Mn-Oxides and Sequestration of Heavy Metals In a Suburban Catchment Basin of the Chesapeake Bay Watershed

Mn-oxides found in soils sequester heavy metals and likely play an important role in regulating pollutants and water chemistry of the Chesapeake Bay watershed. Clasts from a stormwater catchment basin draining a subdivision near George Mason University, Fairfax VA (38º 50.090N 78º19.204W) were investigated for the presence of Mn-oxides and bound heavy metals.

Mn-oxide coatings were identified in the field visually and analyzed by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The coatings were dominantly found on quartzite and shale clasts. XRD results indicate the sample material is dominantly poorly crystalline layered Mn-oxides. SEM imaging showed a variety of textures including anhedral particles, well formed spheres (25-50 µm) and stacked aggregates of hexagonal plates (30 to 100 µm). Energy dispersive spectroscopy (EDS) showed that a number of elements are present. Average major element concentrations found were: Mn (33.38 wt%), Fe (11.88 wt%), Si (7.33 wt%), Al (5.03 wt%), and Ba (0.90 wt%). Average heavy metal element concentrations found were: Zn (0.66 wt%), Pb (0.03 wt%), Co (0.02 wt%) and Ni (0.02 wt%). Other significant elements include Ce (0.08 wt%) and Ti (0.41 wt%). Zn was the most common species of heavy metal found being present in 23 of 40 analyses. Potential anthropogenic sources for heavy metals include batteries, pigments, automotive parts and debris, electrical components, steels, and pesticides.

Field and laboratory observations indicate Mn-oxides occurring on the surface of the clasts can be mechanically mobilized. This is a mechanism for transporting heavy metals into the Chesapeake Bay watershed. However, Mn-oxides can retain heavy metals at typical concentrations of 1 to 2 wt% and serve as a buffer for pollution. Mn-oxide coating are common in the region and act as a control on heavy metal distribution and should be investigated as an environmental technology.