Sorption behavior of heavy metals on birnessite: Relationship with its Mn average oxidation state and implications for types of sorption sites

Sorption mechanisms of heavy metals at the mineral/water interface are largely controlled by the type and number of sorption sites on the mineral surfaces. However, in the case of layered manganese oxides, with a highly reactive interlayer (internal) region, the effects of variable substructures on their sorption sites, sorption capacities and characteristics are still obscure. Sorption experiments at pH 4.5 combined with powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were performed to investigate the sorption characteristics and mechanisms of Pb2 +, Cu2 +, Zn2 + and Cd2 + onto hexagonal birnessites with various Mn average oxidation states (AOS), together with detailed physicochemical characterizations of the birnessites. The results show that a decrease in Mn AOS of birnessites decreased considerably the sorption capacity for heavy metals, despite the fact that the specific surface area increased almost linearly. The sorption capacity for any given birnessite followed the order Pb2 + ≫ Cu2 + > Zn2 + > Cd2 +; for Pb2 + ranging from 1.6 to 3.9 times greater than those of the other metals, while Cu2 +, Zn2 +, and Cd2 + sorbed with similar maxima among them. The large differences between maximum Pb2 + sorption and that of the other metals were approximately of the same absolute magnitude regardless of the specific birnessites compared, as opposed to decreasing in equal relative proportions as would be expected in a homogeneous-site model. This evidence agrees well with previous work on hexagonal birnessites that proposes a general two-site structural model for metal-binding to birnessite, and the sorption maxima differences obtained in this work were used to estimate the concentration of low-energy and high-energy binding sites in the different birnessites. Relating these to previous structural information allowed us to assign these site types roughly to particle edge sites and vacancy sites at interlayers, respectively. In this manner, we found that for the birnessite containing exclusively Mn(IV), the contributions of edge sites and interlayer sites to total Pb2 + sorption were approximately equivalent; but as Mn AOS decreased, the contribution of vacant sites at interlayers sensibly decreased, probably through the increasing presence of layer Mn(III) at the expense of vacant sites. The results presented here of highly distinctive affinity site types are valuable for a fundamental understanding of the behavior of birnessites towards heavy metal sorption, which in turn contributes both to predictions of the geochemical behavior of birnessites and to applications for remediation schemes of metal-contaminated aqueous environments.