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Preconcentration of trace multi-elements in water samples using Dowex
50W-x8 and Chelex-100 resins prior to their determination using
inductively coupled plasma atomic emission spectrometry (ICP-OES)
Philiswa N. Nomngongo a, J. Catherine Ngila a,, Titus A.M. Msagati a, Brenda Moodley b
a Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africab School of Chemistry and Physics, University of KwaZulu Natal, P/Bag X54001, Westville, Durban, 4000, South Africa
a r t i c l e i n f o
Article history:
Available online 28 August 2013
Keywords:
Trace multi-element
Simultaneous preconcentration
Ion exchange resins
Chelex-100
Dowex 50W-x8
Drinking water
a b s t r a c t
This work presents a solid phase extraction (SPE) method for simultaneous preconcentration of trace ele-
ments in water samples prior to their ICP-OES determination. Dowex 50W-x8 and Chelex-100 resins
were used as SPEsorbent materials for preconcentration of trace Cd, Co, Cr, Cu, Fe, Ni, Pb and Zn. The opti-
mum sample pH, eluent concentration and sample flow rates were found to 6, 3.0 mol L1 and
3.0 mLmin1, respectively. In terms of multi-element preconcentration capabilities, Dowex 50W-x8
appeared to be a better sorbent. The recoveries for all the tested analytes were >95%. However,
Chelex-100 showed a better performance in terms of recovery (>95%) towards Cu, Fe and Zn. Under opti-
mized conditions using Dowex 50W-x8, the relative standard deviations for different metals were
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The present study seeks to determine the most suitable cation
exchange resin that will have high metal retention efficiency over
a wide operating pH range. Therefore, performance of Chelex-100
and Dowex 50W-x8 sorbents for simultaneous pre-concentration
of cadmium, cobalt, chromium, copper, iron, nickel, lead and zinc
in aqueous solutions was investigated. Various factors affecting
the cation exchange process, such as sample volume, concentration
of the eluent, sample and eluent flow rates as well as the accuracy
of the method, were investigated.
2. Experimental
2.1. Instrumentation
Analyte metal ions were determined using Spetro Arcos ICP-OES
equipped with Cetac ASX-520 autosampler. Solid phase extraction
was carried out in a VacMaster-24 sample SPE station (Supelco, PA,
USA). The latter was used to control the sample loading and elution
flow rate to 3.0 mL min1.
2.2. Reagents and solutions
All reagents were of analytical reagent grade unless otherwise
stated and Millipore water was used throughout the experiments.
Spectrascan stock solutions (1000 mg L1) of Cd, Co, Cr, Cu, Fe, Ni,
Pb and Zn (Teknolab A/S, Drbak, Norway) were used to prepare
the working solutions for SPE at concentrations of 6 lg L1 (Cr,
Co, Ni), 10 lg L1 (Cd), 12 lg L1 (Pb), 30 lg L1 (Cu, Fe and Zn).
Working solutions, as per the experimental requirements, were
freshly prepared from the stock solution for each experimental
run. A Spectrascan multi-element standard solution at concentra-
tionof 100 mg L1 was used to prepare working standard solutions
at concentrations of 1070 lg L1 for Cd, Co, Cr, Fe, Ni and Pb; and
30180 lg L1 for Cu and Zn in measurements of concentrations of
analytes in all model and sample solutions. Solutions of nitric acid
at concentrations of 0.5, 1.0, 2.0, 3.0 and 4.0 mol L1
used for theelution of the analytes from the column were prepared from ultra-
pure concentrated acid (65%, SigmaAldrich, St. Loius, MO, USA).
The pH adjustments were performed with 1.0 M HNO3 and NaOH
solutions. The cation exchangers used in this study as packing
materials were Chelex-100 and Dowex 50W-x8 (sodium forms)
purchased from Sigma Aldrich (St. Loius, MO, USA).
2.3. Water samples and preparation
Tap water samples were obtained from University of Johannes-
burg (Doornfotein and Kingsway campuses). Effluent wastewater
samples were collected from Johannesburg Water. The wastewater
samples were filtered through a 0.45 lm pore-size Millipore
cellulose nitrate membrane to remove any fine particulate matterpresent. Bottled water samples were obtained from a local
supermarket.
2.4. Column preparation
Supelco polyethylene columns (1.35 cm in diameter and 6.5 cm
in length) with frits were employed for SPE. The columns were
soaked in 5% HNO3 solution and then rinsed successively with
Millipore water. Afterwards, slurries of 1.5 g of Chelex-100 or Dow-
ex 50W-x8 resin in Millipore water were loaded into the columns.
A porous frit was placed at the bottomof the columnand at the top
of the packing material to hold and confine the adsorbent within
the designated capacity/volume. The resin columns were washed
using triple distilled water followed by conditioning with 10 mLammonium acetate buffer (1.0 mol L1, pH 9.0). After each use,
the resin in the column was washed with 20 mL of water followed
by 10 mL of 1.0 mol L1 NaOH. This was done in order to keep the
resin in sodium form.
2.5. Preconcentration procedure
The pH values of the model solutions of Cd, Co, Cr, Cu, Fe, Ni, Pb
and Zn were adjusted to 6. The solutions were then each passedthrough a SPE column packed with either Chelex-100 or Dowex
50W-x8 at a flow rate of 2.0 and 3.0 mL min1, respectively. Metal
ions retained on the resins were eluted with 5.0 mL of HNO3 at a
flow rate of 3.0 mL min1. The metal concentrations in the final
solution were determined using ICP-OES. The same procedure
was applied to the blank solutions. After each run, the columns
were conditioned as per Section2.3.
2.6. Optimization of preconcentration parameters
The SPE system was optimized in order to determine the best
retention/ elution conditionsfor trace metal ion determinationwith
good sensitivity and precision (Soylak, 2004). Several experimental
variables affecting thepre-concentration system suchas eluent con-centrations, sample and eluent flowrates, among other parameters,
wereevaluated andoptimized. To obtainthese conditions, prelimin-
ary tests were performedto investigate factors that exert significant
influence on the retention of the analytes by cation exchange resin.
Thefactorsselected includeeluentconcentration, samplevolume as
well as sample flow rate. In previous study (Soylak, 2004), the max-
imum retention of the analytes onto the cation exchange resin was
observed at pH 6. We decided to use the same pH value at 6 in the
present study, for all the experiments.
The optimization of the sample flow rate was carried out to en-
sure the quantitative retention of the analytes of interest. The ef-
fect of flow rate of the sample solution on the retention of the
studied metal ions on the Chelex-100 and Dowex 50W-x8 resins
was carried out with a column packed with 1.5 g of resin. Samplesolutions were passed through the column at various flow rates
(1.05.0 mL min1). The flow rates less than 1.0 mL min1 were
not studied to avoid long analysis time.
3. Results and discussion
The SPE system was optimized in order to determine the best
retention/elution conditions for trace metal ion determination
with good sensitivity and precision (Soylak, 2004). Several experi-
mental variables affecting the pre-concentration system, such as
eluent concentrations, sample and eluent flow rates, among other
parameters, were evaluated and optimized. The percentage recov-
eries were calculated by relating the final obtained concentration
(Cf) of the analyte to the original concentration (Ci) of the metal
ion in the model solution.
3.1. Effect of pH
Thesample pHfor quantitativepreconcentrationof Cd,Co, Cr,Cu,
Fe, Ni, Pb and Zn in the Dowex 50W-x8 and Chelex 100 columns is
one of the most important factors (Jimnez et al., 2002). This is
because, highly acidic solutions may lead to protonation of resins
functional group while highly alkaline solution may result in the
precipitation of metal ions as hydroxides. This may results in the
underestimation of metal ion concentrations in drinking water
samples. Therefore, the effect of sample pH on the retention of the
analytes onto Dowex 50W-x8 and Chelex 100 resins was carried
out between pH 4 and 10. The influence of the sample pH on the
pre-concentration of Cd, Co, Cr, Cu, Fe, Ni, Pb and Zn is presentedin Fig. 1. It was observed that for both resins, lower recoveries
84 P.N. Nomngongo et al./ Physics and Chemistry of the Earth 66 (2013) 8388
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(
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3.2. Effect of eluent concentration
The desorption/elution of metal ions from Dowex 50W-x8 and
Chelex-100 using various nitric acid concentrations (0.5
4.0 mol L1) has been investigated. From the results in Fig. 2, it
was observed that in order to desorb the metal ions from Dowex
50W-x8, a higher concentration of nitric acid as compared to Che-
lex-100 was used. This implied that Dowex 50W-x8 strongly binds
the metal ions compared to Chelex-100. The results indicated that
metal ions were quantitavely recovered from Chelex-100 when the
concentration of HNO3 was between 1.0 and 2.0 mol L1 while in
the case of Dowex 50W-x8 3.0 mol L1 HNO3was used. This should
be expected because Chelex-100 (iminodiacetic acid functional
group) is a weakly acidic cation exchanger whereas Dowex 50W-
x8 (sulfonic acid functional group) is a strongly acidic cationexchanger.
3.3. Effect of flow rate
The optimization of the sample flow rate was carried out to
ensure the quantitative retention of the analytes of interest. The
effect of flow rate of the sample solution on the retention of the
studied metal ions on the Dowex 50W-x8 resin was carried out
with a column packed with 1.5 g of resin. Sample solutions were
passed through the column at various flow rates (1.0
5.0 mL min1). The flow rates less than 1.0 mL min1 were not
studied to avoid long analysis time. The optimum flow rate for this
work was defined as the rate of flow of the sample solution
through the column at which more than 95% retention of metal
ions takes place. The results (Fig. 3) showed that the optimum flowrate for quantitative sorption of metal ions onto the resin was
between 1.0 and 3.0 mL min1. The increase of flow rate more than
3.0 mL min1 caused a gradual decrease in sorption due to insuffi-
cient contact time between the resin and the metal ions; hence, 2.0
and 3.0 mLmin1 flow rates were chosen as the optimum flow rate
for sample loading onto Chelex-100 and Dowex 50W-x8 resins,
respectively.
3.4. Preconcentration of multi-element
The efficiency of studied cation exchange resins for pre-concen-
tration of multi-elements (concentration of each analyte equal to
10 lg L1) in aqueous solution was investigated under optimum
conditions. The results indicated that the highest retention of theanalytes from aqueous model solutions was observed on Dowex
50W-x8 resin (Table 1). This might be due to the larger exchange
capacity (1.7 meq mL1) and its functional groups (sulfonic acid).
The recoveries of metal ions from Dowex 50W-x8 ranged from
95% to 101%. It can be concluded that the affinity of studied ana-
lytes towards Dowex 50W-x8 was very similar. Therefore, they
could be pre-concentrated with the same efficiency (Pyrzyska
and Joca, 2000). The results in Table 1 indicated that Chelex-
100 was only suitable for the removal of Cu, Fe and Zn at an opti-
mum flow rate of 2.0 mL min1. The rest of the metals were not
quantitatively recovered at this optimum flow rate. It was then
concluded that Chelex-100 was not suitable for pre-concentration
of multi-element in aqueous matrices. Therefore, Dowex 50W-x8
at an optimum flow rate of 3.0 mL min
1
was used for furtheranalysis.
Table 1
Recovery (%) of multi-element in aqueous solution using Dowex 50W-x8 and Chelex-100 SPE methods.
Resins Recovery (%)
Cd Co Cr Cu Fe Ni Pb Zn
Dowex 99.2 1.4 97.4 1.3 96.3 1.2 101 1.2 99.3 4.2 96.4 1.4 95.1 1.2 97.9 2.1
Chelex 88.9 1.2 80.6 3.8 85.3.1 4.0 95.8 2.4 97.5 2.4 78.1 1.2 91.0 1.2 96.5 3.8
Experimental conditions: sample volume = 20 mL; amount of resin = 1.5g; flow rates = 2.0 and 3.0 mL min1 for Chelex-100 and Dowex 50W-x8, respectively; eluent
volume = 5 mL; replicates = 3.
Fig. 4. Effect of sample volume on the recoveries of metal ions. Experimental
conditions: pH 6.0; analyte concentration 10 lg L1; amount of sorbent 1.5 g; flow
rates of sample and eluent 3.0 mL min1; eluent volume 5 mL; replicatesn = 3.
Table 2
Analysis of certified reference materials (mean of 3 replicates; concentration in lg L1).
Cations BCR-713 Effluent wastewater CRM TMDW-500 drinking water
Certified Obtained Recovery Certified Obtained Recovery
Cd 5.1 0.6 5.0 0.8 97.5 1.1 10.0 0.05 9.7 0.7 97.0 2.1
Co NCa 15.3 1.3 25.0 0.1 24.3 0.6 97.2 1.4
Cr 21.9 2.4 22.1 0.7 100.9 0.5 20.0 0.1 19.6 0.3 98.0 1.1
Cu 68.4 3.3 66.8 1.3 97.7 2.4 20.0 0.1 20.1 0.2 100.5 0.9
Fe 398.3 32.0 383.5 3.5 96.3 1.4 100.0 0.5 97.8 0.6 97.8 1.7
Ni 30.6 4.6 29.7 2.1 97.1 1.3 60.0 0.3 57.9 0.9 96.5 1.3
Pb 47.0 4 48.3 1.3 102.8 0.8 40.0 0.2 38.7 0.4 96.8 2.4
Zn 216.2 32.13 213.5 1.8 98.8 3.1 70.0 0.4 70.3 0.1 100.4 0.5
Experimental conditions: sample volume= 100 mL; amount of resin = 1.5 g; flow rates = 3.0 mL min
1
; eluent volume = 5 mL.a NC = not certified.
86 P.N. Nomngongo et al./ Physics and Chemistry of the Earth 66 (2013) 8388
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3.5. Effect of sample volume
The influence of sample volume on the recoveries of analyte
ions on the solid phase was studied in order to obtain high precon-
centration factor (Shishehbore et al., 2011; Aydin and Soylak,
2010). Therefore, the effect of sample volume on the retention of
Cd, Co, Cr, Cu, Fe, Ni, Pb and Zn onto Dowex 50W-x8 resin was
investigated in the range of 501000 mL, while keeping the metal
ion concentration fixed at 10 lg L1. It is seen from theFig. 4that
the retention of metal ions can be achieved quantitatively (P95%)
by sample volume up to 700 mL. Therefore, the highest preconcen-
tration factor was found to be 140 when the adsorbed metal ionswere eluted with 5 mL of 3 mol L1 HNO3. At volumes higher than
700 mL, a decrease in quantitative recoveries of metal ions was ob-
served. This might be due to the saturation of the active sites of the
adsorbent. For further investigations, 100 mL was used.
3.6. Column regeneration
In order to investigate the recyclability of Dowex 50W-x8 col-
umn, successive retention and elution cycles were performed by
passing 20 mL of copper, iron and zinc solutions through the col-
umn. The regeneration of Dowex 50W-x8 column were evaluated
by monitoring the changes in the recoveries of copper, iron and
zinc through 200 retentionelution cycles. The Dowex 50W-x8 col-
umn was reused after regeneration with 20 mL Millipore water and10 mL of 1.0 mol L1 NaOH, respectively. It was found to be stable
up to 150 retention/elution cycles without observable decrease in
the recoveries of copper, iron and zinc (>95%).
3.7. Analytical performances
The analytical performance of the SPE-Dowex 50W-x8 method
under optimum conditions for pre-concentration of metal ions
was evaluated. The dynamic linear range of the method was eval-
uated and obtained as 1070 lg L1 for Cd, Cr, Co, Ni and Pb; 30
160lg L1 for Cu, Fe and Zn. The correlation coefficients (R2) of
the calibration curves were in the range 0.99910.9997. The IUPAC
limit of detection (LOD) and limit of quantification (LOQ) for the
SPE method under optimized conditions were obtained from thesignals of 20 successive measurements of the blank and the slope
(m) of the calibration curve. The LOD of Cd, Co, Cr, Cu, Fe, Ni, Pb,
and Zn were found to be 0.06, 0.08, 0.05, 0.02, 0.01, 0.39 and
0.02 lg L1, respectively; and LOQ were 0.19, 0.26, 0.11, 0.08,
0.05, 1.3 and 0.08 lg L1 for Cd, Co, Cr, Cu, Fe, Ni, Pb, and Zn,
respectively. The LOD and LOQ values obtained in this study can
be improved by increasing the volume of the sample.
The precision (reproducibility) of the SPE method was studiedby performing 15 successive measurements at a concentration le-
vel of 10 lg L1 of multi-element aqueous solution (containing Cd,
Co, Cr, Cu, Fe, Ni, Pb and Zn). The overall reproducibility of pre-con-
centration procedure expressed in terms of relative standard devi-
ation (%RSD) was reasonably good (
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filters or/and adsorbents used and to how often these filters or/and
adsorbents are changed or cleaned. As expected, wastewater con-
tained all the studied metal ions with highest Fe content, followed
by Zn and Cd.
The metal ion concentrations obtained were compared against
the allowed maximum contamination levels (MCLs) by USEPA
(2011), WHO (2008)andSANS 241 (2005)in drinking water. The
MCL values for the analytes of interest are given inTable 4. Based
on the drinking water samples analysed, all samples investigated
in this study showed no pollution Co, Cr, Cu, Fe, Ni, Pb, and Zn
except for TW 4 and TW 5 samples which showed pollution of Cd.
4. Conclusions
In this study, the efficiency of Chelex-100 and Dowex 50W-x8
cation exchange resins for the separation and pre-concentration
of multi-element in aqueous solutions was investigated and the re-
sults demonstrated that Dowex 50W-x8 resin has good capability
and efficiency for the simultaneous preconcentration of metal ions.
In comparison, Chelex-100 showed limited performance (precon-
centration with percentage recovery P95%) to only few metals
namely Cu, Fe and Zn whereas Dowex 50W-x8 had the best overall
performance for a wider range of metals.
The limits of detection (0.010.39 lg L1) and quantification
(0.051.3 lg L1) were relatively low, suggesting that the method
may be applied for trace analysis of these analytes in drinking
water and wastewater samples. The accuracy (% recovery) and
precision (% RSD) of the Dowex 50W-x8 SPE method ranged from
95105% and 1.22.2%, respectively. The proposed procedure was
applied to the determination of trace metals in CRMs drinking
water and wastewater samples. The results revealed that the
method can be used for routine monitoring or spot analysis of
metal ion contaminants in the drinking water supplies. In addition,
the results indicated that all except for TW 4 and TW 5 passed the
drinking water standards (guidelines) for the studied trace metals
set by WHO, USEPA and SANS.
Acknowledgements
Ms P.N. Nomngongo wishes to thank Sasol and NRF for financial
assistance. University of Johannesburg (Spectrau) is acknowledged
for providing ICP-OES facilities.
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