Removal of metal ions Cd (II), Pb (II), and Cr (III) from water by the cashew nut shell Anacardium occidentale L

Ecological Engineering 73 (2014) 514–525

Removal of metal ions Cd (II), Pb (II), and Cr (III) from water by thecashew nut shell Anacardium occidentale L

Gustavo Ferreira Coelho a,*, Affonso Celso Gonçalves Jr. a,1, César Ricardo Teixeira Tarley b,2

, Juliana Casarin a,3, Herbert Nacke c,4, Marcio André Francziskowski a,5

a State University of West Paraná, Center for Agricultural Sciences, Marechal Cândido Rondon, Paraná, Rua Pernambuco, 1777, CEP 85960-000, Brazilb State University of Londrina, Center of Exact Science, Department of Chemistry, Londrina, Paraná, BrazilcUniversity Center Dynamic of Cataracts, Foz do Iguaçu, Paraná, Rua Castelo Branco, 349, CEP:85852-010, Brazil

A R T I C L E I N F O

Article history:Received 9 July 2014Received in revised form 10 September 2014Accepted 29 September 2014Available online xxx

Keywords:BiosorptionWater contaminationSustainabilityRemoval

A B S T R A C T

Current paper analyzes the cashew nut shell (Anarcadium occidentale L.) (CNS1) as a natural adsorbent inthe removal of the metal ions Cd2+, Pb2+, and Cr3+ from contaminated water. Adsorbent was characterizedas to its chemical and structural composition by infra-red spectroscopy (IR2); as to its morphology byscanning electron microscopy (SEM3); and point of zero charge (pHPCZ

4). Best adsorption conditions (pH,adsorbent mass, contact time) were determined. Adsorption kinetics was evaluated by pseudo-firstorder, pseudo-second order, Elovich and intra-particle diffusion mathematic models, whereas adsorptionisotherms were linearized according to mathematical models by Langmuir, Freundlich and Dubinin–Radushkevich (D–R5). The effects of initial concentration, temperature in the process, de-sorption andthe comparison with activated coal were also performed. SEM and IR showed positive characteristics toadsorption. Further, pHPCZ of CNS lay between 3.69 and 4.01. Best adsorption conditions of the ions Cd2+,Pb2+, and Cr3+ were pH 5.0; adsorbent mass: 12 g L�1; equilibrium time 60 min. Pseudo-second order andD–R models suggested the predominance of chemo-sorption process. Adjustment of Langmuir andFreundlich models suggested adsorption in mono- and multi-layers. Thermodynamic study showed thatthe process was spontaneous for Cd2+ at 15 and 25 �C. CNS had high desorption rates for Cd2+ and Pb2+, butlow desorption with Cr3+. CNS has a potential to aggregate economical rate and increase the crop’sproductive chain when the residue is used for the removal of Cd2+, Pb2+, and Cr3+ from water.

ã 2014 Elsevier B.V. All rights reserved.

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Ecological Engineering

journal homepage: www.elsevier .com/ locate /ecoleng

1. Introduction

Metals derived from human activities have been frequentlydetected in sediments, rivers, lakes and others. They cause highly

* Corresponding author. Tel.: +55 45 3228 7924/9972 5614.E-mail addresses: [email protected] (G.F. Coelho),

[email protected] (A.C. Gonçalves Jr.), [email protected](C.R.T. Tarley), [email protected] (J. Casarin), [email protected](H. Nacke), [email protected] (M.A. Francziskowski).

1 Tel.: +55 45 3228 7924.2 Tel.: +55 43 3371 4811.3 Tel.: +55 45 3228 7924.4 Tel.: + 55 45 3523 6900.5 Tel.: +55 45 3228 7924.1 CNS: chasew nut shell.2 IR: infra-red spectroscopy.3 SEM: scanning electron microscopy.4 pHPCZ: point of zero charge.5 D–R: Dubinin–Radushkevich.

http://dx.doi.org/10.1016/j.ecoleng.2014.09.1030925-8574/ã 2014 Elsevier B.V. All rights reserved.

contaminated waters world wide (Peng et al., 2009). Contamina-tion of aquatic environments by metals is a great concern due to itstoxicity, abundance and persistence in the environment, with thesubsequent accumulation in water habitats, micro-organisms,water flora and fauna which, in its turn, enters the food chain andcauses significant effects on human health (Chabukdhara andNema, 2012).

According to Ahmaruzzaman (2011), the efficient removal ofmetal ions from wastewaters is a highly relevant issue nowadays.Methods such as precipitation followed by coagulation or filtrationby membrane have been used for the removal of metals fromwater. These processes become unfeasible since they produce slug,low metal removal rates, and high costs (Ahmaruzzaman, 2011 andHsu, 2009). Among several methods, the adsorption process is oneof the most efficient methods for the removal of heavy metals fromwater solution (Özacar et al., 2008).

Adsorption with activated coal is a highly known method for theremoval of metal ions but high costs restrict its use. Cheaperefficient alternatives should be given priority (Hsu, 2009). One of

Table 1Chemical characteristics of cashew nut shell.

K Ca Mg Cu Fe Mn Zn Cd Pb Cr––––- g kg�1 –––– –––––––––––––––––– mg kg�1––––––––––––––––-

7.65 9.23 1.67 6.73 19.30 52.60 13.40 <0.005 <0.01 <0.01

LQ (quantification limits): K = 0.01; Ca = 0.005; Mg = 0.005; Cu = 0.005; Fe = 0.01;Mn = 0.01; Zn = 0.005; Cd = 0.005; Pb = 0.01; Cr = 0.01.

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these alternatives is the use of natural alternatives in the removalof metals from water since many agro-industries and food plantsproduce great amounts of solid wastes which, in their turn, have ahigh pollution potential and constitute serious problems for finaldisposal Pinto et al. (2006).

Several assays gave promising results with regard to the use ofagro-industry wastes in the de-contamination of water environ-ments: dry biomass of Eichornia crassipes (Gonçalves et al., 2009);sugarcane bagasse (Dos Santos et al., 2010), Moringa oleifera L am.(Gonçalves et al., 2013a and Meneghel et al., 2013a) Crambeabyssinca Hochst (Gonçalves Jr. et al. 2013b and Rubio et al., 2013a,b,b), bark of Pinus elliottii (Gonçalves et al., 2012 and Strey et al.,2013), manioc industrial wastes (Schwantes et al., 2013) andbiomass of the Jatropha curcas (Nacke et al., 2013). The cashew nutis a highly promising crop and actually several assays have beenimplemented for the development of technologies and processesthat would make possible its use Pinho et al. (2011).

The cashew nut is very important in the social and economicalcontext of many regions due to its high nutrition rates and itscommercial potentiality. Intensive work in its production andindustrialization warrant a good income. The cashew nut culture isa relevant source of jobs for many populations Ramos et al. (2011).

Processing of the cashew crop involves the nuts for consump-tion and the mesocarp or cashew nut shell which produces thecashew nut shell liquid (CNSL), or rather, oil for industrial use,namely resins and brakes, and medicine, such as antiseptics andworm killer Mazzeto et al. (2009). However, after the extraction ofCNSL, shells are normally disposed of inadequately in the soil.

Since the shell makes 20% of the cashew nut and, according todata by FAO (2013), world production amounts to 4.28 million tons,it may be stated that the yearly production of shells isapproximately 856 thousand tons. Brazil participates in 54 thou-sand tons of world total. Current investigation aims at using thecashew nut shell, after oil extraction, for the removal of metal ions(Cd2+, Pb2+, and Cr3+) from contaminated water by adsorption. Itshould also be underscored that the use of the above material forenvironment de-contamination may enhance the culture of thespecies aggregating rates to its co-product and may promote amore sustainable development of the cashew nut.

2. Materials and methods

2.1. Adsorbent material

Cashew nuts were harvested in the municipality of Curionóp-olis, in the Pará State, Brazil and transported to the Laboratory ofEnvironmental Chemistry and Instrumental of the State Universityof West Paraná (UNIOESTE), campus Marechal Cândido Rondon,where research was performed.

Shells of the cashew nut (A. occidentale L.) were separated fromthe nuts, ground in an industrial liquefier and dried in a buffer at60 �C for 36 h. The cashew nut shell liquid (CNSL) was extracted bySoxhlet system with n-hexane (C6H14, Nuclear) (IUPAC, 1998) andCNS biomass was obtained. The latter was dried once more in abuffer at 60 �C for 24 h for the total evaporation of n-hexane, asolvent for oil extraction.

No previous treatment was employed during assays so that theadsorbent material could be employed at the same conditions inwhich it was obtained after the industrial processing of cashew nuts.The adsorbent was only sieved (14–65 Mesh–Bertel) to standardizethe particles (between 0.212 mm and 1.18 mm) after drying.

2.2. Characterization of adsorbent materials

The chemical composition of the adsorbent material wasdetermined by nitroperchloride digestion (2:1) (AOAC, 2005),

followed by the determination of K, Ca, Mg, Cu, Fe, Zn, Mn, Cd, Pb,and Cr by Flame Atomic Absorption Spectrometry (FAAS).

The main functional groups in the adsorbent’s structure werealso evaluated due to the fact that they may affect the adsorption ofthe metals Cd, Pb, and Cr. Infra-red spectra were determined in aShimadzu Infrared Spectrophotometer FTIR- 8300 Fourier Trans-form, in the region between 400 and 4000 cm�1, resolution 4 cm�1,in which spectra were obtained by transmittance with KBr pellets.

The material’s surface morphology was also evaluated byScanning Electron Microscopy (SEM) by a microscope FEI Quanta200, at voltage 30 kV. Samples were placed on a double-face carbonadhesive band fixed on a sample support and later metalized withgold up to a thickness of approximately 30 nm by Baltec ScutterCoater SCD 050.

The point of zero charge (pHPCZ) was also determined, or rather,pH in which cations and anions on the adsorbent’s surface wereequivalent, featuring zero charge. Further, 500 mg of the bio-sorbent were added to 50 mL of KCl water solution at 0.05 and0.5 mol L�1 at initial pH rates between 2.0 and 9.0, were adjustedwith HCl and NaOH standard solutions (0.1 mol L�1). After 24 hshaking at 200 rpm, final pH rates were obtained. A graph wasprepared on the variation of initial pH and final pH (DpH), in whichpHPCZ was attributed to the rate where DpH equaled zero (Mimura,2010).

2.3. Determination of best conditions for the adsorption process

Mono-elementary solutions with metal ions Cd2+, Pb2+, and Cr3+

were prepared from cadmium nitrate salts (Cd(NO3)24H2O P.A. � 99%), lead nitrate [Pb(NO3)2 P.A. � 99%] and chromium nitrateIII [Cr(NO3)39H2O P.A. � 99%]. All metal concentrations weredetermined by FAAS at the end of each adsorption test.

The removal of heavy metals by adsorption in a water mediumdepends on several factors, such as amount of adsorbent, pH,contact time and temperature. Tests were performed to verify thebest conditions for adsorption. In the case of tests on the biomass,increasing amounts of adsorbent material (0, 200, 400, 600, 800,1000, and 1200 mg) were employed, at three pH conditions (5.0,6.0, and 7.0), adjusted with standardized solutions of HCl or NaOH(0.1 mol L�1). The biomass used with 50 mL of each metal-contaminated water solution was added to 125 mL Erlenmeyerflasks. The flasks were shaken for 90 min at constant temperatureand stirred (25 �C and 200 rpm) in a thermostatized warm bathDubnoff. Samples were then filtered in quality paper to determinethe final concentrations of metals by FAAS.

Adsorbed amount was calculated from rates obtained for finalconcentration Eq. (1).

Q ¼ C0 � Cf� �

mV (1)

where: Q is the amount of ions per unit of adsorbent in equilibrium(mg g�1); m is the mass of the adsorbent used (g); C0 is the initialconcentration of the ion in the solution (mg L�1); Cf is theconcentration of the ion in the solution (mg L�1); V is the volume ofthe solution used (L). Removal percentage of metal ions wascalculated by Eq. (2):

Fig. 1. IR spectra of the A. occidentale L. shell.

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%R ¼ 100 � Cf

C0� 100

� �(2)

where: %R is the removal percentage of the ion by the adsorbent; Cfis the final concentration of the ion (mg L�1); C0 is the initialconcentration of the ion in the solution (mg L�1).

Further, 600 mg of the adsorbent were added to 50 mL of therespective mono-elementary solutions, strengthened with 10 mgL�1 of Cd2+, Pb2+, or Cr3+, at pH 5.0, in 125 mL Erlenmeyer flasks todetermine equilibrium time for the adsorption process. Sampleswere then removed at intervals (5–180 min) and filtered in qualityfilter paper to determine the final concentration of the metals byFAAS.

The kinetic mechanism which controlled the adsorptionprocess was evaluated by linear model parameters of pseudo-firstorder (Lagergren), pseudo-second order, Elovich and intraparticlediffusion mathematical models (Ho and McKay, 1999; Ibrahim,2010; Han et al., 2010 and Witek-Krowiak et al., 2011).

2.4. Adsorption isotherms

Adsorption isotherms were obtained by the addition of 600 mgof the adsorbent in 125 mL Erlenmeyer flasks to 50 mL of a mono-elementary solution with the metal ions Cd2+, Pb2+, or Cr3+, inincreasing concentrations 5, 20, 40, 60, 80, 100, 120, 140, 160, 180,and 200 mg L�1.

The flasks were shaken at 200 rpm in a thermostatized warmbath at 25 �C for 60 min. Samples were then filtered in quality filterpaper to determine the concentration of metal ions in the solutionby FAAS. The amount adsorbed in equilibrium of metal ions by theadsorbent (Qeq) was calculated by Eq. (1). Isotherms of metaladsorption on the cashew nut shell were linearized from resultsobtained by Langmuir, Freundlich and Dubnin–Radushkevichmathematical models.

Fig. 2. SEM image of the adsorbent material amp

2.5. Desorption

Adsorbent material used in the process was separated from thewater solution by filtration in quality filter paper, washed inultrapure water and dehydrated in a buffer at 60 �C for 24 h.Adsorbent mass obtained after drying was placed in contact with50 mL of HCl solution (0.1 mol L�1) for 60 min at constanttemperature and shaking. The samples were again filtered todetermine the metals’ final concentrations.

De-sorption percentage was calculated by Eq. (3):

D ¼ CeqðdesÞCeqðadsÞ

� �� 100 (3)

where, Ceq(des) (mg L�1) and Ceq(ads) (mg L�1) were respectivelythe concentration desorbed by the adsorbent and the concentra-tion adsorbed in equilibrium.

2.6. Influence of temperature

Tests were performed to verify the effect of temperature on theadsorption process. Therefore, 1 g of the material was added to50 mL of the solution with Cd2+, Pb2+, and Cr3+ at concentrations 50,150, and 50 mg L�1, respectively, adjusted at pH 5.0 in 125 mLErlenmeyer flasks. Samples were then shaken at 200 rpm atdifferent temperatures (15, 25, 35, 45, and 55 �C).

Results were used to calculate the parameters of Gibbs’s freeenergy (DG), enthalpy (DH) and entropy (DS) to evaluate thethermodynamic parameters and investigate the process type (Sariet al., 2007).

2.7. Comparison with commercial adsorbent

Cashew nut shell was compared with powdered commercialadsorbent activated coal A.R. (Synth), with a particle size smallerthan 365 mesh, a commercial adsorbent widely used in theremoval of pollutants (Gonçalves et al., 2007). The same testconditions with regard to adsorption isotherms and desorption forthe adsorbent were employed.

3. Results and discussion

3.1. Characterization of the adsorbent

Table 1 shows the chemical composition of metals in theadsorbent Cd, Pb, and Cr rates above the quantification limits werenot detected by the method employed.

FTIR spectra within the 400–4000 cm�1 interval for the A.occidentale L. shell (Fig. 1) shows anion functional groups(carboxylic, hydroxyl, and amine) on the adsorbent’s surface.

A wide strong band at 3400 cm�1 may be due to the vibrationstretching of the O–H bond and suggests the presence of the

lified 160 (a), 5000 (b) and 12,000 (c) times.

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hydroxyl (OH�) groups in the cellulose, lignin and water (Rubioet al., 2013a; Tarley and Arruda, 2004; Gonçalves et al., 2010) oramine (NH2) and starch groups in the polymer compounds(Munagapati et al., 2010).

According to Rodrigues et al. (2006) and Mazzeto et al. (2009),these polymers may be related to the continuous heating of the oilof the cashew nut shell in the adsorbent, caused at the instance ofextraction by the Soxhlet system in which the polymerizationreactions occurred. Polymerization may have been caused byreactions from the hydroxyl group, followed by oligomerization inwhich polymers could also been obtained (Mazzeto et al., 2009).

Band at 2925 and 1380 cm�1 refers to the vibration stretching ofC–H bonds of the alkane and aliphatic acid groups. Stretching wasalso found in the Crambe abyssinica H. used as adsorbent of Cd2+

(Rubio et al., 2013a) and Cr3+ (Rubio et al., 2013b).Carboxyl and starch groups may also be present in the cashew

nut shell due to 1640 cm�1 band attributed to the vibrationstretching of the C–O bonds in phenols or still the C=C or C=N bondswithin the aromatic region (Ibrahim, 2010; Monier et al., 2010).According to Garg et al. (2009), the 1076 cm�1 band suggests C–O

Fig. 3. Effect of adsorbent’s mass, solution’s pH and final pH of the solution in theremoval of Cd2+ (a), Pb2+ (b), Cr3+ (c).

stretching, related to aromatic groups, due to the –OCH3 groupwhich confirms the lignin structure in the cashew nut shell.

Lignin occurs in great quantities on the cell walls of plants andvegetal residues. It is a natural polymer considered to be the mainagglutinating agent for the components of fibrous plants, with 16to 33% of the plants’ biomass (Guo et al., 2008). It is made up ofseveral functional groups such as hydroxyl, aliphatic, phenolic,carboxylic groups which, according to Wu et al. (2008), empowernatural adsorbents to remove metals in contaminated water byadsorption, ion exchange or complexation.

According to Barka et al. (2010), band with wave lengths lessthan 800 cm�1 may also be due to N with bio-bonds. Vibrationstretching of the C–N bond may also be found within the 667 cm�1

band (Salem and Awwad, 2011).SEM directly demonstrates the micro-structures of different

adsorbents (Kumar and Porkodi, 2007). The micro-porous struc-ture of the cashew nut shell was detected in resolutions 160, 5000,and 120,000� (Fig. 2), in which the adsorbent surface had alamellar, spongy, irregular and heterogeneous structure which,according to Rubio et al. (2013a), favored the adsorption of metalions in the water solution.

The electric charge of a solid surface depends on the solution’spH; the point of zero charge (PCZ) corresponds to pH in which thesolid surface has nil charge, or rather, when it has the same amountof anions and cations. Results from pHPCZ tests indicated rate of thecashew nut shell between 3.69 and 4.01. According to Kumar andPorkodi, (2007) and Araújo et al. (2013), when pH > pHPCZ, theadsorbent’s surface is electronegative and favors the adsorption ofmetal cations such as Cd2+, Pb2+, and Cr3+. On the other hand, whenpH < pHPCZ, the adsorbent’s surface is electropositive. In currentanalysis, H+ ions effectively vie with the metal cations which arerepelled from the surface and a decrease in adsorption ensues.

For studies on pH influence in the adsorption of metal cations, itis important that rates of the solution’s pH are close to those ofpHPCZ. Solutions for pH influence tests were adjusted to pH rates4.0, 5.0, and 6.0. Acid media favored the adsorption of metals byforming soluble cations Sud et al. (2008).

3.2. Influence of solution’s pH and adsorbent mass

Adsorbent mass and pH are relevant parameters within theadsorption process to evaluate the removal capacity of anadsorbent (Garg et al., 2009; Yadla et al., 2012). This fact is partlydue to H+ ions being a strong adsorbate to compete with adsorptionsites and also partly due to the chemical speciation of metal ionsaffected by the solution’s pH. The adsorption of metal ions depends

Fig. 4. Effect of the adsorbent’s contact time on the removal of the metal ions Cd2+,Pb2+, Cr3+.

Table 2Kinetic parameters of models obtained in the study of Cd2+, Pb2+ and Cr3+ adsorption of cashew nut shell for pseudo-first order and pseudo-second order, Elovich, intraparticlediffusion models and quantity of metal absorbed in equilibrium (Qeq(exp.)).

Pseudo-first order Pseudo-second order Elovich

K1 Qeq(cal.) R2 K2 Qeq(cal.) R2 A B R2

(min�1) (mg g�1) (g mg�1min�1) (mg g�1) (mg g�1 h�1) (g mg�1)

Cd2+ �0.0109 0.0233 0.4062 4.8991 0.7166 0.9990 0.6590 0.0121 0.7760Pb2+ �0.0144 0.0855 0.8157 0.6810 0.7690 0.9990 0.6198 0.0283 0.8676Cr3+ �0.0118 0.0551 0.3200 1.5167 0.6856 0.9990 0.4962 0.0403 0.5800

Intraparticle diffusion

–––––––-Kid––––––––- ––––––––Ci–––––––- –––––––-R2––––––––- Qeq(exp.)(mgg�1)(g mg�1min�1/2) (mg g�1)

A B C A B C A B CCd2+ 0.011 0.003 0.003 0.647 0.69 0.679 0.945 0.999 0.462 0.7147Pb2+ 0.028 0.008 0.009 0.595 0.677 0.646 0.368 0.627 0.831 0.7323Cr3+ 0.043 0.003 0.005 0.44 0.667 0.624 0.547 0.567 0.25 0.656

K1: velocity constant of the first order; Qeq: amounts of adsorbates retained per gram of adsorbent in equilibrium; K2: velocity constant of the second order; A: constantindicating the velocity of initial chemo-sorption; B: number of sites which are adequate for adsorption, related to the coverage extension of surface and the activation energyof chemo-sorption; R2: coefficient of determination; Kid: constant of intraparticle diffusion: Ci: suggests thickness of the limiting layer’s effect; R2: coefficient ofdetermination.

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on the type of adsorbent surface and on the ion forms that metalsfind in the water solution (Memon et al., 2008).

According to results in Fig. 3, adsorption behavior within the pHband under analysis was similar for Cd2+ and Pb2+; the percentageremoval was very low at pH 6.0 only for Cr3+ when compared toother pH rates. Cr (III) precipitation may occur in pH rates higherthan 6.0 and becomes unavailable for adsorption. Therefore, alltests should be performed with pH rate at 5.0, at the highest(Yang et al., 2013), so that this does not occur in experiments withCr (III) adsorption. Ercan and Aydin, (2013) also reportedprecipitation of ions Cd2+ and Pb2+ at pH � 6.0. However,adsorption in this experiment was similar among the three pHrates investigated.

After each adsorption test, pH was monitored to investigate theinfluence of cashew nut shell on the final pH of the solution. Fig. 3shows that the solution’s final pH tends towards stabilization whenin contact with the lowest adsorbent mass. Results show that theadsorbent contributes towards the maintenance of an acid solutionand a decrease in the precipitation of metal ions.

Pb2+ is the predominant species up to pH 6.0 band. In higher pHrates, the process is compromised due to the formation of hydroxylspecies of low solubility, causing a decrease in the adsorbedamount (Pehlivan et al., 2009; Feng et al., 2011).

Therefore, pH 5.0 was considered the ideal for adsorption testssince results confirm those obtained by pHPCZ, where, in suchcondition, the adsorbent behaved as an electronegative species,favoring the adsorption of metal cations of the solution. It shouldbe underscored that, in these conditions, the possibility ofprecipitation of these metals was low

According to Kiran et al. (2013), the number of active sitesavailable depends on the amount of the adsorbent. In fact, studieson ideal adsorption mass are highly important. According toMeneghel et al. (2013a) and Rubio et al. (2013a), in certain cases,the amount adsorbed may decrease due to the formation ofagglomerates that reduce the total surface area and, therefore, thenumber of active sites available for the process.

Results in Fig. 3 show that 600 mg of the adsorbent aresufficient to remove the metal ions Cd2+, Pb2+, and Cr3+, sincehigher mass rates had insignificant changes in the metals’ finalconcentration. The establishment of a mass/volume relationshipshows that 12 g L�1 is the ideal quantity of the adsorbent to removethe metals.

3.3. Influence of contact time

Contact time is unavoidably the basic parameter for alltransference phenomena, such as adsorption (Choudhury et al.,2012). It is highly important to investigate its effect on theretention capacity of the metal ions Cd2+, Pb2+, and Cr3+ on thecashew nut shell as adsorbent.

Fig. 4 shows that metal adsorption is fast during the initial timecontact; it slowly increases at larger time contact till reachingequilibrium at 60 min, with insignificant variations thereby.According to Kumar and Porkodi (2007), proportionately to thegradual exhaustion of adsorption sites on the adsorbent’s surface,the amount of metal ions adsorbed is controlled by the velocity bywhich the adsorbate is transported from the exterior to the interiorof active sites of the adsorbent’s particles

The precise point in which the adsorbed quantity reachesequilibrium is the ideal contact time for the adsorption of metalions on the cashew nut shell. Ideal adsorption time in whichequilibrium studies were performed was 60 min. It should beunderscored that greater contact time may be unfeasible whendealing with large scale metal removal.

3.4. Evaluation of the adsorption’s kinetic mechanism

Studies on the kinetic mechanisms of adsorption give an idea onthe transference velocity of metal ions from the water solution tothe solid phase and the required time to reach equilibriumbetween the phases (Boži�c et al., 2013). Kinetics that control theadsorption process was thus evaluated according to the pseudo-first and pseudo-second, Elovich and Intraparticle diffusion models(Table 2).

According to Febrianto et al. (2009), for data interpretation,rates of the coefficients of determination (R2) should have the bestadjustment and Qeq rates should be close to the experimental ones(Qeq(exp.)) (Table 2).

According to the rates of the coefficient of determination (R2),the pseudo-first order model failed to adjust itself to experimentaldata. Actually Qeq(exp.) and Qeq(calc.) rates were not close to eachother. Contrastingly to the pseudo-second order model, R2 = 0.999for ion metals and Qeq(exp.) and Qeq(calc.) rates were very close.Results suggest that the adsorption kinetic behavior of the ions Cd2

+, Pb2+, and Cr3+ follows the pseudo-second order model which

Fig. 5. Pseudo-first order (a), pseudo-second order (b), Elovich (c) and intraparticle diffusion (d) kinetic models for the adsorption of Cd2+, Pb2+, Cr3+ by A. occidentale L nutshell.

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indicates a chemo-sorption as the process’s limiting factor whichinvolves the sharing of valence forces or electron exchangebetween metals and adsorbent (Feng et al., 2011).

Weber–Morris intraparticle diffusion model has been oftenused to determine the limiting stage of the adsorption process.According to Boparai et al. (2011), the limiting stage is determinedby the linear interception of Qeq due to t1/2 passing through theorigin, with results in rates of Ci = 0. However, results of currentstudy show rates of Ci 6¼ 0 (Table 2) and suggest that theintraparticle diffusion model is not the process’s limiting stage.The process may be controlled not only by surface adsorption butalso by intraparticle diffusion (Gundogdu et al., 2009; Boparaiet al., 2011). According to Kavitha and Namasivayam (2007) and

Fig. 6. Effect of initial ion force of the cations Cd2+, Pb2+ and Cr3+ on the CNS andactivated coal (AC) (C0: 5–200 mg L�1; adsorbent mass: 12 g L�1; pH 5.0; contacttime: 60 min; 200 rpm; temperature: 25 �C).

Boparai et al. (2011),Ci interception rate is related to the thicknessof the limiting layer. The bigger the interceptions, the bigger is theamplitude of the surface diffusion in the process’s limiting stage.

Fig. 5 shows the linearity of the kinetic models of the pseudo-first order, pseudo-second order, Elovich and intraparticle diffu-sion models from time tests for the metal ions Cd2+, Pb2+, and Cr3+.

In the case of Qeq, evidence exists that, since t1/2 is multilinear,adsorption process is regulated by two or more stages. Due tomultilinearity (Fig. 5d and Table 2) referring to the adsorption ofthe metal ions Cd2+, Pb2+, and Cr3+ on the cashew nut shell, it maysuggested that in the case of Cd2+ there were two stages in theprocess, respectively represented by straight lines A (R2 = 0.9452)and B (R2 = 0.9989) (Boparai et al., 2011; Wu et al., 2009).

The first stage comprises adsorption on the external surface ofthe bordering layer where the transference of the external volumeto the surface of the adsorbent and Cd2+ is fast adsorbed(Kid = 0.0112 g mg�1min�1/2). The second stage corresponds tothe gradual adsorption stage by diffusion from ion Cd2+ to theadsorbent’s more internal sites, although with a lower adsorptionspeed than the first stage (Kid = 0.0031 g mg�1min�1/2) (Kumar andPorkodi, 2007; Boparai et al., 2011; Carvalho et al., 2010).

3.5. Influence of the initial concentration of the adsorbed

Data obtained from adsorption isotherms were used to evaluatethe removal of cations Cd2+, Pb2+, and Cr3+ under the influence oftheir respective ion forces in solution and report on the efficiencyof the adsorbents cashew nut shell (CNS) and activated coal (AC)after the process’s equilibrium (Fig. 6). According to Al Rub (2004),the study verifies the increase of the transference rate of metal ionson the adsorbent due to the metals’ initial concentration increase.

The behavior of metal ions removal was affected by solution’sinitial concentrations. Fig. 6 shows that the percentage of metal

Fig. 7. Adsorption isotherms of Cd2+, Pb2+, and Cr3+ on the CNS (a) and activated coal (b) (C0: 5–200 mg L�1; adsorbent mass: 12 g L�1; pH 5.0; contact time: 60 min; 200 rpm;temperature: 25 �C).

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removal decreased with the increase in ion force, albeit at constantpH 5.0 and mass at 12 g L�1. Exception comprised Pb2+ on activatedcoal where no behavior changes occurred within the concentrationband under analysis (5–200 mg L�1).

Similar results were reported by Bhattacharya et al. (2006) intheir studies on the adsorption of Zn2+ by different adsorbents.According to these authors, behavior involved the ratio metal ion:adsorbent, or rather, when ratio was low, adsorption comprisedbigger energy sites. As the ratio increased, the bigger energy sitesbecame saturated and the adsorption of smaller energy sitesbegan, with a decrease in adsorption efficiency (Bhattacharyaet al., 2006).

As an adsorbent of metal cations at low concentrations, thecashew nut shell had a high removal percentage; this is similar tothe ions Cd2+ and Cr3+ (Fig. 6) in activated coal. The highestefficiency was shown by Pb2+ on activated coal where removalpercentage reached rates lower than the quantification limit(LQ = 0.01 mg L�1) of the method employed (FAAS).

In order to verify that the CNS has the capacity to remove metalsfrom aqueous solution and the remaining concentration of theseare in within the range allowed by Brazilian lay, the concentrationsof ions Cd2+, Pb2+ and Cr3+ in aqueous solution after removal by CNSvalues were compared with established by CONAMA resolutionsnumber 357/2005 and number 430/2011 and ordinance number2914/2011 of the Ministry of Health (Brazil, 2005, 2011a,b).

The lowest concentration of Cd2+, Pb2+ e Cr3+ remaining in thesolution achieved in removing the CNS were respectively 0.63,0.71, and 0.625 mg L�1, where these values area above the amountsallowed by the laws already mentioned, except for Cr, wich showeda concentration within the allowed by Resolution number 430/2011 (Brazil, 2011a). It is noteworthy that the fortified aqueoussolution with metals only has received treatment for the CNS oncebut can be considered that if applied successive adsorptionprocesses using this biosorbent, the same solution could be within

Table 3Parameters of Langmuir, Freundlich and Dubinin–Radushkevich (D–R) mathematic modand activated coal (AC).

Langmuir’s constants Fr

Qm (mg g�1) b or KL (L mg�1) RL R2 K

CNS Cd2+ 11.233 0.201 0.024 0.989 1.Pb2+ 28.653 0.050 0.093 0.975 1.Cr3+ 8.4211 0.217 0.023 0.998 1.

CA Cd2+ 18.038 5.27 e�3 0.487 0.989 5Cr3+ 9.933 3.20 e�3 0.609 0.999 4

Qm: maximum adsorption capacity; b or KL: constant related to the interaction forces

related to adsorption capacity; n: related to the solid’s heterogeneity; Qd: maximum a

the potability standards established by these resolutions andordinance.

As a general rule, however, when the cashew nut shell wascompared to activated coal, the former was not so efficient. It shouldbe underscored that it is actually a natural adsorbent without anyneed ofany previous treatment,and obtainedfrom the agro-industrywastes. In other words, it is a low-cost and highly available materialwhich contrasts with activated coal, or rather a material resultingfrom physical and chemical modifications and obtained at high costswhich justify its high efficiency in the removal of metal cations.

3.6. Adsorption isotherm

Adsorption isotherms are relevant rate curves that describe thephenomenon that controls retention or mobility of the fluid-derived ions at a solid phase. This process requires constanttemperature and pH. It also needs a contact time which is sufficientfor the concentration of the adsorbed in the fluid to be in dynamicequilibrium in the solid/adsorbent interface (Ghiaci et al., 2004;Foo and Hameed, 2010; Witek-Krowiak et al., 2011). It is highlyimportant to understand the adsorption process of the metal ionsCd2+, Pb2+ and Cr3+ by the cashew nut shell and activated coal(Fig. 7).

The behavior of the adsorption isotherms shows that they maybe classified into classes and subgroups, following Giles et al.(1960). Isotherms of the metal ions Cd2+, Pb2+, and Cr3+ adsorbed bythe cashew nut shell and Cd2+ and Cr3+ adsorbed by activated coal(Fig. 7) were classified at class L since their behavior indicated anavailability decrease of active sites (Giles et al., 1960).

According to Montanher et al. (2005), the most favorable are theconvex shapes of adsorption isotherms since they tend towardsequilibrium, with the saturation of active sites in the adsorbent,following the theory of adsorption in monolayers proposed byLangmuir, in which the maximum adsorption capacity is obtained.

els related to the adsorption process of Cd, Pb and Cr on the cashew nut shell (CNS)

eundlich’s constants D–R parameters

f (mg g�1) n R2 Qd (mol g�1) E (kJ mol�1) R2

365 2.520 0.982 2.89 e�3 10.435 0.989226 1.520 0.985 1.41 e�2 8.513 0.992197 2.036 0.963 6.66 e�3 9.834 0.985.275 1.707 0.865 1.06 e�2 8.518 0.891.028 2.256 0.902 2.15 e�4 15.076 0.967

adsorbent/adsorbed; RL: Langmuir’s constant; R2: coefficient of determination; Kf:dsorption capacity; E: mean sorption energy.

Table 4Comparasion between different values of maximum adsorption capacity (Qm) of Langmuir with others biosorbents for biosorption of metal ions Cd2+, Pb2+, e Cr3+.

Metal Biosorbent Qm (mg g�1) References

Cd Cashew nut shell 11,233 This studyPinus elioti 6301 Strey et al., 2013M. oleifera 7864 Meneghel et al., 2013aC. abyssinica 19,342 Rubio et al., 2013a

Pb Cashew nut shell 28,653 This studyCassava peel plus bagace 24,810 Schwantes et al., 2013Cassava bagace 25,160 Schwantes et al., 2013Cassava peel 29,260 Schwantes et al., 2013

Cr Cashew nut shell 8,4211 This studyRice bran 0130 Oliveira et al., 2005M. oleifera 3191 Meneghel et al., 2013bC. abyssinica 6807 Rubio et al., 2013bSorghum straw 9350 Garcia-Reyes and Rangel-Mendez, 2010Oat straw 12,100 Garcia-Reyes and Rangel-Mendez, 2010

G.F. Coelho et al. / Ecological Engineering 73 (2014) 514–525 521

Within class L isotherms, Cr3+ adsorbed by activated coal(Fig. 7b) had a behavior typical of subgroup 2. Surface saturationoccurred in which the ion Cr3+ had greater preference for theadsorbent surface to the already adsorbed molecules. The class’sother ions (Fig. 7) belong to subgroup 1, featuring slow saturationof the adsorbent’s surface (Giles et al., 1960).

Pb2+ isotherm on activated coal (Fig. 7b) had a differentbehavior and did not fit in the classification by Giles et al. (1960).The above occurred because Pb2+ had a high removal percentage ofactivated coal, with a 100% removal in certain sites, in whichconcentration was <LQ than the method employed (FAAS).

Parameters of Langmuir, Freundlich and Dubinin–Radushke-vich (D–R) mathematical models were linearized by dataobtained from the equilibrium of the adsorption isotherms ofthe ions Cd2+, Pb2+, and Cr3+ by cashew nut shell and activatedcoal (Table 3).

Langmuir, Freundlich and D–R mathematical models employedto describe the adsorption of metals Cd2+, Pb2+ and Cr3+ by thecashew nut shell (CNS) were satisfactory, as R2 rates show(Table 3). The above suggests that within the process there aremany other types of adsorption sites derived from adsorption inmono- and in multi-layers (Gonçalves et al., 2012).

In the case of activated coal (AC), satisfactory results were few:ion Cd2+ was only adjusted by Langmuir and Cr3+ by Langmuir andDubinin–Radushkevich. Monolayer adsorption occurred in theseions. On the contrary, Pb2+ did not adjust itself by any model underanalysis. Good adjustments for CNS occurred due to the bestconditions (pH, adsorbent mass and contact time) adopted by testsin current investigation, contrastingly to AC in which tests wereperformed in the same conditions as those with CNS. These may

Fig. 8. IR spectra before (a) and after (b) the adsorption

not have been the best for AC, with the consequent badadjustment.

Langmuir’s parameter RL has been widely used to determinewhether the adsorption process is favorable. The process isfavorable if rates are between 0 < RL< 1 (Sun et al., 2013), as incurrent study (Table 3). According to the same author, when RL = 1,the isotherm is linear. In fact, Pb2+ CA had a linear trend since its RLrate is close to 1 (RL= 0.997).

According to Febrianto et al. (2009), maximum adsorptioncapacity (Qm) within Langmuir’s model is supposed to coincidewith the saturation of a fixed number of active sites on theadsorbent’s surface. The same authors report that certainbiomasses are affected by several factors, such as the number ofsites in the biosorbent material, accessibility of sites, chemicalstate of the sites (availability) and affinity between the site and themetal (bonding force). They may also be associated with thefunctional groups on the surface.

The Qm of CNS provided satisfactory results for the metals evenwith lower rates than AC for Cd2+ and Cr3+. It should beunderscored that in the case of Pb2+, the Qm of CNS was higherthan other ions (Qm CNS = 28.653). However, these rates over-estimated those found by the isotherms (Fig. 7a). High Pb2+

adsorption on CNS may be related to the strong affinity that it hason the carboxyl groups in the adsorbent (Krishnami et al., 2008).

Langmuir’s parameter b or KL is a constant that expresses theinteraction force between the adsorbent and the adsorbed and ishighly relevant in adsorption studies. According to Table 3, low brates show low bonding energy between metals and adsorbentsand suggest the possibility of high desorption percentage of ionsfor the solution.

of ions Cd2+, Pb2+, and Cr3+ by A. occidentale L. shell.

522 G.F. Coelho et al. / Ecological Engineering 73 (2014) 514–525

Within Freundlich’s parameters, parameter n indicates thereactivity of the adsorbent’s active sites which is entirely related tothe solid’s heterogeneity. Table 3 shows that in all cases, n rateswere higher than l. When n rates are >1, strong evidence exists onthe presence of highly energetic sites. The highest the differencebetween n and 1, the greater is the distribution of the bondingenergy on the adsorbent’s surface. The above rates may alsosuggest the occurrence of cooperative adsorption in which stronginteractions between molecules of the adsorbate are involved(Sodré et al., 2001; Khezami and Capart, 2005).

It should be noted that the application of Langmuir andFreundlich’s isotherms in current study, as shown by Namasivayamand Sureshkumar (2008) and by Abd el-latif and Elkadym (2010), ispossible for adsorption in monolayers and for the heterogeneousenergy distribution of active sites on the adsorbent’s surface,especially for CNS in which the two models had the bestadjustment in the adsorption of ions Cd2+, Pb2+, and Cr3+.

However, Langmuir and Freundlich’s isotherms have a specialcharacteristic: parameters obtained by models are insufficient toexplain the physical and chemical traits of adsorption. Actuallycurrent analysis applied the Dubinin–Radushkevic (D–R) modelwhich is highly employed to describe the system’s adsorptionisotherms with individual ions. Isotherm is applied to equilibriumdata obtained from empirical studies for ion removal anddetermines whether the adsorption process is physical or chemical(Abd el-latif and Elkadym, 2010).

Within D–R parameters, mean sorption energy (E) providesimportant information on the mechanism of adsorption. The freeenergy is involved in the transference of 1 mol of the solution’ssolute to the adsorbent’s surface. If E > 8 kJ mol�1, chemicaladsorption predominates in the system; if E < 8 kJ mol�1, theprocess is physical (Wan Ngah and Hanafiah, 2008). Results inTable 4 demonstrate that there was a predominance of chemicaladsorption of ions Cd2+, Pb2+, and Cr3+ on CNS and AC, since E rateswere higher than 8 kJ mol�1 in all cases.

The Table 4 compare the values of Qmobtained by CNS with otherstudies biosorbents for the ions Cd2+, Pb2+ e Cr3+. It can be seen inTable 4 that for the ions studied Qm of the CNS was not higher thanthat of C. abyssinca for Cd, cassava peel for Pb and oat straw for Cr.

Fig. 8 shows IR spectral of the cashew nut shell after theadsorption of metal ions Cd2+, Pb2+, and Cr3+.

Table 5Qeq rates and thermodynamic parameters of Cd2+, Pb2+, Cr3+ adsorption on thecashew nut shell.

Ion Temperature(�C)

Thermodynamic parameters

Qeq

(mg g�1)DG(kJ mol�1)

DH(kJ mol�1)

DS(J mol�1)

R2

Cd2

+15 3.848 �0.328 �7.847 �26.107 0.99225 3.837 �0.06735 3.810 0.19545 3.790 0.45655 3.760 0.717

Pb2+

15 3.072 3.439 0.794 �9.182 1.00025 3.110 3.53035 3.080 3.62245 3.182 3.71455 3.088 3.806

Cr3+

15 2.185 5.693 7.763 7.188 0.97625 1.852 5.62135 1.775 5.54945 2.528 5.47755 3.061 5.405

Qeq: quantity adsorbed per unit of adsorbent; DG: variation of Gibb’s free energy;DH: variation of enthalpy; DS: variation of entropy.

Fig. 8 reveals no modification in IR spectra after the adsorption ofmetal ions Cd2+, Pb2+, and Cr3+, only there was a decrease in theintensity of the vibrations due to the adsorption process. This factsuggests that they are adsorbed in the functional hydroxyl, aliphatic,phenolic and carboxyl groups which make up lignin, discussed above(Fig. 1) (Argun and Dursun, 2008; Ding et al., 2012).

3.7. Thermodynamics of adsorption

Some thermodynamic parameters for ions Cd2+, Pb2+, and Cr3+

(Table 5) have been analyzed to investigate better the effect oftemperature on the adsorption process of the metal ions by theadsorbent and analyze the process’s traits.

DG rate within thermodynamic parameters mainly indicatesthe system’s spontaneity. Table 5 shows that negative rates of DGfor Cd2+ in low temperatures (15 and 25 �C) indicate thatadsorption process is spontaneous and favorable in theseconditions (Senthilkumar et al., 2012). However, increase intemperature decreases the system’s spontaneity. Rates for ionsPb2+ and Cr3+ are positive but the system tends towardsspontaneity respectively with temperature decrease and increase.

According to the positive enthalpy rates (DH), the system isendothermal (Wan Ngah and Fatinathan, 2010) for ions Pb2+ andCr3+. Negative DH rates for Cd2+ reveal that the process isexothermal for this specific metal (Table 4). Results in Table 4 alsosuggest the possible occurrence of physo-sorption due to the factthat DH rates are lower than 40 kJ mol�1, however, the adsorptionprocess can be coordinated directly by some functional groups byformation of complexes in the inner sphere adsorption (McBride,1989; Dos Santos et al., 2010).

Results differ from those reported for the pseudo-second order(Table 2) and Dubinin–Radushkevich models (Table 3). Positiveentropy rates (DS) for the ion Cr3+ (Table 5) suggest an increase inthe solid-solution interface disorder and indicates an increase ofrandomness in the solid-solution interface. The above may occurdue to the substitution of the solution’s water moleculespreviously adsorbed by the metal ions (Rao and Khan, 2009),different for ions Cd2+ and Pb2+ in which the system’s randomnessis low for these metals.

3.8. Desorption

The recovery and regeneration of adsorbent material arerelevant aspects for studies involving water treatment, since theytest the reversibility of the adsorption process. De-sorption, as theprocess is called, occurs by an increase in the ion force of H+ in thesolution which moves towards the metal cations (Cd2+, Pb2+ andCr3+) adsorbed on the surface of the adsorbent material by ionexchange for the solution (Mimura et al., 2010; Choudhury et al.,2012; Senthilkumar et al., 2012).

Data on Cr3+ isotherms (Table 6) show high adsorption rate(CNS: 73.71; AC: 93.41%), albeit with low desorption capacity forthe adsorbents (CNS: 6.37%; AC: 13.41%). Above results cannot beconsidered satisfactory for possible reutilization of the adsorbentin other adsorption processes.

Within the total adsorbed by CNS, the ions Cd2+ and Pb2+

revealed a good de-sorption rate (Table 6). Cd2+ showed a mean

Table 6Mean adsorption and desorption percentages of ions Cd2+, Pb2+, and Cr3+ by thecashew nut shell (CNS) and activated coal (AC).

Adsorbent ––––––– Adsorption (%)–––––––- –––––- Desorption (%)–––––-

Cd2+ Pb2+ Cr3+ Cd2+ Pb2+ Cr3+

CNS 70.01 87.91 76.51 89.10 96.22 6.37AC 97.88 99.98 93.41 92.75 46.69 13.41

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de-sorption for CNS close to that reported for AC and Pb2+ hadhigher results than those reported for AC. In fact, CNS has a highreutilization potential for the adsorbates Cd2+ and Pb2+.

Several studies reported similar results for Cr3+ desorptionwhich evidenced a strong bonding between Cr3+ and other naturaladsorbents derived from agro-industries. Meneghel et al. (2013b)employed the Moringa oleifera L and AC as adsorbent and reporteda desorption percentage of 16.4% and 4.3% respectively. Rubio et al.(2013b) employed the Crambe abyssinica H and AC as adsorbentand reported a desorption percentage of 6.1% and 4.7%.

HCl in higher concentrations than those in current study (HCl0.1 mol L�1) may be used in these cases, or even new regeneratingsolutions, such as HNO3 or H2SO4 (Meneghel et al., 2013b; Witek-Krowiak, 2013). Some authors suggested the burning of theadsorbent material to be later placed in concrete blocks, bricks andceramics to lower costs and increase the yield of the products.However, the processes may emit toxic gases in the environment(Bezerra et al., 2011; Meneghel et al., 2013b).

According to Lima and Rossignolo (2010), ashes from materialsderived from agro-industrial processes may be used in thecomposition of cement material due to great amounts of silicain materials of organic origin. The same authors undertook a studyto verify whether ashes from the cashew nut shell may beemployed in this case. Results, however, were not satisfactory dueto the great amounts of metals such as Cd, Pb, and Cr in thematerial. According to these authors, the above is related to the useof agrotoxics and fertilizers, soil contamination or the result ofatmospheric disposals. These elements may restrict the use of CNSashes in cement materials since they delay, in the case of Cd and Pb,or accelerate, in the case of Cr, the start and end of cohesion bydecreasing the product’s resistance. A new efficient method,economically feasible, should be discovered to dispose adequatelythe adsorbents with metals after the adsorption process (Meneghelet al., 2013b).

4. Conclusion

Current results show that the best conditions for the adsorptionprocess of ions Cd2+, Pb2+, and Cr3+ on the cashew nut shell were pH5.0; adsorbent = 600 mg; equilibrium time = 60 min.

According to rates from the pseudo-second order model and themean sorption energy rate (E) of Dubinin–Radushkevich, theprocess was predominantly chemo-sorption but enthalpy ratessuggest physio-sorption.

Studies on adsorption isotherms showed good linear adjust-ments for all mathematical models (Langmuir, Freundlich andDubinin–Radushkevich), with the occurrence of adsorption inmono- and in multi-layers.

Desorption percentage after Cd2+ and Pb2+ adsorption showedthe possibility of reusing the adsorbent material. However, low Cr3+ desorption revealed the non-feasibility of the reutilization of thematerial for new adsorption processes.

Acknowledgment

This research was supported by the Brazilian National Counselof Technological and Scientific Development (CNPq) and by theBrazilian Ministry of Science and Technology (MCTI).

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