H

Eo

Fa

b

a

ARRAA

KHWAEKL

1

Ciash((ittt

tsb[

h0

ARTICLE IN PRESSG ModelAZMAT-16167; No. of Pages 9

Journal of Hazardous Materials xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal ho me pa ge: www.elsev ier .com/ locate / jhazmat

quilibrium and dynamic study on hexavalent chromium adsorptionnto activated carbon

. Di Natalea, A. Ertoa,∗, A. Lanciaa, D. Musmarrab

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università di Napoli “Federico II”, Piazzale Tecchio 80, 80125, Napoli, ItalyDipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Università di Napoli, Via Roma 29, 81031, Aversa (CE), Italy

r t i c l e i n f o

rticle history:eceived 27 January 2014eceived in revised form 28 July 2014ccepted 29 July 2014vailable online xxx

eywords:exavalent chromiumater

dsorption

a b s t r a c t

In this work, the results of equilibrium and dynamic adsorption tests of hexavalent chromium, Cr (VI), onactivated carbon are presented. Adsorption isotherms were determined at different levels of pH and tem-perature. Dynamic tests were carried out in terms of breakthrough curves of lab-scale fixed bed columnat different pH, inlet concentration and flow rate. Both the adsorption isotherms and the breakthroughcurves showed non-linear and unconventional trends. The experimental results revealed that chromiumspeciation played a key role in the adsorption process, also for the occurrence of Cr(VI)-to-Cr(III) reductionreactions. Equilibrium tests were interpreted in light of a multi-component Langmuir model supportedby ion speciation analysis. For the interpretation of the adsorption dynamic tests, a mass transfer modelwas proposed.

quilibriumineticsangmuir model

Dynamic tests at pH 11 were well described considering the external mass transfer as the rate con-trolling step. Differently, for dynamic tests at pH 6 the same model provided a satisfying descriptionof the experimental breakthrough curves only until a sorbent coverage around 1.6 mg g−1. Above thislevel, a marked reduction of the breakthrough curve slope was observed in response to a transition to aninter-particle adsorption mechanism.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

In the past decades, the occurrence of hexavalent chromium,r(VI), in wastewater and natural water became a paradigm of

ndustrial pollution [1]. For this reason, chromium is considereds a priority hazardous pollutant and the European Union definedevere environmental regulations to set the maximum level ofexavalent chromium allowed in industrial and civil wastewaters200 �g L−1), as well as in superficial and underground water bodies5 �g L−1). Moreover, because of the occurrence of redox reactionsnvolving the harmful hexavalent chromium and the much lessoxic trivalent one, Cr(III), limiting values were also defined for theotal chromium concentration in waters [2]. These limits are usuallyen times higher than those of hexavalent chromium [3].

Commonly adopted methods to remove chromium from indus-rial wastewaters include precipitation [4], membrane filtration [5],

Please cite this article in press as: F. Di Natale, et al., Equilibrium and dycarbon, J. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2

olvent extraction with amines [6], ion exchange [7], activated car-on adsorption [2,8], electro deposition [9] and biological processes10]. Activated carbon adsorption appears as a reliable process to

∗ Corresponding author. Tel.:+39 081 7682236; fax: +39 081 5936936.E-mail address: aleserto@unina.it (A. Erto).

ttp://dx.doi.org/10.1016/j.jhazmat.2014.07.072304-3894/© 2014 Elsevier B.V. All rights reserved.

remove organic and inorganic pollutants from both wastewatersand groundwater and, more in general, to comply with the waterquality standards [11,12]. In groundwater remediation, it actuallyfind practical application in both pump-and-treat systems and per-meable adsorbing barriers [13,14].

The adsorption of metallic ions on the surface of activated car-bons is the result of the interactions between the aqueous solutionand the different active sites on the carbon surface. The presence ofdifferent surface functional groups with either acid or basic prop-erties is responsible for the surface hydrolysis of the activatedcarbons in aqueous solutions and for the preferential interactionwith cations and anions, respectively. These structures are generi-cally denoted as COxHy and are mainly originated by the activationprocess of the raw material [15]. The presence of surface het-eroatoms and the structure of the graphitic layer itself, as well asthe ash properties, determine the occurrence of active sites actingeither as Lewis acids or as bases, able to capture ions with base oracid character, or as nucleophilic/electrophilic active sites [15,16].Furthermore, some of these groups, such as lactonic and pheno-

namic study on hexavalent chromium adsorption onto activated014.07.072

lic, exploit reducing properties, which are also observed for thesame graphitic layer. Lakatos et al. [17] pointed out the role of sur-face functional groups with reducing properties in determining theadsorption of chromium ions. During an adsorption experiment,

ARTICLE ING ModelHAZMAT-16167; No. of Pages 9

2 F. Di Natale et al. / Journal of Hazardou

Nomenclature

[x] molar concentration of the species x (M)a specific surface areas (1/m)BM dimensionless mass loading (–)c chromium concentration (mg L−1)c0 chromium concentration (mg L−1)D chromium diffusivity (m2 s−1)Dl fixed bed column dispersion coefficient (m2 s−1)�G Gibbs free energy of adsorption (kJ mol−1)dp particle diameter (m)ε bed void fraction (–)K adsorption equilibrium constant in Langmuir model

(1/M)kf fluid-particle mass transfer coefficient (m s−1)� fluid viscosity (kg (ms)−1)m sorbent mass (g)Q liquid flow rate (L h−1)q adsorption capacity in the fixed bed column

(mg g−1)Re particle Reynolds number, Udp�/� (–)� fluid density (kg m−3)�s solid particle density (kg m−3)Sh Sherwood number, kfdp/D (–)T temperature (◦C or K)t time (s)U superficial fluid velocity in the column (m s−1)ω equilibrium adsorption capacity (mg g−1)ωmax maximum adsorption capacity in Langmuir model

(mg g−1)

tm

egspTsw[eaVtea

afo

tcbaiptttr

−1

z column longitudinal coordinate (m)

he occurrence of surface reduction reactions can be observed byeasuring Cr(VI) and total chromium in solution.In general, commercially available activated carbons may be

xpensive and the use of low cost adsorbing materials has arousedreat interest in the pertinent literature [18–20]. For the same rea-on, the optimization of the operating conditions for adsorptionrocesses is of the utmost importance for a correct process design.he analysis of the most recent findings in the pertinent literaturehowed that chromium adsorption kinetics in packed bed columnsas investigated in several papers considering activated carbons

21–23], resins [24] and by-products derived materials [25–29],ither for Cr(VI) or Cr(III) ions. The simultaneous presence of Cr(VI)nd Cr(III) species was only considered in the packed bed tests ofinodhini and Das [29]. The occurrence of Cr(VI) reduction reac-

ions to Cr(III) on the surface of carbons was also observed inquilibrium studies on activated carbon (e.g. [17,19,21]). Similarly,ll the tests were carried out with a constant solution pH over time.

The state of the art can benefit for the development of a modelble to describe equilibrium and kinetic data by taking into accountor the complex ion speciation, the effect of pH and the occurrencef Cr(VI)–Cr(III) redox reactions.

In this work, the results of equilibrium and dynamic tests onhe adsorption of chromium on a commercial granular activatedarbon (GAC) are presented. Equilibrium studies were carried outy determining the adsorption isotherms in stirred batch reactorst different levels of pH and temperature. Equilibrium data werenterpreted in light of a multi-component Langmuir model sup-orted by a ion speciation analysis, which allowed to correlate

Please cite this article in press as: F. Di Natale, et al., Equilibrium and dycarbon, J. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2

he distribution of Cr(VI) and Cr(III) species, the solution pH andhe temperature, with the experimental adsorption capacity. Oncehe adsorption equilibrium was assessed, dynamic tests were car-ied out in terms of breakthrough curves of a lab-scale fixed bed

PRESSs Materials xxx (2014) xxx–xxx

column at different levels of pH, inlet concentration and flowrate. The results of adsorption dynamics allowed determining theintercorrelation between the main process parameters (e.g. flowrate and pH). Since the equilibrium model allowed estimatingthe instantaneous driving force for adsorption rate, an accuratedynamic model was proposed to describe the breakthrough curvesand to address the adsorption rate controlling mechanism in theinvestigated conditions.

2. Materials and methods

2.1. Sorbent properties

The sorbent used in this work is Aquacarb 207EATM GAC, pro-vided by Sutcliffe Carbon. The morphological analysis showed thatAquacarb 207EATM is a mesoporous material with an average porediameter of 467 nm and an average porosity of about 17%. The den-sity of the solid is around 500 kg m−3 and a particle range between0.85 and 1.18 mm was selected by mechanical sieving. Sorbentcharacterization also included the evaluation of the pHPZC [30] andthe analysis of surface functional groups by Boehm’s titration analy-sis [15,31]. The measurement of pHPZC (equal to 8) and the Boehm’stitration method showed that Aquacarb 207EATM has a slightlybasic character and that the acidic functional groups are mainly rep-resented by quinones, phenols and lactones. Additional propertiesof the GAC are reported in Di Natale et al. [32].

2.2. Equilibrium tests

The experimental runs were carried out in batch mode and wereconducted at constant temperature in a proportional-integral-derivative (PID) controlled thermostatic oven. The chromiumsolution was obtained by the dissolution of a Cr(VI) salt (K2Cr2O7,reagent grade) in distilled water. The chromium concentrationin the initial solution varied in the range 5–50 mg L−1, while thesorbent concentration spanned from 2 to 10 g L−1. The pH of thesolution was adjusted in the range 2–11 by addition of either nitricacid or potassium hydroxide. Preliminary kinetic tests showed thata reaction time of 48 h was sufficient to reach equilibrium con-ditions in all the experimental runs. When equilibrium conditionswere achieved, total and Cr(VI) concentrations in solution and totalchromium uptake on the sorbent surface were measured. To thisaim, the solution was filtered in a ceramic filtering Hirsch funnelusing a vacuum pump to assure a very rapid and efficient separationof the granular material. The filtered solution was then analysedfor pH and chromium concentration while the carbon was leachedwith 1 M nitric acid solution to obtain the complete extraction ofsorbed chromium. Following this procedure, a direct measure ofthe total chromium uptake on the solid surface can be determinedand a comparison with the homologous quantity as calculated by amass balance can be made. The accuracy of the experimental runswas checked by allowing a maximum error of 5% in the chromiummass balance.

Blank tests were run in parallel on chromium solutions, withoutaddition of sorbent, for the sake of comparison.

2.3. Dynamic tests

Adsorption kinetics was studied in terms of breakthrough curvesin a 500 mm length and 15 mm ID glass column, fed with differ-ent masses of GAC, from 10 to 40 g, mixed with 0.9 mm inert glass

namic study on hexavalent chromium adsorption onto activated014.07.072

beads. The liquid flow rate was varied in the range 1–11 L h andthree levels of inlet chromium concentration (10, 25, 50 mg L−1)were considered. Tests were performed for two different pH levels(i.e. 6 and 11) at room temperature (about 25 ± 1 ◦C).

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pH=2ω ,

mg

g-1

0

2

4

6

8

10

12pH=2.5 pH=3

pH=6

ω, m

g g-1

0

2

4

6

8

10

12pH=7 pH= 8

c, mg l-10 10 20 30 40 50 60

pH=9.5

-10 10 20 30 40 50 60

ω , m

g g-1

0

2

4

6

8

10

12pH=11

c, m -110 20 30 40 50 60

iffere

2

bt(obCsscltcm

mwv

c, mg l

Fig. 1. Total chromium adsorption isotherms at T = 20 ◦C and for d

.4. Analytical procedures

The total chromium concentration in solution was measuredy means of air/acetylene flame atomic absorption spectropho-ometry (AAS-F), with a Varian SpectrAA-220 spectrophotometerStandard Method 3111B in Clesceri et al. [33]). The concentrationf Cr(VI) was measured by UV–vis absorption spectrophotometryy complexation with diphenilcarbazide (Standard method 3500-r-B, in Clesceri et al. [33]) with a PerkinElmer Lambda EZ-150pectrophotometer. The concentration of trivalent chromium inolution was determined by the difference between total and Cr(VI)oncentration. The amount of sorbed chromium was measured byeaching the solid material with 100 mL of nitric acid aqueous solu-ion (HNO3, 1 M) for 24 h, which assures a complete desorption ofhromium from GAC surface. Then, the solution was analysed byeans of AAS-F.

Please cite this article in press as: F. Di Natale, et al., Equilibrium and dycarbon, J. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2

The accuracy of the experimental runs was checked allowing aaximum error of 5% in the chromium material balance. Each testas repeated three times and the average values of the measured

ariables were considered.

g l

nt pH levels. Dashed lines represent model results (Eqs. (1)–(4)).

3. Results and discussion

3.1. Equilibrium test results

Chromium adsorption isotherms at 20 ◦C and different equilib-rium pH are reported in Fig. 1. All the isotherms in Fig. 1 showedthat equilibrium adsorption capacity, ω, was an increasing func-tion of concentration in the liquid solution, c, and that chromiumadsorption capacity reached its maximum at pH between 6 and 7.

Experimental results on the fraction of trivalent chromium insolution as a function of pH and temperature are shown in Fig. 2.Experiments revealed that Cr(VI) was partially reduced to Cr(III),with a reduction degree having a decreasing sigmoid shape, thatat 20 ◦C, spanned from 100% at pH 3.5 to about 7% at pH 6 (asalready shown in Di Natale et al. [16]). At 55 ◦C no appreciable dif-ferences were observed compared with results at 20 ◦C, while at

namic study on hexavalent chromium adsorption onto activated014.07.072

lower temperature, the reduction at acid pH was lower. For all thetemperatures, the reduction at higher pH levels was negligible.

Blank tests confirmed that the initial chromium concentration,the pH and the Cr(III)/Cr(VI) ratio remained unchanged in absence

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4 F. Di Natale et al. / Journal of Hazardous Materials xxx (2014) xxx–xxx

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14

[Cr(I

II)]/[

Cr to

t]

0.0

0.2

0.4

0.6

0.8

1.0

T= 7°C T= 20 °CT= 55 °C

Fa

otdbbcC

csoeso

FfC

Table 1Chromium speciation reactions.

Chromic acid hydrolysis reactions [3,43]2HCrO4

− = Cr2O72− �G = −8724 kJ/mol

H+ + CrO42− = HCrO4

− �G = −37,120 kJ/molH+ + HCrO4

− = H2CrO4 �G = 1140 kJ/mol

Formation of Cr(III) complexes [3,43]Cr3+ + OH− = CrOH2+ �G = −57,021 kJ/molCr3+ + 2OH− = Cr(OH)2

+ �G = −104,348 kJ/molCr3+ + 3OH− = Cr(OH)3(aq) �G = −136,85 kJ/molCr3+ + 4OH− = Cr(OH)4

− �G = −163,079 kJ/mol

Redox reactions [44]

ig. 2. Fraction of Cr(III) in solution as a function of equilibrium pH and temperaturet c = 9 ± 0.5 mg L−1.

f activated carbon. Hence, the reduction of chromium concen-ration, the pH variation and the occurrence of redox reactionsuring adsorption tests were only related to the presence of the sor-ent. Besides, in these experiments, solution was composed onlyy Cr(VI), K+, NO3

− and water. Therefore, in absence of activatedarbon, there were no reducing agents in solution able to promoter(VI)–Cr(III) reduction, even at low pH.

Experimental tests at different temperature showed thathromium adsorption was slightly increasing with temperature,imilarly to the case of mercury chloride and arsenic adsorptionn the same GAC [12,32], but in apparent contradiction with the

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xothermic character of adsorption phenomena [1,34]. This effect ishown in Fig. 3 for the case of pH 6-7 and equilibrium concentrationf 8 mg L−1.

T, °C

0 10 20 30 40 50 60

ω, m

g/g

0

1

2

3

4

Exper iments Model pred ictions

ig. 3. Total chromium equilibrium adsorption capacity on Aquacarb 207EA as aunction of temperature for a total chromium concentration of 8 mg L−1 and pH 6–7.omparison between experimental data and model predictions.

HCrO−4 + 6H+ + 3e− ⇔ Cr(OH)2+ + 3H2O Eh

◦ = 1.275 VCrO4

2− + 4H+ + 3e− ⇔ CrO2− + 2H2O Eh = 0.945 V

In the pertinent literature, experiments on the effect of solutiontemperature on chromium adsorption reported different results.Some authors found an increase in adsorption with temperature[35–39], while some others observed the opposite trend, for bothCr(VI) and Cr(III) at acid pH [40,41]. In many cases, the increase inadsorption capacity with temperature was related to either kineticeffects due to increased ion diffusivities (which however, imply thatequilibrium conditions were not achieved in the referred experi-ments) or to the “activation” of new adsorption sites on the solidsurface or to the evolution of sorbent porosity at higher tempera-ture. However, these justifications were not clarified and the overalladsorption process was thus referred as apparently endothermic.

In this work, experiments were interpreted in light of themethodological approach and the thermodynamic model devel-oped in the past by our research group. This was successfullyapplied to the case of arsenic, cadmium and mercury adsorption[12,32,42] and described in details in Di Natale et al. [16]. The start-ing point of this model is that adsorption of metallic ions ontoactivated carbon occurs through parallel and consecutive reactionsthat involve the active sites on carbon surface and the ionic speciesof the dissolved metal, while the adsorption of molecular solublespecies is negligible [1,3,16]. This approach involves the analysisof chromium speciation at equilibrium based on mass and elec-tric charge balances coupled with the equations representativeof chemical equilibria of the single chromium species (Table 1).Davies’ procedure [1,3] is used for the evaluation of the activitycoefficients of the ionic species. The occurrence of reaction betweenK+ and Cr(VI) were neglected [43].

Cr(III) and Cr(VI) speciation in solution are shown in Figs. 4 and 5.The ion speciation in all the investigated conditions shows that themain ionic species in solution at pH < 3 is by far the Cr+3 (Fig. 4)while for pH > 7 the CrO4

−2 anion is prevalent (Fig. 5). The inter-mediate values are difficult to analyze because of the simultaneouspresence of Cr(III) and Cr(VI) chromium in experimental conditionswhich cannot be defined of pure equilibrium, due to the pres-ence of slow reduction reactions on the carbon surface [17]. Theincrease in temperature leads to an increasing dissociation gradeof ionic complexes. Thus, the concentration of ions with highernet charge, such as CrO4

−2 and Cr+3, increases with temperature.The activated carbon surface functional groups can be divided intofour categories: acid, basic, nucleophilic and electrophilic. We areassuming that each of these categories is characterized by a uniquevalue of the adsorption energy. Theoretically, this allows apply-ing a Langmuir model to describe the adsorption of each ion oneach active site category. In fact, the description of adsorptionphenomena is carried out adopting a multicomponent Langmuirmodel to take into account the competitive effects between solvent

+ − −

namic study on hexavalent chromium adsorption onto activated014.07.072

ions (H , OH , Cl , etc.) and of the additive effects of the dissolvedchromium ions (CrO4

−2, HCrO−4, Cr2O7−2, Cr+3,CrOH+2, Cr(OH)+2,

etc.), also involving the effect of temperature on their complexationreactions. However, due to the ion speciation in the investigated

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2 4 6 8 10 12

Cr(

OH

) 4- , M

Cr+3

, M

10-9

10-8

10-7

10-6

10-5

10-4

10-3

7°C 20°C 55°C

2 4 6 8 10 12

CrO

H+2

, M

10-9

10-8

10-7

10-6

10-5

10-4

10-3

Cr(

OH

) 2+ , M

pera

cC

am

ω

ω

ω

K

rtasd

i

pH

Fig. 4. Cr(III) ion speciation as a function of pH and tem

onditions, CrO4−2 was used to describe Cr(VI) adsorption, whiler+3 was used to describe Cr(III) adsorption.

The equilibrium model proposes a correlation between totaldsorbed chromium and dissolved Cr(III) and Cr(VI) ions experi-entally determined. The model equations are [16]:

Cr(VI) = ωmaxCr(VI)

K1[CrO4

−2]

1 + K1[CrO4

−2]

+ KOH[OH−](1)

Cr(III) = ωmaxCr(III)

K2[CR+3

]

1 + K2[Cr+3

]+ KH

[H+] (2)

= ωCr(III) + ωCr(VI) (3)

Where the Langmuir equilibrium constants are expressed as:

i = exp(

−�Gi

RT

)(4)

The good accordance between experimental and model results,eported in Figs. 1 and 3, testifies the model accuracy in the descrip-ion of the effects of both pH and temperature on chromiumdsorption. The values of the model parameters and the results of

Please cite this article in press as: F. Di Natale, et al., Equilibrium and dycarbon, J. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2

tatistical analysis are shown in Table 2, and their calculation wasetailed in Di Natale et al. [19,32].

The analysis of experimental data revealed that �Gi is a constantndependent of temperature. This means that the term (T�Si) is

pH

ture for a total chromium concentration of 20 mg L−1.

negligible compared to �Hi. Therefore, since �Gi < 0, it follows that�Hi < 0, thus indicating that the adsorption process is exothermic.

The model results suggest that Cr(VI) adsorption increased withtemperature due to a higher dissociation of Cr(VI) ions, whichdetermines a higher concentration of CrO4

−2 (Fig. 5), the final dis-sociation product of chromic acid (H2CrO4). This ion has a highernet charge, which is expected to lead to a higher adsorption capac-ity on the GAC, especially at pH below the pHPZC. The model alsoallows a correct description of the effect of solution pH: the optimalpH conditions to maximize chromium adsorption capacity occur-ring in the range 6–7 is related to the compensation between higherCr(VI) ionization, favored at higher pH, and lower competition withOH− ions, favored at lower pH.

3.2. Dynamic test results

Breakthrough curves of Cr(VI) were carried out at initial pH of11 and 6, at different chromium concentrations (c0 = 10–50 mg L−1)and liquid flow rates (Q = 1–11 L h−1). These values were chosen asthe typical composition and operational parameters for a fixed-bed treatment of polluted water, either industrial or natural (by

namic study on hexavalent chromium adsorption onto activated014.07.072

pump and treat operation). It is worth noticing that typical super-ficial velocity in packed bed experiments ranges between 0.2 and3 mm s−1 [21–29]. In this paper, we adopted a higher superficialvelocity (i.e. 1.5–17 mm s−1) to operate in a regime of fast external

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6 F. Di Natale et al. / Journal of Hazardous Materials xxx (2014) xxx–xxx

H2C

rO4

, M

10-9

10-8

10-7

10-6

10-5

10-4

10-3

pH

2 4 6 8 10 12

Cr 2O

72-, M

10-9

10-8

10-7

10-6

10-5

10-4

10-3

pH

2 4 6 8 10 12

CrO

42-, M

7° C 20°C 55°C

HC

rO4- , M

Fig. 5. Cr(VI) ion speciation as a function of pH and temperature for a total chromium concentration of 20 mg L−1.

Table 2Mean values of the adsorption model parameters (Eqs. (1)–(4)), their standard errors and a synoptic of the statistical data analysis.

Mean Stand. error T-test Dependency F-test R2 R2adj

Normality test, P

Trivalent chromiumωmax

Cr(III), mg g−1 23 3.2 10 0.98 49.2 0.70 0.69 >0.01

�G2, kJ mol−1 −15.63 1.2 8.49 0.96�GH, kJ mol−1 −16.74 0.25 7.66 0.96

Hexavalent chromiummax −1 0.95

0.960.7

mtwl

w

sl

B

ωCr(VI), mg g 14.04 1.54 7.21

�G1, kJ mol−1 −19.53 0.68 28.66

�GOH, kJ mol−1 −29.70 0.50 36.88

ass transfer, thus allowing the exploitation of intra-particle massransfer rate. A constant sorbent loading in the column (m = 10 g)as adopted, but some tests were carried out with different sorbent

oadings (i.e. 25 and 40 g).In all the tests, the total chromium and the Cr(VI) concentrations

ere measured at the column outflow, as well as the solution pH.The experiments at pH 11 are shown in Fig. 6 in terms of dimen-

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ionless concentration c/c0 as a function of the dimensionless massoading in the column, BM, defined as:

M = Q · c0 · t

m(5)

244 0.85 0.845 >0.01

where t is the test time expressed in hours.Fig. 6 shows that all the experimental data almost collapse on

a single master curve in the plane (BM, c/c0), with slight differ-ences between the tests at different flow rates and inlet chromiumconcentrations: the higher Q and c0, the steeper is the break-through curve. The solution pH remained almost unchanged duringthese tests and, as expected, no reduction of Cr(VI) to Cr(III) was

namic study on hexavalent chromium adsorption onto activated014.07.072

observed.Dynamic tests at initial pH of 6 are shown in Fig. 7, in which the

effects of chromium concentration, liquid flow rate and sorbentloading are reported.

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F. Di Natale et al. / Journal of Hazardous Materials xxx (2014) xxx–xxx 7

BM, -

0 2 4 6 8 10

c/c 0

0.0

0.2

0.4

0.6

0.8

1.0

1 L h-1 - 10 mg L-1

4 L h-1 - 10 mg L-1

7 L h-1 - 10 mg L-1

11 L h-1 - 10 mg L-1

4 L h-1 - 25 mg L-1

Mode l pred iction

Fc

bwCt(tCaaC

taitis(atciop

amaCflMt

tdrea

- B -

BMx 103, -

0 2 4 6 8 10

c/c 0

0.0

0.2

0.4

0.6

0.8

1.0

6.5 l/h, 10 mg/ l, 10 g 6.5 l/h, 25 mg/ l, 10 g 6.5 l/h, 50 mg/ l, 10 g 7.5 l/h, 50 mg/ l, 25 g 1.9 l/h, 50 mg/ l, 25 g

- A -

BMx 103, -

0 15 30 45 60 75 90

c/c 0

0.0

0.2

0.4

0.6

0.8

1.0

6.5 l/h, 10 mg /l, 10 g 6.5 l/h, 25 mg /l, 10 g 6.5 l/h, 50 mg /l, 10 g 7.5 l/h, 50 mg /l, 25 g 1.9 l/h, 50 mg /l, 25 g

(1 − ε) �∂q = k a · c − c (q) (7)

ig. 6. Experimental data (symbols) and model results (lines) of Cr(VI) breakthroughurve at pH 11. T = 25 ± 1 ◦C.

The experiments showed that the solution pH slightly variedetween 7 and 6 with the exception of the very first samples,hich had a pH around 8. Moreover, experiments indicate thatr(VI) reduction to Cr(III) while occurring during equilibriumests (contact time >48 h) was not observed during dynamic testscontact time < 8 h), as already observed in the pertinent litera-ure [17,19,21–29]. This result can be explained considering thatr(III) formation on the carbon surface is a process slower thandsorption. Nothing can be said concerning the oxidation state ofdsorbed chromium, which is expected to include both Cr(III) andr(VI).

The experiments showed that the effect of chromium concen-ration can be well described considering the dependence of c/c0 as

function of BM, but data at different sorbent mass provided signif-cant deviation from a hypothetic master curve (Fig. 7A). Besides,he shape of the curves reveals a marked difference with the exper-ments carried out at pH 11. In fact, all the breakthrough curveshowed a marked change in their slope for a certain value of BMsee also the zoomed diagram in Fig. 7B) corresponding to an aver-ge sorbent loading of about 1.6 ± 0.15 mg g−1 of chromium, for allhe experiments. This loading corresponds to an equilibrium con-entration of chromium in solution of about 1 mg L−1. This changen slope led to a slower adsorption rate which gave rise to valuesf BM at saturation almost ten times higher than those observed atH 11.

A similar result was already observed in the literature. Indeed,vailable experimental data demonstrated that an adsorptionodel based on a single rate controlling mechanism, usually

ddressed by a mass transfer coefficient, is able to describer(III) adsorption on granular activated carbon [22,23]. A deviationrom a single mass transfer rate controlling mechanism, simi-arly to that observed in this study for pH 6, was observed by

alkoc et al. [26] and by Sag and Aktay [28] for Cr(VI) adsorp-ion.

In order to analyze the results of dynamic tests, the break-hrough curves are interpreted assuming a plug flow with axialispersion velocity profile in the column, and an ion adsorption

Please cite this article in press as: F. Di Natale, et al., Equilibrium and dycarbon, J. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2

ate controlled by external mass transfer rate [34,45,46]. To thisnd, the mass balance equations of Cr(VI) ions were taken intoccount.

Fig. 7. Experimental data of Cr(VI) breakthrough curves at T = 25 ± 1 ◦C and pH 6 (A).Zoom of the experimental data at small BM values and application of the adsorptionmodel to the experimental data (B).

Hence, the mass balance over the column height z = [0; H] andtime t = [0; ∞] is:

ε∂c

∂t+ εU

∂c

∂Z= εDI

∂2C

∂Z2− (1 − ε) �s

∂q

∂t(6)

where c(z, t) and q(z, t) are the concentration in the liquid and inthe solid phases of each Cr(VI) ion present, respectively, U is theempty column liquid velocity in the absorber; �s is the solid par-ticle density; ε is the bed void fraction; Dl is the axial dispersioncoefficient.

It is worth to observe that at pH 11 only CrO4−2 species is

present, hence it coincides with total Cr(VI) concentration.The adsorption rate (∂q/∂t) for an external liquid-solid mass

transfer rate controlling mechanism is expressed by the equation:

( )

namic study on hexavalent chromium adsorption onto activated014.07.072

s∂t

f eq

where kf [m s−1] is the mass transfer coefficient between the liquidand a single adsorbent particle; a = 6/dp is the specific area of the

ARTICLE ING ModelHAZMAT-16167; No. of Pages 9

8 F. Di Natale et al. / Journal of Hazardou

Table 3Values of the mass transfer coefficient, kf , and of the Sherwood number, Sh, cal-culated by best fitting of experimental data at pH 11 with the adopted adsorptionmodel.

Q (L h−1) U (mm s−1) Re kf (m s−1) Sh

1 1.49 1.74 7.419 × 10−6 5.98−5

sstta

t

z

z

fitn

Sd1ol

S

w

rtpia

wmoHdatmicTcre

4

iwir

[

[

[

[

[

[

[

4 6.47 7.52 1.462 × 10 11.87 11.6 13.43 2.028 × 10−5 16.3

11 18.2 21.1 2.602 × 10−5 21.0

orbent; and ceq(q) is the concentration in the liquid phase thathould be in equilibrium with the actual sorbent uptake q at anyime and position of the sorbent particle in the column. Therefore,he value of ceq(q) was calculated using the equilibrium model (1)ssuming q = ω at the actual pH at time t of the tests.

Initial and boundary conditions for this system are:

= 0 c (z, t) = 0 q (z, t) = 0 (8)

= H ce (z, t) = c (z, t)∂q (z, t)

∂z= 0 (9)

= 0 c (z, t) = c0 − εDI∂c (z, t)

∂zq (z, t) = 0 (10)

The mass transfer coefficient was estimated through a best-tting analysis of experimental data. The values of kf were assumedo be constant for any chromium concentration and solution pH, butot with liquid flow rate.

For tests at pH 11, they are resumed in Table 3 in terms ofherwood number Sh = kfdp/D, where dp is the particle averageiameter and D is the chromium diffusivity in water assumed equal.44 × 10−9 m2 s−1 [47,48]. The values of kf obtained by best fittingf data are consistent with former models found in the availableiterature (e.g. [45,46]).

This data can be eventually fitted by the following expression:

h = 2 + 0.376 · Re0.55p (11)

here Rep is the particle Reynolds number.Moreover, a comparison between experiments and model

esults is reported in Fig. 6, which clearly indicates that the assump-ion of external mass transfer rate controlling mechanism canroficiently describe the experimental data at pH 11 in the entire

nvestigated range and explains both the effects of liquid flow ratend sorbent mass.

For the experimental data at pH 6, the same adsorption modelith external mass transfer velocity assumed as the rate controllingechanism allowed an effective description of the experiments

nly for the initial part of the breakthrough curves (Fig. 7B).owever, it is worth observing that the model allows a correctetermination of the breakpoint of the curves, which represents

key design parameter for packed bed column. In these calcula-ions, we successfully used the same expression of the external

ass transfer coefficient obtained from tests at pH 11 and reportedn Eq. (11). Differently, the model failed to describe data above theritical time when the change in the curve slope was observed.his slope change can be related to a transition to a inter-particleontrol mechanism as diffusion in macro/micropores or to surfaceeaction kinetic. To date, the experiments are not able to providenough information to address this behavior with the due accuracy.

. Conclusions

Chromium adsorption on a granular activated carbon was stud-

Please cite this article in press as: F. Di Natale, et al., Equilibrium and dycarbon, J. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2

ed in terms of process equilibrium and kinetics. Batch experimentsere carried out to characterize equilibrium adsorption capac-

ty at different pH and temperature of the solution. Experimentalesults suggest that Cr(VI) adsorption increases with temperature

[

PRESSs Materials xxx (2014) xxx–xxx

due to higher dissociation of chromic acid. Furthermore, optimal pHconditions were derived from the compensation between higherCr(VI) ionization, favored at higher pH, and lower competition withOH− ions, favored at lower pH. An additive-competitive Langmuirmodel was used to describe the experiments allowing a properdescription of the effects of both pH and temperature on chromiumadsorption capacity, also accounting for the chromium ionspeciation.

Dedicated dynamic tests were carried out to assess the effect ofchromium concentration and flow rate in a fixed-bed column con-figuration. Dynamic tests at initial pH 11 were properly describedby a mass transfer model in which an external mass transfer washypothesized as rate controlling mechanism. However, at initialpH 6 the same model failed to describe the experimental break-through curves obtained at a sorbent coverage higher than 1.6 mg/gcirca. Above this level, a marked reduction of the breakthroughcurve slope was observed. This slope change can be related to atransition to an inter-particle control mechanism such as diffu-sion in macro/micropores or to surface reaction kinetic. To date,the experiments do not provide enough information to addressthis behavior with the due accuracy. However, this study sug-gests that future developments should include new experimentsat different pH levels and with GAC particle of different sizes, inorder to assess the nature of the intra-particle adsorption ratemechanism. The development of a proper modelling to take intoaccount for the change of controlling mechanism should be alsoconsidered.

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