The impact of abiotic and biogenic mn oxide coatings on contaminant mobility, bioavailability, and attenuation

Abiotic and biogenic manganese oxide coatings significantly impact the mobility and bioavailability of metals in soils and sediments. While sorption to discrete Mn oxides has been investigated in a number of studies, coatings have not received as much attention. Abiotic nanocrystalline (hydrous manganese oxide (HMO)) and crystalline Mn oxides (bimessite and pyrolusite) coated on montmorillonite were studied for formation, structure, stability, and surface properties. Coatings dominated the clay surface characteristics where the surface charge behaved similar to that of the discrete oxide and the local structure was consistent with the pure oxide phase. Furthermore, in sorption studies, Zn and Pb ions formed inner-sphere complexes as tridentate and bidentate corner-sharing structures, respectively. On the other hand, vacancy sites along the oxide surface were occupied by Ni. In biomineralization, a nano-particulate Mn oxide coated the sheathed Leptothrix discophora SP-6 forming a dendritic biofilm structure. The surface charge and the local structure were consistent with that of abiotic HMO where the oxide potentially forms a polymer-like precursor to birnessite consistent with the phyllomanganate family. Surface properties of the coated bacteria are dominated by the oxide as Zn formed inner-sphere complexes resulting in octahedral structures. On the other hand, sorption to the sheathed L. discophora SP-6 involved complexation with phosphoryl (84%) and carboxyl (16%) groups. The slow sorption process, intraparticle surface diffusion, was observed in abiotic and biogenic oxide systems, indicating that Mn oxide present as coatings or discrete particles acts as a sink for metal contaminants