Journal of Soil Contamination. 7(5):559-571 (1998)

Removal of a nonvolatile paraffin oil fromspiked soils using column flotation withcountercUffent bubbles was explored atboth ambient and elevated temperatures.Up to 80% of the contaminant was sepa-rated from the coarse fraction (250 to 800Jim) by flotation at 45°C using aqueoussolutions of anionic and nonionic surlac-tants or alkali salt as collectors. With the75 to 800 Jim fraction, removal efficienciesof up to 65% was achieved. Sodiumdodecyl-sulfate and Triton 100X at 50 ppmconcentrations as well as sodium carbon-ate at pH 10 were found to yield similarremoval efficiencies. Same surlactantswere tested in soil washing experimentsat similar and higher dosages. Removalefficiency by flotation was higher than thoseobtained by soil washing in all cases. Inaddition, as high surfactant dosage arenot used in flotation, unlike in the case ofsoil washing, the problem of fonnation ofstable emulsions was absent. Experimentswith soil polluted by hydrocarbons from acontaminated site demonstrated the fea-sibility of the flotation process for decon-tamination of coarse (250 to 830 Jim) frac-tions. A 70% reduction of petroleumhydrocarbon in soil was achieved as aresult of flotation at 45°C using the abovesurlactants.

Chun-Chiao ChOU,1 Victor Ososkov1. *

Lei Zhangz and P. Somasundaranl1 Center for Environmental Engineering and

Sciences, New Jersey Institute of Technology,Newark, NJ 07102; 2Langmuir Center forColloids & Interlaces, 911 Mudd Building,Columbia University, New York, NY 10027

To whom the correspondence should be addressed.

KEY WORDS: soil cleaning, column flotation, decontamination, remediation, nonvolatile oil,

hydrophobic.

1058-8337/98/$.50<0 1998 by AEHS

559

INTRODUCTION

trOT A nON has been used for decades for the separation of naturallyJ ~drophobic minerals such as graphite, sulfur, and talc from hydrophilicmaterials (Gaudin, 1957; Somasundaran, 1986). This process is also used in thepetroleum industry for separation of bitumen from oil sands (Shaw et al., 1979).It is evident that this proven, relatively inexpensive technology has considerablepotential for the remediation of hazardous waste sites. Obvious candidate contami-nants are nonvolatile hydrophobic compounds, such as P AH, heavy fuel oils, PCB,and some chlorinated pesticides. However, there are only a few reported investi-gations on the removal of organic contaminants from soil by flotation (Clifford,1993; van Rijt, 1993, Wilichowski and Werther, 1995). While flotation is apromising method for the removal of hydrophobic organic compounds from soil,the role of many relevant parameters in soil decontamination is largely unknown.

The feasibility of paraffin oil removal from artificially contaminated soil usingconventional mechanical agitated flotation machine was demonstrated in our pre-vious work (Somasundaran et al., 1997). For soil particles in the range of 75 to830 ~m, flotation with 300 mg/kg of sodium dodecylsulfate (SDS) removed about60% of the contaminant oil. Two-stage flotation under optimal conditions precededby soil attrition using a stirred mill yielded 74% oil removal.

The same artificially contaminated soil with paraffin oil was used in thisinvestigation. Column flotation with bubbles generated using a fine porous frit wastested at ambient and elevated temperatures. Two approaches were explored insome depth. First, oil was removed from the sand fraction of the soil by flotationwith anionic and nonionic surfactant solutions at concentrations well below thecritical micelle concentration (CMC). The second method used flotation with analkaline solution (pH = 10) at elevated temperature. Hot alkaline solutions havebeen used in the petroleum industry for the recovery of bitumen from oil sands byflotation (Dai and Chunk, 1995; Shaw et ai., 1996). Efficiency of the flotationprocess for hydrocarbon removal from soil collected from a contaminated site wasalso investigated.

SOIL CHARACTERIZATION AND SAMPLE PREPARATION

Uncontaminated natural soil was collected on the New Jersey Institute of Techno 1-ogy (NJIT) campus. It was dried at room temperature and sieved to remove largeparticles above 830 ~m (debris, gravel, etc.) The remaining soil was separated bydry sieving into three fractions: less than 75 ~m, 75 to 250 m, and 250 to830 ~m.(Figure 1). The pH of the soil/0.1M KCI slurry (1: 6.7 soil to solution ratio)was 6.5.

For the soil remediation experiments, synthetically contaminated samples wereprepared by mixing solvent refined paraffin oil (Ivax Industries, Inc.) with the

560

Sl.w.59.5%

27.1%

8A

. 250 - 800 m. 7S - 250 m. <75 m

FIGURE 1

(A) Size distribution of the soil (by weight). (8) Distribution of oil in soil fractions.

natural soil collected in NJIT campus. This oil was chosen because of its extremelylow volatility and high purity. Contaminated soil was prepared by stirring 990 g ofwhole or fractionated soil with 2 I of solution containing 109 of paraffin oil inhexane for about 1 h using a 4 in impeller at 300 rpm. The hexane was thenremoved by drying in an oven at 80°C for 3 h.

When the whole soil was spiked with oil, the distribution of oil among varioussize fractions (separated later for various tests) was not uniform. The finest fractionof the soil contained a higher fraction of the oil (Figure 1). Therefore, for furtherstudies the soil was separated into the three fractions, and then each fraction wasspiked with oil. The spiked oil concentration in each fraction was 10 g/kg.

EXPERIMENTAL PROCEDURE

The column flotation experiments were carried out in a glass vessel 12.5 cm indiameter and 20 cm high, fitted with a fine porous glass frit at the bottom.Compressed air introduced into the column through the frit was monitored usinga rotameter. A 45°C pitch-blade impeller (Chemineer Inc.) usually operated at 300rpm installed just above the surface of the frit ensured good distribution of the airand the soil slurry during flotation. Initially, we tried to perform experiments in ataller and thinner vessel. However, we could not achieve good distribution of thesoil slurry and the air throughout the column during flotation. Therefore, we useda short dimension vessel that is not a typical colul1U1. In our opinion it is appropriatefor investigation of the soil flotation feasibility and effect of various parameters onthe effectiveness of the process.

561

20 to 30 g samples of the contaminated soil were mixed with desired volumesof distilled water and the surfactant or other flotation agents to be tested andtransferred into the column. For experiments at above-ambient temperatures, thecolumn and the air inlet tubes were wrapped with electrical heating tape to heatboth the slurried soil and the incoming air and the slurry temperature was moni-tored.

Air bubbles were introduced into the soiVwater slurry through the frit at thebottom of the column. During the flotation process, some fine soil particles wereentrained and trapped in the foam at the top of the column. The bulk of the soilremained as a slurry and settled to the bottom of the column soon after the air flowwas discontinued. After flotation, the following distribution of oil was observed:some oil at the top of the column, floated on fine particles or free, some oil wasin the bulk soil at the bottom of the column and some in the aqueous solution inan emulsified state.

A sample of soil from an industrial site contaminated with hydrocarbons wasalso tested. It was dried at room temperature, particles above 830 ~m wereremoved, and then particle size distribution was determined by dry sieving into lessthan 75 ~m-6%, 75 to 250 ~m-18%, and 250 to 830 ~m-76%. Flotation experi-ments were also performed with 250 to 830 ~m fraction, as described above for theartificially contaminated soil.

Anionic and nonionic surfactants, sodium dodecylsulfate (SDS), and TritonlOOX (Fisher Sci.), respectively, were used as the collector, and Pine oil (ArizonaChemical Co.) was used in some experiments as a foaming agent. The pH of thesoil slurry was adjusted using Na2CO3/NaOH buffer. After each flotation experi-ment, samples of the settled bulk soil were analyzed. In some cases the percent ofsoil accumulated in the foam was also evaluated. The original soil samples werealso analyzed for oil content.

To compare the results obtained by flotation, performance of soil washing wastested. 30 g samples of the same contaminated soil were agitated for 30 min insurfactant solutions in the same flotation column, using the impeller at 500 rpm butwith no air introduced. Similar methods of analysis and calculations were used todetermine the efficiency of this method.

DETERMINATION OF OIL IN SOIL

The soil sample to be analyzed was dried in an oven for I h at 110°C. One hourwas determined to be a sufficient to dry the soil thoroughly but with no loss ofparaffin oil. 20 g of the dried soil was extracted with hexane, the hexane evapo-rated, and the extracted oil determined gravimetrically. The dried soil was shakenvigorously for 5 min with three successive 20-ml portions of hexane. The hexanewashes were combined in a preweighed vial and heated in an oven at 80°C untilthe hexane was completely evaporated. This normally took about 1.5 h. The vial

562

was cooled, weighed, and the mass of extracted oil was detennined from thedifference.

The natural soil has some extractable organic compounds such as humic acidsin it. They were extracted with hexane from an uncontaminated soil, as describedabove, and the weight of the organics taken into account for estimating the removalof the actual contaminant oil from the soil. The content of natural extractable in thewhole soil is 0.2%, in 75 to 250 ~m fraction, 0.03%, and in 250 to 800 ~m fraction,0.015%. This indicates that natural organic compounds are mostly in the fine soilfraction.

When the amount of oil applied to the soil was compared with that extracted itwas found that there was some loss, probably due to irreversible sorption into thepores of the soil particles. The average percentage of oil extracted from the 250 to800 ~m soil fraction used for our experiments was about 80%. To improve thisrecovery for the analytical determination of the oil, Soxhlet extraction was at-tempted. A 20-g sample of soil was extracted in a Soxhlet apparatus with 200 mlof hexane for 16 h and 87% oil removal was achieved in this case. Because thedifference in oil extraction using these two procedures was not significant, thesimpler method of manual shaking with three portions of 20 ml hexane was usedthroughout the experiment.

The efficiency of the flotation process for removal of oil from the 20-g samplesof soil was calculated as follows:

0.2%R= 100

0.2

where: %R = efficiency of the flotation for oil removal, m = mass of oil extractedfrom soil after flotation, k = efficiency of the extraction process, ml = mass ofnatural organics extracted from the soil.

The spiked soil was prepared so that each 20-g sample would contain 0.2 g oil.The naturally occurring extractables, ml' was calculated from the data obtained foreach soil fraction. A similar efficiency of 0.8 was obtained for the extraction of oiland natural organics.

Two analytical methods were used for evaluating the efficiency of removal ofnonvolatile organic compounds from soil of a contaminated site by flotation. In thef1fst method, 5 g of the soil was shaken for 5 min with two successive 100miportions of Freon-1 13. After filtration, total petroleum hydrocarbon (TPH) wasdetermined by IR (Perkin-Elmer 1310) measurement according to EPA method418. In the second method, sox let extraction of untreated and treated samples wasdone with 200 rnI of hexane:acetone (1:1) mixture for 16 h, and the extract wasanalyzed by a gas chromatography (Varian GC-MS, model 3400) according to theEP A method 8270. From the chromatograms obtained, the areas of eight selectedcompounds with relatively large peaks were compared before and after flotation,

563

and the efficiency of the removal calculated. Probable identification of these peaksusing the library of MS spectrum was also carried out.

RESULTS AND DISCUSSION

Flotation of Artificially Contaminated Soil Using Surfactants

Initial flotation experiments were performed using pure sand (particle size 500 to830 Jim) and bentonite clay as soil surrogates. These were contaminated with oilin the same manner as described above for the soil samples. It was found thatflotation is much more effective for the coarse fraction (about 90% of oil wasremoved during 30 rnin .from sand and 45% from clay). Practically no sand wasfound in the foam during flotation, while a significant portion of clay particles waspicked up by the bubbles to the surface.

Anionic (SOS) and nonionic (Triton 100X) surfactants at concentrations of 50ppm in the solution (or 333 mg per kg of soil) and pine oil at 20 ppm (133 mgikg)were evaluated for oil removal. Results obtained were similar for both surfactants.Initially, 20 ppm pine oil was also added, but it was not found to improve the oilremoval significantly; however, it did cause more soil to float and become trappedin the foam at the top and hence the use of pine oil was discontinued in furtherexperiments.

It was concluded that it is not practical to attempt to remove oil from the silt andclay fractions of soil below 75 Jim using column flotation. The removal efficiencyis poor, and a significant portion of the soil is floated along with the oil. Therefore,further tests were focused on the coarser +75 Jim or +250 Jim fraction of the soil.

To determine optimal conditions for the removal of oil from soil using flotationwith surfactants, a series of tests were done as a function of relevant parameters.The results obtained with the coarse soil fraction (250 to 800 ~m) containing 1 %oil by weight are given in Table 1. Kinetics of oil removal using 50 ppm SOS is

TABLE 1Removal Efficiency of Oil by Flotation at

Different Temperature and Soil/Water Ratios

Temperature,°c

Soil/Waterratio (g/ml)

Surfactant50 ppm

Removalefficiency (%)

222245454545

SDSTritonSDSTritonSDSTriton

30/20030/20030/20030/20030/12030/120

626473797078

564

presented in Figure 2. Based on these experiments, a flotation time of 30 min waschosen for further work. Oil removal by flotation for 30 min increased from 61 to67% when a high SDS concentration of 200 ppm was used. However, more finersoil was picked up in the foam. Increase in the pH of the slurry to 10.5 had littleeffect on the oil removal using flotation with SDS as collector.

Because an increase in the temperature will affect by lowering the viscosity ofthe oil being removed, a series of experiments on flotation was done at elevatedtemperatures. The results of flotation experiments with SDS and Triton lOOX, at22 and 45°C, are also given in Table 1. Increase in temperature was found toimprove the removal efficiency, especially when Triton l00X was used as thecollector.

The effect of the soil to water ratio on flotation was next explored. Flotation wascarried out using a 30-g raw sample of soil in 120 and 200 ml of solutions, and therewas little difference observed between the two test results (Table I).

Soil Washing of Artificially Contaminated Soil Using Surfactants

The flotation method was next compared with soil washing. Soil washing wascarried out in the same cell using the impeller, but without an air flow through theporous frit. To improve the agitation, the impeller speed was increased to 500 rpm.

~'-"i~u

]

FIGURE 2

The kinetics of oil removal with 50S.

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Soil washing was carried out using two different surfactant concentrations, 50ppm, as it was used for the flotation process, and 5 g/l. The first concentration iswell below the CMC, while the second is considerably above it. The resultspresented in Table 2 show that efficiency of the soil-washing process even at thehighest surfactant dosage is less than that obtained with flotation under the sameexperimental conditions (Table 1).

Other researchers have reported 80 to 90% removal of such contaminants asPCBs and P AHs from soil using 0.5 to 2% surfactant solutions (Sabatini et a/.,1995). The low values obtained in this work is attributed to the limited solubilityof the highly hydrophobic paraffin oil used in these experiments. When the highersurfactant dosage (5 g/l) was used extremely stable emulsions were formed withno visible breaking of the emulsion even after I week. While after washing orflotation with 50 ppm only unstable emulsions were formed, which settled withina few days without the addition of any emulsion-breaking reagent or with theaddition of acid to pH 3 within few hours.

Flotation of Artificially Contaminated Soil Using Alkaline Solution

Flotation with hot alkaline solutions is used in industry for the separation ofpetroleum from bitumen sands. Usually temperatures near 80°C are used for thispurpose. According to literature (Dai and Chung, 1995; Dai and Chung, 1996), apH value of 10 is optimum for this separation, as more basic solutions formundesirable stable oil in water emulsions. At high pH values both oil and sandminerals, such as silica are negatively charged and are dispersed by charge repul-sion. To our knowledge, this work is the first attempt to test this technology for theremediation of contaminated soils. In initial tests, the pH was raised to lOusingNaOH before air flow was commenced. However, the pH generally dropped toabout 8.5 by the end of the experiment, due to lack of buffering. NaOH wasreplaced by Na2CO3 in later experiments. The carbonate buffered the solution more

TABLE 2Removal Efficiency of Oil from

Soil by Washing with Surfactants

SOSSOSTritonSOSSOS

2222224S4S

SOpSg/ISOpSOpSg/I

30/20030/20030/20030/12030/120

4858456264

566

pm

pmpm

efficiently, and the pH was 9.2 after 30 min of flotation. Unless stated otherwise,experiments were carried out with 250 to 830 ~m fraction of the soil.

Kinetics of oil removal using flotation with N~CO3 solution at soiVwater ratioof 30 g/200 mI and temperature 22°C is presented in Figure 3. Based on theseresults flotation time of 30 min was chosen for further work. It was found that forflotation in alkaline medium oil removal efficiency improved by increasing thetemperature (Figure 4). When the temperature was increased to 45°C, the removalrates 83 and 78% at soil liquid ratios of 30 g/120 ml and 30 g/200 mI, respectively.Oil exhibits adhesive behavior at lower temperatures, and as a result remainattached to sand particles once they are entrapped. When the oil viscosity isreduced by increasing the temperature, oil detaches more readily from particles.The negative zeta potential of silica increases with temperature (Dai and Chung,1995), this also enhancing repulsion between oil and sand particles.

The oil removal was found to be more efficient when a lower volume of waterwas used for the same amount of soil (Figure 4). This may be due to the higherdensity of dispersed air in the slurry, or to the increased contact between theparticles in the slurry. Additional experiments on the effect of soil-to-liquid ratioand the total volume of slurry floated in each experiment were undertaken in anattempt to clarify this point. It was concluded that at a fixed volume of liquid thereexists an optimal concentration of soil that can be cleaned (Table 3). Reduction ofthe volume of liquid from 200 to 120 ml also increased the removal efficiency.

~i'y'510!

FIGURE 3

The kinetics of oil removal with Na2CO3 solution (pH= 1 0).

567

T~ ~C)

FIGURE 4

Removal of oil from soil using flotation with alkali solutions.

TABLE 3Effect of Changes in Soil and Liquid Quantitieson Effectiveness of Flotation at pH 10 and 22°C

2020061

3020068

5020063

3012072

3030058

Soil mass (g)Liquid volume (rnl)Removal efficiency, %

Alkaline hot solutions were also used for flotation of soil containing particles ofa wider range (75 to 830 ~m). As expected, the efficiency obtained was lower(66%) than that obtained for the 250 to 830 ~m fraction (83%). When alkalineflotation was used for the coarse fraction (250 to 830 ~m), almost no soil particleswere levitated by the bubbles in contrast to about 3% for the 75 to 830 ~m soil.Foaming and floating of finer particles were substantially less in the alkalineflotation than when surfactants were used.

Flotation of Artificially Contaminated Soil Using Alkaline Solution

For comparison purposes, soil washing was carried out with a sodium carbonatesolution at pH 10 and 45°C (Table 4), and mixing in the same cell, but without air

568

TABLE 4Comparison of Flotation and

Washing for Oil Removal fromSoil at 45°C, at pH 10 Using Na2CO3, as

Collector and 30 g/120 ml Soil/Water Ratio

Air flow (ml/min) Removal efficiency (%)

768183

0 (washing)13.527

flow and with the impeller rotating at 500 rpm. The removal of oil by washingusing hot alkali solution is more efficient than that using SDS at normal or elevatedtemperature (Table 2). Efficiency of flotation is more than that of washing andincreases with air flow. The efficiencies of oil removal using alkaline flotation andsurfactants were found to be similar. Stability of oil in water emulsions afterflotation or washing with sodium carbonate solution at pH 10 is lower than thatafter treatment with SDS or Triton lOOX even at low (50 ppm) concentrations.Lowering of pH to 3 significantly increases the rate of phases separation.

Flotation of Actual Contaminated Soil

Preliminary experiments were also carried out with soils from a contaminated site.Experiments were performed with a coarse fraction (250 to 830 Jim) at 45°C usingSDS, Triton lOOX, and sodium carbonate as collectors. Results are shown inTable 5. Average removal of the eight compounds determined using GC-MS were

TABLE 5Efficiency of Organic Compounds

Removal from a Contaminated Site by Flotation

Efficiency with Collectors

SDS(50 ppm)

Triton 100X(50 ppm)

Na2CO3(pH = 10)

CompoundNo.

Chemicalformula

7691647560656379

C,oHnO.C"H240C'6HnO.CI6HnO.C,sH3IC~.2~H52~H3IO.

8S716289S3S7S384

9086705955696386

234S678

569

69,72, and 72% for SDS, Triton, and sodium carbonate, respectively. It can be seenthat removal efficiency of compounds #5. 6, and 7 not containing oxygen (morehydrophobic) is lower (in the range of 53 to 65%). The removal efficiency usingTPH determination before and after flotation using Freon extraction and IR mea-surements was also evaluated. Percent TPH removal were 65, 66, and 73% forflotation with SDS, Triton .100X, and sodium carbonate, respectively. These resultsare close to the removal efficiencies determined by GC-MS method.

CONCLUSIONS

The experiments carried out with soils artificially contaminated with paraffin oilindicate that the flotation process can be applied successfully to substantiallyreduce the amount of nonvolatile hydrophobic contaminants in soil. The processis successful only with sandy fractions of soils, suggesting that the soil should befirst separated into particle size fractions, and flotation used preferably withcoarser fractions. Clay particles are usually highly contaminated both with organicand inorganic pollutants. In sandy soil they can be separated by hydrocyclone andremoved to a landfield, while the bulk of the soil can be treated by flotation andused for construction or other purposes.

A removal efficiency of about 80% was achieved, for the 250 to 830 Jim soilfraction, using low dosage of anionic and nonionic surfactants (SDS or Tritonl00X) or sodium carbonate (pH =10) at a temperature 45°C. For a wider particlerange fraction, 75 to 830 Jim, the efficiency was lower (about 65). Efficiencyobtained with the flotation column with bubbles dispersed by porous frit is similarto that obtained with the same soil in mechanically agitated flotation machine(Denver flotation cell) (Somasundaran et a/., 1997). Removal of oil from silt orclay fractions «75 Jim) is low, and a substantial quantity of the soil is entrainedby the rising bubbles. Increase in the temperature and the density of bubblesimproved the separation of oil from the soil by flotation.

Flotation was more efficient than soil washing of the same soil contaminatedwith paraffin oil, even when much higher surfactant dosages were used in thewashing process. Because of the relatively low surfactants dosage used, stable oilin water emulsions. characteristic for the soil washing, did not form in the flotation

process.Feasibility of the flotation process was also demonstrated for the soil collected

from a hydrocarbons contaminated site. Using SDS, Triton l00X or N~CO3 ascollectors, a 70% reduction of total petroleum hydrocarbons content in the 250 to830 Jim fraction of the soil has been achieved.

ACKNOWLEDGMENT

The authors are grateful for the support of the USEP A Northeast HazardousManagement Research Center and the US EP A exploratory research program.

570

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Oai, Q. and Chung, K. H. 1996. Hot water extraction process mechanism using model oil sand. Fuel75, 220-226.

Gaudin, A. M. 1957. Flotation, New York, Mc Graw-Hill.Sabatini, O. A., et al. (Eds.). 1995. Surfactant Enhanced Subsurface Remediation, Emerging Tech-

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for oil sands. Adv. Chern. Ser. 251, 639-657.Somasundaran, P. (Eds.). 1986. Advances in Mineral Processing. New York, Soc. Mining Eng.Somasundaran, P., Zhang, L., Zheng, J. Ososkov, V., and Chou, C. C. 1997. Removal of nonvolatile

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Van Rijt, C. 1993. Qeaning contaminated sendiments by separation on the basis of particle size.Water Sci. Technol. 28, 283-295.

Wilichowski, M. and Werther, J. 1995. Applicability of flotation in the washing of soil. Chern. lng.Tech. 67,760'-763.

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