The Link between Clay Mineral Weathering and the Stabilization of Ni Surface Precipitates

The formation of transition-metal surface precipitates may occur during sorption to clay minerals under ambient soil conditions. This process may lead to significant longterm stabilization of the metal within the soil profile. However, the rates and mechanisms controlling surface precipitate formation are poorly understood. We monitored changes in the reversibility of Ni sorbed to a clay mineral, pyrophyllite, in model batch experiments maintained at pH 7.5 for up to 1 year. The macroscopic sorption and dissolution study was complemented by a time-resolved characterization of the sorbed phase via spectroscopic and thermal methods. We found that nickel became increasingly resistant, over time, to extraction with EDTA. Initially, the sorbed phase consisted of a Ni-Al layered double hydroxide (LDH). With time, the anionic species in the interlayer space of the LDH changed from nitrate to silica polymers transforming the LDH gradually into a precursor Ni-Al phyllosilicate. We believe that this phase transformation is responsible for a substantial part of the observed increase in dissolution resistance. Thus, clay mineral weathering and the time-dependent release of Al and Si ions controlled Ni precipitate nucleation and transformation. Our results suggest a potential pathway for long-term Ni stabilization in soil.