地下水中薄膜積垢物二氧化矽去除之研究

Silica Removal for a RO Brackish Water Source

林政立(Cheng-Li Lin)

指導教授 ： 陳孝行

國立臺北科技大學/工程學院/環境規劃與管理研究所/碩士(2006年)

https://doi.org/10.6841/NTUT.2006.00110

摘要

本研究旨在探討地下水中薄膜積垢物質SiO2之去除，透過實場地下水樣和實驗室模擬水樣進行相關研究，實驗共分四部份，1.利用軟體程式（ROSA），探討SiO2去除率/薄膜回收率/ SiO2飽和度三者間的關係；2.探討前處理程序對於SiO2之去除；3.探討Mg(OH)2及CaCO3對於SiO2之去除及吸附模式模擬；4.應用XRD進行沉積物表面分析探討生成之化合物物種。
本研究結果顯示，經由ROSA模擬去除率/回收率/飽和度之關係，於進流水中（SiO2濃度為21 mg/L），當SiO2去除率為20%時，薄膜回收率可提高至87%，當SiO2去除率提高至40~80％時，則薄膜回收率可提高至90%，且不會有薄膜阻塞的現象發生。針對前處理程序SiO2去除之影響因子結果顯示，當pH大於9時，SiO2的去除率會大幅的增加，特別當pH大於11時，SiO2可至80%的去除效果。此外；透過SiO2去除量對鈣、鎂沉澱量之比較結果，推導其關係式可得Mg(OH)2和CaCO3對於SiO2的去除比為0.045 SiO2 /mg Mg(OH)2 (s)及0.028 SiO2 /mg CaCO3 (s)，於混凝程序去除SiO2上，pH宜控制在10的範圍，SiO2去除率可達80%。當pH 11時，因鋁離子為兩性氫氧化物於高pH下形成帶負電之Al(OH)4-，反而與略帶正電性之Mg(OH)2發生反應，降低Mg(OH)2去除SiO2的能力。
在模擬水樣試驗中，利用不同沉澱量之Mg(OH)2，對水樣中SiO2進行吸附實驗，並將數據代入Freundlich及Langmuir吸附模式中，其結果顯示，在Freundlich模式中Mg(OH)2其n值分別為1.405，R-squared值為0.928。而Langmuir模式中其R-squared值為0.904。根據上述模式模擬之結果可知，在Mg(OH)2吸附去除SiO2上，Mg(OH)2較符合Freundlich吸附模式。
經XRD對沉積物進行表面分析，其化合物物種為CaCO3和Mg(OH)2。而在相關文獻中記載解離之矽酸會與水中鈣、鎂離子形成的Mg2SiO4和Ca2SiO4，透過XRD分析，本研究發現並無此二物種之形成。

關鍵字

二氧化矽 ； 軟化程序 ； 混凝程序 ； XRD ； 薄膜前處理

並列摘要

The objective of this study was to investigate the removal of silica from brackish water by the pretreatment of softening and coagulation. This study was conducted by the following four steps：(1) Development of a removal-saturation-recovery curve to determine the threshold limits of RO recovery and silica removal (2) Determining the enhanced effect for silica removal by pretreatment (3) Determining the adsorption isotherm simulation and the silica removal efficiency test in a pilot test, and (4) Qualitatively determining the effects of Ca and Mg for silica removal and verifying removal mechanism by XRD.
Threshold limit for RO recovery and required silica removal were firstly determined by a removal-saturation-recovery curve. The result shows, in raw water (SiO2 = 21 mg/L) or 20% silica removal case, if the water recovery is high, the % saturation will be exceed 100% and silica scaling will occur. High pH was more effective for silica removal when pH>11 and the silica removal ratio exceeded 80%. The precipitation of Mg(OH)2 has assisted more for silica removal than that of CaCO3 and the ratio of 0.045 mg SiO2/mg Mg(OH)2 and 0.028 mg SiO2/mg CaCO3 were statistically determined.
Silica removal could be enhanced by coagulation. Experimental results show the silica removal was increased as pH was increased. The addition of PACl was capable of enhancing silica removal for all the pHs other than 11.Therefore, the addition of PACl for silica removal can not be applied for pH higher than 10, causing adverse effect for silica removal.
The adsorption pilot tests were performed to assess the rate of silica adsorption by Mg(OH)2, using Langmuir and Freundlich isotherm respectively. Adsorption of silica by Mg(OH)2 was better fitted by Freundlich isotherm, According to the XRD database from the pilot tests, the most prevalent species were identified as Mg(OH)2 and CaCO3. Therefore, no Mg2SiO4 and Ca2SiO4 were observed and were verified by XRD analysis.

並列關鍵字

Silica ； Softening ； Coagulation ； XRD ； Pretreatment

參考文獻

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[4] Roques, H. and S. Altmann, Chemical Water Treatment Principles and Practice. VCH Publishers, New York, 1995.

[6] Alexander, G.B., W.M. Heston, and R.K. Iler, The solubility of amorphous silica in water. J. Phys. Chem., 1954. 58: p. 453-455.