Saturday, 15 July 2006

Sorption of Arsenate on Lithium/Aluminum Layered Double Hydroxide Intercalated by Chloride: Macroscopic and Spectroscopic Studies.

Yu T. Liu1, Ming K. Wang1, and P.M. Huang2. (1) Dept of Agricultural Chemistry, No. 1, Section 4, Roosevelt Rd.,, Taipei, 106, Taiwan, (2) Dept of Soil Science, Univ of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada

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 research, 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. Arsenate sorption on Li/Al LDH-Cl can be divided into the fast and slow reactions described by the second-order rate equation. This biphasic arsenate sorption behavior was partially attributable to: (i) micropore sorption sites on the Li/Al LDH-Cl surfaces, and (ii) Li exposed on planar surfaces and Al exposed on edges of double hydroxyl layers. The rate coefficient of the fast reaction of arsenate sorption was 9 to 14 times higher than that of the slow reaction. The pre-exponential factor value, which is a measure of collision frequency of arsenate with the reactive sites, was more than double in the fast reaction compared with the slow reaction. The activation energy of arsenate sorption on Li/Al LDH-Cl obtained from the Arrhenius equation was less than 42 kJ mol-1, indicating that the rate-limiting step of arsenate sorption was predominantly a diffusion process. Extended X-ray absorption fine structure (EXAFS) analysis of arsenate sorbed on Li/Al LDH-Cl shows that As(V)-Li and As(V)-Al interatomic distances were 2.62 0.02 and 3.08 0.02 and the coordination number of As(V)-Li and As(V)-Al were 1.6 and 2.3 atoms, respectively, indicating that the inner-sphere bidentate mononuclear and bidentate binuclear bonding were the predominant mechanism of arsenate sorption on the planar surfaces and edges of Li/Al LDH-Cl. In addition, the sorption behavior of arsenate on Li/Al LDH-Cl and on 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. Arsenate sorption maximum on Li/Al LDH-Cl was approximately six times higher than that on gibbsite. At pH 4.0 to 9.0, all of arsenate sorption amounts on Li/Al LDH-Cl were more than that on gibbsite. Sorption envelopes of arsenate on Li/Al LDH-Cl showed a significantly pH-sensitive tendency at pH 4.0 7.0, but it was inconspicuous at pH 7.0 to 9.0. From EXAFS analysis, arsenate, sorbed on Li/Al LDH, reacted not only with Al in the edges of Al(OH)3 layers, but also with Li located in the vacant octahedral sites within Al(OH)3 layers below pH 7.0, however, the As(V)-Al complex cannot be observed at pH 9.0. In addition, the reducing intensity of As(V)-Al peaks with increasing pH represented that fewer As(V)-Al complex existed at higher pH. By considering the results of sorption envelopes and EXAFS analysis, the superior sorption capability of Li/Al LDH-Cl to that of gibbsite could be attributed to the Li cations of Li/Al LDH-Cl which made the surface of Al(OH)3 have high affinity to arsenate and served as the permanent sorption sites. Keywords: Arsenate sorption, Li/Al LDH-Cl, Extended X-ray absorption fine structure (EXAFS)

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