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, 1, 2, 5, 10, and 15 tons per acre) to a semi-arid grassland in 1991 (single) or again in 2002 (repeated). In July 2003, soils were obtained from the 0-3, 3-6, and 6-12-inch depths in all plots. Using soil pH, soluble anion and cation concentrations from 0.01 moles per liter calcium chloride extractions, and dissolved organic carbon content, copper and zinc associated with minerals, hydrous ferric oxide, or dissolved organic phases was modeled using Visual Minteq. Scanning electron microscopy and energy dispersive x-ray analysis was also utilized to identify solid phase metal associations present in single and repeated biosolids-amended soils. Based on soil solution chemistry and verified using Visual Minteq, greater than 89 and 96 percent of copper and zinc, and greater than 99 percent of zinc were adsorbed to hydrous ferric oxides in all single or repeated biosolids-applied soils, respectively. However, when detected in the repeated biosolids treatments, only 59-79 percent of copper was adsorbed to hydrous ferric oxides while 21-41 percent was associated with dissolved organic carbon; downward copper movement was associated with dissolved organic carbon. The scanning electron microscopy and energy dispersive x-ray analysis of clay-sized separates from all soil depths led to direct observation of iron-zinc, aluminum-zinc, and aluminum-copper associations. Results implied that even after surface-applying biosolids either once or twice of up to 15 tons per acre, soil solution concentrations, Visual Minteq predictions, and scanning electron microscopy and energy dispersive x-ray analysis suggested minimal shifts occur in phases controlling long-term copper and zinc solubility.