Sorption Dynamics of Formaldehyde on Microplastics and Indoor Materials: Experiments and 1D‑Box Modelling Insights

Formaldehyde remains one of the most critical indoor air pollutants due to its ubiquity, reactivity, and harmful health effects (Salthammer et al., 2010). In recent years, microplastics (MPs) have also been reported as a critical indoor air pollutant (Zhang et al., 2020). Although sorption processes of formaldehyde on indoor surfaces have a significant impact on their persistence and spatial distribution, quantitative data on material-specific adsorption behavior are currently scarce, especially for emerging pollutants like MPs. This study provides a systematic experimental investigation of formaldehyde sorption onto the surface of fresh and O₃-aged MPs, such as Low-density polyethylene (LDPE), polyvinyl chloride (PVC), Polyether ether ketone (PEEK) (PlasticsEurope, 2018; Stober et al.,1984), and also common indoor materials: cement, gypsum, conventional paint, and depolluting paint. Uptake measurements were performed in a flow reactor coupled with selective ion flow tube mass spectrometry (SIFT-MS) to enable real-time monitoring of formaldehyde. Experiments were performed at room temperature across a wide range of formaldehyde concentrations (100–550 ppb), under both dry air and 50% relative humidity. The objective was to determine formaldehyde partition coefficients, K_e, on the surfaces of interest.

The partitioning coefficient (K_e) of formaldehyde to MPs was found to be very low, at least two orders of magnitude lower than that of common indoor materials. Ozone aging and relative humidity influenced formaldehyde uptake, with the extent of this effect varying depending on the type of MP studied. However, in all cases, K_e values for MPs remained significantly below those measured for typical indoor surfaces. Under humid conditions (50% RH), depolluting paint exhibited the highest partitioning capacity, followed by conventional paints, gypsum, and cement. These findings suggest that, despite their growing presence in indoor environments, MPs are unlikely to have a significant contribution in formaldehyde loss compared to conventional building materials.

To evaluate the impact of the experimentally derived K_e-values on indoor air quality and identify dominant loss processes, we implemented them in a modified 1D-box model (Fiorentino et al., 2021) representing a typical room, based on the IRINA (Harb et al., 2016) experimental facility. Simulations considered a pollution episode and included ventilation, gas-phase reactions with atmospheric oxidants, and heterogeneous uptake on room surfaces. Results show that heterogeneous loss dominates formaldehyde removal indoors, with rates over an order of magnitude higher than gas-phase processes. Depolluting paint under 50% RH led to the fastest concentration decline. These results highlight the key role of surface interactions in indoor air quality and the importance of material choice in controlling pollutant levels. The combined experimental–modelling approach facilitates improved predictions of pollutant behavior in indoor environments and promotes the development of more potent passive depollution strategies.

References:

Salthammer, T. et al (2010) Chem. Rev. 110, 2536–2572.

Zhang, Y. et al (2020) Earth-Sci. Rev. 203, 103118.

PlasticsEurope (2018) Plastics – the Facts 2018.

Stober, E. J. et al (1984) Polymer 25, 1845–1852.

Fiorentino, E. A. et al (2021) Geosci. Model Dev. 14, 2747-2780.

Harb, P. et al (2016) Chem. Eng. J. 306, 568-578.