| Summary: | 碩士 === 朝陽科技大學 === 環境工程與管理系碩士班 === 95 === In last few decades, industrializations have made environmental pollution a very serious problem. Accumulation of heavy metals in soils that exceeded regulated concentration is a typical environmental issue in Taiwan. When soil is contaminated by heavy metals, the metals would not only affect the growth of crops, but also have the chance to enter the food chain. The consumption of the crops that contain heavy metals could result in health risk for human being and animals. In order to find a proper solution for such problem, an experimental case study has been conducted by using a multi-heavy-metal contaminated soil sampled from central Taiwan. The site is a paddy farm near Dali High School in Taichung County of central Taiwan where the soil is contaminated with nickel, chromium and copper.
The Chitosan is a chemical that is usually used as heavy metal chelating and fixation agent and to improve heavy metal removal in soil. In the experimental study, different concentrations of Chitosan were manually added to the sample soil, planted with Chinese cabbage to test the effects of Chitosan on the mobility of heavy metals (nickel, chromium, copper) between soil and plant roots as well as the phyto-extraction of heavy metals. The experimental design prepared 5 g/kg-soil, 20 g/kg-soil, 35 g/kg-soil of Chitosan added soils and control group with no Chitosan addition for comparison. Chinese cabbages were planted to the sample soils, and harvested every 9-day period for sampling and analysis.
The results showed that as the concentration of Chitosan increased the aeration of soil reduced, therefore the water and nutrient absorption of Chinese cabbage is stagnated. Especially in the 35g /kg-soil Chitosan added soil, the growth of Chinese cabbage stopped and withered at the end of 27-day growing period. The concentrations of heavy metals in Chinese cabbage tissue were saturated on the 9th day, thus, the heavy metal concentrations in Chinese cabbage were decreased as time went by.
The higher the Chitosan concentration resulted in higher soil pH value. In this study the soil pH values ranged between 6.0 to 7.0.The results also showed that under different Chitosan concentration, the relative ability of heavy metal absorption for Chinese cabbage is nickel﹥copper﹥chromium in order. The result of the control group showed that Chinese cabbage absorption of chromium and nickel was relatively higher than that of the other experimental groups. Though it is not clear but the possible reason may be related to the effect of Chitosan on the obstruction of metal movement due to viscosity formed by the metal chelating mechanism.
According to the experimental data, the addition of Chitosan has positive effect on the absorption of copper and nickel, but insignificant effect on chromium. The root tissues of Chinese cabbage accumulated a relative higher amount of heavy metals than the stems and leaves. It is deduced that the heavy metal pressure accumulated in roots is due to Chitosan viscosity. The results also showed that the rank of heavy metal accumulation in Chinese cabbage tissues was root>stem>leaf, which was consistent with the results of several previous researches.
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