Maxime Dahirel
email: maxime.dahirel@yahoo.fr

Maarten De Cock

Pieter Vantieghem

Dries Bonte

Urbanization-driven changes in web-building and body size in an orb-web spider

Keywords: adaptation, fecundity, foraging, multivariate mixed model, spider web, temperature-size rule

1. In animals, behavioural responses may play an important role in determining population persistence in the face of environmental changes. Body size is a key trait central to many life history traits and behaviours. Correlations with body size may constrain behavioural variation in response to environmental changes, especially when size itself is influenced by environmental conditions.

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. 2. Urbanization is an important human-induced rapid environmental change that imposes multiple selection pressures on both body size and (size-constrained) behaviour. How these combine to shape behavioural responses of urban-dwelling species is unclear.

3. Using web-building, an easily quantifiable behaviour linked to body size, and the garden spider Araneus diadematus as a model, we evaluated direct behavioural responses to urbanization and body size constraints across a network of 63 selected populations differing in urbanization intensity. We additionally studied urbanization at two spatial scales to account for some environmental pressures varying across scales and to obtain first qualitative insights about the role of plasticity and genetic selection. 4. Spiders were smaller in highly urbanized sites (local scale only), in line with expectations based on reduced prey biomass availability and the Urban Heat Island effect. Web surface and mesh width decreased with urbanization at the local scale, while web surface also increased with urbanization at the landscape scale. The latter two responses are expected to compensate, at least in part, for reduced prey biomass availability in cities. The use of multivariate mixed modelling reveals that although web traits and body size are correlated within populations, behavioural responses to urbanization do not appear to be constrained by size: there is no evidence of size-web correlations among populations or among landscapes, and web traits appear independent from each other. 5. Our results demonstrate that responses in size-dependent behaviours may be decoupled from size changes, thereby allowing fitness maximisation in novel environments. The spatial scale at which traits respond suggests contributions of both genetic adaptation (for web investment) and plasticity (for mesh width). Although fecundity decreased with local-scale urbanization, Araneus diadematus abundances

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. were similar across urbanization gradients; behavioural responses thus appear overall successful at the population level.

Introduction

In animals, behaviour is often considered as the first route of adaptation to rapid environmental changes (Wong & Candolin, 2015). Numerous examples of both adaptive and maladaptive behavioural changes in response to human-induced environmental changes have now been recorded [START_REF] Lowry | Behavioural responses of wildlife to urban environments[END_REF]; the potential costs and constraints associated with these behavioural changes are, however, poorly understood. Behaviours may be linked to metabolic and physiological processes that directly impact fitness [START_REF] Bonte | Costs of dispersal[END_REF][START_REF] Debecker | Integrating the pace-of-life syndrome across species, sexes and individuals: covariation of life history and personality under pesticide exposure[END_REF].

When behaviour is correlated to other traits, conflicting selection pressures may hinder adaptation, leading to mismatches between the expressed and optimal behaviours in the new environment [START_REF] Sih | Understanding variation in behavioural responses to human-induced rapid environmental change: a conceptual overview[END_REF]Wong & Candolin, 2015). In particular, many behaviours are correlated to body size (e.g. [START_REF] Gregorič | Does body size predict foraging effort? Patterns of material investment in spider orb webs[END_REF][START_REF] Stevens | A comparative analysis of dispersal syndromes in terrestrial and semiterrestrial animals[END_REF], a key trait that can itself be directly impacted by environmental changes [START_REF] Oliveira | Size and sex-dependent shrinkage of Dutch bees during one-and-a-half centuries of land-use change[END_REF][START_REF] Renauld | Landscape simplification constrains adult size in a native ground-nesting bee[END_REF].

Urbanization is one of the most prominent human-induced environmental changes, with cities now harbouring more than half of the global human population [START_REF] Seto | Global forecasts of urban expansion to 2030 and direct impacts on biodiversity and carbon pools[END_REF]United Nations Population Division, 2015). Direct and indirect impacts of the urbanization process include habitat fragmentation, increased temperatures (the "Urban Heat

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. Island" effect), elevated levels of pollution and changes in resource availability due to the decline of key species and/or the increased availability of anthropogenic food sources [START_REF] Alberti | Eco-evolutionary dynamics in an urbanizing planet[END_REF][START_REF] Parris | Ecology of Urban Environments[END_REF]. This cocktail of environmental changes means cities present novel ecological conditions never encountered before in organisms' evolutionary histories [START_REF] Alberti | Eco-evolutionary dynamics in an urbanizing planet[END_REF][START_REF] Hendry | Human influences on evolution, and the ecological and societal consequences[END_REF][START_REF] Johnson | Evolution of life in urban environments[END_REF]. This drives changes in community taxonomic and functional composition [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF][START_REF] Piano | Urbanization drives community shifts towards thermophilic and dispersive species at local and landscape scales[END_REF] as well as intraspecific phenotypic changes (plastic and/or genetic: [START_REF] Alberti | Global urban signatures of phenotypic change in animal and plant populations[END_REF][START_REF] Brans | The heat is on: Genetic adaptation to urbanization mediated by thermal tolerance and body size[END_REF][START_REF] Johnson | Evolution of life in urban environments[END_REF][START_REF] Lowry | Behavioural responses of wildlife to urban environments[END_REF] at an unprecedented rate [START_REF] Alberti | Global urban signatures of phenotypic change in animal and plant populations[END_REF]. Cities can therefore be seen as key "natural experiments" to understand eco-evolutionary consequences of global change [START_REF] Johnson | Evolution of life in urban environments[END_REF].

Orb web spiders (Araneae, Araneidae) are a unique model for the study of behaviour, as their foraging behaviour is archived in their web [START_REF] Foelix | Biology of Spiders[END_REF]Fig. 1). Detailed quantification of the orb web structure therefore allows inferences on both the implemented foraging strategies (from web architecture) and the energetic investments in web production [START_REF] Sherman | The orb-web: an energetic and behavioural estimator of a spider's dynamic foraging and reproductive strategies[END_REF].

Keeping total investment equal, larger spacing of the capture threads allows the construction of larger webs and an increase in prey interception; conversely, pressures to build small meshes to e.g. increase prey retention [START_REF] Blackledge | Mesh width influences prey retention in spider orb webs[END_REF] come at the cost of reduced capture areas [START_REF] Sandoval | Plasticity in web design in the spider Parawixia bistriata: a response to variable prey type[END_REF]. Spiders can maximise both retention and interception simultaneously only by increasing the quantity of capture silk per web [START_REF] Eberhard | The rare large prey hypothesis for orb web evolution: a critique[END_REF][START_REF] Sherman | The orb-web: an energetic and behavioural estimator of a spider's dynamic foraging and reproductive strategies[END_REF]. Variation in web building behaviour has been recorded in relation to aging and growth (e.g. Witt, Rawlings, & Reed, 1972), and in response to a wide variety of environmental changes. Spiders experiencing lower prey biomass have for instance been shown to increase the capture area of their web to increase its efficiency [START_REF] Mayntz | Nutrient balance affects foraging behaviour of a trapbuilding predator[END_REF]. Spiders may also use information on the size of recently caught prey to

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. optimize the web mesh width and capture area [START_REF] Schneider | The effect of prey type on the geometry of the capture web of Araneus diadematus[END_REF]. The architecture and size of webs are influenced by weather, both through changes in the physical pressures the web has to withstand and through changes in the internal physiological state of the building spider [START_REF] Barghusen | The effects of temperature on the web-building behaviour of the common house spider, Achaearanea tepidariorum[END_REF]Vollrath, Downes, & Krackow, 1997). Web building can also be disturbed by exposure to various chemical pollutants [START_REF] Benamú | Short and long-term effects of three neurotoxic insecticides on biological and behavioural attributes of the orb-web spider Alpaida veniliae (Araneae, Araneidae): implications for IPM programs[END_REF], and constrained by space availability in the current habitat (Vollrath et al., 1997). Most of these environmental drivers are highly altered by urbanization [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF][START_REF] Parris | Ecology of Urban Environments[END_REF] and may either impose constraining or adaptive changes in web building.

Besides these direct responses, environmental changes may also influence spider foraging behaviour indirectly, as web-building is coupled to body size both within and among species.

Larger spiders produce on average larger, and often less densely structured webs [START_REF] Gregorič | Does body size predict foraging effort? Patterns of material investment in spider orb webs[END_REF][START_REF] Heiling | The web of Nuctenea sclopetaria (Araneae, Araneidae): relationship between body size and web design[END_REF]. This may stem from physiological constraints such as the quantity of silk available or the need for larger spiders to capture larger prey to fulfil energy requirements), as well as physical constraints, as spiders seem to use their own body as a "measurement stick" during web building [START_REF] Gregorič | Does body size predict foraging effort? Patterns of material investment in spider orb webs[END_REF][START_REF] Venner | Body-mass-dependent cost of web-building behavior in an orb weaving spider, Zygiella x-notata[END_REF], Vollrath, 1987). Increased temperatures in urban areas are expected to lead to reduced body size in ectotherms through increases in metabolic rates [START_REF] Brans | The heat is on: Genetic adaptation to urbanization mediated by thermal tolerance and body size[END_REF][START_REF] Horne | Temperature-size responses match latitudinal-size clines in arthropods, revealing critical differences between aquatic and terrestrial species[END_REF][START_REF] Sheridan | Shrinking body size as an ecological response to climate change[END_REF]. We may expect urbanization-induced body size reduction to be even stronger when urbanization decreases resource availability during development [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF], as spider adult size is also highly resource-dependent [START_REF] Mayntz | Effects of prey quality and availability on the life history of a trap-building predator[END_REF]. This expected size reduction may thus constrain optimal web-building behaviour, for instance limiting the ability of spiders to increase web size to catch more prey when needed.

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. Behavioural responses to urbanization and other human-induced rapid environmental changes, whether adaptive or not, can result from plasticity and/or genetic changes [START_REF] Sih | Behavior as a key component of integrative biology in a human-altered world[END_REF][START_REF] Tuomainen | Behavioural responses to human-induced environmental change[END_REF]. A formal understanding of the underlying sources of phenotypic (behavioural) variation in response to urbanization can only be achieved by experimental work [START_REF] Johnson | Evolution of life in urban environments[END_REF][START_REF] Merilä | Climate change, adaptation, and phenotypic plasticity: the problem and the evidence[END_REF]. However, the spatial scale of phenotypic variation, relative to the scale of dispersal, may provide first insights. Spiders disperse passively by "ballooning" [START_REF] Bonte | Case study II: Spiders as a model in dispersal ecology and evolution[END_REF]; although very long-distance dispersal events occur, most dispersers are thought to land within a few hundred metres from their departure point [START_REF] Bell | Ballooning dispersal using silk: world fauna, phylogenies, genetics and models[END_REF][START_REF] Croucher | Population structure and dispersal in a patchy landscape: nuclear and mitochondrial markers reveal area effects in the spider Theridion californicum (Araneae: Theridiidae)[END_REF][START_REF] Reed | Levels of gene flow among populations of a wolf spider in a recently fragmented habitat: current versus historical rates[END_REF][START_REF] Reynolds | Ballooning dispersal in arthropod taxa: conditions at take-off[END_REF]. Once airborne, individuals have no control over their trajectory, meaning adaptive habitat choice [START_REF] Edelaar | Non-random gene flow: an underappreciated force in evolution and ecology[END_REF] is unlikely. Gene flow is thus expected to swamp genetic differentiation at small spatial scales; any response to environmental variation with a spatial grain equal or smaller than the typical dispersal event is more likely to result from plasticity. By contrast, evidence of phenotype-environment matching at larger spatial scales (several kilometres) suggests putative genetic adaptation [START_REF] Macdonald | Using connectivity to identify climatic drivers of local adaptation[END_REF][START_REF] Richardson | Microgeographic adaptation and the spatial scale of evolution[END_REF].

Here, we study and quantify shifts in body size and web building in response to urbanization, then use a multivariate mixed modelling approach in order to identify the contribution of size constraints to (adaptive) behavioural variation. We used the garden spider Araneus diadematus as a model; this species is one of the most common species in both urban and non-urban orb web spider communities in western Europe [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF]. A. diadematus adjusts its web-building behaviour depending on abiotic conditions and the availability/ characteristics of potential prey [START_REF] Bonte | Web building flexibility of an orb-web spider in a heterogeneous agricultural landscape[END_REF][START_REF] Schneider | The effect of prey type on the geometry of the capture web of Araneus diadematus[END_REF]Vollrath et al., 1997); webs are recycled and rebuilt daily,

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. allowing spiders to match currently/recently experienced environmental conditions [START_REF] Breed | The fate of the intact orb web of the spider Araneus diadematus[END_REF].

We studied responses to urbanization at two independent spatial scales to obtain indications on the roles of genetic adaptation vs. plasticity, and to account for the fact that environmental correlates of urbanization may be scale-dependent [START_REF] Kaiser | The Urban Heat Island and its spatial scale dependent impact on survival and development in butterflies of different thermal sensitivity[END_REF][START_REF] Mcdonnell | Adaptation and adaptedness of organisms to urban environments[END_REF][START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF]. We used a well-studied network of urban and non-urban sites in which differences in resource availability and temperature have been previously recorded [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF][START_REF] Kaiser | The Urban Heat Island and its spatial scale dependent impact on survival and development in butterflies of different thermal sensitivity[END_REF][START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF]. Although prey numbers are roughly constant across urbanization levels, prey biomass is expected to be lower in highly urbanized sites as the size of the average prey declines by about 18% [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF]. Average temperatures are higher in highly urbanized sites, but this effect is mainly detected when considering urbanization at the most local scale (about +1-2°C) and weaker to non-significant at larger spatial scales [START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF].

In this context, we expected Araneus diadematus individuals to become smaller with urbanization, and the decrease in size to be stronger with local-scale urbanization as the effects of resource reduction and increased temperature cumulate. We expected Araneus diadematus to present adaptive shifts in web-building behaviour in response to urbanizationrelated resource reduction, namely an increase in web investment/ web area to increase interception and a narrowing of mesh width to increase retention. If spiders were unable to display such adaptive behavioural responses, we expected to see negative fitness consequences through lower individual fecundity and/or population success, measured by spider density. Assuming correlations between web traits and body size were important and persisted across urbanization contexts (which may not be the case; [START_REF] Fischer | Plasticity and evolution in correlated suites of traits[END_REF][START_REF] Peiman | Comparative analyses of phenotypic trait covariation within and among populations[END_REF], we expected the mesh reduction response to be facilitated by reduced body size, while a smaller body size should have hindered spiders'

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. ability to build larger webs. Mesh width and web area responses should thus be stronger at the scale at which web trait and body size are the most/ the least strongly correlated, respectively.

Material and Methods

Study species

Araneus diadematus Clerck 1757 is a common orb-weaving spider present across the Holarctic in a wide range of natural and human-altered environments, building its web in shrubs and tall herbaceous vegetation (Lee & Thomas, 2002;[START_REF] Nentwig | Spiders of Europe[END_REF]. Its distinctive dorsal cross pattern makes field identification easy [START_REF] Roberts | The Spiders of Great Britain and Ireland (Vols. 1-2). Stenstrup, Denmark: Apollo Books[END_REF]Fig. 1). Females usually become mature in late summer, and can survive through to late autumn (Lee & Thomas, 2002). In cities, Araneus diadematus mostly settles in gardens and green spaces as opposed to roadsides or close to buildings [START_REF] Van Keer | More than one third of the Belgian spider fauna (Araneae) found within the city of Antwerp: faunistics and some reflections on urban ecology[END_REF].

Study sites

We sampled A. diadematus females in 63 sites (hereafter "populations") across a well-studied network of urban, rural and natural landscapes in northern Belgium [START_REF] Brans | Eco-evolutionary dynamics in urbanized landscapes: evolution, species sorting and the change in zooplankton body size along urbanization gradients[END_REF][START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF][START_REF] Kaiser | The Urban Heat Island and its spatial scale dependent impact on survival and development in butterflies of different thermal sensitivity[END_REF][START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF][START_REF] Piano | Urbanization drives community shifts towards thermophilic and dispersive species at local and landscape scales[END_REF]Fig. 2), one of the most urbanized and densely populated regions in Europe (United Nations Population Division, 2015). Urbanization was studied at two different spatial scales thanks to a 2-step stratified selection design, with 3 × 3 km landscapes ("landscape scale" urbanization) being selected first, then three 200 × 200 m sites ( "local scale" urbanization) chosen within each landscape. We used the percentage of surfaces occupied by buildings as our proxy for

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. urbanization (extracted from the Large-scale Reference Database, a reference map of Flanders; https://www.agiv.be/international/en/products/grb-en). This metric is precise down to the individual building, and thus usable at both spatial scales, but excludes other artificialized surfaces such as roads, railways, parking spaces or pavements; hence percentages higher than 10% already correspond to highly urbanized contexts. We crosschecked this metric with the percentage of artificial surfaces based on the CORINE land cover database (European Environmental Agency, 2016)(Level 1: "Artificial surfaces", excluding classes 1.41 and 1.42 corresponding to human-created urban green spaces). This second metric includes all artificialized surfaces but, due to its coarser resolution (minimum mapping unit 25 ha) cannot be used at smaller spatial scales. Both metrics were highly correlated (N = 21 landscapes, Spearman correlation coefficient = 0.95, p = 5.17 × 10 -6 , Supplementary Figure S1). Twenty-one 3 × 3 km landscapes were selected and sorted into three urbanization levels (7 landscapes by level). High-urbanization landscapes had more than 10% of their area covered by buildings (equivalent to more than 70% of artificial surfaces based on CORINE).

Low-urbanization landscapes had less than 3% of their surfaces occupied by buildings (less than 20% of artificial surfaces based on CORINE); additionally, they were selected so that more than 20% of their surface was covered by so-called "ecologically valuable areas" (areas with rare, vulnerable or highly diverse vegetation based on the Flanders Biological Valuation

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. were separated on average by 1.4 km (range 0.2 to 3.3 km, SD: 0.8 km), while sites belonging to different landscapes were separated on average by 40.33 km (range: 5.6 to 91.3 km, SD: 19.75 km, Fig. 2).

Spider collection and phenotypic measurements

Populations were sampled from 25 August to 5 October 2014, i.e. during the first part of the reproductive period (Lee & Thomas, 2002). One landscape (3 sites) was visited per day; there was no significant link between landscape-level urbanization and sampling date (ANOVA; N = 21 landscapes, F 2,18 = 0.009, p = 0.991). In each visited site, between 7 and 11 adult females per population (N total = 621, average ± SD : 9.86 ± 0.74) were sampled on their webs and stored in 70% ethanol; spiders' cephalothorax width was measured under binocular microscope and used as a proxy for body size [START_REF] Bonte | Web building flexibility of an orb-web spider in a heterogeneous agricultural landscape[END_REF]Fig. 1). Out of these 621 spiders, 193 individuals caught in the 9 landscapes of the Ghent region (Fig. 2) were also dissected and the number of mature eggs recorded. As Araneus diadematus webs are conspicuous and located in similar habitats (grassland-shrub mosaics taken in the broad sense) independently of urbanization levels (Lee & Thomas, 2002;[START_REF] Van Keer | More than one third of the Belgian spider fauna (Araneae) found within the city of Antwerp: faunistics and some reflections on urban ecology[END_REF], we were also able to collect reliable population density data (number of adult female spiders observed per 200 × 200 m site in 4.5 person-hours) in the 62 sites (out of 63) sampled in the community-level study by [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF].

Based on measurements taken in the field (vertical and horizontal diameters of whole web and free central zone, number of sticky silk spirals in each web quadrant, Fig. 1), we estimated two design parameters for the webs belonging to sampled spiders: the web capture area surface (considering orb webs as ellipses, following [START_REF] Herberstein | Evaluation of formulae to estimate the capture area and mesh height of orb webs (Araneoidea, Araneae)[END_REF], and the mesh width (interval between sticky spirals, averaged over the horizontal and vertical axes).

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Statistical analysis

All analyses were carried out using R, version 3.5 (R Core Team, 2018). Body size, web traits and sampling date values were scaled to unit standard deviation prior to inclusion in models (see Table 1 for descriptive statistics), in order to improve model convergence and make results comparable across parameters and variables.

The number of adult spiders per site was analysed using a Poisson generalized linear mixed model. Spider numbers were modelled as a function of local and landscape level of urbanization, as well as sampling date, with a random effect of landscape identity added to account for spatial clustering of populations in landscapes.

We followed broadly Araya-Ajoy and Dingemanse (2014)'s approach and analysed spider traits and their correlations in two steps.

First, each trait (body size, web surface, mesh width, egg number) was analysed separately using univariate mixed models. These models included fixed effects of local and landscape level of urbanization as well as sampling date, with random effects of landscape and site of origin to account for non-independence of observations. Body size, web surface and mesh width were analysed using linear mixed models, egg number was analysed using a Poisson generalized linear mixed model.

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. All models were fitted using the MCMCglmm R package [START_REF] Hadfield | MCMC methods for multi-response generalized linear mixed models: the MCMCglmm R package[END_REF]. We used uninformative default priors for fixed effects, inverse-Wishart priors for the residual (co)variances and parameter expanded priors for among-landscapes and among-population (co)variance matrices. Degrees of belief ν for the (co)variance matrices were set to weakly informative values, i.e. the dimension of the corresponding matrix (1 for univariate models, 4 for the multivariate model). Chains ran for (univariate/ multivariate model) 1000000/ 3300000 iterations, with the first 250000/ 300000 iterations discarded as burn-in and sampling every 750/ 3000 iteration post burn-in, resulting in 1000 samples per chain with serial autocorrelation < 0.1 for all model parameters. We ran three chains per model, checked mixing graphically and confirmed chain convergence using the Gelman-Rubin statistic (using the coda R package; [START_REF] Plummer | CODA: Convergence Diagnosis and Output Analysis for MCMC[END_REF]all were ≤ 1.03). There was no evidence of residual spatial autocorrelation in any of the models; 95% confidence intervals of spline correlograms (fitted using the ncf package; Bjornstad, 2016) overlapped with 0 at all distances and for all response variables. Model diagnostic plots are presented in Supplementary Figures S2 to S30c.

Results

Spider population density

The number of spiders found per site was not influenced by urbanization at either spatial scale (95% Credible Intervals for all model coefficients β overlapped with 0, Table 2). There was also no temporal trend in spider abundance (Table 2).

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. low to high urbanization: -16.62% [-20.81%, -12.12%]). There was no supported effect of landscape-scale urbanization or sampling date on fecundity.

Sources of variation in spider traits: univariate models (

Correlations among traits: multivariate models

Inferences based on fixed effects from this multivariate model were virtually the same as those obtained from the univariate models (see Table 2 vs. Supplementary Table S1 for a comparison of parameters). At the landscape and population levels, none of the estimated trait covariances were different from zero, based on credible intervals (Table 3). At the withinpopulation level, web traits were positively correlated with each other and with body size (Table 3).

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Discussion

Using a standardized hierarchical sampling design and multivariate modelling, we found Araneus diadematus to decrease in size with increasing levels of local urbanization, and independently, to adjust different components of its web-building behaviour. Web-building changes, in particular the reduction of mesh width at the local scale, and the increase in web investment at the landscape scale, appear to be adaptive (i.e. beneficial) and follow predictions based on lower prey biomass in cities. The putative adaptive value of these behavioural changes is supported by the fact that A. diadematus is able to maintain consistent population densities across broad gradients of urbanization, when other orb web spider species in the same communities cannot [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF]. An alternative explanation is that urban habitats are population sinks that are kept afloat by a supply of immigrant ballooning spiders from natural habitats [START_REF] Parris | Ecology of Urban Environments[END_REF]. While we have no reason to anticipate such an asymmetry in dispersal, we should then expect landscape-scale urbanization to decrease local population density, as it would limit this supply. This is contrary to our observations. Interestingly, while web building behaviours are intrinsically connected to body size, as shown by within-population correlations, their responses to urbanization are decoupled (no among-populations or among-landscapes correlations).

Araneus diadematus experienced size reduction (by about 8%) in response to local urbanization only. The temperature-size rule, found in many arthropods, predicts that individuals should be smaller at higher temperatures [START_REF] Atkinson | Temperature and organism size-a biological law for ectotherms?[END_REF][START_REF] Horne | Temperature-size responses match latitudinal-size clines in arthropods, revealing critical differences between aquatic and terrestrial species[END_REF]. As the spatial scale of urban heating matches the spatial scale at which size reduction is observed [START_REF] Kaiser | The Urban Heat Island and its spatial scale dependent impact on survival and development in butterflies of different thermal sensitivity[END_REF][START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF], the Urban Heat Island Effect provides a valid explanation for the observed patterns [START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF], but see Entling, Schmidt-Entling, Bacher, Brandl, & Nentwig, 2010;[START_REF] Puzin | Intra-and inter-specific variation in size and habitus of two sibling spider species (Araneae: Lycosidae): taxonomic and biogeographic insights from sampling across Europe[END_REF]. However, the impact of small temperature changes on spider body size remains to be studied, and resource availability

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during development is a well-documented influence and a potentially stronger driver of spider adult size (DiRienzo & Montiglio, 2016;[START_REF] Kralj-Fišer | Development and growth in synanthropic species: plasticity and constraints[END_REF][START_REF] Mayntz | Effects of prey quality and availability on the life history of a trap-building predator[END_REF]. Higher prey abundance in urbanized spots of the Sydney area was for instance associated with a larger body size in the spider Nephila plumipes [START_REF] Lowe | Urbanisation at multiple scales is associated with larger size and higher fecundity of an orb-weaving spider[END_REF][START_REF] Lowe | Persistence and survival of the spider Nephila plumipes in cities: do increased prey resources drive the success of an urban exploiter?[END_REF]. In our study region, prey size and prey biomass availability decreased with urbanization (by about 18%; [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF]. The among-studies discrepancy in spider body size shifts cannot be easily reconciled with a major role of the Urban Heat Island effect, but is fully consistent with a prominent role of resource availability. Interestingly, we did not detect a spider size reduction in response to landscape-level urbanization (Table 2, Fig. 3), even though prey body size declines equally in response to urbanization at both scales [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF]. This result is in accordance with our prediction that size reduction should be stronger at the spatial scale at which more urbanization-induced environmental changes cumulate, i.e. the local scale. Pollutants such as heavy metals also have a negative impact on body size [START_REF] Ramirez | Relationships between physiological characteristics and trace metal body burdens of banded garden spiders Argiope trifasciata (Araneae, Araneidae)[END_REF], but we expect their influence here to be limited, as pesticide and heavy metal levels appear unrelated to urbanization in our study area [START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF]T. Merckx, pers. comm.). Alternatively, resource loss may be the main mechanism driving size-reduction with urbanization, which is only observed at the local scale because spiders are able to increase their investment to compensate for prey biomass loss at the landscape, but not at the local scale.

Changes in foraging behaviour in response to urbanization were scale-dependent: spiders built smaller webs with smaller mesh width in response to local urbanization, but increased their web surface with urbanization at the landscape scale (Fig. 3). Additionally, web surface responded to urbanization at both scales independently of mesh width variation (surfacemesh correlation ≈ 0; Table 3), meaning that web surface changes can tentatively be interpreted in terms of corresponding silk investment changes. Webs with smaller mesh are

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. considered better at retaining prey [START_REF] Blackledge | Mesh width influences prey retention in spider orb webs[END_REF], at the cost of a smaller web surface and therefore fewer prey intercepted for a given investment in silk production.

When mesh width is held constant, as it is in response to landscape-level urbanization, increased silk investment leads to an increase in web surface (Figs 1, 3) and therefore an increase in the number of prey caught [START_REF] Prokop | Factors affecting the foraging success of the wasp-like spider Argiope bruennichi (Araneae): role of web design[END_REF][START_REF] Venner | Spider webs designed for rare but life-saving catches[END_REF].

Because of their larger diameter, larger webs are also built with longer radial silk threads, meaning they can stop prey with higher kinetic energy (i.e. bigger prey) without breaking down [START_REF] Harmer | Large orb-webs adapted to maximise total biomass not rare, large prey[END_REF]. Although the importance of these large prey in particular for orb web evolution [START_REF] Blackledge | Prey capture in orb weaving spiders: are we using the best metric[END_REF][START_REF] Eberhard | The rare large prey hypothesis for orb web evolution: a critique[END_REF][START_REF] Harmer | Large orb-webs adapted to maximise total biomass not rare, large prey[END_REF] and individual performance [START_REF] Harmer | Large orb-webs adapted to maximise total biomass not rare, large prey[END_REF][START_REF] Venner | Spider webs designed for rare but life-saving catches[END_REF] remains controversial, selective advantages to having webs that maximize prey biomass in general are clear [START_REF] Harmer | Large orb-webs adapted to maximise total biomass not rare, large prey[END_REF]. Temperature can also have a positive effect on silk production and hence web surface [START_REF] Barghusen | The effects of temperature on the web-building behaviour of the common house spider, Achaearanea tepidariorum[END_REF]Vollrath et al., 1997). However, the Urban Heat Island effect is stronger at the local, rather than landscape scale in our study region [START_REF] Kaiser | The Urban Heat Island and its spatial scale dependent impact on survival and development in butterflies of different thermal sensitivity[END_REF][START_REF] Merckx | Body size shifts in aquatic and terrestrial urban communities[END_REF], and is too weak (about 1-2°C) to explain the observed increase in web surface (in Araneus diadematus, a 12°C increase was needed to obtain surface changes similar to those we observed; Vollrath et al., 1997). Prey size (and thus biomass) decreased with urbanization at both scales [START_REF] Dahirel | Intraspecific variation shapes communitylevel behavioral responses to urbanization in spiders[END_REF]. In this context, both the reduction of mesh size at local scales and the increase in capture area at larger spatial scales can be seen, in accordance with our predictions, as adaptive responses to urbanization, respectively increasing prey retention and prey capture efficiency. These adaptations potentially contribute to the persistence of Araneus diadematus across urbanization gradients, and confirm that biotic interactions can be important drivers of phenotypic changes in urban environments [START_REF] Alberti | Global urban signatures of phenotypic change in animal and plant populations[END_REF].

Accepted Article

.

Web-building in orb-weaving spiders has previously been shown to be both size-constrained [START_REF] Bonte | Web building flexibility of an orb-web spider in a heterogeneous agricultural landscape[END_REF][START_REF] Gregorič | Does body size predict foraging effort? Patterns of material investment in spider orb webs[END_REF], but also highly variable depending on the environment [START_REF] Herberstein | Spider webs: evolution, diversity and plasticity[END_REF]; the net effect in the case of environmental changes influencing body size was so far unknown. We show that correlations among our studied traits are only present within populations (residual covariance matrix) and not at the amongpopulation and among-landscape levels (Table 3), implying that traits respond independently to urbanization. This must be interpreted with some caution however, as our ability to accurately detect weak among-population correlations (|r| < 0.3) may be limited [START_REF] Dingemanse | The contribution of developmental experience vs. condition to life history, trait variation and individual differences[END_REF]. Within-population (phenotypic) correlations likely result from physiological and possibly physical constraints on web-building, with larger spiders having more reserves to build large webs and spiders using their own body as measurement during web building [START_REF] Gregorič | Does body size predict foraging effort? Patterns of material investment in spider orb webs[END_REF][START_REF] Venner | Body-mass-dependent cost of web-building behavior in an orb weaving spider, Zygiella x-notata[END_REF], Vollrath, 1987). They may also result from small scale within-site environmental variation influencing both webs and body size independently, for instance if web-building micro-habitats leading to smaller webs for structural reasons [START_REF] Blamires | Habitat selection and web plasticity by the orb spider Argiope keyserlingi (Argiopidae): Do they compromise foraging success for predator avoidance[END_REF]Vollrath, Downes, & Krackow, 1997) are also sites poorer in prey. In any case, our results suggest that while body size is correlated with web-building strategy within populations, it does not substantially limit the possibilities of spiders to adapt their webbuilding behaviour in response to urbanization. The observed changes in web building are thus a direct response to urban selection pressures, and not an indirect consequence of body size shifts. Additionally, these results confirm that the existence of trait correlations/syndromes in one environmental context or at one biological organisation level may not be informative with respect to their maintenance across contexts/ levels (e.g. [START_REF] Fischer | Plasticity and evolution in correlated suites of traits[END_REF][START_REF] Peiman | Comparative analyses of phenotypic trait covariation within and among populations[END_REF].

Accepted Article

.

Changes in web-building behaviour in relation to urbanization may both originate from plasticity and/or evolutionary changes [START_REF] Herberstein | Spider webs: evolution, diversity and plasticity[END_REF]. Disentangling the relative contribution of both is however difficult in observational studies [START_REF] Merilä | Climate change, adaptation, and phenotypic plasticity: the problem and the evidence[END_REF].

We take advantage of our two-spatial scales design, and the fact that plasticity and genetic adaptation are generally expected to appear in response to finer-grained and coarser-grained environmental variation, respectively [START_REF] Richardson | Microgeographic adaptation and the spatial scale of evolution[END_REF], to emit hypotheses on the relative contributions of these two mechanisms. Mesh width varies with urbanization at the local but not at the landscape scale. This would indicate a more important role of plastic responses, in accordance with experimental work demonstrating high within-individual behavioural flexibility in response to food availability and prey spectrum (reviewed by [START_REF] Herberstein | Spider webs: evolution, diversity and plasticity[END_REF]. By contrast, silk investment, reflected in web surface, adaptively increased in response to landscape-scale urbanization. With the caveat that some drivers of web building variation may have been overlooked, this suggests here a putative role of genetic adaptation in urbanization-driven silk production changes. We acknowledge our results only provide first hints in this direction, and that further research will be needed to quantify the contribution of genetic selection relative to developmental and behavioural plasticity.

Costs associated with urban life were especially prominent at the local scale. Indeed, at this scale, body sizes and web capture surfaces were smaller, and lower fecundity was observed despite stable population densities (Fig. 3, Table 2). Egg load did not, however, covary with body size or web traits at either the inter-or intra-population levels. Reduced fecundity is therefore unlikely to be a direct consequence of increased energetic investment in webs or a reduced body size, but rather an additive response to environmental changes associated with urbanization. Food limitation has strong negative impacts on spider lifetime fecundity even in the absence of survival costs [START_REF] Kleinteich | Contributions of juvenile and adult diet to the lifetime reproductive success and lifespan of a spider[END_REF] or body size changes [START_REF] Miyashita | Decreased reproductive rate of the spider, Nephila clavata, inhabiting small woodlands in urban areas[END_REF] [START_REF] Tew | The web repair behaviour of an orb spider[END_REF]. Overall, Araneus diadematus was comparatively much better at dealing with landscape-scale urbanization; indeed, both fecundity and population densities remained unaffected by urbanization at this scale. This suggests that traits that successfully responded at this scale (namely web surface) are more important to prey capture success and biomass gain than the others (mesh width) [START_REF] Blackledge | Functionally independent components of prey capture are architecturally constrained in spider orb webs[END_REF]. Conversely, benefits, costs and trade-offs associated with increased silk production may be detectable in other life-history dimensions than the ones we explored in the present study, such as development time and adult longevity (both expected to increase with resource restriction; [START_REF] Kleinteich | Contributions of juvenile and adult diet to the lifetime reproductive success and lifespan of a spider[END_REF][START_REF] Kralj-Fišer | Development and growth in synanthropic species: plasticity and constraints[END_REF], or dispersal, which is strongly counter-selected in urban fragmented environments for passively dispersing organisms [START_REF] Cheptou | Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta[END_REF].

From an applied perspective, our results confirm the oft-stated importance of maintaining local green spaces in highly urbanized landscapes (e.g. [START_REF] Philpott | Local and landscape drivers of arthropod abundance, richness, and trophic composition in urban habitats[END_REF], as even generalist "winning" species such as A. diadematus may benefit from them. Additionally, even though spider abundance stayed constant, webs were up to 20% smaller in locally urbanized sites. Spiders are important predators in natural and anthropogenic landscapes [START_REF] Birkhofer | Agroecology: Trait composition, spatial relationships, trophic interactions[END_REF][START_REF] Foelix | Biology of Spiders[END_REF]: our results highlight the necessity of accounting for changes in functional, and not only numerical predator responses to accurately Predictions are based on fixed effects from univariate models (Table 2), fixed effects from the multivariate model being virtually identical (Supplementary Table S1). Predictions at one spatial scale are made by averaging over the effects of sampling date and urbanization at the other spatial scale. Grey points represent observed individual site (local scale) and landscape average values.

  Map;Vriens et al., 2011). A third class of "intermediate" landscapes had between 5 and 10% of their surface covered by buildings (between 20 and 70% of artificial surfaces based on CORINE. Within each landscape, three 200 × 200 m sites (one per urbanization level) were selected this time based on the percentage covered by buildings only. Vegetated areas in chosen sites were unforested and dominated by low vegetation (grasslands/lawns with low shrubs and occasional trees in e.g. gardens, parks or hedgerows). Sites were separated on average by 39.4 km (range: 0.2 to 91.3 km, SD: 20.7 km). Sites within the same landscape

  Second, we used a multivariate mixed model[START_REF] Araya-Ajoy | Characterizing behavioural 'characters': an evolutionary framework[END_REF][START_REF] Dingemanse | The contribution of developmental experience vs. condition to life history, trait variation and individual differences[END_REF] to estimate trait covariances/correlations and partition them across hierarchical levels (correlations among landscapes, among populations, and (residual) within populations). Fixed effects, random intercepts and error families for each trait were included as in univariate models.

Figure 1 .

 1 Figure 1. Left: photograph of a female Araneus diadematus showing the distinctive cross

Figure 2 .

 2 Figure 2. Location of the 21 studied landscapes within northern Belgium. Colour denotes

Figure 3 .

 3 Figure 3. Predicted mean Araneus diadematus trait values ± 50% (thick bars) and 95% (thin

Table 2 ,

 2 Fig 3)

	Body size, as measured by cephalothorax width, decreased with local-scale urbanization, with
	spiders being 8.30% smaller [95% Credible Interval: -14.03%, -2.34%] in highly urbanized
	sites compared to populations from low-urbanization sites. There was no effect of landscape-
	scale urbanization or sampling date.
	Spiders built webs with narrower mesh when local scale urbanization increased (low to high
	urbanization: -9.67% [-14.96%, -4.07%]). Mesh width was not influenced by landscape-scale
	urbanization, but increased with sampling date. Web surface decreased with increasing
	urbanization at the local scale (low to intermediate urbanization: -12.93% [-23.83%, -1.62%];
	low to high urbanization: -20.28% [-31.12%, -9.37%]). It increased with landscape-scale
	urbanization (low to intermediate urbanization: +26.19% [+2.25%, +54.63%]; low to high
	urbanization: +25.43% [+0.5%, +54.54%]) and with time.
	Local urbanization (both intermediate and high levels) decreased fecundity (number of eggs
	per spider) at the local scale (low to intermediate urbanization: -9.64% [-14.51%, -4.77%];

  . The lower fecundities observed in locally urbanized sites may therefore

	indicate that spiders were unable to fully compensate for reduced prey biomass at this scale
	despite shifts in web-building, and despite smaller size leading to reduced requirements. Other
	-here unmeasured-stressors related to urbanization may also lead to reduced fecundity and capture area in locally urbanized sites: heat waves (Kingsolver, Diamond, & Buckley, 2013), pollution (fecundity costs: Hendrickx, Maelfait, Speelmans, & Straalen, 2003; web surface costs: Benamú et al., 2013; but see Ramirez et al., 2011), or repeated web destructions and Accepted Article rebuilding following human disturbance
	.

Acknowledgments

We are grateful to Jasper Dierick for his assistance during field sampling, and to Hans Matheve for GIS calculations and site selection. We thank Niels Dingemanse and four anonymous reviewers for their comments on previous versions of this paper. This article is part of the Belspo-funded IAP project P7/04 SPEEDY (SPatial and environmental determinants of Eco-Evolutionary DYnamics: anthropogenic environments as a model). MD is funded by a postdoctoral grant from the Fyssen Foundation.

Data accessibility

Data and R code are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.j886dg0 [START_REF] Dahirel | Data from: Urbanization-driven changes in web-building and body size in an orb-web spider[END_REF].

Accepted Article

. quantify ecosystem services provided by green infrastructures in urban environments. Further comparisons among successful species that differ in their silk production strategy (e.g. A.

diadematus, which destroys and recreates webs regularly, versus Nephila plumipes, which build semi-permanent webs; [START_REF] Lowe | Urbanisation at multiple scales is associated with larger size and higher fecundity of an orb-weaving spider[END_REF] may highlight commonalities and help shed further light on the costs/ benefits balance of adaptation to urban life.

Authors' contributions

DB conceived the study and designed methodology; MDC and PV collected the data; MD and MDC analysed the data; MD led the writing of the manuscript. All authors contributed critically to the draft and gave final approval for publication.

Accepted Article

.

Accepted Article

. Vollrath, F. (1987). Altered geometry of webs in spiders with regenerated legs. Nature, 328(6127), 247-248. doi:10.1038/328247a0 Vollrath, F., Downes, M., & Krackow, S. (1997). Design variability in web geometry of an orbweaving spider. Physiology & Behavior, 62(4), 735-743. doi:10.1016/S0031-9384(97) 3. Trait variance-covariance matrices for each hierarchical level under study, based on the multivariate mixed model (Fixed effects for this model are virtually identical to those from the univariate models presented Table 2, and are presented Supplementary Table S1). In each case, among landscapes/populations/individual variances are given on diagonals, with between-trait covariances below and corresponding betweentrait correlations above the diagonal. Covariances/correlations involving Egg load are on the latent (log) scale for this trait. 95% credible intervals are given in parentheses


Serguei Alejandro

Héctor Valdés
email: hvaldes@ucsc.cl

Marie-Hélène Manéro

Claudio A Zaror

Oxidative regeneration of toluene-saturated natural zeolite by gaseous ozone: The influence of zeolite chemical surface characteristics

Keywords:

Surface acidity of modified natural zeolite is related to its chemical reactivity.

• Brønsted acid sites are associated to toluene adsorption.

• Lewis acid sites could decompose ozone generating surface active oxygen species.

• Infrared spectra evidence active atomic oxygen and oxidation by-product formation.

• 2NH4Z1 sample shows the highest reactivity toward adsorbed toluene.

Introduction

Hazardous air pollutants such as volatile organic compounds (VOCs) are typically generated in chemical industries, posing a high risk to human health [START_REF] Lu | Catalytic oxidation of benzene using DBD corona discharges[END_REF]. Industrial atmospheric emissions have been traditionally controlled using adsorption processes [START_REF] Pires | Adsorption of volatile organic compounds in Y zeolites and pillared clays[END_REF][START_REF] Khan | Removal of volatile organic compounds from polluted air[END_REF]. Once adsorbents are exhausted they need to be disposed or regenerated. Regeneration techniques such as chemical or thermal desorption are normally used by the industry, allowing valuable adsorbate recovery [START_REF] Ghoshal | Selection of appropriate adsorption technique for recovery of VOCs: an analysis[END_REF]. However, when recovery is not possible, further treatments are needed in order to eliminate adsorbed contaminants [START_REF] Ojala | Catalysis in VOC abatement[END_REF].

Recently, advanced oxidation processes, such as gas phase catalytic ozonation, have been proposed as an effective alternative to regenerate spent adsorbents [START_REF] Chao | Potential use of a combined ozone and zeolite system for gaseous toluene elimination[END_REF][START_REF] Einaga | Benzene oxidation with ozone over manganese oxide supported on zeolite catalysts[END_REF]. Adsorption of toluene on synthetic zeolites (FauY, SilZ and ZSM-5) followed by ozonation, has been proposed as a reliable process to oxidise adsorbed toluene on zeolite surface, resulting in a high degree of recovery on adsorption capacity [START_REF] Monneyron | Regeneration of high silica zeolites via advanced oxidation processes-a preliminary study about adsorbent reactivity toward ozone[END_REF][START_REF] Monneyron | A combined selective adsorption and ozonation process for VOCs removal from air[END_REF][START_REF] Brodu | Removal of volatile organic compounds by heterogeneous ozonation on microporous synthetic alumino silicate[END_REF]. However, there is a lack of information related to the effect of chemical surface characteristics of zeolites in these coupled processes. Separated processes for VOCs and gaseous ozone removal from industrial waste gas streams using synthetic and natural zeolites have been applied [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: the role of Bronsted acid sites[END_REF][START_REF] Valdés | Control of released volatile organic compounds from industrial facilities using natural and acid-treated mordenites: the role of acidic surface sites on the adsorption mechanism[END_REF][START_REF] Valdés | Influence of chemical surface characteristics of natural zeolite on catalytic ozone abatement[END_REF][START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF][START_REF] Valdés | Natural zeolite reactivity towards ozone: the role of compensating cations[END_REF][START_REF] Brodu | Role of Lewis acid sites of ZSM-5 zeolite on gaseous ozone abatement[END_REF].

On one hand, Brønsted and Lewis acid sites have been claimed as responsible of VOCs removal by adsorption onto natural zeolites [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: the role of Bronsted acid sites[END_REF][START_REF] Valdés | Control of released volatile organic compounds from industrial facilities using natural and acid-treated mordenites: the role of acidic surface sites on the adsorption mechanism[END_REF]. It has been indicated that weak base aromatic VOC molecules such as benzene, toluene and p-xylene could be adsorbed by a surface mechanism that includes interaction with Brønsted acid sites in the form of proton-donating hydroxyl groups of natural mordenite surface, forming hydrogen bonds; and with Lewis acid sites, generating a Lewis acid-base adduct [START_REF] Valdés | Control of released volatile organic compounds from industrial facilities using natural and acid-treated mordenites: the role of acidic surface sites on the adsorption mechanism[END_REF]. On the other hand, weak and strong Lewis acid sites of natural and synthetic zeolites have been identified as the main active sites responsible of gaseous ozone removal [START_REF] Valdés | Influence of chemical surface characteristics of natural zeolite on catalytic ozone abatement[END_REF][START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF][START_REF] Valdés | Natural zeolite reactivity towards ozone: the role of compensating cations[END_REF][START_REF] Brodu | Role of Lewis acid sites of ZSM-5 zeolite on gaseous ozone abatement[END_REF]. Molecular ozone could be adsorbed via coordinative bonding at weak Lewis acid sites. Moreover, ozone could transform immediately after adsorption on strong Lewis acid sites and decomposes into O 2 and an active atom of oxygen that could participate in catalytic removal of ozone [START_REF] Valdés | Natural zeolite reactivity towards ozone: the role of compensating cations[END_REF][START_REF] Brodu | Role of Lewis acid sites of ZSM-5 zeolite on gaseous ozone abatement[END_REF].

Unfortunately, the influence of chemical surface properties of natural and synthetic zeolites on VOC removal, using a sequential adsorption-ozonation treatment, at room temperature has not been fully studied yet. In this work, a Chilean natural zeolite is used as starting material; and it is chemically and thermally modified in order to generate a zeolite surface reach in a high content of Brønsted acid sites (using ammonium ion-exchange followed by thermal treatment at 623 K) or Lewis acid sites (using ammonium ion-exchange followed by thermal treatment at 823 K). The nature and strength of acidic surface sites resulting from the applied modification treatments are characterised by Fourier transform infrared (FTIR) spectroscopy using pyridine as a probe molecule. Toluene is chosen here as a target VOCs, since it represents a serious threat to human health [START_REF] Win-Shwe | Neurotoxicity of toluene[END_REF]. Additionally, FTIR spectroscopy is employed to clarify the surface reaction mechanism that takes place between toluene and ozone on zeolite samples. Finally, zeolite surface active centres are identified and the natures of their interaction toward toluene and ozone are characterised at a molecular level.

Materials and methods

Materials

Chilean natural zeolite (53% clinoptilolite, 40% mordenite and 7% quartz) was provided by Minera Formas. Zeolite sample was ground and sieved to 0.3-0.425 mm; then was rinsed with ultra pure water, oven-dried at 398 K for 24 h, and stored in a desiccator until further use. Natural zeolite was modified by acid treatment using HCl (2.4 mol dm -3 ), and by ion-exchange using ammonium sulphate (0.1 mol dm -3 ), as reported elsewhere [START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF]. Acid-treated zeolite and ion exchanged zeolite were named ZH2.4 and NH4Z1, respectively. A second ion-exchange treatment was conducted to the NH4Z1 sample and a new zeolite sample with different chemical surface properties was obtained (2NH4Z1), as described in previous work [START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF]. Zeolite samples were thermally out-gassed in an oven at 623 K or 823 K (heating rate 10 K min -1 ) during two hours prior to adsorption-ozonation experiments.

Toluene (99.8% purity) was provided by MERCK. Ozone was produced in situ, from synthetic dry air, using a Traigalaz 5LO ozone generator. The inlet concentration of ozone was fixed at 24.3 g dm -3 in all experiments.

Physical-chemical characterisation of zeolite samples

Surface area of zeolite samples were obtained by nitrogen adsorption at 77 K, after zeolite out-gassing overnight at 623 K and 823 K, under vacuum [START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF]. Bulk chemical composition of natural and modified zeolite samples was obtained by X-ray fluorescence (XRF) using a RIGAKU Model 3072 spectrometer [START_REF] Valdés | Catalytic ozone aqueous decomposition promoted by natural zeolite and volcanic sand[END_REF]. Density measurements of zeolite samples were carried out in an AccuPyc 1330 pycnometer using helium (99.995% purity) and registering the helium pressure change in a calibrated chamber [START_REF] Viana | About pycnometric density measurements[END_REF]. Physical and chemical surface characteristics of natural and modified zeolite samples are listed in Table 1. Surface areas (S) were calculated by applying the Langmuir adsorption model to nitrogen adsorption data. After acid treatment of natural zeolite, the surface area and the Si/Al ratio increase for ZH2.4, as a consequence of a decationisation and de-alumination mechanisms [START_REF] Ates | The effect of various treatment conditions on natural zeolites: ion exchange, acidic, thermal and steam treatments[END_REF]. The high value of ZH2.4 surface area could be related to an enhancement on zeolite microporosity. During the de-alumination process, the exchange of hydrated cations by H + could have risen micropore accessibility, eventually blocked before the acid treatment [START_REF] Ates | The effect of various treatment conditions on natural zeolites: ion exchange, acidic, thermal and steam treatments[END_REF]. Pore opening expansion could result also from a de-cationisation process, generating the so-called "open structures" that could lead to a decrease in diffusion resistance inside the zeolite framework [START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF]. Additionally, surface areas of NZ and ZH2.4 samples decrease after thermal treatment at 823 K. This could be related to the interaction of compensating cations and water in the zeolite framework. During the out-gassing process at 823 K, water molecules leave the zeolite, destabilising the zeolite charge structure. In order to correct this, it has been suggested that compensating cations in the zeolite framework, form bonds with network oxygen atoms, affecting the zeolite structure channels [START_REF] Valdés | Natural zeolite reactivity towards ozone: the role of compensating cations[END_REF]. However, in the ammonium-exchanged zeolites, surface areas are increased after thermal treatment at 823 K. Such increase in the value of surface areas could be related to the elimination of ammonium due to thermal out-gassing at 823 K, thus reducing any blocking effect on microporous structures [START_REF] Valdés | Natural zeolite reactivity towards ozone: the role of compensating cations[END_REF].

Results summarised in Table 1 also show that the content of compensating cations of zeolite samples decreases after the chemical treatments applied here. Although the de-cationisation treatment using ion-exchange with ammonium sulphate effectively reduces the amount of compensating cations (Na, K, Mg, Ca); the Si/Al ratio of the zeolite framework does not show a significant change. A considerable amount of calcium and sodium cations migrates to the solution during chemical modification, while magnesium and potassium cations are exchanged at a lower extent. Magnesium and potassium cations seem to be hard to access due to their large radius (e.g. Mg 2+ has an atomic radius of 0.16 nm; whereas K + , Ca 2+ and Na + have an atomic radius of 0.133 nm, 0.104 nm and 0.098 nm, respectively). Sodium ions in natural clinoptilolite zeolites have been reported to be weakly bonded to zeolite lattice and are easy to remove [START_REF] Allen | Adsorption of sulfur dioxide on chemically modified natural clinoptilolite. Acid modification[END_REF]. Moreover, helium pycnometry technique reveals the true density of zeolite samples. The value of true density of NZ decreases from 2.30 to 2.26 g cm -3 after acid treatment. In the case of ammoniumexchanged zeolites, density values fall to 2.10 and 2.11 g cm -3 for NH4Z1 and 2NH4Z1, respectively.

Acidic properties were determined by Infrared (IR) spectroscopic using pyridine as a probe molecule (99.5% purity supplied by Fluka) in a conventional IR cell of a Nicolet Magna-IR 550 spectrometer, using a pressed disc containing 2 mg of the zeolite sample and 100 mg of KBr, according to experimental procedures reported elsewhere [START_REF] Simon-Masseron | Influence of the Si/Al ratio and crystal size on the acidity and activity of HBEA zeolites[END_REF]. Samples were previously out-gassed at 623 K or 773 K for 2 h, and then pyridine was introduced into the IR cell at 293 K and maintained for 24 h. After that, non-adsorbed pyridine was removed at 423 K, 523 K, 623 K, 723 K or 773 K (heating temperature) under vacuum. This thermal desorption procedure allows to determine the acidity strength of Lewis and Brønsted acid sites by evaluating the amount of remaining adsorbed pyridine as temperature increases. Lewis and Brønsted acid sites that are detected above 623 K are considered as strong acidic sites [START_REF] Brodu | Role of Lewis acid sites of ZSM-5 zeolite on gaseous ozone abatement[END_REF]. Finally, samples were cooled down to room temperature and IR spectra were collected at an average of 64 scans and a resolution of 2 cm -1 . Surface concentrations of Brønsted and Lewis acid sites were calculated using the following equation [START_REF] Emeis | Determination of integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on solid acid catalysts[END_REF]:

q s-Py = A p r 2 pd m w pd × 1000 (1)
where q s-Py represents surface acidic site concentration (mol g -1 ); A p is the integrated absorbance band relative to the adsorbed pyridine (cm -1 ), at Brønsted acid sites (1545 cm -1 ) or at Lewis acid sites (1450 cm -1 ); r pd is the radius of the pressed-disc (cm); m is the pyridine molar extinction coefficient (1.13 cm mol -1 for Brønsted acid sites and 1.28 cm mol -1 for Lewis acid sites [START_REF] Konan | Surface modifications of illite in concentrated lime solutions investigated by pyridine adsorption[END_REF]) and w pd is the mass of the pressed-disc (mg). Chemical surface characteristics of zeolite samples were identified by FTIR spectroscopy, using a Bruker Mod Tensor 27 apparatus. Spectra from untreated samples and those after sequential adsorption-ozonation experiments were collected at an average 100 scans with a resolution of 4 cm -1 , using a pressed-disc (1% zeolite in KBr).

Experimental procedures

Toluene adsorption isotherms were performed using static volumetric flasks, as described in a previous study [START_REF] Brosillon | Mass transfer in VOC adsorption on zeolite: experimental and theoretical breakthrough curves[END_REF]. Sequential adsorption-ozonation experiments were carried out in a fixed-bed flow contactor (45 mm ID), using 50 g of zeolite sample and operating at 293 K and 101 kPa, as described elsewhere [START_REF] Monneyron | Measurement and modeling of singleand multi-component adsorption equilibria of VOC on high-silica zeolites[END_REF]. The inlet concentration of toluene (C Tin ) was fixed by bubbling dry air into pure liquid toluene using a temperature controlled bath (293 K) and diluted to a desired concentration by mixing with a fresh dry air stream. A 4.5 dm 3 min -1 stream containing 22.2 mol dm -3 toluene was continuously supplied over the zeolite bed until saturation was reached. Then, toluene was replaced by ozone (4.5 dm 3 min -1 ; 24.3 g O 3 dm -3 ) and ozone-based oxidative regeneration of toluene-saturated zeolite samples was performed.

Analytical methods

Toluene concentrations at the inlet and outlet streams were monitored on-line by gas chromatography using a VARIAN CP-3800 gas chromatographer equipped with a CP-SIL 8 capillary column (30 m × 0.53 mm ID and a 1.0 m film thickness) and coupled to a flame ionisation detector operating at 523 K. Separations were conducted under isothermal conditions at 403 K, during 4 min. Ozone concentration was registered on-line, using a BMT 963 ozone analyser.

Adsorbed toluene oxidation by-products were desorbed from regenerated zeolite samples after mixing 1 g of zeolite sample with 3 g of methanol in a closed amber glass flask. Samples were shaken during 48 h and oxidation by-products were detected by High Performance Liquid Chromatography (HPLC) in a Thermo Finning AS 1000 XR apparatus at room temperature (using a mixture of acidify water (pH = 2)/acetonitrile under gradient conditions, a Pronto SIL 120-5-C18 column, and UV-detection at 254 nm).

Results and discussion

Effect of zeolite chemical surface characteristics on toluene removal

Table 2 lists toluene adsorption isotherm parameters at 293 K on natural and modified zeolite samples after the out-gassing step at 623 K and 823 K. Adsorption equilibrium data are calculated here as the amount of adsorbed toluene per square meter of surface area of zeolite sample (mol m -2 ). As it can be seen in Table 2, adsorption isotherms are well represented by the Langmuir adsorption model [START_REF] Langmuir | The constitution and fundamental properties of solids and liquids. Part I. Solids[END_REF] with a R 2 > 98%, as follows:

q = q m bC T 1 + bC T ( 2
)
where q is the amount of adsorbed toluene on the zeolite surface at equilibrium (mol m -2 ), C T is the concentration of toluene at equilibrium (mol dm -3 ), q m is the maximum adsorption capacity (mol m -2 ), and b is the adsorption intensity or Langmuir coefficient (dm 3 mol -1 ). The Langmuir sorption model has been widely applied to VOC adsorption on natural and synthetic zeolites [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: the role of Bronsted acid sites[END_REF][START_REF] Valdés | Control of released volatile organic compounds from industrial facilities using natural and acid-treated mordenites: the role of acidic surface sites on the adsorption mechanism[END_REF][START_REF] Brosillon | Mass transfer in VOC adsorption on zeolite: experimental and theoretical breakthrough curves[END_REF][START_REF] Monneyron | Measurement and modeling of singleand multi-component adsorption equilibria of VOC on high-silica zeolites[END_REF]. Moreover, the values of separation factors (R L ), shown in Table 2, are comprised between 0 and 1; indicating the favourability of the adsorption process. For NZ, ZH2.4 and 2NH4Z1 samples, the maximum adsorption capacity values of the out-gassed samples at 623 K are higher than those values obtained for the out-gassed zeolites at 823 K. However, the value of maximum adsorption capacity registered by the out-gassed NH4Z1 sample at 623 K is slightly lower than the value obtained at 823 K. This lower value in the maximum adsorption capacity could be compensated with a higher value in the adsorption intensity coefficient registered by the out-gassed NH4Z1 sample at 623 K. The lowest adsorption capacity is achieved when natural zeolite is used. On the contrary, the highest adsorption capacity among all samples is reached by the out-gassed 2NH4Z1 sample at 623 K. Under such out-gassing conditions, a high content of Brønsted acid sites is generated on 2NH4Z1 zeolite sample as a consequence of the ammonium exchange treatment followed by thermal modification, as it has been indicated in a previous study [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: the role of Bronsted acid sites[END_REF]. Hence, Brønsted acid sites might be responsible for the observed enhancement on the adsorption capacity of 2NH4Z1 zeolite. The role of acidic surface sites toward the adsorption of VOCs on natural zeolites is analysed in detail elsewhere [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: the role of Bronsted acid sites[END_REF][START_REF] Valdés | Control of released volatile organic compounds from industrial facilities using natural and acid-treated mordenites: the role of acidic surface sites on the adsorption mechanism[END_REF].

Experimental results obtained during the ozonation of toluenesaturated zeolite samples are shown in Figs. 1 and2. On one hand, Fig. 1 indicates the evolution of the residual dimensionless concentration of ozone for samples previously out-gassed at 623 K (Fig. 1A) and out-gassed at 823 K (Fig. 1B). On the other hand, Fig. 2 depicts the variation of reactor bed temperature for samples previously out-gassed at 623 K (Fig. 2A) and out-gassed at 823 K (Fig. 2B).

Residual dimensionless concentration of ozone is calculated as a ratio of the ozone concentration at the outlet stream to the ozone concentration at the inlet stream. Results indicate that higher ozone consumptions, are observed in the out-gassed NH4Z1 and 2NH4Z1 samples at 823 K (see Figs. 1A andB). Indeed, the zeolite sample 2NH4Z1 (out-gassed at 823 K) registers the highest ozone consumption. However, the sample ZH2.4 shows the highest temperature change (about 23 K) when out-gassing thermal pre-treatment at 623 K or 823 K is applied (see Figs. 2A andB). The maximum increase on temperature shown by ZH2.4 sample is registered at around 2000 s, which is related to the total ozone consumption observed up to 3000 s. The increase on zeolite microporosity after the acid treatment could lead to multilayer adsorption of toluene on the ZH2.4 sample. Hence, the observed increase on reactor temperature during the oxidative regeneration of saturated ZH2.4 sample might be as a consequence of ozone reaction with multilayer adsorbed toluene.

After the ozone oxidative regeneration of toluene-saturated zeolites a 28%, 57%, 18% and 17% of the initial adsorption capacity is recovered for NZ, ZH2.4, NH4Z1 and 2NH4Z1 samples (previously out-gassed at 623 K), respectively; whereas the recovery of adsorption capacity for the out-gassed zeolite samples at 823 K are 22%, 57%, 28% and 22% for NZ, ZH2.4, NH4Z1 and 2NH4Z1, respectively. The highest degree of recovery on the adsorption capacity for ZH2.4 sample could be associated to a thermal desorption of oxidation by-products during the ozonation step, as a consequence of the observed increase in the reactor bed temperature. When toluenesaturated 2NH4Z1 sample (out-gassed at 823 K) is regenerated with ozone, the highest ozone consumption is registered. Ozone could not only be consumed by the adsorbed toluene but also by a surface decomposition mechanism that could take place at Lewis acid sites [START_REF] Valdés | Influence of chemical surface characteristics of natural zeolite on catalytic ozone abatement[END_REF].

Results shown in Table 3 reveal the presence of Brønsted and Lewis acid sites with different strength, obtained by IR spectroscopic analysis using pyridine as a probe molecule. As the heating temperature increases from 423 K to 773 K, weak acidic sites disappear and only strong acidic sites are detected. In the case of ammonium exchange zeolites out-gassed at 823 K, the results show an increase on strong Lewis acid sites as the temperature increase. Ammonium ion-exchange treatments followed by thermal outgassing at 823 K, could reduce pyridine diffusion resistance inside the zeolite framework, leading to an increase on the detection of adsorbed pyridine at stronger Lewis acid sites in such zeolite samples. The concentration of Brønsted acid sites on modified zeolites is higher than on NZ sample. Lewis acid sites increase from NZ to 2NH4Z1. On one hand, the high ozone consumption registered for the sample 2NH4Z1 (out-gassed at 823 K) could be due to its high concentration of strong Lewis acid sites. On the other hand, the high toluene adsorption capacity observed for the sample 2NH4Z1 (outgassed at 623 K) could be related to its high concentration of strong Brønsted acid sites. Brønsted acid sites could be the main surface sites responsible of toluene adsorption; whereas Lewis acid sites could be mainly related to ozone decomposition and the generation of active oxygen species. During the oxidative regeneration with ozone, surface oxidation reactions could take part among adsorbed toluene at Brønsted acid sites and surface atomic oxygen species generated at Lewis acid sites, reducing the amount of adsorbed toluene on zeolite surface.

FTIR study of natural and modified zeolite samples

IR spectra for NZ, ZH2.4, NH4Z1, and 2NH4Z1 zeolite samples upon the out-gassing pre-treatment at 623 K (and at 823 K) without toluene adsorption, after toluene adsorption at 293 K, and after oxidative regeneration using gaseous ozone; are shown in Figs. 3456, respectively.

As it can be seen in all figures, IR bands around 1100 cm -1 and 795 cm -1 are observed in all spectra of the zeolite samples prior to toluene adsorption (dotted line). The bands near 1100 cm -1 and around 795-800 cm -1 are attributed to Si-O vibrations of amorphous silica normally present in the network of natural zeolites and clays [START_REF] Ates | The effect of various treatment conditions on natural zeolites: ion exchange, acidic, thermal and steam treatments[END_REF][START_REF] Madejová | FTIR techniques in clay mineral studies[END_REF]. These absorption bands are shifted to: 1041 cm -1 (NZ-623 K), 1045 cm -1 (NZ-823 K), 1082 cm -1 (ZH2.4-623 K and ZH2.4-823 K), 1043 cm -1 (NH4Z1-623 K), 1057 cm -1 (NH4Z1-823 K) and 1047 cm -1 (2NH4Z1-623 K and 2NH4Z1-823 K). The highest observed intensity bands at 1082 cm -1 and 795 cm -1 are recorded by ZH2.4 sample. These results are in agreement with those obtained by XRF analyses, showing the biggest silica content of this zeolite sample (see Table 1). Additionally, the absorption band around 1630 cm -1 is observed in all samples; which has been assigned to the deformation band of adsorbed water [START_REF] Valdés | Control of released volatile organic compounds from industrial facilities using natural and acid-treated mordenites: the role of acidic surface sites on the adsorption mechanism[END_REF]. Stretching vibrations at around 3625 cm -1 and 3440 cm -1 could be associated to coordinated water on zeolite surface [START_REF] Serra | FTIR study of toluene adsorption on Csexchanged mordenites[END_REF] Recorded IR spectrum after toluene adsorption (dashed line) on NZ sample (out-gassed at 623 K and 823 K), registers a band at 1420 cm -1 , assigned to C C stretching vibration corresponding to the aromatic ring of toluene molecule. This band is slightly shifted to a lower frequency, compared to the C C stretching vibration of toluene in the gaseous phase (see Figs. 3A andB). Similarly, a band at 1520 cm -1 is registered for ZH2.4 zeolites (see Figs. 4A andB). IR spectra of ammonium-exchanged zeolite samples, NH4Z1 (see Figs. 5A andB) and 2NH4Z1 (see Figs. 6A andB) out-gassed at 623 K and 823 K show a band at 1470 cm -1 after toluene adsorption. An absorption band in the region around 1450-1540 cm -1 with a maximum at 1477 cm -1 has been associated to adsorbed toluene on zeolites [START_REF] Malherbe | Study of Fourier transform infrared-temperatureprogrammed desorption of benzene, toluene and ethylbenzene from H-ZSM-5 and H-beta zeolites[END_REF]. Such experimental results confirm that toluene is chemically adsorbed on zeolite surface. For the out-gassed NH4Z1 samples at 623 K and 823 K an increase on the intensity of an absorption band at 1470 cm -1 could be associated to the C C aromatic ring stretching mode of toluene molecule. This absorption band is shifted to a lower wavenumber compared to that of gaseous toluene at 1506 cm -1 . This band displacement could be attributed to changes in electronic distribution and the symmetry of the aromatic ring when it interacts with the zeolite structure [START_REF] Serra | FTIR study of toluene adsorption on Csexchanged mordenites[END_REF]. Similar trend is observed in the case of benzene interacting with the OH group of zeolites [START_REF] Su | Quantitative characterisation of H-Mordenite zeolite structure by infrared spectroscopy using benzene adsorption[END_REF]. Hence, results obtained here could be related to toluene interaction with a higher content of Brønsted acid sites, as shown Table 3.

Additionally, an IR absorption band around 1420 cm -1 is observed for the out-gassed ammonium-exchanged zeolite samples at 623 K, NH4Z1 (see Fig. 5A) and 2NH4Z1 (see Fig. 6A), prior to the adsorption of toluene. This absorption band can still be seen in these samples after toluene adsorption and even after the oxidative regeneration with ozone. However, this band does not appear in the spectra of the out-gassed ammonium-exchanged zeolite samples at 823 K, NH4Z1 (see Fig. 5B) and 2NH4Z1 (see Fig. 6B). Such behaviour could be related to some ammonium molecules (NH 4 + ) that remain adsorbed on the out-gassed samples at 623 K and are finally desorbed after the thermal out-gassing pre-treatment at 823 K. These results are in agreement with those reported by another research group who associates an IR band around 1430 cm -1 to adsorbed ammonium on zeolite samples [START_REF] Niwa | Ammonia IRMS-TPD study on the distribution of acid sites in mordenite[END_REF].

As it can be appreciated in all figures, the IR spectra after oxidative regeneration with ozone (solid line) show a common absorption band at 1384 cm -1 for all out-gassed zeolite samples. This IR band could be related to active oxygen surface species generated after ozone decomposition on acidic sites. Several studies have assigned the IR absorption band at 1380 cm -1 to active atomic oxygen on zeolites surface, generated after ozone decomposition at Lewis acid surface sites [START_REF] Einaga | Catalytic oxidation of benzene with ozone over aluminasupported manganese oxides[END_REF][START_REF] Einaga | Benzene oxidation with ozone over supported manganese oxide catalysts: effect of catalyst support and reaction conditions[END_REF][START_REF] Kasprzyk-Hordern | Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment[END_REF][START_REF] Dhandapani | Gas phase ozone decomposition catalysts[END_REF][START_REF] Bulanin | IR spectra of adsorbed ozone[END_REF][START_REF] Bulanin | Infrared study of ozone adsorption on CaO[END_REF][START_REF] Li | Mechanism of ozone decomposition on a manganese oxide catalyst. 2. Steady-state and transient kinetic studies[END_REF]. Previous studies have suggested the interaction of surface atomic oxygen with organic compounds such as toluene [START_REF] Chao | Potential use of a combined ozone and zeolite system for gaseous toluene elimination[END_REF][START_REF] Einaga | Catalytic oxidation of benzene with ozone over Mn ion-exchanged zeolites[END_REF]. Moreover, an increase on the intensity of the absorption band observed at 1384 cm -1 , could be associated to a higher content of Lewis acid sites present on the out-gassed samples at 823 K in comparison to the out-gassed samples at 623 K, as shown in Table 3. The density of Lewis acid sites is increased after the out-gassing treatment at 823 K due to that Brønsted acid sites are converted into new Lewis acid sites. Hence, a high content of Lewis acid sites are available for ozone adsorption and transformation into active oxygen species.

Two distinctive bands can be seen at 2851 and 2920 cm -1 , after ozone regenerative oxidation of adsorbed toluene (solid line) on NZ out-gassed at 623 K and 823 K (see Figs. 3A andB), NH4Z1 outgassed at 823 K (see Fig. 5B), and 2NH4Z1 out-gassed at 623 K and 823 K (see Figs. 6A andB). A band at 2957 cm -1 is observed in 2NH4Z1 samples (out-gassed at 623 K and 823 K). Bands at 2899 and 2935 cm -1 have been related to the (C-H) stretching vibrations of formate species [START_REF] Chen | In situ DRIFTS study on the partial oxidation of ethylene over Co-ZSM-5 catalyst[END_REF]. In this study, such IR absorption bands could be shifted to 2851, 2920 and 2957 cm -1 and might be an evidence of the formation of toluene oxidation by-products that remain adsorbed on zeolite surface after ozone regeneration treatment. These IR bands are not registered in the spectra of regenerated ZH2.4 samples; showing that less oxidation by-products remain adsorbed [START_REF] Chen | In situ DRIFTS study on the partial oxidation of ethylene over Co-ZSM-5 catalyst[END_REF]. However, a high amount of toluene remains adsorbed on ZH2.4 sample (out-gassed at 623 K and 823 K) after ozone regeneration, as indicated by HPLC analyses. Such experimental results could be associated to a lower activity of Lewis acid sites of this sample. A similar trend is observed for NH4Z1 outgassed at 623 K, as consequence of its lower content of Lewis acid sites. Aditionally, a band at 1743 cm -1 is observed in the collected spectra for the ozonation of toluene saturated-2NH4Z1 samples. In another publication, the band around 1752 cm -1 has been associated to formic acid [START_REF] Jeon | Mechanistic study on the SCR of NO by C3H6 over Pt/V/MCM-41[END_REF].

HPLC technique allows identifying a narrow band of oxidation by-products that remain adsorbed on zeolite surface after the oxidative regeneration with ozone. As a consequence of ozonetoluene surface reactions; formic acid, acetaldehyde, benzoic acid, benzaldehyde are detected here, as the main intermediate compounds of toluene oxidation. The highest concentration level of formic acid is detected for NZ sample out-gassed at 623 K. Similarly, high concentrations of acetaldehyde, benzaldehyde and benzoic acid are registered in ZH2.4 (out-gassed at 623 K), NZ (out-gassed at 823 K) and NZ (out-gassed at 623 K) samples, respectively. The high concentration levels of formic acid, benzoic acid and benzaldehyde observed in NZ sample (out-gassed at 623 K), could be related to a lower catalytic activity toward ozone, as it has been previously suggested [START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF].

Moreover, the concentration of toluene that remains adsorbed inside zeolite channels after oxidative regeneration with ozone, decreases in the following order: ZH2.4-623 K > ZH2.4-823 K > NZ-623 K > NH4Z1-823 K > NZ-823 K > NH4Z1-623 K > 2NH4Z1-823 K > 2NH4Z1-623 K. The presence of adsorbed toluene after the ozone regeneration process applied here could be related to a combination of factors, which involve a high adsorption capacity of toluene and a low catalytic activity toward ozone of some zeolite samples. Thus, the highest registered concentration of toluene for ZH2.4 sample (out-gassed at 623 K) could correspond to a high toluene adsorptive capacity and a lower catalytic activity toward ozone, as reported elsewhere [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: the role of Bronsted acid sites[END_REF][START_REF] Alejandro | Natural zeolite reactivity towards ozone: the role of acid surface sites[END_REF]. Acetaldehyde and benzaldehyde have been identified as oxidation by-products of toluene reaction with ozone in the presence of synthetic zeolites [START_REF] Chao | Potential use of a combined ozone and zeolite system for gaseous toluene elimination[END_REF]. In addition, formic and benzoic acids have already been reported as the main intermediates of toluene reaction with ozone [START_REF] Franco | BTEX decomposition by ozone in gaseous phase[END_REF]. These results are in agreement with those obtained by FTIR analyses where IR bands registered at around 2851-2920 cm -1 and at 1743 cm -1 are associated to adsorbed oxidation by-products, such as formic acid, acetaldehyde, benzoic acid and benzaldehyde (see Figs. 3A, B, 5B, 6A and B). As discussed before, these oxidation by-products might be responsible for the observed reduction in toluene adsorption capacity after ozone oxidative-regeneration of toluene saturated-zeolite samples.

Results obtained here suggest that a surface reaction mechanism could take place among toluene molecules adsorbed at Brønsted acid sites and surface active oxygen species generated after ozone decomposition at Lewis acid sites. As a consequence of such surface interactions, intermediate products such as formic acid, acetaldehyde, benzoic acid and benzaldehyde are generated. These findings are in agreement with other published research studies [START_REF] Van Durme | Abatement and degradation pathways of toluene in indoor air by positive corona discharge[END_REF][START_REF] Huang | Destruction of toluene by ozone-enhanced photocatalysis: performance and mechanism[END_REF].

Conclusions

Experimental results obtained here show that single ozone gas regeneration would not be suitable to regenerate the adsorption capacity of toluene-saturated natural zeolites. Indeed, the presence of adsorbed toluene oxidation by-products on the zeolite surface impairs the regeneration process. The highest degree of recovery on the adsorption capacity is obtained when acid-treated zeolite is used (ZH2.4), probably due to the desorption of adsorbed species as a consequence of temperature increase during the regeneration step. Moreover, results indicate that acidic surface sites in the form of Brønsted acid sites could be responsible for toluene adsorption, whereas Lewis acid sites could be mainly associated to ozone decomposition and the generation of surface active oxygen species. Toluene molecules adsorbed at Brønsted acid sites seem to react with surface active oxygen species generated by ozone gaseous decomposition at Lewis acid sites. A sequential adsorptionozonation followed by thermal regeneration could be an attractive option for VOCs removal.

Fig. 1 .

 1 Fig. 1. Evolution of residual dimensionless concentration of ozone during ozonation of toluene saturated-zeolites: (A) thermal out-gassed zeolite samples at 623 K; (B) thermal out-gassed zeolite samples at 823 K ( / ) NZ, ( /△) ZH2.4, ( /♦) NH4Z1, (᭹/ ) 2NH4Z1. Experimental conditions: 50 g of zeolite, 4.5 dm 3 min -1 , 101 kPa, 293 K, C O 3 inlet = 23.4 g m -3 .

Fig. 2 .

 2 Fig. 2. Variation on the reactor bed temperature during ozonation of toluene saturated-zeolites: (A) thermal out-gassed zeolite samples at 623 K; (B) thermal out-gassed zeolite samples at 823 K. ( / ) NZ, ( /△) ZH2.4, ( /♦) NH4Z1, (᭹/ ) 2NH4Z1. Experimental conditions: 50 g of zeolite, 4.5 dm 3 min -1 , 101 kPa, 293 K, C O 3 inlet = 23.4 g m -3 .

Fig. 3 .

 3 Fig. 3. FTIR spectra of natural zeolite (NZ): (A) out-gassed at 623 K; (B) out-gassed at 823 K. Dotted line represents NZ sample after thermal treatment, dashed line represents toluene saturated-NZ sample, and solid line represents regenerated NZ after ozonation.

Fig. 4 .

 4 Fig. 4. FTIR spectra of acid-modified natural zeolite (ZH2.4): (A) out-gassed at 623 K; (B) out-gassed at 823 K. Dotted line represents ZH2.4 sample after thermal treatment, dashed line represents toluene saturated-ZH2.4 sample, and solid line represents regenerated ZH2.4 sample after ozonation.

Fig. 5 .

 5 Fig. 5. FTIR spectra of ammonium-modified natural zeolite (NH4Z1): (A) out-gassed at 623 K; (B) out-gassed at 823 K. Dotted line represents NH4Z1 sample after thermal treatment, dashed line represents toluene saturated-NH4Z1 sample, and solid line represents regenerated NH4Z1 sample after ozonation.

Fig. 6 .

 6 Fig. 6. FTIR spectra of twice ammonium-modified natural zeolite (2NH4Z1): (A) out-gassed at 623 K; (B) out-gassed at 823 K. Dotted line represents 2NH4Z1 sample after thermal treatment, dashed line represents toluene saturated-2NH4Z1 sample, and solid line represents regenerated 2NH4Z1 sample after ozonation.

Table 1

 1 Physical-chemical characterisation of zeolite samples.

	Sample	S623K (m 2 g -1 )	S823K (m 2 g -1 )	SiO2 a	Al2O3 a	Na2O a	CaO a	K2O a	MgO a	TiO2 a	Fe2O3 a	MnO a	Si/Al ratio	b (g cm -3 )
	NZ	205	170	75.25	14.1	1.89	4.57	0.74	0.66	0.42	2.31	0.05	5.34	2.30
	ZH2.4	434	369	82.71	11.65	0.43	1.48	0.64	0.39	0.48	2.19	0.04	7.1	2.26
	NH4Z1	181	222	78.07	14.69	0.68	2.36	0.67	0.46	0.46	2.55	0.05	5.32	2.10
	2NH4Z1	171	261	79.26	14.85	0.26	1.82	0.39	0.37	0.47	2.53	0.05	5.34	2.11

a By XRF (% w). b By Helium pycnometry.

Table 2

 2 Parameters of Langmuir adsorption model.

	Sample		qm (mol m -2 )	b (dm 3 mol -1 )	R 2 (%)	RL a ×10 2
	NZ	623 K	1.46	27.35	99	0.16
	ZH2.4		3.27	31.67	99	0.14
	NH4Z1		2.07	30.06	99	0.15
	2NH4Z1		5.22	9.27	98	0.48
	NZ	823 K	0.55	108	99	0.04
	ZH2.4		3.07	19.77	99	0.23
	NH4Z1		2.76	9.02	98	0.50
	2NH4Z1		3.09	19.29	99	0.23
	C

Tin : inlet concentration of toluene. a Separation factor (RL): RL = 1/(1 + bC Tin ).

Table 3

 3 Brønsted and Lewis acidity content of zeolite samples detected by IR at different heating temperatures using pyridine as a probe molecule.

	Sample	Out-gassing	qs-Py (mol g -1 )	Heating
		temperature			temperature (K)
			Brønsted	Lewis
	NZ	623 K	32.7	76.3	423
			52.4	48.1	523
			23.5	60.0	623
			0.7	44.4	723
			0.4	31.1	773
		773 K	18.8	172.9	423
			48.8	93.3	523
			26.2	91.9	623
			9.6	52.5	723
			1.4	42.8	773
	ZH2.4	623 K	433.8	115.8	423
			383.9	104.4	523
			297.3	76.9	623
			125.7	70.2	723
			64.6	72.0	773
		773 K	487.6	114.2	423
			490.1	84.5	523
			368.7	76.6	623
			175.2	68.8	723
			92.9	70.3	773
	NH4Z1	623 K	516.6	47.8	423
			514.5	43.8	523
			509.0	53.8	623
			372.6	60.0	723
			281.2	60.5	773
		773 K	263.2	48.4	423
			312.6	49.4	523
			266.9	91.3	623
			243.5	139.5	723
			189.0	165.0	773
	2NH4Z1	623 K	715.6	96.3	423
			851.2	71.9	523
			723.9	69.1	623
			432.9	162.3	723
			274.5	201.9	773
		773 K	301.4	200.3	423
			312.6	194.1	523
			302.7	207.0	623
			233.1	260.5	723
			179.8	282.8	773

Acknowledgements

The authors wish to thank CONICYT, FONDECYT/Regular (Grant No. 1130560), ANR (Grant No. ANR-10-ECOT-011-01), ECOS/CONICYT Program (Grant No. C11E08), and BASAL PFB-27 for their financial support. S. Alejandro expresses his gratitude to the France Embassy in Santiago de Chile for providing a mobility doctoral scholarship and to Dr. N. Brodu from Université de Toulouse, INPT, UPS, Laboratoire de Génie Chimique for his valuable collaboration. H. Valdés gratefully acknowledges funding under CNRS Délégation Midi-Pyrénées contract 424948.