Ind. Eng. Chem. Res. 2008, 47, 8095–8100
8095
Alumina Nanoparticles for the Removal of Ni(II) from Aqueous Solutions Y. C. Sharma,*,† V. Srivastava,† S. N. Upadhyay,‡ and C. H. Weng§ Department of Applied Chemistry and Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu UniVersity, Varanasi 221 005, India, and Department of CiVil and Ecological Engineering, I-Shou UniVersity, Da-Hsu Township, Kaohsiung 84008, Taiwan
Alumina nanoparticles were developed by the sol-gel method and were used for the removal of Ni(II) ions from aqueous solutions. The nanoparticles were characterized by TEM and XRD. Nanoparticles of alumina were then used for removal of Ni(II) ions from aqueous solutions of nickel. The nanosize of the adsorbent was confirmed by TEM and XRD. Removal (%) was found to be dependent on the initial concentration of nickel, and maximum removal was found to be 96.6% at 25 mg/L Ni(II). The removal increased from 99 to 99.6% by decreasing the initial concentration from 75 to 25 mg/L. Equilibrium time was found to be 120 min. As expected, higher removal was obtained at higher adsorbent dose. The removal was governed by first-order kinetics, and the value of the rate constant of adsorption was found to be 1.83 × 10-2 min-1 at 25 mg/L and 25 °C. The removal was found to be pH dependent, and maximum removal was found to be at pH 8.0. The adsorption process was endothermic in nature. The experimental data fit well the Langmuir and Freundlich isotherms. Constants of the two isotherm equations were determined. Thermodynamic studies for the present process were performed by determining the values of ∆G°, ∆H°, and ∆S° at different temperatures. 1. Introduction Nickel is the 24th element in order of natural abundance in the earth’s crust.1,2 Nickel is a common metal and is frequently used in different industries, viz., electroplating, dyeing, steel manufacture, porcelain enameling, mineral processing, storage batteries, and paint manufacture.3-5 The tolerance limit of nickel in drinking water is 0.01 mg/L, and for industrial wastewater it is 2.0 mg/L.6 However, effluents of different industries contain higher concentrations of nickel than its acceptable limit. Though nickel is an essential micronutrient for animals and takes part in the synthesis of vitamin B12, its higher concentrations cause cancer of the lungs, nose, and bones and it may also cause nausea, rapid respiration, headache, cyanosis, and dry cough.7-9 It is thus necessary to treat industrial effluents rich in Ni(II) before their discharge. Many technologies such as ion exchange, reduction, flocculation, reverse osmosis, membrane filtration, and precipitation have been employed by different scientific workers for the removal of nickel from aqueous solutions and effluents.10,11 However, most of these technologies require high operational and maintenance costs, and also generate toxic sludge.12-14 Due to high expenses these techniques are not suitable for smallscale industries especially in developing nations like India. Adsorption is one of the most promising techniques for the removal of metallic pollutants from industrial effluents.15-17 Different adsorbents have been used for the removal of metallic pollutants, but activated carbon is considered to be the best adsorbent for the removal of a variety of pollutants. Application of nanoparticles for the removal of pollutants has come up as an interesting area of research. Nanoparticles exhibit good adsorption efficiency especially due to higher surface area and greater active sites for interaction with metallic species.18-22 There are many routes for the preparation of nanoparticles. Inert * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Applied Chemistry, Banaras Hindu University. ‡ Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University. § I-Shou University.
gas condensation, pulsed laser ablation, spark discharge generation, ion sputtering, spray pyrolysis, laser pyrolysis, photothermal synthesis, thermal plasma synthesis, flame synthesis, lowtemperature reactive synthesis, flame spray pyrolysis, sol-gel process, mechanical alloying/milling, and electrodeposition are the common methods used for preparation of nanoparticles.23,24 However, among these methods, the sol-gel method seems to be the most promising technique for preparing nanoparticles. This technique gives high-purity products. The sol-gel route offers a degree of control of composition and structure at the molecular level.25,26 Further, according to the available reports alumina nanoparticles involve minimum financial liability for their development. In the present study the sol-gel method has been used to prepare nanoparticles of alumina. This nanoparticle is then used for the removal of nickel from its aqueous solutions. The effect of contact time and initial concentration, adsorbent dose, temperature, pH, etc. on the removal of nickel has been studied. Kinetic, equilibrium, and thermodynamic studies have also been carried out. 2. Experimental Section All the chemicals used in the present studies were of AR/ GR grade and were procured from Merck, Mumbai, India. 2.1. Preparation of Nanoalumina Powder. The nanoalumina used as an adsorbent in these studies was synthesized by using the sol-gel method. A 0.5 M solution of aluminum sulfate was prepared by dissolving it in doubly distilled water. Ammonia solution was slowly added to this solution under continuous stirring on a magnetic stirrer for precipitation. The resulting precipitate was then dried at 80 °C in a hot air oven for 24 h and milled into gel powder. To get nanoalumina powder, dried gel was calcined for 1 h in a muffle furnace (Libratherm instrument PID 300, Naskar & Co.) at 1100 °C followed by natural cooling in the furnace. After calcinations, the calcined powder was milled and sieved. The phase characterization of calcined powder as well as gel powder was carried out by the XRD technique (X-ray diffractometer, Scifert and Co., Model ID-3000). The scanning speed was fixed at 1°/min.
10.1021/ie800831v CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
8096 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
Figure 1. XRD of nanoalumina powder.
Crystallite size was determined by using XRD results through the diffraction peaks using Scherrer formula:27 Xs ) 0.9λ/(fwhm) cos θ
(1)
where Xs ) crystallite size (nm), λ ) wavelength of monochromatic X-ray beam (λ ) 0.154 056 nm for Cu KR radiation), fwhm ) full width at half-maximum for the diffraction peak under consideration (rad), and θ ) diffraction angle (degrees). The surface morphology of the powder was determined by using transmission electron microscopy (TEM). 2.2. Adsorption Studies. Batch adsorption studies were conducted to determine equilibrium time. A stock solution of Ni(II) was prepared by dissolving 4.479 g of NiSO4 · 6H2O in 1000 mL of distilled water. Batch adsorption studies were performed by mixing 0.25 g of nanoalumina powder with 50 mL of Ni(II) solution of varying concentrations in 250 mL stoppered conical flasks. The dose of adsorbents was decided by the experiments. All the adsorption experiments were conducted at 25 °C, at the pH of the working solution, viz., 6.3, and at an agitation rate of 100 rpm on a shaking thermostat. After equilibrium time, 120 min, the adsorbents were separated from the aqueous phase by centrifugation at 10 000 rpm for 10 min. The concentration of Ni(II) in supernatants was determined by a UV-visible spectrophotometer (Spectronic 20, Bausch and Lomb) at 445 nm. The ionic strength of the aqueous solutions was was maintained at 1.0 × 10-2 M NaClO4.
Figure 3. Effect of initial concentration on percent removal of Ni(II).
3. Results and Discussion 3.1. Characterization of the Nanoalumina Particles. Figure 1 shows XRD of powder calcined at 1100 °C. This figure shows the presence of peaks of the R-phase and γ-phase of alumina. The TEM of the powder confirms the average particle size of the powder in the range 15-20 nm (Figure 2). 3.2. Effect of Contact Time and Initial Concentration. Two parameters, namely, contact time and initial concentration, have a pronounced effect on the removal of adsorbate species from aqueous solutions. In the present studies, the effect of initial concentration on the removal of Ni(II) from aqueous solutions was carried out. The removal increased from 96.6 to 99.0% by decreasing the initial concentration of Ni(II) from 75 to 25 mg/L at optimum conditions (Figure 3). It is clear from Figure 3 that the graphs are single and smooth, indicating monolayer coverage of the adsorbent surface by Ni(II). Further, the removal is rapid in the initial stages, decreases slowly, and acquires a maximum at the time of equilibrium, viz., 120 min. 3.3. Effect of Adsorbent Dose. The effect of different doses of the adsorbent on removal of Ni(II) was carried out, and the results have been presented in Figure 4. The amount of adsorbent was varied from 0.25 to 1.0 g while all the variables such as pH, rpm, contact time, and temperature were kept constant. It is clear from Figure 4 that the percent removal of Ni(II) increases
Figure 2. (a) TEM of nanoalumina powder and (b) diffraction pattern of nanoalumina powder.
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8097
Figure 4. Effect of adsorbent dose on percent removal of Ni(II). Figure 6. Effect of pH on percent removal of Ni(II).
Figure 7. Speciation diagram for nickel.
Figure 5. Lagergren’s plot for kinetic modeling of the adsorption process of Ni(II) on nanoalumina. Table 1. Values of Rate Constant of Adsorption temp((0.5°C) 25 35 45
Kad (×10-2 min-1) 1.83 1.95 2.30
by increasing adsorbent dose and it reached 100% when the dose was 1.0 g. This is due to greater availability of active sites. 3.4. Kinetic Studies. The kinetic modeling of the removal of Ni(II) by nanoalumina was carried out by Lagergren’s model:28 log(qe - q) ) log qe - (Kad/2.303)t
(2)
where qe and q (both in mg/g) are amounts of Ni(II) adsorbed at any time and at equilibrium, respectively, and Kad (min-1) is the rate constant of adsorption. The straight line plots of log(qe - q) vs t (Figure 5) confirm that the process of removal is governed by first-order kinetics. The linear plots also demonstrate the applicability of Lagergren’s model for this study. The values of Kad were determined by the slopes of the graphs. The Kad value was calculated from the slope of the plot and was found to be 1.83 × 10-2 min-1 at 25 mg/L concentration and 25 °C.The values of Kad (Table
1) show that nanoalumina can be used for Ni(II) removal from aqueous solutions. 3.5. Effect of pH. The effect of pH on adsorption of Ni(II) was studied at room temperature by varying the pH of aqueous solutions from 2.0 to 8.0 (Figure 6). The adsorption increases from 91.4% to 98.2% by increasing the pH from 2.0 to 8.0. During the current investigations removal was studied in both the acidic and alkaline ranges of pH. Selected values of pH were 2.0, 4.0, 6.0, and 8.0. Removal was significant at all four values of pH, but it was found to be maximum at pH 8.0. Some authors have reported maximum removal at around pH 8.0-9.0.29,30 It is clear from the speciation diagram of nickel that only Ni2+ is the only important oxidation state in aqueous chemistry of nickel and up to pH 8.0 this species is prominent (Figure 7). It is present in aqueous solutions as Ni(H2O)62+.30 It is expected that the protonated surface favors the uptake of Ni(II) from aqueous solutions and it would acquire a maximum in the pH range 8.0-9.0. The same has been borne out in our results as well. The following hydrolysis and adsorption reactions are expected at the solid solution interface. H-OH + Mz+ T H-OM(z-1)+ + H+
(3)
+
S-OH + M T S-OM +H (4) It is clear from experimental data that removal increases slowly and then acquires a maximum in 120 min. The uptake of nickel is rapid in the initial stages, and in later stages it becomes steady. The metal competes with protons for surface sites:30 z+
(z-1)+
S-OH + Ni2+ T S-O-Ni2+ + H+
(5)
8098 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 2. Values of Langmuir and Freundlich Constants for the Removal of Ni(II) Langmuir constants temp(K) 298 308 318
Freundlich constants 2
Q° (mg/g)
b (L/g)
R
30.82 22.69 21.85
0.49 1.57 2.65
0.991 0.999 0.999
n
Kf (L/g)
R2
1.59 2.20 2.40
9.66 12.53 14.68
0.976 0.971 0.975
reported most frequently. The Langmuir model assumes that uptake of metal ions occurs on a homogeneous surface by monolayer adsorption and that there is no interaction between sorbed species. The Langmuir equation is expressed by the following expression: Ce/qe ) 1/Q ° b + Ce/Q°
Figure 8. Effect of temperature on percent removal of Ni (II).
Figure 9. Langmuir’s isotherm plot for the removal of Ni(II) on nanoalumina.
It seems that during adsorption one proton is replaced by one nickel atom. Some amount of surface precipitation at the maximum removal of nickel at around pH 8.0 also cannot be ruled out. At pH >8.0, the Ni(II) ions get precipitated due to hydroxide anions forming a nickel hydroxide precipitate.11,29 3.6. Effect of Temperature. The effect of temperature was investigated in the temperature range 25-45 °C. The experimental results show that, in the present investigations, the removal of nickel increased from 97.0 to 99.0% by increasing the temperature from 25 to 45 °C(Figure 8). Most of the adsorption processes are governed by exothermic processes, but the present study is amongst a few examples of endothermic adsorption. Our findings are supported by other workers for the removal of nickel.31 The temperatures selected are usually encountered in treatment plants, and that was one basis for the selection of the temperatures for the present studies. 3.7. Equilibrium Modeling. Equilibrium modeling of the process of removal of nickel was carried out by using the Langmuir and Freundlich adsorption isotherms.10 Several mathematical models have been applied for describing equilibrium studies for the removal of pollutants by adsorption on solid surfaces. Selection of an isotherm equation depends on the nature and type of the system. Out of several isotherm equations the Freundlich and Langmuir isotherm equations have been
(6)
where Ce (mg/L) is the equilibrium concentration of the solute (mg/L), qe is amount adsorbed at equilibrium (mg/g), and Q° (mg/g) and b (L/mg) are constants related to the adsorption capacity and energy of adsorption, respectively. A plot of Ce/ qe versus Ce (Figure 9) gives a straight line. The values of Q° and b were determined by the slopes and intercepts of Figure 9 and are given in Table 2. The Freundlich model assumes that the uptake of metal ions occurs on a heterogeneous adsorbent surface. The Freundlich equation is expressed as qe ) KfCe1/n
(7)
log qe ) log Kf + 1/n log Ce
(8)
where Kf and 1/n are related to the adsorbent capacity and sorption intensity of the adsorbent, respectively. The values of the Freundlich constants, Kf and 1/n, were determined by slopes and intercepts of Figure 10. It is clear from Table 2 that experimental data obeyed both the Langmuir and Freundlich isotherm models. Table 3 shows the adsorption capacities of various adsorbents.3,4,7,9,10,32-38 It is clear from this table that the adsorption capacity of the nanoparticles used in the present studies is significant. 3.8. Thermodynamic Studies. Thermodynamic studies are used to decipher any reaction in a better way. In the present studies also, thermodynamic studies were performed and the parameters, namely, ∆G°, ∆H°, and ∆S°, were determined at
Figure 10. Freundlich isotherm plot for the removal of Ni(II) on nanoalumina.
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8099 Table 3. Comparison of Adsorption Capacities of Different Adsorbents for the Removal of Ni(II) adsorbent
adsorption capacity Q0 (mg/g)
reference
bagasse fly ash Aspergillus niger granular activated carbon sheep manure waste peat moss coir pith calcium alginate baker’s yeast Thuja orientalis carbon aerogel waste tea Fe(III)/Cr(III) hydroxide nanoalumina powder
0.001 0.03 1.1 1.5 7.20 9.18 9.5 10.5 11.40 12.42 12.875 18.42 22.94 30.0
32 32 33 7 34 35 4 36 3 9 37 10 38 presentstudy
Table 4. Thermodynamic Parameters for Adsorption of Ni(II) temp(K)
∆G° (kcal/mol)
∆H° (kcal/mol)
∆S° (kcal/mol)
-2.276 -2.842 -3.260
+14.59
+0.0566
298 308 318
25, 35, and 45 °C, respectively. The thermodynamic parameters were calculated by using the following equations:39,40 Kc ) Cac/Ce (9) ∆G° ) -RT ln Kc (10) ∆H° ) R(T2T1/T2 - T1) - ln(K2/K1) (11) ∆S° ) (∆H° - ∆G°)/T (12) where Kc is the equilibrium constant and Cac and Ce are the equilibrium concentration of metal ions on the adsorbent (mg/ L) and the equilibrium concentration of metal ions in the solution (mg/L), respectively. The values of Kc increased by increasing the temperature, which indicates the endothermic nature of the process of removal. The values of these parameters have been given in Table 4. Other authors have also reported thermodynamic studies for the removal of different metallic species.41-43 Positive values of entropy change ∆S° and enthalpy change ∆H° also indicate the endothermic nature of adsorption of Ni(II) on nanoalumina powder. Values of ∆G° decreases by increasing temperature. This revealed that a greater adsorption can be obtained at higher temperature. 4. Conclusion Nanoparticles of alumina can be prepared by using the sol-gel method and can be used as an effective adsorbent for the removal of Ni(II) from aqueous solutions. It was found that the removal of Ni(II) is pH dependent and gives maximum removal at pH 8.0.The adsorption was found to be endothermic in nature. The experimental data well obeyed both the Langmuir and Freundlich adsorption isotherms. Negative values of free energy indicate the feasibility of adsorption of Ni(II) on nanoalumina. A 100% removal can be achieved by only 20 g/L adsorbent within 120 min. Acknowledgment The authors are thankful to AICTE, Government of India, for providing financial assistance to V.S. The authors express their thanks to the fellows of MANALAB, I-Shou University, for supporting the TEM analysis. Literature Cited (1) Krishnamurti, C. R.; Vishwanathan, P. Toxic Metals in the Indian EnVironment; Tata McGraw-Hill Publishing Company Ltd.: New Delhi, 1991.
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ReceiVed for reView May 24, 2008 ReVised manuscript receiVed August 8, 2008 Accepted August 8, 2008 IE800831V
