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Solar Energy 82 (2008) 1031–1036

Photocatalytic oxidation of cyanide in aqueous titaniumdioxide suspensions: Effect of ethylenediaminetetraacetate

Khemarath Osathaphan a, Bundhit Chucherdwatanasak a,Pichaya Rachdawong a, Virender K. Sharma b,*

a Department of Environmental Engineering, Chulalongkorn University, Thailandb Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901, USA

Received 15 September 2007; received in revised form 28 February 2008; accepted 30 April 2008Available online 2 June 2008

Communicated by: Associate Editor Gion Calzaferri

Abstract

The kinetics of the photocatalytic oxidation of cyanide in aqueous TiO2 suspensions was investigated as a function of catalyst loading(0.1–5.0 g l�1), air-flow rate (0.2–1.1 l min�1), and the concentration of ethylenediaminetetraacetate, EDTA (0.4–40 mM) at pH 13.0. Thecyanide oxidation rate did not vary with the TiO2 loading while a slight increase in the degradation rate with an increase in the air-flowrate was found. Cyanate (NCO�) was the only product of the cyanide decomposition. The effect of EDTA on the photocatalytic oxida-tion of cyanide was examined at different molar ratios of EDTA to cyanide (0.1–10.5) by keeping the initial cyanide concentration at3.85 mM. EDTA retarded the photocatalytic oxidation of cyanide and the decrease in the oxidation rate was not proportional to themolar ratio of EDTA to cyanide. The first-order rate constant, k (min�1) for the oxidation of cyanide in the presence of EDTA maybe expressed as k = 3.38 � 10�3 � ([EDTA]/[CN�])�0.23. A mechanism of the oxidation of cyanide by a photocatalytic process inabsence and presence of EDTA is presented.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Oxidation; Cyanide; Ethylenediaminetetraacetate; Photocatalysis; TiO2; Degradation

1. Introduction

Cyanide is used or produced in several industries,including gas production, metal plating, pharmaceutical,and mining (Mudder and Botz, 2004; Young, 2001; Zaguryet al., 2004). This has caused the generation of several bil-lions gallons of cyanide wastes in the environment. Goldmining and metal finishing are among the largest industriesfor cyanide consumption. There is an increasing risk to theenvironment from spills such as those at Baia Mare(Romania), Kumtor (Kyrgyzstan), Omai (Guyana), andSummitville (Colorado) (Boening and Chew, 1999; Beebe,

0038-092X/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2008.04.007

* Corresponding author. Tel.: +1 321 674 7310; fax: +1 321 674 8951.E-mail address: vsharma@fit.edu (V.K. Sharma).

2001). Cyanide thus needs treatment before discharginginto the environment. Various treatment procedures suchas physical, adsorption, complexation, and oxidation areknown for treating cyanides (Rowley and Otto, 1980;Gurol and Bremen, 1985; Pak and Chang, 1997; Young,2001; Zagury et al., 2004). The procedures other than oxi-dation give highly concentrated products in which toxiccyanides still exist. The most common method to treat cya-nide is alkaline chlorination. However, improper chlorina-tion of cyanide would give evolution of toxic cyanogenchloride. Chlorination also gives high total dissolved solids(TDS) in the treated water. Comparatively, ferrate as agreen chemical oxidant can address some of the concernsof chlorination in the treatment of cyanides (Sharmaet al., 1998, 2005). Another alternative is the photocatalyticoxidation for wastewater remediation in which the use of

Time, min0 200 400 600

[CN

- ], m

M

0.0

1.0

2.0

3.0

4.0

0.0 g l-1

0.1 g l-1

0.5 g l-1

1.0 g l-1

5.0 g l-1

Fig. 1. Photocatalytic oxidation of cyanide as a function of time atdifferent amounts of Degussa P25 TiO2. (Experimental conditions: pH13.0; air-flow rate = 1.1 l min�1).

1032 K. Osathaphan et al. / Solar Energy 82 (2008) 1031–1036

solar light minimizes the economic burden for practicalapplications (Bahnemann, 2004).

In the past three decades, several studies have demon-strated the use of heterogeneous photocatalytic processes(Chiang et al., 2003; Liu et al., 2007). The degradation ofcyanide in wastewater using titanium dioxide (TiO2) isappealing because of the possible repeated use of anon-toxic catalyst without any significant reduction inphotoreactivity. The photocatalytic oxidation of cyanidehas been studied by several workers (Frank and Bard,1977; Domenech and Peral, 1988; Peral and Domenech,1992; Aguado et al., 2002; Dabrowski et al., 2002;Chiang et al., 2003; Barakat et al., 2004; Kobayashi etal., 2006). However, measurements to see the effect oforganics on the cyanide oxidation are very limited.Organic ligands such as ethylenediaminetetraacetate(EDTA) forms strong complexes with heavy metal ionsand are commonly used in the industrial cyanide platingprocesses. Therefore, wastewater contains both cyanideand EDTA (Cho et al., 2007). No study to determinethe influence of EDTA on the photocatalytic oxidationof cyanide has been made in the literature.

This paper presents the photocatalytic oxidation of cya-nide in aqueous TiO2 suspensions with and without EDTApresent in the reaction mixture. The cyanide oxidation wasstudied at different amounts of TiO2, air-flow rate, andEDTA concentration at pH 13.0. The initial concentrationof cyanide was 3.85 mM and molar ratios of EDTA to cya-nide varied from 0.10 to 10.5. The product of cyanide oxi-dation was also determined. A mechanism for thephotocatalytic oxidation of cyanide is suggested.

2. Materials and methods

Experiments were performed in a borosilicate batchphotoreactor of cylindrical shape having an immersedlow pressure mercury lamp (Philips Cleo 15W). Water cir-culation through a borosilicate jacket cooled the photore-actor. The ports in the upper-section of the photoreactorprovided an opening for gas flow and for sample withdraw-als. Degussa P-25 TiO2 was used as a catalyst in the exper-iments and magnetic stirring gave uniformity in thereacting mixtures.

Solutions were prepared using 0.1 M NaOH. Allreagents were of analytical grade and were used withoutany further purification. Solutions were prepared with de-ionized water that was obtained by passing doubly distilledwater through an 18.2 MX cm Milli-Q water system. Cya-nide solutions were prepared by adding KCN to obtain theCN� concentration of 100 mg l�1, and later the pH of thesolution was adjusted to 13.0 by adding NaOH. Mixturescontaining EDTA and cyanide were prepared by addingcrystals of tetrasodium EDTA to a 100 mg l�1 CN� solu-tion. The pH of the mixed solution was adjusted to 13.0.TiO2 powder was added to the solution and air was bub-bled at atmospheric pressure for 30 min before startingthe irradiation. During the irradiation of the solution, air

was continuously bubbled to achieve a constant amountof oxygen in a particular experiment. Samples were period-ically withdrawn and were immediately filtered through a0.2 lm cellulose nitrate membrane filter. Filtrates wereanalyzed for cyanide and cyanate.

The concentrations of cyanide in the sample were deter-mined by Dionex ICS-2500 ion chromatography equippedwith an ED50 electrochemical detector consisting of a sil-ver working electrode (0.00 V vs Ag/AgCl reference). AnIonPac� AS7 (4 � 250 mm) column was used and Ion-Pac� AG7 (4 � 50 mm) was also used as a guard column.The eluent was an aqueous mixture of sodium acetate(0.5 M), sodium hydroxide (0.1 M), and ethylenediamine(0.5% v/v). The flow rate was 1 ml min�1 and the injectionvolume of the sample was 5 lL. The quantitative determi-nation of cyanate was performed by Dionex ICS-2500 ionchromatography equipped with a conductivity detectorand an ASRS-ULTRA 4 mm suppressor module. An Ion-Pac� AS16 (4 � 250 mm) column with an IonPac� AG16(4 � 50 mm) guard column were used to perform the anal-ysis of cyanate. An aqueous solution of 11 mM KOH wasthe eluent. The flow rate of the eluent was 1 ml min�1. A50 lL volume of the sample was injected into the columnfor analysis.

3. Results

Initially, experiments were conducted to assess the effectof TiO2 loading on the cyanide oxidation rate for a cyanideconcentration (3.6 mM) with catalyst loading varying from0.1 to 5.0 g l�1 (Fig. 1). The results are also compared withno TiO2 in the reaction solution. Fig. 1 clearly demon-strates that when no TiO2 was present in the solution, nosignificant oxidation of cyanide occurred. One of the earli-est studies (Rose and Nanjundiah, 1985; Peral et al., 1990)

K. Osathaphan et al. / Solar Energy 82 (2008) 1031–1036 1033

carried out on cyanide removal also reported almost nooxidation of cyanide in the absence of the photocatalyst.As shown in Fig. 1, the presence of TiO2 in the solutioncaused degradation of cyanide and a decrease of more than99% could be achieved after illumination for 350 min.Interestingly, no significant change in the removal rate ofcyanide occurred by varying the TiO2 loading. The resultsare somewhat similar to previous reports (Peral et al., 1990;Pollema et al., 1992). Only a slight increase in the rate ofcyanide oxidation occurred by increasing the amount ofTiO2 from 0.08 to 0.80 g l�1 (Pollema et al. (1992). The oxi-dation rate did not show any additional increase whenloading of TiO2 increased to 4.0 g l�1. In the present study,optimal oxidation of cyanide was achieved at 0.1 g l�1,hence this loading of TiO2 was chosen for furtherexperiments.

Next, the influence of the oxygen concentration on therate of cyanide oxidation was conducted by bubbling airthrough the solution (Fig. 2A). A complete decompositionof cyanide took place in about 350 min in continuously aer-ated suspensions and only �45% oxidation of cyanidecould occur without air supply under same time period.

Time, min0 100 200 300 400 500 600 700

Con

vers

ion

of C

N -

to C

NO

- (%

)

0

20

40

60

80

100

120Time, min

0 100 200 300 400 500 600 700

[CN

- ], m

M

0.0

1.0

2.0

3.0

4.0

5.0

0.0 l min-1

0.2 l min-1

0.5 l min-1

1.1 l min-1

2.2 l min-1

Fig. 2. Photocatalytic oxidation of cyanide (A) and percentage of cyanideconversion to cyanate (B) as a function of time with different flow rates.(Experimental conditions: pH 13.0; [TiO2] = 0.1 g l�1).

A slight increase in the photocatalytic oxidation of cyanidewith an increase in air or oxygen flow rate was observed(Fig. 2A), similar to earlier studies (Pollema et al., 1992;Dabrowski et al., 2002). However, when the air-flow rateexceeded to 2.2 l min�1, the oxidation rate is decreased.At this flow rate, it was noticed that the air bubbles inthe suspension were so big that it hindered the UV lightpath and thus caused the decrease in cyanide oxidationrate. In the remaining experiments, a 1.1 l air min�1 air-flow rate was therefore used so that the maximum oxida-tion rate could be achieved. As shown in Fig. 2B, cyanate(NCO�) was the only product of photocatalytic oxidationof cyanide and a stoichiometric conversion of CN� toNCO� was observed. Cyanide was fully destructed toNCO�.

Finally, photocatalysis experiments were performed tosee the effect of EDTA on the removal rate of cyanide.The initial concentration of cyanide was 3.6 mM and theinitial EDTA concentration varied from 0.4 to 40.0 mM.As can be seen in Fig. 3, the presence of EDTA in solutiondecreased the removal of cyanide. At the highest concen-tration of EDTA, the removal of cyanide was �50% in350 min – a time at which complete removal of cyanidewas achieved without EDTA (Fig. 3). A reasonably linearrelationship between ln[CN�] and time for the consump-tion of two-thirds of the initial cyanide concentration wasobtained (inset Fig. 3). Values of the first-order rate con-stant, k, obtained at different concentration of EDTA aregiven in Table 1 and are shown as a function of the molarratio of EDTA to cyanide (Fig. 4). Though the kinetics ofcyanide oxidation are complex, the first-order decay of cya-nide has been observed by other workers (Pollema et al.,1992). There was a sharp decrease in the rate of cyanide

Time, min0 200 400 600

[CN

- ]rem

aine

d (%

)

0

20

40

60

80

100

120

0.0 mM0.4 mM1.0 mM5.3 mM10.5 mM

Time, min0 200 400 600

ln[C

N- ] e4

Fig. 3. Percentage of remained cyanide as a function of time during thephotocatalytic oxidation of cyanide with different amounts of addedEDTA. (Experimental conditions: [CN�] = 3.85 mM; [TiO2] = 0.1 g l�1,air-flow rate = 1.1 l min�1).

Table 1Effect of EDTA on the photocatalytic oxidation of cyanide at pH 13.0

EDTA (mM) [EDTA]/[CN�] k, 10�3 min�1

0.0 0.0 6.85 ± 0.411.44 0.4 5.57 ± 0.133.6 1.0 3.44 ± 0.2419.1 5.3 2.32 ± 0.1037.8 10.5 1.97 ± 0.07

[CN�] = 3.6 mM.

[EDTA]/[CN-] 0.0 2.0 4.0 6.0 8.0 10.0 12.0

k,10

-3 m

in-1

2.0

4.0

6.0

8.0

[EDTA]/[CN-]0.0 0.1 1.0 10.0 100.0

k, 1

0-3

min

-1

1.0

10.0

r2 = 0.99

Fig. 4. The first-order rate constant (k) as a function of the molar ratio ofEDTA to cyanide.

1034 K. Osathaphan et al. / Solar Energy 82 (2008) 1031–1036

removal at lower molar ratios, which indicates that EDTAsuccessfully competed with cyanide for oxidizing speciesduring the photocatalytic processes. A separate study hasdemonstrated that EDTA can be photocatalyticallydegraded on TiO2 suspensions (Mansilla et al., 2006).

A strong linear relationship between logk andlog([EDTA]/[CN�]) was found (inset Fig. 4), thereforethe observed rate constant k for the decay of cyanide inthe presence of EDTA may be empirically presented as

k ¼ 3:39� 10�3 � ð½EDTA�=½CN��Þ�0:23Þ ðIÞ

where k has a unit of min�1.

4. Discussion

The photocatalytic oxidation of cyanide by TiO2 photo-catalyst may occur through either direct charge transfer oran indirect pathway (Frank and Bard, 1977; Kogo et al.,1980; Peral et al., 1990; Hidaka et al., 1992; Augugliaroet al., 1997; Young, 1998). In the indirect pathway, the oxi-dation of cyanide is carried out by either the adsorbed ordiffused hydroxyl radical (Serpone et al., 1987). Both heter-ogeneous and homogeneous pathways are presented inReactions (1)–(13). In semiconductor photocatalysis,photo-generated holes ðhþvbÞ, electrons ðe�cbÞ, hydroxyl radi-cals (HO�), superoxide ions ðO��2 Þ, and hydrogen peroxide(H2O2) are generated (Reactions (1), (3), (7) and (12))

TiO2 ! e�cb þ hþvb ð1ÞO2 þH2Oþ 2e�cb ! OH� þHO�2 ð2ÞOH�ads þ hþvb ! HO� ð3ÞHO� þ CN� ! ��OHCN! ��CONH

k4 ¼ 6:9� 109 M�1 s�1 ð4Þ2��CONH! NCO� þ CONH�2 ð5Þ��CONHþO2 ! ��O2CONH ð6Þ��O2CONH! ��O2 þHNCO ð7ÞHNCOþOH� ! H2OþNCO� ð8ÞCN�ads þ hþvb ! CN� ð9Þ2CN� ! ðCNÞ2 ð10ÞðCNÞ2 þ 2H2O! NCO� þ CN� þH2O ð11Þ2��O2 þ 2Hþ ! 2H2O2 ð12Þ2H2O2 ! 2H2OþO2 ð13Þ

Reaction (3) gives the formation of the hydroxyl radicalHO�, which could diffuse into the bulk solution to reactwith the cyanide ion to form the ��CONH radical (Reac-tion (3)). Disproportionation of the radical would yield acyanate ion and formamide (Reaction (5)) (Munoz et al.,2000). In the present study, formamide as one of the prod-ucts was not found and the cyanate ion was the only prod-uct of the photocatalytic oxidation of cyanide (see Fig. 2B).Therefore, the possibility of the occurrence of Reaction (5)could be ruled out. However, in the presence of oxygen, the��CONH radical would form a peroxy radical, ��O2CONH(Reaction (5)) (Munoz et al., 2000). The elimination of theperoxy radical would give cyanic acid (Reaction (6)). AtpH 13.0, cyanic acid can be neutralized by OH� to yieldthe cyanate ion (Reaction (8)). Reactions (6)–(8) suggestthat the oxidation of cyanide would increase with theincrease in the amount of oxygen present, as was foundin this study (see Fig. 2A). However, the increase in oxida-tion rate may also happen because of the elimination of e�cb

due to its reaction with oxygen (Reaction (2)). This reac-tion would reduce the possibility of scavenginge�cb by hþvbðe�cb þ hþvb ! heat=hmÞ and thus increase the pho-tooxidation efficiency of a TiO2 catalyst. If such a possibil-ity exists, formation of cyanate would only occur throughReactions (9)–(11).

Cyanide is oxidized by hþvb to form CN� radicals (Reac-tion (8)). The dimerization of CN� forms cyanogen (Reac-tion (10)), which subsequently hydrolyzes to result incyanate and cyanide ions (Reaction (11)). A combinationof Reactions (9) and (11) [be consistent with the parenthe-ses around reaction numbers] gives one mole of cyanatefrom the consumption of one mole of cyanide in photocat-alytic processes and agrees with the results of this study(Fig. 2B). Overall, whether the formation occurs throughReactions (4) and (6)–(8) or Reactions (9)–(11) cannot bedistinguished in the present study. It should be pointedout that there is low adsorption of cyanide onto the TiO2

surface (Pollema et al., 1992), therefore Reactions (6)–(8)may predominate to give cyanate in the photocatalytic

K. Osathaphan et al. / Solar Energy 82 (2008) 1031–1036 1035

oxidation of cyanide. The H2O2 formed by Reaction (12)rapidly decomposes in the presence of TiO2/UV to giveH2O and O2 (Reaction (13)).

A likely oxidizing species, HO� competes between CN�

and EDTA during photocatalytic oxidation of cyanide inthe presence of EDTA. The second-order rate constantfor the reaction of HO� and EDTA (Reaction (14)) is sim-ilar to the reaction of Reaction (4) (Hobel and Sonntag,1998)

EDTAads þHO� ! Products k14

¼ 6:0� 109 M�1 s�1 ð14Þ

This suggests that the cyanide oxidation rate will be re-duced in the presence of EDTA as has been observedexperimentally (Fig. 4).

Rate for disappearance of cyanide may be expressed as

�d½CN��=dt ¼ k4½CN��½HO�� ðIIÞThe rate for disappearance for HO� in presence of both cya-nide and EDTA is given as

�d½HO��dt ¼ �k3½OH��½hþvb� þ k4½CN��½HO��þ k14½EDTA�½HO�� ðIIIÞ

Since HO� would be in a steady-state, the concentration ofHO� can be determined by

½HO�� ¼ k3½hþvb�=ðk4½CN�� þ k14½EDTA�Þ ðIVÞThe substitution of the concentration of HO� into Eq. (II)gives the following equation:

�d½CN��=dt ¼ k3k4½CN��½hþvb�=ðk4½CN��þ k14½EDTA�Þ ðVÞ

or

1=ð�d½CN��=dt�Þ ¼ 1=ðk3½hþvb�Þ þ ðk14=k3k4Þ� ð½EDTA�=½CN��½hþvb�Þ ðVIÞ

Eq. (VI) suggests a linear relationship between inverse ofoxidation rate of cyanide and molar ratios of EDTA tocyanide. However, analysis of data did not give such line-arity in the relationship. This indicates that cyanide is notonly oxidized by hydroxyl radicals, but also by other sug-gested Reactions (9)–(11).

5. Conclusions

The optimum condition for the photocatalytic oxidationof 3.85 mM cyanide at pH 13.0 was found in 0.1 g l�1 TiO2

loading at a 1.1 l min�1 air-flow rate. The complete decom-position of cyanide to relatively non-toxic cyanate wasachieved after illumination for �350 min. The presence ofEDTA in the reaction mixture reduces the photocatalyticoxidation rate of cyanide and the decrease was not propor-tional to the molar ratio of EDTA to cyanide. A proposedmechanism could explain the photocatalytic oxidation ofcyanide in absence and presence of EDTA.

Acknowledgments

This research was supported by the Graduate School,Chulalongkorn University and the National Research Cen-ter for Environmental and Hazardous Waste Management,Chulalongkorn University, Bangkok, Thailand. Authorswish to thank Dr. Kurt Winkelmann for useful commentson the manuscript.

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