Speciation of Cr(III) and Cr(VI) in Environmental Samples after Solid Phase

Extraction on Amberlite XAD-2000

Celal Duran,a Mustafa Soylak,b* Volkan Numan Bulut,c Ali Gundogdu,a Mehmet Tufekci,a

Latif Elcid and Hasan Basri Senturka

aDepartment of Chemistry, Faculty of Art and Science, Karadeniz Technical University,

61080 Trabzon-TurkeybDepartment of Chemistry, Faculty of Art and Science, Erciyes University, 38039 Kayseri-TurkeycDepartment of Chemistry, Giresun Faculty of Art and Science, Karadeniz Technical University,

28049 Giresun-TurkeydDepartment of Chemistry, Faculty of Art and Science, Pamukkale University, 20020 Denizli-Turkey

A method for speciation of Cr(III) and Cr(VI) in real samples has been developed. Cr(VI) has beenseparated from Cr(III) and preconcentrated as its pyrrolidinedithiocarbamate (APDC) complex by using acolumn containing Amberlite XAD-2000 resin and determined by FAAS. Total chromium has also beendetermined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with KMnO4. Cr(III) has been cal-culated by subtracting Cr(VI) from the total. The effect of pH, flow-rate, adsorption and batch capacityand effect of various metal cations and salt anions on the sorption onto the resin were investigated. The ad-sorption is quantitative in the pH range of 1.5-2.5, and Cr(VI) ion was desorbed by using H2SO4 in acetone.The recovery of Cr(VI) was 97 � 4 at a 95% confidence level. The highest preconcentration factor was 80for a 200 mL sample volume. The adsorption and batch capacity of sorbent were 7.4 and 8.0 mg g–1

Cr(VI), respectively, and loading half time was 5.0 min. The detection limit of Cr(VI) is 0.6 �g/L. The pro-cedure has been applied to the determination and speciation of chromium in stream water, tap water, min-eral spring water and spring water. Also, the proposed method was applied to total chromium precon-centration in microwave digested moss and rock samples with satisfactory results. The developed methodwas validated with CRM-TMDW-500 (Certified Reference Material Trace Metals in Drinking Water) andBCR-CRM 144R s (Certified Reference Material Sewage Sludge, Domestic Origin) and the results ob-tained were in good agreement with the certified values. The relative standard deviations were below 6%.

Keywords: Chromium speciation; Preconcentration; APDC; Amberlite XAD-2000; FAAS.

INTRODUCTION

The large amount of waste disposal has resulted in

serious environmental pollution over the last decades.

Among various pollution sources, heavy metals tend to

cause a series of risks, including carcinogenic effects to hu-

man health, ecological threats and extinction of species.1-3

Studies have shown that the carcinogenesis induced by cer-

tain toxic metals could be further enhanced or inhibited

through their interactions with other metals, which pro-

cesses depend strongly on the chemical forms of the met-

als.3 Nowadays, the growing awareness of the strong de-

pendence of the toxicity of heavy metals upon their chemi-

cal forms has led to an increasing interest in metal specia-

tion analysis.

Toxicological studies have shown that the degree of

toxicity of some elements depends on the chemical form

in which the element is present. Chromium(III), for exam-

ple, is considered an essential micronutrient for humans,

whereas chromium(VI) is a potentially carcinogenic agent.4

It is therefore necessary to control the level of chromium in

wastewater, natural water and drinking water. Many coun-

tries have developed laws along this line, but since the leg-

islation permits a larger content of Cr(III) than Cr(VI),

speciation of chromium in environmental samples is very

important.5 Therefore, the knowledge on the speciation of

chromium is of particular necessity.

Chromium is widely used in various industries, such

Journal of the Chinese Chemical Society, 2007, 54, 625-634 625

* Corresponding author. Tel/Fax: +90-352-4374933; E-mail: soylak@erciyes.edu.tr, msoylak@gmail.com

as plating, tanning, paint and pigment production, and

metallurgy, which possibly contaminate the environment.

Chromium(III) compounds are one of the essential trace

nutrients in human bodies, and play an important role in the

metabolism of glucose and certain lipids, whereas chro-

mium(VI) compounds are toxic and carcinogenic.6-8 The

United States Environmental Protection Agency (USEPA)

has regulated the permissible limit of 0.1 mg L–1 of total

chromium in drinking water. In Japan, the maximum toler-

able concentration of chromium in wastewater is 0.5 and

0.05 mg L–1 for total chromium and chromium(VI), respec-

tively. However, the World Health Organization (WHO)

thinks that the guideline value of 0.05 mg L–1 of chro-

mium(VI) is too high, compared with its high risk of carci-

nogenicity. Consequently, the development of a sensitive

method, as well as the speciation method of chromium in

environmental sample is absolutely essential.

Various analytical techniques have been used for de-

termination of chromium including spectrophotometry,

atomic absorption spectrometry (AAS), inductively cou-

pled plasma mass spectrometry (ICP-MS), etc.9-14 Due to

the levels of chromium species in the natural water samples

being generally at �g L–1 and the high matrix contents of

the samples, separation/preconcentration techniques (sol-

vent extrtaction,15 coprecipitation,16 solid phase extrac-

tion17-19) are necessary, prior to determination of chromium

by an instrumental technique.20

Solid phase extraction is one of the well-known pre-

concentration/separation techniques for trace metals, and

various supports are used in this technique.21-27 Amberlite

XAD-2000, which is a polystyrene-divinylbenzene copol-

ymer, is one of the Amberlite XAD adsorption resins, and

the studies related to this resin are seldom encountered in

the literature.28,29 It has a high adsorption capacity like

other members of the Amberlite XAD resin family.30 It has

been used for the solid phase extraction of biomolecules

and pharmaceuticals.30,31 Because Amberlite XAD-2000 is

not used for the speciation of Cr(III) and Cr(VI) according

to our literature survey and due to its good adsorption prop-

erties, it was selected as an adsorbent for the present work.

In this work, we combined this well known procedure

with the column preconcentration procedure. As the APDC

is very selective for Cr(VI), Cr(VI) has been preconcen-

trated as its pyrrolidinedithiocarbamate complex onto a

column containing Amberlite XAD-2000 and determined

by flame atomic absorption spectrometry. Total chromium

has been determined similarly after oxidizing the Cr(III) to

Cr(VI). The concentration of Cr(III) was obtained by dif-

ference.

EXPERIMENTAL

Instrumentation

A Unicam AA-929 atomic absorption spectropho-

tometer equipped with single element hollow cathode lamps

and an air/acetylene burner (10 cm) was used for the deter-

mination of chromium. The instrumental parameters were

those recommended by the manufacturer. The wavelength

(nm) selected for the determination of chromium was 357.9

nm. A Hanna 211 pH meter with glass electrode was used

for the pH adjustments. A mechanical shaker Nuve SL 350

having speed control was used for batch experiments. A

Milestone Ethos D microwave oven was used for digestion

of certified reference materials (sewage sludge), lichen and

rock samples. A glass mini-column (10 cm length and 1.0

cm diameter), having a porous disk and a stopcock, was

used for preconcentration of the metals.

Reagents

All solutions were prepared using analytical reagent

grade chemicals purchased from Merck and Fluka, unless

otherwise specified, and doubly distilled-deionized water.

Stock solutions of studied metals (Cr(III) prepared Cr(NO3)3

in 0.5 mol L–1 HNO3 and Cr(VI) prepared K2CrO4 in water)

with a concentration of 1000 mg L–1 were used. The model

and standard solutions of the metals were prepared by ap-

propriately diluting the stock solutions. H2SO4 (Merck) and

KOH (Sigma) were used for pH adjustments. Amberlite

XAD-2000 resin (particle size of 20-60 mesh and surface

area of 600 m2g–1) and ammonium pyrrolidinedithiocar-

bamate (APDC) was purchased from Sigma Chem. Co., St.

Louis. 0.1% (w/v) solution of APDC in ethanol was used as

a chelating agent. Trace metal in drinking water standard

(CRM-TMDW-500) from High-Purity Standards, Inc and

Certified Reference Material Sewage Sludge, Domestic

Origin (BCR-CRM 144R s) from the Institute for Refer-

ence Materials and Measurements (IRMM) were used as

standard reference materials.

Sampling

Water samples were taken from the tap, which pro-

vides drinking water for Karadeniz Technical University,

from the stream of Degirmendere River, which supplies

626 J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 Duran et al.

drinking water to Trabzon city, from the stream of the Solakli

River (Trabzon/Of), from mineral spring water (Trabzon/

Bengisu district) and from spring water (Trabzon/Of-

Tashanpazari district). Polyethylene bottles were used for

sample storage and were thoroughly washed with deter-

gent, tap water, 1 mol L-1 HNO3, and distilled-deionised

water, respectively, prior to collection of the water sam-

ples. Finally, the water samples were acidified with conc.

HNO3 in order to adjust pH to ~ 2 and filtered through a

nitrocellulose membrane with a 0.45 �m pore size.

A plant sample (moss), collected from Cayeli (Rize)

in Turkey, and a rock sample (analyzed by ACME Analyti-

cal Lab. (ISO 9002 Accredited Co.) in CANADA) col-

lected from Kumbet Plateau (Giresun) in Turkey were

dried in an oven for 20 hours at 105 �C and finely pow-

dered.

Column Preparation

A glass column (10 cm length and 1.0 cm diameter)

containing 0.3 g of Amberlite XAD-2000 resin (~1.5 cm

bed height) was used for preconcentration of chromium.

XAD-2000 (ground and sieved to 150-200 �m) resin was

washed successively with 1 mol L–1 NaOH, water, 1 mol

L–1 HNO3, water, acetone and water. A glass-wool plug was

rested on the stopcock of the column and another plug of

glass-wool was placed on top of the resin to avoid the distri-

bution of resin during sample passage. The sorbent was

washed with ethanol and 1 mol L-1 HNO3 and was thor-

oughly washed with H2O until the effluents were neutral.

After each use, the resin in the column was washed thor-

oughly with water and sulphuric acid (0.05 mol L-1) in or-

der to condition and clean it, and then it was stored in water

for further applications.

Determination of Cr(VI)

The method was tested with model solutions before

its application to environmental samples. For the optimiza-

tion of column separation and preconcentration method,

100 mL of spiked sample solutions containing 10 �g of

Cr(VI) and 5 �g of Cr(III) were used. pH of the spiked sam-

ple solution was adjusted to the desired value (pH 2) at

which the recovery of Cr(VI) is the highest (Cr(III) < 3%)

with sulphuric acid and then 5 mL 0.1% (w/v) APDC was

added into the model solution. Cr(VI)-APDC solution was

passed through the column at a flow rate of 10 mL min–1.

The retained species (Cr-APDC) on the column was eluted

with 7.5 mL of 0.05 mol L-1 H2SO4 solution in acetone. The

eluent was evaporated over a hot plate to near dryness. The

residue was diluted to 2.5 mL with 1 M HNO3. Chromium

content in the eluate was determined by FAAS.

Determination of Total Chromium

Total chromium was determined as Cr(VI) by the

method described above after oxidizing Cr(III) to Cr(VI)

by the addition of KMnO4 in acid medium. For this pur-

pose, 4 or 5 drops of KMnO4 (0.02 mol L-1) solution and 0.5

mL of concentrated H2SO4 were added into a 250 mL

beaker containing 100 mL of the spiked solution, 0.1 mg

L-1 Cr(VI) and 0.05 mg L-1 Cr(III). The beaker was covered

with a watch glass and heated without boiling (~45 �C) for

about 15 min to complete oxidation. The solution was

cooled and sodium azide solution (2.5%, w/v) was added

dropwise to reduce the excess of KMnO4 (decolorizing the

pink solution), waiting a few seconds after the addition of

each drop. Then, the solution was transfered to a 100 mL

volumetric flask. APDC solution (5 mL 0.1%) was added

and the solution was diluted with water to the mark and

mixed thoroughly. pH of the solution was controlled with a

pH meter and adjusted to the desired value (pH 2). After

oxidation of Cr(III) to Cr(VI) by using KMnO4 in acidic

media, the method was applied to the determination of the

total chromium. The level of Cr(III) is calculated by sub-

tracting Cr(VI) from total chromium.

Analysis of the Real Samples

The water samples (tap water, river waters, mineral

spring water, spring water and CRM-TMDW-500 trace

metal in drinking water certified reference material) ana-

lyzed were collected in pre-washed polyethylene bottles.

The samples, except for certified reference material, were

filtered through a Millipore cellulose membrane of pore

size 0.45 �m, stored in 1 L polyethylene bottles and acidi-

fied to 1% with nitric acid and were subsequently stored at

4 �C in a refrigerator. Before the analysis, the pHs of the

samples (200 mL for tap water, river waters, mineral spring

water, spring water and 50 mL for certified reference mate-

rial, CRM-TMDW 500) were adjusted to 2.0. Then 0.1%

APDC solution was added. The sample was passed through

the column. The APDC chelates adsorbed on the column

were eluted with 0.05 mol L-1 H2SO4 in acetone. The efflu-

ent was evaporated to near dryness and made up to 2.5 mL

with 1 mol L–1 HNO3. The level of chromium was deter-

mined by FAAS.

Prior to the preconcentration step for standard refer-

Speciation of Cr(III) and Cr(VI) J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 627

ence material and solid samples analyzed, sewage sludge of

domestic origin (BCR-CRM 144R s), plant sample (moss),

and rock sample were also microwave digested. Digestion

conditions for a microwave system for the samples were

applied as (35 bar) 1 min for 250 W, 1 min for 0 W, 10 min

for 650 W, 5 min for 250 W, vent: 3 min, respectively.

BCR-CRM 144R s (0.15 g) were digested with 6.0

mL of HCl (37%), 2.0 mL HNO3 (65%) and 0.5 mL HF

(39%) in a microwave digestion system. Plant samples

(moss) (1.00 g) were digested with 6 mL of HNO3 (65%), 2

mL of H2O2 (30%) in a microwave digestion system. Rock

samples (0.5 g) were digested with 4.5 mL of HCl (37%),

1.5 mL HNO3 (65%) and 2 mL HF (39%) in a microwave

digestion system.

After microwave digestion, the suspension was fil-

tered through a blue band filter paper, and the insoluble part

was washed with distilled water. And then the volume of

the sample was made up to 50.0 mL with distilled water.

Blanks were prepared in the same way as the sample, but

omitting the sample. The preconcentration procedure given

above was applied to the samples. The final volume was 2.5

mL.

Batch Capacity of the Resin

100 mL of working solution containing 1 mg Cr(VI)

chelated with APDC was brought to pH 2 and transferred to

a polyethylene bottle and then to which was added 0.1 g of

XAD-2000. After shaking for 1, 3, 5, 10, 30, 60 and 120

min at room temperature separately, the metal-loaded sor-

bent was filtered off, and the metal remaining in the filtrate

was measured by FAAS.

Adsorption Capacity of the Resin

The adsorption capacity is the maximum metal quan-

tity taken up by 1 gram of resin and given by mg metal g–1

resin or meg (miliequivalent gram). In order to determine

this, test solutions of Cr(VI) weighing in the range of 100-

10000 �g were loaded to the column containing 300 mg of

resin and recoveries were investigated. Langmuir iso-

therms were plotted in order to determine the resin capac-

ity. According to the Langmuir isotherm, a plot of Ce/qe

versus Ce shows linearity, hence Langmuir constants qmax

and aL can be calculated from the slope and intercept of the

plot where qe is the amount of metal adsorbed per unit

weight of the resin (mg g–1) at equilibrium, Ce the final con-

centration in the solution (mg L–1), qmax the maximum ad-

sorption at monolayer coverage (mg g–1), and aL the ad-

sorption equilibrium constant which is related to energy of

adsorption (L mg–1). The amount of maximum total chro-

mium (qmax) adsorbed on 1.0 g resin is calculated from the

slope of the plot obtained for chromium.

RESULTS AND DISCUSSION

To determine for quantitative recoveries of chromium

on XAD-2000, the separation/preconcentration procedure

was optimized for various parameters such as pH, sample

volume and eluent type. The percentage of chromium(VI)

adsorbed was calculated from the amounts of chromium(VI)

in the starting sample and the amounts of chromium(VI)

eluted.

Effect of pH

The efficiency of recoveries of Cr(III) and Cr(VI)

were investigated in the pH range 1-12. pH of the solution

was adjusted in a range of 1-12 by using H2SO4 and KOH

and passed through the column (containing 0.3 g of XAD-

2000). According to the recovery results (Cr(VI) 97.0%,

Cr(III) 2.0%) the optimum pH was determined as 2. In ad-

dition, it was revealed that the H2SO4 used for pH 2 did not

have a negative effect, hence the following optimization

work was carried out at pH 2. The change of recovery of

Cr(VI) and Cr(III) with pH is shown in Fig. 1. As can be

628 J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 Duran et al.

Fig. 1. Effect of the pH on the recoveries of chromiumspecies on XAD-2000 (Ligand amount: 5 mg,amount of adsorbent: 300 mg, sample flow rate:10.0 mL min–1, Eluent type and volume: 7.5 mLof 0.05 mol L-1 H2SO4 solution in acetone, sam-ple volume: 50 mL, N = 3).

seen in Fig. 1 quantitative recovery (97%) was found at the

pH 1.5-2.5 with Cr(VI) while the recovery of Cr(III) is

rather low (2%). This could make it possible to separate

Cr(VI) from Cr(III). According to the recovery results the

optimum pH was determined as 2.0 for Cr(VI).

Effect of ligand amount

The effects of the APDC amounts on the adsorption

of Cr(VI) on the resin was investigated. The recoveries of

Cr(VI) were < 8%, when APDC was not added to the solu-

tion. The recovery values increased with the addition of

APDC. The quantitative values were obtained after 3.0 mg

(0.1% (w/v) in ethanol 3 mL) of APDC. After this point the

recoveries were quantitative in all working ranges of

APDC. In all further studies, 5 mg (0.1%, w/v in ethanol 5

mL) of APDC was used.

Effect of amount of adsorbent (bed height)

The influences of amounts of XAD-2000 on the re-

tention of Cr(VI) were investigated. The results are given

in Fig. 2. The recovery values for chromium were not quan-

titative till 250 mg of XAD-2000. Quantitative recovery

values were obtained in the 250-600 mg range of XAD-

2000. All experiments were performed with 300 mg of

XAD-2000.

Influence of flow rate of sample

The rate of the flow of model solutions through the

column is one of the factors affecting the duration of the de-

termination. The rate of the flow through the column is di-

rectly related to the contact of the Cr(VI)-APDC complex

with the resin. Hence, model solutions of 50 mL were

passed through the column with rates in the range of 1-23

mL min–1. It was observed that the recovery values were

quantitative in the flow rate range of 1-12 mL min–1. Thus,

the flow rate 10.0 mL min–1 was chosen for all the subse-

quent experiments.

Eluent type and volume

For the elution of Cr(VI) complex, sulphuric acid, hy-

drochloric acid, nitric acid, acetone, ethanol, methanol and

their combinations have been tested as eluent. As can be

seen in Table 1 the best elution (97% recovery) was ob-

tained by using 7.5 mL of 0.05 mol L-1 H2SO4 solution in

acetone. The volume of 0.05 mol L-1 H2SO4 solution in ace-

tone as eluent was also tested. The recoveries of chro-

mium(VI) were quantitative (> 96%) 7.5-15.0 mL of 0.05

mol L-1 H2SO4 solution in acetone. In all further works, 7.5

mL 0.05 mol L-1 H2SO4 solution in acetone was used.

Effect of sample volume

As the concentrations of chromium in real samples

are too low, by using samples with large volumes, chro-

mium in these volumes should be taken into smaller vol-

umes for determination of these metals with high accuracy.

Speciation of Cr(III) and Cr(VI) J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 629

Fig. 2. Effect of resin amount on the recovery ofCr(VI) (pH: 2.0, ligand amount: 5 mg, sampleflow rate: 10.0 mL min–1, Eluent type and vol-ume: 7.5 mL of 0.05 mol L-1 H2SO4 solution inacetone, sample volume: 50 mL, N = 3).

Table 1. Effect of the type of elution solution on the recovery ofCr(VI)-APDC complex (pH: 2.0, ligand amount: 5 mg,amount of adsorbent: 300 mg, sample flow rate: 10.0mL min–1, Eluent volume: 7.5 mL, sample volume: 50mL, N = 3)

Type of elution solutionConcentration

(mol L–1)Recovery

(%)

HCl in acetone 1.0 80 � 3HCl in methanol 1.0 45 � 1HNO3 in acetone 1.0 91 � 2HNO3 in methanol 1.0 36 � 1H2SO4 in acetone 000.025 89 � 2H2SO4 in acetone 00.05 97 � 3H2SO4 in acetone 0.1 92 � 2H2SO4 in methanol 00.05 36 � 2H2SO4 in water 00.05 < 5HCl in water 1.0 < 5HNO3 in water 1.0 < 5Acetone – 35 � 3Ethanol – 28 � 2Methanol – 12 � 1

Hence, the maximum sample volume was optimized by the

investigation of the recovery of trace metal (10 �g for Cr(VI),

5 �g for Cr(III)) in various sample volumes in the range of

50-300 mL by using the proposed separation and precon-

centration procedure described above. Consequently, the

recovery was found to be stable until 200 mL for Cr(VI)

(97%). The results are depicted in Fig. 3. Above 200 mL,

the recovery decreased with increasing volume of sample

(93% for 225 mL, 84% for 250 mL and 65% for 300 mL). In

this work, because the elution volume was 2.5 mL, the

highest preconcentration factor was 80 for a 200 mL sam-

ple volume, theoretically.

Adsorption and batch capacity of the resin and

loading half-time

The amount of maximum metal (qmax) adsorbed on

1.0 g resin was calculated as 7.4 mg g–1 from Langmuir iso-

therms (Figs. 4 and 5). The result is given in Table 2. The

sorption percentage of the Cr(VI) is recorded as a function

of time in Fig. 6. Batch capacity of the Amberlite XAD-

2000 resin was obtained as 8.0 mg g-1 for Cr(VI). Sorption

was complete within 10 min. for chromium (Fig. 6). The

sorption half-times were 5.0 min. Both half-time and batch

capacity are tabulated in Table 2. As seen from the data in

Table 2, the proposed method developed using Amberlite

XAD-2000 resin has a high sorption capacity, and it is very

fast with a loading half-time, t1/2 of 5.0 min.

Amberlite XAD-2000 is a styrene-divinylbenzene

copolymer. The adsorption of Cr(VI)-APDC chelates on

Amberlite XAD-2000 is based on molecular adsorption.

The desorption of the metal chelates are performed by us-

ing 0.05 mol L-1 H2SO4 in acetone to break down the physi-

cal interactions between resin and metal chelates.

630 J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 Duran et al.

Fig. 3. Influences of sample volume on Cr(VI) reten-tions (pH: 2.0, ligand amount: 5 mg, amount ofadsorbent: 300 mg, sample flow rate: 10.0 mLmin–1, Eluent type and volume: 7.5 mL of 0.05mol L�1 H2SO4 solution in acetone, N = 3).

Table 2. Adsorption and batch capacities and loading half-time(N = 3)

Parameters Chromium(VI)

Adsorption Capacity (mg g–1) 7.4Batch Capacity (mg g–1) 8.0Loading Half-Time (min) 5.0

Fig. 4. Adsorption capacity of the resin for Cr(VI): Ce

vs qe graph (pH: 2.0, N = 3).

Fig. 5. Adsorption capacity of the resin for Cr(VI): Ce

vs Ce/qe graph (pH: 2, N = 3).

Effect of diverse ions

Na+, K+, Ca2+, Mg2+, Cl-, NO3-, PO4

3- and SO42- ions

with various concentrations were added to the preconcen-

tration medium in order to identify the effects of these ions

on trace metal recovery. The efficiency of the recovery was

not affected by whether these ions exist individually or al-

together, defined as mixed containing 10000 mg L–1 Na+,

1000 mg L–1 K+, 1000 mg L–1 Ca2+, 1000 mg L–1 Mg2+,

12500 mg L–1 Cl–, 1000 mg L–1 PO43-, 10000 mg L–1 NO3

–,

500 mg L–1 SO42–, Cu2+, Ni2+, Cd2+, Co2+, Pb2+, Zn2+ (10 mg

L–1) and Fe3+, Mn2+ (20 mg L–1) ions (Table 3). Hence, this

method can be applied to sea water since it was not affected

by high concentrations of Na+ and Mg2+ and environmental

samples.

Detection limit for Cr(VI)

The detection limit (n = 20, blank + 3s, where s is

standard deviation of blank estimation) was found to be

0.6 �g L–1. The precision of the determination of chro-

mium(VI) was evaluated under the optimum conditions

mentioned above. For this purpose, the procedure was re-

peated ten times for chromium(VI). It was found that the

recovery of Cr(VI) was 97 � 4 at the 95% confidence level.

Determination of total chromium

In order to determine total chromium, firstly model

solutions that contain different amounts of Cr(VI) and

Cr(III) were prepared. Then Cr(III) ions in the model solu-

tions were oxidized to Cr(VI) by using KMnO4 in acidic

media. The pH of the solution was adjusted to 2.0 by the ad-

dition of 0.05 mol L–1 H2SO4 carefully. Then the procedure

presented was applied to these solutions. The results are

given in Table 4. The results show that the proposed method

could be applied for the determination of total chromium.

Applications

The proposed method was applied to the speciation of

Cr(VI) and Cr(III) in tap water, river water samples, min-

eral spring water and spring water collected from Trabzon.

The samples were filtered through MFS membrane filters

(pore size 0.45 �m). The Cr(VI) and total chromium were

determined in unspiked and spiked original water samples.

For this purpose, Cr(VI) and Cr(III) were added to all sam-

ples in different amounts and the proposed method was ap-

plied. The obtained results are given in Table 5. As can be

Speciation of Cr(III) and Cr(VI) J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 631

Fig. 6. The rate of Cr(VI) sorption on XAD-2000 resin(pH: 2.0, N = 3).

Table 3. Influences of some ions on the recovery of chromium(VI) on the XAD-2000 (pH:2.0, ligand amount: 5 mg, amount of adsorbent: 300 mg, sample flow rate: 10.0 mLmin–1, Eluent type and volume: 7.5 mL of 0.05 mol L- 1 H2SO4 solution in acetone,sample volume: 50 mL, N = 3)

Ions Concentration (mg L–1) Recovery (%)

Na+ 100000 94 � 2K+ 1000 96 � 3Ca2+ 1000 97 � 2Mg2+ 1000 93 � 4Cl- 125000 95 � 4PO4

3- 1000 98 � 5SO4

2- 0500 93 � 3NO3

- 100000 95 � 2Cu2+, Ni2+, Cd2+, Co2+, Pb2+, Zn2+ 0010 97 � 2Fe3+, Mn2+ 0020 93 � 3Mixed* 95 � 4

* The solution containing ions in Table 3 combined.

seen from Table 5, the method could be applied success-

fully for the separation, preconcentration and speciation of

trace amounts of chromium in original water samples. The

accuracy of the results was quite satisfactory. Relative er-

ror was lower than 6% for both Cr(VI), Cr(III) and total

chromium.

632 J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 Duran et al.

Table 4. Determination of total chromium in spiked test solutions (pH: 2.0, ligand amount: 5mg, amount of adsorbent: 300 mg, sample flow rate: 10.0 mL min–1, Eluent type andvolume: 7.5 mL of 0.05 mol L- 1 H2SO4 solution in acetone, sample volume: 50 mL,N = 3)

Added (�g) Found (�g)

Cr(VI) Cr(III) Total Chromium Total Chromium Recovery (%)

0 15 15 14.4 � 0.2 965 10 15 14.1 � 0.3 947.5 7.5 15 14.2 � 0.3 9510 5 15 14.7 � 0.5 9815 0 15 14.5 � 0.4 97

Table 5. Determination of Cr(III), Cr(VI) and total chromium in tap water and river water (pH: 2.0, ligand amount: 5mg, amount of adsorbent: 300 mg, sample flow rate: 10.0 mL min–1, Eluent type and volume: 7.5 mL of0.05 mol L- 1 H2SO4 solution in acetone, sample volume: 200 mL, N = 3)

Added (�g) Found (�g) Recovery (%)

SamplesCr(III) Cr(VI) Cr(III) Cr(VI)

TotalChromium

Cr(III) Cr(VI)Total

Chromium

– – 0.76 � 0.04 BDL* 0.76 � 0.04 – – –05 10 5.4 � 0.2 10.1 � 0.30 15.5 � 0.40 94 1010 98

KTÜ Tap Water

10 05 10.6 � 0.40 4.9 � 0.2 15.5 � 0.50 99 98 98– – 4.5 � 0.1 BDL 4.5 � 0.1 – – –

05 10 9.3 � 0.3 9.3 � 0.3 18.6 � 0.40 98 93 95DegirmendereRiver

10 05 14.2 � 0.40 4.7 � 0.2 18.9 � 0.50 98 94 97– – 1.10 � 0.03 BDL 1.10 � 0.03 – – –

05 10 5.9 � 0.1 9.9 � 0.4 15.8 � 0.40 97 99 98Solakli River

10 05 10.8 � 0.30 4.8 � 0.2 15.6 � 0.30 97 96 97– – 0.45 � 0.02 BDL 0.45 � 0.02 – – –

05 10 5.6 � 0.2 9.7 � 0.5 15.3 � 0.60 1030 97 99Mineral SpringWater

10 05 10.5 � 0.40 5.1 � 0.2 15.6 � 0.40 1010 1020 1010– – 0.56 � 0.05 BDL 0.56 � 0.05 – – –

05 10 5.4 � 0.3 9.4 � 0.2 14.8 � 0.40 97 94 95Spring Water

10 05 10.5 � 0.50 4.6 � 0.3 15.1 � 0.60 99 92 97

* Below detection limit.

Table 6. The level of total chromium in the standard reference materials after application ofthe presented procedure (pH: 2.0, ligand amount: 5 mg, amount of adsorbent: 300mg, sample flow rate: 10.0 mL min–1, Eluent type and volume: 7.5 mL of 0.05 molL- 1 H2SO4 solution in acetone, sample volume: 50 mL, N = 3)

Sample Found value Certified value Recovery (%)

CRM-TMDW-500* (�g L–1) 19.6 � 0.2 20.0 � 0.1 98BCR-CRM 144R s (�g g–1)** 87.0 � 5.1 90.0 � 6.3 97

* Certified reference material trace metals in drinking water.** Certified reference material sewage sludge, domestic origin.

The accuracy of the proposed separation/preconcen-

tration method was examined by determination of total

chromium in certified reference drinking water materials

(CRM-TMDW-500) from High-Purity Standards, Inc. and

certified reference sewage sludge, domestic origin (BCR-

CRM 144R s) from the Institute for Reference Materials

and Measurements (IRMM). The results are given in Table

6. The results show that this system provided analytical

data within the ranges of the certified values.

The proposed preconcentration method was applied

to the determination of chromium in environmental sam-

ples. Environmental samples, except for the water sample,

Speciation of Cr(III) and Cr(VI) J. Chin. Chem. Soc., Vol. 54, No. 3, 2007 633

Table 7. The level of chromium species in environmental sample from Trabzon, Giresun andRize after application of the presented procedure (pH: 2.0, ligand amount: 5 mg,amount of adsorbent: 300 mg, sample flow rate: 10.0 mL min–1, Eluent type andvolume: 7.5 mL of 0.05 mol L- 1 H2SO4 solution in acetone, sample volume: 50 mL,N = 3)

Concentration (�g L–1)Sample

Chromium(III) Chromium(VI)

Tap water 3.8 � 0.2 BDLDegirmendere River 22.5 � 0.50 BDLSolakli River 5.5 � 0.1 BDLMineral Spring Water 2.3 � 0.1 BDLSpring Water 2.8 � 0.3 BDL

Total Chromium (�g g-1)

Moss 1.01 � 0.05Rock* 10.1 � 0.40

* Rock sample was determined as 10.7 �g/g (RSD: < 5%) by ACME Analytical Laboratory(ISO 9002 Accredited Co.) in CANADA.

Table 8. Comparative data from some recent studies for preconcentration and speciation of chromium using SPE methods and detectionby FAAS

System pH EluentFlow rate,mL min–1 PF

Resin capacity,mg g–1

LOD,�g L–1 Ref.

Melamine-urea-formaldehyderesin

2.0 0.2 mol L–1 NaOH 15 � 15 � 10

C-18 octadecyl silica disks/TEBD

4.0 Methanol 65 3000 225 �g 0.02 17

Amberlite XAD-2010/DDTC 2.5 1 mol L–1 HNO3 in acetone 7.0 25 4.40 1.28 19Amberlite XAD-16/shellacsorbent

4.5 0.05 mol L–1 HCl 1.0 75 0.90 1.0 32

Chromosorb 108/dithizone 8.0 2 mol L–1 HNO3 in acetone 5.0 71 4.50 0.75 33Ambesorb 563/APDC 2.5 1 mol L–1 HCl in acetone 5.0 1250 4.76 2.70 34Amberlite XAD-16/1,5-diphenylcarbazide

1.0 0.05 mol L- 1 H2SO4 inmethanol

1.5 25 0.40 45 35

Amberlite XAD-2/5-palmitoyl-8-hydroxyquinoline

4.5 2 mol L–1 HCl 5.0 � � 58 36

C-18 bonded silica disks/cetyltrimethyl ammonium bromide

7.0-8.0 1 mol L–1 HNO3 + 0.1%methanol (v/v)

25 � 720 �g 15 Cr(VI),20 Cr(III)

37

Ambersorb 563/1,5-diphenylcarbazide

0.05 mol L- 1

H2SO4

Acetone 2.0 30 � 3.4 38

Amberlite XAD-2000/APDC 2.0 0.05 mol L- 1 H2SO4 10.0 80 Column 7.4Batch 8.0

0.6 Thiswork

LOD: Limit of detection, PF: Preconcentration factor, SPE: Solid-phase extraction, TEBD: (E)-N1-((1-thiophen-2-yl)ethylidene)-benzene-1,2-diamine, APDC: Ammonium pirrolidinedithiocarbamate, DDTC: Diethyldithiocarbamate.

were digested in a microwave digestion system. The results

are given in Table 7.

CONCLUSION

The proposed separation and preconcentration method

using XAD-2000 as a sorbent material for FAAS has been

evaluated and demonstrated to be promising for routine

speciation and determination of chromium at low levels in

environmental samples. A 80-fold enrichment factor was

obtained. The methodology proposed has shown adequate

accuracy and selectivity, besides being simple and econom-

ical.

The method was also compared with other precon-

centration/speciation procedures for chromium in the liter-

ature. The suggested procedure has a relatively high pre-

concentration factor, flow rate and resin capacity, and low

LOD when compared to similar methods given in Table 8.

ACKNOWLEDGEMENTS

Authors thank the Unit of the Scientific Research

Projects of Karadeniz Technical University for financial

support.

Received October 30, 2006.

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