cobalt (ii) removal from aqueous solutions by natural hemp fibers: batch and fixed-bed column...

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ARTICLE IN PRESS Model

PSUSC-25956; No. of Pages 7

Applied Surface Science xxx (2013) xxx– xxx

Contents lists available at ScienceDirect

Applied Surface Science

j ourna l ho me page: www.elsev ier .com/ locate /apsusc

obalt (II) removal from aqueous solutions by natural hemp fibers:atch and fixed-bed column studies

avinia Tofana, Carmen Teodosiua, Carmen Padurarua,∗, Rodica Wenkertb

Department of Environmental Engineering and Management, Faculty of Chemical Engineering and Environmental Protection, “Gheorghe Asachi”echnical University of Iasi, 73, Prof. Dr. D. Mangeron Street, 700050 Iasi, RomaniaSoroka University Medical Center, Beer-Sheva, Israel

r t i c l e i n f o

rticle history:eceived 17 April 2013eceived in revised form 14 June 2013ccepted 27 June 2013vailable online xxx

eywords:o(II)

a b s t r a c t

Natural hemp fibers were explored as sorbent for the removal of Co(II) ions from aqueous solutionsin batch and dynamic conditions. The batch Co(II) sorption capacity increased up to pH 5, reached themaximum (7.5–7.8 mg/g) over the initial pH of 4.5–5. As the initial concentration of metal ion increased (inthe range of 25–200 mg/L), the cobalt uptake was enhanced, but the Co(II) removal efficiency decreased.The batch sorption of Co(II) on the tested hemp follows a pseudo-second order model, which relies onthe assumption that the chemisorptions may be the rate-controlling step. The Langmuir model betterdescribed the Co(II) sorption process on the natural hemp fibers in comparison with the Freundlich model.

orptionempatcholumnrocess models

This finding complies with the results of fixed-bed studies which emphasize that the optimal solutionfor describing the behavior of the investigated hemp bed column is provided by the Thomas model. Thesorption capacity of the hemp fibers column (15.44 mg/g) performed better than that of the Co(II)-hempbatch system (13.58 mg/g). The possibility to use hemp fibers as an alternative in the Co(II) wastewatertreatment should be studied under pilot scale applications, so as to complete the studies concerning theremoval efficiencies with technical and economic factors that influence process scale-up.

. Introduction

The increased use of cobalt containing compounds in nuclearnd coal fired power plants and in many industries such as mining,etallurgical, electroplating, paints, pigments, petroleum, elec-

ronics has resulted in Co(II) finding its way into natural bodiesf water [1,2]. Cobalt compounds are classified as class II, whicheans they are not extremely toxic [3]. However, one of theirajor impacts over the receiving water, if the discharges of indus-

rial/municipal effluents are insufficiently treated is the decrease ofhe natural self-purification capacity of surface water, as well as theegative influence on human beings and aquatic ecosystems due tohe “acute” or “chronic” toxicity and the effects on liver, kidney andones [4].

A wide variety of strategies have been developed for the removalf heavy metals from wastewaters, including cobalt ions [5]. Amonghese, sorption has been evolved into one of the most promisingechnologies. In recent years, the applications of low cost sorbents

Please cite this article in press as: L. Tofan, et al., Cobalt (II) removal from aqstudies, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.

or the removal and recovery of heavy metals from industrial andunicipal effluents have been widely studied [6–13]. However,

he necessity for investigating more and more natural and waste

∗ Corresponding author. Tel.: +40 726030153.E-mail address: [email protected] (C. Paduraru).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.06.151

© 2013 Elsevier B.V. All rights reserved.

materials is still very important in order to obtain the best materialfor industrial applications. In this context, the main objective of thisstudy is to evaluate the potential of natural hemp fibers as sorbentfor cobalt (II) ions. A literature search revealed that no work hasbeen reported on the feasibility of hemp use as sorbent for cobaltremoval from aqueous effluents.

Hemp is a fast growing plant almost anywhere and requiresno pesticides or fertilizer. The potential for hemp is vast, includ-ing sustainable biomass (power) and biodiesel (fuel). Hemp alsomakes an excellent source for textile and paper [14]. The fiber ofhemp is one of the inexpensive and readily available bast natu-ral fibers and hemp-fiber reinforced polymer composite productshave gained considerable attention in last years [15–17]. The per-formances of hemp as sorbent for heavy metal ions (Table 1) isbased on its remarkable fundamental features: low cost, availabil-ity, high mechanical strength and porosity, hydrophilic character,fast sorption, tolerance to biological structures, easiness in func-tionalizations, possibility of being used as fibers and filters.

For a complete description of the cobalt (II)-hemp sorption sys-tem behavior and performance, this work presents the results ofbatch and fixed bed column studies. Batch studies were carried out

ueous solutions by natural hemp fibers: Batch and fixed-bed column151

in order to establish the optimum conditions of sorption so as toobtain equilibrium sorption isotherms and to evaluate the sorp-tion capacity of hemp for cobalt (II) ions present in the aqueousphase. However, the information obtained from batch studies is not

dx.doi.org/10.1016/j.apsusc.2013.06.151


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APSUSC-25956; No. of Pages 7

2 L. Tofan et al. / Applied Surface Science xxx (2013) xxx– xxx

Table 1Hemp as sorbent for heavy metal ions from aqueous solutions.

Type of hemp Retained cations Remarks References

Natural hemp fibers Cr(III), Cu(II), Ag(I), Cd(II),Zn(II), Pb(II)

The monolayer sorption capacity was 4. 006, 9.0735, 1.2253, 2.509,21.047 and 25.05 mg/g hemp for chromium(III), copper (II), silver(I),cadmium(II), zinc(II) and lead(II) ions, respectively.

[18–23]

Natural hemp and hypochlorite bleachedhemp fibers physically modified with�-benzoinoxime

Cu(II) The analytical potential of natural and bleached hemp fibers in batchand dynamic retention of Cu(II) has been significantly improved byimpregnation with �-benzoinoxime

[24,25]

Hemp fibers impregnated with alizarine S Cr(III) An increase in solution temperature tends to reduce the maximumCr(III) sorption capacity of hemp impregnated with alizarine S from8.632 mg/g at 4 ◦C to 4.726 mg/g at 40 ◦C

[26]

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lhwTh

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owtt

pu

2

dea(

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Sulphydryl hemp fibers Ag(I), Cd(II), Pb(II)

ufficient for designing a wastewater treatment system for continu-us operation. Taking this fact into account, the dynamic behaviorf the fixed bed column was described in terms of breakthroughurve. Furthermore, the breakthrough experimental results wererocessed by means of Thomas and Yoon–Nelson models.

. Materials and methods

.1. Hemp preparation

In these experiments thick and rigid fibers of hemp witharge surface area and porosity, high swelling capacity and strongydrophilic character have been used, these fibers resulting asaste from a textile factory in the North-East region of Romania.

he main physical properties and chemical composition of the usedemp fibers are listed in Table 2 [26].

Hemp fibers were purified by boiling for 4 h in a solution con-aining soap and soda ash, followed by washing several times withater, rinsing with bidistilled water and drying in an oven at 45 ◦C.

.2. Chemicals

Stock solution of 1130 mg/L were prepared through dissolutionf analytical grade reagent Co(NO3)2·6H2O in bidistilated water andas complexonometrically standardized. The actual Co(II) solu-

ions for testing were prepared through appropriate dilutions ofhe stock solution.

In order to study the effect of medium acidity on the sorptionrocess, a solution of HNO3 with concentration of 10−2 mol L−1 wassed (340-A/SET 1 pH-meter).

.3. Sorption procedure

The sorption experiments were performed under batch con-itions. For this purpose, samples of about 0.25 g hemp werequilibrated with 50 mL of aqueous solution containing a definedmount of Co(II) ions, at desired temperature (25 ◦C) and initial pH1–5).

The mixture was then filtrated and the solution was analyzedor the cation content. The Co(II) concentrations in solutions haveeen determined by atomic absorption spectrometry (210 VGBuck Scientific atomic absorption spectrometer). The parametersharacteristic for Co(II) sorption on hemp were calculated from theifference between the initial and final concentrations of the Co(II)olutions, as follows:

emoval efficiency, R (%) R =[(

C0 − C

C

)]× 100 (1)


0

etained amount of metal ion, q (mg/g) q =[(

C0 − C

G

)]· V (2)

onolayer sorption capacity of chemically modified hemp is, 14.05 and 23.00 mg/g for silver, cadmium and lead ions,ctively, at 18 ◦C.

[27,28]

where C0 = initial concentration of metal ion (mg/L), C = cation con-centration after sorption (mg/L), V = volume of solution (L), andG = weight of hemp fibers (g).

2.4. Fixed bed column studies

The dynamic sorption studies were carried out in a glass columnof 1.5 cm inner diameter and 15 cm in length. A 0.7 g sample ofhemp was packed into the column, achieving a bed height of 7 cm. Alayer of wadding glass was fitted at the bottom of column to supportthe hemp during studies. Co(II) solutions with concentrations of25–50 mg/L were fed from the top of the column. Effluent sampleswere collected from the bottom of the column at different times.The metal ion concentration in the column effluent samples wasdetermined by atomic absorption spectrometry.

3. Results and discussion

3.1. Characterization of the hemp

The morphological structure of natural hemp and Co(II)-hempfibers was studied by scanning electron microscopy, SEM (BrukerAXS-Microanalyse GmbH microscope). Fig. 1 shows the images ofhemp fiber (a) and hemp loaded with Co(II) ions (b). The SEMimages clearly show the morphological changes occurring on thesurface of the hemp fiber after the treatment with a solution con-taining Co(II) ions.

Furthermore, the EDX analysis was performed to highlight thecobalt ion. The elemental composition from the peak areas is cal-culated and it is as follows: oxygen – 79.11%; carbon – 12.14%;nitrogen – 7.58%; cobalt – 1.14%.

3.2. Batch studies

3.2.1. The effect of the initial pHIndustrial wastewaters are characterized by a substantial vari-

ation in pH values, and hence the initial pH of the solution is animportant factor to be considered during the sorption studies [2].The effect of the initial pH on the Co(II) sorption by the testedhemp is shown in Fig. 2. The initial pH of the aqueous solution wasinvestigated over a range of 1–5, since the cobalt hydroxide startsprecipitating from solutions at higher pH values, making the truesorption studies impossible [29].

The Co(II) sorption capacity was minimum at pH = 1 (1.25 mg/g)and increased up to pH 5, reached the maximum (7.5–7.8 mg/g)over the initial pH of 4.5–5.

The decreased degree of the Co(II) sorption at low pH can be


attributed to the positive charge of the hemp fibers and to the com-petition between hydrogen ions and cobalt ions for the active siteson the hemp surface. On the other hand, this behavior may be usedto recover the cobalt ions by desorption with mineral acids from




L. Tofan et al. / Applied Surface Science xxx (2013) xxx– xxx 3

Table 2Characterization of the hemp used in this study.

Specific resistance 1.47 × 102 N/mm2

Specific weight 14.71 N/cm2

Humidity 12%Chemical composition (%) Cellulose Hemicellulose Lignin Waxs

74–77 18.4–15.4 37 4.04Xylans Pectines Proteins Ash3.0–7.0 4.0–8.0 0.5–1.0 0.82

Fi

taghctoe

3

p

and the plots of the liniarized form of the pseudo-second orderequation are given in Table 3, together with the correspondingcoefficients of determination (R2).

ig. 1. SEM images of: (a) natural hemp fibers and (b) hemp fibers loaded with Co(II)ons.

he loaded sorbent [30]. On the other hand, the increased sorptiont higher values of pH may be due to the ionization of functionalroups and to an increase in the negative charge density on theemp surface which significantly improved the attraction of theobalt cations. A similar result of the pH effect was also reported forhe sorption of the cobalt (II) ions on chemically modified and rawrange peel and on sepiolite [1,31]. In this context, further sorptionxperiments were carried out with initial pH 4.5–5.


.2.2. Effect of the contact time and sorption kineticsThe Co(II) sorption process on hemp proceeded in two distinct

hases. The sorption rate was initially very fast, and then become

Fig. 2. The influence of the initial pH on the Co(II) ions retention by natural hempfibers.

almost stable as contact time increased, reaching an equilibrium(Fig. 3).

A similar behavior was reported in literature for the sorptionof Co(II) on other low-cost sorbents [2,4] This two-phases sorptionmay be explained by taken into account the fact that the active sitesin a system is a fixed number and each active site can adsorb onlyone ion in a monolayer, the metal uptake by the sorbent surface willbe rapid initially, showing down as the competition for decreasingavailability of active sites intensifies by the metal ions remainingin solution [32]. The equilibrium was achieved in about 360 min.To ensure that true equilibrium was established, the subsequentsorption experiments were carried out for approx. 6 h.

In order to describe the kinetics of the Co(II) sorption on naturalhemp fibers, the pseudo-first order (Lagergren) and pseudo-secondorder kinetic models were applied to fit the experimental data.These models are based on the equations that are presented inTable 3.

The kinetic parameters obtained from the linear Lagergren plots


Fig. 3. Effect of the contact time on the Co(II) sorption by hemp fibers.





Table 3Kinetic characterization of the Co(II) sorption on the natural hemp.

Kinetic model Equation form Kinetic parameters Obtained value R2

Pseudo-first order model(Lagergren model) [33]

where qe and qt are the amounts ofcation (mg/g) sorbed at equilibriumand at time t, respectively

k1 – rate constant of pseudo-first order modelsorption (min−1)

k1 = 5.29 × 10−3 min−1 0.9876

Pseudo-second order model(Ho model) [34]

1qt

= 1h

+ 1qe

· t k2 – the rate constant of the pseudo-secondorder model, g/mg minh = k2 · q2

e (mg/g min) can be regarded as initialsorption rate constant of thepseudo-second-order sorption (g mg−1 min−1).

k2 = 2.325 × 10−4 g/mg minh = 0.0465 mg/g minqe = 14.14 mg/g

0.9932

Table 4The effect of the initial Co(II) concentration on its sorption by natural hemp.

C0 (mg/L) q (mg/g) R%

25 3.25 82.550 5.75 7675 7.8 70.6

100 9.4 64.6125 11.2 57150 12.8 51.5

Cmmoth

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175 13.6 47.3200 14 41.6

By comparing the R2 values it is obvious that the sorption ofo(II) on the tested hemp follows better the pseudo-second orderodel, which relies on the assumption that the chemisorptionsay be the rate-controlling step [35]. Similarly, the pseudo second

rder kinetics has described the mechanism of cobalt (II) sorp-ion on Ficus religiosa leaf powder (k2 = 4.18 × 10−4 g/mg min) andydroxyapatite (k2 = 2.74 × 10−3 g/mg min) [4,36].

.2.3. Effect of the Co(II) initial concentrationThe effect of the initial metal ion concentration was investigated

n the range of 25–200 mg/L and is shown in Table 4.As it can be seen from Table 4, the increase in the initial metal ion

oncentration enhances the cobalt uptake. This behavior might beue to the increase in the driving force of the concentration gradientroduced by the increase in the initial cobalt concentration. On thene hand, this finding complied with that reported in literatureor the Co(II) sorption by chemically treated Quercus Coccifera shell2]. On the other hand, the Co(II) removal efficiency decreased ashe initial concentration of metal ion was increased (Table 4). Thisrend may be due to the increase in the number of ions competingor the available binding sites in the hemp fibers and also due to theack of binding sites for the complexation of cobalt ions at higheroncentration levels [21]. In this context, it may be concluded thathe studied hemp fibers could be efficiently used in the removal ofhe tested metal ions from wastewaters with low contents in Co(II).

.2.4. Sorption isothermsSorption isotherms are essential data source for practical design

f sorption system and understanding of relation with sorbent andorbate. The Langmuir and Freundlich isotherm equations (Table 5)ere used in this study, despite the fact that these equations have

erious limitation on their usage. The simplicity and easy inter-retability are some of the reasons which would account for thextensive use of these models [39].

The Langmuir and Freundlich isotherm parameters for the Co(II)orption on the tested hemp and a comparison between Langmuirnd Freundlich models are given in Table 5. The R2 values indicatedhat the Langmuir model better described the Co(II) sorption pro-


ess on the natural hemp fibers in comparison to the Freundlichodel. The Langmuir isotherm model is based on the assumptions

hat [40]:

Fig. 4. Breakthrough curves of the Co(II) sorption on the investigated hemp fibersat different influent concentrations (� −50 mg/L; � 25 mg/L).

- the sorption is limited to monolayer coverage;- all surface sites are alike and can only accommodate one adsorbed

species;- the ability of a species to be sorbed on a given site is independent

from its neighboring sites occupancy;- sorption is reversible;- the sorbed species cannot migrate across the surface or interact

with neighboring molecules.

3.3. Fixed-bed column studies

3.3.1. Breakthrough curvesThe shape of breakthrough curves is a very important feature

for the evaluation of the operation and the dynamic response of asorption column.

The resulting breakthrough curves (metal ion effluent concen-tration at time t, Ct, versus effluent volume profile) for the Co(II)dynamic sorption on the tested hemp are depicted in Fig. 4.

As it can be seen from Fig. 4, the breakthrough curves exhibitedan S-shape. However, their shape depends on the influent con-centration of the cation in the tested solution. The characteristic“S” shape for breakthrough curves is due to mass transfer effects[41]. At lower Co(II) influent concentration, the breakthrough curveis dispersed. As influent concentration increases, the slope of theobtained breakthrough curve becomes steeper. This trend may beexplained by the fact that the increase in influent concentrationresults in more sorption sites being covered [42].

These results demonstrate that the change of the concentrationgradient affects the saturation rate and the breakthrough time. Thisbehavior complies with the literature data where the influence ofthe concentration on the breakthrough curves shape is explained


on the basis of the following effect sequence: increase of influentconcentration → increase of sorption rate → decrease of mass trans-fer → limitation in breakthrough zone [43].




L. Tofan et al. / Applied Surface Science xxx (2013) xxx– xxx 5

Table 5Quantitative description of the Co(II)-hemp batch sorption systems on the basis of the Langmuir and Freundlich models.

Sorption isotherm model Equation Isotherm significance Parameters values R2

Langmuir [37] q = KL ·C·q01+KL ·C KL – binding energy (relative sorption affinity)

q0 – maximum capacity of sorptionKL = 1928 L/molq0 = 0.2432 mmol/g = 13.58 mg/g

0.9956

Freundlich [38] log q = log KF + (1/n)log C(linearized form)

KF – binding energy (relative sorption affinity)n – energy of sorption

KF = 1.43n = 1.43

0.9907

F2

3

T

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wectm

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pts

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stdcrajtdr

os

ig. 5. Thomas plots for hemp-Co(II) dynamic system of sorption (C0 = � 50 mg/L; �

5 mg/L).

.3.2. Modeling and analysis of hemp column dataThe experimental breakthrough data were processed using the

homas and Yoon–Nelson models.The Thomas solution is one of the most general and widely used

odel in column performance theory. This model is based on thessumption of Langmuir’s kinetics of adsorption–desorption, with-ut axial dispersion. Its main hypothesis is that the rate drivingorce obeys second-order reversible reaction kinetics [44].

The following linearized form of Thomas equation has been usedn this study [45]:

n(

C0

Ct− 1

)= KT · q0(T) · m

F− KT · · C0

F· V (3)

here C0 is the initial metal ion concentration (mg/L); Ct is thequilibrium concentration (mg/L) at time t (min); kT is the Thomasonstant (L/min mg); F is the volumetric flow rate (L/min); q0(T) ishe maximum column capacity (mg/g), determined by the Thomas

odel; m is mass of sorbent (g) and V is the volume (L).From the slope and the intercept of the straight line obtained by

lotting ln(C0/Ct − 1) versus V, the values of the Thomas parameterskT and q0(T)) can be determined.

The applicability of the Thomas model to the experimental datarovided by the dynamic sorption of Co(II) ions from aqueous solu-ions with different initial concentrations on hemp under study ishown in Fig. 5.

The hemp column sorption parameters derived from the linearhomas plots are listed in Table 6.

From Table 6 it is obvious that the values of Thomas rate con-tant, kT, decrease as the initial metal concentration increases. Athe same time, the hemp maximum sorption capacity of Co(II),etermined by the Thomas model, increased as the initial metaloncentration increased. These trends are in good agreement withecent literature data reporting the sorption of the Co(II) ions fromqueous solution by chaff in a fixed bed column [46]. They can beustified by taking into account the fact that the gradient concen-ration is the driving force of the retention process. Thus, a higherriving force due to an increased concentration of the metal ion


esults in improved performances of the hemp column.Yoon and Nelson have proposed a less complicated model based

n the assumption that the rate of decrease in the probability oforption for each sorbate molecule is proportional to the probability

Fig. 6. Linear Yoon–Nelson plots for the dynamic sorption of Co(II) from solutionswith different initial concentration on natural hemp fibers (C0 = � −50 mg/L; �

25 mg/L).

of sorbate breakthrough on the sorbent [47]. The linear form of theYoon–Nelson model used in this study is represented as follows[48]:

ln(

Ct

C0 − Ct

)= kYN · t − � · kYN (4)

where Ct is effluent concentration at time t (mg/L); C0 is metalion initial concentration(mg/L); kYN is Yoon–Nelson rate constant(min−1); � is time required for 50% sorbate breakthrough; t is samp-ling time (min). A plot of ln(Ct/(Ct − C0)) versus t gives a straight linewith a slope of kYN and intercept of −�·kYN.

According to the Yoon–Nelson model, the amount of metal ionsorbed in a fixed bed is half of the total metal ion entering theadsorption bed within 2� period [49]. In this context, for a givenbed, the column sorption capacity in the Yoon–Nelson model, q0(YN)can be computed with the following equation:

q0(YN) = q(total)

m= (1/2)C0[(r/1000) × 2�]

m= C0 · r · �

1000 · m(5)

where C0 is the initial concentration (mg/L); r is flow rate (mL/min);m is weight of sorbent (g) and � is time required for 50% sorbatebreakthrough.

The characterization of the Co(II) dynamic sorption on the testedhemp by means of the Yoon–Nelson model is illustrated in Fig. 6.

The values of the Yoon–Nelson parameters, kYN and � togetherwith those of the column sorption capacity, q0(YN) for the testeddynamic system of sorption are recorded in Table 6.

It is clear from Table 6 that the values of the Yoon–Nelson rateconstant, kYN, increase as the initial concentration increases. Thisbehavior is similar to that previously reported for the adsorptionof Acid blue 92 and Basic red 29 dyes on a packed bed column pre-pared by Euphorbia antiquorum L. activated carbon [48]. This maybe explained by the fact that the increase in initial concentrationof the metal ion increases the competition between the sorbatespecies for the sorption sites, which ultimately results in a higher


rate of retention. It is known that kYN and � are inversely related,so that, as expected, the time required for 50% breakthrough, �,decreases with increasing the influent concentration of Co(II). Thehemp column sorption capacity, calculated based on the results of





Table 6Description of the Co(II)-hemp dynamic systems by means of the Thomas and Yoon–Nelson sorption parameters.

Initial concentration (mg/L) KT(L/min mg) Q0(T) (mg/g) R2 KYN (min−1) � (min) q0(YN) (mg/g) R2

Thomas model 25.00 8.56 × 10−4 12.49 0.97350.00 3.59 × 10−4 15.44 0.946

Yoon–Nelson model 25.00

50.00

Table 7Comparison of the Co(II) sorption capacity of natural hemp fibers with other low-cost sorbents.

Low-cost sorbent q (mg/g)

Batch Column Reference

Rose waste biomass 27.15 [29]Cells of Saccharomyces cerevisae 0.68 1.56 [50]Lignocellulosic fiber derived from

Citrus reticulata waste biomass52.64 [51]

Brown seaweeds (Sargassum wightii) 20.63 50 [52]Coal fly ash 0.401 [53]Freshwater alga Spirogyra hyalina 12.821 [54]Marine green alga Ulva reticulata 46.1 [55]Almond green hull 45.50 [56]Black carrot residues 5.35 [57]Brown alga Sargassum glaucescens

treated with formaldehyde27.6 [58]

ti

el(pimpia

3C

octtog[

ieccfstt

4

b

[

[

[

[

[

[

[

[

[

Bone char 108.7 [59]Present study 13.58 15.44

he Yoon–Nelson model, q0(YN), increases with the increase of thenitial concentration of Co(II).

To compare the Thomas and the Yoon–Nelson models, thexperimental breakthrough data were statistically processed byinear regression. Higher values of the linear regression coefficientsR2) for the Thomas plots (Table 6) than those for the Yoon–Nelsonlots suggest that the optimal solution for the description of the

nvestigated hemp bed column is provided by using the Thomasodel. This finding complies with the results of batch studies which

ointed out that the sorption of Co(II) ions on natural hemp fiberss very well described by the Langmuir isotherm model and follows

pseudo-second order kinetics.

.4. Comparison of sorption capacity of natural hemp fibers foro(II) with other low-cost sorbents

The values of the Co(II) sorption capacities in batch system andn column are compared in Table 7. It was found that the sorptionapacity of the column performs better than that of the batch sys-em. The increased capacity of the column method can be attributedo a continuously increasing gradient concentration in the interfacef the sorption zone, as it passes through the column, while theradient concentration decreases with time in batch experiments49].

On the other hand, it is very well known that a direct compar-son between the sorbents is difficult due to the use of differentxperimental conditions. However, the maximum Co(II) sorptionapacity of the tested hemp in batch and dynamic conditions isompared in Table 7 with other low-cost sorbents that were usedor the removal of Co(II). It can be seen from Table 7 that the Co(II)orption capacity of hemp is significant and comparable, so thathe hemp can be considered as a valuable alternative for use in thereatment of industrial effluents.


. Conclusions

Natural hemp provided a viable low-cost and eco-friendly sor-ent for the removal of Co(II) ions from wastewaters with low

[

[

0.014 215.85 15.38 0.9780.022 126.045 18.006 0.933

contents in the tested metal ions. The optimum value of the initialpH for the Co(II) ions sorption was found to be 4.5–5.

The sorption kinetics for the Co(II) removal from aqueous solu-tions with the initial pH of 4.5–5 was well described by thepseudo-second order model. The sorption isotherm studies clearlyindicated that the sorptive behavior of the Co(II) ions on naturalhemp fibers under study satisfied the Langmuir assumptions. Theslope of the obtained breakthrough curves was steeper as the Co(II)influent concentration increased.

In compliance with the results of the batch studies, the optimalsolution for the description of the investigated hemp bed columnwas provided by the Thomas model. The sorption capacity of thehemp fibers column (15.44 mg/g) performed better than that of theCo(II)-hemp batch system (13.58 mg/g).

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cobalt (ii) removal from aqueous solutions by natural hemp fibers: batch and fixed-bed column...

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