Speciation of chromium in aqueous samples by solid phase ... - NOPR

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-concentrationseparation techniques including solvent extraction4,

co-precipitation5, cloud point extraction6, ionexchange7 and solid phase extraction (SPE) have been used for the separation of chromium species. The procedures are based on the preconcentration/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

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.

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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 (ICPAES) 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

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

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

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INDIAN J CHEM, SEC A, JULY 2010

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 Cr3+ + 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.

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

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

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].


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 (CrO42-)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 Cr 3+ + 4 NaOBr

Na + OBr- + HBr Na2 CrO4 + 2 NaBr

... (2) ... (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

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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

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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].

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 preconcentration of chromium(III) adsorption. The concentration of interfering ions causing ± 4 % error

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] Ion

Na+ K+ Ca2+ Mg2+ ClNO3SO42PO43Cd2+ Mn2+ Zn2+ Ni2+ Fe3+ Fe2+ Cu2+ Pb2+ Co2+ Ag2+ Cr 6+ a

Added as

NaCl KNO3 CaCl2.3 H2O MgCl2 NaCl KNO3 Na2SO4 KH2PO4 CdSO4 MnSO4 ZnCl2 NiCl2 FeCl3 (NH4)2 (FeSO4)2.6H2O CuSO4 PbSO4 CoCl2 AgSO4 K2Cr2O7

Conc. (mg L-1) 1000 1000 1000 1000 1000 1000 1000 1000 25 25 25 25 50 50 25 25 25 25 25

Cr (III)a (%) MWCNTsb

ACb

97.0 ± 2.0 96.0 ± 3.0 97.0 ± 2.0 96.0 ± 3.0 96.0 ± 2.0 96.0 ± 4.0 98.0 ± 3.0 96.0 ± 2.0 98.0 ± 2.0 97.0 ± 2.0 97.0 ± 2.0 97.0 ± 3.0 98.0 ± 3.0 98.0 ± 3.0 98.0 ± 2.0 98.0 ± 2.0 98.0 ± 2.0 96.0 ± 3.0 95.0 ± 3.0

76.0 ± 2.0 77.0 ± 3.0 76.0 ± 2.0 84.0 ± 3.0 76.0 ± 2.0 78.0 ± 2.0 83.0 ± 2.0 76.0 ± 3.0 78.0 ± 3.0 81.0 ± 2.0 86.0 ± 2.0 79.0 ± 2.0 79.0 ± 2.0 79.0 ± 2.0 78.0 ± 3.0 88.0 ± 3.0 85.0 ± 2.0 79.0 ± 2.0 90.0 ± 4.0

Mean ± standard deviation based on the five replicates. Imprgnated with 0.25 M D2EHPA.

b

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.


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

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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) Cr(III) 0 5 10 15 20 25 a b

Cr(VI) Cr(III) 25 20 4.9 ± 0.2 15 9.8 ± 0.3 10 14.7 ± 0.3 5 19.8 ± 0.5 0 24.6 ± 0.2

Adsorption (%)b

Found (µg L-1) Cr(VI) 24.2 ± 0.2 19.6 ± 0.3 14.8 ± 0.5 9.8 ± 0.3 4.7 ± 0.3 -

Total Cr 24.2 ± 0.2 24.5 ± 0.3 24.6 ± 0.4 24.5±0.3 24.5 ± 0.4 24.6 ± 0.2

pH = 4.5. Mean ± standared deviation based on the five replicates.a

Cr(III) 98.0 ± 2.0 98.0 ± 2.0 98.0 ± 2.0 99.0 ± 1.0 98.4 ± 3.0

Cr(VI) 96.8 ± 2.0 98.0 ± 1.0 98.6 ± 3.0 98.0 ± 2.0 94.0 ± 1.0 -

Rel. error (%) Total Cr Cr (III) 96.8 ± 2.0 98.0 ± 1.0 -2 98.3 ± 3.0 -2 98.0 ± 2.0 -2 96.5 ± 1.0 -2 98.4 ± 3.0 - 1.6

Cr(VI) - 3.2 - 2.0 - 1.0 - 2.0 - 6.0 -

rsd (%) Cr(III) + 4.0 + 3.0 + 2.0 + 2.5 + 1.0

Cr(VI) + 0.8 + 1.5 + 3.3 + 3.1 + 6.3 -


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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 Tap water

Well water

Electroplating wastewater

Added (µg L-1)

Adsorption (%)a

Found (µg L-1)

Rel. error (%) (rsd) (%)

Cr (III) 0

Cr (VI) 10

Cr (III) -

Cr (VI) 9.7 ± 0.2

Total Cr 9.7 ± 0.2

Cr (III) -

Cr (VI) 97.0 ± 2.0

Total Cr Cr (III) 97.0 ± 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

10

0

9.6 ± 0.2

-

9.6 ± 0.2

96.0 ± 3.0

-

96.0 ± 3.0

0

10

-

9.8 ± 0.2

9.8 ± 0.2

-

98.0±3.0

98.0 ± 3.0

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

10

0

9.9 ± 0.5

-

9.9 ± 0.5

99.0 ± 4.0

-

99.0 ± 4.0

-

-

8.4 ± 0.4

10.8 ± 0.7

19.2 ± 0.6

-

0

10

-

20.6 ± 0.6

20.6 ± 0.6

-

98.0 ± 3.0

98.0 ± 3.0

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

10

0

18.2 ± 0.4

18.2 ± 0.4

98.0 ± 4.0

-

98.0 ± 4.0

-8.0 (-2.0) (-4.0 (-1.4) (-2.0) -4.0 (+ 6.2) -1.0 (+5.0) (+ 4.7) -4.0 (+ 4.1) -2.0 (+2.6)

Cr (VI) -2.6 (+2.0) -2.0 (+ 6.1) -1.4 (-) -2.0 (+2.0) -6.0 (+2.1) (+ 6.4) -2.0 (+ 6.1) -2.0 (+ 6.1) -

a

Mean value ± standard deviation based on the five replicates (n = 5).

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


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Fig. 4  SEM images of (a) pure MWCNTs; (b) MWCNTs modified with D2EHPA; (c) MWCNTs modified with (Cr –D2EHPA) complex; (d) pure AC; (e) AC modified with D2EHPA; (f) AC modified with (Cr – D2EHPA) complex.

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). References 1 2 3 4

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