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Solid–water interfaces are crucial for clean water, conventional and renewable energy, and effective nuclear waste management. However, reflecting the complexity of reactive interfaces in continuum-scale models is a challenge, leading to oversimplified representations that often fail to predict real-world behavior. This is because these models use fixed parameters derived by averaging across a wide physicochemical range observed at the molecular scale. Recent studies have revealed the stochastic nature of molecular-level surface sites that define a variety of reaction mechanisms, rates, and products even across a single surface. To bridge the molecular knowledge and predictive continuum-scale models, we propose to represent surface properties with probability distributions rather than with discrete constant values derived by averaging across a heterogeneous surface. This conceptual shift in continuum-scale modeling requires exponentially rising computational power. By incorporating our molecular-scale understanding of solid–water interfaces into continuum-scale models we can pave the way for next generation critical technologies and novel environmental solutions.

 

Understanding nanoscale interfacial reactions unlocks the chemistry controls that are critical for clean water generation. This Catalyst discusses three important roles of chemistry in clean water: Understanding and controlling evolving interfaces induced by nucleation, deciphering and utilizing hidden interfaces in nanoconfined spaces, and harnessing interfaces with functionalized surfaces. Chemically guided developments of new materials and technologies for purifying clean water can bring all water resources back to one H2O, which supports life for all people. 

Much of the large quantity of plastics produced annually is discharged into the environment, where it degrades into tiny plastic debris (e.g., macro-, micro-, and nano-plastics). There are increasing concerns about the adverse effects of these plastics. In particular, nanoplastics are more prone to interacting with surrounding substances, because of their substantially larger surface areas and consequent increased exposure of surface functional groups. However, the oxidative roles of nanoplastics in inducing redox reactions with heavy or transition metals remain poorly understood. In this study, we investigated how Mn2+ was oxidized by the photolysis of polystyrene (PS)-based nanoplastics. We found that peroxyl (ROO•) and superoxide radicals (O2•−) were generated during the photolysis of PS-based nanoplastics, and they were primarily responsible for Mn oxidation. In addition, different plastic particle sizes and functional groups influenced the formation of radicals and the growth and mineral phases of Mn oxide solids. This study provides insights into the occurrence and diversity of Mn oxides in nature. These new findings also enhance our understanding of the oxidative roles of nanoplastics in generating reactive oxygen species (ROS) and how this may apply to the oxidation of other redox-active metal ions and essential chemicals, which could disrupt ecosystems and affect elemental cycling. Moreover, the production of ROS from nanoplastics in the presence of light endangers marine life and human health, and also potentially affects the mobility of the nanoplastics in the environment via redox reactions, which in turn might negatively impact their environmental remediation. 

Manganese (Mn) oxides are abundant in aquatic and terrestrial environments, where they play significant roles in redox cycling and biological metabolisms. We recently observed that Mn oxides were homogenously formed during the abiotic oxidation of Mn2+(aq) to Mn(IV) by O2•− via nitrate photolysis, at a rate comparable to that of biotic Mn oxides formation. On the other hand, for the heterogeneous formation of Mn oxides, the presence of a substrate can alter the required thermodynamic driving force, which may affect their crystalline phases and further influence the oxidative capability of redox cycling in environmental systems. However, little is known about the photochemically-induced heterogeneous formation of Mn oxides on substrates. In this study, we investigated the heterogeneous formation of Mn oxides on a quartz substrate in the presence of two environmentally abundant cations, Na+ and Mg2+. In contrast to homogeneously generated Mn oxides, the heterogeneously formed Mn oxides displayed earlier crystalline phase evolutions and morphological changes over time. Additionally, the coexistence of Na+ and Mg2+ ions greatly affected the initial crystalline phase and the phase evolution, as well as the surface morphologies of the Mn oxides. These discoveries contribute to our understanding of how various Mn oxides form in nature and provide insight into the processes involved in manufacturing specific Mn oxide crystalline structures for engineering applications. 

Every year, large quantities of plastics are produced and used for diverse applications, growing concerns about the waste management of plastics and their release into the environment. Plastic debris can break down into millions of pieces that adversely affect natural organisms. In particular, the photolysis of micro/nanoplastics can generate reactive oxygen species (ROS). However, their oxidative roles in initiating redox chemical reactions with heavy and transition metals have received little attention. In this study, we investigated whether the photolysis of polystyrene (PS) nanoplastics can induce the oxidation of Mn2+(aq) to Mn oxide solids. We found that PS nanoplastics not only produced peroxyl radicals (ROO•) and superoxide radicals (O2•−) by photolysis, which both play a role in unexpected Mn oxidation, but also served as a substrate for facilitating the heterogeneous nucleation and growth of Mn oxide solids and controlling the formation rate and crystalline phases of Mn oxide solids. These findings help us to elucidate the oxidative roles of nanoplastics in the oxidation of redox-active metal ions. The production of ROS from nanoplastics in the presence of light can endanger marine life and human health, and affect the mobility of the nanoplastics in the environment via redox reactions, which in turn may negatively impact their environmental remediation. 

Halide ions are naturally abundant in oceans and estuaries. Large amounts of highly saline efflux are also made and discharged to surface water from desalination processes and from unconventional oil and gas recovery. These highly concentrated halides can generate reactive halogen radicals. However, the redox reactions of halogen radicals with heavy metals or transition metals have received little attention. Here, we report undiscovered fast oxidation of manganese ions (Mn2+) by reactive halogen radicals. Hydroxyl radicals (˙OH) are produced by nitrate photolysis. While ˙OH radicals play a limited role in the direct oxidation of Mn2+, ˙OH can react with halide ions to generate reactive halogen radicals to oxidize Mn2+. In addition, more Mn2+ was oxidized by bromide (Br) radicals generated from 1 mM Br− than by chloride (Cl) radicals generated from 500 mM Cl−. In the presence of Br radicals, the abiotic oxidation rate of Mn2+ to Mn(IV)O2 nanosheets is greatly promoted, showing a 62% increase in Mn2+ (aq) oxidation within 6 h of reaction. This study advances our understanding of natural Mn2+ oxidation processes and highlights unexpected impacts of reactive halogen radicals on redox activities with heavy metals and corresponding nanoscale solid mineral formation in brine. This work suggests a new, environmentally-friendly, and facile pathway for synthesizing MnO2 nanosheets. 

Environmentally ubiquitous manganese (Mn) oxides play important roles in geochemical element redox cycling. They can be formed by both biotic and abiotic Mn2+(aq) oxidation processes. We recently observed photochemically-assisted abiotic oxidation of Mn2+(aq) to δ-MnO2 nanosheets during nitrate photolysis. Mn2+ was mainly oxidized by superoxide radicals, while hydroxyl radicals (•OH) contributed little to Mn oxidation. However, unexpected abiotic Mn2+ oxidation was observed in the presence of tert-butyl alcohol (TBA) that was added to scavenge •OH. TBA, one of the most common •OH scavengers, has been thought to be able to completely scavenge •OH, leaving less reactive products that do not participate in further redox reactions in the system. However, we discovered that TBA was not an inert agent in scavenging •OH. Secondary peroxyl radicals (ROO•) were produced from the chain reactions between TBA and •OH, facilitating the oxidation of Mn2+ to MnO2(s). These findings can also be applied to other alcohol scavengers, such as methanol, ethanol, and propanol. In addition, ROO• can be produced by the reaction between •OH and unsaturated organic matter in natural environments. This study helps understand the occurrences of Mn oxides in the environment, and it provides new insights into the oxidation pathways of other heavy metals ions (Fe2+, As3+, and Cr3+) by ROO•. 

In meeting rapidly growing demands for energy and clean water, engineered systems such as unconventional oil and gas recovery and desalination processes produce large amounts of briny water. In the environment, these highly concentrated halides can be oxidized and transformed to reactive halogen radicals, whose roles in the degradation and transformation of organic pollutants have been studied. However, redox reactions between halogen radicals and heavy metal ions are still poorly understood. In this work, we found that aqueous manganese ions (Mn2+) could be oxidized to Mn oxide solids by reactive halogen radicals generated from reactions between halide ions and hydroxyl radicals or between halide ions and triplet state dissolved organic matter. In particular, more Mn2+ was oxidized by Br radicals generated from bromide ion (Br−) than by Cl radicals generated from chloride ion (Cl−), even though the concentrations of Br− in surface waters are much lower than Cl− concentrations. In addition, the highly concentrated halides greatly increased the ionic strength of the solution, affecting Mn2+ oxidation kinetics and the crystallinity and oxidation state of the newly formed Mn oxides. These newly discovered pathways involving Mn2+(aq) and reactive halogen radicals aid in understanding the generation of abiotic Mn oxide solids and forecasting their redox activities. Moreover, this work emphasizes the critical need for a better knowledge of the roles of reactive halogen radicals in inorganic redox reactions. 

Engineered cyanobacteriumSynechococcus elongatuscan use light and CO₂to produce sucrose, making it a promising candidate for use in co-cultures with heterotrophic workhorses. However, this process is challenged by the mutual stresses generated from the multispecies microbial culture. Here we demonstrate an ecosystem whereS. elongatusis freely grown in a photo-bioreactor (PBR) containing an engineered heterotrophic workhorse (either β-carotene-producingYarrowia lipolytica or indigoidine-producingPseudomonas putida) encapsulated in calcium-alginate hydrogel beads. The encapsulation prevents growth interference, allowing the cyanobacterial culture to produce high sucrose concentrations enabling the production of indigoidine and β-carotene in the heterotroph. Our experimental PBRs yielded an indigoidine titer of 7.5 g/L hydrogel and a β-carotene titer of 1.3 g/L hydrogel, amounts 15–22-fold higher than in a comparable co-culture without encapsulation. Moreover,¹³C-metabolite analysis and protein overexpression tests indicated that the hydrogel beads provided a favorable microenvironment where the cell metabolism inside the hydrogel was comparable to that in a free culture. Finally, the heterotroph-containing hydrogels were easily harvested and dissolved by EDTA for product recovery, while the cyanobacterial culture itself could be reused for the next batch of immobilized heterotrophs. This co-cultivation and hydrogel encapsulation system is a successful demonstration of bioprocess optimization under photobioreactor conditions.