• journal_title：Clays and Clay Minerals
• Contributor：Wei Zhao ; Haojie Cui ; Fan Liu ; Wenfeng Tan ; Xionghan Feng
• Publisher：Clay Minerals Society
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.1346/CCMN.2009.0570501
• journal_abbrev：Clays and Clay Minerals
• issn：0009-8604
• volume：57
• issue：5
• firstpage：513
• section：Articles

The relationship between vacant Mn structural sites in birnessites and heavy-metal adsorption is a current and important research topic. In this study, two series of birnessites with different average oxidation states (AOS) of Mn were synthesized. One birnessite series was prepared in acidic media (49.6–53.6 wt.% Mn) and the other in alkaline media (50.0–56.2 wt.% Mn). Correlations between the Pb²⁺ adsorption capacity and the d₁₁₀ interlayer spacing, the AOS by titration, and the release of Mn²⁺, H⁺, and K⁺ during adsorption of Pb²⁺ were investigated. The maximum Pb²⁺ adsorption by the birnessites synthesized in acidic media ranged from 1320 to 2457 mmol/kg with AOS values that ranged from 3.67 to 3.92. For birnessites synthesized in alkaline media, the maximum Pb²⁺ adsorption ranged from 524 to 1814 mmol/kg, with AOS values between 3.49 and 3.89. Birnessite AOS values and Pb²⁺ adsorption increased as the Mn content decreased. The maximum Pb²⁺ adsorption to the synthetic birnessites calculated from a Langmuir fit of the Pb adsorption data was linearly related to AOS. Birnessite AOS was positively correlated to Pb²⁺ adsorption, but negatively correlated to the d₁₁₀ spacing. Vacant Mn structural sites in birnessite increased with AOS and resulted in greater Pb²⁺ adsorption. Birnessite AOS values apparently reflect the quantity of vacant sites which largely account for Pb²⁺ adsorption. Therefore, the Pb²⁺ adsorption capacity of birnessite is mostly determined by the Mn site vacancies, from which Mn²⁺, H⁺, and K⁺ released during adsorption were derived.