FROM ATOM TO CRYSTAL: A MULTIDISCIPLINARY EFFORT TO UNDERSTAND THE PRECIPITATION OF SOIL-FORMING MINERALS IN AQUEOUS SYSTEMS

The Center for Environmental Kinetics Analysis (CEKA) at Penn State University is a member of the Environmental Molecular Science Institute sponsored by NSF and DOE. CEKA supports research thrusts targeted at those chemical reactions that produce the thin envelope at the Earth's surface known as the “critical zone.” The authors work jointly within CEKA's Precipitation Interest Group, which is focusing on the mechanisms and rates at which dissolved ions in groundwaters evolve into crystalline entities during the process of mineralization. Because these processes occur at low temperatures and pressures, they are characterized by well-known departures from ideality. The kinetic parameters of the reactions often trump thermodynamic factors in the determination of reaction behaviors, and transient intermediate phases typically appear in the sequence between dissolved ion and final precipitate. In addition, polymeric species commonly precede the appearance of distinctly crystalline particles.

Understanding soil-forming reactions requires analytical methods capable of high spatial and temporal resolution, and the diversity of expertise among the members of the Precipitation Interest Group has fostered novel ways of exploring well-traveled systems. Projects that will be described in this poster include: 1) Time-resolved synchrotron X-ray diffraction experiments that have allowed us to monitor the nucleation and growth of Fe- and Ti-oxides in solution, phase transformations in Cu- and Fe-sulfides, and the enzymatically catalyzed bioreduction of Mn oxides; 2) Computational simulation of nanocolloidal surface behaviors using density functional theory; 3) EXAFS analyses of the formation of Al-rich Fe-oxide nanoparticles with and without the presence of quartz substrates; and 4) Chromatographic studies of the evolution of silica particles from monomers to nanocolloids to amorphous precipitates over a range of pH and ionic strengths.