Study effect of different parameters on the sulphate sorption onto nano alumina

Journal of Industrial and Engineering Chemistry 18 (2012) 230–236

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Study effect of different parameters on the sulphate sorption onto nano alumina Reza Katal a, Mohsen Vafaie Sefti a,*, Mehdi Jafari b, Amir Hossein Saeedi Dehaghani a, Seyedmehdi Sharifian c, Mohammed Ali Ghayyem d a

Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran Faculty of Chemical Engineering, Amirkabir University, Tehran, Iran c Faculty of Chemical Engineering, Iran University of Science and Technology, Tehran, Iran d Petroleum University of Technology, Ahwaz, Iran b

A R T I C L E I N F O

Article history: Received 17 December 2010 Accepted 10 March 2011 Available online 4 November 2011 Keywords: Nano alumina Sulphate Kinetic Isotherm Thermodynamic

A B S T R A C T

The aim of this research work is to investigate sorption characteristic of modified nano alumina (n-Al) for the removal of SO42 from aqueous solutions and wastewater. The sorption of SO42 by batch method is carried out. The optimum conditions of sorption were found to be: a sorbent dose of 0.3 g in 100 ml of SO42, contact time of 35 min, pH = 5. In optimum condition, removal efficiency was 85.6% for the SO42. Three equations, i.e. Morris–Weber, Lagergren and pseudo second order have been tested to track the kinetics of removal process. The Langmuir, Freundlich and D–R are subjected to sorption data to estimate sorption capacity. It can be concluded that n-Al has potential to remove SO42 ions from aqueous solutions at different concentrations. It was found that the temperature has positive effect on the process and negative DG values indicated thermodynamically feasible and spontaneous nature of the sorption. Positive value of DS reveals the increased randomness at the solid–solution interface during the fixation of the ion on the active sites of the sorbent. The effect of other anions was studied and it was found the existence of them in the solution has considerable effect on the sulphate removal. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Sulphate is a common constituent of many natural waters and wastewaters [1], which is present as a dissolved compound in seas and oceans or as insoluble salt (e.g., gypsum layers). Industrial wastewaters are responsible for most anthropogenic emissions of sulphate into the environment. Domestic sewage typically contains between 20 and 500 mg/l sulphate [2] while certain industrial effluents may contain several thousands of milligrams per liter. The main source of sulphate in the laboratory wastewaters is the use of sulphuric acid in many routine chemical analyses. Sulphur compounds are also present in wastewaters used in the research activities, such as those from the pulp and paper industry, the food processing industry, and the photographic sector, among others [3]. Sulphate ions are some of the main contributors to so-called water ‘‘mineralisation’’, thereby increasing the conductivity and corrosion potential of receptor bodies. These anions promote the following, among other things: corrosion and scaling in pipes, structures and equipment; fouling and deposition in boilers; and acidification of

* Corresponding author. Fax: +98 2182883314. E-mail address: [email protected] (M. Vafaie Sefti).

soils and blockage of soil pores, retarding irrigation or water drainage [4]. The removal of sulphate ions from such waters is a complex problem due to the high solubility and stability of these anions in aqueous solutions. Partial sulphate ions removal is often accomplished through precipitation with lime [5]. However, this process usually presents a very low practical efficiency because of the high solubility of the produced CaSO4. The main processes for treating sulphate ion-bearing water are as follows: (1) biological degradation [6,7]; (2) membrane filtration (mainly reverse osmosis) [8]; (3) adsorption and/or ion exchange [9]; and (4) chemical precipitation [10]. Many researchers have studied the coagulation performance of polyaluminium salts, even for sulphate ion removal but coagulation performance of PACs is closely related to the Al species distribution, the basicity and the Al concentration [11,12]. Recent years, Pt electrodes are used for sulphate adsorption but high-cost value of this method and electrode consumption are problems of this method [13,14]. In this study, adsorption of SO42 by n-Al is studied. The characteristic of n-Al, effects of dosage of adsorbent, contact time, temperature, and pH value on sorption of SO42 are investigated in detail, also adsorption isotherms, thermodynamic parameters and adsorption kinetics are obtained and then sorption of SO42 in urban wastewater is studied.

1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.012

R. Katal et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 230–236

2. Materials and methods

experimental error was below 4%, the average data were reported. The efficiency of SO42, %Removal, was calculated as:

2.1. Preparation of n-Al % Removal ¼ The precursor solutions for Al2O3 nano powder were prepared by sol–gel method using AlCl36H2O (Merck), Al powder (M.A. University) and HCl (domestic product). At first, the aluminium chloride hexahydrate was mixed with aqueous HCl. The Al powder was then gradually added to the solution. The solution was then stirred using a magnetic stirrer at 95 8C for 4 h to obtain a transparent solution. The obtained gel was dried at 85 8C for 48 h. The dried gel was then ground and calcined in a furnace at different temperatures. The obtained powder was ball milled in ethanol media using a high dense alumina jar and high pure alumina balls. The powder was again dried at 80 8C. 2.2. Instrumentation In this study, scanning electron microscope ((SEM) model XL30) was used to characterize the surface of the n-Al at very high magnification. The n-Al was coated with gold and palladium by a sputter coater with conductive materials to improve the quality of micrograph. The thickness of the coating was 30.00 nm, and the density was 19.32 g/cm3. Functional groups in n-Al were determined by the Fourier transform infrared (FTIR) spectroscopy. Spectra were collected with a spectrometer using KBr pellets. In each case, 1.0 mg of dried n-Al sample and 100 mg of KBr are homogenized using mortar and pestle thereafter pressed into a transparent tablet at 200 kgf/cm2 for 5 min. The pellets are analyzed with a FTIR spectrometer (Shimadzu 4100.) in the transmittance (%) mode with a scan resolution of 4 cm1 in the range 3500–500 cm1. The pH measurements of all aqueous samples were performed following standard methods with SP21 pH meter manufactured by VWR scientific product. The meter was standardized using buffer solutions with the following pH values: pH 4.0, pH 7.0 and pH 10.0. The major elemental species for solubles in the wastewater were quantified with inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Varian Vista Pro ICP-OES instrument operating at a forward power of 1200 W utilizing a Sturman – Masters spraychamber (Varian, Palo Alto, CA). Sulphate was shown by comparison with ion chromatography data (method details follow) to typically represent greater than 90% of the total sulphur content of the wastewater. Accordingly, it was assumed that the sulphur content determined with ICP-OES was a direct measure of the sulphate content of the wastewater (in the absence of biological activity). The XRD diagrams were obtained with a Miscience NH18XHF diffractometer using Fe Ka. Surface area of the n-Al was measured by BET (Brunauer–Emmett–Teller nitrogen adsorption technique) radiation. The density of n-Al was determined by specific gravity bottle.

Ci  C f  100 Ci

where Ci is the initial concentration (mg/l) and Cf is the final concentration (mg/l). q is the amount of metal adsorbed per specific amount of adsorbent (mg/g). The sorption capacity at time t, qt (mg/g) was obtained as follows: qt ¼ ðC i  C t Þ 

V m

where Ci and Ct (mg/l) were the liquid-phase concentrations of solutes at initial and a given time t, v was the solution volume and m is the mass n-Al (g). The amount of adsorption at equilibrium, qe was given by: qe ¼ ðC i  C e Þ 

V m

where Ce (mg/l) was the ion concentration at equilibrium. 3. Results and discussion 3.1. Characterization of n-Al The SEM analysis of n-Al (Fig. 1) revealed that the sorbent does not possess any well defined porous structure (only few pores on the surface). The phase analysis of n-Al was observed by XRD (Fig. 2) and it confirmed g-alumina phase only in the sorbent. The FTIR spectrum of n-Al is shown in Fig. 3. The characteristic region for n-Al powders lies in the wave number range from 500 to 4000 cm1 [15]. The characteristic infrared absorption peak of n-Al was observed at 3465 cm 1. Further, two bands at 470 and 527 cm1 also appear in the spectra, which are the characteristics of Al–O vibration in Al2O3 [16]. An absorption band at ca. 1631 cm1 was also observed which is in accordance with the reported literature that alumina presents an absorption band at ca. 1631 cm1 [17]. The main index of n-Al is shown in Table 1. 3.2. Effect of pH pH is an important controlling parameter in all the adsorption processes. A typical experiment with 0.3 g of adsorbent and at a temperature of 20 8C shows interesting adsorption capacities for a pH range from 3 to 11 (Fig. 4). The adsorption of sulphte increased with increasing pH, reaching a maximum at pH = 5, and then decreased with further increase in pH. This may be due to the competition for the active sites by OH ions and the electrostatic repulsion of anionic nitrate by the negatively charged n-Al surface

2.3. Batch adsorption experiments The adsorption experiments in this work were done to study the effect of experimental conditions on SO42 adsorption and determining the conditions that achieve the maximum amount of SO42 removal. Isotherm, kinetic and thermodynamic evaluations were also conducted in this portion of the study. The adsorption tests were conducted in magnetic mixer. The magnetic mixer was 400 rpm throughout the study with 100 ml of solution prepared from the dilution of 1 g/l stock solutions. Sodium sulphate anhydrous was employed in the preparation of sulphate solutions. At the end of predetermined time intervals, the sorbate was filtered and the concentration of SO42 was determined. All experiments were carried out twice and the adsorbed concentrations given were the means of duplicate experimental results. The

231

Fig. 1. SEM image of n-Al.

R. Katal et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 230–236

232

Table 1 The main characteristics of n-Al. Crystal phase

Purity %

Average size (nm)

Specific surface (m2/g)

Density (cm3/g)

g

>99

<150

12

0.4–0.5

assumed to offer no mass transfer (both external and internal external) resistance to the overall adsorption process. Therefore kinetic can be studied through the residual metal ion concentration in the solution. The study of adsorption kinetics describes the solute uptake rate and evidently these rate controls the residence time of adsorbate uptake at the solid–solution interface including the diffusion process. To investigate the change in the concentration of sorbate onto sorbent with shaking time, the kinetic data of SO42 ions sorption onto n-Al were subjected to Morris–Weber Eq. (1) [19]: qt ¼ K id ðtÞ0:5 þ C

(1) 2

3.4. Kinetics of sorption

where qt is the sorbed concentration of SO4 ions at time ‘t’. The plot of qt versus t1/2 is given in Fig. 6. The value of rate constant of Morris–Weber transport, Kid, calculated from the slope of the linear plot are shown in Fig. 6. The rate constant k = 1.3 min1 was calculated from the slope of the straight line with a correlation factor of 0.981. Internal particle diffusion may involve pore and/or surface diffusion. The intraparticle diffusion plots show multi-linearity in the process indicating that three steps are operational. The first steep stage can be attributed to the diffusion of adsorbate through the solution to the external surface of the adsorbent or the boundary surface diffusion of the sorbate molecules. The second stage describes the gradual sorption, where intraparticle diffusion is rate-limiting, and the third stage is attributed to the final equilibrium due to extremely low sorbate concentration left in solution and the reduction of interior active sites. The three stages in the plot suggest that the sorption process occurs by surface adsorption and intraparticle diffusion. The pseudo first order of the sorption of SO42 ions onto n-Al was evaluated by treating the data to the following form of Lagergren rate expression (2) [20], to determine the rate constant of sorption for SO42 ions–n-Al system.   K (2) t logðqe  qt Þ ¼ log qe  2:303

Various kinetic models, namely Morris–Weber, Lagergren and pseudo second order models have been used for their validity with the experimental adsorption data for the SO42 onto n-Al. It was

where qe is the sorbed concentration at equilibrium and k is the first order rate constant. The linear plot of log(qe  qt) against time ‘t’ (Fig. 7) demonstrates the applicability of the above equation for

Fig. 2. XRD pattern of n-Al.

at higher pH. The decrease in nitrate adsorption is particularly sharp above pH = 6, as the surface charge becomes more negative. Different pHpzc values ranging from 6 to 9 are reported for aluminium based oxides/hydroxide in the literature [18]. Hence, nitrate ions would have to overcome electrostatic forces as there would be a higher density of negative charge very close to the surface, hence greater electrostatic repulsion. 3.3. Effect of contact time Fig. 5 shows the effect of contact time on sorption of SO42 by nAl. For these cases, initial SO42 concentration was of 50 mg/l, pH of 5 was used for SO42. Also n-Al dose of 0.25 g in 100 ml were used. For SO42, sorption rate reaches up to 96.1 when contact time is 35 min, and then little change of sorption rate is observed. This result revealed that adsorption of SO42 is fast and the equilibrium was achieved by 35 min of contact time. Taking into account these results, a contact time of 35 min was chosen for further experiments.

100

removal efficiency (%)

90 80 70 60 50 40 30 20 10 0 0

2

4

6

8

10

12

pH

Fig. 3. FTIR spectrum of n-Al.

Fig. 4. The effect of pH on the removal efficiency (the initial concentration, contact time, volume of solution and amount of adsorbent was 50 mg/l, 35 min, 100 ml and 0.3 g respectively).

R. Katal et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 230–236

233

1

100 80

0.5

70 60

log(qe-qt)

removal efficiency (%)

90

50 40 30

0 0

20

30

40

-0.5 y = -0.061x + 1.165 R² = 0.927

20

-1

10

10

0 10

0

20

30

40

50

-1.5

t(min)

contact me (min) Fig. 5. The effect of contact time on the removal efficiency (the initial concentration, pH, volume of solution and amount of adsorbent was 50 mg/l, 5, 100 ml and 0.3 g respectively).

Fig. 7. Validation of Lagergren plot of SO42 sorption onto n-Al (the initial concentration, pH, volume of solution and amount of adsorbent was 50 mg/l, 5, 100 ml and 0.3 g respectively).

3.5. Effect of n-Al dosage on sorption of SO42 SO42 ions sorption onto ash. The rate constant k = 0.15 min1 was calculated from the slope of the straight line with a correlation factor of 0.92. Pseudo-second order is the most commonly applied model to adsorption kinetic data of different adsorbate–adsorbent systems [21–23]. When this model is applicable to experimental data, adsorption is complying with a second order rate kinetic with respect to availability of adsorption sites of the sorbent instead of adsorbate concentration in the bulk solution [24,25]. The kinetic data of SO42 ions sorption onto n-Al was subjected to pseudo second order Eq. (3) [26]: t 1 t ¼ þ qt Kq2e qe

(3)

The rate constant was calculated from the slope of the straight line (Fig. 8). The rate constant k = 0.015 min1 was calculated from the slope of the straight line with a correlation factor of 0.994 (Fig. 8). The kinetic data indicated that the adsorption process was controlled by pseudo-second-order equation. Also this suggests the assumption behind the pseudo-second-order model that the SO42 uptake process is due to chemisorptions [27]. The assumption behind the pseudo-second-order kinetic model was that the rate-limiting step might be chemisorptions involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate [28].

The effect of n-Al dose was studied for a by varying the dose between 0.05 g and 4 g in 100 ml aqueous. These tests were conducted at a temperature of 20 8C, with optimum pH value for SO42. The initial metal ion concentration was 100 mg/l. The pH value of the solution was adjusted with 0.1 M NaOH or 0.1 M H2SO4. It was observed that the adsorption percentage of SO42 onto the n-Al increased rapidly with the increasing of adsorbent concentration (Fig. 9). This result is expected because the increase of adsorbent dose leads to greater surface area. When the adsorbent concentration was increased from 0.1 g to 0.4 g, the percentage of SO42 ions adsorption increased from 65 to 85.6. At higher concentrations, the equilibrium uptake of SO42 did not increase significantly with increasing n-Al. For subsequent studies, a dose of 0.4 g of n-Al into 100 ml aqueous solution was selected. The data of Fig. 9 were fitted to Langmuir, Freundlich and Dubnin– Randkovich (D–R) models in order to examine the models constants at different temperature adsorption isotherms. 3.6. The isotherm model The adsorption isotherm is based on the assumption that every adsorption site is equivalent and independent of whether or not adjacent sites are occupied. Isotherms show the relationship between metal concentration in solution and the amount of sulphate sorbed on a specific sorbent at a constant temperature.

3

16 15

y = 1.300x + 7.042 R² = 0.981

14 13

2

t/qt

12

qt

y = 0.062x + 0.247 R² = 0.997

2.5

11 10

1.5 1

9 8

0.5

7

0

6 0

1

2

3

4

5

6

7

t0.5 (min) Fig. 6. Morris–Weber plot of SO42 sorption onto n-Al (the initial concentration, pH, volume of solution and amount of adsorbent was 50 mg/l, 5, 100 ml and 0.3 g respectively).

0

10

20

30

40

t(min) Fig. 8. Pseudo second order plot of SO42 sorption onto n-Al (the initial concentration, pH, volume of solution and amount of adsorbent was 50 mg/l, 5, 100 ml and 0.3 g respectively).

R. Katal et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 230–236

234

90

2 1.8

80 1.6

75

logqe

removal efficiency (%)

85

70 65

1.4 1.2

60

1

55

0.8

50 0

0.1

0.2

0.3

0.4

0.7

0.5

0.8

0.9

1

1.1

1.2

1.3

log Ce

amount of adsorbent (g) Fig. 9. The effect of amount of adsorbent on the removal efficiency (the initial concentration, pH, volume of solution and contact time was 50 mg/l, 5, 100 ml and 35 min respectively).

Fig. 11. Freundlich isotherm for SO42 sorption onto n-Al (the initial concentration, pH, volume of solution and contact time was 50 mg/l, 5, 100 ml and 35 min respectively).

3.6.1. The Langmuir isotherm model The Langmuir isotherm model is valid for monolayer adsorption onto surface containing finite number of identical sorption sites which is presented by the following equation (4):

equation, based on sorption on heterogeneous surface, can be derived assuming a logarithmic decrease in the enthalpy of adsorption with the increase in the fraction of occupied sites. The Freundlich equation is purely empirical based on sorption on heterogeneous surface and is given by:

qe ¼

qm K L C e 1 þ K LCe

(4)

where qe is the amount of metal adsorbed per specific amount of adsorbent (mg/g), Ce is the equilibrium concentration of the solution (mg/l), and qm is the maximum amount of metal ions required to form a monolayer (mg/g). The Langmuir equation can be rearranged to linear form for the convenience of plotting and determining the Langmuir constants (KL) as below. The values of qm and KL can be determined from the linear plot of Ceq/qeq versus Ceq: (5)

The equilibrium data were analyzed using the linearized form the Langmuir adsorption isotherm Eq. (5). The Langmuir constants, KL and monolayer sorption capacity, qm were calculated from the slope and intercept of the plot between Ce/qe and Ce (Fig. 10). The results and equations are indicated in Fig. 10 and Table 2. As can be seen, the correlation factor of this equation is negative, and it is not suitable for this process. 3.6.2. The Freundlich isotherm model While Langmuir isotherm assumes that enthalpy of adsorption is independent of the amount adsorbed, the empirical Freundlich 0.6 0.5 0.4

(6)

KF and (1/n) are the Freundlich constant and adsorption intensity, respectively. Equilibrium constants evaluated from the intercept and the slope, respectively, of the linear plot of log qe versus log Ce based on experimental data. The Freundlich equation can be linearized in logarithmic form for the determination of the Freundlich constants as shown below: logðqe Þ ¼ logðK F Þ þ

Ce 1 1 ¼ þ Ce qe qo  K L qo

Ce/qe

qe ¼ K F ðC e Þ1=n

1 logðC e Þ n

(7)

The slope and the intercept correspond to (1/n) and KF, respectively. It was revealed that the plot of log qe and log Ce yields a straight line (Fig. 11). The results are indicated in Table 2. 3.6.3. The Dubinin–Radushkevick isotherm model The Dubinin–Radushkevick (D–R) [29–31] isotherm was used to determine the nature of the adsorption process viz. physisorption or chemisorption. The linear form of this model is expressed by: lnðqe Þ ¼ lnðqm Þ  be2

(8) 2

where qe is the amount of SO4 adsorbed per unit dosage of adsorbent (mg/g), qm is the monolayer capacity, and b is activity coefficient related to mean sorption energy and e is Polanyi potential described as:    1 e ¼ RT ln 1 þ Ce

the the the

(9)

From the plots of ln qe versus e2 (Fig. 12) the values of b, qm were determined by the slope and intercept of the linear plot. The

0.3 0.2

Table 2 The isotherm constant for SO42 adsorption onto n-Al.

0.1 0 5

7

9

11

13

15

17

19

Ce (ppm) Fig. 10. Langmuir isotherm for SO42 sorption onto n-Al (the initial concentration, pH, volume of solution and contact time was 50 mg/l, 5, 100 ml and 35 min respectively).

k 0.53 qm 10.86

Langmuir equation y = 0.022x + 0.698 Freundlinch parameter n 0.65 D–R parameter

b 1  106

r2 0.98 r2 0.88

R. Katal et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 230–236

3

sites on the surface of the adsorbent for lesser number of adsorbate species.

2.5

3.8. Adsorption thermodynamics

2

ln Qe

235

1.5 1 0.5 0 0

200000

400000

600000

800000

1000000

1200000

ε2 Fig. 12. D–R isotherm for SO42 sorption onto n-Al (the initial concentration, pH, volume of solution and contact time was 50 mg/l, 5, 100 ml and 35 min respectively).

statistical results along with the isotherm constants are also given in Table 2. As our results show, adsorption of SO42 by n-Al can be fitted using Langmuir equation also the D–R equation shows considerable correlation factor. Although the Freundlich isotherm provides the information about the surface heterogeneity and the exponential distribution of the active sites and their energies, it does not predict any saturation of the surface of the adsorbent by the adsorbate. Hence, infinite surface coverage could be predicted mathematically. In contrast, D–R isotherm relates the heterogeneity of energies close to the adsorbent surface. If a very small subregion of the sorption surface is chosen and assumed to be approximately by the Langmuir isotherm, the quantity can be related to the mean sorption energy, E, which is the free energy for the transfer of 1 mol of metal ions from the infinity to the surface of the adsorbent [32]. 3.7. Effect of initial concentration of SO42 on the adsorption 2

3.8.1. Effect of temperature on adsorption of SO42 To study the effect of temperature adsorption experiments are carried out at 20–50 8C at optimum pH value of materials and adsorbent dosage level of 0.3 g in to 100 ml of solutions. The equilibrium contact time for adsorption was maintained at 35 min. The percentage of adsorption increases with rise of temperature from 20 to 50 8C. The results were shown in Table 3 and it revealed the endothermic nature of the adsorption process which later utilized for determination of changes in Gibbs free energy (DG), heat of adsorption (DH) and entropy (DS) of the adsorption of SO42 from aqueous solutions. The increase in adsorption with rise in temperature may be due to the strengthening of adsorptive forces between the active sites of the adsorbents and adsorbate species and between the adjacent molecules of the adsorbed phase. 3.8.2. Effect of temperature on thermodynamics parameter on adsorption of SO42 To study the thermodynamics of adsorption of SO42 on n-Al, thermodynamic constants such as enthalpy change DH, free energy change DG and entropy change DS were calculated using Eqs. (10)–(12). The values of these parameters are given in Table 4. Thermodynamic parameter DH, DS and DG for NO3 ions–n-Al system were calculated using the following equations: Kc ¼

F 1  Fe

logðK c Þ ¼

(10)

DH DS þ 2:303RT 2:303R

(11)

DG ¼ RT ln K c 2

To investigate the effect of initial SO4 concentration on SO4 adsorption onto n-Al, batch mode experiments were performed at ambient temperature (20  2 8C). The initial concentration of SO42 solution was varied from 50 mg/l to 250 mg/l with optimum adsorbent dose, contact time and pH (Fig. 13). It is evident from the result that the percentage removal of nitrate decreased from 85.6% to 37.5% for initial SO42 concentration of 50–300 mg/l. The results indicate that there is a reduction in nitrate adsorption, owing to the lack of available active sites required for the high initial concentration of SO42. The higher uptake of nitrate at low concentration may be attributed to the availability of more active

(12) 2

where Fe is the fraction of SO4 ions sorbed at equilibrium. A perusal of Table 4 indicated that the enthalpy change DH is positive (endothermic) due to increase in adsorption on successive increase in temperature. The negative DG values indicated thermodynamically feasible and spontaneous nature of the sorption. The positive value of DS reveals the increased randomness at the solid–solution interface during the fixation of the ion on the surface of the sorbent. Also, at all temperatures DH > TDS, indicating that the SO42 ions adsorption onto n-Al is dominated by enthalpic rather than entropic changes [33]. 3.9. Effect of other ions

100

The adsorption results discussed above were obtained taking nitrate ion only. However, in reality the SO42 contaminated water contains several other anions which can equally compete in the adsorption process. In order to see effect of interfering ions on adsorption of nitrate, a mixture of known quantities of commonly occurring anions in water, viz., phosphate, chloride, carbonate and nitrate were added to nitrate solutions. The initial concentration of SO42 was fixed at 50 mg/l while the initial concentration of other anions was 100 mg/l. The dependence of such ions on adsorption of

removal efficiency (%)

90 80 70 60 50 40 30 20 10

Table 3 The effect of temperature on the removal efficiency.

0 0

50

100

150

200

250

300

inial concentraon (ppm) Fig. 13. The effect of initial concentration on the removal efficiency (the amount of adsorbent, pH, volume of solution and contact time was 0.3 g, 5, 100 ml and 35 min respectively).

Temperature (8C)

Removal efficiency of NO3

20 30 40 50

85.62 88.41 91.55 94.45

R. Katal et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 230–236

236

Table 4 Thermodynamic parameter for SO42 adsorption onto n-Al.

DH kmol) SO42

27.44

(kJ/

DS

(kJ/

T (8C)

DG (kJ/mol)

20 30 40 50

4.34 5.11 6.2 7.3

r2

kmol) 0.107

0.98

Table 5 The concentration of sulphate in the urban wastewater and removal efficiency after treatment by n-Al. Adsorbent

Concentration in the wastewater (ppm)

Removal efficiency using n-Al

n-Al

41.7

83.2

concentration was investigated, it was found that increasing the initial concentration reduce the removal efficiency. The anions reduced the nitrate adsorption in the order of chloride > nitrate > carbonate > phosphate. It was observed that there was an about 47%, 43%, 38% and25% reduction in the percentage removal of SO42 by chloride, nitrate, carbonate and phosphate. Also n-Al was used for the removal of sulphate from real wastewater (urban wastewater) that the result was considerable. Acknowledgements The research upon which this paper is based was supported by a grant from (Pars Gas and Oil Co.) Hooman Taher Rahmati at Tarbiat Modares University is acknowledged for his assistance with experimental design and analysis. References

SO42 at varying concentrations of competitive anions was studied and was found that these anions reduced the adsorption of nitrate appreciably. The anions reduced the nitrate adsorption in the order of, chloride > nitrate > carbonate > phosphate. From above, it is evident that chloride has maximum and phosphate has least effect on the removal of SO42 by n-Al. It was observed that there was an about 47%, 43%, 38% and 25% reduction in the percentage removal of SO42 by chloride, nitrate, carbonate and phosphate. 3.10. Application of n-Al for removal of the SO42 from urban wastewater Upon completion of basic adsorption experiments, the efficacy of n-Al in the removal of SO42 from wastewater was evaluated. To this end, a bulk wastewater sample was obtained from a local urban wastewater (Shahi, Iran). The pH and nitrate concentration of collected wastewater was determined at the beginning of adsorption experiments, are shown in Table 5. The pH, COD, Color (absorbance, at 600 nm) and temperature of wastewater was 5.7, 950 ppm, 0.31, 27 8C. Adsorption was performed on 100 ml of wastewater with ash dose of 0.3 g. The suspensions were stirred at room temperature (25 8C) and 400 rpm. Table 5 shows SO42 removal from wastewater in terms of percent removal. As can be seen, n-Al is an efficient and effective adsorbent for the removal of SO42 from real wastewaters. The main advantages of n-Al for the removal of SO42 from water and wastewater include a high adsorption rate, capacity, and efficacy, as well as a short equilibration time. 4. Conclusions The n-Al showed considerable potential for the removal of SO42 from aqueous solutions. The optimum conditions of sorption were found to be: a sorbent dose of 0.3 g in 100 ml of solution. Contact time of 35 min and pH = 5. The results gained from this study were well described by the theoretical Freundlich. The kinetic data indicated that the adsorption process was controlled by pseudo-second-order equation. The results show the endothermic nature of the adsorption. The negative DG values indicated thermodynamically feasible and spontaneous nature of the sorption. The positive value of DS reveals the increased randomness at the solid–solution interface during the fixation of the ion on the active sites of the sorbent. The effect of sulphate initial

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