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Hypervalent iron intermediates have been invoked in the catalytic cycles of many metalloproteins, and thus, it is crucial to understand how the coupling between such species and their environment can impact their chemical and physical properties in such contexts. In this work, we take advantage of the solvent kinetic isotope effect (SKIE) to gain insight into the nonradiative deactivation of electronic excited states of the aqueous ferrate(VI) ion. We observe an exceptionally large SKIE of 9.7 for the nanosecond-scale relaxation of the lowest energy triplet ligand field state to the ground state. Proton inventory studies demonstrate that a single solvent O–H bond is coupled to the ion during deactivation, likely due to the sparse vibrational structure of ferrate(VI). Such a mechanism is consistent with that reported for the deactivation of f–f excited states of aqueous trivalent lanthanides, which exhibit comparably large SKIE values. This phenomenon is ascribed entirely to dissipation of energy into a higher overtone of a solvent acceptor mode, as any impact on the apparent relaxation rate due to a change in solvent viscosity is negligible. 

Ferrate is a promising, emerging water treatment technology. However, there has been limited research on the application of ferrate in a water reuse paradigm. Recent literature has shown that ferrate oxidation of target contaminants could be improved by “activation” with the addition of reductants or acid. This study examined the impact of sulfite-activated ferrate in laboratory water matrix and spiked municipal wastewater effluents with the goal of transforming organic contaminants of concern (e.g., 1,4-dioxane) and inactivating pathogenic organisms. Additionally, the formation of brominated disinfection byproducts by activated ferrate were examined and a proposed reaction pathway for byproduct formation is presented. In particular, the relative importance of reaction intermediates is discussed. This represents the first activated ferrate study to examine 1,4-dioxane transformation, disinfection, and brominated byproduct formation. Results presented show that the sub-stoichiometric ([Sulfite]:[Ferrate] = 0.5) activated ferrate treatment approach can oxidize recalcitrant contaminants by >50%, achieve >4-log inactivation of pathogens, and have relatively limited generation of brominated byproducts. However, stoichiometrically excessive ([Sulfite]:[Ferrate] = 4.0) activation showed decreased performance with decreased disinfection and increased risk of by-product formation. In general, our results indicate that sub-stoichiometric sulfite-activated ferrate seems a viable alternative technology for various modes of water reuse treatment. 

Ferrate(VI) has the potential to play a key role in future water supplies. Its salts have been suggested as “green” alternatives to current advanced oxidation and disinfection methods in water treatment, especially when combined with ultraviolet light to stimulate generation of highly oxidizing Fe(V) and Fe(IV) species. However, the nature of these intermediates, the mechanisms by which they form, and their roles in downstream oxidation reactions remain unclear. Here, we use a combination of optical and X-ray transient absorption spectroscopies to study the formation, interconversion, and relaxation of several excited-state and metastable high-valent iron species following excitation of aqueous potassium ferrate(VI) by ultraviolet and visible light. Branching from the initially populated ligand-to-metal charge transfer state into independent photophysical and photochemical pathways occurs within tens of picoseconds, with the quantum yield for the generation of reactive Fe(V) species determined by relative rates of the competing intersystem crossing and reverse electron transfer processes. Relaxation of the metal-centered states then occurs within 4 ns, while the formation of metastable Fe(V) species occurs in several steps with time constants of 250 ps and 300 ns. Results here improve the mechanistic understanding of the formation and fate of Fe(V) and Fe(IV), which will accelerate the development of novel advanced oxidation processes for water treatment applications. 

It is inherently difficult to plan water systems for a future that is non-predictive. This paper introduces a novel perspective for the design and operation of potable water systems under increasing water quality volatility ( e.g. , a relatively rapid and unpredicted deviation from baseline water quality). Increased water quality volatility and deep uncertainty stress water systems, confound design decisions, and increase the risk of decreased water system performance. Recent emphasis on resilience in drinking water treatment has partly addressed this issue, but still establishes an adversarial relationship with change. An antifragile system benefits from volatile change. By incorporating antifragility, water systems may move beyond resilience and improve performance with extreme events and other changes, rather than survive, or fail and quickly recover. Using examples of algal blooms, wildfires, and the COVID-19 pandemic, this work illustrates fragility, resilience, and antifragility within physicochemical process design including clarification, adsorption and disinfection. Methods for increasing antifragility, both individual process options and new system design tools, are discussed. Novel physicochemical processes with antifragile characteristics include ferrate preoxidation and magnetic iron (nano)particles. New design tools that allow for systematic evaluation of antifragile opportunities include artificial neural networks and virtual jar or pilot “stress testing”. Incorporating antifragile characteristics represents a trade-off with capital and/or operating cost. We present a real options analysis approach to considering costs in the context of antifragile design decisions. Adopting this antifragile perspective will help ensure water system improved performance during extreme events and a general increase in volatility.