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

Abstract

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. 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. 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. 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.