Sorption studies of Cu(II) on gooseberry fruit ( emblica of fi cinalis ) and its removal
from electroplating wastewater
Rifaqat Ali Khan Rao ? , Shaista Ikram
Environmental Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002 (U.P.) India
Article info
Article history:
Received 4 December 2010
Received in revised form 10 April 2011
Accepted 27 April 2011
Available online 28 May 2011
Keywords:
Sorption
Cu(II)
Equilibrium isotherms
Kinetics
Thermodynamics
Abstract
The sorption of Cu(II) onto waste residue left after extraction of juice from Indian gooseberry (Amla) fruit
(Emblica officinalis ) was studied. Equilibrium isotherms, kinetic data and thermodynamic parameters have been
evaluated. Equilibrium data agreed well with Langmuir, Freundlich, Temkin and Dubinin –Radushkevich (D–R)
isotherm models. The kinetic data followed pseudo-second-order model and it was found that intra-particle
diffusion was not the sole rate-controlling factor. Gibbs free energy showed spontaneous process for all
interactions. The breakthrough and exhaustive capacities were found to be 4.0 and 24.0 mg/g respectively.
Emblica officinalis was shown to be a promising sorbent for Cu(II) removal from aqueous solutions. The practical
utility of the sorbent has been demonstrated by achieving 97.60% removal of Cu(II) from electroplating
wastewater by column process.
? 2011 Elsevier B.V. All rights reserved.
1.Introduction
One of the cardinal demerits of rapid industrialization is the
manifold increase in industries ranging from metalfinishing, electro-
plating, plastics and polymer manufacturing, pigments, textile, pulp
and paper, mining etc. giving rise to air, soil and water pollution [1 –3] .
The uncontrolled discharge of ef fluent from these industrial units
pollutes different aquatic bodies posing serious health and environ-
mental problems [4]. Among the wide variety of toxic compounds
found in wastewater, heavy metals are of utmost signi ficance because
of their high toxicity at ultra trace concentrations and persistent
nature [5]. Due to their non-biodegradability they tend to accumulate
in the vital organs of living organisms causing various diseases and
long term disorders, as well as deleterious ecological effects [6]. Hence
there has been a continuous quest to develop novel process and
techniques over the years to remove these deleterious heavy metal
io n s p resen t in in du s tr ia l wa s te wa t er s . A p ra c ti c a l p ro b le m in
removing out these heavy metal ions from wastewater is their
appreciable solubility in aqueous systems and hence their removal by
ordinary physical and chemical means is very difficult and ineffective
[7].
There is a variety of conventional treatment techniques employed
o remove heavy metals from wastewaters before their final discharge
n to the envi ronm ent. Mos t notab le among the se are c hemic al
recipitation, evaporation, electro deposition, ion exchange, adsorp-
on, membrane separation, coagulation etc. [8]. However, these techniques suffer from either one drawback or the other. Some of
these techniques are either too costly or they are ineffective in terms
of removal of heavy metal ions present at low concentration. Among
all these enlisted conventional techniques, removal of heavy metal
ions by adsorption is found to be highly effective, inexpensive and
ease of operation [9]. The use of sorbent for removing and recovering
heavy metals from contaminated industrial effluents has emerged as a
potential alternative method to the conventional ones [10] . In recent
years, the process involving the removal of heavy metal ions by
passive binding with the dead or living biomass, emerges as one of the
potentially cost-effective techniques[11] . Sorption contains many
passive processes of accumulation such as sorption, ion exchange,
covalent bonding, complexation, chelation, and micro-precipitation
processes. Several studies have shown that the method is effective to
remove heavy metals from industrial wastewaters and could be
employed most effectively in concentration range below 100 mg/L
[12,13].
Cu(II) is one of the most common heavy metals which is essential
for hum an bei ngs, a ni mal s an d m ic ro -or g ani s ms but e xcessi ve of
C u ( I I ) produces many toxic and harmful effects on the living organism
[14] . Acute Cu(II) poisoning may lead to hemolysis, liver and kidney
damage, and influenza syndrome while its local effects give rise to
irritation of upper respiratory tract, gastrointestinal disturbance with
vomiting, and diarrhea and a form of contact dermatitis [15] .The
seriousness of the Cu(II) toxicity and its bioaccumulation property
have led the United States Environmental Protection Agency (USEPA)
to recommend a t hreshold conce ntrat ion l i mi t, de fi ned a s t he
m a x i m um c on ta m i n a n t l ev e l ( MC L ) o f 1. 3 m g /L[16] . C u( I I ) m a in ly
or ig in a te s f r om d ye in g , p a p e r, p e tr ol eu m , c op p er / b ra s s - pl a t in g a n d copp er-ammonium r ayon indus tr ial wastewaters. S amples of wastewater co llected from co pper c lean ing, co pper plating an d
metal p ro cessing industries, have s hown Cu(II ) c oncentrations up t o
100 –120 mg/L which are a bout 1 00 to 200 times gr e a t er than its MC L
[15] . H ence there is a dire need to develop econo mic a ll y feasible and
effective sorbent wh ich c an remo ve Cu(II) thus s aving the life o f
people engaged i n these abov e mentioned industries. Removal o f
heavy metal io ns from the a qu eou s streams b y a gricultural wa ste
ma terials i s a n i nn ovative a nd promising technology . The effi ciency
of the wa ste material depends upon t he capacity, affi nity, and
specifi c it y , i nc l udi n g i t s p h y si co - c hem i ca l n atu re . In r e cen t ye a r s,
bio log ica l mat e ria ls i ncludin g ag ri cult ural and i ndust ri a l so li d
wastes are b ein g used as sor bents for the removal o f heavy metals
[17] . This paper describes the sor ption abil ity o f I ndian go o seberry
( A ml a) frui t ( abb re v i a ted a s EO ) f or t he removal o f C u (I I) f rom
aqueous s olu tio n.
2.Materials and methods
2.1. Sorbent
EO belongs to the familyPhyllanthaceae . The tree is medium sized
reaching 8 –18 m in height and found natively in India. Amla fruit is
used as valuable ingredient in various Ayurvedic medicines in India
and abroad. It contains on an average, moisture 81.8%, protein 0.5%, fat
0.1%, minerals 0.5%, fibers 3.4% and carbohydrates 13.7% per 100 g of
edible part. Its minerals and vitamin contents include calcium, iron,
phosphorous, carotene, thiamine, riboflavin, niacin and vitamin C. The
fruit juice of EO can be extracted and used in many pharmaceutical
industries for the formulation of various drugs. The waste material left
after extraction was used to explore its sorption properties.
2.2. Treatment of sorbent
The waste material left after extraction of juice from EO was
collected from local medicine manufacturing unit at Aligarh. The
biomass was rinsed with water and dried in an oven at 60 °C. The
dried mass was sieved to obtain particle size of 100–300 μm. The
biomass was washed several times again with double distilled water
(DDW), and subsequently dried again in an oven at 60 °C and then
stored in an airtight container in order to avoid moisture and used as
such for the sorption studies.
2.3. Sorbate solution
Stock solution of Cu(II) was prepared (1000 mg/L) by dissolving
the desired quantity of copper nitrate trihydrate (AR grade) in DDW.
The stock solution was diluted to obtain desired Cu(II) concentration.
Solutions of other metal ions were prepared by dissolving their
chlorides or nitrates.
2.4. Determination of active sites
Active sites present on the surface of the sorbent were determined
by acid–base titration method [18] at room temperature (30 °C).
2.5. Sorption studies
Sorption studies were carried out by batch process. 0.5 g sorbent
was placed in a conical flask in which 50 mL solution of metal ion of
desired concentration was added and the mixture was shaken in
shaker. The mixture was then filtered and fi nal concentration of metal
ion was determined in the filtrate by atomic adsorption spectropho-
tometer (AAS) (GBC 902). The amount of metal ions sorbed was
calculated by subtracting final concentration from initial concentra-
tion. All the experiments were carried out in triplicate and mean
concentration was calculated by averaging them. The percent sorptionof sorbate and equilibrium sorption capacity (q
e
), were calculated
using the following relationships.
% Sorption = C
o
– C
e
eT= C
o
×100 e1T
Sorption capacity q
e
eT mg= g eT=Co
–C
e
eT×V= W e2T
whe re , Co
is in it ia l c on c ent ra t ion of so rb at e ( mg/ L ) , C
e
is th e
equilibrium sorbate concentration (mg/L), V is the volume of the
solution (L) and W is the mass of the sorbent (g).
2.6. Effect of pH
The experiment was performed by taking 100 mL of Cu(II) solution
in a beaker and the desired pH of solution was adjusted by adding
either 0.1 N HCl or NaOH solutions. 50 mL of this solution was taken in
a conical flask and treated with 0.5 g of sorbent and after equilibrium,
the final concentration of Cu(II) was determined.
2.7. Effect of contact time
A series of 250 mL conical fl asks, each having 0.5 g sorbent and
50 mL solution (of known metal concentration) were shaken in
temperature controlled shaker incubator at 120 rpm and at the pre
determined intervals (1, 2, 3, 5, 10, 15, 30, 60, 120, 1440 min). The
solution from the speci fi edflask was taken out and the concentration
of Cu(II) was determined by AAS. The amount of Cu(II) sorbed in each
case was then determined as described earlier.
2.8. Effect of concentration and temperature
The effect of concentration on sorption of Cu(II) was investigated
with initial Cu(II) concentrations of 10, 20, 30, 40, 50, 80, 100, and
150 mg/L. The experiments were performed by adding 50 mL solution of each concentration to eight different 250 mLflasks each containing
0.5 g of sorbent. Theflasks were shaken at 120 rpm and 30 °C for 3 h
and the final concentration of Cu(II) was determined by AAS. The
same experiment was repeated at 40 and 50 °C.
Fig. 1. A. Emblica of ficinalis (native). B. Cu (II) adsorbed Emblica of fi cinalis .
Fig. 2. IR spectra of Emblica of ficinalis : (a) before sorption; (b) after Cu(II) sorption.
2.9. Determination of point of zero charge
The pH at point of zero charge (pHpzc) of the sorbent was
determined by the solid addition method[19] . To a series of 250 mL
Stoppard conical fl asks, 40 mL of 0.01 N KNO3was transferred. Theinitial pH (pHi) values of the solutions were roughly adjusted between 2 and 10 by adding either 0.1 N HCl or NaOH solution. The total volume of the solution in each flask was made 50 mL by adding KNO3solution of the same strength. The pHiof the solution was thenaccurately noted and 0.5 g of sorbent was added to eachflask, and theflask was securely capped immediately. The suspensions were then manually shaken and allowed to equilibrate for 24 h with intermittentmanual shaking. Thefinal pH (pHf) values of the supernatant liquidwere no t ed. The difference betwe en the init ial and fi nal pH( ΔpH = pHi ?pHf) was plotted against pHi. The point of intersectionof the resulting curve with the abscissa, at which ΔpH = 0, gave the pH PZC. The same experiment was repeated with DDW.
2.10. Quality assurance and quality control
To make sure that no metal ions were released to the solution from
the sorbent, an equilibrium test was performed using sorbent EO in
DDW. For assuring quality in the sorption studies DDW blank was
included in the experiment. To ensure accuracy after each set of five
samples, a standard was run to ensure that drift had not occurred.
The samples were analyzed in triplicate by AAS. Average of the
values obtained gives mean concentration of the sample. In order to
compare quantitatively the applicability of different models in fitt in g to
data, the percent deviation (P), was calculated. It is generally accepted
that when P value is less than 5, the fit is considered to be excellent [20] .
2.11. Breakthrough capacity
0.5 g of sorbent was taken in a glass column (0.6 cm internal
diameter) with glass wool support. 500 m L of Cu(II) solution of 50 m g/L
initial concentration (Co) was then passed through the column with a
flow rate of 1 mL/min. The effluent was collected initially in 10 mL and
then 50 mL fractions and the concentration of Cu(II) (Ce) was then
determined in each fraction with the help of AAS. The breakthrough
curve was obtained by plotting Ce/Co
versus volume of the effluent.
2.12. Desorption studies
Desorption studies were carried out by batch as well as column
process. In batch process desorption of Cu(II) was carried out by
treating 0.5 g of sorbent with 50 mL of Cu(II) solution (50 mg/L) in a
conical flask. The solution was fi ltered after 24 h and filtrate was
analyzed for Cu(II). The sorbent was then washed several times with
DDW to remove any excess of Cu(II). It was then treated with 50 mL of
0.1 N HCl solutions and then fi ltered after 24 h. The fi ltrate was
analyzed for Cu(II) desorbed. The same procedure was repeated with
different desorbing solutions like sodium chloride, acetic acid, sodium
sulfate, sodium hydroxide and EDTA.
Desorption studies by column process were carried out as follows.
The exhausted column (from breakthrough capacity) was washed
several times with DDW to remove excess of Cu(II) ions from the
column, then 0.1 N HCl solution was passed through the column with a
flow rate of 1 mL/min. The Cu(II) ions eluted were collected in 10 mL
fractions and the amount of Cu(II) ions desorbed in each fraction was
determined by AAS.
2.1
3. Treatment of electroplating wastewater
Electroplating wastewater was collected from a local electroplating
plant in Aligarh city. The pH of the wastewater was noted. It wasfilt er ed to remove suspended matter and diluted and then analyzed for various
parameters. Heavy metal was determined by AAS. 50 mL of the diluted
wastewater was treated with 0.5 g sorbent by batch as well as column
process. In column process wastewater was passed through the column
with a flow rate of 1 mL/min. The effluent collected was again analyzed
and amount of metal ions removed were determined.
2.14. Regeneration and recycling studies
0.5 g of sorbent was taken in a glass column (0.6 cm diameter)
with glass wool support. 50 mL of Cu(II) solution of 50 mg/L initial
concentration (Co) was then passed through the column w ith aflow
rate of 1 mL/min.The ef fluent was collected and the amount of Cu(II)
was then determined as usual. The sorbent was washed several times
with DDW to remove any excess of Cu(II) ions and then 50 mL of 0.1 N
HCl was passed with the same flow rate. The ef fluent was collected in
10 mL fraction and amount of Cu(II) desorbed in each fraction was
then determined. The sorbent was then washed several times with
DDW to remove any excess of HCl and again treated with 50 mL Cu(II)
solution. The same procedure was repeated several times.
3. Results and discussion
3.1. Characterization of sorbent
3.1.1. Scanning electron microscope (SEM) analysis
The SEM was used to examine the surface of the sorbent before and after sorption of Cu(II). The surface of the sorbent appeared to be
irregular and porous. The pores were prominent on the surface of
sorbent before sorption ( Fig. 1 A). After sorption of Cu(II) the pores
werefilled showing adherence of Cu(II) on the surface ( Fig. 1B).
3.1.2. Fourier transform infrared (FTIR) spectroscopy
The FTIR spectra of native and Cu(II) sorbed EO shown in Fig. 2a
and b indicated the presence of several functional groups. The peak 3438 cm? 1 in native sorbent indicates the presence of OH groups. The
peak at 2922 cm? 1
may be due to the carboxylic stretching bonds
[21] . The peak at 1643 cm? 1
may be attributed to C=C and C=O
where as peaks in the region 1506 and 1452 cm? 1
represent OH
bonds. The broad peak at 1038 cm? 1
may be attributed to the C ―OH
stretching vibrations. It has been observed that the peak of ionizable
functional groups was slightly reduced or distorted in the Cu(II)
sorbed sample of EO ( Fig. 2b) showing that Cu(II) was sorbed at these
functional groups.
3.3. Effect of contact time and initial concentration
Sorption of Cu(II) on EO at various initial concentrations was carried out at different intervals of time. The maximum uptake of Cu(II)
at equilibrium was found to be 1.7, 3.1 and 5.8 mg/g at 20, 40 and
80 mg/L initial Cu(II) concentrations respectively. The extent of
sorption increased rapidly in the initial stages and then became slow
at later stages till the equilibrium was attained. Equilibrium time for
sorption of Cu(II) at concentrations 20, 40 and 80 mg/L was found
to be 15, 30 and 60 min. showing that equilibrium time depends
upon the initial concentration of Cu(II) (Fig. 3). The increase in initial
concentration of Cu(II) ions resulted an increase in the sorption
capacity. This is a usual phenomenon observed on various sorbents
since transfer of metal ions from bulk to the surface of the sorbent
increases with increase in concentration of metal ions. This is the basic
p rop er ty of th e s or ben t t o be u tiliz ed f or t he re mo va l o f me ta l i on s f ro m
solution.
3.4. Effect of pH
Fig. 4 indicates that sorption of Cu(II) on EO was strongly pH
dependent. The%sorption was minimum (48%) at pH 2 and reached to
maximum when pH was increased to 3. The sorption of Cu(II) with
respect to pH can be explained by considering the surface charge on
the sorbent and the speciation of Cu(II). The minimum sorption at pH
2 (excess of H+ions) was due to the fact that the surface of sorbent
became positively charged because H+ ions compete with Cu(II) ions
and did not favor the sorption of positively charged Cu2+
because of
the electrostatic repulsion. The sorption of H+
ions resulted in the
protonation of various functional groups present on the surface of the
sorbent decreasing the negatively charged surface sites. However,
when pH was increased to 3, the acidic sites were deprotonated and
attractions of positively charged Cu(II) ions increased. Cu(II) exists as
Cu2+ at pH 3 hence it can be concluded that maximum amount of
copper was sorbed in the form of Cu2+
ions up to pH 3. Above pH 3
sorption increased very slowly due to the formation of other Cu(II)
species like Cu(OH)+
(pH 4 –5) and Cu(OH)2 (pH N 6)[22] . The pH of
the solution also infl uenced the surface charge of the sorbent. The
pH pzc of EO in DDW was found to be 6 (Fig. 5) showing that surface is
positively charged below pH 6. Therefore, at high pH value (above 6) Cu(II) species mainly present as Cu(OH)+and Cu(OH)2
were sorbed (due to micro precipitation). The shifting of pH pzc towards lower pH va lu e (Fi g. 5) in presence of electrolyte (0.01 N KNO3) indicated that
sorption of Cu(II) ions was specific [23] . The effect of electrolyte on the
sorption of Cu(II) is also shown in Fig. 4. The sorption of Cu(II) decreased
appreciably in presence of electrolyte (0.1 N KNO3)atpHN 3.
3.5. Sorption isotherms
The data obtained from studies were tested for their applicability
to the isotherm models namely Langmuir, Freundlich, Temkin and
Dubinin–Radushkevich (D–R) isotherm. The dependence of 1/qe on
1/Ce with varying concentration of Cu(II) ions at different tempera-
ture was observed to be linear indicating the applicability of the
Langmuir isotherm. The values of Langmuir isothermconstants b and
qm were calculated from the slope and intercept of the linear plots of
1/Ce vs. 1/qe [24]. The values of dimensionless constant separation
factor (RL) were plotted at different initial Cu(II) concentrations and
temperatures (Fig. 6). RL values were found to be in the range 0–1in
all experimental systems, which con?rmed the favorable uptake of
Cu(II). The monolayer sorption capacity (qm) of EO was compared
with various agricultural byproducts studied earlier [15,25–35].EO
represented higher Cu(II) sorption capacity than 11 from 14 sorbents,
re?ecting a promising future for EO utilization in Cu(II) removal from
aqueous solution. The values of Freundlich isotherm constants Kf and
n were calculated from the intercept and slope of the freundlich plots
at different temperatures. When ‘n’approached zero, the surface site
heterogeneity increased. The values of nN1(Table 2) at all temperatures
indicated favorable sorption of Cu(II) [36]. The values of Temkin
constants A and B related to sorption potential and heat of sorption
respectively were calculated from the slope and intercept of the plot of
qe versus lnCe. To distinguish between the physical and chemical
sorption, Dubinin–Radushkevich (D–R) isotherm based on the hetero-
geneous nature of the sorbent surface was applied. The values of qm
(maximum sorption capacity) and constant βwere calculated from the
intercept and slope of the D–R linear plots of lnqm vs. ε 2
(Polanyi potential). The constant βgave an idea about the mean free energy (E)
(kJ/mol) of sorption molecule of the sorbate when it was transferred to
the surface of the solid from in?nity and can be calculated using the
following relationship [37].
E=1= 2βeT1= 2e3T
The fitting procedure was performed using R software version 2.10.1 (2009-12-14). The fitness of the data, correlation coef ficients (R2), error analysis (residual standard error (RSE)) and P-values were calculated. The values of constants obtained from different models werefitted and corresponding q e values were calculated from each model (qecal ). The values of q e found experimentally (q exp) were compared with ecalusing chi-square test (χ2). The chi-square test values were calculated from the following relation
χ& =∑q eexp –q ecal hi2 = qe cal e4T
The sum of square error (SSE) values were calculated using the following relation
SSE = ∑q eexp –q ecal 2 = N 1 = 2 e5T
where, N is the number of observations. The lower the value ofχ2 and
SSE the better is the fit. It was clear fromTable 2that r 2
values for all the
isotherms were higher (except D –R isotherm at 50 °C), indicating the
applicability of these isotherms. However, Langmuir and Temkin
isotherms were better obeyed by the system as indicated by their
le ast χ2 and SSE values. These data also indicated that Freundlich
isotherm was obeyed at 40 °C. D–R isotherm was obeyed at 30 and 40 °C
but at 50 °C the data become insigni ficant because of the higher P-value
(P-value N 0.05).
3.6. Sorption kinetics
In order to further explore the sorption mechanism of Cu(II) and
rate controlling steps, a kinetic investigation was conducted. Pseudo-
first-order, Pseudo-second-order and intra-particle diffusion kinetic
models have been used for testing experimental data. The Pseudo-
first-order rate equation of Lagergren [38] was widely used for the
sorption of liquid/solid system on the basis of solid capacity. The linear
form of Lagergren equation is generally expressed as
log q e–qt
eT=?K1
= 2: 303 eT×teT+ logq e e6T
wh er e , qe ( mg/g) a nd q t (m g/g) ar e th e sorption c apa ci ties at eq uilibriu m
an d a t tim e t ( min) res pectively. K1(min?1) is the rate constant of
Pseudo- first-order sorption process. The values of k1 we re determin ed
by plotting log(qe–qt)versustandlistedin Table 3. The values of qe
calculated from the model (qecal) differed from t he experimen tal values
(qeexp) implying that the sorption process did not follow the Pseudo-
first-order sorption rate equation.Pseudo-second-order sorption rate equation [39] may be ex- pressed as
t =qt=1= K2q2e +1= qeeT×t e7T
K2(g/mg/min) is the pseudo-second-order sorption rate constant.
Plots of t/qtversus t for all experimental concentrations gave straight
lines ( Fig. 7), and values of qeand K2 were calculated from the slope
and intercept respectively. The initial sorption rate, h (mg/g/min) is
expressed as [40] .
h=K2q2ee8T
The values of qe,h,K2and r2 are listed in Table 3. The values of
correlation c oef fi ci en ts (r2)werefound tobe verycloseto 1con firming the applicability of the Pseudo-second-order equation. Inaddition, the qe exp
values were very close to qe cal from the model
indicating the better applicability of this model. It can also be seen
from Table 3 that with an increase in initial metal concentration, the
rate constant of sorption (K2) decreased. A similar observation was also reported by earlier researchers[41] .The kinetic data were analyzed using intra-particle diffusion
model to elucidate the diffusion mechanism[42] .
qt=Kid×t1= 2+I e9T
where Kid(mg/g/min) is the intra-particle diffusion rate constant and I
(mg/g) is another constant that gives an idea about the thickness of a
boundary layer, qt is the amount of Cu(II) sorbed (mg/g) at time t (min).
Pl ots of qt ve rs us t 1/ 2 are shown in Fig. 8for different concentrations. The
deviation in the plots from origin for all concentrations indicated that
pore diffusion was not the only sole rate-controlling factor but some
other processes likefilm diffusion were also involved in the sorption
Process.
3.7. Sorption thermodynamics
Temperatures used in this study were 30, 40 and 50 °C. The
equilibrium constant at different temperatures can be calculated with
the following relation.
Kc=CAc=Cee10T
Kc is the equilibrium constant. C
Ac(mg/L) is the amount of Cu(II)sorbed on solid at equilibrium and Ce (mg/L) is the equilibrium concentration of Cu(II) in the solution, respectively. The values of free
energy change (ΔG°) at different temperatures were calculated from
the following relation
ΔG°=?RTlnKce11T
where R is gas constant and T is absolute temperature. van't Hoff
equation was applied to calculate the enthalpy change (ΔH°) and
entropy change ( ΔS°).
lnKc =ΔS°= R –ΔH °=R×1= T eT e12T
ΔS°and ΔH°can be calculated from the intercept and slope of the
linear plot of lnKc
versus 1/T. These values are reported in Table 4. The
negative value of ΔG°indicated the spontaneous nature of the
sorption.ΔG°value was more negative with increase in temperature.
The positive value of ΔH°implied that the sorption phenomenon was
endothermic. The positive value of ΔS°suggested increased random-
ness at adsorbate –adsorbent interface. The magnitude of Ea indicated
that the sorption is chemical in nature.
3.8. Sorption mechanism
In order to investigate themechanism of Cu(II) sorption by EO, the
release of alkali metals (Na+ and K+) and alkaline earthmetal (Ca2+)
present in the sorbent was determined. The released concentrations
of Na+,K+ and Ca2+ during the sorption process were calculated by
subtracting their amount released by washing with DDW (used as
control) from the amount released during acidi?cation of EO with
0.1 N HCl. The total amount of cations released were 0.07 meq/g
(comprise of Ca2+ and Na+ ions) while no K+ was released in either
case. It has been suggested that the total cationic contents can be
considered as ameasure of the approximate cation-exchange capacity
(CEC) of the sorbent [43].
The amount of Na+,K+ and Ca2+ released during the sorption of
Cu(II) was also determined in the supernatant liquid (Table 5). These
results showed that 0.0634 meq of Cu(II) was sorbed and a sum of
0.042 meq Na+ and Ca2+ were released indicating that 0.042 meq
removal of Cu(II) (66.25%) per g of sorbent was by ion-exchange
mechanism with Na+ and Ca2+ therefore it may be concluded that
sorption of Cu(II) occurred mainly through ion-exchange mechanism
involving replacement of Na+ and Ca2+ ions. A similar type of
mechanism has been suggested for mango peel waste [44]. The involvement of ion-exchange process during the sorption of Cu(II)
was also con?rmed by Cu(II) desorption with 0.1 N HCl. About 80% Cu(II) was released by means of exchange of H+ ions.
3.9. Breakthrough capacity
The breakthrough curve (Fig. 9) showed that 40 mL of Cu(II) solution (corresponding to 2.0 mg Cu(II)) could be passed through the column (containing 0.5 g sorbent) without detecting Cu(II) in the
ef?uent. The breakthrough and exhaustive capacities were deter- mined as 4 and 24 mg/g, respectively.
3.10. Desorption studies
In order to make the sorption process more economical, it is important to desorb metal ions from the spent sorbent. Desorption studies were carried out by batch process using NaCl, CH3COOH, EDTA, Na2SO4 and HCl solutions. Desorption of Cu(II) with NaCl, CH3COOH or Na2SO4 was negligible showing that Cu(II) was strongly sorbed. However, N80% desorption could be achieved with 0.1 N HCl. Desorption by column process was carried out by passing 0.1 N HCl through the column saturated with Cu(II) ions (from breakthrough capacity experiment) containing 1.83 mg sorbed Cu(II). Fig. 10 showed that desorption of Cu(II) was rapid and 1.41 mg of Cu(II) (77%) was desorbed within 30 mL of ef?uent.
3.11. Removal of Cu(II) from electroplating wastewater
The applicability of the sorbent was demonstrated by removing
Cu(II) from electroplating wastewater. The analysis of electroplating wastewater is shown in Table 6. The results reported in Table 7 showed that 65% removal of Cu(II) can be achieved by batch process and 97.60% by column process.
3.12. Regeneration studies
For potential practical application, it is important to examine the possibility of desorbing the metal ions from the sorbent for its reuse [44]. 0.1 N HCl was used in this study as a regenerating agent. The results showed slight increase in the sorption of Cu(II) from 98 to 100% in the fourth cycle (Fig. 11). This increase in the sorptionmay be due to the activation of some more surface active sites present on the sorbent surface when it came in contactwith 0.1 N HCl during elution. The recovery of Cu(II) was decreased from 99 to 56.77% in seventh consecutive cycle (Fig. 11). These results showed promising regen- eration potential of the EO. This property of EO may be utilized by small scale commercial units to remove Cu(II) from their discharging
ef?uents in an economical and ef?cient way.
4. Conclusions
The experimental data showed maximum sorption of Cu(II) at
pH 4. Addition of electrolyte in the solution hindered the sorption of
Cu(II) to some extent. The addition of electrolyte also affected the pHpzc. A shift in the pHpzc towards lower pH value indicated speci?c
sorption of Cu(II) ions. The positive value of ΔH°indicated that
sorption increases with increase in temperature. Higher value of E
showed that sorption is endothermic and chemical in nature. The
sorption kinetics followed the pseudo-second-order kineticmodel. On
the basis of intra-particle diffusion values it is concluded that intra-
particle diffusion is not the only rate controlling factor. The Langmuir
and Temkin models yielded a much better ?t than Freundlich model.
The maximum monolayer sorption capacity was found to be 5.2, 5.7,
and 9.5 mg/g at 30, 40, and 50 °C respectively. The RL values showed
favorable sorption of Cu(II). 97.60% removal of Cu(II) is possible from
electroplating wastewater at pH 4.2 by column process. Regeneration
studies showed that EO can be effectively utilized for the removal of
Cu(II) ion up to seven cycles.
Acknowledgements
Authors thank the Chairman, Department of Applied Chemistry,
Z.H. College of Engineering and Technology, AligarhMuslimUniversity,
Aligarh (India), for providing research facilities.
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