The influence of dissolved Si on Ni precipitate formation at the kaolinite water interface: Kinetics, DRS and EXAFS analysis

- The dissolved Si leads to a rapid increase in the Ni sorption.
- The coprecipitation of Si and Ni ions to Ni phyllosilicate occurs.
- The nucleation of a (Ni, Al) phyllosilicate phase was confirmed by DRS and EXAFS analysis.
- Kaolinite act as a nucleating surface for the formation of (Ni, Al) phyllosilicates in the presence of substantial Si.

Unraveling the formation process of Ni precipitates at molecular scale is important for understanding the fate and mobility of Ni species in the real environment. Dissolved Si presents in the natural environment ubiquitously, which can alter Ni sorption as well as incorporation into neoformed precipitates. Batch experiments show that the dissolved Si leads to a rapid increase in the Ni sorption rate and interferes with the formation of Ni precipitates. The results of diffuse reflectance spectroscopy (DRS) and extended X-ray absorption fine structure (EXAFS) spectroscopy analyses suggest that the nucleation of a (Ni,Al) phyllosilicate phase involves a kaolinite-like local structure. Then, the substantial presence of Si affects the initial formation of Ni precipitate nucleation and the resulting crystal growth. Dioctahedral kaolinite may act as a nucleating surface for the heterogeneous formation of trioctahedral (Ni,Al) phyllosilicates under environmentally relevant conditions. This study provides experimental evidence on nucleation and epitaxial growth processes of Ni precipitate on kaolinite and provides insight on the relationship between substrates and precipitation, which is crucial for understanding the physicochemical behavior of Ni on mineral surfaces.