THE RISE OF SYNCHROTRONS AND MOLECULAR ENVIRONMENTAL SCIENCE

Molecular Environmental Science (MES) has emerged as a distinct research field in geosciences over the last ~20 years, driven to a large extent by the establishment of synchrotron X-ray radiation user facilities. In the mid-1980's, the molecular nature of species adsorbed at surfaces, their modes of bonding, and the ability of surfaces to react and reform in the presence of an aqueous solution containing trace solutes was mostly unknown. Several studies at the time pointed out the inability of macroscopic experiments to unambiguously constrain molecular-scale processes that control element partitioning between aqueous and solid phases. Gordon Brown and colleagues sought to apply synchrotron X-ray spectroscopic methods, then emerging from chemistry and physics research, to determine directly the ligand structure, oxidation state, and mode of attachment of surface-adsorbed species. This pioneering step, which in the last ~20 years has included novel methods of interrogating both sorbed species and the fundamental structure of the mineral-water interface, has revealed the rich and complex chemistry that occurs when natural aqueous solutions react with mineral surfaces. Synchrotron X-ray measurements, including X-ray absorption, diffraction, and scattering methods have become essential tools for both the exploration of fundamental mineral surface processes and for the characterization of element speciation in natural sediments, soils, water, and biota. In numerous studies, molecular-scale data have provided definitive information about speciation, structure, and bonding for environmentally important trace elements as both contaminants and nutrients. Still, MES is a young field with enormous potential for growth and application. Areas of current and future research include probing the dynamics of interfacial processes at the molecular scale, understanding mechanisms of exchange and reaction across the biotic-abiotic interface, and integrating molecular constraints with thermodynamic, kinetic, and transport descriptions of field-scale processes to improve predictive capability and advance remediation and mitigation technologies.