Divalent Heavy Metal Removal Using Manganese Oxide Coated Polymeric Media for Storm Water Treatment

Urban storm water runoff mobilizes and transports significant loads of heavy metals. Promulgation of NPDES Phase II regulations has spurred development of Best Management Practices (BMPs) for treatment of urban storm water. Recent BMP designs provide both adsorption and filtration using engineered media such as manganese oxide coated polymeric media (MOPM). In this study, a method was developed to coat spherical polymeric media with an iron oxide scratch coating and a manganese oxide outer coating creating a buoyant polymeric media with a specific gravity less than 1.0. SEM images and BSE thin section images depict a porous MOPM surface with a dense coverage of rough spherically-shaped manganese oxides. The oxide coating increased media specific surface area from less than 0.1-m2/g to 27-m2/g, provided a negatively charged surface (PZC = 4.0), and significantly greater adsorption capacity for the divalent heavy metals studied. Freundlich adsorption isotherms were fit to batch equilibrium data. Results indicate the adsorption onto the MOPM is very sensitive to the pH. The order of adsorption affinity on this manganese oxide coated media for the four divalent heavy metals studied is Pb(II) > Cu(II) > Cd(II) > Zn(II). Triple layer surface complexation model were used to model the adsorption equilibrium of divalent heavy metal for MOPM. The intrinsic surface acidity constants for MOPM were determined using FITEQL-TLM and are log Ka1int = 3.196 and log Ka2int = −5.802. The intrinsic surface reaction constants for divalent heavy metals studied are log KPbint = −1.91, log KCuint = −2.53 and log KZnint = −4.45. Adsorption rates for MOPM in the flowthrough batch system were rapid with over 50% removal in the first 30 minutes and over 90% removal within 5 hours. A chemisorption kinetic model, the Elovich model, was applied successfully to fit batch kinetic data. Results of batch equilibrium and column breakthrough indicated that MOPM had a comparable removal capability to other commercial and research sorptive media. The MOPM can be a viable alternative adsorption medium for heavy metal removal while taking advantage of buoyant media characteristics for designing upflow Best Management Practices (BMPs), such as sorptive buoyant media clarifiers (SBMC).