Pb²⁺ adsorption on birnessite affected by Zn²⁺ and Mn²⁺ pretreatments

Purpose Lead contamination is ubiquitous, and much attention has been paid due to its toxicity. The phyllomanganate birnessite is the most common Mn oxide in soils. The MnO₆ octahedral layers may have significant Mn vacancies in the hexagonal birnessites. Among heavy metal ions, birnessites possess the greatest adsorption affinity and capacity for Pb²⁺. The aim of this study was to understand the relationship between vacant Mn octahedral sites and Pb²⁺ adsorption. Materials and methods Birnessite synthesis was achieved by the reduction of potassium permanganate in a strong acidic medium. Synthetic birnessite was then treated with Mn²⁺ or Zn²⁺ at different concentrations. Isothermal Pb²⁺ adsorption on birnessite before and after treatments was measured at a solid-to-liquid ratio of approximately 1.67 g/L, and Pb²⁺ concentrations ranged from 0 to 10 mmol/L with an ionic strength of 0.1 mol/L NaNO₃. The amount of Pb²⁺ adsorbed and the amount of Mn²⁺ or Zn²⁺ released during the whole adsorption process were obtained by comparison with a control group without adding Pb²⁺. The amount of H⁺ released was determined from the recorded additions of standard HNO₃/NaOH solutions. Results and discussion Mn average oxidation state (AOS) and d(110)-interplanar spacings of the birnessites remained almost unchanged as the concentration of the treating Zn²⁺ increased, indicating an unchanged number of vacant Mn octahedral sites, whereas the maximum Pb²⁺ adsorption decreased from 3,190 to 2,030 mmol/kg due to the presence of Zn²⁺ on adsorption sites. The AOS's of the Mn²⁺-treated birnessites decreased and most of the Mn²⁺ ions added were oxidized to Mn³⁺ ions. The d(110)-interplanar spacing of Mn²⁺-treated birnessites increased from 0.14160 to 0.14196 nm, indicative of a decreased vacant Mn octahedral sites. Moreover, the maximum Pb²⁺ adsorption of Mn²⁺-treated birnessites decreased from 3,190 to 1,332 mmol/kg, the decrease being greater than that for the corresponding Zn²⁺-treated birnessites. Conclusions Most Mn²⁺ was oxidized to Mn³⁺ by birnessite, with a portion of Mn³⁺ located above or below vacant sites, which did not affect the number of vacant sites, and the remaining Mn³⁺ migrating to occupy the vacant sites. In contrast, Zn²⁺ ions are adsorbed only above or below vacant sites. Birnessite Pb²⁺ adsorption capacity is determined largely by the number of vacant Mn sites.