The Influence of Zn2+ and Mn2+ on Pb2+ Adsorption Behaviors of Birnessite

Abstract

The present study comparatively investigated Pb2+ adsorption behaviors of the birnessite with high Mn average oxidation state (AOS) before and after treatment of preadsorption with Zn2+ and Mn2+, respectively. The association of vacant Mn octahedral sites with Pb2+ adsorption was further understood from the variances of birnessite AOS, d(110)-interplanar spacing, maximum Pb2+ adsorption, maximum Zn2+ and Mn2+ release during the Pb2+ adsorption before and after treatments. The birnessite AOS and d(110)-interplanar spacing were almost unchanged as the concentration of Zn2+ increased, indicative of the unchanged vacant Mn octahedral sites, whereas the maximum Pb2+ adsorption decreased from 3190 to 2030 mmol·kg−1 due to occupancy of the treating Zn2+ in adsorption sites. However, the AOS of the Mn2+-treated birnessites decreased and most of the treating Mn2+ were oxidized to Mn3+ and located below or above vacant Mn octahedral sites or migrated into vacant Mn octahedral sites. Increasing Mn2+ concentration from 1 to 2.4 mmol·L−1 increased the d(110)-interplanar spacing of the treated birnessites from 0.1416 to 0.14196 nm but decreased the maximum Pb2+ adsorption of the treated birnessites from 3190 to 1332 mmol·kg−1, indicating the decrease in the amount of vacant Mn octahedral sites, mainly due to the increase of the produced Mn3+ migrating into vacant Mn octahedral sites. Therefore, birnesstie Pb2+ adsorption capacity was largely determined by the number of Mn site vacancies.