| 摘要: | 硝酸根為常見的地下水污染物之一，主要以農地肥料滲入為主要污染來源。目前去除硝酸根的方式有，物理方式、生物方法、化學方式。物理方式實際上並無破壞硝酸根，只是將其濃縮，仍需要再處理。在生物方式則是反應時間太長，以及容易受到有毒物質干擾。整體來說，仍以化學處理為最佳方案，本實驗使用化學程序，以元素鐵配合CO2曝氣去除水中硝酸根。
由基礎實驗得到以下結論，當系統控制CO2流量(100、200、400 mL/min)不同流量下水樣pH、DO、ORP變化相近，決定CO2=200 mL/min為系統控制基本條件；當CO2=200 mL/min條件下，Fe0不同劑量下(0、1、2、4 g/L)，觀察水樣pH、DO、ORP、Fe2+變化，決定以Fe0= 2g/L為基本條件。
基礎條件去離子水1L、NO3-=30 mg/L、CO2=200 mL/min、Fe0= 2g/L，迴流=1L/min。改變CO2流量， 當CO2流量=0 mL/min沒有去除NO3-之反應，可見本系統須在酸性環境下才有利於Fe0還原NO3-反應；有曝氣之條件下，較高流量去除NO3-較快，約30 min去除率達到100%，且高流量反應速率並無大幅度提升。Fe0不同劑量實驗中，高Fe0劑量下去除NO3-較快，約30 min去NO3-達到100%，但是劑量增加下去除NO3-速率並無加倍提升。在QR(0、1000 mL/min)實驗中，有迴流去除率較佳約30 min去NO3-達到100%，在無迴流狀況經60 min反應後仍殘留有NO3-，可見系統固液相均勻混合，有利於反應之進行。
再回收Fe0系統中，經回收後之Fe0處理效率依然很高；但在不添加新的Fe0狀態下，由於Fe0之逐漸流失，反應槽體Fe0總量減少，會降低其NO3-去除率。由實驗資料發現，本系統每批次建議添加新的Fe0劑量為0.25-1 g/L之間。
在改變水質條件實驗中，當添加較高劑量的NO3-，NO3-反應速率較快且較高濃度下相同處理流程可移除較多的NO3-；當腐植酸未添加時去除NO3-約40min可達到100%，當添加0.5mg/L經90min反應僅去除71%，且濃度越高去NO3-除率越低；改變Na2CO3對於去除NO3-並無改變，可知水中若含有Na+或CO32-對處理程序並無影響；在添加NaCl實驗中發現，NaCl有利於反應進行，且濃度反應速率越快，可見Cl-存在水體中將有利於本實驗的進行；實驗中發現，當添加CaCl2•2H2O 300 mg/L as CaCO3有利於反應進行，但是添加50、150mg/L則有抑制效果，由於Cl-可加速反應速率，可見當水體中存在於Ca2+會抑制反應的進行。
Aqueous nitrate is one of the most common pollutants in ground water. It mainly comes from the agricultural fertilizer. There are three means of removing the aqueous nitrate: physical, biological, and chemical. When using the physical approach to remove aqueous nitrate, aqueous nitrate does not be removed. Actually, it was just be concentrated, so it has to be dealt with biological or chemical means. In biological approach, the time of reaction is too long, so it would be bothered by toxicants easily. Given that, physical method is the best one to remove aqueous nitrate; this experiment is tried to remove aqueous nitrate by zero-valent iron coupled with dioxide bubbling in physical way.
We would get following results from the basic test: when the speeds of flowing are 100, 200, 400 mL/min, the pH, DO, ORP are almost the same, so CO2=200 mL/min is the basic condition of this system. When CO2=200 mL/min, using different doses of Fe0 (0, 1, 2, 4 g/L), the pH, DO, ORP, Fe2+ are extremely different, hence Fe0=2 g/L is the basic condition of this system.
The basic condition of this system is that Distilled water is 1L, NO3-=30 mg/L, CO2=200 mL/min, Fe0=2g/L, recycle=1L/min. After changing the flows of CO2 (0, 100, 200, 400 mL/min), we could find that when the flows of CO2 is 0 mL/min, NO3- could not be removed. Thus, Fe0 could reduce NO3- in an acid environment. In different doses of Fe0 (1, 2, 4 g/L) test, the more Fe0 doses, the faster of removing NO3- ( it could be removed totally within 30min). However, when the doses increase doubly, the speed of removing NO3- would not be fast doubly. In QR(0, 1000 mL/min) test shows that the rate of removing NO3- in recycle is better than without recycle condition. When in recycle condition, NO3- would be removed totally in 30 min. And when without recycle condition, NO3- would be remained after 60 min of reacting. Therefore, if sample and Fe0 were well mixed, the speed of removing NO3- would be faster.
In the system of reusing Fe0, the speed of removing NO3- is fast. If we do not add Fe0, the amount of Fe0 would become less and less. Then, the rate of removing NO3- would become slower due to the reduction of Fe0. According to this experimental data, we suggest that the amount of adding Fe0 within 0.25-1 g/L would be better.
In changing the concentration of NO3- (0, 10, 30, 50, 100 mg/L): when adding more NO3-, the speed of reacting would be faster. Besides, the more the NO3-, the more of removing NO3- within the same time. In changing the concentration of humic acid (0, 0.36, 0.55, 0.93, 1.34, 2.29, 4.2 mg/L): when not adding humic acid, NO3- would be removed perfectly in 40 min; when adding 0.55 mg/L of humic acid, NO3- just would be removed 71% in 90 min. For that reason, the more the humic acid, the slower the removal of NO3-. In changing the amount of Na2CO3 (0, 47, 94, 141 mg/L): there is no effect while changing the amount of NaCO3. It proves that there would not affect NO3- in this procedure because of containing Na+ or CO32-. In changing the amount of NaCl (35.5, 106.5, 213 mg/L as CaCO3 ): adding NaCl would profit the reaction of NO3-. Furthermore, when the concentration of NaCl is more, the rate of removing NO3- is faster. It shows that the existence of Cl- is profitable in this experiment. In changing the concentration of CaCl2•2H2O (0, 50, 150, 300 mg/L as CaCO3): 300 mg/L as CaCO3 of CaCl2•2H2O would be the better way to help the reaction of NO3-. However, 50, 150 mg/L as CaCO3 of CaCl2•2H2O would restrain the reaction of NO3-. Cl- could faster the removal of NO3-, so Ca2+’s restraining the removal of NO3- is known. |
