Characterization of RHA PFACFA Adsorbent and Its Equilibrium and Kinetic Studies for Zn2+ Removal

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Caspian Journal of Applied Sciences Research, 1(13), pp. 23-34, 2012

Available online at http://www.cjasr.com

ISSN: 2251-9114, 2012 CJASR

23

Full Length Research PaperCharacterization of RHA /PFA/CFA Adsorbent and Its Equilibrium and KineticStudies for Zn

2+Removal

Haider M. Zwain1, Irvan Dahlan2*

1School of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal,Pulau Pinang, Malaysia

2School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 NibongTebal, Pulau Pinang, Malaysia

*Corresponding Author: Tel.: +604-5996463, Fax: +604-5941013; E-mail: [email protected]

Received 14 September 2012; Accepted 29 October 2012

Zinc removal was investigated by adsorption using a mixture of rice husk ash (RHA), palm oil fuel ash (PFA)

and coal fly ash (CFA). Sol-gel method was used in the preparation of RHA/PFA/CFA adsorbent. In this study,

the physical and chemical characterization of RHA/PFA/CFA adsorbent was investigated to obtain a better

understanding of zinc adsorption process. The particle size distribution of RHA/PFA/CFA adsorbent was found

to have a variation as a result of the reaction during the preparation and during treatment processes. The SEM

micrograph results revealed the structure of RHA/PFA/CFA adsorbent before and after the adsorption

processes. X-ray diffraction analysis indicated the formation of component phases in the sorbent and NaNO 3

became the major content in the RHA/PFA/CFA adsorbent. In addition, various specific surface functional

groups were formed in the RHA/PFA/CFA adsorbent before and after the adsorption processes. Thus, it is

concluded that the physical and chemical characteristics of RHA/PFA/CFA adsorbent affecting the adsorption of

zinc. Also, equilibrium data were adapted to adsorption isotherm and kinetic studies. The equilibrium data were

best described by Freundlich isotherm model. RL value which indicates the type of Langmuir isotherm is

favorable for RHA/PFA/CFA adsorbent. For adsorption kinetic, the adsorption kinetics obeys a pseudo-second-

order model.

Key words: Adsorption isotherms, Adsorption kinetics, Agriculture waste, Characterization, Sol-gel method

1. INTRODUCTION

Few decades ago, pollution towards water source isconsidering a norm. Due to rapid industrialization,heavy metals are among the most importantenvironmental pollutants. High concentration ofheavy metals in the environment can be harmful to

a variety of living species. Immoderate ingestion ofthese heavy metals by humans can cause cancer,accumulative poisoning, nervous system damageand ultimately death (Issabayeva et al., 2008).Many industrial activities such as leather tanning,electroplating, paint manufacturing, batterymanufacturing, steel fabrication and mining, etc,have produced heavy metal contaminants inaqueous waste streams (lvarez-Ayuso et al.,2003).

Among heavy metals, zinc (Zn2+) has receivedspecial attention due to the fact that zinc is

important for many biochemical processes, due toacute toxicity and non-biodegradabilitycharacteristics. Zinc containing solid and liquidwastes is considered as hazardous wastes, it can

have harmful impact when discharged into naturalenvironment at high concentrations (Hui et al.,2005). Various regulatory bodies have set themaximum prescribed limits for the discharge oftoxic heavy metals in the aquatic systems. WorldHealth Organization (WHO, 2011) has set aprovisional limit of 3 mg/l of zinc. Stipulated to

Malaysian standards, the threshold limit/standardhas been set as 1 mg/l of zinc-containing liquidfrom sewage and industrial effluents (DOEMalaysia, 1979).

Various methods are available to remove andisolate these heavy metals from water andwastewater such as ion-exchange, chemicalprecipitation, membrane filtration, adsorption,electrochemical treatment technologies, etc. (Fuand Wang, 2011). Adsorption is one of the safest,easiest and most cost-effective methods because itis widely used in effluent treatment processes

(Balkse and Baltaciolu, 1992). A number ofworkers have used different adsorbent systems,developed from various industrial waste materials,for the removal of heavy metals (Daneshvar et al.,


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Characterization of RHA /PFA/CFA Adsorbent and Its Equilibrium and Kinetic Studies for Zn2+ Removal

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2002; Dimitrova and Mehandgiev, 1998;Galiatsatou et al., 2002; Srivastava et al., 1989).There still exists a need to develop a low cost andefficient adsorbent for the removal of zinc fromwastewater.

The most recent research is using solid wastesas an adsorbent. In agricultural sector, Malaysia iswell known. Therefore, agricultural plants suchrice milling industry and palm oil industry havegenerated a huge amount of biomass wastesannually, i.e. rice husk ash (RHA) and palm oilfuel ash (PFA). In another hand, Malaysia alsoproduce voluminous coal fly ash (CFA) isgenerated from burning coal to generate electricity.Many studies have been utilized the RHA, PFAand CFA as a sorbent to remove pollutants,

however all those studies use one type of ash only(with or without modification) and most of theseashes before utilizing as a sorbent were convertedinto activated carbon. In addition, no work hasbeen carried out to combine these three ashesmaterials as a sorbent.

Therefore, the adsorption characteristics ofthese mixed waste-derived siliceous materials(RHA/PFA/CFA) towards removing heavy metalsare still unclear. Thus, in this study, physical andchemical characterization of RHA/PFA/CFAadsorbent was investigated to obtain a better

understanding about the morphology and surfaceproperties of adsorption process in removing heavymetals. By means of a variety of analyses (ParticleSize Distribution, SEM, XRD and FT-IR) it will be

shown that this synthesis variant allows affectingthe morphological properties of RHA/PFA/CFAadsorbent in such a way to promote promisingproperties for Zn2+ adsorption. Additionally,adsorption isotherms and adsorption kinetics werealso investigated.

2. MATERIALS AND METHODES

2.1. Materials

A mixture of three types of ashes, i.e. rice huskash, palm oil fuel ash and coal fly ash were used inthis study. These ashes were collected fromburning industrial fuels and used in the process ofsynthesizing adsorbent. The raw rice husk ash

(RHA) was supplied by Kilang Beras & MinyakSin Guan Hup Sdn. Bhd., Pulau Pinang, Malaysia.Palm oil fuel ash (PFA) was obtained directly fromUnited Oil Palm Mill, Pulau Pinang, Malaysia.While coals fly ash (CFA) was collected fromStesen Janakuasa Sultan Azlan Shah, Manjung,Perak, Malaysia. Prior to use, all three type ofashes were sieved to obtain less than 63m fineparticle size and oven dried overnight at 110 C.The chemical composition of these raw materials islisted in Table1 (Dahlan and Razali, 2012).

The stock solutions of zinc (synthetic

wastewater) for all experimental studies wereprepared by dissolving proper amount of each ofnoticed material in deionized water usinganalytical reagent grade ZnSO4.7H2O.

Table1: Chemical composition of raw materialsComposition Percentage (wt. %)

RHA PFA CFASiO2 63.31 34.00 31.00C 18.63 25.00 24.00K2O 2.23 2.90 0.68P2O5 0.58 2.00 0.12

CaO 0.43 5.00 6.60MgO 0.42 3.10 3.50Fe2O3 0.26 5.30 10.00SO3 0.20 0.26 0.33TiO2 0.09 0.25 0.48Cl2O 0.18 0.03 traceAl2O3 0.11 5.50 11.00Others 0.01 0.24 0.29LOI 13.54 16.42 12.00

2.2. Adsorbent Preparation

To prepare a combined adsorbent, initially eachtype of these three ashes, i.e. (RHA, PFA, andCFA) were mixed together in same weight ratio of1:1:1. The preparation of adsorbent was carried out

by modification of sol-gel method according toAdam and Chua (2004) and Adam et al. (2006).

About 15 g of each ash (RHA, PFA, CFA) weremixed and stirred in 100 ml of nitric acid (HNO365%) for 24 hr. Then, it was filtered and rinsedwith distilled water until the pH of the rinse


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became constant. The mixture was subsequentlydried in an oven at 110C for one day. About 15 gof the acidified RHA/PFA/CFA adsorbent weredissolved in 250 ml of 6M NaOH, stirred for 12 hr,and filtered to remove undissolved particles. Thefiltrant was titrated with 3M nitric acid whichcontained 10% (w/w) Al3+ [Al (NO3)3.9H2O inHNO3]. A black suspension was observed whenthe pH reached 11.5. Titration was carried out untilthe pH ~ 5. The RHA/PFA/CFA adsorbent wasthen aged for 6 days. The soft gel formed wasfiltered and dried at 110C for 24 hr.

2.3. Absorbent characterization

The RHA/PFA/CFA adsorbent were prepared from

a mixture of coal fly ash (CFA), palm oil fuel ash(PFA) and rice husk ash (RHA) using sol-gelmethod. These prepared sorbents were tested (inbatch experiment) to remove zinc (Zn2+) fromaqueous solutions (synthetic wastewater). Theconcentration of Zn2+ in aqueous solutions beforeand after adsorption was determined using a DR2500 spectrophotometer (Shimadzu, Japan).

Surface analysis of the RHA/PFA/CFAadsorbent was examined using Mastersizer 2000.BET specific surface area was analyzed usingAutosorb-1 Quantachrome analyzer. The scanning

electron microscopy (SEM) examinations wereperformed through Leo Supra 35 VP ScanningElectron Microscope. The analysis was carried outfor the selected RHA/PFA/CFA adsorbent beforeand after Zn2+ uptake to obtain the surfacemorphologies and to verify the presence ofporosity. X-ray diffraction (XRD) spectrum wasrecorded using X-ray diffractometer to investigatethe various phases present in the adsorbent. Inaddition, the surface functional groups of theadsorbent were detected using Fourier transforminfrared (FTIR) spectroscopy. The spectra were

recorded from 4000 to 400 cm1.

2.4. Adsorptions isotherm

Adsorption isotherms were performed in a set of 5Erlenmeyer flasks (250 ml), where solutions ofheavy metal (100 ml) with initial concentrations152 mg/l of Zn2+ were place in these flasks. Theoriginal pH (6) of the solutions was used. Differentmasses, i.e. (0.25-4 g) of RHA/PFA/CFAadsorbent were added to heavy metal solutions, themixtures were then kept in SK-600 horizontalshaker for 2 hr to reach equilibrium at 200 rpmshaking rate. In another hand, removal of Zn2+ ionwas observed for shaking speed varied from 50 to

250 rpm, therefore, the value of 200 rpm presentthe optimum shaking rate in this experiment. Theflasks were then removed from the shaker, and thefinal concentration of heavy metal in the solutionwas measured using DR 2500 spectrophotometer.The amount of adsorption at equilibrium time t, qe(mg/g), is calculated by

W

VCCq eoe

)((1)

where Co and Ce (mg/l) are the liquid-phaseconcentrations of Zn2+ at initial and equilibrium,respectively; Vthe volume of the solution (l); Wisthe mass of dry RHA/PFA/CFA adsorbent used(g). Adsorption isotherm study was conducted onthree isotherm models: the Langmuir, Freundlich,and Temkin isotherm models. The suitability of the

isotherm models to the adsorption study done wascompared by judging the correlation coefficients,R

2 values.

2.4.1. Langmuir isotherm

Langmuir isotherm takes for granted monolayeradsorption onto a surface containing a finitenumber of adsorption sites of uniform strategies ofadsorption with no transmigration of adsorbate inthe plane of surface (Langmuir, 1918). The linearform of Langmuir isotherm equation is given as:

o

e

oe

e

Q

C

bQq

C

1(2)

where Ce is the equilibrium concentration ofZn2+ (mg/l), qe is the amount of adsorbate adsorbedper unit mass of adsorbent (mg/g), Q0 and b areLangmuir constants related to adsorption capacityand rate of adsorption, respectively.

2.4.2. Freundlich isotherm

Freundlich isotherm in the other hand takes forgranted heterogeneous surface energies, in whichthe energy expression in Langmuir equation variesas a function of the surface coverage (Freundlich,1906). A linear form of the Freundlich expressionwill yield the constants KF and n is uttered as

ee Cn

Kq ln1

lnln F (3)

where Ce is the equilibrium concentration ofZn2+ (mg/l), qe is the amount of adsorbate adsorbedper unit mass of adsorbent (mg/g), KF and n areFreundlich constants.


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2.4.3. Temkin Isotherm

Temkin isotherm (Temkin and Pyzhev, 1940) isexpressed in its linear form by the followingequation:

ee CBKBq lnln 1T1 (4)

where Ce is the equilibrium concentration ofZn2+ (mg/l), qe is the amount of adsorbate adsorbedper unit mass of adsorbent (mg/g), KT and B1areTemkin constants.

2.5. Adsorptions kinetic

The procedures of kinetic experiments are

basically identical to those of equilibrium tests.The aqueous samples were taken at present timeintervals, i.e. (0.5-2.5 hr) and the concentrations ofheavy metal were similarly measured. The amountof adsorption at time t, qt(mg/g), is calculated by

W

VCCq tot

)((5)

where Co and Ct (mg/l) are the liquid-phaseconcentrations of Zn2+ at initial and any time t,

respectively; Vthe volume of the solution (l); Wisthe mass of dry RHA/PFA/CFA adsorbent used(g). Simplified kinetic models were selected tostudy the mechanism of the adsorption process.Foremost, the kinetics of adsorption was examinedby the pseudo-first-order equation given(Lagergren, 1898) as:

tk

qqq ete303.2

log)log( 1 (6)

where qe and qt are the amounts of Zn2+adsorbed (mg/g) at equilibrium and at time t (hr),respectively, and k1 (1/hr) is the rate constantadsorption. Additionally, the pseudo-second-orderequation based on equilibrium adsorption (Ho andMcKay, 1999) is expressed as:

eet q

t

qkq

t

2

2

1(7)

Where k2 (g/mg hr) is the rate constant ofsecond-order adsorption.

3. RESULT AND DISCUSSION

3.1. Adsorbent efficiency

Based on the adsorption studies (Dahlan andZwain, 2012), the adsorption efficiency for Zn2+was 96-98%. Based on the results obtained, it wasshowed a high percentage of Zn2+ removal.Optimum amount of adsorbent and shaking rate

were selected from previous study (Dahlan andZwain, 2012) to ensure that all of the adsorbentmix together accordingly. In this study, sol-gelmethod involves a series of complicated steps andusing with more than one chemical component.Observation on steps used in the experimentprocess, sol gel method showed high adsorptionefficiency. Adsorbent preparation involvestreatment with concentrated HNO3 which helps toreduce the impurities and leach out the metaloxides by forming nitrates which were easilydissolved in water. Second, sodium silicate

solution was produced dissolving in NaOH. Silicagel with the metal ion chemically incorporated intothe silica matrix was produced by neutralization ofsodium silicate solution using HNO3 containing themetal to produce.

3.2. Adsorbent characteristics

3.2.1. Particle Size Distribution

Selected RHA/PFA/CFA adsorbent wascharacterized for particle size distribution before

and after Zn+2 removals. In this analysis, twosamples of prepared and spent RHA/PFA/CFAadsorbent were chosen to undergo the analysis.The particle size distribution of the selectedRHA/PFA/CFA adsorbent was examined and theresults are shown in Fig.1.


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Fig. 1: Particle size distribution of prepared and spent RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio in removing ofzinc (Zn2+).

The prepared RHA/PFA/CFA adsorbent, aswell as the spent sorbents was shown to havebimodal particle size distribution (Fig.1). Thebimodal particle size distribution of the preparedRHA/PFA/CFA adsorbent might be resulted froma process involving breakup of multiple sources ofparticles or variable growth mechanisms during thepreparation method.

3.2.2. Surface morphology

SEM was used to observe the surface morphologyof the prepared and spent RHA/PFA/CFAadsorbent. Using this analysis, the structural of theparticles made up of the sorbent was clearlyobserved. In this analysis, two samples werechosen, one for prepared sorbent and one for spent

sorbent after Zn+2

removal. Figs.2 and Fig.3 showthe surface morphology of the (selected) preparedand spent RHA/PFA/CFA adsorbent respectively.

Fig. 2: Scanning electron micrograph of prepared RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.1 1 10 100 1000 10000

Volume%

Particle Size (m)

Prepared adsorbent

Spent adsorbent


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Fig. 3: Scanning electron micrograph of spent RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio in removing of zinc(Zn2+).

Based on the observation of Fig.2, the surfaceof RHA/PFA/CFA prepared adsorbent has anirregular-shaped particle that unevenly scattered.After the adsorption of Zn2+, the sorbent was foundto have more compact structures covered by roughirregular-shaped particles on the surface and insidethe sorbent. On the other observation of spentRHA/PFA/CFA adsorbent, it was observed fromFig.3 that the surface of adsorbent particle wascovered by a layer like structures which is mostprobably the zinc ions that covered the externalsurface of sorbent.

3.2.3. X-ray diffraction

X-ray diffraction studies were carried out foradsorbent before and after treatment of Zn2+removal to investigate the various phases presentin the sorbent. As it can be seen from Fig.4, thespectra of adsorbent before Zn2+ removal indicatethe presence of Sodium Nitrate NaNO3 as majorconstituents and quartz low SiO2 at 2=26.8 and29.5 respectively. As raw materials the majorconstituent was SiO2, but due to preparation ofadsorbent using sol gel method the Sodium NitrateNaNO3 became the major content and its maybeone of the more efficient constituents for Zn2+removal.

Fig. 4: X-ray diffraction of prepared RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio.

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50 60 70 80 90 100

Lin(Counts)

2-Theta-Scale


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Although, X-ray diffraction (XRD) wasemployed to investigate the Zn2+ contain inRHA/PFA/CFA spent adsorbent. Due to a smallquantity of adsorbed Zn2+ to the RHA/PFA/CFAadsorbent, only a small peaks of these crystalphases was detected by XRD analysis. Fig.5indicates the presence of silica as major

constituents with iron, calcium, nitrite, aluminumand sodium as minor constituents. As a result ofZn2+ adsorption process, zinc constituent was foundcombined with many metal oxide (as trace orminor constituents) like zinc iron oxide, zincaluminum iron oxide and zinc oxide.

Fig. 5: Spent RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio in removing of zinc (Zn2+).

3.2.4. Surface functional groups

The FT-IR spectrum (Fig.6) of prepared and spentsorbents was used to identify functional groupspresent on the RHA/PFA/CFA adsorbent thatcould be responsible for uptake of Zn2+. Thespectrum of the sorbent was measured within therange of 4000-400/cm wave number. FTIR spectrafor both the sorbents show five main broad bandregions around 408-465, 794-833, 1053, 1384-1419 and 3436-3467 cm1. The absorption peak

around 3450 cm1

indicates the existence of O-Hgroups on the adsorbent surface due to OHstretches (Abou-Mesalam, 2003). The 13601420cm1 bands may be attributed to the aromatic CHand carboxyl-carbonate structures (Ricordel et al.,2001). Meanwhile an intensive peak around 1050cm1 is assigned to Si-O-C stretch (Stuart, 2004).The peaks observed at 794-833 cm-1 can beassigned to C-H group. The weak bands around465 and 408 cm1 can be attributed to theoctahedral SiOAl bending stretching (Hu, 2007;Pabn et al., 2004).

In contrast, after the sorbent was treated withZn2+ removal, the peak 1384 cm1 show noticeable

changes. The vibration peaks of CH groupsbecome weaker, which indicated the amount ofalkenes decreased and could be indicator of theoccurrence of adsorption process. Furthermore, thepeak at 1053 cm1 became sharper due to increaseof Si-O-C stretch.

3.3. Adsorption isotherms

The adsorption isotherm shows how the adsorptionmolecules distribute between the solid phase and

the liquid phase when the adsorption processreaches an equilibrium state. The analysis of theisotherm data by preparing them to variousisotherm models is an important step to find theappropriate model that can be used for designpurposes (El-Geundi, 1991). Adsorption isothermis essentially significant to describe how solutesinteract with adsorbent, and is critical inoptimizing the use of adsorbent.

The Langmuirs constants b and Q0 werecalculated from (Eq.2) and their values are shownin Table2.Ce/qe was plotted versus Ce, a straight

line with slope of 1/Q0 was obtained, as shown inFig.7.

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50 60 70 80 90 100

Lin

(Counts)

2-Theta-Scale


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Fig. 6: FTIR image for Prepared and Spent RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio in removing of zinc(Zn2+).

Fig. 7: Langmuir plot for the adsorption of Zn2+ by RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio: pH 6, adsorbentdosage 2.5-10 g/l, contact time 2 hr and 200 rpm.

The necessary characteristics of the Langmuirisotherm can be expressed in terms of adimensionless equilibrium parameter (RL) (Webiand Chakravort, 1974), which is defined by:

bCR

o

1

1L

(8)

where b is the Langmuir constant and C0 is thehighest Zn2+ concentration (mg/l). The value ofRLindicates the form of the isotherm to be either

favorable (0 1), linear(RL = 1) or irreversible (RL = 0). The value ofRLwas found to be 0.146, which indicates favorableadsorption for adsorption of Zn

2+ onto theRHA/PFA/CFA adsorbent under the conditionsused in this study.

As shown in (Eq.3), the Freundlich constants n

and KF were calculated. The value of n giving anindication of how favorable the adsorption processand KF (mg/g (l/mg)1/n) is the adsorption capacity

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0.24

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Ce/qe(g/L)

Ce (mg/L)


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of the adsorbent. The slope of 1/n ranging between0 and 1 is a measure of adsorption surface intensityor heterogeneity, becoming more intensity as itsvalue gets closer to one (Haghseresht and Lu,1998).

A value for 1/n below one shows a normalLangmuir isotherm while 1/n above one is showingof cooperative adsorption (Fytianos et al., 2000).KF can be explained as the adsorption ordistribution coefficient and represents the quantity

of Zn2+ adsorbed onto RHA/PFA/CFA adsorbentfor a unit equilibrium concentration. The plot oflnqe against lnCe gave a straight line with slope of1/n with value of 0.883 (Fig.8), indicating a normalLangmuir isotherm. The R2 value of 0.994indicated that the adsorption data of Zn2+ removalfrom synthetic wastewater were best fitted to theFreundlich isotherm model. The constants KF and nwere also calculated and are listed in Table2.

Fig. 8: Freundlich plot for the adsorption of Zn2+ by RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio: pH 6, adsorbentdosage 2.5-10 g/l, contact time 2 hr and 200 rpm.

Another equation (Eq.4) used in the analysis ofisotherms was proposed by Temkin. Temkin tookinto account the effects of indirectadsorbate/adsorbate interactions on adsorptionisotherms. The heat of adsorption of all themolecules in the layer would decrease linearly with

coverage due to adsorbate/adsorbate interactions(Temkin and Pyzhev, 1940).The Temkin constantsKt and B1 were calculated and their values areshown together with the R2 values in Table2. Aplot of qe versus lnCe yielded a linear line, asshown in Fig.9.

Fig. 9: Temkin plot for the adsorption of Zn2+ by RHA/PFA/CFA adsorbent with 1:1:1 mixing ratio: pH 6, adsorbentdosage 2.5-10 g/l, contact time 2 hr and 200 rpm.

-1.5

-1

-0.5

0

0.5

1

1.5

2

-3.00 -2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50lnqe

ln Ce

-1

0

1

2

3

4

5

6

-3.00 -2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50

qemg/g

ln Ce


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Table 2: Comparison between the adsorption rate constants, Langmuir, Freundlich and Temkin isotherm modelconstants associated with isotherm model equations

Langmuirisotherm

Q0 (mg/g) b (l/mg) R2 RL

16.95 0.39 0.923 0.146Freundlich

isotherm

KF(mg/g (l/mg)1/n) n R2

4.92 1.13 0.994Temkinisotherm

B1 Kt R2

1.65 15.2 0.911

3.4. Adsorption kinetics

Understanding the mechanism of metal ionsinteraction is essential to the control of theadsorption process. Several researchers havedescribed the reaction order of solute sorption ontoa sorbent using various kinetic models. Therefore,implementing a model can predict and describe

sorption kinetics for any value of the initialconcentration. In another hand, the kinetics ofmetal ions sorption is a significant parameter for

designing sorption systems and its essential forselecting the optimum operating conditions forfull-scale batch metal removal process (Liu et al.,2009).

In order to examine the mechanism of sorption,characteristic constants of sorption weredetermined using pseudo-first order equation(Eq.6) and pseudo-second order equation (Eq.7).

To conduct the kinetics of adsorption by pseudo-first-order equation, the value ofk1 was calculatedfrom the plots of log (qe-qt) versus t(Fig.10).

Fig. 10: Pseudo-First-Order reaction plot forthe adsorption of Zn2+ by RHA/PFA/CFA adsorbent with 1:1:1 mixing

ratio: pH 6, adsorbent dosage 10 g/l, contact time 0.5-2.5 hr and 200 rpm.

The R2 values obtained were relatively smalland the experimental qe values did not agree withthe calculated values obtained from the linear plots(Table3). But then, the kinetics of adsorption bypseudo-second-order equation was calculated bythe linear plot oft/qtversus t, as shown in (Fig.11),yielded R2 values that were 0.999 showed a goodagreement between the experimental and thecalculated qe values, indicating the applicability ofthis model to describe the adsorption process of

Zn2+ ions onto the prepared RHA/PFA/CFAadsorbent.

The parameters of kinetic studies, i.e. k1, k2 arecalculated and shown in Table3. Pseudo-first ordermodel was not as efficient as the pseudo-secondorder model in describing the kinetic data, with thecorrelation coefficients obtained lower than thoseobtained from the pseudo-second order model.Based on the results of correlation coefficients, atinitial concentrations 15 mg/l of Zn2+, theadsorption kinetics obeys a pseudo-second-ordermodel.

-2.5

-2

-1.5

-1

-0.5

0

0 0.5 1 1.5 2 2.5

log(q

e-qt)

Time (hr)


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Fig. 11: Pseudo-Second-Order reaction plot for the adsorption of Zn2+ by RHA/PFA/CFA adsorbent with 1:1:1 mixingratio: pH 6, adsorbent dosage 10 g/l, contact time 0.5-2.5 hr and 200 rpm.

Table 3: Comparison between the adsorption rate constants, qe, estimated and correlation coefficients associated withpseudo-first-order and to the pseudo-second order rate equations

pseudo-first-order rate equation pseudo-second-order rate equationK1 (1/hr) qe (mg/g) R K2 (g/mg hr) qe (mg/g) R1.07 0.06 0.955 52.03 1.46 0.999

4. CONCLUTION

This investigation study showed thatRHA/PFA/CFA adsorbent was effective precursor

in removing Zn2+ from aqueous solutions. Theparticle size distribution of RHA/PFA/CFAadsorbent was found to have a variation as a resultof the reaction during the preparation and duringtreatment processes. The SEM micrograph resultsrevealed the structure of RHA/PFA/CFA adsorbentbefore and after the adsorption processes. X-raydiffraction analysis indicated the formation ofcomponent phases in the sorbent and NaNO3became the major content in the RHA/PFA/CFAadsorbent. In addition, various specific surfacefunctional groups were formed in theRHA/PFA/CFA adsorbent before and after theadsorption processes. Through this analysis, itshowed that the adsorption process was affected bythe physical and chemical characteristics ofRHA/PFA/CFA adsorbent.

In another hand, equilibrium data were adaptedto adsorption isotherm and kinetic studies. Theequilibrium data were best described by Freundlichisotherm model, with maximum monolayeradsorption capacity of 16.95 mg/g. RL value whichindicates the type of Langmuir isotherm is

favorable for RHA/PFA/CFA adsorbent. Foradsorption kinetic, based on the results ofcorrelation coefficients, at initial concentrations 15

mg/l, the adsorption kinetics obeys a pseudo-second-order model.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the financialsupport from the Universiti Sains Malaysia (ShortTerm Grant A/C. 60310014).

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Characterization of RHA PFACFA Adsorbent and Its Equilibrium and Kinetic Studies for Zn2+ Removal

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