Dynamic foaming capacity (min) 0.0
- Department of Chemical Engineering, National Cheng Kung University,
1 Ta-Hsueh Road, Tainan 701, Taiwan
23 April 2014 Accepted
1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
j o u r n a l h o m e p a g e :
Journal of the Taiwan Institute of Chemical Engineers
Contents lists available at
E-mail address: (C.-H. Chang).
6 2757575x62671; fax: +886 6 2344496.
ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
This study demonstrated the abilities of negatively charged biosurfactants, surfactin and rhamnolipid, to remove adsorbed copper and cadmium ions from sand surfaces with the foam-enhanced solution flushing technique. A popular anionic surfactant, sodium dodecylsulfate (SDS), was used for the purpose of removal efficiency comparison. The role of surfactant foaming ability in the flushing approach was then identified. It was found that the surfactant solution flushing could only result in limited removal efficiency of 3–10% and 13–36% for copper ion and cadmium ion, respectively, after 24-pore volume (PV) flushing due to the channeling effect. As compared to the surfactin solution, a less pronounced channeling effect was detected for the rhamnolipid or SDS solution. With the presence of foam in the flushing approach, the channeling effect could be inhibited, and one could obtain improved removal efficiency of 10–30% and 20–46% for copper ion and cadmium ion, respectively, after 24-PV flushing. The removal efficiency was higher for cadmium ions than for copper ions, which could be explained by the significant adsorption of the cadmium ions in the inter-particle pore regions. Moreover, the cumulative removal efficiency variations with the foam-enhanced solution flushing could be correlated with the dynamic foam capacity of the surfactant solutions.
Keywords: Biosurfactant Foam-enhanced solution flushing Surfactant solution flushing A B S T R A C T
26 April 2014 Available online
13 February 2014 Received in revised form
I N F O Article history: Received
I C L E
A R T
2 June 2014
by Pseudomonas aeruginosa Rhamnolipid in water possesses a negatively charged characteristic and may be particularly effective in remediating soils contaminated with metal ions that are less sensitive to ion exchange processes
subtilis with negatively charged characteristic in rod micelle form Rhamnolipid is produced
Applications of biosurfactants in the field of environmental protection have received much attention due to their biodegrad- ability, low toxicity, effectiveness in enhancing biodegradation, and ability to solubilize hydrophobic compounds Surfactin is a biosurfactant produced by various strains of Bacillus
Several remediation technologies have been developed for removing heavy metal ions from contaminated soils. A foam-enhanced solution flushing technique has been applied to remediate soils containing metal ion contaminants, and one was able to improve the migration of surfactant solutions with the presence of foam during the solution flushing process The foam could inhibit the channeling effect of solution flow by increasing the resistance of the solution flow and thus by forcing the solution to homogeneously flow throughout the medium. This would enhance the removal efficiency of the soil remedia- tion By using a foam-enhanced solution flushing technique, the removal efficiency was increased even in a heterogeneous porous medium . The effectiveness of foam generation is influenced by surfactant concentration and this technique is attractive due to the low usage of surfactants .
and the sand surfaces involving the liquid phase. The inner-sphere type is the interaction between metal ions and functional groups on the surfaces, which does not involve a liquid phase or water molecules between the ions and the surfaces Natural sands may have macropores and mesopores, and the porosity is mostly influenced by particle size, grain shape, and rock type The porosity can be classified into inter-particle porosity and intra- particle porosity. Pores cause not only great surface area, but also high selectivity in adsorption The interaction between metal ions and sand surfaces will affect the desorption behavior of the metal ions in a remediation process.
. The outer-sphere type is the interaction between metal ions
1. Introduction Metal ions in a solution flowing over sand particles may adsorb onto the particle surfaces with various interaction mechanisms
Foam-enhanced removal of adsorbed metal ions from packed sands with biosurfactant solution flushing Bode Haryanto, Chien-Hsiang Chang
- Corresponding author. Tel.: +886
B. Haryanto, C.-H. Chang / Journal of the Taiwan Institute of Chemical Engineers
When a 100-mL aqueous solution containing 50-ppm metal ion was mixed with 100-g sand, the metal ions would adsorb onto the sand surfaces and the adsorption density was increased with time until the adsorption equilibrium was reached The metal ions in the solutions may interact with the sand surfaces with outer-sphere and inner-sphere types . After drying the sand particles with N
tion of
5 critical micelle concentration (cmc) and SDS solution ion-adsorbed sand-packed column. The effluent from the packed column during the flushing operation was collected every
4 pore- volumes (PVs, one PV is about 2.2 mL), and the metal ion concentration in the effluent was analyzed by atomic absorption spectrometry.
Images of the cleansed sand surface were obtained by using a scanning electron microscope (SEM) (JOEL JSM-6700F, Japan). The surface tension-lowering abilities of surfactin and rhamno- lipid in aqueous phase were evaluated with a Wilhelmy plate tensiometer (CBVP-A3. Kyowa Interface Science Co. Ltd., Japan). The zeta potentials of surfactin and rhamnolipid micelles or aggregates were measured by using a zeta potential analyzer (model 3000HS, Malvern Instrument, UK).
3. Results and discussion
3.1. Sands with adsorbed metal ions
The sand surface morphology was observed by SEM and a typical image is shown in
The porous characteristic of the
sand surfaces with the presence of intra-particle regions was demonstrated in the SEM image and was expected to affect the metal ion adsorption density.
2 gas, adsorption densities of cadmium
2. Materials and methods Surfactin was produced from Bacillus subtilis ATCC 21332 with purity about 90%, and rhamnolipid was produced from P.
and copper ions on the sand surfaces were found to be 5.85 and 13.45 mg/kg, respectively (
). With the N
2
drying treatment, the adsorbed metal ions with the outer-sphere interaction type on inter-particle sand surfaces would be removed, and the adsorbed metal ions with the inner-sphere interaction type were expected to remain on the sand surfaces.
3.2. Physical properties of surfactant solutions
The abilities of two biosurfactants, surfactin and rhamnolipid, to lower surface tension of aqueous phase are demonstrated in
. Based on the surface tension data, the critical micelle
concentrations of surfactin and rhamnolipid were estimated to be about 20 mg/L and 40 mg/L, respectively. The surface tensions of surfactin and rhamnolipid aqueous solutions at corresponding critical micelle concentrations were 31 mN/m and 35 mN/m, Fig.
1. SEM image of the sand surface.
45 (2014) 2170–2175 2171
2 gas. Biosurfactant solutions with a concentra-
The objective of this study is to demonstrate and compare the abilities of two biosurfactants, surfactin and rhamnolipid, to remove strongly adsorbed metal ions from sand surfaces with the foam-enhanced solution flushing technique. The key role of surfactant foaming ability in the flushing approach is then identified. Sands with adsorbed metal ions were prepared by an adsorption process and then were dried to allow the metal ions to interact with the sand surfaces mainly through the inner-sphere interaction. Physical properties of the biosurfactant solutions were characterized, and the removal efficiency of the biosurfactant solutions without or with foam for the adsorbed metal ions from the sand surfaces was evaluated. The removal efficiency obtained by using a popular anionic surfactant, sodium dodecyl- sulfate (SDS), solution was also determined for the comparison purpose, and the importance of the surfactant foaming ability on the removal efficiency of the foam-enhanced solution flushing approach was discussed.
10
V cm was obtained from a Milli-Q
aeruginosa J4 with purity about 63% Sodium dodecylsulfate
(SDS), a popular anionic surfactant, (purity 99.0%) was purchased from Sigma–Aldrich, Japan. Research-grade copper (II) sulfate pentahydrate (Cu
2 SO
4
5H
2 O) and cadmium chloride (CdCl
2
) purchased from Showa Chemical Co. Ltd., Japan, were chosen as the sources for adsorbed metal ions on sand surfaces. Purified water with a resistivity of
18.2 M
plus purification system (Millipore, USA) and was used in all experiments.
m were used as the porous medium in the column. All experiments were performed at room temperature. Surfactin solution was prepared in a
The sands were cleansed with purified water and were adsorbed by copper and cadmium ions through mixing the cleansed sands with the metal ion solutions. 100-g sand was mixed with 100 mL of 50-ppm metal ion solution, and the mixture was shaken for 24-h with 150 rotations per minute. Sands with adsorbed metal ions were dried by introducing N
2 gas to remove
the aqueous phase with the metal ions adsorbing through the inner-sphere interaction . After the sands were completely dried, the metal ion concentration in the solution was analyzed by GBC Scientific Equipment Pty Ltd., Australia) and the adsorption density of the metal ions on the sand surfaces was determined.
For a foam-enhanced surfactant solution flushing operation, a glass column with a length of 5 cm and an outside diameter of
3.5 cm was used as the foam-generator with inlets for surfactant solution and N
2
gas, respectively . The continuous flow of surfactant solution into the foam generator was controlled by using a peristaltic pump, and the dynamic foam capacity of a surfactant solution was assessed when the volume of foam in the foam generator was constant. The dynamic foam capacity was determined by dividing the constant volume of foam (mL) by the flow rate of N
2 gas (mL/min). A glass column with a length of
7.5 cm and an outside diameter of 1.5 cm was used in the sand- packed column experiments to simulate the soil remediation condition. Sand particles with an average diameter of 320
m
3 M phosphate buffer
with a pH value of 8.0 Rhamnolipid was dissolved in pure water with a pH of 5.6 to prepare the aqueous solution The SDS solution was also prepared with pure water of pH = 5.6. Flow rates were fixed at 2 mL/min for surfactant solution and at 20 mL/min for N
B. Haryanto, C.-H. Chang / Journal of the Taiwan Institute of Chemical Engineers
50
10 Adsorption density (mg/kg)
5
10
15
20 Fig.
2. Adsorption densities of copper (*) and cadmium (*) ions on the sand surfaces.
Biosurfactant concentration (mg/L) 120 160 240 200
80
40 Surface tension (mN/m)
10
20
30
40
60
30
70
80 Fig.
3. Surface tensions of surfactin (*) and rhamnolipid (*) aqueous solutions.
5x cmc 5x cmc 2.5x cmc SDS Rhamnolipid Surfactin Dynamic foaming capacity (min) 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 Fig.
4. Dynamic foam capacity of surfactin, rhamnolipid, and SDS aqueous solutions with solution and N 2 gas flow rates of 2 mL/min and 20 mL/min, respectively.
2172 45 (2014) 2170–2175
20
respectively. With a higher concentration of
5 critical micelle concentration (cmc), the surface tensions of surfactin and rhamnolipid aqueous solutions were about the same with a value of 29 mN/m. The cmc of the popular anionic surfactant SDS in water at
2.5 cmc because an SDS solution with a concentration of
22
8C was 8.0 mM with a surface tension of 38 mN/m and we found that the surface tension was decreased to 32 mN/m at a concentration of
5 cmc. The charged characteristics of surfactin and rhamnolipid micelles or aggregates in aqueous phase were then evaluated by zeta potential measurements. Surfactin micelles at a concentration of
5 cmc and pH 8 possessed zeta potentials in the range of 60 to 90 mV. For rhamnolipid molecules in aqueous phase, a variety of aggregates including micelles and vesicles might exist and the zeta potential of rhamnolipid micelles or aggregates at a concentration of
5 cmc was found to vary between 17 to 56 mV. The negatively charged characteristic of SDS micelles has been generally accepted and the zeta potential of SDS micelles was about
18 mV It has been proposed that anionic surfactants with tension- lowering ability and negatively charged characteristic in aqueous phase could facilitate metal ion removal from contaminated sands istics of micelles or aggregates of the two biosurfactants were clearly demonstrated from the zeta potential data, and thus the biosurfactants have the potential of removing adsorbed metal ions from sand surfaces in a solution flushing process.
The foaming ability of the biosurfactant solutions was then evaluated with a continuous foam generation system , and the dynamic foam capacity was determined . The dynamic foam capacity of the biosurfactant solutions was determined with solution and gas flow rates of 2 mL/min and 20 mL/min, respectively.
shows the dynamic foam capacity of the
biosurfactant solutions at a concentration of
5 cmc. For the comparison purpose, the dynamic foam capacity of SDS solution at a concentration of
2.5 cmc is shown in
. The SDS
concentration was set at
5 cmc would produce foam over the capacity of the foam generator. It has been reported that surfactin was able to produce foam at a low concentration with excellent foam stability . However, rhamnolipid and SDS, especially SDS, possessed better foaming ability in comparison with surfactin under the experimental conditions.
50
For surfactin, a phosphate buffer was used as the solvent to control the aqueous phase at pH
8. The presence of buffer ions would affect the micelle rigidity of an anionic surfactant When micelles are rigid, they may not quickly dissociate to supply monomers, limiting the ability of the surfactant molecules to adsorb onto the gas–water interface of foam and resulting in low foaming ability This may explain the low dynamic foam of the lower concentration of surfactin than rhamnolipid or SDS adopted for determining the dynamic foam capacity, the excellent foaming ability of surfactin could be assured.
3.3. Removal efficiency for metal ions
Sands with adsorbed copper and cadmium ions mainly through the inner-sphere interaction were packed into a column. The potential of biosurfactants, surfactin and rhamnolipid, on remov- ing the adsorbed metal ions from the sand surfaces was then investigated and was compared with that of SDS, a popular anionic surfactant. The sand-packed column was flushed with biosurfac- tant solutions at a concentration of
5 cmc, and the removal efficiency for the metal ions was compared with that obtained by using a SDS solution.
show the removal efficiency for
the metal ions by using the solution flushing technique without foam and with foam, respectively.
3.3.1. Solution flushing without foam
The removal efficiency for adsorbed copper ions from the sand surfaces by using a solution flushing approach is plotted in
a.
It was found that the surfactin solution could remove the adsorbed copper ions from the sand surfaces only with initial 4-PV solution flushing and with the efficiency of 3%. However, rhamnolipid or
Adsorption time (hour)
60
40
B. Haryanto, C.-H. Chang / Journal of the Taiwan Institute of Chemical Engineers
12
40
30
20
10
4 Cumulative removal efficiency (%)
8
16
Amount of effluent (pore volume)
20
24
Amount of effluent (pore volume)
(a) (b) Fig.
4 Cumulative removal efficiency (%)
50 Surfactin Rhamnolipid SDS
40
50 Surfactin Rhamnolipid SDS
24
20
20
45 (2014) 2170–2175 2173
6. Cumulative removal efficiency of solution flushing with foam for adsorbed (a) copper and (b) cadmium ions.
(b) Fig.
50 Surfactin Rhamnolipid SDS (a)
40
30
20
10
4 Cumulative removal efficiency (%)
8
12
16
30
4 Cumulative removal efficiency (%)
5. Cumulative removal efficiency of solution flushing without foam for adsorbed (a) copper and (b) cadmium ions.
to 24-PV solution flushing with the cumulative removal efficiency of about 10%.
The ability for biosurfactant solution flushing to remove adsorbed cadmium ions from the sand surfaces is depicted in
The adsorbed copper ions in the intra-
. The efficiency of the surfactin solution
The removal efficiency for adsorbed metal ions from the sand surfaces by using solution flushing approach with foam is demonstrated in
3.3.2. Solution flushing with foam
It should be noted that the effect of surfactin solution flow rate on the removal efficiency for adsorbed copper ions from sand surfaces in a solution flushing operation with the similar operation conditions has been investigated and it was found that for solution flow rates of 2–6 mL/min, a similar channeling effect was detected.
During the flushing by surfactant aqueous solutions, the channeling effect of the surfactant flow was expected to occur, that is, the solutions could not homogeneously spread throughout the sand-packed column . The channeling effect would limit the efficiency of the solution flushing approach because the medium with a low contact area between the solution and sand surfaces. In view of the cumulative removal efficiency variations during the solution flushing operations, it seemed that the channeling effect of the surfactin solution was more pronounced than that of the rhamnolipid or SDS solution.
particle pore regions would be difficult to interact with the surfactant micelles or aggregates during the flushing operation. It is noted that water alone could only remove 5 and 12% of copper
cadmium ions in the inter-particle pore regions were expected to easily interact with the negatively charged surfactant monomers or micelles during the flushing operation. However, copper ions seemed to adsorb not only in the inter-particle pore regions but also in the intra-particle pore regions with a comparatively high adsorption density (
a).
The adsorbed
ion adsorption would affect the removal efficiency of the surfactants. A significant number of cadmium ions might adsorb only in the inter-particle pore regions, as judged from the comparatively low adsorption density
), could affect the flushing efficiency. The location of metal
The porous characteristic of the sand surfaces with the presence of inner-particle pore regions, as demonstrated in the SEM image (
With 24-PV solution flushing, one could find that the ions was about 13% and higher removal efficiency of 23% was found for rhamnolipid solution. For SDS solution, much higher removal efficiency of 36% was detected. In comparison with the removal efficiency for copper ion, higher removal efficiency was generally found for cadmium ions with solution flushing. It was noted that after 4-PV flushing, cadmium ions could be still removed with further solution flushing even with the surfactin solution.
flushing with foam for removing the adsorbed copper ions was about 10%, which was much higher than that obtained by the surfactin solution flushing without foam. However, as compared to rhamnolipid and SDS, surfactin could only remove a comparatively small amount of copper ions with 24-PV solution flushing
For removing adsorbed copper ions from the sand surfaces by using the rhamnolipid solution flushing with foam, the efficiency was increased from 15% with 4-PV effluent to 23% after 24-PV solution
8
40
12
16
20
24
Amount of effluent (pore volume)
50 Surfactin Rhamnolipid SDS
30
Amount of effluent (pore volume)
20
10
8
12
16
20
24
10
B. Haryanto, C.-H. Chang / Journal of the Taiwan Institute of Chemical Engineers
2174 45 (2014) 2170–2175
[10]
[9]
[7] [8]
[6]
[4] [5]
[3]
[2]
efficiency was increased from 23% with 4-PV effluent to 30% with 24-PV effluent. Apparently, the presence of foam in a solution flushing approach could increase the ability of the surfactant solutions to remove the adsorbed metal ions from the sand surfaces in a sand-packed column (
[1]
References
The authors wish to express sincere gratitude to Professor Yao- Hui Huang for the help on using the atomic absorption spectrometer and would like to thank Dr. Wei-Ta Li and Mr. An-Tsung Kuo for editing the manuscript.
4. Conclusions This study demonstrates the potential of applying two biosurfactants, surfactin and rhamnolipid, on removing strongly adsorbed copper and cadmium ions from sand surfaces with a foam-enhanced solution flushing approach. The important role of surfactant foaming ability in the efficiency of the flushing approach is then identified. The results indicated that surfactin and rhamnolipid with negatively charged characteristics had the ability to interact with the adsorbed metal ions on the sand surfaces and could be adopted in the solution flushing process. However, only limited removal efficiency for the adsorbed metal ions was found by using the surfactin or rhamnolipid solution flushing due to a low contact area between the solution and the ions, resulting from the channeling effect of the solution flow in a sand-packed medium. With the foam-enhanced surfactant solu- tion flushing approach, the channeling effect of the solution flow could be inhibited with the presence of foam by increasing the solution flow resistance and thus forcing the solution to homogeneously flow throughout the sand-packed medium, resulting in significantly improved removal efficiency for the adsorbed metal ions. It is noted that the removal efficiency was higher for cadmium ions than copper ions, which could be explained by the significant adsorption of the cadmium ions in the inter-particle pore regions. Moreover, the cumulative removal efficiency variations for the adsorbed metal ions by using the foam- enhanced solution flushing approach could be correlated with the dynamic foam capacity of the surfactant solutions rather than the tension-lowering ability and aggregate charge characteristic of the surfactants. Acknowledgements
The high dynamic foam capacity of the rhamnolipid solution or SDS solution may explain the continuous increase in the cumulative removal efficiency of rhamnolipid or SDS up at least to 24-PV solution flushing for the adsorbed metal ions during the flushing process. The solution flushing with foam demonstrated the good performance of the biosurfactants for removing adsorbed metal ions from the sand surfaces, and the due to a low usage of biosurfactant and short treatment time Moreover, the channeling effect is expected more significant with smaller sand particle size and higher inner porosity. Under such conditions, the advantage of applying the foam-enhanced solution flushing operation to remove adsorbed metal ions from sand surfaces would be more pronounced.
In comparison with surfactin, rhamnolipid molecules formed micelles or aggregates with a less negatively charged characteris- tic, but higher dynamic foam capacity of the rhamnolipid solution was detected. With the comparatively high dynamic foam capacity, rhamnolipid micelles or aggregates would be ease to reach not only the inter-particle pore regions but also the intra- particle pore regions to interact with more adsorbed metal ions. As a result, higher removal efficiency enhancement for the adsorbed copper ions by foam was found for rhamnolipid than for surfactin. For SDS with a small micelle structure and the SDS solution with the highest dynamic foam capacity, the possibility for SDS monomers or micelles to interact with the adsorbed metal ions even in the intra-particle pore regions would be increased.
In view of the removal efficiency data, the removal of adsorbed metal ions by using a surfactant solution flushing approach was apparently enhanced with the presence of foam. Moreover, the cumulative removal efficiency variation for the adsorbed metal ions was strongly affected by the foaming ability of the surfactants. A high foam capacity could inhibit the channeling effect of the solution flow and increase the possibility of the surfactant aqueous solution to penetrate into the porous regions of the sands, thus improving the contact between the surfactant solutions and adsorbed metal ions on the sand surfaces. Among the three surfactant solutions, the SDS solution possessed the highest dynamic foam capacity, followed by the rhamnolipid solution and surfactin solution, under the experi- form micelles with the most pronounced negatively charged characteristic, but the lowest dynamic foam capacity was found for the solution, leading to the lowest removal efficiency enhancement by foam for the adsorbed metal ions.
spreading in the sand-packed column and increase the contact between the negatively charged surfactant micelles or aggregates and the adsorbed metal ions, resulting in enhanced desorption of the metal ions from the sand surfaces. The metal ions were interacted with the surfactants by forming complexes on the sand surfaces and then were moved into the aqueous phase by associating with the surfactant micelles or aggregates .
The presence of foam would thus improve the solution
Applying surfactant solution flushing with foam could inhibit the channeling effect of the solution flow by increasing the resistance of the solution flow and thus by forcing the solution to homogeneously flow throughout the sand-packed medium
b). SDS still demonstrated the highest removal efficiency among the three surfactants for the adsorbed metal ions. The removal efficiency of rhamnolipid was improved in the presence of foam but was slightly lower than that of SDS.
For the cumulative removal efficiency after 24-PV solution flushing, 20%, 40%, and 46% were found for surfactin, rhamno- lipid, and SDS solutions, correspondingly (
For adsorbed cadmium ions, in comparison with the solution flushing without foam, solution flushing with foam could also increase the removal efficiency of the three surfactant solutions.
[11]
[29]
[30]
[31] [32]
[33]
[34]
[25] [26]
[35]
[36]
[37] [38]
[39] [40] [41]
[27] [28]
[24]
[12]
[17]
[13]
[14]
[15]
[16]
[18] [19]
[20]
[21]
[22]
[23]
B. Haryanto, C.-H. Chang / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2170–2175 2175