Coprecipitation of trace metal ions during the synthesis of hectorite

Physicochemical sequestration of transition metal ions is increasingly promoted as a cost-effective remediation technique for heavily contaminated soils and sediments. In enhanced stabilization/solidification strategies, this sequestration is hypothesized to result from the neoformation of various oxides, hydroxides and silicate minerals. However, more detailed information on the conditions that facilitate these neoformations or on the resulting metal sequestration is needed. In the present experimental study, the fate of selected transition metal ions (Cd2+, Cu2+, Pb2+, and Zn2+) is monitored during the precipitation of a synthetic smectite (hectorite) in a salt solution at 100 °C. X-ray diffraction (XRD) patterns and Fourier transform infrared (FT-IR) spectra confirm the synthesis of well-crystallized hectorite. Electron paramagnetic resonance (EPR) spectroscopy, coupled to NH3 vapor treatment, is used to determine the chemical form and structural location of Cu2+ in coprecipitates with hectorite. A quick extraction with EDTA probes the structural location of Cd2+, Cu2+, Pb2+, and Zn2+, and provides information on the fraction of the transition metal ions that are loosely associated with the external surfaces of the clay mineral. As indicated by the spectroscopic and chemical data, and as expected from the Pauling radii of the four transition metal ions, Cu2+ and Zn2+ are substituted in the mineral structure more significantly than are Cd2+ and Pb2+. The latter metal ions may be too large to be accommodated within layer silicate structures, and therefore represent a greater challenge for stabilization by coprecipitation in clay minerals. The results obtained in this research provide some of the theoretical foundation needed to improve the design of stabilization/solidification technologies.