Use of Ricinus communis leaves as a low cost adsorbent for removal of Cu(II) ions from aqueous...

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Research on Chemical Intermediates ISSN 0922-6168 Res Chem IntermedDOI 10.1007/s11164-013-1029-z

Use of Ricinus communis leaves as a low-cost adsorbent for removal of Cu(II) ionsfrom aqueous solution

M. Makeswari & T. Santhi

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Use of Ricinus communis leaves as a low-cost adsorbentfor removal of Cu(II) ions from aqueous solution

M. Makeswari • T. Santhi

Received: 16 October 2012 / Accepted: 5 January 2013

� Springer Science+Business Media Dordrecht 2013

Abstract The study was conducted to compare removal of Cu(II) from aqueous

solutions by water-washed raw leaves of Ricinus communis (RLRC) and by activated

carbon prepared, with microwave assistance, from zinc chloride-treated leaves of

R. communis (ZLRC). The ZLRC preparation conditions were: radiation power

100 W, radiation time 8 min, were mixed with the ZnCl2 concentration of 30 % by

volume, and impregnation time 24 h. The RLRC and ZLRC were characterized by

FTIR, SEM-EDAX, and XRD analysis. The effects of different conditions, for example

solution pH, initial metal ion concentration, contact time, adsorbent dose, and presence

of other ions were studied by use of batch-mode experiments. Maximum adsorption of

Cu(II) was observed at pH 5.4 for RLRC (50 %) and at pH 6.3 for ZLRC (64.25 %).

The Langmuir, Freundlich, Temkin, and Dubin–Radushkevich isotherm models were

used to analyze the equilibrium data. The data were also fitted to the pseudo-first order,

pseudo-second order, intra particle, and Elovich kinetic models. The adsorption

equilibrium data were well fitted by the Langmuir model. Kinetic studies showed that

adsorption followed pseudo-second order and intra particle diffusion models. The

adsorption capacity of ZLRC was greater than that of RLRC. According to the

experimental results, the adsorbent derived from this material is expected to be an

economical product for metal ion remediation of water and waste water.

Keywords ZnCl2 � Microwave activation � Adsorption � Isotherms �Kinetics � Cu(II)

Introduction

The contamination of water by toxic heavy metals by discharge of industrial

wastewater is a worldwide environmental problem [1]. The presence of metal ions

M. Makeswari � T. Santhi (&)

Department of Chemistry, Karpagam University, Coimbatore 641021, Tamil Nadu, India

e-mail: [email protected]

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Res Chem Intermed

DOI 10.1007/s11164-013-1029-z

Author's personal copy

in streams and lakes has been responsible for several health problems with animals,

plants, and human beings. Numerous metals, for example Sb, Cr, Cd, Cu, Pb, and Hg,

have toxic effects on humans and the environment [2]. Because copper is widely used

there are many actual or potential sources of copper pollution. Copper may be found as

a contaminant in food, especially shellfish, liver, mushrooms, nuts, and chocolate.

Briefly, any processing or container containing copper material may contaminate such

products as food, water, or drink. Copper is essential to human life and health but,

similar to all heavy metals, is potentially toxic also. For example, continual inhalation

of copper-containing spray is linked with an increase in lung cancer among exposed

workers [3]. A limit of 2 mg/L has been proposed by the World Health Organization as

provisional guideline value for the copper content of drinking water, and intake of

excessively large doses of copper by man leads to severe mucosal irritation and

corrosion, widespread capillary damage, hepatic and renal damage, and central

nervous system irritation followed by depression. Severe gastrointestinal irritation

and possible necrotic changes in the liver and kidney can occur [4–6].

It is, therefore, urgent to remove copper metal from wastewater. Although heavy

metal removal from aqueous solutions can be achieved by conventional methods,

including chemical precipitation, oxidation and/or reduction, electrochemical

treatment, evaporative recovery, filtration, ion exchange, and membrane technology,

these methods may be ineffective or costly, especially when metal ion concentrations

in solution are in the range 1–100 mg/L [7, 8]. Recently, adsorption technology has

become and alternative treatment on either laboratory or industrial scale [9, 10].

Many adsorbents are in use. The different types of activated carbon (AC) are very

effective adsorbents, because of their high porosity, large surface area, variable

surface chemistry, and high surface reactivity [11]. Because of high production costs,

however, these materials tend to be more expensive than other adsorbents. This has

led to growing research interest in the production of activated carbon from renewable

and cheaper precursors. The choice of precursor largely depends on its availability,

cost, and purity, but the manufacturing process and intended applications of the

product are also important considerations [12]. Several suitable agricultural by-

products (lignocellulosics) including fruit stones [13], olive waste cake [14, 15], pine

bark [16], rice husks [17], pistachio nut shells [18], and wheat bran [19] have recently

been investigated as activated carbon precursors, and are still receiving attention.

Two methods are used for preparation of AC, physical and chemical activation.

During physical activation, the raw material is first carbonized at high temperature

then activated by CO2 or steam under pressure to increase the porosity and surface

area of the AC. In chemical activation carbonization and activation occur

simultaneously; the raw material is first impregnated with activating chemical then

carbonized at the desired temperature, which varies in accordance with the

activating chemical used [20]. Development of a porous structure is better for

chemical activation [21]. Chemical activation is conducted in the presence of such

dehydrating reagents as KOH, K2CO3, NaOH, ZnCl2, and H3PO4, which affect

pyrolytic decomposition and inhibit tar formation. In chemical activation the carbon

yield is higher and the temperature used is lower than in physical activation. The

behavior of the reagents during chemical activation has different effects on the final

product. ZnCl2 is widely used as activating reagent, because it results in high

M. Makeswari, T. Santhi

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surface areas and high yield [22, 23]. When ZnCl2 is used the activated carbon

obtained has larger surface area and greater micro pore structure [24, 25]. To

prepare activated carbon, conventional heating is usually adopted, the energy being

produced by use of an electrical furnace. Recently, microwave heating has been

used in the production of activated carbon because it enables rapid and uniform

heating [23].

Application of microwave (MW) heating for regeneration of industrial waste

activated carbon has been investigated, with very promising results [26, 27]. The

main difference between MW devices and conventional heating systems is the

manner of heating. Microwaves supply energy directly to the carbon bed. Energy

transfer is not by conduction or convection as in conventional heating, but

microwave energy is readily transformed into heat inside the particles by dipole

rotation and ionic conduction [26–28]. Microwave energy has recently been widely

using in several applications, in both research and industrial processes. Although

use of microwave energy substantially changes the properties of carbonaceous

materials, there are relatively few publications describing use of microwaves for

production and regeneration of activated carbon. Nabais et al. studied modification

of the surface chemistry of activated carbon fibers by microwave heating, which

was found to be very effective. Microwaves are being used in a variety of fields to

heat dielectric materials, because treatment time and energy consumption are

substantially reduced.

Because the leaves of Ricinus communis are available free of cost, only

carbonization of its leaves is involved in waste water treatment. The capacity of

classically activated epicarp of Ricinus communis for adsorption of MG dye from

aqueous solution has previously been investigated [29]. The capacity of microwave-

activated zinc chloride-treated leaves of Ricinus communis for removal of nickel(II)

ions from aqueous solution has also been investigated [30].

The purpose of this work was to investigate the capacity of a low-cost activated

carbon sorbent prepared from Ricinus communis leaves by microwave-assisted

chemical activation, using zinc chloride as activating agent, for adsorption of copper

from aqueous solution.

Optimization of process conditions and characterization of both the optimum

activated carbon and of water-washed leaves of Ricinus communis were first

investigated. Thereafter, adsorption of the copper ions was undertaken. The

adsorption isotherms, kinetics, and column studies were investigated.

Materials and methods

Preparation of adsorbents

Preparation of activated carbon from zinc chloride-treated leaves of Ricinuscommunis (ZLRC)

Ricinus communis leaves were obtained from a farm in Tirupur district (Tamil

Nadu). They were air-dried then powdered in a grinder. Dried Ricinus communis

Use of Ricinus communis leaves as a low cost adsorbent

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leaves (6 g) were mixed with 30 mL ZnCl2. The slurry was kept at room

temperature for 24 h to ensure access of the ZnCl2 to the R. communis leaves. After

mixing, the slurry was placed in a MW heating apparatus (MW71E; Samsung).

After exposure to microwave heating power of 100 W for 8 min the carbonized

samples were washed with 0.5 M HCl, hot water, and cold distilled water until the

pH of the washing solution reached 6–7, filtered, and finally dried at 110 �C.

Preparation of water-washed raw leaves of Ricinus communis (RLRC)

Powdered leaves of Ricinus communis were washed with distilled water several

times to remove water-soluble matter, for example pigments. After washing, the

leaves were dried at 110 �C in a hot air oven. The RLRC was then stored in an air-

tight container for later experimental use.

Preparation of stock solution

Copper sulfate pentahydrate (CuSO4�5H2O), nickel sulfate hexahydrate

(NiSO�6H2O), and potassium chromate (K2CrO4) were obtained from Aluva,

Edayar (Spectrum Reagents and Chemicals). All other chemicals used in this study

were analytical grade and also purchased from Aluva, Edayar (Spectrum Reagents

and Chemicals).

A stock solution (1000 mg/L) was prepared by dissolving accurately weighed

CuSO4�5H2O in 1000 mL double-distilled water. Working copper solutions were

prepared just before use by appropriate dilutions of the stock solution.

Characterization of the adsorbent

Physicochemical characteristics of the adsorbent

The characteristics yield, iodine number, moisture content, surface acidity, and

basicity of adsorbents prepared from leaves of Ricinus communis were determined.

The yield of the prepared carbon samples was estimated by use of the equation:

Y ¼ M

M0

� 100; ð1Þ

where M is the weight of RLRC and ZLRC and M0 is the weight of air dried Ricinuscommunis leaves.

Iodine is regarded as a probe molecule for assessing the adsorption capacity of

adsorbents for solutes of molecular size \10 A. Iodine number (mg/g adsorbent),

was determined by use of standard 0.1 M iodine solution. The titrant used was

0.1 M sodium thiosulfate.

Surface acidity and basicity was estimated by mixing 0.2 g adsorbent (RLRC and

ZLRC) with 25 mL 0.5 M NaOH and 0.5 M HCl in a closed flask, the flask was

agitated for 48 h at room temperature (28 �C). The Suspension was decanted and

the remaining NaOH or HCl was titrated with 0.5 M HCl or 0.5 M NaOH, using

phenolphthalein as indicator.


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Zero surface charges—analysis of the characteristics of RLRC and ZLRC

The zero surface charge of RLRC and ZLRC were determined by use of the solid

addition method [31]. The experiment was conducted in a series of 250-mL glass

stoppered flasks; 50 mL NaNO3 solution of different initial pH and 0.2 g RLRC or

ZLRC was placed in each flask. The pH of the NaNO3 solutions was adjusted

between 2 and 9 by addition of either 0.1 M HNO3 or 0.1 M NaOH. The

suspensions were then sealed and shaken for 2 h at 150 rpm. The final pH of the

supernatant liquid was noted. The difference between the initial pH (pH0) and final

pH (pHf) (pH = pH0 - pHf) was plotted against pH0. The point of intersection of

the resulting curve with the abscissa gave pHZPC.

Functional group analysis

Functional groups in RLRC and ZLRC were determined by use of Fourier-transform

infrared spectroscopy, at room temperature, by use of a Shimadzu, IR Affinity-1

spectrometer. KBr pellets were prepared from the samples, and the scanning range

was 4000–400 cm-1.

Batch equilibrium studies

Batch experiments were performed to study the effect of conditions such as adsorbent

dose, dye concentration, and solution pH on removal of adsorbate by RLRC and

ZLRC. Stock solutions of CuSO4�5H2O were diluted to 50–200 mg/L for these

experiments. pH was adjusted by addition of 0.1 M HCl or 0.1 M NaOH to solutions

of known initial metal content. Batch adsorption experiments were conducted by use

of 250-mL stoppered flasks containing 0.2 g adsorbent and 50 mL metal solution of

different concentration (50, 100, 150 and 200 mg/L) at pH 5. The flasks were agitated

by use of a mechanical orbital shaker, and maintained at room temperature for 2 h at a

fixed shaking speed of 120 rpm until the equilibrium was reached. The suspensions

were filtered and metal concentrations in the supernatant solutions were measured by

use of a Digital photo colorimeter (model number 313). From the initial and final

concentrations, percentage removal can be calculated by use of the formula:

% of Removal ¼ ðC0 � Cf ÞC0

� 100; ð2Þ

where C0 is the initial concentration of Cu(II) ions and Cf is the final concentration

of Cu(II) ions (both mg/L). The results obtained in batch mode were used to cal-

culate the equilibrium metal-uptake capacity. Equilibrium uptake of Cu ions by

RLRC and ZLRC at (qe) was calculated by use of the mass–balance relationship:

qe ¼C0 � Ceð Þ

W� V; ð3Þ

where qe is the equilibrium uptake capacity (mg/g), V is the sample volume (L), C0

is the initial metal ion concentration (mg/L), Ce the equilibrium metal ion con-

centration (mg/L), and W is the dry weight of adsorbent (g).


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Adsorption isotherms

The adsorption isotherm is extremely important information indicating how

adsorbate molecules are distributed between the liquid and solid phases when the

adsorption process reaches equilibrium. Knowledge of when the system is at

equilibrium is of importance for determining the maximum sorption capacity of

RLRC and ZLRC for the metal in solution. Equilibrium data are basic requirements

for design of adsorption systems and adsorption models, which are used for the

mathematical description of the adsorption equilibrium of the metal ion. The results

obtained for adsorption of Cu(II) ions were analyzed by use of well-known models

given by the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich, iso-

therms. For the sorption isotherms, initial metal ion concentration was varied

whereas solution pH and amount of adsorbent were held constant. The sorption

isotherms for Cu were obtained for RLRC at solution pH 5.4 and ZLRC at pH 6.3.

Langmuir isotherm

The Langmuir model is based on the assumption of a structurally homogeneous

adsorbent on which all adsorption sites are identical and energetically equivalent.

The Langmuir model is used for fitting of a monolayer and/or chemical adsorption.

It is represented as follows [32]:

Ce

qe¼ 1

qmaxbþ Ce

qmax

; ð4Þ

where qe (mg/g) is the amount of phosphate adsorbed at equilibrium, Ce (mg/L) is the

liquid-phase phosphate concentration at equilibrium, qmax (mg/g) is the maximum

adsorption capacity of the adsorbent, and b (L/mg) is the Langmuir adsorption constant.

Freundlich isotherm

The Freundlich model is used to describe a heterogeneous system characterized by a

heterogeneity factor of 1/n. This model describes reversible adsorption and is not restricted

to the formation of a monolayer. The Freundlich model is expressed as follows [33]:

log qe ¼1

nlogðCeÞ þ log K; ð5Þ

where Ce (mg P/L) is the liquid-phase phosphate concentration at equilibrium, K is the

Freundlich isotherm constant, and 1/n (dimensionless) is the heterogeneity factor.

Temkin isotherm

Temkin adsorption isotherm is expressed as:

qe ¼ B ln Aþ B ln Ce; ð6Þ


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where A is Temkin constant representing adsorbate–adsorbate interactions and B is

another constant related to the heat of adsorption [34].

The Temkin isotherm takes into account adsorbing species–adsorbent interac-

tions. Isotherm constants A and B can be determined from plot of qe against ln Ce.

Dubinin–Radushkevich (D–R) isotherm

To determine whether adsorption is physical or chemical in nature, the equilibrium

data were applied to D–R model [35]. The linearized form of the D–R model is:

ln Cads ¼ ln Cm � Ye2; ð7Þ

where Cads is the amount of metal ions adsorbed on the surface of the adsorbent (mg/L),

Cm is the maximum adsorption capacity (mg/g), Y is the activity coefficient related to

mean adsorption energy (mol2/J2), and e is the Polanyi potential (kJ2 mol2).

The Polanyi potential [36] can be calculated by use of the equation:

e ¼ RT ln 1þ 1

Ce

� �ð8Þ

The mean adsorption energy, E (kJ/mol) is calculated by use of equation:

E ¼ 1ffiffiffiffiffiffiffiffiffiffi�2Yp : ð9Þ

Batch kinetic studies

Kinetic experiments were performed by using a procedure similar to that in the

equilibrium studies. Studies were conducted in 250-mL shaking flasks at a solution

pH of 7. Adsorbent (0.2 g) was thoroughly mixed with 50 mL copper solution

(100 mg/L) and the suspensions were shaken at room temperature for appropriate

times. The solutions were filtered and analyzed for residual copper ion concentra-

tion. To determine the kinetic model best fitting the experimental adsorption data,

pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion mod-

els were examined.

Pseudo-first-order kinetic model

The Lagergren pseudo-first-order rate expression is given by the equation [37]:

ln ðqe � qtÞ ¼ ln qe � k1t; ð10Þ

where qe (mg/g) is the amount of metal ions adsorbed at equilibrium, qt (mg/g) is the

amount of nickel ions adsorbed at time t, and k1 (min-1) is the rate constant of the

pseudo-first-order adsorption model.

Pseudo-second-order kinetic model

The pseudo-second-order model is given by the equation [38]:


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t

qt¼ 1

k2q2e

þ 1

qet; ð11Þ

where k2 (g mg-1 min-1) is the rate constant of the pseudo-second order kinetic

model. In the pseudo-second-order model chemical sorption is the rate-limiting step

[39]. In reactions involving chemisorption of adsorbate on to a solid surface without

desorption of products, adsorption rate decreases with time because of increased

surface coverage.

Intra particle diffusion model

The Intra particle diffusion can be described by three consecutive steps:

1 Transport of adsorbate from the bulk solution to the outer surface of the

adsorbent by molecular diffusion.

2 Internal diffusion—transport of adsorbate from the surface of the particles into

interior sites.

3 Adsorption of the solute particles by the active sites of the interior surface of the

pores.

The effect of intra particle diffusion resistance on adsorption can be determined

by use of the relationship [40]:

qt ¼ kipt1=2 þ C; ð12Þ

wherekip is the intra particle diffusion rate constant (mg g-1 min-1/2). If the intra

particle diffusion model is obeyed, a plot of qt against t1/2 is linear with slope kip.

The simplified Elovich equation

The simplified Elovich equationis:

qt ¼1

b lnðabÞ þ1

b ln t; ð13Þ

where a (mg g-1 min-1) is the initial adsorption rate constant and b (g mg-1) is

related to the extent of surface coverage and the activation energy for chemisorption

[41, 42]. The values of a and b can be calculated from the plot of qt against 1/ln t.

Results and discussion

Characterization of adsorbents

Physicochemical characteristics of RLRC and ZLRC

Characteristics of the adsorbents RLRC and ZLRC , for example yield, iodine value,

moisture content, pH, pHZPC, and surface acidity and basicity, were determined. The

results summarized in Table 1 indicate that yield and iodine number can be


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correlated with ability to adsorb low-molecular-weight substances and provide a

measure of surface area or capacity available to small molecules. The higher the

yield and iodine value, the higher will be the adsorption capacity of the adsorbents.

It has been reported that the higher the moisture content of activated carbon, the

lower the adsorption capacity, so more carbon is needed [43]. The results indicated

that pHZPC of RLRC and ZLRC depended on the raw material and the activated

carbon. The zero point charge (pHZPC; 5.06 for RLRC and 6.04 for ZLRC) is below

the solution pH (pH 5.4–6.3) and hence the negative charge density on the surface of

RLRC and ZLRC increased, which favors adsorption of metals. This proves the

adsorption capacity of adsorbents and the higher adsorption capacity of RLRC

compared with ZLRC.

Scanning electron microscopic studies (SEM)

The surface structure of the adsorbents RLRC and ZLRC before and after

adsorption was analyzed by scanning electron microscopy (SEM). SEM images for

the samples of RLRC and ZLRC before and after metal ion adsorptions are shown in

Fig. 1a–d. As shown in Fig. 1a, the RLRC material has more fibre and more active

sites. It can be seen from the micrographs (Fig. 1a, c) that the external surface of

ZLRC is full of cavities compared with RLRC, and quite irregular as a result of

activation, and the pores were different sizes and different shapes. According to the

micrograph, it seems that the cavities resulted from evaporation of ZnCl2 during

carbonization, leaving the space previously occupied by the ZnCl2. It is clear that

the adsorbent has many heterogeneous pores where there is a good possibility for

metal to be trapped and adsorbed. Figure 1b and d shows the adsorbent is covered

with Cu(II) ion, which proves adsorption of Cu(II) ions by the ZLRC surface.

EDAX elemental analysis

Results from EDAX elemental analysis of the RLRC and ZLRC before and after

adsorption are presented in Fig. 2. The main elements are carbon and oxygen;

carbon content is higher than oxygen content for both materials. The presence of Zn

in the ZLRC is because the Leaves of Ricinus communis were impregnated with

Table 1 Physicochemical characteristics of RLRC and ZLRC

Characteristic RLRC ZLRC

Yield (%) 40.66 58.18

Iodine value (mg/g) 1307.59 1797.939

Moisture content (%) 5.498 3.65

Zero point charge (pHZPC) 5.06 6.04

pH 6.28 6.45

Surface acidity (mmol/g) 4.75 4.87

Surface basicity (mmol/g) 3.68 2.56


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ZnCl2. The elements and their percentage masses in RLRC and ZLRC before and

after adsorption are summarized in Table 2a, b.

X-ray diffraction analysis

Adsorption may lead to changes in the molecular and crystalline structure of the

adsorbent and, hence, the molecular and crystalline structures of the adsorbent, and

changes thereof, provide valuable information about the adsorption reaction. Hence,

XRD patterns of the adsorbent before and after adsorption of Cu(II) ions were

studied.

As a representative example, the XRD patterns of RLRC and ZLRC before and after

treatment with Cu(II) ions are shown in Fig. 3a, b, respectively. The results indicated

that the diffraction profiles of RLRC and ZLRC before and after adsorption of Cu(II)

ions contained broad peaks; the absence of sharp peaks was indicative of a

predominantly amorphous structure, the broad peaks seem to occur at approximately

2h = 21�, 26�, and 29�, similar to the peaks of crystalline carbonaceous structures

such as graphite. It is evident from the figure that the XRD patterns of RLRC and

ZLRC loaded with Cu(II) ions are no different from those of the unloaded materials,

which suggests that the Cu(II) ions diffuse into micropores and are adsorbed mostly by

physisorption without altering the structure of the adsorbent. From XRD analysis of

RLRC and ZLRC we concluded that activation was complete in the preparation of

Fig. 1 a and b SEM images of RLRC. c and d SEM images of ZLRC


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RLRC and ZLRC as activated carbon. This observation was in good agreement with

results from batch sorption experiments.

Functional group analysis

Surface functional groups were detected by Fourier-transform infrared spectros-

copy. Figure 4 shows the FTIR spectra of RLRC and ZLRC before and after

adsorption of Cu(II) ions from aqueous solution. The functional groups of both the

adsorbents and the corresponding infrared absorption frequencies are shown in

Table 3 and Fig. 4. The spectra contain a broad band between 3499.5 and

3447 cm-1. This indicates the presence of hydrogen-bonded OH groups. The

intense band in the region 2916.16–2933.13 cm-1 for the precursor was attributed

to asymmetric and symmetric vibration modes of methyl and methylene groups

(C–H groups) [44]. The peak at 1715 cm-1 is assigned to carbonyl groups, C=O,

group present in aldehyde, ester, ketone, and acetyl derivatives. The peak at

Fig. 2 a and b EDAX analysis of RLRC before and after adsorption. c and d EDAX analysis of ZLRCbefore and after adsorption


123


approximately 1647 cm-1 can be assigned to symmetric and asymmetric stretching

vibrations of the C=C group. The peak at 1030 cm-1 is related to lignin. Therefore

it is possible that cellulose, hemicelluloses, and lignin, with many OH groups in

their structure, make up most of the absorbing layer. The peak present at 824 cm-1

indicates the presence of aromatic heterocyclic molecules.

Table 2 Elements present in (a) RLRC-B and RLRC-Cu, (b) ZLRC-B and ZLRC-Cu

Element App conc. Intensity corrn. Weight % Weight % sigma Atomic %

(a)

RLRC-B

C K 59.82 1.1993 54.16 1.54 62.00

O K 19.64 0.4952 43.06 1.50 37.00

S K 0.41 0.9601 0.46 0.10 0.20

K K 0.43 1.0514 0.45 0.11 0.16

Ca K 1.69 0.9763 1.87 0.16 0.64

Totals 100.00

RLRC-Cu

C K 40.88 1.0961 50.92 1.13 59.06

O K 18.03 0.5381 45.72 1.15 39.81

Si K 0.30 0.8939 0.46 0.11 0.23

S K 0.27 0.9488 0.39 0.11 0.17

Ca K 1.00 0.9789 1.39 0.16 0.48

Cu K 0.61 0.7458 1.11 0.35 0.24

Totals 100.00

(b)

ZLRC-B

C K 44.29 0.9674 51.68 1.08 61.10

O K 19.90 0.5388 41.69 1.10 37.00

Al K 0.28 0.7801 0.40 0.11 0.21

Si K 0.67 0.8619 0.88 0.13 0.44

Ca K 0.49 0.9809 0.57 0.12 0.20

Zn K 3.18 0.7484 4.79 0.58 1.04

Totals 100.00

ZLRC-Cu

C K 46.11 0.9899 46.33 0.97 55.59

O K 28.68 0.6005 47.50 0.99 42.78

Mg K 0.25 0.6517 0.38 0.12 0.23

Ca K 0.83 0.9841 0.83 0.12 0.30

Cu K 1.19 0.7544 1.57 0.36 0.36

Zn K 2.56 0.7504 3.40 0.51 0.75

Totals 100.00

RLRC-B and ZLRC-B means before adsorption of Cu(II) ions

RLRC-Cu and ZLRC-Cu means loaded with Cu(II) ions


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Effect of pH on adsorption, desorption, and possibility of recycling

The zeta-potentials of the RLRC and ZLRC particles in water were measured at

different pH. It was found that the RLRC and ZLRC particles are positively charged

(a) (b)RLRC ZLRC

0 10 20 30 40 50 60 70 80 900

500

1000

1500

2000

2500

3000

3500

4000

RLRC-B

RLRC-CU

Inte

nsity

Angle Two Theata

0 10 20 30 40 50 60 70 80 900

500

1000

1500

2000

2500

3000

3500

4000

Inte

nsity

ZLRC-Cu

ZLRC-B

Angle Two Theata

Fig. 3 XRD analysis of a RLRC and b ZLRC before adsorption and loaded with Cu(II) ions

4500 4000 3500 3000 2500 2000 1500 1000 500

1/cm

RLRC-B

% T

RLRC-Cu

ZLRC-Cu

ZLRC-B

Fig. 4 FTIR spectra of RLRCand ZLRC before and afteradsorption of Cu(II) ions

Table 3 Assignment of peaks in FTIR spectra of RLRC and ZLRC

IR peak Frequency (cm-1) Assignment

1 3447 Bonded –OH groups

2 2916 Aliphatic C–H groups

3 1715 C=O stretching

4 1647 Symmetric and asymmetric stretching vibrations of the C=C group

5 1030 –C–C group

6 824 Aromatic heterocyclic molecules


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at low pH and negatively charged at high pH, having a point of zero charge (pHZPC)

at pH 5.06 for RLRC and 6.04 for ZLRC. It can be expected that positively charged

metal ions are likely to be adsorbed by the negatively charged RLRC and ZLRC

particles at a pH [ pHZPC for RLRC and ZLRC.

The pH of the aqueous solution has been identified as the most important

variable governing metal adsorption by adsorbents. This is partly because

hydrogen ions themselves are a strongly competing adsorbate and because

solution pH affects the ionization of surface functional groups. To establish the

effect of pH on adsorption of copper(II), batch equilibrium studies at different pH

were conducted in the range 2–9 (Fig. 5). As solution pH increased from 2.0 to

9.0, adsorption capacity for copper(II) increased up to pH 6.0 for RLRC and up to

pH 7.0 for ZLRC, and then decreased. The increase in the amount adsorbed as pH

increased from 2.0 to 7.0 may be because of the presence of negative charge on

the surface of the adsorbent, which may be responsible for metal binding.

However, as the pH is reduced, hydrogen ions compete with the metal ions for the

sorption sites; the overall surface charge on the adsorbent becomes positive and

hinders binding of positively charged metal ions [45]. At pH [ 6.0, precipitation

of insoluble metal hydroxides occurs, restricting true adsorption studies [46]. So,

optimized pH of 5.40 for RLRC and 6.30 for ZLRC were used in all adsorption

experiments. The effect of pH may be explained on the basis of the interaction of

Cu(II), Cu(OH)?, and Cu(OH)2 with surface functional groups present in activated

carbon, as follows:

R�OH2þ $ R�OHþ Hþ; ð14Þ

R�OH$ R�O� þ Hþ; ð15Þ

R�O� þ Cu2þ $ R�OCuþ; ð16Þ

R�O� þ CuðOHÞþ $ R�OCuðOHÞ; ð17Þ

where R represents the surface sites of activated carbon; R–OH2?, R–OH, and

R–O– represent protonated, neutral, and ionized surface hydroxyl functional groups;

and R–OCu? and R–OCu(OH) represent the complexes formed. It can be seen that

at low pH H? competes with Cu ions for the active surface sites and, moreover,

fewer functional groups, e.g. R–O–, are ionized (deprotonated) in this region, so it is

difficult for them to form Cu complexes. Because Cu2? and Cu(OH)? are the

dominant species involved in adsorption below pH 6.0, other species, for example

Cu(OH)2 and Cu(OH)3-, were not taken into account in the formation of surface

complexes [47].

In wastewater-treatment systems using adsorption processes, regeneration of the

adsorbent and/or disposal of the loaded adsorbent are very important. Desorption

studies were conducted for both RLRC and ZLRC by use of batch methods; the

results are shown in Fig. 5. Maximum desorption of 38.0 % occurred in neutral

medium at pH 2.27 for RLRC and of 54.4 % at pH 3.12 for ZLRC. The results

indicate that Cu(II) ions adsorbed by both RLRC and ZLRC can be recovered by use

of distilled water, and that the amount retrieved from ZLRC was higher than that

from RLRC. After desorption, the adsorbents were further used for adsorption of


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Cu(II) ions. Percentage removal of Cu(II) ions was found to be 30.0 % for RLRC at

pH 5.3 and 43.75 % for ZLRC at pH 6.3 (Fig. 5).

Effect of contact time on adsorption

Agitation time was evaluated as one of the most important factors affecting

adsorption efficiency. The optimum time for copper removal was 35 min for RLRC

and 40 min for ZLRC (Fig. 6). These experimental studies showed that highly

efficient adsorption of copper can be achieved rapidly.

Effect of amount of adsorbent

In copper removal it was found that efficiency of adsorption increased as amount of

adsorbent (RLRC and ZLRC) was increased, until the amount reached 1000 mg/

50 mL. This increase in efficiency can be explained by the increasing surface area

Fig. 5 Effect of Initial pH on adsorption of Cu(II) ions by RLRC and ZLRC

Fig. 6 Effect of contact time onadsorption of Cu(II) ions byRLRC and ZLRC


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on which adsorption can occur. As seen in Fig. 7, optimum amounts of sorbent for

copper removal were 200 mg/50 mL.

Effect of initial metal ion concentration

The effect on the adsorption capacity of RLRC and ZLRC of initial metal ion

concentrations in the range 25–200 mg L-1 was investigated for 0.2 g carbon

(50 mL)-1, pH 5.4 for RLRC and 6.3 for ZLRC, contact time 2 h, and temperature

303 K; the results are shown in Fig. 8. Metal uptake per unit weight of adsorbent

increases with increasing initial metal ion concentration, with maximum Cu

adsorption capacity of 7.14–127.27 mg/L. This is because at higher initial

concentrations the ratio of initial number of moles of metal ions to the available

adsorption surface area is high. This may be attributed to an increase in the driving

force of the concentration gradient with increasing the initial metal concentration

[48].

Adsorption isotherm

In this investigation, the equilibrium data were analyzed by use of the Langmuir,

Freundlich, Temkin, and Dubinin–Radushkevich isotherm models. To optimize the

design of an adsorption system, it is important to establish the most appropriate

Fig. 7 Effect of amount ofadsorbent on adsorption ofCu(II) ions by RLRC and ZLRC

Fig. 8 Effect of Cu(II) ion concentration on adsorption by RLRC and ZLRC


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isotherm model. The mono-component equilibrium characteristics of adsorption of

Cu(II) ions by RLRC and ZLRC were described by these four different isotherm

models. Experimental equilibrium adsorption data were obtained by varying the

concentration of Cu(II) ions and use of 0.2 g/50 mL RLRC and ZLRC.

The adsorption data obtained by fitting the different isotherm models with the

experimental data are listed in Table 4, with the linear regression coefficients, R2.

RLRC and ZLRC have a homogeneous surface for adsorption of metal ions. The

Langmuir isotherm equation is therefore expected to best represent the equilibrium

adsorption data. The R2 values for the Langmuir model are closer to unity than those

for the other isotherm models for both RLRC (R2 = 0.9930) and ZLRC

(R2 = 0.9950). Therefore, equilibrium adsorption of Cu(II) ions by RLRC and

ZLRC can be represented appropriately by the Langmuir model in the concentration

range studied.

Adsorption kinetics

Adsorption of Cu(II) can be well fitted by use of the pseudo-second order rate

constant for both RLRC and ZLRC. The kinetic data are given in Table 5. The qe

value (253) obtained from the second-order kinetic equation for RLRC was close to

the experimental qe value (195.83), and the linear regression coefficient R2 (0.936)

obtained for pseudo-second-order kinetics was closer to unity than the R2 value

(0.841) for first-order kinetics. This indicates that adsorption of Cu(II) ions by

RLRC follows pseudo-second-order kinetics. The qe value (333) obtained from the

second-order kinetic equation for ZLRC was close to the experimental qe value

(307.27) and the linear regression coefficient (R2 = 0.966) obtained for pseudo-

second-order kinetics was closer to unity than the R2 value (0.836) for first-order

Table 4 Adsorption isotherm data for adsorption of Cu(II) ions by RLRC and ZLRC

Isotherm model Variable RLRC ZLRC

Langmuir Qm (mg g-1) 250 333.3

b (L mg-1) 0.0279 0.0319

R2 0.9930 0.9950

Freundlich 1/n 0.480 0.479

Kf (mg g-1) 19.142 24.71

R2 0.9680 0.971

Temkin a (L g-1) 0.1208 1.5262

b (mg L-1) 48.33 55.77

b 52.12 45.17

R2 0.9860 0.979

Dubinin–Radushkevich Qm (mg g-1) 144.33 167.01

K (910-5 mol2 kJ-2) 1.0000 0.6000

E (kJ mol-1) 223.71 288.68

R2 0.8210 0.770


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kinetics. This indicates that adsorption of Cu(II) ions by ZLRC follows pseudo-

second-order kinetics.

In the intra particle diffusion model, values of qt were found to be linearly

correlated with values of t1/2. The kdif values were calculated by use of correlation

analysis. kdif = 27.88, R2 = 0.971 for RLRC and kdif = 30.91, R2 = 0.984 for

ZLRC. The R2 values were closer to unity for RLRC than for ZLRC, indicating the

application of this model is a better fit for ZLRC than for RLRC. When the Elovich

equation was used, the linear coefficient for RLRC was found to be 0.925 whereas it

was 0.930 for ZLRC. The Elovich constant AE was 0.0174 mg/g min for RLRC and

0.0277 mg/g min for ZLRC. These values were also better for ZLRC than for

RLRC, which proves the suitability of the Elovich equation for ZLRC. This reveals

the presence of an intra-particle diffusion process in RLRC and ZLRC.

Effect of other ions from binary and ternary metal solutions

Effects of the presence of Ni(II) and Cr(III) ions on adsorption of Cu(II) ions were

investigated by varying the concentrations of Ni(II) and Cr(III) ions from 10 to

40 mg/L. Percentage adsorption of Cu(II) ions at equilibrium for solutions with

Cu(II) ions only present and in the presence of increasing concentrations of Ni(II)

and Cr(III) ions is compared in Fig. 9.

The concentration of the Cu(II) ion solution was kept as 100 ppm. The

concentration of Ni(II) and Cu(II) ions was varied as 10, 20, 30, and 40 ppm. Each

solution was placed in a bottle with RLRC and ZLRC and the pH was adjusted to 5.4

for RLRC and 6.3 for ZLRC. After shaking for 50 min percentage adsorption was

calculated. Percentage adsorption of Cu(II) ions decreased as the concentration of the

Ni(II) and Cr(III) solutions was increased. This indicated competitive adsorption was,

to some extent, occurring between the Cu(II) ions and the Ni(II) and Cr(III) ions.

Table 5 Comparison of the correlation coefficients of kinetic data for adsorption of Cu(II) ions by

RLRC and ZLRC

Model Variables RLRC ZLRC

Pseudo first-order model k1 (min-1) 0.0690 0.1059

qe (mg/g) 194.98 396.27

R2 0.8410 0.836

Pseudo second-order model k2 (g/mg/min) 1.3986 7.272

qe (mg/g) 253.0 333

h 0.2389 1.8514

R2 0.9360 0.9660

Intra particle diffusion model kdif (mg/(g min1/2)) 27.88 30.91

C 20.46 32.79

R2 0.971 0.984

Elovich model AE (mg(g/min)) 0.0174 0.0277

b (g/mg) 146.43 169.76

R2 0.9250 0.930


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RLRC and ZLRC contain a finite number of surface binding sites, some of which

would be expected to be saturated by the competing metal solutions, preventing

adsorption of Cu(II) ions. The decrease in sorption capacity of the same activated

carbon in metal solution containing more than one metal ion compared with solution

containing a single metal may be ascribed to the availability of fewer binding sites.

In binary and ternary solutions, binding sites are competitively divided among the

different metal ions.

Conclusion

Leaves of Ricinus communis were a good raw material for preparation of activated

carbon of high surface area. This investigation showed that ZLRC was a more

effective adsorbent than RLRC for removal of Cu(II) ions from aqueous solutions.

The surface morphology and functional groups of the adsorbents were determined

by measurement of pHZPC and by SEM-EDAX, XRD, and FTIR analysis.

Adsorption capacity of the adsorbents was highly dependent on the initial

concentration of metal ion, amount of carbon, contact time, and solution pH.

According to the pHZPC obtained for RLRC and ZLRC, the pH of the adsorbate

solution must be 5.4 for RLRC and 6.3 for ZLRC to ensure that the surface of both

the adsorbents will be more favorable for adsorption of positively charged metal

ions. The efficiency of Cu(II) adsorption increased with increasing amount of

adsorbent but decreased with increasing initial concentration of adsorbate solution.

The adsorption data were well fitted by the Langmuir isotherm model; this is

indicative of monolayer adsorption by RLRC and ZLRC. Among the kinetic models

tested, the adsorption kinetics for both adsorbents were best described by the

pseudo-second order equation. The adsorption process was found to be controlled

by intra particle diffusion. Adsorbed Cu(II) ions can be desorbed from both

adsorbents by use of double distilled water—38 % of adsorbed Cu(II) ions were

recovered from RLRC and 54.4 % from ZLRC, at pH 2 and pH 3, respectively.

Percentage adsorption of Cu(II) ions on RLRC and ZLRC was higher in single-ion

(a) (b)RLRC ZLRC

Fig. 9 Effect of other ions on adsorption of Cu(II) ions from binary and ternary systems by a RLRC andb ZLRC


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systems (Cu(II) ions only) than in binary and ternary systems (containing Cu(II),

Ni (II), and Cr(III) ions), which is indicative of competitive adsorption among the

metal ions. These experimental studies showed that leaves of Ricinus communiscould be used as an alternative, inexpensive, and effective material for removal of

large amounts of Cu(II) ions from aqueous solutions.

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