TiO2 담지 활성탄을 이용한 Microcystin-LR의 흡착 및 광촉매 산화분해

강민수·정익상·이동근·김성우*

경상대학교 대학원 환경보전학과, 경상대학교 환경보전연구소,

*삼협자원개발(주)

Adsorption and Photocatalytic Oxidation of Microcystin-LR with TiO2-coated Activated Carbon

Min-Su Kang·Ik-Sang Chung·Dong-Keun Lee

·Sung-Woo Kim*

Department of Environmental Protection, Research Institute of Environmental Protection, Gyeongsang National Univ., *Samhyup Resource Development Co.

Introduction

The worldwide appearance of toxic cyanobacterial blooms in drinking water supplies has raised concerns about systemic effects on human health. One of the most acutely toxic cyanobacterial toxins is microcystin-LR which is a hepatoxic material produced by several cyanobacteria general including Microcystis, Anabaena and Planktothrix, which are increasingly found in water bodies at high densities(water blooms) as a result of eutrophication1). The microcystin-LR is a cyclic heptapeptide containing 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid(ADDA), with leucine(L) and arginine(R) in the variable positions. Microcystin-LR has caused the death of both animals and humans as a result of ingestion of contaminated water. It is also believed that long-term exposure to sublethal levels of microcystins may promote primary liver cancer by disruption of protein phosphates 1 and 2A.

Conventional water treatment methods of chemical coagulation, flocculation and sand filtration are poor at removing low concentrations of cyanotoxins2). Treatment using chlorination requires high doses and long contact times, and there is the possibility of generating  toxic byproducts such as trihalomethanes. Biological methods typically requires a reaction time of hours to days. Thereby special treatment methods need to be developed.

Different sources of activated carbon have been investigated for their ability to adsorb microcystin-LR. Wood-based products were found to be must effective because of their high mesopore volume. It was found that treatment with 25□/L of wood-based powdered activated carbon, with a contact time of 30 minutes, could reduced the concentration of microcystin-LR from 50 to <1□/L. Granular activated carbon filters were found to be effective in reducing microcystin-LR levels from 20 to 1□/L3). Although both powdered and granular activated carbon have proven effective, the presence in the water of other organic pollutants that could be adsorbed by the activated carbons needs to be considered. Due to frequent replacement or regeneration the activated carbon adsorption method might become expensive.

In order to make the environmental application of TiO2 photocatalysis more practical immobilization of TiO2 on a certain suitable substrate is inevitably required. Granular activated carbon can be a successful substrate due to its adsorption ability of microcystin-LR and easy separation from water. When combining the roles of both the adsorption and photocatalytic destruction, the TiO2-immobilized granular activated carbon is expected to show successful performance for the removal of microcystin-LR from waster.

In the present study TiO2 was coated mainly at the exterior surface in the vicinity of macropore entrances of the granular activated carbon, and this TiO2-coated activated carbon was employed for the adsorption followed by photocatalytic decomposition of microcystin-LR.

Experimental

1. Materials

Microcystin-LR was purchased from Sigma-Aldrich Co.. Granular activated carbon was prepared by exposing carbonated coconut shell to steam at 900□. The average particle size was adjusted to be 1□ through crushing process. Titanium tetraisopropoxide [Ti(OCH(CH3)2)4] and isopropyl alcohol, obtained from Merk, were used as the precursor of TiO2 and solvent, respectively.  All solutions were prepared in Milli-Q water, and all other organic solvents used were analytical grade.

2. TiO2 coating procedures

In order to locate TiO2 mainly at the exterior surface in the vicinity of macropores of the activated carbon, a modified sol-gel preparation method was employed. Twenty grams of the activated carbon were placed in a flask of a rotary evaporator. After filling the pores of the activated carbon with pure isopropyl alcohol at 20□, the temperature was dropped to -4□ immediately. The mixture of 15mL titanium tetraisopropoxide and 15mL water was then introduced into the activated carbon drop by drop during 24h under continuous stirring. The prepared samples were then dried in vacuo at 50□ for 24h. The dried samples were finally treated in a microwave oven with a microwave power generator(Hitachi, 600W) for 3h.

3. Adsorption and photocatalysis

The adsorption and photocatalysis of microcystin-LR were performed with a water-jacketed borosilicate glass vessel incorporating a quartz window for ultraviolet(UV) light illumination. The light source was a 4-Watt black light lamp (370□, F4T5-BLB). When the lamp was switched on, the lamp irradiance was 0.6 □/□, as determined with a radiometer(Optronic Laboratories, Inc., Model OL730C). This UV intensity was selected so that test results could be relevant to solar-excited treatment. The average intensities of UV-A(330-390□) in sunlight in Seoul, Korea on the surface of rivers were 4.3 □/□ and 0.6 □/□ on cloudless days at 2p.m. and cloudy days at 9 a.m., respectively. The water cooling jacket provided a constant temperature of 20□5□ in the vessel. The vessel contained the granular activated carbon without TiO2(GAC) or the granular activated carbon with TiO2(GAC-Ti) completely dispersed in an aqueous phase with microcystin-LR. The initial concentration of microcystin-LR(200□/mL) used in this investigation was considerably higher than might be expected to occur in natural environment4),5). However, this enabled direct analysis of toxin by HPLC without multistep processing that would be necessary to quantify the much lower levels found in the environment.

Results and Discussion

1. Characterization of GAC-Ti

Figure 1 shows the SEM micrograph of the TiO2-coated granular activated carbon used in this study. As can be seen in this figure, pores are well developed and their average diameter was estimated to be 1.0□.

When looking the pores in more detail with TEM, fine particles of TiO2 are known to be dispersed at the exterior surface in the vicinity of the pores(Figure 2).

2. Adsorption of microcystin-LR with GAC

The concentrations of microcystin-LR remaining in the solution over time in the absence and presence of GAC and in the absence and presence of light are shown in Figure 3. It is clear that essentially no decrease in microcystin-LR concentration occurs in the absence of GAC, while in the presence of GAC more than 50% initial microcystin-LR disappeared within 30 min. The presence of light, however, does not give any additional detectable effects on the removal efficiency of microcystin-LR. These results support the hypothesis that an adsorption process by GAC accounts for the reduction in concentration of microcystin-LR and homogeneous photolysis of microcystin-LR is not a significant process. The adsorption of microcystin-LR on GAC proceeded slowly and more than 12h exposure was required for the complete removal of microcystin-LR.

3. Photocatalytic degradation of microcystin-LR with GAC-Ti

Figure 4 demonstrates the effects of GAC-Ti on the removal of microcystin-LR in the presence and absence of light. Microcystin-LR disappears very rapidly on exposure to GAC-Ti and light. This rapid disappearance, however, is not observed in the absence of light. Almost complete removal of microcystin-LR under illumination can be achieved in approximately 20 min. This extremely rapid removal rate of microcystin-LR on TiO2-coated activated carbon(GAC-Ti) might be due to the photocatalytic destruction of microcystin-LR on the surface of TiO2 particles. As could be seen in Figure 2, fine TiO2 particles were located mainly in the vicinity of the pore entrances of the activated carbon. Microcystin-LR might adsorb onto the surface of TiO2 particles and/or on the surface of the activated carbon. The adsorbed

microcystin-LR on TiO2 particles is believed to be degraded quickly. Microcystin-LR was already reported to be degraded photocatalytically by TiO2 powder at very fast rate.

Conclusions

Microcystin-LR in water could successfully be degraded into non-toxic materials by using TiO2-coated granular activated carbon. Most of microcystin-LR was degraded within 20 min under UV light. Microcystin-LR, adsorbed to the surface of the activated carbon, migrated continuously onto the surface of TiO2 particles locating near the entrance of macropores in the activated carbon. Continuous migration and subsequent photocatalytic degradation on the surface of TiO2 accelerated microcystin-LR removal efficiency greatly, and made the application of the TiO2-coated granular activated carbon more practical.

References

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2. A.J. Feitz, T.D. Waite, G.J. Jones, B.H. Boyden and P.T. Orr, Photocatalytic degradation of the blue green algal toxin microcystin-LR in a natural organic aqueous matrix, Environ. Sci. Tech., 33, 243-249(1999).

3. M. Drikas, Removal of cyanobacterial toxins by water treatment processes, in: Toxic cyanobacteria- a global perspective, Australian Centre for Water Quality Research, Australia, 30-44(1994).

4. S.E. Hrudrg, T.W. Lambert, S.L. Kenefick, Health risk assessment of micrcystins in drinking water supplies, in: Toxic cyanobacteria- a global perspective, Australian Centre for Water Quality Research, Australia, 7-12(1994).

5. G. James, Toxic algae study summary, Manitoba Department of Environment, Canada, (1996).