Interactions of Chromium Ions with Starch Granules in an Aqueous

Article pubs.acs.org/JPCB

Interactions of Chromium Ions with Starch Granules in an Aqueous Environment Jadwiga Szczygieł,† Krystyna Dyrek,† Krzysztof Kruczała,*,† Ewa Bidzińska,† Zuzanna Brozė k-Mucha,†,‡ Elzḃ ieta Wenda,† Jerzy Wieczorek,§ and Joanna Szymońska∥ †

Faculty of Chemistry, Jagiellonian University, 3 Ingardena Str., 30-060 Krakow, Poland Institute of Forensic Research, 9 Westerplatte Str., 33-031 Krakow, Poland § Department of Agricultural and Environmental Chemistry, Agricultural University, 21 Mickiewicza Ave., 30-120 Kraków, Poland ∥ Department of Chemistry and Physics, Agricultural University, 122 Balicka Str., 30-149 Kraków, Poland ‡

ABSTRACT: In this study, interactions of dichromate ions with potato starch granules in highly acidic aqueous solutions and at different temperatures were investigated. It was found that the process underwent a reduction of Cr2O72− to Cr3+ accompanied by the formation of intermediate Cr5+ ions detected by electron paramagnetic resonance (EPR) spectroscopy. The reactions took place after the attachment of dichromate anions to the granules and resulted in a lowering of the Cr2O72− initial content in the solution. The newly formed Cr3+ ions were both accumulated by the granules or remained in the solution. It was observed for the first time that the quantity of such ions taken by the granules from the solution was noticeably higher than that delivered by trivalent chromium salt solution. It was revealed by scanning electron microscopy coupled with energydispersive X-ray spectroscopy (SEM-EDX) that the chromium ions were not only adsorbed on the granule surface but also introduced into the granule interior and evenly distributed there. An activation energy of the reduction reaction equal to 65 kJ·mol−1 and the optimal parameters of the process were established. The proposed mechanism could be useful for the bioremediation of industrial effluents polluted by hexavalent chromium compounds.

1. INTRODUCTION Hexavalent chromium compounds, owing to their strong oxidizing properties, are considered to be highly toxic to living organisms1,2 and aqueous ecosystems.3 Biosorption using biodegradable low-cost materials, e.g. carbohydrate polymers,4−9 is one of the most promising methods of Cr(VI) anion removal.4−12 Starch is a natural semicrystalline polysaccharide consisting of glucose-based polymers, i.e., amylose and amylopectin, in the form of amorphous and ordered layers.13 This substance is abundant in the environment, fully biodegradable, and inexpensive. Because of these properties and the inner structure of its granules, starch could be considered to be a suitable biosorbent of various chemical species. In particular, starch could attach metal ions via cation−hydroxyl group interactions, as previously reported for metal ions.14−16 Also, chemically modified starches, mainly the cross-linked cationic ones, were found to be effective binders of ions, especially anions from aqueous solutions.17−19 Usually, the transition-metal cations interact with the functional groups of macromolecules by the formation of complexes20−22 or in situ reduction (Au, Ag) leading to the incorporation of metal nanocrystals or nanoparticles into the polymer structure.23,24 The same has already been reported for interactions of numerous transitionmetal cations with polysaccharides.15,16,25 Thus, the hydroxyl and carbonyl groups present in starch could act as the ligands as © 2014 American Chemical Society

well as reducers of the metal cations. The previous function is additionally supported by the phosphoric moiety typical of the granular potato starch applied in our work. Much controversial information regarding the mechanism of biosorbent−chromium anion interactions can be found in the literature. In some studies, physical sorption as the main method of hexavalent chromium interaction with the biosorbent has been taken into account.9 Other authors assumed that Cr(VI) ions, due to their high oxidation potential, could be involved in redox reactions.26 The adsorption-coupled reduction mechanism of the interactions of Cr(VI) compounds with functional groups of biomaterials was also postulated.5 Verification of the existing hypotheses is required for scientific and practical reasons. Our study was undertaken with the aim of determining the mechanism and the experimental parameters of the effective Cr2O72− ion reduction in aqueous solutions by means of granular potato starch. Various spectroscopic methods such as electron paramagnetic resonance (EPR), scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDX), and ultraviolet−visible spectroscopy (UV−vis) were applied for the identification of the reaction products and their distribution in the system. Our Received: February 26, 2014 Revised: May 30, 2014 Published: May 30, 2014 7100

dx.doi.org/10.1021/jp502028d | J. Phys. Chem. B 2014, 118, 7100−7107

The Journal of Physical Chemistry B

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findings help us to understand the process, which could be useful in the bioremediation of Cr(VI)-contaminated wastewater.

investigated samples with that of the standards containing known amounts of the paramagnetic centers.30 The Cr(III) reference sample was prepared by shaking 5 g of the starch with Cr(NO3)3 solution (5 g of Cr·L−1) for 24 h at room temperature and then filtering and drying. The total content of Cr(III) (electron configuration 3d3) determined by AAS in such a prepared standard sample was equal to 5.57 mg of Cr(III) ions per 1 g of starch. The VOSO4·5H2O diluted with diamagnetic K2SO4 (5 × 1019 spins·g−1) was used as a standard for Cr(V) ions exhibiting a 3d1 configuration.30 2.5. Electron Imaging and X-ray Microanalysis. The Cr-loaded starch samples were prepared by mixing 5 g of starch with 22.5 cm3 of Cr(VI) solution (5g·L−1, pH 1 or 2) at 21 °C for 1 week. Filtered and dried samples were embedded in an epoxy resin and then air hardened for 24 h and cut into 10-μmthick slices using an HM355 microtome (Microm International GmbH, Walldorf, Germany). Three slices per sample were randomly chosen from the inner fragments of the resin cast and then covered with a graphite layer using an SCD-050 sputter coater (BAL-TECH, Lichtenstein). The granule cross sections covered with a graphite layer were examined by means of the JSM-6610LV scanning electron microscope (Jeol Ltd., Tokyo, Japan) coupled with energy-dispersive X-ray spectrometry (SDD S-Max80/INCA Energy software, Oxford Instruments Ltd., Abingdon, Oxfordshire, U.K.). The examinations were performed at an acceleration voltage of 20 kV, a working distance of 10 mm, and a typical magnification of 100−2000×. All presented images were obtained with the use of a secondary electron signal. The chromium concentrations (in wt %) in the granule slices were assessed by means of the SEM-EDX technique, taking into account the element profiles collected for solid standard materials, provided by the instrument producer, and a cobalt standard for the electron beam current calibration. The quantitative elemental analysis was performed for nine granule cross sections chosen within the sample obtained at pH 1 and for seven granule cross sections within the one obtained at pH 2. 2.6. Zeta Potential Measurements. Zeta potentials of the native and Cr-loaded potato starch granules suspended in distilled water were examined in the range of pH values from 1 to 10 by means of a Zetasizer Nano S combined with the MPT2 Autotitrator (Malvern Instruments, U.K.).

2. MATERIALS AND METHODS 2.1. Materials. Native potato starch used in the study was manufactured according to a standard procedure.27 The pH values of the chromium(VI) solutions prepared from the potassium dichromate and chromium(III) solutions prepared from chromium(III) nitrate were adjusted by using sulfuric acid and sodium hydroxide. In all experiments, Sigma-Aldrich puregrade chemicals were applied. 2.2. Batch and Kinetics Experiments. Batch experiments were carried out in order to find the influence of the solution pH, contact time, temperature, the Cr(VI) initial concentration, and starch dose with respect to the efficiency of the Cr(VI) ion reduction in aqueous solutions. Samples of the granular potato starch (0.5−7.5 g) were mixed in conical flasks with 25 mL of Cr(VI) solution containing 25−150 mg of Cr·L−1. The pH values adjusted to the levels of 1−11 were controlled with a pH meter. The closed conical flasks were shaken in a water-bath shaker (Elpin 357) at a given temperature (21, 35, or 45 °C) for a certain period of time (0.5−24 h). Then, the suspensions were filtered and washed with 30 mL of distilled water. Starch samples were dried in air and then stored in a desiccator, while the supernatants were kept in falcon tubes for further investigations. In the kinetic experiments 7.5 g of the granular potato starch was shaken in the Cr(VI) solutions (100 mg·L−1) at pH 1 and a temperature of 21, 35, or 45 °C. 2.3. Chromium Analysis. Quantitative measurements of the Cr(VI) content were performed both in the processed aqueous solutions (supernatants) and in the solid samples. In the case of supernatants, the UV−vis colorimetric measurements at wavelength 352 nm were carried out by using the Spectroquant Pharo 300 spectrometer (Merck, Germany). The method was chosen because the Cr(VI) concentrations of the investigated solutions (in the range of 25 mg of Cr·L−1 to 5 g of Cr·L−1) were much higher than recommended for using the 1,5-diphenylcarbazide colorimetric method. The results obtained for the supernatants by both methods differed from each other by less than 5%. The solid samples (0.1 g) were wet mineralized with 20 mL of concentrate HNO3, and the total chromium content was determined by using the Solaar M6 spectrometer (Thermo Scientific, U.S.A) with the standard procedure of the flame atomic absorption spectrometry method (AAS).28 Samples were prepared and analyzed in duplicate. 2.4. EPR Spectroscopy. The starch samples for the EPR measurements were prepared in duplicate by the same procedure as described in section 2.2 but using different quantities of reagents. In this case 5 g of starch was mixed with 22.5 mL of Cr(VI) or Cr(III) solution (5 g of Cr·L−1) at different pH values, and the contact time varied from 0.5 h to 1 week. After the experiments the suspensions were filtered, dried, and stored in a desiccator. The starch samples containing chromium ions were examined by EPR spectroscopy. The measurements were carried out at room temperature (22 ± 1 °C) with a Bruker Elexsys E-500 spectrometer (Karlsruhe, Germany) operating at the X band (9.8 GHz) at modulation amplitude 0.1 mT and microwave power 3 mW. EPR parameters of the paramagnetic chromium species were determined by a simulation procedure using the program EPR SIM 32.29 The number of spins was calculated by comparison of the integral signal intensity of the

3. RESULTS AND DISCUSSION 3.1. Effect of pH. In aqueous solutions chromium(VI) could form hydrochromate HCrO4−, chromate CrO42−, and dichromate Cr2O72− ions. The ion equilibrium is governed by the chromium(VI) concentration and pH of the solution.10 Depending on the Cr(VI) concentration the HCrO4− and Cr2O72− ions could be present in the solution of pH value in the range of 1 to 6.5, whereas the CrO42− ions predominate in solutions with a pH value higher than 6.5. Results of our experiments regarding the effect of pH on the Cr(VI) content in the solutions after contact with the starch are shown in Figure 1. It could be seen that pH 1 was most favorable for lowering the Cr(VI) concentration as only 55% of the initial Cr(VI) amount left in the solutions, whereas at pH ≥4 more than 80% of the Cr(VI) ions remained in the solution. Probably under very acidic conditions (pH 1) the dichromate ions could readily join a positively charged interfacial double layer at the starch granule surface and subsequently undergo the reduction. This hypothesis is corroborated by our results of the zeta potential 7101



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two subsequent steps of the process could be recognized. The first temperature-independent step occurred very fast in the initial 0.5 h, whereas the second one was much slower and noticeably reliant on the processing temperature. The sharp decrease visible in the beginning of all of the curves could be explained by spontaneous Cr anion gathering in the positively charged interfacial solution/granule surface layers and simultaneous ion reduction by the polysaccharide hydroxyl groups. After a longer contact time, the process became controlled by temperature-dependent reduction, which can be seen in Figure 2. The temperature increase resulted in the significant decrease in the number of chromium(VI) ion amounts in the solution. Differences in both the efficiency and the reaction route were especially visible for data collected at 21 and 35 °C. The slopes of the curves obtained at 35 and 45 °C were steeper compared to that for 21 °C. All of the observations pointed to the complexity of interactions between the starch granules and dichromate ions in aqueous solution and the importance of temperature and the contact time for the process effectiveness. The total Cr content in the starch in relation to temperature and the contact time is presented in Figure 3. As could be seen, the number of chromium ions accumulated by the granules enlarged linearly with the contact time (Figure 3a), reaching 16 and 20% after 24 h for 21 and 35 °C, respectively. It could thus be concluded that the higher temperature facilitated the chromium ion incorporation into the starch granules. Quantity of total chromium in the starch as a function of processing time obtained at 45 °C is shown in Figure 3b. In this case an increase (about 15%) in the amount of chromium incorporated into the granules, observed during the first 5 h of the process, was followed by an almost steady level of metal content at the longer contact time (Figure 3b). It should be stressed that lowering the amount of Cr(VI) in the solutions observed for the process duration (Figure 2) did not correspond quantitatively to its total content found in the granules. This difference could be explained by the transformation of some quantity of hexavalent chromium to the trivalent form which remained in the solution. Therefore, it may be concluded that the higher temperature supported the Cr(VI) to Cr(III) reduction (Figure 2), and the sorption of chromium ions by the granules reached equilibrium (Figure 3b). 3.3. Effect of Amount of Starch and Initial Cr(VI) Concentration. Effect of starch dosage on Cr(VI) concentration in the supernatant solutions was demonstrated in Figure

Figure 1. Effect of pH on Cr(VI) concentration in the solution at 21 °C. Conditions: contact time = 2 h, initial Cr(VI) concentration = 100 mg·L−1, volume = 25 mL, starch dose = 2.5 g.

measurements (vide infra) and the data reported for other biosorbents.5,6 3.2. Effect of Temperature and Contact Time. The influence of temperature and the contact time on the Cr(VI) concentration in aqueous solutions after interaction with the starch is presented in Figure 2. This graph indicates that the

Figure 2. Effect of contact time and temperature on the Cr(VI) concentration in the solution. Conditions: pH 1, initial Cr(VI) concentration = 100 mg·L−1, volume = 25 mL, starch dose = 2.5 g.

Figure 3. Effect of contact time on total Cr concentration in starch after the process at 21 and 35 °C (a) and 45 °C (b). Conditions: pH 1, initial Cr(VI) concentration = 100 mg·L−1 (100%), volume = 25 mL, starch dose = 2.5 g. 7102



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4. The dependences obtained at all examined temperatures showed a similar course. A greater amount of starch used as a

Figure 6. Effect of contact time and temperature on Cr(VI) concentration diminishing in solution. Conditions: pH 1, initial Cr(VI) concentration = 100 mg·L−1, volume = 25 mL, starch dose = 7.5 g.

Figure 4. Effect of starch content on Cr(VI) concentration in solution. Conditions: pH 1, contact time = 5 h, volume = 25 mL, Cr(VI) initial concentration = 100 mg·L−1.

biosorbent resulted in a higher efficiency of decreasing the chromium content in the solution. The initial Cr(VI) concentration had no significant effect on the ion residue in the solutions after the processing, as shown in Figure 5. This is mainly visible in the samples with higher

Figure 7. Kinetic curves obtained according to the pseudo-first-order kinetic model.

Figure 8. Arrhenius equation plot.

ln cCr(VI) = ln cCr(VI)in − kt

(1)

where cCr(VI) is the chromium(VI) concentration after the process [mg·L−1], cCr(VI)in is the initial chromium(VI) concentration [mg·L−1], t is the contact time [s], and k is the rate constant [s−1]. The plots of ln cCr(VI) versus contact time for the given temperatures are shown in Figure 7. The calculated values of the appropriate k constants and R2 coefficients are presented in Table 1. The values of the k constant for each temperature were used to estimate the reaction activation energy according to the Arrhenius equation

Figure 5. Effect of initial Cr(VI) concentration on the Cr(VI) content in the processed solutions. Conditions: pH 1, contact time = 5 h, volume = 25 mL, starch dose = 2.5 or 7.5 g.

dosages of starch (i.e., 7.5 g). Probably in the studied range of the Cr(VI) concentrations the used amount of starch provides a sufficient number of active sites participating in the hexavalent chromium reduction. However, in the case of a smaller starch quantity (i.e., 2.5 g) the influence of temperature on the process efficiency was more pronounced. 3.4. Kinetic Results. Results of the kinetic experiments are shown in Figures 6−8. It was assumed in the study that the Cr(VI)−starch reaction occurs according to an adsorptioncoupled reduction mechanism.5 The application of a large amount of the starch (7.5 g) allowed us to take its concentration as constant and justified the assumption of a pseudo-first-order reaction. The fitted model follows the equation

Table 1. Values of k Constants and R2 Calculated According to the First-Order Kinetic Model

7103

T [°C]

k [s−1]

R2

21 35 45

6.64 × 10−5 1.20 × 10−4 2.10 × 10−4

0.97 0.87 0.96


The Journal of Physical Chemistry B ln k = B −

E RT

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significant increase in the Cr(V) signal intensity and the simultaneous broadening of the Cr(III) line (ΔBpp = 51 mT). However, the quantitative data, calculated as the area under the absorption curve, indicated the domination of Cr(III) ions adsorbed in the starch granules (Table 2). The signal of Cr(V) exhibits an anisotropic line (Figure 10) with the g values, equal to gxx = 1.976, gyy = 1.979, gzz = 1.984,

(2)

where B is the intercept and E is the activation energy [J· mol−1]. High values of the R2 coefficient (nearly 0.99) found at 21− 45 °C validate the proposed kinetic model. The E/R coefficient equal to 7.8 × 103 obtained from eq 2 was used to compute the reaction activation energy, and a value of 65 kJ·mol−1 was obtained. Values of the activation energy in the range of 33−75 kJ·mol−1 were reported by other authors as typical of the Cr(VI) to Cr(III) reduction.31−33 Such activation energy together with the temperature-dependent reaction rate corroborates with the assumption that the process occurring in the solution between the starch granules and Cr(VI) ions was in fact a chemical reaction. This conclusion could be supported by the results of the EPR as well as the AAS experiments presented in section 3.5 and Table 2. Table 2. Amouns of Total Chromium, Cr(III) and Cr(V), Determined by AAS and EPR after 1 Week of Contact Time at pH 1 and 2

pH

total amount of chromium per 1 g of starch (determined by AAS)

1

8.96 mg of Cr

2

6.12 mg of Cr

number of Cr(III) ions per 1 g of starch (from EPR spectra)

number of Cr(V) ions per 1 g of starch (from EPR spectra)

10.4 × 1019 Cr(III) ions = 9.01 mg of Cr(III) 6.2 × 1019 Cr(III) ions = 5.36 mg of Cr(III)

0.1 × 1017 Cr(V) centers = 0.001 mg of Cr(V) 5.6 × 1017 Cr(V) centers = 0.05 mg of Cr(V)

Figure 10. EPR spectrum of the starch granules after the accumulation of chromium from the Cr(VI) solution. Inset: HFS lines related to Ayy of the 53Cr isotope. Conditions: pH 2, initial Cr(VI) concentration = 5 g·L−1 (100%), volume = 22.5 mL, starch amount = 5 g.

found by spectrum simulation.29 The main line originated from the nCr isotopes (n = 50, 52, 54; I = 0). Moreover, a partially resolved hyperfine structure (HFS) derived from the 53Cr isotope (nuclear spin I = 3/2, natural abundance = 9.55%) was observed in the EPR spectrum (inset of Figure 10). Among 12 expected hyperfine satellites provided by the 53Cr isotope, only 2 lines assigned to Ayy = 3.87 mT were seen in our experimental spectra. The EPR parameters obtained in the study were similar to those already reported for Cr(V) generated via Cr(VI) reduction by coir pith.35 The quantities of Cr(III) and Cr(V) ions accumulated in the starch granules determined by EPR spectroscopy and atomic absorption spectrometry are presented in Table 2. The total amount of chromium in the starch granules processed at pH 1 was found to be equal to the Cr(III) ion content determined by the EPR method. This indicated that all of the bound chromium occurred in the trivalent form while the total amount of chromium identified in the sample processed at pH 2 was somewhat greater than the detected Cr(III) amount. This could be a sign that under such pH conditions the Cr(VI) → Cr(III) reduction was less effective and a certain number of EPR silent Cr(VI) ions were also present in the starch. In both cases, the number of bound Cr(V) ions was negligible. Our results led to the conclusion that in the studied system the hydroxyl groups of the glucose units (St−OH, where St denotes starch) play a major role as Cr(VI) reducers and undergo oxidation to carbonyl or carboxyl functional groups according to the scheme:

3.5. EPR Results. The EPR spectra of the processed starch granules were collected in order to recognize the paramagnetic chromium species possibly appearing as a result of the Cr(VI) reduction. In the spectrum of the sample treated at pH 1 for 1 week (Figure 9), two signalsa broad one (ΔBpp = 44 mT) with a g factor equal to 1.988 and a sharp line (ΔBpp = 1.4 mT) with g = 1.978were observed.

Figure 9. EPR spectrum of the starch granules after the accumulation of chromium from Cr(VI) solution at pH 1 and 2. Conditions: initial Cr(VI) concentration = 5 g·L−1 (100%), volume = 22.5 mL, starch dose = 5 g, contact time = 1 week.

On the basis of the EPR parameters the first signal was assigned to Cr(III) while the second one was assigned to Cr(V) ions.34 The presence of both signals confirmed our hypothesis that the interactions of Cr(VI) ions with starch granules resulted in chromium reduction, most probably via the adsorption-coupled reduction mechanism postulated by Park et al.5 The signals of the same ions were also observed in the EPR spectra of the sample treated at pH 2 (Figure 9). In this case, the line related to Cr(III) was hardly visible due to the

St−OH → St−CHO → St−COOH

(3)

At the same time the dichromate ions act in line with their oxidizing properties, which are especially significant at pH 1: Cr2O7 2 − + 14H+ + 6e− = 2Cr 3 + + 7H 2O

(4)

The reduction of Cr(VI) to Cr(III) proceeds as a three-electron redox reaction, whereas the formation of Cr(V) requires the 7104



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transfer of one electron only. The formation of the Cr(IV) transient species by two electron transfers is also possible,36 but these species are less stable than the Cr(V) species and in aqueous media undergo the following reactions: Cr(IV) + Cr(VI) → 2Cr(V)

(5)

2Cr(IV) → Cr(V) + Cr(III)

(6)

Thus, the Cr(III) and Cr(V) paramagnetic species, detected by EPR, were recognized as the final stable products of the processes taking place in the aqueous suspensions of dichromate ions and potato starch granules. The latter ions could be stabilized in the polysaccharide structure by their oxygen atoms. Numerous Cr(V) complexes with oxygencontaining ligands have already been isolated. An important role in the stabilization of Cr(V) species is played by hydroxy acids37 and diol ligands.36 The results of the quantitative determination of the spin numbers related to the particular chromium species present in the starch granules are shown in Figure 11.

Figure 12. Sorption of the Cr(III) ions generated by the reduction of Cr2O72− or supplied directly from Cr(NO3)3 solution on starch granules at pH 1.

created in statu nascendi by the reduction of K2Cr2O7 were accumulated by the granules in a greater quantity than those that originated from the chromium nitrate solution. The content of chromium ions newly reduced at pH 1 linearly increased, while that of another type decreased with sorption time. These observations could be explained on the basis of the Cr(III) ion hydration status in the aqueous solution. Those originating from the dissolving salt are present in the form of hexaaqua complexes ([Cr(H2O)6]3+)38 and exhibit the most stable hydration structure among the other transition metals.39 However, the freshly formed Cr(III) ions are free of coordinated water molecules and so could be easily attached to the starch granule surface. Our observations allowed for the conclusion that the Cr(VI) → Cr(III) reduction was an important step in the hexavalent chromium elimination from aqueous solutions. The reaction delivered not only the “bare”, nonhydrated, dynamic Cr(III) ions but also the new carbonyl and/or carboxyl functional groups, which are known for their formation of complexes with transition-metal ions.40,20 3.6. Electron Imaging and X-ray Microanalysis. The starch granule slices obtained in the manner described in section 2.5 were investigated by means of the SEM-EDX method. In the obtained SEM images both the unbroken and sectioned starch granules immobilized in the epoxy resin were visible (Figures 13 and 14). A chemical and morphological examination of the intact and cross-sectioned starch granules was performed. The SEM images (Figure 13) have shown that the reaction of the granules with dichromate ions had a negligible influence on the granule surface structure. A quantitative EDX plot (Figure 14), achieved by the line scanning of the electron beam along and across the 10 randomly chosen granule cross

Figure 11. Number of Cr(III) and Cr(V) ions accumulated in starch granules after the process. Conditions: initial Cr(VI) concentration = 5 g·L−1 (100%), volume = 22.5 mL, starch dose = 5 g, contact time = 24 h.

The largest number of Cr(III) ions was accumulated by the granules at pH 1, while the quantity of Cr(V) bound to the starch was 2 orders of magnitude smaller, irrespective of the pH value. The obtained EPR results together with those of the other experiments presented in the paper support the conclusion that pH 1 was the most favorable for the effective conversion of Cr(VI) to less-toxic Cr(III) ions in aqueous solutions. It was also observed in our study that the quantity of Cr(III) ions incorporated into the starch granules was significantly dependent on the ion origin, i.e., if they were introduced into the solution by the dissolution of nitrate salt or were formed in statu nascendi by the dichromate ion reduction. The appropriate results obtained for solutions of the same initial chromium concentration are presented in Figure 12. As could be seen, the amount of Cr(III) bound to the starch after 30 min of treatment at pH 1 was 2 times greater for the freshly reduced chromium compared to that supplied by chromium(III) nitrate solution. Moreover, significant differences in the sorption ability of two kinds of ions were also noticed during the prolonged treatment. The Cr(III) ions

Figure 13. SEM image of the starch granules after reaction with Cr(VI) ions at pH 1, magnification 1800× (a) and at pH 2, magnification 1400× (b). 7105



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The process, occurring with H3O+ participation, led to the formation of Cr(III) and Cr(V) species (Figures 9−11) and simultaneously the carbonyl or/and carboxyl groups. These groups became new coordination sites for reduced chromium ions, enabling their even distribution in the starch granules as shown by SEM (Figure 14). The calculated activation energy of the process, equal to 65 kJ·mol−1 (Figure 8), indicated that the studied interactions were controlled by a chemical reaction. The quantitative results concerning Cr(VI) in the solutions (Figures 1, 2, and 4−6) and Cr(III) in the solid samples achieved by means of AAS and EPR spectroscopy (Table 2) pointed to the Cr(VI) reduction reactions as a dominant process over the Cr(VI) sorption under the applied experimental conditions. It was also experimentally evidenced that the Cr(III) ions generated in statu nascendi by the Cr(VI) reduction are exceptionally active in coordinating to the starch functional groups compared to those delivered by dissolving of the chromium(III) salts (Figure 12).

Figure 14. Exemplary SEM image (magnification 1400×) of the starch granule cross section with a marked line of the electron beam scan and the appropriate linear chromium distribution.

sections, revealed the even distribution of chromium in the interiors of the processed granules. In order to determine the amount of chromium in the granules, the X-ray spectra were collected from the rectangular areas selected from each granule cross section. The average chromium concentrations assessed by the SEM-EDX method were approximately 0.7 ± 0.1 and 0.5 ± 0.1 wt % for pH 1 and 2, respectively. The results remain in good agreement with those obtained by our AAS and EPR measurements (Table 2). 3.7. Starch Granule Zeta Potential Measurements. The results of the granule surface zeta potential measurements are shown in Figure 15. The similar influence of the solution pH,

4. CONCLUSIONS The presented study has shown that Cr(VI) anions could interact with the granules of potato starch in an aqueous environment, which resulted in the considerable lowering of the hazardous ion concentration in the solution. It was experimentally evidenced that the reduction of Cr(VI) to Cr(III) and Cr(V) ions dominated the anion sorption, which is contrary to the published data on this subject. It was documented for the first time that the Cr(III) ions generated in statu nascendi by the Cr(VI) ion reduction were coordinated in a larger quantity to the active sites of potato starch granules compared to those ions supplied by dissolving the Cr(III) salts. Additionally, chromium was found to be uniformly distributed in the starch granule interior. The obtained results could be useful in the low-cost bioremediation of Cr(VI)-contaminated industrial effluents.

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AUTHOR INFORMATION

Corresponding Author

Figure 15. Granule zeta potential in relation to the solution pH for native and Cr-loaded potato starch.

*Phone: (+48 12) 663 2918, (+48 12) 663 2074. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

that is, the increase in the zeta potential with the lowering of the pH, was observed in all of the samples despite the differences in their treatment. In neutral and alkaline environments, the granule zeta potential was found to be negative. However, in an acidic medium (pH 3 for native and pH 3.5 for Cr-loaded starch) these values became positive due to the surface binding of H3O+ ions present in excess in the solution. Under such conditions the negative dichromate ions could be easily attached to the granule surface and thus could undergo the reduction by the starch hydroxyl/carbonyl groups. This is in accordance with our observations that the Cr(VI) → Cr(III) reduction occurred mainly in solutions with low pH values (Figures 1 and 11). 3.8. Reaction Mechanism. The experimental data obtained in the study allowed for the following elucidation of the interactions between the Cr(VI) ions and potato starch granules in an aqueous environment. In a highly acidic medium (pH 1), the positively charged surfaces of the starch granules (confirmed by the zeta potential measurements, Figure 15) could readily attach the dichromate anions which are simultaneously reduced by the polysaccharide hydroxyl groups.

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ACKNOWLEDGMENTS We thank the Laboratory of Analytical Atomic Spectrometry (Faculty of Chemistry, Jagiellonian University) for performing the AAS measurements and students Ewa Nawolska and Natalia Ogrodowicz for their help with some experimental work. Some of the research was carried out with equipment purchased via the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (ATOMIN, contract no. POIG.02.01.00-12-023/08).

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REFERENCES

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Interactions of Chromium Ions with Starch Granules in an Aqueous

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