Elsevier

Building and Environment

Volume 188, 15 January 2021, 107433
Building and Environment

The indoor fate of terpenes: Quantification of the limonene uptake by materials

https://doi.org/10.1016/j.buildenv.2020.107433Get rights and content

Highlights

  • Effective and reliable methodology for partitioning coefficient (K) determination.

  • Characterization of the reversibility of limonene uptake onto building materials.

  • Quantitative ranking of real building materials regarding limonene uptake.

  • Evidence for building material behavior as sinks and/or secondary sources of terpenes.

Abstract

Beyond their indoor emission by various sources, another aspect of the presence of VOCs in confined environments involves their indoor fate. Terpenes, because of their ubiquity, source variety and reactivity, are VOCs whose contributions to the indoor air quality may largely exceed primary emissions because of possible secondary processes such uptake and secondary emissions. Limonene is flagged as a species typifying the behavior of terpenes, and a selection of representative indoor materials is proposed. Limonene uptake characterization of selected surfaces is performed under typical indoor conditions using a FLEC-based experimental setup allowing (i) limonene partitioning coefficient determination and (ii) quantification of the reversible nature of this interaction. Interestingly, the materials of interest exhibit highly differentiated affinities for limonene, evidencing the contrast in their surface contributions to terpene loss. Glazing is confirmed as a non-significant sink, while cotton fabric and gypsum board are major contributors to limonene surface loss and exhibit high surface uptake capacities. This work allows a quantitative ranking of the selected materials from minor to major limonene sinks. Reversibility quantification of the uptake process provides key insights into further secondary limonene emissions. The major sink materials are highlighted such as inducing irreversible limonene uptake, thus creating indoor surface pools of reactive organics possibly available for further oxidation processes.

Introduction

The indoor concentrations of volatile organic compounds (VOCs) result from various contributions, mainly involving the (i) air renewal rate, (ii) emissions from materials and activities in the indoor environment, (iii) reactive processes, either homogeneous or heterogeneous, inducing the conversion of primary VOCs into secondary ones, and (iv) VOC exchange with indoor surfaces [1]. Among these phenomena, the interactions between VOCs and materials, such as emission and uptake processes, have been demonstrated as being drivers of the temporal dynamics of the VOC concentration in indoor environments [2]. If indoor materials have first been addressed as sources to be controlled [[3], [4], [5]], they can also be regarded as sinks [[6], [7], [8]]. However, the contribution of heterogeneous VOC losses has neither quantitatively nor temporally been assessed, while it has been identified by several authors as a key driver. Indeed, Weschler [9] in 2011 suggested that the interactions between VOCs and the surfaces of indoor materials impose a more notable impact than the gas-phase reactivity on indoor VOC concentrations. However, the diversity of materials present in indoor environments should be considered and addressed. As a first insight, Gunschera et al. [10] conducted a comprehensive study of the sorption-desorption processes of VOCs on building materials. The physical and chemical properties of surfaces as well as the morphology of the materials considerably impact their heterogeneous behaviors towards the uptake of VOCs.

The description of the exchange processes between VOCs and materials requires the preliminary definition of the relevant physical and chemical concepts. Three processes contribute to the VOC-material interactions [11]: (i) the transport of VOCs in the boundary layer formed in the vicinity of the surface of the materials, (ii) the adsorption and/or desorption of VOCs onto the surface of the materials, and (iii) the diffusion of VOCs into the bulk structure of the materials. Adsorption and desorption are defined by the orientation of the concentration gradient of the VOCs in the boundary layer. The concept of sorption encompasses multiple interactions between VOCs and surfaces. Physisorption refers to weak electrostatic interactions between sorbent and sobate molecules. Thus, the physisorbed fraction is mostly reversibly adsorbed at ambient temperature. Hence, a slight variation in the VOC concentration gradient in the boundary layer or a slight increase in temperature subsequently induces the desorption process. Chemisorption corresponds to stronger VOC/material interactions, such as covalent bonds with the surface. Chemisorption generally leads to an irreversible sorptive process. This may also result in the modification of the adsorbed VOC structure. Uptake is not a permanent state, indeed the adsorbate can return to the gas phase, depending on the environmental conditions. Therefore, it may contribute to the gas phase dynamic of pollutants again. These aspects refer to the reversible or irreversible nature of the uptake. Indeed, if a material acts as a sink, to what extent is it a long-lasting sink? Should it be considered as a secondary source later on? As a complement to the concept of sorption, diffusion corresponds to the migration of a species in the bulk structure of the material. This phenomenon becomes all the more significant at a large material thickness, and the considered species are characterized by high diffusion coefficients (Kd) in the material. The review conducted by Haghighat et al. [12] in 2002 on the VOC diffusion coefficient in typical building materials points to the fact that diffusion coefficients vary considerably according to the considered material-VOC system. Nonetheless, gypsum board and highly porous concrete characteristically exhibit the highest diffusion coefficients with any VOC. Similarly, Meininghaus et al. [13] and Blondeau et al. [14] in 2003 reported VOC diffusion coefficients of gypsum boards that were higher than those of other indoor materials. Nevertheless, diffusion coefficients can greatly vary from one VOC to another since they are dependent on their chemical structures [12].

When uptake is mostly attributed to physisorption interactions, the Langmuir model can be applied to typify the interactions between VOCs and materials. The Langmuir isotherm describes the evolution of the amount of VOCs taken up by the material as a function of the gas-phase concentration. The typical profiles of this isotherm can be described by two regimes. At the lowest concentrations, typically in the ppb range, the taken-up quantity linearly increases with increasing gas-phase concentration. This behavior was confirmed by Thevenet et al. [8] in 2018 under typical indoor conditions for toluene and formaldehyde onto various types of gypsum boards. At higher concentrations, the isotherm is characterized by a saturation regime. However, typical indoor conditions are not associated with ppm-level concentration ranges, and only the linear section of the isotherm needs to be determined [15]. The corresponding slope of the isotherm within the ppb range is named the partitioning coefficient K, expressed in m. It describes the uptake capacity of a given material for a specific VOC. The concept of uptake represented by the partitioning coefficient encompasses both sorption and diffusion phenomena, and it characterizes the loss of a given VOC to a certain material under specific temperature (T) and relative humidity (RH) conditions.

Among the various indoor sources of pollutants, the activities of building occupants such as interior renovation or decoration, smoking, cooking, and intense cleaning activities constitute major sources of contaminants [16]. Cleaning is commonly performed by occupants to increase hygiene, improve esthetics, and preserve materials [17]. Many available cleaning products are scented” because a pleasant odor provides the perception of a clean environment. Despite all the benefits provided by cleaning activities, cleaning product use presents many associated risks. Terpenoid VOCs are widely adopted as natural fragrances in household cleaning products and air fresheners marketed as green and healthy. Therefore, natural essential oils or synthetic terpenes are widespread, these VOCs commonly occur indoors. A review on the ubiquity of terpenes in indoor environments was performed by Angulo Milhem et al. [18] in 2020. They addressed the current issues related to terpenes and air quality in confined spaces. Adapted from the work of Angulo Milhem et al. [18], Fig. 1 shows the occurrence percentages of the 19 main terpenes identified in the liquid composition of 450 household cleaning products. Aggregated data are retrieved from 14 different references published between 2001 and 2015, i.e., [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]]. Each of these works highlights the significance and recurrence of household cleaning products as indoor sources of terpenes. The occurrence percentages of the main individual terpenes range from 18% for α-terpinolene to 72% for limonene (Fig. 1). The occurrence of limonene as a fragrance compound in household cleaning products clearly exceeds that of any other terpene (Fig. 1). Moreover, the application of select household cleaning products under realistic scenarios in large-scale experimental setups demonstrated that the indoor limonene concentration may even exceed 250 μg/m3. Similar concentration ranges were reported by Missia et al. [33] and Krol et al. [34], thus identifying limonene as a representative terpene of indoor environments.

Beyond the monitoring of the terpene concentrations in indoor air, the fate of these VOCs in indoor environments is assessed in two aspects: (i) their gas-phase reactivity and (ii) their deposition on surfaces. Long et al. [35], Sarwar et al. [36] and Langer et al. [37], reported the formation of ultrafine particles from terpenes in the presence of ozone under indoor conditions. More specifically, the use of - detergent containing limonene in schools is reported as a source of secondary organic aerosols (SOAs) by Morawska et al. [38]. This observation is confirmed by Rossignol et al. [39]. In addition to this gas phase reactivity, Waring et al. [40] showed that limonene sorbed onto various surfaces highly contributes to the generation of SOAs as well. This finding suggests that the impact of limonene as a heterogeneous source of secondary pollutants may largely exceed its emission time span from household products.

Limonene adsorbed onto the surface of materials may represent a pool of reactive contaminants that could subsequently impact the indoor air quality. This aspect was emphasized by Destaillats et al. [41], who suggested that the consumption of ozone by the adsorbed terpenes may exceed the removal of ozone by ventilation systems. This point was supported by Springs et al. [42], who showed that the reaction rates of 3-carene and limonene adsorbed onto glass and polyvinylchloride (PVC) are 10–100 times higher than the reaction kinetics in the gas phase. In addition, Shu and Morrison determined the surface reaction rate of α-terpineol on 3 typical indoor surfaces: (i) glass, (ii) PVC and (iii) paint. For all evaluated surfaces, the heterogeneous reaction kinetics were higher than those in the gas phase by a factor of 25 [43]. Consequently, the lain sinks of terpenes in indoor environment have to be identified first. These materials may potentially represent emission sources of secondary products from the heterogeneous oxidation of terpenes.

The objective of this study is to characterize the uptake of limonene, as a model terpene, onto ten representative indoor materials. This investigation aims to address the uptake behavior of limonene under realistic indoor conditions and materials to provide an overview of the corresponding partitioning coefficients and identify the main indoor sinks of terpenes. Beyond these two main tasks, this work aims to provide a more precise understanding of the reversible or irreversible nature of the limonene uptake process to identify materials representing long-term pools of adsorbed terpenes in indoor environments.

Section snippets

Selection and characterizations of the indoor materials

To address the variety of building products in indoor environments, materials from five main categories were selected, including glazing materials, construction materials, textiles, paints, and wood-based materials. Table 1 lists the ten materials selected with their characteristics. Despite the interest of this study in the uptake process, materials recognized as potential sources of VOCs have been selected, such as paints or wood-based materials, and this point is further addressed below.

The

Results and discussion

The interactions of limonene with the ten selected indoor surfaces is experimentally addressed. To provide a first detailed insight into the uptake results, the behavior of the limonene + natural pine-wood system is primarily described and examined. Thereafter, the partitioning coefficients K for the various materials are presented and assessed. Finally, the reversible nature of the uptake process is reported and evaluated in terms of the impacts on the indoor air quality.

Conclusion

The determination of the partitioning coefficient of limonene for a selection of representative indoor materials allows an effective and quantitative ranking of the main sinks of terpenes in indoor environments. Even though all materials exhibit linear relations of the limonene uptake amount within the investigated indoor concentration range, the results reveal different contributions of the selected materials in terms of the limonene uptake amount. Indeed, the partitioning coefficients range

Funding

This work was financially supported by l’Agence de l’Environnement et de la Maîtrise de l’Energie (ADEME), IMT Lille Douai and the Centre Scientifique et Technique du Bâtiment (CSTB) and the Labex CaPPA, funded by the ANR through the PIA under contract ANR-11-LABX-0005-01.

Role of the funding source

ADEME provided financial support for the conduct of research and for the postdoctoral grant of Pamela Harb. IMT Lille Douai and CSTB provided funding for the PhD grant of Shadia Angulo Milhem. Labex CaPPA provided funding or the Master grant of Siveen Thlaijeh.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge Agence de l’Environnement et de la Maîtrise de l’Energie (ADEME) for the provided financial support of research project ESSENTIEL within the framework of the CORTEA research program. In particular, Isabelle AUGEVEN-BOUR and Souad BOUALLALA are acknowledged for their benevolent and constructive care of the project. Pamela HARB thanks ADEME for the provided postdoctoral grant. Siveen THLAIJEH acknowledges the Labex CaPPA, funded by the ANR through the PIA under contract

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