Properties and structure of manganese oxide-coated clay

In the environment, heavy metals are important contaminants that sorb to and accumulate in soils and sediments. Dominant minerals in the subsurface are oxides and clay, which occur as discrete particles and heterogeneous systems; these surfaces can significantly impact the mobility and bioavailability of metals through sorption. To better understand heterogeneous systems, amorphous (hydrous manganese oxide (HMO)) and crystalline manganese oxides (birnessite and pyrolusite) were coated on montmorillonite. However, the montmorillonite substrate potentially inhibited crystallization of the pyrolusite coating, and also resulted in a poorly crystalline birnessite. Mineralogy and morphology of the coated systems suggest an amorphous structure for HMO and uniform coverage for HMO and birnessite coatings; the presence of Si and Al indicates uncoated areas along intraplanar surfaces. The coating surface charge behaved similarly to that of discrete oxides and clay where the pH _znpc of HMO- and birnessite-coated clay were 2.8 and 3.1, respectively. Surface area of the coated systems increased while the pore size distribution decreased as compared to the external surface area and pores of montmorillonite. X-ray absorption spectroscopy (XAS) revealed the local structural environment of Mn in the HMO- and birnessite-coated clay was consistent with the pure phase oxides: for HMO-coated clay 3.1 atoms of oxygen at 1.89±0.02 Å in the first shell and 2.7 atoms of manganese at 2.85±0.02 in the second shell; and, for birnessite-coated clay 6 atoms of oxygen at 1.91±0.02 Å in the first shell and 6 atoms of manganese at distance 2.99±0.02 Å in the second shell. Overall, the surface properties suggest that the coating behaves like that of discrete oxides, an important sink for metal contaminants.