Removal of Cadmium Using Cocos Nucifara.l Equilibrium and Kinetic Studies

8/3/2019 Removal of Cadmium Using Cocos Nucifara.l Equilibrium and Kinetic Studies

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REMOVAL OF CADMIUM USING COCOS NUCIFARA.L: EQUILIBRIUM

AND KINETIC STUDIES

Document By: Bharadwaj

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ABSTRACT:

This investigation comprises the equilibrium and kinetics on biosorption of Cadmium ions from aqueous solutions using `Coconut husk powder’ in a batch

process. The results indicate that biosorption of Cadmium is increased with anincrease in adsorbent dosage and decrease in adsorbent size. A significant increasein percentage removal of Cadmium is observed as pH value is increased from 1 to 7and the percentage removal is decreases beyond pH 7. Increased initialconcentration of Cadmium in the aqueous solution results in lower percentage of

biosorption. Freundlich and Langmuir isotherm models describe the present datavery well indicating favourable biosorption. The biosorption follows pseudo-second-order kinetics.Key words: Adsorption, Isotherms, Kinetics of Cadmium adsorption

1. INTRODUCTION

Rapid industrialization generating large quantities of liquid effluents withvarying quantities heavy metals. Cadmium is one among them and is toxic when it ispresent in small quantities. The effluent based cadmium take different roots for itsdispersion including fish, algae and other aquatic flora and fauna. Removal of thismetal is vary much essential. Several studies have been conducted with varying

degree of success. Toxic heavy metals include cadmium, mercury, silver, lead, tin,and chromium, although several nutrient metals, notably zinc, copper and nickel, canalso be toxic at elevated concentrations. Waste water of industrial and domestic, if not properly managed, is responsible for severe damage to the environment andadversely affecting the health of the people.

In recent years, increasing attention has been placed on removal of toxicmetals from industrial wastewaters, not only for pollution control aspect but also toenable the reuse of water and toxic materials themselves in some cases. Theindustries where toxic metals are found in the effluent are shown in Table-I. Amongthese heavy metals, Cadmium represents major hazardous waste in the environment.Cadmium can be taken into the body through the pulmonary system fromcontaminated air or cigarette smoke or via the digestive system through water orfood contamination from plant Cadmium uptake. Cadmium can cause problem such

as hypertension, emphysema, renal cancer, and prostrate cancer and kidney disease.Several processes are available for the removal of cadmium from industrial

effluents. These include adsorption, bio-sorption with organisms like bacteria, yeast,or algae, precipitation, filtration through lime stone beds, ion exchange, reverseosmosis, evaporation, and electrodialysis

The present investigation looks into a specific process, for the removal of toxicheavy metal, cadmium by adsorption using an economically viable low costadsorbent developed from an easily, freely and abundantly available Coconut HuskPowder.

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2. MATERIALS AND METHODS

The matured Coconut Husk Powder is collected from Visakhapatnam region. They aredried at room temperature in a shade followed by oven at 110 0C till the constantweight of the sample is attained. The resulting husk is sieved to different size

fractions using Rotap sieve shaker. The size fractions are 53, 75,106 and 212μm arepreserved in glass bottles for use as adsorbent.

2.1Metal solution

3CdSO48H2O is used in the preparation of cadmium stock solution. All the requiredsolutions are prepared with double-distilled water. 2.132gm of 3CdSO48H2O isdissolved in 1L of distilled water to produce 1000 mg/l of cadmium stock solution.Samples of different concentrations of Copper are prepared from this stock solutionby appropriate dilutions. 81ml of 1000mg/l cadmium stock solution is taken in a 1000ml volumetric flask and is made up to the mark with distilled water.

2.2Analysis

These samples are analyzed in Atomic Adsorption Spectroscopy (AAS) at 326.1nm toobtain final concentrations of Cadmium. The same experimental procedure is

repeated with other adsorbent sizes varied as 75, 106 and 212μm.

3. RESULTS AND DISCUSSION

3.1Effect of agitation time:

Agitation time has influence on metal uptake. Percentage copper removedwas calculated by the expression [(Co-Ct)/Co] * 100. Time versus concentration of metal in solution data is obtained. The percentage removal of cadmium against theagitation time was shown in figures 1 for different dosages (0.5, 1.0, 1.5 and 2.0gm).

The % adsorption is found to increase upto 50 min and there after, negligibleincrease in percentage adsorption is noticed with agitation time. It is noticed that therate of adsorption is fast in the initial stages because adequate surface area of the

adsorbent is available for the adsorption of Cadmium. As time increases, moreamount of Cadmium gets adsorbed onto the surface of the adsorbent and the surfacearea available for adsorption decreases. The adsorbate, normally, forms a thin onemolecule thick layer over the surface. When this monomolecular layer covers thesurface, the adsorbent capacity is exhausted. The maximum percentage of adsorption is attained at 50 min of agitation. The percentage removal of cadmiumbecomes almost constant after 50 min. indicating the attainment of the equilibrium.

Therefore all other experiments are conducted at this agitation time.

3.2 Effect of adsorbent size:

The variations in percentage removal of cadmium from the aqueous solutionwith particle size (53μm, 75μm, 106μm and 212μm) are obtained at differentadsorbent dosages- namely at 0.5, 1.0, 1.5 and 2.0gm. The results are drawn as

figure 2 as percentage removal of cadmium versus adsorbent size. The percentageremoval of cadmium is increased as the adsorbent particle size decreased. Thisphenomenon is expected, as the size of the particle decreases, surface area of theadsorbent increases, thereby the number of active sites on the adsorbent are moreand better exposed to the adsorbate.

3.3Effect of adsorbent dosage:

The percentage removal of cadmium is drawn against adsorbent dosage for



different adsorbent sizes (53μm, 75μm, 106μm and 212μm) and shown in figure 3. Itis evident from the plots that the percentage removal of metal from the aqueousphase increases with increase in the adsorbent dosage. For the adsorbent size53μm, percentage removal of cadmium increases from 72.84% to 91.98%, as dosageis increased from 0.5 to 2.0 gm. Such behavior is obvious because the number of active sites available for metal removal would be more as the amount of the

adsorbent increases.

3.4Effect of initial concentration of cadmium in the aqueous

solution:

A graph is drawn as the percentage removal of cadmium versus initialconcentration of cadmium and is shown in fig.4. The percentage removal of cadmium is decreased from 93.48% to 86.17% as the initial concentration of cadmium in the aqueous solution increases from 23 mg/l to 188 mg/l. Such behaviorcan be attributed to the increase in the amount of adsorbate to the unchangingnumber of available active sites on the adsorbent (since the amount of adsorbent iskept constant.

3.5Effect of pH of the aqueous solution:

pH influences the surface charge of the adsorbent, the degree of ionizationand the adsorbate. So pH is an important factor controlling the process of adsorption.It is shown in fig 5. In the present investigation, adsorption data are obtained in thepH range of 1 to 14 for cadmium initial concentration of 81 mg/l and 2 gm of adsorbent dosage, 53μm size adsorbent. The effect of pH of aqueous solution onpercentage removal of cadmium is drawn and shown in figure 8. The percentageremoval of metal is increased from 69.14%to 93.83% as pH is increased from 1 to 7.

The percentage removal is decreased from 83.95% to 70.99% as pH increases from 8to 14.

In the present investigation, the maximum percentage removal of cadmium isobtained for 2 gm of 53μm size adsorbent at equilibrium. The principal driving force

for metal ion adsorption is the electrostatic interaction (i.e) attraction betweenadsorbent and adsorbate. The greater the interaction, adsorption of heavy metal willbe more. With an increase in interaction, the cadmium ions replace H+ ions boundto the adsorbent for forming part of the surface functional groups such as – OH, -COOH etc.

4. EQUILIBRIUM STUDIES

4.1Freundlich isotherm for adsorption of cadmium:

The Freundlich relationship is an empirical equilibrium relationship. It does notindicate a finite uptake capacity of the adsorbent and can thus only be applied incase of low and intermediate concentration ranges. However, it is easier to handlemathematically as it is a simple relationship.

The freundlich isotherm is given byqe = Kf Cen ---(3)

Taking logarithms on both sides, we getlogqe = logKf + n logCe ---(4)

Where Kf and n are known as Freundlich constants obtainable from the plotsof logqe versus log Ce on the basis of the linear form of the equation (4)Freundlichisotherm is drawn between log Ce and log qe shown in figure 6. The resulting lineshave the correlation coefficient of 0.9862. The following equations are obtained fromthe plot drawn in fig.10.



log qe = 0.6880log Ce – 0.3368 at 303K The slopes (n) of the above equations are varied between 0.6880. The n value satisfies the condition of 0< n< 1 indicating favorable adsorption.

4.2Langmuir isotherm for adsorption of cadmium:Since the chemical forces fall off very rapidly with distance, it is probable that

chemisorption does not extend beyond a single layer of adsorbate on the surface of the solid. It can be anticipated as first pointed out by Langmuir that chemisorbedadsorbate layers may be only one molecule thick. The Langmuir isotherm is the mostwidely used two-parameter equation. The relationship is of a hyperbolic type form:

qe/qm= bCe / (1+bCe) --- (5)Where Ce is the concentration of the adsorbate at equilibrium, qe is the

amount adsorbed at equilibrium per unit mass of the adsorbent, qm is the maximumamount adsorbed per unit mass of the adsorbent and b is the coefficient related toaffinity. Equation (5) can be rearranged as

(Ce/qe) = 1/bqm + Ce/qm --- (6)From the plots between (Ce/qe) and Ce, we can calculate the slope (1/qm) and

the intercept (1/b). Further analysis of the Langmuir equation is made on the basis of separation factor, RL defined as

RL = 1/(1+bCe) --- (7)

0< RL<1 indicates Favorable adsorptionRL > 1 indicates Unfavorable adsorptionRL = 1 indicates Linear adsorptionRL = 0 indicates Irreversible adsorption

Langmuir isotherms drawn in figure 7 have good linearity (correlationcoefficient, R~ 0.9856) indicating strong binding of cadmium ions to the surface of Coconut husk powder. The separation factor (RL), obtained is 0.7167 showsfavourable adsorption.

The following equation obtained from fig.7.

(Ce /qe) = 0.1562Ce + 2.57 at 303K

The isotherm constants for cadmium – Coconut husk powder interactions at303K, t = 50min, Co = 81mg/l, dp = 53μm and w = 2gm are shown in Table-1.

5. KINETICS OF ADSORPTION

The order of adsorbate adsorbent interaction has been described by usingvarious kinetic models. Traditionally, the pseudo first order model derived byLagergren finds wide application. On the other hand, several authors have shownthat pseudo second order kinetics can also describe these interactions very well incertain specific cases.

In the case of adsorption preceded by diffusion through a boundary, thekinetics in most cases follows the pseudo first order rate of equation of Lagergrenand graph is shown in fig 8.

(dqt/dt) = K ad (qe – qt) --- (8)

Where qe and qt are the amounts adsorbed at t, min and at equilibrium andK ad is the rate constant of the pseudo first order adsorption process. Equation (8) isthen written as

∫ (dqt / (qe – qt)) = ∫ K ad dt --- (9)



After applying the initial condition qt = 0 at t = 0 and integration, we get

log (qe – qt) = log qe – (K ad/2.303) t --- (10)Plot of log (qe – qt) vs t gives a straight line for first order kinetics, which allows

computation of the adsorption rate constant, K ad. If the experimental results do notfollow equations (8) and (10), they differ in two important aspects:

K ad (qe – qt) then does not represent the number of available adsorption sitesand log qe is not equal to the intercept of the plot of log (q e – qt) against t.In suchcases, pseudo second order kinetics, given by

(dqt/dt ) = K (qe – qt)2 --- (11)

is applicable, where K is the second order rate constant and its shown in fig 9. Theplots (t/qt) versus t for the present data are indicated in figs.15, 16, 17 and 18. Thelinearity of the plots (R=0.999) confirms the suitability of pseudo second order rateequation.

The following second order rate equations are obtained from the graphs:From fig.15, the following pseudo second order rate equations are obtained.For w = 0.5, 1.0, 1.5 and 2.0gm of dp = 53μm

(1) (t/qt) = 0.1678t + 0.1891

(2) (t/qt) = 0.3147t + 0.2640

(3) (t/qt) = 0.4501t + 0.4377

(4) (t/qt) = 0.5304t + 0.6529

6. CONCLUSIONS

The following conclusions are drawn from the studies

1. The optimum agitation time for the metal adsorption is 50 minutes.2. The percentage removal of cadmium from the aqueous solution increases with a

decrease in the particle size of the adsorbent.3. The percentage removal of cadmium from aqueous solution is augmented with

increase in weight of the adsorbent.

4. Higher the concentration of cadmium in the aqueous solution, the percentageremoval of cadmium from the aqueous solution is decreased.5. Percentage removal of cadmium from the aqueous solution is increased

significantly with increase in pH value from 1 to 7 and removal decreases forpH beyond 7.

6. In the range of variables studied, percentage removal is increased from 50.62 %to 91.98 %.

7. The data are well represented by Freundlich and Langmuir isotherms indicatingfavorable adsorption of cadmium by the adsorbent.

8. The kinetic studies show that the adsorption of cadmium is better described bypseudo- second- order kinetics.

Graphs:



W (gm) Co (mg/L) d p pH

Fig 1. Effect of agitation time on percentage removal of Cadmium

Agitation time , t (min)

0 20 40 60 80 100 120 140

% R e m o v a l

o f C a d m i u m

50

60

70

80

90

100

0.5 81 50 53 4.1

1.0 81 50 53 4.1

1.5 81 50 53 4.1

2.0 81 50 53 4.1

W ,gm Co, mg/l V, ml dp, µm

pH

Fig 3 Effect of adsorbent dosage on percentage removal of Cadmiu

Adsorbent Dosage , W (gm)

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

% R e m o v a l o f C a d m i u m

65

70

75

80

85

90

95

53 81 50 50 4.1

75 81 50 50 4.1

106 81 50 50 4.1

212 81 50 50 4.1

dp,µm Co, mg/l V, ml t, min pH

Fig 2. Effect of adsorbent size on percentage removal of Cadmioum

Adsorbent Size (µm)

40 60 80 100 120 140 160 180 200 220


60

70

80

90

100

110

120

0.5 81 50 4.1

1.0 81 50 4.1

1.5 81 50 4.1

2.0 81 50 4.1

W, gms Co, mg/l V, ml pH

Fig 4. Effect of initial concentration of aqueous solution onpercentage removal of CAdmium

Initial concentration of Aqueous Solution (mg/l)0 20 40 60 80 100 120 140 160 180 200


84

86

88

90

92

94

96

W, gms dpp,µm pH

2.0 53 4.1

Fig 5. Effect of pH of the aqueous solution onpercentage removal of cadmium

pH of the Aqueous Solution0 2 4 6 8 10 12 14 16


65

70

75

80

85

90

95

100

Co, ppm W, gms dpp,µm

81 2.0 53

Fig 8. First order kinetics for adsorption of Cadmium

Agitation time , t ,(min)

0 10 20 30 40 50

l o g ( q

e - q t

)

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

l o g ( q e - q t ) = - 0 . 3 2 4 1 t - 0 . 2 0 9 5

r2

=0.981

,

Co, ppm W, gms dpp,µm pH

81 2.0 53 4.1



Fig 6. Freundlich isotherm for adsorption of Cadimum w.r.t concentrationusing Coconut husk powder

log Ce

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

l o g q e

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

l o g q e = 0 . 6 8 8 0 l o g

C e - 0 . 3 3 6 8

r2

=0.9862

W, gms dpp,µm pH

2.0 53 4.1

, , ,

. .

Fig 9. Second order kinetics for adsorption of C ad

Agitation time , t , (min)

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0

t / q t ( g m - m i n / m g )

0

10

20

30

40

50

60

70

0.5 81 50 53 4.1 303

1.0 81 50 53 4.1 303

1.5 81 50 53 4.1 303

2.0 81 50 53 4.1 303

t/ q t = 0. 1 6 7

8 t + 0. 1 8 9 1

t / q t = 0 . 3 1 4

7 t + 0 . 2 6

4 0 t / q t =

0 . 4 5 0 1

t + 0 . 4 3 7 7

t / q t =

0 . 5 3 0 4 t

+ 0 . 6 5 2 9

W, gms Co, mg/l V , ml dp,m pH T, K

r2

= 0.999

Fig7. Langmuir isotherm for adsorption of Cadmium w.r.t concentrationusing Coconut husk powder

Ce(mg/L)

0 5 10 15 20 25 30

C e

/ q e

( g m / L )

2

3

4

5

6

7

C e / q e =

0 . 1 5 6 2 C

e + 2 . 5 7

r2

=0.9856

, , ,

. .

, ,,

. .

, , ,

. .

, , ,

. .

, ,,

. .

, , ,

. .

, ,,

. .

, ,,

. .

, ,,

. .

W, gms dpp,µm pH

2.0 53 4.1

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Document By: Bharadwaj

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Removal of Cadmium Using Cocos Nucifara.l Equilibrium and Kinetic Studies