Kd

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Chemical Engineering Journal 179 (2012) 193– 202

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

inetic and thermodynamics of chromium ions adsorption onto low-costolomite adsorbent

hmad B. Albadarina,∗, Chirangano Mangwandia, Ala’a H. Al-Muhtasebb,c, Gavin M. Walkera,tephen J. Allena, Mohammad N.M. Ahmada,b

School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, Northern Ireland, UKPetroleum and Chemical Engineering Department, Faculty of Engineering, Sultan Qaboos University, P.O. Box 33, Al-Khod, OmanDepartment of Chemical Engineering, Faculty of Engineering, Al-Hussein Bin Talal University, Ma’an, Jordan

r t i c l e i n f o

rticle history:eceived 27 June 2011eceived in revised form 24 October 2011ccepted 24 October 2011

eywords:olomitedsorption

a b s t r a c t

The chromium bearing wastewater in this study was used to simulate the low concentration dischargefrom a major aerospace manufacturing facility in the UK. Removal of chromium ions from aqueoussolutions using raw dolomite was achieved using batch adsorption experiments. The effect of; initialCr(VI) concentration, amount of adsorbent, solution temperature, dolomite particle size and shakingspeed was studied. Maximum chromium removal was found at pH 2.0. A kinetic study yielded an opti-mum equilibrium time of 96 h with an adsorbent dose of 1 g/L. Sorption studies were conducted overa concentration range of 5–50 mg/L. Cr(VI) removal decreased with an increase in temperature (qmax:

◦ ◦ ◦ ◦

exavalent chromiuminetic studyhermodynamic parameters

20 C = 10.01 mg/g; 30 C = 8.385 mg/g; 40 C = 6.654 mg/g; and 60 C = 5.669 mg/g). Results suggest thatthe equilibrium adsorption was described by the Freundlich model. The kinetic processes of Cr(VI) adsorp-tion onto dolomite were described in order to provide a more clear interpretation of the adsorption rateand uptake mechanism. The overall kinetic data was acceptably explained by a pseudo first-order ratemodel. Evaluated �Go and �Ho specify the spontaneous and exothermic nature of the reaction. Theadsorption takes place with a decrease in entropy (�So is negative).

. Introduction

Chromium is one of the heavy metals present in effluentsroduced from the aerospace, electroplating, leather, mining, dye-

ng, fertilizer and photography industries [1,2]. Cr(VI) exists asxtremely soluble and highly toxic chromate ions (HCrO4

− orr2O7

2−) which can transfer freely in aqueous environments [3].ersistent exposure to Cr(VI) causes cancer in the digestive tractnd lungs, and may cause other health problems for instancekin dermatitis, bronchitis, perforation of the nasal septum, severeiarrhea, and haemorrhaging [4,5]. Whereas, Cr(III), ions are com-aratively stable, have low solubility and mobility in soils andquifers, and are usually considered as a much less dangerousollutant [6,7]. The rate of Cr(VI) reduction into Cr(III) in contam-

nated soil is highly important due to the remarkable differencesn the physical and chemical characteristics of these two typesnd benign nature of Cr(III) [4]. The discharge of Cr(VI) ions on

urface water is regulated to below 0.05 mg/L, whereas that ofotal Cr (Cr(VI) and Cr(III)) is regulated at less than 2 mg/L [8,9].or that reason, the amount of chromium in effluents must be

∗ Corresponding author. Tel.: +44 0 7787540865.E-mail address: aalbadarin01@qub.ac.uk (A.B. Albadarin).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.10.080

© 2011 Elsevier B.V. All rights reserved.

reduced to an acceptable limit before releasing them into streamsand rivers to protect human health and the environment [7].Several methods are utilized to remove Cr(VI) from the indus-trial wastewater. Conventional separation techniques for instance;electrochemical precipitation, slow sand filtration, ion exchange,reverse osmosis and solvent extraction have many disadvantagesfor example; the high cost, possible production of secondary toxiccompounds and the generation of sludge leading to high disposalcosts [10,11]. Adsorption separation has been widely used in envi-ronmental chemistry, owing to its relatively low cost, simplicityof design/operation and pollutant removal to low concentrations[12]. The benefits of adsorption as a treatment step has led to thedevelopment of other materials with properties and characteris-tics which may demonstrate a high capacity for chromium. Thus, anumber of low cost and readily available raw materials have beeninvestigated as potential adsorbents for the removal of Cr(VI) fromwastewater [13]. Dolomite has been a subject of interest for oversix decades [14]. Dolomite material is available very cheaply andin abundance around the world (important occurrences are foundin India, Indonesia, Turkey and China) [15]. Dolomite which has

similar properties to that of limestone, is sometimes known asmagnesium-limestone in industry [16]. Its crystal structure con-sists of alternative layers of magnesium and calcium carbonate. Itis therefore considered to be a concentrated source of chemical

1 ginee

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2

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94 A.B. Albadarin et al. / Chemical En

gO and magnesium ions, especially for agricultural and pharma-eutical applications. It is also used in metallurgy for extractingertain metals from their ores. Dolomite has received attentionesulting in several studies considering its various applicationsor water treatment due to its ability to absorb certain toxic sub-tances [15,17–20]. The sorption of cations onto dolomite takeslace mainly via a physical adsorption mechanism but can also takelace by surface precipitation or ion exchange between the mag-esium and calcium ions within the dolomite and metal cations inqueous solutions [21]. Research conducted by our group [22,23]n the treatment of hazardous shipyard waste and dyes usingolomitic sorbents, found that a 98% reduction in COD level in thehipyard effluent could be achieved. However, the physiochemi-al mechanisms have been attributed to a combination of surfacehemical reaction and surface precipitation. There is limited datavailable on Cr(VI) removal using dolomite.

The purpose of this investigation therefore is to obtain an under-tanding of the equilibrium adsorption of Cr(VI) on dolomite bytudying the influence of different experimental parameters onhe adsorption process. Moreover, the Langmuir, Freundlich andubinin–Radushkevich isotherm equations will be used to study

he equilibrium data. Experimental kinetic data will be investigatedsing the pseudo first-order, pseudo second-order, intraparticleiffusion and Boyd adsorption kinetic equations.

. Experimental and analytical methods

.1. Dolomite adsorbent

Dolomite employed in this study was extracted from a deposit ino. Fermanagh, Northern Ireland. Dolomite is typically composedf 44% MgCO3 and 53% CaCO3 (impurities are mostly SiO2 and iron)23]. The surface area of the dolomite was measured by N2-BET

ethod technique (NOVA Gas adsorption Analyser, Quantachrometd., UK). Also, the t-method was used for the determination of thexternal surface area and volume distributions for microporousaterial [24]. Morphology in addition to the structural ordering

f the dolomite was analyzed by energy dispersive X-ray anal-sis (EDAX) and scanning electron microscopic (SEM), using aEOL-JSM 6400 scanning microscope. The surface structure of thedsorbent was explored with Fourier Transform Infrared Spectro-copic (FT-IR) to identify alterations on the dolomite surface beforend after adsorption (using a KBr disc technique in the range of00–4000 cm−1).

.2. Adsorption experiments

With the aim of assessing the effect of pH on the adsorptionapacity of the dolomite, batch studies were carried on each of 7H values between 2 and 12. Also, the effect of dolomite dose onemoval efficiency was studied and various amounts of dolomiteere added to different Cr(VI) solutions with initial concentra-

ion 50 mg/L. For isotherm studies, 0.05 g adsorbent was addedo 50 mL of Cr(VI) solution with altering Cr(VI) initial concentra-ion (5–50 mg/L) in 100 cm3 screw-cap conical flasks at a choice ofemperatures (20–60 ◦C) and kept in a water bath/shaker (with:H 2; time = 4 days). After equilibrium was reached, the sam-le solutions were filtered through a 0.45 �m membrane filter.hromium concentrations were measured according to standardethodology demonstrated by Gilcreas et al. [25] using a UV/Vis

pectrophotometer (Perkin Elmer LAMBDA 25, UK) (at wavelength,

= 540 nm).

For the kinetics investigations, the mixtures were shaken at aate of 100 rpm to ensure good mixing. At timed intervals, sam-les (1 mL) were withdrawn. This small sample volume guarantees

ring Journal 179 (2012) 193– 202

that the reactor volume stays essentially constant throughout thecourse of the experiment. The pH of the experimental solutionswas adjusted by adding 1 M NaOH or HNO3 using a micro-pipetteas required. Duplicate samples were considered and standard errorwas less than 5%. The effect of initial Cr(VI) concentration, particlesize, and speed of shaking was studied. After equilibrium, the con-centrations of Cr(VI) in the equilibrium solution was determined.The amount of Cr(VI) per unit mass of adsorbent q in (mg/g) wascalculated using Eq. (1):

q =[

C0 − Ce

M

]× V (1)

where Co and Ce (mg/L) are the concentration of Cr(VI) at initial andequilibrium, respectively, M is the mass of dolomite used (g) and Vis the volume of the solution (L).

Subsequent to the adsorption experiment, the dolomite wasseparated by filtration and washed with deionised water to removeany remaining Cr(VI) on the surface. After that, dolomite was con-tacted with 25 mL of desorbent solutions: deionised water, 0.1 MKOH and NaOH, 0.1 M H2SO4 and HNO3 and 5% HCHO. Desorptionpercentage can be calculated using the equation:

Desorption (%) = amount of Cr(VI) desorbedamount of Cr(VI) adsorbed

× 100

2.3. Kinetic and equilibrium studies

The kinetics equation proposed by Lagergren has been used todescribe the adsorption of an adsorbate from an aqueous solution[26]. The pseudo first-order model is described by the followingequation (Eq. (2)) [27]:

q = qe(1 − e−k1t) (2)

where k1 (1/h) is the adsorption rate constant of pseudo first-order.The pseudo second-order kinetic model (Eq. (3)) [28] is given

as:

q = q2e k2t

1 + qek2t(3)

where k2 (g/mg h) is the second order adsorption rate constant.Intraparticle diffusion, often the rate controlling step, is the

possible method of movement of the ions from solution into theadsorbent [29]. To determine possible mechanisms and the effectof rate controlling steps on the process, the results were fitted tothe Weber intraparticle diffusion model [30] and Boyd [31] model.The Weber intraparticle diffusion model:

q = kdifft0.5 (4)

where kdiff (mg/g min0.5) is the intraparticle diffusion rate constantdetermined from the slope of the plot q versus t0.5. The Boyd modelprovides information as to whether the rate controlling step resultsfrom film diffusion (boundary layer), or particle diffusion (diffusioninside the pores) [32].

The film diffusion model of Boyd is given as:

F = qt

qe= 1 − 6

�2

∞∑n=1

1n2

exp(−n2Bt) (5)

where F is the fractional achievement of equilibrium, at alteredtimes, t and Bt is a mathematical function of F. Eq. (5) can be rear-ranged and simplified to:

Bt = −0.4977 − ln(1 − F) (6)

The value of Bt is calculated for each qt value and then plot-ted against t. From this plot, it is achievable to find out whetherintraparticle diffusion or external transport control the rate of

A.B. Albadarin et al. / Chemical Engineering Journal 179 (2012) 193– 202 195

Table 1Characteristics of dolomite.

Measurement method

BET method t-Method

Total surface area (m2/g) 4.63 Total surface area (m2/g) 6.35

aidpd

B

ws

uiai

q

wacd

R

TRv

q

wc

sW

2

icb(

TPc

W

Solid phase density (kg/m3) 1281

Total pore volume (cm3/g) 0.0064

Pore radius (Å) 15.97

dsorption. The rate controlling step is pore diffusion if this plots linear and passes through the origin. Otherwise surface (film)iffusion is the rate controlling step [33]. The slopes found fromiecewise linear regressions (B) were utilized to obtain the effectiveiffusion coefficient, Di (cm2/s) using the equation:

= �2Di

r2(7)

here r is the radius of the adsorbent particle assuming a sphericalhape.

The equilibrium data of chromium adsorption were exploredsing the isotherm models of Langmuir, and Freundlich isotherms,

n order to determine the correlation between solid phase andqueous concentrations at equilibrium. The Langmuir adsorptionsotherm is:

e = qmaxbCe

1 + bCe(8)

here qmax (mg/g) and b (L/mg) are Langmuir constants associ-ted with the capacity and energy of adsorption. The fundamentalharacteristics of the Langmuir isotherm can be expressed byimensionless separation factor, RL, defined by:

L = 11 + (qmax × b)Co

(9)

he RL parameter indicates the shape of the isotherm as follows:L > 1, unfavourable; RL = 1, linear; 0 < RL < 1, favourable; RL = 0, irre-ersible.

The Freundlich equation is:

e = KF C1/ne (10)

here KF and 1/n are the Freundlich model constants related to theapacity and intensity of the adsorption respectively.

The model parameters were evaluated by non-linear regres-ion analysis using POLYMATH® 5.1 software (Polymath Software,

illimantic, CT, USA).

.4. Thermodynamics

The original concepts of thermodynamics assumed that in an

solated system, where energy cannot be gained or lost, the entropyhange is the driving force [34]. The heat of adsorption of the adsor-ents, �Ho (kJ/mol), the free energy �Go (kJ/mol) and entropy �So

kJ/mol K) for the adsorption process can be calculated by fitting

able 2seudo first-order, pseudo second-order and intraparticle diffusion model constants and

oncentrations.

Co (mg/L) qe,exp Pseudo first-order model

qe,cal k1 R2

10 3.661 3.570 6.04 0.989

20 5.920 6.453 4.70 0.998

30 6.882 7.237 3.56 0.996

40 8.833 9.391 3.29 0.995

50 9.982 10.11 4.52 0.994

here Co , mg/L; qe , mg/g; k1, 1/h (×102); k2, g/mg h (102); kdiff , mg/g h1/2.

External surface area (m2/g) 0.71Micropore volume 0.0026

the Langmuir constant, b, to the Van Hoff equation using the fol-lowing Eqs. (11)–(13) [35]. The b constant is simply recalculated asdimensionless by multiplying it by 55.5 (no. moles water):

�Go = −RT ln(55.5)b (11)

where R is the gas constant (8.314 J mol−1 K−1) and T is the temper-ature.

The relationship between �Go, �Ho and �So is expressed bythe following equation:

�Go = �Ho − T�So (12)

For the determination of �Ho and �So the equation above canbe written as:

ln b = �So

R− �Ho

RT(13)

From the Van’t Hoff plot between ln b versus 1/T the valuesof �Ho and �So can be determined from the slope and interceptrespectively.

3. Results and discussion

3.1. Dolomite characterisation

The dolomite was ground, sieved and then used directly foradsorption experiments without pre-treatment. The solid phasedensity, total pore volume and the specific surface area of dolomitemeasured by the N2-BET method, are presented in Table 1. Thetotal surface area of dolomite used in this study was higher thanthose reported in other studies [23,36,37]. Comparable resultsfor the total pore volume and average pore were reported byStaszczuk et al. [37], for dolomite originating from Otdrzychowice-Romanowo deposits (Poland). Typically, conventional porous solidshave low adsorption capacities and act in a kinetically slow man-ner [38]. However, on the basis of results in Table 1, the dolomiteadsorbent is expected to be a useful material for the removal ofmetal ions.

3.2. Kinetic studies

3.2.1. Effect of contact time and initial concentration

The plots of qt versus t at different initial Cr(VI) concentrations at

pH 2.0 are shown in Fig. 1A. It has been found that Cr(VI) adsorptionrate was relatively fast for the first 30 h with the plateau occur-ring at approximately 70 h. After this time the amount of adsorbed

correlation coefficients for Cr(VI) adsorption onto dolomite at various initial Cr(VI)

Pseudo second-order model Intraparticle diffusion model

qe,cal k2 R2 kdiff R2

4.375 1.45 0.985 0.402 0.8999.113 0.23 0.987 0.723 0.9879.724 0.31 0.990 0.845 0.96212.71 0.32 0.991 1.074 0.97312.93 0.34 0.990 1.159 0.951

196 A.B. Albadarin et al. / Chemical Enginee

Fig. 1. Effect of contact time on Cr(VI) adsorption on dolomite; (A) (- - -) fitting ofpseudo first-order model, (B) (–) fitting of pseudo second-order model and (C) thefractional approach to equilibrium with time. Experimental conditions: Initial Cr(VI)concentration: (10–50 mg/L); dolomite ratio: 1.0 g/L; shaking speed: 100 rpm; andtemperature: 22 ◦C.

ring Journal 179 (2012) 193– 202

Cr(VI) ions onto dolomite continued to increase slightly. The datareveal that the adsorption uptake q (mg/g) increases with increasedinitial concentration and the adsorption rate declines and reachesthe equilibrium position over time. This may be attributed to ahigher chance of collision between Cr(VI) ions and the adsorbentsurface and a better driving force, which lowers the mass trans-fer resistance [39]. The long time required to establish equilibriummay indicate that the adsorption process is governed by diffusioncontrolled physical adsorption [40].

The adsorption rate was quite fast at low initial concentration,on the other hand, it can be seen that the adsorption is slow at ini-tial concentrations higher than 10 mg/L, especially at time periods20–60 h. Slow diffusion regions which are associated with pores,are of a comparable size to the diffusing molecules and in whichdiffusion is retarded.

The values of constants of kinetic models obtained from the plotsfor adsorption of Cr(VI) on to dolomite at 20 ◦C are shown in Table 2.The data showed good agreement with both the pseudo first andsecond-order kinetic models (R2 > 0.985). However, the values ofthe determination coefficient (R2) indicate the applicability of thepseudo first-order model for describing the experimental results toa higher degree of accuracy (R2 > 0.989) for all studied chromiumconcentrations. In addition, Fig. 1A and Table 2, show that the qvalues (qe,cal) determined from the pseudo first-order model werecloser to the experimental q values (qe,exp) than those determinedfrom the pseudo second-order model. From Table 2, it was foundthat k1, the rate constant, declines from 6.04 to 3.29 × 10−2 (1/h)when the Cr(VI) initial concentration increases from 10 to 40 mg/L.This observation is corroborated by examining the plots in Fig. 1C.Although, further increase in initial concentration to 50 mg/L causesk1 to reverse the trend and increase to 4.52 × 10−2 (1/h).

The nonlinear relations between the initial Cr(VI) concentra-tion and the rate constant indicates that mechanisms such as; ionexchange, chelation and physical adsorption are involved in theadsorption process. However, a larger k1 value suggest that adsorp-tion systems with low concentrations will required a shorter timeto achieve a specific fractional uptake [39]. The sorption rate con-stant k2 and the equilibrium sorption capacity, qe, of the pseudosecond-order model are given in Table 2. Fig. 1B shows the pseudosecond-order model for the Cr(VI) adsorption onto dolomite. Itwas found that (k2) the rate constant of the pseudo second-orderdeclines gradually with an increase in the Cr(VI) initial value, thiscan be due to competition between higher levels of Cr(VI) ions forthe adsorbent active sites. Values of determination coefficient R2

are higher than 0.985, suggesting that the model gives a good fit.Hameed and El-Khaiary [39] proposed that there is uncertainty inrelation to how the process variables make one model describe anadsorption process more favourably compared to others. Khezamiand Capart [41] suggest that process kinetics are affected by theproperties of the adsorbent. They also report that the pseudo kineticmodels should be considered as empirical equations that do notprovide an accurate picture of the chemical and physical processeswhich are taking place.

Nevertheless, the qe versus t plots do not provide a clear indica-tion regarding how close the adsorption system at time t is towardsits equilibrium state. Therefore, the temporal approach to equilib-rium can be presented by a plot of the fractional uptake f againsttime t, where f = q/qe [39]. Fig. 1C illustrates that the time requiredto attain equilibrium increases with increased initial Cr(VI) con-centration. It illustrates that the fractional uptake f declines withincreased initial Cr(VI) concentration as well.

3.2.2. Adsorption mechanismThe intraparticle diffusion coefficient for the sorption of Cr(VI)

onto dolomite was obtained from the slope of the plot between q(mg/g), the amount of Cr(VI) sorbed, and t0.5 (h0.5) (Fig. 2). For metal

A.B. Albadarin et al. / Chemical Engineering Journal 179 (2012) 193– 202 197

0

2

4

6

8

10

108.575.542.51

q t (m

g/g)

t 0.5

10 mg/L 20 mg/L 30 mg/L

40 mg/L 50 mg/L

I

I Film diffusionII Intraparticle diffusionIII Binding on active sites

II

III

Fi

iimmbwfibsotafiTicmctl(fisd(elaifio

3

pu2tbbaob

-0.5

0

0.5

1

1.5

2

2.5

3

100806040200

Bt

10 mg/L 20 mg/L 30 mg/L

40 mg/L 50 mg/L

demonstrates the pseudo first and second-order, and intraparticlediffusion kinetic models at various shaking speeds. In Table 4 it canbe also seen that at all stirring rates, the pseudo first-order modeldid exhibit a good fit with experimental data (R2 > 0.995).

0

2

4

6

8

10

12

q t (m

g/g)

100 rpm 150 rpm 200 rpm

ig. 2. Intraparticle diffusion plots for Cr(VI) adsorption onto dolomite at differentnitial concentrations.

on adsorption from aqueous phase to solid phase, the ions transfers a multi-step process which typically consists of three types of

echanisms as follows: bulk diffusion, which involves the move-ent of metal ions from aqueous phase through the hydrodynamic

oundary layer film of the solid and then intra-particle diffusion,here the metal ions move through the interior solid surface andnally, adsorption at the active sites. The third step is considered toe very rapid and therefore it cannot said to be the rate controllingtep. Thus, the overall rate of adsorption is controlled by either filmr intraparticle diffusion, or a mixture of both. Fig. 2, illustrateshe pore diffusion plot of the adsorption of Cr(VI) onto dolomitet room temperature. The values of kdiff, and the correlation coef-cient R2 obtained from intraparticle diffusion plots are given inable 2. In general, kdiff was found to increase while increasing thenitial Cr(VI) concentration, which can be due to the greater con-entration driving force [42]. In Fig. 2, it can be seen that there areainly three linear regions. The calculated intercept values indi-

ate that intraparticle diffusion is not the rate controlling step andhat film diffusion controls the initial rate of adsorption. The secondinear region is related to intraparticle diffusion. The Boyd modelEq. (6)) establishes whether the rate controlling step of mass trans-er is film diffusion or particle diffusion. If the plot of Bt against ts linear and passes through the origin, then the rate controllingtep in the adsorption process is the pore diffusion, other wise, filmiffusion will be the rate controlling step [43]. The plots obtainedFig. 3) illustrate the non-linearity over the period studied, how-ver, the plots are linear in the initial period of adsorption and theinearized data do not pass through the origin (intercept valuespproximately −0.4977). This indicates that external mass transfers the initial controlling process in the adsorption system. Similarndings were reported by Karthikeyan et al. [44] for the adsorptionf Cr(VI) onto Hevea brasilinesis sawdust.

.2.3. Effect of particle sizeThe particle size can have a strong influence on the sorption

roperties of the adsorbent [45]. Experiments were performedsing different particle sizes of the adsorbent (0.180–2.0 mm) at0 ◦C, and initial concentration of 50 mg/L. Generally, the smallerhe adsorbent particles, the greater the amount of metal ions coulde adsorbed. Comparatively higher adsorption with smaller adsor-

ate particles may be due to the fact that smaller particles give

larger external surface area. Also, for a large particle size somef the interior pores of the dolomite could be unapproachabley the Cr(VI) ions. Access to all pores is facilitated through small

t (hour)

Fig. 3. Boyd plots for the adsorption of Cr(VI) onto dolomite.

size particles [46]. The R2 values for the linear plots being >0.989confirmed that kinetic data fitted the pseudo first-order kineticmodel (Table 3). The kinetic constants for the Cr(VI) adsorption ontodolomite at different particle size are shown in Table 3, and specifythat the amount of Cr(VI) adsorbed onto dolomite decreases withan increase in the particle size of the adsorbent.

3.2.4. Effect of shaking speedThe distribution of the adsorbate ions in the aqueous solution

is generally affected by agitation speed. The effect of the shakingspeeds (100, 150 and 200 rpm) on the adsorption of Cr(VI) ontodolomite was performed and the results are shown in Fig. 4. Therate of shaking has a clear effect on the amount of Cr(VI) uptakewhich may indicate that mass transfer is a key factor in controllingthe rate. The amount of Cr(VI) adsorbed increased from 10.05 to10.67 mg/g when the rate of shaking increased from 100 rpm to200 rpm. This can be attributed to the increase of the mobility ofthe system and decrease in the film resistance to mass transfersurrounding the adsorbent particle [47]. As demonstrated by Dottoand Pinto [48], this occurs because, the stirring rates from 100 to200 are in the average range of agitations rates, and usually in thisrange a slight change in adsorption behaviour take place. Table 4

100806040200t (hour)

Fig. 4. Effect of shaking speeds on Cr(VI) adsorption onto dolomite.

198 A.B. Albadarin et al. / Chemical Engineering Journal 179 (2012) 193– 202

Table 3Pseudo first and second-order model and intraparticle diffusion constants and correlation coefficients for Cr(VI) adsorption onto dolomite at various particle sizes.

dp. (mm) qe,exp Pseudo first-order model Pseudo second-order model Intraparticle diffusion model

qe,cal k1 R2 qe,cal k2 R2 kdiff R2

0.180 15.13 15.32 4.07 0.990 19.23 1.92 0.980 1.642 0.9480.335 13.20 13.91 3.85 0.989 18.52 1.84 0.981 1.575 0.9420.710 11.84 13.16 3.01 0.992 18.36 1.30 0.987 1.341 0.9542.000 10.27 10.83 3.55 0.991 14.70 2.00 0.984 1.183 0.942

Where Co , mg/L; qe , mg/g; k1, 1/h (×102); k2, g/mg h (×103); kdiff , mg/g h1/2.

ae(cptsat[

3

3

pafdso(Ah

TP

Fig. 5. Effect of pH on adsorption Cr(VI) onto dolomite.

To determine the mass transfer steps in various shaking rates, qt

s a function of t0.5 was plotted. The plots show multi linearity, andach segment corresponds to a distinct mass transfer mechanismnot shown here). It was observed that film diffusion and intraparti-le diffusion were simultaneously operating during the adsorptionrocess, but, film diffusion was the rate-limiting process. However,he film diffusion effect decreases with an increase in the shakingpeed, and as a result, the increases intraparticle diffusion effect. At

higher degree of agitation, the effect of mass transfer is present athe beginning of the process and diminished with passage of time13].

.3. Equilibrium studies

.3.1. Effect of pHCr(VI) is often present at a concentration ranging from parts

er billion up to several hundred parts per million, and are oftencidic in nature (i.e. below pH 4) [49]. At low pH the oxo-anionicorm of Cr(VI) is predominant [50]. The process was found to be pHependent (Fig. 5) which indicates that ion exchange and electro-tatic interactions might be involved in the adsorption mechanism

f Cr(VI) on dolomite. The amount of Cr(VI) adsorbed qe on dolomitemg/g) decreased from 10.1 mg/g at pH 2 to 1.79 mg/g at pH 12.t low pH values (below 4.0), the surface will be surrounded byigh quantities of hydronium ions (H+). The hydroxyl groups are

able 4seudo first and second-order model and intraparticle diffusion constants and correlation

Shaking speed (rpm) qe,exp Pseudo first-order model

qe,cal k1 × 102 R2

100 10.05 10.17 4.54 0.996

150 10.28 10.34 5.15 0.995

200 10.67 10.68 6.26 0.997

protonated and as such are positively charged, which will electro-statically attract the negatively charged Cr(VI) ions. Dolomite has arough surface and is primarily composed of MgCO3 and CaCO3; thisincreases the possibility of solid contact resulting in chemisorptionof chromium ions. This may also enhance the adsorption of Cr(VI)onto dolomite in the form of Ca(CrO4) and Mg(CrO4) even at lowpH values [51,52].

The relatively low Cr(VI) removal on dolomite at high pH can bedue to the existence of a negative charge on the dolomite surfaceover the pH range of 8–12. Thus the adsorption capacity decreasesdue to the repulsion between negatively charged chromium ionsand OH− ions [21]. These findings are supported by the measure-ments of the point of zero charge (PCZ) of dolomite, which was8.55. It was also found that the basic characteristic of the dolomiteis higher than its acidic characteristic (0.471 and 0.035 mmol/grespectively) (results are not shown here). It is worth mentioningthat the pH of the solution increased after the adsorption of Cr(VI),which may be due to the increase in the HCO3

− concentrations. AtpH 2, the HCO3

− ions decompose from the dolomite surface increas-ing the negative charge and thus the pH of the solution [53]. Therelatively low Cr(VI) removal onto dolomite can be caused by itshigh basic properties and HCO3

− hydrolysis.

3.3.2. Effect of amount of adsorbentUnder experimental conditions; Cr(VI) initial concentration

50 mg/L and pH 2, the effect of adsorbent dosage on adsorption ofchromium onto dolomite was carried out and the results are pre-sented in Fig. 6. As can be seen from this figure, adsorption capacitywas found to decrease proportionally with a decrease in the amountof dolomite. In contrast, the Cr(VI) removal efficiency of dolomiteincreased with increased adsorbent amount, which can be due tothe large number of vacant adsorption sites and the greater surfacearea hence favouring more Cr(VI) adsorption.

The decrease in adsorption capacity is attributable to the split-ting effect of the concentration gradient between sorbate andsorbent with increased dolomite concentration causing a decreasein amount of Cr(VI) adsorbed onto unit weight of dolomite [54]. Thisphenomenon was also found by Anupam et al. [55] using a pow-dered activated carbon, for the removal of hexavalent chromiumfrom aqueous solution. As the initial Cr(VI) concentration is fixed,Cr(VI) ions can occupy only a certain amount of active sites. Thus,

an additional raise in the number of active sites of dolomite doesnot influence the total Cr(VI) ions adsorbed. It can be concludedthat for each ion concentration there is a corresponding adsorbentdose at which adsorption equilibrium will be established [56].

coefficients for Cr(VI) adsorption onto dolomite at various shaking speeds.

Pseudo second-order model Intraparticle diffusion model

qe,cal k2 × 103 R2 kdiff R2

12.93 3.41 0.990 1.166 0.95312.94 4.01 0.989 1.176 0.92812.94 5.21 0.989 1.182 0.887

A.B. Albadarin et al. / Chemical Engineering Journal 179 (2012) 193– 202 199

Table 5Adsorption isotherm constants values for Cr(VI) on dolomite at different temperatures.

Temp (◦C) Langmuir isotherm constants Freundlich isotherm constants

qmax (mg/g) b (L/mg) RLa R2 KF 1/n R2

20 10.01 0.272 0.007 0.974 3.171 0.304 0.99130 8.385 0.247 0.009 0.978 2.513 0.306 0.98940 6.654 0.200 0.015 0.986 2.392 0.319 0.99260 5.669 0.144 0.024 0.973 1.449 0.333 0.982

a Calculated at Co = 50 mg/L.

2

4

6

8

10

12

14

16

0

20

40

60

80

100

211815129630

q e(m

g/g

dolo

mite

)

Chr

omiu

m re

mov

al (%

)

adsorbent dose g/L

Removal (%)

Adsorption capacity

3

teinctT

twcaTawsoti

t

0

2

4

6

8

10

4536271890q e

(mg/

g)

Equilibrium concentartion (mM)

20 °C 30 °C 40 °C 60 °C

3.3.4. Comparison of adsorption capacity with various adsorbents

TC

Fig. 6. Dependence of Cr(VI) ions adsorption on the amount of adsorbent.

.3.3. Equilibrium isothermsAdsorption isotherms are essential to determining the adsorp-

ion mechanism of metal ions onto adsorbent surfaces [57]. Thequilibrium data obtained in the current investigation was exam-ned with two isotherm models to find out the best fitting isotherm,amely: Langmuir and Freundlich isotherms. The determinationoefficient, R2 and isotherm constants for these models for sorp-ion of Cr(VI) onto dolomite at different temperatures are shown inable 5.

The Freundlich isotherm represents a particular adsorption sys-em when the adsorption takes place on a heterogeneous surfaceith interaction between the adsorbed ions [58]. The results indi-

ate that the Freundlich isotherm fits the experimental data forll solution temperatures as the R2 value is higher than R2 > 0.982.he value of 1/n obtained from the Freundlich isotherm for thedsorption of Cr(VI) on dolomite was found to be less than one,hich specifies that the adsorption is favourable. Similar conclu-

ions were found by Stefaniak et al. [21] who studied the effectf different parameters on the adsorption of Cr(VI) on raw andhermally treated dolomite such as; initial metal concentration and

onic strength.

On the other hand, the Langmuir model suggests that adsorp-ion is monolayer and that the dolomite surface is composed of sites

able 6omparison of adsorption capacities of the adsorbents.

Serial no. Adsorbents Adsorption capacity fo

1 Ceria nanoparticles 26.81

2 Carbon slurry 15.24

3 Treated sawdust 3.60

4 Organobentonite 5.12

5 Modified tannery waste 177

6 Coconut husk fibers 29.0

7 Activated olive bagasse 88.59

Raw dolomite 10.01

Fig. 7. Effect of solution temperature on Cr(VI) uptake onto dolomite at differentinitial concentrations.

possessing the same energy, hence, the adsorption energy is con-stant. The RL values for the adsorption of Cr(VI) onto dolomite arein the range of 0.007–0.024 (see Table 5) which indicates that theadsorption is a favourable process. The qmax for Cr(VI) adsorptiononto dolomite decreased from 14.4 to 6.76 mg/g with the increasein temperature from 20 to 60 ◦C. This may be due to a decreasein the degree of freedom of adsorbed species and a decrease inavailable adsorption active sites [59]. This effect implies that theadsorption mechanism associated with the removal of Cr(VI) ontodolomite involves a physical process. However, the values of R2 forLangmuir analysis are comparatively lower than those for the Fre-undlich isotherm. Fig. 7 illustrates the Cr(VI) adsorption uptake, qe

(mg/g) onto dolomite against the solution temperature at variousinitial concentrations (5–50 mg/L). The Cr(VI) adsorption uptakewas found to decrease with an increase in solution temperature forall initial concentration. This indicates that the adsorption reactionis exothermic.

The comparison of maximum adsorption capacity of thedolomite with that of various adsorbents is represented in Table 6.As represented in this table the adsorption capacity of dolomite for

r Cr(VI) (mg/g) Best fit isotherm Ref.

Langmuir [60]Langmuir/Freundlich [48]Langmuir [61]Langmuir [62]Langmuir [63]Langmuir [64]Langmuir [65]Freundlich Present study

200 A.B. Albadarin et al. / Chemical Engineering Journal 179 (2012) 193– 202

Fig. 8. SEM micrographs of (A) dolomite and (B)

Table 7Thermodynamic parameters for adsorption of Cr(VI) onto dolomite.

Temp (◦C) Thermodynamic parameters

�Go (kJ/mol) �Ho (kJ/mol) �So (J/mol K)

20 −6.617 −13.21 −22.47

Cctta

3

a�cwes

ap

3

d[a

q

ε

w(i(

pTc

3.4.1. SEMIn order to inspect the morphology of dolomite, scanning elec-

tron microscopy (SEM) is equipped with an energy dispersive X-ray

Table 8Dubinin–Radushkevich isotherm parameters for Cr(VI) adsorption onto dolomite.

Temp (◦C) Dubinin–Radushkevich isotherm parameters

KDR (mol2/kJ2) × 102 qs (mg/g) R2 EDR (kJ/mol)

30 −6.59540 −6.26460 −5.763

r(VI) is comparable to that of other adsorbents. The adsorptionapacity varies and depends mainly on the initial Cr(VI) concentra-ion and characteristics of the individual adsorbent. Nevertheless,he current experiments were carried out to find the technicalpplicability of the cheaply available adsorbents to treat Cr(VI).

.3.5. Adsorption thermodynamicThe values of �Ho, �So are calculated from the plot of ln b

gainst 1/T; a straight line with a slope and intercept of �Ho/R andSo/R respectively is obtained (R2 = 0.984). The negative enthalpy

hange �Ho value obtained showed that the adsorption processas exothermic in nature. The negative value of the standard

ntropy �So obtained reflects the decreased randomness at theolid/solution interface during the adsorption process (Table 7).

The Gibbs free energy change �Go values were found to be neg-tive. The negative �Go values with temperature suggest that therocess is feasible and spontaneous.

.3.6. Evaluation of adsorption energyFor the determination of adsorption energy, the equilibrium

ata were further analyzed by Dubinin–Radushkevich equations66]. The Dubinin–Radushkevich adsorption isotherm is expresseds:

e = qsexp(−KDRε2) (14)

The parameter ε can be found from Eq. (15):

= RT ln[

1 + 1Ce

](15)

here ε is the Polanyi potential, qs is the monolayer capacitymg/g), KDR is a constant related to adsorption energy (mol2/kJ2), Rs the gas constant (8.314 J/mol K) and T is the absolute temperatureK).

Therefore, the Dubinin–Radushkevich isotherm can be used toredict equilibrium adsorption data over a range of temperatures.he mean free energy of adsorption EDR (kJ/mol) is the free energyhange when one mole of an ion is transferred from a solution to

EDXA analysis for Cr(VI)-loaded dolomite.

the surface of the sorbent [58]. EDR is calculated from the value ofKDR using Eq. (16):

EDR = 1√2KDR

(16)

The Dubinin–Radushkevich parameters and mean free energiesare given in Table 8. The maximum capacities, qs, obtained using theDubinin–Radushkevich isotherm model for adsorption of Cr(VI) arein the range of 3.97–7.35 mg/g by dolomite (dose 1 g/L) (Table 8),and decreased with an increase in the solution temperature asexpected.

The amount of EDR is practical for estimating the type of adsorp-tion reaction; the values of EDR calculated using Eq. (16) are shownin Table 8 and range in between 4.368 and 4.867 kJ/mol. Typically,EDR values for ion-exchange mechanisms are ranged between 8 and16 kJ/mol, suggesting a physisorption process.

3.3.7. Desorption studiesThe Cr(VI) loaded adsorbents may create a environmental dis-

posal issue because they are hazardous in nature if desorption andleaching occur. Cr(VI) desorption from the adsorbent may solvethis problem to some extent. The percentage of Cr(VI) desorbedfor deionised water, KOH and NaOH, H2SO4 and HNO3 and HCHOwere; 1.66%, 6.23% and 9.01%, 2.01% and 0.0% and 1.19% respec-tively. The Cr(VI) striped from dolomite was relatively low. The Crions left irreversibly bound to the dolomite sites could be consid-ered as responsible for the low recovery percentage. The presenceof mainly Cr(VI) in the stripping solution during desorption stud-ies showed the presence of electrostatic interaction between Cr(VI)ions and dolomite.

3.4. Surface characteristics of dolomite

20 2.11 7.350 0.821 4.8730 2.43 6.463 0.852 4.5340 2.51 5.256 0.871 4.4660 2.62 3.974 0.824 4.37

A.B. Albadarin et al. / Chemical Enginee

Ft

adEtTabcCOf1Tbc

3

tFdrdWeiacfwrnssBbottttcwt

[

[

[

[

ig. 9. FT-IR spectra of dolomite sorbent (A) before and (B) after chromium adsorp-ion.

nalysis (EDXA). Fig. 8A shows the SEM for the micrograph ofolomite and revealed its complex and porous surface texture. TheDXA analysis (Fig. 8B) of the dolomite-Cr(VI) loaded reveals thathe dolomite structure changed after adsorption of the Cr(VI) ions.he micrograph of the dolomite exhibited considerable differencefter adsorbing of Cr(VI), which is not clearly understood. It mighte due to Cr(VI) adsorption and solvent induced morphologicalhange. The results of chemical analysis of the fresh dolomite were:: 18.26%; O: 46.96%; Mg: 7.60%; Ca: 25.38%; Si: 0.58%; Fe: 1.22%.n the other hand, Cr-loaded dolomite chemical analysis were as

ollows: C: 19.04%; O: 47.40%; Mg: 7.46%; Ca: 22.38%; Si: 1.73%; Fe:.55% and Cr: 0.44% as obtained using semi-quantitative EDXA data.he existence of chromium on the dolomite surface proves that it isound to the surface, as predicted from the solution concentrationhange.

.4.2. FTIR analysisFTIR spectra analyses for the dolomite before and after adsorp-

ion of chromium were undertaken and results are illustrated inig. 9. Dolomite FTIR analysis shows main absorption bands ofolomite at 3466.5, 1437.1, 881 and 728 cm−1 [67]. The currentesults are in good agreement with the absorption frequencies,emonstrated by previous researchers [14,68,69]. According toahab et al. [70] bands around 3440 cm−1 are related to the pres-

nce of bonded hydroxyl groups (OH). The weak band at 1040 cm−1

ndicates the presence of silicate phases (Si–O vibrations). The bandt 1437.1 cm−1 may be assigned to HCO3

− group. The FTIR spectraonfirm that there is a shift of some functional group bands. Dif-erences in the spectra would indicate bonding between the metalith active sites on the dolomite due to adsorption or a chemical

eaction. Dolomite is a double salt, in contact with water, the phe-omena of dissolution and hydration will occur. The charge of theurface is therefore the consequence of the formation of the ionicpecies to the solid–liquid interface, which is a function of pH [36].elow the PZC, the surface of dolomite is positively charged causedy a high concentration of the positively charged species. Changesccurring on the dolomite after adsorption of Cr(VI) are reflected inhe broad band present at 3466.5 cm−1, which is perhaps assignedo the electrostatic attraction between Cr(VI) ions and the pro-onated OH groups at pH 2. Cr(VI) ions are possibly adsorbed on

he >(Ca, Mg)OH2

+ sites in the form of >(Ca, Mg)Cr2O7−. After

ontact with Cr(VI) solutions the dolomite exhibited FTIR spectraith a clear decrease of peak intensities. This could be attributed

o an interaction between Cr(VI) species and dolomite function

[

ring Journal 179 (2012) 193– 202 201

groups. The stretching intensity is much weaker than that of freshdolomite, which indicates that the hydrophilic property of Cr(VI)-loaded dolomite is noticeably weakened. The intensity of the peakrepresenting the HCO3

− group decreased and shifted from 2627.9to 2625.3 cm−1, which could be attributed to Cr(VI) ion insertion,which replaces the HCO3

− ions [53]. The increase in the pH of theCr(VI) solution taking place throughout adsorption is evidence ofthe possibility of a Cr(VI)-carbonate ion exchange process.

4. Concluding remarks

Kinetic studies on the adsorption of Cr(VI) onto dolomiterevealed that experimental data fitted the pseudo first-orderkinetic model and that film diffusion initially controls the sorp-tion process. Low pH rendered the dolomite surface more positiveand increased the removal efficiency of Cr(VI) in the aqueous phase,given that the binding of anionic Cr(VI) ion species with positivelycharged groups was enhanced. FTIR analysis confirmed the involve-ment of –OH− and HCO3

− groups in Cr(VI) adsorption by dolomiteadsorbent.

The Freundlich isotherm model gave the best fit to equilibriumexperimental data. Thermodynamic data indicate the adsorptionprocess was exothermic and non-spontaneous. The adsorptionstudies and the values of thermodynamic parameters; �Go, �Ho

and �So indicate that the rise in the solution temperature did notsupport the Cr(VI) removal by dolomite.

Acknowledgement

This work was funded by the Queen’s University EnvironmentalScience and Technology Research Centre (QUESTOR).

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