Pergamon

Chemosphere, Vol. 33, No. 8, pp. 1531-1542. t996 Copyright © 1996 Published by Elsevier Science Ltd

Printed in Great Britain. All rights reserved PII: S0045-6535(96)00291-3 0045-6535/96 $15.00+0.00

PILOT-PLANT PHOTOMINERALIZATION OF DICHLOROMETHANE AND

TETRACHLOROETHENE IN AQUEOUS SOLUTION, BY PHOTOCATALYTIC MEMBRANES

IMMOBILIZING TITANIUM DIOXIDE AND PHOTOPROMOTERS ~

Franco Gianturco and Luca Vianelli Chimia Prodotti e Processi, 1-20124 Milan, Italy

Luca Tatti, Fabrizio Rota, Paolo Bruzzi, Laura Rivas and Ignazio Renato Bellobono* Department of Physical Chemistry and Electrochemistry, University of Milan; via C. Golgi, 19, 1-20133 Milan, Italy

Michele Bianchi and Herbert Muntau Commission of the European Communities, Joint Research Centre, 1-21020 Ispra (VA), Italy

(Received in Germany 24 April 1996; accepted I1 July 1996)

ABSTRACT

The TiO2-mediated photomincralization of 5.0x10-6-5.0x10 -7 M aqueous solutions of dichloromethane

(DCM), tewachlorocthene (PCE), and mono-, di-, and trichloroethanoic acids (MCEA, DCEA, and TCEA

respectively) was studied at 296 + 2 K, using PHOTOPERM® CPPI313 membranes containing irranobilized 30 +

3 wt.% TiO 2, and, in some of the runs with PCE, 7 wt.% of photocatalytie promoters based on Co III, V V, and

Fe III organometallic compounds. Disappearance of both substrate and total organic carbon (TOC) was examined,

as well as chloride ion formation. The apparent reaction order being unit in the range of concentration considered,

half times for these three kinetic processes have been measured in a PHOTOPERM® WW membrane module,

fitted with 1 m 2 of membrane, with an absorbed radiating power of 31 W, and saturated with air. With PCE, the

marked decrease of rate (to 1/25) with decreasing 0 2 concentration (to 2-3 ppm), and the positive effects of

promoters or of applying an anodic bias (enhancement of rate up to about 3/1) were ascertained. The role of

oxygen as scavenger for photogenerated electrons is much preferable to that of adsorbed substrate to scavenge

0 Part 51 of the series "Photosynthetic Membranes".

1531

1532

these conduction band electrons, both to explain the efficient action of photopromoters as oxidant scavengers or

oxygen transporters, and redox processes involving formation of chlorinated intermediates, such as DCEA, during

radical reactions, as TOC disappearance from DCEA, MCEA, and TCEA proceeds at a much slower rate than that

experimentally observed for PCE. Copyright © 1996 Published by Elsevier Science Ltd

INTRODUCTION

In 1972 Fujishima and Honda discovered the photocamlytic splitting of water on TiO 2 electrodes (1). This

event marked the beginning of a new era in heterogeneous photocatalysis. Twenty years later (2), it has been

proved that immobilization of TiO 2 onto microporous membranes prepared by photografting is able to maintain

substantially unaltered the electrochemical behaviour of immobilized photocatalyst, the anndic membrane, coupled

with a hydrogen or oxygen cathode yielding the same energy efficiency as that of the Fujishima -Kahayakawa -

Honda - type cell (3). Besides applications related to energy renewal and storage (4), in the last fifteen years

applications of heterogeneous photocamlysis, driven by semiconductors, to environmental cleanup have been one of

the most active areas of research. In previous papers of this series (5, 6), it has been shown that industrial

applicability of these processes has become effective only when photocatalytic systems could be convenientiy

immobilized in massive amounts into m i ~ s membranes, and in such a way as to remove kinetic drawbacks of

thin film immobilization, that is by allowing full ~ b i l i t y and very high fluxes through the supporting structure,

while retaining completely active surface area of inunobilized photocatalyst. By this way, quantum yields at least

two order of magnitude greater than those relative to thin films of immobilized semiconductor have been obtained

(7) in pilot-plant-scale studies relative to photomineralizatiou of trichloroethene (TCE) and atrazine.

In a recent study (8), implications for the use of photocatalysis for the treatment of groundwater

contaminated by polychlorinated alkanes or alkenes, such as TCE or tetrachloroethene (PCE), have been discussed,

due to the possibility that dichloroethanoic acid may result as degradation intermediate, this latter or other potential

chlorinated intermediates being possibly more toxic than the starting chlorinated hydrocarbons themselves. A

photoreductive pathway, based on a direct reaction of substrates with conduction band electrons has also been

proposed (8).

In the present paper, systematic research on model compounds having environmental impact is being

continued, by means of pilot-plnnt-scale studies, onto photosynthetized membranes iron~lizing TiO 2 and other

photocatalytic promoters, such as those employed in previous investigations (9, 10), both with the aim of

confirming the full suitability of this emerging technology, bypassing any unfitted use of suspended photocatalyst,

the inconveniences of which have been amply underlined (5), and with the purpose of examining critically the

problem of chlorinated intermediates, of their origin, and of their fate.

EXPERIMENTAL

Materials

Dichtoromethnne (DCM), tetrachloroethene (PCE), and mono-, di-, and trichioroethanoic acids (MCEA,

DCEA, and TCEA, respectively) were obtained from Aldrich (purity greater than 99.5%). They were used as

received with no further purification. Solutions were prepared with ultrapure water (maximum contents of Na + and

1533

heavy metal ions 0.02 and 0.004 mg Kg -1 respectively): this was obiained by cross-flow ul~afillrafion on composite

memlxanes immobiH~ng active carbon and nuclear grade ion exchange resins, as de~'ibed (11). As pilot-plant

operations of the present paper were intended to study the removal of chloro-organics from solutions simulating

water for drinking purposes, DCM, PCE, MCEA, DCEA, and TCEA degradation kinetics were examined at

concentrations in the range 5.0x10-6-5.0x10 -7 M; so that no buffe¢ system was added to solutions. Initial pH was

6.7-7 and it dropped to no lower than 5.1 when produced HCI content was greater than 95% with respect to

equivalent initial concentration of substrate.

Photocatalytic membranes

Standard photocatalytic memiranes used in pilot-plant-seale experiments were the same as those already

described (5).

In some of the blends, however, prior to membrane manufacture, varying amounts of proprietary

photocatalysts (I-VII) (by Chimia Prodotti • Proccssi, Milan, I) were added in order to sensitize the semiconductor

outside its optical absorption range at wavelengths greater than that corresponding to its bend gap. These

photucatalysts included stabilized preparations containing the following organometallic compounds: p-peroxo-bis

[N,N'-ethylenc-bis (salicylideneiminato) cobalt(Ill)] (I), p-peroxo-bis [N,N'-ethylene-bis (salicylidenciminato)

dimethylformamide cobalt(HI)] (lI), triethylvanadate(V) (111), oxo-(diquinolyloxo)vunadic(V)acid (IV),

wis(phenylsilyl)vanadium(V) (V), syne~iistic mixture of tri-(t-butyl)- and tri-(i-propyl)vanadate(V) (VI), and

iron(m) potassium oxalate (VII). As the only information available on composition of these photocatalyfic systems

was their nominal concentration of photoactive species, they were used as supplied, on the basis of this nominal

concentration.

Pilot plant and procedures

The pilot plant eraployed in this work was the PHOTOPERM® WW unitary module, described previously

(5), using a low pressure mercury arc lamp (emission at 254 nm 31 :i: 1 W, as measured actinometrically). Kinetic

procedures were also the same (5). The disappearance of chloro-orgunics was followed gas-chromatographically,

either by head space analysis and an electron capture detector, or by electron impact ionization with purge and trap

pre-treatment and internal calibration standards. Nephelome~,ic or microcoulomme~c analysis of chloride ions was

carried out simultaneously to check chlorine minerali~fion (7). Total organic carbon (TOC) analysis was also

carried out as described (5).

The solutions being processed were continuously saturated by air, by employing the saturation column

making part of the PHOTOPERMO WW unit, in series with the 1.5 m 3 reservoir tanL The whole apparatus was

gas tight, in order to avoid any leakage of volatile chloro-organics towards the exterior. The mean temperature of

the reservoir was 296 + 2 K.

In order to examine the influence of oxygen concentration in solution, this latter, which was continuously

monitored by an oxymeter (Amel, Milan, l), was decreased, in some of the runs, to very low values (less than 0.8

ppm 02) by substituting the air flow with pure nitrogen

Hnally, in some of the experiments for complete mineralization of PCE, at initial concentration of 1.0xl0 "6

M, an anodic bias of +0.8 V was applied to the membranes, as was done in a lm:Vions paper during mineralization

of TCE (7). To this p ~ the standard membcuncs were photografted onto a highly perforated metal network,

the surface of which was appropriately treated, to facilitate chemical bonding (6). A cotmne~al 99.9% titanium

1534

network was used as support for membrane preparation. It was first thermally oxidized at 1300-13500C, until the

dioxide film thickness was about 5 pm. The resulting TiO 2 film was successively sprayed on both sides of the

network with 80-~1 g.m -2 of the standard prepolymerie blend (5), containing the semiconductor without any other

photocatalytic promoter. Only a small region was left bare in order to provide the electrical connections. Then

photografting was carried out by the usual technology. This same procedure was followed to prepare membranes

containing VI as photocatalytie promoter, to be subjected to biasing experiments, in the same way as the standard

membranes.

RESULTS AND DISCUSSION

Rates of substrate disappearance, of chloride ion producttion, and of total organic carbon mineralization

For integral photodegradation runs carried out with 5.0x10-6-5.0x10 -7 M aqueous solutions of DCM, PCE,

MCEA, DCEA, and TCEA saturated by air, the apparent reaction order was always substantially unit, due to the

range of very low concentrations examined. This had also been found in laboratory-scale experiments relative to

DCM and PCE, MCEA, DCEA, and TCEA (10, 12) as well as in pilot-plant-male runs relative to "ICE (10) and

phenol (5). The dependence of the apparent reaction order on concentration of substrate (unit order at relatively

low concentrations and zero order at relatively high concentrations, with an intermediate transition) is a direct

consequence (5) of the role played by the two experimental parameters, k and K, expressing the linearity of the

double-reciprocal plot of initial rate (tO) vs.initial concentration (C0):

(1/r O) = (l/k) + (I/kKC O) (1)

Following the Langmuir-Hinshelwood interpretation to one of these (k) a kinetic significance of zero order

reaction, which occurs at the interface between photocatalyst and solution, is attributed, the other one being usually

considered, formally at least, a thermodynamic constant (h0 of the adsorption equilibrium of the substrate onto the

semiconductor surface itself. Recent experimental observations have thrown some light on the influence of several

variables on the "thermodynamic" parameter K. It has thus been ascertained that this parameter depends on

radiation kind or radiation wavelength, on flow rate, and on concentration of oxidizing chemicals employed during

photodegradation (5, 10). On the basis of these considerations as well as by heeding the dependence of apparent

reaction order on initial concentration, as stated above, to both of these parameters a predominantly kinetic

significance should be given, in full accordance with a well established critical assessment regarding the mechanisms

of semiconductor-mediated photocatalysis (13).

Half times tll 2 for disappearance of substrate, for chloride ion production, and for disappearance of total

organic carbon by transformation into carbon dioxide have been measured in the PtlOTOPF.RM® WW pilot unit,

with DCM, PCE, MCEA, DCEA, and TCEA as subs~ates, in aqueous solutions, onto standard photocatalytic

membranes (without any photocatalytic promoter) at various values of initial concentrations of substrates. Results

are reported in Table 1.

Some experimental facts readily appear, by examining data of Table 1. First of all, as observed above, the

unitary reaction order for all specific rates which may be calculated from these data, within the range of

concentrations tested in this work.

More importantly, the following remarks may be inferred:

1535

i) With the exception of DCM and MCEA, rates of chlorine mineralization differ from those of substrate

disappearance, these latter exceeding them in value, by a factor ranging roughly from 2 to 3. Even if detection

and/or analysis of chlorinated intermediates was outside the scope of the present paper, this is perfectly consistent

with the fact that chlorinated intermediates have been isolated during semiconductor-mediated photocatalysis, in

most (13, 14) though in not all (15) investigated cases. Furthermore, in case studies, such as that of TCE

photodegradation (8), in which dichloroacetaldehyde and DCEA have been isolated, their combined yields were so

low (1-4%), that there is no possibility of detecting their presence by direct kinetic measurements of substrate

disappearance and of chloride ion production (10), as the uncertainty of the latter overcomes their eventual

difference. In other words, it certainly means that in these cases 95% or more of the photocatalytie degradation

paths exclude the routes via chlorinated intermediates.

As regards data of the present paper, it may be easily checked that, while for DCM half times for substrate

degradation and chloride ion formation coincide with confidence within the range of experimental uncertainty, for

MCEA a very small difference seems to exist, even if nearly overlapping with the experimental error. On another

side, for these substrates, the formation of chlorinated intermediates, is objectable also on the basis of chemical

considerations.

ii) For all the substrates tested in this work, with the exception of DCM, rates of full mineralization are

slower than those of chloride ion formation, with a deceleration factor up to about 2. This is notoriously due to the

formation of less reactive oxidized intermediates, prior to their final oxidation to CO 2. For DCM, on the contrary,

the formation of formic acid, which is probable on chemical grounds, does not seem to affect the rate, presumably

owing to the fact that formic acid is more easily attacked by oxidizing radical species, such as .OH, than the starting

substrate molecule.

iii) Reaetivities of MCEA, DCEA, and TCEA are about one order of magnitude lower than those of PCE.

If, in the experimental conditions of the present study, yields of DCEA as intermediate would have been in the

range of 10-15%, as those reported during photodegradation of PCE in the presence of TiO 2 suspension at pH 8

(8), total organic carbon oxidation times would have been at least 7 times higher, owing to the apparent first order

dependence of rate on concentration. This should mean either that DCEA or other chloroethanoic acids are not the

chlorinated intermediates in the conditions of the present study, or that they could be; but in this circumstance their

isolation as intermediates during kinetic runs implies their formation as ground state levels from more reactive

radical intermediates, which by themselves would not have given rise to these chlorinated species other than by

quenching. As a matter of fact, kinetic samples analysed gas-chromatographically did reveal the presence of DCEA,

even if in maximum amounts not exceeding 6-7%. Besides this experimental evidence, the second alternative looks

much preferable to the former, also when considering that during photocatalyzed degradation of phenol (5) TOC

removal proceeds at a rate which is slightly more than one order of magnitude less than that of disappearance of

phenol and seven times slower than that with which opening of the poly-hydroxylated aromatic ring occurs. These

rate ratios between rate of photodegradation of the starting substrate more susceptible to radical attack by oxidising

radicals and rates of the more sluggish alkanoic acids with two or more carbon atoms is confirmed by measurements

of the present work. The much lower rate factor observed for TOC disappearance with respect to that of chlorine

mineralization for PCE should consequently receive a different explanation, along the line suggested above.

1536

TABLE 1

Half fanes tl/2 (min) a for disappcaran~ of substratc, for chloride production, and for total organic carbon (TOC)

disappearance, relative to photocatalyzed degradation of 1 nO of DCM, PCE, MCEA, DCEA, and TCEA aqueous

solutions, over TiO 2 ~ onto photograft~ ~ (1 m 2 of apparent gcomc~cal surface), by

operation carried in the WW pilot plant, in which 31 + 1 W of radiating energy at 254 nm were intcgrally absorbed,

as a function of initial concentration C O of substrate.

C O . 107 (M) substmte disappearance chloride ion production T(X~ disappearance

tl/2.10 -2 (min) tl/2.10 2 (min) tl/2 . 10 -2 (min)

~chlorome~e(DCM)

5.0 2.9±0.3 3.5±0.5 3.6±0.4

10 3.1±0.3 3.4±0.3 3.3±0.4

20 3.3±0.3 3.5±0.2 3.6±0.5

35 3.2±0.3 3.3±0.2 3.3±0.3

50 3.2±0.2 3.4±0.2 3.5±0.3

~trachloro¢~e~0aCE)

5.0 2.4±0.2 5.8±0.3 I0±I

I0 2.2±0.2 5.9±0.3 9.4±0.5

2.6±0.3 5.7±0.2 9.7±0.4

35 2 .3i0 .2 6.0±0.3 9.5±0.4

2.5±0.3 6.1±0.4 9.6±0.5

Monochloroethanoic acid (MCEA)

I0 31 +2 36+4 82+7

50 33 ±I 35 + 3 86 + 6

Dichlotoethanoic acid (DCEA)

10 24:1: I 41 +4 68±4

50 25:1:2 44+3 70:1:4

Trichlorocthanoic acid (TCEA)

10 15:t: 1 32:t:3 56+5

50 17 ± 3 35 + 2 54:1:4

a Uncertainty, expressed as probable error, was obtained from at least 3 experimental runs at each concentration,

the mean value of which is reported in this Table.

1537

As the formation of polychloreethanoic acids has been attributed to a mechanism involving direct reduction

of substrate by conduction bend eluclrons of irradiated semiconductor (8, 15), further investigation was carried out

in the present paper, by examining', a) the influence of some known photocatalytic promoters (9, 10) co-

immobilized with the semiconductor into the membrane,, b) the influence of oxygen diffusion rate, and c) the effect

of applying an aondic bias. These studies are motivated by some well ascertained facts: on one side,

photoelectrocbemical properties of semiconductors (2, 16) are strictly related to their behaviour in photocatalysis

and in photucatalytic membranes; on another side, it is known since long time in radiation and radical chemistry (17,

18) that .O2H is a moderately reactive oxidising radical, even if at a lower potential than that of the .OH/O 2 couple

(19), while its conjugate base .O 2- may act as a reducing agent by electron transfer.

Influence of promoting photocatalysts

Photocatalytic degradation of PCE was studied, at a concentration of 1.0xl0 -6 M, onto photosynthetic

membranes immobilizing 7 wt.% of photocatalytic promoters (I-VI~. Results are reported in Table 2, in the form

of ratios R between half times for disappearance of substrate, for chloride ion production, and for total organic

carbon transformation into CO2, in the presence and in the absence of (I-VID respectively. It may be observed that

a more or less marked increase of reactivity clearly appears, as shown by the R values lower or much lower than

unity.

TABLE 2

Ratios R (mean values of three runs) between half times for disappearance of substrate, for chloride ion production,

and for total organic carbon (TOC) disappemanee, in the presence of 7 wt.% of photucatalytic promoters (I-VH)

co-irmnobilized in the membranes, and those in their absence, relatively to photucatalyzed degradation of 1 m 3 of

1.0xl0 "6 M PCE, in aqueous solution, over TiC 2 i ~ onto photografted memlmmes (I m 2 of apparent

geometrical surface), by operation carried in the grt$' pilot plant, in which 31 :t: 1 W of radiating energy at 254 nm

were integrally absorbed.

Photucatalytic promoter

substrate disappearance R

chloride ion production TOC disappearance

I 0.84 0.89 0.86

l I 0.73 0.77 0.80

m 0.59 0,62 0.65

IV 0.71 0.75 0.73

v 0.55 0.48 0.52

v I 0.34 0.29 0.31

v i i 0.48 0.45 0.49

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As the uncertainty of data in Table 2 is of the order of 0.03-0.06, it clearly appears from these data that

acceleration factors on rates are substantially the same, within the limits of error, for all the three kinetic processes

analysed experimentally,independently on the chemical nature of the photopromoter and of kind of ligands present

in the organometallic compounds. On the contrary, the relative effects are strongly dependent on the central atom

and on the nature of the coordinated ligands. These effects are anyway lower than those observed for TCE in

previous studies ((9, 10), for which an acceleration factor amounting to about 20 (for VI) was observed, when

irradiation was carried out by a medium pressure mercury are lamp, while a maximum acceleration factor of about 3

is observed for the same photopromoter in the present investigation relative to PC'E, with a low pressure mercury

arc lamp irradiation. The reason is due to the fact that an important mechanism of action of these additives (10) is

to sensitize and photoactivate the semiconductor dioxide outside its optical absorption range. As a consequence the

quantitative efficacy and outcome depends on the spectrum of the irradiating source, and on the available energy as

a function of wavelength.

Another possible mechanism of action of these promoters, which has been proposed (6, 10), may most

seemingly be due to their nature of effective oxygen transporters, able to enhance rate of scavenging of electrons in

the conduction band of the irradiated semiconductor, thus decreasing the probabifity of their recombination with

holes, and facilitating the production of .O 2- and .O2H radical species.

Influence of oxygen concentration and diffusion rate

In order to check the possible occurrence of this latter kind of mechanism during promoted photocatalysis,

two kind of kinetic runs were carried out with PCE as substrate, in the experimental conditions of Table 2. In the

first kind of runs, standard photocatalytie membranes have been used, with no addition of promoting

photocatalysts. In the second kind of experiments, membranes containing 7 wt.% of VI have been employed. In

both kinds of runs, however and contrarily to operating situation of the runs of Tables 1 and 2, oxygen

concentration in solution was decreased down to at least 2-3 ppm, by acting as described in the experimental

section. This oxygen concentration was anyway in stoichiometrie excess, given the very low PCE content which

was present.

In these conditions, rate of TOC disappearance, when using standard photocatalytic membranes, decreased

to very low, almost unmeasurable, values, about 1/25 with respect to those reported in Table 1. The same was

observed for rates of chloride ion production and of substrate disappearance. This behaviour is perfectly in line, on

one side with the observed function and currently discussed models about diffusion of oxidizing species, oxygen

particularly, as rate limiting step in these photoeatalytic processes (13, 15), and on the other side with

photoelectrochemistry of TiO 2 (1-4, 16) requiring a fast and effective scavenging action of the oxidizer towards

conduction band electrons, or alternatively an electrochemical bias, in order that photolysis of water may occur, in

proficient competition with electron-hole recombination. In other words, if scavenging of conduction band electrons

proceeds by retarded speed (oxygen concentration seems to play a critical role, following the results of the present

study), photocatalytic processes are strongly inhibited.

When, in the same operating conditions, the standard photoeatalytic membrane was replaced by that

containing photoeatalytic promoter VI, rate of photoeatalytic processes was restored to almost the expected value,

even ff oxygen deficiency was maintained, with respect to the saturation value. As a matter of fact, in these

circumstance a value of R equal to 0.75 + 0.08 was measured, corresponding to a rate which was only about one

half, with respect to that observed in the presence of oxygen saturation (see Table 2). This should mean that

photocatalytic promoter VI is particularly efficient to act directly as electron scavenger at the surface of the

1539

irradiated semiconductor, but a mechanism based on facilitated oxygen transport is also present, since the effect of

oxygen concentration is also clearly noticeable, as has been evidenced experimentally.

From this latter point of view, it precisely appears that the photocatalytic promoter VI, and not in anyway

the substrate itself, is able to drive the photodegradation mechanism via the oO 2" and oO2H radical species,

because, as stated above, and in accordance with all the known principles and mechanisms regarding photocatalysis

on TiO2 surfaces (20), without this possible route no production of .OH radicals is conceivable. To add more,

reductive mechanisms, able to explain the formation of some chlorinated intermediates, as have been proposed (8,

14), do not need necessarily a scavenging action by the subslrate, which should be experimentally demonstrated (no

evidence has been indicated or revealed for it), but simply imply the reducing aptitude of .O 2- radical, which is

known and has been proved in many cases (17, 18). This nmy hold particularly in relatively basic media, in which

the acid-base equilibrium of the .02- / .02H radical couple is displaced towards the former, thus inhibiting the

oxidizing action of the latter. Differently speaking, to understand the electron scavenging processes by oxygen

during the photooxidation of organics, the role of .O 2- radical ions formation has been highlighted in the literature

(20), photoadsorption and photodesorpfion of these or other labile oxygen species being the basis to explain all

redox processes occurring.

Influence of anodic bias

Immobilization of semiconductors in a membrane structure not only affords the possibility of realizing an

almost continuous and thick film, thus removing drawbacks of the use of suspensions, but offers additional

advantages. One of these has been shown in the effect of co-immobilizing photocatalytic promoters. Another one

stems out from electrochemically assisted photocatalysis on thin films (21, 22) and on raembranes with massive

amounts of semiconductors (7). In a slurry, semiconductor particles behave as short-circuited microelectrodes

under band gap excitation, thus promoting the redox processes on the same particle. In this latter system, however,

degree of recombination between pbotogenerated charge carriers is high, and quantum yields consequently low. In a

particulate film, in contrast, one can drive away the photogenerated electrons from the semiconductor particle with

the application of an electric field. By an anodic bias of +0.8 V, an acceleration factor of 2.8 has been measured for

1.0xl0 -4 M solutions of TCE (7). In the present paper, with 1.0xl0 -6 M solutions of PC'E, in the experimental

conditions of Table 2, with standard photocatalytic membranes, an acceleration factor of 3.3 ¢ 0.4 was obtained,

roughly equivalent to that brought about by photocatalytic promoter VI (see Table 2). By replacing the standard

membrane with that containing VI, no further enhancement of rate was noticed, outside the limits of experimental

uncertainty, when the anodic bias was applied. This should mean that, either by anedic biasing or by promoter

addition, some sort of saturation in rate improvement was reached, given the experimental conditions employed

(membrane configuration and surface, flow rate, radiation energy and kind, concentration of oxidizer). This again

speaks in favour of the short-circuited electrode behavionr of irranobilized semiconductor under excitation, during

which the semiconductor exhibits the same interesting photoelectrocbemical properties as those of TiO 2 electrodes,

and by which the complex photodegradation reactions take place.

To summarize all these experimental facts, some observations rmy be drawn, regarding particularly the role and the fate of chlorinated intermediates produced by degradation of chlorinated aliphatics onto photocatalytic

membranes, also in consideration of the possible toxicologic influence of chlorinated intermediates during

treatments by this photocatalyfic technology for water potabilization.

1540

For model molecules such as DCM and TCE, in the present as well as in previous studies (7, 10), rates of

substrate disappearance and of chloride ion formation substantially coincide, at least to practical purposes, and for

DCM they coincide also with rate of TOC disappearance. For PCE, rate of chlorine mineralization is intermediate

between that of substrate disappearance and that of total carbon mineralization. This is consistent with the

formation of chlorinated intermediates, such as DCEA, which have also been detected, but not with the much

slower kinetics of photomincralization of DCEA or other chloroethanoic acids, if considerable amounts of these

intermediates should take origin, as has been underlined above. The explanation of this apparent contradiction may

stem by the suggestion of chain radical mechanisms occurring during the course of photndegradation, which, if

quenched during the runs, for analysis or other causes, give rise to the isolated intermediates containing organic

chlorine, or otherwise proceed towards successive products, such as oxygenated aliphatics, and finally carbon

dioxide. A possible cause for rate decrease after disapcearance of substrate and for relative accumulation of

chlorinated intermediates, such as chloroethanoic acids or chloroaldchydes from PCE, may be, indeed, oxygen

depletion and rate determining oxygen diffusion, particularly in experiments in which no efficient saturation is

provided for.

Oxygen deficiency during the runs, as simply given by the lack of continuous saturation, if air or oxygen are

employed as oxygen suppliers, may give easily origin to a strong decrease in rate, as has been verified in the present

work, and to an enhanced formation of chlorinated intermediates, such as DCEA from PCE. This should not be

taken as a proof of the existence of these intermediates during the straightforward runs carried out in oxygen

saturated solutions, but rather considered as an indication of how the photodegradation mechanism may be

perturbed by diffusional effects on rate (13), and in any case as an important confimuaion of the role of

photoadsorbed oxygen (20), and of the labile radical species deriving from it during band gap irradiation of

semiconductor. Partial charge transfer from the surface adsorption site to the oxygen molecule was found to play an

essential role for oxygen adsorption on "riCh, as well as for the formation of *Of radical species during

photoexcitation (20). It is consequently very hard to accept that the substrate molecules, which are present at a

much lower concentration, and the scavenging power of which towards conduction band electrons has not yet

found any experimental evidence, may be strong competitors for this function.

CONCLUSIONS

i) Photocatalysis by titanium dioxide and promoting species, immobilized in massive amounts by membranes

photografted onto convenient supports, is a very useful and promising method for degrading microcontaminants,

chloro-organics particularly, in waters for drinking purpose, given also the effect of simultaneous debacterization.

ii) When using sufficiently high flow rates of recycle, such as those employed in the pilot plant used in its

operating conditions, no mass transfer control of kinetic data by oxygen diffusion seems to be effectual, if

continuous saturation by air or oxygen is carried out. Critical conditions, on the contrary, appear below 5 ppm of

02., able to decrease rate by more than one order of magnitude, with respect to the saturation conditions.

iii) Complete mineralization of chloro-hydrocarbons may be carried out satisfactorily, also in conditions of

oxygen deficiency, if suitable promoting photucatalysts, such as VI ( a synergistic mixture of tri-(t-butyl)- and tri-(i-

propyl) vanadate (V)), are co-immobilized with TiO2 into the membrane.

iv) Application of an anodic bias, by photografting the membrane onto a suitable titanium support, gives rise

to an acceleration factor on rate, roughly equivalent to that of photucatalytic promoter VI, which shows the best

performance, among the organo-metallic compounds I-VII examined.

1541

v) In the range of concentration, which has been the object of the present work, the apparent reaction order

has been found unitary. Even if no ultimate or absolute conclusions may be drawn from variation of reaction order

with concentration of .substrate, the fitting of the Langmuir-Hinshelwood equation does not imply that the

underlying mechanism of substrate adsorption and surface reaction is adequate, as it has been shown (13) that any

other purely kinetic interpretation of parameters is compatible with this same kind of experimental behaviour, and

that the apparent adsorption constant depends on operative parameters of the photoeatalytie system and of the

photoeatalytie reactor (5).

vi) Results of this study are compatible with the generally accredited action of *OH radicals prndueed by

hole-trapping of surface hydroxyl groups, as primary oxidizing agents, and of oxygen as a scavenger for

photogenerated electrons.

vii) A suggested mechanism for the positive effects brought about by the co-immobilization of

photocatalytie promoters I-VH may derive from their role of oxygen transporters, as well from their possible direct

intervention as electron scavengers. Both routes seem to be necessary for interpretation of results.

ACKNOWLEDGEMENTS

The Milan University group kindly acknowledges financial contribution by the Italian Ministry for University

and Scientific and Technological Research (MURST), in the frame of a national project for photochemistry and

applied photochemistry.

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