Selective extraction of trace metals associated with hydrous aluminum oxides

Abstract

In soil/sediment analysis, subdivision of trace metal content into different categories is usually based on selective extraction schemes. To assess the disposition of metal ions bound to aluminum hydrous oxides in such schemes, suspensions of Al(OH)3gel, gibbsite, or alumina were loaded with up to 5 μ mol of Cu, Pb, Cd, or Zn ions prior to being extracted for 24 h with one of fifteen different chemical solutions. The percentage of sorbed ion retrieved varied along the reagent sequence: NaCl, CaCl2 < MgCl2, NH4NO3 < CH3COONH4, Na citrate, Na4P2O7 < EDTA, DTPA < CH3COOH, H2C2O4, HCl, HNO3. In each system, the recovery value varied with the initial surface loading (a function of sorption pH) and reflected changes in metal species form, e.g., bonded M2+, (MOH+), M(OH)2. With low loading levels up to 40% was displaced by salt solutions; with 1 to 2 μ mol sorbed, as little as 10% was displaced by acids or complex formers but this increased to ~90% with higher loadings. The relationship between sorption pH, amount sorbed, and extraction value was complex, and since in selective extraction schemes classification is based on recovery values, changes in initial retention parameters (e.g. system pH) lead to varying fractions of the different metal ions being classified as "ion exchangeable", "chemisorbed", and "incorporated in the lattice".