An investigation into the mechanism by which synthetic zeolites reduce labile metal concentrations in soils

The addition of synthetic zeolites and similar materials to metal contamina ted soils has been shown to reduce soil phytotoxicity and to improve the qu ality of plant growth on such amended soils. To gain an understanding of th e mechanism by which the phytotoxicity of contaminated soils is reduced whe n treated with synthetic zeolites, sequential extraction procedures and soi l solution techniques have been used to identify changes associated with me tal speciation in amended soils. Sequential extraction data and changes in soil solution composition are presented for three different contaminated so ils, amended with three synthetic zeolites (P, 4A and Y) at concentrations of 0.5%, 1% and 5% w/w, or lime at 1%. The soils were collected from the si te of a metal refinery, an old lead zinc mine spoil tip and from a field wh ich had been treated with sewage sludge. After incubation of the zeolite tr eated soils for between one and three months, results showed a reduction in the metal content of the ammonium acetate fraction between 42% and 70%, de pending on soil, zeolite and rate of addition, compared with the unamended soils. In addition, soil solution experiments indicated that synthetic zeol ite amendments were more efficient at reducing metal content than comparabl e lime treatment. The mechanism by which synthetic zeolites reduce metal bi oavailability in contaminated soils is discussed and compared to other amen dments.