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Enhanced removal of mixed metal ions from aqueous solutions using flotation by colloidal gas aphrons stabilized with sodium alginate
Accepted Manuscript Enhanced removal of mixed metal ions from aqueous solutions using flotation by colloidal gas aphrons stabilized with sodium alginate Aiza Gay Corpuz, Priyabrata Pal, Fawzi Banat, Mohammad Abu Haija PII: DOI: Reference:
S1383-5866(17)34067-4 https://doi.org/10.1016/j.seppur.2018.03.043 SEPPUR 14463
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 December 2017 20 March 2018 20 March 2018
Please cite this article as: A. Gay Corpuz, P. Pal, F. Banat, M. Abu Haija, Enhanced removal of mixed metal ions from aqueous solutions using flotation by colloidal gas aphrons stabilized with sodium alginate, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.03.043
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Enhanced removal of mixed metal ions from aqueous solutions using flotation by colloidal gas aphrons stabilized with sodium alginate Aiza Gay Corpuza, Priyabrata Pala,*, Fawzi Banata,*, Mohammad Abu Haijab a
Department of Chemical Engineering, Khalifa University of Science and Technology, SAN Campus, Abu Dhabi, United Arab Emirates
b
Department of Chemistry, Khalifa University of Science and Technology, SAN Campus, Abu Dhabi, United Arab Emirates *Email of the corresponding authors: [email protected], [email protected]
Abstract Anionic surfactant sodium dodecyl benzenesulfonate (SDBS) was used to prepare colloidal gas aphrons (CGAs). Bio-polymeric sodium alginate was added to enhance the removal efficiencies of mixed metal ions such as lead (Pb2+) and copper (Cu2+) from aqueous solutions via flotation. Stirring speed of 3000 rpm was maintained to produce CGAs containing 500 mL surfactant solutions. The effects of concentration of metal ions, volume of liquid in the flotation column, CGAs loading rate, and pH of solution on the removal of heavy metals were examined. CGAs loading rate of 6.1 cm/min (flow rate 120 cm3/min) to the flotation column containing 2.0 mM of mixed metal ions (0.4 mM of Pb2+ and 1.5 mM of Cu2+ ions) at pH 5.35 gave the maximum removal of 96% for Pb2+ ions and 81% for Cu2+ ions. However, the net amount of copper ions removed was much higher than the lead ions. Addition of calcium chloride to the retentate solution further increased the removal to be 99% for Pb2+ and 92% for Cu2+ by producing alginate gel with metal ions that precipitated out from the solutions. Interestingly, sodium alginate stabilized CGAs improved significantly the removal of metal ions from aqueous solutions by flotation.
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Keywords: Surfactant; colloidal gas aphrons; sodium alginate; flotation; heavy metal ions 1. Introduction The Emirate of Abu Dhabi is located in an arid region with extreme natural water scarcity. It relies mainly on groundwater, desalinated water and treated wastewater. Groundwater and desalinated water account for 60% and 35% respectively of water use in the emirate, while the recycled water contribution is just 5% (Al Madfaei, 2017). Dependence on already scarce groundwater resources and the depletion of its level is a serious issue. While, sea water desalination is challenged by high sea water salinity, elevated sea water temperatures and risks associated with oil spills and marine pollution. Clean water is a necessity for life; thereby disposal of untreated wastes in water is not an option (Grady et al., 2014). Untreated wastewater may impose health risks to individuals, aquatic animals and environment as well. Thus, lot of works focus on ways to dispose wastewater in such a way that no detrimental effects to the population and environment should arise (Bramih et al., 2018; Oller et al., 2011; Dakiky et al., 2002; van Dijk et al., 1997). Organic (organic manure and detergents) and inorganic chemicals (heavy metals, radioactive metals, nonmetals, etc.), normally from industrial sites, that are present in wastewater can be treated physically, chemically, or biologically (De and Mondal, 2012). It was showed that metal contaminated water is a serious threat to living organisms as heavy metals such as lead, copper, cadmium, nickel, cobalt, and zinc tend to accumulate inside the body and are highly toxic even at low concentrations (Gautam et al., 2015). It is very important to minimize the concentration of heavy metals before discharging into receiving waters. One of the wastewater treatment techniques that could address the removal of heavy metal ions using colloidal gas aphrons (CGA) is by flotation method (Rubio et al., 2002). CGAs
2
are defined as 10-100 µm diameter bubbles generated by intense stirring (5000-10000 rpm) of a surfactant solution (Sebba, 1987). These micro foams are not like regular foams as regular foams consist of a gas inner core covered by a single layer of surfactant film (Larmignat et al., 2008). Sebba (1987) have defined the structure of CGA, an inner core surrounded with two thin layers of surfactant film and a third surfactant layer stabilizes the structure. Several properties of CGAs include large interfacial area, relatively high stability, similar flow properties with water, and easy separation from bulk liquid phase (Jauregi et al., 1997; Molaei and Waters, 2015). Aphrons can be produced from anionic, cationic, and non-ionic surfactants and the charge of CGAs retain the same charge as the surfactant solution (Wang et al., 2001; Hashim et al., 2012). These properties make them of interest in several applications such as removal of metals (Grimes, 2002), recovery of proteins (Fuda et al., 2004; Noble et al., 1998; Fuda and Jauregi, 2006; Jarudilokkul et al., 2004), removal of organic acid (Spigno et al., 2010) and recovery of enzymes (O'Connell and Varley, 2001; Zidehsaraei et al., 2009). Moreover, CGAs can be easily pumped and have significant cost effectiveness compared to membrane and chromatographic separations (Molaei and Waters, 2015; Ahmadun et al., 2009). Also, selection of surfactant is essential to facilitate the adhesion of targeted material on CGAs surface (Scamehorn and Harwell, 1991). Sodium dodecyl benzenesulfonate (SDBS) is a widely used anionic surfactant to generate CGAs (Gai et al., 2016; Yangi et al., 2002; Mansur et al, 2004) and can bind easily to positively charged metal ions (Tan et al., 2008). Stability of CGAs is another most concerned parameters and polymer plays a significant role in stabilizing CGAs (Samuel et al., 2012). Water-soluble polymers can aid foam stabilization by increasing bulk viscosity of the film, thus decreasing the film drainage rate (Prud'homme and Khan, 1996). It is well known that sodium alginate is a natural anionic
3
polymer extracted from a brown seaweed. Sodium alginate consists of β-D-mannuronic acid, αL-guluronic acid and free carboxyl groups, which has merits of biocompatibility, abundance, and less costly (Pal and Banat, 2014). It is well-known that sodium alginate has high adsorption capacity and can effectively eliminate metal ions (Papageorgiou et al., 2008). Thus, sodium alginate might be one of the biopolymer could be used to stabilize CGAs. In this study, CGA was generated and used for the removal of lead and copper ions in semi batch process by flotation. Novelty of this work lies in the fact CGAs were generated with stirrer having much lower rpm than previously reported literatures of 5000-10,000 rpm. Sodium alginate was added for the first time as a stabilizing agent, to enhance the metal ions removal process. The effect of various parameters such as concentration of metal ions, volume of the liquid in flotation column, CGAs loading rate, and pH of the solutions were studied with/without the addition of sodium alginate to surfactant solution to find the optimum conditions for the ions removal. Calcium chloride was used to produce the calcium alginate gel in the retentate in order to advance the removal of the remaining metal ions.
2. Materials and methods 2.1. Materials Copper (II) and lead (II) solutions were prepared from copper nitrate trihydrate (Cu(NO3)2.3H2O) and lead nitrate (Pb(NO3)2) (Sigma Aldrich; USA), respectively. Commercially available analytical grade anionic surfactant sodium dodecyl benzenesulfonate (SDBS; Merck, Germany) was used to generate CGAs. The concentrations of the surfactants used in this study was 1.4 mM. A well-known water soluble polymer, sodium alginate (Sigma Aldrich; USA), was used to stabilize the CGAs. Nitric acid (0.1 M) and sodium hydroxide (0.1
4
M) were used to change the pH of the solutions. Calcium chloride (4 weight %) was used to produce the calcium alginate gel in the retentate. All aqueous solutions were prepared using deionized water. 2.2. Methods 2.2.1. Preparation of metal ion solutions Different concentrations of copper and lead ions were prepared by dissolving definite amount of salts in 500 mL deionized water. To ensure that the salts were completely dissolved, the samples were subjected to magnetic stirring (3000 rpm) for 30 minutes at room temperature (23°C). The pH of mixed salt solution was found to be 5.35. 2.2.2. Generation of CGA CGAs were generated using overhead stirrer (Daihan Scientific, Korea; model HS-120A). The diameter of the impeller blade was 90 mm having stainless steel shaft length of 1500 mm. The speed of the stirrer was increased gradually up to 3000 rpm. The surfactant solutions of 500 mL was stirred continuously for 30 minutes until the white creamy CGAs were produced. Sodium alginate (1.0 weight %) was used to produce CGAs with surfactant solutions. After the foam has reached the maximum height, the overhead stirrer was stopped to measure the foam height. 2.2.3. Flotation study The flotation column was filled using the metal ion solutions. Generated CGAs was fed to the column using peristaltic pump. The height of the flotation column is 40 cm having diameter 5.0 cm and could occupy maximum 600 mL solutions. Initially, the metal ion solution was poured on top of the vertically positioned flotation column. A schematic representation of the experimental setup is shown in Fig. 1. The CGAs were pumped into the column and the
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metal ions that attached to the rising bubbles were carried upwards. The foamate containing entrapped metal ions were collected at the top of the column using a beaker. The time taken to pass whole CGA solutions were 90 minutes. The foaming occurred even within less than 15 minutes of the experimental run as reported by other researchers also (Mukherjee et al., 2015). Once CGAs flow rate were stopped, retentate was collected from the bottom of the flotation column to analyze for metal ions concentrations. All the experiments were conducted twice and their average values were taken.
Fig. 1. Schematic diagram of the experimental setup.
2.2.4. Analysis of metal ions
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The equipment used to determine the concentration of the metal ions in original solutions and retentate was ICP-OES (Optima 8000, Perkin Elmer). The air flow rate 25 L/min, nitrogen flow rate 1.4 L/min and argon flow rate 13.0 L/min were used to analyze the mixed metal (lead and copper) ions. The calibration curve was prepared using mixed metal ions standards (Perkin Elmer, USA). The % removal of metal ions from aqueous solutions was calculated as: (1) Where,
is the metal ion concentration in solution (mM) and
is the metal ion
concentration in retentate (mM). 2.2.5. Characterization of calcium alginate gel The surface morphology of the calcium alginate gel was carried out using SEM (FEI Quanta, FEG250, USA) equipped with energy-dispersive X-ray analysis (EDX) at accelerating voltages of 10-20 kV.
3. Results and discussion 3.1. Formation of CGAs The formation of CGAs with SDBS alone and SDBS with sodium alginate is shown in Fig. 2 (a). The formation of CGAs was determined by recording the foam height with time. To enhance CGAs formation, sodium alginate was added to the surfactant solution. With its watersoluble property, sodium alginate dissociates in water by detaching the sodium as cation and polymer chains as anion (Sriamornsak and Sungthongjeeh, 2007). The white creamy CGA was generated within 4 min. by adding sodium alginate on SDBS solutions. The CGA generated without the addition of sodium alginate have reached the same foam height at 18 min. While, the drainage of liquid for both the SDBS and SDBS with sodium alginate follow the same trend (Fig. 7
2 (b)). According to previous research (Matsushita et al., 1992), the half-life of 6.8 min. for 1.4 mM SDBS justified the CGAs formation.
(a)
(b)
Fig. 2. Formation (a) and drainage (b) of CGAs with SDBS alone and SDBS in presence of sodium alginate.
FoamScan apparatus has been used to determine bubble characteristics of CGAs. The foam images were analyzed using Cell Size Analysis (CSA) software. The film camera took several images at every 10 s and then, by using CSA software mean number of bubbles, mean bubble radius, minimum, and maximum bubble radius were calculated. Table 1 shows the CGAs bubble characteristics.
Table 1. The CGAs bubble characteristics. Experiment
Mean Number
Mean bubble
Minimum bubble
Maximum bubble
of bubbles
radius, mm
radius, mm
radius, mm
8
1.
133
0.052
0.017
0.160
2.
88
0.049
0.017
0.140
3.
91
0.049
0.017
0.170
4.
90
0.051
0.017
0.170
5.
84
0.054
0.019
0.180
6.
80
0.056
0.017
0.191
7.
75
0.059
0.017
0.197
8.
73
0.061
0.017
0.202
9.
67
0.061
0.018
0.186
10.
64
0.065
0.019
0.192
11.
63
0.065
0.018
0.194
12.
64
0.065
0.022
0.195
13
59
0.066
0.019
0.199
14.
55
0.067
0.019
0.206
3.2. Removal of lead ions To investigate the effect of the addition of sodium alginate, a comparison on the removal of lead ions using CGA generated using SDBS and SDBS with sodium alginate were performed. The concentration of lead ions in the solution was kept at 2.4 mM. The amount lead ions removed from retentate solution using SDBS alone was found to be 0.6 mMol corresponds to 53.9% removal as shown in Fig. 3 (a and b). While, the amount of lead ions removed using SDBS and sodium alginate was increased to 1.0 mMol with percentage removal of 83.7%. Complexation between lead and alginate ions enhance the removal efficiency (Tahtat et al., 2017).
9
(a)
(b)
Fig. 3. Effect of sodium alginate for the (a) amount and (b) % removal of lead ions having CGAs loading rate 6.1 cm/min and temperature 23°C.
3.3. Removal of mixed metal ions with SDBS alone and SDBS in presence of sodium alginate Lead and copper ion solutions were mixed together in the di-metal ions tests. The amount of metal ions removed as well as % removal was compared using CGAs generated using SDBS alone versus SDBS with alginate using total 10.2 mM of mixed lead (2.4 mM) and copper (7.8 mM) ion solutions as shown in Fig. 4 (a and b). Using the CGAs generated from SDBS, the % removal obtained for lead and copper ions were 51.2% and 34.6%, respectively. On the other hand, the % removal obtained using the CGAs formed from SDBS and sodium alginate were 84.3% and 62% for lead and copper ions, respectively (Fig. 4 (b)). The removal for both lead and copper ions have increased substantially. It was observed that the % removal of lead ions was higher than copper ions. However, the molar concentration of lead (2.4 mM) and copper (7.8 mM) ions varied significantly in 10.2 mM mixed metal ion solution. Thus, the net amount of
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copper ions removed was much higher than the lead ions removed in both the experiments having CGAs generated alone and CGAs generated in presence of alginate (Fig. 4 (a)). Comparatively, copper is a transitional metal having lower molar mass than lead. Thus, chelation of copper ions is easier to attach to the negatively charged CGAs formed from SDBS (Zhao et al., 2009). Again, the ionic radii of copper is 0.73 Å while that of lead is 1.19 Å (Burrows et al., 2013). The smaller the ionic radii and higher the charge of the metal ions, the stronger is the attraction towards oppositely charged SDBS molecules. Thus, the net amount of copper ions removed was much higher than the lead ions.
(a)
(b)
Fig. 4. Effect of adding sodium alginate on SDBS of (a) amount and (b) % removal of metal ions at pH 5.35, CGAs loading rate 6.1 cm/min and temperature 23°C.
3.4. Effect of metal ion concentration
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One of the parameters that affect the removal of the metal ions is the concentration of mixed metal ions in solution. Experiments were conducted at different concentrations of mixed metal ions (1.0 and 2.0 mM) to understand the effect of initial metal ions concentration on % removal. Sodium alginate (1.0 weight %) was added to the surfactant solution to produce CGAs and all the succeeding experiments. Fig. 5 (a and b) show the effect of metal ion concentration on the amount and percentage removal of lead and copper ions. It was observed that with increase in initial metal ion concentrations removal of lead was increased from 89% to 96% while for copper it was increased from 71% to 81%. However, the molar concentration of lead (0.2 mM) and copper (0.7 mM) ions varied significantly in 1.0 mM mixed metal ion solutions. Thus, net amount of copper ions removed (0.2 mMol) was much higher than lead ions (0.1 mMol) from the retentate solution and it was almost double for 2.0 mM mixed metal ions solution. When the concentration is high, more ions were carried by negatively charged CGAs resulting an increase in the percent removal (Wang et al., 2001). Again, simple ion flotation experiments was conducted using 2.0 mM mixed metal ion solution having 1.2 L/min nitrogen flow rate and keeping other parameters constant. The removal of only 46% lead and 11% copper ions justified the effectiveness of CGAs process.
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(a)
(b)
Fig. 5. Effect of (a) amount and (b) % removal of metal ions at CGAs loading rate 6.1 cm/min, pH 5.35 and temperature 23°C.
3.5. Effect of liquid volume To investigate the effect of liquid volume in the flotation column, four different volumes (200, 300, 400, and 500 mL) was kept in the column to check the removal efficiencies of metal ions. The metal ion concentration was kept at 2.0 mM. The CGAs loading rate 6.1 cm/min was passed to the column. The removal of metal ions remain almost constant (96% for lead ions and 81% for copper ions) in the retentate solution with increase in the liquid volume (Fig. 6 (b)). While, the amount of lead and copper ions removed decreased with change in liquid volume as shown in Fig. 6 (a). It can be observed that the amount copper ions removed was 0.6 mMol for 500 mL and decreased to 0.2 mMol for 200 mL solutions. Same decreasing trend is also 13
observed for lead ions. With change in solution volume from 200 mL to 500 mL, the height of liquid level was also changed from 12.7 cm to 30.7 cm in the flotation column. The increase in height resulted longer retention times of the CGA bubbles in the flotation column (Koutlemani et al., 1995). The mean residence time of the CGAs is the ratio of liquid height to CGAs loading rate. Therefore, more metal ions can be taken by CGAs from the solution to enhance metal ions removal.
(a)
(b)
Fig. 6. Effect of liquid volume on (a) the amount and (b) % removal of mixed metal ions having pH of the solutions 5.35, CGAs loading rate 6.1 cm/min and temperature 23°C.
3.6. Effect of CGAs loading rate In order to determine the effect of CGAs loading rate (volumetric flow rate/cross sectional area of the flotation column) on removal of metal ions, the experiments were conducted at different
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CGAs loading rate of 0.5, 2.5, 6.1, and 12.7 cm/min (flow rates 10, 50, 120 and 250 cm3/min) with mixed metal ion concentration (2.0 mM). From Fig. 7 (a), it is observed that increasing the CGAs loading rate the removal of lead ions in the retentate solution is quite high. A significant increment in copper ion removal is observed from 0.5-6.1 cm/min. Increasing the CGAs loading rate beyond led to reduction in the removal of copper ions (Fig. 7 (b)). If CGAs loading rate is too high, the liquid entrained by CGA phase increased causing decrese in copper ions concentration in the foamate (Wang et al., 2001). This explains the sudden decrease of removal of copper ions at 12.7 cm/min (flow rate 250 cm3/min). At a loading rate of 6.1 cm/min, both the removal of lead and copper ions from the retentate solution was significant.
(a)
(b)
Fig. 7. Effect of CGAs loading rate on the removal of (a) lead ions and (b) copper ions having pH of the solutions 5.35 and temperature 23°C. 15
3.7. Effect of pH The effect of pH on the mixed solution has been investigated. The pH of 2.0 mM mixed metal ion solution having concentration of lead ions 0.4 mM and copper ions 1.5 mM was found to be 5.35. Removal of lead and copper was studied at pH 2-7. Fig. 8 (a and b) shows that varying pH at acidic range, total amount of lead ions removed from 500 mL mixed metal ions solution remained low whereas the amount of copper ions removed was increased significantly from the retentate solutions. Within the acidic pH range, % removal of lead was quiet high and reduced beyond pH 7.0. In acidic conditions, the attachment of lead ions with CGAs are improved due to deprotonation of carboxyl groups present in alginate (Cheraghali et al., 2013). While, the removal of copper ions increased with increase in pH of the solution. As the pH was increased above 5.5, the hydrolysis of copper ions take place to produce respective hydroxides to enhance the removal of copper ions (Molaei and Waters, 2015).
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(a)
(b)
Fig. 8. The removal of (a) lead ions and (b) copper ions at different pH having CGAs loading rate 6.1 cm/min and temperature 23°C.
3.8. Effect of addition of calcium chloride on retentate The retentate from 2.0 mM mixed metal ions solution was collected to determine if there is any additional amount of sodium alginate present in the solutions. Calcium chloride solution (4 weight %) was added on the retentate solution to obtain hydrogels as colloidal form (Fang et al., 2007; Grant et al., 1973). Thus, Ca2+ have cross-linked with the sodium alginate bio-polymer to produce gels in the solution. The retentate solution was kept for 24 h to settle down the gels. The clear solution was analyzed using ICP-OES to determine the remaining amount of lead and copper ions in the solution. The amount of both the metal ions removed before and after addition of calcium chloride solution is shown in Fig. 9 (a). Also, it was observed (Fig. 9 (b)) that the 17
percent removal was increased by 1.7% for lead ions and 15.5% for copper ions, respectively. This indicates that lead and copper ions were adsorbed on the gel from the retentate solution resulting in more ions removal.
(a)
(b)
Fig. 9. Effect of calcium chloride addition on (a) amount of metal ions removed from retentate and (b) % removal having CGAs loading rate 6.1 cm/min at pH 5.35 and temperature 23°C.
Fig. 10 (a) shows the surface morphology and Fig. 10 (b) shows the elemental compositions of the dried hydrogel. Semi-quantitative EDX analysis shows that calcium, copper and lead are there on the dried hydrogel.
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(a)
(b)
Fig. 10. (a) SEM analysis of dried hydrogel (b) elemental compositions using EDX analysis
4. Conclusion In this work, the use of sodium alginate as CGA stabilizer was investigated. The use of SDBS was effective in separating the individual ions. With the addition of sodium alginate to SDBS, a higher removal of lead and copper ions was obtained compared with using SDBS alone. The experimental parameters like metal ion concentration, CGAs loading rate and pH of the solutions played a significant role for the removal of metal ions. Maximum removal of 96% for Pb2+ and 81% for Cu2+ was obtained from retentate solution using total mixed metal ion concentrations of 2.0 mM at pH of 5.35 having CGAs loading rate 6.1 cm/min. However, the net amount of copper ions removed (0.6 mMol) was much higher than the lead ions removed (0.2 mMol) due to higher initial concentration of copper ions present in solutions. With the addition of calcium chloride to the retentate, the gels were formed and adsorbed most of the remaining
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lead and copper ions in the solution which resulted an improved removal of 99% for Pb2+ and 92% for Cu2+.
Acknowledgment The work is funded by The Petroleum Institute Research Center (Grant # LTR14013) and AARE 2017 (AARE17-084) Abu Dhabi, United Arab Emirates.
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Spigno, G. Dermiki, M., Pastori, C., Casanova, F., Jauregi, P., 2010. Recovery of gallic acid with colloidal gas aphrons generated from a cationic surfactant, Sep. Puri. Tech., 71, 56-62. Sriamornsak, P., Sungthongjeeh, S. 2007. Modification of theophylline release with alginate gel formed in hard capsules, AAPS PharmSciTech, 8, E1-E8. Tahtat, D., Bouaicha, M. N., Benamer, S., Nacer-Khodjaa, A., Mahlousa, M., 2017. Development of alginate gel beads with a potential use in the treatment against acute lead poisoning, Int. J. Bio. Macro. 105, 1010-1016. Tan, X., Fang, M., Chen, C., Yu, S., Wang, X., 2008. Counterion effects of nickel and sodium dodecylbenzene sulfonate adsorption to multiwalled carbon nanotubes in aqueous solution, Carbon, 46, 1741-1750. van Dijk, L., Roncken, G. C. G., 1997. Membrane bioreactors for wastewater treatment: The state of the art and new developments, Wat. Sci. Tech., 35, 35-41. Wang, Y., Wen, H., Huang, Y., Dai, Y., 2001. Separation of Cu(II) from an aqueous solution by using colloidal gas aphrons, J. Chem. Eng. Jap., 34, 1127-1130. Yangi, H., Yundong,
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Colloidal gas aphrons stabilized with sodium alginate.
Low stirring speed of 3000 rpm was maintained to produce CGAs.
Removal efficiencies of mixed metal ions (Pb2+ and Cu2+) were increased in presence of alginate.
Addition of calcium chloride to the retentate solution further increased the removal.
25
S1383-5866(17)34067-4 https://doi.org/10.1016/j.seppur.2018.03.043 SEPPUR 14463
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 December 2017 20 March 2018 20 March 2018
Please cite this article as: A. Gay Corpuz, P. Pal, F. Banat, M. Abu Haija, Enhanced removal of mixed metal ions from aqueous solutions using flotation by colloidal gas aphrons stabilized with sodium alginate, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.03.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced removal of mixed metal ions from aqueous solutions using flotation by colloidal gas aphrons stabilized with sodium alginate Aiza Gay Corpuza, Priyabrata Pala,*, Fawzi Banata,*, Mohammad Abu Haijab a
Department of Chemical Engineering, Khalifa University of Science and Technology, SAN Campus, Abu Dhabi, United Arab Emirates
b
Department of Chemistry, Khalifa University of Science and Technology, SAN Campus, Abu Dhabi, United Arab Emirates *Email of the corresponding authors: [email protected], [email protected]
Abstract Anionic surfactant sodium dodecyl benzenesulfonate (SDBS) was used to prepare colloidal gas aphrons (CGAs). Bio-polymeric sodium alginate was added to enhance the removal efficiencies of mixed metal ions such as lead (Pb2+) and copper (Cu2+) from aqueous solutions via flotation. Stirring speed of 3000 rpm was maintained to produce CGAs containing 500 mL surfactant solutions. The effects of concentration of metal ions, volume of liquid in the flotation column, CGAs loading rate, and pH of solution on the removal of heavy metals were examined. CGAs loading rate of 6.1 cm/min (flow rate 120 cm3/min) to the flotation column containing 2.0 mM of mixed metal ions (0.4 mM of Pb2+ and 1.5 mM of Cu2+ ions) at pH 5.35 gave the maximum removal of 96% for Pb2+ ions and 81% for Cu2+ ions. However, the net amount of copper ions removed was much higher than the lead ions. Addition of calcium chloride to the retentate solution further increased the removal to be 99% for Pb2+ and 92% for Cu2+ by producing alginate gel with metal ions that precipitated out from the solutions. Interestingly, sodium alginate stabilized CGAs improved significantly the removal of metal ions from aqueous solutions by flotation.
1
Keywords: Surfactant; colloidal gas aphrons; sodium alginate; flotation; heavy metal ions 1. Introduction The Emirate of Abu Dhabi is located in an arid region with extreme natural water scarcity. It relies mainly on groundwater, desalinated water and treated wastewater. Groundwater and desalinated water account for 60% and 35% respectively of water use in the emirate, while the recycled water contribution is just 5% (Al Madfaei, 2017). Dependence on already scarce groundwater resources and the depletion of its level is a serious issue. While, sea water desalination is challenged by high sea water salinity, elevated sea water temperatures and risks associated with oil spills and marine pollution. Clean water is a necessity for life; thereby disposal of untreated wastes in water is not an option (Grady et al., 2014). Untreated wastewater may impose health risks to individuals, aquatic animals and environment as well. Thus, lot of works focus on ways to dispose wastewater in such a way that no detrimental effects to the population and environment should arise (Bramih et al., 2018; Oller et al., 2011; Dakiky et al., 2002; van Dijk et al., 1997). Organic (organic manure and detergents) and inorganic chemicals (heavy metals, radioactive metals, nonmetals, etc.), normally from industrial sites, that are present in wastewater can be treated physically, chemically, or biologically (De and Mondal, 2012). It was showed that metal contaminated water is a serious threat to living organisms as heavy metals such as lead, copper, cadmium, nickel, cobalt, and zinc tend to accumulate inside the body and are highly toxic even at low concentrations (Gautam et al., 2015). It is very important to minimize the concentration of heavy metals before discharging into receiving waters. One of the wastewater treatment techniques that could address the removal of heavy metal ions using colloidal gas aphrons (CGA) is by flotation method (Rubio et al., 2002). CGAs
2
are defined as 10-100 µm diameter bubbles generated by intense stirring (5000-10000 rpm) of a surfactant solution (Sebba, 1987). These micro foams are not like regular foams as regular foams consist of a gas inner core covered by a single layer of surfactant film (Larmignat et al., 2008). Sebba (1987) have defined the structure of CGA, an inner core surrounded with two thin layers of surfactant film and a third surfactant layer stabilizes the structure. Several properties of CGAs include large interfacial area, relatively high stability, similar flow properties with water, and easy separation from bulk liquid phase (Jauregi et al., 1997; Molaei and Waters, 2015). Aphrons can be produced from anionic, cationic, and non-ionic surfactants and the charge of CGAs retain the same charge as the surfactant solution (Wang et al., 2001; Hashim et al., 2012). These properties make them of interest in several applications such as removal of metals (Grimes, 2002), recovery of proteins (Fuda et al., 2004; Noble et al., 1998; Fuda and Jauregi, 2006; Jarudilokkul et al., 2004), removal of organic acid (Spigno et al., 2010) and recovery of enzymes (O'Connell and Varley, 2001; Zidehsaraei et al., 2009). Moreover, CGAs can be easily pumped and have significant cost effectiveness compared to membrane and chromatographic separations (Molaei and Waters, 2015; Ahmadun et al., 2009). Also, selection of surfactant is essential to facilitate the adhesion of targeted material on CGAs surface (Scamehorn and Harwell, 1991). Sodium dodecyl benzenesulfonate (SDBS) is a widely used anionic surfactant to generate CGAs (Gai et al., 2016; Yangi et al., 2002; Mansur et al, 2004) and can bind easily to positively charged metal ions (Tan et al., 2008). Stability of CGAs is another most concerned parameters and polymer plays a significant role in stabilizing CGAs (Samuel et al., 2012). Water-soluble polymers can aid foam stabilization by increasing bulk viscosity of the film, thus decreasing the film drainage rate (Prud'homme and Khan, 1996). It is well known that sodium alginate is a natural anionic
3
polymer extracted from a brown seaweed. Sodium alginate consists of β-D-mannuronic acid, αL-guluronic acid and free carboxyl groups, which has merits of biocompatibility, abundance, and less costly (Pal and Banat, 2014). It is well-known that sodium alginate has high adsorption capacity and can effectively eliminate metal ions (Papageorgiou et al., 2008). Thus, sodium alginate might be one of the biopolymer could be used to stabilize CGAs. In this study, CGA was generated and used for the removal of lead and copper ions in semi batch process by flotation. Novelty of this work lies in the fact CGAs were generated with stirrer having much lower rpm than previously reported literatures of 5000-10,000 rpm. Sodium alginate was added for the first time as a stabilizing agent, to enhance the metal ions removal process. The effect of various parameters such as concentration of metal ions, volume of the liquid in flotation column, CGAs loading rate, and pH of the solutions were studied with/without the addition of sodium alginate to surfactant solution to find the optimum conditions for the ions removal. Calcium chloride was used to produce the calcium alginate gel in the retentate in order to advance the removal of the remaining metal ions.
2. Materials and methods 2.1. Materials Copper (II) and lead (II) solutions were prepared from copper nitrate trihydrate (Cu(NO3)2.3H2O) and lead nitrate (Pb(NO3)2) (Sigma Aldrich; USA), respectively. Commercially available analytical grade anionic surfactant sodium dodecyl benzenesulfonate (SDBS; Merck, Germany) was used to generate CGAs. The concentrations of the surfactants used in this study was 1.4 mM. A well-known water soluble polymer, sodium alginate (Sigma Aldrich; USA), was used to stabilize the CGAs. Nitric acid (0.1 M) and sodium hydroxide (0.1
4
M) were used to change the pH of the solutions. Calcium chloride (4 weight %) was used to produce the calcium alginate gel in the retentate. All aqueous solutions were prepared using deionized water. 2.2. Methods 2.2.1. Preparation of metal ion solutions Different concentrations of copper and lead ions were prepared by dissolving definite amount of salts in 500 mL deionized water. To ensure that the salts were completely dissolved, the samples were subjected to magnetic stirring (3000 rpm) for 30 minutes at room temperature (23°C). The pH of mixed salt solution was found to be 5.35. 2.2.2. Generation of CGA CGAs were generated using overhead stirrer (Daihan Scientific, Korea; model HS-120A). The diameter of the impeller blade was 90 mm having stainless steel shaft length of 1500 mm. The speed of the stirrer was increased gradually up to 3000 rpm. The surfactant solutions of 500 mL was stirred continuously for 30 minutes until the white creamy CGAs were produced. Sodium alginate (1.0 weight %) was used to produce CGAs with surfactant solutions. After the foam has reached the maximum height, the overhead stirrer was stopped to measure the foam height. 2.2.3. Flotation study The flotation column was filled using the metal ion solutions. Generated CGAs was fed to the column using peristaltic pump. The height of the flotation column is 40 cm having diameter 5.0 cm and could occupy maximum 600 mL solutions. Initially, the metal ion solution was poured on top of the vertically positioned flotation column. A schematic representation of the experimental setup is shown in Fig. 1. The CGAs were pumped into the column and the
5
metal ions that attached to the rising bubbles were carried upwards. The foamate containing entrapped metal ions were collected at the top of the column using a beaker. The time taken to pass whole CGA solutions were 90 minutes. The foaming occurred even within less than 15 minutes of the experimental run as reported by other researchers also (Mukherjee et al., 2015). Once CGAs flow rate were stopped, retentate was collected from the bottom of the flotation column to analyze for metal ions concentrations. All the experiments were conducted twice and their average values were taken.
Fig. 1. Schematic diagram of the experimental setup.
2.2.4. Analysis of metal ions
6
The equipment used to determine the concentration of the metal ions in original solutions and retentate was ICP-OES (Optima 8000, Perkin Elmer). The air flow rate 25 L/min, nitrogen flow rate 1.4 L/min and argon flow rate 13.0 L/min were used to analyze the mixed metal (lead and copper) ions. The calibration curve was prepared using mixed metal ions standards (Perkin Elmer, USA). The % removal of metal ions from aqueous solutions was calculated as: (1) Where,
is the metal ion concentration in solution (mM) and
is the metal ion
concentration in retentate (mM). 2.2.5. Characterization of calcium alginate gel The surface morphology of the calcium alginate gel was carried out using SEM (FEI Quanta, FEG250, USA) equipped with energy-dispersive X-ray analysis (EDX) at accelerating voltages of 10-20 kV.
3. Results and discussion 3.1. Formation of CGAs The formation of CGAs with SDBS alone and SDBS with sodium alginate is shown in Fig. 2 (a). The formation of CGAs was determined by recording the foam height with time. To enhance CGAs formation, sodium alginate was added to the surfactant solution. With its watersoluble property, sodium alginate dissociates in water by detaching the sodium as cation and polymer chains as anion (Sriamornsak and Sungthongjeeh, 2007). The white creamy CGA was generated within 4 min. by adding sodium alginate on SDBS solutions. The CGA generated without the addition of sodium alginate have reached the same foam height at 18 min. While, the drainage of liquid for both the SDBS and SDBS with sodium alginate follow the same trend (Fig. 7
2 (b)). According to previous research (Matsushita et al., 1992), the half-life of 6.8 min. for 1.4 mM SDBS justified the CGAs formation.
(a)
(b)
Fig. 2. Formation (a) and drainage (b) of CGAs with SDBS alone and SDBS in presence of sodium alginate.
FoamScan apparatus has been used to determine bubble characteristics of CGAs. The foam images were analyzed using Cell Size Analysis (CSA) software. The film camera took several images at every 10 s and then, by using CSA software mean number of bubbles, mean bubble radius, minimum, and maximum bubble radius were calculated. Table 1 shows the CGAs bubble characteristics.
Table 1. The CGAs bubble characteristics. Experiment
Mean Number
Mean bubble
Minimum bubble
Maximum bubble
of bubbles
radius, mm
radius, mm
radius, mm
8
1.
133
0.052
0.017
0.160
2.
88
0.049
0.017
0.140
3.
91
0.049
0.017
0.170
4.
90
0.051
0.017
0.170
5.
84
0.054
0.019
0.180
6.
80
0.056
0.017
0.191
7.
75
0.059
0.017
0.197
8.
73
0.061
0.017
0.202
9.
67
0.061
0.018
0.186
10.
64
0.065
0.019
0.192
11.
63
0.065
0.018
0.194
12.
64
0.065
0.022
0.195
13
59
0.066
0.019
0.199
14.
55
0.067
0.019
0.206
3.2. Removal of lead ions To investigate the effect of the addition of sodium alginate, a comparison on the removal of lead ions using CGA generated using SDBS and SDBS with sodium alginate were performed. The concentration of lead ions in the solution was kept at 2.4 mM. The amount lead ions removed from retentate solution using SDBS alone was found to be 0.6 mMol corresponds to 53.9% removal as shown in Fig. 3 (a and b). While, the amount of lead ions removed using SDBS and sodium alginate was increased to 1.0 mMol with percentage removal of 83.7%. Complexation between lead and alginate ions enhance the removal efficiency (Tahtat et al., 2017).
9
(a)
(b)
Fig. 3. Effect of sodium alginate for the (a) amount and (b) % removal of lead ions having CGAs loading rate 6.1 cm/min and temperature 23°C.
3.3. Removal of mixed metal ions with SDBS alone and SDBS in presence of sodium alginate Lead and copper ion solutions were mixed together in the di-metal ions tests. The amount of metal ions removed as well as % removal was compared using CGAs generated using SDBS alone versus SDBS with alginate using total 10.2 mM of mixed lead (2.4 mM) and copper (7.8 mM) ion solutions as shown in Fig. 4 (a and b). Using the CGAs generated from SDBS, the % removal obtained for lead and copper ions were 51.2% and 34.6%, respectively. On the other hand, the % removal obtained using the CGAs formed from SDBS and sodium alginate were 84.3% and 62% for lead and copper ions, respectively (Fig. 4 (b)). The removal for both lead and copper ions have increased substantially. It was observed that the % removal of lead ions was higher than copper ions. However, the molar concentration of lead (2.4 mM) and copper (7.8 mM) ions varied significantly in 10.2 mM mixed metal ion solution. Thus, the net amount of
10
copper ions removed was much higher than the lead ions removed in both the experiments having CGAs generated alone and CGAs generated in presence of alginate (Fig. 4 (a)). Comparatively, copper is a transitional metal having lower molar mass than lead. Thus, chelation of copper ions is easier to attach to the negatively charged CGAs formed from SDBS (Zhao et al., 2009). Again, the ionic radii of copper is 0.73 Å while that of lead is 1.19 Å (Burrows et al., 2013). The smaller the ionic radii and higher the charge of the metal ions, the stronger is the attraction towards oppositely charged SDBS molecules. Thus, the net amount of copper ions removed was much higher than the lead ions.
(a)
(b)
Fig. 4. Effect of adding sodium alginate on SDBS of (a) amount and (b) % removal of metal ions at pH 5.35, CGAs loading rate 6.1 cm/min and temperature 23°C.
3.4. Effect of metal ion concentration
11
One of the parameters that affect the removal of the metal ions is the concentration of mixed metal ions in solution. Experiments were conducted at different concentrations of mixed metal ions (1.0 and 2.0 mM) to understand the effect of initial metal ions concentration on % removal. Sodium alginate (1.0 weight %) was added to the surfactant solution to produce CGAs and all the succeeding experiments. Fig. 5 (a and b) show the effect of metal ion concentration on the amount and percentage removal of lead and copper ions. It was observed that with increase in initial metal ion concentrations removal of lead was increased from 89% to 96% while for copper it was increased from 71% to 81%. However, the molar concentration of lead (0.2 mM) and copper (0.7 mM) ions varied significantly in 1.0 mM mixed metal ion solutions. Thus, net amount of copper ions removed (0.2 mMol) was much higher than lead ions (0.1 mMol) from the retentate solution and it was almost double for 2.0 mM mixed metal ions solution. When the concentration is high, more ions were carried by negatively charged CGAs resulting an increase in the percent removal (Wang et al., 2001). Again, simple ion flotation experiments was conducted using 2.0 mM mixed metal ion solution having 1.2 L/min nitrogen flow rate and keeping other parameters constant. The removal of only 46% lead and 11% copper ions justified the effectiveness of CGAs process.
12
(a)
(b)
Fig. 5. Effect of (a) amount and (b) % removal of metal ions at CGAs loading rate 6.1 cm/min, pH 5.35 and temperature 23°C.
3.5. Effect of liquid volume To investigate the effect of liquid volume in the flotation column, four different volumes (200, 300, 400, and 500 mL) was kept in the column to check the removal efficiencies of metal ions. The metal ion concentration was kept at 2.0 mM. The CGAs loading rate 6.1 cm/min was passed to the column. The removal of metal ions remain almost constant (96% for lead ions and 81% for copper ions) in the retentate solution with increase in the liquid volume (Fig. 6 (b)). While, the amount of lead and copper ions removed decreased with change in liquid volume as shown in Fig. 6 (a). It can be observed that the amount copper ions removed was 0.6 mMol for 500 mL and decreased to 0.2 mMol for 200 mL solutions. Same decreasing trend is also 13
observed for lead ions. With change in solution volume from 200 mL to 500 mL, the height of liquid level was also changed from 12.7 cm to 30.7 cm in the flotation column. The increase in height resulted longer retention times of the CGA bubbles in the flotation column (Koutlemani et al., 1995). The mean residence time of the CGAs is the ratio of liquid height to CGAs loading rate. Therefore, more metal ions can be taken by CGAs from the solution to enhance metal ions removal.
(a)
(b)
Fig. 6. Effect of liquid volume on (a) the amount and (b) % removal of mixed metal ions having pH of the solutions 5.35, CGAs loading rate 6.1 cm/min and temperature 23°C.
3.6. Effect of CGAs loading rate In order to determine the effect of CGAs loading rate (volumetric flow rate/cross sectional area of the flotation column) on removal of metal ions, the experiments were conducted at different
14
CGAs loading rate of 0.5, 2.5, 6.1, and 12.7 cm/min (flow rates 10, 50, 120 and 250 cm3/min) with mixed metal ion concentration (2.0 mM). From Fig. 7 (a), it is observed that increasing the CGAs loading rate the removal of lead ions in the retentate solution is quite high. A significant increment in copper ion removal is observed from 0.5-6.1 cm/min. Increasing the CGAs loading rate beyond led to reduction in the removal of copper ions (Fig. 7 (b)). If CGAs loading rate is too high, the liquid entrained by CGA phase increased causing decrese in copper ions concentration in the foamate (Wang et al., 2001). This explains the sudden decrease of removal of copper ions at 12.7 cm/min (flow rate 250 cm3/min). At a loading rate of 6.1 cm/min, both the removal of lead and copper ions from the retentate solution was significant.
(a)
(b)
Fig. 7. Effect of CGAs loading rate on the removal of (a) lead ions and (b) copper ions having pH of the solutions 5.35 and temperature 23°C. 15
3.7. Effect of pH The effect of pH on the mixed solution has been investigated. The pH of 2.0 mM mixed metal ion solution having concentration of lead ions 0.4 mM and copper ions 1.5 mM was found to be 5.35. Removal of lead and copper was studied at pH 2-7. Fig. 8 (a and b) shows that varying pH at acidic range, total amount of lead ions removed from 500 mL mixed metal ions solution remained low whereas the amount of copper ions removed was increased significantly from the retentate solutions. Within the acidic pH range, % removal of lead was quiet high and reduced beyond pH 7.0. In acidic conditions, the attachment of lead ions with CGAs are improved due to deprotonation of carboxyl groups present in alginate (Cheraghali et al., 2013). While, the removal of copper ions increased with increase in pH of the solution. As the pH was increased above 5.5, the hydrolysis of copper ions take place to produce respective hydroxides to enhance the removal of copper ions (Molaei and Waters, 2015).
16
(a)
(b)
Fig. 8. The removal of (a) lead ions and (b) copper ions at different pH having CGAs loading rate 6.1 cm/min and temperature 23°C.
3.8. Effect of addition of calcium chloride on retentate The retentate from 2.0 mM mixed metal ions solution was collected to determine if there is any additional amount of sodium alginate present in the solutions. Calcium chloride solution (4 weight %) was added on the retentate solution to obtain hydrogels as colloidal form (Fang et al., 2007; Grant et al., 1973). Thus, Ca2+ have cross-linked with the sodium alginate bio-polymer to produce gels in the solution. The retentate solution was kept for 24 h to settle down the gels. The clear solution was analyzed using ICP-OES to determine the remaining amount of lead and copper ions in the solution. The amount of both the metal ions removed before and after addition of calcium chloride solution is shown in Fig. 9 (a). Also, it was observed (Fig. 9 (b)) that the 17
percent removal was increased by 1.7% for lead ions and 15.5% for copper ions, respectively. This indicates that lead and copper ions were adsorbed on the gel from the retentate solution resulting in more ions removal.
(a)
(b)
Fig. 9. Effect of calcium chloride addition on (a) amount of metal ions removed from retentate and (b) % removal having CGAs loading rate 6.1 cm/min at pH 5.35 and temperature 23°C.
Fig. 10 (a) shows the surface morphology and Fig. 10 (b) shows the elemental compositions of the dried hydrogel. Semi-quantitative EDX analysis shows that calcium, copper and lead are there on the dried hydrogel.
18
(a)
(b)
Fig. 10. (a) SEM analysis of dried hydrogel (b) elemental compositions using EDX analysis
4. Conclusion In this work, the use of sodium alginate as CGA stabilizer was investigated. The use of SDBS was effective in separating the individual ions. With the addition of sodium alginate to SDBS, a higher removal of lead and copper ions was obtained compared with using SDBS alone. The experimental parameters like metal ion concentration, CGAs loading rate and pH of the solutions played a significant role for the removal of metal ions. Maximum removal of 96% for Pb2+ and 81% for Cu2+ was obtained from retentate solution using total mixed metal ion concentrations of 2.0 mM at pH of 5.35 having CGAs loading rate 6.1 cm/min. However, the net amount of copper ions removed (0.6 mMol) was much higher than the lead ions removed (0.2 mMol) due to higher initial concentration of copper ions present in solutions. With the addition of calcium chloride to the retentate, the gels were formed and adsorbed most of the remaining
19
lead and copper ions in the solution which resulted an improved removal of 99% for Pb2+ and 92% for Cu2+.
Acknowledgment The work is funded by The Petroleum Institute Research Center (Grant # LTR14013) and AARE 2017 (AARE17-084) Abu Dhabi, United Arab Emirates.
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Colloidal gas aphrons stabilized with sodium alginate.
Low stirring speed of 3000 rpm was maintained to produce CGAs.
Removal efficiencies of mixed metal ions (Pb2+ and Cu2+) were increased in presence of alginate.
Addition of calcium chloride to the retentate solution further increased the removal.
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