Plasma degradation of contaminated PPE: an energy-efficient method to treat contaminated plastic waste

The study addresses the urgent problem of safely treating the surge of contaminated PPE waste, particularly disposable polypropylene (PP) surgical masks, driven by the COVID-19 pandemic. Conventional treatments such as incineration pose environmental and economic challenges (hazardous APC residues, toxic flue gases, high costs, transport risks) and do not align with circular economy goals. The research explores whether low-power air plasma can provide an effective, energy-efficient, and safer alternative for on-site decontamination and controlled degradation of PPE, potentially enabling sterilisation for reuse and/or facilitating recycling, while mitigating environmental risks such as microplastics generation.

Plasma technologies are widely used for plastics surface modification, etching, and sterilisation. Cold atmospheric pressure/plasma treatments can inactivate a broad range of pathogens (e.g., ESKAPE pathogens, E. coli, Candida albicans, S. aureus including MRSA, S. epidermidis) and various viruses; they can also be used for skin disinfection. Plasma produces reactive species and UV, both useful in decontamination. Prior work shows rapid microbial reduction (e.g., MRSA from 3×10^7 to ~600 colonies in a few seconds). The rate of polymer etching/degradation increases with plasma power. These precedents suggest plasma could be a viable, fast, and green method for PPE sterilisation and degradation, motivating its application to PP mask materials.

Materials: Commercial 3‑ply PP surgical masks (Fisher Scientific) were cut into 1.5 ± 0.1 cm × 5.3 ± 0.4 cm strips; elasticated ear loops were cut into 2.5 cm segments. A bulk PP sheet (Goodfellow, 1 mm thick) was cut into 1 × 1 cm squares, cleaned with distilled water and air-dried. Handling used gloves/tweezers; samples transported in closed containers. Plasma irradiation: Treatments were conducted in a Henniker HPT‑200 glow discharge unit (stainless-steel cylindrical chamber, diameter 15 cm, length 28 cm, volume 4.95 L) operating at 40 kHz RF. Protocol: evacuate to ~0.30 mbar; introduce ambient air at 15 sccm to 0.70 mbar; ignite plasma. Unless otherwise noted, power was set to 200 W. Exposure times were in 30 min increments up to 4 h. For 3‑ply strips, the outer (blue) layer faced the discharge. After each interval the chamber was vented to lab atmosphere to remove the sample for analysis. Each treatment was repeated ≥3 times for reproducibility. Additional tests on bulk PP explored power dependence (60–200 W; see Supplementary for details). Gravimetric analysis: Mass changes were measured with a five significant-figure microbalance (Avery Berkel); errors reported as standard deviations. SEM: Zeiss Merlin FE-SEM, 2.5 kV acceleration voltage, InLens detector; samples sputter-coated with thin Pd/Au. ATR-IR: ThermoFisher Nicolet iS5 with iD5 ATR, 500–4000 cm⁻¹. XPS/ARXPS: PHI Quantera II (monochromatic Al Kα). Charge neutralisation applied; survey and high-resolution C 1s, N 1s, O 1s at 26 eV pass energy with samples tilted to 90°. Spectra referenced to adventitious C 1s at 284.4 eV. TGA/DSC: TA Instruments SDT Q600. ~5 mg sample heated to 300 °C at 5 °C min⁻¹ under N₂ (75 mL min⁻¹). DSC melting peak integrated to calculate ΔfusH; crystallinity determined using literature ΔfusH for 100% crystalline PP: 170 ± 3 J g⁻¹. WAXS: Panalytical Empyrean diffractometer (Cu anode, 45 kV, 40 mA), reflection geometry; treated samples on null-reflecting Si. Diffraction patterns fitted with amorphous (Gaussian) and crystalline (pseudo-Voigt) components to compute crystallinity: 100·(ΣA_ci)/(ΣA_aj + ΣA_ci).

- Mass loss kinetics at 200 W air plasma (0.70 mbar): - 3‑ply mask strip (unfolded): 0.26 ± 0.03% mass loss per minute; 63.34 ± 7.76% total after 4 h. - 3‑ply mask (folded): 0.20 ± 0.02% per minute; 45.50 ± 2.65% after 4 h (reduced surface area exposure). - Bulk PP (solid sheet): 0.01 ± 0.01% per minute (slowest; low surface-area-to-volume ratio). - Elasticated ear loop: 0.04 ± 0.01% per minute; 8.75 ± 1.5% after 4 h. - Power dependence: bulk PP weight-loss rate increased markedly between 60–200 W, showing apparently exponential behavior (Supplementary). - Individual mask layers (separately exposed, 4 h): - Middle filter (melt-blown PP): 99.0 ± 1.5% weight loss (fastest degradation). - Outer blue (spun-bond PP): 87.4 ± 6.7% weight loss. - Inner white (spun-bond PP): degraded less than middle layer; trends consistent with outer/inner layers being more resistant than melt-blown middle. - Morphology (SEM): - Early surface pitting detectable after ~0.5 h; between 1–2 h fibers shorten and thin fibers are removed. - By 4 h, extensive etching with localized melting/fusion; apparent increase in average fiber width due to loss/fusion of thinner fibers. - Middle layer exhibited pronounced pore formation before subsequent melting/fusion; outer/inner layers showed less pronounced etching over same durations. - Surface chemistry (ATR-IR): - Growth of carbonyl band at ~1714 cm⁻¹ (C=O) and broad O–H stretch ~3300 cm⁻¹ with irradiation time; rapid increase within first hour then slower growth (apparent surface saturation trend). - Decrease in C–H stretch intensity; bulk PP shows similar trends with band shifts; ear loops showed minimal spectral changes, consistent with low mass loss. - Surface chemistry (XPS): - New C 1s components at ~286.6 eV (C–OH) and ~288.7 eV (O–CO), and O 1s at ~533.7 and ~532.8 eV, confirming hydroxyl and carboxyl functionalities. - Emergence of N 1s features at ~401.3 and ~400.9 eV (N–(C=O)–, C–NR₂), attributable to nitrogen incorporation from air plasma; additional N 1s at ~397 and 404–405 eV and C 1s at ~290 eV suggest possible metal nitrides/nitrates and metal carbonate (potentially from residual polymerisation catalysts). - Thermal/crystallinity (WAXS, TGA/DSC): - All layers showed decreased crystallinity after irradiation (WAXS) and lowered melting temperatures (DSC), consistent with chain scission/oxidation. - TGA indicated increased low-temperature weight loss (100–200 °C) post-irradiation; middle layer increased by >3%, suggesting formation of smaller, more volatile organics. - Energy comparison: A typical muffle furnace (similar volume) for pyrolysis consumes ~4000 W vs 200 W used here, indicating substantially lower energy input for cold plasma processing. - Mechanistic insight: Data support oxidation-driven surface modification (OH and O atom initiated) producing oxygenated functionalities that precede and accompany fragmentation; expected gaseous products include short-chain oxygenates (aldehydes, ketones, acids), CO, CO₂, and H₂O.

Findings demonstrate that low-power air plasma rapidly oxidizes and degrades PP microfibers in surgical masks, with kinetics strongly dependent on morphology and processing history. The melt-blown middle layer (higher surface area, broader diameter distribution, lower molecular weight, less chain orientation) is markedly more susceptible than spun-bond outer/inner layers, which retain greater structural integrity over the same treatment. Surface analysis (IR/XPS) shows oxygen functionalization (hydroxyl, carbonyl, carboxyl) preceding and accompanying etching, consistent with O/OH radical chemistry and cool-flame type oxidation pathways. Thermal and crystallinity measurements corroborate chain scission and reduced crystallinity/Tm, producing smaller, more volatile fragments. Practically, the approach offers two pathways: short exposures for sterilisation (literature indicates <1 h effective decontamination), enabling potential reuse/circularity; longer/higher-power exposures for substantial mass reduction with lower energy input and potentially less toxic emissions than incineration. Controlled plasma degradation may also mitigate uncontrolled release of microplastics by managing fiber erosion in a contained system. Compared with incineration, cold plasma has lower power demands and may enable capture of value-added oxygenates from the off-gas, especially if integrated with catalytic upgrading. Differences in response among mask components suggest that manufacturing processes and additives influence degradability, informing future design of recyclable PPE.

The study shows that cold air plasma (200 W) can effectively and energy-efficiently degrade PP surgical masks: 3‑ply masks lost ~63% mass in 4 h, with the melt-blown middle layer reaching ~99% loss and pronounced surface oxidation (C=O, O–H). Morphological, spectroscopic, and thermal analyses consistently indicate oxidation-induced chain scission, reduced crystallinity, and formation of smaller volatile organics. The approach is promising both for rapid sterilisation (enabling reuse) and for degradation with reduced energy and potentially cleaner emissions than incineration. Future work should focus on scale-up via dedicated high-power reactors, optimization of power/time for throughput, full characterization and capture of gaseous products (with potential catalytic upgrading), engineering of PPE (e.g., middle layer via spun-bond) for improved recyclability, and integration of plasma treatment into on-site waste management workflows.

- Scale and power: The HPT-200 unit is a small, low-power system not designed for large-scale waste processing; throughput and scalability remain to be demonstrated. Higher powers are likely needed for faster rates. - Off-gas characterization: The composition and yields of gaseous degradation products were not fully quantified; environmental and safety assessments of off-gases and capture/abatement systems are needed. - Material heterogeneity: Unknown minor components (dyes, plasticisers, metals) may affect degradation rates and product speciation; their identities and impacts were not resolved. - Configuration effects: Mask folding reduces effective surface exposure and slows degradation; real-world handling may require process adaptations (e.g., agitation, unfolding, or flow dynamics) to ensure uniform treatment. - Component variability: Elasticated ear loops degraded slowly; non-PP components will require tailored strategies or pre-processing. - Energy comparison caveat: The simple 200 W vs 4000 W comparison is illustrative; comprehensive life-cycle energy/emissions analyses are needed for fair benchmarking.