Adsorption of CD(II) and Pb(II) From Aqueous Solutions on Saw Dust and Neem Bark

8/3/2019 Adsorption of CD(II) and Pb(II) From Aqueous Solutions on Saw Dust and Neem Bark

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Adsorption of Cd(II) and Pb(II) from aqueous solutions on Saw dust and neem bark

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Abstract

The ability of saw dust and neem bark as natural adsorbent was investigated foradsorptive removal of Cd(II) and Pb(II) ions from aqueous solutions. Various physico-chemical

parameters such as pH, initial metal ion concentration, and adsorbent dosage level and

equilibrium contact time were studied. The optimum solution pH for adsorption of Cd(II) andPb(II) from aqueous solutions was found to be 5. Kinetics data were best described by pseudo-

second order model. The effective particle diffusion co-efficient of Cd(II) and Pb(II) are of the

order of 10-10 m2/s. The equilibrium adsorption data for Cd(II) and Pb(II) were better fitted to

Langmuir adsorption isotherm model. The thermodynamic studies indicated that the adsorptionwas spontaneous and exothermic for Cd(II) adsorption and endothermic for Pb(II). The sorption

energy calculated from Dubinin-Radushkevich isotherm were 9.6855 kJ/mol and 10.1015

kJ/mol for the adsorption of Cd(II) and Pb(II) respectively which indicated that both theadsorption processes were chemical in nature.

Key words: Saw dust, Neem bark, Pseudo second order, Effective diffusivity, Langmuir,

Desorption, Sorption energy

Introduction

Environmentalist is primarily concerned with the heavy metals due to their toxicity andimpact on human health and environment. These heavy metals introduced into natural water

resources by waste water discharged from industries such as metal plating, cadmium-nickel andlead storage batteries, mining, galvanizing, paints, pigments, insecticides, cosmetics, stabilizer

and alloy manufacturing. The health effects of Cd(II) on human include nausea, vomiting,diarrhea, muscle cramp, salivation, reduction of red blood cells, damage of bone marrow,

hypertension, kidney failure following oral ingestion, lung irritation, chest pain, and loss of

sense of smell after inhalation [1-3]. Pb(II) is a highly toxic substance, exposure to which canproduce a wide range of adverse health effects for both adults and children. At high levels of

exposure, a child may become mentally retarded, fall into a coma, and even die from lead

poisoning. In adults, lead can increase blood pressure and cause fertility problems, nervedisorders, muscle and joint pain, irritability, and memory or concentration problems [4]. The

tolerance limits for heavy metal concentration in potable water and discharge into inland surface

waters is shown in Table 1 [5-7].Precipitation, ion exchange, electrochemical precipitation, solvent extraction, membrane

separation, concentration, evaporation, reverse osmosis and bio-sorption and emulsion per

traction technology are the conventional methods for the removal of heavy metals from the

aqueous solution [8-11]. These technologies suffer from cost effectiveness and ineffectivenesswhen the metals are dissolved in large volume of solution of relatively low concentration.

Adsorption process seems to be most versatile and effective method for removal of heavy metal
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if combined with appropriate regeneration steps. It solves the problem of sludge disposal and

renders the system more viable. Considering the cost economics, recent research has focused onthe development of low-cost readily available alternatives using various agricultural, industrial,

natural/biological waste materials including synthetic, modified model adsorbents. Several

contributions have made in this area utilizing number of materials like sunflower stalks, spentgrain, oak sawdust, cassava waste, baggage fly ash, wheat bran, olive pomace, carrot residues,

tree fern, sugar beat pulp, grape stalk waste etc. Present study deals with a series of batch

adsorption experiments to investigate the capability of sawdust and neem bark as low cost

natural biosorbent for removal of metal ions from aqueous solutions. Saw dust and neem barkcontain lingo-cellulose based materials, readily available and found to have potential surface

adsorption properties.

2. Materials and methods

Sawdust of teakwood origin was collected from a saw mill at Dum Dum near Kolkata,

West Bengal, India. Neem bark was collected from a local Neem tree near Kolkata, WestBengal, India.

Sawdust was collected from a local sawmill. It was of teakwood origin. After collection

it was washed thoroughly with distilled water to remove muddy materials and then with 0.1NNaOH to remove lignin based color materials followed by 0.1N H2SO4. Finally it again washed

with distilled water several times and dried in an oven at 105 50 C for 6 hr and cooled to room

temperature in desiccators. Particle size of 250-350 m were used for adsorption studies.

Neem bark after collection washed thoroughly with distilled water to remove muddy materialsand then sun dried. Then it was cut into small pieces, ground and homogenized. It was then

dried in an air oven at 105 5 0C for 5-6 hr to remove the adherent moisture. Then aftercooling to room temperature it was washed several times with distilled water to remove dust

and color. Finally it was dried again in the air oven at 105 5 0C and cooled in a desiccators.All the necessary chemicals used in the study were of analytical grade and obtained from

Merck Limited, Mumbai, India. Stock solution of Cd(II) and Pb(II) were made by dissolvingexact amount of respective metal salt. The range of concentration of the metal components

prepared from stock solution was varied between 3 mg/L to 300 mg/L. The test solutions were

prepared by diluting 1 g/L of stock metal solution with double distilled water.Atomic adsorption spectrophotometer (Varian Spectra AA 55, USA) was used to

determine the Cd(II) and Pb(II) content in standard and treated solutions after adsorptionexperiments. The pH of the solution was measured with a 5500 EUTECH pH Meter using FET

solid electrode calibrated with standard buffer solutions.Required amount of adsorbent was taken in a 250 ml stopper conical flask containing

100 ml of desired concentration of the test solution for the batch adsorption studies at the

desired pH value. pH of the solution was monitored in a 5500 EUTECH pH Meter using FETsolid electrode calibrated with standard buffer solutions. Necessary amount of adsorbent was

then added and contents in the flask were shaken for the desired contact time in an electrically

thermo stated reciprocating shaker at 300C. The contents of the flask were filtered through filterpaper and the filtrate was analyzed for remaining metal concentration in the sample using

Atomic Absorption Spectrophotometer (VARIAN SPETRA AA 55, USA) as per procedure laid

down in APHA, AWWA standard methods for examination of water and wastewater, 1998edition [12].

3. Results and discussion

3.1 Effect of pHThe effect of pH on the adsorption of Cd(II) by saw dust and neem bark were studied by

varying pH of the solution over the range of 1-8. Suitable range for adsorption of Cd(II) was

evaluated from the studies of solubility product. The solubility product equilibrium constant

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(Ksp) demonstrated that the best pH range of 2-8 for Cd(II) for adsorption [13]. Removal

efficiency increased with increase pH for the adsorption of Cd(II) on saw dust and neem bark(Figure 1). Although a maximum uptake was noted at a pH of 8, as the pH of the solution

increased to >7, Cd(II) started to precipitate out from the solution. Therefore experiments were

not conducted over pH 7. The increased capacity of adsorption at pH >7 may be a combinationof both adsorption and precipitation on the surface of the adsorbent. It can be inferred from the

studies that both saw dust and neem bark had maximum adsorption capacity at pH 5, if the

precipitated amount is not considered in the calculation. Therefore, the optimum pH for Cd(II)

adsorption is 5. The same trend has also been reported in the removal of Cd(II) ions by otheradsorbent materials such as Chitosan-Coated Perlite Beads [14].

The pH range for adsorption of Pb(II) on saw dust and neem bark was chosen as 3-7 inorder to avoid precipitate in the form of lead chloride and lead hydroxides, which has beenestimated to occur at pH6.5 for Pb(OH)2 [15]. The effect of pH on

adsorption efficiencies are shown in Figure 1. Removal of Pb(II) increases with increasing

solution pH and a maximum value was reached at an equilibrium pH of around 5 in each of thecases if the precipitated amount is not considered. The same trend has also been reported in the

removal of Pb(II) ions by other vegetable materials such as spent grain [16] and crop milling

waste [17].The metal ions in the aqueous solution may undergo solvation and hydrolysis. The

process involved for metal adsorption is as follows [18],

M2+ + nH2O = M (H2O)n2+ (1)

M(H2O)n2+ = [M(H2O)n-1(OH)]+ + H+ (2)

M 2+ + nH2OaK

[M(H2O)n-1(OH)]+ + H+ (3)

The pKa value for Cd(II) and Pb(II) are 10.1 and 7.7 respectively. Perusal of the

literature on metal speciation shows that the dominant species is M(OH)2 at pH > 6.0 and M2+

and M(OH)+ at pH < 6.0. Maximum removal of metal was observed at pH 5 for adsorption. Onfurther increase of pH adsorption decreases probably due to the formation of hydroxide of

cadmium and lead because of chemical precipitation [19]. The optimum pH value for adsorption

was found to be 5.

3.2 Effect of adsorbent type and its concentration

Experiments have been conducted using constant initial pH 5 for adsorption of bothCd(II) and Pb(II) and constant equilibrium contact time (240 min and 180 min for adsorption of

Cd(II) and Pb(II) respectively). The effect of adsorbent dosage on adsorption of Cd(II) on saw

dust and neem bark are shown in Figure 2. It is observed that the adsorbed amount of metal ionswas increased by increasing the adsorbent dosage for each case. As the adsorbent dosage

increases, the number of adsorbent particles surrounded by metal ions or ratio of the number of

the adsorbent particles to metal ions increases in the solutions result more ions attached to the

adsorbent surfaces. It is clear that, the optimum removal efficiency have been achieved at anadsorbent dosage level of 10 g/L for the adsorption on both the adsorbents. The variation in

sorption capacities between the two adsorbents could be related to the type and concentration of

surface group responsible for adsorption of metal ions from solution [20]. Different magnitude

of attraction power of the metal ions by saw dust and neem bark, and different ability ofcomplex formation of Cd(II) with the functional groups present on the surface of the adsorbents

surfaces are responsible [21].Effect of adsorbent dosage on the percent removal of Pb(II) are shown in Figure . These

figures indicated that the adsorption increases from 1g/L adsorbent dosage to 7.5 g/L adsorbent

dosage for adsorption on both saw dust and neem bark. It is also seen from the figure (Figure 2)

that a further increase in the adsorption dosage (greater than 7.5 g/L) does not affect theadsorption greatly on the adsorption on both the adsorbents. These results can be explained on a

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consequence of a partial aggregation, which occurs at higher adsorbent dosage [22]. Therefore,

adsorbent dosage is selected as 7.5 g/L for both the adsorbent during further experiments.

3.3 Effect of contact time

Experiments have been conducted using constant adsorbent dosage (7.5 and 10 g/L foradsorption of Cd(II) and Pb(II) respectively) and constant initial pH 5. Variation of percentage

removal of metal ion with contact time was studied in this section. Figure show the effect of

contact time on the adsorption of Cd(II) on saw dust and neem bark. It is seen from figures

(Figure 3) that the adsorption yield of Cd(II) steeply increases with rise in time up to 60 min.Between 60 min to 120 min there is no considerable increase in adsorption. After 120 min of

contact time major adsorption process has been completed. Accordingly, all further batchexperiments have been conducted for a contact time of 3 h to ensure the equilibrium is reached.

Effect of contact time on adsorption is depicted in Figure. The curve rose steeply within

45 min and reached almost constant after the 90 min of contact time in case of both the

adsorbents. After 90 min of contact time major adsorption processes have been completed.Accordingly all the batch experiments have been conducted for a contact time of 2 h to ensure

equilibrium is achieved.

3.4 Adsorption kinetics study

The rate kinetics of Cd(II) and Pb(II) adsorption on the saw dust and neem bark have

been analyzed using pseudo first-order [23], pseudo-second order [24] and intraparticle

diffusion models [25]. The conformity between experimental data and the model predictedvalues is expressed by correlation coefficients, r2 and chi square, 2.

Lagergren model

The integral form of the Lagergren (1898) model generally expressed in Equation (3.3)as follows,

( )303.2

loglogtK

qqq adee = (4)

The plot of log(qe qt) versus tgive a linear relationship from whichKad and qe can bedetermined. The values of rate constants, r2 and 2 which can be obtained from equation

(Equation 5) for both the adsorbents are shown in Table 2.

( )

2

2 t tmt

et

q q

q

= (5)

Pseudo second order model

The kinetics of adsorption process may also be analyzed by pseudo second order rateequation (Ho and McCay, 1998). The linearized form of Equation (3.7), which represents the

pseudo second order model, is expressed as

2

2

1 1

t e e

tt

q k q q= + (6)

Figure 1 show pseudo second order plots. The values of pseudo second order rate constants

along with correlation coefficients, r2 and chi square, 2 are shown in Table 2. The pseudo

second order rate constant, k2 is decreased while h2 and qe values increase with increase in initialCr(VI) ion concentration.

Intraparticle diffusion model

The intraparticle diffusion model is based on the theory proposed by Weber and Moris

(1963). According to this theory the model Equation 7 can be expressed as0.5

t idq K t C = + (7)

Data points are related by two straight lines - first straight portion depicting the

macropore diffusion and second representing the micropore diffusion. The deviation of straight

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lines from the origin may be due to difference in rate of mass transfer in the initial and final

stage of adsorption. Further, such deviation of straight line from the origin indicates that thepore diffusion is not sole rate-controlling step. The values of rate constants and correlation

coefficients for each model are shown in Table 2.

It is observed from the Table 2 that pseudo second order model fits best with theexperimental data than the other models.

3.5 Determination of effective diffusion coefficient for adsorption

Adsorption data obtained from kinetic study could be described well by the models given byBoyd et al. [26]. Diffusion found to be rate controlling in the adsorption of Cd(II) and Pb(II)

onto the particles of spherical shape. Effective diffusion co-efficient for the solution of divalentexchangeable ion can be determined using the equation (Equation)

( )

2

2 2

1ln

1e

a

D tF t R

=

(8)

Where plot of ln[1/(1-F2(t))] versus t provide a line from whose slope 2De /r2 the

diffusion coefficient,Decan be calculated. The value of diffusion co-efficient as calculated from

the equation is tabulated in Table 3. The value of De, fall well within the values reported in

literature, especially for chemisorptions system (10-9 - 10-17 m2/s) [27].

3.5 Adsorption isothermsThe adsorption isotherm for the removal of metal ion was studied using initial

concentration of between 10 and 300 mg/L at an adsorbent dosage level of 5.0 g/L for Pb(II)30oC.

Langmuir isotherm

Equilibrium adsorption of Pb(II) assuming monolayer adsorption onto a surface with afinite number of indentical sites is represented by Langmuir adsorption isotherm model [28]

maxmax

1

q

C

bqq

C e

e

e+= (9)

Linear plots ofCe/qe vs. Ce (Figure 5) were employed to determine the value ofqmax (mg/g) and

b (L/mg). The data obtained with the correlation coefficients (r2) and2

e

obtained using

equation (Equation) are listed in Table 4.

( )2

2 e em

e

em

q q

q

= (10)

Freundlich isotherm

The Freundlich adsorption isotherm [29] is an empirical equation employed to describe

heterogeneous systems, in which it is characterized by the heterogeneity factor, n. The linear

form of Freundlich adsorption isotherm takes the following form

efe Cn

Kq log1

loglog += (11)

The values for Freundlich constants and correlation coefficients (r

2

) and

2

e for the adsorptionprocess are presented in Table 4. The values of n between 1 and 10 (i.e.1/n less than 1) represent

a favorable adsorption The values of n, which reflects the intensity of adsorption, also reflectedthe same trend. The n values obtained for the adsorption process represented a beneficial

adsorption.From the Table 4, it is seen that experimental data for both Cd(II) and Pb(II)

adsorption are better fitted to Langmuir than Freundlich adsorption isotherm. Therefore uptakeof both Cd(II) and Pb(II) ion preferably follows the monolayer adsorption process.

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Dubinin-Radushkevich (D-R) isothermThe D-R isotherm [30] was employed in the following linear form:

2ln lnabs mC X = (12)

The Polanyi potential [31], , can be expressed as,

1ln(1 )

e

RTC

= + (13)

A plot of ln Cabs vs 2 in Figure 6 gave a straight line from which values of for Cd(II) and

Pb(II) was evaluated. Using the calculated value of , it was possible to evaluate the mean

sorption energy, E, from

1

2

E

=

(14)

The value of E for the adsorption (Table 5) are in the range of 9-16 kJ/mol which indicate that

adsorption of Cd(II) and Pb(II) on both the adsorbents are to be chemical in nature [32].

4. Conclusions

The obtained results can be summarized as follows,

(1) Maximum adsorption at pH 5-and at higher pH precipitation of hydroxyl species onto theadsorbents .

(2) Maximum uptake was obtained at adsorbent dosage of 7.5 g/L and 10 g/L for adsorption ofCd(II) and Pb(II) respectively, which may be considered as optimum adsorbent dosage level at

the specified conditions.(3) The equilibrium time for adsorption of Cd(II) and Pb(II) from aqueous solutions was

achieved within 3 h and 2 h of contact time respectively.

(4) The experimental data were better described by pseudo 2nd order model as evident from

correlation co-efficient (r2) and2 values.

(5) The effective diffusivity calculated using Vermeulens approximation was indicate that the

interaction between both Cd(II) and Pb(II) on saw dust and neem bark are chemical innature.

(6) Langmuir adsorption isotherm models were better fitted than Freundlich adsorption isotherm

model for all the cases of adsorption.(7) Sorption energy calculated from Dubinin-Raduskevich (D-R) isotherm indicated that the

adsorption processes are chemical in nature for both the metal ion.

Nomenclature

b = Langmuir constant (Lmg-1)

C0 = Initial concentration of metal ion in solution, (mgL-1)

Ct = Concentration of metal ion after time t, (mgL-1)

E = Mean sorption energy, (kJmol-1)

K2 = Pseudo-second-order rate constant of adsorption [(mg/g) min]Kad = Lagergren rate constant (min

-1)Kf = Measure of adsorption capacity (mg/g)

Kid = Intra-particle rate constant [(mg/g) min1/2]

m = Amount of adsorbent added in gm

n = Freundlich constants, intensity of adsorptionq = Amount adsorb per gm of the adsorbent (mg/g)

qe = Amount adsorb per gm of the adsorbent at equilibrium

qmax = Maximum adsorption capacity(mg/g)

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qt = Amount adsorb per gm of adsorbent at time t (min)

qtm = Amount of metal ion adsorbed per gm of adsorbent according to kinetic model(mg/g).

qem = Amount of metal ion adsorbed per gm of adsorbent according to isotherm model

(mg/g).r2 = Correlation coefficient

t = Time, minuteV = Volume of the solution in mL

F(t) = Amount adsorbed per gm of adsorbent at time/Amount adsorbed per gmof adsorbent at equilibrium

Xm = Maximum adsorption capacity, (mmol/g)Greek letter

= constant related to energy (mol2/kJ2) = Polanyi potential (kJ 2/ mol2)

References

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http://www.cpcb.nic.in/News%20Letters/Latest/cadmium/contentCADMIUM.html ,

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[9] M. Sekar, V. Sakthi, S. Rengaraj, Kinetics and equilibrium study of lead(II) ontoactivated carbon prepared from coconut shell, J. Col. Int. Sci. 279 (2004) 307-313

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(2007) 9-16.[11] D. C. K. Ko, J. F. Porter, G. Mckay, Mass transport model for the fixed bed

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coated perlite beads Ind. Eng. Chem. Res. 45 (2006) 5066-5077[15] C. F. Baes, R.E. Mesmer, The hydrolysis of cations, Wiley, New York, 1976

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and Sargassum sp. for the biosorption of Cu(II) from aqueous solution,Biores. Technol.98

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from aquous solution onto Harro River Sand, Ads. Sci. Tech. 24(6) (2006) 475-485

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Table 1 Tolerance limits for heavy metal concentration in drinking water and discharge into

inland surface waters

Heavy

Metal

IS 10500 1992 [5] EPA [6] WHO [7]

Drinkingwater

mg/L

Discharge ininland surface

watermg/L

Dischargein public

sewersmg/L

Drinkingwater

mg/L

Drinkingwater

mg/L

Cd(II) 0.01 2.00 1.00 0.005 0.003

Pb(II) 0.05 0.10 0.10 0.015 0.01

Table 2 Rate Kinetics for adsorption of Cd(II) and Pb(II) by sawdust and neem bark

Adsorbents Metal

ion

Lagergren 1st Order Pseudo 2nd Order Weber and Moris

Kad x102

(min-1)

r2 2 K2

(g.mg-1.min-1)

r2 2 Kid x102

(mg.g-1.min-1/2)

r2 2

Saw Dust Cd(II) 4.09 0.995 10.765 0.0448 0.995 0.001 6.31 0.961 0.199Saw dust Pb(II) 4.145 0.996 2.111 0.1077 0.997 0.042 15.35 0.996 0.538

Neem bark Cd(II) 3.63 0.991 8.976 0.156 0.999 0.014 6.77 0.991 0.335

Neem bark Pb(II) 6.190 0.997 0.989 0.125 0.996 0.092 19.32 0.971 0.843

Table 3 Determination of effective diffusion coefficient

Metal Saw dust Neem bark

De 1012

m2/s

r2 De 1012

m2/s

r2

Cd(II) 369.433 0.998 260.399 0.9897

Pb(II) 239.269 0.995 249.403 0.9912

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Table 4 Langmuir and Freundlich adsorption isotherm constants for adsorption________________________________________________________________________

Langmuir FreuindlichAdsorbents Metal ion qmax b

(mg g-1) (L mg-1) r2 2 Kf n r2 2

Sawdust Cd(II) 26.73 0.053 0.990 0.090 1.62 1.54 0.977 0.805Sawdust Pb(II) 88.49 0.025 0.978 0.179 2.13 1.18 0.997 0.355

Neem bark Cd(II) 25.57 0.056 0.984 0.438 1.65 1.52 0.975 0.632

Neem bark Pb(II) 83.33 0.023 0.988 0.084 2.03 1.16 0.996 0.468

________________________________________________________________________

Table 5 Dubinin-Radushkevich (D-R) isotherm parameter and activation energy for adsorption

Metal

ion

Adsorbents DubininRadushkevich

Constant

Activation

energykJ/mol

Correlation

coefficientr2

Cd(II) Saw dust -0.00533 9.415 0.9989

Cd(II) Neem bark -0.00488 9.911 0.9907

Pb(II) Saw dust -0.0049 9.911 0.9907

Pb(II) Neem bark -0.0051 9.950 0.9967

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0 2 4 6 8 1020

40

60

80

100

Figure 1 Effect of pH on adsorption

Perc

entageremovalofmetalion,

%

initial pH

Symbol Adsorbents Metal IonSawdust Cd(II)

Neembark Cd(II)

Sawdust Pb(II)

Neembark Pb(II)

0 7 14 21 28 3560

80

100

Figure 2 Effect of adsorbent dosage on adsorption

Pe

rcentageremovalofmetalion,

%

Adsorbent dosage (g/L)

Symbol Adsorbents Metal Ion

Sawdust Cd(II)

Neembark Cd(II)Sawdust Pb(II)

Neembark Pb(II)

0 80 160 240 32020

40

60

80

100

Figure 3 Effect of contact time on adsorption

Percentageremovalofmetalio

n,

%

contact time, min


Sawdust Zn(II)

Neembark Zn(II)

Sawdust Cd(II)

Neembark Cd(II)

0 80 160 240 3200

100

200

300

400

Figure 4 Pseudo scond order plot for adsorption


Sawdust Cd(II)Neembark Cd(II)

Sawdust Pb(II)

Neembark Pb(II)

t/q

time,min

0 20 40 60 80 1000

1

2

3

4

5

Figure 5 Langmuir plot for adsorption


Sawdust Cd(II)

Neembark Cd(II)Sawdust Pb(II)

Neembark Pb(II)

Ce

/qe

Ce

0 300 600 900 1200-13

-12

-11

-10

-9

-8

Figure 6 D-R plot for adsorption

lnC

abs

2


Sawdust Cd(II)

Neembark Cd(II)

Sawdust Pb(II)Neembark Pb(II)

11

Documents

Adsorption of CD(II) and Pb(II) From Aqueous Solutions on Saw Dust and Neem Bark