Removal and recovery of Cr(VI) from water using Li/Al layered double hydroxide

• Main Authors: Liang-Ching Hsu, 許良境
• Other Authors: 鄒裕民
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/36862970063045022547

碩士 === 中興大學 === 土壤環境科學系所 === 94 === Chromium is one of transition metals widely distributed in geologic and soil environments. Although weathering may release Cr-rich minerals or rocks into the environment, anthropogenic sources of Cr are the major concerns due to its spread very closed to ecosystems. Chromium exits predominantly in two oxidation states, Cr(VI) and Cr(III). The toxicity and mobility of these two Cr species are quite different. Chromium (VI) is toxic to both plants and animals. Especially, the exposure to Cr(VI) may increase a risk of contracting cancer or developing hypersensitivity of the skin or respiratory system. Moreover, because Cr(VI) has high mobility in most neutral to alkaline soils, it poses a great threat to the surface water or groundwater. On the contrary, Cr(III) is comparable less toxic and relative immobile in soils. Therefore, the development of a technique to remove and recover Cr(VI) from Cr-contaminated media become more vital. Recently, a unique mineral known as layered double hydroxides (LDHs) has received much scientific attention because of their high anon exchange capacity and large surface areas. [LiAl2(OH)6+]Cl-•xH2O(Li/Al-LDH) belongs to one of LDH categories which has been shown a potential adsorbent for scavenging anionic contaminants, such as Cr(VI) according to the preliminary experiments. However, the detail information of the influences of temperatures on kinetic adsorption of Cr(VI) on Li/Al LDH is unclear. Additionally, a novel treatment/recycle technique will be developed on the basis of the unique characteristics of Li/Al LDH. In this study, the adsorption of Cr(VI) by Li/Al-LDH was first investigated at 10, 25, 40, 60 and 90oC using a batch mode. The results showed that Cr(VI) was rapidly adsorbed through ion-exchange reaction with interlayered Cl-. However, a portion of adsorbed Cr(VI) was released over the reaction time, particularly at higher temperature. For instance, 4, 12, 20, 53 and 60% of adsorbed Cr(VI) were released at 10, 25, 40, 60 and 90oC, respectively after 6 h reaction. Accompanied with Cr(VI) desorption, Li+ was also detected in the solution. Therefore, Li de-intercalation was assumed a major factor causing Cr(VI) desorption. To clarify this, a system was designed to evaluate the Li de-intercalation from Li/Al LDH as influenced by the temperatures with or without Cr(VI). The results indicated that Li/Al LDH was not stable in a solution with an elevated temperature. Chromium(VI) was not a key factor resulting in Li de-intercalation because the reaction could be observed even in the absence of Cr(VI). Without Cr(VI), the rate of Li de-intercalation followed the first order kinetic, and the kinetic parameters increased from 9.00 x 10-6 to 1.06 x 10-2 s-1 with increasing temperature from 25 to 90oC. With Cr(VI), the release of adsorbed Cr(VI) was also accompanied with Li+ de-intercalation, but in a lower rate than that in Cr(VI) free system. Li de-intercalation resulted eventually in the transformation of Li/Al-LDH into gibbsite. The properties of thermal unstablility and high adsorption ability of Li/Al LDH may lead to the development of an innovative technique for the removal of Cr(VI) from Cr(VI)-containing wastewater. That is, Li/Al LDH may be used as an effective adsorbent for the adsorption of Cr(VI) under an ambient environment. Following the adsorptive process, the adsorbed Cr(VI) may be released using heated water to treat the Cr(VI)-containing Li/Al LDH particles. Through this hydrothermal treatment of the used adsorbent, Cr(VI) is recovered and the solid product, i.e., gibbsite, can be recycled for further uses.