現地土壤重金屬去除之研究

Heavy Metal Removal from Field Soil

黃政鵬(Cheng P. Huang)

指導教授 ： 林勝雄

元智大學/工程學院/化學工程與材料科學學系/碩士(2001年)

摘要

重金屬在現今的工業上使用很普遍，一旦造成污染可能經年累月也無法改善。由於重金屬不像有機物會分解且消滅，使本來的毒害作用完全消失，雖然經過物理及化學變化，重金屬仍是重金屬。因此對於受重金屬污染的土壤，我們必須尋求一個有效且經濟的處理對策。
近年來，土壤污染案例層出不窮，例如桃園縣蘆竹鄉中福社區的鎘污染，乃是當年基力化工廠含重金屬廢水處理不當的結果，而本文乃是針對此一污染土壤，提供一個復育土壤的技術。在中福土壤中，重金屬鎘及鋅均超過標準值甚多，且大部分以水溶性及交換性形態居多，而此兩種形態最易為農作物吸收而威脅人類健康，因此欲處理亦需以此兩種形態物種為主要目標。
在批次實驗中，我們使用了三種不同形式的萃取劑（HCl、EDTA、CaCl2）來去除中福土壤中的鎘及鋅，探討何種萃取劑對去除效果最好，結果發現CaCl2溶液萃取出的鎘量最多，可能是因為Ca2+與土壤的吸附選擇性大於H+與土壤的吸附選擇性的結果，且經由氯化鈣溶液處理過後的土壤不會破壞土壤結構，仍可繼續種植。而在恆溫去除實驗方面，經驗式的中Freundlich 比 Langmuir 較適切描述此等溫去除現象，而修正的Langmuir 及 Jossens et al. 也都有不錯的契合效果。在質傳模式的探討上，以簡單的平方動力模式對於三種試劑皆能適切的表示出質傳現象。
在連續淋洗實驗上是採用管柱淋洗方式，主要以交換性萃取劑為主，探討的變因包括萃取劑的種類、pH、通量、濃度、價數、土壤粒徑、回收液淋洗次數及萃取劑純度。在理論方面，則使用General Logistic、Hill、Weibull等方程式模擬其淋洗去除貫穿曲線，發現三種模式皆能適切的表示出淋洗系統的行為。
在使用小規模管柱淋洗後，我們將規模放大為長30公分寬30公分的管柱，實際模擬現場操作行為，探討是否會因規模放大而對重金屬的去除效率有所影響，結果發現，氯化鈣及氯化鈉皆有不錯的效果。而在後續的土壤廢液處理問題上，離子交換法及沈澱法也都能有效的將重金屬自淋洗廢液中分離出來，此流程可作為日後處理類似土壤污染問題的一個參考指標。

關鍵字

重金屬 ； 氯化鈣 ； 淋洗 ； 土壤污染

並列摘要

Heavy metals are widely used in various in industrial processes nowadays. However, these industrial processes very often generate different types of wastes that contain toxic heavy metals. Unlike the organic or inorganic compounds that can be decomposed, heavy metals remain the polluted environment for quite long time. Therefore, effective and economic methods are needed to tackle the heavy metal pollution problem.
In the past decade, the farmland in Chung Fu area, Lu Tsu, Taoyuan Country was found to be seriously contaminated by cadmium and zinc. Such a heavy metal contamination of soil stemmed from improper handling of heavy metal-containing wastewater produced by Chi-Lee chemical plant. Various methods have been proposed for cleaning up the contaminated soil. In the present study, acid and calcium salt solution washings were explored as potential methods for effective removal of heavy metal from the contaminated farmland. Experimental tests were conducted to determine the heavy metal removal efficiencies under different operating conditions.
In the batch tests, three kinds of extractants (HCl, EDTA, CaCl2) were examined to remove cadmium and zinc in contaminated soils. Both soil artificially contaminated with heavy metals as prepared in the laboratory and contaminated soil obtained from the field were tested in the extraction tests. It was found that calcium chloride was more efficient than the other two extractants in removing cadmium from the contaminated soil. This may be due to that the adsorptive selectivity of Ca2+ was stronger than H+. Extensive equilibrium extraction tests were performed to collect data for the equilibrium adsorption of heavy metal in soil. Different adsorption isotherms were attempted for describing the equilibrium adsorption relationship. The empirical Freundlich model was found to represent the observed data better than the Langmuir model. The modified Langmuir and Jossens models also were to describe the equilibrium adsorption well. In the mass transfer aspect, a simple model based on the sequared-driving principle was adopted and found to describe well the overall mass transfer process.
Continuous column washing tests were also conducted to determine the effects of operating variables, including the type of extractant, pH, flow rate, extractant concentration, valence, soil particle size, extraction time, etc. on the heavy metal removal efficiency. Various empirical or semi-empirical equations, like general logistic, Hill and Weibull models, were employed to represent the soil washing breakthrough curves. The column models were be helpful in accurate estimation of the washing tome.
After soil washing tests using a small column, test runs were also conducted in a much bigger pilot column of 30 cm (W) x 30 cm (L) x 100 cm (H) to simulate the in-situ operation and to determined effectiveness of two calcium salts, i.e. calcium chloride and sodium chloride. One important aspect considered in the present investigation was the reuse of washing solution. The washing solution after soil washing contained significant amounts of heavy metals and chemicals. Chemical precipitation offerred good heavy metal removal from wastewater and the treated washing solution can be readily recycled for reuse in the washing process.

並列關鍵字

Heavy Metal ； Calcium Chloride ； Leaching ； Soil Pollution

參考文獻

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Alloway, B. J. ,Heavy Metals in Soils, John Wiley and Sons, New York (1990)

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Cline, S. R. and Reed, B. E., “Lead Removal from Soils via Bench-scale Soil Washing Techniques,” Journal of Environmental Engineering, ASCE, 121,700-705（1995）

Davis, A. P. and Hotha, B. V., “Washing of Various Lead Compounds from a Contaminated Soil Column,” Journal of Environmental Engineering, ASCE, 124, 1066-1075（1998）