Léa Lorrain-Soligon

Frédéric Robin

Xavier Bertin

Marko Jankovic

Pierre Rousseau

Vincent Lelong

François Brischoux

Lorrain-Soligon Léa

Robin Frédéric

Bertin Xavier

Jankovic Marko

Brischoux François

Long-term trends of salinity in coastal wetlands: Effects of climate, extreme weather events, and sea water level

Keywords: Brackish water, Marine submersion, Precipitation, Salinization, Seasonal variations, Temperature

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Introduction

Coastal wetlands are situated at the boundary between the land and the sea [START_REF] Janas | Importance of benthic macrofauna and coastal biotopes for ecosystem functioning-Oxygen and nutrient fluxes in the coastal zone[END_REF]. Despite a restricted extent of the interface between oceanic and terrestrial biomes, coastal wetlands represent a significant surface area worldwide [START_REF] Hopkinson | Coastal wetlands: A synthesis[END_REF]. While they play significant ecological roles [START_REF] Davis | Water resource development and high value coastal wetlands on the lower burdekin floodplain, Australia[END_REF][START_REF] Hobohm | Coastal habitats, shallow seas and inland saline steppes: Ecology, distribution, threats and challenges[END_REF][START_REF] Hopkinson | Coastal wetlands: A synthesis[END_REF], sheltering a large diversity of species and habitats [START_REF] Janas | Importance of benthic macrofauna and coastal biotopes for ecosystem functioning-Oxygen and nutrient fluxes in the coastal zone[END_REF][START_REF] Maynard | Coastal Wetlands[END_REF] and serving a role of carbon sequestration and protection of the terrestrial environment [START_REF] Barbier | Coastal ecosystem-based management with nonlinear ecological functions and values[END_REF][START_REF] De Battisti | The resilience of coastal ecosystems: A functional trait-based perspective[END_REF][START_REF] Nelson | Salt marsh as a coastal filter for the oceans: changes in function with experimental increases in nitrogen loading and sea-level rise[END_REF], coastal wetlands are currently affected by a large variety of anthropogenic threats such as eutrophication, chemical pollution, land use, invasive species, and ongoing modification of climatic conditions [START_REF] Barua | Coastal zone and wetland ecosystem: Management issues[END_REF][START_REF] Martínez-Megías | Biodiversity impacts by multiple anthropogenic stressors in Mediterranean coastal wetlands[END_REF]. Among these threats, changes in salinity regimes (environmental salinization) across spatial and temporal scales relevant to coastal wetlands have been less studied to date, despite the major role of this environmental parameter in the functioning of these ecosystems [START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF].

Coastal ecosystems are naturally exposed to spatial and temporal variations of salinity [START_REF] Barua | Coastal zone and wetland ecosystem: Management issues[END_REF]. Such natural variations of salinity (i.e. primary salinization, [START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF] are linked to the specific geographic position of coastal ecosystems, and notably the exchanges between terrestrial and oceanic environments [START_REF] Xue | Modeling ocean circulation and biogeochemical variability in the Gulf of Mexico[END_REF]. The natural processes which affect variations of salinity are primarily linked to landward aerial transport of dry or wet oceanic salts [START_REF] Zaman | Guideline for salinity assessment, mitigation and adaptation using nuclear and related techniques[END_REF], such as sea-sprays [START_REF] Benassai | Sea-spray deposition in Antarctic coastal and plateau areas from ITASE traverses[END_REF], and groundwater intrusions and exchanges [START_REF] Alcolea | Hydrogeological modelling for the watershed management of the Mar Menor coastal lagoon (Spain)[END_REF][START_REF] Casamitjana | Modeling the salinity fluctuations in salt marsh lagoons[END_REF][START_REF] Coluccio | Groundwater discharge rates and uncertainties in a coastal lagoon using a radon mass balance[END_REF][START_REF] Erostate | Groundwater dependent ecosystems in coastal Mediterranean regions: Characterization, challenges and management for their protection[END_REF][START_REF] Kløve | Groundwater dependent ecosystems. Part I: Hydroecological status and trends[END_REF][START_REF] Knighton | Tidal-creek extension and saltwater intrusion in northern Australia[END_REF][START_REF] Meredith | Effects of morphology and sediment permeability on coastal lagoons' hydrological patterns[END_REF][START_REF] Tamborski | Temporal variability of lagoon-sea water exchange and seawater circulation through a Mediterranean barrier beach[END_REF][START_REF] Visschers | Accelerated migration of mangroves indicate large-scale saltwater intrusion in Amazon coastal wetlands[END_REF]. As a consequence, coastal ecosystems are characterized by spatial and temporal heterogeneity of salinity [START_REF] Estévez | Drivers of spatio-temporal patterns of salinity in Spanish rivers: a nationwide assessment[END_REF][START_REF] Fu | Sea surface salinity estimation and spatial-temporal heterogeneity analysis in the gulf of Mexico[END_REF][START_REF] Ghalambor | Ecological effects of climate-driven salinity variation in the San Francisco estuary: Can we anticipate and manage the coming changes?[END_REF][START_REF] Ranjbar | Spatio-temporal mapping of salinity in the heterogeneous coastal aquifer[END_REF]. The spatial determinants of salinity are mainly linked to the proximity of salt sources (i.e. distance to the coastline; [START_REF] Wu | Prediction of soil salinity in the Yellow River Delta using geographically weighted regression[END_REF] while salinity can also change temporally from short-(hours) to longer temporal scales (years) [START_REF] Ghosh | Short-and long-term temporal variations in salinity and the oxygen, carbon and hydrogen isotopic compositions of the Hooghly Estuary water, India[END_REF][START_REF] Gutierrez | Temporal variations of natural soil salinity in an arid environment using satellite images[END_REF]. Complementarily, climatic conditions (e.g. temperature, rainfall, wind regimes) interact with salt deposition and can influence variations of salinity [START_REF] Morcillo | Salinity in marine atmospheric corrosion: its dependence on the wind regime existing in the site[END_REF][START_REF] Perigaud | Impact of interannual rainfall anomalies on Indian Ocean salinity and temperature variability[END_REF]. For instance, salinity increases when evaporation exceeds precipitation [START_REF] Hassani | Global predictions of primary soil salinization under changing climate in the 21st century[END_REF], which might naturally occur seasonally [START_REF] De Luis | Seasonal precipitation trends in the Mediterranean Iberian Peninsula in second half of 20th century[END_REF].

Coastal ecosystems are also susceptible to secondary salinization, the increase in salt concentration caused by the direct actions of humans or their activities [START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF][START_REF] Izam | Freshwater ecosystem: a short review of threats and mitigations in Malaysia[END_REF], because of several processes operating at different temporal scales. First, ongoing climatic changes are expected to induce a significant increase in sea level [START_REF] Church | Sea-level rise from the late 19th to the early 21st century[END_REF][START_REF] Domingues | Improved estimates of upper-ocean warming and multi-decadal sea-level rise[END_REF][START_REF] Gornitz | Sea-level rise: A review of recent past and near-future trends[END_REF][START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF]. Second, current climatic changes are expected to increase the intensity and frequency of extreme events, such as marine storms and associated surges [START_REF] Dettinger | Climate change, atmospheric rivers, and floods in California -A multimodel analysis of storm frequency and magnitude changes[END_REF]IPCC, 2022;[START_REF] Trenberth | Attribution of climate extreme events[END_REF]. Third, alterations of hydric cycles due to changes in the magnitude and timing of precipitation [START_REF] Martínez-Megías | Biodiversity impacts by multiple anthropogenic stressors in Mediterranean coastal wetlands[END_REF][START_REF] Neubauer | Global change and tidal freshwater wetlands: scenarios and impacts[END_REF] should affect freshwater inputs and change evaporation rates [START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF][START_REF] Jeppesen | Salinization increase due to climate change will have substantial negative effects on inland waters: a call for multifaceted research at the local and global scale[END_REF][START_REF] Mills | Paleoclimate studies and natural-resource management in the Murray-Darling Basin II: unravelling human impacts and climate variability[END_REF]. Finally, other anthropogenic activities can directly lead to an increase in salinity, such as groundwater pumping or water abstraction which have been shown to increase sea water inundation and saline intrusion [START_REF] Peters | Groundwater pumping causes salinization of coastal streams due to baseflow depletion: Analytical framework and application to Savannah River, GA[END_REF][START_REF] Reid | Emerging threats and persistent conservation challenges for freshwater biodiversity[END_REF]. All of these processes and the associated increased salinity may severely transform coastal areas [START_REF] Knighton | Tidal-creek extension and saltwater intrusion in northern Australia[END_REF][START_REF] Visschers | Accelerated migration of mangroves indicate large-scale saltwater intrusion in Amazon coastal wetlands[END_REF], affecting habitat structure through landward habitat shifts [START_REF] Visschers | Accelerated migration of mangroves indicate large-scale saltwater intrusion in Amazon coastal wetlands[END_REF], and biodiversity through changes in geographic distribution [START_REF] Brischoux | Osmoregulatory ability predicts geographical range size in marine amniotes[END_REF][START_REF] Gunter | Some relations of faunal distributions to salinity in estuarine waters[END_REF], and in turn community structure [START_REF] Anufriieva | Diversity of fauna in Crimean hypersaline water bodies[END_REF][START_REF] Cunillera-Montcusí | Freshwater salinisation: a research agenda for a saltier world[END_REF][START_REF] Ersoy | Experimental assessment of salinization effects on freshwater zooplankton communities and their trophic interactions under eutrophic conditions[END_REF][START_REF] González-Sansón | Estuarine fish diversity as indicator of natural environmental gradients[END_REF][START_REF] Hart | Ecological risk to aquatic systems from salinity increases[END_REF][START_REF] Kendall | Too much freshwater, not enough, or just right? Long-term trawl monitoring demonstrates the impact of canals that altered freshwater flow to three bays in SW Florida[END_REF]. However, despite the major ecological impacts of salinization, the spatio-temporal variations of salinity in coastal areas, and more importantly, their recent changes in response to climatic modifications remain poorly known. Particularly, if salinity variations in underground coastal water [START_REF] Colombani | Origin and pattern of salinization in the Holocene aquifer of the southern Po Delta (NE Italy)[END_REF]Mastrocicco et al., 2021;Mastrocicco and Colombani, 2021) or large water bodies such as lagoons [START_REF] Casamitjana | Modeling the salinity fluctuations in salt marsh lagoons[END_REF][START_REF] Meredith | Effects of morphology and sediment permeability on coastal lagoons' hydrological patterns[END_REF] have been investigated, the impact on smaller water bodies, relevant to coastal freshwater biodiversity such as ponds, and considering different processes, is poorly understood. In order to better understand these variations in salinity, it is essential to investigate the effects of environmental factors that might influence salinization with high temporal resolution time series of salinity measurements [START_REF] Lee | Recognizing salinity threats in the climate crisis[END_REF], such precise measurement lacking to date.

Such investigations are currently of crucial importance because they will allow assessment of the extent of salinity variations in coastal environments (either natural or induced by climate change), and thus to understand the consequences of these changes on the resilience of these ecosystems.

In this study, we investigated the spatio-temporal variations of salinity across longterm (decades) and fine temporal (monthly records) and spatial (varying distance to the coastline) resolutions in water bodies of two typical temperate coastal wetlands situated on the Atlantic coast of France. On both sites, water salinity was recorded each month (for 24 years on 24 ponds in one site, and for 10 years on 31 ponds in the other site). Based on these detailed data sets, our objectives were multiple. First, we analysed the seasonal and spatial variations of salinity. We hypothesized that salinity should be greater during summer because of reduced precipitation and higher temperatures, and that salinity should be higher closer to the seashore because of landward deposition of seaspray as well as exchanges (especially groundwater exchanges) with the marine environment. Second, we analysed the long-term changes in salinity and related these changes to concomitant variations in precipitation and temperature in order to highlight potential effects of ongoing climate change on this parameter. We hypothesized that increased temperatures and decreased precipitation over time [START_REF] Briffa | Wet and dry summers in Europe since 1750: evidence of increasing drought[END_REF][START_REF] Schär | The role of increasing temperature variability in European summer heatwaves[END_REF] should increase salinity levels across our time series. We also hypothesized that these longterm changes should interact with the spatial variations of salinity, with proportionally greater salinity increase closer to the seashore.

Finally, we complemented our analyses with models of sea water levels computed in front of both sites across 2000-2020 in order to investigate whether fluctuation in this parameter can contribute to further salinization through local inundations. We hypothesized that sea water levels which occasionally provoke landward inundations should increase in frequency and intensity over time. At MO, salinity of 24 ponds was measured (distance to the ocean: mean=564.52 m ± 59.99, min=57.69 m, max=1079.24 m, Figure 1) from January 1996 to December 2019.

Methods

At MY, the salinity of 31 ponds was measured (distance to the ocean: mean=564.29 m ± 24.95, min=186.83 m, max=836.76 m, Figure 1) from January 2010 (corresponding to the sea water intrusion by storm Xynthia) to December 2019. These represent small body of still water (<5000 m 2 ), presenting similar characteristic of habitat types, surrounding and aquatic vegetation, and depth (mean depth of ponds ranging from ~0.5m to 1.5m). Some details on pond characteristics are given in Appendix A . On each pond, the salinity was recorded once per month during 24 years (at MO) and 10 years (at MY). The salinity was recorded using a portable reflactometer until 2000 (Cond 330i), and then using a conductimeter (YSI Professional Plus, ProQuatro

Multiparameter Meter, and Multi 340i with TetraCon 325 probe), and was repeatedly measured at the same location for each pond (20 cm from the pond edge, 10 cm below the water surface).

Meteorological parameters

We included yearly and monthly data of temperature and precipitation using the easyclimate package [START_REF] Moreno | Spatial downscaling of European climate data[END_REF]) on R 3.6.3 (R Core Team 2020).

This package allows extraction of maximum temperature, which is expected to influence evaporation, and total precipitation on a daily scale. We extracted these data from La Rochelle meteorological station (46°9'0.684"N, 1°8'43.08"W) situated at 17.4 km and 26.6 km from MY and MO respectively, between 1996 and 2020. The variations of these data across months are presented in Appendix B.

Estimates of the water levels across 20 years

To investigate the possible impact of changes in sea level on coastal wetland salinization, we simulated the hydrodynamic circulation in the study area using the numerical model SCHISM [START_REF] Zhang | Seamless cross-scale modeling with SCHISM[END_REF], which solves the primitive equations on unstructured grids using advanced numerical methods. We used a hydrodynamic model because sea water levels strongly vary spatially while the only tide gauge available over the studied period (La Pallice, see Figure 1) is located about 20 and 30 km to the North of both study sites. This spatial variability of water levels is due to spatial variations of tides and storm surges. Thus, [START_REF] Dodet | Characterization of sea-level variations along the metropolitan coasts of France: Waves, tides, storm surges and long-term changes[END_REF] showed that, over the modelled domain and over the period 1998-2018, the mean tidal range varied from 3.28 and 3.83 m and the maximum storm surge varied from 1.15 and 1.57 m. To a lesser extent, the tide gauge data has some gaps while the model is continuous over the studied period. SCHISM was implemented in 2DH (two dimensional horizontal, which means that the model uses one single layer on the vertical) over the 'Pertuis Charentais' area with a spatial resolution ranging from 2000 m along the open boundary to 100 m near the shoreline, as described in [START_REF] Savelli | Impact of chronic and massive resuspension mechanisms on the microphytobenthos dynamics in a temperate intertidal mudflat[END_REF]. The model starts at mean sea-level and uses a ramp of 1 day. A sensitivity analysis reveals that the model converges when using longer ramps, which is due to the small dimensions of the modelled domain. The model is forced by amplitudes and phases of the 18 main tidal constituents, linearly interpolated from the regional tidal model of [START_REF] Bertin | Importance of wave age and resonance in storm surges: The case Xynthia, Bay of Biscay[END_REF] and fields of sea-level pressure and 10 m winds originating from the CFSR reanalysis [START_REF] Saha | The NCEP climate forecast system reanalysis[END_REF]. The hydrodynamic model used in this study is well-established and was previously developed to compute sediment transport and morphological changes [START_REF] Guerin | Wave control on the rhythmic development of a wide estuary mouth sandbank: A process-based modelling study[END_REF] and bed shear stress and biofilm resuspension in front of MO [START_REF] Savelli | Impact of chronic and massive resuspension mechanisms on the microphytobenthos dynamics in a temperate intertidal mudflat[END_REF]. The model was run over the period 2000-2020 and hourly time series of water levels were extracted in front of each studied wetland. Due to the 2DH (two dimensional horizontal) barotropic configuration, the model cannot capture longterm changes in sea levels due to large-scale circulation patterns and global warming.

In order to account for these processes which are relevant at the scale of the studied period (2000-2020), yearly mean sea levels observed at the nearby tide gauge of La Rochelle (data available at https://data.shom.fr/) were computed and linearly added to model outputs. This approach was validated against the available water level data at La Rochelle tide gauge, which are reproduced with a root mean squared discrepancy of 0.13 m (see Appendix C). Extreme water levels are only substantially underestimated during Xynthia storm, mostly because the effects of short waves were not accounted for in the simulation, as discussed in [START_REF] Bertin | A modeling-based analysis of the flooding associated with Xynthia, central Bay of Biscay[END_REF].

Statistical analyses

All statistical analysis were performed using R 3.6.3 (R Core Team, 2020) and Rstudio v1.1.419. We computed Linear Models (LMs) or Linear Mixed Models (LMMs) using the lme4 [START_REF] Bates | Fitting linear mixed-effects models using lme4[END_REF] and lmerTest [START_REF] Kunzetsova | lmerTest package: Tests in linear mixed effect models[END_REF] packages. For all test computed, models accuracy was tested using the check_model function from the performance package [START_REF] Lüdecke | Performance: assessment of regression models performance[END_REF]. When models did not fit the data, the response variable was log transformed or log+1 transformed.

Seasonal variations of salinity

We investigated seasonal variations of salinity (all years combined), by computing LMMs with each value of salinity recorded as dependent variables (log+1 transformed), month and its quadratic form as covariates, and pond as a random effect. We also investigated the climatic drivers of these variations by computing LMMs with mean salinity (measured for each month and each pond, all years combined) as the dependent variable (log transformed), mean temperature or precipitation as covariates, and pond as a random effect. As temperature and precipitation were highly correlated (cor=-0.145, p-value<0.001) these variables were tested in two separate models.

For these analyses, we calculated a mean temperature for each month (across the study period) and a mean precipitation (mean of the sum of precipitations recorded during each month across the study period).

Spatial variations of salinity

We analysed salinity records according to distance to the ocean of the ponds where salinity was recorded, by computing LMMs with each value of salinity recorded as dependent variables, distance to the ocean as the covariate (log+1 transformed), and pond as a random effect.

Long term changes in salinity

Across the whole study period (January 1996 to December 2019 at MO, and January 2010 to December 2019 at MY), we evaluated long-term temporal trends of salinity by computing LMMs with each value of salinity recorded as the dependent variable (log+1 transformed), year as the covariate, and pond as a random effect.

Additionally, we investigated trends of maximum and minimum salinity (for all ponds and all years: one value of maximum and minimum salinity for each pond and each year), using either minimum or maximum salinity as dependent variables, year as the covariate, and pond as a random effect. We also investigated the climatic drivers of salinity trends, by computing LMMs with mean salinity (one value of mean salinity for each pond and each year) as the dependent variable, mean temperature across the year or total precipitation as covariates, and pond as a random effect. Because previous analyses demonstrated that salinity recorded in 2010 at MY was exceptionally high due to the marine flooding driven by storm Xynthia (Lorrain-Soligon et al., 2021), we removed this year from our analysis at MY, in order to investigate only variations of salinity according to environmental parameters under normal conditions.

Finally, to investigate whether the spatial gradient of salinity (distance to coastline) changes across the study period, we extracted, for each year, the slope of the relationships between salinity and distance to the ocean (see Appendix D), and computed LMs with these slope as dependent variables, and year, temperature (mean temperature of the year) or precipitation (sum of the precipitations of the year) as a covariate.

Estimates of the high tides' water level

To investigate the variations of estimated water levels across the study period (2000-2020 for both sites), we computed yearly-mean values of water levels exceeding 2.4 m/MSL (Mean Sea Level, the water level is thus given with respect to mean sea level), which corresponds to spring tides. Indeed, under neap tides, the sea level is too low to induce any flooding and salinization of the studied ponds. Although this value is somehow arbitrary, it has no impact on inter-annual variability and upward trend of sea levels.

We then computed LMs with these estimated water levels at high tides as dependent variables, and year as a covariate.

Results

Seasonal variations of salinity

Across the whole study period, salinity varied seasonally (monthly variation), according to a quadratic form, with maximum salinity attained in summer on both sites (Figure 2, Table 1), and maximum values also attained in October and November in MO (Figure 2, Table 1). In both sites, these variations were significantly correlated to temperature, with salinity increasing with increasing temperature (MO:

Estimate=0.002, SE<0.001, t-value=14.

Spatial variations of salinity

At MO and MY, salinity decreased with increasing distance to the ocean (MO:

Estimate<-0.001, SE<0.001, t-value=-3.41, p-value=0.003; MY: Estimate=-0.001, SE<0.001, t-value=-2.048, p-value=0.050).

Long term changes in salinity

Across the whole study period, salinity decreased at MY, considering either mean, minimum, or maximum values (Table 1). At MO, results were more complex with a significant increase in minimal values of salinity across time (Table 1), and no significant trends for maximal and mean values (Table 1). The long-term variations of salinity were linked to precipitation, with salinity decreasing with increasing precipitation at both sites (MO: Estimate<-0.001, SE<0.001, t-value=-3.562, p-value<0.001; and MY: Estimate<-0.001, SE<0.001, t-value=-3.282, p-value=0.001), but not to temperature (at MO: Estimate=-0.011, SE=0.016, t-value=-0.695, p-value=0.487;

and at MY: Estimate=0.066, SE=0.044, t-value=1.524, p-value=0.130).

Across years, the spatial gradient of salinity (slope of the relationships between salinity and distance to the coastline) increased at MO (Estimate<-0.001, SE<0.001, t-value=-7.449, p-value<0.001, Figure 3A), an effect that was marginally related to decreased precipitation (Estimate<0.001, SE<0.001, t-value=1.881, p-value=0.073), but not to temperature (p-value=0.238). At MY, there was no difference in the strength of the spatial gradient of salinity (Estimate<0.001, SE<0.001, t-value=0.327, p-value=0.752, Figure 3B), and no relationships with temperature or precipitation (both p-values>0.269).

Estimates of the high tides' water level

In both sites, water levels at high tides increased over time (at MO: Estimate=0.002, SE<0.001, t-value=4.735, p-value<0.001; at MY: Estimate=0.002, SE<0.001, t-value=4.330, p-value<0.001; Figure 4). These data show that open water surface can exceed the height of the protective coastal sand dunes during significant amount of time (9 days at MO and 7 days at MY).

Discussion

Several studies have investigated contemporary long-term salinity changes in marine environments and highlighted contrasted patterns linked to variable freshwater inputs and evaporation patterns [START_REF] Li | Long-term variation in the salinity of the Southern Yellow Sea Cold Water Mass, 1976-2006[END_REF][START_REF] Lin | Long-term variations of temperature and salinity of the Bohai Sea and their influence on its ecosystem[END_REF][START_REF] Meredith | Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20th century[END_REF][START_REF] Omstedt | Modeling the variations of salinity and temperature in the large Gulfs of the Baltic Sea[END_REF][START_REF] Samuelsson | Interannual salinity variations in the Baltic Sea during the period 1954-1990[END_REF][START_REF] Tukenmez | Long-term variations of air temperature, SST, surface atmospheric pressure, surface salinity and wind speed in the Aegean Sea[END_REF]. Very few studies have investigated such variations for freshwater or continental water bodies [START_REF] Benjankar | Analyses of spatial and temporal variations of salt concentration in waterbodies based on high resolution measurements using sensors[END_REF][START_REF] Collins | Toxicity of road salt to Nova Scotia amphibians[END_REF][START_REF] Dugan | The ecosystem implications of road salt as a pollutant of freshwaters[END_REF]. To our knowledge, our study is the first to investigate long-term changes in salinity in coastal wetlands susceptible to salinization through natural processes (e.g. seaspray, [START_REF] Benassai | Sea-spray deposition in Antarctic coastal and plateau areas from ITASE traverses[END_REF] and ongoing climate perturbations (e.g. marine surges linked to extreme weather events, [START_REF] Nordio | Frequent storm surges affect the groundwater of coastal ecosystems[END_REF]. Our detailed data set allowed for highlighting that salinity in ponds varied seasonally, being higher during summer months as well as until mid-autumn in one site, but also spatially, as salinity increase closer to the seashore. Importantly, this spatial gradient of salinity (which might be linked to landward salt deposition or groundwater intrusions) strengthened through time. These variations of salinity are linked to both increased temperature and reduced precipitations. Complementarily, in both sites, water levels at high tides increased through time, a pattern which can induce an additional source of salinization.

At both sites, salinity varied seasonally, with lowest salinity in winter and highest salinity in summer and until mid-autumn. Such seasonal variation was expected given the concomitant variations in climatic conditions (temperature and precipitation) in temperate areas. Indeed, we showed that the seasonal variation of salinity is driven both by temperature and precipitation, as salinity increased with increasing temperature and decreasing precipitation (see also Appendix B). Both variables are likely to affect evaporation patterns interactively [START_REF] Hassani | Global predictions of primary soil salinization under changing climate in the 21st century[END_REF], which naturally occur seasonally [START_REF] De Luis | Seasonal precipitation trends in the Mediterranean Iberian Peninsula in second half of 20th century[END_REF]. Additionally, because water bodies are highly dependent on groundwater [START_REF] Erostate | Groundwater dependent ecosystems in coastal Mediterranean regions: Characterization, challenges and management for their protection[END_REF][START_REF] Kløve | Groundwater dependent ecosystems. Part I: Hydroecological status and trends[END_REF], these seasonal variations can be explained due to groundwater discharge and water recirculation which are known to affect surface water bodies [START_REF] Casamitjana | Modeling the salinity fluctuations in salt marsh lagoons[END_REF][START_REF] Rodellas | Groundwater-driven nutrient inputs to coastal lagoons: The relevance of lagoon water recirculation as a conveyor of dissolved nutrients[END_REF][START_REF] Sadat-Noori | Intermittently Closed and Open Lakes and/or Lagoons (ICOLLs) as groundwater-dominated coastal systems: Evidence from seasonal radon observations[END_REF][START_REF] Santos | The driving forces of porewater and groundwater flow in permeable coastal sediments: A review[END_REF], and are also known to vary seasonally [START_REF] Sadat-Noori | Intermittently Closed and Open Lakes and/or Lagoons (ICOLLs) as groundwater-dominated coastal systems: Evidence from seasonal radon observations[END_REF][START_REF] Tamborski | Temporal variability of lagoon-sea water exchange and seawater circulation through a Mediterranean barrier beach[END_REF].

As expected from landward deposition of seaspray and groundwater exchanges, we found that, overall, salinity was higher in coastal ponds situated closer to the coastline, despite the small spatial extent of our study sites (situated from ~50m to 1000m from the coastline). Importantly, we found that, over time, the strength of this spatial gradient of salinity increased significantly on one of our study sites (MO).

That is, the slope of the relationship between pond salinity and distance to the coastline increased through time. This effect was mainly driven by increased salinity closer to the seashore rather than by decreased salinity farther from the seashore, which might be due to increase groundwater infiltration over time [START_REF] Colombani | Origin and pattern of salinization in the Holocene aquifer of the southern Po Delta (NE Italy)[END_REF]Mastrocicco et al., 2021;Mastrocicco and Colombani, 2021). Such a result highlights a remarkable spatial heterogeneity in the susceptibility of these coastal ponds to salinization over a relatively small spatial scale. Complementarily, this result reinforces the notion that ongoing climate change is currently inducing significant salinization, and that this process strongly alters the spatial gradient of salinity. Despite the strengthening of the spatial gradient of salinity, we did not found a consistent trend of salinity over time, but found contrasted responses in our two study sites. Indeed, in one of our study sites (MO), salinity (maximum and mean) remains relatively stable across time, but minimum values of salinity gradually increased through time (at a mean of 0.09 g.l -1 /year). This site was spared from marine submersion linked to storm Xynthia due a protection dune [START_REF] Lorrain-Soligon | Slight variations in coastal topography mitigate the consequence of storm-induced marine submersion on amphibian communities[END_REF], but minimal values still seems to be affected. The second site however (MY), was completely submerged by storm Xynthia [START_REF] Lorrain-Soligon | Slight variations in coastal topography mitigate the consequence of storm-induced marine submersion on amphibian communities[END_REF]. Our salinity recordings started at the onset of this marine submersion and documented the temporal dynamics of this parameter following a massive and brutal intrusion of sea water. We showed that salinity was maximum following storm Xynthia (due to sea water intrusion and increased salinity) and progressively decreased through time to reach lower and temporally stable values at ~3 years poststorm (see [START_REF] Lorrain-Soligon | Slight variations in coastal topography mitigate the consequence of storm-induced marine submersion on amphibian communities[END_REF] for some salinity values recorded before Xynthia). These lower values remained stable through time, although we emphasize that the following 7 years are likely too short of a time scale to investigate potential long-term changes in salinity. Importantly, other studies have shown that effects of this marine submersion were long-lasting both for fluxes of marine elements (salt, sediments, and organic matter) through the food web [START_REF] Lorrain-Soligon | Can stable isotopes assess habitat use in complex coastal wetlands? A case study in an amphibian species[END_REF] and for the direct negative impact of salinization on biodiversity [START_REF] Lorrain-Soligon | Slight variations in coastal topography mitigate the consequence of storm-induced marine submersion on amphibian communities[END_REF] In this study, while we emphasize a decrease in salinity following storm Xynthia, the salinity levels recorded 7 years after the storm (7.23 g.l -1 in 2017 and 4.16 g.l -1 in 2018) are well beyond salinity levels recorded before the storm on the same area (2.53 g.l -1 in 2007, see [START_REF] Lorrain-Soligon | Slight variations in coastal topography mitigate the consequence of storm-induced marine submersion on amphibian communities[END_REF]. We thus highlighted gradual as well as brutal changes in salinity in these coastal wetlands, which might both impact coastal ecosystems [START_REF] Delaune | Impacts of salinization on aquatic communities: Abrupt vs. gradual exposures[END_REF]. Indeed, chronic salt exposure to gradual increase in salinity might gradually transform coastal ecosystems through acclimatory processes or epigenetic changes [START_REF] Venâncio | Seawater intrusion: an appraisal of taxa at most risk and safe salinity levels[END_REF], while brutal changes can induce species displacements or direct mortality (Lorrain-Soligon et al.,

2021).

Long-term variations in salinity are hypothesized to be mainly due to low precipitation and excess evaporation linked to increased temperature [START_REF] Al-Shammiri | Evaporation rate as a function of water salinity[END_REF][START_REF] Mills | Paleoclimate studies and natural-resource management in the Murray-Darling Basin II: unravelling human impacts and climate variability[END_REF][START_REF] Obianyo | Effect of salinity on evaporation and the water cycle[END_REF]. Our results suggest that salinity increase may be related to precipitation, while we failed to detect any influence of thermal conditions, despite the strong effect of both parameters on seasonal variations of salinity (see above). These results suggest that long-term salinization is primarily driven by changes in the magnitude and the timing (seasonality) of precipitation, the effects of which are strengthened by climate change and, notably, the increased frequency of droughts [START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF][START_REF] Jeppesen | Salinization increase due to climate change will have substantial negative effects on inland waters: a call for multifaceted research at the local and global scale[END_REF][START_REF] Mills | Paleoclimate studies and natural-resource management in the Murray-Darling Basin II: unravelling human impacts and climate variability[END_REF].

Finally, the numerical modelling of sea levels and the computation of yearly-mean values at high tides also shows a significant increase over time at both our study sites (0.002 m.year -1 through a 20 years' period, in both sites). This pattern is explained firstly by the nodal tidal cycle (18.6 year period, which can influence sea water levels, [START_REF] Baart | The effect of the 18.6-Year lunar nodal cycle on regional sea-level rise estimates[END_REF][START_REF] Royer | High-latitude oceanic variability associated with the 18.6-year nodal tide[END_REF][START_REF] Yndestad | Lunar nodal tide effects on variability of sea level, temperature, and salinity in the Faroe-Shetland Channel and the Barents Sea[END_REF] to increase with global changes in several areas of the globe [START_REF] Dettinger | Climate change, atmospheric rivers, and floods in California -A multimodel analysis of storm frequency and magnitude changes[END_REF][START_REF] Knutson | Tropical cyclones and climate change[END_REF][START_REF] Mclean | Coastal zones and marine ecosystems[END_REF][START_REF] Nicholls | Increasing flood risk and wetland losses due to global sea-level rise: regional and global analyses[END_REF][START_REF] Trenberth | Attribution of climate extreme events[END_REF], even though they have not been formally tested in temperate areas. A previous study have emphasized the importance of coastal sand dunes as a protection of coastal wetlands against sea water intrusions (Lorrain- [START_REF] Lorrain-Soligon | Slight variations in coastal topography mitigate the consequence of storm-induced marine submersion on amphibian communities[END_REF], but these natural barriers may well prove inefficient in the long-term, given that by the end of this century, global mean sea levels will most likely rise between 0.29m and 1.1m (IPCC, 2022).

Whatever the underlying causes, salinization is known to negatively affect the rich biodiversity of coastal ecosystems [START_REF] Bradley | Animal Osmoregulation[END_REF][START_REF] Evans | The cellular stress response in fish exposed to salinity fluctuations[END_REF][START_REF] Hellebusi | Osmoregulation[END_REF][START_REF] Schultz | Euryhalinity in an evolutionary context[END_REF]. Indeed, salinity higher than 4 g.l -1 might transform fungal and microbial communities [START_REF] Canhoto | Pulsed vs. chronic salinization effects on microbial-mediated leaf litter decomposition in fresh waters[END_REF], as well as amphibian communities [START_REF] Lorrain-Soligon | Some like it salty: Spatio-temporal dynamics of salinity differentially affect anurans and caudates in coastal wetlands[END_REF], and salinities higher than 7 g.l - 1 can be detrimental for invertebrate communities [START_REF] Venâncio | Active emigration from climate change-caused seawater intrusion into freshwater habitats[END_REF] But salinization will also affect hydrology by destabilizing the water column [START_REF] Dugan | The ecosystem implications of road salt as a pollutant of freshwaters[END_REF][START_REF] Koretsky | Redox stratification and salinization of three kettle lakes in southwest Michigan, USA[END_REF] biogeochemistry (e.g. lack of oxygenation, [START_REF] Ladwig | Impact of salinization on lake stratification and spring mixing[END_REF][START_REF] Dugan | The ecosystem implications of road salt as a pollutant of freshwaters[END_REF], and soils characteristics [START_REF] Neubauer | Saltwater intrusion into tidal freshwater marshes alters the biogeochemical processing of organic carbon[END_REF]. Taken together, these various consequences of coastal wetland salinization suggest that salinity fluctuations are likely to dramatically transform -and impact the functioning of -coastal ecosystems [START_REF] Cañedo-Argüelles | Salinisation of rivers: An urgent ecological issue[END_REF][START_REF] Cunillera-Montcusí | Freshwater salinisation: a research agenda for a saltier world[END_REF][START_REF] Denny | Biodiversity and wetlands[END_REF][START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF][START_REF] Visschers | Accelerated migration of mangroves indicate large-scale saltwater intrusion in Amazon coastal wetlands[END_REF]. Our study allows to emphasize that even ponds situated farther from the seashore are at risk of salinization and should be considered when planning conservation measure to mitigate this global change. Despite growing interest on the outcome of human-induced salinization, we emphasize that the consequences of these processes at large spatial and temporal scales are still to be investigated [START_REF] Cunillera-Montcusí | Freshwater salinisation: a research agenda for a saltier world[END_REF], and can be site-specific. Further research should focus on the impacts on biodiversity in relation to the spatiotemporal salinity gradients. In addition, forecasting models should incorporate more information on critical factors that might influence salinity changes, across large spatial and temporal scales, not only for water bodies but also for soil, as terrestrial salinization is a critical but overlooked process [START_REF] Ayub | Salinity and its tolerance strategies in plants[END_REF][START_REF] Greenway | Mechanisms of salt tolerance in nonhalophytes[END_REF][START_REF] Rath | Salt effects on the soil microbial decomposer community and their role in organic carbon cycling: a review[END_REF][START_REF] Singh | Microbial and enzyme activities of saline and sodic soils[END_REF].

Conclusion

We highlighted both natural long-term changes in salinity in coastal wetlands (linked to climatic conditions), but also the consequences of a marine submersion, inducing brutal increase in salinity. Additionally, we demonstrated that salinity increased closer to the seashore, probably due to exchanges with the marine environment (through groundwater infiltration or sea-sprays), and this spatial gradient of salinity is strengthen through time. Models of water levels at high tides further highlight a temporal increase, suggesting a higher susceptibility to marine intrusion. We emphasize the importance of recording long-term series for a large variety of locations, as the salinity trends can be highly variable and site-specific.

  , which reached a minimum in 2006 and maximum in 2015. The minimum values observed in 2003-2005 also correspond to a period characterized by a weaker storminess in the region (Chaumillon et al., 2019). Over the studied period, water levels increased by about 0.10 m. This increase is explained mostly by long term sea-level rise and, to a lesser extent, by an increase in storminess from 2003-2005 to 2014-2017. More specifically, these data show that open water surface can exceed the height of the protective coastal sand dunes during significant amount of time. Such data highlight additional risks of salinization through sea water inundations, which frequency has been shown

Figure 1 :

 1 Figure 1: (A) Map of the study area relative to Western France, including open boundary of the model run for estimated sea water levels, and (B) map representing the two study sites (MO and MY) and the study ponds (blue points), At MO, position of the study ponds ranged from 58 m to 1080 m from the seashore. At MY, position of the study ponds ranged from 187 m to 840 m from the seashore. La Pallice , where the tidal gauge and point use for climatic data are situated, is represented by a star. Bathymetry is indicated with the colour scale. MO (« Réserve Naturelle Nationale de Moëze-Oléron ») and MY (« Réserve Naturelle Nationale du marais d'Yves »).

Figure 2 :Figure 3 :

 23 Figure 2: Variations of salinity by month at MO (A) and MY (B). Mean (24 years in MO and 10 years in MY) ± SE, and linear trend line ± SE. MO : « Réserve Naturelle Nationale de Moëze-Oléron » and MY : « Réserve Naturelle Nationale du marais d'Yves »

Table 1 :

 1 Effects of temporal scales (year, month and month²) on salinity at MO and MY (absolute values of salinity are log+1 transformed, not minimum and maximum values). MO : « Réserve Naturelle Nationale de Moëze-Oléron » and MY : « Réserve Naturelle Nationale du marais d'Yves »

						t-	p-
	Time scale Site	Variable	Covariate Estimate SE	value	value
		MO	Salinity	Month	0,006 <0,001 13,14 <0,001
	Seasonal	MO MY	Salinity Salinity	Month 2 -0,0003 <0,001 -10,41 <0,001 Month 0,008 <0,001 13,47 <0,001
		MY	Salinity	Month 2 -0,0006 <0,001 -12,31 <0,001
		MO Minimum salinity	Year	0,09	0,031 2,935 0,003
		MO	Mean salinity	Year	<0,001 <0,001 0,991 0,322
	Inter-annual	MO Maximum salinity MY Minimum salinity	Year Year	-0,033 -0,481	0,055 -0,606 0,545 0,042 -11,49 <0,001
		MY	Mean salinity	Year	<-0,001 <0,001 -30,48 <0,001
		MY Maximum salinity	Year	-1,6161 0,106 -15,18 <0,001

Acknowledgments

The authors thank the staff from the "Réserve Naturelle Nationale de Moëze-Oléron" and the « Réserve Naturelle Nationale du marais d'Yves ».

Funding

Funding was provided by the CNRS, La Rochelle Université, the LPO, the Agence de l'Eau Adour-Garonne, the Conseil Départemental de la Charente-Maritime, the ANR PAMPAS (ANR-18-CE32-0006), and the Beauval Nature association.

Data Availability Statement

The data that support the findings of this study will be made available upon request.

Conflict of interest statement

The authors declare that they have no conflict of interest.

Author contributions

Léa Lorrain-Soligon: Conceptualization ; Methodology; Formal analysis; Data


Claire Dazon
email: dazonclaire@gmail.com

Christine Taviot-Gue

Vanessa Pre ´vot

Layered double hydroxides: where should research stress on for massive scaling up?

For more than half a century, layered double hydroxides (LDHs) have attracted great attention from the scientific community. These materials belong to a rare anionic clay group and have demonstrated amazing physicochemical properties, leading to disruptive, cost-effective and eco-friendly applications for health, environment and agriculture. However, LDHs are not widely used in daily items, nor are they massively applied in industry despite their very good potential for large-scale development. This study looks at the current shortcomings preventing the industrial implementation of LDH powders while evidencing and discussing the strategies to encourage research to bridge the gaps on this topic.

Introduction

Health, environment, and agriculture are essential areas that have been made significantly fragile by human activities. The recent COVID-19 pandemic and global warming are signs of a pessimistic outlook for the future. As mentioned in the latest NIC report about the global trends in 2040, ''avoidance'' strategies addressing the aforementioned domain issues are not the best ones, and our societies should turn to ''adapting'' behaviours to fight against numerous troubles. 1 In this view, there is good news to highlight; in particular, materials science could be a great resource to bring smart, efficient, and maybe transcendent solutions, provided the applications have available raw materials, eco-friendly design criteria, and are costeffective. From this perspective, layered double hydroxides (LDHs) are undoubtedly materials of choice for development and expansion.

LDH particles belong to a natural and rare anionic clay group, but several synthesis methods are relatively well controlled today to compensate for this natural scarcity and allow vast possibilities of chemical compositions and textural properties for numerous applications in strategic fields. [2][3][4] LDHs have great potential for use as drug delivery systems, controlled release pesticides for agriculture, adsorbents of heavy metals and pollutants for water decontamination, and promising applications in the controlled release of fertilizers, CO 2 capture, CO 2 conversion and green electrochemical energy storage devices are being seriously considered. [3][4][START_REF] Feng | Materials Advances Perspective[END_REF][START_REF] Kano | Advanced Sorption Process Applications[END_REF][START_REF] Yan | [END_REF][8][9] Notably, LDHs display a 2D structure, allowing them to be insertion systems for active molecules, pollutants, and so forth, promising numerous smart host-guest materials. For each of these fields, considerable progress has been made recently as highlighted by Kameliya et al. 10 with eco-friendly LDH synthesized today for application as catalysts, and in photoelectrocatalysis with amazing results 11 with a strong proof of concept for water decontamination, 12,13 and the sensor-biosensor perspectives are also stemming from these long years of research. Another field where LDHs can be the original material for good solutions is in corrosion protection. 14 Co-precipitation and mechanosynthesis are well-known LDH synthesis methods that can produce several kg of LDH powder using available precursors. [15][16][17][18] We did not notice a massive spread of LDH in applications and at the industrial scale in the aforementioned sectors, even if some reduced-scale studies have emerged, especially in the environmental field 19,20 and for new buildings in the case of concretes reinforced with LDH particles. 21 Since LDHs encompass the required criteria for sustainable applicative development and industrial scale production (the availability of the precursors and their relative competitive prices, cost-effective production through proven methods, and a non-toxic nature for numerous chemical compositions), one can wonder finally, why these particles remain insufficiently applied, despite a significant number of patents (226 patents registered at the beginning of 2022 since the 1960s 22 ). Looking closely, most of them indicate numerous processes to scale up LDH (nano)composites, with these being less versatile for the applications than the powder form, and hardly 16% of them directly address the raw LDH powder synthesis. None of these patents mention the use of LDH in their dry form (supposing the powder) for further implementation. Nonetheless, the LDH powders could provide great opportunities for disruptive applications, using them as produced, such as aerosol therapy for drug delivery, fillers for innovating textiles or material buildings, and fertilizers for smart agriculture, to quote a few. The LDH powders are practical for packaging and transportation, weighing, or handling at the industrial scale, and can no doubt pop up, concerning the highly pragmatic features they offer. So, what are the shortcomings that prevent their large-scale application? Where should research make efforts to overcome them? Given the urgency of issues our societies should solve in health, the environment, agriculture, or energy areas, maybe it is time to boost the scalability of LDH powder. This article compares coprecipitation and mechanosynthesis for the massive production of raw LDH powder and explores the possibilities that could be proposed to unlock the agglomeration/aggregation issues of LDH particles as synthesized, which we have pointed out as an important one hindering their further scale-up in items or industry. The ideas we propose cover the raw LDH, with their (nano)composites being out of our scope.

Co-precipitation and mechanosynthesis methods: the relevant industrial production of LDH powders

Anionic clays are rare natural inorganic materials that are, organized in 2D structures, resulting in superposed hydroxide sheets between which the anion inserted is exchangeable. layered double hydroxides (LDH) belong to the anionic clay group and can be described as brucite (Mg(OH) 2 )-like structured materials surrounded by water molecules, with the general formula

[M II 1Àx M III x (OH) 2 ] x+ (A nÀ x/n )ÁyH 2 O
where M II is a divalent metallic cation surrounded by 6 OH À (octahedral organization). Trimetallic LDH compositions can also be obtained by replacing part of the M II or M III with other metals. 23 The case of a monovalent cation instead of M II is possible with Li + , with these remaining quite rare. The x in the general formula corresponds to the charge density often between 0.2 and 0.33, and the anion A nÀ , compensates for the resulting positive charge. Water is also located between the layers. The first LDH structure described in 1915 was hydrotalcite, 24 a powder similar to talc with the exact chemical formula Mg 6 Al 2 (OH) 16 CO 3 Á4H 2 O. Since then, many studies have shown that it is possible to obtain so-called ''hydrotalcite-like'' compounds when they have the LDH-type structure. Indeed, the general chemical formula of LDH makes it possible to consider numerous metal-anion associations 1. In the case of LDH containing rare earth elements or tetravalent cations such as Ti, Zr, and Sn, a deep characterization using structural local probe techniques is highly recommended. This is of great importance because it very often appears that the so-called LDH materials are composite materials [START_REF] Intissar | [END_REF] and not the expected pure brucite-like inorganic structure as described above.

For more than half a century, chemists have managed to rebuild LDH materials with available precursors and involving different synthesis methods, mainly soft chemistry as coprecipitation, anionic exchange, and the urea method, but other synthesis routes are possible, such as mechanosynthesis (solid-state chemistry). The large-scale and sustainable production of LDH requires a quite long list of criteria, depending on the chemical composition. However, for any LDH composition given, one can mention general requirements inspired by the work of Patino-Ruiz et al. 28 regarding the composite production of Fe 3 O 4 magnetite nanoparticles: (i) the availability of the precursors, (ii) satisfying production yield by batch, (iii) an excellent control process at any stage of the synthesis, and (iv) eco-friendly design both on the material and the process (possibility of recycling, limited waste, reduced occupational and environmental risks). Regarding these criteria, coprecipitation and mechanosynthesis are relevant candidates for comparison and discussion for large-scale LDH production. Most of the patents related to LDH synthesis involve coprecipitation, but mechanosynthesis is within reach on a larger scale. Note that the other non conventional routes (urea, hydrothermal, sonochemical, sol-gel methods) all involve a co-precipitation step during synthesis. That is why it is worth taking a deep look at the purpose of industrial scalability for the general co-precipitation approach and mechanosynthesis. They are significantly different from each other in the whole material production cycle regarding cost, energy demand, precursors, solvent, and water consumption to quote a few. To begin this comparison, in Fig. 1, we display the pros and cons for each of these methods regarding key parameters for the scale-up process.

Co-precipitation and mechanosynthesis have common points that are advantageous for the industrial-scale development of LDH powders, which also fall within the green chemistry concepts. Co-precipitation implies the dissolution of salts (sulfate, nitrate, chloride, etc.) containing the M II /M III cations, most of the time in aqueous solutions, and the continuous mixing of the precursors, with anions for intercalation or proton scavengers. 2,4 The LDH is immediately formed in solution and is, therefore, ready to be used, except that it requires several washing and centrifugation cycles with water or sometimes other solvents. Mechanosynthesis consists of grinding the hydroxide (Mg(OH) 2 , Al(OH) 3 ) or salt precursors in a planetary ball mill (mortar and pestle otherwise), sometimes assisted by the addition of a reduced solvent (water or organic solvent). Post-treatment can be considered after synthesis but generally, the LDH powder composition is directly obtained after short milling times, less than 60 min. Washing steps can be implemented but with a limited quantity of water or solvent regarding the co-precipitation. Both processes described here are valid for raw LDH compositions as well as hybrid materials, 3,4,16 even if in this latter case, adjustments can be made to produce intercalated LDH with organic molecules. 27 Taking a closer look at the advantages and shortcomings of each of the methods based on Fig. 1, one can see that both are ready for industrial scalability with a clear attractiveness of mechanosynthesis to reduce water consumption and the generation of basic liquid effluents.

Co-precipitation and mechanosynthesis are well-known approaches for the important production of other types of inorganic (nano)materials, 16,[30][31][32][33][34] and yields of several kilograms (410 kg) are achievable goals for satisfying LDH production, [9][10][11][12][13][14][15][16][17][18]30 taking into account the continuous improvement of reactor design and controlled parameters benefiting from past feedback. One can also note the relatively reduced energy demand involved during the first step of coprecipitation as compared to mechanosynthesis, the first occurring generally at low temperature (typically at room temperature), contributing to a cost-effective approach. 16,34 Mechanosynthesis requires much more energy to start the solid-state reaction. , F À , Cl À , Br À , I À , ClO 4 À , OH À , anionic complexes, macromolecules, polymers. . . Fig. 1 Comparison of co-precipitation and mechanosynthesis in view of the large-scale production of LDH powders, taking into account the basic principles of green chemistry. 29 Another point in favour of the large-scale development of LDH is the availability of the salts or hydroxide precursors. This does not seem problematic regarding the chemical compositions considered in the different studies. The majority of LDH materials synthesized for health, environmental and agricultural perspectives show a clear predominance of common salts precursors, with hydroxide, nitrate, chloride, sulfate, carbonate compounds (Mg(OH) 2 , Mg(NO 3 ) 2 , MgCl 2 , Mg(SO 4 ), and their aluminium equivalents for instance. . .) for co-precipitation as well as for mechanosynthesis. For the latter, 45% of the studies chose cheap precursors that were aluminium based, 30% Mgbased, the rest being shared among iron, zinc, calcium, nickel, and copper, which are more expensive precursors. 16,30 Moreover, given the reduced costs of primary materials, it is possible to use raw metal oxides, as well as recycled effluents containing the metallic precursors. 35 The main drawbacks in co-precipitation and mechanosynthesis generally include more than four synthesis parameters to control. 2,4,[9][10][11][12][13][14][15][16][17] Synthesis times for co-precipitation vary between a few minutes to more than 8 h, depending on the LDH composition, the nature of intercalation compounds and above all, the crystallization degree expected. The synthesis time for mechanosynthesis is longer, typically more than 12 hours, although studies have reported milling times of less than 1 hour. For co-precipitation, temperature, pH, solvent, concentration, surfactant addition, speed of precursors addition, and stirring are the crucial parameters to be controlled, whereas mechanosynthesis calls for meticulous attention to materials, such as beads and reactors for grinding, milling speed, the nature of solvent added, if any, and the atmosphere of the synthesis. 9,[15][16][17] Post-treatment may be necessary for LDH since this step strengthens good crystallization, achieves the targeted particle sizes, and eliminates residual impurities remaining after washing. The post-treatment can take different forms, typically with thermal sequences like hydrothermal treatments, calcination, or supplementary grinding and washing. This adds extra time and energy costs for the final LDH product but when considering the whole synthesis cycle, co-precipitation remains less consuming in terms of energy demand. Nonetheless, on looking closer at the apparent shortcomings of co-precipitation and mechanosynthesis, these are not specific to these compounds. For several years, the industry has managed to synthesize numerous other inorganic and hybrid (nano)particles with these methods with good production yields, and pure materials were obtained, thanks to cost-effective strategies stemming from continuously adapted procedures. LDH should not be specific in this view. However, one major point, and not the least, separates the two approaches and could increase LDH powder production. Mechanosynthesis promotes a solvent-free LDH method or a limited solvent use compared to co-precipitation that uses an aqueous phase, and sometimes other solvents as mentioned before in the case of hybrid synthesis. 15,16,36,37 In this case, the quantity of water (or solvent) is much higher as compared to mechanosynthesis. With the worrisome management of water worldwide today, mechanosynthesis appears to be a more suitable method for LDH industrial eco-scalability and should attract and motivate research to develop and scale up the benchtop mechanosynthesis that is currently available. More room should be provided for such work. Indeed, papers dealing with LDH powders produced by mechanosynthesis at the laboratory are fewer than those on co-precipitation, however, very promising LDH compounds are ready to be scaled up massively with mechanosynthesis, such as pigments or intercalated LDH, as reviewed recently. 16 Concerning the pilot plant studies, according to our knowledge, none have been carried out to date for LDH produced by mechanosynthesis. Co-precipitation, carried out using separate nucleation and aging steps method (SNAS), sometimes followed by hydrothermal treatment is more in advance on this, with emerging pilot plants or meso scale studies for the continuous production of LDH. 9,17,33,[37][38][39][40] However, interestingly, production plants are already underway in China in Shandong Vansinvena Material Technology Co., Ltd., 38 which use both co-precipitation and mechanosynthesis with production capacities of 1500 t.a À1 and 3000 t.a À1 , respectively, without more information on the LDH chemical composition, purity and other physicochemical properties for supposedly confidential reasons. Other high-production sites located in the same country are implementing patented coprecipitation methods. 41 Also, a patent assigned to Sud-Chemie [START_REF] Eisgruber | Method for producing hydrotalcites[END_REF] describes a method for the synthesis and manufacture of hydrotalcite where one of the starting compounds or both are not used in the form of a solution. The mixture is subjected to intensive wet grinding, and further processing can lead to either a crystalline phase or an amorphous phase of a high-quality hydrotalcite. The patent suggests starting compounds including unreactive MgO, Al(OH) 3 and CO 2 in the form of carbonated water, gas or dry ice. Sigma-Aldrich has proposed a synthetic hydrotalcite (product No. 652288), supposedly obtained by co-precipitation, but official information on this cannot be obtained due to confidential reasons. The scaled-up LDH diversity is low, whereas a myriad of compositions make it possible to go from the benchtop to the industrial scale. On the other hand, for mechanosynthesis, the diversity of LDH studied is not as important as for co-precipitation. Mechanosynthesis remains attractive for direct LDH powder production, eventually followed by post-treatment and/or the insertion of organic molecules, with the whole process interestingly organized with the same site production of powders, post-treatment and direct packaging for further transportation or use. The variety of LDH studied through mechanosynthesis is limited but it can be supposed that the production of pure LDH with mechanosynthesis (i) does not gather all the energetic conditions to start the chemical reactions between the precursors, (ii) works in a dry state most of the time, preventing the intimate mixing of the precursors and calling for longer synthesis time, and (iii) the set-up is not easy. However, some compositions can be obtained with very good purity, such as the Cu-Al LDH explored by Qu et al., 30 leading to the exploration of more LDH structures, including the design of inorganicorganic compounds. The MgAl 43,44 and CaAl 35 LDH are also studied compositions. Isupov et al. 43 used a planetary mill to perform the mechanochemical synthesis of magnesium-aluminium double hydroxides starting with a mixture of magnesium hydroxide and aluminium salts, also from magnesium hydroxide, aluminium hydroxide and sodium bicarbonate. 44 Ferencz et al. 45 used a mixer mill with 20 mm diameter stainless steel grinding balls and a minute and controlled amount of water to produce Ca-Al LDH.

Thus, apart from a lack of studies on the meso/industrial scale production of LDH powders by mechanosynthesis, and thinking about improving water management for coprecipitation, both methods can be readily considered in this view, increasing the large deployment of applications with LDH powders.

So, what prevents this large LDH scale-up worldwide if the powder production is operational? Why do we not notice more use in the applications? A possible answer (but not a unique one) could be found in the systematic agglomeration/aggregation phenomenon of LDH powders, which is likely to degrade their intrinsic properties and very often entails re-dispersion in liquids before use. This adds extra steps in the application processes or impedes them due to solvent compatibility with the particles and/or unwanted interactions. As will be explained in the next section, the control of particle size (well separated in the media, agglomerated or aggregated) is one of the key parameters likely to unlock industrial-scale development.

Insight into the LDH agglomeration/ aggregation

The literature exploration of LDH processed by co-precipitation and mechanosynthesis presents a common phenomenon for raw LDH powders or their hybrids, namely the agglomeration/ aggregation of the particles. This disturbing element entails a loss of the key properties provided by LDH in its applications. An agglomerate is defined as a non-rigid assembly of the smallest pieces of matter often designed as unit, individual or primary particles. An aggregate refers to a strong link created between the unit particles, making the clear identification of the individual components difficult. 46 Agglomerates and aggregates are composed of at least two individual particles. To clearly illustrate both definitions with the simplest shape (spherical), the typical case of SiO 2 particles produced by the co-precipitation method is displayed in Fig. 2. 47 This agglomeration/aggregation phenomenon is exacerbated when small particles (typically the nanosized ones) are synthesized. Even if considerable efforts are made to limit this well-known behaviour, an important parameter to control is the particle size distribution (the unit particles, or their agglomerates/aggregates) and well characterize this as explained by Weibel et al. 48 If the size distribution is well known, it is possible to consider the particles in appropriate applications providing they are stable.

LDH produced by co-precipitation in solution at room temperature follows the well-known nucleation-growth mechanism encountered for bottom-up synthesis approaches. The necessary aging step subsequent to co-precipitation leads to agglomeration/aggregation of the LDH particles. 49 To avoid this, one approach is to consider coprecipitation on the surface of silica beads. However, although interesting, the transposition of the conditions used on the industrial scale seems difficult. For mechanosynthesis, LDH production involves a top-down approach, sometimes assisted by solvent to improve the chemical reaction resulting in agglomeration/aggregation, depending on the nature of the intrinsic forces between the particles (van der Waals, electrostatic, capillary forces). 15,16 Even if considerable efforts were made to limit it, the final dry particles remain strongly agglomerated. However, the main issue is still the size distribution control of the agglomerates as explained by Xu et al. 50 and Zhou et al. 51 In an attempt to understand what rules the agglomeration process of LDH, the authors 51 highlighted that zeta potential, crystallinity and surface defects are important parameters to monitor, given LDH scalability. In any case, the LDH agglomeration results in dramatic consequences for the applications using dry powders for which it is necessary to have a well-known size distribution (painting formulation, composite elaboration). For instance, in the building field, LDH materials demonstrated very good potential to preserve structures from corrosion phenomena estimated at several millions of US$ annually, representing between 2.5% and 3.5% of the growth domestic product (GDP). 21,52 When agglomerated, typical Mg-Al-CO 3 -LDH dispersed in concrete does not affect concrete durability against corrosion, which is particularly accelerated in the presence of chloride species, 21 with a loss of anti-corrosion properties varying between -21% and À14% (most of the time, due to a lack of size distribution control of the raw LDH powder (agglomerated) and concentration in the concrete). The compressive strength can also be impacted, with LDH giving better mechanical properties to cementitious structures. The LDH particles are used as received, i.e., in a dry state and

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View Article Online agglomerated, and are hardly dispersed in the concrete; no supplementary steps were implemented to improve the dry particle dispersion before incorporation into the final bulk, and there was no size control at any step. Another example of bad results can be highlighted when LDH particles are used in fire retardancy. 53,54 The use of dry LDH particles leads to the dispersion of polydisperse agglomerates randomly in the polymer matrix, and the temperature degradation is the same as that of the raw polymer. A good LDH dispersion results in the homogeneous occupation of the particles in the whole polymer matrix, leading to better insulation and mechanical strength. Inhomogeneous LDH agglomeration causes ''holes'' in the polymer matrix, disequilibrating the properties achieved. Therefore, in the case of dry LDH powder use, an initial size distribution control is required even if the agglomeration/ aggregation phenomenon cannot be limited.

Another drawback in LDH agglomeration/aggregation is their inappropriate use as drug carriers, for aerosol therapy. [55][56][57] Among the prerequisites for drug delivery by aerosol therapy is good particle dispersion, which should be managed with a narrow and stable particle size distribution of the unit/primary particles, and is more challenging in this type of application. Poor particle dispersion hinders the delivery efficiency of the therapeutic molecules due to unsuitable particle diameters passing through the respiratory tracts. Moreover, other parameters should be considered that can impact the LDH dispersion for aerosol therapy, such as the relative humidity or the design of aerosol delivery. Finally, depending on the applications, the LDH dispersion should be accurately checked, either in the agglomerated or unit particle state but with a stable and unique size distribution.

However, in an industrial approach, the powder form is practical and is a favoured way to produce, package, transport, and deliver raw LDH for further commercialisation and application. As described in recent works on inorganic nanoparticles massively encountered in industry (TiO 2 , SiO 2 , ZnO), 47,58,59 the powder appears as the main form produced. It seems that the dry particle state could not be avoided but can be optimized in terms of the control of particle dispersion. Regarding what we described above, the main challenge lies in implementing coprecipitation and mechanosynthesis to produce relatively good dry LDH particles with stable size dispersion at the end of the process. According to Zhou et al., 51 the implementation of powder rheology requires a good understanding of the relationship between influencing parameters such as surface charge during synthesis, particle size, surface area, and processing equipment to achieve the desired particle size distribution in the dry state. This could lead to, promising ideas for LDH powder rheology.

Can powder rheology inspire LDH dispersion?

To face the agglomeration/aggregation phenomenon of LDH powders, efforts should be concentrated on laboratory and meso-scale studies aimed at LDH deagglomeration during or after synthesis, with a focus on the eco-friendliest LDH and their hybrids (magnesium, aluminum, calcium, and iron LDH based). To date, surfactants have largely been used to help LDH dispersion, but this requires meticulous and smart combinations of the chemical composition of the surfacing agent and those of the LDH, including the medium used. This approach does not offer great versatility for dry applications of LDH and adds complex processing, including the removal of surface agents. An interesting solution that is hardly explored today and directly inspired by powder rheology is using particles with a bigger size to ''dilute'' LDH powders with smaller diameters during synthesis or post-treatment. This concept is illustrated in Fig. 3.

Typically, the particles with the biggest sizes are mixed in the dry state with the smallest to disperse them, then, after a mechanical stimulus, the smallest particles are more isolated and the biggest ones are removed through a sieving step. [55][56][57][60][61][62] To correctly achieve this dispersion, the bigger particles should stand against mechanical constraints imposed by the fluidizing device as well as being chemically inert. The smallest particles have less probability of interacting with others because of the presence of the bigger particles blocking their dynamic behaviour. In this case, the smallest particles maintain their high surface reactivity at the origin of their tendency to agglomerate and will be oriented around the biggest particles characterized by a lower surface tension. This approach is simple; reducing impurities, allowing the particles to disperse, and getting rid of the deagglomerating agents is easy due to the numerous sieving systems. Scattering the smallest LDH particles with micrometric ones has recently been explored for mechanosynthesis. 63 SiO 2 microparticles (5 mm sized) have been added to the reactor to improve the deagglomeration of the Zn-Al LDH formed during milling, on top of the zirconia beads. Adding ZnO or SiO 2 improved the LDH adsorption capacity regarding methyl orange by more than 20%. The positive effect observed is attributed to the better dispersion of LDH, which is applicable in other areas such as fire retardancy. 53,54 The nature and bead size can be directly exploited to also obtain good dispersed LDH as tested recently with zirconia or glass beads with different sizes ranging from 200 mm to more than 2 mm, 64 followed by tri-sodium citrate treatment to improve the dispersion quality. In both cases, LDH is less agglomerated, even if there are some remaining clusters. Note that mechanosynthesis is appropriate for such exploration because the process itself involves ''big particles'' through the beads used but more studies should be proposed in this view. For co-precipitation, the possibility of adding a ''diluter'' during the nucleation and growth process is a harder task since a significant risk of a chemical reaction is possible in the co-precipitation medium. Nonetheless, coprecipitation can implement the powder rheology approach as a supplementary step leading to the final LDH powder. We can thus imagine setting fluidized bed equipment following the co-precipitation and washing. This adds an extra device cost but, results in an appropriate and controlled size distribution of the LDH powders. The fluidized bed pulses up the particles that settle on the bottom of a tank with the diluting particles, giving them extra mobility and very good dispersing behaviour.

Given the quite simple strategy proposed by the sole sieving step to remove the diluters, this task should not prevent the scalability of LDH through co-precipitation synthesis, nor impose supplementary energy demand. Are there other unlocking strategies for better LDH dispersion during synthesis? Some studies have indicated the possibilities for the accurate control of the co-precipitation parameters and post-treatment, 65,66 favouring reduced temperature and aging time, and the use of the zeta potential as a key indicator of the agglomeration state to manage the process. However, these suggestions have already been considered for co-precipitation development without significant improvement on a large scale for LDH dispersion. Undoubtedly, it is worth inputting the very little studied LDH powder rheology at the laboratory and meso-scales due to the obvious economic, availability and already operational set-up it can offer. LDH-controlled powder dispersion is a big challenge, but inspiration from powder rheology is a potential smart response to address it and lead to massive scaling up, possibly making strides in laboratory studies, igniting efficient and sustainable solutions, and motivating more public-private partnerships to progress on this issue.

Conclusions

In the coming years, the world will face global cascading challenges regarding health, the environment agriculture, etc. Materials sciences should be one of the most relevant tools poised to bring about disruptive and resilient solutions for adapting and reorganizing our societies. There is a clear need for the development of eco-friendly, cost-effective and efficient materials as quickly as possible. Among the tremendous possibilities are layered double hydroxide (LDH) materials, a rare natural anionic clay group with amazing potential. However, we have not seen the large-scale use of LDH structures, even with the relatively good management suggested by coprecipitation and mechanosynthesis methods to produce significant amounts of LDH powder particles.

Why so few LDH powder applications at the industrial scale? Are we ready to scale up LDH powders worldwide? Should research explore more strategic points to unlock the different shortcomings? Is it worth pursuing this development? The response is certainly ''yes''. One of the big problems inherent to LDHs is their tendency to aggregate, which causes them to lose their powder or dry particle properties for certain applications. Here, research can offer considerable advances to overcome this drawback. This is a key point that public-private efforts should focus on. First, concentrating the efforts on strategic chemical compositions for raw LDH encompassing economic and ecological criteria should be favoured. Mg and Al LDH-based materials, as well as their hybrid derivatives combined with polymers or biomolecules for innovative applications, have proven their really good potential for future use in drug delivery, slow-release pesticides, as well as catalysis applications. More room should be given to accelerating the studies inspired by powder rheology to deagglomerate and control the particle size distribution and scale up the resulting process. As such, there should be new initiatives and experiments for coprecipitation and mechanosynthesis, diluting LDH particles in their medium or through a post-synthesis step by bigger-sized ones such as micrometric silica, which is a promising approach. It is important to systematically think about the transfer to the industrial scale during the development and study of new promising materials, suggesting a close exchange between private companies and academic structures. If this general concept is smartly organized for LDH, we can cultivate hope to see more and more of these materials integrating the industrial area, with sustainable development and numerous applications within reach.

Fig. 2

 2 Fig.2Illustration of the agglomerate and aggregate definition in the case of SiO 2 powder particles obtained by a co-precipitation method at the industrial scale (scanning electron microscopy micrographs from Dazon et al.47 ).

Fig. 3

 3 Fig. 3 Schematic route to obtaining an efficient dispersion of LDH powder, ''diluting'' the LDH with smallest dimensions with bigger ones.

  [2][3][4][START_REF] Feng | Materials Advances Perspective[END_REF] as summarized Her main research interests focus on the synthesis of nanostructured layered materials for various applications. Since 2016, she has been Vice-President of the French Clay group. She is Associate Editor of Applied Clay Sciences (Elsevier).

	Vanessa Pre ´vot heterogeneous catalysts. in Table Dr Vanessa Pre ´vot is currently a CNRS Researcher Scientist at the Chemistry Institut of Clermont-Ferrand (ICCF-France). She completed her PhD at the Blaise Pascal University (1999) in Material Science. Then, she joined the Materials Science Institute of Madrid (CSIC) as a Postdoctoral Fellow in Prof. Ruiz-Hitzky's team before working at the Institut of Research on Catalysis (IRC-Lyon France) on

Table 1

 1 Some possibilities of cations and anions for LDH chemical composition (from Cavani et al.,2 Duan et al.,[START_REF] Duan | Chapter 18 -Functional Host-Guest Materials[END_REF] and Seliverstov et al.

	À , CO 3 2À , SO 4 2À

27

)

M(+II)

Be, Mg, Cu, Ni, Co, Zn, Fe, Mn, Cd, Ca M(+III) Al, Si, Ga, Ni, Co, Fe, Mn, Cr, Ti, Ce, Eu, La, Ru, Y Anion Organic species (sulfate, acetate, sulfonate, phosphate), NO 3

© 2023 The Author(s). Published by the Royal Society of Chemistry

Acknowledgements

This work was not funded by any grant or other financial support. In this view, the authors acknowledge sincerely all their colleagues for their cheerful encouragements for writing this article. The author thank also gratefully all the research teams worldwide feeding the references of this paper and letting the Layered Double Hydroxides being great materials to study, understand and apply for progress.

Her PhD project was supported by the French National Institute for Research and Safety within an OCDE pre-normative research framework on the dustiness of nanomaterial powders. She received a thesis award from the French National Centre for Scientific Research for the significant impact of her studies on nanoparticles and their occupational risks. Since then, she has been involved in the research and development of inorganic (nano)particles, with one year of experience as a Post-doctoral fellow (IRSN 2020; CEA, 2021), of layered materials, hybrid derivatives and composites. She is a fellow of the French Chemical Society (SCF).

Conflicts of interest

The author declares no conflicts of interest.