Heavy Metal Extractability in Long-Term Sewage Sludge and Metal Salt-Amended Soils
- Paul F. Bell,
- Bruce R. James * and
- Rufus L. Chaney
Heavy metals applied to agricultural soils as soluble salts are usually considered more plant available in the short-term than equivalent amounts of metals applied to soil in sewage sludge. Slow soil chemical processes over several years, however, may change the forms, extractability, and bioavailability of metals. To determine the long-term changes in forms of Cd added to a soil, Cd and other heavy metals were extracted with several solutions from a Christiana fine sandy loam (Typic Paleudult; clayey, kaolinitic, mesic) Ap horizon 8 or 9 yr after application of sewage sludge or CdCl2. Cadmium was applied at the rate of 21 kg ha−1 either as a component of sewage sludge or as CdCl2 in 1978 and 1979, respectively. In 1987, the distribution of Cd and other metals among soil fractions was operationally defined using the following selective dissolution and total metal analyses techniques: 10 mM Ca(NO3)2 (soluble); 5 mM DTPA (diethylenetriaminepentaacetate)-10 mM CaCl2-100 mM triethanolamine (TEA), pH 7.5 (exchangeable/organically complexed); 100 mM NH2OH·HCl-10 mM HNO3 (pH 2) (Mn oxide bound); 100 mM NH2OH·HCl-1M HNO3 (paracrystalline Fe oxide bound); and aqua regia dissolution (total soil metal). The percentages of total soil Zn, Mn, and Fe extracted by DTPA and Ca(NO3)2 from the sludge or salt-amended soils were much less (<4%) than the percentages for Cd (47–60%), indicating greater potential mobility for Cd than for the other metals, regardless of the form of Cd added originally. A larger percentage of total soil Cd was extracted with the paracrystalline iron oxide fraction of the sludge-amended soil (35%) than in the salt-amended soil (23%). This greater apparent Cd association with Fe oxides in the sludge-amended soil may have been due to the greater quantity of paracrystalline Fe oxides in that soil (843 vs. 616 mg Fe kg−1 for the sludge and salt-amended soils, respectively). Compared to the sludge-amended soil, the salt-amended soil had a higher percentage of Cd in the soluble and exchangeable fractions (60 vs. 47%) and nearly equal percentages in the residual fraction (15%), suggesting, at the same total soil Cd concentration (5 mg kg−1 in 1987), plant-available Cd would be greater from the salt-amended soil. The sludge addition apparently enhanced retention of Cd in an iron oxide pool, relative to the soluble and exchangeable forms.
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