(458d) Adsorptive Ozonation of Organic Pollutants in Zeolite Monolith: a Kinetic Study

Abstract

The
enhancement of oxidation rate of persistent organic pollutant by adsorptive
ozonation process with a zeolite monolith was demonstrated in this study.
  The dissolved ozone was concentrated in the micropores of the
zeolite monolith as well as persistent organic pollutants such as acetaldehyde,
and their reaction rates were increased significantly.   Furthermore,
the organic pollutants could be mineralized in this process, which was
difficult in the bulk phase ozonation.   On the basis of the
experimental outcomes, a numerical model was developed for predicting the
decomposition of organic pollutants in the zeolite monolith packed column.
Using this model, the optimum operation condition was discussed.

1. Introduction

Ozonation
of organic pollutants in aqueous media have been extensively investigated by
many researchers [1,2,3].   However, the low solubility of ozone in
water was found to be a major difficulty in the ozonation
processes.   Advanced oxidation processes (AOPs) have been suggested
[4-6] to enhance the decomposition rate of persistent organic
pollutants.   In AOPs, OH radical was generated from O₃ by
adding H₂O₂ or radiating UV light etc., and as a result,
the decomposition rate in AOPs was increased significantly, because of the high
oxidation potential of OH radical.   However, OH radical also could
react with non-targeted substrates, and as a result, undesirable by-products
were produced in some cases.

In
our previous study, a novel ozonation process using zeolite adsorbents,
adsorptive ozonation, was proposed to resolve those problems [7].  
The organic pollutants and ozone could be adsorbed in the micropores. As a
result, those concentrations become extremely higher than in bulk
phase.   Therefore, the apparent reaction rate of them could be
increased.   The feasibility of adsorptive ozonation for
trichloroethene decomposition was verified in our previous study [7]. However,
the behavior of decomposition products was not clear.   In addition,
the contact method of ozone or organic pollutants with the zeolite was demanded
to improve.   To resolve this, the zeolite monolith was proposed instead
of the zeolite.   The objective of this study is to clarify the
decomposition kinetics of persistent organic pollutants in a zeolite monolith,
and to estimate the deterioration factors in the process.  
Acetaldehyde was used as a model persistent organic pollutant in this study.

2. Materials and methods

2.1
Materials

The
adsorbents employed in this study were powdery high silica zeolites, HiSiv 3000
(SiO2/Al2O3 > 1000, UNION SHOWA K. K., Japan),
and a zeolite monolith.   The zeolite monolith was prepared by
coating the HiSiv 3000 powder on a cordierite honeycomb monolith support (IWAO
JIKI KOGYO Co., Ltd., Japan)( 10 mm x10 mm x
10 - 25 mm).   These adsorbents were treated with NH₄Cl
aqueous solution prior to the experiments [7].   Analytical grade
acetaldehyde was supplied from Wako Pure Chemical Industries Ltd (Japan).  
Ozone gas (concentration from 0.2 mol O₃ m^-3 to 1.45 mol
O₃ m^-3) was produced from pure oxygen with an ozone
generator (Toshiba, WOR-0.5, Japan).

2.2 Bulk phase ozonation

The reactor used in
these experiments was a 250 mL gas washing bottle.   250 mL of 2.0
mol C m^-3 acetaldehyde water solution was added into the bottle and
ozone gas (1.45 mol O₃ m³) was supplied through a
diffuser equipped in the bottle continuously.   The concentrations of
residual acetaldehyde and its decomposition products were measured by LC/RI
method described below.

2.3 Adsorption
equilibrium

Batch adsorption
experiments were performed to investigate the adsorption behaviors of
acetaldehyde to the powdered HiSiv 3000 and the zeolite monolith prepared from
HiSiv 3000.   A precisely weighed 0.004 - 0.500 g of each adsorbent
and 50 mL of 2 mol C m^-3 acetaldehyde water solution were added into
50 mL glass vial and shaken on a rotary shaker (R-II, TAIYO) in 120 rpm under
20 ^oC.   After 5 hrs, acetaldehyde concentrations were
measured by LC/RI method described below.

2.4 Adsorptive ozonation

An
experimental apparatus was used for adsorptive ozonation
experiments.   This apparatus consists of a part (A) to feed a dissolved
ozone solution, a part (B) to feed an acetaldehyde water solution, and a part
(C) as a reactor for adsorptive ozonation.   In the part (A), the
dissolved ozone solution was prepared by bubbling ozone gas into distilled
water in a 1 L glass tank through a glass diffuser.   In the part
(B), a 7 mol C m^-3 acetaldehyde stock solution was stocked in a 1 L
glass tank.   In the part (C), the HiSiv 3000 or the zeolite monolith
was packed and the filling weight was 0.38 g - 1.30 g.   The
dissolved ozone solution in the part (A) and the acetaldehyde water solution in
the part (B) were fed into the part (C) through a mixer.   The
concentrations of dissolved organic carbon (DOC), acetaldehyde and its
decomposition products such as acetic acids, and dissolved-ozone were measured
at inlet and outlet of the zeolite-packed column periodically until these
concentrations were not changed any more, i.e. it was judged to be a steady
state.

2.5 Analytical procedure

The
concentrations of acetaldehyde and its decomposition products were measured by
a liquid chromatograph system (LC-6A, Shimadzu, Japan) equipped with Shodex
SH1011 column (300 mm in length and 8 mm in diameter) (Shodex, Japan) and an RI
detector (RID-6A, Shimadzu, Japan).   0.1 N sulfuric acid aqueous solution
was used as mobile phase.   The retention time of acetic acid and
acetaldehyde were 11.2 min and 13.3 min, respectively.   DOC was
measured by a Total Organic Carbon analyzer (TOC-5000A, Shimadzu,
Japan).   Dissolved ozone
concentration in the aqueous solution was measured by the potassium indigo
trisulfonate method [8].

3 Results and Discussion

3.1 Bulk phase ozonation

The
initial concentration of acetaldehyde in the bulk phase ozonation experiment
was 2.0 mol C m^-3 and has decreased below 0.1 mol C m^-3
after 90 min.   The 2nd order rate constant of acetaldehyde ozonation
was estimated as 2.29 x 10³ mol C^-1 m³ s^-1
on the basis of the time courses of acetaldehyde and dissolved ozone
concentrations.    The increasing in acetic acid concentration
was accompanied by decomposing acetaldehyde.   About 90% acetaldehyde
was oxidized to acetic acid, meaning that acetic acid was a main decomposition
product of acetaldehyde in bulk ozonation.

3.2 Adsorption
equilibrium

The
adsorption isotherm of acetaldehyde in the HiSiv 3000 was obtained by a series
of batch adsorption experiments.   The amount adsorbed acetaldehyde
in HiSiv 3000 was linearly increased with the increasing in equilibrium
concentration of acetaldehyde when the acetaldehyde concentration was below 2.0
mol C m^-3.   The Henry adsorption constant of acetaldehyde
was estimated as 0.95 m³ kg^-1 from the slop between
amount adsorbed and equilibrium concentration.   Because of the fact
of acetaldehyde adsorption in the HiSiv 3000, it was considered that the
adsorptive ozonation was possible.

3.3 Adsorptive ozonation

On
condition that retention time was below 0.6 min, the residual ratio of
acetaldehyde in outlet of column with HiSiv 3000 was below than 5%.  
The apparent decomposition rate of acetaldehyde by adsorptive ozonation with
HiSiv 3000 was significantly faster than that in the bulk phase ozonation
described above.   Furthermore, the outlet concentration of DOC was
decreased with the increasing in retention time.   It was considered
that the acetaldehyde could be mineralized in this process, which was not
observed in the bulk phase ozonation at all.   The adsorptive
ozonation process was thus more efficient to decompose acetaldehyde than bulk
phase ozonation.

3.4 Numerical model

A
numerical model was developed for predicting the decomposition property of
acetaldehyde in the zeolite monolith.   The decomposition rate
constant and the adsorption constant were estimated on the basis of the results
of bulk phase ozonation experiment and the batch adsorption experiment
described above.  Then, the predictability of the model was verified
against the data obtained in the adsorptive ozonation experiment.  
According to this model, the enhancement of acetaldehyde decomposition could be
explained by concentrating acetaldehyde and ozone in HiSiv 3000.

4 Conclusions

(1) The rate constant of acetaldehyde ozonation in bulk
solution was 2.29 x 10³ mol C^-1 m³ s^-1
and the main product was acetic acid.

(2) The retention time for 5% residual ratio of acetaldehyde
in adsorptive ozonation was about 1/100 times shorter than ozonation in bulk
solution and acetaldehyde was possible to be mineralized.   The
adsorptive ozonation process, therefore, was more efficient than ozonation in
bulk solution.

(3) A simulation model was developed for predicting the
decomposition rate of acetaldehyde by this process, which was verified against
experimental data.

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