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

Air–water interfaces, ubiquitous in atmospheric, biological, and synthetic systems, can accelerate chemical reactions by orders of magnitude compared with bulk water. Recent studies highlight that reactive oxidative species (ROS) reactions are significantly enhanced at droplet and bubble interfaces, affecting oxidation of atmospheric trace gases. Harnessing such interfaces for engineered applications could overcome bulk-phase kinetic and thermodynamic constraints that limit ROS generation and effectiveness in Fenton and Fenton-like systems. Highly reactive radicals such as HO• and SO₄•⁻ can mineralize organic pollutants, yet bulk reactions suffer from low steady-state ROS concentrations and scavenging side reactions. Microbubble interfaces present a promising platform to accelerate Fenton-like chemistry, but direct evidence for ROS formation and behavior at dynamic microbubble surfaces has been lacking due to challenges in probing interfacial species and in transporting oxidants from bulk to the interface. This study addresses these gaps by using an amphiphilic single-Co-atom catalyst (Co@SCN) that attaches to microbubbles and shuttles PMS to their surfaces, enabling in situ generation and interrogation of interfacial SO₄•⁻ and demonstrating its application to ultrafast VOC oxidation.

Prior work established that chemical reactions can be greatly accelerated at air–water interfaces, including oxidation of NOx and VOCs on cloud droplet surfaces and other heterogeneous atmospheric processes. Conventional Fenton/Fenton-like systems (e.g., with H₂O₂ or PMS) have been improved via catalyst design and energy inputs but remain limited by bulk-phase kinetics, thermodynamics, and scavenging, leading to low steady-state ROS concentrations. Microbubbles and related gas–liquid microstructured interfaces have been proposed to enhance ROS processes and pollutant degradation, and interfacial radical chemistry has been implicated in photocatalysis and bubble-collapsing radical generation. However, the existence, localization, and reactivity of ROS specifically at microbubble interfaces under catalytic PMS activation remained unclear due to difficulties in direct interfacial probing and oxidant delivery to the interface. This study builds on these insights by combining an amphiphilic single-atom catalyst with microbubbles to overcome transport limitations and directly visualize and quantify interfacial ROS.

• Synthesis of amphiphilic single-Co-atom catalyst (Co@SCN): Monodispersed SiO₂ nanospheres (~50 nm) were coated with polydopamine (PDA), then treated with cyanamide and Co(NO₃)₂ solution under ultrasonic conditions, followed by stirring, washing, drying, and calcination at 600 °C under N₂ to form Co@SCN core–shell nanospheres. Structural characterization was performed by SEM, HRTEM/SAED, XRD, HAADF-STEM, EDS mapping, XPS, XANES, EXAFS, and WT-EXAFS to confirm atomically dispersed Co coordinated to N (Co–N, five-fold coordination) on an N-doped graphitic carbon shell over a SiO₂ core.
• Interfacial attachment assessment: Dispersibility tests in polar/nonpolar solvents and formation of stable oil-in-water Pickering emulsions evidenced amphiphilicity. FTIR confirmed reduced –OH (SiO₂ core) and emergence of graphitic features (hydrophobic shell). A force balance model quantified adhesive vs detachment forces as a function of particle size and contact angle, predicting robust microbubble trapping for 40–80 nm particles. Reflected-light video microscopy directly visualized Co@SCN films at the microbubble surface.
• ROS generation and identification: In Co@SCN–PMS systems with/without microbubbles, ESR/EPR spectroscopy detected HO•, SO₄•⁻, and O₂•–; coumarin probe fluorescence quantified ROS-derived 7-hydroxycoumarin. Radical scavengers (TBA for HO•, EtOH for HO•/SO₄•⁻, TEMP for ¹O₂) parsed contributions. Epifluorescence microscopy visualized spatiotemporal SO₄•⁻ generation around microbubbles. XPS probed Co 2p binding-energy shifts to assess electron-transfer enhancement at interfaces. Representative conditions (Fig. 4): [Co@SCN] = 0.01 g L⁻¹, [PMS] = 0.1 g L⁻¹, [DMPO] = 100 μM, [Coumarin] = 1.0 mM (20 μM for imaging), [EtOH] = [TBA] = [TEMP] = 10 mM, pH 5.6.
• Quantification of interfacial vs bulk SO₄•⁻: From fluorescence and ESR/EPR analyses, interfacial SO₄•⁻ concentrations were estimated and compared to bulk values and literature Fenton-like systems.
• Theory and simulation: Ab initio molecular dynamics (AIMD) computed the free energy profile for SO₄•⁻ transitioning from gas to interface to bulk, and radial distribution functions for SO₄•⁻ interactions with H₃O⁺/H₂O at interface vs bulk. Time evolution of OSO₄⁻–H₃O⁺ distances probed hydrogen bond stability. DFT with CI-NEB evaluated activation barriers for SO₄•⁻-induced electron transfer from toluene at gas, interface, and liquid phases. Transition state theory estimated rate constants using k = kBT/(hP)·e^(−ΔG/kBT). Meta-dynamics and NVT simulations refined free energy calculations.
• VOC oxidation experiments: A Co@SCN–PMS–microbubble reactor was constructed (toluene-filled microbubbles traversing catalyst suspensions) to couple absorption at the interface and oxidation by interfacial SO₄•⁻. Outlet toluene was monitored; MIMS quantified dissolved toluene in bulk. Parameters varied included pH (3–11) and bubble size (≈300–1500 μm). Mass-transfer coefficients (KLa) were measured with/without Co@SCN–PMS. Long-term stability was evaluated via multiple PMS additions (2 g L⁻¹ every 120 min) with continuous toluene microbubble feed (inlet 30 ppmv) and CO₂ monitoring. Additional VOCs (ethyl acetate, chlorobenzene, benzene, styrene) were tested. ESR tracked SO₄•⁻ over time.
• Controls: Systems with microbubbles only, PMS–microbubble, and Co@SCN–microbubble were compared; hydrophilic (CoOx@SiO₂) and hydrophobic (CoOx@C₃N₄) catalysts were tested for interfacial ROS enhancement in the presence of microbubbles.

• Amphiphilic Co@SCN forms robust interfacial films on microbubbles, balancing hydrophilic SiO₂ cores and hydrophobic N-doped graphitic shells; Co exists as isolated Co–N single-atom sites on the shell.
• Microbubbles markedly enhance ROS generation in Co@SCN–PMS systems; scavenger tests identify SO₄•⁻ as the dominant oxidant for coumarin oxidation (EtOH suppresses fluorescence by ~90%; TBA by ~15%; TEMP negligible).
• Epifluorescence microscopy shows rapid, localized SO₄•⁻ formation at microbubble surfaces within 10–60 s, while bulk Co@SCN–PMS shows negligible fluorescence up to 15 min.
• Co 2p XPS shifts indicate stronger electron transfer when Co@SCN reacts with PMS at microbubble interfaces (+0.9 eV) versus bulk (+0.6 eV), supporting accelerated one-electron PMS activation at the interface.
• Interfacial SO₄•⁻ concentration reaches 4.48 × 10⁻11 M versus 2.33 × 10⁻12 M in bulk (~20-fold increase), exceeding most reported Fenton-like systems (10⁻15–10⁻12 M).
• AIMD reveals a minimum free energy for SO₄•⁻ at the air–water interface (thermodynamic preference) and stronger, stable hydrogen bonding with interfacial H₃O⁺ (OSO₄⁻–H₃O⁺ distance ~1.7 Å at interface vs ~1.9 Å initial and >4.5 Å unstable in bulk), explaining interfacial enrichment.
• Ultrafast toluene oxidation: >99% removal achieved despite ~7 s residence time; no detectable dissolved toluene by MIMS during 2 h operation with Co@SCN–PMS–microbubbles. Controls without full system show minimal removal.
• Kinetic superiority at interfaces: Rate constants for SO₄•⁻ + toluene are 8.59 × 10¹⁰ M⁻1 s⁻1 (interface), 7.78 × 10⁸ M⁻1 s⁻1 (liquid), and 2.37 × 10⁶ M⁻1 s⁻1 (gas). DFT barriers: 0.22 eV (interface) < 0.29 eV (liquid) < 0.46 eV (gas).
• Predicted decomposition: Interfacial SO₄•⁻ achieves ~99% toluene decomposition within ~2 s; bulk SO₄•⁻ requires ~42.3 min.
• Process robustness: 99% toluene removal sustained over 600 min with repeated PMS dosing; concurrent CO₂ production indicates mineralization; SO₄•⁻ ESR signal remains stable.
• Broad applicability and operating windows: >97% removal for ethyl acetate, chlorobenzene, benzene, and styrene. Performance is highest at acidic pH (3–6.5) and diminishes in basic media (70% at pH 9; 30% at pH 11) due to interfacial OH⁻ scavenging and reduced interfacial H₃O⁺/H₂O stabilization. Smaller bubbles increase KLa and enhance oxidation (e.g., at ~300 μm, KLa increases from 0.0087 s⁻1 to 0.0635 s⁻1 with Co@SCN–PMS).

By attaching an amphiphilic single-atom catalyst to microbubble interfaces and shuttling PMS from bulk to the air–water boundary, the study directly demonstrates that ROS—especially SO₄•⁻—are generated and enriched at the interface. Enhanced interfacial electron transfer (observed via Co 2p shifts) accelerates PMS activation, increasing local SO₄•⁻ concentrations by about 20-fold compared to bulk. AIMD establishes a thermodynamic preference for SO₄•⁻ at the interface and reveals strong, persistent hydrogen bonding with interfacial H₃O⁺, which stabilizes radicals and prevents rapid diffusion into bulk water. This unique microenvironment also lowers activation barriers for electron transfer to hydrophobic VOCs like toluene, yielding interfacial rate constants orders of magnitude larger than in liquid or gas phases. The combined effects—co-localization of reactants at the interface, favorable thermodynamics and kinetics, and intensified electron transfer—explain the ultrafast, near-complete VOC oxidation observed within the short residence time of microbubbles. These findings validate the hypothesis that harnessing air–water interfaces can overcome bulk-phase limitations in ROS chemistry and provide a mechanistic foundation for interface-enabled environmental remediation.

This work establishes a catalyst-enabled strategy to generate, stabilize, and exploit interfacial SO₄•⁻ at microbubble air–water interfaces for ultrafast pollutant oxidation. Amphiphilic Co@SCN furnishes stable microbubble attachment and PMS shuttling, leading to accelerated interfacial PMS activation, elevated local SO₄•⁻ concentrations, and rapid VOC mineralization. Mechanistic insights from spectroscopy and AIMD/DFT show that thermodynamic preference and strong interfacial hydrogen bonding localize SO₄•⁻ at the interface, while reduced activation barriers and enhanced electron transfer enable high reaction rates with hydrophobic VOCs. The approach achieves >99% toluene removal within seconds, long-term stability, and broad applicability to other VOCs. Future work could optimize catalyst structures and amphiphilicity, explore other oxidants and ROS pathways, tailor bubble size distributions and interfacial electric fields, expand pollutant scope and gas compositions, and develop scalable reactor designs for practical air and water treatment.

• Performance is pH dependent, with markedly reduced efficiency under basic conditions (e.g., ~70% at pH 9 and ~30% at pH 11) due to diminished interfacial stabilization of SO₄•⁻ and OH⁻ scavenging.
• Efficacy depends on microbubble size and residence time; smaller bubbles enhance mass transfer and interfacial enrichment, indicating scale-up must control bubble distributions.
• The system relies on a specific amphiphilic single-atom catalyst (Co@SCN) and PMS oxidant; generalization to other catalysts/oxidants requires validation.
• Most detailed oxidation tests focused on toluene (with additional VOCs shown in supplementary), and real-world complex mixtures or high humidity/temperature fluctuations were not reported.
• Direct, quantitative in situ measurement of absolute interfacial radical concentrations remains challenging; estimates combine spectroscopic signals and probes, potentially introducing uncertainties.