Effect of adsorbents coated with titanium dioxide on the photocatalytic degradation of propoxur

Chtmqhere, Vol. 38, No. 3, pp. 617-627, 1999 Q 1998 Elsevier Science Ltd. All rights reserved W5-6535/99/ $ - see front matter

Pergamon

PII: SO0456535(98)00204-S

EFFECT OF ADSORBENTS COATED WITH TITANIUM DIOXIDE ON THE PHOTOCATALYTIC DEGRADATION OF PROPOXUR Ming-Chun

Lu ‘,

Jong-Nan

Cheu*, and Kuo-Tai Chang3

1. Department of Environmental Engineering and Heulth Chia Nan College of Pharmacy and Science Tainan, Taiwan 71710, ROC 2. Institute of Environmental Engineering National Chiao Tung University Hsinchu, Taiwan 30039, ROC (Received in Germany 13 February 1998; accepted 15 May 1998)

ABSTRACT Photocatalytic

oxidation

of pesticides

in aqueous media irradiated

of research.

Therefore,

the treatment

technology

titanium dioxide coated on the supports was performed oxidizing

in this research.

propoxur

for degradation

such as activated

Results

show

because of its adsorption

agent.

than that with Ti02IGAC complexing can improve the photocatalytic

is the best complexing

The others follow the sequence: rate of propoxur

But the mineralization

01998

using

with plain TiOz

agent

for

plain TiOz > is higher than

rate of propoxur with plain TiOz is lower

it can be concluded

Elsevier

(an insecticide)

carbon, zeolite, brick, quartz and glass beads.

properities.

agent. However,

efficiency

of propoxur

that GACiTiOl

glass beads > zeolite > brick > quartz. The degradation that with TiOz/GAC complexing

by UV light is a rapidly growing field

Science Ltd.

that using GAC as the support

All rights reserved

KEY WORDS

Photocatalytic

oxidation;

supported titanium dioxide; propoxur;

insecticide

INTRODUCTION

Most pesticides

are non-selective;

addition to applying pesticides, considered. pesticides.

Photocatalytic Studies

have

they are also harmful

the pollution

oxidation shown

that

prevention

is a promising illumination

to ecological of pesticides

technology

of semiconductor 617

systems

as well. Therefore.

used in our environment in treating powders

wastewater in aqueous

in

has to be containing solutions

618 containing

organic compounds

TiOz catalyzed

and oxygen induces oxidative

the oxidation

of organic pollutants

equal to or greater than the band-gap

energy

electronic

solution

processes,

reactor

design,

decomposition

in the presence

of UV radiation

The quantum efficiency composition,

[I-S].

of organic compounds

at photon energies

of the process

the organic

substrate.

is influenced

light intensity

by and

surface interactions.

In the last few years, much attention has been devoted to the heterogeneous fixed-bed

reactors

reaction

is initiated

adsorbability efficiency

[3,6,7].

It is agreed that the critical

with hydroxyl

of the pollutants

Matthews

on suitable

[8] suggested

reaction.

adsorbents

inner

surface

to concentrate

agent in dilute aqueous

of a glass

tube.

behavior,

reaction [3-51. However,

indicating

zeolite,

brick.

of pesticides

reaction. Propoxur,

an insecticide,

the

factor in evaluating

the

efficiency,

photocatalyst

(Germany). San-Yu

with TiOz coated on the

of pesticides

follows

may play an important

(Millipore)

the Langmuir-

role in the oxidation

the reaction occurs, at the surface or in the bulk, the

glass

with titanium beads

dioxide

expression

[ 1 I].

Therefore.

coated on supports.

to investigate

the

characteristics

such as of the

was used as a test chemical.

AND METHODS

P25, 50 m*/g, 30 nm mean particle

The supports,

Company

surface.

with TiOl / activated

a herbicide,

Propoxur was obtained from Bayer Company with purity of 96.3%. The photocatalyst, study was Degussa

it may load

scaling up of the process.

was investigated

oxidation

process and

[l-3].

in the

Therefore,

on the

form as the Langmuir-Hinshelwood

quartz

oxidized

with

property of TiOl.

it was noted that wherever

is to study the photocatalytic

MATERIAL

pollutants

oxidation

Kato et al.. [IO] also used the porous alumina

that adsorption

this research

photocatalytic

surface

is an important

of propyzamine.

The photocatalytic

have the same analytical

carbon,

compounds

the decomposition the

solution.

oxidation

rate reactions

activated

surface

the photomineralization

studies, photocatalytic

Hinshelwood-type

on the photocatalyst

To promote

ceramic as the supporter to study the photocatalytic

In our previous

step of organic

that the use of TiOz coated on sand permits economic

Uchida et al., [9] demonstrated carbon complexing

produced

onto the photocatalyst

of the photocatalytic

photocatalyst

radicals

photocatalytic

Ti02 , used in this

from the Degussa

Company

activated carbon, zeolite. brick. quartz and glass beads. were obtained from the

(Taiwan).

and has a resistivity

without further purification.

size obtained

The water used for experiments greater than 18 MO/cm.

was purified

All other chemicals

by a Milli-Q/R0

system

used are of reagent grade

619 The supports were coated with TiOz by preparing minutes),

adding supports

to the sonicated

a TiOz suspension

suspension

in 100 ml water (sonicated

and evaporating

The amounts of TiOz and supports added in this procedure

depended

to dryness in an oven at 100 “c’.

on the experimental

coated supports were then heated at 550 “C for half an hour to produce the complexing

The photocatalytic

reactor is shown in Fig. 1. The reaction

was conducted

vessel with a quartz cover: the volume of reaction mixture was I liter. shorter wavelength

of IJV light.

The cut-off wavelengths

w low pressure

mercury lamps with a major emission

conditions.

The

agents.

in a i.2-liter

double-jacked

Quartz cover was required for the

for quartz are less than 200 nm.

254 nm light can pass through the cover to excite the TiOz.

for a few

The UV radiator was composed

Therefore. by three 10

at 254 nm (Sankyo Ltd., Japan) and was located

above the reactor.

?

0.

-

i

agnetic Stirrer

k+M Gas

Flow Meter

Fig. I, Photocatalytic

For all experimental

runs performed

During experiments,

the reactant solution

reactor used in this study

in this study. the pH of the reacting mixture was ad.justed initially. was agitated continuously

with a magnetic

stirrer and oxygen

(99.9 % purity) was bubbled into the solutions at a constant flow of 50 mlimin to insure oxygen saturation. The samples

were taken from the solution

HPLC (Waters LC module

1). total organic carbon (TOC) was detected

Model 2001), and surface area was determined

Potassium

ferrioxalate

with certain time intervals.

solution

Propoxur

was analyzed

with a

with a TOC analyzer (ASTKO

with a ASAP 2000 BET analyzer.

was used to measure the photon flux in the reactor. The quantum yield

(QY) can be defined as the ratio of the number of molecules on reaction with TiOl - free actinometry

solutions

quantum yield for ferrous ion formation

in the ferrioxalate

reacting to that the photons supplied.

Based

for the reactor. the flux rate was 7.2 ‘* I OeJ Mimin if the solution

is taken

to be 1.28

1121.

620

RESULTS

Photocatalytic

Choosing

AND DISCUSSION

degradation

of propoxur with different complexing

a proper support with a better ability of adsorption

since the support can concentrate g TiOz coated were immersed was also dispersed

organic chemicals

agents

can improve the photocatalytic

on its surface.

Since 3g complexing

in aqueous solutions to start the photocatalytic

efficiency

agent with 0.15

reaction, 0.15 g plain TiOz

in the reaction solution.

Table 1.

Surface areas of coated and uncoated supports

Support and Complexing

agent*

Surface Area (m*/g)

Plain TiOz

50

GAC

902

Coated GAC

745

Quartz

4.90

Coated Quartz

4.57

Zeolite

8.90

Coated Zeolite

4.95

Glass Beads

5.70

Coated Glass Beads

2.95

*: 1 g support was coated with 0.05g of Ti02

Table 1 lists the surface areas of supports coated and uncoated Ti02. The coating process eliminated surface areas of complexing supports. comparing

As shown

agent. The extent of surface area reduction depended

in Fig.2,

GAC/

TiO2 is the best complexing

the surface areas among the complexing

factor affecting on the propoxur

except

disappearance

GAC,

in this reaction

coated

with TiOa

did not improve

system. The reason is that the illumination

to find the proper support

for improving

surface area of GAC induces not only propoxur

By

agent of glass beads has a low surface area, but

the efficiency

agents except GAC/ Ti02. the efficiency

of propoxur

area was reduced when TiOl

was coated on the support surface and the supports can not adsorb propoxur this study is

removal.

of

agents, it is obvious that surface area is not the only

removal. The complexing

the supports

on the characteristics

agent for propoxur

the propoxur removal with glass/ Ti02 is high than that with other complexing Clearly,

the

efficiently.

of propoxur

but also TiOl adsorbs on GAC.

However,

removal.

In fact. TiOl

since

The large is a fine

621 particle that can disperse efficiency

of quantum

mentioned

above,

experiments,

in the bulk solution,

yield because

the GAC

but if coated on the supporter

of surface area reduction.

is still worthy

investigating

surface, will result in a low

Based on the the experimental

in more

details.

Thus.

resulted

in the following

GAC was chosen as the medium.

0.8

0.6 t

Fig.2

Propoxur removal after 60 min illumination Experimental complexing

conditions:

with different-wmplexing

[propoxur]=4.78x10‘4

agents

M: pHo=4;

agent, 3g

Effect of support diameter

To evaluate the effect of support size on propoxur study. diameters

oxidized,

Fig. 3 shows the time course of propoxur of GAC/ TiOz

disappearance. GACI TiOl

complexing

However,

agents.

The smaller

of GAC were used in this

in the photocatalytic

reaction

GACi TiOl diameter.

over various

the faster propoxur

it is difficult to separate the amount of propoxur adsorbed and oxidized on the

surface because the disappearance

the same time.

removal

several diameters

Therefore,

we compared

of propoxur

is attributed

the propoxur remainings

to oxidation

in the GAC/ TiOl

and adsorption

solutions with and Table 2

without UV illumination

to find the effect of GAC diameter on the disappearance

of propoxur.

indicates

of GAC diameter

with and without

the influence

illumination.

on the disappearance

of propoxur

in

UV

622 In general, the rate of propoxur UV illumination oxidation TiOz

because

disappearance

with UV illumination

GAC is an excellent

adsorbent

rate. Thus, UV light can not make a significant

solutions.

However,

according

is slightly higher than that without

resulting

in a higher

effect on the propoxur

to Table 2, it is proved that oxidation

adsorption removal

in the GACi

reaction really happened

its rate was much lower than that of adsorption.

-0-l

0-2Omesh

Effect of GAC diameter on the photocatalytic

oxidation

0.6 -

Time (mitt)

Fig.3

Experimental complexing

Table 2

Effect of

conditions:

[propoxur]=4.78x10-4

of propoxur

M; pHa=4;

agent, 3g

GAC diameter on the adosrption

and oxidation of propoxur*

Propoxur remaining(%)

Diameter

mesh

mm

70-40

0.21-0.42

2.5

1.2

40-25

0.42-0.707

8.6

8.1

25-20

0.707-0.841

11.5

8.3

20-10

0.841-2

31

20

>lO

>2

39.9

40

*Propoxur

remainings

without illumination

are compared at 15 min of reaction time.

rate than

with illumination

, and

623 Effect of Ti02 loading

TiOz loading is another important rate increases

with increasing

blocked by catalyst itself.

factor for propoxur

the TiOz loading

However,

In this experiment,

oxidation

of propoxur.

induces the increase of oxidation

rate.

the rate will be reduced because UV light is

four kinds of TiOJGAC

0.2g and 0.25g TiO2 were coated on lg GAC, separately. the photocatalytic

With plain TiOz , the photocatalytic

oxidation.

Increasing

Fig.4 indicates the effect of TiOs loading on

the amount of TiOz coated on the GAC surface

However, the results shows that the oxidation

0.2 g TiO2 can almost perform as well as that with 0.25 g TiO2 the reason that

the illumination

010

and adsorption

As GAC is an excellent the adsorption complexing

020

oxidation

[propoxur]=4.78x10‘4

agent, lg; illumination

of propoxur

M; pHa=4;

time, 5 hr

of propoxur on the TiOzIGAC surface

adsorbent,

and oxidation

conditions:

0 25

loading (g)

Effect of TiOz loading on the photocatalytic Experimental

Oxidation

The present instance is assumed to be

06

Ti02

complexing

rate with 0.15 and

area do not increase due to a higher catalyst concentration.

0 05

Fig. 4

were prepared; O.OSg, 0. I g, 0.15g.

therefore,

the disappearance

reaction; propoxur

agent surface and then

oxidized.

and its oxidation According

of propoxur

in the solution resulted from

intermediates

were concentrated

on the

to this characteristics

of the complexing

agent,

624 it was found that the disappearance adsorption.

In this experiment.

and then we hardly distinguished For this reason, the adsorption

Fig.5 shows the propoxur

of propoxur

the oxidation

rate on the complexing

of propoxur on the complexing

remaining

complexing

experiment

during the adsorption

than that of oxidation. process.

described

agent for three hours.

phase is attributed to oxidation

phase was mainly

attributed

to

agent surface can not be measured

the amount of propoxur oxidized and adsorbed in the combined reaction.

propoxur adsorbed on the surface of complexing photodegradation

from the aqueous

agent surface was furthur studied.

reaction.

It was found that the amount

agent reached a saturation

after 3 hours.

below were carried out with a preadsorption

It is assumed that the disappearance

with a preadsorption

The reduced amount of propoxur

of propoxur

of propoxur

oxidation

of propoxur

from the aqueous phase is assumed

the

on the

from the aqueous

process because the rate of adsorption

Fig.5 also shows the photocatalytic

Therefore,

of

is much higher

with a preadsorption

to be resulted from the

oxidation reaction.

m

Fig.5 Adsorption Experimental complexing

Comparison

of photocatalytic

To compare the catalytic GAC and dispersing

an Tme(min) and photocatalytic conditions:

oxidation

M; pHo=4;

agent, 1g

reactions onto the surfaces of plain Ti02 and Ti02/GAC

activity of plain TiOz and TiO$GAC.

directly

of propoxur

[propoxur]=8,6xlO‘”

in the solution,

respectively.

0.25 g ‘Ii01 was used for coating on lg

In the TiOziGAC

system,

1 g complexing

625 agent was immersed

in a solution containing

for saturating the complexing photocatalytic

reaction.

The initial concentration

the initial propoxur

oxidation

followed a first-order

solution

of propoxur

was 4.78X lOAM.

In the plain TiOz

was also 4.78X 10e4M. In these two reaction

systems,

propoxur

kinetic behavior. Table 3 lists the kinetic data of propoxur oxidation.

oxidation rate of propoxur with plain TiOz But the mineralization

reaction

agent surface. After three hours, the UV light was turned on to initiate the

solution.

complexing

9.56X 10e4M propoxur to proceed the preadsorption

rate of propoxur

is slightly higher than that with TiOz /GAC compiexing with plain TiOz is somewhat

The agent.

lower than that with TiO$GAC

agent.

Table 3

Rate constants of propoxur oxidation

Catalyst

Rate constant( 1Ihr) Mineralization

Oxidation

TiOziGAC

0.024

0.096

TiOz

0.018

0.102

Without preadsorption

process,

higher than that containing

propoxur

removal

rate in the solution

plain the TiO2 because

containing

GAC is an excellent

TiOz/GAC

adsorbent

was much

that can adsorb

propoxur on its surface in a short period of time. From the HPLC analysis, there were less intermediates found in the TiOIlGAC

system.

It is assumed

adsorbed onto the surface of complexing the reaction remained

the concentration

propoxur

of intermediates

intermediates conclusion,

are oxidized the complexing

as the target substrate

In the plain TiO2 system. the intermediates

agent.

of solution-phase

of a dilute concentration

dissolution

intermediates.

radicals

occurs

very fast as compared

to the suspended

into solution

produced

in

TiO2 surfaces.

of collection

phase onto the complexing

on the GAC can not ultilize

propoxur

and intermediates

and dissolved

propoxur

and

when they collide with TiOz

of degradation agent. as TiO2

all the photons

because

of dissolved

The use of plain ‘TiOz allows the

due to low the adsorbability,

agent has a large capability

and

of the adsorbed

with the collision

further to give a variety of other intermediates

from the solution

TiOl supported for degrading

were

of propyzamide

In this study. the supply

reactor with plain TiO2 can be almost absorbed by the photocatalyst reactor.

in this system

supports can ehhance the rate of mineralization

to the GAC/ TiOz interface

propoxur

formed

in the aqueous phase that can be detected by TOC analysis. Trimoto et d.[ 121 have

also reported that the use of adsorbent reduce

that the intermediates

intermediates

The photons dispersed

entering

TiOr is concentrated

as well

entering

completely

In

the

in the

the reactor to produce on the surface of 1 g

626 GAC

However.

the oxidation

efficiency

with GAC/ TiOz is almost as that with plain Ti02 even the

GACi TiO2 system utlized less amount of photons. support can improve the photocatalytic

efficiency

Therefore,

it can be concluded

that using GAC as the

and it is necessary to optimum light utlization

for GAC/

TiO2 system.

CONCLUSIONS

The rates of photocatalytic supports.

TiOzlGAC

oxidation

of propoxur

is the best complexing

properties.

The others follow the sequence:

degradation

rate of propoxur

However,

the mineralization

complexing associated

agent.

agent for oxidizing

rate of propoxur coated

on the blunt

with plain TiO2

Therefore,

because

of its adsorption

supports

eliminates

the problem

of the decomposition

efficiency

design

to permit

optimum

of filtration The use of

for the pollutants.

in TiO2 /GAC system is due to the small illumination

the reactor

agent.

TiO2 is lower than that with TiOz/GAC

but the reaction rate is mass transfer limited.

TiO2 coated on the GAC permits the promotion efficiency

propoxur

of TiO2 coated

is higher than that with TiO2 /GAC complexing with suspended

with the use of catalyst suspensions

chief loss of quantum

in the presence

aked Ti02 > glass beads > zeolite > brick > quartz. The

with plain TiOz

The catalyst

have been determined

light adsorbed

The

area comparing by TiO2 /GAC is

necessary.

ACKNOWLEDGMENT

This research was supported

by the National Science Council, Republic of China (Grant NSC 85-221 I-E-

009-015)

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