Effect of organic Cr(III) complexes on chromium speciation

Chemical Speciation and Bioavailability (2005), 17(2)

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Effect of organic Cr(III) complexes on chromium speciation S˛afak Uluçinara and A. Nur Onarb* a

Gazi University, Faculty of Education, Teknikokullar, 06500 Ankara, Turkey Ondokuz Mayis University, Art and Science Faculty, Chemistry Department, Kurupelit 55139 Samsun, Turkey

b

ABSTRACT Chromium speciation in the presence of organic chromium(III) complexes was investigated using solid-phase extraction. The adsorptions of Cr(VI) and Cr(III) on alumina and pumice powder were studied. Maximum sorption of Cr(VI) was obtained by alumina (90.22%), while Cr(III) was highly adsorbed onto pumice powder (86.65%). This result shows that pumice may be a new and promising adsorbent for Cr(III). The experimental equilibrium data for Cr(VI) adsorption onto alumina and Cr(III) sorption onto pumice were analysed using Langmuir and Freundlich isotherms. The separation and adsorption of Cr(VI), Cr(III) and five organic chromium(III) complexes onto pumice and alumina at different pH values were evaluated. Ethylenediaminetetraacetate (EDTA), oxalate, citrate, glycine, alanine and 8-hydroxyqinoline were used as ligands. Sorption of alanine and ethylenediaminetetraacetate complexes was higher onto alumina than pumice at pH43. The enhancement of adsorption of chromium(III) complexes onto pumice was achieved by surface modification of pumice using a surfactant, namely hexadecyltrimethylammoniumbromˇr (HDTMA). The presence of surfactant enhanced the adsorption of Cr(III) citrate, oxalate, glycine and 8-hydroxyquinoline complexes onto pumice. However, the adsorption of EDTA and alanine complexes decreased, with ratio of 13.40% and 4.00% respectively. Here we demonstrate that chromium speciation methods depending on adsorption onto various adsorbents including alumina may lead erroneous results. Analytical measurements were performed by flame AAS, data were obtained by standard addition method. Keywords: chromium adsorption, chromium speciation, chromium(III) complexes, organoclay, pumice

INTRODUCTION Although chromium is able to exist in several oxidation states, the most stable and common forms are trivalent Cr(III) and hexavalent Cr(VI) species, which display quite different geochemical, biological and toxicological properties. Cr(VI) is considered the most toxic form of chromium. This is attributed to the ability of Cr(VI) to penetrate the biological membrane, increasing the intracellular chromium concentration. Whereas Cr(III) is an essential trace element in the metabolism of lipids and proteins, and is vital for maintenance of a normal glucose tolerance factor (Nieboer et al., 1988). Cr(VI) is a strong oxidising agent and in *To whom correspondence should be addressed. E-mail: [email protected] w.w.w.scilet.com

presence of organic matter is reduced to Cr(III); this transformation is faster in acid environments such as acidic soils. However, high levels of Cr(VI) may overcome the reducing capacity of the environment and thus persist as a pollutant. In addition, chromium(III) may be also oxidized to chromium(VI) in the presence of an excess of oxygen, being transformed again to the more toxic form (Nieboer et al., 1988). Cr(VI) is usually associated with oxygen as chro2 mate (CrO2 4 ) or dichromate (Cr2O7 ) ions. In contrast, Cr(III) in the form of oxides, hydroxides or sulfates, is much less mobile and exists mostly bound to organic matter in soil and aquatic environments. Free Cr(III) metal ions would quickly become adsorbed or hydrolyzed and precipitated in the absence of soluble complexing ligands. The presence of organic ligands kept Cr(III) in solution and

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Effects of organic Cr(III) complexes on chromium speciation

prevented removal of Cr at pH45:5 (James et al., 1983). There are many published methods for chromium speciation. Coupled methods combining liquid chromatography with AAS detection, atomic emission spectrometry, inductively coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry and visible spectrometry have been developed (Posta et al., 1993; Li et al., 1996; Byrdy et al., 1995; Padarauskas et al., 1998). Ion chromatography, reversed-phase liquid chromatography and ion-pairing chromatography are applied as chromatographic methods (Poboz’y et al., 1996; Padarauskas et al., 1998). The most widely employed procedures for chromium speciation by atomic spectrometry are based on selective preconcentration of Cr(VI) and Cr(III). Prolonged sample manipulation may affect the chromium species distribution significantly, because of the redox equilibrium between Cr(VI) and Cr(III). For both speciation and removal of chromium, solid phase extraction has been very often employed due to its high speed and minimal sample treatment requirement. Many investigations have been carried out to improve selective retention with different partitioning mechanisms using different solid sorbents, such as ion exchangers, chelating resins or immobilized functional group on solid sorbent materials, alumina and TiO2 (Baffi et al., 1992; Sule et al., 1996; Cespon-Romero et al., 1996; Sperling et al., 1992a; Manzoori et al., 1996; Marque´s et al., 2001; Vassileva et al., 2000; Yu et al., 2001). Cr(VI) is retained by using columns with melamine-formaldehyde, C18 bonded silica reversed phase sorbent with diethyldithiocarbamate as complexing agent, zirconium (IV) oxide modified silica, Amberlite and several other anion-exchange resins (Demirata et al., 1996; Sperling et al., 1992b; Li et al., 2002; Dyg et al., 1994; Naranjit et al., 1979). Retention of Cr(III) has been carried out using chelating resins and cation-exchange resins such as Chelex 100 or Lewatit TP207 (Cespon-Romero et al., 1996; Pasullean et al., 1995; Baffi et al., 1992). Simultaneous preconcentration of Cr(VI) and Cr(III) has been achieved with alumina, Dionex columns, TiO2, Sephadex A-25 columns (Sperling et al., 1992a; Sperling et al., 1992b; Manzoori et al., 1996; Marque´s et al., 2001; Gammelgaard et al., 1997; Vassileva et al., 2000; Yu et al., 2001; Hiraide et al., 1989). The determinations of Cr(VI) and Cr(III) in these cases are based on the use of different conditions, especially pH, for retention and elution of both species. Many procedures disregard the chromium(III) that is complexed or adsorbed to organic ligands. However, a significant percentage of dissolved chro-

mium in river water was found to be non-ionic and probably organically bound (Li et al., 2001). Cr(III) bound to natural organic ligands contributed significantly to total chromium concentrations and was found to be critical in the speciation and solubility of chromium in soils (James et al., 1983; Li et al., 2001). Walsh et al. (1996) assessed three chromium speciation methods in relation to possible interferences from Cr(III)-organic complexes. The methods were namely 1, 5-diphenyl-carbazide spectrophotometry, organic extraction with methyl isobutyl ketone and coprecipitation with iron and bismuth salts. They concluded that expressing the dissolved Cr(III) content of an aqueous sample simply as total dissolved chromium minus Cr(VI) is no longer adequate in any speciation study. Although many investigations have been carried out to improve selective sorbent for chromium species, to our knowledge, none of them were interested in separation of organically bounded Cr(III) species. The aim of the present work is to investigate the effect of Cr(III) complexes on the separation of chromium species by adsorption. Alumina and pumice were selected as adsorbents for separation of Cr(III) and Cr(VI). Ethylenediaminetetraacetate (EDTA), oxalate, citrate, glycine, alanine, 8-hydroxyquinoline were chosen as ligands for Cr(III) complexation. Furthermore surface modification of pumice by a surfactant, namely hexadecyltrimethylammoniumbromu¨r (HDTMA) was performed to enhance the adsorption of chromium(III) species.

METHODOLOGY Reagents Distilled and deionised water was used for all procedures. Hexavalent and trivalent chromium standard stock solutions were prepared using potassium dichromate and chromium(III) chloride hexahydrate at 1000 mg mL1 concentration. These chemicals were purchased from Merck. Surfactant used for surface modification was hexadecyltrimethylammoniumbromu¨r (HDTMA), purchased from Aldrich. Organic complexes of trivalent chromium were prepared using disodium EDTA (Carlo Erba), ammonium oxalate (Pancreac), sodium citrate (Horasan, Turkey), glycine (Merck), alanine (Merck), 8-hydroxyquinoline (Merck). Analyticalreagent grade chemicals (HCl, NaOH) (Merck) were used for adjusting pH. Alumina (Merck), kaolin (Sigma), activated carbon (Merck) and ion-exchange resins were all commercial products. Anion exchange resin (Ionac) was from Baker Analyser, cation exchanger was

S˛afak Uluçinar and A. Nur Onar

Dowex 500 W-X8 and Amberlite XAD-7 was used as non-ionic exchange resin. The pumice used in this work was purchased from the market as ‘‘callus stone’’ with a brand name ‘‘Morgan’’ (Sultanlar Company, Turkey). EDXRF analysis of pumice samples showed that it contained 55.0% SiO2; 29.0% CaO and 3.6% Al2O3 (Onar et al., 1996). Instrumentation The analytical measurements were performed by flame AAS (Unicam 929). The hollow cathode lamp for chromium was operated at 9 mA and the spectrometer was set to 357.9 nm with bandwidth of 0.5 nm. An air-acetylene flame was used for determination of chromium. Measurements of pH were performed by Jenway 3040 ion analyser. Deionised water was obtained from Elga water purification system. The adsorption experiments were carried out by using Clifton Temperature Controlled Shaker at 150 cpm. Procedure Adsorption experiments were performed by using batch technique. Solid adsorbents were suspended in 50 mL of solution of chromium species. These test solutions were stirred for required period at desired temperature in shaker. Supernatants were filtered (0:45 mm membrane filter) and then analysed by AAS. Standard addition method was used to obtain the data with four standard solution additions and unadsorbed chromium concentrations were found using linear regression analysis. The amounts of chromium species adsorbed on adsorbents were then calculated by subtracting from initial solution contents. Alumina was activated by treating with 5 M HNO3 for two hours while agitating on a magnetic stirrer. Pumice samples were powdered and then sieved to use 18 – 35 mesh fractions. Then alumina and pumice powder were both washed with deionised water, dried at 100 C for one hour. For surface modification, 1 g pumice was treated with 50.0 mL aqueous solution of HDTMA (0.05 M). Adsorbent and surfactant mixture was agitated overnight on a magnetic stirrer at room temperature with low speed. After filtration from Whatman 41 filter paper, surfactant coated adsorbent material was dried in oven for an hour at 100 C. FTIR spectra with 4 cm1 resolution and 100 scans were collected for pumice pellets before and after modification using FTIR spectrometer (Mattson) with a DTGS detector. In the FTIR spectrum of modified pumice, two peaks at 2944 cm1 and 2868 cm1 those arising from C-H stretching of respectively methyl and methylene groups of HDTMA were used to prove the surface

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modification as described in our previous paper (Akbal et al., 2000). Syntheses of organic Cr(III) complexes were made as described previously by Walsh et al. (1996).

RESULTS AND DISCUSSION Adsorption of Cr(III) and Cr(VI) Adsorption behaviours of Cr(III) and Cr(VI), without pH arrangement, onto alumina, pumice powder, kaolin and activated carbon are presented in Figure1. Maximum sorption of Cr(VI) was obtained by alumina (90.22%). Cr(III) is adsorbed highly onto pumice powder (86.65) % and activated carbon (91.28%) with 90 min contact time. We preferred to study with pumice powder for Cr(III) because activated carbon was previously investigated (Leyva-Ramos et al., 1995). So pumice might be a promising new sorbent for Cr(III). Uptake of Cr(III) and Cr(VI) by alumina and pumice powder as a function of pH is shown in Figure 2. For Cr(III) pH range was restricted to pH 2 – 7, due to the possibility of precipitation Cr(III) hydroxide at higher pH values. Up to pH 6, the adsorption of Cr(III) increased with pH for both sorbents. Then the adsorption onto alumina slightly decreased (47.7%) at pH 7.00, while it showed an opposite trend for pumice powder (96.39%). Cr(VI) adsorption onto pumice did not significantly differ between pH 4 – 8 (38:91% 1:11). Maximum Cr(VI) adsorption onto alumina was obtained at pH 5 – 7 range (80:17% 4:04). Cr(III) was reported to exhibit typical cationic sorption behaviour onto alumina, its adsorption increased with pH. On the other hand, Cr(VI) was reported to show typical anionic sorption behaviour onto alumina, its adsorption decreased with increasing pH (Sperling et al.,

Figure 1 Adsorption of Cr(III) and Cr(VI) onto alumina, pumice, kaolin and activated carbon using 10 mg L1 Cr solution, 0.1 g adsorbent and 90 min contact time.

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end of 120 minutes for Cr(III) onto pumice and 90 minutes for Cr(VI) onto alumina. Langmuir and Freundlich adsorption isotherm models were used to analyse adsorption data of Cr(III) onto pumice and Cr(VI) onto alumina. The adsorption isotherm parameters are listed in Table 1 with correlation coefficients (R2 ). Experimental adsorption isotherm data of Cr(III) onto pumice fits better to Langmuir with a correlation coefficient R2 40:99. The Langmuir isotherm theory assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface. The distribution of active sites for Cr(III) on the pumice surface seems to be homogenous. The Q0 (mg g1) defines the maximum capacity of the pumice and a large ‘‘b’’ (mg L1) value implies strong binding of Cr(III) onto the pumice. The favourable nature of adsorption can be expressed in terms of dimensionless parameter RL that is given by the equation: RL ¼

1 1 þ bC0

Where b is the Langmuir constant (0.40 mg L1) and C0 is the initial Cr(III) concentration. RL values between 0 and 1 indicate favourable adsorption. All RL values obtained for Cr(III) adsorption onto pumice were between 0.11 and 0.03 in concentration range of 20 – 70 mg L1, indicating favourable adsorption(McKay et al., 1991). On the other hand, the Freundlich model seems to provide better fittings for Cr(VI) adsorption onto alumina. The Freundlich isotherm may be used to describe heterogeneous systems (McKay et al., 1991). According to the adsorption theory, the parameter n represents the heterogeneity of the adsorbents and characterizes the shape of the site energy distribution, the value K is commonly interpreted as a ‘‘capacity’’ parameter. It has been shown that n values between 2 and 10 indicate beneficial adsorption. In the present study, the value of n (2.38) is depicting favourable adsorption and because it is higher than one, it indicates that bond energies decrease with surface density. Figure 3 shows the effect of temperature on sorption. Sorption of Cr(VI) onto alumina decreased while increasing temperature to 98 C. Adsorption of Cr(III) onto pumice rises with an increase of

Figure 2 Effect of pH on the adsorption of (a) Cr(VI); (b) Cr(III) 5 mg L1 chromium solutions, 0.1 g adsorbent alumina and pumice, 60 min contact time.

1992a). Our results for alumina are in agreement with literature and there is a similarity between the adsorption behaviour of pumice and alumina especially for Cr(III) retention. Sperling et al. (1992a) reported a method depending on selective sorption, using acidic activated alumina column with Clark-lubs buffer systems at pH 7 for Cr(III) and at pH 2 for Cr(VI). Their retention efficiency for Cr(III) was only up to 80% and for Cr(VI) 90%, which means separation was incomplete. Our data shown in Figure 2 indicate that for all examined conditions, alumina adsorbs both chromium forms, hence their separation is impossible. According to our preliminary experiments, equilibrium of adsorption process was established at the

Table 1 Freundlich and Langmuir adsorption isotherm constants and correlation coefficients Freundlich constant Adsorbent Pumice Alumina

ð1Þ

Langmuir constant

Cr species

n

K

R2

Q0

b

Cr(III) Cr(VI)

0.39 2.38

5.02 1.17

0.82 0.96

14.42 8.88

0.40 0.21

R2 0.99 0.82


Figure 3 Effect of temperature on the adsorption of 50 mg L1 Cr(III)and Cr(VI) solutions, 0.1 g adsorbent and 60 min contact time.

Table 2 Adsorption capacities (mg g1) of pumice and alumina in batch system Adsorbent

Species

Pumice

Cr Cr Cr Cr

Alumina

(III) (VI) (III) (VI)

Adsorption capacity (mg g1) 9820 190 3130 220 2885 500 7330 770

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Figure 5 Cr(III)-citrate adsorption onto pumice and alumina at different pH values, 0.1 g adsorbent, 60 min contact time.

value of alumina for Cr(VI) (7330 770 mg g1, Table 2) was in agreement with the value of Vassileva et al. (2000), (6587 mg g1), our data for Cr(III) (2885 500 mg g1, Table 2) were much less than their batch capacity, which had been 5822 mg g1.

temperature to 80 C. The increase in adsorption suggests that the active surface centres available for adsorption increase with temperature. However, the adsorption efficiency decreases above 80 C, suggesting that adsorption process onto pumice involves different steps. The adsorption capacities of pumice and alumina for Cr(III) and Cr(VI) were determined in batch system (Table 2). Although the batch capacity

Adsorption behavior of Cr (III) complexes Oxalate, citrate, EDTA complexes are negatively charged, while glycine and alanine complexes are positively charged. Adsorption of Cr(III) complexes onto pumice and alumina at different pH values were investigated. In Figure 4, it can be seen that the uptake of Cr(III)-oxalate, by pumice is higher than alumina below pH 5. However, another negatively charged complex ion, Cr(III)-citrate adsorption onto pumice is slightly more than alumina above pH 3 (Figure 5). The sorption of Cr(III)-glycine, a positively charged complex ion is higher on pumice than alumina at all pH values (Figure 6). On the other

Figure 4 Cr(III)-oxalate adsorption onto pumice and alumina at different pH values, 0.1 g adsorbent, 60 min contact time.

Figure 6 Cr(III)-glycine adsorption onto pumice and alumina at different pH values, 0.1 g adsorbent, 60 min contact time.

Agitation period: 60 minutes, Adsorbent amount: 1.0 g.

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Figure 7 Cr (III)-alanine adsorption onto pumice and alumina at different pH values, 0.1 g adsorbent, 60 min contact time.

hand, Cr(III)-alanine sorption is more on pumice only in the pH 1 – 3 range (Figure 7). Similar data were obtained for Cr(III) – EDTA complex (Figure 8). Our results show that it is not possible to explain the adsorption behaviour of complexes plainly by surface charges of sorbents. Although since the early 1980s there have been research papers pointing out the high percentage of organically bounded Cr(III) (ca 60% in seawater) and Cr(III) complexes, chromium speciation methods depending on ion-exchange mechanism have been appearing in the literature (Nakayama et al., 1981; Collins et al., 1997; Stasinakis et al., 2003; Srivastava et al., 1998). Separation of chromium species via ion exchange would fail due to simultaneous retention of Cr(VI) and negatively charged complexes of Cr(III). Methods developed using anion column was reported to result in a concentration estimate too high for Cr(VI) and too low for Cr(III) ( Sule et al., 1996).

Figure 8 Cr (III)-EDTA adsorption onto pumice and alumina at different pH values, 0.1 g adsorbent, 60 min contact time

Figure 9 Comparison of sorbents, pumice, organopumice, alumina for adsorption of Cr(VI), Cr(III) and Cr(III) complexes at pH 6.5 mg L1 chromium solutions, 0.1 g adsorbent, 60 min contact time.

In nature, the negative charge of layer silicate clays is balanced by abundant alkali and alkali earth metal ions such as Naþ, Kþ and Ca2þ. Heavy metals can be adsorbed by clays via ion exchange reactions and by silanol groups, which dissociate even at low pH to provide negative charges for binding. Clays are often ineffective in removing organic compounds from water, because the hydration of native inorganic exchange cations creates a hydrophilic environment at the clay mineral surfaces and interlayers. Recently, attention is being paid to the use of non-ionic surfactants for preconcentration and separation of metal chelates, organic compounds and proteins in analytical chemistry and separation sciences (Li et al., 2000). Replacement of inorganic cations by organic cations changes the clay surface and interlayer environment from hydrophilic to hydrophobic. Previous studies have described the sorptive characteristics of ‘‘organoclays’’, the clays modified with quaternary ammonium cations such as HDTMA (Sheng et al., 1999). The presence of quaternary ammonium cations on the exchange complex may greatly decrease the adsorption ability of clays for heavy metals. However, organoclays may be useful for preconcentration of metal complexes. Hence Manzoori et al. (1996) investigated the preconcentration of Cr(VI) as chromium diphenyl carbazone on surfactant coated alumina. Sheng et al. (1999) used a quaternary ammonium salt having carboxyl functional group, carboxydecycltriethylammonium (CDTEA), for modification of montmorilonite, with the purpose of production a dual function sorbent for both metals and organics. In the present work, in order to increase the sorption of Cr(III) complexes, pumice surface was modified using HDTMA. Produced organopumice has been


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Table 3 Synthetic water sample analysis

Adsorption Efficiency Recovery for Cr(VI) Recovery for Cr(III)

Series 1 Alumina þ pumice mixture

Series 2 Pumice mixture þ alumina

97.49% 1.51 76.03% 13.98 4.38% 1.43

96.09% 0.60 12.36% 5.03 12.87% 4.85

investigated for preconcentration of Cr(III) complexes. Figure 9 shows the adsorption of species at pH 6 onto alumina, pumice and organopumice. The presence of surfactant enhanced the adsorption of Cr(III) citrate, oxalate, glycine and 8hydroxyquinoline complexes onto pumice as expected. The highest increase was for Cr-oxalate. Cr-oxalate was adsorbed onto pumice and organopumice with ratio of 19.75% and 35.23% respectively. The least enhancement was for Cr-citrate, it increased from 43.86% to 45.00%. However, the adsorption of EDTA and alanine complexes decreased, with ratio of 13.40% and 4.0% respectively. On the other, hand alumina uptakes EDTA, glycine and alanine complexes much more than pumice and organopumice at pH 6. Cr(III) adsorption increased to 85.94% from 76.18% onto organopumice at pH 6. Chromium is reported to form several hydroxo species including 4þ 0 CrOH2þ, Cr(OH)þ 2 , Cr(OH)3 , Cr(OH)4 , Cr2(OH)2 5þ and Cr3(OH)4 . When polynuclear species are ignored because their contribution are considered insignificant and in the absence of organic ligands, the total concentration of Cr(III) in equilibrium with Cr(OH)3(s) can be expressed as (Sperling et al., 1992a): CrðtotalÞ ¼½Cr3þ þ ½CrðOHÞ2þ þ bCrðOHÞþ 2c þ bCrðOHÞ03 c þ bCrðOHÞ 4c

ð2Þ

Between pH 6.5 to 11.0, non-ionic hydroxo complex is the predominant species. This distribution of inorganic chromium may explain the increase of Cr(III) uptake by pumice after surface modification. For the elution of Cr species, HCl, NH3, EDTA, HNO3, methanol, NaOH solutions at different concentrations were investigated with batch system experiments. Optimum recovery for Cr(III) was obtained using 2.0 mol L1 HCl and for Cr(VI) with 2.0 mol L1 NH3. Increased solution concentrations caused dissolution of the oxide structure of adsorbent during elution resulting turbid solutions. Besides the ions passed through the eluent have negative effects during the subsequent determination step. Sperling et al. (1992a) found that 1.0 mol L1 HNO3 was optimum for Cr(III) elution from alumina

and eluted 95% of sorbed chromium. Their recovery was 90 – 106% for Cr(VI) and Cr(III). However, our best recovery results were 49.69% for Cr(III) and 48.9% for Cr(VI), although miscellaneous experimental conditions were investigated including the application of heat. The elution of chromium complexes from pumice and organopumice were insufficient either. Shah et al. (1990) used a polyhydroxamic acid resin for preconcentration of Cr(III) and they could not achieve complete recovery even with prolonged elution, either. Due to the increasing behaviour of adsorption with temperature (Figure 3) and low yields in recovery imply that adsorption of Cr(III) onto pumice is a chemisorption. For the analysis of a synthetic water sample containing Cr species: Cr(III), Cr(VI), Cr(III)oxalate, Cr(III)-alanine each at concentration of 10 mg L1, Cr(III)-8 hydroxyquinoline 2 mg L1, two columns were used. One of the columns was filled with a mixture of pumice and organopumice (0.5 g each) and the other column was filled with alumina (1.0 g). Two series of experiments were conducted and five replicate analyses were made for each. In the first series, alumina was used as the first column; pH of the sample solution was adjusted to pH 6.5. In the second series experiments, pH of the solution was adjusted to 3.0 and the solution was passed through pumice-organopumice mixture column in the first place. Table 3 presents the data obtained for synthetic water analysis. Although the adsorption efficiencies were 96.10% and 97.50%, the recoveries were insufficient.

CONCLUSIONS Here in this work our results reveal that Cr(III) can be efficiently adsorbed onto pumice. Pumice may be used as a low-cost, natural and favourable alternative adsorbent for Cr(III) adsorption. The equilibrium data for Cr(VI) and Cr(III) adsorption have been analysed using Langmuir and Freundlich equations. Adsorption isotherm parameters and correlation coefficients have been determined. Freundlich model fitted for Cr(VI) adsorption onto alumina. On the other hand, for Cr(III) sorption on pumice Langmuir isotherm provided a better correlation.

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Natural aqueous systems containing inorganic or organic complexing agents have various distributions of complexed Cr(III) species. In speciation studies, the behaviour of chromium (III) complexes should be taken into account. Since some of these complexes have no charge, positive or negative charge, speciation procedures depending on ion exchange andyor adsorption are not adequate and may give erroneous results. Besides, Cr speciation studies performed by complex formation strategy should be questioned because in many cases natural Cr(III) complexes may constitute kinetically stable species. Surface modification of pumice enhances the adsorption of chromium (III) and its complexes; however, alumina is also able to uptake high amounts of complexes. Altering the pH of the medium is useless for separating chromium species in the presence of organic chromium (III) complexes. Incomplete recovery of Cr species from alumina and pumice is another main problem for analytical method development. Although there are published data with high recoveries, in our experiments the recoveries did not exceeded 76% for either Cr(VI) or Cr(III).

ACKNOWLEDGEMENTS This work was financially supported by Ondokuz Mayis University, Research Project No: F182.

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