For nearly 20 years, John Zachara led insightful and impactful research into the geochemistry, biochemistry, and environmental fate of technetium-99 (⁹⁹Tc), a long-lived (half-life: 211,100 years), environmentally mobile contaminant in the subsurface at the U.S. Department of Energy’s (DOE’s) Hanford Site and other sites world-wide. John authored ~20 papers on technetium biogeochemistry, resulting in over 1,000 citations to date. John’s research ranged from ⁹⁹Tc colloid genesis (and biogenesis) to stabilization of ⁹⁹Tc in subsurface minerals. His work on ⁹⁹Tc biogeochemistry not only informed the understanding of ⁹⁹Tc redox chemistry in the environment, but it also provided mechanistic insights into biogeochemical redox reactions with minerals. John also advanced the use of X-ray absorption spectroscopy to probe ⁹⁹Tc redox chemistry and chemical speciation to inform mechanistic reactive transport models. John’s insights into the mechanisms and kinetics of the reduction of pertechnetate, ⁹⁹Tc(VII)O₄^-, by ferrous iron and metal-reducing bacteria (see figure below), particularly the role of unstable intermediates and the formation of nanoparticles, has informed similar investigations into the reduction and bioremediation of other metals and radionuclides, such as uranium.

Electron micrographs of thin sections from <sup>99</sup>Tc(VII)O<sub>4</sub><sup>-</sup> and ferrihydrite (Fh) suspensions incubated with hydrogen (H<sub>2</sub>) and <i>Shewanella oneidensis</i> (A, B) and <i>Anaeromyxobacter dehalogenans</i> (C, D), uninoculated Fh incubated under identical conditions (E), and <i>Geobacter sulfurreducens</i> incubated with <sup>99</sup>Tc(VII)O<sub>4</sub><sup>−</sup> and H<sub>2</sub> in the absence of Fh (F). From Plymale et al. 2011, <i>Environ. Sci. Technol</i>. 245(3):951-957.

Electron micrographs of thin sections from ⁹⁹Tc(VII)O₄^- and ferrihydrite (Fh) suspensions incubated with hydrogen (H₂) and Shewanella oneidensis (A, B) and Anaeromyxobacter dehalogenans (C, D), uninoculated Fh incubated under identical conditions (E), and Geobacter sulfurreducens incubated with ⁹⁹Tc(VII)O₄⁻ and H₂ in the absence of Fh (F). From Plymale et al. 2011, Environ. Sci. Technol. 245(3):951-957.

Andrew Plymale

Appearances