Removal of Cadmium (II) from Aqueous Solution by Ion Flotation

Article pubs.acs.org/IECR

Removal of Cadmium(II) from Aqueous Solution by Ion Flotation Using Rhamnolipid Biosurfactant As an Ion Collector Atefe Bodagh,†,‡ Hamid Khoshdast,§ Hakimeh Sharafi,† Hossein Shahbani Zahiri,† and Kambiz Akbari Noghabi†,* †

National Institute of Genetic Engineering and Biotechnology (NIGEB), P.O. Box 14155-6343, Tehran, Iran Islamic Azad University, Karaj Branch, Karaj, Iran § Mining Engineering Department, College of Mining and Industry, Shahid Bahonar University, Zarand 77611-56391, Iran ‡

ABSTRACT: In this research, the potential of cadmium removal from aqueous solution by foam flotation with a rhamnolipid biosurfactant sample was studied. The effects of different major operating conditions, such as rhamnolipid and cadmium concentrations, solution pH, aeration rate, and frother type and concentration, on the cadmium removal were investigated. The selectivity coefficients in the presence of rhamnolipid maintained definite orders as follows: cadmium > zinc, cadmium > copper, and zinc > copper. The selectivity coefficient of cadmium over copper was the highest one. The maximum removals of Cd from clean and Zn- and Cu-contaminated solutions were about 57%, 36%, and 48%, respectively. Kinetic studies indicated that ion flotation of cadmium follows a first-order equation with a kinetic rate of 0.0071 min−1. Although the removals are rather low, it seems that the use of rhamnolipid biosurfactant can be promising in heavy-metal removal from wastewaters by foam flotation with some modifications. rhamnose are linked to one or two molecules of βhydroxydecanoic acid, are the best-studied glycolipids10 (Figure 1). Because of their high binding capacity with metal ions, rhamnolipid biosurfactants are widely used in the remediation of heavy-metal-contaminated soils.12−16 Although there is a rather large body of literature on the use of synthetic collectors in the foam flotation of heavy-metal ions, few scientific studies involving biosurfactants have been published.17,18 The aim of this work was to investigate the possible applicability of rhamnolipid biosurfactants as ion collectors in wastewater treatment using an ion flotation process. In addition, the effects of different operating parameters on the selectivity, kinetics, and overall performance of the removal process were studied.

1. INTRODUCTION Many common heavy metals are released into the environment by a wide variety of household and industrial effluents. For example, cadmium, although it is rarely found in natural materials, is becoming increasingly prevalent because of its release from various sources including metal processing, batteries, plating, paints, fertilizers, waste disposal, and fuel burning. Currently, numerous treatment methods and techniques exist to remove heavy-metal ions from aqueous solutions. Among them, ion flotation is a well-known process in which removal is achieved through a reaction between metal ions and an oppositely charged ionic surface-active reagent (collecting surfactant).1 The ability of synthetic surfactants to bind with heavy-metal ions has been reported by researchers who have conducted experiments involving water and wastewater treatment with different synthetic surfactants.1−7 In comparison to synthetic surfactants, relatively little information is available for biologically produced surfactants (biosurfactants), but their application in bioremediation processes might be more acceptable from a social point of view because they occur naturally. Potential advantages of biosurfactants relative to chemically synthesized surfactants include their unusual structural diversity, which could lead to unique properties; the possibility of cost-effective production; biodegradability; low toxicity; higher selectivity; and better activity under conditions of extreme temperature or pH.8,9 Pseudomonas aeruginosa produces glycolipid molecules, namely rhamnolipids, which comprise a well-studied family of microbial surfactants. Rhamnolipids, like other surfactants, are amphiphilic compounds with both hydrophobic and hydrophilic portions. These molecules have the ability to decrease the surface tension, critical micelle concentration, and interfacial tension. Rhamnolipids, in which one or two molecules of © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Production of Biosurfactant. The rhamnolipid biosurfactant applied in this study was produced using P. aeruginosa MA01 grown on a mineral salt medium at 30 °C. In this regard, potential bacteria-polluted samples were collected from wastewater from different types of food industries (e.g., dairy, meat processing, edible oil processing, soft drinks, canned foods). Molasses, raw soybean oil, malt extract, raw milk, raw cream, honey, and spoiled fruits were used as natural sources. Screening of biosurfactant-producing microorganisms was carried out using sunflower oil as the sole carbon source according to the method of Kitamoto et al.19 with some modifications. Briefly, samples were transferred to 50 mL falcon tubes containing 7.5 mL of selective medium consisting of Received: Revised: Accepted: Published: 3910

January 8, 2013 February 19, 2013 February 19, 2013 February 19, 2013 dx.doi.org/10.1021/ie400085t | Ind. Eng. Chem. Res. 2013, 52, 3910−3917

Industrial & Engineering Chemistry Research

Article

Figure 1. Chemical structures of rhamnolipid homologues produced by Pseudomonas aeruginosa.11

NH4NO3 (3.0 g/L), KH2PO4 (0.3 g/L), MgSO4·7H2O (0.3 g/ L), yeast extract (0.5 g/L), and sunflower oil (100 g/L). The medium was autoclaved at 121 °C and 1.5 atm for 20 min. Sunflower oil was separately sterilized under the same conditions. Cultures were incubated on a rotary shaker at 200 rpm and 30 °C for 7−10 days. When a nearly stable emulsion had been produced, serial dilutions of cultures were prepared and then spread on nutrient agar plates. The resultant colonies were then purified as single colonies and examined for their ability to produce biosurfactant. The production medium was composed of NaNO3 (3.0 g/L), KH2PO4 (0.25 g/L), MgSO4·7H2O (0.25 g/L), yeast extract (1.0 g/L), and soybean oil (10 g/L). Seed cultures were prepared using a seed culture medium containing glucose (40 g/L), NaNO3 (3.0 g/L), KH2PO4 (0.25 g/L), MgSO4·7H2O (0.25 g/L), and yeast extract (1.0 g/L) by overnight incubation on a rotary shaker at 200 rpm and 30 °C. Seed cultures were inoculated (2% v/v) in 250 mL flasks containing 50 mL of production medium and then incubated at 200 rpm and 30 °C for 4−7 days. Production of biosurfactant was evaluated qualitatively and quantitatively. Cell-free supernatant was used for rapid detection of biosurfactant production using the oil displacement test.20 Because the soybean oil was used as the sole carbon source, the lipase activity of isolated microorganisms probably degraded triacylglycerols to free fatty acids and mono- and diacylglycerols. These compounds have surfactant properties that can lead to a positive oil displacement test. Therefore, extraction of biosurfactant was carried out by the acid precipitation and solvent extraction method.21 Finally, potent biosurfactantproducing isolates were selected and maintained on nutrient agar slants for further studies. 2.2. Structural Characterization of Biosurfactant. The rhamnolipid compounds were analyzed by thin-layer chromatography (TLC) using a silica gel plate. For this purpose, chromatograms were developed with chloroform/methanol/ acetic acid (5:1:0.16 v/v/v) and visualized by TLC reagents, namely, iodine vapors for lipid staining and α-naphthol/H2SO4 (5−8 drops of H2SO4 were added to a 10% solution of αnaphthol in 96% ethanol, Molisch’s reagent) for sugar detection. Further purification was performed by means of column chromatography. To load the biosurfactant on silica gel 60, the partially purified biosurfactant (∼1 g) was dissolved in 1 mL of chloroform in a 50 mL round balloon, and silica gel 60

(35−70 mesh, particle size 0.2−0.5 mm, Merck, Darmstadt) was added, making sure the silica gel particles did not stick to the balloon flask. The solvents were removed by rotary evaporator (Rotavapor, Büchi RE 111, Büchi 461 water bath, Flawil, Switzerland), and the obtained green silica gel particles, which contained the biosurfactant, were applied to a chromatography column (57 cm ×1.5 cm) packed with 45 cm of silica gel 60 up to 51 cm. The column was eluted in a stepwise fashion with chloroform, chloroform/ethyl acetate (9:1, v/v), chloroform/ethyl acetate (50:50, v/v), ethyl acetate, and chloroform/methanol (9:1, v/v). To obtain the purified fractions, TLC was performed on 10 mL of the eluent fractions using silica gel plates by developing a solvent of chloroform/ methanol/acetic acid (5:1:0.16, v/v/v) and Molisch’s reagent to investigate the sugar stain separation. All fractions with the same sugar stains were pooled, and the solvents were removed by rotary evaporator to obtain the purified fractions. The chemical structures of the purified fractions were analyzed by electrospray ionization-mass spectrometry (ESIMS) from m/z 950 to m/z 150. The acquisition parameters were: ion polarity, negative; ion source type, ESI; dry gas flow rate, 6 L/min; nebulizer pressure, 30 psi; temperature, 325 °C; capillary exit voltage, −127.3 V. The trap drive values were close to 50, indicating an intermediate stability for our compounds. 2.3. Chemicals and Synthetic Wastewater. The following chemicals, purchased from Merck as analytical grade, were used: cadmium sulfate (CdSO4·8H2O), copper sulfate (CuSO4·5H2O), and zinc sulfate (ZnSO4·H2O) as the source of competing ions. The produced rhamnolipid biosurfactant was applied as an ion collector, and pure ethanol (C2H5OH) and methyl isobutyl carbonyl (MIBC, C6H13OH) were used as frothers. The solution pH was adjusted using NaOH and nitric acid (HNO3) when necessary. The metal salts were used as 1000 ppm stock solutions after the required amounts had been dissolved in doubly distilled water. Heavy-metal stock solutions (metal ion concentration of 44 × 10−3 mM) were prepared by dissolving analytical-grade reagents of copper sulfate and cadmium sulfate with deionized water. Each test solution was prepared by combining the required amount of metal salt stock solution, biosurfactant solution, and the necessary frother stock solution with deionized water to make up 1 L of mixture and then stirring 3911

dx.doi.org/10.1021/ie400085t | Ind. Eng. Chem. Res. 2013, 52, 3910−3917


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3.2. Effect of Rhamnolipid and Cadmium Concentrations. The effect of the crude biosurfactant concentration on the Cd removal efficiency is shown in Figure 2. As can be

with a magnetic stirrer for 15 min. The initial concentrations of rhamnolipid were carefully selected according to its critical micelle concentration such that no precipitation occurred in the bulk solution. 2.4. Flotation Apparatus and Experiments. Flotation experiments were carried out in a glass column with a height of 90 cm and an internal diameter of 4 cm. Bubbles were generated by sparging air through a sintered frit at the bottom of the column through a controlled in-line air filter (0.2−0.4 μm). To avoid sparger blockage, a rotameter was used to control the air flow rate at 300 mL/min. A port 8 cm above the sparger was also used for sampling. Solution drained from the frit was collected using a valve at the base of the column. For each batch flotation test, 300 mL of initial solution containing the requisite amounts of rhamnolipid biosurfactant and frother (ethanol or methyl isobutyl carbinol) was prepared in deionized water. The pH of the solution was adjusted to the desired value, and consistent mixing of all reagents was ensured. Bubble and foam production was begun just after the addition of the solutions into the column and continued for 90 min. Samples were collected at 10, 30, 45, 60, and 90 min. 2.5. Analysis Methods. The concentrations of metals in the outlet solution were measured by atomic absorption technique (Varian model SpectrAA 220, Mulgrave, Victoria, Australia) and the ion removal efficiency, R, was then calculated as1 R=

1 − Cr × 100 Ci

Figure 2. Effect of rhamnolipid concentration on cadmium removal.

seen, the removal efficiency initially increased with increasing rhamnolipid concentration up to 50 ppm and then decreased at higher concentrations. In an aqueous environment, carboxyl groups in the rhamnolipid structure (Figure 1) are dissociated and give an anionic character to the rhamnolipid molecules. In an ion flotation system, rhamnolipid anions interact with cadmium cations, and then, the resulting ionic clusters are removed from the column by attaching to rising bubbles. Because of a higher charge value, each divalent cadmium cation needs to interact with at least two single rhamnolipid anions. Thus, the removal efficiency is expected to be enhanced at higher rhamnolipid concentrations. At excess concentrations, micelles can form, so the number of free rhamnolipid molecules that can adsorb at bubble surfaces decreases.22 The critical micelle concentration of the used rhamnolipid product is about 47.5 mg/L. In addition, as the rhamnolipid concentration increases, the froth stability and, thus, water recovery will increase, and the cadmium removal will also decrease. Figure 3

(1)

where Ci and Cr are initial and residual concentrations of heavymetal ion, respectively. The selectivity of ion flotation was also calculated using the equation18 ln cA = SBA ln c B + C

(2)

where SAB is the selectivity coefficient; cA and cB are the concentrations of counterions A and B, respectively; and C is a constant value.

3. RESULTS AND DISCUSSION 3.1. Biosurfactant Production and Characterization. The crude extract of biosurfactant was analyzed by analytical TLC as explained in the preceding section. Essentially two major components with retention factors (Rf) of 0.31 and 0.73 were observed, and a very small contamination of highly hydrophobic substances was observed at the solvent front. The two spots reacted with the anthrone reagent, indicating its carbohydrate nature. After the structural characterization by ESI-MS, it was concluded that the Rf = 0.31 component corresponds to dirhamnolipid and the Rf = 0.73 component corresponds to monorhamnolipid. The TLC results showed that the crude biosurfactant was a mixture of mono- and dirhamnolipids in a 1:1 ratio. Results from ESI-MS indicated the presence of three major monorhamnolipid species, namely, R1C10C10, R1C10C12:1, and R1C10C12, as well as three major dirhamnolipid species, namely, R2C10C10, R2C10C12:1, and R2C10C12. Among these compounds, R1C10C10 and R2C10C10 (for n = 6 in Figure 1) were the major components of the rhamnolipid produced by the MA01 strain. These two compounds are named L-rhamnosyl-β-hydroxydecanoyl-βhydroxydecanoate (R1C10C10) and L-rhamnosyl-L-rhamnosylβ-hydroxydecanoyl-β-hydroxydecanoate (R2C10C10). The crude rhamnolipid product had a purity of about 97%.

Figure 3. Effect of initial cadmium concentration on removal efficiency.

shows the cadmium removal for solutions with different initial cadmium concentrations. At a certain rhamnolipid concentration, as the initial Cd concentration increases, fewer rhamnolipid anions are available to interact with the Cd2+ cations to adsorb at bubble surfaces. Thus, it would be expected that the removal efficiency should decrease as the cadmium concentration is increased. 3912



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3.3. Effect of Rhamnolipid Structure. Figure 4 shows the effect of rhamnolipid structure on the Cd removal efficiency.

Figure 6. pH-sensitive conversion of molecular aggregates of rhamnolipids.24

Figure 4. Cadmium removal efficiency for rhamnolipid homologues.

Dirhamnolipid was found to exhibit ion-collecting properties than monorhamnolipid and crude biosurfactant. Di- and monorhamnolipid homologues were shown to have hydrocarbon chains with equal lengths. These hydrophobic groups interact at the bubble surface, and the rhamnolipid molecules are oriented at the air/water interface such that the hydrophilic rhamnosyl groups point toward the water phase. These hydrophilic head groups further interact with each other through numerous hydrogen bonds.22,23 Dirhamnolipid molecules form a more compact adsorbed layer at the bubble surface because of the presence of more rhamnosyl group in their structure compared to monotype molecules. Therefore, at a certain concentration, more dirhamnolipid molecules adsorb at the air/water interface (bubble surface), thereby increasing the ion-loading capacity of the bubbles and the ion removal efficiency. 3.4. Effect of Initial Solution pH. The effect of solution pH on cadmium removal efficiency is reported in Figure 5,

rhamnolipid aggregates follows the order micelle > lamella > vesicle. The decrease in surface activity of rhamnolipids also leads to a decrease in frothing capacity. In addition, as shown in Figure 6, the surface charge of the aggregate changes from negative in a micelle to neutral in a vesicle. Therefore, the adsorption of positively charged cadmium ions by rhamnolipid anions is expected to decrease at acidic pH values. Alkaline pH values decrease the adsorption density of rhamnolipids at the air−water interface (bubble surface) because of the electrolytic effect of sodium cations (dissociated from NaOH used as a pH regulator). Carboxyl groups give the rhamnolipid molecules their anionic character, which is strongly affected by the ionic strength of the solution. The majority of the carboxyl groups of rhamnolipids are dissociated to form carboxylate groups in the absence of an electrolyte. With the addition of an electrolyte, the ionic strength of the medium increases, and the carboxylate groups are shielded by a diffuse layer of counterions.28 This, in turn, increases the head diameter of the adsorbed molecules, forming a monolayer at the bubble surface and resulting in a decreasing adsorption density of molecules. Thus, fewer Cd2+ ions can be adsorbed and carried out of the solution by the bubbles. In alkaline solutions, formation of metal hydrate can occur, instead of complexation with rhamnolipid, making it difficult for the adsorption process to continue. 3.5. Effect of Aeration Rate. In a process driven by the presence of a rising stream of bubbles, such as flotation, the interfacial area of the bubbles present in the gas−liquid dispersion is important for process effectiveness. This interfacial area, in turn, is related to the amount of gas being held up in the dispersion and is regulated by its volumetric flow rate. Therefore, given the physicochemical conditions, one of the potentially important parameters in dispersed-air flotation is the aeration rate. Figure 7 shows the effect of aeration rate on the Cd removal. As the air flow rate increases, more bubbles pass through the solution, removing more ion−surfactant clusters. At higher aeration rates, the water removal by hydraulic entertainment is also increased because of the enhanced bubble distribution in the column, and this leads to a drop in flotation efficiency. Another possible reason for this recovery loss is the turbulence induced by the stream of bubbles rising through the column: The circulation velocity induced by the bubble swarm rising through the column increases the turbulence at the froth/ collection zone interface, and some cadmium ions and water

Figure 5. Effect of solution pH on cadmium removal efficiency.

which shows a nonlinear trend in removal efficiency upon pH variation, with a maximum value at pH 6. It is well-known that the maximum surface activity of rhamnolipids is achieved at pH values of around 6.5−7.24−27 As the pH decreases, the critical micelle concentration of the rhamnolipid molecules decreases, and this negatively influences the surface activity of the rhamnolipid biosurfactant. The effect of pH variations below neutral pH on the molecular morphology of rhamnolipid aggregates is shown in Figure 6. The surface activity of 3913



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3.7. Selective Adsorption of Ions at Air/Surfactant Solution Interfaces. Selectivity in mixed-metal solutions is also important in ion flotation. For example, the target metal might not be removed from a solution that contains a large excess of other metallic ions that compete with target metal in the electrical attraction for available surfactant binding sites. This problem can be overcome by selective complexation between a specific surfactant and the target metal. The complex itself could then function as a surfactant. It could also provide a clue to the metal’s recovery.18 The selectivity for ion flotation was observed in this work, and the selectivity coefficients of cadmium over zinc, cadmium over copper, and zinc over copper in the presence of rhamnolipid biosurfactant were measured. Concentrations and removals of cadmium and zinc in a competitive system are presented in Figure 9a. It can be

Figure 7. Effect of aeration rate on cadmium removal efficiency.

are entrained back into solution. Thus, a limit is probably reached at some aeration rate where these two rates, that of attachment and removal and that of drainage and backentrainment, become equal so that no further effect of air flow rate can be noticed.29,30 3.6. Effect of Frother Type and Concentration. Rhamnolipid biosurfactant has a significant frothability [its dynamic frothability index (DFI) is ∼600000 s·dm3/mol], thus increasing water recovery.17,31 This enhanced water recovery can decrease the efficiency of ion flotation. Therefore, it would be possible to improve the removal efficiency if the frothability of rhamnolipid were moderated by applying a suitable frother. The effects on cadmium removal of the two frothers ethanol and methyl isobutyl carbinol (MIBC), commonly used in foam separation practices, were also studied. The results are shown in Figure 8. As can be seen, frother addition was found to increase

Figure 9. Ion flotation of Cd and Zn in a competitive system: (a) concentrations and removals as functions of flotation time and (b) selectivity coefficient determination. Figure 8. Effect of frother type and concentration on cadmium removal efficiency.

observed that Cd recovery was more effective than Zn recovery. The highest recovery of Cd was about 36%, and the residual Cd2+ concentration decreased to 0.028 mM. The highest recovery of Zn was about 29%, and the residual Zn2+ concentration reached 0.031 mM. Figure 9b presents a plot of ln [Cd2+] versus ln [Zn2+], where the square brackets represent concentrations (see eq 2). From this plot, the selectivity coefficient of Cd over Zn, SCd Zn , was determined to be 1.61, with a coefficient of determination, R2, of 0.9854. Figure 10a shows the results for the ion flotation of cadmium and copper from a solution containing an equimolar mixture of the two metal ions. It can be seen that Cd was removed more efficiently than Cu. In addition, it was observed that Cd2+ was removed much more rapidly than Cu2+ and eventually reached a lower concentration. The maximum Cd removal was about

the removal efficiency, to a point, because of the controlled foaming characteristics of the solution. After that, the removal decreased upon further frother addition because of increasing solution frothability and, consequently, increasing water recovery. Figure 8 also shows that MIBC improved the Cd removal more efficiently at much lower concentration than ethanol. The dynamic frothability index (DFI) is a reliable measure for describing the frothing power of a frother. The DFI values for MIBC and ethanol are about 34000 and 1000 s·dm3/ mol, respectively.32 These values indicate that MIBC is more powerful than ethanol in improving the frothability of a solution at lower dosages. This is important when the cost of the process is taken into account. 3914



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Figure 11. Ion flotation of Zn and Cu in a competitive system: (a) concentrations and removals as functions of flotation time and (b) selectivity coefficient determination.

Figure 10. Ion flotation of Cd and Cu in a competitive system: (a) concentrations and removals as functions of flotation time and (b) selectivity coefficient determination.

It has been shown that, when the ratio of the ionic radii of two competing covalent ions exceeds 1, the corresponding experimental selectivity coefficient is also higher than 1. Then, one can conclude that, in ion flotation, ions with larger crystalline radii have a preference for reacting with the surfactant and being removed.1,5,6,18,33,34 The ratios of the ionic radii for the competitive systems investigated in this study are 1.28, 1.32, and 1.03 for Cd/Zn, Cd/Cu, and Zn/Cu, respectively. These data are in accord with the corresponding Cd selectivity coefficient values, namely, SCd Zn = 1.61, SCu = 3.05, and Zn SCu = 1.68. Therefore, it can be concluded that the order of selectivity of these metals in ion flotation in the presence of rhamnolipid is Cd2+ > Zn2+ > Cu2+. 3.8. Kinetic Studies. Flotation kinetics indicates the variation of the floated concentration with flotation time. Studies of flotation kinetics are useful in elucidating the mechanism of the process and serve as predictive tools in the implementation of flotation technology. Chemical kinetics principles have been used in the modeling of flotation processes, particularly in the formulation of the basic rate equation.35,36 Studies of the kinetics of ion flotation have been carried out by different investigators, and the process has been shown to follow the first-order equation36−39 c − cr c − cr k=− i ln c it c i − cr (3)

48%, and the remaining Cd2+ concentration in the solution reached about 0.022 mM. The maximum recovery of Cu was 18.6%, and the remaining Cu2+ concentration reached 0.036 mM. Figure 10b shows a plot of ln [Cd2+] versus ln [Cu2+] for the experimental value of the selectivity coefficient. As can be seen, the selectivity coefficient of Cd over Cu, SCd Cu, was determined to be 3.05, with R2 = 0.9593. Figure 11a shows experimental data for ion flotation in a zinc−copper system. It can be seen that the removal of Zn was better than the removal of Cu. The maximum floatability of Zn was about 29%, and the residual Zn2+ concentration decreased to about 0.03 mM. The maximum removal of Cu was about 19%, and the residual concentration of Cu2+ reached about 0.04 mM. Using the data plotted in Figure 11b, the selectivity coefficient of Zn over Cu, SZn Cu, was determined to be 1.677, with R2 = 0.9672. The flotation process can be explained by two mechanisms: adsorption and coagulation. In adsorption, the collectors gather at the gas−liquid interface and then form complexes with metal ions through electrostatic adsorption or association. In coagulation, complexes form with the metal ions in the liquid at first and then assemble into bubbles. Therefore, two viewpoints on the mechanism of selectivity of ions are accepted. First, it is assumed that the ions in solution undergo exchange reactions until equilibrium is reached, and then the surfactant molecules with their metal ions adsorb onto the rising bubbles. Second, it is assumed that the ion-exchange process occurs after the surfactants and metal ions have adsorbed onto the rising gas bubbles.18

where k is the kinetic rate constant; t is the flotation time; and ci, cr, and c are the initial, residual, and time-dependent ion concentrations in solution, respectively. The rate constant, k, can be estimated by a correct data fitting of c values against 3915



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flotation time. The flotation rate constants for the metals investigated in this work were found to follow the order kCd (0.0071 min−1) > kZn (0.0044 min−1) > kCu (0.0028 min−1). From Figures 9−11, it can be observed that the order of the kinetic constants of these three metals corresponds to their selectivity order. This result can be explained as follows: The ion with higher selectivity interacts first with the rhamnolipid collector and is thus carried out to the froth zone faster than the other metal−rhamnolipid clusters.

(9) Vilinska, A.; Rao, K. H.; Forssberg, K. S. E. Microorganisms in Flotation and Flocculation of MineralsAn Overview. Presented at the 24th International Mineral Processing Congress, Beijing, China, Sep 24−28, 2008. (10) Abbasi, H.; Hamedi, M. M.; Lotfabad, T. B.; Zahiri, H. S.; Sharafi, H.; Masoomi, F.; Movahedi, A. A. M.; Ortiz, A.; Amanlou, M.; Noghabi, K. A. Biosurfactant-Producing Bacterium, Pseudomonas aeruginosa MA01 Isolated from Spoiled Apples: Physicochemical and Structural Characteristics of Isolated Biosurfactant. J. Biosci. Bioeng. 2012, 113, 211. (11) Heyd, M. Continuous Production of Rhamnolipids by Means of Process Integration. Ph.D. Dissertation, Fridericiana Karlsruhe University, Karlsruhe, Germany, 2009. (12) Mulligan, C. N.; Yong, R. N.; Gibbs, B. F. Removal of Heavy Metals from Contaminated Soil and Sediments Using the Biosurfactant Surfactin. J. Soil Contam. 1999, 8, 231. (13) Mulligan, C. N.; Yong, R. N.; Gibbs, B. F. Surfactant-Enhanced Evaluation of Contaminated Soil: A Review. Eng. Geol. 2001, 60, 371. (14) Mulligan, C. N.; Wang, S. L. Remediation of a Heavy MetalContaminated Soil by a Rhamnolipid Foam. Eng. Geol. 2006, 85, 75. (15) Hong, K. J.; Tokunaga, S.; Kajiuchi, T. Evaluation of Remediation Process with Plant-Derived Biosurfactant for Recovery of Heavy Metals from Contaminated Soils. Chemosphere 2002, 49, 379. (16) Dahrazma, B.; Mulligan, C. N. Investigation of the Removal of Heavy Metals from Sediments Using Rhamnolipid in a Continuous Flow Configuration. Chemosphere 2007, 69, 705. (17) Zouboulis, A. I.; Matis, K. A.; Lazaridis, N. K.; Golyshin, P. N. The Use of Biosurfactants in Flotation: Application for the Removal of Metal Ions. Miner. Eng. 2003, 16, 1231. (18) Yuan, X. Z.; Meng, Y. T.; Zeng, G. M.; Fang, Y. Y.; Shi, J. G. Evaluation of Tea-Derived Biosurfactant on Removing Heavy Metal Ions from Dilute Wastewater by Ion Flotation. Colloids Surf. A: Physicochem. Eng. Aspects 2008, 317, 256. (19) Kitamoto, D.; Akiba, S.; Hioki, C.; Tabuchi, T. Extracellular Accumulation of Mannosylerythritol Lipids by a Strain of Candida antarctica. Agric. Biol. Chem. 1990, 54, 31. (20) Youssef, N. H.; Duncan, K. E.; Nagle, D. P.; Savage, K. N.; Knapp, R. M.; McInerney, M. J. Comparison of Methods to Detect Biosurfactant Production by Diverse Microorganisms. J. Microbiol. Methods 2004, 56, 339. (21) Wei, Y. H.; Chou, C. L.; Chang, J. S. Rhamnolipid Production by Indigenous Pseudomonas aeruginosa J4 Originating From Petrochemical Wastewater. Biochem. Eng. J. 2005, 27, 146. (22) Khoshdast, H.; Abbasi, H.; Sam, A.; Akbari Noghabi, K. Frothability and Surface Behavior of a Rhamnolipid Biosurfactant Produced by Pseudomonas aeruginosa MA01. Biochem. Eng. J. 2012, 64, 127. (23) Sánchez, M.; Aranda, F. J.; Espuny, M. J.; Marqués, A.; Teruel, J. A.; Manresa, A.; Ortiz, A. Aggregation Behaviour of a Dirhamnolipid Biosurfactant Secreted by Pseudomonas aeruginosa in Aqueous Media. J. Colloid Interface Sci. 2007, 307, 246. (24) Ishigami, Y.; Gama, Y.; Nagahora, H.; Yamaguchi, M.; Nakahara, H.; Kamata, T. The pH Sensitive Conversion of Molecular Aggregates of Rhamnolipid Biosurfactant. Chem. Lett. 1987, 275, 763. (25) Champion, J. T.; Gilkey, J. C.; Lamparski, H.; Retterer, J.; Miller, R. M. Electron Microscopy of Rhamnolipid (Biosurfactant) Morphology: Effects of pH, Cadmium and Octane. J. Colloid Interface Sci. 1995, 170, 569. (26) Ö zdemir, G.; Peker, S.; Helvaci, S. S. Effect of pH on the Surface and Interfacial Behavior of Rhamnolipids R1 and R2. Colloids Surf. A: Physicochem. Eng. Aspects 2004, 234, 135. (27) Benincasa, M.; Marqués, A.; Pinazo, A.; Manresa, A. Rhamnolipid Surfactants: Alternative Substrates, New Strategies. In Biosurfactants; Sen, R., Ed.; Springer: New York, 2010; Vol. 672, pp 170−184. (28) Helvaci, S. S.; Peker, S.; Ö zdemir, G. Effect of Electrolytes on the Surface Behavior of Rhamnolipids R1 and R2. Colloids Surf. B: Biointerfaces 2004, 35, 225.

4. CONCLUSIONS The potential of rhamnolipid for removing cadmium, zinc, and copper from wastewater was investigated. The effects of operating parameters including rhamnolipid structure and concentration, initial Cd concentration, frother type and concentration, initial solution pH, and removal selectivity were examined and shown to have a significant influence on the performance of ion flotation. This investigation also showed that the flotation separation of cadmium from zinc and copper could be performed in competitive systems. The order of selectivity for the studied metal ions was found to be Cd2+ > Zn2+ > Cu2+. The flotation rate constants for these metals were found to be in the order kCd (0.0071 min−1) > kZn (0.0044 min−1) > kCu (0.0028 min−1). These results indicate that biosurfactants such as rhamnolipid could play an important role in ion flotation and yield good environmental benefits.

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Corresponding Author

*E-mail: [email protected]. Tel.: 0098-44580352. Fax: 009844580386. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was technically supported by the National Institute of Genetic Engineering and Biotechnology (NIGEB), Tehran, Iran. Funding for this project was provided by the Iran National Science Foundation (INSF) (GNo:9000339) to K.A.N.

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REFERENCES

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Removal of Cadmium (II) from Aqueous Solution by Ion Flotation

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