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Aug 30, 2012 - aqueous solutions of both QPP and QSA were stable for over a period of 1 year. Quercetin and these derivatives were subsequently utiliz...
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Reduction of Hexavalent Chromium Using Naturally-Derived Flavonoids Veronica A. Okello, Samuel Mwilu, Naumih Noah, Ailing Zhou, Jane Chong, Michael T. Knipfing, David Doetschman, and Omowunmi A. Sadik* Department of Chemistry Center for Advanced Sensors & Environmental Systems (CASE), State University of New York at Binghamton, P.O Box 6000 Binghamton, New York 13902, United States S Supporting Information *

ABSTRACT: Quercetin is a naturally occurring flavonoid that is known to form complexes with metals; a process that reduces the environmental availability of toxic metals such as chromium. We hereby report the first evidence of the removal of Cr(VI) from environmental samples using quercetin (QCR) and two synthetic derivatives: namely quercetin pentaphosphate (QPP) and quercetin sulfonic acid (QSA). We successfully synthesized both QPP and QSA using simple procedures while characterizing them with UVvis spectroscopy, H1-NMR, 13C NMR, 31P-NMR, and LC-MS techniques. The solubility of QPP was found to be 840 mg/mL and aqueous solutions of both QPP and QSA were stable for over a period of 1 year. Quercetin and these derivatives were subsequently utilized for the reduction of Cr(VI) and QCR was found to have a higher reduction efficiency of 99.8% (30 min), followed by QPP/ palladium nanoparticles mixture (PdNPs) at 96.5% (60 min), and finally QSA/PdNPs mixtures at 91.7% (60 min). PdNPs catalyst increased the efficiency by ∼36.5% while a change in operating temperature from 25 to 45 °C improved the efficiency by ∼46.8%. Electron paramagnetic resonance spectroscopy was used to confirm the presence of Cr (III) in the reaction products. This reduction approach was validated in environmental (Binghamton University) BU and standard reference material (BRS) soil samples. Results showed that the analysis could be completed within one hour and the efficiency was higher in BU soil than in BRS soil by 16.1%. QPP registered the highest % atom economy of 94.6%. This indicates enhanced performance compared to bioremediation approach that requires several months to achieve about 90% reduction efficiency.

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processes.9 The successful removal of Cr (VI) depends on the formation and stability of Cr (III) precipitates. Several in situ and ex-situ chemical treatment methods have also been reported for the reduction of Cr (VI). These include hydrogen sulfide,10 sodium dithionite (Na2S2O4),11,12 sodium metabisulfite (NaHSO3), calcium metabisulfite (CaHSO3), ferrous sulfate (FeSO 4 ), calciumpolysulfide (CaS 5 ), 13 Iron(II) (Fe2+),14 zerovalent iron (Fe0),15,16 and tin(II) chloride (SnCl2). At the Hard Chrome Superfund Site in Vancouver, Washington, chromate concentration was reported to have decreased from 900 μg/L to less than 8 μg/L,11 and from 4500 μg/L to less than 20 μg/L17 respectively. However, sodium dithionate may undergo hazardous decomposition, condensation or polymerization, reacting violently with water to emit toxic gases that can cause skin and respiratory tract irritation.17 On the other hand, chronic inhalation of excessive concen-

INTRODUCTION Chromium can exist in various oxidation states from 0 to VI with Cr(III) and Cr(VI) being the most prevalent. Chromium is used in many industrial applications such as pigmentation, textile dyeing, chrome plating, wood preservation, industrial pulp, and tanning among others. The effluent resulting from these processes, if not well treated, will contain a high amount of hexavalent chromium, Cr(VI).1,2 In particular, the tanning process generates about 30%-40% of Cr pollutant to the environment.3,4 A number of methods have been employed to remove toxic metal ions from aqueous solutions. These include biological processes, chemical precipitation, ion exchange, reverse osmosis, membrane processes, evaporation, solvent extraction and adsorption.1,5,6 Of these, chemical precipitation is the most commonly used method. Many parameters affect the process of chemical precipitation. These include the type of precipitating agent, the pH, the velocity of precipitation, sludge volume, time of mixing, and nature of complexing agents.1,7,8 Biological processes, on the other hand, require several days for effective treatment to be achieved. In recent studies, nanoparticlesespecially palladium nanoparticles (PdNPs)have been used to speed up the rate of biological remediation © 2012 American Chemical Society

Received: Revised: Accepted: Published: 10743

March 23, 2012 July 2, 2012 August 30, 2012 August 30, 2012 dx.doi.org/10.1021/es301060q | Environ. Sci. Technol. 2012, 46, 10743−10751

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recorded on AC600 at 121.5 MHz using DMSO-d6 and chloroform-d6 solvent with respect to 85% HPO3 external reference. Other instruments are as follow: Labconco Protector Atmosphere controlled Glove Box from Labconco Corporation, Laboratory stirrer/hot plate from Corning PC-420D, Mexico. Combiflash Companion Flash Chromatography System (Teledyne Isco combiflash) equipped with a prepacked silica column which was used for purification of QPP intermediate. Electron Paramagnetic Resonance spectrometer (Bruker EleXsys E680 EPR spectrometer) was used for confirmation of Cr(III) in reaction product. Synthesis of QSA. The synthesis of QSA followed previously reported procedure but with some modifications, see Supporting Information (SI).29 Synthesis of QPP. The synthesis of QPP followed the procedure described in literature, however in this case, catalytic hydrogenation at ambient temperature under atmospheric pressure of hydrogen was used for debenzylation purposes.30 Synthesis of Palladium Nanoparticles (PdNPs). PdNPs were synthesized as reported in literature31 and were stored in the refrigerator at 4 °C awaiting further use. Reaction of Cr (VI) with QCR, QPP, and QSA. The procedure employed by Hosseini et al.28 for the reaction of QCR with Cr(VI) was adapted with some slight modification. Briefly, a working solution of QCR in 1-pentanol was prepared daily due to the rapid degradation of QCR. Both QPP and QSA were dissolved in deionized water. 0.5 M HCl was used to adjust the acidity of the solutions. In each reaction, equal volumes of the reductant and the oxidizing agents were used. To eliminate auto-oxidation of QCR, the reactions involving the reduction of Cr (VI) using QCR were carried under vacuum conditions in a glovebox. Unless otherwise stated, other experiments were carried out under ambient conditions. In general, to determine the extent of Cr(VI) reduction by QPP, QSA, or QCR, different experimental setups were employed. These included (i) a Cr(VI) solution with neither QPP, QSA nor QCR as the control; (ii) a reaction mixture containing QCR only to monitor the effect of pH change; (iii) a blank solution containing HCl only; (vi) a reaction mixture of QPP, QSA, or QCR and Cr(VI) at different acidic strengths. The absorbance spectra were obtained using either an HP UV/ vis spectrophotometer or synergy HT Multi-Mode Microplate Reader, Biotek, VT. Jobs Method of Continuous Variation. The composition of the Cr(III)-quercetin complex was studied through Job’s method of continuous variation using equimolar (2.0 × 10−5 mol/L) solutions of Cr(VI) and quercetin at pH 2.00.32 Similar experiments were carried out for both QSA and QPP.

trations of iron oxide fumes or dusts may result in development of a benign pneumoconiosis.18 We have reported various approaches for removing Cr(VI) using formic acid, sulfur, poly (amic) acid (PAA)/PdNPs19−21 with promising results recorded. However, atmospheric concentrations as low as 32 mg/L of formic acid may be corrosive and sulfur may cause catalyst poisoning.22 We hereby propose the use of quercetin and other flavonoids for safe and effective reduction of Cr(VI). Quercetin (QCR) is a naturally occurring flavonoid that can act as a free radical scavenger when in contact with bioorganic materials.23 The mechanism of oxidation of QCR24 and the intermediates formed, as well as its interaction with a wide range of transition metals have been extensively studied.24 Yet, there are limited studies on the interaction of QCR or its intermediates with hexavalent chromium. The current work is based on the premise that the more water-soluble QCR derivatives should provide a greener approach to removing Cr(VI) from the environment while reducing or completely eliminating the use of organic solvents and in turn exhibiting high % atom economy. Consequently, we report the synthesis of a new water-soluble QCR derivative, hereby referred to as Quercetin pentaphosphate (QPP). We subsequently showed that QCR, QPP and another well-known; water-soluble flavonoid Quercetin sulfonic acid (QSA) can provide the reduction of Cr(VI) from environmental samples. Most studies have reported the interation of QSA with noble metals,25 Cd, Hg, and Pb.26 However known reports of QSA with Cr(VI) have mainly focused on in vitro studies.27 Also, speciation analysis of Cr(III)/Cr(VI) have been reported28 but limited application has been reported due to QCR insolubility in water ( QPP > QSA under ambient conditions. The calculated initial

Figure 1. Characterization of QCR, QPP and QSA. (A) UV-visible spectrum of QSA and QPP in deionized water. QCR in pentanol. Inset: Molecular structure of QSA; R = OH, R* = SO3H, QPP; R = OPO(OH2), R* = H, QPPI; R = OPO(BnO2), R* = H and QCR; R = OH, R* = H. (B) Negative ESI-MS of quercetin-5′- sulfonic acid. Blank was deionized water. (C) H NMR of QSA. 10745

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rate of reduction of Cr(VI) by QSA was 0.147 AU/s, QPP was 0.0795 AU/s and QCR was 0.0515 AU/s. The higher efficiency with QCR with Cr(VI) was attributed to the fact that QCR was predominantly present in the organic phase and therefore it was able to extract more of the Cr(III) into this phase thus driving the reaction further to the formation of the products. Removal of Cr(VI) Using the Flavonoids. The UV/vis spectra of free QSA and QPP have been given in Figure 1. As shown QSA and QPP have two characteristic peaks similar to those of QCR at 257 nm, 366 nm, 266 nm, and 360 nm, respectively, that are due to π → π* transitions. Cr(VI) on the other hand has a characteristic absorbance at 351 nm which nearly overlaps with the cinnamoyl band of both systems. Hence, when Cr(VI) was reacted with either QSA or QPP, there was initial observation of a sharp enhancement of the peak at 351 which gradually decays with time. As is shown in Figure 3A, when Cr(VI) was added to QSA, the absorption peak intensities at 257 and 366 nm decreased with time, whereas a new peak and a band were observed at ∼295 and 450 nm, respectively. They both gradually increased in intensity with time. The decrease in peak intensity at 257 and 366 nm indicated destruction of both cinnamoyl and benzoyl ring systems due to oxidation of the QSA. Also, three isosbestic points were observed at 274, 319, and 408 nm, which provided evidence that a new complexation species was forming during the competitive interaction between the QSA and the Cr(VI). The absorption band at ∼295 nm is the characteristic for the oxidized form of quercetin earlier identified as quinone.33 The second absorption band at ∼450 nm occurs at a range

Figure 2. (A) Stability of aqueous solution of QSA (1.33 × 10−4 M) and QPP (4.09 × 10−3 M) monitored for 1 year. Inset: stability study of QCR aqueous solution. (B) Interaction of QCR (pink), QPP(blue), QSA(green), and Cr(VI) in 0.5 M HCl. The control (red) contains all the solutions except QPP, QSA, or QCR.

Figure 3. UV-vis spectra of (A) QSA 1.33 × 10−4 M reaction with Cr(VI) 3.3 × 10−5 M in the absence of PdNPs and HCl at 25 °C. pH of QSA dissolved in deionized water was 2.5. (B) QPP 1.25 × 10−4 M reaction with Cr(VI) 3.3 × 10−5 M, 0.5 M HCl in the absence of PdNPs at 25 °C. pH of QPP dissolved in deionized water was 6.8. [C] Time based analysis of QCR with Cr(VI) [D] Time based analysis of QPP reaction with acidified Cr(VI) pH at ambient room conditions. Inset QSA reactions (see SI Table S2 and S3 for complete description of experimental conditions 1−4). 10746

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characteristic of wavelength for the complex formed between the flavonoid and the respective metal ion. Similar results were observed for the reaction between QPP and Cr(VI) shown in Figure 3B. However, it was observed that the reduction of Cr(VI) was possible even in the absence of HCl when QSA was used. This was attributed to the fact that the aqueous solution of QSA was acidic with a pH of 2.50. Its corresponding pKa value was 4.07 ± 0.04.29 The reaction of QCR and Cr (VI) in acidic medium also showed similar results. In the aqueous and organic phase extracts, a new peak was observed at 290 and 440 nm (SI Figures S3A and S3B), respectively. This was a clear indication of the formation of new products, that is, quercetin−Cr (III) complex. This reaction was confirmed to be a redox reaction taking place between Cr(VI) and quercetin. The Cr(VI), being a strong oxidizing agent, oxidized quercetin and in turn was reduced to Cr(III). Confirmation experiment was achieved by oxidizing quercetin with hydrogen peroxide before reacting it with Cr(VI) solution. No observable change was noted under this reaction conditions. However no isosbestic points were observed in either of the aqueous or the organic phase products due to the complexity to quickly separate the two phases before analysis. From the results listed in SI Tables S2 and S3, it is evident that doubling the concentration of Cr(VI) produced an increase in the rate of reaction by a factor of 2 indicating that the reaction is a first order with respect to the concentration of Cr(VI). Plots of ln[Cr(VI)] versus time yielded linear graphs (0.955 > r2 < 0.994) with the rate constants as shown in SI Table S4. A comparative study of our method with other reductants is presented in SI Table S4. This result compares well to the existing results in literature.19,21,34−36 In particular, the rate constant obtained for the naturally derived flavonoids were very low with QPP being the lowest at 0.0259 mol−1.L.S1−. However, there was no significant change in the rate of reaction when the concentration of QCR, QPP, or QSA was doubled indicating a zero order dependence with respect to the concentration of the reductants. In the case of QCR reaction, when the concentration of HCl was increased to 1 M, the rate of reaction increased to 1.24 × 10−5 mol/min which is a 14% increase in the reaction rate (Figure 3C). Similar results were obtained for QPP and QSA Figure 3D. From this, HCl can be considered to act as a catalyst in the reaction kinetics. The electron paramagnetic resonance spectroscopy (EPR), being a technique sensitive to paramagnetic molecules with unpaired electrons, was further used to confirm the presence of Cr(III) in the reaction products. This was advantageous in preventing the photoreduction of Cr(VI) unlike XPS technique which required high vacuum conditions. EPR was thus expected to respond to either Cr(III), d3, or Cr(V), d1, but not to Cr(VI), d0. From the results in Figure 4A, it was evident that Cr(III) was present in the reaction products. Xu et al.37 reported that the complex formation in the acidic medium was between the QCR and the Cr(III). However, our studies repudiated this claim after we investigated the reaction between QCR and a series of Cr(III) standard solution using both UV-vis and LCMS techniques under acidic conditions. Therefore, we believe that complexation occurred during charge transfer between QCR and Cr (VI), that is, the later being a strong oxidizing agent, this should readily accept electrons from QCR, thus resulting in complex formation between the oxidized form of QCR (quinone (Q)) and Cr(III). The same principle applies for the reduction of Cr(VI) using either QPP or QSA. Our results were in agreement with those

Figure 4. (A) EPR of the aqueous phase extract of the reaction between QCR and Cr(VI) (B) Job’s method of continuous variation for the reaction of Cr(VI) and QCR; equimolar concentration of 2.0x 10−5 mol/L used, pH 2.0. (C) Negative ion ESI-MS of QCR organic phase in acidic media treated with Cr(VI), inset QCR in pentanol only.

of Hosseini et al.28 which attributes the UV/vis absorption bands observed between the reaction products of quercetin 10747

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Figure 5. (A) Effect of the acidity content on the response of 3.33 × 10−5 M Cr(VI) in the presence of 1.25 × 10−4 M QPP. (B) Temperature dependence exponential decay curve of the concentration of Cr(VI) monitored at 350 nm for 30 min at 25 °C, 30 °C, 35 and 45 °C. 1.5 × 10−3 M Cr(VI) after treatment with 5 × 10−6 M QCR and 0.5 M HCl. (C) UV-vis spectra depicting effect of PdNPs on the rate of reduction of Cr(VI) using QPP [Cr(VI)] 3.3 × 10−5 M, [QPP], [HCl] 0.5M, PdNPs 1.5 mg, temperature 45 °C. (D)] QSA 1.33 × 10−4 M in the presence/absence of PdNPs (1.5 mg) and HCl (0.5 M). * and ** represent reactions carried out at 25 and 45 °C, respectively. (E) UV-vis spectra of (A]) Cr(VI) 3.3 × 10−5 M, (B) Cr(VI) + 0.5 M HCl, (C) 1.33 × 10−4 M QSA + Cr(VI) + HCl t = 0 min, (D) 1.25 × 10−4 M QPP + Cr(VI) + HCl t = 0 min, (E) 1.33 × 10−4 M QSA + Cr(VI) + HCl + PdNPs t = 30 min, (F) 1.25 × 10−4 M QPP + Cr(VI) + HCl + PdNPs t = 30 min, (G) 1.33 × 10−4 M QSA + Cr(VI) + HCl t = 60 min, ([H) 1.25 × 10−4 M QPP + Cr(VI) + HCl t = 60 min, (I) 1.33 × 10−4 M QSA + Cr(VI) + HCl + PdNPs t = 60 min, and (J) 1.25 × 10−4 M QPP + Cr(VI) + HCl + PdNPs t = 60 min. The reactions were carried out at 45 °C.

(Cr·2H2O), whereas the m/z 950.96 represented the dimer for the complex with m/z 475.17. The negative ESI-MS/MS fragments presented in SI Figure S1A and Table S5, revealed the formation of quinone m/z 299. As shown in SI Figure S1C, the formation of quinone was also observed from the structural analysis of the aqueous phase extract. Chelation of QCR in acidic medium has been extensively studied using Al33 and Fe(II) metals38 with reports showing that the ratios 2:1 and 1:1 were the preferred stoichiometry.33 In this work, the Jobs

with Cr(VI) to quercetin intermediates and the newly formed Cr(III). We have used the experimental LCMS spectra (Figure 4C) to generate the possible structures for Q-Cr (III) (SI Figure S2). For QCR-Cr reactions, the results obtained in Figure 4C indicated that the formation of the complex was in the organic phase and not in aqueous phase. Hence the complex isolated could have the following stoichiometry: m/z 475.17 = QCR. 2Cr·4H2O, m/z 688.9 = 2QCR.Cr·2H2O, m/z 776.9 = 2QCR.2 10748

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between QCR, QSA, QPP, and Cr(VI) occurs in an acidic medum. The pH of aqueous QSA even before the addition of HCl was found to be 2.50. The pH varied from ∼0.921 to 2.00. Effect of Temperature. The effect of temperature on the reduction of Cr(VI) was investigated by treating 1.36 × 10−4 M of the Cr(VI) with QCR in 1-pentanol at 25 °C, 30 °C, 35 and 45 °C, respectively. Figure 5 shows the UV-vis spectra obtained. These results indicate that an increase in temperature enhances the reduction of Cr(VI). An increase in temperature from 25 to 45 °C results in a corresponding increase in the rate of reaction and over 98% reduction in the efficiency was achieved by 10 min at 45 °C. Starting with an initial concentration of 1.36 × 10−4 M [Cr(VI)] at t = 0 min, a percentage reduction in efficiency of 36.91% and 86.76% was obtained after 5 min (t5) at 25 and 45 °C respectively. Subsequent rates of reaction were calculated to be 1.0 × 10−5 M·min−1 and 2.36 × 10−5 M·min−1 at 25 and 45 °C, respectively. The absence of a distinctive peak above 45 °C after 30 min was attributed to an almost complete reduction of the Cr(VI), that is, over 99.8% reduction in efficiency was achieved. Consequently, the optimum temperature of the reaction was concluded to be at 45 °C for the reaction of Cr(VI) with either QPP or QSA. Effect of PdNPs. Nanoparticles are used as catalysts in many reactions, for example, polymer production, chemical refineries, biofuels, fast chemical reactions. How well these nanoparticles perform as catalysts for these reactions depend on how well their crystal phases are exposed to the reaction media. The PdNPs that we synthesized had particle sizes ranging from 3 to 20 nm.20 The effects of PdNPs were monitored by analyzing a control with PdNPs only in the absence of the reductants. When 1.5 mg of PdNPs was added to the reaction media containing 3.3 × 10−5 M Cr(VI) and 1.25 × 10−4 M QPP, the efficiency of the reduction increased by 36.7% as shown in Figure 5C and E. Similarly, when 1.5 mg of PdNPs were added to 3.3 × 10−5 M Cr(VI) and 1.33 × 10−4 M QSA, the reduction efficiency also increased by a similar magnitude 36.1% (Figures 5D and E). These results show the ability of PdNPs to catalyze the rapid conversion of Cr(VI) to Cr(III) using QPP or QSA. The amount of PdNPs added (1.5 mg) was previously determined in our experiments involving the reduction of Cr(VI) with Poly (amic) acid.20 The overlaid UV-vis spectrum comparing the reduction efficiency of both QSA and QPP is given in Figure 5E. The overall reduction efficiency of QPP and QSA in the presence of PdNPs at 45 °C over a period of 1 h was calculated using eq 4 and was found to be 96.5% and 91.2% respectively with initial concentration of Cr(VI) being 3.3 × 10−5 M. In both cases studied, there was a 50% increase in reduction efficiency when the temperature was increased from 25 to 45 °C.

method of continuous variation was further used to ascertain the stoichiometry of the reaction products. The corresponding graph of the absorbance at 440 nm and the mole fraction of the metal are shown in Figure 4B. Since the top of the spectrum is not sharp, this indicates the formation of a complex mixture. However, the complexation of both QPP and QSA with Cr (III) ions showed a 3:1 ratio (SI Figure S4). Equations 1−3 describe the general chemistry of the reaction occurring between the flavonoids and Cr (VI). 3QH 2 → 3Q + 6H+ + 6e−

(1)

Cr2O7 2 −(aq) + 14H+ + 6e− → 2Cr 3 +(aq) + 7H 2O(l) (2)

Cr2O7 2 −(aq) + 3QH 2(s) + 8H+ → 2Cr 3 +(aq) + 3Q(s) + 7H 2O

(3)

Where QH2 refers to either QCR, QPP, or QSA, and Q refers to the respective quinone formed. Flavonoids are weak polybasic acids that tend to protonate especially in the basic media. A high pH favors more complex species although this could lead to additional side reaction such as the hydrolysis of the metal ions to form hydroxo-complexes. Although complexation at lower pH is difficult due to the predominant presence of their undissociated form, these complexes with higher coordination number can form in acidic or neutral pH (but rarely in basic media) because transition metals with high oxidation states can readily act as strong oxidants. This explains why the reduction of Cr (VI) by QCR was possible in acidic media. From eqs 1−3, Cr (VI) acts as an electrophile that readily accepts electrons from the flavonoid, it is then reduced to Cr (III) and in the process oxidizes the flavonoid to a quinone, finally resulting in complexation between the Cr(III) and the generated quinone. The efficiency of the reduction of Cr(VI) with the selected flavonoids was further accessed by calculating % atom economy. By considering the molar mass of all the atoms utilized divided by the molar mass of all atoms in the reaction, and multiplied by 100, the % atom economy for QCR, QSA, and QPP was calculated to be 89.8%, 90.8%, and 94.6%, respectively. The higher % atom economy by QPP can be attributed to its high solubility in water. Optimization of the Reduction Conditions for Cr(VI). In order to obtain the effective reduction efficiency of Cr(VI) using either QSA, QPP, or QCR, a number of parameters were investigated, which include concentration, acidity content, temperature and the amount of catalyst. The results obtained are discussed as follows. Acidity Effect. Adjustment of pH is very important in order to optimize the electrostatic surface interactions between the Cr(VI) anionic species and the electron donors such as QCR, QPP, and QSA which enhance the formation of Cr(III) complexes or hydroxides. The complex formation of QCR derivatives with Cr(VI) was investigated at HCl concentration range of 0.1 - 0.6 M. As observed in Figure 5A, the concentration of the residual Cr(VI) decreased with an increase in the acidity up to 0.5 M HCl, 0.3 M HCl, and 0.07 M HCl for QCR, QPP, and QSA, respectively, after which the values leveled off. Therefore, 0.5 M HCl was adapted as the ideal concentration of HCl required to achieve maximum reduction of Cr(VI) under the given experimental conditions. From these results, it is sufficient to conclude that the complex formation

E R = 100

(Co − Ct ) Co

(4)

Time dependence studies were carried out using both QSA and QPP. From the results in SI Figure S5, it was noted that the rate of reaction was higher during the initial 20 min then it gradually leveled off. Therefore, subsequent studies were carried out for 20 min in order to minimize assay time. When the concentration of QSA was varied and the concentration of HCl, Cr(VI) and the amount of PdNPs held constant, the peak at 295 nm increased progressively, indicating that the concentration of quinone formed was 10749

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Figure 6. Histograms showing application of QCR and its derivatives (QPP and QSA) in Cr(VI) remediation of 5 g of soil samples at a temperature of 45 °C . The blank is soil sample with deionized water while the control contains the soil sample with Cr(VI) only, at a temperature of 45 °C.

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dependent on the concentration of the reductant, in this case QSA (SI Figure S6). Environmental Application. BRS and BU soil samples were used to validate the reduction of Cr(VI) with QCR, QPP and QSA. The BRS soil sample was used as is while the BU soil sample was finely grinded before incubating it with Cr(VI) solution. Briefly, 5 g of the soil samples were incubated overnight with 4 mL of 3.3 x10−5 M Cr(VI). After 24 h, 0.5 M of HCl, 4 mL of QPP and 3 mg PdNPs were added and the reaction mixture was placed in a water bath at 45 °C for 1 h while vigorously stirring at 1050 rpm. The resulting products were filtered through 0.2 μm acrodisc filter membrane. The residual Cr(VI) was determined by studying the initial and final absorbance values at 380 nm and the results are as shown in Figure 6. From the results in Figure 6, it is clear that the QCR had the highest % efficiency (95.6% BU soil) followed by QPP (90.5%) and finally QSA (89.1%). Likewise, the presence of PdNPs in the reaction composite increased the overall efficiency by ∼3%. QPP/PdNPs from the BRS soil registered 71.5% compared to 90.5% efficiency in the BU soil, whereas QSA/PdNPs composite registered 64.2% and 89.1% efficiency in BRS and BU soil, respectively. Similar trend was observed with the QCR. The higher % reduction of Cr(VI) in BU soil was attributed to the possible reduction of Cr(VI) by other reducing agents in the soil such as humic acid which is commonly present in soils. The highest % reduction efficiency of QCR could be attributed to its higher hydrophobicity as compared to QPP and QSA thus will have a stronger affinity to Cr(III). The cost per gram of soil for QPP is 2.3e−4 g = 0.0079 $ whereas the cost for QSA is 4.612e−4 $/1 g of soil. Moreover, the synthesis of both QSA and QPP from quercetin as the starting material is very simple, and given that these derivatives are very stable, their use in environmental remediation will be very practical. Future work will focus on comparing the advantages (including cost analysis) of this green remediation approach for Chromium VI compared to conventional methods.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (607) 777-4478; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Science Foundation (DMR 1007900) for funding. Dr. Jürgen Schulte is acknowledged for NMR analysis. The Regional NMR Facility (600 MHz instrument) at Binghamton University is supported by NSF (CHE-0922815).

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REFERENCES

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S Supporting Information *

Additional information on the structural characterization of QCR, QSA and QPP; LC-MS of reaction products and UVvisible spectrum and time dependence analyses data (Figures S1−S4; Tables S1 and S2). This material is available free of charge via Internet at http://pubs.acs.org. 10750

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Environmental Science & Technology

Article

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dx.doi.org/10.1021/es301060q | Environ. Sci. Technol. 2012, 46, 10743−10751