Harnessing air-water interface to generate interfacial ROS for ultrafast environmental remediation

The air-water interface is a unique microenvironment that can dramatically accelerate chemical reaction rates, particularly those involving reactive oxidative species (ROS). This phenomenon has been observed in atmospheric chemistry, where the oxidation of trace gases is significantly enhanced at the surface of cloud water droplets. Harnessing this effect for practical applications, such as environmental remediation, holds significant potential. ROS, such as hydroxyl (HO•) and sulfate (SO₄⁻) radicals, are highly reactive and capable of completely oxidizing various organic pollutants. Fenton and Fenton-like reactions are commonly used to generate ROS, but their efficiency is limited by kinetic and thermodynamic constraints in the bulk phase. The rapid consumption of ROS by side reactions further limits their effectiveness. Microbubbles offer a promising approach to overcome these limitations by providing a large air-water interface area. However, directly probing ROS behavior at the dynamic microbubble interface and efficiently transporting oxidants to this interface have posed significant challenges. This study addresses these challenges by developing a novel approach using an amphiphilic single-atom catalyst to generate and investigate interfacial ROS, exploring its application in ultrafast environmental remediation.

Existing literature highlights the accelerated reaction rates at air-water interfaces, particularly for ROS-mediated reactions. Studies on atmospheric chemistry demonstrate the significant enhancement of oxidation kinetics of trace gases like NOx and VOCs at the surface of cloud water droplets. Research into Fenton and Fenton-like reactions, while demonstrating their effectiveness in generating ROS for pollutant degradation, also points to the limitations of bulk-phase reactions in achieving high ROS concentrations due to kinetic and thermodynamic constraints and scavenging effects. Previous investigations have suggested the potential of ROS generated at the air-water interface of microbubbles for environmental remediation, but a comprehensive understanding of their generation, behavior, and application remained elusive due to the difficulty of directly probing the dynamic interface and the limited transport of oxidants to the microbubble surface.

This study employed an amphiphilic single-Co-atom catalyst (Co@SCN) synthesized through a multi-step process involving SiO₂ nanospheres, dopamine hydrochloride (PDA), and cyanamide. The detailed synthesis is described in the supplementary methods. The morphology and structure of the Co@SCN were characterized using SEM, HRTEM, XRD, SAED, HAADF-STEM, and XAS. The amphiphilic nature of Co@SCN, crucial for its attachment to the microbubble interface, was confirmed by its dispersion in various solvents and its ability to stabilize oil-in-water Pickering emulsions. The interaction forces between the Co@SCN and microbubbles were analyzed, highlighting the robust trapping capability. The catalytic generation of ROS at the air-water interface was investigated using EPR spectroscopy, fluorescence measurements with coumarin as a probe molecule, and scavenging experiments to identify the dominant ROS species. In-situ epifluorescence microscopy directly visualized SO₄⁻ generation at the microbubble surface. XPS was used to assess the electron-transfer capacity of the Co atom before and after the reactions. Ab initio molecular dynamics (AIMD) simulations were conducted to investigate the enrichment mechanism of interfacial SO₄⁻, focusing on free energy variations and hydrogen bonding interactions. The oxidation performance of interfacial SO₄⁻ was evaluated using a custom-designed system for VOC purification. The removal efficiency of toluene was assessed, and the reaction kinetics and rate constants were calculated. DFT calculations were performed to investigate the activation barriers for toluene oxidation by SO₄⁻ in different phases.

The synthesized Co@SCN catalyst showed a uniform spherical morphology with a core-shell structure, confirmed by various microscopic and spectroscopic techniques. The single-atom dispersion of Co on the N-doped graphitic carbon shell was verified by HAADF-STEM. The amphiphilic nature of Co@SCN, with both hydrophilic SiO₂ core and hydrophobic carbon shell, ensured its stable attachment to the microbubble interface. In the presence of microbubbles, the Co@SCN-PMS system showed a significantly enhanced generation of ROS, particularly SO₄⁻, with a concentration 20 times higher at the interface (4.48 × 10⁻¹¹ M) compared to the bulk (2.33 × 10⁻¹² M). In-situ epifluorescence microscopy and AIMD simulations provided direct evidence that SO₄⁻ preferentially accumulates at the air-water interface due to its lowest free energy in this location and strong hydrogen bonding interactions with H₃O⁺. Interfacial SO₄⁻ exhibited remarkably high oxidation reactivity toward toluene, with a rate constant of 10¹⁰ M⁻¹s⁻¹, exceeding that of bulk reactions by over two orders of magnitude. This high reactivity resulted in >99% toluene removal within a short retention time (approximately 7.0 s) in the Co@SCN-PMS-microbubble system. The system showed long-term stability, maintaining high removal efficiency over 600 min with multiple PMS additions. DFT calculations confirmed the lower activation barrier for toluene oxidation by interfacial SO₄⁻ compared to the bulk phase and gas phase. The reaction rate constant for interfacial SO₄⁻ was calculated to be 8.59 × 10¹⁰ M⁻¹s⁻¹, significantly higher than that in the bulk phase (7.78 × 10⁸ M⁻¹s⁻¹).

The findings demonstrate a novel strategy for harnessing the air-water interface to significantly enhance the efficiency of ROS-mediated reactions for environmental remediation. The amphiphilic Co@SCN catalyst effectively transports the oxidant PMS to the microbubble interface, where the strong intrinsic electric field accelerates the Fenton-like reaction and increases the concentration of SO₄⁻. The preferential localization of SO₄⁻ at the interface, driven by thermodynamic stability and strong hydrogen bonding, and its significantly higher reactivity contribute to the ultrafast oxidation of pollutants. The observed long-term stability and high efficiency across multiple VOCs highlight the broad applicability and practical potential of this approach. The results provide new insights into the fundamental mechanisms governing ROS chemistry at the air-water interface and demonstrate its potential for developing advanced oxidation technologies for environmental applications.

This study successfully demonstrated a promising strategy for generating high concentrations of highly reactive interfacial sulfate radicals using an amphiphilic single-atom catalyst and microbubbles. The resulting ultrafast oxidation capability offers significant potential for environmental remediation, particularly for the treatment of gaseous pollutants. Future research could explore other amphiphilic catalysts, investigate different types of pollutants, and optimize the system for scalability and practical applications. Further investigation into the influence of microbubble size and solution pH on reaction efficiency would also be valuable.

The study primarily focused on toluene as a model VOC pollutant. While other VOCs were tested, a more comprehensive study involving a wider range of pollutants would strengthen the generalizability of the findings. The long-term stability testing was performed under specific controlled conditions; further investigation is needed to assess the stability under more diverse and realistic environmental conditions. The cost-effectiveness and scalability of this approach for industrial applications requires further investigation.