Irene Lara-Lbeas

Claire Trocquet

Rouba Nasreddine

Christina Andrikopoulou

Vincent Person

Béatrice Cormerais

Stéphanette Englaro

Stéphane Le Calvé

BTEX near real-time monitoring in two

Keywords: BTEX, toluene, indoor air schools, CO2 levels, real-time, portable device

Introduction

During the last decades, Indoor Air Quality (IAQ) is a growing concern in the scientific community due to the harmful effects of airborne pollutants on human health. According to World Health Organization (WHO), household air pollution led to more than 4 million premature deaths annually, among which the large majority concerns children. Children are more vulnerable than adults to the adverse health effects of air pollution due to their higher inhalation rates, the continuing development of their tissues and organs, their immature immune system, and the longer periods of time they spend indoors [START_REF] Mendell | Do indoor pollutants and thermal conditions in schools influence student performance? A critical review of the literature[END_REF]. A large number of epidemiological studies have reported an association between exposure to criteria air pollutants and several morbidity and mortality outcomes in children [START_REF] Buka | The effects of air pollution on the health of children[END_REF].

Between the age of 3 and 16, schools are the most common places were children spend almost 30% of their day, 70% of which inside classrooms [START_REF] Daisey | Indoor air quality, ventilation and health symptoms in schools: an analysis of existing information[END_REF]. Therefore, indoor air quality in schools is very critical for children's health. Indeed, an increasing amount of scientific evidence has demonstrated that the physical environments in which children spend their time have a profound impact not only on their health but also on their ability to learn [START_REF] Mendell | Do indoor pollutants and thermal conditions in schools influence student performance? A critical review of the literature[END_REF][START_REF] Simoni | School air quality related to dry cough, rhinitis and nasal patency in children[END_REF]. Indoor air quality in schools is assessed generally by its comfort parameters (such as relative humidity, temperature and CO2 levels), ventilation rates, and particulate matter levels and by gaseous pollutants concentrations.

Even if CO2 is not defined as an air pollutant by the WHO, its concentration level is commonly adopted as an indicator of ventilation rates inside the building [START_REF] Satish | Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance[END_REF] or in other words a confinement indicator. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), if CO2 is controlled to less than 700 ppm, then it is generally thought that indoor environment receives sufficient quantities of outdoor air to dilute human metabolic odors (and some other odors) to acceptable levels. A number of researches have shown that CO2 concentrations above 1000 ppm can affect students' performance [START_REF] Apte | Associations Between Indoor CO2 Concentrations and Sick Building Syndrome Symptoms in U.S. Office Buildings: An Analysis of the 1994-1996 BASE Study Data[END_REF][START_REF] Twardella | Effect of classroom air quality on students' concentration: results of a clusterrandomized cross-over experimental study[END_REF][START_REF] Muscatiello | Classroom conditions and CO2 concentrations and teacher health symptom reporting in 10 New York State Schools[END_REF]. Many studies of IAQ in classrooms, reported CO2 levels between 500 and 4000 ppm [START_REF] Alves | Air quality in a school with dampness and mould problems[END_REF]) and most frequently above 3000 ppm, especially in naturally ventilated schools. These high levels of CO2 exceed the ASHREA recommended threshold value, demonstrating inadequate ventilation for the majority of schools [START_REF] Griffiths | Control of CO2 in a naturally ventilated classroom[END_REF][START_REF] Turanjanin | Indoor CO2 measurements in Serbian schools and ventilation rate calculation[END_REF][START_REF] Bari | Source apportionment of indoor and outdoor volatile organic compounds at homes in Edmonton, Canada[END_REF][START_REF] Hänninen | Analysis of CO2 monitoring data demonstrates poor ventilation rates in Albanian schools during the cold season[END_REF].

Numerous studies have been carried out in order to monitor the indoor/outdoor levels of Volatile Organic Compounds (VOCs) in schools (see Table 1). Among the indoor air contaminants, BTEX (Benzene, Toluene, Ethylbenzene and Xylenes isomers) have been founded to be the most abundant in school indoor air [START_REF] Kotzias | Exposure to Multiple Air Contaminants in Public Buildings, Schools and Kindergartens -The European Indoor Air Monitoring and Exposure Assessment (AIRMEX) Study[END_REF][START_REF] Geiss | The AIRMEX study -VOC measurements in public buildings and schools/kindergartens in eleven European cities: Statistical analysis of the data[END_REF]. The presence of BTEX in indoor air presents harmful effects for children's health leading to the development of serious pathologies such as asthma [START_REF] Arif | Association between personal exposure to volatile organic compounds and asthma among US adult population[END_REF]. Among BTEX, benzene is the most dangerous since it was classified as human carcinogenic according to the World Health Organisation (WHO) and IARC (International Agency for Research on Cancer) in 1986. Therefore, the European Union has established guidelines for benzene in indoor air where a threshold level is fixed to 5 µg m -3 (1.6 ppb) in public buildings, requiring sensitive analytical methods.

Table 1 presents some studies carried out during the last few years focused on indoor air quality in schools. All reported studies except for one were carried out using classical methods: indoor/outdoor air was sampled on commercial cartridges for a duration time varying between few hours to few days depending on the type of sampling method, i.e. active or passive. In both cases, once sampling was achieved, the commercial cartridges were thermally desorbed or extracted with the appropriate solvent and then analyzed using a gas chromatograph equipped with a flame ionization detector (FID) or a mass spectrometer (MS). Even if these techniques are very sensitive and can analyze tens of molecules at a single run, the obtained results represent only an average concentration of the detected molecules over the considered sampling time.

Therefore, these off-line analysis are not adapted for real-time monitoring and concentration-time profiles determination which can be a key information to identify the sources of pollutants in a given indoor environment. In the literature, only one study was performed using real-time devices aimed to monitor BTEX in indoor air of schools over a long period. This study, conducted by our research group, reported toluene levels for 2 consecutive weeks in an unoccupied classroom using a laboratory prototype of a miniaturized GC [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF] whereas other BTEX were not detected.

In addition, simultaneous passive measurements in indoor and outdoor air are often performed in order to identify if the studied pollutants are emitted inside or outside the investigated building by determining commonly the ratio between indoor and outdoor concentrations [START_REF] Madureira | Source apportionment of CO2, PM10 and VOCs levels and health risk assessment in naturally ventilated primary schools in Porto, Portugal[END_REF][START_REF] Romagnoli | Indoor air quality at life and work environments in Rome, Italy[END_REF][START_REF] Zhong | Volatile Organic Compounds (VOCs) in Conventional and High Performance School Buildings in the U[END_REF][START_REF] Marzocca | Indoor Air Quality Assessment and Study of Different VOC Contributions within a School in Taranto City, South of Italy[END_REF]. Nevertheless this type of passive sampling provides only an average value (usually one measurement per week in the case of schools) which does not allow highlighting the temporal phenomena that are demonstrated by inducing peaks of pollution.

In this work, we report for the first time the accurate temporal variation of all BTEX concentrations in two primary schools and one kindergarten over 5 weeks and 1 week respectively, using pre-industrial BTEX analyzers based on the miniaturized GC/PID reported previously by [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF] [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF]. This field campaign was performed in the city of La Rochelle, in the framework of Impact'Air project aiming at monitoring French indoor regulated air pollutants (formaldehyde, benzene and carbon dioxide), at quantifying the relative emissions sources of air pollutants (building materials vs furniture for example) and at identifying the efficiency of various ventilation scenario on indoor air quality. Note that formaldehyde measurements will be published in a separate paper.

One of the interesting aspects of this study lies in the fact that the real-time measurement of BTEX combined with the measurement of CO2 serves the same purpose as an additional measurement of outdoor air pollution. Indeed, the continuous measurement of CO2 permits to precisely identify the windows' opening periods of the classroom, which are visible when the CO2 concentration rapidly decreases to values close to 400 -500 ppm. As a result, during this study, parallel outdoor pollution monitoring was not conducted but the correlation of pollutants concentrations with outdoor or indoor sources was possible. As soon as the windows were open (as indicated by CO2 values), an increase in the pollutants' concentration in the room can be attributed to external pollution in the absence of controlled mechanical ventilation. On the other hand, when the windows were closed, an increase in the concentration in the room can be imputed to indoor pollution.

Methodology

Site description

The study was conducted in La Rochelle city (east of France 46°09'33.9"North 1°09'21.6" West). Two public primary schools hereinafter school 1 (S1) and school 2 (S2), and one kindergarten hereinafter school 3 (S3) were selected considering as representative schools of the city. Fig. 1 shows the sites of the three selected schools. S1 is a primary school built in 1965 and located at a residential suburban area at about 150 m from the highway and at 1.2 km far from La Rochelle airport. S2 was constructed in 1974 and it is also located at a residential suburban area next to the city park. The kindergarten S3 was also built in the seventies and was located about 300 meters from the same highway as S1.

One classroom was chosen on each site according to the possibility 1) to install the analytical materials inside an adjacent room to limit noise as much as possible and thus avoid disturbing the courses and 2) to limit the sampling line length. In the first one (S1), the classroom occupied 187 m 3 and presented 4 west facing windows and 2 doors: one was connected to an adjoining office and the second to a hallway. During the activity period, the classroom was occupied by 25 students aged between 9 and 11 years. In S2, the classroom volume was equal to 198 m 3 and also equipped with 4 windows west sides and 3 doors. This classroom was occupied by 16 students of 7-9 years. In S3, the classroom volume was similar to the other schools (180 m 3 ) and had 4 large south-east facing bay windows and 3 doors connecting the classroom with the corridor, the playground and the library. During school days, the classroom was occupied by 23 students between the age of 3 and 6 years.

The investigated rooms in S1 and S2 had similar characteristics i.e. tiled floor, wallpaper, a blackboard and two corkboards. The only differences were the type of the ceiling, covered by ceiling plates in S1 while it was painted in S2 and the presence of a whiteboard in S1.

On the other hand, the classroom studied in S3 had a linoleum floor, painted walls, ceiling covered of ceiling plates, a blackboard and a corkboard.

All the investigated classrooms were naturally ventilated and equipped with heating radiators operating from 7 am to 5 pm in weekdays to keep an average temperature of 20°C during the day and 16°C during the night.

Analytical instruments

In order to monitor in real time the three French regulated pollutants, different analytical instruments were used: home-made HCHO analyzers, pre-industrial BTEX analyzers and CO2 detectors i.e. LUM'Air (Cairpol) or Q-Trak TM IAQ monitor (TSI Incorporated, St. Paul, Minnesota, USA). During the whole campaign, Temperature, Relative Humidity and Carbon Dioxide were measured using Q-Trak TM IAQ in all schools. In addition, LUM'Air was used to alert when threshold levels were exceeded.

Lumair CO2 detector

In both S1 and S2, the LUM'Air CO2 detector was used. This later is equipped with three different LED (green, orange and red) which are switched on depending on the CO2 concentration. For concentrations below 1000ppm, the green LED is on, meaning atmosphere is not confined. When levels are higher than 1000ppm (first alert threshold), light switched to orange indicating windows and/or doors should be opened. If concentration exceeds 1700 ppm (second alert threshold), light switchers to red and windows and/or doors must be open quickly to renew indoor air.

In the kindergarten (S3), temperature, humidity and CO2 levels were also measured continuously every 10 minutes with Q-Trak TM IAQ monitor (TSI Incorporated, St. Paul, Minnesota, USA). The BTEX results were correlated to CO2 values to determine exactly when indoor air was renewed by opening windows and/or doors.

Pre-industrial BTEX analyzer

Gaseous BTEX in schools' indoor air were continuously analyzed by two novel pre-industrial BTEX analyzers (see Fig. 1). These devices were constructed based on the laboratory prototype µGC developed by [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF] [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF]) and used for real-time toluene monitoring in a previous study (Nasreddine et al. (2015b). The pre-industrial µGC operates according to three steps: i) sampling, ii) separation and iii) detection as shown in Fig. 2. The detailed operating principle is described elsewhere [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF]) so that only a brief description is given below.

In a previous study (Nasreddine et al., 2015a), the optimal experimental conditions of BTEX separation were determined according to the calculated Height Equivalent to the Theoretical Plate (H.E.T.P.) and the total analysis time for a single run, which was set to a maximum of 10 minutes. Using the optimal conditions, the detection limit, stability and repeatability of the analytical system were assessed. A detection limit of 1 ppb was found for benzene and toluene whereas it was less than 3 ppb for other monoaromatic compounds. The pre-industrial BTEX analyzer was standalone and fully controlled by an embedded software running on Linux which provides visualization of real-time chromatograms, automatic integration of obtained BTEX peaks and calculation of the corresponding concentrations. The pre-industrial BTEX analyzer system shown in Fig. 3. can fit into a plastic case that measures 32.4 (l) × 28.9 (w) × 14.5 (h) cm and weights around 5.5 kg (e.g. battery and screen included) The micro-device can be powered either by a 12 V DC external adapter or a 12V battery pack. A built-in 7-inch touch panel screen is used to control all GC parameters (sampling time, oven temperature, number and frequency of analysis and integration parameters). This powerful instrument provides a full BTEX analysis within only 10 minutes.

The laboratory prototype of this micro-device was also deployed in a junior high school for two weeks during a previous indoor field campaign (Nasreddine et al. 2016a). The obtained results validated and demonstrated its ability to highlight rapid changes of atmospheric BTEX concentrations in the range between 1-20 ppb. In addition, the toluene concentrations measured with our new micro-device were in very good agreement with those provided by another commercial BTEX analyzer (GC8900, Baseline).

Calibration and detection limits of BTEX analyzer

During this previous campaign, it was highlighted that all devices equipped with a photoionization detector may need to be recalibrated during field campaign occurring several weeks. A long interruption due to a long transport also implies to make in-situ a full calibration or at least a partial one to check the previous calibration curves made under laboratory conditions.

For these reasons, in the present study, both pre-industrial portable BTEX analyzers were in-situ calibrated at the beginning and during the field campaign using the calibration procedure detailed below.

Stainless steel canisters were previously cleaned in our laboratory within a standard cycle of three steps using Model 3100A Canister Cleaner (ENTECH INSTRUMENTS, INC.). Cleaned canisters were then filled with a standard mixture of BTEX purchased from Messer (France). The initial concentration of every compound was equal to 100 ppb with a 10 % of uncertainty.

Once the instruments were installed in the schools, the aforementioned canisters were connected to calibration set-up described elsewhere [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF], this later being connected to the sampling inlet of the BTEX analyzer. The different BTEX concentrations were obtained by dilution in nitrogen (99.999 %, Messer). Each generated BTEX concentration and its corresponding analysis was repeated twice. This set-up allowed us to calibrate easily the system during field campaign. For both pre-industrial devices, BTEX concentrations ranged from 0 to 50 ppb and calibration curves were obtained by plotting the peak area for each species versus the corresponding gaseous concentrations. The equations of the calibration curves and limits of detection are presented in Table 4.

Every week, a single point calibration with a BTEX mixture at 20 ppb was performed for each instrument. This allowed the detection of any significant change of the response factor of the BTEX analyzer. Finally, these pre-industrial micro-devices were recalibrated twice during the course of our field campaign and deviations between 3 and 15% compared to the initial calibration were recorded depending on the gaseous compound and on the BTEX analyzer, excepting for ethylbenzene in the preindustrial microdevice 1 that reached a deviation of 26 %.

Sampling conditions and ventilation scenario

The field campaign was conducted during winter for 5 consecutive weeks from 15 th of February 2016 to 20 th of March 2016 for schools 1 and 2 whereas the school 3 was only investigated for the week 4.

All instruments excepting CO2 probes were installed in an adjacent room and indoor air was continuously sampled through 6mm Teflon tube. This sampling tube measured 4 to 6 meters and its inlet reached approximately the middle of the classroom at 50 to 100 cm under the classroom ceiling. Each instrument had its own sampling line. The sampling flow rate used for the portable analyzers was fixed at 20 mL min -1 which is a sufficient rate to ensure the air renewal in the sampling line in less than 3 minutes. Each analysis was completed in only 10 min, the next sampling starting 3 min before the end of the previous analysis, and the pre-industrial BTEX analyzer was programmed to work continuously for a full week. Simultaneous indoor and outdoor BTEX measurements in real time was not feasible in this case because of the number of analytical devices available and the number of schools investigated. Nevertheless, CO2 concentrations were continuously monitored thus providing information on the windows status (open or closed) and enabling the investigation of possible correlation of indoor BTEX concentrations with outdoor sources.

The various ventilation scenario used in this field campaign have been summarized in Table 3. For the first two weeks of the campaign, the two investigated classrooms of schools S1 and S2 were not ventilated and occupied neither by students nor by teachers. The windows were always closed whereas the doors were maintained closed as much as possible and nobody was allowed to enter inside the investigated classroom. During the first week from 15 th to 21 st of February 2016, the chosen classrooms were also fully empty without any furniture in order to estimate the concentrations levels which could be attributed to emissions from building materials.

The second week, the furniture was added to the classrooms at the very beginning of the second week and the pollutants concentrations were measured from 22 nd to 28 th of February 2016. Note that in S1, the ramp of the stairs leading to the hallway was repainted at the beginning of the second week which distorted a part of the results.

For the weeks 3 to 5, students and their teachers were also present in the classrooms and they followed different ventilation instructions each week. In S1, during the third week (from 1 st to 6 th March 2016), usual ventilation was followed, where no specific instructions were given to the teachers, so they followed its subjective criteria for ventilation. During the fourth week (from 7 th to 13 rd of March 2016), ventilation was made according to the CO2 level reached in the room, inducing the lighting of LED on the CO2 detector, and the fifth week (from 14 th to 20 th of March 2016) ventilation instructions given by the French Indoor Air Quality Observatory were followed. According to these latter instructions windows were opened an average of 30 minutes in the following cases: before and after the classes, every two hours of class and after the cleaning events. All the doors of the classroom remained open during the night. The time schedules for both usual ventilation and the ventilation made according the instructions given by the French Indoor Air Quality Observatory are detailed in Table S1.

In S2 the ventilation scenario previously explained were also applied but in a different order because of the availability of a single LUM'AIR CO2 detector. Consequently, ventilation following the recommendations given by the French Observatory of Indoor Air Quality was carried out during the fourth week (from 7 th to 13 rd of March 2016) whereas the occupants used the lighting of LED on the CO2 detector during the fifth week (from 14 th to 20 th of March 2016), in opposite to S1.

In S3, measurements were conducted only over the fourth week, i.e. from 7 th to 13 rd of March 2016, during the presence of students and when no specific ventilation constraints were followed, so that teachers and children opened doors/windows as usually.

Results and discussion

The data obtained in this work are summarized in Table 4. Results obtained for school 3 are not included in this table since no BTEX were detected.

The temperature inside the studied room varied between 14.9 and 24.4°C and 17.4 and 23.5°C for S1 and S2, respectively. The relative humidity ranged from 41.8 to 69.6% in S1 and from 35.3 to 67.3% in S2.

CO2 concentration

During the week 1, the weekly average concentrations of CO2 were found to be 576 and 460 ppm for S1 and S2 respectively with the maximum concentrations of 972 and 661 ppm recorded during the sampling lines installation. After the addition of furniture in the classroom but in absence of people the maximum CO2 concentrations reached 918 and 958 ppm respectively, and the weekly mean levels of CO2 were 444 and 418 ppm.

As expected, significant higher values of carbon dioxide were recorded in both S1 and S2 when the occupants were present in weeks 3 to 5. The highest concentrations were monitored when the occupants followed the usual ventilation, with average values equal to 1296 and 1221 ppm and maximum levels up to 3743 and 3565 ppm for S1 and S2 respectively. The lowest values recorded in presence of occupants correspond to the week where the occupants opened the windows and doors according to the CO2 level reached in the room which induced the lighting of LED on the CO2 detector. During this week, average CO2 values of 820 and of 682 ppm were recorded for S1 and S2 respectively limiting at the same time the corresponding maximum CO2 concentrations equal to 2211 and 1805 ppm.

Intermediate average levels of CO2 were finally monitored during the week where occupants complied with ventilation instructions given by the French Indoor Air Quality Observatory: 1066 ppm (S1) and 756 ppm (S2). The corresponding maximum concentrations were then 3752 and 2974 ppm which are close to those recorded during the week 3.

As expected, our recorded CO2 levels were higher in the occupation period than those detected in the previous weeks in absence of people. Our CO2 concentrations were often above 1000 ppm when children were present confirming the results of many studies in classrooms reporting CO2 levels between 500 and 4000 ppm and most frequently above 3000 ppm, especially in naturally ventilated schools, demonstrating inadequate ventilation for majority of schools [START_REF] Griffiths | Control of CO2 in a naturally ventilated classroom[END_REF][START_REF] Turanjanin | Indoor CO2 measurements in Serbian schools and ventilation rate calculation[END_REF][START_REF] Bari | Source apportionment of indoor and outdoor volatile organic compounds at homes in Edmonton, Canada[END_REF].

In S3, average CO2 level was equal to 459 ppm, significantly lower than those found in primary schools during the occupation periods. In this school, the maximum CO2 concentration recorded was 1406 ppm.

BTEX monitoring in both schools

General results

Once all instruments installed and calibrated, indoor air sampling began for 5 consecutive weeks and instruments worked continuously, except when they were stopped for calibration or moved to another school as in the case of the pre-industrial micro device 2 (moved from S2 to S3 during the fourth week). Data collected were analysed daily since our novel BTEX analyser operates in near real-time with a time resolution of 10 minutes. The BTEX concentrations ranges obtained during the whole campaign are given in Table 4. Results obtained in S3 are not presented in the table because none of BTEX were detected in the chosen classroom of the kindergarten.

Empty room

The results reported in Table 4 show the absence of BTEX in indoor air in both schools S1 and S2 during the first week where classrooms were empty, except traces of toluene in S1 lower than 1.6 ppb. This fact demonstrates that building materials in both schools do not emit any benzene, ethylbenzene and xylenes. Very low emissions of toluene from building materials were only recorded in S1.

Addition of furniture

When the furniture was added into the classroom at the beginning of the second week, the BTEX concentrations were again below their corresponding detection limits in S2 for the whole week and in S1 at the very beginning (see Table 4). Later on, in School 1, benzene and ethylbenzene were all the time absent or below the detection limit; however, higher xylenes levels were recorded at the same specific period, as an unplanned repainting of the ramp of the stairs leading to the floor where the classroom was located. Concentrations of xylenes suddenly increased up to 59.5 and 41.4 ppb for m/pxylenes and o-xylene respectively, and then slowly decreased from 59.5 ppb on February 26 th at 11 a.m. to 27.5 ppb on February 27 th and down to 15.6 ppb on February 28 th . Finally, on February 29 th at 6 a.m. the levels of xylenes were below the detection limit of the instrument or equal to zero. This may be related to the fact that at 6 AM, the cleaning of the classroom was usually performed, maintaining the windows opened and renewing the air inside the classroom. Toluene levels ranged between 0-3.2 ppb. The latter were recorded in the same period as xylenes and they followed the same decreasing trend from February 26 th until February 29 th, suggesting that the toluene may also be originated by the use of paints near the classroom. To support this observation, correlation analysis was performed between toluene and o-xylene concentration versus m,p-xylenes concentration. Correlation coefficients ranging between 0.62 and 0.97 indicate that all of them were emitted by the same source, i.e. paints application in this case. Moreover, the increase of VOC concentration due to the use of paints has also be reported in other studies [START_REF] Srivastava | Volatile organic compounds in indoor environments in Mumbai, India[END_REF]. To evaluate if the toluene concentrations before and after the painting were statistically different, p values using standard student tests were calculated between the first week and the three groups of measurements conducted the second week. As expected, for the series of measurements performed in week 2 before the painting, there was no statistical difference with the concentrations measured during the first week (p = 0.866). On the contrary, p values calculated for the two next series of measurements obtained after the painting were lower than 0.0001 indicating that the painting strongly influenced toluene concentration in the investigated classroom. In conclusion, the furniture equipping the classroom do not constitute a significant emission source of BTEX in S2 whereas for S1 no certain conclusions could be made about the furniture emission since results were distorted by the unplanned painting. However, the absence of BTEX in S1 just after the installation of furniture, at the very beginning of week 2, can considered as an indication of no significant emission from furniture.

Presence of occupants

The results obtained during school occupation by students and teachers showed considerable variations of BTEX levels. During the third week when children followed the usual ventilation procedure, most of BTEX were detected at different levels in both schools, except benzene in S2 and ethylbenzene in S1. If we do not consider xylenes concentrations recorded in S1 during the second week due to the paint emission, the values recorded during this third week were always higher than those found for weeks 1 and 2. During the fourth week in S1, corresponding to ventilation according to the CO2 level, benzene and toluene were detected in the ranges 0-12.1 and 0-5.5 ppb respectively although other BTEX were not present. All xylenes emitted during the paint episode were not detected from week 4 indicating the corresponding emissions were significantly reduced. Note that no measurements were conducted in S2 during week 4 because the instrument was moved to S3, the choice to move this instrument instead of the other installed in S1 being motivated by the presence of hazardous benzene level in S1. During the fifth week in S1, corresponding to ventilation instructions given the French Indoor Air Quality Observatory, only benzene and toluene were found to be present similarly to week 4 and varied in the ranges 0-5.7 and 0-17.6, respectively. In S2 where they followed ventilation according to the CO2 level, the BTEX concentrations were slightly lower than those found during week 3 suggesting that this ventilation mode could be more efficient. Contrary to the results obtained by [START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF] showing no significant difference in BTEX levels between absence and presence of occupants in schools, the BTEX concentrations measured in this work were higher when the students and the teachers were present in both schools, except for all xylenes isomers in S1, whose elevated levels during week 2 were related to the fresh painting of the hallway nearby. For both schools S1 and S2, Student tests were conducted between the BTEX concentrations measured in weeks 3 4 and 5, where different ventilation scenarios were applied, week 3 being considered as the reference since the standard ventilation mode was used. The results will be presented below.

Benzene concentrations time profiles

During this field campaign, benzene was only present in S1 which is close to the airport of La Rochelle and near to a busy road which may lead to high VOCs concentrations in the area as suggested in other studies [START_REF] Madhoun | Levels of benzene concentrations emitted from motor vehicles in various sites in Nibong Tebal, Malaysia[END_REF][START_REF] Rad | Levels and sources of BTEX in ambient air of Ahvaz metropolitan city[END_REF]Liu et al. 2016b). During the first and the second week when people were absent, benzene concentrations in both schools were zero or below the detection limit of our instruments (see Table 4) indicating that building construction materials and school furniture do not constitute an emission source of benzene. During the three weeks of children occupation, benzene was not detected in S2. These results are in agreement with those encountered in other studies of air quality in schools, where benzene concentrations varied between 0.06 and 1 ppb (see Table 1), levels that are below the detection limit of our devices. In School 1, on the contrary to S2, benzene was always present, but its concentration was not constant and varied between 0 and 12.1 ppb. Most of the time benzene concentration was below our detection limit of 1.1 ppb and then considered equal to zero but sometimes exhibited some maximum peaks in specific periods. In S1, benzene concentrations during week 5 were significantly different (p < 0.001) when compared to those of week 3 although this was not the case when compared data of weeks 3 and 4 (p = 0.24). For example, Fig. 4 highlights benzene and CO2 concentration time profiles inside the studied classroom in S1 during the fifth week. In March 15 th , a benzene peak up to almost 5 ppb can be clearly identified during the night. Other scattered values up to 6 ppb were recorded in March 16 th and 17 th around 12:00. It should be underlined that these upper values are higher than the threshold of 1.6 ppb set as an annual average value by the European Union. Comparison of benzene concentrations and CO2 levels shows that they are not correlated. Indeed, the main peak of benzene concentration was recorded in March 15 th when CO2 level decreased indicating the window opening. Benzene started to disappear when CO2 concentration started to increase once the windows closed as shown in Fig. 4. This observation indicates that the main emission source of benzene was not related to the students' activities, but was more probably due to the road resurfacing work occurred in close proximity of the School 1 in the same period. Despite the windows closed during the night, an unusual event of great pollution influenced significantly the indoor air quality of the classroom. Nevertheless, the other scattered concentrations recorded in March 16 th and 17 th around 15:00 were correlated to CO2 peaks indicating that this compound may be emitted inside the classroom by a noncontinuous source such as related to students activities.

Toluene concentrations time profiles

During the first two weeks, toluene was detected only in S1 and its concentrations ranged from 0 to 3.2 ppb showing that the building materials and maybe the furniture constitute an indoor emission source of toluene in this school. Both schools have similar characteristics except for the presence of a whiteboard in S1. This may constitute a source of toluene since markers release toluene as it was demonstrated in other studies (Liu et al., 2016;[START_REF] Madureira | Indoor air quality in schools and its relationship with children's respiratory symptoms[END_REF][START_REF] Pegas | Outdoor/indoor air quality in primary schools in Lisbon: a preliminary study[END_REF]. The ceiling was also different in both schools but the ceiling plate of S1 is not known to be a source of toluene. When occupants were present (weeks 3 to 5), toluene was present in both schools, unlike benzene, and its concentrations varied in the ranges 0-29.5 and 0-3.5 ppb in Schools 1 and 2, respectively. In S1, contrary to benzene, toluene concentration were found to be significantly different in week 4 where ventilation was done according to the CO2 level (p < 0.001) when compared to those of week 3 (standard ventilation) whereas this was not the case when compared weeks 3 and 5 (p = 0.77). In S2, the difference was also significant (p < 0.001) between week 5 (ventilation based on CO2 level) and week 3 (standard ventilation). Concentrations time profiles of toluene and CO2 are presented in Fig. 5 for measurements performed in S1 during the fifth week (fig. 5a) and in S2 during the third week (fig. 5b). In S1, toluene concentrations ranged from 0 to 17.6 ppb during the fifth week (fig. 5a). Similarly to benzene during the same week discussed above, most of the time toluene concentration were below our detection limit of 1.1 ppb and then considered equal to zero but sometimes exhibited some maximum peaks, more or less at the same time (around noon) every day from Monday 14 th to Friday 18 th . Fig. 5 shows that benzene and CO2 concentration time profiles inside the studied classroom in S1 during the fifth week were strongly correlated. Indeed, the toluene concentration was low (2 to 4 ppb) or equal to zero when CO2 levels were around 1000 ppm. Higher concentrations of toluene were recorded (8 -17.6 ppb) when the levels of CO2 were higher than 1500 ppm. The decrease of CO2 level from 3500 ppm to less than 1000 ppm indicates that air inside the classroom was renewed either by opening windows and/or doors. Interestingly, the same observation was noted in School 2 on March 1 st and 4 th (see Figure 5b). This obvious correlation tends to prove that toluene emission is related to students' activities such as arts and craft or the use of glue and paints as it was noticed in other previous studies [START_REF] Pegas | Outdoor/indoor air quality in primary schools in Lisbon: a preliminary study[END_REF][START_REF] Madureira | Indoor air quality in schools and its relationship with children's respiratory symptoms[END_REF]Liu et al. 2016a). In order to confirm if the variation of toluene levels is related to students' activities, the measured concentrations during the occupation condition in S1 i.e. the third, fourth and fifth weeks were overlapped and presented in Figure 6. This figure shows clearly that concentration time profiles of toluene over three weeks present a similar behavior from one week to another with maximum concentrations always recorded between 11:30 and 12:00. Such observation reinforces the idea that toluene emission is directly related to students' activities. Furthermore, in both schools, toluene levels were lower during the week where the CO2 indicator was used, following the same trend as CO2 concentration. As it was discussed above, the use of this device may limit the confinement of the classroom and may be also a way to reduce the presence of pollutants issued from an internal source.

Ethylbenzene and xylenes concentrations time profiles

In school 1, ethyl benzene was never detected. Except during and after the paint event (weeks 2 and 3), xylenes were also absent in S1. Indeed, once the painting was finished and classroom was ventilated, their concentrations decrease to reach zero in the beginning of the third week and they were totally absent in the last two weeks. In S2, the presence of ethylbenzene and xylenes was limited to few days. Their concentrations varied between 0 and 10 ppb and from 0 to 4 ppb for xylenes and ethylbenzene, respectively. In S2, for ethyl benzene and m,p-xylenes the difference was significant (p < 0.001, Student test) between week 5 (ventilation according to CO2 level) and week 3, used as reference (standard ventilation). Fig. 7 represents the variations of ethylbenzene and xylenes concentrations obtained in School 2 during the third week i.e. 29 th of February to 5 th of March. Figure 7 highlights that both compounds were rarely detected in the investigated classroom and that increase of their concentrations were not correlated to high CO2 levels. Indeed, Fig. 7a shows an increasing of ethylbenzene concentration from 2 to 4.5 ppb when CO2 level decreases from 3500 to 1000 ppm and then, its concentration decreases from its maximum to reach its initial value around 1.5 ppb at the same time the CO2 level increases from 1000 ppm to 2500 ppm. Similar behavior was observed with m/p-xylenes where its concentration increases from 2 to 10 ppb when CO2 level decreases from 2700 to 1000 ppm, and then, the m/p-xylenes concentration decreases from its maximum to reach 1.5 ppb when CO2 level increases and reaches 2500 ppm as shown in Fig. 7b. These two concentrations time profiles indicate clearly that ethylbenzene and xylenes were only presented when CO2 levels decrease which is related to air exchange inside the classroom by opening windows and/or doors. This behavior suggests that the main emission source is located outside the investigated classroom in S2 and more probably outdoor.

Comparison with literature data

Benzene

Indoor benzene maximum level of 12.7 ppb measured in this study, is higher than those reported in most of the previous studies of air quality in schools (see Table 1). As mentioned in paragraph 3.2.2, outdoor air pollution can influence significantly the indoor air quality of the classroom. It was the case in School 1 in March 15 th where benzene peak up to almost 5 ppb was recorded during the night. This observation is consistent with other studies where benzene concentration in indoor air depends strongly of outdoor concentration [START_REF] Godoi | Healthy environmentindoor air quality of Brazilian elementary schools nearby petrochemical industry[END_REF][START_REF] Guo | Source characterization of BTEX in indoor microenvironments in Hong Kong[END_REF]. High levels of benzene were measured in a recent study carried out in 8 well ventilated high schools in Johor Bahru, Malaysia [START_REF] Norbäck | Volatile organic compounds (VOC), formaldehyde and nitrogen dioxide (NO2) in schools in Johor Bahru, Malaysia: Associations with rhinitis, ocular, throat and dermal symptoms, headache and fatigue[END_REF]. In this study, where the windows of the studied classrooms were always opened, the average concentration of benzene found in indoor air was 2.3 ppb, the same concentration was measured in outdoor air, and the maximum measured indoor was 9.9 ppb. The I/O ratio of benzene was equal to 0.93, indicating that the main benzene source was the external environment as suggested in our present study. Similarly to schools, the ratio I/O for benzene was often below to 1 for other indoor environments indicating that the main source is frequently the outdoor environment. Consequently, indoor benzene concentrations followed similar trends as the outdoor levels. This observation can be noticed in several recent studies [START_REF] Bari | Source apportionment of indoor and outdoor volatile organic compounds at homes in Edmonton, Canada[END_REF][START_REF] Madureira | Source apportionment of CO2, PM10 and VOCs levels and health risk assessment in naturally ventilated primary schools in Porto, Portugal[END_REF][START_REF] Romagnoli | Indoor air quality at life and work environments in Rome, Italy[END_REF][START_REF] Hazrati | Preliminary assessment of BTEX concentrations in indoor air of residential buildings and atmospheric ambient air in Ardabil, Iran[END_REF][START_REF] Yurdakul | Spatial variation of VOCs and inorganic pollutants in a university building[END_REF][START_REF] Zhong | Volatile Organic Compounds (VOCs) in Conventional and High Performance School Buildings in the U[END_REF][START_REF] Marzocca | Indoor Air Quality Assessment and Study of Different VOC Contributions within a School in Taranto City, South of Italy[END_REF].

Toluene

If we do not consider the high concentrations recorded during the second week in S1 and generated by the fresh paint applied to the steps ramp, toluene was the major benzene derivative found in the investigated classroom of school 1. This is consistent with most of school measurements reported in the literature (see Table 1) [START_REF] Guo | Source characterization of BTEX in indoor microenvironments in Hong Kong[END_REF][START_REF] Godwin | Indoor air quality in Michigan schools[END_REF][START_REF] Stranger | Comparative overview of indoor air quality in Antwerp, Belgium[END_REF][START_REF] Phatrabuddha | Ambient and personal exposure levels of BTEX among school children residing near the oil refineries in Chonburi Province[END_REF][START_REF] Godoi | Healthy environmentindoor air quality of Brazilian elementary schools nearby petrochemical industry[END_REF]Nasreddine et al. 2016b;[START_REF] Romagnoli | Indoor air quality at life and work environments in Rome, Italy[END_REF][START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF][START_REF] Yurdakul | Spatial variation of VOCs and inorganic pollutants in a university building[END_REF][START_REF] Zhong | Volatile Organic Compounds (VOCs) in Conventional and High Performance School Buildings in the U[END_REF] and with results obtained in other type of indoor environments such as residential buildings [START_REF] Bari | Source apportionment of indoor and outdoor volatile organic compounds at homes in Edmonton, Canada[END_REF][START_REF] Romagnoli | Indoor air quality at life and work environments in Rome, Italy[END_REF][START_REF] Hazrati | Preliminary assessment of BTEX concentrations in indoor air of residential buildings and atmospheric ambient air in Ardabil, Iran[END_REF].

Similarly to benzene, indoor toluene maximum level of 29.5 ppb measured in this study, is higher than those reported in most of the previous studies of air quality in schools (see Table 1). However, toluene concentration was lower than the values ranging between 0 and 106.3 ppb found in other indoor environments such as universities [START_REF] Can | Indoor air quality assessment in painting and printmaking department of a fine arts faculty building[END_REF][START_REF] Yurdakul | Spatial variation of VOCs and inorganic pollutants in a university building[END_REF], or urban and rural dwellings [START_REF] Liu | Serious BTEX pollution in rural area of the North China Plain during winter season[END_REF][START_REF] Bari | Source apportionment of indoor and outdoor volatile organic compounds at homes in Edmonton, Canada[END_REF]. Most of studies indicate that main sources of toluene are indoor ones [START_REF] Ilgen | Aromatic hydrocarbons in the atmospheric environment: Part I. Indoor versus outdoor sources, the influence of traffic[END_REF][START_REF] Kotzias | Exposure to Multiple Air Contaminants in Public Buildings, Schools and Kindergartens -The European Indoor Air Monitoring and Exposure Assessment (AIRMEX) Study[END_REF]. For example, [START_REF] Can | Indoor air quality assessment in painting and printmaking department of a fine arts faculty building[END_REF] reported toluene concentrations close to 80 ppb in the Faculty of Fine Arts of Anadolu University due to the use of oil-based paintings, varnish and adhesives related to art activities. As shown in paragraph 3.2.3, toluene emission was found to be related to people's activities such as arts and craft or the use of glue or paints as it was noticed in other previous studies [START_REF] Pegas | Outdoor/indoor air quality in primary schools in Lisbon: a preliminary study[END_REF][START_REF] Madureira | Indoor air quality in schools and its relationship with children's respiratory symptoms[END_REF]Liu et al. 2016a). In addition, in dwellings, toluene can be present at high concentrations varying between 0.10 and 99.79 ppb but the sources are typically household products, combustion processes or wall coverings [START_REF] Bari | Source apportionment of indoor and outdoor volatile organic compounds at homes in Edmonton, Canada[END_REF].

Ethyl benzene and xylenes

For ethylbenzene and xylenes, our results are consistent with those obtained by previous studies reported in Table 1, excepting those found for xylenes in several studies ( [START_REF] Madureira | Source apportionment of CO2, PM10 and VOCs levels and health risk assessment in naturally ventilated primary schools in Porto, Portugal[END_REF][START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF][START_REF] Norbäck | Volatile organic compounds (VOC), formaldehyde and nitrogen dioxide (NO2) in schools in Johor Bahru, Malaysia: Associations with rhinitis, ocular, throat and dermal symptoms, headache and fatigue[END_REF]). Norback et al. ( 2017) have reported very high levels of xylenes, reaching 118 ppb for the three isomers which may be explained by the infiltration of outdoor air. In fact, during the study the windows of the investigated classroom were always opened, in addition to the presence of high levels of xylenes outdoors. Results for m/p-xylenes concentrations found by [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF] and [START_REF] Madureira | Source apportionment of CO2, PM10 and VOCs levels and health risk assessment in naturally ventilated primary schools in Porto, Portugal[END_REF]) ranged between 0.27-82.56 and 0-95.98, respectively, did not agree with our results. This difference can be due to the presence of additional internal sources of xylenes, since an I/O ratio of 1.25 and 1.8, respectively by Madureira and Canha was determined.

Conclusion

In this study, our pre-industrial portable devices were able to detect all BTEX compounds in both primary school classrooms. The devices provided measurements at the same temporal variations of 10 minutes, while working continuously for five consecutive weeks. The simultaneous continuous monitoring of CO2 level was found to be very useful to indicate if the windows were open or closed in presence of students. The comparison between CO2 and BTEX concentrations permits us to highlight correlation or anti-correlation which was proven very helpful to identify the origin of their main emission source whether it was internal or external.

All BTEX were present in the investigated classrooms at different levels excepting in the kindergarten where all BTEX levels were lower than the detection limit of the device. Benzene was only present in S1 where its concentration reached up 6 ppb which is almost four times higher than the threshold value fixed by the European Union. The comparison with the CO2 measurements showed that its main emission source is the outdoor environment. The same conclusion was obtained for ethylbenzene and xylenes which were rarely present in both schools and their concentrations were limited to few tens of ppb. However, toluene was the most abundant compound in both schools and its concentrations were very variable. The higher measured toluene concentration was 30 ppb. In addition, its comparison with CO2 levels measured in both schools has shown a clear correlation indicating that is was emitted inside the classroom. More precisely, our real-time measurements contrary to traditional passive sampling on Radiello cartridges allowed us to observe the repetitive patterns in toluene temporal variation related to students' activities.

This study showed that naturally ventilated schools present high confinement index as shown the CO2 measurements where its concentration reached 3500 ppm when windows and doors are closed. This high value of CO2 could have side effects on occupants' health. During the occupation period over 3 weeks, CO2 concentration was lower in both schools during the week where the CO2 indicator was used, indicating that this device may be an efficient method to limit the confinement of the classroom. In addition, the external environment of the school presents a significant effect on indoor air quality. On the one hand, opening windows or doors could dramatically decrease the CO2 level improving afterwards the air quality by reducing the confinement. On the other hand, the outdoor air may constitute an additional source of pollutants as is the case for benzene, ethylbenzene and xylenes leading to degrade further the indoor air quality. This field campaign showed the significance of a portable device able to provide in-situ answers in nearreal time. Our devices were able to detect the BTEX temporal variations in two schools in different occupation conditions and during the presence of students and teachers. We succeed to detect every single variation and to determine how the BTEX concentrations varied during school days as well as to identify exactly when higher concentrations of BTEX are emitted, which allow us to determine the origin of their sources. This result could not be obtained with classical methods because of the sampling duration. In the case of schools occupied by children, the comparison and correlations with CO2 realtime monitoring were found to be an alternative solution to the widely used quantification of indoor/outdoor air ratio for a given pollutant, in order to determine its origin, i.e. indoor or outdoor one.

Acknowledgments

This work has been conducted in the framework of IMPACT'AIR project financially supported by ADEME (Agence De l'Environnement et de la Maîtrise de l'Energie), the city of La Rochelle and the Ligue contre le cancer.


Irene Lara-Ibeas

Cristina Megías-Sayago

Benoît Louis

Stéphane Le Calvé
email: slecalve@unistra.fr

ADSORPTIVE REMOVAL OF GASEOUS FORMALDEHYDE AT REALISTIC CONCENTRATIONS

Keywords: formaldehyde, adsorption, zeolites, indoor air, gas analysis, realistic concentrations

Due to analytical reasons, adsorption capacity is often determined at very high concentrations which is not representative of a real environment. The aim of this work is to provide experimental data of formaldehyde adsorption capacity at realistic levels (~ 164 ppb) thanks to a near real-time powerful formaldehyde analyser recently developed in our laboratory. Gaseous formaldehyde breakthrough experiments were conducted on a broad spectrum of materials including carbon, zeolites, metal organic framework and mesoporous silica in order to determine their adsorption capacity. Among these materials, HKUST-1 exhibited the highest adsorption capacity (504 µg /g ads) followed by MgZSM-5 (35 µg /g ads), SBA-16 (29 µg /g ads), ZSM-5 (10 -26 µg /g ads) and Carbopack® B (4 µg /g ads).

In the case of ZSM-5 zeolites, the results revealed that formaldehyde adsorption was related to their aluminium content. The presence of aluminium implies more bridging hydroxyl groups Si-(OH)-Al that act as Brønsted acid sites interacting with formaldehyde molecules. Despite the moderate adsorption capacity of ZSM-5 zeolites, they can be considered as promising candidates for gas analysis applications, for example where a good analytical blank is essential for accurate measurements.

Introduction

Formaldehyde is one of the most common toxic pollutants found in indoor air [START_REF] Salthammer | Formaldehyde in the Indoor Environment[END_REF]. In closed environments, a broad spectrum of sources has been identified as responsible for its emission as candles, paints or wood-based materials as well as sources related to the occupants' activity like smoking or cooking [START_REF] Salthammer | Formaldehyde sources, formaldehyde concentrations and air exchange rates in European housings[END_REF]. These sources usually lead to indoor formaldehyde concentrations from 2 to 15 times higher than those measured outdoors [START_REF] Wang | Characteristics of carbonyls: Concentrations and source strengths for indoor and outdoor residential microenvironments in China[END_REF]. Formaldehyde has adverse effects on human health, being considered carcinogenic (Group 1) by IARC since 2006. Short-term exposure can provoke throat, nose and eye irritation and headache whereas the long-term exposure effects are more damaging with the appearance of pneumonia, asthma and nasopharyngeal cancer [START_REF] Hauptmann | Mortality from Solid Cancers among Workers in Formaldehyde Industries[END_REF][START_REF] Suresh | Removal of formaldehyde on carbon -based materials: A review of the recent approaches and findings[END_REF][START_REF] Marsh | Work in the metal industry and nasopharyngeal cancer mortality among formaldehyde-exposed workers[END_REF]. Consequently, new legislations concerning formaldehyde have been developed in several countries in order to limit human exposure and protect public health. In the European Union, a short-term guideline of 100 μg•m -3 formaldehyde concentration is recommended which should not be exceeded at any 30 min interval during a day [START_REF] Europe | Air Quality Guidelines for Europe[END_REF]. Nevertheless, in some countries, as is the case of France, the legislation concerning indoor air quality in public buildings became stricter. According to the Decree no. 2011-1727 of December 2011, a limit value of 30 μg•m -3 was fixed for formaldehyde long-term exposure from 2015, value that will be decreased to 10 μg m -3 from 2023 [START_REF]1727 du 2 décembre 2011 relatif aux valeurs-guides pour l'air intérieur pour le formaldéhyde et le benzène[END_REF]. Motivated by this stringent regulation, the development of both sensitive analytical devices able to monitor formaldehyde as well as effective low cost techniques to remove it even at low concentrations have attracted great interest in the last years [START_REF] Suresh | Removal of formaldehyde on carbon -based materials: A review of the recent approaches and findings[END_REF][START_REF] Guglielmino | Development of microfluidic analytical method for on-line gaseous Formaldehyde detection[END_REF].

Adsorptive methods are usually employed for both gaseous pollutants removal and sample preconcentration in gas analysis. However, common adsorbents are often not effective for formaldehyde adsorption due to the high polarity and small size of this molecule. In this regard, consequent efforts have been made to investigate formaldehyde adsorption properties over a wide variety of commercially available and synthesized materials [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF][START_REF] Carter | Gas-Phase Formaldehyde Adsorption Isotherm Studies on Activated Carbon: Correlations of Adsorption Capacity to Surface Functional Group Density[END_REF][START_REF] Kalantarifard | Formaldehyde Adsorption into Clinoptilolite Zeolite Modified with the Addition of Rich Materials and Desorption Performance Using Microwave Heating[END_REF][START_REF] Nomura | Amine-Functionalized Porous Silicas as Adsorbents for Aldehyde Abatement[END_REF][START_REF] Wang | Diamine-appended metal-organic frameworks: enhanced formaldehyde-vapor adsorption capacity, superior recyclability and water resistibility[END_REF] (summarized in Table 1). Nevertheless, most of these works were performed at rather unrealistic formaldehyde levels (1 to 150 ppm) [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF][START_REF] Carter | Gas-Phase Formaldehyde Adsorption Isotherm Studies on Activated Carbon: Correlations of Adsorption Capacity to Surface Functional Group Density[END_REF][START_REF] Nomura | Amine-Functionalized Porous Silicas as Adsorbents for Aldehyde Abatement[END_REF][START_REF] Wang | Diamine-appended metal-organic frameworks: enhanced formaldehyde-vapor adsorption capacity, superior recyclability and water resistibility[END_REF], thus rendering those experimental results not fully representative of the performances in real environments. Indeed, the adsorption capacity depends on the gas partial pressure, this relationship being not linear in all the partial pressure range. According to Henry's law [START_REF] Kapoor | Correlation of equilibrium adsorption data of condensible vapours on porous adsorbents[END_REF], it can be assumed that the adsorption capacity increases linearly with the partial pressure at very low concentrations, however, previous studies carried out over different materials have demonstrated that formaldehyde adsorption follows a non-linear tendency [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF][START_REF] Carter | Gas-Phase Formaldehyde Adsorption Isotherm Studies on Activated Carbon: Correlations of Adsorption Capacity to Surface Functional Group Density[END_REF]. Carter et al. [START_REF] Carter | Gas-Phase Formaldehyde Adsorption Isotherm Studies on Activated Carbon: Correlations of Adsorption Capacity to Surface Functional Group Density[END_REF] studied formaldehyde adsorption in the ppm range over different activated carbonaceous materials. In the high-pressure region (concentration ~ 25 ppm), ACF (activated carbon fibers) exhibited the highest adsorption capacity followed by GAC1 (all-purpose granular AC) and GACF (commercial granular AC for HCHO gas-phase removal). However, a completely different behaviour was observed at lower concentrations (~ 3.5 ppm) where the highest adsorption capacity was ascribed to GACF followed by ACF and GAC1. Similar results were reported by Bellat et al. [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF] over AC (activated carbon), MIL-53 (metal organic framework) and SBA-15 (mesoporous silica). In this case, AC showed poor adsorption capacity compared to MIL-53 at formaldehyde partial pressure of 2 hPa (~ 2000 ppm). However, almost identical adsorption capacities were registered when formaldehyde partial pressure approached more realistic levels. These results reveal the importance of using concentrations as close as possible to the real ones to accurately assess the adsorption performance for real-world applications. In this regard, there is an obvious lack of experimental data in the literature on formaldehyde adsorption at ppb levels, which are necessary to properly evaluate the materials for either pollutant removal or potential gas analysis applications. Therefore, the objective of this study is to provide accurate information on the adsorption performance of a series of materials at realistic formaldehyde concentrations.

In this work, formaldehyde adsorption was investigated over a series of porous materials at ppb levels. On the one hand, three commercial adsorbents differing in their textural properties and chemical composition were evaluated, named Carbopack® B, HKUST-1 and SBA-16, respectively. On the other hand, three synthetic H-ZSM-5 zeolites with Si/Al molar ratios (SAR) of 198, 132 and 79 were assessed which additionally allow evaluating the influence of the aluminium content on the adsorption process.

Finally, commercially available CBV3020E zeolite (Zeolyst, SAR = 15) was exchanged with Mg and both materials were tested in formaldehyde adsorption. The obtained experimental data were compared with those found in the literature.

Materials and methods

Commercial materials and synthesis

Commercially available Carbopack® B (60-80 mesh, SUPELCO), SBA-16 (ACS Materials),

HKUST-1 (Basolite® C300, Sigma-Aldrich) and ZSM-5 zeolite (CBV3020E, Zeolyst) were used as received for adsorption tests.

Fluoride-mediated ZSM-5 synthesis was adapted from previous studies [START_REF] Bleken | Conversion of methanol into light olefins over ZSM-5 zeolite: Strategy to enhance propene selectivity[END_REF][START_REF] Losch | H-ZSM-5 zeolite model crystals: Structure-diffusion-activity relationship in methanol-to-olefins catalysis[END_REF]. In a typical preparation, the aluminium source (different amounts to achieve 200, 150 and 100 Si/Al mole ratios, respectively) was placed in a 150 mL-Erlenmeyer flask where 50 mL of distilled water were added under vigorous stirring (700 rpm, room temperature). Subsequently, tetrapropylammonium bromide (TPABr) (0.498 g) and NH4F (1.103 g) were added. Finally, 1.607 g of SiO2 (Aeroperl 300/30, Evonik) was slowly added during 5 min. The as-prepared gel was stirred during 2h and finally autoclaved at 443 K for 144 h. The acidic form was obtained after calcination at 823 K during 15 h. The samples were labeled as ZSM_198, ZSM_132 and ZSM_79, respectively; the number indicates the experimentally determined SAR.

MgZSM-5 zeolite was synthetized from commercial H-ZSM-5 zeolite (CBV3020E) following a procedure adapted from [START_REF] Maia | Ni-ZSM-5 catalysts: Detailed characterization of metal sites for proper catalyst design[END_REF][START_REF] Kazansky | A new charge alternating model of localization of bivalent cations in high silica zeolites with distantly placed aluminum atoms in the framework[END_REF]. To this purpose, 2 g of commercial ZSM-5 zeolite was dispersed in 100 mL of distilled water and stirred vigorously. Then, the appropriate amount of Mg(NO3)2 (4 wt.% nominal value) was added and the mixture stirred at 70 °C for 1 h. Afterwards, the solution was dried in a rotavapor at 50 °C under vacuum until complete evaporation of water. The solid was then dried at 110 °C overnight and calcined at 550 °C for 5 h.

Materials characterization

X-ray diffraction patterns were recorded on a Bruker D8 Advance diffractometer, with a Ni detector side filtered Cu Kα radiation (1.5406 Å) over a 2θ range of 5-60°.

Scanning electron microscopy (SEM) images were acquired in a ZEISS GEMINI SEM 500 microscope using an electron high tension (EHT) voltage ranging from 2 to 6 kV. EDX and mapping analyses were performed in the same microscope by using an EDAX SDD detector.

Nitrogen adsorption isotherms for all the adsorbents previously presented were recorded at 77 K using a Micromeritics ASAP 2420 equipment. The specific surface area and pore volume were calculated using the Brunauer-Emmett-Teller (BET) method. Prior to analysis, SBA-16, Carbopack® B and HKUST-1 samples were pre-treated in-situ at 180°C for 5 h under vacuum. Zeolites were pretreated at 250°C under vacuum during 10 h.

Zeolite chemical compositions were determined by X-ray fluorescence (XRF) using a XEPOS (AMETEK) device equipped with Rh radiation tube.

Temperature programmed ammonia desorption (NH3-TPD) was used to evaluate the zeolite acid site density. The experiments were carried out in a Micromeritics AutoChem II 2920 chemisorption analyzer equipped with a TCD detector. Samples were pre-treated in-situ at 550°C during 1 h. Once cooled down to room temperature, 5% NH3/He flow was passed through the reactor until saturation.

Physisorbed ammonia was removed by flowing an inert gas during a certain time, after which the temperature was gradually increased to 550°C.

Formaldehyde monitoring

Formaldehyde was on-line monitored using a novel device recently developed and marketed.

This instrument was already detailed in our previous studies [START_REF] Guglielmino | Development of microfluidic analytical method for on-line gaseous Formaldehyde detection[END_REF][START_REF] Guglielmino | On-line gaseous formaldehyde detection by a microfluidic analytical method based on simultaneous uptake and derivatization in a temperature controlled annular flow[END_REF][START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF] so that only a brief description is given. As shown in Figure S1, this analytical method is also based on three highly coupled steps: 1) the uptake of gaseous formaldehyde into an aqueous acetylacetone solution at room temperature through the obtention of an annular diphasic flow, gas and liquid flow rates being fixed at 20 Nml/min and 17 µL/min, respectively; 2) the chemical reaction between formaldehyde and acetylacetone via the Hantzsch mechanism [START_REF] Ulli | Chemistry of dihydropyridines[END_REF] enhanced at 65°C for 3.5 min; and 3) the on-line fluorescence detection of the reaction product, i.e. France) has a temporal resolution of 2 s, a response time of 10 min and a detection limit of 1 µg/m 3 (0.81 ppb).

Formaldehyde adsorption

The adsorption experiments were carried out in copper tubes (1/8-inch outer diameter) where each sample was packed and fixed separately by using two quartz wool plugs. These tubes were mounted in the experimental setup schematized in Figure S1. Gaseous formaldehyde (164 ppb) was produced in situ by using a home-made formaldehyde generator. The operating principle of the generator is based on permeation. In order to have the desired formaldehyde concentration, a first synthetic air flow at 10

NmL/min was passed first through a formaldehyde aqueous solution (0.0925 wt.%) maintained at 20°C through a Peltier module and then diluted by using a second synthetic air flow at 390 NmL/min. A resulting formaldehyde gaseous flow of 164 ppb was then generated [START_REF] Guglielmino | On-line gaseous formaldehyde detection by a microfluidic analytical method based on simultaneous uptake and derivatization in a temperature controlled annular flow[END_REF][START_REF] Becker | Development and Optimization of an Airborne Formaldehyde Microfluidic Analytical Device Based on Passive Uptake through a Microporous Tube[END_REF]. This generator of known gas concentrations of formaldehyde was previously calibrated by using the conventional sampling method on 2,4-dinitrophenylhydrazine (DNPH) tubes followed by HPLC/UV analysis [START_REF] Allou | Henry's law constant measurements for formaldehyde and benzaldehyde as a function of temperature and water composition[END_REF][START_REF] Marchand | Concentrations and determinants of gaseous aldehydes in 162 homes in Strasbourg (France)[END_REF]. The resulting overall relative uncertainty of the generated gaseous concentration was calculated to be in the range 9-11%.

During the dynamic adsorption experiments, a part of the gaseous formaldehyde mixture was pumped and injected into the tube containing the adsorbent at a flow rate of 15 NmL/min (mass flow controller 1, see Fig. 1). Since the formaldehyde analyser employed to monitor the gaseous effluent was very sensitive (Limit Of Detection, LOD, ~ 0.81 ppb), the tube outlet was diluted with 30 NmL/min of synthetic air (mass flow controller 2) prior to the analysis in order to be in the linear calibration range and avoid the detector saturation. Afterwards, the effluent gas was continuously sampled and analysed using a formaldehyde analyser.

Prior to each adsorption experiment, the generated gaseous flow was passed through the bypass, diluted and analysed. The obtained intensity corresponds to the initial concentration (C0) and, therefore, it was used as an indicator to determine the necessary time to reach the saturation, i.e. when the outlet concentration was equal to the inlet concentration (C = C0).

Results and discussion

Samples characterization

Both the commercial adsorbents and as-prepared zeolites were fully characterized in terms of structure, morphology, physicochemical properties, textural properties and acidity in our former studies [START_REF] Lara-Ibeas | Adsorbent screening for airborne BTEX analysis and removal[END_REF][START_REF] Megías-Sayago | Volatile organic compounds (VOCs) removal capacity of ZSM-5 zeolite adsorbents for near real-time BTEX detection[END_REF]. Consequently, these data are only briefly summarized in Table 2.

All as-prepared zeolites exhibit similar pore volume and BET surface thus enabling the solely evaluation of the aluminium content in HCHO adsorption. As expected, the acid site density decreases when SAR (Si/Al mole ratio) increases since a higher aluminium content implies more bridging hydroxyl groups Si-(OH)-Al that act as Brønsted acids. Commercial CBV3020E zeolite exhibits similar specific surface area and slightly higher pore volume which decrease when exchanged with Mg. As previously reported, a decrease in both the specific surface area and pore volume is observed when the zeolite charge compensation cation H + is replaced by a metal cation [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF]. This decrease can be due to the larger size of magnesium cation compared to the proton. Reasonably, the acid site density (BrØnsted acidity) also drops, as a result of the 2H + /Mg 2+ exchange. As CBV3020E and Mg/CBV3020E samples are reported here for the first time, the corresponding XRD and SEM / EDX analyses are presented in Figure 1. The diffraction patterns of both samples correspond to the sole presence of the MFI structure, characteristic of ZSM-5 zeolites. Neither peaks related to the presence of Mg precursor, nor MgO phase could be observed which somehow accounts for the successful Mg 2+ cationic exchange within the zeolite frame, thus excluding MgO presence as segregated phase. Magnesium is uniformly distributed along the zeolite (Fig. 1c) having an experimental metal loading of 3 wt.% determined by EDX.

Evaluation of formaldehyde adsorption capacity of different adsorbents

Dynamic adsorption experiments were performed to obtain the corresponding breakthrough curves. These breakthrough curves represent the evolution of the adsorbate concentration in the effluent leaving the adsorbent bed as a function of time. In air treatment, the breakthrough time is usually defined at the time in which 5% of the feed concentration (C0) is leaving the adsorbent bed and represents the usable capacity of the reactor. Additionally, breakthrough curves allow determining the total adsorption capacity of each material. This capacity can be calculated from each curve using the following equation:

a) b) c) 𝑞 = 𝑄 𝐶 0 m ∫ (1 - 𝐶 𝐶 0 ) 𝑑𝑡 𝑡 𝑠 𝑡 0 (1)
where 𝑞 is the dynamic adsorption capacity per gram of adsorbent, 𝑄 is the gas flow rate, 𝑚 is the mass of adsorbent, 𝑡 0 is the initial time, 𝑡 s is the saturation time, 𝐶 0 is the initial concentration, and 𝐶 is the outlet concentration at a given time.

The error on the calculation of the dynamic adsorption capacity has been estimated as follows:

∆𝑞 𝑞 = ∆𝑄 𝑄 + ∆𝐶 0 𝐶 0 + ∆𝑡 𝑡 + ∆𝑚 𝑚 (2)
where ∆𝑞 is the error on the adsorption capacity. ∆𝑄 is the error on the flow rate, ∆𝐶 0 is the error on the initial concentration and ∆𝑚 is the error on the adsorbent mass.

Mesoporous silica, graphitised carbon and MOF

The breakthrough curves obtained for SBA-16, Carbopack® B and HKUST-1 are presented in As observed in Figure 3a, Carbopack® B and SBA-16 exhibit comparable adsorption profiles while HKUST-1 demonstrates an outstanding adsorption performance despite the smaller adsorbent quantity used. However, Figure 3b provides a detailed view of the first 150 min of the experiment in which an appreciable difference can be observed between Carbopack® B and SBA-16 curves.

Carbopack® B shows immediate breakthrough and the outlet concentration sharply increases until the saturation point which is reached after 26 min. This behaviour might be related with the surface chemistry of the graphitised carbon black. Indeed, based on its hydrophobic character, the adsorbateadsorbent interactions should depend solely on London forces. For a small molecule such as formaldehyde, these forces are very weak thus the adsorption of this compound in Carbopack® B is almost negligible. In contrast, SBA-16 needs 34 min to reach the breakthrough most probably due to its higher specific surface area as well as more favourable adsorbate-adsorbent interactions. Formaldehyde molecules can interact with silanol groups (Si-OH) at the inner surface of SBA-16 mesopores via hydrogen bonding [START_REF] Nomura | Amine-Functionalized Porous Silicas as Adsorbents for Aldehyde Abatement[END_REF], thus promoting their adsorption. As a result, the adsorption capacity of SBA-16 is much higher than that obtained for Carbopack® B (Table ).

HKUST-1 showed the best adsorption performance among the investigated commercial adsorbents. Despite the HKUST-1 mass employed during the experiment remained the lowest, the breakthrough and saturation points were reached much later. This remarkable capacity can be explained by several factors. HKUST-1 has a very high specific surface area that provides a large amount of adsorption sites and pores with diameters of 5.4 and 6.9 Å suitable for formaldehyde adsorption (kinetic diameter = 2.5 Å). Additionally, as reported by Dutta et al. [START_REF] Dutta | Metal-organic framework and Tenax-TA as optimal sorbent mixture for concurrent GC-MS analysis of C1 to C5 carbonyl compounds[END_REF], a coordination bond between the copper atoms present in the MOF structure and the formaldehyde's carbonyl oxygen can be expected thus contributing to a better adsorption performance. Despite HKUST-1 exhibited an extraordinary adsorption capacity, a significant decrease in formaldehyde adsorption capacity might be expected in humid environments due to its hydrophilic character [START_REF] Zhao | Competitive adsorption and selectivity of benzene and water vapor on the microporous metal organic frameworks (HKUST-1)[END_REF][START_REF] Küsgens | Characterization of metal-organic frameworks by water adsorption[END_REF]. 2 5.5mg sample; 3 3 mg sample; 4 1.7 mg sample; 5 the quoted error is calculated according to the eq.( 2).

Zeolites

Formaldehyde dynamic adsorption experiments were carried out over three synthetic zeolites with different SAR (Figure 3). The shape of the obtained breakthrough curves is fairly similar in all cases. The adsorption of formaldehyde was complete during the first minutes, the breakthrough point being reached at 11, 12 and 21 min for ZSM-5_198, ZSM-5_132 and ZSM-5_79, respectively. Zeolites with higher SAR exhibited earlier breakthrough most probably due to the difficulty of formaldehyde molecules to find adsorption sites. By contrast, ZSM-5_79 almost doubling the acid sites showed a longer breakthrough. After the breakthrough point, two different slopes can be distinguished thus informing about a different kinetic behavior. Firstly, a sharp increase in the outlet concentration is observed which indicates fast kinetics followed by a continuous but slow increment of the concentration until the initial concentration is reached, suggesting that the adsorption kinetics close to the adsorbent saturation is much slower. Adsorption capacities were assessed by numerical integration of the area above the breakthrough curve and are also presented in Table . It can be concluded therefore that zeolites with higher acid density, i.e. lower Si/Al ratio, exhibit greater adsorption capacity. Based on the latter observation, it appears that the adsorption of formaldehyde molecules is directly related to the aluminium content and thus to the amount of BrØnsted acid sites. In order to illustrate this relationship, adsorption capacities of the investigated ZSM-5 zeolites including the commercial CBV3020E (Si/Al = 15) were plotted versus SAR (Figure 5). A linear correlation is evidenced suggesting that formaldehyde adsorption is directly related with Al concentration. These results are in line with earlier IR spectroscopic studies of formaldehyde adsorption on ZSM-5 zeolites in which it was demonstrated that formaldehyde interacts with Si-OH-Al groups by hydrogen bonding [START_REF] Kukulska-Zaja̧c | Transformations of Formaldehyde Molecules in Cu-ZSM-5 Zeolites[END_REF]. Therefore, the adsorption process seems to be ruled by the interactions between the formaldehyde's carbonyl function and the bridging Si-(OH)-Al zeolite groups. On the other hand, the reduction of the specific surface area observed when SAR decreases appears to have a negligible effect on formaldehyde adsorption. This observation reinforces the idea that formaldehyde adsorption is primarily dominated by surface chemistry. Therefore, it can be concluded that, at low concentrations, a more effective formaldehyde adsorption can be achieved by using zeolites with high aluminium content. In order to further evaluate the influence of surface chemistry on formaldehyde adsorption, breakthrough experiments were conducted over commercial CBV3020E and Mg/CBV3020E (Figure 6). Apparently, both curves are very similar however it should be taken into account that the employed masses for the tests were slightly different, 1.8 and 1.6 mg for CBV3020E and Mg/CBV3020E, respectively. Therefore, a slight difference between the curves could represent a great variation in the adsorption capacity. Breakthrough times obtained for pristine and metal loaded zeolite were 7 and 4 min, respectively. The shorter breakthrough time of Mg/CBV3020E could be attributed to both the lower mass of the sample and the lower specific surface area. Regarding kinetics, both zeolites showed similar behaviour until C/C0 reached ~ 0.5. From this point to the saturation, Mg/CBV3020E exhibited slower kinetics, presumably due to the diffusional limitations induced by the presence of large magnesium cations. A positive variation in the adsorption capacity (34%) is evidenced after Mg cationic exchange.

This enhancement in the capacity suggests that formaldehyde establishes stronger interactions with Mg 2+

than with H + , results that are in line with those reported by Bellat et al. [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF]. They observed that the presence of different cations such as Na + , K + or Cu 2+ in FAU-type zeolites led to an improvement in the formaldehyde adsorption capacity. As aforementioned for as-prepared zeolites, a decrease in the specific surface area does not significantly affect the adsorption ability, providing a compelling evidence of the importance of surface chemistry on formaldehyde adsorption.

Comparison between different adsorbents

Motivated by its occurrence in indoor air and the harmful effects that causes, formaldehyde adsorption has been studied over a wide variety of different materials such as MOF, zeolites, AC and mesoporous silica (see Table 1). As shown in Figure 7, most studies are conducted at formaldehyde concentrations from 3.65 to 150 ppm (corresponding to partial pressures from 0.015 to 13.7 Pa), thus being not representative of ambient conditions where formaldehyde is usually present at several tens or few hundreds of ppb. Moreover, comparing experimental data obtained at different partial pressures can result in an inaccurate evaluation of the adsorption performance. In order to minimize the possible errors, the Henry constant (𝐾 𝐻 ) is commonly employed for comparison purposes instead of adsorption capacity [START_REF] Ahmed | Performance comparison of MOF and other sorbent materials in removing key odorants emitted from pigpen slurry[END_REF][START_REF] Na | High-performance materials for effective sorptive removal of formaldehyde in air[END_REF][START_REF] Szulejko | Seeking the most powerful and practical real-world sorbents for gaseous benzene as a representative volatile organic compound based on performance metrics[END_REF][START_REF] Vellingiri | Metal-organic frameworks for the adsorption of gaseous toluene under ambient temperature and pressure[END_REF]. 𝐾 𝐻 is a relevant parameter used to explain the gas-solid adsorption behaviour [START_REF] Ahmed | Performance comparison of MOF and other sorbent materials in removing key odorants emitted from pigpen slurry[END_REF] that allows to evaluate the adsorption affinity of a material for a certain adsorbate. However, there are some limitations in the use of this constant. Henry's law establishes that the adsorbate should neither undergo any chemical reaction with the sorbent nor dissociation when adsorbed. This is not always the case in formaldehyde adsorption where chemisorption frequently takes place due to the presence of the carbonyl group. In other cases, polymerization reactions [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF] or self-catalytic oxidation-reduction reactions have been observed as a consequence of extremely low adsorption kinetics which can lead to several days to reach the equilibrium [START_REF] Nakayama | Adsorption of formaldehyde by polyamine-intercalated α-zirconium phosphate[END_REF][START_REF] Zhang | Studies on adsorption of formaldehyde in zirconium phosphateglyphosates[END_REF]. Besides, this parameter is considered valid only at relatively low pressures (< 100 Pa corresponding to ~1,094 ppm), where it is assumed that the adsorption capacity increases linearly with adsorbate partial pressure. Hence, only in this interval, Henry's law can be applied [START_REF] Szulejko | Seeking the most powerful and practical real-world sorbents for gaseous benzene as a representative volatile organic compound based on performance metrics[END_REF] and 𝐾 𝐻 (mol/kg Pa) estimated by plotting the adsorption capacity versus partial pressure according with the following equation:

𝐾 𝐻 = 𝑞 𝑃 𝑓 𝑀 𝑓 (3)
where q (kg/ kg adsorbent) is the adsorption capacity, 𝑃 𝑓 is the formaldehyde partial pressure (Pa) and 𝑀 𝑓 is the molecular weight of formaldehyde (kg/mol).

In most studies, Henry constant is not provided; it has therefore been calculated whenever possible from the published data available. In Table 1, experimental data are listed in decreasing order of Henry constant. The determination of 𝐾 𝐻 values also enables to draw some general conclusions about the most crucial factors impacting the formaldehyde adsorption. Similarly to the trend observed in zeolites, SSA (specific surface area) does not seem to be one of the most important factors on formaldehyde adsorption capacity as elucidated by a relatively low 𝐾 𝐻 of MIL-101 (Cr) (SSA = 2,367 m 2 g -1 ) and the high 𝐾 𝐻 obtained for S-AP (SSA = 422 m 2 g -1 ). Considering the pore size, it appears to be relevant when it is in the same order as formaldehyde molecule diameter (Dp = 0.25 nm) due to the steric hindrance that can be caused, as observed in zeolite 3A (Dp = 0.3 nm) [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF]. However, it does not appear to be significant when it is much larger, as confirmed by the different 𝐾 𝐻 values obtained for NaY (Dp = 0.74 nm, KH = 0.135 mol/kg Pa) and NaX (Dp = 0.74 nm, KH = 4.373 mol/kg Pa), thus confirming that surface chemistry i.e. adsorbate-adsorbent interactions governs the adsorption of formaldehyde.

As evidenced by 𝐾 𝐻 values, activated carbon fibers and some of metal loaded FAU zeolites exhibited an outstanding affinity to formaldehyde attributed to the formation of strong adsorbateadsorbent interactions. Therefore, higher formaldehyde adsorption capacity is expected even at low concentrations, making these materials potential candidates for pollutant removal purposes.

Nevertheless, these metal loaded zeolites are considerably hydrophilic therefore exhibiting high selectivity for water at low concentrations as recently reported [START_REF] Bellat | Selective adsorption of formaldehyde and water vapors in NaY and NaX zeolites[END_REF]. Consequently, their use in humid environments is conditioned upon adding an upstream device for water trapping. Concerning gas analysis applications, these adsorbents are probably not suitable since a complete desorption is required for an accurate formaldehyde quantification. Related to this, Bellat et al. [START_REF] Bellat | Capture of formaldehyde by adsorption on nanoporous materials[END_REF] observed that the adsorbed formaldehyde can polymerize into paraformaldehyde in the cages of NaX and NaY zeolites . The complete polymer desorption becomes much more difficult due to its size, being only possible at 400 °C under dynamic vacuum, which is not compatible with gas analysis applications. Formaldehyde desorption from AC fibers was not investigated but the strong interactions of this compound with the acidic groups could potentially result in higher desorption temperatures. Therefore, for these applications, moderate adsorbate-adsorbent interactions are preferred. Hence, adsorbents showing 𝐾 𝐻 values from 0.1 to 1.5 mol/kg Pa are expected to be more appropriate. These adsorbents exhibited a non-negligible formaldehyde adsorption capacity, thus being suitable for pollutant removal and enabling a quantitative sampling for gas analysis applications. Furthermore, the intermediate strength of the interactions may presumably lead to less extreme conditions for desorption compared to the metal loaded FAU zeolites, thereby facilitating adsorbent regeneration. In this context, HKUST-1 could be a suitable candidate for this application. In this MOF, water molecules are adsorbed by interactions with copper sites and the carboxylic groups of the linkers and, since formaldehyde is a molecule of similar structure and polarity, comparable interactions are expected in formaldehyde adsorption. It may be reasonable to think that formaldehyde desorption can be accomplished at moderate temperatures (~120 °C), as in the case of water [START_REF] Zhao | Competitive adsorption and selectivity of benzene and water vapor on the microporous metal organic frameworks (HKUST-1)[END_REF].

Finally, KH values lower than 0.1 mol/kg Pa are found for considerably hydrophobic materials such as ZSM-5 zeolites [START_REF] Serrano | Characterization of adsorptive and hydrophobic properties of silicalite-1, ZSM-5, TS-1 and Beta zeolites by TPD techniques[END_REF], certain mesoporous silica [START_REF] Serrano | Adsorption and Hydrophobic Properties of Mesostructured MCM-41 and SBA-15 Materials for Volatile Organic Compound Removal[END_REF] and Carbopack® B [START_REF] Tolnai | Evaluation of Carbopack B adsorbent for the tubetype diffusive sampling of volatile organic compounds at ambient concentration[END_REF] due to the weakness of the adsorbate-adsorbent interactions. It is obvious that these materials are not suitable for pollutant removal on large scale because of their limited adsorption capacity at low concentrations. However, some of them, such as Mg loaded and low SAR ZSM-5 zeolites or mesoporous silica (SBA-15 and SBA-16), can be used as a formaldehyde scrubber to be integrated in the formaldehyde analyser employed in this work. As illustrated in Figure S1, in this instrument a DNPH cartridge (see Figure S1) Where t is the lifespan (min), qt is the total capacity of the cartridge (µg), C is the formaldehyde concentration in the air flow (µg/m 3 ) and Q is the flow rate (mL/min). Using the abovementioned conditions, the lifespan of a DNPH commercial tube has been calculated to be 13 days. Instead of using these commercial cartridges, a similar cartridge can be filled with 10 g of Mg/HZSM-5 giving a total formaldehyde adsorption capacity of 349 µg per cartridge. Under the same conditions, this consumable could be used for 60 days thus largely improving the lifespan of the commercial one. Additionally, since the price of ZSM-5 zeolites is extremely low (1-3 €/kg, Zeolyst International) compared to other adsorbent materials, the cost of these consumables could be drastically reduced.

Conclusions

Thanks to the recent development of a new formaldehyde analyser operating in near real time and enabling to quantify very low concentrations of gaseous formaldehyde [START_REF] Guglielmino | On-line gaseous formaldehyde detection by a microfluidic analytical method based on simultaneous uptake and derivatization in a temperature controlled annular flow[END_REF][START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF], the formaldehyde adsorption capacities of a series of materials differing in structure, porosity and chemical composition have been evaluated at realistic concentrations. Among them, HKUST-1 demonstrated to be the most efficient adsorbent for formaldehyde capture at ppb levels. This outstanding capacity is presumably ascribed to the strong adsorbate-adsorbent interactions between copper centres and formaldehyde's carbonyl group. However, as formaldehyde molecules, water molecules can be easily coordinated on the unsaturated copper sites of HKUST-1, therefore, a significant decrease of the formaldehyde adsorption capacity in humid environments could be envisaged, limiting its use in these conditions. In gas analysis, this disadvantage can be partially overcome by adding a water-trapping device prior to the gas sampling on the adsorbent bed, as can be for example a Nafion tube. To avoid the need to integrate water-trapping devices, more hydrophobic adsorbents such as ZSM-5 zeolites could be preferred. Formaldehyde adsorption over these zeolites is directly related to the aluminium content suggesting that formaldehyde adsorption occurs on Brønsted acid centres. Despite the moderate adsorption capacity of ZSM-5 zeolites compared to HKUST-1 that could prevent their use for pollutant removal on large scale, zeolites can be considered for gas analysis applications. In this work, it has been demonstrated that this material can be employed to manufacture formaldehyde scrubbers with a 60 days lifespan in polluted environments (CHCHO = 164 ppb). This result represents a considerable improvement with respect to the lifespan of the current scrubber (13 days) and can be manufactured at a highly competitive price.

  3.5-diacetyl-1.4-dihydrolutidine (DDL), DDL being excited by a LED centered at 415 nm and fluorescence being collected on a photomultiplier (Hamamatsu) coupled to a 530 ± 40 nm band pass filter. The commercial formaldehyde analyser (µ-F1, In'Air Solutions, Strasbourg,

Figure 1 .

 1 Figure 1. Scheme of the experimental set-up for formaldehyde breakthrough experiments.

Figure 2 .

 2 Figure 2. a) X-Ray diffraction patterns of commercial CBV3020E and Mg/CBV3020E. b) and c)

Figure 3 .

 3 Figure 3. It is noteworthy that, to enable better graphical comparison of the breakthrough curves, 5.5

Figure 3 .

 3 Figure 3. (a) Breakthrough curves of gaseous formaldehyde on Carbopack® B, SBA-16 and HKUST-

Figure 4 .

 4 Figure 4. Breakthrough curves of gaseous formaldehyde over ZSM-5 zeolites with different Si/Al ratios (formaldehyde concentration = 164 ppb, flow rate = 15 NmL/min).

Figure 5 .

 5 Figure 5. Formaldehyde adsorption capacities obtained over ZSM-5 zeolites with different Si/Al ratios.

Figure 6 .

 6 Figure 6. Breakthrough curves of gaseous formaldehyde over CBV3020E and CBV3020E-Mg zeolites (C0 = 164 ppb, flow rate = 15 NmL/min).

Figure 7 .

 7 Figure 7. Formaldehyde adsorption capacities of diverse materials at different formaldehyde concentrations

  is employed to generate zero formaldehyde air flow required for the blank. This cartridge costs ~15€ and has a formaldehyde adsorption capacity of 75 µg (Waters Corporation, Milford, MA, USA). If formaldehyde ambient concentration is equal to 164 ppb (201.4 µg/m 3 , T = 25 °C) and the analyser operates with a sampling flow rate of 20 mL/min, the lifespan can be calculated as follows:

Table 1 .

 1 Formaldehyde adsorption capacities of various materials obtained in previous studies classified in ascending order of KH value.

	Adsorbent	Adsorbent Type	SBET (m 2 /g)	Vtotal (mL/g)	Pore size (nm)	Concentration (ppm)	Partial pressure (Pa)	Temperature (°C)	Adsorption Capacity (mg/g ads)	KH (mol/kg Pa)	Reference
	ACF	AC	860	0.34	0.53	3.65	0.334	26	52.30	5.221	(Carter et al., 2011)
	NaX	Zeolite	690	n. a.	0.74	10	0.914	25	120	4.373	(Bellat et al., 2015)
	CuX	Zeolite	653	n. a.	0.74	10	0.914	25	90	3.279	(Bellat et al., 2015)
	NaY	Zeolite	749	n. a.	0.74	10	0.914	25	90	3.279	(Bellat et al., 2015)
	AC fiber	AC	1,084	0.41	0.53	3.65	0.334	26	28.2	2.815	(Carter et al., 2011)
	M-CLZ	Zeolite	663.2	0.298	n. a.	46	4.204	25	300.5	2.380	(Kalantarifard et al., 2016)
	Clinoptilolite	Zeolite	436.3	0.27	n. a.	46	4.204	25	194.5	1.541	(Kalantarifard et al., 2016)
	HKUST-1	MOF	1,733	0.89	0.54 / 0.69	0.164	0.015	23	0.504	1.120	This work
	Faujasite (Y)	Zeolite	415.2	0.136	n. a.	46	4.204	25	123.7	0.980	(Kalantarifard et al., 2016)
	KY	Zeolite	703	n. a.	0.74	10	0.914	25	24	0.875	(Bellat et al., 2015)
	Zeolite A	Zeolite	301.6	0.107	n. a.	46	4.204	25	106.7	0.845	(Kalantarifard et al., 2016)
	S-AP	MS	422	0.72	7	88	8.042	r. t.	140.2	0.581	(Nomura and Jones, 2013)
	Modernite	Zeolite	253.3	0.064	n. a.	46	4.204	25	70	0.555	(Kalantarifard et al., 2016)
	All-purpose granular AC (GAC1)	AC	869	0.34	0.5	3.65	0.334	26	5.30	0.529	(Carter et al., 2011)
	ED-MIL-101 (Cr)-3	MOF	382	0.34	< 2.3	150	13.708	r. t.	164.9	0.401	(Wang et al., 2016)
	ED-MIL-101 (Cr)-2	MOF	n. a.	n. a.	< 2.3	150	13.708	r. t.	164.0	0.398	(Wang et al., 2016)
	S-MAP	MS	401	0.69	6.6	88	8.042	r. t.	80.1	0.332	(Nomura and Jones, 2013)
	ED-MIL-101 (Cr)-1	MOF	764	0.58	< 2.3	150	13.708	r. t.	131.5	0.320	(Wang et al., 2016)

Table 2 .

 2 Textural properties of the investigated adsorbents

	Sample	SBET (m 2 /g)	Vtotal (cm³/g)	Vmeso (cm³/g)	Vmicro (cm³/g)	Dpore (nm)	SAR	Acid site density a (mmol H + / g zeolite)
	Carbopack® B	112	-	-	-	-	-	-
	SBA-16	572	0.72	0.68	-	3.4/6.0	-	-
	HKUST-1	1733	0.89	0.14	0.68	0.54/0.69	-	-
	ZSM_198	367	0.17	-	0.07	-	198	0.132
	ZSM_132	352	0.18	-	0.08	-	132	0.133
	ZSM_79	348	0.18	-	0.11	-	79	0.249
	CBV3020E	369	0.26	-	0.11	-	15	0.361
	Mg/CBV3020E	296	0.20	-	0.10	-	15	0.280

a Determined by NH3-TPD.

Table 3 .

 3 Breakthrough time and adsorption capacities of studied materials towards gaseous formaldehyde

	Adsorbent	C-B 1,2	SBA-16 2	HKUST-1 3 ZSM 198 2 ZSM 132 2	ZSM 79 2	CBV3020E 4	CBV3020E Mg 4
	Breakth. time (min)	2	32	67	11	12	21	7	4
	Ads.
	Capacity 4	4.5 ± 0.6	29.0 ± 4.1 503.7 ± 86.8 10.2 ± 1.4 14.8 ± 2.0 21.6 ± 3.0	26.0 ± 5.6	34.9 ± 8.0
	(µg/g ads)

1 C-B: Carbopack® B,

Acknowledgments

This project has been supported by the European Union's through the LIFE SMART IN'AIR -Smart indoor air monitoring network to reduce the impacts of pollutants on environment and healthunder grant number LIFE17 ENV/FR/000330.