윤활유로 오염된 토양의 계면활성제를 이용한 처리에 관한 연구

김철현, 김용수, 공성호

한양대학교 화학공학과

A Study on the Detergency of Soils Contaminated with Lubricating Oils using Different Surfactants

Cheol-Hyun Kim, Yong- Soo Kim, and Sung-Ho Kong

Department of Chemical Engineering, Hanyang University

Introduction

Lubricating oils are the most frequently spilled oils because of their widespread and unregulated use with vehicles and machinery.  Soils and sludges contaminated with lubricating oils are difficult to treat since the waste compounds are strongly sorbed.  For lubricating oils, transformation process, such as biological treatment or advanced oxidation process, is often controlled by the rate of desorption.  Therefore, enhancing desorption of oils through processes such as soil washing or thermal desorption could be important.  One of the most common processes, soil washing, is based on using water or water-surfactant solutions to wash contaminants.  Surfactants, which are commonly used as detergents, are long-chain amphibolic compounds.

Surfactants tend to migrate to interface between contaminants and substrates to lower the interfacial tensions.  This often results in the formations of emulsions, thus greatly increases the interfacial area.  Another characteristic of surfactant is their tendency to form micelles or aggregates of surfactant molecules when critical micelle concentration (CMC) is surpassed. Hydrocarbon compounds could be desorbed and solubilized easily above the CMC.

The term detergency, as used to describe a property of surfactants, has a special meaning; as a general term, if means cleaning power, but no surfactant can cleanse a surface alone.  The term detergency, when applied to a surfactant, means the special property it has of enhancing the cleaning power of a liquid.  This it accomplishes by a combination of effects involving adsorption at interfaces, alteration of interfacial tensions, solubilization, emulsification, and the formation and dissipation of surface charges.

The objectives of this research are to investigate the effects of various surfactants on the lubricating oil removal, to find optimal removal conditions (concentration, temperature, and cleaning time).

Experiment

Detergency

Detergency tests were performed to measure the relative ability of surfactants to remove lubricating oils.  Commonly used surfactants, such as sodium laurate (SL), sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), and tween 20, were selected according to solubility in water and biodegradability.  Table 1 shows the CMCs of the selected surfacants.

Table 1. The CMC of different surfactants

CMC	SL	SDS	SDBS	Tween 20
mM	28.1	8.2	1.19	4.9×10-2
g/L	6.25	2.36	0.41	0.06

First, the fine gravel samples (2-3 mm in diameter) were contaminated with lubricating oils (20g oils/kg gravel).  One hundred gram of oil-contaminated gravels and 100 ml of surfactant solution were placed in erlenmeyer flask.  The surfactant solution concentrations were based on CMCs of different surfactants tested. Erlenmeyer flasks were shaked at 200 rpm using G24 environmental incubator shaker (New Brunswick Scientific Co., U.S.A.).  Controls were also run using distillated water instead of surfactant solutions.  After 10 hr of shaking, the solutions in erlenmeyer flasks were decanted.  Extracting lubricating oils remained on the surface of gravels was done using n-hexane, and detergency was calculated with following equation:

[eqn: rm{Detergency~= ~A-B over A TIMES 100}]

A : Amount of extracted lubricating oils from surface of gravels before washing

B : Amount of extracted lubricating oils from surface of gravels after washing.

Interfacial tension

The oil/water interfacial tension was measured to understand primarily detergency results.  The measurement was performed using Digital K10ST Tensionmeter (Kruss, Germany).

Emulsifying Effects

The relative emulsifying effects of surfactants were determined by measuring oil drop size through a optical microscope (Nikon, Japan, 800X magnifications).  The oil drop size showed by a optical microscope became a criterion to the emulsifying effect of surfactant.

Result and discussion

Detergency

Figures 1, 2, and 3 were results acquired from conditions shaken at 25℃ for 10hr.  Figure 1 showed that the SDS has higher detergency than other surfactants above CMC, and SDS was appeared to be an optimal surfacant.  When same concentration (2.4 g/L) of surfactants was used, SDS also showed better detergency than others (Fig. 2).  Sodium laurate was not tested due to very high CMC value and high CMC will cause high treatment cost.  Increase in detergency was slowed down above CMC since effects of surfactants also reduced above CMC.   Optimal temperature and cleaning time to remove lubricating oils were determined at 35℃ and 8hr when SDS was used (Fig. 4)

.

Fig 1. Effects of different surfactant solutions (CMC) on detergency

Fig 2. Effects of different surfactant solutions (same wt%) on detergency

Fig 3. Effect of concentration using SDS

Fig 4. Effect of temperature and cleaning time using SDS

Interfacial tension

This test was performed to understand primary function (wetting) of surfactants into surface of contaminants.  The results did not exactly coincide with detergency tests; however, since all surfacants efficiently lower the interfacial tension, it was estimated that they are efficient to play primary function of detergency (Fig. 5).

Fig 5. The interfacial tension between lubricating oils and surfactant solutions

Emulsifying effects

In general, the smaller drop size showed higher emulsifying effect.  This coincided with results of detergency tests indicating the emulsifying effect plays a major role in detergency.

(a)

(b)

(c)

(d)

Fig 6. The drop size of lubricating oil/surfactant solutions emulsion, (a) SL, (b) SDS, (c) SDBS, (d) Tween 20

Conclusion

Almost all the useful functions of surfactants are necessary for detergency, but, on the contrary, surfactants with the best detergency are not necessarily most superior in each and every function.  In this investigation SDS showed the best detergency compared to SL, SDBS, and Tween 20.  Optimal operating conditions were temperature of 35℃ and shaking time of 8 hr.  Emulsifying effects appeared to be a major factor in detergency.  Further studies are suggested for an actual application at the sites using pilot scale tests.

Reference

1. Takehiko Fujimoto, "New Introduction to Surface Active Agents", Sanyo Chemical Industries, Ltd., 1981

2. Richard J. Watts, "Hazardous Wastes", John Willey & Sons, Inc., 1997

3. Milton J. Rosen, □Surfactants and Interfacial Phenomena", John Wiley & Sons, Inc., 1978

4. Lenore S. Clesceri, Arnold E. Greenberg, Andrew D. Eaton, "Standard Methods for the Examination of Water and Wastewater", 1998