Evaluation of performance properties of surfactants applied in compounds intended for the removal of oil-based substances
Zuzanna ŚLOSORZ, Katarzyna RADWAN, Joanna RAKOWSKA ? Centrum Naukowo-Badawcze Ochrony Przeciwpożarowej im. J. Tuliszkowskiego, Państwowy Instytut Badawczy (CNBOP-PIB), Józefów
Please cite as: CHEMIK 2013, 67, 8, 725?732
Abstract:
The article presents the results of the tests of agents comprising surface active compounds (SAC) that serve to remove oil-based contaminants from the surface of solids. Nonionic and anionic surface active compounds have been selected for tests. The results obtained confirm the effectiveness of the examined SACs as agents facilitating the elimination of hydrocarbon contaminants.
Keywords: degreasing agents, dispersants, surfactants, oil-based contaminants
Introduction
Most toxic substances pass over to the ecosystem in heavily industrialised areas as well as in the vicinity of traffic routes. Adequate protective measures of both the health and life human and the natural environment [2] must be in place due to the diversity of threats caused by the release of hazardous materials [1]. It should be borne in mind that all road elements are exposed to contamination with oil-based substances upon emergencies involving chemical agents, not limited to traffic lanes but extended also to road shoulders and related objects. Contamination also has an effect on soil and water drawn from surface sources as well as groundwater. The methods and measures of the elimination of the aftermath of such an occurrence shall be adjusted to the site of an emergency action [3]. In view of the threats caused by hazardous substances, it is essential to monitor and identify the technical processes devised for the mitigation of the extent and outcome of both industrial and natural disasters [4]. Natural purification of water and ground proceeds very slowly. Biodegradation of oil-based compounds is a multi-step process that occurs in both aerobic and anaerobic conditions and involves many different microorganism groups, whose actions often present a synergistic effect.
The oil-based substance contamination level as well as the applicable remedies are largely consequent upon the properties of the ground that was contaminated. Oil-based contaminants that penetrate the environment may be removed by mechanical measures such as the collection of spilled oil or the application of the adsorbents of substances of kerosene origin. Unfortunately, the solutions indicated are not optimal and the residue of hazardous compounds on incorrectly or poorly cleaned surfaces pose a genuine threat to both the environment and the local ecosystem. If mechanical or adsorptive methods prove ineffective or insufficient, other measures of mitigation of the outcome of an ecological disaster are used such as oil dispergation. The cleaning and removal of contaminants produced by either normal industrial processes or failure is implemented as a process of remediation. Various types of remediation serve land reinstatement, i.e., the restoration of utility and natural properties of brownfield land as well as the revitalisation of biological life on waste lands. A noteworthy type of remediation is bioremediation that uses biological processes of degradation of kerosene-based substances. Bioremediation facilitated the removal of less volatile organic contaminants, e.g., fuel oils [5].
Cleaning surfaces that are contaminated with substances of high viscosity, e.g., fuel oils, lubricants, petroleum usually requires water solution of washing agents with an additional component of surface active compounds (SAC). This process reduces the viscosity of contamination and increases its mobility inside ground pores or on hardened surfaces. It is characteristic of all surface active compounds to be capable of adsorbing at interphase contact surfaces, modifying the properties of the surface layers of solutions that contain surfactants. Agents intended for the removal of oil-based substances should make it possible to disperse oil particles in water and obtain an emulsion, thus increasing the mobility of contaminants in ground pores and fissures and facilitating their effective collection by mechanical measures. The addition of surfactant in the washout process of oil-based substances induces a reduction in interphase tension, improvement of the wettability of ground particles and the rate of detachment of hydrophobic contaminants from ground particles. The compound particles are transferred to the liquid phase by way of emulgation and solubilisation. The resultant emulsion makes it possible to disperse a significant volume of hydrophobic contaminants in the surfactant solution [6].
Moreover, it is vital that the application of SACs contributes to the conditions of soil bacteria activity as they help provide access to oxygen and nutrients within the contamination zone. This, in turn, enhances bacterial activity and accelerates the biological degradation of undesirable substances. The cleaning agent may propagate and penetrate below the layer of oil, detaching it from the ground. The addition of water causes the further detachment of fragmented oil fractions [7]. The ability of a surface active compound to decrease the total free energy of a system by its adsorption at the interphase contact or the aggregation of particles that leads to the formation of micelles is defined as the surface activity of the said substance. The surface activity of a surfactant is due to the surface properties of all the elements of the system interacting with one another at the interface of respective phases. The parameters that determine the surface activity of a surfactant may be derived from the measurements of such parameters as surface tension, surface potential as well as surface viscosity [8]. The effects of SACs with a high surface activity involve the intense solution foaming, wetting or emulsifying of immiscible phases. The cleaning effect of oil contaminants is attributable to micelles that form emulsions of the O/W type, consisting of water as the continuous phase and oil as the dispersed phase. It is typical of the structure of the O/W emulsion to possess an extensive area of mass exchange, directly determining the amount of the organic compound to be dissolved. The fragmentation (dispersion) of the dispersed substance, namely, oil in water, should be sufficiently high for the mixture to show the physical properties of a homogenous substance. Degreasing formulations contain anionic and nonionic solutions of surface active compounds with an aryl group (e.g., TX-1, LAS). The said solutions manifest a potent emulsifying and foaming effect. The mixtures of surfactants were gaining an increasingly significant role in many branches of industry as they often prove more efficient (in terms of synergism) than individual components [9, 10]. Most marketed products that contain surface active compounds are derived from more than one surfactant. The application of agents that accelerate the degradation of the eliminated biological oil-based contaminations is also worth considering. To this end, an oxidant, a reducer, an enzymatic or biological substance may be used [11].
In practice, emergency services implement extinguishing and washing agents to remove oil-based substances that do not meet the ever rising requirements concerning the elimination of oil contamination. Since their intended use is different, the formulations of such agents often contain potent foaming agents that lead to the formation of a slippery layer on the cleaned surfaces. The difference between dispersants and either foaming agents or washing is that foaming and washing agents serve the purpose of stable water foams, while agents intended for the removal of oil-based substances should only facilitate the dispersion of oil particles in water and the formation of emulsion, contributing to greater contamination mobility and, hence, the possibility to remove the undesirable substance by mechanical means as well as its greater biodegradability [11].
Empirical study
Materials
The following technical surface active compounds were used in tests:
? decaethylene glycol oxyl phenyl ether diisobutyl phenoxy ethoxy ethyl dimethyl benzyl ammonium chloride (designated as TX-100), with the density of d20°C = 1.057 g/cm3, viscosity of 40.29 mPa?s, and 100% of the active substance manufactured by Sigma-Aldrich
? fatty alcohols with alcoxylic groups C12?14 (designated as APG), with the density of 0.977 g/cm3, viscosity of 1.92 mPa?s, and 99.7% of the active substance, manufactured by Cognis
? sodium linear alkylbenzene sulphate (designated as LAS), with the density of d20°C=1.065 g/cm3, viscosity of 40.09 mPa?s, and 50% of the active substance manufactured by PCC Rokita
? polyoxyethylene lauryl ether (designated as POE), with the density of d50°C=0.980?1.000 g/cm3, viscosity of 23.48 mPa?s, and 100% of the active substance manufactured by PCC Rokita
? alcohol mixture C8-C10 (designated as AL), with the density of d20°C=0.834 g/cm3, viscosity of 1.66 mPa?s, and 99.8% of the active substance, manufactured by SASOL.
In order to ensure a micelle concentration sufficient for the solubilisation of the contaminants as well as to compare the effectiveness of the investigated SACs, water solutions with the concentration of 5% [11÷13] were applied in the examined systems comprising technical agents with the content of active substances ranging from 50% to 100%. Distilled water showing a surface tension of 70 mN/m was used to prepare the solutions. Emulsions were obtained with the use of fuel oil Ekodiesel Ultra, manufactured by PKN Orlen with the cold filter plugging point < 0°C, density of 0.845 g/cm3 and viscosity of 4.08 mPa?s, compliant with PN-EN 590 [14].
Methodology
The investigation of surface tension and of interphase tension ?Du N oüy ring method
The tests of equilibrium surface tension were conducted by means of duNoüy ring method. The said method involves the measurement of the maximum value of force required for the detachment of Pt-Ir alloy ring from the surface of the examined liquid [15]. Interphase tension at the contact between the SACs water solution/fuel oil has also been tested. The said method consists in the measurement of the maximum value of force required for the detachment of a ring from the surface of two immiscible liquids. Equilibrium surface tension and interphase tension were measured with a K9 ET tensiometer manufactured by Krüss. The measurements of respective surface active compounds water solutions were conducted at the temperature of 20?1°C.
The investigation of emulsion stability
The stability of the emulsion derived from 5% SACs water solution and fuel oil was assessed during the 60s period of homogenisation performed at the rotational velocity of 14000 rev./min. The evaluation was completed on the basis of the observations of alterations in the volume of emulsion and water solutions of respective SACs. The observation lasted 30 minutes. The measurements were conducted at the temperature of 20?1°C.
Viscosity test
The viscosity test of emulsion derived from 5% water solutions of surface active compounds with fuel oil were conducted by means of a rotational viscometer VT 550 manufactured by Haake. The emulsion was prepared in a closed homogeniser at the rotational velocity of 14000 rev/min for 60s. The samples were tested at the shear rate 375 s-1. The measurements were taken at the temperature of 20?1°C.
Results
The measurements of equilibrium surface tension of 5% water solutions of the examined surface active compounds were provided in Figure 1. Of all solutions analysed, AL and APG proved to be the most effective at reducing surface tension. Degreasing formulations should comprise surface active compounds of AL and APG type, as evidenced by the comparative analysis of the surface tension reduction effect exerted by 5% solutions of respective technical agents. POE solution shows the least effectiveness in terms of the ability to reduce the parameter examined. Other surfactants show similar moderate surface properties.
Of all 5% solutions of surface active compounds analysed, TX-100, APG, LAS proved to be the most effective at reducing interphase tension in systems with fuel oil. The measured value of interphase tension for the said systems amounted to less than 1mN/m. The value indicates high efficiency in the dispersion of oil in water. AL solution (approx. 4.5 mN/m) showed the least effectiveness in terms of the ability to reduce interphase tension of all examined surface active compounds. Following the measured values of the investigated parameter in the system water solution/fuel oil, AL cannot be counted
among effective dispersants of oil in water (Fig. 2). The rationale behind its application should be reconsidered on account of its insufficient effect on the increase in contaminant bioavailability and its subsequent degradation.
Stability test results have been presented in Figures 3 and 4. Basing on the test outcome provided in Figure 3, it may be concluded that a stable emulsion may be derived from APG surfactant.
Whereas, other emulsions were observed to be unstable during the tests. The investigated surface active compounds proved to yield emulsions with varying effectiveness. The first group comprises surface active compounds that practically do not emulsify fuel oil (LAS and AL), the second ? that form minor amount of an unstable emulsion (TX-100 and POE), and the third ? the potent emulsifier of stable emulsions APG. The low volume and decreasing amount of emulsion in case of solution/oil mixture for respective agents is the
effect of the discharge of liquid phase from the emulsion obtained. Stable O/W emulsions are desirable as they ensure longer availability of the dispersed contaminants to microorganisms involved in biodegradation. It is for this reason that APG is noteworthy. It forms a stable emulsion with fuel oil.
During an emergency operation conducted by chemical and ecological recovery team [16] the water solution discharged from the washout agent causes the oil to surface from the pores and fissures of the hardened ground, thus making it possible to remove oil by mechanical measures as well as sorbents. In such a case, the effect of the discharge of the marketed product solution from the oil/water emulsion, consequent upon the instability of the emulsion (Fig. 4), is a desirable property. Otherwise, however, the applied agent intended for the removal of oil contamination must be considered to be ineffective as it impedes biological degradation. Of the surface active compounds examined, the systems of emulsifying surfactants TX-100, POE and LAS gradually discharge water solution from the emulsion obtained. In systems comprising AL only immiscible layers of solution and oil were seen. The volume of layers remained identical throughout the period of observation
Another parameter examined in terms of the evaluation of the functional properties of the agents intended for the removal of oilbased substances is the viscosity of the resultant emulsion. The tests of emulsion viscosity were carried out on systems of fuel oil/water solution. It has been underscored in many studies [17÷20] concerning the ability to remove oil contaminants that low emulsion viscosity is essential for the improvement of contamination mobility. Figure 5 features the results of the tests of the viscosity of the emulsions of the systems of 5% surfactant solutions with fuel oil. The results indicate a significant discrepancy between the values of viscosity of the emulsions obtained. Emulsions with APG and AL show the lowest viscosity of all the systems investigated, while the ones with TX-100 and LAS ? the highest. The low value of viscosity of oil/ SACs water solution emulsion contributes to a higher rate of oil contaminant washout from a porous surface. It is for this reason that TX-100 and LAS fail to prove efficient in agents intended for the removal of oil-based substances from soils and brownfield areas.
Conclusion
The increasing demand for measures of disposal of oil-based contaminants in many branches of industry has contributed to the development of the existing methods and the creation of novel solutions. Now there are multiple disposal means to adopt, adjusted to the location and situation. Some of the solutions applied for the removal of oil-based substances feature oil dispersion by means of surface active compounds. The addition of surfactants to oil-contaminated surface cleaning agents increases the mobility and bioavailability of contaminants. It may be concluded form the conducted examinations that APG and AL ensure the lowest viscosity of oil/SACs water solution emulsion that is a warranty of the mobility of contaminants. However, AL proves to be a poor dispersant in view of its high value of interphase tension. The property has also been confirmed in the emulsion stability test, demonstrating that AL water solutions fail to emulsify fuel oil. The formation of a stable oil emulsion facilitates oil-based substance biodegradation. Of the examined surface active compounds, APG yielded a stable solution. Degreasing agents applied during recovery operations after oilbased substance contamination must have the property of discharging water solution from the emulsion (POE, LAS, TX-100), which, in turn, allows the oil to collect at the surface where it can be readily collected by mechanical measures from road infrastructure and soils. The investigations performed provide grounds for the conclusion that the solutions of POE, AL, TX-100, APG may be applied in the formulations for the removal of oil substances. The comparison between the investigated surface active compounds indicates that the samples comprising APG, TX-100 and POE proved effective as the emulsifiers of the tested oil-based substances, used for chemical and ecological recovery. The said surfactants demonstrate a good emulsifying effect and facilitate the discharge of oil-based contaminants from the pores and fissures of the contaminated soil thanks to a limited stability of emulsion.
It must be borne in mind that the attempt at the elimination of oil spillage may lead to the introduction of yet other agents that may prove hazardous to the environment [21, 22]. Such substances include surface active compounds that are the components of surface cleaning agents. The application of the said agents must be accompanied by careful selection of surfactants in terms of their harmfulness to the environment and the accelerating effect on biodegradation.
Acknowledgements
Research conducted as part of the project O ROB 000401/ID4/3, funded by the National Centre for Research and Development.
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Zuzanna ŚLOSORZ ? M.Sc., graduated from the Faculty of Chemical Technology of the Poznan University of Technology (2011). Currently, she is employed at CNBOP ? PIB at the Laboratory of Fire and Chemical Testing Team. She has co-authored national and international publications, posters and conference presentations. Her field of interest covers the effect of extinguishing agents on materials as well as other electrochemical phenomena. e-mail:
Katarzyna RADWAN ? M.Sc., graduated from the Faculty of Mathematics and Natural Sciences of the Jan Kochanowski University in Kielce (former University of Humanities and Sciences), specialisation: Chemistry (2007). Currently, she is employed at the Centre of Science and Research for Fire Protection at the Laboratory of Fire and Chemical Testing (Centrum Naukowo-Badawcze Ochrony Przeciwpożarowej ? Państwowy Instytut Badawczy). Specialisation ? technology of extinguishing agents. She has co-authored monographs, publications, presentations and posters at both national and international conferences. e-mail:
Joanna RAKOWSKA ? Ph.D., ( Eng), graduated from the Faculty of Materials Science and Engineering of the Warsaw University of Technology. In 2011 she was awarded the title of the Doctor of Technical Sciences in the field of chemical technology at the Poznan University of Technology. Currently, she is employed at CNBOP ? PIB at the Laboratory of Fire and Chemical Testing. Specialisation: technology of extinguishing agents. She has co-authored over 50 national and international publications in specialist journals, posters and conference presentations as well as 6 patents. e-mail: