RETENTION OF NI, ZN AND CD BY SI-ASSOCIATED GOETHITE

Synthetic goethite used to study the effects of reaction time and temp erature on the pH-dependent sorption of Ni, Zn and Cd was associated w ith amorphous silica. Ni interacted with dissolved Si and formed a Ni/ Si precipitate on the goethite surface. Individual metals added at a c oncentration of 0.5 mumol g-1 and sorbed during a reaction period of 5 04 hours (21 days) at 35-degrees-C were extracted by 0.7 M HNO3 for 14 days. At the end of this period 11, 28 and 40 percent of Ni, Cd and Z n, respectively, were not extracted whereas 20 percent of the total Fe content of the goethite and 39 percent of the associated Si were diss olved. During the sorption process metals became immobilized in the go ethite particles. This effect can be related to a diffusion of metal i ons into micropores. A total mobilization of sorbed metals can only be achieved by a complete dissolution of the goethite. The strong fixati on of Ni, Zn and Cd by goethite suggests that additions of this Fe oxi de could be used to ameliorate highly contaminated sludges or soils.