以巨觀實驗及光譜分析探討砷酸鹽吸持在夾層為氯離子之鋰鋁層狀雙氫氧基化合物之機制

Abstract

砷為人類的致癌物且常見於自然環境中。吸持作用是環境中可用來控制砷分布的最重要的化學反應之一；層狀矽酸鹽、金屬氧化物及有機物質可以藉由形成內圈或外圈錯合物來吸附重金屬，成為生態圈中重金屬的重要匯池。本文是以夾層為氯化物之鋰鋁層狀雙氫氧基化合物作為吸附材料，從巨觀及微觀的角度探討砷酸鹽在夾層為氯化物之鋰鋁層狀雙氫氧基化合物上的吸持行為。夾層為氯化物之鋰鋁層狀雙氫氧基化合物是以氯化鋰插入層狀的Al(OH)3而合成的，直到1977年才被廣泛的研究，本實驗是第一次將夾層為氯化物之鋰鋁層狀雙氫氧基化合物應用在環境復育上，即利用其來吸持毒性甚大的砷酸鹽。 第一部份的實驗，其目的在觀察在278, 288, 298及308 K且pH 5.0下砷酸鹽在夾層為氯化物之鋰鋁層狀雙氫氧基化合物之動力吸附行為。實驗結果發現，其動力吸附行為可分為快反應及慢反應，且都符合二級的動力方程式。推測造成兩相吸附行為的原因部份是由於夾層為氯化物之鋰鋁層狀雙氫氧基化合物上吸附位置的異質性。從X光射線吸收邊緣延續光譜細微構造的分析結果可知，砷酸鹽會與夾層為氯化物之鋰鋁層狀雙氫氧基化合物中的鋰與鋁分別以雙螯單核型與雙螯雙核型的鍵結形成內圈錯合物。在第二部份的實驗中，目的為藉著等溫吸附、吸附邊緣實驗及X光射線吸收邊緣延續光譜細微構造的技術比較砷酸鹽在夾層為氯化物之鋰鋁層狀雙氫氧基化合物與三水鋁石上 (α-Al(OH)3) 的吸附行為，進而了解位於層狀Al(OH)3中之氯化鋰對於夾層為氯化物之鋰鋁層狀雙氫氧基化合物吸附砷酸鹽之貢獻。實驗結果顯示，砷酸鹽在夾層為氯化物之鋰鋁層狀雙氫氧基化合物上的最大吸附量為在三水鋁石上的六倍且夾層為氯化物之鋰鋁層狀雙氫氧基化合物在pH 4.0到pH 9.0下對砷酸鹽的吸附量都大於三水鋁石。由X光射線吸收邊緣延續光譜細微構的分析中可知，砷酸鹽不僅與夾層為氯化物之鋰鋁層狀雙氫氧基化合物中之鋁鍵結也與鋰鍵結，鋁對於吸附砷酸鹽的貢獻會隨著pH值的升高而減弱；不同於鋁，鋰提供一不受pH值影響之吸附位置且會提高層狀Al(OH)3表面對砷酸鹽的親和力。

Arsenic is a commonly occurring toxic metal in natural ecosystems and a known carcinogen in humans. Sorption, however, is one of the most important chemical processes to control the distribution of arsenic in the environment. Phyllosilicates, metal (hydr)oxides, and humic substances adsorb heavy metals by forming of inner- or outer- sphere sorption complexes, creating important sinks for these metals in ecosystem. Lithium / aluminum layered double hydroxide intercalated by chloride, as the sorbent for arsenate in this research, was formed by treatment with lithium chloride intercalated into the host structure of Al(OH)3. Li/Al LDH-Cl has not been well studied until 1977 and it is the first time that Li/Al LDH-Cl was used in environmental remediation for removing toxic anions. In this thesis, sorption of arsenate on Li/Al LDH-Cl was studied through sorption kinetics, isotherms, envelopes and mechanisms of arsenate sorbed on Li/Al LDH-Cl by extended X-ray absorption fine structure (EXAFS). The kinetics of arsenate sorption at pH 5.0 was studied at 278, 288, 298 and 308 K in the first research. As the results showed, arsenate sorption on Li/Al LDH-Cl could be divided into the fast and slow reactions described by the second-order rate equation. This biphasic arsenate sorption behavior was partially attributable to the heterogeneity of sorption sites. From the EXFAS analysis, inner-sphere complex occurred between arsenate and Li as well as Al of Li/Al LDH-Cl by bidentate mononuclear and bidentate binuclear configurations, respectively. In the second research, the sorption behavior of arsenate on Li/Al LDH-Cl and gibbsite (α-Al(OH)3) was studied to define how the intercalated lithium chloride participated in the sorption of arsenate through sorption isotherms, envelopes and EXFAS analysis. The sorption maximum of Li/Al LDH-Cl was approximately 6 times higher than that of gibbsite and the amounts of arsenate sorbed on Li/Al LDH-Cl at pH 4.0 – 9.0 were always higher than that on gibbsite. The reason of superior sorption capability of Li/Al LDH-Cl was sorption sites diversity of Li/Al LDH-Cl. The EXAFS analysis showed that arsenate sorbed on Li/Al LDH not only bonded with Al based on edges of Al(OH)3 layers but with Li located in the vacant octahedral sites with in Al(OH)3. However, the reaction between arsenate and Al would diminish with raised pH. In contrast with Al, Li served as permanent sorption sites participating in arsenate sorption and make the surface of Al(OH)3 had high affinity to arsenate.