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Colloids and Surfaces B: Biointerfaces 117 (2014) 51–59

Contents lists available at ScienceDirect

Colloids and Surfaces B: Biointerfaces

journal homepage: www.elsevier .com/ locate /colsur fb

Preparation of magnetic, macro-reticulated cross-linked chitosan for

tetracycline removal from aquatic systems

N.A. Oladoja a,∗, R.O.A. Adelagun b, A.L. Ahmad c, E.I. Unuabonah d, H.A. Bello a

a Department of Chemistry, Adekunle Ajasin University, Akungba Akoko,Nigeriab Department of Chemistry, The Federal University,Wukari, Nigeriac School of Chemical Engineering, Universiti SainsMalaysia, Penang, Malaysiad Environmental and Chemical Processes Research Laboratory, Department of Chemical Sciences, Redeemer’sUniversity,Nigeria

a r t i c l e i n f o

Article history:

Received 9 August2013

Received in revised form 3 February 2014

Accepted 6 February 2014

Available online 14 February 2014

Keywords:

Chitosan

Tetracycline

Magnetite

Cross-linked chitosan

Pore forming agent

a b s t r a c t

A novel adsorbent, magnetic, macro-reticulated cross-linked chitosan (MRC) was synthesised for the

removal of tetracycline (TC) from water using a source of biogenic waste (gastropod shells) as a pore-

forming agent. The insertion of crosslinks into the chitosan frame was confirmed by FTIR analysis, while

the stability of the MRC was demonstrated via a stability test performed in an acidic solution. The

enhanced porosity of the MRC was confirmed by the evaluation of its porosity, a swelling test and the

determination of its specific surface area. The time–concentration profile of the sorption of TC onto the

MRC demonstrated that equilibrium was attained relatively quickly (120 min), and the data obtained

fitted a pseudo second order (r 2 > 0.99) kinetic equation better than a pseudo first order or reversible

first order kinetic equation. The optimisation of process variables indicated that the sorption of TC onto

the MRC was favoured at a low solution pH and that the presence of organics (simulated by the addi-

tion of humic acid) negatively impacted the magnitude of TC removal. The area of coverage of TC onthe

MRC (2.51 m2/g) was low compared to the specific surface area of the MRC (47.95 m2/g). The value of

the calculated energy of adsorption of TC onto the MRC was 100 kJ/mol, which is far above the range of

1–16 kJ/mol stipulated for physical adsorption.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Globally, pharmaceuticals are among the most widely con-

sumed materials, and tetracycline (TC) is one such widely

consumed pharmaceutical. TC comprises a suite of antibiotics that

exhibit broad-spectrum antimicrobial activity against a variety

of disease-producing bacteria. TC, chlortetracycline and oxytet-

racycline are antibiotics that are commonly used in animal

feed as growth promoters and prophylactics for swine and cat-

tle production [1]. Pharmaceutically active compounds undergo

metabolic processes within organisms, and significant portions of

the metabolised compounds are absorbed into the physiological

systems of these organisms. Nevertheless, significant fractions of

the parent compounds are also excreted in their unmetabolised

form and ultimately find their way into wastewater systems.

Considering recent reports from different parts of the world

[2,3] that conventional wastewater treatment plants (WWTP) are

∗ Corresponding author. Tel.: +234 8055438642.

E-mail address: [email protected] (N.A. Oladoja).

inadequately equipped to remove TC from their aquatic matrices,

efforts are now being focused on the occurrence and behaviour

of pharmaceuticals in WWTPs, which are considered as a sink for

these compounds. Consequently, advanced treatment procedures

are currently beingevaluatedas tertiary treatment options thatcan

improve the performance of the WWTPs in this regard. Ozonation

has beenreportedto be effective for theremovalof pharmaceuticals

from municipalWWTPsin Japan [4,5] and Germany [6]. Nanofiltra-

tion (NF) and reverse osmosis (RO) membrane filtration, applied at

the bench, pilot and full treatment plant scales [7–12], have also

been found to be effective options. Amongst the tertiary treatment

options being assessed, adsorption has been found to be an eco-

nomic, effective and attractive technology that can be employed

effectively due to the operational simplicity and the ubiquity of

potentialsorbents. Theadsorption of TC on sorbents based on metal

or metal oxides [1,13], magnesium–aluminium hydrotalcite [14],

hydrous oxides of aluminium and iron [15], iron oxides and iron

oxide-rich soils [16] have been reported.

In environmental engineering, polymers have been used

for different purposes, but the use of natural polymers has

attracted considerable attention from the perspectives of cost,

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52 N.A. Oladoja et al. / Colloids andSurfaces B: Biointerfaces 117 (2014) 51–59

environmental concern, and safety. Chitosan is one of the most

prominently studied natural polymers because of its interest-

ing inherent properties, such as its hydrophilicity, non-toxicity,

biodegradability, and excellent adsorption capacity. Caroni et al.

[17], while assessing the potential use of chitosan particles as

a controlled drug release system, did a kinetic analysis of the

sorption of TC on chitosan and reported that it is a promising

sorbent for TC. In its original form, chitosan is a relatively weak

base (pK a∼6.2) and thus soluble in acidic media (pH<6.0), which

limits its use as a sorbent in acidic media.

The chemical modification of chitosan by inserting crosslinks

has been used to decrease its solubility in acidic media [15,18].

However, while the insertion of crosslinks into the chitosan frame

confers stability in acidic media, this modification significantly

reduces its adsorption capacity because of the reaction of the

crosslinkerwith the aminegroup of chitosan [19,20], whichreduces

the number of binding sites for available sorption. The porosity

of the cross-linked adsorbent is also negatively impacted because

of the new three-dimensional structure, which increases the mass

transfer resistanceduring the sorption process. It is our opinion that

the reduction in the porosity of the adsorbent could be avoided by

incorporating a pore-forming agentduring the synthesis of this chi-

tosan sorbent. Theincrease in porosity that is derived from thehigh

void space of the pores formed, which offers a promising approach

for increasing the specific surface area and the diffusion coefficient

for substance transfer, has been confirmed previously [21]. It is

also noteworthy that a plethora of inexpensive adsorbents with

high porositieshave been developed,but manysuffer the disadvan-

tage of requiring tedious centrifugation and filtration steps in the

separation and reuse processes [22,23]. Thetediousnessof this pro-

cess has been reduced by the incorporation of magnetic particles

into this material during synthesis [13,24,25]. The resulting mag-

netic resins have been found to be easily recoverable by applying

an external magnetic field after the sorption process, operationally

inexpensive andare often highlyscalableand amenableto automa-

tion [26].

This current study explored a novel and economical method for

the preparation of magnetic macro-reticulated cross-linked chi-tosan (MRC) adsorbent for the removal of TC from aquatic systems.

Gastropod shells (GS) are used as the pore-forming agent, mag-

netite as the magnetic-inducing particle and glutaraldehyde (GA)

as thecrosslinker. The TC sorption parameters were derived during

the optimisation of the kinetic, isothermal and process variables of

the procedure.

2. Materials and methods

2.1. Materials

The GS used as a pore-forming agent was derived from the

shells of the African land snail (SS) ( Achatina achatina). Some of

the physicochemical parameters of the SS have been determined

and reported in an earlier treatise, and the SS were prepared

as described previously [27,28]. Samples of chitosan flakes with

average molecular weights of 105–106 g/mol and a deacetylation

percentage of approximately 87.94% that were prepared from the

shells of prawns were purchased from Chito-Chem (M) Sdn. Bhd.,

Malaysia. A TC hydrochloride (Fig. 1) that was purchased from a

registered pharmacy was used as the source of TC.

2.2. Magnetite preparation

The modified Massart method [20,29] was adopted for the

preparation of magnetite, viz: 250mL ofa 0.2M FeCl3 solution was

mixed with 250mL ofa 1.2M FeSO4 solution, and 200 mL o fa 1 . 5 M

Fig.1. (a)Chemicalstructureof TCin thehydrochlorideform.(b) Chemicalstructure

of glutaraldehyde cross-linked chitosan.

NH4OH solution was addedwhile stirring vigorously.The black pre-

cipitate that formed was allowed to crystallise for 30min under

magnetic stirring. The precipitate was filtered off and washed with

deoxygenated water by magnetic decantation until the pH of the

suspension decreased to below 7.5. The precipitate was dried in

an oven and tested with a magnetic bar to confirm its magnetic

property.

2.3. Preparation of macro-reticulated cross-linkedmagnetic

chitosan

A gram of chitosan was dissolved in 50mL of 25% aqueous

acetic acid. 12mL of a 25% glutaraldehyde solution was added toa known amount of magnetite and SS powder in a round bot-

tom flask and the mixture was heated in a water bath for 2 h at

50 ◦C. The contents of the flask were then added to the chitosan

solution and stirred until the solution became homogenous; the

solution was then heated at 70◦C for 6 h. The gel produced from

this process was washed repeatedly with a 0.5 M NaOH solution

followed by distilled water and dried at 70◦C for 8h before being

ground and sieved. The sieved material was then soaked in 0.5M

HCl to dissolve the SS and produce air bladders (CO2). The MRC

that was produced was collected by filtration, washed thoroughly

with deionised water, dried in the oven and stored until its use.

For the characterisation of the MRC, a reference sample was also

prepared following the same process, except for the HCl soaking

step.



N.A. Oladoja et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 51–59 53

2.4. Characterisation of the MRC

The surficial functional groups were examined using Fourier

transform infra-red spectrophotometery (FTIR), and the point of

zero charge(PZC) was determined using the solid addition method.

The porosity was determined by adopting the method of Chat-

terjee et al. [30], viz: A gram of the MRC was added to a beaker

containing 100mL of distilled water and allowed tosit for 24h. The

weight of the wet sorbent after 24h was determined, and then the

MRC wasdriedto constant weightto determinethe dryweight. The

porosity (, %)ofthe MRCwasdetermined using Eq. (1), accounting

for the water content within the pores of the MRC

ε =

(W w−W D)w

wDmat

+ (W w−W D)

w

× 100% (1)

where W W (g) is the weight of the wet MRC before drying; W D (g)

is the weight of the MRC after drying; w is the density of water,

1.0g/cm3, andMat is the density of the MRC in g/cm3. Thistestwas

performed for both the MRC and the MC.

The swelling rate of the sorbent in distilled water was deter-

mined by monitoring the weight gain of the sorbent in water. Dry

samples were immersed in distilledwater for24 h at room temper-

ature. The degree of swelling (S w) at equilibrium was calculated asfollows [31]:

Sw =W −W 0

W 0(2)

where W and W 0 denote the weight of wet MRC and the dry MRC,

respectively.

The stability of the MRC in an acidic medium was assessed via a

protocol described by Khoo and Ting [32], viz: Three acid solutions

with a pH of 1.99, 3.65 and 6.12, respectively, were prepared using

HCl, and a gram of the MRC was soaked in each of the solutions

for 24h at room temperature. Thereafter, the sorbent was thor-

oughly rinsed with deionised water and dried in a desiccator until

no further change in weight was detected.

Thespecific surface areasof theMRC andthe MCwere comparedvia a modified method of measuring specific surface based on the

theory of the single-layer molecular absorption of methylene blue

(MB) on a solid surface. The monolayer sorption capacity (qmax)

derived from a linear plot of the Langmuir equilibrium isotherm

model was used to determine the specific surface area by multiply-

ing the value of the saturation adsorption capacity (qmax) by 2.45

[33].

2.5. Sorption studies

A stock 2000mg/L TC solution was prepared with laboratory-

grade water, and working solutions of different TC concentrations

(25–200mg/L) were prepared from the stock by serial dilution.

The kinetic parameters of the sorption of TC onto MRC werederived by adding 2.0g of TC into a litre of a TC solution for con-

centrations that ranged between 25 and 200 mg/L. Samples were

withdrawn at intervals between 0 and 4h of sorption. The sor-

bent was separated by using an external magnetic field, and the

supernatant TC concentration was determined in each case.

The equilibrium isotherm analysis of the sorption process was

evaluated by initiating contact between 50mL of a solution of

a known TC concentration (within the range between 25 and

200 mg/L) and 0.1 g of sorbent. The mixture was stirred at 200rpm

in a thermostatic shaker for 2h. After shaking, the samples were

removed and the sorbent was separated by using an external mag-

netic field. The supernatant was then analysed for residual TC.

The influence of some process variables on the sorption process

were evaluated. The effect of pH on the sorption process was

Fig. 2. FTIR analysis of rawchitosan(↓) and MRC (↑).

investigated by varying the pH of the initial phosphate solution

between pH 3 and pH 10. An organic interference was simulated

by the addition of humic acid (HA) at concentrations that ranged

between 10 and 80mg/L.

The method of Caroni [17] was adopted for the quantification of

the residual TC concentration in the supernatant by using a UV–Vis

spectrophotometer at a wavelength of 276nm. A calibration curve

created fromanalysingTC solutions of different concentrations was

used. All of the concentrations considered in this work resulted in

absorbance values that were <2 and followed Lambert–Beer’s law.

The amount of TC sorbed per unit mass of adsorbent (in mg/g) was

calculated using the mass balance equation, viz:

qe =(C I − C f )v

m (3)

where, C I and C f are the initial and final sorbate solution concen-

trations, respectively; v is the sorbate volume and m is the mass of

sorbent used.

3. Results and discussion

3.1. Sorbent characterisation

The FTIR analysis of the chitosan powder (Fig. 2) showed a char-

acteristic band at 3404.47cm−1 that was attributed to the –OH and

–NH2 groups overlaying each other and the stretching vibrations

of C=O groups were observed at 1755.28cm−1 [34]. The bending

vibrations of the C–H bond in –CH2 were located at 2922cm−1

and those of the –CH3 group were located at 2877cm−1 [34]. The

band at 1656.91 cm−1 wasattributed to the stretching vibrations of

amide group carbonyl bonds C=O, and the band at 1558.540cm−1

was ascribed to the stretching vibrations of amine groups [35]. The

bending vibrationsof methylene andmethyl groups were located at

1377.22 and 1421.58cm−1

, respectively. Thespectrum in the range




of 1153–1030cm−1 was attributed to stretching vibrations of C=O

groups [36].

Vibrations similar to those observed in theraw chitosan particle

were detected in the MRC, but with different intensities and shifts

in peak positions (Fig. 2). There was a substantial reduction in the

number of peaks, but a few new peaks were also detected in the

MRC spectra. The absorption band of the –NH2 and –OH groups

that overlayed each other at 3404.47 were shifted to 3441.12 in

the MRC. The spectrum of the MRC displayed peaks at 2856.67 and1635.69 cm−1, which were ascribed to the C–H stretching of the

secondary alcoholic groups and the C=N stretching of the imine

groups, respectively. This result clearly indicates that the cross-

linking reaction with glutaraldehydeoccurredon the NH2 groups in

the chitosan [37]. The peak at 1712.85 wasascribed to the presence

of free aldehydes group, which did not react with amino groups

in the raw chitosan. It is known that bifunctional glutaraldehyde

molecules do not necessarily have both aldehyde groups reacted

with chitosan, and unreacted aldehyde functions may be available

in the final cross-linked matrix [38].

The porosity increased from 21.34% for the chitosan particle

to 44.26% for the MRC, which demonstrates the increase in the

porosity of the material caused by the addition of the pore-forming

agent (SS) and its incorporation into the MRC when soaked in

dilute HCl. The insertion of crosslinks into the chitosan framework

is expected to cause a substantial reduction in the ability of the

material to swell because of the presence of three-dimensional

networks should now hold the linear polymer chains tightly in

place andrestrictchain motion. Consequently, the swelling rates of

the MRC and the chitosan particles were found to be 1.83 and 0.82,

respectively. Theincrease in theswellingrate of theMRC compared

with the chitosan particle was caused by the increase in porosity,

which promotes fluid transfer and the movement of water into the

MRC.

The effects of the insertion of crosslinks on the stability of the

MRC were tested against that of the raw chitosan particles. A min-

imal loss in the weight of the MRC (<10%) was observed, whereas

a complete dissolution of the chitosan particle was obtained. The

minimal loss in the weight of the MRC was ascribed to the inser-tion of crosslinks in the chitosan framework, which hindered its

dissolution in acidic media. The observed minimal loss in weight

could have arisen from the migration of some of the constituents

of the MRC (magnetite and residual SS) from the solid phase into

the acidic medium.

The surface charge density of the MRC, tested via the determi-

nationof thePZC, revealed that the PZCvalue was7.30.The specific

surface area of the MRC was 47.95 m2/g while that of the chitosan

particle was 23.98m2/g. The larger specific surface area of the MRC

shouldbe attributed tothe inclusionof thepore-forming step in the

MRC preparation process, which endows the MRC with a greater

porosity than the chitosan particle, which was prepared without

the pore-forming step.

3.2. Time–concentration profile of TC sorption on MRC

The time–concentration profile of the sorption of TC by MRC

(Supplementary Fig. 1(SIF. 1)) was studied for 4 h at different ini-

tial solution TC concentrations (25–200 mg/L). The removal of TC

by the MRC occurred gradually until equilibrium was approached

at 120 min in all the initial solution TC concentrations studied. The

relatively short duration until theattainment of equilibrium (2 h) is

an indication that sorption of TC is a kinetic rather than a diffusion-

controlled process [39]. Consequently, the time–concentration

profile data of the sorption process were tested with differ-

ent kinetic models to derive the sorption kinetic parameters,

viz:

The uptake of TC by the MRC is represented by

MRC + TC k1−→k2

MRC ≡ TC (4)

where k1 is the forward reaction rate constant andk2 the backward

reaction rate constant.

To obtain the reversible-first-order kinetic parameters, the

equation for the sorption rate of TC by MRC is represented as

[40,41]:

d[TC]

dt = k1[TC]t + k2[MRCTC]t (5)

The [TC] and [MRCTC] represent the aqueous TC concentration

and solid phase (i.e., the MRC adsorbed) TC concentrations, respec-

tively, at time t . The migration of the TC moieties from the liquid

phase to the MRC surface, which caused a reduction in the aqueous

TC concentration, can be represented by: [MRCTC]t = [TC]0 − [TC]t .

Consequently, the integration of Eq. (5) yields

[TC]t − [TC]e = [TC]0 − [TC]e exp[−(k1 + k2)t ] (6)

where [TC]e is the concentration of aqueous TC at equilibrium.

Given that, under equilibrium conditions, d[TC]/dt = 0 , we may

define an equilibrium constant,K , and rewrite Eq. (5) thus:

K =k1

k2=

TC0 − TCe

TCe(7)

Rearranging Eq. (5) in terms of k2 and substituting it into Eq. (4)

results in the following:

k1t =

TC0 − TCe

TC0

ln

TC0 − TCe

TCt − TCe

(8)

The gradient of a plot of the term on the right hand side of Eq.

(8) versus time affords an estimate of the first order forward rate

constant for the reaction. Thus, an estimate of the reverse reac-

tion constant may be obtained from Eq. (7). The linearity of the

reversible first order kinetic model was relatively high (r 2 range:

0.9519–0.9872) (Table 1). The overall reaction rate, K (min−1),

decreased from 0.508 to 0.172 with an increase in the TC solution

concentration, and the rate of forward reaction,k1,was slower thanthat of the backward reaction, k2 (min−1), at all the concentrations

studied (Table 1). The results show that the uptake of TC by the

MRC is a reversible process.

The system response time ( I resp), defined as the reciprocal of

the summed rate constants, was used to evaluate the impact of

concentration on attaining equilibrium.

(9)

where I resp represents the time required for the system to reach

the new equilibrium with varying concentrations [42]. When the

initial TC concentration increased from 25 to 200mg/L, I resp

increased from 29.50 to 42.55min (Table 1). This indicates that the

extent of sorption increased with an increase in the solution TCconcentration. The tendency of the response time to increase with

an increase in concentration is an indication that theincreasein the

concentration gradient of the TC at higher solution concentrations

promotes themigration of TC towards the binding sites on the MRC

and causes an increase in the sorption potential.

To further elucidate the kinetics of the sorption process, the

inclusion of the instantaneous uptake of sorbate as a precursor to

the first-order reaction, as suggested by Martino et al. [42], was

employed to analyse the kinetic data viz:

MRC + TCK D0−→MRC ≡ TC; MRC + TC

k1−→k2

MRC ≡ TC

In this case, sorption sites, MRC, on the MRC are immediately

occupied as MRC particles are introduced into the system, an effect




T a b l e

1

R e s u

l t s o

f t h e

k i n e t i c m o

d e

l a n a

l y s i s o

f s o r p t i o n o

f T C o n M R C

.

I n i t i a l c o n c .

( m g

/ L )

R e v e r s i b

l e fi r s t o r d e r

P s e u

d o

fi r s t o r d e r

P s e u

d o s e c o n

d o r d e r

K

( m i n −

1 )

k 1

( m i n −

1 )

k 2

( m i n −

1 )

I r e s p

K D 0

( m L / g )

r 2

q e 1

( m g

/ g )

k 1

( g / ( m g m

i n ) )

r

2

q e 2

( m g

/ g )

k 2

( g / ( m g m

i n ) )

h ( m g

/ ( g m

i n ) )

r 2

2 5

0 . 5

0 6

0 . 0

1 1 4

0 . 0

2 2 5

2 9

. 5 0

1 1

. 4 3 3

0 . 9 5 1 9

3 . 9

0 3

0 . 0

3 4

0

. 9 5 1 9

4 . 4

6 0

0 . 0

7 7

1 . 5

3 2

0 . 9

9 9 6

5 0

0 . 3

7 6

0 . 0

0 5 3

0 . 0

1 4

5 1

. 8 1

6 . 9

1 8

0 . 9 5 7 1

4 . 3

4 8

0 . 0

1 9

0

. 9 5 7 1

7 . 3

0 5

0 . 0

6 0

3 . 2

0 2

0 . 9

9 9 7

1 0 0

0 . 2

3 3

0 . 0

0 3 3

0 . 0

1 4

5 7 . 8

0

4 . 5

0 7

0 . 9 5 4 8

7 . 5

6 3

0 . 0

1 8

0

. 9 5 4 8

1 0

. 5 4 9

0 . 0

3 4

3 . 7

8 4

0 . 9

9 1 7

1 5 0

0 . 2

1 3

0 . 0

0 5

0 . 0

2 3

3 5 . 7

1

2 . 0

5 2

0 . 9 8 2 7

1 3

. 0 8 9

0 . 0

2 9

0

. 9 8 2 7

1 5 . 0

8 3

0 . 0

3 5

7 . 9

6 2

0 . 9

9 1 6

2 0 0

0 . 1

7 2

0 . 0

0 3 5

0 . 0

2 0

4 2

. 5 5

2 . 4

0 1

0 . 9 8 7 2

1 1

. 7 0 0

0 . 0

2 3

0

. 9 8 7 2

1 6

. 3 4 0

0 . 0

4 4

1 1

. 7 4 8

0 . 9

9 7 7 thatis quantified by an instantaneous distribution coefficient,(K D)0

(mL/g). This constant is calculated from the difference between the

aqueous concentration of the sorbate at the start of the experi-

ment, [TC]0 and the concentration at the “effective” start of the

experiment, [TC]

0, which is defined as the first measurement in

the adsorption time course.

K D0 =[TC]0 − [TC]0

[TC]0[m] (10)

where [m] represents the amount of MRC particles in the reac-

tor in g/mL. The value of the instantaneous distribution coefficient,

(K D)0 (mL/g), decreases with an increase in the initial concentration

of TC (Table 1).

The Lagergren pseudo first-order model [43] represented by Eq.

(11) and the pseudo second-order model [44] represented by Eq.

(12) were also employed to analyse the time–concentration data.

log[qe − qt ] = log[qe] −

k1

2.303

t (11)

t

qt =

1

kqe+

1

qet (12)

where the initial sorption rate, h, for the pseudo second-order

kinetic model is represented by

h = k2q2e (13)

The results from fitting the data obtained from the sorption

process into the aforementioned kinetic models are presented in

Table 1. The values of the pseudo first-order equilibrium capac-

ity (qe) and the pseudo first-order sorption rate increased with

an increase in initial solution concentrations of TC (Table 1). The

results of the pseudo second-order parameters (Table 1) also

demonstrated that the amount of TC uptake (qe) and the initial

sorption rate (h) increased with an increase in the initial TC con-

centration. The qe value increased from 4.460 to 16.340 (mg/g) and

the k2 value decreased from 0.077 to 0.044 (g/(mg min)), while h

increased from 1.532 to 11.748 (mg/(g min)). According to Ho and

McKay, [45] if the sorbate uptake is chemically rate controlled, thepseudo second-order constants will be independent of the particle

diameter andflow rate andwill depend on the concentration of the

ions in a solution.

3.3. Effects of pH on TC sorption onto MRC

The results presented in Fig. 3 show that the magnitude of the

TC sorbed by the MRC decreased as the initial solution pH (pHi)

increased from 3 to 10. This revealed that the sorption of TC onto

the MRC was favoured at lower values of pHi. The determination of

the equilibrium pH (pHe), at different pHi levels, revealed a slight

0

1

2

3

4

5

6

7

8

9

10

0

5

10

15

20

25

30

30 1 2 4 5 6 7 8 9 10 11

E q u i l i b r i u m p

H

q e ( m g / g )

Inial pH

qe final pH

Fig. 3. Effect of pH onthe sorptionof TC onto MRC.




increase in the pHe at pHi 3 and pHi 4, whereas a reduction in the

values of pHe was recorded at higher pHi levels (pH = 6–10) (Fig. 3).

A previous examination of the sorption of TC onto different

sorbents has revealed that the process is pH dependent because

the speciation of TC in an aquatic system and the surface chemistry

of the sorbent, which both strongly influence the chemistry of

sorbate–sorbent interactions, are both pH dependent (SIF 2) [46].

Reports [46] have shown that TC exists predominantly as a cation

(+00) below pH3.3 because the dimethyl ammonium group is

protonated, as a zwitterion (+−0) between pH 3.3 and 7.3 due to

the loss of protons from the phenolic diketone moiety, and as an

anion, (+−−) o r ( 0−−), above pH 7.3 because of the loss of protons

from the tricarbonyl system and phenolic diketone moiety. In

addition to the speciation profi le of TC, Caroni et al. [17] also

opined that the hydrochloride form (Fig. 1) of TC (HTC), the species

of TC used in the present study,also deprotonates in aquatic media.

The high tendency of the amino groups on the chitosan parti-

cle to become protonated at low pH has been reported previously

[37]. Consequently, the proton obtained from the deprotonation

of the HTC is beneficial for the protonation of the amino group on

the chitosan phase of the MRC, as presented in Eq. (15). The higher

sorption capacity exhibited by the MRC for TC at a low solution pH

(pH = 3.0) was ascribed to concurrent adsorption of TC moiety via

interactions based on hydrogen bonding, permanent and induced

dipoles [17], electrostatic interactions and cation exchange. The

occurrence of electrostatic interactions, despite the existence of

TC as a cationic species at pH< 3.33, and the value of the pHPZC

of the MRC obtained (pHPZC = 7.30), which is expected to preclude

anyform of electrostatic interaction, is illustrated in Eqs. (14)–(16),

viz:

R-NH2 +TC-HCl = R-NH3+Cl−+TC+ (14)

R-NH3+Cl−+ TC++R-NH3

+Cl− TC+ (15)

and/or

R-NH3+Cl−+ TC++R-NH3

+Cl hydrogen bonding TC+ (16)

The electrostatic interactions between the Cl− species, whichare partly released by the deprotonation of the HTC, and posi-

tively charged protonated amino groups on MRC can occur (Eq.

(14)), which ultimately create avenues for another possible elec-

trostatic interaction between the negative Cl− end and the cationic

TC species in solution (Eq. (15)).

The possibility of cation exchange (Eq. (17)) between the pro-

tons of the protonated amino groups on the MRC and the cationic

TC species present in solution at the initial solution pH (pH = 3.0)

that resulted in the greatest TC removal from the solution was

tested via the determination of the pHe. This test was premised on

the assumption that, if cation exchange actually occurred, a reduc-

tion in the pHe value is imminent because of the increase in the

hydrogen ion concentration of the solution that would result from

the contribution of the exchangeable protons (H+) from the MRCsurface to the aquatic system, viz:

R-NH2 +TC-HCl = R-NH3++TC+ +Cl− (17)

R-NH3++TC++R-NH2 TC++H+ (18)

An increase in solution pH was observed from a pHi of 3.0 to a

pHe of 3.26, not a reduction in the pH value. This increase in the

pH value could be ascribed to the low solution pH, which promotes

the release of some of the residual Ca2+, which is a constituent of

the SS used as a pore forming agent, from the MRC. The released

Ca2+ ions counteract the effect of the contribution from the H+ ions

contributed from the cation exchange process and, depending on

the amount of Ca+ released, could cause an increase in the pH of

the solution.

Fig. 4. FTIR spectra of MRC (↓) and MRC–TC(↑).

Above a pH of 3.26, a reductionoccurredin themagnitude of the

TC removal bythe MRC (Fig.3), buta significant drop in the amount

of TC sorption occurred above pH 6.0. The decrease in the amount

of TC removal was attributed to the reduction in the number of

removal mechanisms in operation as the value of pHi increases,

a result of the change in the species of TC in the aquatic system

from a cationic to a zwitterionic form. It is also noteworthy that

the decrease in the amount of TC removed, which was of greater

significance at pHi > 6, could also be attributed tothe predominance

of TCin the anionic format a pH> 7.3.Thishigher pHsystem, which

coincides with the pH region above the isoelectric point of chargeof the MRC, promotes the repulsion between the predominantly

negatively charged surface and the predominantly anionic species

present in solution.

The evidence for an interaction between the MRC and the sor-

bate (i.e., the TC) was acquired using FTIR, and the results obtained

are presented in Fig. 4. Premised on the report that the most char-

acteristic peaks of TC are those in 1200–1800cm−1 [15], peaks

synonymous with TC were sought within this region of the TC

laden MRC (i.e., MRC–TC) FTIR spectra. The identifiable TC peaks

withinthis regionare theamide I peak at 1641.48cm−1 andthe C–C

stretching vibration at 1442.8 cm−1. Some of the peaks observed in

theMRC also appeared in theMRC–TC,but there wasa slightshiftin

the peaks from 1712, 2359, 2939 and 3441cm−1 in the MRC spec-

tra to 1722, 2343, 2937 and 3416cm−1 in the MRC–TC, which is an

indication of an interaction between the MRC surface and the TC

molecule.

3.4. Effects of organic load on the sorption of TC onto MRC

The results presented in Fig. 5 show the effects of the presence

of organic load, simulated by the addition of HA, on the TC sorp-

tion by MRC. The magnitude of the TC removed decreased with an

increase in the initial HA concentration. For the initial HA concen-

tration range from0 to 80(mg/L), the amount of TC removal (mg/g)

decreased from14.340 at an initial HA concentration of 0 mg/L to

0.699 at 80mg/L HA.This resultindicates the occurrence of compe-

tition between the TC molecules and the HA molecules for sorption

sites on the MRC.




0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80 90

q e

( m g / g )

Humic Acid conc. (mg/L)

Fig. 5. Effect of organic load on the sorption of TContoMRC.

A typical humic substanceis a mixture ofmany moleculeslinked

together, and some of these molecules are based on a motif of aro-matic nuclei with phenolic and carboxylic substituents. Anirudhan

etal. [37] concluded that the carboxylic andphenolic groupson the

HA are deprotonated in weakly acidic to basic media, thereby, con-

ferring a negative charge to the HA molecule. Depending upon the

PZC value of the MRC, the negatively charged HA species can inter-

act electrostatically with the predominantly positively charged

MRC surface at the solution pH studied, which was below PZC of

the MRC. This electrostatic interaction thus caused a reduction in

the magnitude of TC uptake by the MRC.

3.5. Equilibrium isotherm analysis of the sorption of TC onto the

MRC

The results of theLangmuirand Freundlich isotherm analyses of the equilibrium isotherm studies are presented in Table 2. The lin-

ear form of both the Langmuir and Freundlich isotherm equations

are presented in Eqs. (19) and (20), respectively:

C eqe

=1

qmC e +

1

K aqm (19)

where C e is the equilibrium concentration of TC in solution (mg/L),

qe is the amount of TC adsorbed at equilibrium (mg/g), qm is the

theoretical maximum monolayer sorption capacity (mg/g), and K ais the Langmuir constant (L/mg). When C e/qe is plotted against C e,

a straight line with a slope of 1/qm and an intercept of 1/qmK a is

obtained.

log qe = log K f +1

n

log C e (20)

where C e and qe have been previously defined, K f is the Freundlich

constant (mg/g)(L/mg)1/n and1/n is the heterogeneity factor. There-

fore, a plot of log(qe) versus log(C e) enables the determination of

the constant K f and the exponent 1/n.

The conformity of the experimental data to the two isotherm

equations tested, as indicated by the values of the correlation

Table 2

Equilibrium isotherm parameters for theremoval of TC by MRC.

Langmuir isotherm Freundlich isotherm

qm =20.704 n=1.872

K a =0.0133 K f = 1.111

r 2 = 0.963 r 2 =0.9916

0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100 120 140 160 180

q e

( m g / g )

Ce (mg/L)

qeLang qeFreund qeexp

Fig. 6. Equilibrium isotherm modelling of thesorptionof TC onto MRC.

coefficients (r 2) of the linear plots, indicate that the Freundlichisotherm model provides a better description (r 2 =0.9916) of the

sorption of TC onto the MRC, as compared to Langmuir isotherm

model (r 2 =0.963), which is an indication of the heterogeneous

nature of the MRC surface available for TC binding. The hetero-

geneous surface, with varying affinities of the MRC for TC, could

be ascribed to the multicomponent nature of the adsorbent (i.e.,

the presence of SS, chitosan, and magnetite) and the contribu-

tion of each of the constituents to TC sorption. The roles of these

different components of the MRC in TC abstraction from aquatic

systems have beenreported.Differentiron rich materialshave been

reported to remove TC from aquatic systems [15,16] and the mech-

anism has been attributed to a surface complexation mechanism

that is attributed to the interaction between divalent anion species

and the oxide surface. The presence of Ca in soil matrices has alsobeen reported toenhance TC removal viacation bridging andcation

exchange [24].

To further assess the correspondence between different

isothermsand the experimental data, the theoretical plotsobtained

fromeach isotherm analysis were fittedwith the experimental data

for the sorption of TC ontothe MRC (Fig. 6). The graph is plotted as

theamountofTCsorbedperunitmassofsorbent,qe (mg/g)graphed

against the concentrationof TC remainingin solution,C e (mg/L). The

results indicate that the two isotherm models provide fair predic-

tions of the experimental data, but the Langmuir isotherm model

provides a better fit to the experimental data than the Freundlich

isotherm model. This result indicates that the mechanism for the

interactions between the TC and the MRC surface did not strictly

follow one particular isotherm model butcould be describedas the

monolayersorption of TC taking placeupon heterogeneous surfaces

or that MRC surfaces support sites with varying affinities for TC.

3.6. Estimation of area of coverage of TC on MRC

Duarte et al. [47] opined that TC is a very adaptive molecule,

capable of easily modifying itself through tautomerism in response

to various chemical environments. The generally accepted con-

formations are extended and twisted ones. In neutral to acidic

solutions, TC adopts a twisted conformation, while in basic solu-

tions the equilibrium is shifted to the extended conformations

[47–50]. Considering the medium in which our experiment was

carried out, the reported dimensions of entirely protonated TC in

a twisted conformation are 12.9 A long, 6.2 A high and 7.5 A thick,




and this form of the TC molecule occupies a cross sectional area of

97 A2 [49]. Considering the monolayer coverage of TC on the MRC

particle surface, (estimated from the Langmuir equation), the pro-

jected cross sectional area of the TC molecule ( A), N , is theAvogadro

number (6.02×1023); m is the molecular weight of the adsorbate

(in this case 480.93 for TC hydrochloride), and the TC coverage per

unit gram of MRC (S , m2/g) can be calculated using Eq. (21):

S =

qmNA

m (21)

Based on the equation shown above, the area occupied by the

TCon theMRC was 2.51m2/g, which is an indication that the max-

imum specific surface area of MRC occupied by TC binding was far

below the MRC specific surface area (47.95m2/g).

3.7. Estimation of energy of sorption of TC on MRC

The energy of sorption of TC onto the MRC was estimated by

using the data obtained from the equilibrium isotherm studies to

solve the Dubinin–Radushkevick (D–R) equation [51,52].

lnqe = lnqm − kDR ε2 (22)

ε = RTln

1 +

1

c e

where ε is the Polanyi potential, qm is the D–R adsorption capac-

ity (mol/kg), andK DR is a constant related to the adsorption energy

(mol2/kJ2). The qm and K DR parameters, evaluated from the inter-

cepts and slopes of the plots of ln qe versus ε2, were 11.62 and

5×10−5. Themeanfreeenergy ofadsorption(E DR )isthefreeenergy

change when 1 mol of ion is transferred to the surface of the adsor-

bent from infinity in thesolution[51,52] and it wascalculated using

Eq. (23):

E DR = (−2K DR )−1/2 (23)

The E DR (kJ/mol) value of the sorption of TC by the MRC was100 kJ/mol. Akkaya, and Ozer [53] suggested that the magnitude of

E DR could be used for estimating the type of adsorption reaction

and, if the value of E DR is between 1 and 16kJ/mol, then physi-

cal adsorption prevails. If the value is more than 16kJ/mol, then

chemisorption prevails. The value of the EDR obtained from the

present sorption process is far above the range given for a phys-

ical adsorption mechanism and, thus, indicates that chemisorption

was the dominant mechanism for the interactions between the TC

and the MRC surface. The dominant effect of chemisorption as the

mechanism for interaction between TC and MRC is also confirmed

by the conformance of the time–concentration profile data to the

pseudo second-order kinetic model.

4. Conclusions

• MRC can be prepared using gastropod shell as a pore-forming

agent.• The sorption of TC onto MRC is more a kinetic- than a diffusion-

controlled process.• The interactions between TC and the MRC surface occurred via a

monolayer sorption of TC onto heterogeneous surfaces of varied

affinities.• TC removal by the MRC was favoured at lower pH values, and the

presence of organics had a negative impact on TC sorption.• TheareaoccupiedbyTContheMRC wasverylowwhen compared

with the specific surface area of the MRC.• Chemisorption wasthe dominant mechanismof TC sorption onto

the MRC.

Appendix A. Supplementary data

Supplementary data associated with this article can be

found,in theonline version, at http://dx.doi.org/10.1016/j.colsurfb.

2014.02.006.

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