| Abstract
| - The formation of transition-metal surface precipitates mayoccur during sorption to clay minerals under ambientsoil conditions. This process may lead to significant long-term stabilization of the metal within the soil profile.However, the rates and mechanisms controlling surfaceprecipitate formation are poorly understood. We monitoredchanges in the reversibility of Ni sorbed to a clay mineral,pyrophyllite, in model batch experiments maintained atpH 7.5 for up to 1 year. The macroscopic sorption anddissolution study was complemented by a time-resolvedcharacterization of the sorbed phase via spectroscopic andthermal methods. We found that nickel became increasinglyresistant, over time, to extraction with EDTA. Initially,the sorbed phase consisted of a Ni−Al layered doublehydroxide (LDH). With time, the anionic species in the interlayerspace of the LDH changed from nitrate to silica polymerstransforming the LDH gradually into a precursor Ni−Alphyllosilicate. We believe that this phase transformationis responsible for a substantial part of the observed increasein dissolution resistance. Thus, clay mineral weatheringand the time-dependent release of Al and Si ions controlledNi precipitate nucleation and transformation. Our resultssuggest a potential pathway for long-term Ni stabilization insoil.
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