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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

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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.

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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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