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Indian Journal of Chemistry
Vol. 49A, July 2010, pp. 882-890
Speciation of chromium in aqueous samples by solid phase extraction using
multiwall carbon nanotubes impregnated with D2EHPA
S Vellaichamy & K Palanivelu*
Centre for Environmental Studies, Anna University Chennai, Chennai 600 025, India
Email: [email protected]
Received 12 February 2010; revised and accepted 14 June 2010
A solid phase extraction system has been developed for the speciation of chromium(III) and chromium(VI). This
method is based on the adsorption of chromium(III) on D2EHPA impregnated with multiwall carbon nanotubes and has
been compared with commercially available activated carbon impregnated with D2EHPA. The chromium concentration has
been determined by inductively coupled plasma atomic emission spectroscopy. The effect of parameters such as pH of the
aqueous solution, amount of adsorbent, contact time, initial ion concentration, sample volume, eluent type and D2EHPA
concentration have been investigated. The results indicate that the maximum adsorption of chromium(III) is at pH 4.5 ± 0.1
on the multiwall carbon nanotubes. Desorption studies have been carried out with 0.25 M Br2 in 1.0 M NaOH wherein
quantitative recovery of the chromium(III) has been observed. The adsorption capacity of MWCNTs- D2EHPA is found to
be 0.96 mg g -1 for chromium with detection limit of chromium 0.05 µg mL-1. The highest pre-concentration factor of 60
could be obtained for 300 mL of sample volume. The developed method has been applied for the speciation of chromium in
natural water sample and the pre-concentration method is satisfactory (recoveries > 98 %, rsd < 10 %, n = 5) for chromium
species determination.
Keywords: Solid phase extraction, Speciation studies, Chromium, Carbon nanotubes, Nanotubes, ICP-AES
Heavy metals are one of the major pollutants in the
environment because of the toxic nature of industrial
wastes discharged into the environment1. Among the
heavy metal pollutants, the interest in chromium is high
as it poses the highest risk to human health2. Chromium
is a major water pollutant, usually as a result of
industrial pollution including leather tanning,
metallurgical, wood preservation and industrial
electroplating, etc. Chromium species exist mainly in
two different oxidation states, chromium(III) and
chromium(VI). The properties of these species are
different from each other. Chromium(III) is considered
to be an essential trace element for effective
functioning of insulin, whereas chromium(VI) is
reported to be toxic. Due to the varied nature of
chromium species, their accurate determination is a
very important issue in analytical chemistry3.
Except for some like electroanalytical methods,
direct and simultaneous determination of chromium
species is difficult by instrumental techniques like
flame or graphite furnace atomic absorption
spectrometry (AAS) and inductively coupled plasma
atomic emission spectrometry (ICP-AES). To solve
problem, generally various pre-concentration-
separation techniques including solvent extraction4,
co-precipitation5, cloud point extraction
6, ion-
exchange7 and solid phase extraction (SPE) have
been used for the separation of chromium
species. The procedures are based on the pre-
concentration/separation of chromium(III) and
chromium(VI)8.
SPE is an important technique to determine
speciation of heavy metals. The consumption of
reagents is lower and more importantly, it is
environment friendly9 and several analytes can be
enriched and separated simultaneously. The main
properties of the solid phases for solid phase extraction
should be high surface area, their high purity and good
sorption properties including porosity, durability and
uniform pore distribution. A literature survey covering
the most recent information on the chromium metal ion
separations revealed that organophosphorous acids are
often the chosen extractants10
. Di-(2-ethyl hexyl)-
phosphoric acid (D2EHPA) and cyanex 272 allow an
efficient recovery of chromium(III) ions by solvent
extraction11
. D2EPHA has been used for the separation
of chromium(III) from spent tanning liquors. Within 2
min, the yield of chromium(III) extraction from
aqueous phase at pH 5 exceeded 95 % or 86 % with
D2EHPA or a mixture of D2EHPA and its ammonium
VELLAICHAMY & PALANIVELU: SPECIATION OF Cr(III) AND Cr(IV) BY SPE WITH MWCNT + D2EHPA
883
salt, respectively12
. Islam and Biswas13
have reported
that chromium(III) could not be stripped with sulphuric
acid from the loaded organic phase containing
D2EPHA. They further observed that the extraction of
chromium(III) with D2EHPA depends upon the
concentration of the extractant. The optimum pH for
stripping of chromium(III) with D2EHPA14
was
4.5-5.0. In contrast, Pandey et al.15
found that mineral
acid (8 M HCl) was required to obtain 80 % recovery
in 30 min at room temperature.
Several new solid phase extraction materials have
been suggested for chromium speciation16
. Since
carbon nanotubes (CNTs) have additional advantages
over charcoal despite the higher adsorption capacity
of the latter, the interactions are highly irreversible
and unspecific. In this context, CNTs have been
proposed as a novel solid phase extractor for various
inorganic and organic materials at trace levels. CNTs
are one of the most commonly used building blocks of
nanotechnology. With 100 times the tensile strength
of steel, thermal conductivity better than all others but
the purest diamond, electrical conductivity similar to
copper and large surface area of carbon, CNTs show
good characteristics of the adsorption processes in
solid sorbent pre-concentration procedures17
. Herein,
a simple methodology is proposed for the solid phase
extraction of chromium speciation. The present
method is based on the adsorption of chromium(III)
on D2EHPA impregnated within multiwall carbon
nanotubes (MWCNTs) and has been compared with
commercially available activated carbon (AC)
impregnated with D2EHPA. As only chromium(III)
can be preconcentrated on impregnated MWCNTs
and AC with D2EHPA, the total chromium can be
estimated by reducing the solution containing
chromium(VI) to chromium(III). The effects of
variable such as, sample volume, pH, amount of
adsorbents and concentration of diverse ions as well
as chromium(III) have been studied by ICP-AES for
chromium determination. The validity of the proposed
method has been applied to the determination of
chromium in spiked water samples and electroplating
wastewater. Characterization of adsorbents before and
after pre-concentration has also been done using
FT-IR and SEM.
Materials and Methods
The standard solutions of chromium(III) and
chromium(VI) were prepared from respectively
Cr(NO)3.9H2O and K2Cr2O7 (Merck, Mumbai, India)
respectively. Stock solution (1000 mg L-1) was used to
prepare working solutions by appropriate dilution.
D2EPHA extractant was supplied by (Merck,
Darmstadt, Germany) and was dissolved in hexane
(CDH, New Delhi, India) and used as diluent in this
study. H2O2 (AR grade, 30 % (v/v), sodium hydroxide,
hydroxylamine hydrochloride and Br2 were supplied by
Merck, Mumbai, India. MWCNT was obtained from
Nanokarbon, South Korea and other chemicals used
were of analytical reagent grade.
The total chromium was determined by inductively
coupled plasma atomic emission spectrometry (ICP-
AES) using a Thermo Electron Corp - IRIS intrepid II,
XSP (UK) instruments. The instrumental and operating
conditions for ICP-AES measurements were as
follows: RF power: 1.14 kW; plasma gas: 16 L Ar min-1;
auxillary gas: 1.5 L Ar min-1
; nebulizer gas:
0.75 Ar min-1
; measurement mode: time scan-axial
mode and analytical line of Cr: 267.716 nm. PTFE
tubing (0.5 mm id) was used for assembling the flow
lines in a flow injection pretreatment system. The pH
was measured with a combined electrode pH meter
(WTW-197, Germany). The IR spectrum of
chromium–D2EHPA complex was recorded using a
Perkin-Elmer spectrometer (RX1, USA). Scanning
electron microscope (SEM) images were obtained from
JEOL/EQ JSM instrument (model 6360).
Before impregnation, the multiwall carbon
nanotubes and activated carbon powder were washed
with 1 M NaOH to remove acidic impurities, by
agitating in a mechanical shaker for 1 h and filtered
through a Whatman 41 filter paper. After that, the
carbon material was further washed with 1 M HNO3 to
remove base impurities, with subsequent agitation and
filtration. Finally, the material was washed with double
distilled water until excess acid was removed (neutral
pH of solution), dried at 105 °C and stored for
impregnation purposes. The MWCNTs (10 g) and AC
(20 g) were poured into a solution (500 mL) containing
D2EHPA with hexane as a diluent with constant
stirring for 30 min at room temperature and was
allowed to settle for 10 min. Then the solvent was
evaporated completely for 24 h and dried in a hot air
oven at 75 °C. After the carbon was dried, it was stored
for future experimental studies.
Procedure
The aqueous solution (100 mL) containing up to
500 µg of chromium(III) was adjusted to the desired
pH value by adding dilute H2SO4 solution. This pH
adjusted solution was transferred into a clean beaker
containing 500 mg of MWCNTs-D2EHPA and
INDIAN J CHEM, SEC A, JULY 2010
884
1000 mg of AC-D2EHPA and shaken on a
mechanical shaker (agitation speed 200 rpm) for 1 h
at room temperature (27±1 ºC). Then the solutions
were filtered through Whatman 41 filter paper. The
adsorbent containing chromium(III) was desorbed
using 10 mL of 0.25 M Br2 in 1.0 M NaOH. The
desorbed chromium(VI) was determined by ICP-AES.
In order to determine the total chromium, the
solution pH was adjusted to 4.5 and then by adding
2 mL of 0.04 M hydroxylamine hydrochloride, the
chromium(VI) ions were reduced to chromium(III)
(Eq. 1)18
. The solution containing chromium ions were
then determined as per the procedure given above for
chromium(III). The difference in total chromium and
chromium(III) will give chromium(VI).
2Cr2O72-+ 3NH2OH + 13H
+ 4 Cr
3+ + 11H2O +3NO2
-
... (1)
Results and Discussion Pre-concentration studies
Preliminary experiments were conducted at
27±1 ºC to study the effects of pH of the aqueous
solution, contact time, agitation speed, carrier
concentration, MWCNTs and AC amounts on the
adsorption of chromium(III) and chromium(VI). The
test was conducted in conical flasks and the initial as
well as final concentrations of aqueous solution
before and after adsorption were determined. The
experimental results reveal that chromium(III) and
chromium(VI) adsorption on MWCNTs and AC
before modification with D2EHPA was around 75 %,
while after modification with D2EHPA,
chromium(III) adsorption was found to be 95 % and
chromium(VI) was found to be only around 5 %. This
may be because D2EHPA complexes with
chromium(III) rather than with chromium(VI). With a
view to develop a speciation model for chromium(III)
and chromium(VI), detailed investigations were
carried out. All the pre-concentration experiments
were conducted in triplicate and analysis of chromium
were done five times.
Effect of pH
Influence of the pH on the pre-concentration of
both chromium(III) and chromium(VI) with multiwall
carbon nanotubes and activated carbon has been
investigated. The pH of the solution was adjusted in a
range of 2-7. The adsorption of chromium ion was
studied with 500 mg of MWCNTs-D2EHPA and
compared with 1000 mg of AC-D2EHPA. As the
surface area occupied by MWCNTs is higher than
that of AC per unit weight, lower amount of
MWCNTs (500 mg) than AC (1000 mg) was taken
for the study. Figure 1 shows the adsorption behavior
of both chromium(III) and chromium(VI) on the
batch, as a function of pH. As can be seen from the
figure, chromium(III) was complexed with D2EHPA
impregnated adsorbents quantitatively (95 %) at pH
4.5±0.1 on MWCNTs, but the chromium(III)
complexed with D2EHPA on AC was found to be less
than 65 %. The pre-concentration of chromium(VI)
was poor with less than 5 % adsorption at the same
pH on MWCNTs and AC. In order to determine the
chromium(VI), it was reduced to chromium(III) with
hydroxylamine hydrochloride followed by adsorption
at same pH. It is evident that in both the cases,
chromium(III) and reduced chromium(VI) species
were quantitatively adsorbed at pH 4.5±0.1 on the
multiwall carbon nanotubes (95 %) and activated
carbon (65 %). Based on this, further works were
carried out at pH 4.5. The results obtained in this
study are supported by the work done by Pandey et al.15
in which 99.8 % chromium(III) was extracted by
D2EHPA at pH 4.5±0.1. It is also reported that above
and below pH 4.5±0.1, the chromium(III) extraction
was not appreciable. However, at pH > 4.5, the
adsorption of chromium(III) decreased to 88 %,
possibly due to decrease in the cationic charge for
chromium due to the formation of various hydroxide
chromium species such as Cr(OH)4- and Cr(OH)3. It
was also confirmed that, chromium(III) was
quantitatively adsorbed in the pH range of 4-8, which
is the pH range for the formation of positively
charged chromium(III) species. Therefore, pH 4.5±0.1
Fig. 1 Effect of pH on the adsorption (%) of chromium(III)-
MWCNTs and AC impregnated with D2EHPA [Cond: Conc. of
Cr(III) and Cr(VI): 0.5 mg L-1. Curve 1, Cr(III)+MWCNTs;
2, Cr(III)+AC; 3, Cr(VI)+MWCNTs+AC].
VELLAICHAMY & PALANIVELU: SPECIATION OF Cr(III) AND Cr(IV) BY SPE WITH MWCNT + D2EHPA
885
was selected for all subsequent works. Persual of
literature on chromium speciation diagram shows that
in the pH range of 3-8, the possible chromium species
are Cr3+
, Cr (OH)2+
, Cr (OH)2+ etc. The reason for the
maximum retention of chromium(III) in acidic pH
range possibly due to the exchange of various cationic
forms of chromium(III) with H+ ions of phosphoric
acid functional groups present in the mass, whereas, in
the pH range of 3-8 chromium(VI) is present mainly in
the anionic forms of (HCrO4-) and (CrO4
2-)
19.
Desorption studies
Different types of eluents were used for the
stripping of chromium(III) from D2EHPA20
. In the
present study, varying concentrations of NaOH (0.25
and 0.5 M), 0.5 M NaOH in H2O2, 0.25 M Br2 in
0.5 M NaOH and 0.25 M Br2 in 1.0 M NaOH were
evaluated for the desorption of chromium(III). The
results reveal that the maximum quantitative recovery
of chromium(III) was obtained with 0.25 M Br2 in
1.0 M NaOH eluent while the minimum was obtained
with 0.25 M NaOH and 0.5 M NaOH in H2O2 eluent.
The maximum recovery of chromium in the case of
0.25 M Br2 in 1.0 M NaOH may be due to the alkaline
condition; sodium hydroxide reacts with bromine
water to produce sodium hypobromite, which is a
powerful oxidizing agent when compared to H2O2. In
the presence of such a powerful oxidizing agent,
chromium(III) is readily converted to chromium(VI),
which makes it desorbed and eluted (Eqs 2 and 3 )21
.
NaOH + Br2 Na
+ OBr
- + HBr ... (2)
Cr 3+
+ 4 NaOBr Na2 CrO4 + 2 NaBr ... (3)
Effect of D2EHPA concentration
The concentration of D2EPHA loaded on the batch
was varied from 0.025-0.50 M. The concentration of
chromium(III) was maintained at 500 µg in a 100 mL
aqueous sample volume. The results revealed that, the
quantitative adsorption of chromium(III) could be
achieved in the range of 0.25-0.30 M. The maximum
quantitative adsorption of chromium(III) 95 % was
obtained at 0.25 M of D2EHPA. For concentrations
above 0.25 M, there was no significant increase in the
pre-concentration of chromium(III). This might be
due to saturation of the interface between the aqueous
and solid phase by the carrier. Hence, the adsorption
was constant above 0.25 M carrier concentration.
Therefore, further studies were performed with
0.25 M of D2EHPA impregnated with MWCNTs and
activated carbon for 100 mL of aqueous solution
containing 500 µg chromium(III) at pH 4.5.
Effect of amounts of MWCNTs and AC
In order to determine the effect of amount of
adsorbent on the pre-concentration of chromium(III),
the amount of adsorbents was varied in the range of
100-500 mg for MWCNTs and 200-1000 mg for
activated carbon. The aqueous sample volume was
100 mL containing 500 µg of chromium(III) and the
solution pH was adjusted to 4.5. The results show that
the maximum chromium(III) ion quantitatively
adsorbed in the batch experiments was with optimum
dosage of 500 mg of MWCNTs (95 %) and 1000 mg
of AC (70 %). The minimum adsorption (75 %) was
found to be with 100 mg of MWCNTs and 200 mg of
AC (50 %) respectively. Based on the above, 500 and
1000 mg of MWCNTs and AC impregnated with
0.25 M D2EHPA was used for the further
experiments.
Effect of contact time
The equilibrium time was studied from 0-150 min
and the effect of contact time determined by plotting
the percentage adsorption of chromium(III) against
contact time. The results reveal that the adsorption
was very fast for the first 45 min, but it gradually
becomes slower until equilibrium was attained22
in
90 min (Figure not presented). It was also observed
that, the adsorption of chromium(III) was saturated23
between 90 and 150 min. For the MWCNTs,
equilibrium adsorption was established at about
90 min when the quantitative adsorption of 95 %
chromium(III) was achieved. The above results
indicate that MWCNTs with highest binding sites
require shorter time to achieve a high chromium(III)
metal adsorption when compared to that of AC.
Effect of initial ion concentration
Pre-concentration of chromium(III) was studied by
varying the concentration from 0.1-0.6 mg. Figures 2
and 3 show the effect of initial ion concentration of
chromium(III) on MWCNTs and AC. From the
figures it is evident that, at concentrations below
0.6 mg L-1
, the adsorption of chromium(III)
concentration increased. This is due to interaction of
chromium(III) ion in the solution with the binding
sites of MWCNTs impregnated with D2EHPA.
Above 0.6 mg L-1
, there was no significant increase in
chromium(III) metal adsorption. Comparison between
INDIAN J CHEM, SEC A, JULY 2010
886
Figs 2 and 3 shows that MWCNTs were 95 % a better
adsorbent than AC (65 %) and also had high
adsorption capacity.
Effect of sample volume
The influence of aqueous sample volume on the
adsorption of chromium(III)-impregnated MWCNTs
and AC was investigated in the range of 25-700 mL.
The results reveal that the chromium(III)–D2EHPA
complex was quantitatively adsorbed with sample
volume in the range of 25-300 mL; above 300 mL the
adsorption of chromium(III) decreased gradually.
With increase in sample volume, the concentration of
analyte decreased. This is probably due to excess
analyte loaded over the capacity of MWCNTs. The
highest pre-concentration factor of chromium(III) was
60 and 40 for MWCNTs and AC respectively.
Effect of diverse ions
In order to determine the effect of diverse ions,
various ions were added individually to an aqueous
solution containing 500 µg of chromium(III). The
metal ion can be adsorbed through organic phase of
D2EHPA-MWCNTs and the carrier extraction of
D2EHPA caused metal complex formation with
various metal ions such as Zn, Cd, Ni, Pb and Fe. The
interference effect of calcium, magnesium, other
alkali and alkaline earth metal ions are presented in
Table 1. At higher concentration of Zn, Cd, Ni, Pb
and Fe metal ions, there was an interference with pre-
concentration of chromium(III) adsorption. The
concentration of interfering ions causing ± 4 % error
in the determination of chromium(III) was set as the
tolerance limit. Except Fe2+
, the adsorption of
chromium(III) was found to be quantitative in the
concentration range of the other metal ions investigated.
Since the ions that are commonly present in water
samples did not affect the adsorption of chromium(III)
species; this method can be applied to water samples.
Fig. 2 Effect of initial ion concentration on the adsorption of
chromium(III) on MWCNTs modified with D2EHPA. [Cond: pH
4.5, conc. of Cr(III) (1, 0.1; 2, 0.2; 3, 0.3; 4, 0.5; 5, 0.5; 6, 0.6 mg L-1);
MWCNTs: 500 mg; conc. of D2EHPA: 0.25 M].
Fig. 3 Effect of initial ion concentration on the adsorption of
chromium(III) on AC modified with D2EHPA. [Cond: pH 4.5;
conc. of Cr(III) (1, 0.1; 2, 0.2; 3, 0.3; 4, 0.5; 5, 0.5; 6, 0.6 Mg L-1);
AC: 1000 mg; conc. of D2EHPA: 0.25 M].
Table 1 Effect of diverse ions on the adsorption of
chromium(III). [Chromium: 500 µg; pH = 4.5; vol: 100 mL]
Cr (III)a (%) Ion Added as Conc.
(mg L-1) MWCNTsb ACb
Na+ NaCl 1000 97.0 ± 2.0 76.0 ± 2.0
K+ KNO3 1000 96.0 ± 3.0 77.0 ± 3.0
Ca2+ CaCl2.3 H2O 1000 97.0 ± 2.0 76.0 ± 2.0
Mg2+ MgCl2 1000 96.0 ± 3.0 84.0 ± 3.0
Cl- NaCl 1000 96.0 ± 2.0 76.0 ± 2.0
NO3- KNO3 1000 96.0 ± 4.0 78.0 ± 2.0
SO42- Na2SO4 1000 98.0 ± 3.0 83.0 ± 2.0
PO43- KH2PO4 1000 96.0 ± 2.0 76.0 ± 3.0
Cd2+ CdSO4 25 98.0 ± 2.0 78.0 ± 3.0
Mn2+ MnSO4 25 97.0 ± 2.0 81.0 ± 2.0
Zn2+ ZnCl2 25 97.0 ± 2.0 86.0 ± 2.0
Ni2+ NiCl2 25 97.0 ± 3.0 79.0 ± 2.0
Fe3+ FeCl3 50 98.0 ± 3.0 79.0 ± 2.0
Fe2+ (NH4)2 (FeSO4)2.6H2O 50 98.0 ± 3.0 79.0 ± 2.0
Cu2+ CuSO4 25 98.0 ± 2.0 78.0 ± 3.0
Pb2+ PbSO4 25 98.0 ± 2.0 88.0 ± 3.0
Co2+ CoCl2 25 98.0 ± 2.0 85.0 ± 2.0
Ag2+ AgSO4 25 96.0 ± 3.0 79.0 ± 2.0
Cr 6+ K2Cr2O7 25 95.0 ± 3.0 90.0 ± 4.0
a Mean ± standard deviation based on the five replicates. bImprgnated with 0.25 M D2EHPA.
VELLAICHAMY & PALANIVELU: SPECIATION OF Cr(III) AND Cr(IV) BY SPE WITH MWCNT + D2EHPA
887
Determination of total chromium
In order to determine the total chromium, spiked
test solutions containing different amounts of
chromium(III) and chromium(VI) were prepared.
Then chromium(III) ions in the spiked test solutions
were oxidized to chromium(VI) by using 0.25 M Br2
in 1.0 M NaOH and chromium(VI) ions reduced to
chromium(III) by hydroxylamine hydrochloride
during the pre-concentration stage. This procedure
was applied to spiked test solutions containing
chromium(III) and chromium(VI). The results show
that the proposed method of pre-concentration
procedure could be applied for the determination of
total chromium and its species (Table 2).
Adsorption capacity of MWCNT and AC
The adsorption capacity of MWCNTs and AC was
studied. MWCNTs (500 mg) and AC (1000 mg) were
added to 100 mL of solution containing 1000 µg of
chromium(III) and chromium(VI) at pH 4.5. After
shaking for 90 min, the mixture was filtered to study
the adsorption capacity of the nanotubes as well as the
activated carbon. The supernatant solution (10 mL)
was diluted to 100 mL and analysed by inductively
coupled plasma atomic emission spectroscopy. The
adsorption capacity of MWCNTs and AC for
chromium species was found to be 0.96 and
0.84 mg g-1
respectively, which was stable for
10 cycles. Beyond 10 cycles, there was a reduction in
the adsorption of chromium(III) in the nanotubes and
activated carbon. MWCNTs and AC washed with
hexane and dried at 75 °C, followed by
reimpregnation with D2EHPA was still found to be
effective for adsorption of chromium(III).
Detection limit
The detection limit was calculated under optimal
conditions after the application of pre-concentration
procedure to spiked test solutions. The limit of
detection for chromium(III) and chromium(VI) based
on the three times the standard deviations of the blank
(k = 3, n = 5) was 0.05 µg L-1
. The determination of
chromium(III) and chromium(VI) was done as per the
procedure given in experimental section. The
procedure was repeated five times for chromium(III)
and chromium(VI). It was found that the adsorption of
chromium(III) was 98.0±3.0 at 95 % confidence level. Analysis of real sample
The present proposed method was applied to tap
water, well water and electroplating industrial
wastewater. The pre-concentration and separation of
chromium(III) and chromium(VI) was determined in
the form of total chromium. The concentration of
chromium metal ions in the samples was determined
with MWCNTs impregnated with D2EHPA only. The
studies conducted with AC were found to be less
efficient in adsorption of chromium(III) than
MWCNTs. Therefore, further studies were conducted
by MWCNTs. The results are given in Table 3. The
recovery of chromium was found to be quantitative
(95 %) (rsd < 10 %).
Characterization of adsorbents
Due to the existence of electron donating oxygen of
OH group, as well as sulphur of SH group and
phosphorous of PO group, D2EHPA is expected to
form stable chromium metal ion complex. The FT-IR
spectra for pure D2EHPA and MWCNTs as well as
those loaded with Cr (D2EHPA) show that, for pure
D2EHPA, seen for P-O-C group intense absorption
band is around 1031 cm-1
. This group appears to have
two stretching frequencies, one primarily due to the
stretching of the P-O bond and the other due to the
O-C band stretching. However, it was not possible to
specifically distinguish between the two since P-O-H
Table 2 Determination of total chromium of spiked test solutions.a [Vol. of sample:100 mL; n = 5]
Added (µg L-1) Found (µg L-1) Adsorption (%)b Rel. error (%) rsd (%)
Cr(III) Cr(VI) Cr(III) Cr(VI) Total Cr Cr(III) Cr(VI) Total Cr Cr (III) Cr(VI) Cr(III) Cr(VI)
0 25 - 24.2 ± 0.2 24.2 ± 0.2 - 96.8 ± 2.0 96.8 ± 2.0 - - 3.2 - + 0.8
5 20 4.9 ± 0.2 19.6 ± 0.3 24.5 ± 0.3 98.0 ± 2.0 98.0 ± 1.0 98.0 ± 1.0 - 2 - 2.0 + 4.0 + 1.5
10 15 9.8 ± 0.3 14.8 ± 0.5 24.6 ± 0.4 98.0 ± 2.0 98.6 ± 3.0 98.3 ± 3.0 - 2 - 1.0 + 3.0 + 3.3
15 10 14.7 ± 0.3 9.8 ± 0.3 24.5±0.3 98.0 ± 2.0 98.0 ± 2.0 98.0 ± 2.0 - 2 - 2.0 + 2.0 + 3.1
20 5 19.8 ± 0.5 4.7 ± 0.3 24.5 ± 0.4 99.0 ± 1.0 94.0 ± 1.0 96.5 ± 1.0 - 2 - 6.0 + 2.5 + 6.3
25 0 24.6 ± 0.2 - 24.6 ± 0.2 98.4 ± 3.0 - 98.4 ± 3.0 - 1.6 - + 1.0 -
a pH = 4.5. b Mean ± standared deviation based on the five replicates.a
INDIAN J CHEM, SEC A, JULY 2010
888
group may overlap in the same frequency. The P=O
stretching frequency has been assigned at 1229 cm-1
.
The bands corresponding to alkyl moieties have been
identified at 2959–2874, 1463 and 1380 cm-1
. When
the D2EHPA interacts with any metal the phosphoryl
bond is highly affected. The FT–IR spectra of the free
D2EHPA and chromium show that the P=O band
occurred at 1212 and 1205 cm-1
respectively. The
intensity of the P=O band corresponding to the free
D2EHPA increases with increase in chromium(III)
concentration in the organic phase. The relative
intensity of the band for free D2EHPA (P-O-H) at
1034 cm-1
was slightly shifted to 1032 cm-1
, since there
are two groups (P-O-C and P-O-H) overlapping at
1032 cm-1. The decrease in absorbance seems to be
related to the P-O-H group because chromium is
extracted with liberation of hydrogen in the aqueous
phase, while P-O-C group is unchanged. According to
Mansur24
, the limit experimentally corresponds to 68 %
loading of free D2EHPA in the organic phase, so the
complexation reaction will continue in order to
consume the remaining P-O-H bonds available in this
phase. However, as no change on absorbance is
observed, it seems that the band at 1034 cm-1 after the
limit has been reached may correspond to the P-O-C
group, which remains unchanged with the degree of
loading. This indicates that the free P-O-H bonds were
reduced. After stripping chromium from the loaded
organic phase it was analyzed by FT-IR. The spectrum
show bands at 888 and 1463 cm-1
for P-O-H and P=O
respectively. This reveals that the organic phase was
not affected by the stripping agent and it was also
further confirmed by reuse of organic phase. Similar
results were observed for Zn2+
/D2EHPA system24
.
SEM images of MWCNTs and AC modified with
and without D2EHPA are presented in Fig. 4. The
SEM images clearly show that the chemical
modification of carbon adsorbents surface is extremely
irregular; the adsorbent impregnated with D2EHPA has
the appearance of an agglomerate of globular and
cylindrical elements with diameters and lengths of
~ 5 µm or less. The rugosity and irregularity of the
surface prevent an accurate measurement of the
impregnated adsorbents. However a comparison of
micro-photographs obtained from the impregnated and
non-impregnated adsorbents indicate that the width of
the MWCNTs-D2EHPA/AC-D2EHPA layers ranges
between 10–20 µm. However, the irregular surface can
be considered as a desirable feature, since it increases
the effective surface area of the adsorbent, and
provides a faster adsorption and desorption of analytes.
The surface modification on SEM images reveals that
Table 3 Determination of chromium(III), chromium(VI) and total chromium of some water samples using MWCNTs impregnated with
D2EHPA. [Vol. of sample: 100 mL; pH :4.5]
Sample Added (µg L-1) Found (µg L-1) Adsorption (%)a Rel. error (%)
(rsd) (%)
Cr (III) Cr (VI) Cr (III) Cr (VI) Total Cr Cr (III) Cr (VI) Total Cr Cr (III) Cr (VI)
0 10 - 9.7 ± 0.2 9.7 ± 0.2 - 97.0 ± 2.0 97.0 ± 2.0 - -2.6
(+2.0)
5 5 4.6 ± 0.2 4.9 ± 0.3 9.5 ± 0.3 92.0 ± 2 98.0 ± 2.0 95.0 ± 3.0 -8.0
(-2.0)
-2.0
(+ 6.1)
Tap water
10 0 9.6 ± 0.2 - 9.6 ± 0.2 96.0 ± 3.0 - 96.0 ± 3.0 (-4.0
(-1.4)
-1.4
(-)
0 10 - 9.8 ± 0.2 9.8 ± 0.2 - 98.0±3.0 98.0 ± 3.0 -
(-2.0)
-2.0
(+2.0)
5 5 4.8 ± 0.3 4.7 ± 0.1 9.5 ± 0.2 96.0 ± 2.0 94.0 ± 5.0 95.0 ± 3.0 -4.0
(+ 6.2)
-6.0
(+2.1)
Well water
10 0 9.9 ± 0.5 - 9.9 ± 0.5 99.0 ± 4.0 - 99.0 ± 4.0 -1.0
(+5.0)
-
- - 8.4 ± 0.4 10.8 ± 0.7 19.2 ± 0.6 - - -
(+ 4.7)
-
(+ 6.4)
0 10 - 20.6 ± 0.6 20.6 ± 0.6 - 98.0 ± 3.0 98.0 ± 3.0 - -2.0
(+ 6.1)
5 5 13.2 ± 0.2 15.7 ± 0.3 28.9 ± 0.5 96.0 ± 5.0 98.0 ± 2.0 97.0 ± 4.0 -4.0
(+ 4.1)
-2.0
(+ 6.1)
Electroplating
wastewater
10 0 18.2 ± 0.4 18.2 ± 0.4 98.0 ± 4.0 98.0 ± 4.0 -2.0
(+2.6)
-
aMean value ± standard deviation based on the five replicates (n = 5).
VELLAICHAMY & PALANIVELU: SPECIATION OF Cr(III) AND Cr(IV) BY SPE WITH MWCNT + D2EHPA
889
the D2EHPA covering the surface of MWCNTs and
AC is responsible for their good adsorption of
chromium(III).
Conclusions
The pre-concentration and separation procedures for
chromium speciation in MWCNTs impregnated with
D2EHPA are superior in terms of selectivity, detection
limit and enrichment factor. Chromium(VI) can be
determined after reduction using hydroxylamine
hydrochloride to chromium(III). The maximum
adsorption of chromium(III) 95 % was achieved with
MWCNTs impregnated with 0.25 M D2EHPA at
an equilibrium pH of 4.5. The developed
pre-concentration procedure allows the specific
determination of chromium(III) and chromium(VI) in
real samples.
Acknowledgement SV is grateful to Council of Scientific and Industrial
Research (CSIR), New Delhi, India, for providing
financial assistance under senior research fellowship
scheme(No. 9/468/(369)/2007-EMR-I).
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