Studies on the adsorption mechanism of Ni(II) ions by

Studies on the adsorption mechanism of Ni(II) ions by a new activated carbon A. Shajudha Begum*, A. Jafar Ahamed, A. John Amalraj, C. Pragathiswaran

Studies on the adsorption mechanism of Ni(II) ions by a new activated carbon A. Shajudha Begum 1*, A. Jafar Ahamed2, A. John Amalraj1, C. Pragathiswaran1 1

PG and Research Department of Chemistry, Periyar E.V.R College (Autonomous) Tiruchirapalli, Tamil Nadu, India

2

PG and Research Department of Chemistry, Jamal Mohamed College (Autonomous) Tiruchirapalli, Tamil Nadu, India

E-mail : [email protected]

Abstract The efficiency of activated carbon prepared form Adathoda vasica stem (AVAC) for the adsorption of Ni2+ ions from aqueous solutions has been studied as a function of contact time, adsorbent dosage, initial metal ion concentration, temperature and pH of adsorbate solution. The optimal conditions for the adsorption have been arrived at and experiments were conducted to determine the Langmuir constants, Freundlich parameters and thermodynamic parameters such as ∆G, ∆H and ∆S. Desorption studies were also carried out for the recovery of both adsorbent and metal ion. The adsorption of Ni 2+ ions on to the surface of AVAC has been confirmed by the analysis of IR spectra, XRD and SEM images before and after Ni2+ adsorption.

Key words: Adathoda vasica stem, Activated carbon, Nickel ions, Adsorption isotherm, Equilibrium, Kinetic and Thermodynamic parameters, Intraparticle diffusion, Regeneration pattern. INTRODUCTION Water is life, it is the soul of nature and hope of future [1]. Clean drinking water is essential to human and other life forms. Heavy metal pollution in water has always been a serious 65

Int J Nano Corr Sci and Engg 4(1)(2017)65-80


environmental problem, because heavy metals are not biodegradable and can be accumulated in living tissues [2]. Rapid industrialization has seriously contributed to the release of toxic heavy metals to water streams. Some metals such as Fe, Cu, Zn & Ni can be harmful above certain limits and Certain metals such as Hg, Pb& Cd are toxic and very harmful to living beings. In order to solve heavy metal pollution in the environment, it is important to bring applicable solutions. Many techniques has been focussed for the removal of heavy metal and among them adsorption process is preferred because of their local availability, technical feasibility and cost effectiveness [3]. In the present investigation, the efficiency of the activated carbon prepared from the Adathoda vasica for the removal of Ni(II) ions from its aqueous solution has been studied and the applicability of the kinetic and equilibrium models for the Ni(II)-AVC system has also been discussed.

EXPERIMENTAL SECTION

Adsorbent Carbon was prepared by treating air-dried Adathoda vasica stem with concentrated sulphuric acid in a weight of ratio of 1:1. The resulting black product was kept in a furnace, maintained at 500˚C for 12 h followed by washing with water until from excess acid and dried at 150±5 ˚C. The carbon thus obtained was ground well and the portion retained between 0-75µm sieves was used in all the experiments. All the chemicals used were of Analytical Grade. Batch equilibrium method Batch mode experiments were carried out at different temperatures namely 35, 40, 45 and 50˚C. A known weight of adsorbent was agitated in a temperature controlled mechanical shaker with a known volume and concentration of the adsorbate with a pH of 6.0. The initial concentration was varied from 100mg/L to 400 mg/L. The flask containing the sample was withdrawn from the shaker at a predetermined time interval, filtered and the residual concentration of the metal ion was estimated by photoelectric colorimeter using 0.5% DMG reagent and 520nm filter. Effect of variable parameters Experiments were carried out to determine the effect of dosage of the adsorbent (1.0-5.0 g/L), effect of different initial concentrations of Ni(II) ions (ranging from 50 to 300 mg/L), effect of contact time on the removal of the copper ions, effect of initial pH of the adsorbate solution (pH ), effect of other ions such as Ca2+ (using CaSO4) and Cl- (using NaCl) ions during the adsorption of Ni(II) and the effect of solution temperature (35-50 ˚C) at 5 ˚C intervals with an accuracy of ±0.5 ˚C. Titration studies 66



Literature survey has revealed that [4], only strong acidic carboxylic acid groups are neutralized by sodium bicarbonate, whereas those neutralized by sodium carbonate are thought to be lactones, lactol and carboxyl group. Only the weakly acidic phenolic groups react with sodium hydroxide. Therefore, by selective neutralization using bases of different strength, the surface acidic functional groups in AVC can be characterized the amount of surface basic groups such as pyrones and chromenes. The results indicated that the activated carbon used may possess acidic oxygen functional group of their surface and this is supported well by their respective zero point charge values. The results obtained from the above characterization studies are given in the Table-1 Regeneration studies The regeneration of the adsorbed carbon was done by using 0.2 M mineral acids and sodium chloride solutions. Analytical Measurements The IR spectra, SEM images and XRD diagrams of the adsorbents before and after adsorption were recorded at the CECRI, Karaikudi, South India. Results and Discussion Characterization of the adsorbent The physico-chemical properties of the chosen adsorbent, AVC, were carried out by standard method [5-7] and are listed in the following Table-1. S. No.

Control Test

AVAC

1.

Bulk density (g/cc)

0.42

2.

Moisture (%)

0.092

3.

Ash (%)

99.32

4.

Matter soluble in water (%) 1.842

5.

Matter soluble in acid (%)

3.2514

6.

pH

5.5

7.

Surface area (m2 / g)

3.1

Effect of dosage of AVAC The adsorption of the Ni2+ ions on AVAC was studied by varying the carbon concentration (1.0 to 5.0 g/L) for the copper ion concentration of 1.0g/L. The percentage of adsorption 67



increased with increase in the carbon concentration (Figure-1). This may be due to the increased carbon surface area and availability of more adsorption sites [8, 9]. Effect of contact time and initial metal ion concentration The experimental results for the adsorption of Ni2+ ions on the activated carbon at various concentration (50,100, 150, 200, 250, 300 mg/L) with contact time presented in Table-2 which reveals that, the present adsorption decreased with increase in initial metal ion concentration, but the actual amount of Ni2+ ion adsorbed per unit mass of carbon is found to be increased with increase in metal ion concentration. It means that the adsorption is highly dependent on initial concentration of metal ion. It is because of the reason that at lower concentration, the ratio of the initial number of metal ion to the available surface area is low, subsequently the fractional adsorption becomes independent of initial concentration. However, at high concentration, the available sites for adsorption become fewer and hence the percentage removal of metal ion is dependent upon initial concentration. Equilibrium is established at 35 minutes for all concentrations. The plot of percent Ni(II) adsorbed against contact time for the initial Ni2+ ion concentration of 100 mg/L, 5.5 pH and a temperature of 35 ˚C is given in figure 2 which reveals that the curve is single, smooth, and continuous, leading to saturation, suggesting the possible monolayer coverage of the metal ion on AVAC [8, 9].

80

70

70 65

60 50

1.0g/L 2.0g/L 3.0g/L 4.0g/l 5.0g/L

40

% removal of Ni(II)

% removal of Ni(II)

60

55

30 50 20 0

50

100

150

200

250

300

45 50

100

150

200

250

Contact time,t,min.

C 0,mg/L

Fig-1 Effect of contact time on Ni(II) removal

Fig-2 Effect of initial concentration on the removal of Ni(II)

300

68



TAB LE-2 EQUILIBRIUM PARAMETERS FOR THE ADSORPTION OF Ni(II) ONTO AVAC Ni(II)]ini., mg/L 50

Ce, mg/ L 40˚ 45˚

50˚

35˚

40˚

45˚

15.6

16.1 3

16.83

17.5 3

64.1026

61.996 3

59.4177

57.0450

28.5

29.8

31.1

32.4 5

35.0877

33.557 0

32.1543

30.8166

35.17

36.0

37.6

39.8 3

28.4333

27.777 7

26.5957

39.8

41.4

43.5

46.3

25.1256

24.154 6

46.0

47.8

51.5

54.8

21.7391

20.920 5

54.9

56.9

58.7

64.4

18.2149

17.574 7

35˚

100 150 200 250 300

x, mg/ L 50˚

35˚

x/m, mg/ g 40˚ 45˚

50˚

35˚

18.8

17.75

16.3 5

14.95

18.8

42.9

40.4

37.8

35.1

42.9

25.1067

79.6 5

78.0

74.8 5

70.35

22.9885

21.5983

120. 4

117.2

113.0

19.4175

18.2482

158. 0

154.5

15.5279

189. 3

186.3

17.0358

1.9

% Metal adsorbed 40˚ 45˚

50˚

56.3380

61.1620

66.8896

24.7525

26.4550

28.4900

79.6 5

12.8205

13.3600

14.2146

107.4

120. 4

8.5324

8.8495

9.31099

147. 0

140.5

158. 0

6.4725

6.8027

7.1174

182. 7

171.3

189. 3

5.3677

5.4735

5.8377

0.07

1.8 0.06

1.7 0.05

1/qe

log( x/m)

1.6

1.5

0.04

0

35 C 0 40 C

1.4

0

35 0 40 0 45 0 50

0.03

0

45 C 0

50 C

1.3

0.02

1.2

0.01 0.00

1.1 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

0.01

0.02

0.03

0.04

0.05

0.06

0.07

l/Ce

logC e

Fig-4 Langmiur Isotherm

Fig-3 Freundlich isotherm

69



Adsorption isotherm The experimental data were analyzed in the light of Langmuir [10] and Freundlich adsorption isotherm [11]. The Langmuir [10] isotherm is 1/qe = 1/Qm b Ce + 1/Qm Where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g) and Qm and b are Langmuir constants related to adsorption efficiency and energy of adsorption, respectively. The plots of 1/qe versus 1/Ce suggest linearity and the applicability of the Langmuir isotherm to the Ni2+-AVAC adsorption system (Figure 4).

Values of Qm and b were

determined from slope and intercepts of the plots and are presented in Table-3. From the results, it is clear that the value of adsorption efficiency, Qm, and adsorption energy, b, of the carbon increases on increasing the temperature. From the values, we can conclude that the maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on adsorbent surface. The Freundlich isotherm [11] is log qe = log KF + 1/n log Ce Where qe is the amount of copper ion adsorbed (mg/g). Ce is the equilibrium concentration of metal ion in solution (mg/L) and KF and n are constants incorporating all factors affecting the adsorption capacity and intensity of adsorption respectively. Linear plot of log (x/m) versus log Ce shows that the adsorption of copper ion follows the Freundlich isotherm (Figure 3). Values of KF and n were determined and are given in the Table-5, which reveals that an increase in the negative charge on the surface that enhances the electrostatic force like van der Waal’s force between the carbon surface and metal ion, thereby increases the adsorption of copper ions. The values clearly indicate that there is dominance in adsorption capacity. The intensity of adsorption is an indicative of the bond energies between metal ion and adsorbent and the possibility of slight chemisorption rather than physisorption. The possibility of multilayer adsorption of metal ion through the percolation

70



process cannot be ruled out. However, the values of n are greater than one indicating the adsorption is much more favourable [12-15].

Correlation Co-efficient

S. No.

Temp., ˚C

Qm (mg/g)

b (L/mg)

1.

35

63.78

0.0173

0.990

1.8003

2.0080

0.972

2.

40

65.06

0.0184

0.987

2.0565

2.2523

0.983

3.

45

67.16

0.0206

0.988

1.9620

2.0790

0.972

4.

50

71.69

0.0218

2.0057

2.0367

0.974

Correlation KF n Co-efficient (mg/g) (L/mg)

0.988 TABLE-3

LANGMIUR AND FREUNDLICH ISOTHERM CONSTANTS FOR ADSORPTION OF Ni(II) ON AVAC TABLE-4 VALUES OF RLFOR Ni(II) ADSORPTION ON AVAC [Ni(II)]ini., C0, mg/L

RL at different temp., ˚C 35

40

45

50

50

0.5362

0.5168

0.4926

0.4785

100

0.3663

0.3484

0.3268

0.3145

150

0.2782

0.2628

0.2445

0.2342

200

0.2242

0.2109

0.1953

0.1866

250

0.1878

0.1762

0.1626

0.1550

300

0.1615

0.1513

0.1393

0.1326

Kinetics of adsorption The contact-time experimental results can be used to study the rate-limiting step in the adsorption process, as shown by Weber and Morris [16, 17]. Since the particles are vigorously agitated during the adsorption period, it is probably reasonable to assume that 71



the rate is not limited by mass transfer from the bulk liquid to the particle’s external surface and one might then postulate that the rate-limiting step may be either film or intra-particle diffusion. As they act in series, the slower of the two will be the rate-determining step [16]. The rate constant for intra-particle diffusion is obtained using the equation q = kid t1/2 + C

Where, kid (mg/g/min) is the intra-particle diffusion rate constant. The kid values obtained from the slope of the linear portions of the curves for different metal ion concentrations at 35 ˚C are given in Table-5 and Fig. 6. The kid values increased with increase in the copper ion concentration, which reveals that the rate of adsorption is governed by the diffusion of adsorbed copper ion within the pores of the adsorbent. S.No.

1. 2. 3. 4. 5.

Dose of the adsorbent m, g/L 1.0 2.0 3.0 4.0 5.0 TABLE-5

kid 1.537 1.043 0.678 0.551 0.507

Effect of temperature The adsorption capacity of the carbon increased with increase in the temperature of the system from 35-50 ˚C. Thermodynamic parameters such as change in free energy (∆G), enthalpy (∆H) and entropy (∆S) were determined using the following equations [12] -∆H 1 + log10 b1 --------- ---2.303 R T log 1 = ∆G . 1 ----------- ---T 2.303R T ∆S = ∆H -∆G ---------T

log10b =

Where, b = Langmuir constant ∆H = apparent enthalpy change of adsorption ∆G =apparent free energy of adsorption and ∆S =apparent entropy change of adsorption 72



In the present work, the plot of log10 b against the reciprocal of temperature shown in Figure and that of log 10 1/b against the reciprocal of temperature are found linear. From the slopes of these linear plots, ∆H and ∆G were determined and subsequently, ∆Swas calculated. The values of ∆H and ∆G are found to be 14630.3 and 14009.96 KJmol-1 respectively. The term ∆H represents the enthalpy associated with the overall adsorption process. It is not possible to arrive at any conclusion regarding the heat of adsorption for any of the different processes expected to be involved in the adsorption process. The positive ∆H value found in the present investigation indicates that Ni(II)-AVAC, adsorption process is endothermic. Thermodynamically, the value of ∆G is positive and the value of ∆H is positive, suggesting that the adsorption is spontaneous at higher temperature [18]. The positive values of ∆S showed that the increasing randomness at the solid/liquid interface during the process [19].

Effect of pH The solution pH plays a major role in determining the amount of Nickel ions absorbed. Adsorption was studied over the range of pH 1-7 and the results are shown in Figure. The initial metal ion was kept constant. Adsorption of copper ions increased appreciably times) with increase of pH from 1to 7 and consistent with results obtained by others.

(2 The

increase is partly attributed to the formation o different hydroxo species with rise in solution

73



pH. Based on the hydrolysis constants of metals ions as defined in and taking only primary metal species expected to be formed in the working pH range into consideration, the species distribution diagram for copper ion is constructed and are given in Figure-. It is evident that Ni2+ and its monohydroxo species are the predominating species up to pH

7 for Nickelion.

Since maximum adsorption of Nickel ions was achieved at pH 5.5, it may safely be stated that the removal of Nickel ion was mostly due to adsorption and not precipitation.

45

21 44

y = 0.6322x + 10.847 R² = 0.9613

20

x/m, q, mg/g

% removal of Ni(II)

43 42

41

19 18 Linear (Dose=2.0g /L)

17

16 40

15 39

14 5 1

2

3

4

5

6

7

pH Fig--5Effect of pH on Ni(II) removal

10

15

t1/2, min1/2 Fig-6 Intraparticle diffusion plot

74



Desorption Studies Desorption studies help to elucidate the nature of adsorption and recycling of the spent adsorbent and the metal ion. If the adsorbed metal ion can be desorbed using neutral pH water or by very dilute acids, then the attachment of the metal ions onto the surface the adsorbent is by weak bonds. If sulphuric acid or alkaline water desorbs the metal ions, then the adsorption is by ion exchange. If organic acids like acetic acid can desorb the metal ion, then the metal ion is held on the adsorbent through chemisorptions [20]. The effect of various reagents used for desorption studies reveals that hydrochloric acid is a better reagent for desorption, because we could get 100 % removal of adsorbed metal ions. The desorption of metal ions by dilute mineral acids and alkaline medium indicates that the metal ion was adsorbed onto the activated carbon through physisorption [21]. Evidences for adsorption The IR spectra of the raw activated carbon and after adsorption of metal ion are shown in Figures 7a & 7b respectively. The band at the region of 2323.3 cm-1 shows the presence of carboxylate ion. The band at 1611.59 and 1604.8 cm-1 represents the aromatic ring vibration assigned to aromatic carbonyl and carbonyl motion in carboxylic and with intermolecular hydrogen bonding. The bands at 1598.09, 1587.48 show the presence of C=C stretching vibrations of cyclo alkenes. The (S=O) Symmetric stretching frequency of organic sulphates stretching frequency of organic sulphates occurs in the region of 1408.10 cm-1 The bands between 782.17 and 669.33 cm-1 likely results from NO2 bending vibrations. The band at 592.17 cm-1 represents the S-S Stretching vibration of disulphide. The C-C bending vibrations of normal alkenes occur in the region of 458.11Cm-1 Thus the FTIR results correlated by the amount of COOH, Carbonyl groups, Nitro, Sulphide groups on the material indicated that the adsorbents containing the functional groups such as OH, COOH, CO, NO2, S2 that could be the potential sites for interaction with the Ni(II) ions. It could be seen that there is a slight reduction in the stretching vibration adsorption bands. This clearly indicates the adsorption of metal ion on the adsorbent by physical forces [2, 16].

75



XRD spectrum of adsorbents before and after adsorption shown in Fig. 8a & 8b. The intense main peak shows the presence of highly organized crystalline structure of raw activated carbon, after the adsorption of metal ion, the intensity of the highly organized peaks is slightly diminished. This has attributed to the adsorption of metal ion on the upper layer of the crystalline structure of the carbon surface by means of physisorption [22]. SEM Photographs of the selected adsorbents before and after adsorption at 1000 and 500 magnifications shown in Fig. 9a & 9b. SEM images of adathoda stem shows that the adsorbent have a rough surface with almost non-compact structure. It is obvious that the sorbent have considerable number of pore spaces, where appropriate conditions exist for Cu(II) ions to be trapped and adsorbed into these pores. The SEM photos also illustrate that the metals can be homogeneously adsorbed on the surface of adsorbent.

It is revealed from the

SEM figures that the surface of AVAC is found to be not so random but rough in such a way to adhere the solute specie on to the surface of the adsorbent.

Therefore the adsorptive

characteristics of adathoda stem are expected to be highly effective [22].

The bright spots,

shows the presence of tiny holes on the crystalline structure of raw activated carbon, after treatment with metal ion the bright spots became black shows the adsorption of the metal ion on the surface of the carbon by means of physisorption [23].

FIG. 7a IR SPECTRUM OF AVAC BEFORE Ni(II) ION ADSORPTION 76



FIG. 7b IR SPECTRUM OF AVAC AFTER Ni(II) ION ADSORPTION

FIG. 8a XRD SPECTRUM OF RAW AVAC

77



FIG. 8b XRD SPECTRUM OF AVAC AFTER Ni(II) ION ADSORPTION

FIG. 9a SEM PHOTOGRAPH OF RAW AVAC

78



FIG. 9b SEM PHOTOGRAPH OF AVAC AFTER Ni(II) ADSORPTION CONCLUSION The experimental data correlated reasonably well by the Langmuir and Freundlich isotherms and the isotherm parameters were calculated. The amount of metal ion adsorbed increased with increase in pH if the medium. The amount of metal ion adsorbed slightly decreased with increasing ionic strength and increased with increase in temperature.

The dimensionless

separation factor (RL) showed that the activated carbon can be used for the removal of metal ion from aqueous solution.

The values of ∆H, ∆G and ∆S results show that the carbon

employed has a considerable potential as an adsorbent for the removal of metal ions. Acknowledgment The authors thank the Principal and members of the Management Committee of Jamal Mohamed College for providing necessary facilities. REFERENCES 1. Kulshreshtha, S.N., Wat. Res. Management, 1998, 12:167. 2.Regine, H.S.F., Vieira Boya Volesky, Int. Microbial, 2000, 3: 17. 79



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