ZINC EXTRACTABILITY AS A FUNCTION OF pH IN ORGANIC WASTE-AMENDED SOILS

Abstract

The distribution of various forms of heavy metals after addition to soils needs study to improve predictions of metal solubility, especially when the metal is added to soils with large quantities of organic carbon that oxidize over time. The extractability of added zinc (Zn) was determined in a laboratory study using sequential extractions as a function of pH and organic matter (OM as biosolids) additions to A and B horizon samples of three soils from the Coastal Plain of Maryland. The study used Galestown loamy sand (Psammentic Hapludults), Bertie silt loam (Typic Hapludults), and Othello silt loam (Typic Endoaquults), chosen to represent a typical toposequence in this physiographic province. Watersoluble Zn in the six horizons, with and without biosolids added, decreased with increasing pH from 4.0 to 7.0 and increasing cation exchange capacity (CEC) from 2.2 to 106 mmol c /kg soil. Although this decrease with increasing pH was expected, exchangeable levels of Zn were positively correlated with the CEC only up to pH 5.8 but were negatively correlated at pH > 5.8 in all the soils, suggesting that CEC was not a major contributor to Zn exchangeability at pH > 5.8. The biosolids amendment increased the solubility of Zn in all soils at pH 5.8 in all of the soils. The results indicate that pH was a controlling variable for the redistribution of water-soluble, exchangeable, and nonexchangeable Zn affected by biosolids addition. The higher level of nonexchangeable Zn in the Bertie soil compared with other soils, about 0.80 mmol/kg soil for the biosolids treatments and 0.47 mmol/kg soil for the soils without biosolids at pH 7.0, was attributed to the presence of higher native organic C levels and Fe(III) and Mn(III, IV)(hydr) oxide contents that enhance surface-induced hydrolysis and chemisorption of Zn. The effect of biosolids addition on nonexchangeable Zn retention was more pronounced in B horizons of all the soils, especially at pH > 5.8. The results have implications for predicting the mobility and bioavailability of Zn in soils in which pH and organic C levels are modified purposefully for the disposal of heavy metal and C-rich wastes.