Investigation of UV-Induced decomposition of hexacyanoferrate(II) and -(III) by capillary...

Pergamon Chemosphere, Vol. 39, No. 14, pp. 2467-2478, 1999 © 1999 Elsevier Science Ltd. All rights reserved

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INVESTIGATION OF UV-INDUCED DECOMPOSITION OF HEXACYANOFERRATE(U D

AND -(HI) BY CAPILLARY ELECTROPHORESIS

Marian Gu~ek, Robert Susi~ and Boris Pihiar

Faculty of Chemistry and Chemical Technology, University of Ljubljana,

A~ker6eva 5, SI-1000 Ljubljana, Slovenia

(Received in Switzerland 23 February 1999; accepted 4 May 1999)

ABSTRACT

UV-degradation processes of hexacyanoferrate(II) and hexacyanoferrate(III) h aqueous solutions were

studied using capillary electrophoresis (CE). A variety of solutions were UV irradiated. In each case an

intermediate appears, aquapentacyanoferrate(II) and aquapentacyanoferrate(RI), respectively. The

polycyanoferrate species were separated using 30 mM phosphate buffers with different pHs and applying

negative voltage. They were identified by synthetic standards. The migration time of

aquapentacyanoferrate~H) was 7.6 rain at pH=12, while aquapentacyanoferrate(IH) migrated after 6.8 rain at

pH= 10. Concentration profiles of polycyanoferrates were constructed fxom the CE electrophorograms of

the appropriate aliquots sampled at regular intervals. The final products ofphotolytic degradation depend on

the pH of the solutions, producing Prussian blue in acidic media, and hydrated iron oxides and cyanides in

alkaline media. © 1999 Elsevier Science Ltd. All rights reserved

Keywords: hexacyanoferrates, capillary electrophoresis, photodegradation, aquapentacyanoferrates

1. INTRODUCTION

It is known that complex metal cyanides decompose under the influence of UV and/or visible light to form

fxee cyanide, which is highly toxic to living organisms [1, 2]. In spite of the well known cyanide toxicity,

cyanide metallurgy is still an indispensable basis for precious metals processing. In the course of gold

production, additional complexed cyanides are formed, including hexacyanoferrate(II) and

hexacyanoferrate(IH). Since both transform and decompose photochemically, it is suggested that solar

2467

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power could be effectively employed to treat waste waters from various metallurgical processes [3]. Free

cyanide can be further photocatalytically oxidised to nitrate (NO3-) via a cyanate (OCN-) route, thus

converting toxic cyanide into less toxic species, in a form acceptable to be discharged into the environment

[4,5].

Although the photodecomposition of hexacyanoferrates has been thoroughly investigated [2, 3, 6-9],

much debate remains regarding the mechanism and the intermediates which are formed. It is established that

free cyanide is produced during UV irradiation and that the main intermediary species of the photolytic

decomposition of hexacyanoferrate(II) and hexacyanoferrate(III) are aquapentacyanoferrate(II) and

aquapentacyanoferrate(l]]), respectively.

Other studies on hexacyanoferrate(II) photochemistry have focused on photoelectron production [10-12],

postulating two excited state species. The first, directly leads to hexacyanoferrate(I]I) ions, while in the

second, a cyanide ion is expelled, followed by aquation (the replacement of a ligaud by water molecule) and

leads to the formation of aquapentacyanoferrate(II). The ensuing steps in the degradation process are less

obvious and require more research.

Up to now the analytical methods employed to study the photolytic decomposition of hexacyanoferrate

species include monitoring of pH changes, UV-spectrophotometry, and more recently, ion chromatography

(IC) [3]. However, these techniques (except IC) fail to separate and identify certain ion species and so cannot

be reliably used to follow the decay/production of polycyanoferrate species. For this reason we decided to

apply capillary electrophoresis (CE), a technique that already shows promise for metal speciation studies

[13]. Previous studies on the hexacyanoferrate system employing CE include monitoring of

hexacyanoferrates in electroplating solutions [14], determination of various metallo-eyanides after a

concentration step on supported liquid membranes [15, 16], investigations of gold processing solutions [17]

and leaching solutions from catalytic converters [18].

In our study, we use both CE and UV-spectrophotometry to follow the degradation of

hexacyanoferrate(II) or -(III) during the irradiation of aqueous solutions at different pH values. Additionally,

the production of intermediates and their subsequent transformation and degradation is also monitored.

2. EXPERIMENTAL

2.1. Instrumentation

Separation was achieved using the 270A-HT Capillary Electrophoresis System (Applied Biosystems, Perkin

Elmer, USA), powered by Turbochrom CE Plus System (Perkin Elmer) software, fitted with a fused-silica

capillary (72 cm × 50 Jam i.d., 50 cm effective length). Injections were performed hydrodynamically with a

sampling time of 3 s. Direct UV detection at 214 nm was used.

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The UV measurements were performed on a UV/VIS Spectrometer Lambda2 (Perkin Elmer) using an 1

cm quartz cell.

The samples of hexacyanoferrates were irradiated either in a water-cooled reactor using a 100 W high

pressure Hg lamp (Applied Photophysics 100LQ, Germany), or in a home-made cylindrical quartz cell using

a 50 W high pressure Hg lamp (Osram Ultra-vitalux, Germany). The intensity of the incident light inside the

photoreactor, measured employing the ferrioxalate actinometry, was 9.0.10 -6 and 3.5.10 -6 Einstein min 1,

respectively.

2.2. Cartier electrolytes

All experiments were performed using 1 mM solutions of polycyanoferrate species in a series of phosphate

buffers with different pH values, but with the same phosphate concentration (30 raM). The buffers were

prepared from analytical grade reagents: Na3PO4.121-120, Na2HPO4.2H20, NaH2PO4.2H20 (Kemika, Zagreb,

Croatia) and H3PO4 (Merck, Darmstadt, Germany) using deionlsed water purified on a Milh'pore Milli-Q

(Bedford, MA, USA) water treatment system. No electroosmotic flow modifiers were required since

sufficient separation of various polycyanoferrate species was achieved using only the phosphate buffers.

2.3. Preparation of aquapentacyanoferrates(II) and-(IU)

Sodium aquapentacyanoferrate(II) and -(III) were prepared according to Hoffmann [19] and modified by

Ag-perger et al. [20]. Disodium pentacyanonitrosylferrate (Na2[Fe(CN)sNO].2H20, Riedel-de Hahn,

Hannover, Germany) was dissolved in a solution of sodium hydroxide (Merck, Darmstadt, Germany).

Hydroxylamine hydrochloride (NH2OH.HCI, Merck, Darmstadt, Germany) was added and left overnight to

precipitate the aquapentacyanoferrate(II) salt that was additionally purified by subsequent recrystallisation in

water-ethanol. Sodium aquapentacyanoferrate(lII) was prepared by bromine-water oxidation of

aquapentacyano ferrate(II).

The purity of both products was controlled by UV spectroscopy and elemental analyses. The complexes

were decomposed in an H2SO4-acidic medium under heating. The cyanide was determined argentometrically

and the iron was precipitated as the hydroxide and weighed as oxide [19]. The molar ratio between iron and

cyanide was found to be 1:5.

2.4. Procedures

Initially, the influences of reaction modifiers on the UV oxidation of hexacyanoferrate(II) were studied in

a cylindrical quartz cell. Ten millilitres of 1 mM solutions of K4[Fe(CN)6].3H20 were placed into the cell

and UV irradiated for 5 min. Reaction modifiers that might influence the photolytic decay were added in

equimolar quantities of potassium cyanide, sodium benzoate (both Riedel-de Hahn, Hannover, Germany),

dimethyl sulphoxide (DMSO), hydrogen peroxide (both Merck, Darmstadt, Germany) and iso-propyl alcohol

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(i-PrOH, Rathbum Chemicals, Walkerburn, Scotland). The solutions were analysed by CE prior to and after

irradiation.

When monitoring the decomposition of hexacyanoferrate(II) and -(m) (K4[Fe(CN)6] and K3[Fe(CN)6],

Alkaloid, Skopje, Macedonia), 1 mM solutions of one of the hexacyanoferrate in phosphate buffers at

different pH values (between 5 and 12), were placed into a water-cooled reactor, whereas, the UV-lamp was

submerged directly into the solution. Aliquots of the irradiated hexacyanoferrate solution were taken at

regular time intervals and promptly analysed by CE.

3. RESULTS AND DISCUSSION

3.1. Optimising CE parameters for the polycyanoferrate species separation

In the initial experiments, an optimal separation of hexacyanoferrate(II) and -(III) ions is achieved using a

30 mM NaH2PO4fNa2HPO4 buffer at a pH value of 7. The applied voltage was negative (- 15 kV) because the

vectors of electrophoretic mobilities oppose the direction of the electroosmotic flow and exceed its

magnitude. These complexes were injected at the cathode and detected at the anode as described by Aguilar

et al. [14]. Samples containing both liexacyanoferrate(II) and hexacyanoferrate(flI) show two well separated

peaks at 3.6 and 3.1 rain, respectively. The limit of dete~tiun is relatively high (10 --6 M) and is a result of

short path length (50 Bin) of the UV detector [21], which represents the main disadvantage of CE as an

analytical tool for monitoring hexacyanoferrates photodegradation processes.

In the course of the photolytical experiments, buffers at pH values different from the optimum were

employed. Sufficient separation of polycyanoferrate(II) and -(III) species can be achieved by selecting a

negative voltage regardless of the buffer employed.

3.2. The influence of reaction modifiers on the UV oxidation ofhexacyanoferrat~II)

Aqueous solutions (1 mM in hexacyanoferrate(II)) were irradiated for 5 rain without/with reaction

modifiers and promptly analysed by CE. When a 'pure' solution is irradiated, two new peaks appear in the

electrophorogram, belonging to hexacyanoferrate(III) and aquapentacyanoferrate(H). The identity of both

peaks was established by the standard addition method (see below). When solutions were purged with argon,

the degradation rate substantially decreases, showing that dissolved oxygen plays an important role in the

process of photooxidation. When an equimolar quantity of hydrogen peroxide is added, the 5 rain UV

irradiation leads to a complete decomposition of polycyanoferrate species. The same result is also obtained

in the dark, but over a longer period (approximately 80 rain). In both cases, hydrogen peroxide represents an

abundant source of hydroxyl radicals, accelerating the photodecomposition. This is important to determine

2471

to what extent OH" radicals are responsible for the photooxidation in 'pure' hexacyanoferrate(II) solutions.

Therefore, we repeated the experiments and added i-PrOH or DMSO, which are known to be good OH'

radical scavengers. The results show no difference in UV decomposition rate, which agrees with the results

reported by Mitra et al. [22] and Daintun et al. [23].

We believe that cyanide radicals play the major role since the addition of sodium benzoate completely halts

photooxidation by scavenging cyanide radicals formed from the cyanide ions. We surmise, that CN" radicals

are the main driving force in the I N oxidation of hexacyanoferrate(II) and not OH'.

3.3. Monitoring the degradation ofhexacyanoferrate(II) and -(III) under IN-light

When aqueous solutions of hexacyanoferrate(lI) or -(III) are I N irradiated, an intermediate appears in

each case, which transforms/decays after prolonged irradiation. The electrophorograms of irradiated

hexacyanoferrates are shown in Figures la and lb. Under electrophoretic conditions used, only highly

negatively charged species are detected. The migration time of the intermediate at 7.6 rain in Figure la

suggests a lower electrophoretic mobility compared to the 'parent' ion. The identity of the ion was

established by adding aquapentacyanoferrate(II) standard. The same situation occurs during the irradiation of

hexacyanoferrate(III) (see Figure 2b), where an intermediate, identified as aquapentacyanoferrate(IlI),

appears at 6.8 min.

20

10 ferrate(III) A

I I I I

1 2 3 4

ferrate(H)

I

5 6

time [min]

a

intermediate

A

I I I

7 8 9 10

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2 0

10.

ferrate(m)

. . . . . . j

b

ferrate(n) intermediate

0 1 2 3 4 5 6 7 8 9 time [rain]

10

Figure 1: Electrophorograms of hexacyanoferrates (1 mM solution in 30 mM phosphate buffer,

hydrodynamic sampling, 3s, UV detection at 214 nm): (a) record of UV irradiation of hexaeyanoferrate~ll)

at pH=12, (b) record of UV irradiation ofhexacyanoferrate(llI) at pH=10. The duration of irradiation was 5

rain in both cases.

To elucidate the photolysis of polyeyanoferrates, the concentration profiles of negatively charged

polycyanoferrates were measured in dependence of the time of exposition of the solutions to UV, using

buffers with pH values from 5 to 12. The migration times vary with the buffer employed, thus, voltages had

to be selected accordingly (see Table 1). The cartier electrolyte is the same buffer as that used for irradiation

experiments, to reproduce authentic conditions during the electrophoretic analyses. Namely, we believe that

changes can occur in the composition of polycyanoferrate species in the sample when they interact with a

carrier electrolyte with different pH. To avoid this, the carrier electrolyte was the same as the solution that

was being irradiated.

Table 1: Capillary electrophoretic conditions and migration times of polycyanoferrate species. Other

experimental conditions were fixed: 30 mM phosphate buffers, UV detection at 214 nm, hydrodynamic

sampling of 3 s.

Migration time [rain]

Conditions ferrate(II) ferrate(llI) aquaferrate(II) aquaferrate(IlI)

pH = 5, E = -15 kV 2.7 2.2 3.3 3.0

pH = 7, E = -15 kV 3.6 3.1 5.1 4.7

pH = 10, E = -25 kV 4.2 3.2 7.2 6.8

pH = 12, E = -25 kV 5.5 3.7 7.6 7.1

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Under the reported CE conditions, only species with a high negative charge can be detected. We attempted

to detect other species by using positive voltages, however, at 214 nm no peaks were observed within a

migration period up to 30 minutes. It is also to be noted that because of the different molar absorptivities of

the species at 214 nm a direct quantitative determination of the concentration ratios is not possible and must

be made using standard addition.

The concentration profiles are shown in the figures as follows: Figure 2a irradiation ofhexacyanoferrate(II)

at pH=5 and Figure 2b irradiation of hexacyanoferrate(II) at pH=12. There is a considerable difference

between the overall rate of decay in acidic and alkaline media, the former being much more rapid than the

latter, 90% of the initial hexacyanoferrate(II) decays in one hour at pH=5, whereas at the pH 12, the same

level is reached in six hours. The photodegradation of hexacyanoferrate(lII) shows a similar decay pattern

(Figure 2c and Figure 2d).

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0,2

0.1

0

1

--o-- hexacyanoferrate(II)

--o- hexacyanoferrate(HI)

- × - aquapentacyanoferrat e(H)

20 40 60 80 100

t|min]

I

X

120

2 4 7 4

l

0.9

0.8

0.7

0.6

~ " 0.5

0.4

0.3

0.2

0.1

0

--o-- hexacyanoferrate(ID

- n - hexacyanoferrate(III)

- x - aquapentacyanoferrate(II)

b

6o 1:0 ]80 t lmin] :40 300 36o 4:0

11 0.9

0.8

0.7

0.6

~ " 0.5

0.4

0.3

0.2

0.1

- o - - hexacyanofeTrate(l])

---0-- hexacyanoferrate (m)

- x - aquapentacyanoferrate([II)

10 20 30 40 50 60 70 80

t [minl

C

I

0.9 --o--- hexacyanoferrate(H)

0.8 --o- hexacyanoferrate(HI)

0.7 --g-- aquapentacyanoferrate(HI)

d

0.6

0.5

0.4 X

0.3

0.2

0.1

0 X

0 30 60 90 120 150 180 210 240 270 300

t[min]

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Figure 2: Concentration profiles of UV irradiated hexacyanoferrate(II) or -(IH), at different pH values of

phosphate buffers: (a) hexacyanoferrate(H) at pH = 5, (b) hexacyanoferrate(ll) at pH = 12, (c)

hexacyanoferrate(RI) at pH = 5 and (d) hexaoyanoferrate(RI) at pH = 10

No change in pH is observed during irradiation. Nevertheless, when solutions of hexacyanoferrates were

irradiated without the buffer, an increase in pH is observed [2] as a consequence of the hydrolysis of the

released cyanide which was confirmed also in our lab by monitoring the levels of free cyanide. During the

experiments, the appearance of Prussian blue and hydrated iron oxides is observed. The photolytic

degradation is strongly pH dependant, leading to Prussian blue in acidic media or to ferric hydroxide in

alkaline media, regardless of whether hexacyanoferrate(H) or-(m) is illuminated.

3.4. The photodegradation mechanism ofhexacyanoferrate(ll) and -(HI) ions

The experiments show that there is a general route for the photodecomposition of hexacyanoferrate~H) or

(HI), regardless of the oxidation state. In the case ofhexacyanoferrate(H):

A rapid first step is the photoaquation

[Fe(CN)6] 4- + H20 ~ [Fe(CN)5H20] 3" + CN (photoaquation) (1)

giving aquapentacyano species,

2476

[Fe(CN)sH20] 3 - - [Fc(CN)sH20]2"+ e" (photooxidation) (2)

which readily reacts with other polycyanoferrate species [24].

[Fe(CN)6] 4 + [Fe(CN)sH20] 2- - ~ [Fe(CN)6] 3" + [Fe(CN)sH20] 3 (3)

Thus, one can observe repetitive patterns in the concentration profiles:

[Fe(CN)sH20] a + CN - - [Fe(CN)6] 4 + H20 (4)

Aquapentacyanoferrate(]II) is formed by the photooxidation of aquapentacyanoferrate(II) (2). The

resulting species reacts immediately with hexaoyanoferrate(II) giving hexacyanoferrate(HI) and

aquapentacyanoferrate(II) (3). Furthermore, aquapentacyanoferrate(H) can react with released cyanide ions

(4). Reactions (1) to (4) show why there is a repetitive pattern in the concentration profiles. In this particular

case, aquapentacyanoferrate(lll) is not observed due to the mass action law (the concentration of ferrate(II)

is very high), thus, only one intermediate is observed, namely aquapentacyanoferrate(II).

One can also observe that the overall decomposition is faster in acidic media, as a result of the protonation

of released cyanide ions. Since there are few flee cyanide ions to react with aquapentacyanoferrate species

(equation 4), further steps take place. In alkaline media, the complete decomposition is moderated by the

higher concentration of f~ee cyanide. It is interesting to note that the intermediates are longer-lived in the

acidic media, due to cyanide ions which once protonated cannot react with aqua species to form

hexacyanoferrates.

Further decomposition is less understood. Under the electrophoretic conditions used, it was not possible to

detect any other iron cyanide complexes, thus, no information about other possible intermediates could be

obtained. We surmise that because of the relatively long persistence of the aquapentacyanoferrate species,

the rate determining step of the overall decomposition follows the above described reactions.

5. CONCLUSIONS

In our investigation of UV-indnced decomposition of hexacyanoferrates, CE proved to be a robust and

promising analytical technique to follow negatively charged species produced by photolytic processes. Using

CE, we can explain some interesting phenomena taking place at the be~dnning of the photolysis of

hexacyanoferrates. We identified two intermediates, aquapentacyanoferrat e(II) and

2477

aquapentacyanoferrate(m) which form during UV irradiation. This study shows that polycyanoferrate

species can react with each other resulting in a repetitive pattern in the concentration profiles. After a

sufficient time the final products of photolytie degradations are observed, namely Prussian blue in acidic and

hydrated iron oxides and cyanides in alkaline media.

Unforttmately, it was not possible to detect any other intermediates by using CE, thus, the complete

degradation pathway is still not completely resolved and it is under a further investigation.

Acknowledgements

Financial support by the Ministry of Science and Technology of the Republic of Slovenia is gratefully

acknowledged.

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