particular attention in the second task. The presence of dispersed oil complicates this issue even further. Significant effort, both from the fundamental and practical point of view, has been made to optimize froth stability in the presence of dispersed oil. These results together with results from Task 1 have then been applied in Task 3 of the project.

Cleanup of contaminated soil using flotation technology requires that two consecutive steps be accomplished: release/displacement of oil from the soil, and separation of dispersed oil from the water phase. These two steps have been studied in concert during Task 3. In this part of the research, solution chemistry and hydrodynamic conditions were investigated during the decontamination process to reach a compromise between efficient release of oil from soil, achieving a hydrophobic state for oil droplets/agglomerates to be floated, and the required froth stability for the system.

The efficient one-step separation process using ASH-flotation technology was found to be a possible but difficult task and, to implement these findings in full-scale operation, further research is required. Nevertheless, the research completed through this grant has had immediate and significant impact on fundamental understanding of the physicochemistry of these systems and on the application of ASH flotation in the protection of our environment. Of particular significance, as a result of research conducted in this project, ASH-based flotation technology has been commercially developed for treatment of oil-contaminated wastewater. This ASH technology for the treatment of oil-contaminated water currently is researched in collaboration with ZPM, Inc., which has now installed more than 20 ASH-based industrial systems for wastewater treatment. It is expected that findings, obtained during this research also can be used for other environmentally oriented applications like plastic and paper recycling. The technical findings associated with this research program are summarized below.

Summary/Accomplishments (Outputs/Outcomes):

Task 1: Selection of Surface and Solution Chemistry Conditions for Oil Flotation

To select proper conditions for cleaning of oil-contaminated soil, it was necessary to characterize interaction forces between the various components of model systems. It has to be mentioned that at the beginning of this project fundamental aspects of interaction forces between hydrophobic surfaces were not sufficiently understood nor reported in the literature. For direct measurement of interaction forces, the atomic force microscope colloidal probe technique has been employed. To implement this technique for characterization, a procedure to prepare micrometer size spherical particles from aliphatic hydrocarbon polymers has been developed. Using surface spectroscopy techniques (FTIR and XPS) it was found that the preparation procedure had no significant chemical effect on the surface properties of the microsphere material. Prepared polyethylene spheres were found to be strongly hydrophobic and were used as a model for oil droplets and/or air bubbles in AFM experiments. A series of measurements have been performed for the characterization of interaction forces between these particles and different surfaces. These studies were performed to determine the: (1) influence of surface hydrophobicity, roughness, and heterogeneity on hydrophobic forces; (2) influence of ionic strength of the medium, effect of presence of surface tension lowering solvents (e.g., acetone, ethanol), and influence of dissolved gas on these forces; and (3) influence of surfactant adsorption on hydrophobic forces. It has been found that long-range attractive forces exist between the hydrocarbon sphere and various hydrophobic surfaces. The range and magnitude of these forces increase with an increase in the hydrophobicity of the interacting surfaces, as measured by water contact angle. It has been established that the presence of gas (air) dissolved in water, existence of nano-roughness and chemical heterogeneity (non-uniform coverage) of the surface increase the attraction between hydrophobic surfaces even further. An increase in the ionic strength of the solution, despite significant reduction in the zeta potential of the hydrocarbon particles, has a negligible effect on the range and magnitude of attraction between hydrocarbon surfaces, while a decrease in surface tension of the medium by adding water-soluble alcohols or ketones causes the interaction forces to decrease. These experiments confirm formation of gas/vapor cavities between hydrocarbon surfaces as a most important mechanism to account for long-range attraction. These findings are important in the cleaning of oil-contaminated soil, where we deal with rough mineral grains covered with oil, interacting with each other in the aqueous phase. Also, it has been found that the aqueous phase during ASH flotation usually is oversaturated with air, which can intensify cavitation between hydrophobic surfaces and increase the attraction between them. The addition of alcohol or acetone to the washing solution can decrease the attractive forces between hydrophobic particulates; however, to significantly affect these interactions, higher concentrations of ethanol or acetone are required.

The most important part of this basic research was to determine the influence of surfactant adsorption on the interactions between the various phases. The effect of cationic (DDAH), anionic (SDS) and nonionic (fatty alcohol ethoxylates C12E7,10) surfactants on interactions between hydrocarbon surfaces was studied using AFM, zeta potential, and contact angle measurements over a wide range of concentrations. From direct force measurements using AFM, it was found that all these surfactants affect interaction forces between hydrocarbon surfaces in a similar way, decreasing the hydrophobic attraction with an increase in concentration. Nonionic and anionic surfactants significantly decrease the hydrophobic attraction at much lower concentrations than the cationic surfactant. Above a certain concentration, only strong repulsive forces were noted. Based on our recent studies of the structure of adsorbed surfactants at hydrophobic surfaces, using AFM soft-contact imaging, it has been found that surface formation of hemimicellar, cylindrical molecular aggregates with a diameter of approximately 5 nm and a length of up to several hundreds of nanometers occurs at concentrations much lower than the surfactant CMC in the bulk solution. After formation of such surface structures, the surface becomes completely hydrophilic. Based on these measurements plus zeta potential and wetting measurements in the presence of surfactants, the decrease in the hydrophobic force was attributed to the increased exposure of the hydrophilic headgroups of adsorbed surfactants.

Interaction forces and work of adhesion between hydrocarbon and various mineral surfaces (quartz, fluorite, apatite, calcite, mica, orthoclase) in the presence of different concentrations of nonionic surfactant have been calculated using contact angle and interfacial tension measurements, and in the case of quartz and fluorite, these results compared with AFM measurements of the interaction and adhesion forces. A significant decrease in the work of adhesion between hydrocarbon and mineral surfaces has been generally observed in the presence of surfactants. AFM measurements revealed a significant increase in repulsion between the hydrocarbon probe and a mineral surface and a decrease in the small, attractive component observed in pure water. After contact of the surfaces, the adhesion force was measured and a slight increase in the work of adhesion between a quartz surface and the hydrocarbon probe was found, which was not the case for a fluorite surface where a decrease in the work of adhesion was observed. These results suggest adsorption of fatty alcohol ethoxylate molecules on the quartz surface by hydrogen bonding between the ethoxy chain segment and the hydroxyl groups at the quartz surface. The observed increase in the adhesion force as measured by AFM was not confirmed by wetting and interfacial tension measurements, which may suggest some kind of dynamic effect related to the AFM measurement itself and rearrangement of surfactant molecules during hard contact of spherical hydrocarbon probe with the sample. These studies suggest the use of primarily nonionic surfactants for oil removal from the mineral surfaces, with the eventual addition of an anionic surfactant to increase the electrostatic repulsion. A significant effect can be obtained at concentrations below the bulk CMC, when formation of hemimicelles at the surface is observed. Although AFM experiments were performed with solid surfaces, our findings also should be applicable, to a great extent, for deformable hydrophobic surfaces like oil droplets and air bubbles. It has to be expected that the use of higher concentrations of surfactant during the digestion/displacement stage results in excellent removal of oil from the mineral surface, but causes hydrophilization of suspended droplets and decreases attractive interactions between oil and air during the flotation stage. To compromise these two processes, careful control of surfactant addition and use of additional cationic polymers for flocculation and partial re-hydrophobization of the oil surface appears to be necessary.

During the first phase of this project, additional effort was committed to characterize the kinetics of oil transfer from a mineral surface to an air bubble and spreading of the oil at the air/water interface. The microscopic observations of bitumen transfer from a bitumen-coated quartz plate to a gas bubble surface in aqueous alkaline solutions revealed a complexity of spreading phenomena for the multi-component oil, involving fractionation of the bitumen. It was found that during bitumen spreading a bulk layer follows the formation of thin bitumen films (precursor films); for example, bulk layer spreading occurs at the bubble surface with a velocity of 0.007 mm/second after a precursor film spreading with a velocity of about 0.02 mm/second, at 25°C and 75 percent bubble coverage. As expected, the spreading kinetics of bitumen films was significantly improved at elevated temperatures. The activation energy for spreading of the bulk layer over the air bubble surface was calculated and found to vary from 66 to 123 kJ/mole depending on the extent of bubble coverage. On the other hand, the activation energy for spreading of the precursor film did not vary appreciably with the extent of coverage and was found to be about 105 kJ/mole. The spreading of small hydrocarbon droplets (pentane, heptane, dodecane, hexadecane) on the air/water interface was recorded using a high-speed video system. The results showed that the spreading of low-viscosity hydrocarbons on a water surface is a very spontaneous process and usually is completed in 10-15 mseconds for drops with a diameter of 3-4 mm. It was found from the recorded images that the kinetics of a hydrocarbon droplet spreading on a deionized water surface follows a time n-power law: D~tn, where D is the lens diameter, t is the time, and n=0.4-0.5. The kinetics of spreading for hexadecane droplets decreased and the n value was reduced to n=0.36-0.39 when deionized water was replaced by surfactant (SDS) solutions. These results suggest that the kinetics of hydrocarbon spreading are affected by the molecular arrangements of surfactant at the water-fluid interfaces and that the presence of surfactant significantly slows down filming of air bubbles by oil during the flotation stage.

Task 2: Understanding and Control of Froth Stability in Relation to Oil Flotation

Oil separation efficiency by ASH flotation requires appropriate froth stability. In this regard, substantial efforts were made to investigate the influence of oil on foam stability. The desired froth, which could be sustained in the high shear force field of the ASH and easily collapsed on discharge, was obtained at polyglycol surfactant concentrations exceeding 10-4 M, or a combination of nonionic and anionic surfactants. Consideration was given to the fact that the foaming compounds can originate from the oil phase or can be added purposely.
Oil was found to reduce the froth stability; however, an increase in frother concentration can compensate for such an effect. A stable, oil-loaded froth under dynamic conditions, and less stable under static conditions can be obtained for a polyglycol surfactant concentration beginning from 5 x 10-5 M. The froth collapse rate depends on the flotation machine being used (ASH, stirred areated reactor or flotation column) and is particularly essential for process control during ASH flotation.

Task 3: Specification of Operating Variables for Cleanup of Oily Soil Using ASH Flotation Technology

First, it should be noted that in order to benefit from the high specific capacity of the ASH, the oil-contaminated soil must be thoroughly prepared. Early investigation on the recovery of bitumen from Utah oil sands showed the importance of complete oil displacement from the mineral surfaces to obtain a clean sand product in the underflow of the ASH and an oil-rich froth in the overflow. Generally, undigested oil-sand aggregates (approximately 0.5 - 1 mm in size), present in the slurry, were found to report to the underflow, because the buoyancy force of the attached gas bubbles was not able to overcome the pronounced centrifugal force developed in the ASH. However, bitumens and agglomerates (of comparable or larger size), which contain more of the oil phase at the surface than undigested particles, were evenly distributed to the underflow and the overflow. Free bitumen droplets reported to the overflow. To float oils and aggregates a larger diameter ASH (e.g., 6" diameter) was recommended in which smaller centrifugal forces counteract the bubble buoyancy forces. As a result of this experience with oil sands, more emphasis was placed on wetting and colloidal issues in our research program than was originally intended. Review of the literature indicates that fundamental surface chemistry aspects of soil washing still need further elucidation.

Specification of operating variables and evaluation of ASH technology for the treatment of oily soil were considered with respect to both oil displacement from particulates and dispersed oil separation from the aqueous phase. The natural hydrophobicity of the oil phase and the density difference between the oil and the sand should facilitate the ash flotation separation. However, the separation results become unpredictable if the oil firmly adheres to sand particles, such was the case for insufficiently digested oil sand slurry.

Processing conditions were established to separate oil from the finer part of the sand fraction (0.038-0.6 mm) using one step flocculation of dispersed oil and fine sand. It was found that in the centrifugal force field of the ASH, the flocculated phases split. Thus, flocculated oil (presumably as aeroflocs) reported to the froth phase and the flocculated sand to the underflow providing high oil recovery to the overflow. Finely dispersed oil (1-60 microns), stabilized by surfactants used in the washing process, did not float effectively without prior flocculation.
The data clearly show the dependence of oil displacement efficiency on the contact time of the oil phase with the sand during preparation of the model system. Soil cleaning from engine oil was much more effective for sand exposed to oil for less than 24 hours, and very difficult for sand which remained in contact with oil for more than 3 months. Batch tests using paraffin oil indicated that oil recovery deteriorated from 54 to 17 percent when the contact time with oil was extended from 1 day to 240 days.

Another important aspect of the research program is the combined action of temperature and surfactant concentration on the efficiency of soil cleaning. Oil displacement during the conditioning of sand slurry is two times faster when the temperature of the process is raised 10?C. Of course, heating of the entire mass of the sand is not possible from an economical point of view. This situation can be compensated for, in some degree, by an increase in the surfactant concentration.

The polyglycol surfactant performed better than sodium dodecyl sulfate surfactant in displacing oil from sand particles. Flotation tests in a laboratory cell indicated that addition of a polymeric cationic flocculant enhanced the oil recovery. The batch processing tests allowed for over 60 percent oil recovery, with 0.2 percent of residual oil in a clean sand on a dry basis (90 percent efficiency).

Perhaps the most significant practical development associated with this research program was commercialization of the bubble accelerated flotation (BAF) system for treatment of oil-contaminated wastewater in which the ASH serves as a bubble chamber from which the aerated stream is discharged to a gravity separation tank. This development was the result of a continuous research collaboration with ZPM Inc., of California.

Currently, there are more than 20 BAF systems installed within the continental United States. The advantages of the system are, of course, the small footprint, high performance, high solids loading in the sludge, and low amount of treatment chemicals used. The technology is particularly efficient in the removal of free and emulsified fats, oils, and grease. Following successful flocculation, the BAF system is used to remove low density submicron particles. The BAF system has a much shorter response time to changes in chemistry (seconds as opposed to hours in clarifiers or in dissolved air flotation, DAF systems). Such a characteristic is very useful in wastewater treatment processes as the incoming water often changes in composition. Numerous approaches were used to coagulate and flocculate particulates in wastewater prior to the BAF treatment. The pH of the suspension usually is adjusted close to the pH of the isoelectric point. The residual charge then is partially neutralized with either inorganic coagulants or low molecular weight cationic polymers (polyamines, polyDADMACs, etc.). Dual polymer flocculation with high molecular weight cationic and anionic polyacrylamide flocculants (PAMs) is then accomplished, which yields large, stable aeroflocs that are efficiently removed by the BAF system.

Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 9 publications 3 publications in selected types All 3 journal articles
Type Citation Project Document Sources
Journal Article Drelich J, Miller JD. Spreading kinetics for low viscosity n-alkanes on a water surface as recorded by the high-speed video system. Annales Universitas Mariae Curie-Sklodowska Lublin-Polonia 1999/2000;LIV(LV.7):105-115. R825396 (Final)
not available
Journal Article Nalaskowsk J, Drelich J, Hupka J, Miller JD. Preparation of hydrophobic microspheres from low-temperature melting polymeric materials. Journal of Adhesion Science and Technology 1999;13(1):1-17. R825396 (Final)
not available
Journal Article Nalaskowski J, Veeramasuneni S, Hupka J, Miller JD. AFM measurements of hydrophobic forces between a polyethylene sphere and silanated silica plates-the significance of surface roughness. Journal of Adhesion Science and Technology 1999;13(12):1519-1533. R825396 (Final)
not available

Supplemental Keywords:

soil, oil, cleaning, remediation, flotation, air-sparged hydrocyclone., RFA, Scientific Discipline, Waste, Water, Contaminated Sediments, Physics, Remediation, Environmental Chemistry, Chemistry, Hazardous Waste, Engineering, Hazardous, sediment treatment, flotation, contaminant transport, surface chemistry, contaminated sediment, contaminated soil, hydrology, oil spills, oil removal, air sprayed hydrocyclone

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Progress and Final Reports:

Original Abstract
  • 1997
  • 1998
  • 1999