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Journal of Environmental Chemical Engineering 2 (2014) 22802288

Comparative cadmium adsorption study on activated carbon preparedfrom aguaje (Mauritia flexuosa) and olive fruit stones (Olea europaea L.)

Daniel Obregn-Valencia a, Mara del Rosario Sun-Kou b,*a Instituto de Corrosin y Proteccin, Pontificia Universidad Catlica del Per, Av. Universitaria 1801, Lima 32, Perub Seccin Qumica, Departamento de Ciencias, Pontificia Universidad Catlica del Per, Av. Universitaria 1801, Lima 32, Peru

A R T I C L E I N F O

Article history:Received 5 June 2014Accepted 5 October 2014

Keywords:AdsorptionCadmiumAguaje stonesActivated carbonsOlive stonesAdsorption of heavy metals

A B S T R A C T

This study assesses the capacity of activated carbons, prepared from Mauritia flexuosa (AG series) andOlea europaea L. (OL series) fruit stones, to adsorb cadmium ions. These carbons were activatedchemically through phosphoric acid solution, using impregnation ratios of 0.75, 1.0,and 1:5gH3PO4=gprecursor. The impregnated precursor material was subsequently activated at 400,500 and 600 C. The physicochemical characteristics of precursors and activated carbons were analyzedby thermogravimetric analysis (TGA), nitrogen adsorptiondesorption isotherm (SBET), Boehms titration,and scanning electron microscopy (SEM). Kinetic assays were evaluated from solutions containing10 ppm of cadmium (Cd(II)). Within each series, the activated carbons with higher adsorption capacityturned out to be AG0.75_600 and OL1_600 with 8.14 and 9.01 mg g1, respectively at pH 2. Among26.33 and 24.83 mg g1, respectively at pH 5. Both activated carbons were characterized by the highestmesoporous area and acidic surface functional group compared to other activated carbons (of eachseries). The obtained isotherm correlations fit better according to a Langmuir model and this wasconfirmed with a RedlichPeterson model (with G values close to 1). The adsorption process of cadmiumions took place mostly in adsorption sites of uniform energies.

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Introduction

Cadmium is one of the most prevailing heavy metals with one ofthe highest environmental toxicities [1]. Cadmium enters soil,water, and air from mining, industry, and burning coal andhousehold waste. Fish, plants, and animals take up cadmium fromthe environment [2].

Among the techniques that can currently be applied to removeheavy metals from aqueous solutions, there are chemical precipita-tion, conventional coagulation, reverse osmosis, ion-exchange,ultrafiltration, and adsorption, among others [3]. In industry, theadsorption process is widely adopted for water purification becauseof its efficiency, versatility, and ease of implementation. Activatedcarbon (AC) is a carbonaceous material and it is one of the mostapplied adsorption materials due to its high surface area andchemical reactivity [4]. It can be prepared from renewable and lowcost sources (precursors) such as lignocellulosic materials which canbe obtained from agroindustrial waste (fruit stones and peels)[510]. The mainobjectof thisstudyisto analyze and comparetheACadsorption capacity prepared from two different precursors: aguaje

* Corresponding author. Tel.: +51 1 626 2000; fax: +51 1 6262853.E-mail address: [email protected] (M.d.R. Sun-Kou).

http://dx.doi.org/10.1016/j.jece.2014.10.0042213-3437/ 2014 Elsevier Ltd. All rights reserved.

(Mauritia flexuosa) and olive (Olea europaea L.) fruit stones in orderto set the influence of precursor nature and preparation conditionson the adsorbent properties of the activated carbons.

Materials and methods

Activated carbon preparation

Aguaje and olive fruit stones from the Peruvian provinces ofUcayali and Arequipa were used as precursor materials. Thesematerials were previously washed until all organic residues wereeliminated, oven-dried at 80 C and grounded until less than10 mm particles were obtained.

The sample (50 g) was mixed with a particular concentration ofphosphoric acid solution (initial concentration H3PO4 85%, Merck)in order to obtain three impregnation ratios of 0.75, 1.0, and1.5 calculated as gram weight ratio of phosphoric acid per gram ofprecursor. First, this mixture was vacuum dried and thenoven-dried at 85 C for 24 h.

The impregnated and dried material was later carbonized in astainless steel tubular reactor, which was placed in an oven at aheating speed of 10 C min1 until the set activation temperaturewas reached. Three different activation temperatures (400 C,
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D. Obregn-Valencia, M.R. Sun-Kou / Journal of Environmental Chemical Engineering 2 (2014) 22802288 2281

500 C, and 600 C) were considered and these were maintained for60 min. During the activation process, a 100 cm3min1 nitrogenflow was used through the reactor to create an inert atmosphereand to remove the formed vapor by entrainment.

In this study the sample nomenclature assigned is as follows:AG = aguaje stones and OL = olive stones, follow by the impregna-tion ratio, and the activation temperature in Celsius degree (C).For example, the sample AG0.75_600 was prepared from aguajestones, with an acid impregnation ratio of 0:75gH3PO4=gprecursor, andan activation temperature of 600 C.

Characterization

Thermogravimetric analyses (TGA) and differential thermalanalyses (DTA) of starting materials (precursor material) wereperformed in a thermogravimetric analyzer (LINSEIS STA-PT model1600) under an argon flow. For this analysis 25 mg of each materialwere weighted. They were heated at a rate of 5 C min1 until550 C were reached in an argon atmosphere with 4 L h1 flow.

The textural properties were studied by N2 adsorptionmeasurements at liquid nitrogen temperature using MicromeriticsGEMINI VII, model 2390t equipment. Prior to adsorption experi-ments, samples were degassed in vacuum at 250 C for 2 h. Therange of relative pressures used was from 0.013 to 0.99. The specificsurface area was calculated according to the BET method.Microporous surface was calculated by the t-plot method.Mesoporous area was evaluated by the difference of the totalarea and microporous area.

The quantitative determination of the acidic surface groups wasdone according to Boehms method. About 25 mg of activatedcarbon was added to 50 mL solutions of NaOH 0.1 N standard andwas shaken at room temperature for 24 h. Then, the activatedcarbon samples were separated by filtration. The quantity of acidicsurface group on AC samples was determined by back titration offiltered solutions with HCl 0.1 N standard.

The activated carbons were characterized with an electronicmicroscope of scanning (SEM) Phillips model 505DX. In order toappreciate the morphology with more clarity all samples were re-

60045030015000

20

40

60

80

100

Mas

s lo

ss (%

)

T (C)

TGA

AGUAJE STON E

380155 307

6004503001500-100

-80

-60

-40

-20

0

dm/d

t

T (C)

DTA

AGUAJE STONE

155 307380

Fig. 1. Thermogravimetric analysis and differentia

covered with a gold film to provide the sample of electricalconductivity.

Adsorption measurements

Adsorption equilibrium information is the most important tounderstand an adsorption process. Kinetics tests were conductedat room temperature (20 C) with a cadmium solution with aninitial concentration of 10 ppm and pH 2 (which was adjustedusing 0.1 N HCl and 0.1 N NaOH). About 15 mg of activated carbonwere added to different vials with 20 mL of the aqueous solutionand kept under agitation. The contact time in each vial wasdifferent (between 5 min and 180 min). The amount of cadmiumions adsorbed increases sharply during the first 30 min., and itreaches its equilibrium value within 1 h. The activated carbonsprepared with an activation temperature of 600 C reach theirhighest adsorption capacity within each series.

Adsorption isotherms were determined using a series of solutionsof cadmium in which the initial concentration was increased from2 to 80 ppm using the activated carbons prepared with an activationtemperature of 600 C. The adsorbentadsorbate time of contact was4 h in order to ensure stationary conditions. The adsorption capacityis the amount of cadmium ions adsorbed at equilibrium, qe (mg g1),and it was calculated according to Eq. (1):

qe Co CeV

W(1)

where Co and Ce are the respective initial and equilibriumconcentration of cadmium ions (mg L1); V is the volume ofaqueous phase (L); and W is the weight of the activated carbon (g).The cadmium ion concentration was analyzed using a PerkinElmermodel 3110 spectrophotometer with acetylene flame at awavelength of 228.8 nm.

Results and discussion

TGA and DTA curves for both precursors show four differentstages of weight loss (Fig. 1). For both precursors, the first stage

60045030015000

20

40

60

80

100

Mas

s lo

ss (%

)

T (C)

TGA

OLIVE STONE

380333257155

6004503001500-10 0

-80

-60

-40

-20

0380

333

dm/d

t

T (C)

DTA

OLIVE STONE

257155

l thermal analysis of aguaje and olive stones.

2282 D. Obregn-Valencia, M.R. Sun-Kou / Journal of Environmental Chemical Engineering 2 (2014) 22802288

appears by 40163 C, with a weight loss about of 12% of the initialweight. This stage is associated with the loss of water physisorbedon the precursors. The second stage for the aguaje stones extendsby 163307 C with a weight loss of about 47%, due mainly tohemicelluloses degradation (Fig. 1 left). The olive stones (Fig. 1 right) showed consecutive weight losses by 155257 C and257333 C of about 11 and 22%, respectively, attributed tohemicelluloses and lignin degradation. In DTA curves for olivestones, the inflexion point at 257 C shows a sharp exothermic peakdue to volatile evolution (CO2, CH4 and some hydrocarbon) frombiopolymer degradation of hemicelluloses and lignin. These valuesare significantly higher compared to aguaje stones, which indicatesthat there is a high content of these biopolymers in the olives stones.

The third stage for the aguaje stones shows up by 307377 Cwith a weight loss of about 12%. At this stage the cellulose is mainlydegraded. The olive stones were degraded at 334380 C with aweight loss of about 7%.

Lignin degradation occurred in a wide temperature range (100900 C) due to its structure. The last stage is attributed to lignindegradation. The aguaje stones showed a weight loss by391570 C of about 13.83% with a remainder weight of 14.36%,and the olive stones showed a weight loss by 380570 C of about14% with a remainder weight of 31%. This difference is due to thehigher lignin content in olive stones, a fact which agrees with thelignin content of 6.5% for aguaje stone and 19.5% for olive stone, asdetermined by adapting the analysis method from ASTM D1106-96(2007) [11]. The thermogravimetric results were compared withYang et al. [12] who made a thermogravimetric study of thecellulose, hemicelluloses and lignin, and establishing thereby thetemperature degradation range for each biopolymer.

The results for the surface area, mesoporous distribution,surface acidity, and adsorption capacity are shown in Table 1 [13].In both series, the surface area decreases as the pyrolysistemperature is increased, due to a thermal contraction effect onthe porous structure. The same behavior was observed by othersauthors [1416]. This contraction is more evident in the AG seriesand it affects its micro and mesoporous areas as well.

Both series of activated carbons with an activation temperatureof 600 C show that while the surface area increases with theimpregnation ratio, its mesoporous area decreases. This holds truefor both series and especially for the AG series. As for the OL series,an exception is found for sample OL0.75, given that its mesoporousarea is lower than that of the rest of the OL series. The increase insurface area is attributed to phosphoric acid decomposition(Eq. (2)) [1417] which favors the formation of water and enhancesthe formation of various gaseous compounds, such as CO2(g), CO(g),H2(g) (Eqs. (3) and (4). These compounds are released from theprecursor forming the porous structure of the activated carbon[17].

Table 1Characterization and adsorption capacities of the activated carbon.

Sample Surface area(m2g1)

Surfac(mmo

Total Mesoporous

AG1_400 1202 109 1.42 AG1_500 1077 36 1.98 AG1_600 1014 77 2.24 AG0.75_600 906 140 2.43 AG1.5_600 1623 32 1.40 OL1_400 1283 145 1.44 OL1_500 1242 135 1.80 OL1_600 1169 125 2.37 OL0.75_600 800 10 2.36 OL1.5_600 1565 75 0.54

H3PO4! P2O5(S) + 3H2O(g)(2) (2)

C(S) + H2O(g)! H2(g) + CO(g)(3) (3)

CO(g) + H2O(g)! H2(g) + CO2(g)(4) (4)Acidic surface groups were determined by Boehms method.

The increase in activation temperature generated the increase insurface acidity (Table 1); this property is closely related toabsorbentadsorbate interactions that take place on the carbonsurface. Puziy et al. [18] found that, as the activation temperature isincreased, the surface enrichment with oxygen progressivelyexpands to deeper layers of carbon due to the action of phosphoricacid as an oxidizing agent. In both series, the sample with higherimpregnation ratio (AG1.5_600 and OL1.5_600) showed a lowacidic surface. Thus, although phosphoric acid favors the formationof carbonoxygen bonds, in large quantity the phosphorouscompounds avoid the formation of oxygen-containing functionalgroups on the carbon surface. This behavior was observed instudies on AC prepared from cellulose, hemicelluloses, and lignin[1921]. For OL series the surface acidity is very similar betweenOL1_600 and OL0.75_600 decreasing significantly in OL1.5_600.

Fig. 2 shows the SEM micrographs of the four activated carbonswith higher adsorption capacity at a magnification of 1010X. In AGseries, the sample AG0.75_600 (Fig. 2 top-left) shows pores with anaverage size of 8 mm on the surface of the activated carbon. SampleAG1_600 (Fig. 2 top-right) shows pores with an average size of5 mm. OL series show more compact fibers, furthermore, theporous structure is not so developed as in AG series. The sampleOL0.75_600 (Fig. 2 bottom-left) shows pores with an average sizeof 3.5 mm. Finally, the sample AC1_600 (Fig. 2 top-right) shows anirregular surface with pore sizes from 2 to 0.5 mm.

Adsorption capacity

For both series of activated carbons, the capacity for cadmiumion adsorption increases with higher activation temperature, asshown in Table 1. Furthermore, the adsorption capacity decreaseswith the increase of impregnation ratio. The higher adsorptioncapacities in samples AG0.75_600 and OL1_600 were associated totheir high surface acidity and high mesoporous area (Table 1). BothAC samples with high adsorption capacity (AG0.75_600 andOL1_600) differ in their impregnation ratio. This fact is attributedto differences in their precursor source. The higher lignin contentand compact fibers in olive stones required a high ratio ofimpregnation in order to get a high superficial acidity and suitablemesoporous structures in favor of cadmium ion adsorption.

e acidityl H+ g1)

Adsorption capacity(mg g1)

Removal efficiency(%)

0.60 52.92 227.28 558.14 614.11 310.68 53.03 239.01 683.00 232.66 20

Fig. 2. SEM micrographs of activated carbons at a magnification of 1010X.


On the other hand, while sample OL0.75_600 shows a similarsurface acidity as OL1_600, its surface area was not completelydeveloped and its low mesoporous area was unfavorable to theadsorption capacity (3 mg g1).

Fig. 3. Effect of contact time on the removal of cadmium ion from d

In accordance with the results, the surface acidity and thesurface area developed (high mesoporous structure) were themain important factors in the adsorption capacity of cadmium.

ifferent AC prepared with an activation temperature of 600 C.

Table 2Pseudo first order, pseudo second order and Elovich parameters.

Sample Pseudo first order Pseudo second order Elovichs model

qe(mg g1)

k1(min1)

r2 qe(mg g1)

k2(g mg1min1)

r2 Dqe (%) (1/b) ln (ab)(mg g1)

1/b(mg g1)

r2

AG1_400 0.25 0.03 0.69 0.63 0.20 0.99 11.08 0.09 0.11 0.84AG1_500 0.52 0.01 0.64 2.88 0.10 0.99 12.91 2.12 0.13 0.64AG1_600 1.45 0.03 0.78 7.38 0.06 1.00 2.53 4.21 0.19 0.83AG0.75_600 2.58 0.03 0.91 8.35 0.03 1.00 2.29 3.77 0.97 0.89AG1.5_600 0.56 0.05 0.80 4.15 0.21 1.00 2.27 3.06 0.24 0.77OL1_400 0.20 0.02 0.87 0.70 0.36 0.99 3.68 0.32 0.08 0.89OL1_500 0.82 0.02 0.86 3.04 0.07 0.99 13.85 1.90 0.20 0.84OL1_600 3.53 0.07 0.98 9.22 0.04 1.00 17.70 5.67 0.75 0.97OL0.75_600 0.93 0.02 0.94 3.06 0.08 0.99 23.75 1.52 0.31 0.95OL1.5_600 1.60 0.06 0.84 2.84 0.05 0.99 14.84 0.45 0.51 0.92


Kinetic studies

The adsorption kinetic describes the rate at which Cd(II) isadsorbed onto the activated carbon surface and the time requiredto reach its equilibrium. The results were assessed with pseudofirst and second order models, and the Elovich equation describedby:

Pseudo first order [22]:

log qe qt log qe k1

2:303

t (5)

Pseudo second order [23]:

tqt

1k2q2e

1qet (6)

where k1 (min1) and k2 (g mg1min1) are the first and secondrate constant of adsorption respectively, qe and qt (mg g1) are theamount adsorbed at equilibrium and the amount adsorbed at timet. A linear plot of log(qe qt) vs. t and t/qt vs. t is used to calculatedk1 and 1/qe (slops) and log (qe) and 1/kqe2 (intercepts), respectively.

Elovichs model [24]:

qt 1b

lnab 1

blnt (7)

where a is considered the initial adsorption rate (mg g1min1)and b is related to the area of surface coverage and activationenergy for chemisorptions (g mg1). A linear plot of qt vs. ln (t) isused to calculated 1/b (slope) and (1/b)ln (ab) (intercept). The 1/bvalues are related to the vacancy sites available on the AC surfaceand (1/b)ln (ab) values are related to the adsorption amount at zerotime.

All kinetics curves of activated carbon with an activationtemperature of 600 C are shown in Fig. 3. The equilibrium statewas reached within the first hour. The kinetic model parameters

Table 3Comparison of intraparticle diffusion constants from activated carbon.

Sample Intraparticle diffusion model

kdiff1(mg g1min0.5)

kdiff2(mg g1min0.5)

kdiff3(mg g1min

AG1_400 0.09 0.07 0.02 AG1_500 1.05 0.04 0.07 AG1_600 2.15 0.51 0.07 AG0.75_600 2.14 0.61 0.15 AG1.5_600 1.43 0.17 0.02 OL1_400 0.18 0.05 0.01 OL1_500 1.03 0.06 0.11 OL1_600 3.06 0.44 0.07 OL0.75_600 0.85 0.17 0.08 OL1.5_600 0.49 0.33 0.04

are shown in Table 2. The kinetic curves fitted better with pseudosecond order model with r2 values as 0.99 which means thatadsorption occurred by two mechanisms: physical and chemicalabsorption over the activated carbon surface.

A similar behavior was established by other authors [2527]. Therate constant, k2, is lower in the AC with higher adsorption capacity.It can be established that if the diffusion rate is higher in other AC, itdoes determine the adsorption capacity. Furthermore, the Elovichparameters show that adsorption capacity is proportional to theavailable areas according to 1/b (mg g1) values.

Adsorption mechanism

The intraparticle diffusion model was also evaluated to identifythe mechanism of adsorption based on Weber and Morriss theory[28]. They established that the adsorption mechanism depends onthe physicochemical characteristics of the adsorbents as well as onthe mass transport or diffusion. The intraparticle diffusioncoefficient, kdiff, is given by:

qt kdiff t0:5 C (8)where qt (mg g1) is the adsorption capacity at time t (min), kdiff(mg g1min0.5) is the intraparticle diffusion constant and C(mg g1) is related to the thickness of boundary layer.

The intraparticle parameters are shown in Table 3. Theintraparticle diffusion constant values decrease from the firststage, or external surface adsorption, to the second stage whereintraparticle diffusion is the limiting rate. Comparing theparameters on the second stage, in both series the higher valueswere found in AG0.75_600, AG1_600 and OL1_600. None linearportion of second and third stage are extrapolated through theorigin, this indicates that particle diffusion is involved but is notthe only limiting rate mechanism (Fig. 4).

In brief, kinetic curves fitted in pseudo second order model withmajor influence of intraparticle diffusion, which leads to establish

0.5)C1 C2 C3 r1

2 r22 r3

2

0 0.11 0.41 1.00 0.69 0.710 2.29 2.04 1.00 0.41 0.740 4.11 6.52 1.00 0.78 0.990 3.95 6.52 1.00 0.77 0.910 3.04 3.94 1.00 0.68 0.650 0.35 0.52 1.00 0.75 0.960 2.18 1.82 1.00 0.98 0.940 5.90 8.27 1.00 1.00 0.670 1.67 2.16 1.00 0.80 1.000 0.48 2.26 1.00 0.91 0.61

1210864200

3

6

9

12

kdiff 3kdiff2

q t(m

g g-

1 )

t1/2 (mi n1/2)

AG1_600 AG0.75_600 AG1_500 AG1.5_60 0 AG1_400

kdiff1

1210864200

3

6

9

12

kdiff 3kdiff 2

q t(m

g g-

1 )

t1/2 (mi n1/2)

OL1_600 OL0.75_600 OL1_500 OL1.5_600 OL1_400

kdiff 1

Fig. 4. Intraparticle diffusion model plot for the adsorption of cadmium ion from different activated carbons.


that ion cadmium adsorption is governed by two types ofinteractions, physical and chemical, over the activated carbonsurface.

Adsorption isotherms

An adsorption isotherm curve describes the phenomenongoverning the retention or mobility of a substance from theaqueous solution to a solid-phase at a constant temperature (20 C)and pH 2. The experimental values are related to the followingequilibrium isotherm models: Langmuir, Freundlich, RedlichPeterson, DubininRadushkevich and Temkin [29].

Langmuir isotherm model can be represented as:

Ceqe

1KLqmax

ceqmax

(9)

Freundlich isotherm model can be represented as:

logqe logKF 1nlogCe (10)

where qe (mg g1) is the adsorption capacity at equilibrium and Ce(mg L1) is the solute concentration in the solution at equilibrium.The KL (L mg1) constant is related to the adsorption heat and theKF (mg1 1/n L1/ng1) constant is a parameter related to theadsorption capacity. The maximum capacity value is representedby qmax (mg g1) and 1/n value (dimensionless) represents theadsorption intensity.

806040200

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4

6

8

10

12

14

q e (m

g g-

1 )

C (mg Le-1)

OL0.75_600

OL1_600

OL1.5_600

-1

Fig. 5. Adsorption isotherms of cadmium ion from different activat

The RedlichPeterson isotherm [30] is an empirical isotherm. Itcombines elements from both the Langmuir and Freundlichequations and the mechanism of adsorption is a hybrid. Thereforeit can be applied either to homogeneous or heterogeneoussystems. The linearized equation is expressed as:

ln ARPCeqe

1

GlnCelnBRP (11)

where qe (mg g1) is the adsorption capacity at equilibrium, Ce (mgL1) is the solute concentration present in the solution atequilibrium; ARP,BRP and G (heterogeneity grade) are RedlichPeterson parameters. In the limit, it approaches Freundlichisotherms model at high concentration (as G values tend to 0)and is in accordance with the low concentration limit of the idealLangmuir condition (as G values are all close to 1).

DubininRadushkevich (DR) isotherm assumes that there is asurface area in which the adsorption energy is homogeneous. TheDR isotherm is expressed as:

lnqe lnqs Kade2 (12)where e = RTln [1 + 1/Ce], qs is the theoretical isotherm saturationcapacity (mg g1), Kad is related to the mean free energy of sorptionper mole of the adsorbate when it is transferred to the surface ofthe solid from the solution and this energy can be computed usingthe following relationship:

E 1ffiffiffiffiffiffiffiffiffiffiffi2Kad

p (13)

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4

6

8

10

12

14

q e (m

g g

)

C (mg Le-1)

AG0.75_600

AG1_600

AG1.5_600

ed carbons prepared with an activation temperature of 600 C.

Table 4Comparison of Frendlich, Langmuir, RedlichPeterson, DubininRadushkevich, and Temkin model adsorption parameters at pH 2.

Model Sample

AG0.75_600 AG1_600 AG1.5_600 OL0.75_600 OL1_600 OL1.5_600

FreundlichKF (mg g1) 6.08 3.83 1.90 1.70 5.78 2.291/n 0.18 0.23 0.32 0.30 0.20 0.22r2 0.81 0.80 0.88 0.98 0.76 0.92Dqe (%) 1.52 5.92 2.94 158.24 17.03 3.69

Langmuirqmax (mg g1) 11.87 9.47 6.76 6.87 11.86 6.57KL (L mg1) 0.74 0.48 0.23 0.14 0.83 0.11r2 1.00 0.99 1.00 0.99 0.99 0.97Dqe (%) 0.03 3.56 0.76 3.87 10.40 1.39

RedlichPetersonBRP (L mg1)G 0.77 0.98 0.98 0.43 2.34 0.25G 0.99 0.97 0.95 0.96 0.98 0.97r2 1.00 0.99 0.99 0.99 1.00 0.99Dqe (%) 0.03 0.72 0.39 1.08 5.36 0.62

DubininRadushkevichQm (mg g1) 11.1 8.1 5.2 4.7 10.0 5.1Kad 0.29 0.15 0.52 0.53 0.05 3.62E (kJ mol1) 1.32 1.80 0.98 0.97 3.06 0.37r2 0.96 0.95 0.90 0.80 0.96 0.61Dqe (%) 0.03 0.72 0.39 1.08 5.36 0.62

TemkinA (L g1) 69.79 31.73 4.56 3.54 142.84 4.10B (kJ mol1) 1.67 1.98 2.12 2.32 1.83 2.41r2 0.87 0.93 0.98 0.97 0.90 0.88Dqe (%) 1.13 4.39 1.07 0.50 5.98 0.39

Table 5Comparison of Frendlich, Langmuir, RedlichPeterson, DubininRadushkevich, andTemkin model adsorption parameters at pH 5.

Model Sample

AG0.75_600 OL1_600

FreundlichKF (mg g1) 12.03 12.591/n 0.19 0.18r2 0.95 0.91Dqe (%) 0.73 1.85

Langmuirqmax (mg g1) 26.33 24.83KL (L mg1) 0.46 0.78r2 0.99 1.00Dqe (%) 8.27 14.50

RedlichPetersonBRP (L mg1)G 1.69 3.17G 1.00 1.00r2 1.00 1.00


The Temkin isotherm assumes that the decrease in the heat ofadsorption is linear rather than logarithmic, as implied in theFreundlich equation. It is expressed as:

qe RTblnAT RTb lnCe (14)

where constant RT/b is related to the heat of adsorption, R theuniversal gas constant (J mol1 K1), T the temperature (K), b thevariation of adsorption energy (J mol1) and AT is the equilibriumbinding constant (L mg1) corresponding to the maximum bindingenergy.

The isotherms curves are shown in Fig. 5. All activated carbons,except for AG1_600 and OL1_600, are concave type and are similarto curve L type according to Giless classification [31], meanwhile,AG1_600 and OL1_600 are similar to H type curve. This type ofcurve represents a strong adsorption of the adsorbate which isreflected in the high initial slope. In all cases, except forAG0.75_600, there is a progressive increase of adsorption capacityas the equilibrium concentration increases.

Thecorrelationof theexperimentalvalueswiththemathematicalisotherm models are shown in Table 4. The isotherm constants forRedlichPeterson were evaluated using a trial-and-error optimiza-tion method to calculate the isotherm constants through maximiza-tion of the coefficient of determination (r2). Both activated carbonseries (AG and OL) isotherm curves fit the Langmuir and RedlichPeterson isotherm models (r2 0.99 for all the tested samples. Thenormalized standard deviation values were calculated by [32]:

Dqe% 100 X ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiqeexp qepred=qepred2

N

s(15)

where qe(exp) is the experimental qe and qe(pred) is the correspond-ing predicted qe according to the equation under study with bestfitted parameters, N is the number of measurements.

The G value close to 1 in RedlichPeterson model confirms thatthe adsorption process corresponds to a Langmuir model.

The Langmuir isotherm model shows that the monolayeradsorption value (qmax) decreases with high impregnation rates. InAG series, these values decrease from 11.87 to 6.76 mg g1. In OLseries, the maximum adsorption capacity is obtained forOL1_600 and the least for OL1.5_600 with 11.86 and 6.57 mg g1,respectively.

Furthermore, the adsorption constant, KL, is higher in theactivated carbons with higher adsorption capacities in both series;this means that adsorption process requires a high energy to retainthe cadmium ion over the carbon surface. This can be explained bypH solution condition (pH 2) which can produce a competitiveeffect between cadmium ions and protons H+ in the solution.

Dqe (%) 1.22 76.13


Furthermore, the total surface charge will be on average positive[4] in a solution with pH lower than the point zero charge (pHPZC)of the adsorbent (in previous work, it was found that pHPZC was onaverage 2.55) which generates electrostatic repulsion between theAC surface charge and the cadmium ion.

The adsorption capacity of Cd increased at pH 5 (Table 5), forexample, according with Langmuir model, the sample AG0,75 _600reached a qmax = 26.33 mg g1 and OL1_600 a qmax = 24.83 mg g1.These results were compared with Rao et al. [33] which obtained aqmax = 19.59 mg g1. At this pH, the carbon surface is negativelycharged, it gets more nucleophile and this allows a greaterattraction of the adsorbate to the surface of the adsorbent. Thisindicates that the adsorption process would be associated with anelectrostatic interaction.

Conclusions

The parameters with greater influence in the adsorptionproperties of the activated carbons are the precursor nature, theactivation temperature, and the impregnation ratio. Aguaje stonesoffer a promising raw material for the production of activatedcarbon for water treatment purposes.

The activated carbon chemical and textural characteristics arehighly related to the precursor nature and generate differentadsorption capacities, although both precursors (aguaje and olivefruit stones) were prepared under the same conditions.

The activated carbons with the highest adsorption capacitieswere: from AG series, AG0.75_600 (26.33 mg g1) and from OLseries OL1_600 (24.83 mg g1) at pH 5. Both activated carbonspresented a high developed mesoporous area (140 y 125 m2g1,respectively) and a high superficial acidity (2.43 y 2.37 mmolH+ g1, respectively). The high lignin content and natural fibercompaction in olive stones required a higher impregnation ratio toget a suitable superficial acidity and mesoporous area to increasethe cadmium ion adsorption.

The kinetic curve fitted suitably to pseudo second order model,which allows to establish that the adsorption process is the resultof two types of interactions, chemical and physical, between themetal ion and the activated carbon surface.

The isotherm model fits suitably to Langmuir model. Inaccordance with RedlichPetersons G parameter near to 1, theadsorption process mostly takes place in homogeneous active sitesover the activated carbon surface.

Conflicts of interest

The authors declared that there is no conflict of interest.

Acknowledgement

The authors thank Vicerrectorado de Investigacin (VRI-PUCP)for the partial support under Proyecto Interdisciplinario DGI 2010-0099.

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