In recent years,contamination with arsenic and cadmium metals in drinking water and in natural resources has become an important environmental issue that requires serious attention in many part of world. In addition,arsenic and cadmium have been reported to cause renal disturbances,neural problems,lung problems,bone lesions,digestive problems,and hypertension in humans. Also,both metal ions are non-degradable and responsible for severe problems like cancer in humans as well as in animals [1, 2]. Therefore,removal of these toxic heavy metal ions from aqueous solutions has gained more attention over the past few years.
Presently,adsorption is the most economical process for the removal of targeted metal ions from waste water due to its strong affinity and high efficiency . Adsorption by using superabsorbent materials is key to removing toxic species from natural resources and waste water . Superabsorbent hydrogels may offer the desired properties and potential applications in this research area . Several attempts have been made to modify the properties of superabsorbent polymeric materials,from macro to micro to nano particles. Although much more attention has been focused on the improvement of the swelling ability,gel strength, and mechanical and thermal stability of superabsorbents; the adsorption capability of heavy metals by superabsorbents is very important for the selection of a suitable adsorbent for metal removal from aqueous solutions [6, 7].
The present study deals with increasing the adsorption and equilibrium swelling of hydrogels by decreasing the particle size of the hydrogel to increase surface area. In this study,superabsorbent poly (NIPAAm/AA/N-allylisatin) nanohydrogels were synthesized, characterized and used as an adsorbent material for the removal of As(V) and Cd(II) metal ions from aqueous solutions. Finally,the equilibrium removal efficiency was analyzed according to the Langmuir and Freundlich adsorption isotherm model. 2. Experimental
Inverse microemulsion polymerization was employed to synthesize the superabsorbent nanohydrogels with a specified amount of N-isopropylacrylamide (NIPAAm) and by varying the acrylic acid (AA) and N-allylisatin content. Initially,for the continuous phase,0.5 g of AOT was added to 5 mL of toluene and stirred in dry N2 for 30 min. The temperature of the flask was maintained at 60℃ using a temperature controller.
The disperse phase was prepared by dissolving the required amount of NIPAAm with differing amounts of AA and N-allylisatin. The solution was stirred under N2 until a homogeneous solution was obtained. The disperse phase was then added dropwise into a continuous phase to form a W/O microemulsion. A cross linking agent,EGDMA,was added,followed by the addition of AIBN as a surface active initiator. Total conversion was obtained after 7 h of reaction.
Formed hydrogels were then transferred to a 1 L beaker containing double distilled water and left for 2-3 days by changing water every 4 h in order to remove the unreacted monomers and other reactants. The swollen gel was dried using acetone in order to ensure that the desired porosity was generated during solvent drying. The process was repeated until the dry hydrogel was obtained. Finally,the hydrogel was kept in a vacuum oven to constant weight. The feed compositions and relative percentage swelling of the nanohydrogels are given in Table 1.
Percentage swelling of the nanohydrogel in double distilled water and metal ion solutions with respect to time is shown in Fig. 1. Nanohydrogels were immersed for two days in distilled water and metal solutions (200 ppm) until hydrogel mass ceased to change.
|Fig. 1.Percentage swelling of nanohydrogel VNH 06 in (▲) distilled water and in 200 ppm solutions of (□) Cd(II) and (■) As(V) metal ions.|
Fig. 1 shows that percentage swelling of nanohydrogel increases with time until a certain point,after which a constant value was achieved. Nanohydrogel shows maximum swelling in double distilled water and minimum swelling in the As(V) metal ions solution. In general,water diffusion into the hydrogel network gives more swelling in water,while metal ions bonding with the functional groups of the hydrogel,which decrease the ionic pressure of hydrogel,leading to lower swelling in metal ions solution. At the end of the swelling studies,it was concluded that hydrogels treated with the As(V) metal ion solution show low swelling due to more binding capacity with the nanohydrogel,and percentage of swelling proceeded in the order of: water - > Cd(II) > As(V). 3.2. Removal study of As(V) and Cd(II) metal ions
In general,adsorption of metal ions on the nanohydrogel is mainly due to electrostatic interactions and hydrophobic interactions. Electrostatic interactions occur between the ionizable groups of the nanohydrogel and the specific charge of metal ions. Hydrophobic interactions involving hydrogen bonds are expected to occur between the oxygen atoms (oxy anions) of metal ions with amine,methyl and methine groups on the monomer unit of the nanohydrogel. The possible complexation process between nanohydrogel and metal ions is shown in Scheme 1.
|Scheme 1.Possible complexation process between nanohydrogel and metal ions.|
The effect of treatment time on the maximum metal ion uptake was investigated to compare the relative performance of such nanohydrogels and to determine the time at which equilibrium adsorption takes place . The effect of treatment time for As(V) and Cd(II) metal ions adsorption on the nanohydrogel VNH 06 was presented in Fig. 2. The saturation time for metal uptake of the nanohydrogel was obtained by plotting removal efficiency (RE) of the nanohydrogel with time,keeping the initial metal concentration (1000 ppm) and pH constant. The maximum adsorption occurred in the first 8 h,after which absorption slows down and finally levels off after 24 h. The variation of contact time experiments showed that the maximum removal efficiency for As(V) was ~88% and that for Cd(II) metal ions was ~77%. As expected,this result parallels the result of the equilibrium percentage swelling of nanohydrogel experiments.
|Fig. 2.Effect of treatment time on metal ions removal (initial metal ions concentration: 1000 ppm,adsorption dose: 25 mg).|
To determine the effect of initial metal ions concentration on the removal efficiency,the nanohydrogel was equilibrated with a series of metal ion solutions of gradually increasing concentrations . Removal efficiency of the nanohydrogel toward As(V) and Cd(II) ions with different initial ions solution concentrations varying from 50 ppm to 1000 ppm are given in Fig. 3. All measurements were performed at their optimum pH value. First, the removal efficiency increases rapidly with increasing initial metal ion concentration,then it reaches saturation and gives a plateau after 87% for As(V) and 73% for Cd(II) metal ions. After that, increasing the initial metal ions concentration caused no more effective change in RE (%) in the nanohydrogels was observed.
|Fig. 3.Effect of initial metal ion concentration (treatment time: 8 h,adsorption dose: 25 mg).|
In general,pH affects the RE (%) by shifting the equilibrium of the coordination reaction and/or ion-exchange ability in two ways: changing the concentration of active ligands and/or the concentration of soluble metal ions . The effect of pH on RE (%) is shown in Fig. 4. Very low removal efficiency was observed when the metal ion solution pH was below 4. This was because low pH led to protonated forms of the carboxylic acid groups,which are much less capable of forming complexes than the salt form. When the metal ion solution pH was 6 or greater than 6,the salt form of the carboxylic groups dominated,and the maximum removal efficiency was obtained until a pH of 10. Removal efficiency decreased at very high pH due to a significant change in the speciation of metal ions from the free metal ion to the corresponding metal hydroxide,which is usually less soluble.
|Fig. 4.Effect of pH on metal ions removal (initial metal ions concentration: 1000 ppm,treatment time: 8 h,adsorption dose: 25 mg).|
Changes in removal efficiency of As(V) and Cd(II)metal ions with different amounts of the adsorbent are shownin Fig. 5. Results show that increasing adsorbent content from 10 mg to 50 mg for 50mL As(V) and Cd(II) metal ions solution (1000 ppm) at optimum pH, time,and temperature,increased the removal of As(V) and Cd(II) metal ions. In addition,further increasing the adsorbent content had a negligible effect on the removal of metal ions.
|Fig. 5.Effect of adsorption dose on metal ions removal (initial metal ions concentration: 1000 ppm,treatment time: 8 h).|
Metal ions removed by poly (NIPAAm/AA/N-allylisatin) nanohydrogel can be released by using various kinds of eluents . In this work,0.1 mol/L HNO3 was used for the desorption of metal ions from the nanohydrogel. This was carried out after subjecting 0.025 g nanohydrogel to As(V) and Cd(II) aqueous solutions,each having an initial metal concentration of 100 ppm. Results revealed that the nanohydrogel remained active after three runs,and uptake percentages decreased from 97.14% to 24.38% for As(V) metal ions and from 95.89% to 36.78% for Cd(II) metal ions. This was also explained by the fact that removal of As(V) metal ions is greater than Cd(II) metal ions by the nanohydrogel. Also,weight loss occurs during each run of metal ion removal and desorption. 3.4. Adsorption isotherms models
To have a quantitative means of comparing metal removal strength and to design the removal process effectively,it is useful to employ mathematical models predicting the metal ion removal . It is well known that the Langmuir isotherm model is applied for homogeneous performance of different adsorbents,while the Freundlich isotherm model assumes a heterogeneous surface with a non uniform distribution of heat of adsorption. When the Langmuir and Freundlich isotherm models were applied to the removal of As(V) and Cd(II) metal ions,it was observed that As(V) metal ion removal follows the Freundlich isotherm while Cd(II) metal ion removal follows the Langmuir isotherm. A detailed explanation is given in Supporting information (Fig. S1 and S2),and the Langmuir and Freundlich isotherms constants were given in Table 2.
Table 2 represents the Langmuir and Freundlich parameters for As(V) and Cd(II) metal ions removal by the nanohydrogel. These parameters are determined from the slope and intercept of the linear plots of 1/qe vs. 1/Ce (Fig. S1) and the values of Kf and n (Fig. S2),as determined from the intercept and slope of the linear plot of log qe vs. log Ce. As seen from Table 2,deviation from the Langmuir model has severe shortcomings but shows the higher adsorption . According to these studies,it was concluded that As(V) metal ion removal was well defined by the Freundlich isotherm model while Cd(II) metal ions removal can be explained by the Langmuir isotherm model. In all results,the constants of the Langmuir and Freundlich isotherms models show the excess removal of metal ions by the VNH 06 nanohydrogel. 4. Conclusion
Superabsorbent poly (NIPAAm/AA/N-allylisatin) nanohydrogel has been synthesized and successfully utilized for the removal of As(V) and Cd(II) metal ions from aqueous solutions. The effects of various parameters such as treatment time,initial metal ion concentration,pH of solution,and adsorption dose on metal ion removal were investigated. The swelling behaviors of the nanohydrogels in water and in metal ion solutions were determined. The results show that the nanohydrogel is able to remove ~94% of As(V) and ~83% of Cd(II) metal ions from water. The higher removal of As(V) compared to Cd(II) metal ions was also confirmed by FT-IR,TGA,and EDX analysis for both of the metal ions. According to adsorption studies,it was concluded that removal of As(V) metal ions was well defined by the Freundlich isotherm model,while removal of Cd(II) metal ions was explained by the Langmuir isotherm model. Acknowledgments
The authors gratefully acknowledge the Head of the Department of Chemistry at Sardar Patel University,for research facilities. Also,appreciation is expressed for the studies in the Sophisticated Instrument Center for Applied Research and Testing [SICART] and Vallabh Vidyanagar for FT-IR,TGA,TEM,and EDX analysis. Financial support for this work was provided by the University Grant Commission,New Delhi (Project No. F.39-685/2010(SR)),to whom researches are gratefully acknowledged. Appendix A. Supplementary data
Supplementary material related to this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.01.031. )
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