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.  

References

[1]   Benitez, F. J.; Beltran-Heredia, J; Acero, J. L.; Rubio, F. J. J. Hazard. Mater. 2000, B79, 271.

[2]   Kuo, C. H.; Huang, C. H. J. Hazard. Mater. 1995, 41, 31.

[3]   Hoigne, J.; Bader, H. Water Res. 1983, 17, 173.

[4]   Echigo, S.; Yamada, H.; Matsui, S.; Kawanishi, S.; Shishida, K. Water Sci Technol. 1996, 34, 81.

[5]   Hirvonen, A.; Tuhkanen, T.; Kalliokoski, P.; Water Sci Technol. 1996, 33, 67.

[6]   Kuo, C-H.; Yuan, F.; Hill, D.; Ind. Eng. Chem. Res. 1997, 36, 4108.

[7]   Fujita, H.; Izumi, J.; Sagehashi, M.; Fujii, T.; Sakoda, A. Water Res. 2004, 38, 159.

[8]   Standard methods for the examination of water and wastewater 17th edition; APHA-AWWA-WPCF, Washington, DC, 1989.