Copper and zinc speciation in a biosolids-amended, semiarid grassland soil.

Predicting trace-metal solid-phase speciation changes associated with long-term biosolids land application is important for understanding and improving environmental quality. Biosolids were surface-applied (no incorporation; 0, 2.5, 5, 10, 21, and 30 Mg ha) to a semiarid grassland in 1991 (single application) and 2002 (repeated application). In July 2003, soils were obtained from the 0- to 8-, 8- to15-, and 15- to 30-cm depths in all plots. Using soil pH, soluble anion and cation concentrations from 0.01 mol L CaCl extractions, dissolved organic C (DOC) content, and an estimate of solid phase humic and fulvic acids present, Cu and Zn associated with minerals, hydrous ferric oxides (HFO), organically complexed, electrostatically bound to organic matter (OM), or DOC phases was modeled using Visual Minteq. Scanning electron microscopy and energy-dispersive X-ray analysis (SEM-EDXRA) was also used to identify solid-phase metal associations present in single and repeated biosolids-amended soils. Based on soil solution chemistry in all depths, as modeled using Visual Minteq, >90% of the Cu and >95% of the Zn from the single or repeated biosolids-applied soils were sorbed electrostatically or as mono- or bidentate solid-phase OM complexes. Up to 10 and 5% of the Cu and Zn, respectively, was associated with HFO, with negligible amounts associated with DOC. The SEM-EDXRA of clay-sized separates from all soil depths led to direct observation of Fe-Cu and Fe-Zn associations. Results implied that after surface-applying biosolids either once or twice with up to 30 Mg ha, some shifts occurred in phases controlling Cu and Zn solubility, but solution concentrations remained below drinking water standards.