발광다이오드와 저서미세조류를 이용한 부양양화 연안퇴적물의 식물환경복원에 관한 연구

Abstract

In a semi-enclosed bay, the input of pollutants cause environmental pollution such as eutrophication, algal bloom and hypoxic water. Also, active aquaculture cause a self-pollution in the coastal environment. To improve these problems, dredging is considered to be the most practical method. However, this method have led to secondary problems such as silt diffusion, prohibition of agricultural work, treatment of dredged sediment, and high costs. Therefore, ideal remediation methods should be eco-friendly methods. In order to remediate eutrophic coastal sediments, phytoremediation using light-emitting diode (LED) and benthic microalgae was used in this study. In other words, if the wavelength of light, which benthic microalgae could grow but not harmful algae, is able to illuminate on the eutrophic sediments, benthic microalgae may be able to remediate because of oxygen production and nutrient uptake. Therefore, this study examined whether phytoremediation using benthic microalgae and LED can remediate eutrophic sediments caused by promoting growth of benthic microalgae. To evaluate the eutrophication of sediment in Sujeong Bay, attached to Masan Bay, in Korea, field observations were conducted at a mussel farm near St. 1 and at the mouth of the Sujeong Bay near St. 2 from April to September 2010. The eutrophication cleanup index (CIET) values were mean 17±2 at St. 1 and mean 10±2 at St. 2, which greatly exceeded the criterion of 6. Thus, eutrophication of sediment in Sujeong Bay is proceeding seriously, and sustained efforts to remediate this eutrophication are required. To remediate eutrophic sediment using benthic microalgae, it is necessary to identify the physiological characteristics of benthic microalgae. Therefore, benthic microalage Achnanthes lanceolata, Halamphora veneta, Navicula minima and Nitzschia microcephala were isolated from the surface sediment of Sujeong Bay. To identify more eurythermal and euryhaline species for phytoremediaion, the effects of temperature and salinity on the growth of 4 benthic microalgae were examined. Maximum growth rate (μmax) obtained under the combined temperature and salinity conditions was observed at 25℃ and 25 psu for A. lanceolata, at 15℃ and 25 psu for H. veneta, at 20℃ and 30 psu for Na. minima, at 20℃ and 25 psu for Ni. microcephala. Considering there results of temperature and salinity conditions required for optimum growth (≧70% of maximum specific growth rate), Na. minima and Ni. microcephala were characterized as eurythermal and euryhaline species. On the other hand, A. lanceolata and H. veneta characterized as stenothermal species. To improve the culture efficiency of benthic microalgae in mass culture, the effects of substrate size on the growth of 4 benthic microalgae were examined using glass beads of different sizes. The glass beads used in this study were 0.09～0.15 mm (G.B 1), 0.25～0.50 mm (G.B 2), 0.75～1.00 mm (G.B 3) and 1.25～1.65 mm (G.B 4). No addition of glass bead used as control. Specific growth rate (μ) and maximum cell density of 4 benthic microalgae increased as the size of the glass beads decreased. Futhermore, the control experiment without added attachment substrates showed the lowest μ and maximum cell density. Therefore, suitable attachment substrates for mass culture of benthic microalgae appear to be important in order for phytoremediation using benthic microalgae. To identify the optimal wavelength and irradiance for phytoremediation, the effects of 4 wavelengths of light(blue LED; 450 nm, yellow LED; 590 nm, red LED; 650 nm and fluorescent lamp; mixed wavelength) on the growth of 4 benthic microalgae and the harmful dinoflagellate Alexandrium tamarense which was isolated from the surface water, were examined. μ of 4 benthic microalgae and A. tamarense were highest under blue LED, followed by fluorescent lamp, red LED, yellow LED. In particular, 4 benthic microalgae could grow under all irradiance conditions of 4 wavelengths. However, the growth of A. tamarense was stimulated under blue LED, and was suppressed under yellow and red LED to less than 70 μmol m-2 s-1. Therefore, yellow and red LED at irradiance levels less than 70 μmol m-2 s-1 could be important information for phytoremediation. Benthic microalgae used for the remediation of eutrophic sediments should possess a high capacity for the uptake and storage of nutrients. Therefore, the effects of wavelengths on the nutrient uptake and growth kinetics of 4 benthic microalgae were examined. Maximum uptake rate (ρmax) and maximum specific uptake rate (Vmax) for nitrate and phosphate was highest under blue LED, followed by fluorescent lamp, red LED, yellow LED. Among the 4 benthic microalgae, Ni. microcephala had the highest ρmax and Vmax. These results suggest that Ni. microcephala have a high capacity for nutrient storage and uptake. Further, photosynthetic rate of benthic microalgae used for the remediation of eutrophic sediments should be high. Therefore, the effects of wavelengths on the photosynthetic rate based on the oxygen production and carbon assimilation rate of 4 benthic microalgae were examined. Oxygen production and carbon assimilation rate was highest under blue LED, followed by fluorescent lamp, red LED, yellow LED. A photosynthetic quotient (PQ), defined as the ratio between oxygen production and carbon assimilation, with the change of wavelength was showed as 1.27±0.09 for A. lanceolata, 1.24±0.10 for H. veneta, 1.62±0.11 for Na. minima and 1.62±0.13 for Ni. microcephala. There was no significant change in the PQ values with change in wavelength. Among the 4 benthic microalgae, Na. minima and Ni. microcephala showed the relatively high oxygen production. Thus, on the basis of laboratory experiments, Ni. microcephala was identified as a useful species for phytoremediation of eutrophic coastal sediments. However, if the growth of Ni. microcephala will be inhibit under high ammonium and hydrogen sulfide, Ni. microcephala may not be able to remediate eutrophic sediments. Therefore, the possible phytoremediation of eutrophic sediments obtained from the Sujeong Bay was confirmed using microcosm experiments in a 60 L water tank with LEDs and Ni. microcephala. In watertank experiments with no light, environmental factors, such as cell density of Ni. microcephala, chlorophyll a (Chl. a), chemical oxygen demand (COD), acid volatile sulfide (AVS), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), showed no significant difference. However, in water tank experiments with LED, cell density and Chl. a increased, whereas AVS, DIN and DIP concentrations decreased. The removal rates of AVS, DIN and DIP were highest under blue LED at 41%, 32% and 33%, respectively, followed by the fluorescent lamp, red LED, and yellow LED. Further, the removal flux of DIN and DIP was highest under blue LED at 2.59 mgN m-2 day-1 and 0.30 mgN m-2 day-1, respectively, followed by the fluorescent lamp, red LED, and yellow LED. Thus, red LED may be the most appropriate light for remediation of eutrophic sediments during spring and summer when toxic dinoflagellate A. tamarense would be abundant, whereas blue LED may be the most appropriate during other seasons. A field application experiment was conducted at St. 1 from September 9 to December 3, 2011. Two chambers were used, a control site, which did not contain LED lamp and replanted Ni. microcephala on the surface sediment, and an experimental site, which included a red LED (650 nm) lamp and replanted Ni. microcephala. Chl. a concentration increased at experimental site, and it was 2 times the concentration at the control site. AVS concentration decreased at the experimental site, and the removal rate of AVS was as high as 28%. In addition, the removal rate of DIN and DIP were as high as 19% and 24%, respectively. The removal fluxes for DIN and DIP were 2.02 mgN m-2 day-1 and 0.22 mgP m-2 day-1, respectively, at the experimental site, and 0.88 mgN m-2 day-1 and 0.10 mgP m-2 day-1, respectively, at the control site. These changes indicate that oxygen produced by the replanted Ni. microcephala may have enhanced aerobic bacterial activity, and the nutrients may have been taken up by Ni. microcephala. On the basis of these results, the basic concepts of phytoremediation for eutrophic coastal sediments could be summarized as follows. If the light is illuminate on the non-illuminated surface sediments using LED, benthic microalgae may have produced oxygen through photosynthesis. The increased oxygen may have changed the anaerobic condition to aerobic condition in the surface sediments. The aerobic condition may have increased aerobic bacteria activity. The decomposition of organic matter by aerobic bacteria might lead to an increase the inorganic nutrients in the interstitial water. However, increased nutrients may be taken up by the benthic microalgae. Thus, phytoremediation using benthic microalgae and LED shows potential as a novel and eco-friendly method for the remediation of eutrophic coastal sediments. However, in order to commercialize this technique, the additional study will be necessary as follows. First, the effect of wavelength on germination of cyst should be examined, because the most harmful algae, such as A. tamarense, is able to form cyst. In addition, the possibility of heavy metal removal by benthic microalgae should be examined because the oxidation of AVS could be release heavy metals. Finally, the technique of this study may lead to remediation by promoting growth of benthic microalgae. Therefore, if the growth of benthic microalgae could be promoted by mixed or intermittent light of LED, it may lead to remediation of eutrophic sediment by promoting growth of benthic microalgae.