利用超臨界流體搭配CMC分散劑製備奈米零價鐵及複合雙金屬還原地下水中六價鉻之研究

Using Supercritical Fluid with Dispersant to Prepare Nanoscale Zero-valent Metals and Multimetallics for Reducing Cr(VI) in Water

湯世亨(Shi-Han Tang)

指導教授 ： 劉敏信

朝陽科技大學/理工學院/環境工程與管理系/碩士(2012年)

摘要

重金屬鉻在工業上是一種常見的原料，舉凡電鍍、鞣革、製藥等工業都會產生含鉻之廢水，六價鉻是常見的污染物。零價金屬具有強大的還原能力，又以零價鐵最為被廣泛使用於處理地下水中污染技術。超臨界流體具有高度的萃取能力、低表面張力、低污染、高傳達、高擴散性的特性，近年來常用於食品成分萃取、中藥萃取、廢棄物處理及奈米材料之製備。
　　本研究乃利用超臨界流體搭配CMC(Sodium cellulose glycolate)分散劑製備零價鐵，選擇以FeCl3．6(H2O)及NaBH4溶液配製，於製備合成前後分別以溶液重量比各添加1%、5%、15%及30%的CMC，另外以二氧化碳作為流體，在特定壓力及溫度的超臨界狀態下，製備出零價鐵，其效果以還原地下水中六價鉻作為測試。測試中分別探討不同pH、鐵粉添加量、污染濃度、反應液體體積等參數對六價鉻降解效率之影響；並利用比表面積(BET)及場發射掃描式電子顯微鏡(FE-SEM)分析零價鐵特性。本研究結果顯示，添加CMC能有效提升零價鐵比表面積6~7倍；同時試驗中發現pH越低降解效率越佳，所以後續試驗pH以3作為固定參數。在六價鉻降解試驗中首先以固定添加市售鐵粉0.2 g對不同鉻濃度(10、20、30、40、50、100 mg/L)，固定體積50 mL，反應時間30 min進行實驗，六價鉻去除量均為0.361 mg。而不同體積(10、20、50、100 mL)，固定濃度100 mg/L，反應時間30 min進行實驗，六價鉻去除量則均為0.171 mg。在添加不同CMC濃度試驗中可發現，以濕式合成法合成零價鐵後添加5%及30%CMC濃度，以零價鐵0.05 g在反應時間4小時下，六價鉻降解效率均有70%。而添加1%CMC於超臨界流體合成法中，因多次試驗控制參數皆高於二氧化碳的超臨界值，所以不同壓力及溫度下所合成出的粒徑大小大致相同，對於六價鉻的降解率約在50%左右。而利用傳統濕式合成法製備之鐵鎳複合雙金屬以無添加CMC的降解效率反而大於添加CMC合成之鐵鎳複合金屬，主要是在合成中將CMC置入鐵鎳雙金屬水溶液時，將會產生塊狀物，此乃團聚現象所造成。若將濕式合成法添加5%CMC合成之零價鐵予以酸洗，當鐵粉量與六價鉻莫耳比從原來的4.37(0.893 mmol：0.204 mmol)增加至17.5 (3.571 mmol：0.204 mmol)時，在10 min即可對六價鉻的降解效率由50%增加至近100%。而超臨界流體法添加1%CMC合成之零價鐵，相同之鐵粉量與六價鉻莫耳比4.37(0.893 mmol：0.204 mmol)增加至17.5 (3.571 mmol：0.204 mmol)時，亦有近100%的降解效率。在常溫25

關鍵字

零價鐵 ； 六價鉻 ； 超臨界流體 ； CMC分散劑 ； 奈米

並列摘要

Chromium, as one of the widely used heavy metals and a common raw material in many industrial applications such as electroplating, leather tanning, pharmaceutical and others will certainly produce Chromium contaminated wastewater. Thus, hexavalent chromium has been frequently identified in the wastewater generated by factories and in the groundwater nearby. Zero-valent metals, with its remarkable reducing capability like zero-valent iron, have been widely introduced as an agent of removing contaminants found in wastewater and groundwater. Supercritical fluid, with its outstanding characteristics of high extraction capacity, low surface tension, relatively low pollution, high transmittance, high diffusion, has been put to work on extraction of food ingredients, herb medicine extraction, waste treatment and preparation of nano-materials in recent years.

This study was to utilize supercritical fluid with supplement of sodium carboxymethyl cellulose (CMC) dispersant to prepare zero-valent iron. The zero-valent iron was made by using FeCl3‧6(H2O) and NaBH4. Under the supercritical condition of varied pressure and temperature combination (100 bar, 40°C; 100 bar, 50°C; 150 bar, 50°C; and 150 bar, 60°C) and with carbon dioxide as fluid, variable weight ratio of 1%, 5%, 15% and 30% of CMC were introduced to prepare zero-valent iron. The characteristics of zero-valent iron were then examined by reducing hexavalent Chromium under parameters such as varied pH value, dosage of iron powder, concentration of pollutants, and reaction molar ratio. To characterize the property of zero-valent iron, the analysis conducted by specific BET surface area analyzer and Multi-functional Field-Emission Scanning Electron Microscope (FE-SEM) were further implemented. The results suggested, with participation of CMC, the specific surface area of zero-valent iron can be effectively improved up to 6 or 7 times. And the study also found chloride ion in the water does not render impact on the reduction of hexavalent Chromium. The parameter of pH value was set as 3 for all the remaining studies due to the understanding of better performance by lower pH value. In the experiments of hexavalent Chromium reduction, 0.361 mg chromium was removed with varied concentration of chromium (10, 20, 30, 40, 50, 100 mg/L) mixed with 0.2 g commercial iron powder under the condition of volume 50 mL and 30 minutes reaction time. And 0.171 mg chromium was removed with varied volume (10, 20, 50, 100 mL), fixed concentration 100 mg/L of chromium and 30 minutes reaction time. To study the effect of varied CMC dosage, zero-valent iron prepared by wet chemical synthesis was then mixed with concentration 3% and 5% of CMC, 70% reduction of chromium was achieved by 0.05 g of zero-valent iron with reaction time 4 hours. For experiments of adding 1% CMC in supercritical fluid synthesis, the reduction of chromium was around 50% owing to relatively identical particle size synthesized in the condition of varied pressure and temperature which were higher than supercritical parameters of carbon dioxide. The efficiency of zero-valent iron-nickel bimetallic particles prepared by wet chemical synthesis showed better performance without CMC than with because product was coagulated when CMC was introduced into the solution of iron-nickel bimetal. If acid washing was conducted onto zero-valent iron prepared by wet chemical synthesis with 5% CMC, the molar ratio of iron powder to hexavalent chromium was raised from 4.37 (0.893 mmole:0.204 mmole) to 17.5 (3.571 mmole:0.204 mmole), the reduction of hexavalent chromium can be achieved from 50% to 100% within 10 minutes. Likewise, zero-valent prepared by supercritical fluid method, under the same increasing rate of molar ratio as aforementioned, the reduction of hexavalent chromium can reach 100%. Under the condition of 25°C, 0.05 g of zero-valent iron prepared by wet chemical synthesis reacted with varied concentration of hexavalent chromium (25, 50, 100, 200 mg/L) in solution volume 500 mL, the rate of reduction can be defined as a second order reaction with respect to kinetic calculation. The constant of reaction rate was 97.63, 2.5×10-2, 3.6×10-3, 2.3×10-4 (mM．min)-1 for varied concentration of 0.511, 1.046, 2.091, 3.997 mM, respectively.

並列關鍵字

Zero-valent iron ； Supercritical Fluid ； Hexavalent chromium ； CMC dispersant ； Nanoscale

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