Optimization of sulfate removal from wastewater using magnetic multi-walled carbon nanotubes by response surface methodology

As clean water resources are vital and are becoming low, water pollution is a critical topic for human existence and ecosystems. Several media have declared that water contamination is the main reason for the death of more than 14,000 people daily (Water Standards & World Health Organization 2006; Alimohammadi et al. 2017). Wastewater treatment serves two main objectives: protecting the environment and conserving freshwater resources (Bukhari 2008). Sulfate anions are hazardous ions when they exceed certain limits in the water. Sulfates are discharged into the water from mines and smelters and from kraft pulp and paper mills, textile mills, and tanneries (Hassan et al. 2010). The damage caused by sulfate emissions is not direct since sulfate is a non-toxic compound. However, the high sulfate concentration can unbalance the natural sulfur cycle (Li et al. 2011). Sulfate is a major anion occurring in natural water and industrial effluents, such as inorganic chemical industry wastewater and acid mine drainage (Cao et al. 2011). In the wastewater purification, little consideration has been focused on the mitigation of dissolved sulfate; this may be attributed to its lower environmental risks and regulatory standards when compared to those for acidity and dissolved metals. Consuming drinking water containing sulfate concentrations in the excess of 600 mg/l commonly results in laxative effects. The taste threshold for the most prevalent sulfate salts ranges from 250 to 500 mg/l. While the World Health Organization does not propose a health-based guideline for sulfate in drinking water, it does recommend that health authorities are notified if sulfate concentrations exceed 500 mg/l (Haghsheno et al. 2009). When discharged untreated, it poses a threat to the quality of freshwater resources and consequently the well-being of humans and the environment at large (Singh et al. 2011). Established methods for the removal of sulfate from industrial effluents include chemical precipitation, biological treatment, reverse osmosis, dialysis and ion exchange and adsorption technologies. Chemical precipitation, for example, to add barium or calcium salts, is rapid and effective, but it may produce another kind of pollution and secondary treatment for the solid phase is necessary (Haghsheno et al. 2009; Cao et al. 2011). Biological sulfate reduction processes have been used to treat a wide range of sulfate- rich wastewaters including acid mine drainage, tannery and textile wastewater, and landfill leachate. The biodegradation of phenolic and other persistent organic contaminants by sulfate-reducing bacteria culture has also been studied (Rasool et al. 2013). Although reverse osmosis has a high performance for wastewater treatment, the accumulation of matter may occur during the membrane filtration of surfactants, resulting in severe flux decline (Mai et al. 2016). Selecting the wastewater treatment method is usually based on the type of wastewater, removal rate, the waste concentration and cost of the treatment (Haghsheno et al. 2009). The adsorption method may be preferred for its rapid and high selectivity, and sulfur can be recovered (Cao et al. 2011). Adsorption is proven to be an economical and effective method for the removal of a number of contaminants from wastewater (Perić et al. 2004; Deliyanni et al. 2007; Liu et al. 2008; Yang et al. 2009; Mobasherpour et al. 2012). Adsorption methods are the most applied in the removal of sulfate from wastewaters since they can produce high-quality water and also are economically feasible processes (Tait et al. 2009). Zero-valent iron (ZVI) is a reductive material that can serve as an electron donor for both sulfate reduction and methanogens. ZVI has been used as a permeable reactive barrier in the underground water treatment to simultaneously immobilize heavy metals by enhancing sulfate reduction and mineralize organochlorine pesticides through enhanced methanogens (Zhang et al. 2011). Multi-walled carbon nanotubes (MWCNTs) have been widely used for the removal of various contaminants from wastewater due to their large surface area, light mass density, high porosity and hollow structure, small size, and strong interaction between the pollutant molecules and MWCNTs (Di et al. 2004; Chen & Wang 2006; Wang et al. 2007; Chen et al. 2009; Kabbashi et al. 2009; Shao et al. 2010; Ren et al. 2011; Zhang et al. 2012; Al-Hakami et al. 2013; Gu et al. 2013; Laoui et al. 2015; Al Amer et al. 2016). Among the various technologies, magnetic adsorption is a new method to remove sulfate from industrial wastewaters, since it can produce high-quality water and also is a process that is economically feasible (Madrakian et al. 2011). The MWCNTs have received more and more consideration for their unique structure and excellent mechanical, electrical, and thermal properties. The MWCNTs have attracted researchers' interest as a new type of powerful solid-phase extraction adsorbent (Qu et al. 2008). The MWNTs/Fe₃O₄ nanocomposites could respond rapidly to a permanent magnet, and after adsorption, the nanocomposites could be separated from solution easily by an external magnet (Song et al. 2011). Also, iron oxide nanoparticles that are attached to MWCNTs do not separate from the magnetic multi-walled carbon nanotubes (MMWCNTs) during blending with wastewater and the presence of iron on the MWCNT does not cause the removal of sulfate by the Fenton reaction (see Appendix, available with the online version of this paper). In this paper, we wish to report an eco-friendly method for the removal of sulfate using MMWCNTs extract without any stabilizer or surfactant. We synthesized and characterized the magnetic carbon nanotubes and investigated the adsorption behavior of the MWCNTs/Fe₃O₄ for the removal of sulfate in wastewater. Thus, the aim of this study was to report systematically the effect of the pH and D/C ratio (adsorbent dosage per initial concentration of pollutant (mg_adsorbent/(mg/l)_initial)) on the removal of sulfate by MMWCNTs. In addition, the response surface methodology (RSM) was applied as a systematic experimental design method to predict the maximum percent of sulfate removal. The targeted usage of MMWCNT is completely new and promising for this field of investigation.

The MWCNTs (the lengths were a few micrometers and the average outer diameter was 10–30 nm) were used in this study. MWCNTs were bought from Nano Port Co. Ltd (Shenzhen, China). FeCl₃.6H₂O, FeCl₂.4H₂O, NH₃.H₂O and Na₂SO₄ of analytical grade were obtained from Merck and Fluka. A Metrohm model 713 pH-meter was used for pH measurements. All glassware were soaked in dilute nitric acid for 24 h and finally washed three times with distilled water before use. A neodymium magnet was used in producing the magnetic field for sulfate- water separation (maximum field strength theory: 13.2 kG).

The MWCNTs were dispersed in concentrated sulfuric acid and concentrated nitric acid (a 3:1 volume ratio) to modify the MWCNTs surface by ultrasonication for 4 h. This acid mixture containing MWCNTs was diluted to 20% of its original concentration. The oxidized MWCNTs were filtered with a polytetrafluoroethylene filter membrane (Alltech, 0.45 μm pore size) with the aid of a vacuum pump; then it was washed by double-distilled water until the filtrate was neutral. MWCNTs were dried in the oven at 80 °C for 12 h. Thirty-five milligrams of functionalized MWCNTs were dissolved in 22 ml of distilled water by ultrasonic irradiation for 36 min. Then 25 mg of FeCl₃.6H₂O was added under stirring. After the mixture was stirred vigorously for 30 min under N₂ atmosphere, 37 mg of FeCl₂.4H₂O was added and the mixture stirred under N₂ atmosphere for 25 min. Six millilitres of concentrated NH₃.H₂O diluted with 16 ml of distilled water was added to the mixture drop by drop. The adding of NH₃.H₂O aqueous solution was finished in 90 min and then the solution was heated to 100 °C and reacted for 3 h. The whole process must be under N₂ atmosphere. The reaction mixture was then filtered, washed with distilled water and dried at 60 °C for 24 h (see Figure S1 and Scheme S1 in Appendix, available with the online version of this paper).

The chemical structure of γ-Fe₃O₄ nanoparticle functionalized with MWCNT was investigated by X-ray diffraction (XRD). The XRD measurements were carried out using a Philips powder diffractometer type PW 1373 goniometer (Cu Kα = 1.5406 Å). The Raman, Fourier transform infrared (FTIR) and magnetic properties analysis are shown in the Appendix. Transmission electron microscopy (TEM) studies were carried out on a JEOL JEM 2200FS field emission electron microscope (equipped with an energy-dispersive spectrometer) at 200 kV. The specific surface area and pore size of MWCNTs and MMWCNTs were determined by nitrogen adsorption/desorption porosimetry at (77.4 K) using a porosimeter (Bel Japan, Inc.).

Sulfate stock solution containing 147.9 mg Na₂SO₄/l was prepared by dissolving sulfate salt (analytical reagent grade) in distilled water that contains 100 ppm of SO₄. The pH value of the sulfate working solution was adjusted with 0.1 M HCl and 0.1 M NaOH solutions before adsorption experiments.

Phase investigation of the crystallized product was performed by powder XRD measurements and the powder diffraction pattern of MWCNTs is presented in Figure 1(a). The diffraction peaks at 2θ = 25.55° and 44.49° are consistent with (002), (100) planes of the MWCNTs. Figure 1(b) represents the XRD pattern of MMWCNTs, which contains all the carbon nanotube and iron oxide nanoparticle peaks. The peaks at 2θ = 29.74, 35.25, 43.09, 57.09 and 62.67° were identified as the (220), (311), (400), (511) and (440) crystal planes of the spinel phase of Fe₃O₄ (JCPDS No. 65-3107), respectively (Wang et al. 2014). The XRD results confirm that MWCNTs/Fe₃O₄ nanocomposites were successfully synthesized by a facile hydrothermal method. No crystalline impurities were detected and the existence of Fe₃O₄ nanoparticles in the nanocomposites was confirmed.

Figure 2 shows the TEM images of the magnetic carbon nanotubes. Most of the carbon nanotubes were loaded with iron oxide nanoparticles. Figure 2(a) indicates that no separated nanoparticles were present, which suggests that the unattached nanoparticles have been completely removed from the product. The image of MWCNTs/Fe₃O₄ nanocomposites (Figure 2(b)) illustrates that some nanoparticles assembled on the MWCNTs but still adhered stably on the tube surface. It indicates the high efficiency of our synthesis method.

Specific surface area (SSA) is analyzed commonly by the Brunauer, Emmett, and Teller (BET) method. The SSA of the sample is measured by the physical adsorption of a gas on the surface of the solid and by measuring the amount of adsorbed gas corresponding to a monomolecular layer on the surface (Madrakian et al. 2011). The BET measurements found that the SSA of MWCNTs and MWCNT/Fe₃O₄ was 127.91 m²/g and 112.26 m²/g, respectively. When the Fe₃O₄ particles are added to the MWCNTs, the surface area of the MMWCNTs decreases slightly (Qu et al. 2008).

The simultaneous effects of two independent variables, pH and D/C, on sulfate removal were analyzed. The responses corresponding to the contour plots of the second order predicted model indicated that, for high pH, the percent of sulfate removal increases with increasing D/C (Figure 3). Equation (4) is used to construct the response surface and contour plots. Figure 3 proves that the pH and D/C are important parameters affecting the removal of sulfate. However, sulfate removal increases as pH is increased to 5.96, then decreases as pH further increases to 10. At pH = 10 and D/C = 10, the removal of sulfate reaches a minimum value (29%), while at pH = 5.96 and D/C = 24.35, the removal reaches a the maximum value.

The regression model and perturbation plot of two independent factors (pH and D/C) are indicated in the Appendix (Figure S5, available with the online version of this paper).

The pH and D/C of a solution are important parameters during the adsorption process. The pH affects the surface charge of the adsorbent and the degree of ionization and specification of adsorbate. In order to establish the effect of pH, the equilibrium studies at different pH and D/C values were carried out in the range of 3–10 and 10–30, respectively. As shown in Figure 4(a), with the increase of D/C at pH = 5.96, the removal of sulfate increased from 57% to 93.28%. Figure 4(b) shows the pH of the solution has a significant effect on adsorption capacity. With an increase of pH from 3 to 10, the removal increased from 81% to 93.28% at D/C = 24.35 and pH = 5.96. The maximum removal of sulfate by MMWCNT was 93.28% at pH = 5.96 and D/C = 24.35. Therefore, the pH and D/C values of the wastewater are important factors for sulfate removal.

The effect of equilibrium concentration on the adsorption of sulfate was investigated at pH 5.96 and D/C 10–30. Results are presented in Figure 4(c). It could be concluded that the maximum adsorption capacity of MMWCNT increased up to approximately 56.94 mg/g.

This can be due to the negative charge of the surface of MMWCNT in a high pH range. The negative charge of the surface of the nanoparticles was also confirmed from the data on the zeta potential (Legodi & De Waal 2007). At the higher pH, the negatively charged parts generally increased, but the electrostatic repulsion from the negatively charged parts of the adsorbent does not favor absorption of sulfate (Alimohammadi et al. 2016). The results indicate that the nanocomposite is more effective in the acidic pH than high pH.

Result validations of the final optimal point in the multi-responses optimization are presented in Table S3 in the Appendix.

The isotherms of sulfate adsorption by MMWCNTs in wastewater can be investigated using the Langmuir and the Freundlich adsorption models (Li et al. 2015). In order to optimize the use of MMWCNTs adsorbents, it is important to establish the most appropriate adsorption isotherm. Thus, the correlation of equilibrium data by either theoretical or empirical models is essential for practical operation. Langmuir and Freundlich equations were used to analyze the experimental data of the MMWCNTs adsorbents for sulfate in our work.

The adsorption constants calculated according to the Langmuir and Freundlich adsorption models are shown in Table 4. It can be observed that the sulfate adsorption isotherms of MWCNTs/Fe₃O₄ nanocomposites followed the Langmuir adsorption model better than the Freundlich adsorption model. Initial concentration range of 3.33–10.00 mg/l) is assumed. As shown in Table 4, the experimental data of sulfate removal fit the Langmuir adsorption isotherm well with a correlation coefficient of 0.9862 (0.99).

A comparison with other adsorbents that have been used for removal of sulfate by previous researcher is shown in the Appendix.

In conclusion, the authors have indicated an environmentally friendly method for the removal of sulfate from wastewater. The MMWCNTs were shown to be effective adsorbents, which could be employed in wastewater treatment. In addition, the prepared magnetic adsorbent could be well dispersed in the wastewater and can be easily separated magnetically from the solution after adsorption. Modeling and optimization of the removal of sulfate and its stability were studied, using RSM–CCD. Two parameters, namely pH and D/C, were the control factors in this study. One quadratic model for sulfate removal was developed. ANOVA results confirmed that there was a significant agreement between the models and the experimental data. The results show that pH and D/C were the factors with statistically highly significant effect. The optimum point was found at pH = 5.96 and D/C = 24.35 with the predicted removal of 93.28%. MMWCNT is a superior absorbent for the removal of sulfate from wastewater. Both the Langmuir and Freundlich isotherm models were investigated to describe the sulfate adsorption data, showing a higher suitability of the Langmuir isotherm model. Overall, the properties of easy regeneration and stable adsorption behavior enable MMWCNTs to be an excellent adsorbent material to use in wastewater purification.