Removal of Ni(II) from aqueous solution by using micellar enhanced ultrafiltration

Various types of inorganic and organic pollutants can be found in the wastewater and discharges of metal plating factories, textile, tanneries and photographic processing industries. These pollutants are not only dangerous in terms of aesthetic sense but also pose serious threats to the health of both human beings and sea life, if their concentration goes beyond a certain limit. Hence, the removal of such pollutants from wastewater is vital prior to its disposal into the main stream river or sea. Environmental pollution due to hazardous metals is of major concern in modern societies (Liehr et al. 1994; Matheickal & Yu 1996; Chong et al. 2000). Some of the conventional methods for the efficient removal of metal ions from wastewater include boiling, adsorption (Crini 2006), biodegradation (Pearce et al. 2003), coagulation and flocculation, distillation, ion exchange, ozone disinfection, reverse osmosis, microfiltration (Venkiteshwaran & Belfort 2010), nanofiltration (Hafiane et al. 2000), ultrafiltration (Malaviya & Singh 2011) and the membrane separation process (Koyuncu et al. 2004). The removal of contamination by boiling the wastewater is the simplest way of purification, particularly if the contaminants are protozoan, parasites or bacteria. However, this method is quite energy consuming and not suitable for large-scale water treatment. In contrast, adsorption is the least expensive, useful, concise, easy to handle and efficient method (Iqbal & Ashiq 2007). However, due to being intrinsically slow in nature, the efficiency of this process is limited by the equilibrium. Similarly, the biodegradation method is also quite unsuitable for the efficient removal of metal ions from the contaminated water. So far, the membrane-based purification processes have proved to be the most suitable alternative to the aforementioned techniques (Zaghbani et al. 2007).

Among some of the commonly used membrane processes are reverse osmosis and nanofiltration. However, both of these techniques require thick membranes, which eventually lead to low permeate flux. In contrast, ultrafiltration is found to be quite efficient and gives maximum flux at low pressure. This process combines the power of high selectivity of reverse osmosis and high flux of ultrafiltration together in one technique. More recently, it is seen that by adding an appropriate surfactant solution, the efficiency of ultrafiltration can be further enhanced. The modified technique is, therefore, termed as micellar enhanced ultra-filtration (MEUF). Nowadays, MEUF is considered as one of the most promising techniques for the surfactant based removal of metal ions from wastewater. The choice of appropriate surfactant in MEUF process is of prime importance and depends on the nature of contaminant to be removed. Surfactant molecules join each other, beyond critical micelle concentration (Rosen 1978). Electrostatic attraction causes metal ions to adsorb at the surface of anionic micelle and, thus, true solution of metals ions is converted to colloidal solution. These colloidal sized micelles having entrapped/solubilized metal ions can be easily rejected using ultra membranes of suitable molecular weight cutoff (MWCO), while the permeate passes through membrane pores along with the traces of surfactant monomers and non-solubilized metal ions (Purkait et al. 2004; Zaghbani et al. 2007; Bade & Lee 2011; Khosa & Shah 2011).

The present study reports the rejection of Ni(II) by using the micellar solutions of two different anionic surfactants viz. dioctyl sodium sulfosuccinate (DSS) and sodium dodecyl sulfate (SDS). The major objective of the present study was to determine the concentration dependent removal efficiency of MEUF process to effectively remove the Ni(II) ions from aqueous solution. Figure 1 shows the structures of DSS and SDS surfactants used in this study.

Ni(II) chloride and surfactants viz. DSS and SDS, having 99% purity, were purchased from Sigma-Aldrich and used without further purification. Distilled water was used throughout the experimental work.

The ultrafiltration experiments were carried out in a stirred cell (Amicon 8400, Millipore, USA). Table 1 displays the characteristics of membranes used in this study. Before starting the ultrafiltration process, the membranes were washed with deionized water to remove soluble impurities (Khosa & Shah 2011). A transmembrane pressure of 1.5 bar was maintained at room temperature during filtration. The concentrations of metal ions in feed and permeate solution was determined using UV–visible spectrophotometer (U-2800, Hitachi).

The efficiency of MEUF process was assessed by calculating the rejection coefficient (R%) and permeate flux (J) of the Ni(II) ion containing solution.

where the C_P and C_F are the concentration of pollutants in permeate and feed solutions, respectively.

where V is the volume of permeate solution, t is the time taken by ultrafiltration, and A is the effective area of membrane used.

Figure 2(a) displays the trend of change in rejection coefficient of Ni(II) ions by DSS solution within the concentration range of 0.73 mM to –5.9 mM using 30,000 MWCO and 10,000 MWCO membranes, respectively. The tendency to remove metal ions corresponds with stern layer of micelles which attracts oppositely charged metal ions via electrostatic force of interaction. We found that the maximum rejections of Ni(II) ions without surfactant were 44.11% and 51.86%, while in the presence of DSS the same increased to 83.24% (using 30,000 MWCO membranes) and 84.3% (using 10,000 MWCO membranes), respectively. There was a rapid increase in the rejection of Ni(II) ion at the start, which eventually leveled off with the passage of time due to membrane plugging. A plausible explanation of this phenomenon may be offered that an increase in the surfactant concentration has produced increasing number of micelles which, in turn, engaged more and more metal ions via electrostatic force of attraction. The results suggest that the maximum removal of Ni(II) ions was obtained upon using membrane of 10,000 MWCO, at low transmembrane pressure (1.5 bars). These results can be explained that in the pre-micellar region, the high rejection rate can be ascribed to the adsorption of metal ions, surfactant monomers, dimers and multimers while, in the post-micellar region the observed high rejection rate could be due to the adsorption of admicelles onto the membrane surface. Furthermore, the effect of DSS concentration on Ni(II) removal was more apparent, when membrane of 30,000 MWCO was used. Purkait et al. (2004) have suggested that the membrane having such a low MWCO can efficiently reject contaminants even without surfactants but such membranes are very expensive and efforts are being made to use less expensive membranes of high MWCO and enhance their efficiency by MEUF.

As shown in Figure 2(b), the permeate flux (J) of Ni(II) ions decreases with DSS concentration (57.79–20.92 L/h m² using membranes of pore sizes MWCO 30,000 and 48.5–15.17 L/h m² using membranes of pore sizes MWCO 10,000) at fixed operating pressure of 1.5 bar. The decreasing trend of permeate flux (J) with increasing surfactant concentration is due to membrane plugging. The combined effect of solubilization and micellization prohibit Ni(II) ions and surfactant monomers/micelles to pass through the membrane pores, which has caused the value of J to decrease (Khosa & Shah 2011).

Figure 3(a) shows the effect of SDS concentration on the rejection of Ni(II) ion using membranes of different pore size viz. 30,000 MWCO and 10,000 MWCO, respectively. The figure displays an increase in the values of rejection coefficient (R%) with concentration of SDS. Moreover, the highest values for Ni(II) ions rejection was found to be ∼80.60% and 85.52% in the presence of SDS surfactant and upon using membranes of 30,000 MWCO and 10,000 MWCO, respectively, while these values were only 54.77% (for 30,000 MWCO membrane) and 44.55% (for 10,000 MWCO membrane) in the absence of surfactants. It is clear that an increase in the concentration of surfactant has led toward higher concentration of micelles with a concomitant effect of large amount of metal ions solubilized in the micelles (Purkait et al. 2004). It is found that in MEUF process, metal ions can be removed to greater extent by using membrane of 10,000 MWCO, which can be understood from the combined effect of metal ions adsorption, surfactant monomers and dimers or trimmers on membrane surface and presence of micelles having adsorbed Ni(II) ions being retained by the membrane. The effect of SDS concentration on the metal ions removal became more apparent upon using 30,000 MWCO pore sized membrane. A rapid increase in the values of rejection coefficient (R/%) was observed at lower SDS concentration, while at higher surfactant concentration, the feed solution got saturated with the micelles and metal ion solubilization attained almost a constant value (Syamal et al. 1997).

Figure 3(b) displays the trend of changes in the values of permeate flux (J) for removal of Ni(II) ions as a function of SDS concentration. We can see that the values of permeate flux decreased from 28.22 L/h m² to 10 L/h m² and 34.67 L/h m² to 18.67 L/h m² with SDS concentration using membranes of pore sizes MWCO 30,000 and MWCO 10,000, respectively. This trend of decrease in permeate flux suggest that the Ni(II) ions might get entrapped in membrane pores and cause membrane plugging. The mechanism behind this phenomenon is attributed to the high retention rate due to formation of a deposited layer onto the surface of membrane, which increases the resistance against the solvent flux through the membrane and consequently lower down the values of permeate flux (Purkait et al. 2004). Jadhav et al. (2001) reported similar trends in their study for the removal of phenol and aniline by the MEUF process.

As clear from Figures 2(a) and 2(b) and 3(a) and 3(b) the values of retention coefficient (R/%) for Ni(II) ions removal increased up to 80.60% and 85.52% in the presence of SDS surfactant using membranes of 30,000 MWCO and 10,000 MWCO, respectively. Meanwhile, DSS has increased the same values up to 83.24% and 84.75% using 30,000 MWCO and 10,000 MWCO membranes, respectively. This indicates that SDS has better efficiency for Ni removal because it has lower critical micellization concentration (CMC) than DSS. SDS has 12 carbon atoms in hydrocarbon chain while in DSS both chains each contain eight C-atoms. Although the total number of C-atoms in DSS are greater (i.e. 16) than in SDS (i.e. 12) but in DSS, C-atoms are in two chains both attached to same hydrophilic group, while in SDS, 12 C-atoms are in a single chain. According to Rosen (1978) introduction of the hydrophilic group within hydrophobic groups causes significant increase in CMC. Therefore, SDS undergoes micellization more easily than does DSS and thus causes greater removal of Ni ions. Another factor is chain rearrangement during micellization; the SDS has single hydrophobic chain and can form micelle quite easily, while it is relatively difficult for DSS surfactant to undergo the same level of micellization due to its two hydrophobic chains attached to the same hydrophilic moiety (Rosen 1978).

Figure 4 shows the adsorption of Ni(II) ions onto the surface of SDS and DSS micelles due to the electrostatic force of attraction between the stern layer of counter ions on micelle surface and metal ions.

In the present study, micellar enhanced ultrafiltration (MEUF) technique was employed for the efficient removal of Ni(II) ions from the aqueous solution. By following a systematic series of investigations, the micellar solutions of two anionic surfactants viz. SDS and DSS were used with two different membranes (30,000 and 10,000 MWCO) to observe the effect of nature and concentration of the surfactants and membrane pore size in MEUF process. The effect of solution concentrations was observed in the range of 15–29 mM for SDS and from 0.73 to 5.9 mM for DSS. The experimental results revealed that when SDS surfactant was used, the values of retention coefficient (R%) showed an increase from 77.38% to 80.60% and 79.95% to 85.52% with membranes of 30,000 MWCO and 10,000 MWCO, respectively. Whereas, upon using DSS surfactant, the values of (R%) showed an increase from 71.93% to 83.24% and 80.75% to 84.75% for membranes of 30,000 MWCO and 10,000 MWCO, respectively. Our study suggests that the micellar solution of SDS is a better choice compared with DSS for the efficient removal of heavy metal ions from aqueous solutions. Moreover, the efficiency of MEUF process depends on the selection of the appropriate membrane, and in the present case 10,000 MWCO was found to be more efficient due to its relatively small pore size.

The data presented here originated from the Master's thesis of Mr Mansoor Khalid at Department of Chemistry, Government College University Faisalabad (Pakistan). All authors contributed at various stages of planning, execution and write-up of this manuscript. The authors are grateful to Dr Muhammad Arshad Khosa (University of Alberta, Canada), Mr Muhibullah (QAU), Mr Muhammad Ajmal, Dr Muhammad Zubair (GCUF) and Dr Muhammad Faizan Nazar (UOG) for providing theoretical and experimental support. Dr Usman Ali Rana is thankful to the Deanship of Scientific Research at King Saud University for its funding through the Research Group Project no. RGP-VPP-345.