PREFACE: American Journal of Science, Vol. 318, November, 2018

Research in fluid-solid interaction processes has expanded tremendously over the past few decades, with key fronts ranging from fundamental understanding of reaction kinetics to detailed predictions involving the release, migration, and retention of environmentally important components. This special issue of AJS showcases the diversity and progress of this research in a series of invited papers that also illustrate core problems and solutions.
A central theme is the challenge involved in the integration of reaction processes over length and time scales that span many orders of magnitude. At one end of this spectrum, both theoretical and modeling approaches have evolved to describe the very brief interactions at the molecular scale, allowing key insights into details of reaction mechanism. At the other, modeling approaches have focused on the longer reaction times and lengths that characterize macroscopic systems. Experimental and analytical observations are also now able to map the dynamics and reactivity of reacting surfaces at the pore scale and above, providing these modeling approaches with essential feedbacks towards validation of predictions and identification of aspects where improvement is needed.
The strong coupling between the two “worlds” of experimental and simulation approaches is perhaps the most important result of the last years of research in our field. A primary motivation for this special issue was to highlight this productive interaction. A second motivation was to mark the 60th birthday of Professor Andreas Lüttge. Starting his scientific work at Tübingen University, followed by appointments at Yale University, Rice University, and now at the University of Bremen, he has continued to pursue the productive synergy of these two activities via pioneering work combining kinetic Monte Carlo simulations with complementary observations of reacting mineral surfaces.

This special issue is published in two parts. This first part begins with theoretical work by Bender and Becker, involving the kinetics of interactions between redox-sensitive plutonyl species, iron, and hydroxyl radical. This work nicely illustrates a divide-and-conquer approach, elucidating the stepwise reaction sequence involved in the formation and configuration of various complexes. In so doing, it also provides a potential framework for approaching related problems in the context of interactions at mineral surfaces. The second contribution, by Churakov and Prasianakis, combines thermodynamic calculations and kinetic simulations. Here the authors use the scale of the pore itself as a central connector to elegantly link the atomistic description of mineral surface reactivity with structural and compositional heterogeneities of real materials. The third contribution, by Kim, Marcano, Ellis, and Becker, presents experimental data on the photocatalytic role of TiO2 nanoparticles in uranyl reduction, using a diverse array of organic ligands as electron donors. This study demonstrates the importance of understanding the environmental specificity of reactions at surfaces, documenting the sensitivity of reduction efficiency to both ligand and UV wavelength. The last paper in this special issue’s first part, by Gebauer, Raiteri, Gale, and Cölfen, provides insight into current discussions concerning the birth of crystal nuclei during homogeneous precipitation in solution. They provide a nice summary of the ongoing debate, and stimulate further examination of true nature of these processes, arguing that the “critical” aspect of these clusters lies not in their size, but in their dynamics.
This last point also bears on a larger fundamental problem: how to resolve our new and increasing knowledge of the kinetics of these microscopic interactions, with the conventional thermodynamic framework that has long guided our interpretations of interactions at the mineral-fluid interface, but which is also largely macroscopic in origin. In the near future, we will announce the second part of this special issue, with a specific focus on new experimental results that challenge conventional model predictions.