Journal of Hazardous Materials 286 (2015) 466–473

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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Removal of trace Cd2+ from aqueous solution by foam fractionation Jian Lu, Ying Li ∗ , Sen Zhang, Yange Sun Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, South Road of ShanDa, Jinan, Shandong 250100, PR China

h i g h l i g h t s • • • •

Foam properties of a kind of novel anionic–nonionic surfactant AEC were studied. AEC foam was used to remove Cd2+ from diluted solution. The Cd2+ removal rate of AEC foam could be 99.8% under optimum conditions. The Zeta potential and ITC were utilized to study the mechanism.

a r t i c l e

i n f o

Article history: Received 22 September 2014 Received in revised form 3 December 2014 Accepted 10 January 2015 Available online 13 January 2015 Keywords: Foam fractionation Heavy metal ions Removal rate Surfactant

a b s t r a c t In recent years, aqueous foam was known as an efficient technique with high potential on being used to remove heavy metal ions from the polluted water, not only because of the low cost, simple operation, but also ascribed to the high removal efficiency of trace heavy metal ions and would not cause secondary pollution to the environment. In this paper, the removal of Cd2+ from aqueous solution by aqueous foam stabilized by a kind of novel anionic–nonionic surfactant sodium trideceth-4 carboxylate (AEC) was investigated. The effect of conditions such as surfactant/metal ions molar ratio, surfactant concentration on the removal efficiency was studied. In large concentration range of surfactant, the removal rate was higher than 90%, and could reach up to 99.8% under the optimum conditions. The Zeta potential of gas bubbles in the AEC solutions was determined to verify the combination between the negative charged group heads of surfactant molecules and heavy metal ions, and isothermal titration calorimeter (ITC) determination was utilized to demonstrate the interaction, which helped to understand the mechanisms more clearly. © 2015 Published by Elsevier B.V.

1. Introduction With the rapid development of industries, such as metal plating facilities, mining operations, batteries, and paper industries etc., heavy metal pollution is nowadays getting more and more seriously, because heavy metal ions are directly or indirectly discharged into the environment in the production and application process. Unlike organic contaminants, heavy metal ions are not biodegradable and will be accumulated in living organisms too easily [1]. Being worse, many heavy metal ions are known to be toxic or carcinogenic. So heavy metal pollution has generated a profound impact on the human health and environmental security. Along with the increasing emergency for setting up of stringent state laws and regulations, it is extremely urgent to develop effi-

∗ Corresponding author at: School of Chemistry and Chemical Engineering, Shandong University, 27 Shanda Nanlu, Jinan, Shandong, 250100, PR China. Tel.: +86 531 88362078; fax: +86 531 88364464. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jhazmat.2015.01.029 0304-3894/© 2015 Published by Elsevier B.V.

cient techniques to treat the heavy metal pollution in water and the soil. Cadmium has been classified by U.S. Environmental Protection Agency as a probable human carcinogen. Exposure levels of 30 ∼ 50 mg Cadmium per day have been estimated to severe risk for human health of adults, corresponding to increasing risk of bone fracture, cancer, kidney dysfunction and hypertension [2]. Some physicochemical methods for cadmium removal from water have been motivated, such as precipitation, ion exchange, chemical oxidation and reduction, filtration, and electrochemical treatment [3,4]. The application of the above methods is limited, because of the restricted conditions, high cost, or complex operation, and the removal of trace metal ions from water is extremely not easy to be achieved. Adsorptive bubble separation technique for removing a wide variety of substances from wastewater is becoming increasingly dramatic [5], one of which is foam fractionation [6]. In foam fractionation process, gas is introduced into the system to generate bubbles, and surface active solutes adsorb preferentially at the bubble–liquid interface. Foam fractionation process offers many

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Fig. 1. Structure of the surfactant sodium trideceth-4 carboxylate (AEC). Sodium and oxygen were connected by ionic bond. All the remaining ions were connected by covalent bond.

advantages for the treatment of industrial wastewater compared to other techniques, including low space and energy requirements, simple plant design and operation, easy scale-up and the low operating costs [7,8]. So far, using foam fractionation to remove heavy metal ions from aqueous solution has been reported [9–12]. The commonly used surfactants in the literature were sodium dodecyl sulfate (SDS) and biosurfactants, such as glycolipid, but the efficiency was still not satisfied, and the mechanism was not very clear. In this study, a novel anionic–nonionic surfactant sodium trideceth-4 carboxylate (AEC) was used to generate foam to remove Cd2+ from aqueous solution. The effect of separation condition such as surfactant/metal ions molar ratio, surfactant concentration on the removal efficiency was studied. It was found that the removal rate was higher than 90% in large concentration range of surfactants, and could reach up to 99.8% under the optimum conditions. ITC and Zeta potential measurement were utilized to verify the combination between the anionic head groups of surfactant and heavy meal ions, which revealed that the high Cd2+ removal efficiency of AEC foam comes from the strong combination interaction between the negatively charged head group of AEC and the Cd2+ , shown in Fig. 15. The combined Cd2+ was bounded to the surfactant interface layer and was carried by the foam film, thereby being extracted from the bulk solution. The amphiphilic character of AEC molecules actuate the interfacial adsorption tendency and the foamability, while the special nonionic–anionic bipolar head structure determines the salt-tolerance and the carrying capacity of AEC foam for cations at the same time. 2. Methods and materials 2.1. Reagents Cd(NO3 )2 ·4H2 O (A.R.), obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Sodium trideceth-4 carboxylate (AEC), 99% purity, was obtained from NIKKOL GROUP Co. The chemical structure of AEC was represented in Fig. 1. The critical micelle concentration (CMC) of AEC was measured using the conductivity method, which was about 2.9 mmol/L. Deionized water was used to prepare the solutions in all the experiments (Fig. 2). 2.2. Experimental methods The concentration of cadmium ion in aqueous solution was determined by graphite furnace atomic adsorption spectrometry (GFAAS) using ICE3400 (Thermo Fisher Scientific Inc., USA). Test wavelength was 228 nm, slit width was 0.7 nm, electric current was 5 mA. The data was processed with SOLAAR software.

Fig. 2. Sample cell used in Zeta potential determination, Type GT-2, K factor was 67, T = 298 K,Voltage = 200 V, full scale.

Zeta-Meter System 4.0 (Ankersmid Ltd., Netherlands) was used to measure the Zeta potential of the colloids and bubbles in the solutions. The prepared solution was injected into the cell tube (Fig. 1) at first by a needle, then a capillary injection device was used to produce several separated bubbles in the cell tube, and the Zeta potential of the bubbles was determined. The Molybdenum Cylinder Anode (+) was screwed into the left chamber (with the serial number facing you) and the Platinum Rod Cathode (−) was screwed into the right chamber. The sensor was inserted into the solution to measure the temperature and conductivity. The parameters such as volts, scale, temperature, electrodes, K factor and mode (EM or ZP) were adjusted to make the bubbles move in a reasonable rate. For example, if the bubbles moved too slowly, we could change the scale from full to 1/8 or improve the voltage; if the foam moved too fast, we could change the scale from 1/8 to full or reduce the voltage. For determination in this paper, parameters were set as that the type was GT-2, K factor was 67, temperature was 298 K, voltage was 200 V, scale was full, mode was ZP, electrodes were Molybdenum cylinder anode and platinum rod cathode. Isothermal titration calorimetry (ITC) (MicroCalTM iTC200 , USA) was used to verify the interaction between the hydrophilic groups of surfactant and heavy meal ions. The cadmium nitrate solutions were titrated into the AEC solution in multiple injections manner. Parameters were set as follows: total injection volume was 20 ml, temperature was 298.0 ± 0.1 K, reference power was 5 ␮cal/s, initial delay was 60 s, syringe concentration was 0.08%wt, cell concentration was 7.5 mmol/L, stirring speed was 1000 RPM, feedback mode was high, volume was 2 ␮l, duration was 4 s, spacing was 150 s, filter period was 5 s. The foam properties, such as foamabiliy, foam stability, drainage, bubble size distribution etc., were characterized through Foamscan (Teclis Co., France). The change of the state of bubbles in the foam was observed by a CCD (Charge-coupled Device) camera which photographed every 2 s after N2 flow was stopped putting through. CCD camera was only used to obtain the pictures of foam. The pictures were analyzed with CSA (Cell Size Analysis) software, which gave out the size and distribution of the bubbles. In the measurements of this paper, the foams were generated by blowing nitrogen at the required flow rate 200 ml/min through a porous glass filter at the bottom of a glass tube in which 60 ml solution was previously put. The variation of the liquid content of the foam was measured by five pairs of electrodes located along the glass column, labeled as the first, the second, the third, the forth, and the fifth pair of electrode from bottom to the top, respectively. All of the electrodes were made from stainless steel materials, except for measuring the liquid content of the foam, they were also used to record the foam volume in real time. In all the experiments, when the foam volume reached 200 ml, the input of nitrogen gas was stopped and the evolution of the foam was analyzed [13]. Schematic diagram of a Foamscan instrument was showed in Fig. 3 [14]. Fourier transform infrared (FT-IR) measurements of the aqueous solutions were performed by Thermo Scientific Nicolet iS5 with iD5 ATR accessory (ZnSe, 45◦ ) with the attenuated total reflectance (ATR) method [15]. Unlike the case of the KBr pellet method, this method did not require sample preparation, a few drops of solution were placed directly on the ZnSe ATR crystal plate. The diameter of ZnSe crystal plate was about 2 mm. Based on the refractive index of ZnSe, the average penetration depth for a single reflection was 2.0 microns at 1000 cm−1 . ATR-FTIR spectra was recorded

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Fig. 3. Schematic diagram of the Foamscan used in the paper to determine the foam properties.

Fig. 4. The graphical schematic representation of ATR accessory in FT-IR used in the paper. ATR crystal was ZnSe material, incident angle was 45◦ , active sample area was 1.5 mm in single bounce.

between 600 ∼ 4000 cm−1 , at an optical resolution of 4 cm−1 [16]. An average of 16 scans was collected for each assay. Air was used as background before each scan. Baseline correction and spectra smoothening were performed by using Nicolet Omnic software. The schematic diagram of ATR accessory was shown in Fig. 4 [17]. A PHS-3C pH meter (Shanghai Precision & Scientific Instrument Co., Ltd., China) was used to measure the pH of solution. Temperature of all the above measurement was set to be 298.0 ± 0.1 K. 2.3. Foam fractionation procedure The device shown in Fig. 5 was designed and set up in our lab. The bubble column was formed in the cylindrical glass tube by blowing gas through a silica sand core located at the bottom of the col-

umn, and continued to flow through a “U” tube connected with the cylindrical tube by a ground joint union, by which the foam column could be removed and be analyzed at any time as required. Nitrogen was used as gas agent, the gas flow was kept blowing into the column till no more foam could be formed. A float valve type gas flow meter was used to control the flow rate of gas. In this paper, the flow rate of gas was fixed at 200 ml/min according to the Ref. [18]. The temperature was fixed at 298.0 ± 0.1 K. The removal efficiency of cadmium ion was calculated at the end of each experiment. The concentration of Cd2+ in the solutions before and after the foam fraction was determined by GFAAS. The removal efficiency of trace Cd2+ was evaluated using the removal rate (R), which was defined according to Eq. (1): R=

mi − me × 100% mi

(1)

mi and me were the mass of cadmium ion (g) in the initial and residual solutions, respectively, translated from the concentration of Cd2+ and the solution volume [19]. 3. Results and discussions 3.1. Foam properties of AEC solution with and without Cd2+ Foam properties of AEC solution with and without Cd2+ , such as foamability, foam stability etc., were determined, the results were shown as below. The concentration of AEC solution was 7.5 mmol/L. The concentration of Cd2+ in AEC solution was 270 ppm.

Fig. 5. The schematic diagram of device used to proceed foam fractionation in the paper.

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Fig. 6. Foam volume as a function of time determined by foamscan. (), the volume of foam generated by 7.5 mmol/L AEC solution, ( ) the volume of foam generated by 7.5 mmol/L AEC solution containing 270 ppm Cd2+ .

3.1.1. Foamability and foam stability As shown in Fig. 6, the times needed generating 200 ml certain volume of foam from 60 ml solution were 76 s and 75 s for AEC solution and AEC solution containing cadmium nitrate, respectively, which were very similar. And the decay curves of the foams formed from AEC solution and AEC solution containing cadmium nitrate were very similar, too. When the decay time was 600 s, foam

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Fig. 7. Variation of the liquid volume in foam as a function of time determined by foamscan. (), the liquid volume in foam generated by 7.5 mmol/L AEC solution; ( ), the liquid volume in foam generated by 7.5 mmol/L AEC solution containing 270 ppm Cd2+ .

volumes of the foam formed from AEC solution and AEC solution containing cadmium nitrate were 153.9 ml and 152.4 ml, respectively. So there were no significant differences on the foamability and foam stability for foams formed from AEC solution with and without Cd2+ , which was very precious because the salt-tolerance of most of the common anionic surfactants were poor and could not maintain their foamability with multivalent cations existing.

Fig. 8. Variation of the liquid content of foam generated by 7.5 mmol/L AEC, solution without Cd2+ () and with 270 ppm Cd2+ ( ) as a function of time detected by the different electrodes. (a), detected by the first pair of electrode; (b), detected by the second pair of electrode; (c), detected by the third pair of electrode; (d), detected by the forth pair of electrode.

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Table 1 The surface tension of AEC solution with and without Cd2+ . Solution

Surface tension/mN m−1

Water 7.5 mmol/L AEC 2+ 7.5 mmol/L AEC + 291 ppm Cd

72.10 32.11 27.05

3.1.2. Liquid carrying capacity of the foams As seen from Fig. 7, the liquid contents of the foams formed from AEC solutions with or without Cd2+ were both high, which meant that their liquid carrying capacities of the foam film were very good comparing to the common surfactant systems, which favored the foam stabilization and might also benefit the removing of Cd2+ from the solution. The drainage of the foam was determined by the electrode sensors arranged in the class tube, as shown in Figs. 7–8. Fig. 7 showed the liquid fraction in foam as a function of time, and Fig. 8 showed the liquid fraction in foam at each electrode as a function of time. In Fig. 8, from a to d, the percent of liquid fraction in foam decreased gradually. The less percent of liquid fraction in foam was, more dry the foam would be. According to the Figures, the liquid fraction of foam film formed from AEC solution containing Cd2+ was lower than that without Cd2+ , and the difference increased from bottom to top, which illustrated that the foam containing Cd2+ drained liquid out more quickly, the drainage curve be almost a straight line as showed in Fig. 8c and d. If the Cd2+ was combined with the negative groups of AEC, the charge of the surfactant head groups should be shielded, the interface potential of the foam bubble would decrease, and the decrease of the electric repulsion between the two interfacial layers of the foam film would lead to a larger loss of liquid in the foam. In this case, the foam films containing Cd2+ would undoubtly get thinner to a certain degree comparing with that without Cd2+ . Fig. 9. Bubble size distribution of the foam generated by solution at 100 s after

3.1.3. Bubble size statistic analysis According to the images of the foam column, the bubble size distribution at different time was analyzed. The smaller the bubble size was, the better the foam stability should get. Fig. 9 showed the results got at 100 s after foam started to generate and they were fitted with a Gaussian method. Two parameters, the most probable diameter of bubbles Xc and the half-peak width Whalf , were used to quantitively describe the bubble size distribution. The Xc of the foam formed from AEC solution and AEC solution containing Cd2+ at 100 s was 0.028 and 0.030 mm, respectively. While the Whalf was 0.025 and 0.028 mm, respectively. By comparing these two parameters, we could draw the conclusion that the bubble size of the foam formed from the AEC solution containing Cd2+ was larger than that without Cd2+ . On one hand, corresponding to the results in Fig. 9, when the Cd2+ combined to the carboxylate group of AEC, the thickness of the foam film decreased, the penetration of the gas molecules through the foam film would be strengthen, which would result in the enhance of the Ostwald effect and the increase of average bubble size. On the other hand, the combination of Cd2+ to the carboxylate group of AEC would also induce the increase of the adsorption amount of AEC at gas/water interfaces, the interfacial layer of surfactants in the foam film could be more compacted, and the penetration of gas molecules could be retardant, so the coalescence of the bubbles in the foam with Cd2+ existing was not severe, the foam stability was maintained nicely. The surface tension of AEC solution with and without Cd2+ was shown in Table 1, the surface tension decreased when there was Cd2+ in the solution, which agreed very well with the above discussion.

solution began to generate foam, ( ) represented the relative frequency distribution as a function of the bubble size, ( ) represented the Gauss fit of relative frequency to the bubble size. (a), 7.5 mmol/L AEC solution; (b) 7.5 mmol/L 2+ AEC solution containing 270 ppm Cd .

3.2. Effect of several conditions on the removal efficiency of Cd2+ 3.2.1. Effect of pH When the pH of AEC solution was adjusted to be 7 or less, the solubility of the surfactant decreased, and the foaming capability was poor. When the pH was higher than 8, the solution containing Cd2+ became immediately turbid because of precipitation. So the pH of all AEC solutions remained at pH 7.5 in the study. 3.2.2. Effect of surfactant concentration When the concentration of AEC in solution was below the CMC (2.9 mmol/L), the foaming capability of the surfactant solution was not good, which was found to go against achieving high removal efficiency. So the concentration of AEC in the used solutions was all above 3.0 mmol/L in this study. The foam fraction was processed using the device shown in Fig. 5. Nitrogen was used as gas agent, the gas flow was kept blowing into the column till no more foam could be formed. As the AEC solution with different concentration being used, the time of the whole process was slightly different with each other. According to Fig. 10, the removal efficiency of trace Cd2+ with AEC foam got 94.2% when the AEC concentration was equal to 3.0 mmol/L, and the removal efficiency increased with the increase of the surfactant concentration. When AEC concentration was above 7.5 mmol/L, the

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Fig. 10. Effect of surfactant concentration on the removal efficiency of trace Cd2+ . Concentration of Cd2+ was consistent in 84 ppm, pH 7.5, T = 298 K, gas flow rate was 200 mL/min. The removal efficiency of trace Cd2+ with AEC foam got 94.2% when the AEC concentration was equal to 3.0 mmol/L, and the removal efficiency increased with the increase of the surfactant concentration.

removal rate got to 99%, which was regarded as a very ideal removal efficiency. When the surfactant concentration got higher than 7.5 mmol/L, the removal efficiency decreased a little. Considering the cost and the property, it was obviously that the concentration of AEC was not the larger the better. 3.2.3. Effect of the molar ratio of surfactant and metal ions The effect of the molar ratio of surfactant and metal ions on the removal efficiency of trace Cd2+ in aqueous solution was tested, the results were shown in Fig. 11. As seen from Fig. 11, the removal rate increased with the increase of the molar ratio of surfactant/metal ions from 2 to 10. When the molar ratio of surfactant and metal ions was 10, the removal rate could reach 99.8%. And when the molar ratio of surfactant and metal ions was 12, the removal rate decreased slightly, but it was still above 96%. When the molar ratio of surfactant and metal ions was below 2, the solution was turbid because of precipitation, and the foamability and the foam stability were poor. According to the above results, the molar ratio of surfactant and metal ion should be between 5 ∼ 10, the removal

Fig. 11. Variation of the removal efficiency of trace Cd2+ by AEC foam as a function of molar ratio of surfactant and metal ion. Surfactant concentration was consistent in 7.5 mmol/L, pH 7.5, T = 298 K, gas flow rate was 200 mL/min. The removal rate increased with the increase of the molar ratio of surfactant/metal ions from 2 to 10, having more and more binding sites of surfactant to combine with Cd2+ , result in the higher and higher removal rate.

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Fig. 12. Zeta potential of the foam bubbles formed in 7.5 mmol/L AEC solution as a function of the amount of the added cadmium nitrate into the AEC solution. Abscissa represented the amount of the added cadmium nitrate into the AEC solution, its unit was gram, short for “g”. The larger the absolute value of Zeta potential, the better the stability of the colloid in foam bubbles.

efficiency increased when the molar ratio of surfactant and metal ions got larger in this range. 3.3. Mechanism of removal of metal ions by foam fractionation 3.3.1. Zeta potential of foam bubbles The Zeta potential of foam bubbles generated in AEC solution containing Cd2+ or not was determined. Cadmium nitrate was continually added into the AEC solution, the variations of the Zeta potential of the foam bubbles as the function of the amount of the added salt were represented in Fig. 12. Bubbles formed in AEC solu-

Fig. 13. The results of isothermal titration calorimetry (ITC) measurement when cadmium nitrate solution was added to the AEC solution. Concentration of cadmium nitrate was 0.08% wt, concentration of AEC solution was 7.5 mmol/L.

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tion without Cd2+ were with a considerable negative charge, while with the increase of the concentration of Cd2+ , the value of Zeta potential of the bubbles decreased. It meant that the Cd2+ got to combine with the hydrophilic group of surfactants at the interface of bubble, and the charge of the bubble was neutralized gradually. 3.3.2. ITC measurement With isothermal titration calorimeter (ITC), the energetics associated with chemical reactions or physical variations occurring at constant temperature could be determined via heat effects. Thermodynamic analysis of the observed heat effects then permitted quantitative characterization of the variation processes associated with the binding interaction [20]. As seen from Fig. 13, enthalpy H = 2866 cal mol−1 , which demonstrated the electrostatic binding of negatively charged head group of AEC surfactant molecules to the Cd2+ . This reaction was conducted spontaneously at room temperature, which was an entropy-driven reaction, S = 97.7 cal mol K−1 . The possible reason was that as the hydrophilic head group combined to Cd2+ , the hydrophilicity of the head groups of AEC decreased, resulting in an increase of the disorder degree of the hydrophobic chains and the increase of entropy of the system. 3.3.3. FT-IR spectra The FT-IR(ATR) spectra of the AEC solution with or without Cd2+ in the 900–2000 cm−1 region was shown in Fig. 14 It could be

Fig. 14. ATR FT-IR spectra in the 900–2000 cm−1 region, transmittance as a function represents the complexation of wave number, represents the AEC solution, of AEC solution and cadmium nitrate.

found that the stretching vibration peak of carboxylic group moved from 1600 cm−1 to 1585 cm−1 with Cd2+ existing in the solution, which derived from the complex interaction of the cations with the COO− groups, the electron of the carboxyl group cloud transferred

Fig. 15. The schematic mechanism of foam fractionation, Red represents heavy metal ions Cd2+ ; Blue represents the surfactant AEC, in which curve represents the hydrophobic chain, circle represents hydrophilic group (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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to the unoccupied orbital of the heavy metal ions, which caused the red shift of the infrared peak [21,22]. The value of red shift was 15 cm−1 . This phenomenon illustrated the strong interaction between the AEC and Cd2+ . According to the above discussion, the mechanism of removal of Cd2+ from aqueous solution by foam fractionation was clarified and was shown in Fig. 15. The hydrophilic head groups of AEC could be combined with heavy metal ions by electrical interaction. When gas was injected into the solution, foam bubble was generated, the surfactant molecules adsorbed at the gas/water interface, the combined Cd2+ got into the interface layer, and was carried by the foam film, being extracted from the bulk solution. Thereby an ideal removal result of the foam fractionation was achieved [23]. 4. Conclusions The removal of trace Cd2+ from aqueous solution by foam fractionation was investigated in this study. A novel anionic–nonionic surfactant AEC was used as the foam stabilizer. The effects of surfactant concentration, pH, molar ratio of surfactant and metal ions on the removal rate were determined. It was found that under the optimum conditions, when the molar ratio of surfactant and metal ions was about 10, the surfactant concentration was 7.5 mmol/L, the removal rate could be 99.8%. The Zeta potential and ITC methods were utilized to verify the combination between the negative charged hydrophilic groups of AEC and heavy metal ions, which revealed that the combined Cd2+ was bounded to the surfactant interface layer and was carried by the foam film, thereby being extracted from the bulk solution. Acknowledgments The funding from National Science Fund of China (Grant 21173134, 21473103) and the National Municipal Science and Technology Project (Grant 2008ZX05011-002) is gratefully acknowledged. References [1] F.L. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manage. 92 (2011) 407–418. [2] S. Satarug, J.R. Baker, S. Urbenjapol, A global perspective on cadmium pollution and toxicity in non-occupationally exposed population, Toxicol. Lett. 137 (2003) 65–83.

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Removal of trace Cd2+ from aqueous solution by foam fractionation.

In recent years, aqueous foam was known as an efficient technique with high potential on being used to remove heavy metal ions from the polluted water...
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