Introduction

Somasundaran et al. [19] investigated the feasibility of using flotation process to remove non-volatile hydrophobic compounds (paraffin oil) from artificially contaminated soil (particle size 0.075–0.83 mm) using sodium dodecyl sulfate (SDS). Their study indicated that soil washing through flotation with 0.1 %-mass SDS solution is effective in reducing the amount of oil in soil comparing with the ex situ soil washing method, which showed 50 % less oil removal at concentration of 0.5 %-mass. Limited literature has shown that significant amount of petroleum oil may be removed from contaminated soil by flotation process [2, 19, 23, 25].

Soil contamination with spillage lubricant oils generally results from leaking underground storage tanks, pipelines and accidental spills [4, 16]. Air-flushing technique involves introducing forced air into the artificially contaminated soil to encourage removal of contaminants. Air flushing is a cost-effective, time-efficient system for the remediation of volatile organic compounds (VOCs), particularly dissolved petroleum hydrocarbon and/or biodegradable contaminants [14, 15, 18]. Two mechanisms suggested for air flushing to reduce the dissolved organic compounds in subsurface area: (1) physical stripping (volatilization) as air moves through the aquifer and (2) aerobic biodegradation of VOCs through increased oxygen supply [7].

Surfactants enhance organic contaminant act by two mechanisms. First, surfactants reduce the interfacial tension between water and contaminants that slow the mobility of the organic components, thereby, surfactants can be able to transfer the hydrophobic organic compounds (HOCs) to the mobile phase [1]. Secondly, surfactants are capable of forming aggregates known as micelles, thus solubilizing HOCs. Numerous studies have indicated that surfactants can enhance recoveries of non-aqueous phase liquids [6, 17].

Urum et al. [22], study removal of crude oil from soil using air flushing assisted stirred tank reactors. Two surfactants (rhamnolipid and SDS) were tested and the effects of different parameters (i.e., temperature, surfactant concentrations, washing time, volume/mass ratio) were investigated under varying washing modes namely, stirring only, air flushing only and the combination of stirring and air flushing.

Surfactant-enhanced air flushing was conducted to remove perchloroethene (PCE) sources from laboratory flow chambers packed with sand. The resident water was supplemented with an anionic surfactant, (SDBS), to reduce the water’s surface tension, and then sparged with nitrogen gas at a constant flow rate of 0.12 min−1 [9, 10].

The present work aimed to applying air flushing as remedial technology for removing the waste-lubricant oil from soil in presence of nonionic surfactant polyethylene glycol nonylphenyl ether (NPEO9.3). In this respect three different concentrations (3, 5 and wt 7 %) poured individually into laboratory glass column packed with contaminated sand. Two air injection modes continuous (direct injection) and pulsed (on/off interval mode) were applied and the effect of surfactant, pollutant concentrations, washing time and pressure had been studied. The results discussed based on application of surfactant enhancing air flushing techniques as the most suitable and cheapest remedial technology for removal of oil polluted soil.

Materials and methods

Materials

  1. 1.

    Coarse sand with diameter range from 0.5 to 1 mm with porosity equal 25 % was used as porous medium.

  2. 2.

    Commercial grade nonionic surfactant polyethylene glycol nonylphenyl ether (NPEO9.3) was purchased from Egyptian market and used as received, and its physical properties are given in as following; number of moles E.O is 9.3, HLB is 13.0, average molecular weight is 629.37 g mol−1, surface tension, dynes/cm (0.01 % aq., 25 °C) is 32, specific gravity, g ml−1 is 1.05–1.07, and its biodegradability is 90.

  3. 3.

    Waste-lubricant oils were collected from different factories and companies and were used as pollutant. The physical properties of the used lubricating oils are as the following, the specific gravity at 20 °C is 0.875, flash point (close cup Pensesky Martin) is 140 °C, water and sediment is 1.28 (vol%), water content [Dean and Stark method is 0.79 (vol%)], Viscosity at 37.8 °C is 209.235cst, ash content is 0.714 wt%, asphaltene content is 4.995 wt%.

  4. 4.

    n-Hexane supplied by Aldrich Chemical Co. and used without further treatment.

Determination of surface tensions and CMC

Surface tensions of surfactant solutions were determined with Wilhelmy plate Krüss K100 Tensiometer instrument that operates based on DuNouy principle. The critical micelle concentration (CMC) was determined by measuring the surface tension versus surfactant concentration.

Solubilization of lubricant oils in surfactant solutions

The ability of surfactant solutions to solubilize the waste-lubricant oil was investigated by mixing 30 ml surfactant solutions at concentrations ranging from 0.01 to 0.05 % with 0.5 ml waste-lubricant oils. Then the solutions were continuously shaken at 25 °C for 48 h. The sample vials were allowed to settle at 24 h for phase separation. After that, the aqueous phase was withdrawn and extracted with chloroform to determine the concentration of solubilized lubricant oil using ultraviolet spectroscopy.

Soil contamination

According to Urum et al. [21], a fixed mass of 1 kg of coarse sand (0.5–1 mm) was artificially polluted and mixed well with waste-lubricant oils at different concentrations of 10, 25 and 50 wt%, to give soil oil blend.

Experimental set-up and procedure

As shown in Fig. 1, a cylindrical Plexiglas column having dimension (105 × 5 × 4.5 cm), was packed with 31 cm height of 10 wt% contaminated sandy soil. The outlet end of the column was fitted with a fine wire mesh screens (50 μm diameter) to prevent soil wash out. The column was cautiously packed with coarse sand (diameter range 0.5–1 mm) to insure better distribution of injected air during air flushing and preventing the sand particles from clogging the orifice where air was introduced. Then the column was filled with nonionic surfactant solutions (NPEO9.3), prior to packing sand to avoid air bubble entrapment. Two air injection modes were applied in this study, the first one is continuous (direct injection), the second is pulsed (on/off interval mode), both modes are carry out to increasing contaminant mass removal. Air was injected at the side arm of the column into the coarse sand saturated with surfactant solution using opening air compressor system, as described by Kommalapati [11] Air delivery was controlled with a low-pressure regulator and flow meter. The flow rates ranged between 6 L min−1. The air inlet stream was directed to create turbulent air current and to provide better distribution of the injected air with similar and strong focusing at all points of soil surface. The procedure used for soil packing was reproduced with 25 and 50 wt% contaminated sandy soil without substantial variation in the characteristics of the packed soil.

Fig. 1
figure 1

The laboratory experimental model

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Soil remediation by surfactant flushing

The experiment was conducted to study the pollutant removal with nonionic surfactant solutions, and then study the effect of water washing cycles for eliminate the residual oil entrapped in soil. In this respect, a fixed 300 mL of (NPEO9.3) at 3, 5 and 7 wt% concentrations were poured into cylindrical plexiglas column containing 10 % contaminated soil. Then a compressed air at a pressure of 2 bar (2 kg/cm2) and flow rate 6 L min−1 was introduced through the side arm of the column. The pressure was monitored using a pressure gauge as shown in Fig. 1. All experiments were carried out at 25 ± 1 °C. Experiments were also conducted in downflow (gravity-stable). During the downflow flushing experiments, surfactant solutions were poured individually into the top, and the oily waste was recovered from the bottom of the soil column. To rinse off the washed soil, the remaining waste oil was improved by different water cycles. Then the effluent was collected in 50 ml centrifuge tubes, for both surfactant and water runs, and the oil remaining in soil was determined.

Analysis of effluent oily waste

The collected effluent distinct into two phases: oil phase and an aqueous phase. The oil phase was recovered by centrifuging the effluent at 10,000 rpm for 10 min and the supernatant aqueous phase was pipette out. The soil and oily waste mixture was separated and weighted. Finally, the oils remaining in soil was extracted by shaken laterally 1 g of soil for 5 min with 10 cm3 of n-hexane, the washing with n- hexane was continued until nearly all the oily waste was removed from the soil. The n-hexane/oils extract was collected into one volumetric flask up to 50 cm3 with n-hexane and oil percentage was determined using ultra violet spectroscopy.

Determination of oil removing percentage

The oils removed from soil using single air flushing in absence and presence of surfactant was determined and the results are given in Tables 1 and 4.

Table 1 Removed oil % @ 10 wt% polluted soil using continuous air flushing
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Results and discussion

Waste-lubricant oils includes crankcase (engine) oil, brake fluid, automatic transmission fluid, power steering fluid, liquid and semi-solid gear, chain, and ball bearing lubricants, and hydraulic fluid. Waste-lubricant oils are not considered hazardous waste unless it is mixed with a hazardous waste such as a chlorinated solvent. The problem of soil and groundwater pollution was widely recognized in recent years. The flushing air creates air bubbles where the oil will adhere.

Surface tension and CMC of surfactant

The surface tension of nonionic surfactant solutions was determined using 0.01 % of (polyethylene glycol nonylphenyl ether) in pure distille water @ 25 °C. It is found that the surface tension of surfactant solutions is 32 m N m−1, and its CMC have spans between 0.13 and 0.2 g L−1 or (0.2–0.31 mmol L−1).

Apparent solubility

The surfactant concentrations used in current study were 3 %, 5 % and 7 wt%, that equivalent to 15, 25 and 35 times more than its CMC. It was found that the apparent waste-lubricant oil solubility increased linearly with increasing surfactant concentrations above their CMC. The increased solubility may increase the potential removal of trapped waste-lubricant oil droplets. At low concentrations in aqueous solution, single molecules are present. The use of surfactants enhances the solubility of oil significantly by partitioning it into the hydrophobic cores of surfactant micelles. However, beyond a CMC, the surfactant molecules will aggregate, form micelles and reduce the thermodynamic energy in the system. Thereby, surfactants act to reduce the free energy of the system by replacing the bulk molecules of higher energy at an interface.

Air flushing technology and its application

With stress emphasis on air injection mode its recognized that there are two air injection mode well-known, the first one is continuous (direct injection), the second is pulsed (on/off interval mode), both modes are carry out to increasing contaminant mass removal.

Effect of continuous air flushing on removed oil

The compressed air has been injected into the saturated sand column by opening air compressor as illustrated in Fig. 1; in this respect, 1 kg of coarse sand contaminated with 10 % waste oil was subjected to applying continuous air flushing via separate two steps as the following: (1) flushing the contaminated soil with air only for 2,700 s as a blank, (2) flushing the contaminated soil with different surfactant solutions enhanced with air injection followed by washing several times with pure water to eliminate the residual surfactant solutions. The removal rate of 10 wt% waste lubricant oils using continuous air injection was given in Table 1. The results showed that the quantities of waste oil repel out from soil due to action of air flushing only is 27 wt%, and the consuming time during such process is 2,700 s. The flushing time applying and selected depending on no effluent was passing. After that the action of surfactant solutions enhanced with air injection were apply on the same model and the results are recorded in Table 2. The results revealed that the values of removed oil are raised up to 93 %, 96 %, and 97 wt% after flushing with 3 %, 5 % and 7 wt% surfactant solutions at 600, 200, and 90 s, respectively. This can be explained by, during injection of air flushing enhanced by surfactant solutions, the waste lubricant oils entrapped in soil pores were migrate because the interfacial tension between waste lubricant oils and the surfactant solution became low enough to force the blobs or ganglia of waste lubricant oils to move away. Sequentially, it can rapidly remove a large fraction of pollutant mass entrapped in the soil. It is also shown in Table 2, that the water washing cycles in presence of air injection are succeeded for removing the residual oil, this means that during water washing, the percentage of oil removing increased significantly, and then became nearly constant at the end of the process. Therefore, the use of water in the remediation of highly contaminated soil as polishing treatment solution was shown of great interest. Our results proved the performance of water in the remediation of hydrocarbon-contaminated soil as found by Fernandez et al. [3].

Table 2 Removed oil % @ 10 wt% polluted soil using continuous air flushing with pollutant, different surfactant solution, and water
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Effect of flushing time and surfactant concentrations on removed oil

Figure 2 exhibits the effect of continuous air flushing time for remediation 10 % polluted soil in absence and presence of surfactant solutions, it is clear a linear relationship between removing oil with surfactant concentrations and inverse relationship with flushing time. This means that the efficiency of removed oil increases as the surfactant concentrations increased. The drastic decrease in time is attributed to decrees of pollutant concentration, which results from cleaning or opening of air channels due to increasing of surfactant concentrations from 3 % to 7 wt%.

Fig. 2
figure 2

Effect of continuous air flushing for remediation of 10 % polluted soil in absence and presence of surfactant

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Surfactant concentrations used in current study are above its CMC. A good relationship between surfactant concentrations and percentage of removing oil is shown in Fig. 3. It is clear that the solubility of waste-lubricant oil being dependent on surfactant concentrations also surfactants reduce the interfacial tension between water and contaminants that slows the mobility of the organic components. In addition, surfactants are capable of forming aggregates known as micelles, thus solubilizing waste-lubricant oil. Therefore, in order to overcome the difficulty of removal of non-volatile constituents like lubricating oil, the author advise by adding nonionic surfactant solution to enhance its recovery by solubility or desorption and subsequently accelerate the solubilization of the contaminated soil in presence of air flushing.

Fig. 3
figure 3

Effect of air flushing for remediation 10 wt% polluted soil in presence of different surfactant concentrations

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Effect of pulsing air flushing and time on removed oil

In this part the treatment of contaminated soil with different concentration 10 %, 25 % and 50 wt% (100, 250 and 500 g/kg) was carried out by pulsing air flushing in presence of surfactant solution followed by six water washing cycles. This step was performed at (28.37 cf/min) to evaluate the effectiveness of surfactant concentration versus different pollutant concentrations. The results of this treatment process are given in Table 3. The results reveal that in the case of 10 wt% pollutant the removed oil are super grade and exceed to 93 wt% within 40 s.

Table 3 Effect of time on removed oil% @ 10 wt%, and 25 wt% and 50 wt% polluted soil using pulsing air flushing @ 3 wt% surfactant
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The injection of air sparging (by pulsed mode) in presence of 3 wt.% surfactant help us to understand airflow pattern in the case of 10 wt% of pollutant concentration. Therefore, the movement and distribution of air through the aquifer in laboratory models have been studied to investigate airflow patterns in saturated soils at 25 wt% pollutant up to 50 wt%.

In the case of 25 % pollutant concentration Table 3, the efficiency of removed oil after first air flushing is 63 wt% and increased sharply tells to 91 wt% at seventh air flushing, this can attributed to air injection is temporarily turned off the water flows into the air channels and mixed with entrapped oil and formed emulsion. This mixing occurs in both microscale and macroscale air channels in sparging zones.

In the case of highly saturated soil up to 50 wt% pollutant concentration the date given in Table 3 reveals that, only 7 wt% of pollutant is removed under 2 kPa @ 2,100 s, after that the pressure was decreased and fixed at 1.5 kPa. This can attribute to that the oil exists as free phase. Free phase or mobile oil exists when the saturation is high enough to form pore-to-pore connections over a large area producing a continuous fluid. Furthermore, to recover the free phase of the waste lubricant oils in the treatment process it must be disposed of in accordance with applicable requirements. As well as it is necessary to use free-product recovery methods, which include each of vacuum or pumps to capture as much of the free flowing oil as possible.

The sum of all results illustrated that for a well-defined plume, a pump and treat system can be considered as an effective first line of defense in preventing further migration and in removing the bulk of free products. However, if the results of these studies are applied at relatively high airflow rates, the contaminant removal efficiency would be increased but the costs of air injection would also be significantly increased.

Effect of pulsing air flushing on removed oil

The treatment of contaminated soil was carried out using two steps: (1) injection by single air as marker, (2) washing with surfactant solution followed by washing with many water cycles. The removed oil using pulsing air injection is shown in Table 4.

Table 4 Removed oil % @ 10, and 25 and 50 wt% polluted soil using pulsing air flushing @ 3 wt% surfactant
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The results reveal that in the case of the 10 % pollutant, the percentage of oil removal is super grade and exceeded 96 wt%; this means that the waste lubricant oil entrapped in soil pores migrated by the action of surfactant solution and forced the blobs or ganglia to move away.

In the case of the 25 wt% pollutant concentration the removed oil is nearly 79 wt%; this means that the residual waste lubricant oil existing and entrapped in soil pores drained off leaving behind some amount of liquid trapped by capillary forces that holds a liquid to a solid surface.

In case of 50 wt% pollutant concentration the oil removed is not satisfactory and reached 7 wt% only, this can be attributed to the high degree of saturation, and the residual waste lubricant oil can exist as free phase or as mobile non-aqueous phase liquid. This means that the free phase of non-aqueous phase liquid exists when the saturation is high enough to form pore-to-pore connections over a large area, producing a continuous fluid capable of flowing under an imposed gradient or its own gravitational potential. Furthermore, to recover the free phase of the non-aqueous phase liquid in the treatment process unit it must be disposed of accordance with applicable requirements, as well as its necessary to use free-product recovery methods which include each of vacuum trucks or pumps to capture as much of the free-fluid oil as possible. Generally, to solve the problem of how to remediate the high pollutant concentration, first, it must be capture the free phase by mechanical treatment as first remediation defense, followed by chemical treatment.

The advantage of pulsed over continuous air flushing

In this section a comparison study was carried out between pulsed and continuous air flushing mode conducted for remediation of soil contaminated with lubricant oil. Such comparison was carried out to determine which air flushing mode will consumed lower energy. Both of continuous (direct injection) and pulsed air flushing modes (on/off intervals) were applied to remediation course soil contaminated with 10 wt% waste lubricant oils in absence and presence of 3 wt% surfactant concentration. The results found in Tables 2 and 3 shows that both air flushing modes are succeeded to remove the same percentage from pollutant (93 wt%) at different time 600 and 40 s for continuous and pulsed air flushing, respectively, this means the performance of using the pulsed air flushing mode over the continuous mode for soil remediation. Therefore, the pulsed air flushing mode was applied with coarse soil contaminated with 25 and 50 wt% waste lubricant oils in presence of 3 wt% nonionic surfactant.

Several mechanisms have been postulated to explain why pulsed operation improves contaminant removal by air flushing. Induced surfactant solution flow and surfactant solution mixing might be the two dominating mechanisms.

First, as air is introduced into contaminated soil, air displaces flushing solution in the largest pores and creates temporary flushing solution flow around flushing point [12].When an air-flushing system achieves steady state, preferential airflow pathways consisting of the largest network of pores are formed in the soil, and the induced local flushing solution flow ceases [20]. Pulsing air injection frequently creates non-steady-state conditions and induces flushing solution circulation as the air channels form and collapse during each cycle. The induced flushing solution flow created by the pulsed air flushing substantially enhances the contaminant and oxygen mass transfer in the soil [5]. Second, contaminants in the immediate vicinity of air channels can be removed within few time of the start of flushing, but contaminated present at a greater distance from the air channels is less treated because of the limited mass transport (i.e., diffusion) [8].

The advantage of pulsed over continuous air flushing is that the pulsed can be used with shallow soil contaminated with saturated hydrocarbons. In addition, an optimum pulsing frequency can be determined based on the observed time for the hydrocarbon removal and airflow rate until reach steady state. Yang et al. [24] evaluated the field performance of pulsed air flushing in a short-term pilot test and during long-term system operation. Based on their successful results for short-term pilot test, the air flushing system was set to operate long term under pulsed conditions at the selected optimum pulsing frequency. This innovation resulted in higher reduction rates of dissolved benzene, toluene, ethylbenzene, and xylenes (BTEX) than those observed during the continuous operation. Performance monitoring of the air flushing system at 2, 8, and 12 months of pulsed operation indicated an increase in the hydrocarbon removal rate by a factor of up to three as compared to continuous operation, resulting in cost savings from shorter treatment time and less energy usage.

Master’s thesis by Lambert [13], evidence that pulsed air flushing, otherwise known as pulsed bioflushing, is more effective than continuous air flushing because pulsing enhances treatment by inducing groundwater movement and mixing.

The cost of surfactant and air flushing operating mode

One liter commercial grade nonionic surfactant nonylphenol ethoxylate (NPEO9) that purchased from Egyptian market is approximately equal 3US$, and 1,000 ml of 3 % surfactant solution can remediate 100 kg soil contaminated by 10 % waste lubricant oils. i.e., one tone soil polluted with 10 % hydrocarbons need 30US$ for complete remediation. Meanwhile, the amount of energy of the compressors used is 1.6 kWh/ton at 220 V and DC power supplier, where the energy cost is 1 U$/ton.

Conclusion

The removal of poured waste lubricant oils from soil using air flushing with and without surfactant were carried out using laboratory scale model. The effects of surfactant concentrations, water washing cycles and washing time were also studied. The results discussed based on application of surfactant enhancing air-sparging technique as the most suitable and cheapest remedial technology for removal of oil-polluted soil. Reducing the surface tension was found to promote airflow through the preferential air channel. These observations support the use of surfactant to improve air flushing of contaminated zones. This technique, in fact, was found to accelerate the remediation process for NAPL-contaminated soils. Flow time was measured at constant pressure (2 kPa) the evaluated surfactant concentration varied from 3, 5, and 7 wt%, to ensure the micellar solubilization process.