heterogeneous photocatalysis of Cr(VI) in the presence of citric acid over TiO2 particles relevance of Cr.pdf

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APCATB-9813; No of Pages 7

Heterogeneous photocatalysis of Cr(VI) in the presence of citric

acid over TiO2 particles: Relevance of Cr(V)–citrate complexes

Jorge M. Meichtry a, Marta Brusa b, Gilles Mailhot c, Marıa A. Grela b, Marta I. Litter a,d,*a Unidad de Actividad Quımica, Centro Atomico Constituyentes, Comision Nacional de Energıa Atomica,

Av. Gral. Paz 1499, 1650 San Martın, Prov. de Buenos Aires, Argentinab Departamento de Quımica, Universidad Nacional de Mar del Plata, Funes 3350, B7602AYL Mar del Plata, Argentina

c Laboratoire de Photochemie Moleculaire et Macromoleculaire, UMR 6505 CNRS, Universite Blaise Pascal, 63177 AUBIERE Cedex, Franced Escuela de Posgrado, Universidad de Gral. San Martın, Peatonal Belgrano 3563, 18 piso, 1650 San Martın, Prov. de Buenos Aires, Argentina

Received 30 May 2006; received in revised form 14 August 2006; accepted 1 September 2006

www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 71 (2006) 101–107

Abstract

TiO2-photocatalytic reduction experiments of Cr(VI) (0.8 mM) under near UV (366 nm) irradiation in the presence of citric acid (0 � [citric

acid] (mM) � 40) were performed at pH 2 under air bubbling. Addition of citric acid facilitates Cr(VI) reduction, hindering the electron-shuttle

mechanism taking place in pure water. TOC monotonously decreases until all Cr(VI) was reduced. The maximum rate of Cr(VI) reduction was

attained for an initial citric acid/Cr(VI) molar ratio, R, equal to 1.25, a further increment in R being detrimental; however, Cr(VI) decay in the

presence of citric acid was always faster than in its absence. Cr(VI) reduction takes place through Cr(V) species, readily complexed by citrate and

detected by EPR spectroscopy. Quantitative EPR determinations indicate that an important fraction (nearly 15%) of the reduced Cr(VI) is

transformed to Cr(V)–Cit, which also undergoes a photocatalytic transformation. The detrimental effect taking place at high conversions for

R > 1.25 can be ascribed to secondary steps, i.e., the competition between Cr(VI) and Cr(V) complexes for conduction band electrons or to the

competition of Cr(V)–Cit and Cit for holes.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Heterogeneous photocatalysis; TiO2; Cr(VI); Cr(V); Citric acid

1. Introduction

Industrial processes such as leather tanning, paint making

and others make chromium(VI) to be present as a frequent

pollutant in wastewaters [1,2]. Due to its acute toxicity and high

mobility in water, Cr(VI) is in the list of priority pollutants of

many countries in the world [3–5]. Its treatment is performed

generally by transforming Cr(VI) to the less noxious Cr(III),

which is considered non-toxic and an essential trace metal in

human nutrition. Furthermore, Cr(III) can be precipitated and

removed as a solid waste. Among various reductive methods,

photocatalysis with TiO2 or similar photoactive materials has

been the object of various studies in recent times [1 and

* Corresponding author at: Unidad de Actividad Quımica, Centro Atomico

Constituyentes, Comision Nacional de Energıa Atomica, Av. Gral. Paz 1499,

1650 San Martın, Prov. de Buenos Aires, Argentina. Tel.: +54 11 6772 7016;

fax: +54 11 6772 7886.

E-mail address: [email protected] (M.I. Litter).

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.09.002

Please cite this article as: Jorge M. Meichtry et al., Heterogeneous photo

Relevance of Cr(V)–citrate complexes, Applied Catalysis B: Environm

references therein, 6–10]. Many industrial and natural waters

contain also organic compounds such as oligocarboxylic acids,

and it has been largely demonstrated that the addition of organic

donors able to act as hole scavengers or to chelate to the TiO2

surface accelerates Cr(VI) reduction in photocatalytic systems,

the synergy being dependent on the nature of the reducing agent

[11–23].

In previous works [2,24] we experimentally demonstrated

for the first time the formation of Cr(V) complexes in the

heterogeneous photocatalytic reduction of chromium(VI) over

TiO2 in the presence of oxalic acid and of EDTA. It was

proposed that successive one-electron steps passing through

reduced chromium species compose the general pathway for

Cr(VI) photocatalytic reduction. Moreover, ligands able to

form complexes with Cr(III) and Cr(V), an already proved

intermediate in the reduction of Cr(VI) in many processes, play

an especial role in the process.

On the other hand, Cr(V) chemistry has recently received a

lot of attention since their complexes with biomolecules are

catalysis of Cr(VI) in the presence of citric acid over TiO2 particles:

ental (2006), doi:10.1016/j.apcatb.2006.09.002

mailto:[email protected]

http://dx.doi.org/10.1016/j.apcatb.2006.09.002


J.M. Meichtry et al. / Applied Catalysis B: Environmental 71 (2006) 101–107102

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implicated in the mechanism of Cr(VI)-induced genotoxicity

and carcinogenicity [25,26].

Citric acid (2-hydroxy-propane-1,2,3-tricarboxylic acid,

Cit) is a model compound of several natural systems due to

its presence in plants and soils, and also a frequent domestic and

industrial chelating agent, used in the food industry and in

detergents [11].

In the present paper, we analyze the influence of the addition

of citric acid to the Cr(VI) photocatalytic system. Some features

of the system have been reported in previous papers

[11,13,21,27], but a more profound study of the system

appears to be necessary to provide explanations for solar-light

promoted processes in aquatic environments and for the use of

photocatalysis as wastewater treatment in the case of combined

mixtures.

2. Experimental

2.1. Materials and methods

Degussa P-25 was a commercial sample supplied by

Degussa AG, Germany, and used as provided. K2Cr2O7 (Carlo

Erba or Merck), citric acid (C6H8O7�H2O, Riedel de Haen),

2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPO (Aldrich) and

all other chemicals were of analytical reagent grade. Water was

purified with a Millipore Milli Q equipment (resistivi-

ty = 18 MV cm).

2.2. Photocatalytic runs

Photocatalytic irradiations were carried out in a recirculat-

ing reactor equipped with a quartz immersion well, 160 mm

length, 61.3 mm i.d., where a 125 W medium pressure mercury

lamp was placed. The lamp emitted predominantly at 365–

366 nm with smaller emissions at lower wavelengths and with

a significant emission in the visible region. A double-walled

quartz jacket filled with water, interposed between the lamp

and the well, allowed refrigeration and filtration of IR

radiation. The TiO2 suspension (500 mL) was continuously

recirculated (2 L min�1) to the photoreactor from a glass

reservoir by means of a peristaltic pump. Air was bubbled at

1 L min�1 into the reservoir, which served also to take samples

for analysis and to control pH. The irradiated volume in the

well was 285 mL. The whole setup was thermostatted at 298 K.

The total incident photon flux, P0, determined by potassium

ferrioxalate actinometry [28], was 3.3 � 10�5 einstein s�1,

using the same conditions of flow rate and volume as in the

experiments.

In all runs, 1 g of the catalyst was suspended in 800 mL of

Milli Q water, with the addition of some drops of 1 M

perchloric acid to reduce the agglomeration, and the suspension

was ultrasonicated for 4 min. Then, 118 mg of K2Cr2O7 and the

corresponding amount of solid citric acid to give the desired

final concentration were added to the suspension, adjusting pH

to 2 with diluted HClO4. The suspension was diluted to 1 L with

water, giving a final Cr2O72� concentration of 0.4 mM (0.8 mM

Cr(VI)). Prior to irradiation, suspensions were recirculated in



the dark at 298 K for 30 min to ensure substrate–surface

equilibration. The concentration of Cr(VI) after equilibration

was taken as the initial concentration, to discount changes in the

dark (which varied between 0 and 7% in the explored

conditions). Changes after prolonged stirring (60–80 min) gave

similar results and then dark reaction during the photocatalytic

runs was considered negligible. Reactions under irradiation in

the absence of TiO2 yielded also negligible Cr(VI) transforma-

tion (see later, Fig. 3 for the case R = 5). At least two

photocatalytic runs were carried out for each condition,

averaging the results.

During irradiation, 1 mL samples were periodically with-

drawn, filtered through 0.2 mm cellulose acetate filters and

diluted to 10 mL for quantitative analysis. Changes in Cr(VI)

concentration were followed by UV spectroscopy at 352 nm

[29] or by the spectrophotometric method of the diphenylcar-

bazide at 540 nm [30]. Both techniques gave similar results,

indicating no presence of Cr(V). UV–vis absorption measure-

ments were performed employing a Hewlett-Packard diode

array UV–vis, model HP 8453 A or a UV2101 Shimadzu

spectrophotometer. TOC was measured with a Shimadzu 5000-

A TOC analyzer in the NPOC (non-purgeable organic carbon)

mode.

2.3. Detection of intermediates by EPR spectrometry

EPR spectra were obtained at 298 K using a Bruker ER 200

X-band spectrometer (Bruker Analystische Messtechnik

GMBH, Germany). TEMPO (g = 2.0051) was used as a

concentration standard and as a standard for determination of g

factors, as recommended elsewhere [31,32]. Typical instru-

mental conditions were: central field, 3480 G; sweep width,

100 G; scans 1–20; microwave power, 43 mW; modulation

frequency, 100 kHz; time constant, 1–50 ms; sweep time, 1–

5 s; modulation amplitude, 1.25 Gpp; receiver gain, 1 � 105.

Appropriate volumes of stock solutions of 1 mM K2Cr2O7

and 40 mM citric acid were added to 0.1 g L�1 aqueous TiO2

suspensions previously sonicated for 5 min, and pH was

adjusted to 2 with diluted HClO4. Samples were stirred in

the dark for 10 min and then irradiated in a quartz

thin cylindrical tube inside the EPR cavity, with a 400 W,

medium pressure metal halide lamp (Phillips, HPA 400),

emitting light predominantly between 300 and 450 nm. The

output passed successively through a 10 cm water IR

filter and a long-pass glass filter, in order to isolate l �340 nm. For this setup, we determined an incident photon

flux of 5 � 10�9 einstein s�1 cm�2 using ferrioxalate as acti-

nometer [28]. In some experiments, a combination of fine

mesh metal screens of different transmittance was used to

attenuate the photon flux, rendering a value of 1 � 10�9

einstein s�1 cm�2.

EPR amplitude signals were transformed in radical

concentrations by comparing the area under the EPR

first derivative spectrum of the sample with that of a

standard aqueous solution of TEMPO, recorded at the same

microwave power, modulation amplitude and amplification

gain.




J.M. Meichtry et al. / Applied Catalysis B: Environmental 71 (2006) 101–107 103

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3. Results and discussion

3.1. Chromium(VI) decay

Curves (a) and (b) in Fig. 1 show the time profiles of Cr(VI)

decay during the irradiation of 1 g L�1 aqueous TiO2

suspensions containing 0.8 mM Cr(VI) in the absence and in

the presence of 2 mM citric acid (pH 2, continuous air

bubbling). At this concentration and pH (which did not vary

during the photocatalytic run), the main Cr(VI) species is

HCrO4� [33]. In the absence of citric acid, the reaction arrives

to an arrest, ascribed to a short-circuiting due to the continuous

reduction and reoxidation of chromium species by holes or

hydroxyl radicals (Eq. (5)); this short-circuiting can be

suppressed by the addition of a strongly competing chelating

(sacrificial) species D (Eq. (6)). In a simplified way:

CrðVIÞ þ ecb� ! CrðVÞ (1)

CrðVÞ þ ecb� ! CrðIVÞ (2)

CrðIVÞ þ ecb� ! CrðIIIÞ (3)

H2O þ hvbþ ! HO� þ Hþ (4)

CrðVÞ=CrðIVÞ=CrðIIIÞ þ hvbþðHO�Þ

! CrðVIÞ=CrðVÞ=CrðIVÞðþHO�Þ (5)

D þ hvbþ ! D�þ (6)

In our case, D is citric acid, which, besides suppressing the

short-circuiting, forms a very stable complex with Cr(V) [34–

38]:

Cit þ hvbþ ! Cit�þ (7)

CrðVÞ þ Cit ! CrðVÞ�Cit (8)

Similarly to our previous findings with oxalate and EDTA,

citric acid addition favors chromium(VI) elimination. For the

conditions in Fig. 1 (i.e., initial Cit/Cr(VI) molar ratio R = 2.5),



Fig. 1. Cr(VI) evolution with time under continuous irradiation of 1 g L�1

aqueous TiO2 aerated suspensions containing initially 0.4 mM K2Cr2O7

(0.8 mM Cr(VI)) (a) in the absence and (b) in the presence of 2 mM citric

acid at pH 2. Curve (c): TOC decrease for the experiment containing citric acid.

Curve (d): TOC decrease for the experiment containing citric acid in the

absence of Cr(VI). Total incident photon flux, P0 = 3.3 � 10�5 einstein s�1.

we noticed a five-fold enhancement of the initial rate of Cr(VI)

reduction. It should be stressed that Cr(III) was invariably found

in the spectra of the filtered solution at long irradiation times.

Fig. 2 shows the spectra of the filtered solutions of the

photocatalytic experiment at t = 0 and after 60 min of irradiation

in the 400–600 nm range. The peak around 575 nm can be

assigned to the absorption of the Cr(III)–Cit complex [34], and

this is confirmed by the comparison with the spectrum of a

solution prepared with 0.45 mM Cr(III) and excess citric acid at

pH 2, which roughly coincides in the 450–600 nm range. The fact

that the observed absorption is lower than that expected if all

initial Cr(VI) would have been transformed to Cr(III)–Cit

(0.8 mM) can be attributed to two main reasons: (1) degradation

of Cit and formation a less absorbing Cr(III) species with

oxoglutaric acid or some other intermediate of citric acid

degradation, this substantiated by a peak at 290 nm (Fig. 2(b)), in

agreement with the conclusions of Hug and Laubscher [39] for a

similar system (photoinduced Cr(VI) reduction by Fe(III) in the

presence of citrate); (2) adsorption of Cr(III) onto the TiO2



Fig. 2. (a) Spectra at t = 0 and after 60 min under continuous UV irradiation of

1 g L�1 aqueous TiO2 aerated suspensions containing initially 0.4 mM K2Cr2O7

(0.8 mM Cr(VI)) and 1 mM citric acid at pH 2 in the 400–600 nm range. The

spectrum of a solution containing 0.45 mM Cr(NO3)3 and 1.125 mM citric acid,

pH 2 is also shown. (b) Same in the 250–350 nm range. Total incident photon

flux, P0 = 3.3 � 10�5 einstein s�1.



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Table 1

Initial apparent pseudo-first-order rates for Cr(VI) reduction at different citric

acid concentrations ([Cr2O7K2]0 = 0.4 mM, 0.8 mM Cr(VI))

Citric acid (mM) R k0 (�104 s�1)

0 0 6.0

0.1 0.125 18.7

0.4 0.5 26.3

1 1.25 31.4

2 2.5 27.4

4 5 23.8

10 12.5 21.4

40 50 15.4

Conditions as in Fig. 1, R: initial citric acid/Cr(VI) molar ratios.

surface, evidenced by a greenish color on the catalyst surface

after irradiation [19].

TOC evolution in the absence (curve (d)) and the presence

(curve (c)) of Cr(VI) is also shown in Fig. 1. It is interesting to

observe that TOC decrease is higher in the presence of the metal

in the first stages (cf. �15% with �10% at 20 min in both

conditions), but at longer irradiation times (120 min) there is an

inhibition in TOC degradation. We propose that this may reflect

the deactivation of the photocatalyst attributed to Cr(III)

adsorption.

We reasoned that higher initial citric acid/Cr(VI) molar

ratios, R, would always favor Cr(VI) depletion, eventually

reaching a saturation limit, after which a further addition of the

donor would be meaningless. However, as shown in Fig. 3, we

found that increasing R above 1.25 was detrimental. This

behavior has been observed in other systems, e.g., Cr(VI) plus

salicylic acid [11,12] but not completely analyzed. Table 1

summarizes these results by giving the initial apparent pseudo-

first-order rate constants, calculated from the plots, as a

function of R. However, rates were always higher than in the

absence of citric acid.

In an attempt to rationalize the above findings, the evolution

of the primary intermediate, Cr(V) species, was analyzed. In

homogeneous systems, the formation of chromium(V) com-

plexes with citric acid has been previously observed by UV [34]

and EPR spectroscopy [37,36]. Although the structure of

citrate, a 2-hydroxycarboxylato ligand, makes the Cr(V)–Cit

complexes quite stable [38], and their detection by UV–vis

spectroscopy has been reported (absorption at l = 350 nm,

e ffi 2000 M�1 cm�1 [34]), they cannot be easily detected in

complex reaction mixtures. Conversely, Cr(V)–Cit can be

readily observed by EPR spectroscopy due to the specificity of

the technique and its high sensitivity. Fig. 4 shows the spectrum

obtained in a typical experiment performed under irradiation of

a 0.1 g L�1 TiO2 suspension containing initially 0.4 mM

K2Cr2O7 (0.8 mM Cr(VI)) and 4 mM citric acid at pH 2 in

the cavity of the EPR spectrometer. The signal is centered at



Fig. 3. Cr(VI) evolution with time under continuous UV irradiation of 1 g L�1

aqueous TiO2 aerated suspensions containing initially 0.4 mM K2Cr2O7

(0.8 mM Cr(VI)) and several citric acid concentrations. R: initial citric acid/

Cr(VI) molar ratios. Total incident photon flux, P0 = 3.3 � 10�5 einstein s�1.

giso = 1.977 with four 53Cr hyperfine satellites (coupling

constants 18.7 G). The spectra is almost identical to that

reported in the above references and has been ascribed to

CrO(CitH2)2�.

Irradiation experiments of TiO2 suspensions in the EPR

cavity in the same conditions as before but with variable citric

acid concentration were performed in order to obtain the time

profiles of the Cr(V)–Cit complex. In this case, the field was

fixed at g = 1.977 to follow the evolution of the signal

amplitude. A small paramagnetic signal of Cr(V)–Cit com-

plexes appeared immediately after mixing the solutions, either

in the presence or in the absence of TiO2. It is worthwhile to say

that we carefully checked that the initial amplitude remained

constant in the dark and even under irradiation in the absence of

the semiconductor, in the time range of the EPR experiments

(typically 10–20 min). The high sensibility of the EPR

technique allows us to determine initial rates of Cr(V)–Cit

formation at low, negligible Cr(VI) conversion (i.e., without the

influence of secondary steps or intermediates). Fig. 5 shows that

the time evolution of chromium(V) species is nearly

independent of Cit above R � 1.25. These observations could

simply reflect the fact that the trapping of chromium(V) species

is not kinetically controlled and that the increase of Cit

concentration above 1 mM (R = 1.25) does not modify the

initial rate of Cr(V) formation (Cr(VI) reduction). Thus, the

observations in Fig. 3, i.e., the inhibition at high R values,

should be ascribed to some secondary step, for example the

competition between Cr(V)–Cit complexes and Cr(VI) for

conduction band electrons at higher conversions (reactions (1)

and (9)):

CrðVÞ�Cit þ ecb� ! CrðIVÞ (9)

or to the competition of Cr(V)–Cit and Cit for holes (reactions

(7) and (5)). Further work on this issue is in progress.

In the experiments carried out in the recirculating reactor

(Fig. 1), we proposed that the amount of Cr(V) and Cr(IV) was

very low, a result different from that obtained for Cr(V) during

the EPR experiments shown in Fig. 5. It is noteworthy to

remember that the irradiation intensity was much higher in the

experiments of Fig. 1 and that the experiments in Fig. 5 involve

negligible Cr(VI) conversion.

We performed a set of EPR experiments with lower

chromium concentrations and the higher photon flux attainable

in this setup in order to observe the time evolution of the





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Fig. 4. EPR spectra of the Cr(V)–Cit complex formed under continuous UV irradiation of a 0.1 g L�1 TiO2 aerated suspension at pH 2. Conditions: [citric

acid] = 4 mM, [K2Cr2O7] = 0.4 mM (0.8 mM Cr(VI)). Central field: 3483 G, modulation amplitude: 1.25 Gpp, gain: 1.25 � 105. The inset shows the structure of the

complex proposed in Ref. [36]. Incident photon flux = 5 � 10�9 einstein s�1 cm�2.

complex at measurable Cr(VI) conversions, and to quantify the

relevance of Cr(V)–Cit complexes in the mechanism. Fig. 6

shows the results of an irradiation experiment of a TiO2

suspension (0.1 g L�1, 50.5 mM initial K2Cr2O7 = 101 mM

Cr(VI), R = 5, pH 2). Curve (a) shows the profile obtained under

continuous irradiation, while curve (b) describes the behavior

observed in a separate irradiation experiment in which the light

was turned off when the signal was at its maximum. The

persistence of the signal in the dark is consistent with the high

stability of the complex. Spectrophotometric measurements



Fig. 5. Time dependence of [Cr(V)–Cit] obtained under continuous UV

irradiation conditions of a 0.1 g L�1 aqueous TiO2 suspension containing

initially 0.4 mM K2Cr2O7 (0.8 mM Cr(VI)), pH 2 and variable citric acid

concentrations. R: initial citric acid/Cr(VI) molar ratios. Incident photon

flux = 1 � 10�9 einstein s�1 cm�2.

indicated that, after 2 min of irradiation, 40% of Cr(VI) was

reduced, which corresponds to 40.4 mM. Thus, the amount of

complex at the maximum represents nearly a 15% of Cr(VI)

losses, indicating that an important fraction of chromium

reduction occurs through the Cr(V)–citrate complex. This result

sharply differs from our previous findings with oxalic acid or

EDTA, where only a small amount of complexes was detected

[2,24].

To reinforce our conclusions, a comparison of the oxalic acid

and citric acid systems under similar conditions (0.1 g L�1



Fig. 6. Time dependence of [Cr(V)–Cit]. Curve (a): profile obtained under

continuous UV irradiation of a 0.1 g L�1 aqueous TiO2 suspension containing

initially 50.5 mM K2Cr2O7 (101 mM Cr(VI)) and 0.5 mM citric acid, R = 5, pH

2. Incident photon flux = 5 � 10�9 einstein s�1 cm�2. Curve (b): same as (a)

but light was turned off when the signal was at its maximum.



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Fig. 7. Comparison of [Cr(V)–Ox] and [Cr(V)–Cit] generation under continuous irradiation of a 0.1 g L�1 TiO2 aerated suspension. Initial conditions: [citric acid] or

[oxalic acid] = 1 mM, [K2Cr2O7] = 0.2 mM (0.4 mM Cr(VI)), pH 1.5, T = 298 K. Incident photon flux = 1 � 10�9 einstein s�1 cm�2.

TiO2, 1 mM citric acid or oxalic acid, 0.4 mM Cr(VI), pH 1.5)

was performed. pH 1.5 was chosen due to the instability of

Cr(V)–oxalate at higher pH [2]. The time evolution of both

Cr(V)–L complexes is shown in Fig. 7. It is apparent that a

much higher amount of Cr(V) is accumulated in the presence of

citric acid either under irradiation or in the dark (t = 0). It is

worthwhile to notice that the monotonous increase of Cr(V)–

Cit complexes observed in Fig. 7 is a mere consequence of their

rate of formation being much higher than their rate of

destruction in the conditions of these EPR experiments, which

involve very low Cr(VI) conversions. However, the differences

observed in curves (a) and (b) of Fig. 6 clearly indicate that the

removal of the citrate complex is indeed feasible upon

irradiation in the photocatalytic system.

4. Conclusions

Addition of excess citric acid facilitates Cr(VI) TiO2

photocatalytic reduction in comparison with the same system in

the absence of the donor. This synergy helps the reaction by

hindering the electron-shuttle mechanism that occurs in pure

water. The rate of hexavalent chromium transformation is a

function of the amount of citric acid present in the medium: the

maximum rate was attained for an initial Cit/Cr(VI) molar ratio

equal to 1.25. Higher citric acid amounts are detrimental, but

the efficiency is always better than in pure water.

According to EPR experimental evidences, Cr(VI) reduction

is initiated by an one-electron process forming Cr(V), which in

the presence of citric acid is readily trapped as the very stable

CrO(CitH2)2� species. Quantitative EPR determinations

indicate that an important fraction (nearly 15%) of the reduced

Cr(VI) could be detected as Cr(V)–Cit during the course of an

irradiation experiment. The photocatalytic decomposition of

Cr(V)–Cit was ascertained by interrupting the light after a



certain fraction of the complex was formed and observing that

the amplitude of the paramagnetic signal was constant, at

variance with the results obtained under continuous irradiation.

Based on the fact that above R = 1.25 Cr(V)–Cit formation

rates at extremely low conversions are nearly independent of

citric acid concentration, we proposed that the detrimental

effect determined at higher conversions (for R > 1.25) should

be ascribed to secondary steps, i.e., the competition for

conduction band electrons between Cr(VI) and Cr(V) com-

plexes or to the competition of Cr(V)–Cit and Cit for holes.

Because of the stability of the complex, a large fraction of the

reduced Cr(VI) accumulates in this form, a fact different from

that observed with EDTA or oxalate.

The distinctive behavior of the Cr(VI)/citric acid photo-

catalytic system here reported in contrast with others could

explain processes in natural waters. The possibility of the

formation of very stable and carcinogenic Cr(V)–Cit com-

plexes should be taken into account when using TiO2

photocatalysis for remediation of systems containing simulta-

neously Cr(VI) and citric acid. However, this paper evidences

that, although the persistence of Cr(V)–Cit may be long and it

should be continuously evaluated during the treatment, it finally

photocatalytically decays, ending in less noxious Cr(III)

species.

Acknowledgements

This work was performed as part of Comision Nacional de

Energıa Atomica P5-PID-36-4 Program, CONICET/CNRS

grant for cooperation between Clermont Ferrand and Argentine

laboratories, Agencia Nacional de la Promocion de la Ciencia y

la Tecnologıa, PICT03-13-13261 and PICTO 02-006-11307.

MIL and MAG are members of CONICET. JMM thanks

CONICET for a doctoral fellowship.





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http://www.lboro.ac.uk/departments/cg/Projects/2002/gill/Res_Spec_Diagram.htm

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