environmental remediation of cyanide solutions by photocatalytic oxidation using au/cds...

Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx

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JIEC-1676; No. of Pages 6

Environmental remediation of cyanide solutions by photocatalyticoxidation using Au/CdS nanoparticles

E.S. Aazam *

Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

A R T I C L E I N F O

Article history:

Received 20 October 2013

Accepted 3 November 2013

Available online xxx

Keywords:

Au loading

CdS

Visible photocatalyst

Oxidation of cyanide

A B S T R A C T

CdS nanoparticles were prepared by a hydrothermal method, and Au was deposited onto the surface of

CdS via a photoassisted deposition method. The resulting samples were characterised by X-ray

diffraction, ultraviolet and visible spectroscopy, photoluminescence emission spectroscopy, transmis-

sion electron microscopy, X-ray photoelectron spectroscopy, and surface area measurements. The

catalytic performance of the samples in the photocatalytic oxidation of cyanide under visible light was

determined. The UV–vis analysis indicated that a red shift occurred after Au was loaded onto CdS

nanoparticles. The maximum photocatalytic oxidation efficiency was 100%, which was obtained using

0.2 wt% Au/CdS after a reaction time of 40 min.

� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Cyanide originates from metal finishing, ore extraction, andhydrometallurgical industries. It is used in the production oforganic chemicals such as nitrile, nylon, and acrylic plastics. Otherindustrial applications utilising cyanide include electroplating,metal processing, steel hardening, and synthetic rubber produc-tion. Many common plants contain the natural form of cyanide,cyanic glucoside. Its presence may be the product of evolution, as itdeters animals and insects from consuming the entire plant. Mostanimals can tolerate digesting small amounts of cyanic glucoside,but during a drought, the amount of the chemical in plantsincreases. As a result, animals could poison themselves by eatingplants with high concentrations of cyanic glucoside. The traditionaltreatment method of cyanide is the chlorination of cyanide, but thedrawback of this method is the production of cyanogens gas(CNCl), a known carcinogen. The produced gas is poisonous,harmful, and essentially causes environmental pollution thatrequires further treatment. Heterogeneous photocatalysis could beconducted on gas phases, pure organic liquid phases, or aqueoussolutions. Photocatalysis is the method of choice used profes-sionally for the treatment of polluted water. Currently, the TiO2

photocatalyst is most commonly used for the removal of pollutants[1–3]. Because photocatalytic reactions occur on the surface ofTiO2, the enhancement of its photocatalytic activity can beachieved by either the alteration of the surface properties of the

* Tel.: +966 26400000; fax: +966 2 6952292.

E-mail addresses: [email protected], [email protected]

Please cite this article in press as: E.S. Aazam, J. Ind. Eng. Chem. (20

1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2013.11.020

catalyst itself [4–7] or the immobilisation of the catalyst onto asubstrate that improves its surface properties [8–17]. Thesubstrate needs a large surface area for more of the catalyst tobe involved in photocatalytic reactions and to easily allow the masstransfer of the pollutant and degraded products. The supportingmaterial should have a definite affinity for pollutants to provideefficient trapping and degradation. The substrate material shouldbe transparent or semitransparent to allow UV and visibleradiation to pass through it and the superior surface of thecatalyst to be activated. Among the various semiconductormaterials, cadmium sulfide (CdS) has received considerableattention due to a large number of technical applications likephotocatalysis, gas-sensing, photo-sensitive field emissionswitches and so on [18–20]. Especially, proper band potentials ofCdS thermodynamic conditions for photocatalytic redox reaction,make it an efficient photocatalyst [21–23]. Recently, Wu et al.reported that CdS showed the highly efficient photocatalyticperformance on reducing 4-nitroaniline [24]. Also, Luo et al.studied the photocatalytic activity of CdS hollow spheres andfound they exhibited excellent photocatalytic activity for thephotodegradation of Rhodamine B under UV irradiation [25]. Ingeneral, the photocatalytic activity of CdS is strongly dependent onits nanostructure. Owing to the efforts from many research groups,a variety of methods have been used to obtain CdS nanostructures,such as thermal evaporation method, solvothermal method, andthermal decomposition of a single-source precursor method andsolid-state reaction [26–29]. However, improved photocatalyticactivity is still necessary. Herein, we present the synthesis of a Au-doped CdS photocatalyst. The effect of Au content on thephotocatalytic activity of CdS has been investigated in detail. It

13), http://dx.doi.org/10.1016/j.jiec.2013.11.020

ing Chemistry. Published by Elsevier B.V. All rights reserved.


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908580757065605550454035302520

Inte

nsity

(a.u

.)

2 Thetha (Degree)

CdS

0.1 wt % Au-CdS

0.15 wt % Au-CdS

0.2 wt % Au-CdS

0.25 wt % Au-CdS

Fig. 1. XRD pattern of CdS and Au/CdS nanoparticles.

E.S. Aazam / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx2

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was found that the presence of Au significantly improved thephotocatalytic performance of CdS in the cyanide degradationcatalysed by visible light. We hope this study will provide guidancefor the improvement of other Chalcogenide family photocatalysts.

2. Experimental

2.1. Preparation of CdS

CdS photocatalyst was prepared by a hydrothermal method. Ina typical run of synthesis, 2.5 mmol CdCl2 and 10 mmolthioacetamide (TAA) were dissolved in 40 mL deionized water.The solution was transferred to a 50 mL Teflon-lined stainless steelautoclave and maintained at 200 8C for 24 h. The product wascollected by centrifugation, followed by washing thoroughly withdeionized water and absolute ethanol in order. The final productwas dried at 80 8C in vacuum for 3 h.

2.2. Preparation of Au/CdS

PAD-Au/CdS (containing 0.1, 0.15, 0.2, and 0.25 wt% of Aumetal) were synthesised using the following a photo-assisteddeposition (PAD) route: Au metal was deposited on CdS with anaqueous solution of HAuCl4 under UV-light irradiation. Thesamples were dried at 60 8C and were treated via H2-reduction(20 mL min�1) at 150 8C for 2 h.

2.3. Characterisation techniques

X-ray diffraction (XRD) analysis was carried out at roomtemperature with a Bruker axis D8 using Cu Ka radiation(l = 1.540 A). The specific surface area was calculated from N2-adsorption measurements, which were obtained using a Nova2000 series Chromatech apparatus at 77 K. Prior to themeasurements, the samples were treated under vacuum at120 8C for 2 h. The band gap of the samples was identified by UV–visible diffuse reflectance spectroscopy (UV–vis-DRS), which wasperformed in air at room temperature in the wavelength range of200–800 nm using a UV/vis/NIR spectrophotometer (V-570,JASCO, Japan). Transmission electron microscopy (TEM) wasconducted with a JEOL-JEM-1230 microscope, and the sampleswere prepared by suspension in ethanol, followed by ultra-sonication for 30 min. Subsequently, a small amount of thissolution was placed onto a carbon-coated copper grid and driedbefore loading the sample in the TEM. X-ray photoelectronspectroscopy (XPS) studies were performed using a ThermoScientific K-ALPHA, XPS, England. Photoluminescence (Pl)emission spectra were recorded with a Shimadzu RF-5301fluorescence spectrophotometer.

2.4. Photocatalytic activity

The application of the synthesised nanoparticles for thephotodegradation of cyanide was investigated under a visiblelight irradiation. Experiments were carried out using a horizontalcylinder annular batch reactor. The photocatalyst was irradiatedwith a blue fluorescent lamp (150 W, maximum energy at 450 nm)which is doubly covered with a UV cut filter. The intensity data ofUV light is confirmed to be under the detection limit (0.1 mW/cm2)of a UV radiometer. In a typical experiment, a desired weight of thecatalyst was suspended into a 300-mL, 100 mg/L potassiumcyanide (KCN) solution. Ammonia solution was used to set thepH of the experiment at 10.5 to avoid evolution of HCN gas. Thereaction was carried out isothermally at 25 8C and samples of thereaction mixture were analysed at different intervals time for atotal reaction time of 1 h. The CN� concentration in samples was


estimated by a volumetric titration with AgNO3, using potassiumiodide to determine the titration end-point [21]. The removalefficiency of CN� was measured by applying the followingequation:

% removal efficiency ¼ Co � C

Co� 100 (1)

where Co is the initial concentration of the uncomplexed CN� in thesolution and C is the concentration of unoxidised CN� in thesolution.

3. Results and discussion

3.1. Structural, morphological and compositional characterisations

The XRD patterns of each parent CdS and Au/CdS nanoparticleare compared in Fig. 1. The structural characteristics of CdS andAu/CdS were mainly composed of CdS (JCPDS Card: 10-0454),which indicated that the CdS structure remained after thephotoassisted deposition (PAD) method was performed. Howev-er, the diffraction peaks of Au were not observed in the patternsof the Au/CdS samples due to the low Au doping content.Moreover, the data imply that Au was well dispersed within theCdS phase. Au played a prominent role in the crystallisationprocess, as observed in the XRD patterns, which showed that thecharacteristic CdS phase diffraction peaks became broad and theintensity of the diffraction peaks decreased with an increase inthe Au loading. The average crystallite sizes of CdS werecalculated using Scherer’s equation and the full width at halfmaximum of the X-ray diffraction peaks at u = 26.178, whichcorrespond to the most intense CdS peaks. The crystallite size ofCdS, 0.10 wt% Au/CdS, 0.15 wt% Au/CdS, 0.20 wt% Au/CdS and0.25 wt% Au/CdS was 22, 18, 12, 9 and 7 nm respectively.Therefore, the crystallite size of CdS became smaller as the Auconcentration increased.

XPS spectres of 0.20 wt% Au/CdS nanoparticles are shown inFig. 2. The XPS spectra (Figs. A and B) demonstrated that Cd specieswere present in +2 oxidation states, while S species were present in�2 state, corresponding to binding energy of 405.2 and 161.2 eV inCd3d5/2 and S2p levels, respectively. The peak observed at 87.7 eVand 84.0 eV were ascribed to metallic gold (Fig. 2C).

The TEM images of CdS and Au/CdS nanoparticles are shown inFig. 3. The results revealed that Au was dispersed on the surface of



400 405 410 415 420

Cd3dA

Inte

nsity

(a. u

.)

Binding energy (eV)155 160 165 170 175

S2PB

Inte

nsity

(a.u

.)

Binding energy (eV)

90 88 86 84 82

Au 4f7/2Au 4f5/2

Inte

nsity

(a.u

.)

Binding energy /eV

C

Fig. 2. XPS spectres for 0.20 Au/CdS, where (A) for Cd 3d; (B) for S 2p and (C) for Au 4f.

E.S. Aazam / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx 3

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the CdS nanoparticles and the diameter of Au was 4, 5, 7 and 9 nmfor 0.10 wt% Au/CdS, 0.15 wt% Au/CdS, 0.20 wt% Au/CdS and0.25 wt% Au/CdS, respectively.

3.2. Surface area analysis

The surface area and other data calculated from the t-plot areshown in Table 1. The SBET values were 80, 78, 75, 73 and 71 m2/gfor CdS, 0.10 wt% Au/CdS, 0.15 wt% Au/CdS, 0.20 wt% Au/CdS and0.25 wt% Au/CdS, respectively. Furthermore, the total porevolume of CdS was higher than that of Au/CdS due to theblocking of pores by the deposition of Au metal. The SBET and St

values were similar in most samples, indicating the presence ofmesopores.

3.3. Optical characterisation

The UV–vis spectra of CdS and Au/CdS nanoparticles aredisplayed in Fig. 4. Compared to a wavelength of approximately436 nm for CdS, the loading of Au onto CdS caused a red shifttowards higher wavelengths from 452 to 498 nm for differentloadings of Au. The direct band gap energy for CdS and Au/CdSnanoparticles was calculated from the UV–vis spectra based on amethod described by Mohamed et al. [1], and the results aretabulated in Table 2. As shown in the table, the band gap decreased


with an increase in the wt% of Au. The band gap values were 2.80,2.74, 2.65, 2.54 and 2.50 eV for CdS, 0.10 wt% Au/CdS, 0.15 wt% Au/CdS, 0.20 wt% Au/CdS and 0.25 wt% Au/CdS, respectively.

Photoluminescence (Pl) emission spectra have been used tostudy the transfer of photogenerated electrons and holes and tounderstand the separation and recombination of photogeneratedcharge carries. To investigate the photoelectric properties of theprepared samples, the Pl spectra of different samples excited at321 nm were obtained at room temperature. As shown in Fig. 5.,the Pl intensity greatly decreased with an increase in the Auloading. Au acts as a trapping site to capture photogeneratedelectrons from the conduction band, separating photogeneratedelectron–hole pairs. The incorporation of noble metal nanoparti-cles into semiconductor-based catalysts is considered to enhancethe absorption of light by catalysts in the visible light region, whichleads to a shift in the absorption edge towards longer wavelengths,indicating that the band gap energy decreases and that morephotogenerated electrons and holes can participate in photo-catalytic reactions. However, Au appears to modify the interface ofCdS in a way that alters the mechanism of the recombination orsurface reactions of photogenerated charge carriers, which forcesCdS to become more active in the visible region. The observed shiftin the emission position may be attributed to charge transferbetween the Au-generated band and the conduction band of CdS asa semiconductor.



Fig. 3. TEM images of CdS and Au/CdS nanoparticles with Au contents 0.0 wt% (A),

0.1 wt% (B), 0.15 wt% (C), 0.2 wt% (D) and 0.25 wt% (E).

200 250 300 350 400 450 500 550 600 650 700

Abs

orba

nce,

a.u

.

Wavelenght (nm)

CdS 0.10 wt % Au-CdS 0.15 wt % Au-CdS 0.20 wt % Au-CdS 0.25 wt % Au-CdS

Fig. 4. UV–vis absorption spectra of CdS and Au/CdS nanoparticles.


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3.4. Photocatalytic activity

The photocatalytic activity is known to be dependent on thecrystallinity, surface area and morphology and it may be improvedby slowing the recombination of photogenerated electron–holepairs, extending the excitation wavelength to a lower energy range,and increasing the amount of surface-adsorbed reactant species. Ingeneral, the process for photocatalysis begins when supra-bandgap photons are directly absorbed consequently generatingelectron–hole pairs in the semiconductor particles. This is followedby diffusion of the charge carriers to the surface of the particlewhere the interaction with water molecules would produce highlyreactive species of peroxide (O2

�) and hydroxyl radical (OH�)responsible for the degradation of adsorbed organic molecules.

Table 1Texture parameters of CdS and Au/CdS nanoparticles.

Sample SBET (m2/g) St (m2/g) Total VP (mL/g) r (A)

CdS 80.00 83.00 0.182 43.00

0.10 wt%Au-CdS 78.00 79.00 0.178 45.00

0.15 wt%Au-CdS 75.00 77.00 0.175 48.00

0.20 wt%Au-CdS 73.00 75.00 0.173 52.00

0.25 wt%Au-CdS 71.00 73.00 0.170 70.00

Where SBET, BET surface area; St, surface area derived from Vl–t plots; r�, mean pore

radius; Vp, total pore volume.


Step-reactions in the photocatalytic process leading to thedegradation of CN� are presented in the following sequence:

CdS þ hn ! e� þ hþ

e�ðAuÞ ! etr�ðAuÞ

etr� þ O2ðadsÞ ! O2

�

2etr� þ 2Hþ þ O2 ! H2O2

H2O2þ etr� ! OH� þ OH�

hþ þ H2O ! Hþ þ OH�

2CN� þ 4OH� ! 2 N2þ 2CO2þ 2H2

In the prepared Au/CdS enhancement of the photocatalyticactivity of the catalyst is realised through two mechanisms. Thefirst being prevention of the recombination of the electron-holepair by Au atoms in the Au/CdS. Doped metal atoms on asemiconductor often act as electron traps. The second reason forthe improved photocatalytic activity is the reduced band-gabenergy that allows absorption of photons in the visible range.

Fig. 6 shows the photocatalytic degradation of cyanide solutionwith CdS catalyst with different wt% of Au upon visible lightillumination under the following conditions: pH 10.5, 300 mL of100 ppm KCN, and 0.20 g of catalyst. The figure indicates that thephotocatalytic efficiency of pure CdS does not exceed 50% after one

Table 2Band gap energy of CdS and Au/CdS nanoparticles.

Sample Band gap energy (eV)

CdS 2.80

0.10 wt% Au-CdS 2.74

0.15 wt% Au-CdS 2.65

0.20 wt% Au-CdS 2.54

0.25 wt% Au-CdS 2.50



0 5 10 15 20 25 30 35 40 45 50 55 600

10

20

30

40

50

60

70

80

90

100

0.10 g 0.15 g 0.20 g 0.25 g 0.30 g

Pho

toca

taly

tic d

egra

datio

n of

cya

nide

, %

Reacti on t ime, min

Fig. 7. Effect of the loading of 0.20 wt% Au/CdS on the photocatalytic oxidation of

cyanide.

400 450 500 55 0 600 650

ex = 32 1 nm

Inte

nsity

(a. u

.)

Waveleng th (nm)

CdS0.10 wt % Au-Cd S0.15 wt % Au-Cd S0.20 wt % Au-Cd S0.25 wt % Au-Cd S

Fig. 5. Pl spectra of CdS and Au/CdS nanoparticles.

0

20

40

60

80

1003rd

Phot

ocat

alyt

ic o

xida

tion

of c

yani

de ,

% 1st 2nd 4th 5th

No of cyc les

Fig. 8. Reuse of photocatalysts in the photocatalytic oxidation of cyanide.

E.S. Aazam / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx 5

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hour reaction time. The low efficiency of the catalyst in anexperiment where a visible light is shined on the catalyst-KCNsolution is explained by the wide band gab of the catalystcompare to Au/CdS nanoparticles as shown in Table 2. As theband gap decreases with the increased doping wt% of Au theefficiency of the photodegradation increases sharply along withrapid decrease in time needed for complete reaction. Looking atFig. 6 again, one can choose 0.20 wt% Au/CdS as optimumcombination for it contains enough Au to achieve 100%degradation in less than 50 min.

Effect of different catalyst loading weights on the photocatalyticdegradation of cyanide is shown in Fig. 7. The graph shows that theincrease in loading of the catalyst from 0.1 to 0.20 g increasescyanide removal efficiency from 82% to 100% after 50 min reactiontime, respectively. Further increase in catalyst loading from 0.2 to0.25 g decreases the reaction time from 40 to 30 min. However, atphotocatalyst dosages above 0.25 g, the time required to oxidisecyanide increased again due to the blocking of light penetration byexcessive photocatalyst. Therefore, the optimum condition forloading of catalyst is 0.25 g.

0 10 20 30 40 50 600

10

20

30

40

50

60

70

80

90

100

Pho

toca

taly

tic d

egra

datio

n of

cya

nide

, %

Reaction time, min

Au 0.10 wt % Au/CdS 0.15 wt % Au/CdS 0.20 wt % Au/CdS 0.25 wt % Au/CdS

Fig. 6. Effect of the catalyst type on the photocatalytic oxidation of cyanide.


3.5. Recycling of the photocatalyst

The recycling of catalysts is a key step for assessing the practicalapplication of photocatalysts and developing heterogeneousphotocatalysis technology for wastewater treatment. Thus, thephotocatalytic activity of recycled 0.20 wt% Au/CdS was evaluated.The efficiency of the photocatalytic oxidation of cyanide was equalto 100% during the first five cycles (Fig. 8). The results revealed thatphotocatalyst separation is effective and the photocatalyst is stableand promising for environmental remediation applications.

4. Conclusions

1. Au/CdS photocatalysts were successfully synthesised andproven to be promising catalysts due to their high oxidationefficiency of cyanide under visible light.

2. Red shift phenomena were observed in the UV–vis spectra ofCdS and Au/CdS and were observed to depend on the wt% of Audeposited on CdS.

3. Photocatalytic measurements in the photocatalytic oxidation ofcyanide indicated that Au/CdS nanoparticles with 0.20 wt% Auexhibited the highest catalytic activities, efficient photocatalytic




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properties in water purification and may find potentialapplications in related fields.

4. The reaction conditions were optimised, and the use of 0.25 g of0.20 wt% Au/CdS in 1000 mL of a 100 mg/L KCN solution yieldeda cyanide oxidation efficiency of 100% within 40 min ofirradiation of visible light.

5. The present photocatalyst remained effective and active afterfive cycles, which indicated that the Au/CdS photocatalystdisplayed promising recyclability.

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