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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. 

Dissolved natural organic matter (DOM) is a complex matrix of organic matter that is ubiquitous in natural aquatic environments. So far, substantial research has been conducted on the DOM adsorption on Mn oxides as well as the reduction processes of Mn oxides by DOM. However, little is known about the oxidative roles of DOM in oxidizing Mn2+(aq) to Mn(III/IV) oxide solids. Sunlight-driven processes can initiate the degradation of DOM accompanied by the formation of photochemically produced reactive intermediates, including excited triplet state DOM (3DOM*), hydroxyl radical (•OH), superoxide radical (O2•−), hydrogen peroxide (H2O2), and singlet oxygen (1O2). Further, in the presence of halide ions, reactive halogen species can be generated by reactions between 3DOM* and halide ions, and by reactions between •OH and halide ions. In this study, we found that the solution pH controlled the oxidation of Mn2+(aq) to Mn oxide solids during photolysis of DOM. Among the reactive oxygen species, Mn2+(aq) was found to be oxidized to Mn oxide solids mainly by O2•−. The DOM with different quantities of aromatic functional groups affected its oxidative capability. With the addition of bromide ions (Br−), Mn2+(aq) oxidation was promoted further by formation Br radicals, which can also oxidize Mn2+(aq) to Mn oxide solids. These findings can help us better understand the oxidative role of DOM in the formation of Mn oxide solids in organic-rich surface water. In addition, this study assists in comprehending the impacts of the photolytic reactions between DOM and halide ions and their resulting reactive oxygen and halogen species on the oxidation and reduction processes of other transition metal oxides in the environment. 

Manganese (Mn) oxide solids widely exist in nature, serving as both electron donors and acceptors for a variety of redox reactions. Previous studies have highlighted the adsorption of dissolved organic matter (DOM) on Mn oxides, as well as the reduction of Mn oxides by DOM. Here, we show the underappreciated roles of photolytic reactions of DOM in Mn2+(aq) oxidation and its consequential formation of Mn oxide solids. During the photolysis of DOM, reactive intermediates including excited triplet state DOM (3DOM*), hydroxyl radical (•OH), superoxide radical (O2•−), hydrogen peroxide (H2O2), and singlet oxygen (1O2) can be generated. Among them, we found that O2•− was responsible for Mn oxidation. In addition, in the presence of bromide ions (Br−), the photolytic reactions between DOM and Br− formed reactive bromide radicals and facilitated the oxidation of Mn2+(aq) to Mn oxide solids. Moreover, the composition of DOM affected its oxidative capability. When DOM contained more aromatic functional groups, we observed more oxidation of Mn2+ to Mn oxides. These new findings advance our knowledge of natural Mn2+ oxidation and Mn(III/IV) oxide formation, as well as the hitherto overlooked oxidative role of DOM in the oxidation of metal ions in surface water under sunlight illumination. 

All solid materials are created via nucleation. In this evolutionary process, nuclei form in solution or at interfaces, expand by monomeric growth and oriented attachment, and undergo phase transformation. Nucleation determines the location and size of nuclei, whereas growth controls the size, shape, and aggregation of newly formed nanoparticles. These physical properties of nanoparticles can affect their functionalities, reactivities, and porosities, as well as their fate and transport. Recent advances in nanoscale analytical technologies allow in situ real-time observations, enabling us to uncover the molecular nature of nuclei and the critical controlling factors for nucleation and growth. Although a single theory cannot yet fully explain such evolving processes, we have started to better understand how both classical andnonclassical theories can work together, and we have begun to recognize the importance of connecting these theories. This review discusses the recent convergence of knowledge about the nucleation and growth of nanoparticles.