Absorção de Cadmio e Zinco CCA

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Available online at www.sciencedirect.com

Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 172–184

Removal of cadmium(II) and zinc(II) metal ionsfrom binary aqueous solution by rice husk ash

Vimal Chandra Srivastava ∗, Indra Deo Mall, Indra Mani Mishra

Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India

Received 6 November 2006; received in revised form 25 April 2007; accepted 27 June 2007

Available online 29 June 2007

Abstract

The present study reports the competitive adsorptive removal of cadmium (Cd(II)) and zinc (Zn(II)) ions from binary systems using rice husk ash (RHA), a waste obtained from the rice husk-fired furnaces, as an adsorbent. The initial pH (pH 0) affects significantly the capacity of RHA

for adsorbing the metallic ions in the aqueous solution. The pH 0 ≈ 6.0 is found to be the optimum for the removal of Cd(II) and Zn(II) ions by

RHA. The single ion equilibrium adsorption from the binary solution is better represented by the non-competitive Redlich–Peterson (R–P) and the

Freundlich models than by Langmuir model in the initial metal concentration range of 10–100mg/l. The adsorption of Zn(II) ion is more than that

of Cd(II) ion, and this trend is in agreement with the single-component adsorption data. The equilibrium metal removal decreases with increasing

concentrations of theother metal ion and thecombined effect of Cd(II) and Zn(II) ions on RHAis generally found to be antagonistic. Non-modified

Langmuir, modified Langmuir, extended-Langmuir, extended-Freundlich, Sheindorf–Rebuhn–Sheintuch (SRS), non-modified R–P and modified

R–P adsorption models were tested to find the most appropriate competitive adsorption isotherm for the binary adsorption of Cd(II) and Zn(II)

ions onto RHA by minimizing the Marquardt’s percent standard deviation (MPSD) error function. The extended-Freundlich model satisfactorily

represents the adsorption equilibrium data of Cd(II) and Zn(II) ions onto RHA.

© 2007 Elsevier B.V. All rights reserved.

Keywords: Binary adsorption; Rice husk ash; Multi-component isotherms; Cadmium(II); Zinc(II)

1. Introduction

The main techniques used to remove heavy metal ions

from aqueous streams include ion-exchange chromatogra-

phy, reverse-osmosis, chemical precipitation, and adsorption.

Adsorption as a wastewater treatment process has aroused con-

siderable interest during recent years. The use of commercially

available activated carbon as an adsorbent is limited, especially

in developing countries, because of its relatively high cost and

the difficulties associated with its regeneration. As a result, sev-

eral non-conventional adsorbents have been used by a numberof investigators. Rice husk is an agricultural waste, accounting

for about one-fifth of the annual gross rice production (545 mil-

lion metric tonnes) of the world [1]. Rice husk is used as a fuel

by a number of industries to produce steam, thus, conserving

both energy and resources. During the burning of rice husk, the

∗ Corresponding author. Tel.: +91 1332 285889;

fax: +91 1332 276535/273560.

E-mail address: [email protected] (V.C. Srivastava).

residue ash, called rice husk ash (RHA) is collected from the

dust collection device attached upstream to the stacks of rice

husk-fired boilers and furnaces. RHA has good adsorptive prop-

erties and has been used previously for the adsorptive removal

of metal ions [2] and dye [3], andfiltration of arsenic from water

[4]. RHA, obtained from heating rice husk at 300 ◦C, has been

shown to adsorb more gold-thiourea than the conventionally

used activated carbon [5].

Heavy metals exist in aqueous waste streams of many indus-

tries like metallurgical/metal manufacturing and electroplating,

chemical manufacturing,printing, dye and paint industry, paper,

textile, refinery and petrochemicals, leather goods manufactur-

ing, fertilizerand pesticides industry, etc. [6]. Heavy metals have

cumulative effects and tend to accumulate in the living organ-

isms causing various diseases and disorders. Cadmium [Cd(II)]

is a non-essential and non-biodegradable metals which slowly

accumulates in the human body, usually from food-chain. The

body slowly releases adsorbed Cd(II) which has a biological

half-life of more than 10 years. As a consequence, the Cd(II)

content of the kidney increases throughout life. Ingested Cd(II)

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.colsurfa.2007.06.048

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V.C. Srivastava et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 172–184 173

Nomenclature

aij competition coefficients of component i by com-

ponent j

aR constant of Redlich–Peterson isotherm (l/mg)

C e residual concentration of the single-component at

equilibrium (mg/l)C e,i residual concentration of each component in the

binary mixture at equilibrium (mg/l)

C 0 initial concentration of adsorbate in solution

(mg/l)

C 0,i initial concentration of each component in solu-

tion (mg/l)

K i individual extended Langmuir isotherm constant

of each component (l/mg)

K F mono-component (non-competitive) constant of

Freundlich isotherm of the single component

((mg/g)/(l/mg)1/ n)

K F,i individual Freundlich isotherm constant of eachcomponent ((mg/g)/(l/mg)1/ n)

K L constant of Langmuir isotherm (l/mg)

K L,i individual Langmuir isotherm constant of each

component (l/mg)

K R constant of Redlich–Peterson isotherm (l/g)

m mass of adsorbent in the adsorbate solution (g/l)

MPSD Marquardt’s percent standard deviation

n mono-component (non-competitive) Freundlich

heterogeneity factor of the single component

ni individualFreundlichheterogeneityfactorofeach

component

nm number of measurements

nP number of parameters N number of data points

N i(Q) number of sites having energy Q

pH0 initial pH of the solution

qe equilibrium single-component solid phase con-

centration (mg/g)

qe,i equilibrium solid phase concentration of each

component in binary mixture (mg/g)

qe,cal calculated value of solid phase concentration of

adsorbate at equilibrium (mg/g)

qe,exp experimental value of solid phase concentration

of adsorbate at equilibrium (mg/g)

qm maximum adsorption capacity of adsorbent(mg/g)

qmax constant in extended Langmuir isotherm (mg/g)

Q adsorption energy (J)

R universal gas constant (8.314 J/K mol)

RHA rice husk ash

t time (min)

Greek symbols

αi constant in SRS model for each component

β constant of Redlich–Peterson isotherm (0


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174 V.C. Srivastava et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 172–184

Table 1

Mono- and multi-component isotherm models

Reference

Multi-component isotherm models

Freundlich: qe = KFC1/ne

Langmuir: qe =qmKLCe

1+KLCe

Redlich–Peterson: qe =KRCe

1+ aRCβe

[12]

Multi-component isotherm models

Non-modified Langmuir model: qe,i =qm,iKL,iCe,i

1+N

j =1KL,j Ce,j

[13]

Modified Langmuir isotherm:

qe,i =qm,iKL,i(Ce,i/ηL,i)

1+N

j =1KL,j (Ce,i/ηL,i)

[14]

Extended Langmuir isotherm: qe,i =qmaxKEL,iCe,i

1+N

j =1KEL,j Ce,j

Extended Freundlich isotherm: qe,1 =KF,1C

(1/n1)+x1e,1

Cx1e,1 + y1Cz1e,2

;

qe,2 =

KF,2C(1/n2)+x2

e,2Cx2e,2 + y2C

z2e,1

[15]

Sheindorf–Rebuhn–Sheintuch (SRS) model:

qe,i = KF,iCe,i

N j =1

aij Ce,j

(1/ni)−1 [16]

Non-modified Redlich–Peterson model:

qe,i =KR,iCe,i

1+N

j =1aR,j C

β,j e,j

Modified Redlich–Peterson model:

qe,i =KR,i(Ce,i/ηR,i)

1+N

j =1aR,j (Ce,j /ηR,j )

β,j

adsorption. The Freundlich isotherm is derived by assuming a

heterogeneous surface with a non-uniform distribution of heat

of adsorption over the surface. Whereas in the Langmuir theory,

the basic assumption is that the sorption takes place at specific

homogeneous sites within the adsorbent. The R–P isotherm

[12] incorporates three parameters and can be applied either in

homogenous or heterogeneous systems.

Various multi-component isotherm equations have been

developed by various researchers. Non-modified competitive

Langmuir model is the extension of the basic Langmuir model.

Individual adsorptionconstants maynotdefineexactlythemulti-

component adsorption behaviour of metal ion mixtures. For

that reason, better accuracy may be achieved by using mod-ified isotherms related to the individual isotherm parameters

and the correction factors. An interaction term, ηL,i, which

is a characteristic of each species and depends on the con-

centrations of the other components, has been added in the

competitiveLangmuir model by BellotandCondoret [13] to for-

mulate the modified competitive Langmuir isotherm. Similarly,

the competitive non-modified R–P model is modified, using an

interaction term ηR,i, to the obtain modified competitive R–P

model. Assuming that the surface sites are uniform, and that all

the adsorbate molecules (ions) in the solution compete for the

same surface sites, Yang [14] extended the mono-component

Langmuir equation to give extended Langmuir isotherm for

multi-component systems. Similarly, Fritz and Schluender [15]

extended the mono-component Freundlich equation to give

extended Freundlich isotherm for binary systems. Sheindorf

et al. [16] derived a Freundlich-type multi-component adsorp-

tionisothermknown as theSheindorf–Rebuhn–Sheintuch (SRS)

equation, to represent the experimental data. The competition

coefficients aij in the SRS model describe the inhibition to the

adsorption of component i by component j, and can be deter-

mined from the thermodynamic data, or more likely, from the

experimental sorption data of multi-component systems. The

SRS equation assumes that (i) each component individually

obeys the Freundlich isotherm; (ii) that for each component in a

multi-component adsorption system, there exists an exponential

distribution of site adsorption energies, i.e.:

N i(Q) = αi exp

−βiQ

RT

(1)

where αi and βi are constants; and (iii) the coverage by each

adsorbate molecule (or ion) at each energy level Q is given by

the multi-component Langmuir isotherm equation:

θ i(Q) =KiCe,i

1+N

j =1Kj Ce,j (2)

where

Kj = K0j exp

Q

RT

(3)

Integration of N i(Q)θ i(Q) over energy level in the range of −∞

to +∞ yields Eq. (2) and thecompetition coefficientsaredefined

as aij = K 0 j / K 0i and thus a ji =1/ aij.

The isotherm parameters of all the multi-component models

can be found by minimizing Marquardt’s percent standard devi-ation (MPSD) [17]. MPSD has been used extensively [18–20]

to test the adequacy and accuracy of various isotherm model to

represent the experimental equilibrium sorption data. MPSD is

given as

MPSD = 100

1nm − np

ni=1

N i=1

qe,i,exp−N

i=1qe,i,cal

N

i=1qe,i,exp

2i

(4)

Here, the subscripts ‘exp’ and ‘calc’ mean the experimental and

calculated values, nm the number of measurements and np is the

number of parameters in the model.

The qe,i, individual adsorption yield ( Ad i%) and total adsorp-tion yield ( Ad Tot%) can be calculated by using the following

expressions:

qe,i =(C0,i − Ce,i)V

w

mg of adsorbate

g of adsorbent

(5)

Ad i% = 100×C0,i −Ce,i

C0,i(6)

Ad Tot% = 100×

(C0,i − Ce,i)

C0,i(7)

where V is the volume of the adsorbate containing solution (l)

and w is the mass of the adsorbent (g).


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3. Experimental

3.1. RHA

RHA was used as obtained from a nearby paper mill (Bar-

nala paper mill, Punjab, India) without any pretreatment for the

removalofmetal ions from syntheticaqueoussolutionsin a batch

treatment process. Detailed physico-chemical characteristics of

the RHA have already been presented elsewhere [2].

3.2. Chemicals

All the chemicals used in the study were of analytical reagent

(AR) grade. Cadmium sulphate octahydrate (3CdSO4·8H2O),

zincsulphateheptahydrate (ZnSO4·7H2O),NaOHandHClwere

obtained from S.D. Fine Chemicals, Mumbai. Stock solutions

having concentrationsof 1 g/lof Cd(II)andZn(II)were prepared

by dissolving exactamounts of CdSO4·8H2O and ZnSO4·7H2O

in double-distilled water (DDW), respectively. Test solutions

were prepared by diluting the stock solutions with DDW to havethe component concentration in the range of 10–100mg/l.

3.3. Batch adsorption studies

For each experimentalrun, 100mlaqueous solution ofknown

concentration ofCd(II), Zn(II)or binarymixtureof thesecompo-

nentswastaken ina 250mlconical flask containing fixedamount

of RHA. These flasks were agitated at a constant shaking rate of

150rpm in a temperature controlled orbital shaker (Remi Instru-

ments, Mumbai) maintained at 30 ◦C. The initial pH (pH0) of

the adsorbate solution was adjusted using 1N (36.5g/l) HCl or

1N (40 g/l) NaOH aqueous solution without any further adjust-ment during the sorption process. The samples were withdrawn

from the flasks at different time intervals to check whether equi-

librium has been attained. The samples were centrifuged using

Research Centrifuge (Remi Instruments, Mumbai) at 5000 rpm

for 5 min and then the supernatant liquid was analyzed for

residual concentration of metal ions using an atomic adsorption

spectrophotometer (GBC Avanta Instrument).

3.4. Effect of initial pH (pH 0)

The effectof pH0 on thesorption was studied by adjusting the

pH0 in the range of 2–8. In these experiments, the RHA loading

was kept at 10 g/l of solution containing 100 mg/l each of Cd(II)and Zn(II) at 30 ◦C. The contact time (t ) was kept as 5 h, since

equilibrium was found to have been attained in 5 h contact time.

3.5. Adsorption isotherm experiments

For single metal-ion-RHA systems, initial metal ion con-

centration was varied from 10 to 100 mg/l. In binary metal ion

mixture-RHA systems, for each initial concentration of Cd(II)

solution: viz., 10, 20, 30, 50, and 100 mg/l, the Zn(II) concentra-

tion was varied in the range of 10–100mg/l (viz., 10, 20, 30, 50,

and 100 mg/l). In all cases, the pH0 of the solution was main-

tained at 6.0. This pH0 was found to be the optimum on the

basis of batch tests conducted to determine the effect of pH0 on

adsorption capacity of RHA for metal ions.

3.6. Desorption studies

For batch desorption experiments, a series of 250 ml Erlen-

meyer flasks containing 50ml of DDW or aqueous solution of

HCl, H2SO4, HNO3 and CH3COOH of known concentration

were contactedwith metal-loadedRHA(0.5 g) at 30± 1 ◦C.The

mixtures were agitated at 150 rpm for 5 h in the orbital shaker.

Thereafter, the mixture was centrifuged and the supernatant was

analyzed for metal ions released into the solvent.

3.7. Analysis of cadmium(II) and zinc(II)

The concentration of Cd(II) and Zn(II) in the sample was

determined by flame atomic absorption spectrophotometer

(GBC Avanta, Australia) with the detection limit of 0.009 and

0.008 mg/l at the wavelength of 228.8 and 213.9nm, for Cd(II)

and Zn(II), respectively, by using air–acetylene flame. Beforethe analysis, the sample was diluted with DDW to have the con-

centration in therangeof 0.2–1.8 and0.4–1.5 mg/l forCd(II)and

Zn(II), respectively. Metal ion concentrations were determined

with reference to appropriate standard metal ion solutions.

4. Results and discussion

4.1. Effect of initial pH (pH 0)

It is known that metal species [M(II) = Cd(II), Zn(II)] are

present in deionized water in the forms of M2+, M(OH)+,

M(OH)20

, M(OH)2(S), etc. [21]. At pH≈

5.0, the solubility,C T,M, of the M(OH)2(S) is very high, so the M2+ is the main

species present. With an increase in the pH value, the C T,M of

M(OH)2(S) decreases. At pH≈ 10.0, the C T,M of M(OH)2(S) is

much smaller and the main species in the solution is M(OH)2(S).

It is obvious that the adsorption of M(II) must be higher in alka-

line solution. But at higher pH, the precipitation as M(OH)2(S)plays the main role in removing the M(II) ions. Therefore, all

the experiments were conducted at pH0≤ 8.0. In addition to the

speciation of metal ions, the pH0 of the adsorbate solution is

the most important parameter governing sorption on different

adsorbents.

The influence of the pH0 of metal ion solution on the extent

of adsorption of Cd(II) and Zn(II) ions onto RHA is shown inFig. 1. Theadsorption of metal ions increaseswith an increase in

pH0. Up to pH0 4, the increase in adsorption is gradual, which,

however, increases drastically at pH0 > 4. At higher pH0 (≥6.0),

Cd(II) and Zn(II) ion adsorption becomes nearly constant.

During the initial stage of the sorption process up to 30min,

the pH of the solution was found to rise sharply. Thereafter, the

pH of the solution remained invariant with time. The system pH

changes during the adsorption process. Fig. 2 shows the final

pH values (pHf ) as a function of pH0 in Fig. 2. The pHf values

are higher than the pH0 values for pH0 < 7.5 for C 0 = 100 mg/l.

Although themetalremoval increasessharplywith anincrease in

pH0,thepHf valueswere almostconstant for6≤ pH0≤ 7.5. The


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Fig. 1. Effect of pH0 on the removal of cadmium(II) and zinc(II) ionsfor mono-component adsorbate aqueous solution by RHA. T = 30 ◦C, t =5h,

C 0 =100mg/l, and RHA dosage = 10 g/l.

pHf values are, therefore, considered to indicate the equilibrium

pH values [22]. The increase in the solution pH during sorption

process appears to be the combined result of the simultaneous

and, perhaps competitive adsorption of metal ions and H+ ions

onto adsorbents [6], and the hydrolysis of adsorbates to release

basiccations in solution.Thedegree of pHvariation wassmallin

higherpH0 solutionsbecause of theirhigher buffering capacities

[23] (Fig. 2).

Cd(II) and Zn(II) ion adsorption at lower pH0 (pH0≤ 6.0)

is lower than that at higher pH0 (≥6.0). This is because thesurface charge developed at low pH0 is not favourable to adsorp-

tion. To understand the adsorption mechanism, it is necessary

to determine the point of zero charge (pHPZC) of the adsor-

bents. Adsorption of cations is favoured at pH > pHPZC, while

Fig. 2. The variation in equilibrium system pH f with pH0 during the sorption

of Cd(II) and Zn(II) onto RHA.

Fig. 3. Effect of RHA dosage on the removal of Cd(II) and Zn(II). T =303K,

t = 5 h, and C 0 = 100 mg/l.

the adsorption of anions is favoured at pH < pHPZC. The spe-

cific adsorption of cations shifts pHPZC towards lower values,

whereas the specific adsorption of anions shifts pHPZC towards

higher values. The pHPZC for RHA is 8.3 [2]. For pH0 below 6,

a significantelectrostatic repulsion exists between the positively

charged surface of theRHA andthe metal ions. Besides, a higher

concentration of H+ in the solution competes with Cd(II) and

Zn(II) for the adsorption sites, resulting in the reduced uptake of

metal ions. As the pH0 of the system increases, the number of

positively charged sites decreases and the number of negatively

charged sites increases. A negatively charged surface site on theRHA favours the adsorption of metal ions due to electrostatic

attraction. However, it was found that with pH0 6.0, the pH of

the solution rises sharply and stablizes at pH 7.6 and 7.0, for

Cd(II) and Zn(II), respectively. Therefore, it may be concluded

that the H+ in the solution competes with Cd(II) and Zn(II) for

the adsorption sites of RHA at pH0 6.0. Also, the rise in pH

for Zn-RHA system is lower than that of Cd-RHA system. It

means that the amount of H+ adsorption is less in Zn-RHA sys-

tem leading to a slight increase in the system pH, and therefore,

the amount of Zn(II) ions adsorption is more than that of Cd(II)

ions. Similar results have been reported by other researchers, for

the uptake of Zn, Cd and Cu onto activated carbon [24] and onto

a selected mixture of mosses [25].It is also clear that the amount of adsorbed Zn(II) is larger

than that of Cd(II). This may be due to the smaller ionic size

of zinc ion (0.74 Å) than that of cadmium ion (0.83 Å) [26].

Ma and Tobin [27] also reported higher sorption of Zn(II) than

that of Cd(II) onto peat. This preference/affinity of Zn(II) is

because of the chemical characteristics of the ions and the ionic

radius.

The theoretical monolayer surface coverage ratio as calcu-

lated from the specific surface area of RHA (36.44m2 /g) and the

ionic size (0.83 and 0.74 Å) and molecular weight (112.51 and

65.39 g/mol) of the Cd(II) and Zn(II) ions were found to be 5.70

and4.17 mg/g for cadmium andzinc, respectively. However, the


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adsorption order is found to be in the order of increasing molec-

ular weight and ionic radius, i.e. Zn(II) > Cd(II). The micro- and

meso-pore size distribution of the RHA and the shape of the

pores coupled with the ionic size of the two metal ions play

important roles in their sorption uptake by RHA. While con-

sidering adsorption characteristics, the minimum ionic size of

the metal ion in one-dimension is critical for an adsorbent with

slit shaped pores. In the case of an adsorbent having cylindrical

pores with circular cross-section, the minimum ionic sizes in

two-dimensions need to be considered.

The monolayer surface adsorption occurs when the adsorp-

tion is physical in nature. However, in the present case, the

adsorption is chemisorptive in nature. Hence, the results cannot

be interpreted based on monolayer surface coverage ratio.

4.2. Effect of adsorbent dosage (m)

The effect of m on the uptake of Cd(II) and Zn(II) ions onto

RHA was studied and is shown in Fig. 3. This figure reveals thattheremoval of metal ions increaseswith an increase in theadsor-

bent dosage from 1 to 10 g/l. The removal remains unchanged

above 10g/l of RHA dosage. An increase in the adsorption with

the adsorbent dosage can be attributed to the larger availability

of greater surface area and more adsorption sites. At m 7 g/l, the incremental metal ions removal becomes very low,

and at about m = 10 g/l, the removal efficiency becomes almost

constant. Maximum removal of metal ions at C 0 = 100mg/l was

found to be 62.5% for Zn(II) and 29.8% for Cd(II).

Different investigators have reported different values of opti-

mum m (mopt) for the sorptive removal of metallic ions by

different adsorbents. Table 2 summarizes the optimum exper-

imental conditions and the results for the adsorptive removal of

Cd(II) and Zn(II) by various adsorbents. Cho et al. [28] used fly

ash for Cd(II) and Zn(II) removal with a mopt value of 10 g/l at

pH0 6.0 and C 0 =100mg/l; Rao et al. [29] f ound mopt = 25g/l

with zeolites and bentonite while Shukla and Pai [30] found

mopt =20 g/l with unmodified dye loaded and oxidized Jute.

4.3. Effect of contact time (t)

Aqueous metal ion solutions with C 0 = 100 mg/l were kept in

contact with the RHA for 24 h. The residual concentrations at

5 h contact time were found to be higher by a maximum of ∼1%

than those obtained after 24 h contact time. Therefore, after 5h

contact time, a steady state approximation was assumed and aquasi-equilibrium situation was accepted. Accordingly all the

batch experiments were conducted with a contact time of 5 h

under vigorous shaking conditions. Fig. 4 shows the effect of t

on the uptake of Cd(II) and Zn(II) ions from aqueous metal ion

solutions. The rate of metal ion removal is found to be very rapid

during the initial 15 min, and thereafter, the rate of metal ion

removal decreases. No significant change in metal ion removal

is observed after about 120min. During the initial stage of sorp-

tion, a large number of vacant surface sites are available for

adsorption. After lapse of some time, the remaining vacant sur-

face sites are difficult to be occupied due to repulsive forces

Table 2

Optimum experimental parameters and isotherm constants for adsorption of cadmium and zinc ions on various adsorbents as reported in literature

Adsorbent Adsorbate pH T (◦C) C 0 (mg/l) m (g/l) t (h) K F ((mg/g)/(mg/l)1/ n) 1/ n qm (mg/g) K L (l/mg) Reference

Rice husk ash Cd 6 30 10–100 10 5 0.96 0.26 3.04 0.19 Present study

Rice husk ash Zn 6 30 10–100 10 5 1.31 0.38 5.88 0.19 Present study

Bagasse fly ash Cd 6 30 10–100 10 5 0.85 0.47 6.19 0.09 [11]

Bagasse fly ash Zn 6 30 10–100 10 5 1.43 0.42 7.03 0.18 [11]

Fly ash Zn 6 25 10–400 10 – 0.45 0.76 – – [28]

Fly ash Cd 6 25 10–400 10 – 0.12 0.71 – – [28]

Zeolite 4A Cd 6 30 25–100 25 1.5 1.09 0.23 30.27 0.55 [29]

Zeolite 13X Cd 6 30 25–100 25 1.5 0.91 0.27 29.59 0.36 [29]

Bentonite Cd 6 30 25–100 25 1.5 0.75 0.30 28.47 0.22 [29]

Zeolite 4A Zn 6.5 30 25–100 25 1.5 1.75 0.45 42.82 0.30 [29]Zeolite 13X Zn 6.5 30 25–100 25 1.5 1.29 0.43 38.31 0.19 [29]

Bentonite Zn 6.5 30 25–100 25 1.5 1.04 0.42 35.17 0.14 [29]

Jute unmodified Zn 5.87 35 38–212 20 2 0.17 0.56 3.55 0.02 [30]

Jute dye loaded Zn 5.87 35 38–212 20 2 0.45 0.50 5.95 0.03 [30]

Jute oxidized Zn 5.87 35 38–212 20 2 0.16 0.66 8.02 0.01 [30]

Vermiculite Cd 6.2 25 112–1120 2.5 12 – – 63.34 1.10 [31]

Vermiculite Zn 5.6 25 65–650 2.5 12 – – 71.89 1.61 [31]

Bentonite Zn 6 25 12.5–200 – 2 2.68 0.47 30.7 0.92 [32]

Na-enriched bentonite Zn 6 25 12.5–200 – 24 12.13 0.36 57.43 8.27 [32]

TAa immobilised ACb Cd 5.7 RTc 1–10 2 1 1.33 0.35 2.46 1.67 [33]

TAa immobilised ACb Zn 5.6 RT 1–10 2 1 0.89 0.36 1.80 1.29 [33]

a Tannic acid.b Activated carbon.c

Room temperature.


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Fig. 4. Effect of contact time on the adsorption of Cd(II) and Zn(II) by RHA.

T =303K, C 0 =100mg/l, and m =4g/l.

between the solute molecules adsorbed on the solid surface and

the bulk phase. Besides, the metal ions are adsorbed into the

meso-pores that get almost saturated with metal ions during the

initial stage of adsorption. Thereafter, the metal ions have to tra-

verse farther anddeeper into thepores encountering much larger

resistance. This results in the slowing down of the adsorption

during the later period of adsorption.

da Fonseca et al. [31] have reported 12 h equilibrium contact

time for theremoval of Cd(II)andZn(II)by vermiculiteat 25 ◦C.

However, a contact time of only 2 h was required to attain the

equilibrium adsorption of Zn(II) ion onto bentonite [32].

4.4. Single and binary adsorption of cadmium(II) and

zinc(II) ions

The equilibrium uptakes and the adsorption yields obtained

for single component (Cd(II) and Zn(II)) solution at pH0 6.0 are

shown in Figs. 5 and 6, and reported in Table 3. As seen from

the figures and the table, an increase in the initial metal con-

centration up to 100 mg/l increases the equilibrium uptake and

decreases the adsorption yield of both the components. When

the initial ion concentration increases from 10 to 100 mg/l, the

loading capacity of RHA increases from 0.91 to 2.88mg/g for

Cd(II), and from 0.95 to 5.37mg/g for Zn(II). The initial con-

centration provides the necessary driving force to overcome theresistances to themass transfer of Cd(II)andZn(II) ions between

the aqueous and the solid phases. The increase in initial concen-

tration also enhances the interaction between the metal ions in

the aqueous phase and the RHA [20]. Therefore, an increase

in initial concentration of metal ions enhances the adsorption

uptake of the Cd(II) and Zn(II) ions. Also, it is observed that the

adsorption capacity of RHA for Zn(II) is greater than that for

Cd(II).

The simultaneous adsorption of Cd(II) and Zn(II) ions from

binarymixtureswas also investigatedatpH0 6.0. In thefirst stage

of adsorption studies, while initial Cd(II) concentration was

changed from 0 to 100 mg/l, at each initial Zn(II) ion concentra-

Fig. 5. Comparison of the equilibrium adsorption isotherms of cadmium(II)

ion at varying concentrations of zinc(II) ion. pH0 6.0, T = 30◦C, t =5h,

C 0[Cd(II)] = 10–100mg/l, and RHA dosage = 10 g/l.

tion of 0, 10, 20, 30, 50, and 100 mg/l. The non-linear adsorption

isotherms of Cd(II) ions in the absence and presence of increas-

ing concentrations of Zn(II) ions are shown in Fig. 5. It is seen

that the equilibrium Cd(II) uptake increases with an increase

in the initial Cd(II) concentration up to 100 mg/l at all Zn(II)

ion concentrations. The equilibrium uptake of Cd(II) decreases

continuouslywith increasing Zn(II) ionconcentration. Theindi-

vidual and total adsorption equilibrium uptakes and yields of

Cd(II) and Zn(II) ions on RHA as obtained at different Cd(II)

concentrations in the absence of Zn(II) or the presence of Zn(II)

ions with increasing concentrations are also listed in Table 3.In general, an increase in Zn(II) ion concentration decreases the

individual adsorption yield of Cd(II) and total adsorption yield

for each experimental run. The results also show that the equi-

Fig. 6. Comparison of equilibrium adsorption isotherms of zinc(II) ion

at varying concentrations of cadmium(II) ion. pH0 6.0, T = 30◦C, t =5h,

C 0[Zn(II)] = 10–100 mg/l, and RHA dosage = 10g/l.


8/13


Table 3

Comparison of individual and total adsorption equilibrium uptake and yields found at different cadmium(II) concentrations at varying concentrations of zinc(II) ions

onto rice husk ash

C 0,Cd C 0,Zn C e,Cd C e,Zn qe,Cd qe,Zn Ad Cd% Ad Zn% Ad Tot%

0 0 0 0 0 0 0 0 0.00

0 10 0 0.49 0 0.95 0 95.10 95.10

0 20 0 2.16 0 1.78 0 89.20 89.20

0 30 0 4.81 0 2.52 0 83.97 83.970 50 0 13.25 0 3.68 0 73.50 73.50

0 100 0 46.30 0 5.37 0 53.70 53.70

10 0 0.87 0 0.91 0 91.29 0 91.29

10 10 2.23 1.46 0.78 0.85 77.70 85.39 81.54

10 20 3.23 4.76 0.68 1.52 67.70 76.20 73.37

10 30 4.23 8.81 0.58 2.12 57.70 70.63 67.40

10 50 4.90 19.00 0.51 3.10 50.96 62.00 60.16

10 100 5.21 53.30 0.48 4.67 47.91 46.70 46.81

20 0 5.08 0 1.49 0 74.58 0 74.58

20 10 6.82 2.69 1.32 0.73 65.90 73.10 68.30

20 20 8.82 6.34 1.12 1.37 55.90 68.30 62.10

20 30 9.99 10.30 1.00 1.97 50.04 65.67 59.42

20 50 11.01 21.58 0.90 2.84 44.96 56.84 53.45

20 100 11.74 56.50 0.83 4.35 41.29 43.50 43.13

30 0 11.42 0 1.86 0 61.93 0 61.93

30 10 13.20 4.09 1.68 0.59 56.00 59.10 56.78

30 20 14.60 8.10 1.54 1.19 51.33 59.50 54.60

30 30 16.70 13.10 1.33 1.69 44.34 56.33 50.34

30 50 17.84 25.54 1.22 2.45 40.54 48.92 45.78

30 100 18.90 60.80 1.11 3.92 37.01 39.20 38.69

50 0 26.71 0 2.33 0 46.59 0 46.59

50 10 27.80 4.47 2.22 0.55 44.40 55.32 46.22

50 20 29.75 9.56 2.03 1.04 40.50 52.20 43.84

50 30 31.85 15.01 1.82 1.50 36.30 49.97 41.43

50 50 33.75 27.55 1.63 2.25 32.50 44.90 38.70

50 100 35.55 65.50 1.45 3.45 28.90 34.50 32.63

100 0 71.24 0 2.88 0 28.76 0 28.76

100 10 72.60 5.22 2.74 0.48 27.40 47.81 29.26

100 20 75.90 11.24 2.41 0.88 24.10 43.81 27.39

100 30 77.90 18.11 2.21 1.19 22.10 39.63 26.15

100 50 80.90 32.08 1.91 1.79 19.10 35.84 24.68

100 100 83.60 70.30 1.64 2.97 16.40 29.70 23.05

librium uptake of Cd(II) ion decreases with increasing initial

Zn(II) ion concentration. At 100mg/l initial Cd(II) concentra-

tion, without Zn(II) ions and in the presence of 100mg/l Zn(II)

ion concentration, adsorbed Cd(II) quantities at equilibrium are

found to be 2.88 and 1.64 mg/g, respectively. Fig. 6 depicts the

variations in the uptake of Zn(II) at equilibrium with increasing

initial Zn(II) concentrations (from 0 to 100 mg/l) at a constant

initial Cd(II) concentration (10–100mg/l) at pH0 6.0. Similaradsorption patterns are observed both in the individual-Zn(II)

ion and binary Ni(II)–Zn(II) ion systems; Zn(II) ion equilib-

rium uptake increases with an increase in the initial Zn(II) ion

concentration up to 100 mg/l. Increase in Cd(II) concentration

decreases the equilibrium uptake of Zn(II) ion. Theresults given

in Table 3 indicate that the presence of Cd(II) retards the equi-

librium uptake of Zn(II) ions. With no Cd(II) present in the

solution, equilibrium uptake of Zn(II) is found to be 5.37 mg/g

at 100 mg/l initial Zn(II) ion concentration. When the Cd(II)

concentration is kept at 100mg/l at the same initial Zn(II)

ion concentration, the equilibrium Zn(II) uptake decreases to

2.97mg/g.

In general, multi-component adsorbates–adsorbents gener-

ally exhibit three possible types of behaviour: synergism (the

effect of the mixture is greater than that the single compo-

nents in the mixture), antagonism (the effect of the mixture is

less than that of each of the components in the mixture) and

non-interaction (the mixture has no effect on the adsorption

of each of the adsorbates in the mixture) [11]. The combined

effect of the binary mixture of Cd(II) and Zn(II) seems to beantagonistic. To analyze the antagonistic sorption behaviour

of the two components, the adsorption yields of single- and

binary component systems were also compared. For instance,

from Table 3, it was expected that the total adsorption yield

must be equal to 41.23% for the total metal concentration of

200 mg/l containing equal (100 mg/l) concentrations of Cd(II)

and Zn(II) in the mixture [ Ad Tot%=41.23=100× [(28.76 mg/l

Cd(II)+ 53.70mg/l Zn(II) ion)/200 mg/l initial total concentra-

tion]. However, the experimental total adsorption yield was

found to be only 23.05% [ Ad Tot%=23.05=100× [(16.40 mg/l

Cd(II)+ 29.70mg/l Zn(II) ion)/200 mg/l initial total concentra-

tion]. Thus it becomes clear that the binary metal ion solution


9/13


Table 4

Isotherm parameters values for the removal of cadmium(II) and zinc(II) by rice husk ash

Adsorbate K L (l/mg) qm (mg/g) R2

Langmuir constants

Cadmium(II) 0.1870 3.0392 0.9963

Zinc(II) 0.1903 5.8809 0.9942

Adsorbate K F ((mg/g)/(mg/l)1/ n) n R2

Freundlich constants

Cadmium(II) 0.9630 0.2631 0.9988

Zinc(II) 1.3095 0.3836 0.9971

Adsorbate K R (l/g) aR (l/mg) β R2

Redlich–Peterson constants

Cadmium(II) 9.8638 9.3577 0.7613 0.9999

Zinc(II) 7.3196 0.6785 0.6785 0.9998

of Zn(II) and Cd(II) exhibits inhibitory (antagonistic) effects on

adsorption resulting in lower sorption yield.

The experimental equilibrium sorption data obtained forthe single component and the binary systems indicate that the

adsorption capacity of RHA for Cd(II) is, in general, less than

that of Zn(II). There are possible interaction effects between

different species in the solution and, in particular, potential

interactions on the surface depending on the adsorption mech-

anism. The factors that affect the sorption preference of an

adsorbent for different kinds of adsorbates may be related to

the characteristics of the binding sites (e.g. functional groups,

structure, surface properties, etc.), the properties of the adsor-

bates (e.g. concentration, ionic size, ionic weight, ionic charge,

molecular structure, ionic nature or standard redox potential,

etc.) and the solution chemistry (e.g. pH, ionic strength, etc.).However, it is difficult to identify a common denominator from

the physical and chemical properties of Cd(II) and Zn(II) ions

which explains the interactive mechanism and the increase in

the selectivity for sorption of an adsorbate from the binary

mixtures.

4.5. Single-component adsorption isotherm models

The individual Langmuir, Freundlich and R–P adsorption

isotherm parameters for Cd(II) and Zn(II) at pH0 6.0 obtained

from the fitting of experimental data are listed in Table 4 along-

with the regression coefficients, R2. The R2 values are closer to

unity for the R–P and the Freundlich models in comparison tothat for the Langmuir model. Therefore, theequilibrium adsorp-

tion data of Cd(II) and Zn(II)-RHA can be represented more

appropriately by the R–P and the Freundlich models in the stud-

iedconcentration range.Thedata in Table4 alsoindicate that the

amount of Zn(II) ions per unit weight of RHA for the complete

monolayer surface coverage was higher than that of Cd(II). A

large value of K L also implies the strong affinity of Zn(II) ions

to RHA. K F and n, the single-component Freundlich constants,

indicate theadsorptioncapacityandadsorptionintensity, respec-

tively. Higher thevalue of n, the higherwillbe the affinity and the

heterogeneity of the adsorbent surface. It is found from Table 4

that the RHA shows greater heterogeneity for Zn(II) than that

for Cd(II) ions. Since n < 1; both the Cd(II) and Zn(II) ions are

favourablyadsorbed by RHA. Themagnitude of K F also showed

the higher uptake of Zn(II) than that of Cd(II) ions by RHA atpH0 6.0. It is noted that R–P constant, β normally lies between 0

and 1, indicating favourable adsorption. The comparison of the

single component experimental equilibrium adsorption uptake

and the predicted uptake (qe) from the Langmuir, Freundlich

and R–P models for Cd(II) and Zn(II) onto RHA at pH0 6.0 are

presented in Table 5 along with the MPSD values. In view of

the lower MPSD values, the R–P and Freundlich models show

better fit to the experimental adsorption data than the Langmuir

model.

Several authors have reported Freundlich and Langmuir con-

stants for adsorption of Cd(II) and Zn(II) on various adsorbents

under different environmental conditions, as given in Table 2.It may be seen that the isotherm parameters differ widely in

their values for different adsorbents. It is, therefore, necessary

to be cautious while using these isotherm parameter values in

the design of adsorption systems.

Table 5

Comparison of the experimental and calculated qe values evaluated from the

mono-component Langmuir, Freundlich and Redlich–Peterson models for the

individual adsorption of cadmium(II) and zinc(II) to rice husk ash

C 0 (mg/l) C e (mg/l) qe,exp (mg/g) qe,calc (mg/g)

Langmuir Freundlich R–P

Cadmium(II)10 0.87 0.91 0.43 0.93 0.91

20 5.08 1.49 1.48 1.48 1.51

30 11.42 1.86 2.07 1.83 1.85

50 26.71 2.33 2.53 2.29 2.29

100 71.24 2.88 2.83 2.96 2.91

MPSD 31.94 2.47 1.33

Zinc(II)

10 0.49 0.95 0.50 1.00 0.95

20 2.16 1.78 1.71 1.76 1.83

30 4.81 2.52 2.81 2.39 2.49

50 13.25 3.68 4.21 3.53 3.58

100 46.30 5.37 5.28 5.70 5.46

MPSD 29.43 5.88 2.51


10/13


Table 6

Multi-component isotherm parameter values for the simultaneous removal of cadmium(II) and zinc(II) by rice husk ash

Non-modified Langmuir model Non-modified R–P model

MPSD 37.649 53.891

Adsorbate Modified Langmuir model Extended Langmuir model Modified R–P model

ηL,i K i qmax ηRP,i

Cadmium(II) 1.465 0.080 4.324 1.820

Zinc(II) 1.846 0.120 0.128

MPSD 24.708 30.233 26.689

Adsorbate Extended Freundlich model SRS model

x i yi zi aij aij

Cadmium(II) 0.613 0.744 0.559 1 0.820

Zinc(II) 0.603 0.862 0.645 1.744 1

MPSD 16.195 21.404

4.6. Multi-component adsorption isotherm models

The simultaneous adsorption data of Cd(II) and Zn(II) on

the RHA have been fitted to the multi-component isotherm

models, viz., non-modified, modified and extended Langmuir

models; the extended Freundlich and the SRS models and

non-modified and modified R–P models. The parametric val-

ues of all the multi-component adsorption models are given

in Table 6. The MPSD values for the model fit of the experi-

mental data set of Cd(II) and Zn(II) are also given in Table 6.

The comparisons of the experimental and calculated qe values

of Cd(II) and Zn(II) ion in mixtures are also presented in the

parity plots (Figs. 7 and 8). Since most of the data points are

distributed around the 45

◦

line, this indicates that all the multi-component isotherm models could represent the experimental

adsorption data for the binary systems with varying degree of

fit.

Fig.7. Comparisonof theexperimental andcalculated qe valuesof cadmium(II)

ions in a binary mixture of cadmium(II) and zinc(II) ions.

The multi-component non-modified Langmuir model shows

a poor fit to the experimental data (MPSD= 37.6). All the mod-ified Langmuir coefficients (ηL,i) estimated were much greater

than 1.0 indicating that non-modified multi-component Lang-

muir model related to the individual isotherm parameters could

not be used to predict the binary-system adsorption. However,

the use of the interaction term, ηL,i, in the modified Langmuir

model (MPSD= 24.7) clearly increased the fit of non-modified

Langmuir model. The use of interaction term, ηR,i, for modified

R–P model (MPSD= 26.7), similarly, improved the fit of the

non-modified R–P model (MPSD= 53.9).

The use of the extended Langmuir model (MPSD= 30.233)

also does not improve the fit to the binary adsorption data

of metal ions onto RHA. The K i values, reflecting the affin-ity between RHA and the metals in the binary system are:

0.080l/mg for Cd(II) and 0.120l/mg for Zn(II). The overall

total metal ions uptake (qmax) by RHA is 4.324 mg/g. This

value is considerably lower than the sum of the maximum

Fig. 8. Comparison of the experimentaland calculatedqe values of zinc(II) ions

in a binary mixture of cadmium(II) and zinc(II) ions.


11/13


total capacities of Cd(II) and Zn(II) ions resulting from the

single component adsorption. For that reason, the adsorption

sites of Cd(II) and Zn(II) in the binary system onto RHA

may likely be partially overlapped. It may also imply that

there may be a variety of binding sites on RHA showing par-

tial specificity to the individual metal ions. The information

obtainedfrom themaximumcapacities seemsto violate thebasic

assumptions of the Langmuir model, i.e. the entire adsorbent

surface is homogeneous and that there is no lateral interac-

tion between the adsorbate molecules, and thus the affinity

of each binding site for the adsorbate molecules should be

uniform.

The SRS model fitted to the binary adsorption data of Cd(II)

and Zn(II) onto RHA reasonably well (MPSD = 21.4). The

multi-component SRS model applies to those systems where

each component individually obeys the single-component Fre-

undlich isotherm. The isotherm coefficients can be determined

from the mono-component isotherm except for the adsorp-

tion competition coefficients, aij, which have to be determined

experimentally. The competition coefficients, aij, describe theinhibition to the adsorption of component i by component j. The

two components individually were found to obey the single-

component Freundlich model. The competition coefficients aijand a ji were estimated from the competitive adsorption data of

Cd(II) and Zn(II) ions by using MS EXCEL 2002 program. A

comparisonof thecompetitioncoefficients shows that theuptake

of the more favourably adsorbed Zn(II) was strongly affected by

the presence of Cd(II) (a21 =1.74), while the inhibition exerted

in the reverse situation was less (a12 = 0.82). The competition

coefficients seem to prove that the sorption of Cd(II) and Zn(II)

ions onto RHA was inhibited by the presence of either one. For

the SRS model, RHA shows different capacities, K F,i, for Cd(II)andZn(II)and competitioncoefficients duringtheircoexistence.

This suggests that the surface sites of the RHA are heteroge-

neous, and some of the sites may be specific to certain metals

[34].

A comparison of MPSD values for different isotherm models

shows that the extended Freundlich model best-fits the exper-

imental adsorption data of Cd(II) and Zn(II) ions from binary

systems onto RHA.This isexpectedas RHAhasa heterogeneous

surface andthe adsorption of thesinglemetal ions havealso been

well represented by the Freundlich isotherm equation. It is evi-

dent that the modification of the Freundlich equation as given

by extended Freundlich model takes into account the interactive

effects of individual metal ions between and among themselvesand also the adsorbent RHA reasonably well. Therefore, the

binary adsorption of metal ions onto RHA can be represented

satisfactorily and adequately by the extended Freundlich model.

The SRS model, which is also based on Freundlich model, also

fitted the equilibrium binary metal adsorption data reasonably

well.

Three-dimensional (3D) adsorption isotherm surfaces are

used to evaluate the performance of the binary metal ions

adsorption system [35,36]. A 3D graphical representation of the

sorption isotherm plot for the binary metal adsorption system

is given in Fig. 9. In this plot, the experimental data points are

shown along with the predicted isotherms using the extended

Fig. 9. Binary adsorption isotherms cadmium(II)–zinc(II) onto RHA. The sur-

faces are predicted by the extended-Freundlich model and the symbols are

experimental data. (a) Cadmium(II) uptake and (b) zinc(II) uptake.

Freundlich isotherm equations. As can be seen, the predictions

are found to be satisfactory.

4.7. Desorption study and disposal of spent RHA

The regeneration of the adsorbent and/or disposal of the

adsorbate-loaded adsorbent (or spent adsorbent) is very impor-

tant. For the desorption experiments, several solvents (acids,

bases and water) have been used. Batch desorption experiments

were carried out and the desorption efficiencies are compared

in Fig. 10. The use of deionized water resulted in only a lim-

ited amount of metal ion desorption (


12/13


Fig. 10. Desorption of metal ions from metal-loaded RHA by solvents.

T = 30◦

C, t =5h, C 0 (solvent concentration) = 0.1N, and m = 10g/l.

(≈26%) for Cd(II). However, for Zn(II), H2SO4 proved to be

the best. Hydrogen ions released from the acids replace metal

cations on the RHA. Overall, any of the mineral acids can be

selected as the optimal eluting agent for the system studied,

however, the desorption efficiency is not very significant. Sev-

eral authors have demonstrated that the sorption process of trace

metals is not completely reversible. Several explanations have

beenproposedforsuchobservations, includingdiffusionof trace

metals within oxide particles or into micro-pores [37–39], pre-

cipitation [40], incorporation of metals into oxides [41], and

re-adsorption [42]. For the present study, it seems that the

chemisorptive adsorption of metal ions onto RHA hinders the

desorption of metal ions from the spent RHA.

RHA has a heating value of about∼9.68MJ/kg [2,26]. It can

be utilised for making blended fuel briquettes that could be used

as a fuel in the furnaces. The bottom ash obtained after its com-

bustion can be blended with the cementitious mixtures. Setting

and leaching tests on the cementitious mixtures have shown that

the bottom ash can be incorporated into the cementitious matri-

ces to a great extent (75 wt.% of total solid) without the risks of

an unacceptable delay in cement setting and an excessive leach-

ability of heavy metals from solidified products [43]. Further

studies on the disposal of metal loaded RHA is in progress in

our laboratory.

5. Conclusion

The present study shows that the rice husk ash (RHA) is an

effective adsorbent for the removal of Cd(II) and Zn(II) metal

ions from aqueous solution. Maximum sorption for both Cd(II)

and Zn(II) metal ions was found to occur at pH0 6.0. Higher

percentage of metal ion removal was possible provided that

the initial adsorbate concentration in the solution was low. Fre-

undlich and Redlich–Peterson isotherms show very good fits

with the experimental single component adsorption equilibrium

data. The affinity of RHA for Zn(II) ions was greater than that

for Cd(II), for both single component and the binary solutions

undersimilar experimental conditions. The net interactive effect

ofCd(II)andZn(II)ionsontheadsorptionofCd(II)ionsbyRHA

was found to beantagonistic. Based onMPSD error function, the

extended-Freundlichadsorption isothermmodelshowedthe best

fit to thebinaryequilibriumadsorption data. It maybe concluded

that the RHA may be used for the individual and simultane-

ous removal of Cd(II) and Zn(II) ions from metal-containing

effluents.

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Absorção de Cadmio e Zinco CCA

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