Removal of Nickel ( II ) from Aqueous Solutions by Adsorption with Modified ZSM-5 Zeolites

The sorptive removal of nickel ion from aqueous solutions using modified ZSM-5 zeolites was investigated. Experiments were carried out as a function of solute concentration and different temperatures. Mesoporous material of ZSM-5 zeolite was modified with phosphoric acid by wet method. The modified zeolite was converted to Na form using aqueous NaHCO3 solution. The Na form of modified zeolite, represented as PNa2--ZSM-5 was characterized by XRD, BET, SEM and AAS techniques. It was then tested for ion exchange with aqueous Ni(SO4) solution. The Ni 2+ content of the solution was analyzed by AAS. Phosphoric acid modified PNa2--ZSM-5 zeolite shows higher adsorption capacity than the parent Na-Y zeolite. Equilibrium modeling data were fit to linear Langmuir model then the Freundlich model. These parameter confirmed that sorption of Ni is feasible spontaneous and endothermic.


Introduction
Several environmental and health problems, associated with the metal contamination of the natural systems (soil and water) are arising from mining industries, smelting, brass, metal coating, silver refineries, electroplating and several other industrial activities [1][2] .The main symptoms of nickel causes headache, dizziness, nausea and vomiting, chest pain, tightness of the chest, dry cough and shortness of breath, rapid respiration, cyanosis and extreme weakness 3 .Among the methods such as precipitation, oxidation, ultrafiltration, reverse osmosis and elcetrodialysis, ion exchange on natural zeolites seems to be more attractive method.Zeolites have been intensively studied recently because of their applicability in removing trace quantities of heavy metal ions from aqueous solution by utilizing the ion exchange phenomenon [4][5][6] .
Zeolites are naturally occurring hydrated alumino silicate minerals.They belong to the class of minerals known as "tectosilicates".The structure of zeolites consists of three dimensional frameworks of SiO 4 and AlO 4 tetrahedra.The aluminum ion is small enough to occupy the position in the centre of the tetrahedron of four oxygen atom, while the isomorphous replacement of Si 4+ by Al 3+ produces a negative charge in the lattice 7 .But hitherto only unmodified zeolites have been examined for ion exchange capacity, in which one mole of sodium ion is exchanged for every mole of Al 3+ present in the framework of zeolite.There have been reports of modification of zeolite with H 3 PO 4 , for catalytic application.In the present study, ZSM-5 zeolite has been modified for removal of Ni 2+ ion from aqueous solution.Each mole of phosphoric acid introduced into the zeolite provides two moles of exchangeable protons, which can be replaced by Na + ions using mild base like NaHCO 3

8
. The results obtained from this study are important for ion exchange and water softening applications.

Experimental
HZSM-5 zeolite was purchased from Sud-Chemie India Ltd, Mumbai, India.Nickel Sulphate was obtained from Qualigens Fine Chemicals Ltd., Mumbai, India.All other chemicals used were of analytical reagent grade, procured from Rankem Fine Chemicals Ltd, New Delhi, India.

Preparation of modified zeolite
About 4 g of HZSM-5 zeolite in 40 mL of double distilled water was mixed with 0.385 g of phosphoric acid.The mixture was kept under vigorous stirring at 60 °C for 3 h and then evaporated to dryness in an air oven at 120 °C.Phosphoric acid modified zeolite was contacted with 30 mL of aqueous NaHCO 3 solution under vigorous stirring at 60 °C for 3 h to obtain the disodium form of an PNa 2--ZSM-5 zeolite.It was then filtered, washed with water and dried in air oven at 120 °C overnight before use.Figure 1 illustrates the sequence of occurrences in the above treatment.The Na-ZSM-5 zeolite was prepared by adding NaHCO 3 solution directly with H ZSM-5 zeolite.

Ion-exchange equilibrium experiments
The ion exchange capacity of Ni 2+ ion with modified Na-ZSM-5 and PNa 2--ZSM-5 zeolites were studied using a batch method.The batch experiments were carried out with 20 mL of 50 m mol Ni 2+ solution.About 65 mg of the adsorbent was added and the uptake of the zeolite was determined.The pH of the solution was measured using a pH meter (Elico Model LI-120, Hyderabad, India).Adjustments of pH were made with 0.1 M HCl and 0.1 M NaOH solutions.The screw cap bottle containing the adsorbate and adsorbent were placed in the thermostatic orbital shaker (Neolab, Mumbai, India) and were shaken at a constant speed of 200 rpm.After equilibrium time, the samples were centrifuged and the filtrates were analyzed.The kinetic studies were carried out in magnetic stirrer 50 m mol Ni 2+ solutions.About 50 mg of the adsorbent was added.The samples were withdrawn from the stirrer at a regular time intervals and the samples were centrifuged.The absorbance of supernatant solution was measured using Atomic absorption spectrophotometer (SHIMADZU, AA-6300).The uptake and the amount of Ni 2+ ion exchanged by the modified zeolites were computed using the following equations.
Where, q e is the amount of exchanged Ni 2+ ions (m mol/g), C o and C e are the initial and equilibrium concentration of Ni 2+ ion in solution (m mol/L) respectively V is the solution volume and m is mass of the adsorbent (g).

Characterization of modified zeolite
The BET surface areas of both modified Na-ZSM-5 and PNa 2--ZSM-5 zeolites were measured using Smart Sorbs 92 Surface area analyzer where N 2 gas was used as adsorbate.The surface areas of modified Na-ZSM-5 and PNa 2--ZSM-5 zeolites were 385 and 422 m 2 /g respectively.The XRD analysis of the modified Na-ZSM-5 and PNa 2--ZSM-5 zeolites was measured using D-Max / 111 -VC Model with nickel filtered and Cu Kα radiation (λ=1.5406Ǻ).The X-ray analysis reveals the absences of structural degradation during modification are shown in Figure 2. Scanning electron microscope (SEM) images were taken to study the surface morphology of zeolite particles using a Joel Jsm-6360 scanning electron microscope.The SEM pictures of the parent H-ZSM-5, Na-ZSM-5 and PNa 2--ZSM-5 zeolites are shown in Figure 3 (a, b and c) respectively.The modification of PNa 2--ZSM-5 zeolite might not have resulted any structural degradation of the parent zeolite.

Effect of pH
The effect of pH of the suspending medium on nickel removal was studied by performing equilibrium sorption experiments at different pH values.The results are illustrated in Figure 4.For the PNa 2 -ZSM-5 zeolite the uptake efficiency gradually increases as the pH increases from 3 to 4 in Ni 2+ .But above pH 4 Ni 2+ uptake was observed which could be attributed to precipitation.The Na-ZSM-5 zeolite also exhibited similar the behavior.Hence optimum pH range for the removal of Ni 2+ was found to be 4.The PNa 2 -ZSM-5 zeolite showed higher uptake than the Na-ZSM-5 zeolites as shown in Figure 4 as it contains more exchangeable Na + ions than latter.

Sorption isotherms
The sorption isotherms for the removal of Ni 2+ at different temperatures were studied.It provides information on uptake capabilities and reflects the usual equilibrium process behavior.The Langmuir isotherm could be written 9 as: where Q o is the monolayer adsorption capacity (m mol/g) and b is the constant related to the free energy of adsorption.The logarithmic form of Freundlich equation is given 10 as: where K f is the relative adsorption capacity of the adsorbent (m mol/g) and 1/n is the constant indicative of the intensity of the adsorption process.The Langmuir isotherm correctly fitted than the Freundlich.The Langmuir as shown in Figure 5.The corresponding Langmuir and Freundlich parameters along with their correlation coefficients are given in Table 1.
The Langmuir constants Q o and b are increases with increase of temperature showing that the sorption capacity and intensity of the sorption are enhanced at higher temperatures.Hence, the active surface available for sorption has increased with increase of temperature .Recently there have been many reports on the sorption of Ni 2+ with different adsorbents and the results are presented in Table 2.   0.41 Kaolinite 12 1.70 Na-Montmorillonite 13 3.63 Exfoliated Clay 11 5.91 Clay treated with HC l4 10.9 Natural zeolite 12 12.5 Clay treated with NaC l4 14.5 Natural zeolite 12 19.

Kinetic studies Pseudo-first order kinetic model
The linear form of Lagergren's first order expression 15 is written as: where, k 1 (min -1 ) is the first order adsorption rate constant, q e is the amount of metal adsorbed at equilibrium and q t is the amount adsorbed at time't'.The first order equation of Lagergren does not fit well with the whole range of contact time and is generally applicable over the initial stage of the sorption processes 16 .The first order rate constant is given in Table 3.

Pseudo second order kinetic model
The linear form of pseudo-second -order rate model and 17 is expressed as: Where, plotting t/q t against "t" for Ni 2+ removal at different temperatures is presented in Figure 6.The relationship is linear, and the correlation between the parameter also explains that the process of sorption of each ion follows pseudo second order kinetics.The product k 2 q e 2 is the initial sorption rate represented as h = k 2 q e 2 from Table 3.It can be shown that the values of the initial sorption rate 'h' and the rate constant 'k 2 ' increase with in increase in temperature.The correlation coefficient R 2 has an extremely high value, and its calculated equilibrium sorption capacity 'q e ' is consistent with the experimental data.The results suggest that the pseudo-second order sorption mechanism is predominant and that the over all rate constant of each ion appears to be controlled by the chemisorption process 17 .

Conclusions
From this study it is concluded that phosphoric acid modified ZSM-5 zeolite can be a better adsorbent than the parent zeolite.The modification can also be extended to other zeolites for enhanced sorption properties.The removal efficiency increases with the increase in temperature and hence sorption process is endothermic in nature.The adsorption isotherm data fit well with Langmuir isotherm while the kinetic data were represented by pseudosecond order kinetic model.This study forecasts that treatment of wastewater, particularly heavy metal ion removal and softening of hardwater, could be better carried out with the modified zeolite.The results of this study cleary envisage that such modified zeolites can very well be recommended for wastewater treatment and control of environmental pollution.

Table 1 . 2 L
Langmuir and Freundlich isotherm plots for the sorption of Ni 2+ ions from aqueous solutions onto Na-ZSM-5 and PNa 2 -ZSM-5 zeolites.Langmuir isothermFreundlich isotherm Temp (K) Q o , m mol/g b, L/m mol R

Figure 6 .
Figure 6.Pseudo-second-order kinetics plots for the sorption of Ni 2+ ions from aqueous solutions onto (a) Na-ZSM-5 (b) PNa 2 -ZSM-5 zeolites.The correlation coefficient R 2 has an extremely high value, and its calculated equilibrium sorption capacity 'q e ' is consistent with the experimental data.The results suggest that the pseudo-second order sorption mechanism is predominant and that the over all rate constant of each ion appears to be controlled by the chemisorption process17 .

Table 2 .
Adsorption capacities Q o (mg/g) for some zeolite materials

Table 3 .
The calculated parameters of the pseudo-first order and pseudo-second order kinetic models of Ni 2+ ions from Na-ZSM-5and PNa 2 -ZSM-5 zeolites.