Wednesday, November 15, 2006 - 10:15 AM
279-4

Inorganic Iron and Organic Carbon Links to Cadmium Stability in Biosolid-Soil.

Mark Chappell, Kirk Scheckel, Aaron Williams, and James Ryan. U.S. Environmental Protection Agenc, 809 Heavenly Lane, Cincinnati, OH 45238, United States of America

EPA's 503 rule governing the land application of treated municipal wastes (biosolids) has long been debated as a means for mobilizing toxic heavy metals in natural systems.  Critics state that the high carbon contents of biosolids promote the formation of soluble, metal-organic complexes in soil.  However, research has consistently shown heavy metals in long-term, biosolid-amended soils are far less bioavailable to plants and less mobile in soil than the same metals added as salts.  Recent micro-XRF mapping showed Cd was spatially associated with Fe domains in a biosolid material.  Yet, it is unclear how the presence of soil carbon (the phase viewed as contributing to metal mobility) affects this Cd-Fe association once the biosolid is applied to soil. 

 

In this work, we present investigations into the role of iron and organic phases in biosolid-amended soils on Cd sorption/stability.  We constructed Cd sorption isotherms using field-representative levels of biosolid-soil Cd (20 – 120 mg Cd) and two soils with histories of biosolid amendments. As expected, Cd sorption was greater on both biosolid-soils over the control soils. Cd-loaded biosolid-soils underwent density-separations using a 1.5 g cm-3 sodium polytungstate (SPT) solution.  The lower-density fraction, comprising 30% of the total soil material, was enriched in organic C and Fe, and contained ~80% of adsorbed Cd.  SEM/EDX mapping showed C and Fe phases in the lower-density fraction soil particles as spatially separate and distinct – an observation markedly different from associations observed in the initial biosolid materials, where C and Fe phases were homogenously “mixed”.  Ongoing SEM/EDX analyses will determine Cd distribution in biosolid-soils relative to C and Fe, and thus help illuminate the mechanism responsible for the long-term stability of Cd in biosolid-soils.