Removal of Iron and Manganese from Aqueous

Removal of Iron and Manganese from Aqueous Solutions Using Carbon Nanotubes Filters E.M.I. Elsehly1,2,*, N.G. Chechenin1, K.A. Bukunov1, A.V. Makunin1, A.B. Priselkova1, E.A. Vorobyeva1, H.A. Motaweh2 1

Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Russia, 2 Faculty of Science, Damanhour University, Egypt *Corresponding author, e-mail:[email protected] Abstract.

Carbon nanotubes (CNTs) became a focus of attention of many scientists and companies worldwide. CNT-based filters have a prospective advantage in comparison to the commercial filters already in operation because they are light weight and do not require electricity to operate. This investigation handles the filtration efficiency of manganese and iron from aqueous solution using commercial multiwalled carbon nanotubes MWCNTs (Taunit). Effects of different parameters such as CNT filter mass, concentration of manganese and iron in aqueous solution, pH of aqueous solution on removal of these heavy metals are determined. From these investigations, the removal efficiency of manganese and iron could reach 71.5% and 52% respectively for concentration 50ppm, suggesting that Taunit is an excellent adsorbent for manganese and iron removal from water. A significant increase in removal efficiency at pH=3 for manganese and pH= 8 for iron. The effect of oxidation on the structural of MWCNTs was characterized by SEM and EDS techniques to investigate the functionalization with oxygen-containing and outer diameter distribution. It was found that functionalized CNT-based filters are more efficient to remove manganese and iron from aqueous solutions. Oxidized MWCNTs may be a promising candidate for heavy metal ions removal from industrial wastewater. Keywords: MWCNT, Manganese, Iron. Introduction Water crisis is one of the greatest challenges of our time. Water demand is growing rapidly as a result of increasing population and rapid urbanization. However, water resources are limited in populated areas and arid regions. The shortage of water resources calls for efficient technologies for waste water reclamation and seawater desalination (Sheikholeslami 2009). Heavy metal pollutants like manganese and iron have been a major preoccupation for many years because of their toxicity toward aquatic-life, plants, animals, human beings and the environment (Shannon et al. 2008; Macedonio et al. 2012). Manganese forms a very common compound that can be found everywhere on earth. It is present in the atmosphere as suspended particulates resulting from industrial emission, soil erosion and volcanic emissions. Manganese is an essential metal for the human system and many enzymes are activated by manganese, but, on the other hand, it is toxic when too high concentrations are present inhuman body. Manganese effects occur mainly in the respiratory tract and in the brains (Forstner and Wittmann 1983). Manganese affects the appearance and imparts a metallic taste to the water.

Also, iron is one of the most abundant metals of the Earth's crust. It occurs naturally in water in soluble form as the ferrous iron or complex form like the ferric iron, or have an industrial origin; mining, iron and steel industry, metals corrosion. Commonly iron is blamed for much of the water-quality problems, as it makes the water’s aesthetic appearance undesirable, gives a metallic taste to water making it unpleasant for consumption. It can also be at the origin of corrosion in drains sewers, due to the development of microorganisms and the ferrobacteries. Iron bacteria do not cause health problems, but cause odors, corrode plumbing equipment, reduce well yields (Choo et al. 2005). Following the above-mentioned facts, higher concentrations of iron and manganese in water cause failure of water supply systems operation, water quality deterioration and the reduction of pipe flow cross-section (Akpomie and Dawodu 2014). The common methods for removing these metals from water like ion exchange, reverse osmosis, and electrodialysis have proven to be either too expensive or inefficient to remove heavy metal ions from aqueous solutions (Oehmen et al. 2006; Penate and GarciaRodriguez 2012; Ganesan et al. 2013). At present, chemical treatments are not used due to disadvantages like high costs of maintenance. The advent of nanotechnology for water purification brought a lot of hope. The success of CNTs membranes as a filter is based on their unique properties which include high surface areas, and good mechanical and thermal stability ( Corry 2008; Smart et al. 2006). Despite having smaller pores, CNTs have high permeability and less pressure is required to pump water through the filter, possibly due to smooth CNT interiors (Long and Yang 2001). The main advantage of this is reduced costs through energy savings. So their applicability for removal of hazardous pollutants from aqueous streams have been studying extensively (Ren et al. 2011; Stafiej and Pyrzynska 2008). The surface of carbon nanotubes is originally inert and solvophobic, thus the practical effect of the application of CNTs, especially in filtration of solutions, is lower than expected (Chen et al. 2012). A possible solution to this problem is to modify the surface of carbon nanotubes by oxidation. The amorphous carbon and catalyst particles introduced by the CVD preparation process were removed during the course of oxidation treatment. The functionalization of CNTs surfaces with oxygen-containing groups in the present study was carried out by treatment with potassium permanganate in acidic medium. It was found that the degree of functionalization of Taunit-M is somewhat higher than that of Taunit, this explained as a result of its smaller number of carbon atom layers and higher specific surface area. So, in this work, we study the effects of oxidation of (Taunit-M) with potassium permanganate (Dyachkova et al. 2013), investigate the filtration efficiency of manganese and iron by MWCNT (Taunit-filter), the effects of pH, ion concentrations and CNTs dose on the filtration process. Methods Characterization and modification of the filter. The material of the present study is an industrial carbon nanotubes Taunit.“Taunit” and “Taunit-M” CNTs produced by “Nanotech Center” (Tambov, Russia).This material is a loose black powder, composed of grainy agglomerates of MWCNTs with a size of several micrometers. Their properties are listed in Table 1. To perform the oxidation of MWCNT filter, 0.5 g of Taunit-M dispersed in a flask with 20 ml of 0.5 M sulfuric acid by ultrasonic vibration, 0.5 g and 1 g of potassium permanganate were dissolved in 20 ml of 0.5 M sulfuric acid and added to the flask drop by drop. Then the reaction was kept at 80 ◦C for 2 h. The resulting suspension was filtered, washed with deionized water, filtered, rewashed with concentrated HCl to remove the produced MnO2.

Table 1. General characteristics of CNTs used in this work. Parameter/CNTs

“Taunit”

“Taunit-M”

External diameter, nm

(20-70)

(30-80)

Internal diameter, nm

(5-10)

(10-20)

Length, µm

2 and more

20 and more

Specific surface area, m2/g

(120-130 )

(180-200)

Bulk density, g/cm3

(0.4-0.6)

(0.03-0.05)

Filter design and stock solution preparation. MWCNT filters were prepared by sandwiching of compressed “Taunit” and “Taunit-M” CNTs between two pieces of glassy fiber filter with a cotton layer as a filter substrate, to keep insuring that CNTs cannot penetrate through the filter, and putted into a syringe (Fig 1). The 1000 ppm manganese and iron stock solutions were prepared by dissolving 1 g of KMnO4 and 0.18 g of FeSO4∙7H2O in 1000ml of distilled water then diluted to the desired concentration. The pH of the stock solutions is adjusted using buffer solutions. Experimental procedure. Table 2 shows the experimental parameters and their variations. The removal efficiency (R) is defined as follows:

R=

KO  K 100 KO

Where KO and K are the aqueous solution conductivity, initial and after filtration (μS/cm), K=CΛ, Λ is the molar conductivity in μS/(cm∙ mol) and C is the concentration of ions in the solution in mol-1.

Fig 1. Filter design and filtration process. Filtered solution becomes colorless (right photo).

Table 2. Experiment parameters and its variation. Parameter

Variation Low Medium

1. Concentration (ppm) Manganese Iron 2. pH Manganese Iron 3. Filter dosage (g/50ml) Raw Taunit Raw and oxidized Taunit-M

High

50 50

200 100

800 200

3 3

7 6

10 8

0.4 0.1

0.8 0.2

1.2 0.3

Results and discussion Surface analysis of oxidized MWCNTs Morphology of the pristine and oxidized MWCNTs was characterized by SEM(TESCAN). A clear decrease in nanotube diameters was observed as shown in Fig 2. It follows from EDS analysis that the oxygen content was increased with a higher (KMnO4)/(CNTs) mass ratio. Also, it is clearly seen that the Fe impurity removed after oxidation, Fig 3.

60nm 20nm

(a)

(b)

Fig 2. SEM images of MWCNTs showing the outer diameter (a) pristine, (b) after oxidation.

(b) (a) (c) Fig 3. EDS analysis showing the oxygen content: (a) before oxidation, (b) oxidation with 0.5 g KMnO4/0.5 g CNT, (c) oxidation with 1 g KMnO4/0.5 g CNT.

Effect of manganese concentration Table 3 shows that the removal efficiency, R, of manganese decreases as the concentration of aqueous solution increase. One can see R is only 37% with 1.2 g Taunit and 47% for 0.3 g Taunit-M at the concentration of 800 ppm. Whereas at 50 ppm concentration, R is around 61% for Taunit and 71% for Taunit-M, proving that the removal efficiency of Taunit-M is better than Taunit.

Table 3. Removal efficiency at different concentration of KMnO4 solution at pH=7.

C (ppm) 800 200 50

K (μS/cm) before filtration 762 191 49

K (μS/cm) after filtration (Taunit-M) 402 72 14

K (μS/cm) after filtration (Taunit) 480 94 19

R (%) (Taunit-M) 47.24 62.30 71.43

R (%) (Taunit) 37.01 50.79 61.22

Effect of iron concentration in aqueous solution A comparative study of the filtration efficiency of raw and oxidized Taunit-M is performed of iron removal as a function of iron concentration in aqueous solution. From Fig 4 one can see as the concentration of iron aqueous solution increase the removal efficiency decreases. At 200 ppm concentration, R is only 8.75% for 0.3 g raw Taunit-M but for 0.3 g oxidized Taunit-M the efficiency is 22%. Whereas at low concentration of 50 ppm, R of oxidized Taunit-M is around 52%. This may indicate that the adsorption interaction between the oxidized MWCNTs and Fe(II) ions was mainly of ionic interaction nature which is in agreement with an ion exchange mechanism, as illustrated in Fig 5. High concentration of iron limits its transfer to oxidized MWCNT surfaces.

removal efficiency (%)

removal efficiency vs concentration of iron ions 60 50 40 30 20 10 0

raw Taunit- M oxidized Taunit- M 0

50 100 150 200 250 concentration of iron in aqueous solution (ppm)

Fig 4. Variation in removal efficiency with concentration of iron at pH=6.

Fig 5. Schematic diagram for the interaction of iron and manganese with oxidized MWCNTs. Effect of pH of manganese and iron solutions As pH decreased, the surface charge of CNTs became more positive because of the deposition of more hydrogen ions on the CNT surface. In the present investigation, filtration data are obtained in the pH range (3-10) for 800 ppm manganese solution and with 1.2 g Taunit and 0.3 g Taunit-M. As illustrated in Fig 6, removal efficiency of manganese is decreased as pH is increased. The graph reveals that, R increased significantly at pH=3, as it is around 63% for Taunit-M and 50% for Taunit, whereas at pH =10 decreases to 26% for Taunit-M and 19% for Taunit. At low pH the surface of Taunit become more positive and the attraction force to MnO4- increases. As the pH of the solution increases, the surface charge of CNTs becomes more negative probably because of the deposition of more hydroxide ions, increasing iron ion exchange. From an electrostatic interaction point of view, filtration of iron was favored at high pH. In the present investigation, filtration data are obtained in the pH range of (3-8) for iron initial concentration of 200 ppm and with 0.3 g raw and oxidized Taunit-M. From Fig 7, the removal efficiency of iron is increased as pH is increased. The graph reveals that for oxidized Taunit-M at pH=3, R is around 5%, whereas at pH=8, R increases to 32%. The effect of pH was studied for Ni(II) removal by oxidized MWCNT (Chungsyingand and Chunti 2006), this study showed maximum removal efficiency at pH range 8-11.


removal efficiency vs intial pH of manganese solution 80 60 40

Taunit M

20

Taunit

0 0

5

pH

10

15


Fig 6. Variation in removal efficiency with pH for manganese aqueous solution. removal efficiency vs pH of iron solution 40 30 20

raw Taunit- M

10

oxidized Taunit- M

0 2

4

6

pH

8

10

Fig 7. Variation in removal efficiency with pH of iron solution. Effect of CNT dosage on manganese removal efficiency

removal efficiency(%)

removal efficiency vs dosage CNTs filter 50 40 30 20 10 0


With an increase in dosage of adsorbent the removal increases. From Fig 8 one can see that Taunit-M filter is more efficient than Taunit. This is due to the catalyst impurities in Taunit, and a high purity of Taunit-M. This phenomenon implied that the filtration depended on the availability of binding sites. removal efficiency vs dosage CNTs filter 50 30 10

-10

0

0.1

0.2

0.3

0.4

Weight of Taunit M (g)

0

0.4

0.8

1.2

1.6

Weight of Taunit (g)

(a) (b) Fig 8. Variation in removal efficiency of manganese with dosage of Taunit-M (a) and Taunit (b) at pH=7 and manganese concentration 800 ppm in the aqueous solution.

Manganese solution flux in MWCNT filters The flux of 800 ppm manganese solution in Taunit and Taunit-M was calculated as follow:

Flux (kg / m s)  2

T m  Am

Flux ( Kg/m2.S)x102

Where m is the mass of the permeate collected over the time  =10 min, Am was the active membrane area (3.14x10-4 m2), and T is the temperature correction factor calculated using the relation αT = -0.575 lnT+2.85, where T is the temperature in Celsius (Sourirajan 1970). Fig 9 demonstrates that the solution flux through Taunit is higher than through Taunit-M. The flux in Taunit is higher than Pore-filled Membranes for salt rejection (Jiang et al. 2003), although the flux in Taunit obtained just at the atmospheric pressure(in contrast with Jiang experiment).

15 10 Taunit M

5

Taunit Taunit M

Taunit

0

Type of filter

Fig 9. Variation of flux with Taunit and Taunit-M filters.

Conclusion The application of carbon nanotubes for the removal of heavy metal ions from liquid solutions is one of the pioneer studies. In this study the application of both raw and oxidized CNTs were investigated as potential filters to remove manganese and iron from solutions. Some parameters were found important to determine the removal efficiency: the concentration of metals in aqueous solution, pH and the filter mass. All of the parameters used in the filter experiment were significant and have direct impacts on the removal efficiency which was identified from the regression analysis. Basing on the data obtained, the predominant ion exchange mechanism involving surface functional groups of oxidized MWCNTs was presumed. From the characterization of the Taunit-filter, it was found that the interaction of iron with oxidized CNTs is higher compared to raw CNTs. This is due to the fact that oxidation provides oxygen containing groups, reduce diameter of carbon tubes and remove impurities. It was also noted that the key factors that favor the removal efficiency of manganese are low pH and low initial concentration, while for iron, high pH and low initial concentration. Thus, MWCNTs provides high expectations on the development of wastewater treatment and environmental contamination reduction.

References: Akpomie K. G. and Dawodu F. A. (2014). Simultaneous adsorption of Ni(II) and Mn(II) ions from aqueous solution unto a Nigerian kaolinite clay. Journal of Materials Research and Technology, 3(2),129–141. Chen J., Chen Q., and Ma Q. (2012). Influence of surface functionalization via chemical oxidation on the properties of carbon nanotubes. Journal of Colloid and Interface Science, 370( 1), 32–38. Choo K. H., Lee H. and Choi S. J. (2005). Iron and manganese removal and membrane fouling during UF in conjunction with prechlorination for drinking water treatment. Journal of Membrane Science, 267(1-2),18-26. Chungsying Lu., Chunti Liu. (2006). Removal of nickel(II) from aqueous solution by carbon nanotubes. J.Chem.Technol.Biotechnol, 81(12),1932–1940. Corry B. (2008). Designing Carbon Nanotube Membranes for Efficient Water Desalination. J. Phys.Chem.B, 112 (5), 1427–1434. Dyachkova T. P., Melezhyk A. V., Gorsky S. Yu., Anosova I.V. and Tkachev A. G. (2013). Some aspects of functionalization and modification of carbon nanomaterials. Nanosystems: Physics, Chemistry, Mathematics, 4(5), 605–621. Forstner U. and Wittmann G. T. W. (1983). Metal Pollution in the Aquatic Environment. Springer-Verlag, Berlin, Heidelberg, New York, Tokyo. Ganesan P., Kamaraj R., Sozhan G. and Vasudevan S. (2013). Oxidized multiwalled carbon nanotubes as adsorbent for the removal of manganese from aqueous solution. Environmental Science and Pollution Research, 20(2), 987-996. Jiang W., Childs R. F., Mika A. M. and Dickson J. M. (2003). Pore-filled Membranes Capable of Selective Negative Rejections. Nature and Science,1(1),21-26. Long Q. R. and Yang R. T. (2001). Carbon nanotubes as superior sorbent for dioxin removal. Chem.Soc.J, 123, 2058–2059. Macedonio F., Drioli E., Gusev A. A., Bardow A., Semiat R. and Kurihara M. (2012). Efficient technologies for worldwide clean water supply. Chemical Engineering and Processing, 51, 2–17. Oehmen A., Viegas R., Velizarov S., Reis M. A. M. and Crespo J. G. (2006). Removal of heavy metals from drinking water supplies through the ion exchange membrane bioreactor. Desalination, 199(1-3), 405–407. Penate B. and Garcia-Rodriguez L. (2012). Current trends and future prospects in the design of seawater reverse osmosis desalination technology. Desalination, 284,1–8.

Ren X., Chen C., Nagatsub M. and Wang X. (2011). Carbon nanotubes as adsorbents in environmental pollution management, A review. Chem.Eng.J,170,395–410. Shannon M. A., Bohn P. W., Elimelech M., Georgiadis J. G., Marinas B. J. and Mayes A. M. (2008). Science and technology for water purification in the coming decades. Nature, 452(7185), 301-310. Sheikholeslami R. (2009). Strategies for future research and development in desalination challenges ahead. Desalination, 248 (1-3), 218–224. Smart S. K., Cassady A. I., Lu G. Q. and Martin D. J. (2006).The biocompatibility of carbon nanotubes. Carbon,44,1034–1047. Sourirajan S.(1970) Reverse Osmosis. Academic Press, New York. Stafiej A. and Pyrzynska K. (2008). Adsorption of heavy metal ions with carbon nanotubes.Sep.Purif.Technol,58, 49–52.