以連續流動河道模擬系統探討河川中NTA之存留性及其與底泥中重金屬之交互作用

Abstract

NTA為清潔劑中三聚磷酸鈉(STPP)效果較佳之替代品，雖能有效控制優養化，但因NTA會與底淤泥中之重金屬形成可溶解性的錯合物，而使重金屬釋放至水體中，對於飲用水源及水體生態系造成危害。臺灣地區目前污水下水道接管普及率仍然偏低，因此，就目前實際情況而言，NTA在承受水體中行為之重要性不容忽視。本研究之底淤泥採自受污染河川，利用連續式河道試驗來模擬流動的河水，以探討NTA於水體中之生物分解作用及其對底泥中重金屬之釋出能力;並探討重金屬釋出行為與NTA生物分解行為對於反應動力模式之適用性。 NTA生物馴養實驗初期，底淤泥對NTA會有初始吸附作用，導致實驗初期之重金屬隨NTA錯合物吸附於底淤泥，而使釋出量減少，造成重金屬釋出行為改變，以致降低模式之適用性。NTA生物馴養實驗呈現兩階段之生物馴養／分解之現象，第一階段為NTA、Ca-NTA與Mg-NTA等生物穩定性較小者之生物分解作用；第二階段為Zn-NTA之生物分解作用。NTA生物分解實驗殯並無兩階段生物馴養／生物分解，主要是因為系統中已經具有能分解各種NTA錯鹽之微生物，因此，即使系統中含有高量之重金屬，NTA仍能迅速地分解。河水中之微生物經NTA馴養後，若再有NTA之污染時，NTA之生物分解初期適合以零階反應來模擬，而之後其生物分解以一階反應描述較合適。 NTA生物分解期之重金屬濃度會因生物分解作用而降低。經NTA馴養過之河川若再度遭受NTA之污染時，重金屬之釋出以初期較為嚴重，同一種重金屬之最大移動率，隨NTA初始濃度增加而增加。若重金屬釋出最後達到平衡，則多重一階反應模式能適用於較長之反應時間，而擴散模式與二常數模式僅適用於重金屬釋出達平衡之前。雖然二常數模式僅適用於重金屬釋出達平衡之前，但在釋出反應之初期，二常數模式之預測值與多重一階反應模式及擴散模式比較下之誤差皆較小。

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.