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.