Synthesis of Nano Calcium Oxide from a Gastropod Shell and the

ARTICLE pubs.acs.org/IECR

Synthesis of Nano Calcium Oxide from a Gastropod Shell and the Performance Evaluation for Cr (VI) Removal from Aqua System N. A. Oladoja,* I. A. Ololade, S.E. Olaseni, V. O. Olatujoye, O. S. Jegede, and A. O. Agunloye Department of Chemistry, Adekunle Ajasin University, Akungba Akoko, Nigeria ABSTRACT: A spongy, porous nano calcium oxide (NC) with point zero charge of 11.80 was synthesized using the shell of a Gastropod (Achatina achatina) via the sol
gel technique. The ability of the NC to adsorb Cr (VI) from aqueous solution was assessed systematically in a batch process via isothermal, kinetic, and variable process optimization procedure. The sorption data had the best fitting for the Langmuir isotherm model and the monolayer sorption capacity (qm, mg/g) value of 125 mg/g was obtained. The initial solution pH had no palpable effect on the monolayer sorption capacity. The amount of sorbed Cr (VI) increased with contact time and initial Cr (VI) concentration and attained equilibrium within 120 min. The fitting of the different kinetic models to the sorption data, by error analysis, using the linear coefficient determinations (r2) and the Chi-square statistical analysis (χ2), showed that the mechanism of the sorption process is better described by the pseudo second order kinetic model. Thermodynamic evaluation showed that the sorption process was spontaneous (ΔGo:
15.27;
14.87 and
14.45 kJ/mol at 303, 313, and 333 °C, respectively), exothermic (ΔH0 =
22.568 kJ/mol) and increased disorder (ΔS0 = 0.0244) appeared on the NC
solution interface during the adsorption process.

1. INTRODUCTION Chromium exists in several valence states of which the trivalent, Cr (III), and hexavalent, Cr (VI) forms are common. Cr (VI) is highly toxic, mobile, and speciate in environmental media in the form of dichromate (Cr2O72
), hydrogen chromate (HCrO4
), or chromate (CrO42
) all of which are pH dependent. Thus, a repulsive electrostatic effect is created between any of the Cr (VI) anionic species and either the negatively charged soil particles or colloidal substrate in aqua system, hence, they transfer freely in the environment. It is, therefore, worthwhile to develop an effectual protocol for the abstraction of these different species of chromium from industrial effluents and other aqua environmental systems. Among all of the available processes (e.g., electrochemical precipitation, ion exchange, adsorption, co-precipitation, and membrane filtration) for Cr (VI) abstraction in aqua stream, adsorption has been adjudged to be the more popular and economically feasible route.1 Numerous adsorbents such as aluminum magnesium mixed hydroxide,2 Neem sawdust,3 biowaste materials4 activated carbon modified with nitric acid,5 chemically treated olive pomace6 etc. have been reported as promising sorbents for Cr(VI) abstraction from the aqueous solution. The most studied adsorbent for Cr (VI) abstraction is activated carbon, derived from various sources.7 Activated carbon adsorption is an attractive choice due for its exceptionally high surface areas, well-developed internal microporosity structure, as well as the presence of a wide spectrum of surface functional groups e.g. carboxylic group.8 However, the low adsorption capacity of Cr (VI) on activated carbon has restricted its wide application9 in this regard. Thus, many researchers have focused their efforts on optimizing adsorption and developing novel alternative adsorbents with high adsorptive capacity and low cost. On the basis of the basic nature of many metal oxides, it has found application as adsorbent in pollution control. The r 2011 American Chemical Society

abatement of many gas pollutants characterized by significant acidities, such as carbon, sulfur, nitrogen, and arsenic oxides and hydrogen halides and sulfide, can be adsorbed or absorbed into basic metal oxides. Alkaline-earth oxides, mostly calcium and barium-based oxides are useful for the purification of hot gases.10 Nanocrystalline alkaline earth metal oxides have also attracted significant attention as effective chemisorbents for toxic gases, HCl, and chlorinated and phosphorus-containing compounds.11 Destructive sorption takes place not only on the surface of these oxide materials but also in their bulk. It is important to note that the efficiency of the destructive sorption increases with a decrease in the size of the MgO crystallites. MgO works most efficiently in the destructive sorption reaction when its particle size is on a nanometer scale. The high efficiency of the nanoparticle oxides is caused not only by their high surface area but also by the high concentration of low coordinated sites and structural defects on their surface.11 As the particle size is scaled down to a few nanometers, the constituting atoms exhibit highly defective coordination environments. Most of the atoms have unsatisfied valencies and reside at the surface. In short, microstructural features such as small grain size, large number of interfaces and grain boundary junctions, pores, and various lattice defects that result from the chosen routes for their synthesis contribute significantly to the unique physical and chemical properties of nanomaterials. In the water industry, the alkaline metal oxide that is widely used is CaO and its use has been restricted to pH correction and sometimes as coagulant aid in the coagulation/flocculation process. In the present studies, nanosize CaO (NC) shall be synthsised from the shell of a gastropod (African land snail Received: June 3, 2011 Accepted: November 12, 2011 Revised: September 25, 2011 Published: November 12, 2011 639

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(Achatina achatina)), and tested as a sorbent for Cr (VI) abstraction from aqua system. The choice of the shell of this phylum of mollusca as a precursor to the synthesis was predicated on the fact that snail shell (SS) has got the same mineralogical assemblage and basic construction as other Mollusk shells. Basically, the SS consists of mainly CaCO3, as well as various organic compounds. It consists of three layers thus: the Hypostracum, which is the innermost layer; the Ostracum, which is the shell-building layer and the Periostracum, the outermost layer.12 The Hypostracum is a form of Aragonite, a type of CaCO3. The Ostracum is built by several layers of prism-shaped CaCO3 crystals with embedded proteid molecules. The Periostracum, the outermost shell layer, is not made of CaCO3, but of an organic material called Conchin, a mixture of organic compounds, mostly of proteids. Conchin not only makes the outer shell layer, but also embedded between the CaCO3 crystals of deeper layers. The nano CaO (NC) shall be synthesized from the shell of African land snail via the sol
gel method and its properties characterized. The ability of the NC to adsorb Cr (VI) from aqueous solution shall be tested via batch process while different isotherm and kinetic models shall be employed to predict the mechanism and dynamics of the sorption process.

2.4. Adsorption Studies. For the pH-edge experiment, the pH of the Cr (VI) solution was maintained within the range of 3
10. The adsorption isotherm was studied by 50
300 mg/L Cr (VI) in contact with the NC systems at ambient temperature for 120 min. The isothermal data were fitted by different isotherm equations. For adsorption kinetic experiments, the initial Cr (VI) concentration was 200 mg/L to react with the NC. The suspension was shaken at 3000 rpm for 240 min. Subsamples were taken at different times viz: 5, 10, 20, 30, 60, 90, 120, 180, and 240 min.

3. RESULTS AND DISCUSSION 3.1. NC Characterization. The PZC is an important property of colloidal particles because the sign and magnitude of the surface charge of colloidal particles in contact with aqueous solution affect the stability of colloidal dispersions, their rheological properties, and their ability to adsorb ions from solution.17 Metal oxides show pH-dependent surface charging, that is, the sign and the magnitude of the surface charge can be adjusted by pH adjustment, and the PZC is defined as a point on the pHscale. The result of the PZC of NC was found to be 11.80. This value of the PZC obtained in the present study is comparable but slightly higher than the value reported (11.00) for CaO by Panday, et al.18 The difference in the value of the PZC could be ascribed to the difference in sources of the two CaO and possibly the presence of other metallic ions (e.g., K, Na, Cl, Si, and C), as proven by the EDX results, in the precursor of the synthesized CaO in the present studies. The value of the PZC obtained implied that below the pH of 11.80, the surface of the synthesized adsorbent is predominantly positively charged. The result of the EDX analysis is presented in Figure 1 and the different elements present on the surface are as shown. The dominance of calcium ion in the synthesized material was confirmed by the prominence of the peaks and the presence of other elements (K, B, Al, Si, and C), in minor quantities, were also ascertained. The presence of chloride could be ascribed to the residual chloride from the CaCl2 solution. The surface micro structural analysis, using SEM, revealed that the NC is a sponge like and foamy product with large agglomerates of very fine particles (Figure 2). It is a porous and agglomerates of polycrystallite nanoparticles. The particle size estimation, using SEM (Figure 2), showed that the size of the particulates of the NC is within the nanoscale. 3.2. Sorption Studies. The uptake of the Cr (VI) species by the NC was confirmed by comparison of the results of the EDX analysis of both the virgin and the spent NC (Figure 1 and 3, respectively). The EDX diffractogram of the spent NC showed the presence of the Cr species in addition to the other elements present on the surface of the virgin NC. The evidence of the uptake of the Cr (VI) also manifested in the alteration of the surface microstructure of the NC as evidenced by the SEM results presented in Figures 2 and 4. The spongy and foamy nature of the surface became more pronounced and the pore size became irregular and narrower with the uptake of the Cr (VI) species. 3.2.1. Effect of pH on Adsorption. Solution pH is one of the most important variables affecting adsorption characteristics. In order to understand the effects of solution pH on the uptake of Cr (VI) ion, isotherm experiments were conducted at different pH values (3, 5, 8, and 10). Adsorption isotherms are basic requirements to understand the mechanism of adsorption. Thus

2. MATERIALS AND METHODS 2.1. SS Preparation and Characterization. The SS (purchased from a snail farm) was first washed with tap water then rinsed thoroughly with deionized water, dried in the oven and ground in a wooden mortar and pestle. The ground SS was finally made into powder using laboratory grinding machine and sieved with a laboratory sieve of known mesh size. The physicochemical characteristics of the SS were reported in our earlier treatise.12 2.2. Synthesis and Characterization of the NC. The NC was synthesized by the sol
gel method13,14 using SS as a precursor viz: Calcium chloride solution was prepared by the dissolution of known weight of the SS in a dilute HCl (AR, BDH) (9:1 = H2O: HCl) and the reaction was assumed to have gone into completion when the effervescence ceases. A known volume of NaOH (1M) (Reagent grade, BDH) was added slowly into the resultant CaCl solution while stirring, to initiate the growth of Ca (OH)2. The precipitate was aged for 180 min in the parent solution at ambient temperature typical of a tropic region. The suspension was centrifuged at 3000 rpm for 5 min to obtain the Ca (OH)2 gel, washed several times with distilled water, and dried in the oven at 60 °C for 24 h. The dried powder was finally calcined in air at 1000 °C for 4 h and the NC were such made. The NC was characterized thus: point of zero charge (PZC) was determined by the solid addition method,15 energy dispersive X-ray (EDX) was employed to determine the surface composition, while the surface microstructure was determined using the scanning electron microscope (SEM) (EVO MA/10 model of Carl Zeiss SEM). 2.3. Preparation and Analysis of Cr (VI) Concentration. A stock solution of hexavalent chromium (1000 mg/L) was prepared in deionized double-distilled water using potassium dichromate (K2Cr2O7) (A.R., B.D.H.). All working solutions of varying concentrations were obtained by serial dilution. The residual Cr (VI) concentration in the solution was determined spectrophotometrically at 540 nm using a UV
visible spectrophotometer (UV-1601, Shimadzu), following the 1,5diphenylcarbazide (G.R., Merck) protocol.16 640

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Figure 1. Results of the EDX analysis of the virgin NC.

Figure 2. Results of the SEM analysis of the virgin NC.

The parameters of the isotherm analysis are presented in Table 1. The values of the monolayer sorption capacities, qm (mg/g), obtained from the Langmuir plot of the sorption data were in the same range (112
125 mg/g), despite the divergence in the initial solution pH. The magnitude of Langmuir constant, Ka, which is largely determined by the heat of sorption, steadily increased with increase in the initial solution pH (Table 1). The higher the magnitude of the Ka, the higher the heat of sorption and the stronger the bond formed.

the data obtained from the isotherm experiment at varying pH were tested with each of the isotherm models presented below: Langmuir : Ce =qe ¼ 1=qm Ce þ 1=K a qm

ð1Þ

Freundlich : log qe ¼ log K f þ 1=nlog Ce

ð2Þ

Temkin : qe ¼ B1 ln K T þ B1 InCe

ð3Þ 641

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Figure 3. Results of the EDX analysis of the spent NC.

Figure 4. Results of the SEM analysis of the spent NC.

Table 1. Isotherm Parameters of Cr (VI) Sorption at Varying pH on NC Langmuir initial pH

qm

3

125.00

5 8

121.95 113.64

10

112.36


3

Ka  10

Freundlich 2

r

kf

7.81

0.9607

8.21 9.2

0.9849 0.9912

9.4

0.9716

Temkin

n

2

r

kT

BI

r2

2.026

1.399

0.9615

0.089

25.545

0.9989

2.274 2.511

1.454 1.502

0.9763 0.9732

0.094 0.098

24.713 24.122

0.9893 0.9946

2.419

1.483

0.9553

0.098

24.117

0.9987

642

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The Freundlich constants, kf and n are related to adsorption capacity and sorption intensity respectively. The values of these constants, obtained from the plot of log qe vs log Ce, are presented in Table 1. The values of the reciprocal of, n, (i.e., 1/n), which were less than unity (0.6657
0.715), at all the pH studied, is an indication of the favorable nature of adsorption of Cr (VI) on NC. The Temkin constants (Table 1) were obtained from the plot of qe vs lnCe. The comparison of the Langmuir parameter, qm (mg/g), obtained from the present studies with the values obtained by other investigators19
26 is presented in Table 2. An evaluation of the fittings of the data obtained from the isotherm studies, at different initial Cr (VI) solution pH, to each of the isotherm models was tested using the linear coefficient of determination, r2. The coefficient of determination, r2, represents the percentage of variability in the dependent variable that has been explained by the regression line. The value of the coefficient of determination may vary from zero to one. A coefficient of determination of one indicates that 100% of the variation of qe has been explained by the regression equation. The linear coefficient of determination, r2, found from evaluation of data by linear model, was calculated with the aid of the equation: r2 ¼

S2XY Sxx Syy

Where Sxx is the sum of squares of x; Syy is the sum of squares of y and Sxy is the sum of squares of x and y: The results obtained revealed that all the isotherm models showed higher correlation of the isotherm data (Table 1). In order to further assess the different isotherms and their correlation with experimental results, the theoretical plots obtained from each isotherm analysis is shown with the experimental data for the sorption of Cr (VI) ions onto NC at the actual solution pH of Cr (VI) solution and temperature of 303 K in Figure 5. The graph is plotted in the form of metal ions sorbed per unit mass of NC, qe (mg/g), against the concentration of metal ions remaining in solution, Ce (mg/L). Both the Langmuir and Freundlich isotherm models showed better description of the sorption process than the Temkin isotherm model, despite its high r2 values, as it is clearly shown in the conformance of the predicted (i.e., theoretical, qe,) values to the experimental values. In order to be able to evaluate the fit of the different isotherm models to the experimental data, the optimization procedure requires an error function to be defined. Owing to the low dependence of the sorption affinity of the NC for the Cr (VI) oxoanions on initial Cr (VI) solution pH, as shown in Table 1, the fitting of the sorption data to the sorption isotherms used to describe the sorption process was also examined using nonlinear chi-square statistical test, χ2, at the actual solution pH of Cr (VI) solution. The χ2 test statistic is basically the sum of the squares of the differences between the experimental data and data obtained

ð5Þ

Table 2. Comparison of Sorption Capacity (qm mg/g) of Cr (VI) on Various Adsorbents adsorbent

qm (mg/g)

Table 3. χ2 Error Analysis of the Isotherm Data of the Sorption of Cr (VI) on NC

refs

Neem leaves

62.97

Babu and Gupta .,19

initial conc.

Langmuir: qe =

Freundlich:

Temkin: qe =

Trichoderma Fungal

48.9

Liliana and Eliseo,20

(mg/L)

(GmbCeG)/(1 + b Ce)

qe = KfC1/n e

(RT/b)ln(KTCe)

weed Salvina cucullata

13

Soroj and Surendra,21

Black tea leaves

45.5

Mohammed and Mikio et al.,22

χ2

χ2

χ2

23

Alligator weed

82.57

Xue Song Wang et al.,

50

0.0319

0.1350

25.8492

reed lignin

52.6

Yong Sun et al.,24

100

0.1271

0.3640

63.4027

activated lignin

75.75

Tazerouti and Amrani,25

150

0.0043

0.1858

91.3258

sugar cane bagasse nano calcium oxide

13.4 125

Shamah and Forster,26 Present studies

200 300

0.0455 0.0033

0.0010 0.3200

111.0065 129.1559

Figure 5. Comparison of isotherm model with experimental values. 643

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Figure 6. Effects of initial pH on the sorption of Cr (VI) onto CN.

by calculating from models, with each squared difference divided by the corresponding data obtained by calculating from models. The equivalent mathematical statement is as follows: χ2 ¼

∑ðqe
qe, m Þ2 qe, m

above this pH a significant reduction in the extent of adsorption was observed. The pH dependence of metal adsorption is largely related to the metal chemistry in the solution and the properties of the adsorbent.34 Cr (IV) exists in different ionic forms in solution and the most important Cr (IV) states in solution are chromate (CrO42
), dichromate (Cr2O72
) and hydrogen chromate (HCrO4
), depending on the solution pH and total chromate concentration.34 The following are the important equilibrium reactions.36,37

ð6Þ

If data from models are similar to the experimental data, then χ2 will be a small number and if they are different, χ2 will be a bigger number. A comparison of the nonlinear chi-square, χ2, of the four isotherm models for Cr (VI) sorption onto NC was made and listed in Table 3. The results of the χ2 analysis showed that the Langmuir predictions of the qe values were the best in terms of conformance of the theoretical qe values with the experimental qe. The χ2 analysis has also been determined to be best error fitting method for the biosorption of lead onto palm kernel fiber,27 cadmium onto coconut copra meal,28 and Congo red onto palm kernel seed coat.29 The results presented in Table 1 and Figure 6 showed that pH has no palpable effects on the adsorption of Cr (VI) on NC. On the basis of the apparent fact that CaO is a basic oxide and the similitude in the values of the qe (mg/g), got from the analysis of the data obtained at varying pH with Langmuir isotherm model, the final solution pH at the end of each sorption procedure was determined and the results obtained are presented in Figure 6. This was done to gain an insight into the effect of the NC on the solution pH which is known to be a deciding factor on the speciation of the sorbate of interest. The final solution pH after the sorption process ranged between 10.43 and 11.50 despite the correction of the solution initial pH to values that ranged between 3 and 10. The effects of pH on the sorption of the adsorbate observed in the present studies were at variance with the reports of the different studies on the effects of pH on Cr (IV) abstraction from aqua system using different adsorbents. Li et al.,30 Huang et al,31 Kumar and Chakraborty,32 and Kyzas et al.,33 using different sorbents, have reported the optimum pH for Cr (IV) abstraction from aqueous solution which ranged between 2 and 4 and that

H2 CrO4 ¼ Hþ þ HCrO4


ð7Þ

HCrO4
¼ Hþ þ CrO4 2


ð8Þ

2HCrO4
¼ Cr2 O7 2
þ H2 O

ð9Þ

Since the distribution of Cr(VI) species is dependent on both pH and total Cr(VI) concentration, based on thermodynamic database listed in Butler36 and Stumm and Morgan,37 HCrO4
and CrO42
are the two possible most predominant species at the experimental total Cr(VI) concentration (i.e., < 300 mg/L Cr (VI) ion). For pH lower than 6.8, HCrO4
is the dominant species of hexavalent chromium, and above pH 6.8, only CrO42
is stable. Considering the fact that the adsorbent is a metal oxide, the mechanism of anion adsorption on NC is similar to that of such adsorbent as alumina, gibbsite, serpentine, and clay minerals. The anion adsorption sites on these materials are aquo groups (
M
OH2+) and hydroxo groups (
M
OH). The surface chemistry of an oxide in contact with an aqueous solution is determined, to a large extent, by deprotonation or a hydroxyl association reaction: M-OH þ Hþ ¼ -M-OH2þ

ð10Þ

-M-OH þ OH- ¼ -M-O- þ H2 O

ð11Þ

Equation 10 is favored at lower pH while eq 11 is favored at the higher pH range.38 For the anion to be adsorbed onto the NC 644

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Figure 7. Effects of contact time and initial Cr (VI) concentration on the sorption process.

surface, a source of protons is required to protonate the surface to create a potential between the adsorbent surface and the adsorbate molecule. The next two eqs 12 and 13 show the ligand exchange scheme proposed for the adsorption of an anion such as Cr (VI) on a metal oxide surface:39 -M-OH þ HX ¼
MOH þ ---Xð12Þ

The association between the positively charged surface of NC and the HCrO4
anions may be written as follows: NC-OH2 þ þ HCrO4
f NC-OH2 þ :::HCrO4


Despite the difference in the initial pH of the Cr (VI) solution, the final pH of the Cr (VI) solution determined after the sorption process showed that the final pH of the solution were in the same range which showed that all the sorption experiment took place at the same pH range (10.43 to 11.50), which could be ascribed as the reason for the similitude in trends of the Langmuir maximum NC sorption capacity, qe (mg/g), at different initial solution pH. 3.2.2. Effect of Contact Time and Initial Cr (VI) Concentration. The effect of contact time between the NC and the sorbate was determined by conducting kinetic experiment at fixed sorbent dosage (0.1 g), initial sorbate pH(5.7), temperature (303K) but varying sorbate dosages (50, 100, 150, 200, 250, and 300 mg/L) and contact time (5, 10, 20, 30, 60, 90, 120, 180, and 240 min). The results obtained (Figure 7) showed that the amount of adsorbed Cr (VI) increased with contact time for any initial Cr (VI) concentration and attained equilibrium within 120 min, showing that the adsorption occurred quickly. The reason for the rapid adsorption of the Cr (VI) ion might be that the CN colloidal nanoparticles have a small average particle diameter and little internal diffusion resistance. However, the absolute amount of Cr (VI) adsorbed per unit mass of NC increased with increase in the initial Cr (VI) concentration. Kinetic modeling not only allows estimation of adsorption rates but also leads to suitable rate expressions characteristic of possible reaction mechanisms. In this respect, several kinetic models including the pseudofirst-order equation (eq 16),42 pseudosecond-order equation (eq 17),43 Power function (eq 18),44 and simple Elovich equations (eq 19)44 were tested thus:

2

-MOH2 þ -----------X- þ A - ¼ -M-A þ X- þ H2 O

ð15Þ

ð13Þ




Where, A is the anion adsorbate. Some evidence also suggests that anions can be adsorbed by a ligand exchange mechanism even though the surface is neutral:38 -M-OH þ A - ¼ -M-A þ OHð14Þ In the absence of specific adsorption of ions, the PZC determined by potentiometric titration matches the isoelectric point (IEP) determined by electrokinetic measurements. The pH at which the ζ potential equals zero is called the isoelectric point (IEP) and it can be used to qualitatively assess the adsorbent surface charge and characterize the protonation and deprotonation of the amphoteric surface functional groups. At values below the IEP, the hydrated surface of the NC is protonated and therefore has an acquired positive charge. At pH values higher than the IEP, the hydrated surface of NC is deprotonated, thereby acquiring a negative charge. The IEP of CN is pH 11.85 as measured by the solid addition method. Therefore, at values below the IEP, there may be two possible mechanisms for Cr (VI) adsorption onto CN thus: electrostatic attraction or surface complexation. Electrostatic attraction could occur between the highly charged CrO42
species which predominates at the prevailing pH and the predominantly positively charged surface. Surface complexation, has been suggested for anion adsorption onto hydrous solids such as metal oxides, hydroxides and clay minerals. The formation of surface complexes in anion/hydrous solid systems, such as CrO42
/γ-Al2O3, CrO42
/α-FeOOH and CrO42
/MgO, was verified by previous studies [refs 40,35 and41]. 645

log½qe
qt  ¼ log½qe 
½k1 =2:303t

ð16Þ

t=qt ¼ 1=kqe þ 1=qe t

ð17Þ

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Table 4. Kinetic Parameters of Sorption of Cr (VI) on NCa pseudo first order

initial conc.

power function

simple Elovich

qe1

K1  10
2

r2

qe2

K2  10
3

r2

b

a

r2

A

B

r2

50

25.88

6.68

0.852

16.75

3.79

0.998

0.336

2.940

0.933

0.671

3.199

0.967

100

35.73

4.49

0.850

33.44

2.09

0.999

0.327

6.240

0.902

0.187

6.221

0.966

150

64.00

4.92

0.836

48.31

1.16

0.997

0.337

8.230

0.951

2.529

9.145

0.975

200

67.73

4.35

0.959

55.87

1.31

0.985

0.328

10.020

0.968

2.788

10.677

0.977

300

101.32

4.51

0.842

75.76

0.53

0.991

0.361

10.630

0.976

8.667

14.449

0.962

(mg/L)

a

pseudo second order

k1 = min
1; qe1 = mg/g; qe2 = mg/g; k2 = g/mg/min; B (g/mg); A = (mg/g min).

Table 5. Error Analysis for the Kinetic Data of the Sorption of Cr (VI) on NC initial conc. pseudo first order = qe = qe(1
e
k1t) pseudo second order = qe = (tk2q2e )/(1+tk2qe) power function = qe = atb simple Elovich = qe = A + B lnt χ2

r2

χ2

r2

χ2

r2

χ2

r2

50

0.2427

0.9332

0.0002

0.9566

0.5674

0.9998

0.4095

1.0000

100 150

0.7605 5.5944

1.0000 0.9999

0.0004 0.4569

0.9848 0.9957

1.0637 1.5246

0.9997 0.9997

0.2913 1.3150

1.0000 1.0000

200

4.5533

0.9996

0.1610

1.0000

0.9236

0.9998

1.1374

1.0000

300

11.2578

1.0000

1.0030

1.0000

1.1995

0.9999

4.7000

1.0000

(mg/L)

log q ¼ log a þ blog t

ð18Þ

q ¼ A þ 2:303Blog t

ð19Þ

Table 6. Thermodynamic Parameters of the sorption of Cr (VI) on NC

The results from the analysis of the data obtained from the sorption process are presented in Table 4.The fitting of the different kinetic models to the sorption data was also tested by error analysis using the linear coefficient determinations (r2) and the χ2 statistical analysis (Table 5) 3.2.3. Thermodynamic Evaluation. In environmental engineering practice, both energy and entropy factors must be considered in order to determine which process will occur, spontaneously.45 The Gibbs free energy change, ΔGo is the fundamental criterion of spontaneity. Reactions occur spontaneously at a given temperature if ΔGo is a negative quantity. The free energy of the sorption reaction, considering the sorption equilibrium constant, Ka, is given by the equation presented below: ΔG0 ¼
RT ln K a

303

118.32

0.00824

428.48


15.27

313 333

111.44 102.21

0.00582 0.00356

302.64 185.12


14.87
14.45

ΔG0 ¼ ΔH 0
TΔS0

ð24Þ

A plot of Gibbs free energy change, ΔG0, versus temperature, T, was found to be linear (results not shown for brevity) and the values of ΔH0 and ΔS0 were determined from the slope and intercept of the plots. The results of the thermodynamic analysis are presented in Table 6. The negative values of the Gibbs free energy change (
15.27,
14.87,
14.45) kJ/mol at 303, 313, and 333 °C, respectively) indicated that the adsorption process is spontaneous; the negative ΔH0 value (
22.568 kJ/mol) indicated the exothermic nature of Cr (VI) adsorption onto NC. The entropy change was found to be positive (ΔS0 = 0.0244); meaning that increased disorder appeared on the NC
solution interface during the adsorption process.

ð20Þ

ð21Þ

4. CONCLUSIONS (1) A spongy porous NC can be synthesized from a Gastropod shell via the sol
gel technique. (2) The high PZC (11.80) value of the NC made it a promising adsorbent for the Cr (VI) oxoanions from aqueous.

ð22Þ

where Y is a constant. Equation 22 can be rearranged to obtain:
RTln Ka ¼ ΔH 0
TRY

ΔG0

0 Substituting eqs 21 and 22, the Gibbs free energy change, ΔG , can be represented as follows:

After integration, the integrated form of eq 21 becomes: ΔH 0 þ Y RT 2

Ka (L/mol)

ΔS0 ¼ RY

where ΔG is the standard free energy change (J), R the universal gas constant, 8.314 J/molK, and T is the absolute temperature (K). Considering the relationship between free energy and equilibrium constant, change in equilibrium constant with temperature can be obtained in the differential form as follows:46

ln Ka ¼

Ka (L/mg)

Let

0

dlnka ΔH 0 ¼ dT RT 2

qm(mg/L)

T (K)

ð23Þ 646

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(3) The evidence of the uptake of the Cr (VI) manifested in the alteration of the surface microstructure of the NC; the spongy and foamy nature of the surface became more pronounced and the pore size became irregular and narrower with the uptake of the Cr (VI) species. (4) The NC is a basic oxide, hence the effect of pH is neutralized and pH had no palpable influence on the NC sorption capacity for the Cr (VI), irrespective of the initial sorbate solution pH. (5) The sorption data fitted more to the Langmuir sorption isotherm and monolayer sorption capacity of 125 mg/g was obtained for the Cr (VI) sorption from aqueous solution. (6) The mechanism of the sorption process was better described by the pseudo second order kinetic model. (7) The uptake of Cr (IV) by NC was spontaneous and exothermic and increased disorder appeared on the NC
solution interface during the adsorption process.

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: +2348055438642. E-mail: [email protected] ().

’ ACKNOWLEDGMENT We acknowledge the efforts of Miss Olalekan Deborah and Dr. Kana of Physics Advanced Research Laboratory, Sheda Science and Technology Complex (SHETCO), Abuja, Nigeria for the SEM and EDX analyses. ’ REFERENCES (1) Lazaridis, N. K.; Asouhidou, D. D. Kinetics of sorptive removal of chromium(VI) from aqueous solution by calcined Mg-Al-CO3 hydrotalcite. Water Res. 2003, 37, 2875–2882. (2) Li, Y.; Gao, B.; Wu, T.; Sun, D.; Li, X.; Wang, B.; Lu, F. Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide. Water Res. 2009, 43, 3067–3075. (3) Vinodhini, V.; Das, N. Packed bed column studies on Cr (VI) removal from tannery wastewater by neem sawdust. Desalination 2010, 264, 9–14. (4) Vinodhini, V.; Das, N. Biowaste materials as sorbents to remove chromium (VI) from aqueous environment—a comparative study. J. Agric. Biol. Sci. 2009, 4 (6), 19–23. (5) Huang, G.; Jeffrey, X. S.; Langrish, T. A.G. Removal of Cr(VI) from aqueous solution using activated carbon modified with nitric acid. Chem. Eng. J. 2009, 152, 434–439. (6) Martin-Lara, M. A.; Pagnanelli, F.; Mainelli, S.; Calero, M.; Toro, L. Chemical treatment of olive pomace: effects on acid-basic properties and metal biosorption capacity. J. Hazard. Mater. 2008, 156, 1–3. (7) Mohan, D.; Pittman, C. U., Jr. Activated carbons and low cost adsorbents for remediation of tri- and Cr (VI) from water. J. Hazard. Mater. 2006, 137 (2), 762–811. (8) Chingombe, P.; Saha, B.; Wakeman, R. J. Surface modification and characterisation of a coal-based activated carbon. Carbon 2005, 43 (15), 3132–3143, DOI: 10.1016/j.carbon. 2005.06.021. (9) Hezami, L. K.; Capart, R. Removal of chromium (VI) from aqueous solution by activated carbons: kinetic and equilibrium studies. J. Hazard. Mater. 2005, 123, 223–231. (10) Busca, G. Bases and basic materials in industrial and environmental chemistry: A review of commercial processes. Ind. Eng. Chem. Res. 2009, 48, 6486–6511. (11) Mishakov, I. V.; Bedilo, A. F.; Richards, R. M.; Chesnokov, V. V.; Volodin, A. M.; Zaikovskii, V. I.; Buyanov, R. A.; Klabunde, K. J. 647

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