| Summary: | 碩士 === 國立交通大學 === 環境工程與科學系 === 84 === Nitrilotriacetic acid (NTA) used as a substitute of builder in detergent for sodiumtripolyphosphate (STPP) has solved the problem of eutrophication of rivers and lakes. It may form chelated complexes with heavy metals from sediments. The released heavy metals may increase the potential hazard to drinking water supplies and aquatic ecosystems. Biodegradation of NTA is rapid in activated sludge systems. However, the population served by wastewater treatment plants is small in Taiwan, and the importance of behaviuor of NTA in receiving waters is not negligible.
Sediment sample of this study was sampled from a contaminated river. In a continuous-flow channel system used to simulate the flowing river water, the biodegradation of NTA and its effect on the mobilization of heavy metals from sediments in natural waters have been studied. In addition, the agreement of experimental data with models has also been studied.
At the beginning of the experiment of acclimation at the initial concentration of 10 mg/L, removal of NTA from liquid phase by sediments was attributed to adsorption. The mitial adsorption decreased the mobilization of heavy metals and changed the released behaviour of heavy metals. It made worse agreement between models and experimental data. The experiment of acclimation to NTA included two-stage of adaptation/degradation periods. The former was the biodegradation of NTA, Ca-NTA and Mg-NTA, and the latter was the biodegradation of Zn-NTA. Due to the existence of the bacteria that could degrade metal-NTA complexes, two-stage of adaptation/degradation periods have not been observed in the experiment of NTA biodegradation at the respiking of NTA. Therefore, NTA could be degraded rapidly in the experiment of NTA biodegradation even if the levels of heavy metals were high. When the NTA- degrading bacteria have been completely acclimated in rivers, the pattem of NTA biodegradation showed two stages which could be well described by the zeroorder kinetics model and the first-order kinetics model , respectively.
In the experiment of NTA biodegradation at the respiking of NTA, decrease in heavy metals content of liquid phase was attributed to the biodegradation of NTA. If the NTA degrading bacteria have been completely acclimated in rivers, the release of heavy metals would be significant at the beginning of contamination with NTA. The maximal mobilization of heavy metals increased with the increasing initial concentration of NTA. When the released period of metals was long enough to reach the equilibrium concentration, multiple fist-order reaction model can be applied well for a longest term than diffusion and two constant models. Though two constant model was only suitable for the initial period before equilibrium of release, the error between its predicted value and experimental data is least among three models.
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