Effects of freezing-thawing and wetting-drying on heavy metal leaching from biosolids.

The goal of this study was to evaluate the effects of freezing-thawing and wetting-drying on heavy metals leaching from biosolids. Biosolid samples were irrigated with water at two flow rates and three flow stop events in 24 hr intervals. During the period of flow stop, biosolids were subjected to different temperatures, water contents, or freezing-thawing. Leachates were analyzed for heavy metals. The concentrations of metals in biosolids ranged from lower than detection limits (for Pb) to 1,039 mg/kg (for Zn). The leaching percentage of metals ranged from 0% (Pb, Ag, Cs) to 25% (Ni). Lower flow rate with longer residence time induced more metal leaching compared with higher flow rate with shorter residence time. At each flow rate, flow stop caused enhanced metal leaching. Higher drying temperature enhanced metal leaching. Water content or freezing-thawing had no significant effects on metal leaching. We expect that intermittent irrigation or rainfall would enhance the risk of metals leaching from biosolids after land application. However, freezing of biosolids during winter will likely not cause an enhanced leaching of metals in spring when biosolids and soils thaw. Application of biosolids in fall should therefore not cause enhanced leaching of metals out of land-applied biosolids. PRACTITIONER POINTS: Lower flow rate with longer residence time induced more metal leaching compared with higher flow rate with shorter residence time. Flow stop or higher drying temperature enhanced metal leaching from biosolids. Water content or freezing-thawing had no significant effects on metal leaching.

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