Carbon nanotubes (CNTs) have become the 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). The effects of different parameters such as CNT filter mass, concentration of manganese and iron in aqueous solution and 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 50 ppm, suggesting that Taunit is an excellent adsorbent for manganese and iron removal from water. There was 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 scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques to investigate the functionalization with oxygen-containing and outer diameter distribution. It was found that functionalized CNT-based filters are more efficient at removing manganese and iron from aqueous solutions. Oxidized MWCNTs may be a promising candidate for heavy metal ion removal from industrial wastewater.

INTRODUCTION

The 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 wastewater 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 in the human body. Manganese effects occur mainly in the respiratory tract and in the brains (Forstner & 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 forms like the ferric iron, or has 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, giving a metallic taste to water and 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 ferrobacterias. Iron bacteria do not cause health problems, but cause odors, corrode plumbing equipment, and 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 & Dawodu 2014).

The common methods for removing these metals from water, such as 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 & Garcia-Rodriguez 2012; Ganesan et al. 2013). At present, chemical treatments are not used due to disadvantages such as high costs of maintenance. The advent of nanotechnology for water purification has brought much hope. The success of carbon nanotube (CNT) membranes as filters is based on their unique properties, which include high surface area, and good mechanical and thermal stability (Smart et al. 2006; Corry 2008). 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 & Yang 2001). The main advantage of this is reduced costs through energy savings. So their applicability for removal of hazardous pollutants from aqueous streams has been studying extensively (Stafiej & Pyrzynska 2008; Ren et al. 2011). 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 chemical vapour deposition (CVD) preparation process were removed during the course of oxidation treatment. The functionalization of CNT 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 may be a result of the 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 multiwalled carbon nanotube (MWCNT) (Taunit-filter), and the effects of pH, ion concentrations and CNT dose on the filtration process.

METHODS

Characterization and modification of the filter

The material of the present study is an industrial carbon nanotube Taunit. ‘Taunit’ and ‘Taunit-M’ CNTs are produced by ‘Nanotech Center’ (Tambov, Russia). This material is a loose black powder, composed of grainy agglomerates of MWCNTs with a size of several micrometres. Their properties are listed in Table 1. To perform the oxidation of the MWCNT filter, 0.5 g of Taunit-M was dispersed in a flask with 20 ml of 0.5 mol/L sulfuric acid by ultrasonic vibration, then 0.5 and 1 g of potassium permanganate were dissolved in 20 ml of 0.5 mol/L sulfuric acid and added to the flask drop by drop. 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

‘Taunit-M’‘Taunit’Parameter/CNTs
(30–80) (20–70) External diameter, nm 
(10–20) (5–10) Internal diameter, nm 
20 and more 2 and more Length, μm 
(180–200) (120–130) Specific surface area, m2/g 
(0.03–0.05) (0.4–0.6) Bulk density, g/cm3 
‘Taunit-M’‘Taunit’Parameter/CNTs
(30–80) (20–70) External diameter, nm 
(10–20) (5–10) Internal diameter, nm 
20 and more 2 and more Length, μm 
(180–200) (120–130) Specific surface area, m2/g 
(0.03–0.05) (0.4–0.6) Bulk density, g/cm3 

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 ensure that the CNTs could not penetrate through the filter, and put into a syringe (Figure 1). The 1,000 ppm manganese and iron stock solutions were prepared by dissolving 1 g of KMnO4 and 0.18 g of FeSO4·7H2O in 1,000 ml of distilled water, then diluted to the desired concentration. The pH of the stock solutions was adjusted using buffer solutions.
Figure 1

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

Figure 1

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

Experimental procedure

Table 2 shows the experimental parameters and their variations. The removal efficiency (R) is defined as follows: 
formula
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.
Table 2

Experimental parameters and their variation

 Variation
ParameterLowMediumHigh
1. Concentration (ppm) 
 Manganese 50 200 800 
 Iron 50 100 200 
2. pH 
 Manganese 10 
 Iron 
3. Filter dosage (g/50 ml) 
 Raw Taunit 0.4 0.8 1.2 
 Raw and oxidized Taunit-M 0.1 0.2 0.3 
 Variation
ParameterLowMediumHigh
1. Concentration (ppm) 
 Manganese 50 200 800 
 Iron 50 100 200 
2. pH 
 Manganese 10 
 Iron 
3. Filter dosage (g/50 ml) 
 Raw Taunit 0.4 0.8 1.2 
 Raw and oxidized Taunit-M 0.1 0.2 0.3 

RESULTS AND DISCUSSION

Surface analysis of oxidized MWCNTs

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

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

Figure 2

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

Figure 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.

Figure 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 increases. One can see that R is only 37% with 1.2 g Taunit and 47% for 0.3 g Taunit-M at a 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 concentrations of KMnO4 solution at pH = 7

C (ppm)K (μS/cm) before filtrationK (μS/cm) after filtration (Taunit-M)K (μS/cm) after filtration (Taunit)R (%) (Taunit-M)R (%) (Taunit)
800 762 402 480 47.24 37.01 
200 191 72 94 62.30 50.79 
50 49 14 19 71.43 61.22 
C (ppm)K (μS/cm) before filtrationK (μS/cm) after filtration (Taunit-M)K (μS/cm) after filtration (Taunit)R (%) (Taunit-M)R (%) (Taunit)
800 762 402 480 47.24 37.01 
200 191 72 94 62.30 50.79 
50 49 14 19 71.43 61.22 

Effect of iron concentration in aqueous solution

A comparative study of the filtration efficiency of raw and oxidized Taunit-M is performed on iron removal as a function of iron concentration in aqueous solution. From Figure 4 one can see that the concentration of iron aqueous solution increases as 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 an ionic interaction nature, which is in agreement with an ion exchange mechanism, as illustrated in Figure 5. High concentration of iron limits its transfer to oxidized MWCNT surfaces.
Figure 4

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

Figure 4

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

Figure 5

Schematic diagram for the interaction of iron and manganese with oxidized MWCNTs.

Figure 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 Figure 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 it 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 Figure 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 (Chungsying & Chunti 2006 ), this study showed maximum removal efficiency at pH range 8–11.
Figure 6

Variation in removal efficiency with pH for manganese aqueous solution.

Figure 6

Variation in removal efficiency with pH for manganese aqueous solution.

Figure 7

Variation in removal efficiency with pH of iron solution.

Figure 7

Variation in removal efficiency with pH of iron solution.

Effect of CNT dosage on manganese removal efficiency

With an increase in dosage of adsorbent, the removal increases. From Figure 8 one can see that the Taunit-M filter is more efficient than Taunit. This is due to the catalyst impurities in Taunit, and the high purity of Taunit-M. This phenomenon implied that the filtration depended on the availability of binding sites.
Figure 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 aqueous solution.

Figure 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 aqueous solution.

Manganese solution flux in MWCNT filters

The flux of 800 ppm manganese solution in Taunit and Taunit-M was calculated as follows: 
formula
where m is the mass of the permeate collected over the time τ = 10 min, Am was the active membrane area (3.14 × 10−4 m2), and αT is the temperature correction factor calculated using the relation αT = −0.575 lnT + 2.85, where T is the temperature in Celsius (Sourirajan 1970). Figure 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 was obtained just at the atmospheric pressure (in contrast with the Jiang experiment).
Figure 9

Variation of flux with Taunit and Taunit-M filters.

Figure 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 to be 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. Based 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, it reduces the diameter of the carbon tubes and removes impurities. It was also noted that the key factors that favor the removal efficiency of manganese are low pH and low initial concentration, and for iron, high pH and low initial concentration. Thus, MWCNTs provide high expectations for the development of wastewater treatment and environmental contamination reduction.

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