Ferrate(VI) and ferrate(V) oxidation of cyanide, thiocyanate, and copper(I) cyanide

ARTICLE IN PRESS

Radiation Physics and Chemistry 77 (2008) 761–767 www.elsevier.com/locate/radphyschem

Ferrate(VI) and ferrate(V) oxidation of cyanide, thiocyanate, and copper(I) cyanide Virender K. Sharmaa,, Ria A. Yngarda, Diane E. Cabellib, J. Clayton Bauma a

Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901, USA b Department of Chemistry, Brookhaven National Laboratory, Long Island, Upton, NY 11973, USA Received 18 January 2007; accepted 16 November 2007

Abstract Cyanide (CN), thiocyanate (SCN), and copper(I) cyanide (Cu(CN)3 4 ) are common constituents in the wastes of many industrial processes such as metal finishing and gold mining, and their treatment is required before the safe discharge of effluent. The oxidation of VI 2 V 3 CN, SCN, and Cu(CN)3 4 by ferrate(VI) (Fe O4 ; Fe(VI)) and ferrate(V) (Fe O4 ; Fe(V)) has been studied using stopped-flow and premix pulse radiolysis techniques. The rate laws for the oxidation of cyanides were found to be first-order with respect to each reactant. The second-order rate constants decreased with increasing pH because the deprotonated species, FeO2 4 , is less reactive than the 3 5  protonated Fe(VI) species, HFeO 4 . Cyanides react 10 –10 times faster with Fe(V) than with Fe(VI). The Fe(V) reaction with CN proceeds by sequential one-electron reductions from Fe(V) to Fe(IV) to Fe(III). However, a two-electron transfer process from Fe(V) to  Fe(III) occurs in the reaction of Fe(V) with SCN and Cu(CN)3 4 . The toxic CN species of cyanide wastes is converted into relatively  non-toxic cyanate (NCO ). Results indicate that Fe(VI) is highly efficient in removing cyanides from electroplating rinse water and gold mill effluent. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ferrate(VI); Ferrate(V); Premix pulse radiolysis; Cyanides; Oxidation; Removal

1. Introduction Cyanide is used or produced in several types of industries that include gas production, metal plating, pharmaceutical, and mining (Young, 2001; Zagury et al., 2004). Speciation of cyanide determines its degree of toxicity, where cyanide exists as free cyanide (HCN, CN) and metal-complex species. Metal-complex cyanides are classified as weak-acid dissociables (WADs) and strong-acid dissociables (SADs). WADs are relatively unstable complexes of cyanide with transition metals such as Cd, Cu, Ni, and Zn that dissociate under neutral or mildly acidic conditions. SADs form strong complexes with metals such as Fe, Co, Ag, and Au that are dissociable only under very acidic conditions. Thiocyanate (SCN) is considered in its own category although it is a WAD. The free cyanide form is the most toxic form. WADs are less toxic than free cyanide while Corresponding author. Tel.: +1 321 674 7310; fax: +1 321 674 8951.

E-mail address: vsharma@fit.edu (V.K. Sharma). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.11.004

SADs are relatively non-toxic (Wild et al., 1994; Shirfin et al., 1996). Various treatment procedures such as physical, adsorption, complexation, and oxidation are known for treating cyanides (Rowley and Otto, 1980; Gurol and Holden, 1988; Beattie and Polyblanc, 1995; Pak and Chang, 1997; Young, 2001; Zagury et al., 2004). The methods other than oxidation give highly concentrated products in which toxic cyanides still exist. Chlorine, hypochlorite, hydrogen peroxide, ozone, sulfur dioxide, and ammonium bisulfite are common oxidants for cyanide destruction. Alkaline chlorination, although adequate, has many disadvantages such as high chemical cost, formation of cyanogens, chloride contamination and incomplete decomposition of some metal cyanide complexes (Young, 2001). Hydrogen peroxide is successful in oxidizing cyanides but it is not effective for SCN. Ozone has been studied extensively for the destruction of cyanide (Rowley and Otto, 1980; Gurol and Holden, 1988). The pH of the cyanide solution must be above pH 11 to avoid the formation of hydrocyanic gas.

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However, ozonation becomes less efficient at pH higher than 11 because ozone is decomposed by hydroxide ion. Both sulfur dioxide and bisulfite have been applied to treat cyanide. These procedures are most efficient at pH 9 and lime is added for pH control of the reactions, but this produces gypsum, generating sludge. Alternate methods, addressing environmental considerations, are needed for the remediation of cyanides in wastewater (Young, 2001). Ferrate(VI) (FeVIO2 4 ; Fe(VI)) is a suitable candidate as a new chemical oxidant for the treatment of cyanides. Ferrate(VI) is a strong oxidizing agent, which can be seen from the reduction potentials of reactions (1) and (2) in acidic and alkaline solutions, respectively (Wood, 1958). FeO4 2 þ 8Hþ þ 3e ! Fe3þ þ 4H2 O

E1 ¼ þ2:20 V (1)

FeO4 2 þ 4H2 O þ 3e ! FeðOHÞ3 þ 5OH

E1 ¼ þ0:72 V (2)

The spontaneous reduction of ferrate(VI) in water forms non-toxic by-products, molecular oxygen and iron(III) (Eq. (3)) (Sharma, 2002a), suggesting that ferrate(VI) is an environment-friendly oxidant. 2FeO4 2 þ 5H2 O ! 2FeðOHÞ3 þ 32O2 þ 4OH

(3)

Ferric(III) hydroxide, produced from ferrate(VI), acts as a coagulant for the removal of metals, non-metals, radionuclides, and organics (Potts and Churchwell, 1994; Sharma, 2002a). Ferrate(VI) has been proposed as an alternative to chlorine for the disinfection of water and wastewater since it is also very effective in inactivating bacteria and viruses (Sharma, 2002a, 2007b; Sharma et al., 2005a). Ferrate(VI) can oxidize a large number of inorganic and organic pollutants in the aquatic environment. Studies have shown that oxidation of nitrogen- and sulfur-containing pollutants by ferrate(VI) can be accomplished in seconds to minutes with the formation of less harmful

products (Sharma, 2002a, 2004). Ferrate(VI) also has the potential to oxidize emerging contaminants such as pharmaceuticals, hormones, and endocrine disrupting compounds (Sharma et al., 2006; Sharma, 2007a). Studies of ferrate(VI) remediation of cyanides are presented below, followed by a discussion of the ferrate-cyanide reaction stoichiometries and mechanisms involving both ferrate(VI) and ferrate(V).

2. Cyanide removal The gold mining and metal finishing industries are among the largest consumers of cyanide due to the high affinity of gold for cyanide. Cyanide is used in more than 85% of the gold mine operations for the extraction of gold from ores, after which the cyanides are released into the environment as effluents and as solid mine tailing. Each year, more than 1 billion tons of gold ore are leached with cyanide. There is an increasing risk to the environment from spills such as those at Baia Mare (Romania), Kumtor (Kyrgyzstan), Omai (Guyana), and Summitville (Colorado) (Boening and Chew, 1999; Beebe, 2001). Effective treatment of effluents must take place in order to achieve a zero discharge level of undesirable contaminants. The metal finishing industry defines the term zero discharge as no measurable cyanide in effluent wastewater and no environmental impact resulting from the discharged wastewater. According to the US EPA, the Drinking Water Quality Standard total cyanide is 0.2 ppm. Comparatively, the Aquatic-Biota Water Standard is 0.05 ppm. Experiments on the removal of free cyanide from Corpus Christi Army depot rinse water by Fe(VI) were conducted in diluted rinse water (Sharma et al., 1998). The results are shown in Fig. 1A and are compared with cyanide removal by Fe(VI) in deionized water at pH 9.0 (the control). It was found that an excess of 2.5 times the molar amount of

80

250 Control Rinse Water

60 [Cu(CN)43-], μM

[Cyanide]residual, μM

200 150 100

40

20

50 0

0 0

200

400 600 [Fe(VI)], μM

800

0

200 400 [Fe(VI)], μM

600

Fig. 1. Removal of cyanide from rinse water, pH 8.6 (A) and gold mine wastewater, pH 9.0 (B). The rinse water was collected from a metal electroplating facility, which usually contains cadmium, copper, iron, and silver in the concentration range varying from 20 to 200  106 M. The control was cyanide in deionized water at pH 9.0 (Sharma et al., 1998). A synthetic gold mine wastewater sample was prepared by adding salts of KSCN, KCN, and K3Cu(CN)4 to filtered water and adjusting the pH of the sample to 9.1 (Sharma et al., 2005b).

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106

105

103

102 Cyanide 101 Thiocyanate

100

10-1 7

8

10 pH

9

11

12

13

Fig. 2. Dependence of second-order rate constants on pH for oxidation of cyanides by Fe(VI).

3. Ferrate(VI)

1.0

d½FeðVIÞ ¼ k½FeðVIÞ½P, (4) dt where [Fe(VI)] and [P] are the concentrations of Fe(VI) and cyanide, and k is the overall reaction rate constant (Sharma et al., 1998, 2002, 2005b). The reactions of Fe(VI) with the cyanides were found to be first order for each reactant. The reaction rate constants were determined as a function of pH, and the rate of the reaction increases with a decrease in pH (Fig. 2). The pH dependence was interpreted based on the protonation of FeVIO2 4 (Eq. (5)) (Sharma et al., 2001a). 2 þ HFeO 4 3H þ FeO4

pK a;HFeO4 ¼ 7:23 at 25  C

(5)

The fractions of the two species (ai) are given by Eq. (6) and are shown in Fig. 3. þ

½H  ; ½Hþ  þ K a;HFeO4

aðFeO2 4 Þ ¼

K a;HFeO4 . ½Hþ  þ K a;HFeO4

(6) 

Cyanide also has two forms, HCN and CN (Eq. (7)), and both can react with the two forms of Fe(VI) (Eqs. (8)–(11)) (Verhoeven et al., 1990). HCN3Hþ þ CN

pK ¼ 9:3

(7)

HFeVI O4  þ HCN ! Products

(8)

HFeVI O4  þ CN ! Products

(9)

-

CN-

HFeO4

0.8 Fraction of Species

The rate expression for the oxidation of cyanides (HCN and CN, SCN, and Cu(CN)3 4 ) by Fe(VI) can be expressed as

aðHFeO 4Þ ¼

Copper(I) Cyanide

104

k, M-1s-1

Fe(VI) was required to oxidize all the cyanide in the rinse water. This is not surprising due to competing reactions between Fe(VI) with cyanide, complexed cyanide, and other organic and inorganic components present in the rinse water. Removal of copper(I) cyanide by Fe(VI) was measured (Sharma et al., 2005b) in a synthetic sample representing the characteristics of gold mine wastewater (Rowley and Otto, 1980). Increasing amounts of Fe(VI) were added and the remaining copper(I) cyanide was determined after the completion of the reaction. Total removal of copper(I) cyanide was achieved (Fig. 1B). The slope of the linear fit to the plot was 0.1970.01, which is similar to the observed stoichiometric ratio of Cu(CN)3 4 to Fe(VI) in water. This is not surprising because Fe(VI) reacts most rapidly with Cu(CN)3 4 among the constituents of synthetic gold mine wastewater. This reaction should not be affected by the presence of other cyanides and thiocyanate in the wastewater. Cyanate (NCO) was determined to be the major product of the Fe(VI) reaction in the wastewater. This is in agreement with the oxidation product of cyanides by Fe(VI) found in other experiments.

763

0.6 pKa,HCN =9.30

pKa,HFeO4 = 7.23 0.4

2-

0.2

HCN

FeO4

0.0 5

6

7

8

9

10

11

12

pH Fig. 3. Speciation of Fe(VI) and cyanide as a function of pH.

FeVI O4 2 þ HCN ! Products

(10)

FeVI O4 2 þ CN ! Products

(11)

Interpretation of the kinetic data was carried out using the equilibria for HFeO 4 and HCN. However, the data could be fitted by using only the protonated cyanide 2 (HCN) reactions with HFeO (Eqs. (8) and 4 and FeO4 (10)) (Sharma et al., 1998). The reactions of CN with the two forms of Fe(VI) did not contribute to the overall rate constants at different pH. The rate constants determined for reactions (8) and (10) are given in Table 1. Thiocyanate and copper(I) cyanides have no protonation in the alkaline pH range; therefore, the pH dependence of

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the rates is related to the difference in the rates of oxidation of SCN and Cu(CN)3 with the two species of Fe(VI) 4 (Eqs. (12)–(15)).

The stoichiometry of the Fe(VI) oxidation of cyanides was determined at pH 9.0. Stoichiometric Fe(VI):cyanide ratios for the reactions were obtained as 1.1870.04, 4.270.3, and 5.370.8 for CN, SCN, and Cu(CN)3 4 , respectively. The stoichiometric ratio of Fe(VI) to cyanate (NCO), one of the products of the reactions, was determined as 0.8370.03, 3.8570.29, and 1.1970.07 for the oxidation of CN, SCN, and Cu(CN)3 4 , respectively. The reaction of SCN with Fe(VI) gave sulfate (SO2 4 ) as another product of the reaction. The decrease in the SCN concentration was nearly equal to the amount of sulfate formed. The final product of Fe(VI) was Fe(III); ferric hydroxide precipitated after the reaction. The stoichiometries and reaction products suggested the net reactions of cyanides with Fe(VI) are

HFeVI O4  þ SCN ! Products

HFeVI O4  þ CN þ H2 O ! FeðOHÞ3 þ NCO þ OH

Table 1 Rate constants for oxidation of cyanide by Fe(VI) at 15 1C (Sharma et al., 2005b) Pollutant

Reaction

k (M1 s1)

Cyanide

HFeO 4 +HCN  FeO2 4 +CN

9.4970.02  102 4.3770.01  102

Thiocyanate

 HFeO 4 +SCN

2.0670.04  103

Copper(I) cyanide

3 HFeO 4 +Cu(CN)4 3 FeO2 4 +Cu(CN)4

8.8070.90  107 0.5970.06  104

VI

Fe O4

2



þ SCN ! Products

(12) (13)

þ 14O2

(16)

4HFeVI O4  þ SCN þ 5H2 O ! 4FeðOHÞ3 þ SO4 2 þ NCO

HFeVI O4  þ CuðCNÞ4 3 ! Products

(14)

FeVI O4 2 þ CuðCNÞ4 3 ! Products

(15)

In fitting the kinetic data for the oxidation of thiocyanate by Fe(VI), HFeVIO 4 was found to be the only reactive species (Table 1). Similar results were obtained for the oxidation of copper(I) cyanide (Table 1). As demonstrated in Fig. 3, the concentration of the protonated form of Fe(VI) (HFeVIO 4 ) decreases with an increase in pH, and therefore the rate decreases with a decrease in acidity. This is also consistent with the decreased rates for the spontaneous decomposition of Fe(VI) with an increase in pH (Carr et al., 1985). The partial radical character (FeVI ¼ O2FeV–Od) may be proton stabilized and increase the reactivity with cyanides. It has been stated that HFeVIO 4 has a larger spin density on the oxo ligands VI  than FeVIO2 4 increasing the oxidation ability of HFe O4 (Shiota et al., 2003). The reactions of Fe(VI) with cyanides were also studied as a function of temperature (5–45 1C) (Sharma et al., 1998). The activation energies obtained from linear plots of log k versus 1/T (K1) were 38.971.0, 32.772.1, and 17.071.5 kJ mol1, for cyanide, thiocyanate, and copper(I) cyanide, respectively (Sharma, 2003). The reaction rate law and observed rates at different pH values for the oxidation of cyanides by Fe(VI) can be utilized to determine the halflives of the oxidation processes. The rate constants obtained using the rate laws were 1.1270.05  106 M1 s1, 2.1470.10  102 M1 s1 and 4.3270.08  101 M1 s1 at pH 9.0 and 15 1C for Fe(VI) oxidation of Cu(CN)3 4 , cyanide (HCN/CN), and thiocyanate (SCN), respectively. For a concentration of Fe(VI) five-fold higher than the three cyanide substrate concentrations (100 mM), the half-lives for the oxidation by Fe(VI) were 1.8 ms, 9.3 s, and 46.3 s for copper(I) cyanide, cyanide, and thiocyanate, respectively, at pH 9.0 and 15 1C.

þO2 þ 2OH

(17)

5HFeVI O4  þ CuðCNÞ4 3 þ 8H2 O ! 5FeðOHÞ3 þ Cu2þ þ 4NCO þ 32O2 þ 6OH

(18)

As stated earlier, iron(III) hydroxide formed in the reactions is a good coagulant and has been shown to give superior performance over other known coagulants in precipitating metals, non-metals, radionuclides, and organics from solution (Potts and Churchwell, 1994; Sharma et al., 2005b). A study of the removal of Copper from solution in the reaction of Fe(VI) with Cu(CN)3 4 demonstrated a decrease in the copper concentration with an increase in the concentration of Fe(VI) (Sharma et al., 2005b) until complete removal of copper from the solution was achieved. Fe(VI) first oxidizes Cu(CN)3 4 to copper(II), which is subsequently removed from the solution by iron(III) hydroxide. The further reaction of cyanate produced in the oxidation of cyanides (Eqs. (16)–(18)) with Fe(VI) gives nitrite (NO 2 ). The rate constants for nitrite oxidation by Fe(VI) at 25 1C at different pH vary from 1.15  101 M1 s1 at pH 8.05 to 5.8  102 M1 s1 at pH 9.9 (Sharma et al., 1998). The rates for nitrite oxidation by Fe(VI) are approximately 2 orders of magnitude slower than cyanide oxidation rates. Nitrate as a by-product was identified during the oxidation of nitrite by Fe(VI) (McLaughlin, 1984), suggesting that the stoichiometry of the reaction is 2FeO4 2 þ 3NO2  þ 5H2 O ! 2Fe3þ þ 3NO3  þ 10OH (19) Overall, destruction of cyanides with Fe(VI) gives simple by-products that are less harmful to the environment. A free radical mechanism has been proposed for the oxidation of cyanides (CN, SCN and Cu(CN)3 4 ) by FeVIO2 (Sharma et al., 1998, 2002, 2005b). The rate4

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determining step for the reactions is the formation of FeVO3 4 and a radical via a one-electron transfer process (Eqs. (20)–(22)):

CH2OH at near diffusion-controlled rates (Bielski and Thomas, 1987; Sharma, 2002b). Fe(IV) complexes with simple inorganic ligands (P2O4 7 ) have been formed from the oxidation of Fe(III) by dOH/O radical in alkaline solutions (Menton and Bielski, 1990). In performing a study of ferrate(V) with cyanide, a Fe(VI)-cyanide solution was pulsed with radiation in which FeVIO2 4 reacted with a radical to give FeVO3 4 (Eq. (23)) (Sharma et al., 2001b)

HFeVI O4  þ CN þ OH ! HFeV O4 2 þ d CONH (20)

HFeVI O4  þ SCN ! HFeV O4 2 þ d SCN

765

(21)

HFeVI O4  þ CuI ðCNÞ4 3 ! HFeV O4 2 þ CuII ðCNÞ4 2

FeVI O4  þ d CONH2 ! FeV O4 3 þ ProductðsÞ

(22)

k ¼ 2:670:3  109 M1 s1 at pH 9:0

Because of the involvement of ferrate(V) in the reaction mechanism, the reactivity of ferrate(V) with cyanides has been studied independently and the results are summarized below.

V

(23)

O3 4

ions then react with cyanide (Sharma et al., Fe 2001a, b). Spectra obtained in this reaction mixture of Fe(VI)-cyanide following the pulse are shown in Fig. 4A. These spectra show the decay of the Fe(V) (lmax ¼ 380 nm) and concomitant growth of an absorption band (lmax ¼ 430 nm) characteristic of Fe(IV) in NaOH and pyrophosphate (Fig. 4B). The Fe(IV) further reacts with cyanide to give Fe(III) (Fig. 4B). The results with HCN/ CN demonstrate the sequential one-electron reductions of iron(V) to iron(IV) to iron(III) (Eqs. (24) and (25)) (Sharma et al., 2001a).

4. Ferrate(V) The formation of Fe(V) has been observed in hightemperature melts, and the alkali and alkaline derivatives of iron(IV) have also been prepared (Temple and Thickett, 1973; Kopelev, 1997). Both Fe(V) and Fe(IV) have been generated in aqueous solution by pulse radiolysis in which d  Fe(VI) reduces to Fe(V) by radicals such as e aq, CO2 , and

FeV O4 3 þ CN þ OH ! FeðIVÞ þ d CONH

(24)

Fe(IV)

0.02

DELTA O.D.

^

0.01

Fe(V) 0.00

-0.01

300

350

400 450 500 Wavelength, nm

600

0.000

0.02

-0.004

0.01

-0.008 0.00

Fe(IV) -0.012 0.0

0.5

1.0 1.5 Time, ms

2.0

DELTA O.D.440nm

0.03 Fe(V)

DELTA O.D.300 nm

DELTA O.D.440 nm

0.004

550

0.02

Fe(IV)

0.01 0.00 -0.01 0.000 0.005 0.010 0.015 0.020 Time, sec

Fig. 4. Spectra (A) and traces (B) obtained upon pulse radiolysis of solutions during Fe(V) oxidation of cyanides. Experimental conditions: [Fe(VI)] ¼ 1.00  104 M, [Cyanide] ¼ 9.60  103 M; pH 10.9 (Sharma et al., 2001a, b, c).

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FeðIVÞ þ CN þ OH ! FeðIIIÞ þ d CONH

(25)

The reaction rates of Fe(V) and Fe(IV) with cyanides were determined by measuring the disappearance of Fe(V) and Fe(IV) under pseudo-order conditions in which the cyanide concentrations were in excess. The plots of the pseudo-first-order rate constants versus cyanide concentrations were found to be linear, and the second-order rate constants for the reactions of Fe(V) and Fe(IV) with cyanide were obtained as 1.870.12  105 and 1.4170.20  103 M1 s1, respectively, at pH 10.9 (Sharma et al., 2001b). Comparatively, the rate constant for the reactivity of Fe(VI) with cyanide is 7  100 M1 s1 at pH 10.9. Fe(V) and Fe(IV) are thus approximately 4 and 2 orders of magnitude more reactive than Fe(VI), respectively.

0.010 1.18 ms 4.72 ms 11.24 ms

0.005

ΔO.D.

0.000

-0.005

-0.010

-0.015

-0.020 300

350

400

450

500

550

600

λ, nm 0.025

The kinetic traces and spectral studies of the thiocyanate reaction with Fe(V) using pulse radiolysis are shown in Fig. 5. The spectra display the decay of Fe(V) during the reaction. The characteristic spectrum of Fe(IV) was not observed as was found in the reaction of Fe(V) with cyanide (see Fig. 4). This suggests that the reaction of Fe(V) with thiocyanate proceeds via a concerted twoelectron oxidation, which converts Fe(V) to Fe(III) (Sharma et al., 2004). Similarly, the reaction of Fe(V) with CuI(CN)3 4 did not give the one-electron reduction species, Fe(IV), and the reduction of Fe(V) to Fe(III) therefore occurs by a two-electron step (Sharma et al., 2005b). 5. Conclusions Fe(VI) oxidation of free cyanide and complexed cyanide occurs rapidly in seconds to minutes. Comparatively, most current treatment processes, such as alkaline chlorination, sulfur-based treatments, hydrogen peroxide oxidation, and ozonation require much longer time (Gurol and Bremen, 1985; Young, 2001). Products of the reactions are cyanate, sulfate, Fe(III), and molecular oxygen, without harmful residuals or by-products that are associated with other treatment technologies. Therefore, Fe(VI) is a suitable candidate as a new chemical oxidant for the metal finishing and gold mining industry. The reduction of Fe(V) by CN involves sequential oneelectron reductions of Fe(V) to Fe(IV) to Fe(III) in aqueous media with a reactivity in the order k(Fe(V))4k (Fe(IV))4k(Fe(VI)). A study of the oxidation of pollutants by Fe(IV) can be carried out by using a stopped-flow spectrophotometer on-line with a Van de Graff accelerator. The solutions containing Fe(VI) and cyanide can pass through the electron beam to generate Fe(IV) in isolation prior to mixing with the solution of pollutant in the stopped-flow spectrophotometer. The radiolysis of water results in the formation of the hydrated electron, and a conduction band electron is formed in the photocatalytic processes (Sharma et al., 2001c; Sharma and Chenay, 2005). The use of ionizing radiation and the photocatalytic technique (Sharma, 2002b) in the presence of Fe(VI) would therefore form Fe(V) and may have synergic effects in the oxidation of pollutants in the aquatic environment.

O.D.300nm

0.020

Acknowledgments

0.015 0.010

Pulse radiolysis studies were carried out at the Center for Radiation Chemical Research at BNL and supported by the US Department of Energy (DE-AC02-98CH10086).

0.005 0.000 -0.005 0.000

References 0.005

0.010

0.015

0.020

Time, 10-3s Fig. 5. Spectra (A) and trace (B) obtained upon pulse radiolysis of solutions during Fe(V) oxidation of SCN. Experimental conditions: [Fe(VI)]=1.00  104 M, [Thiocyanate]=1  103 M; pH 10.7 (Sharma et al., 2004).

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