Camille Berrier

Nathalie Girard

The construction of the meaning of work for livestock farmers engaged in the agroecological transition

Keywords: agroecological transition, psychosocial processes, livestock farmers, farming practices, professional transition

In the context of agricultural ecologisation, some farmers are undertaking major changes in their practices, which have an impact on the performance of their farm, their work organisation, as well as their identity and the meaning given to their work. This last dimension has so far received little attention, even though studying it is essential to accompany these deep changes in professions and new ways of working. In order to contribute to this question, our research in social psychology aims to explore the psychosocial processes at play in the reconstruction of the meaning of work for farmers engaged in the agroecological transition. We rely on a theoretical framework that allows us to jointly explore the subjective and social dimensions of behaviours at work. This communication proposes the results of an exploratory study conducted in 2018 via ten semi-directive interviews with livestock farmers in a French network that promotes the use of semi-natural vegetation to feed ruminants. We analysed these interviews by studying both personalisation and socialisation processes that we divided into seven axes of analysis. The results show a strong willingness of these farmers to personalise their practices by experimenting, mobilising their senses and emotions, anchoring their practices on their intrinsic motivations and comparing their system with others, in search of freedom and autonomy. The results also show the need for livestock farmers to share their practices within a community of peers. When discussing the reasons for their choices with their peers, they find the support necessary to reassure themselves, to collectively build the meaning of what they are doing, and to strengthen their professional identity. This study confirms the relevance of jointly studying the processes of personal construction and socialisation of work changes in the agricultural world. It opens up prospects for more indepth exploration of their diversity, with the aim of improving the support of farmers engaging in the agroecological transition.

Introduction

In an ever-changing world, farmers redesign their production systems to adapt to various changes and uncertainties, including climate change, political and economic fluctuations, and societal pressures which deeply transform their work. Livestock farms are particularly affected by societal expectations of sustainable development, challenging production and business models. Livestock farmers can either resist and remain within the standards prescribed by conventional agriculture, or move towards more innovative practices like agroecological ones, at the risk of feeling marginalised. Regardless of their choices, livestock farmers as well as all agricultural stakeholders must deeply rethink their work [START_REF] Coquil | Questioning the work of farmers, advisors, teachers and researchers in agroecological transition, A review[END_REF] at the economic, technical and social levels. However, many questions remain open: How do farmers and those who support them personally experience these mutations and changes? What do these mutations psychologically generate for them in their relationship to work? For extension agents, these more intimate dimensions related to identity and the meaning given to work [START_REF] Kling-Eveillard | Working conditions on livestock farms, a personal and multifaceted topic: Some recommenations to help farmers' advisers[END_REF] are more difficult to address than technical or economic problems. Moreover, farming is a professional environment in which practitioners have to be strong and courageous, leading to the difficulty for them to talk about what they are experiencing, as some farmers have testified in the media ("I'm constantly putting on a brave face", The Guardian, 2020). These personal dimensions of work have been addressed by studies of psychosocial risks [START_REF] Fraser | Farming and Mental Health Problems and Mental Illness[END_REF][START_REF] Hagen | Research trends in farmers' mental health: A scoping review of mental health outcomes and interventions among farming populations worldwide[END_REF] since agriculture is a sector particularly affected by these problems (Torske et al., 2016). For example, [START_REF] Kolstrup | International Perspectives on Psychosocial Working Conditions, Mental Health, and Stress of Dairy Farm Operators[END_REF] conducted a literature review showing that "dairy farm operators [...]

are faced with

Conceptual framework

We based our research on social and work psychology concerning psychosocial transitions, occupational transitions, and occupational health and well-being. In particular, we drew on the concept of interstructuring between individuals and institutions [START_REF] Malrieu | La notion d'interstructuration du sujet et des institutions[END_REF], which has proven useful to understand professional transitions [START_REF] Croity-Belz | Rôle des relations interpersonnelles lors d'une phase de transition professionnelle[END_REF] such as the agroecological one. According to [START_REF] Malrieu | Le projet personnel et l'innovation sociale[END_REF], individual socialisation is plural because it takes place in different places, groups and at different moments of life. This plurality puts individuals under tension, even in conflict with others and with themselves. They then develop innovative strategies to individually position and assert themselves by relying on their surrounding social environment. Our ambition is therefore to explore these psychosocial processes within livestock work. We focused our research on livestock farmers who are engaged in the agroecological transition. Our hypothesis was that this transition opens multiple opportunities for farmers for the development of their project. These dynamics are experienced as being innovative, both at the individual and social levels, leading them to rebuild the meaning they give to their system. We chose to focus our study on livestock farmers. We hypothesize that their relationship with animals is one of the driving forces leading them to change their practices. We hypothesize that such a reconstruction takes place through psychosocial

Methodological approach

This research is based on a partnership between INRAE and a SCOP (Cooperative and Participative Association) dedicated to training and advising in the sectors of agriculture and the environment. This SCOP is designed to facilitate the Patur'Ajuste network, which brings together livestock farmers who are heterogeneous in terms of their geographical location as well as their livestock systems (cattle, sheep, goat, milk/meat), and pedo-climatic and socio-economic contexts (supply chain, technical support, social environment, origin of the farmers, etc.). What brings these farmers together is a common desire to make the most of semi-natural vegetation to feed their herds. We chose medium-sized farms for which the farm managers are also those who implement the practices themselves. We conducted interviews with ten farmers in 2018 (Table 1). Using an active listening posture centred on the person (Rogers, 1942) and the "autobiographical statements" method (Malrieu, 2003), we addressed the following topics during the interviews: the definition and description of their profession, the practices they have chosen and, above all, why, and the professional links they maintain with the Pâtur'Ajuste network and/or other networks. Our objective was to understand how these farmers decide to engage in agroecological practices even though this is not the dominant professional standard. The interviews were recorded and transcribed. This corpus was structured in a database using Nvivo Computer-Aided Qualitative Data Analysis Software. It was then coded and analysed in an iterative way, between a descriptive coding that formatted the data, making it possible to describe and retrieve the information collected later, and an analytical coding (or "axial coding" according to Corbin and Strauss (2014)), making it possible to assign conceptual categories to text segments of the corpus. Going back and forth between the corpus, this analytical coding and our conceptual framework, we were able to build an interpretation of the farmers' discourses.

Farmer's code

French department

Age range

WS 8 Innovations and adaptation to changes

Results

We identified seven axes describing the psychosocial processes involved in the construction of new practices, related to personalisation or socialisation (Table 2).

Personalisation

P1

Deliberately experimenting with new ways of doing things

P2

Using their senses and emotions towards the living environment to adjust their practices P3

Designing their practices on the basis of their representation of other systems P4

Designing their system on the basis of their intrinsic motivations Socialisation

S1

Building the meaning of their practices within a community of peers

S2

Finding the support of peers necessary to reassure themselves about their practices S3

Strengthening their identity by widely communicating about their practices Table 2. A grid to describe the behaviours and representations of farmers involved in the agroecological transition.

P1: Deliberately experimenting with new ways of doing things

The interviewed farmers often emphasized the importance of trying out, experimenting with and testing new practices. For them, they have to find ways to adapt their practices to the specificity of their farm: "Since each one is from a different area of France, it allows us to say that, depending on the land, there are different reactions. Everyone has different problems deep down. 1 They also talk about the need to experiment in order to grasp the variability linked to working with living objects, to "what cannot be learned at school". For some of them, experimentation is a way to solve their own problems or frustrations. To set up these experiments, they proceed by trial and error, assessing the reasonable risk from their point of view and considering that failure is part of learning: "it's a philosophy of learning". These behaviours are inherent to innovative farmers but this is probably accentuated by agro-ecology because current practices are complex and uncertain because they must be in harmony with living objects. They fully integrate this risk, which is less important than not trying and then having regrets: "It is better to do something and fail than not to do it and regret not doing it". Some say that this allows them to make up their own minds by "seeing with their own eyes" the result of new practices. It also gives them a sense of pride in finding something they did not think existed. Moreover, they are proud of thinking, acting and doing things on their own, without following prescribed rules and building their own standards. They present experimentation as a means of discovering, learning and developing new knowledge, and they express the pleasure and satisfaction they derive from doing it: "I'm enjoying it", "it's great", "I'm having fun", "I love it".

P2: Using their senses and emotions towards the living to adjust their practices

Farmers often talk about the relationship they have with living objects: "We play with it, we are one with the living world". This relationship with the living world implies that they deal with situations that are unexpected and uncontrollable: "It's living things! One day it can go very well, the next day it can go very badly! Even from one animal to another". In this respect, they say, "Especially the living environment, you can't make it up! It's really a science of nature" and they must know how to "use all that the earth offers through natural means".

WS 8 Innovations and adaptation to changes

They speak of their interest in respecting nature, of loving it: "It is a love of nature", in a relationship of reciprocity, whether positive, "by being closer to nature, it reciprocates", or negative, "nature responds violently to violent acts"! In order to be able to work, they even express some need to see, touch and feel what is happening in the interactions they have, especially with animals. Livestock farmers use their senses in their work: "It's a bit of an innate gift that we have, to be breeders, to feel things. "When I go to see my sheep, I go there every day... and then I pet them. I can approach them with no problem." Breeders also use their emotions by transposing what they feel as human beings and what they imagine their animals feel. One farmer's wife said that her husband relies on his own feelings to choose his practices: "My husband, when he sees the animals locked up, it's like it's him... because he doesn't like being locked up, he doesn't lock up the animals". Some go as far as to attribute human behaviour to their animals, such as the faculty of speech "they talk to us too", positive emotions ("At first they are very happy, for an hour. And then they look for dandelions"), or negative ones ("Goats are awful, they complain for nothing, in fact. As soon as you catch them, they cry"). Farmers thus use their emotions to understand the reactions of living objects with which they interact in their daily activity. A form of anthropomorphism is then established between the farmers and their animals, which are sometimes considered as members of the family ("cows are part of the family"). This enables them to understand the reactions of the animals and to adjust their practices, with the aim of mutual well-being.

P3: Designing their practices on the basis of their representation of other systems

Many farmers also rely on the representation they have of other systems to build and customise their practices. They look at what other farmers are doing in order to form an opinion and build their own system ("There are times when it's good to look at others and test it, but there are times when you have to go back home and say, 'ah yeah [...] it's not so bad at home, it suits us"). Most farmers compare their system to the conventional one to express the fact that, for them, it makes no sense to do so, as expressed in the following examples: "In the conventional system, they delegate a lot and they become dependent on others... they become a link in the agro-industry".

Most of them say that they do not want to invest, especially in mechanisation and buildings "because those who are in trouble are the ones who have made too many investments, [...]. We've stayed in our little thing". Moreover, they feel that some standards prescribed by the conventional model are no longer efficient today. The practices implemented by farmers who have decided to work differently then become a source of inspiration for them: "When I was at his place, when I saw how he worked in the midmountain area, I said to myself: 'wow!'". And he seemed super happy!".

P4: Designing their system based on their intrinsic motivations

The interviewed farmers also expressed very clearly that they had thought up their system based on their intrinsic motivations, whether it be "to earn a living according to my aspirations" and/or to their personal convictions, for example, "to try not to get sick because of what we do". It sometimes put them in the state of cognitive dissonance that they are fully aware of. They recognise that they face difficulties such as low pay, little time off, exhausting physical labour and risk of failure. However, they prefer to accept these constraints in order to be consistent with their vision of their professional activity and, more broadly, with their vision of life ("I had a lot of advantages but, in fact, I realised that the advantages [...] that's not what I wanted... [...] it was to love what I actually do!"). They go so far as to say that, for them,

WS 8 Innovations and adaptation to changes

6 their profession is a passion, which suggests that their emotional state prevails over reason. One interviewed farmer even said that passion is necessary to do his job ("Our job, [...] we do it as passionate people, otherwise we could not do it"). This passion is based in particular on their love of nature (cf. P2), but also on the pleasure of having various activities. Most of them emphasize freedom and autonomy in particular, which seem to be important criteria in their choice. This professional vision contrasts with those who decide to follow the standards of conventional agriculture and to specialise their production. It leads them to "prefer to do a little less [in terms of quantity of production] and do it a little better. It's more fulfilling than be one of many doing a repetitive task" rather than seeking benefits such as days off, better pay, etc., which are the accepted work standards in the professional world (link to P3). Overall, these four processes illustrate a strong willingness of these breeders to personalise their practices. It can be interpreted as a desire to reappropriate their own work, to regain a form of freedom and decision-making autonomy. Their practices of experimentation, of using their senses and emotions, as well as of anchoring their system and practices on their intrinsic motivations, could also be interpreted as a desire to deconstruct the prescriptions advocated in their professional sector.

S1: Building the meaning of their practices within a community of peers

This axis expresses the construction of practices through the confrontation and exchange of experiences and ideas ("they are exchanges of experiences with natural vegetation as well as with ideas"). This exchange takes place within a community of breeders perceived as being different ("we have the same job but different products"), but with the same state of mind ("It is the Pâtur'Ajuste spirit"). Such a spirit is beyond a simple transposition: it is a way of opening themselves up to new practices, of recognising themselves in others ("there is a family side to the network, we're all a bit crazy"), and of relying on the experiences of others to build their own practices ("you call a friend [from the network], then you form your own opinion and you give it a try") (link with P3).

S2: Finding support from peers to reassure themselves about their practices

Even if their system is consistent with their motivations (cf. P4), these choices marginalise them. As a result, they express the need to find people like themselves: "What I found good was that we spoke pretty much the same language, that around here, we didn't talk because we were considered eccentric". They need to know that they are not alone in practicing differently, even if they claim the singularity of their situation (cf. P1). In the network, they seek not only technical advice, to be reassured about the efficiency of their system, but they also seek moral support to be reassured as a person ("we can still help each other, […] we have occasionally experienced something similar, we must not give up, it will happen again"). Within this network, they find it possible to talk and listen to each other without judgment, contrary to what happens with their neighbours "We can't share in our local area, whereas when we meet others [in the Pâtur'ajuste network], we reassure each other"). This possibility to exchange freely and with kindness seems essential for them to face the difficulties they encounter. One of them even mentioned that it was a way for him to get out of a burnout and that, according to him, the fact of being able to talk and exchange could prevent suicides.

S3: Strengthening their identity by widely communicating about their practices

Many breeders expressed the importance for them to be able to communicate and exchange ideas about their work beyond their peers, i.e., with consumers, future breeders, but also with society in general. They need to explain what they do, how they work and what impact this has on the quality of WS 8 Innovations and adaptation to changes their production. For those who sell their products directly to consumers, interacting with them is of utmost importance to explain how their practices shape their products from an organoleptic point of view ("You also need to teach your customers how to eat this meat. It's a little more chewy, it's a little more tasty"). Some interviewees also take the time to explain their practices from an environmental point of view ("I have to provide information to tell them that the plots are not visually aesthetic, but that they are ecologically even richer [...] we have to change people's mentalities"). Some interviewed farmers also expressed their desire to pass their experiences on to other farmers. They welcome trainees, help new farmers to establish themselves, or organise open houses, to make them aware that there are different ways of working. These farmers also talked about their need to communicate more widely with society, to counteract the information disseminated in the media ("We have to explain to people! Everything they hear in the media... Junk food"). For them, this can be done through various means such as : Visits to their farms, commercial events and participating in local initiatives. For them, all these communication initiatives are a way to "spread ideas", to "enhance the value of the profession" and to give meaning to what they do ("we talk to people [...]

it always gives meaning to what we do. It gives us more confidence in our practices").

Discussion and perspectives

This description by process thus highlights personalisation and socialisation as two major orientations in the construction of the meaning given to practices. However, it dissociates these two processes, whereas they are intertwined, as shown in Malrieu's (2003) autobiographical accounts of individuals' behaviours. In our research, we observed that personalisation and socialisation co-construct practices both for the farmer's development and for the development of new practices within a professional community, or even to respond to social issues. For example, the breeders expressed the pleasure of experimenting autonomously (P1), while saying they need to compare their experiments with their peers to feel supported (S2). These two processes thus feed into each other. They enable breeders to assert themselves as individuals by relying on their motivations (P4) while building on the meaning that their peers give to the innovative practices that they share (S1). This study has shown that these livestock farmers engaged in agroecological practices rebuild their practices through different processes of personalisation and socialisation [START_REF] Dufour | Diversité des cheminements en agriculture biologique : le sens du métier en question ?[END_REF]. They thus seek a coherence between their personal aspirations and the collective representations of their social environment, and this coherence enables them to give meaning to what they do. In particular, we highlighted the pleasure that these breeders feel when experimenting with new ways of doing things (P1), close to the experiential learning of Kolb (1984). In this case, individuals have to go through their own experiences (in the sense of Dewey, 1938) in order to achieve self-fulfilment, whether in failure or success (Clot, 1999). It is thus a question of human agentivity (Bandura, 1986), i.e., of the capacity of individuals to be active subjects [START_REF] Almudever | Sujet proactif et sujet actif : deux conceptions de la socialisation organisationnelle[END_REF] of their own lives. Such a capacity may be carried out by exercising control and regulation of their actions, thus promoting the individual's motivation to learn (cf. P4). As we have shown, farmers learn through their view of other people's ways of doing things and they go beyond these models by generating new skills and behaviours to adapt their practices and systems to their particular situation (link P1-S1). The national days of the Pâtur'Ajuste network give them the opportunity to draw on the social representations (Michel-Guillou and Weiss, 2007) they have of other systems in order to build their own practices (P3). This can be by social comparison [START_REF] Festinger | A Theory of Social Comparison Processes[END_REF] that may attest to a need to compare themselves to others in order to know their own value in the absence of standards. By experimenting with new practices (P1), farmers

WS 8

Innovations and adaptation to changes also rely on their senses and emotions (P2), as described by Salmona (2010). In the context of agricultural industrialisation, this relational know-how, close to the emotional skills studied by [START_REF] Gendron | Emotional capital: the set of emotional competencies as professional and vocational skills in emotional works and jobs, Vocational Education and Training An international Perspective[END_REF], and this "engagement with the living environment" are forgotten dimensions of the farmer's profession, as shown by [START_REF] Blanc | Relational knowledge and commitment with the living environment: the forgotten dimensions of sheep farming practice[END_REF]. In these farms, their intrinsic motivations (P4), in the sense of Deci and Ryan (1985), explain their commitment to agroecological practices. This self-determination is a way for farmers to recognise themselves in what they do and to be satisfied with their work, even though their choices are far from the main professional standards. At the same time, they seek a form of social support (S2) in interpersonal relationships [START_REF] Almudever | Sujets pluriels : la construction de la personne à l'articulation de différents milieux et temps de socialisation[END_REF]

Conclusion

This first study, grounded in [START_REF] Malrieu | La socialisation[END_REF]Malrieu's ( , 1989) ) theoretical model of plural socialisation, has shown that within the framework of the agroecological transition, farmers build their own professional practices by relying on their own knowledge and conception of the profession, as well as by drawing inspiration from the different living and professional environments that surround them. It is promising to understand the professional transition that agro-ecology may represent in relation to the orientations of the conventional agriculture dominant until now. We thus confirm that in the agricultural domain as in other professions [START_REF] Croity-Belz | Recherche d'information, conduites d'innovation et interdépendance des domaines de vie : les modalités et les déterminants d'une participation active des nouveaux recrutés à leur socialisation organisationnelle[END_REF], it is important to study the interactions between the processes of personal construction and the processes of social transformation related to changes in the professional world [START_REF] Almudever | Sujets pluriels : la construction de la personne à l'articulation de différents milieux et temps de socialisation[END_REF]. It will be a first step before considering the use of these processes by agricultural advisers who support farmers in their transition.

We plan to study how the commitment level to these changes is related to farmers' possibilities of thinking about a new project for their system that is in line with their values and motivations. Still based on the model of plural socialisation (Malrieu, op cit.), we propose to carry out a psychosocial analysis according to the four levels of Doise (1982): intrapsychic, inter-individual, positional and ideological. We will first focus on perceived social support [START_REF] Cazals | Social support, coping strategies and psychological well-being amoung young people awaiting employment[END_REF] and the feeling of self-efficiency [START_REF] Almudever | Activity System Model, Perceived Self-Efficacy, and Newcomer Integration Behaviour[END_REF][START_REF] Faurie | Sentiments d'efficacité personnelle et dynamique du projet professionnel[END_REF][START_REF] Faurie | Sentiment d'efficacité personnelle et dynamique capacitaire dans les choix d'orientation atypique[END_REF], as well as on motivational (Levy-Leboyer, 2006) and emotional (Van Dam and Nizet, 2015) mechanisms. Our objective is twofold:

-to identify the effects of the commitment to new practices on the professional development of each breeder through the integration of practices consistent with who they are as a person; -to characterise the perceived effects of this commitment on the development of the profession.

The results we expect from this research would allow us to undertake an operational reflection on how to take these processes in the farmer-counsellor relationship into account and to thus grasp the more personal dimensions of work linked to people's identities and the meaning given to work (Dockès et al.,

2

  nd International Symposium on Work in Agriculture Thinking the future of work in agriculture March 29 th -April 1 rst , 2021 Clermont-Ferrand (France)

  with their peers. Finally, they look for recognition of their knowledge and skills by comparing them with others (De Gaulejac, 2012), such as consumers, neighbours and institutions. These first results helped us to open new research avenues: -To what extent can the agroecological transition open perspectives for farmers to build their own professional standards in the face of the uncertainties they face? To what extent is the agroecological transition a means for them to claim their opposition to the prescriptive standards advocated by industrial agriculture? -To what extent can agro-ecology enable farmers to reintegrate the more subjective dimensions into their relationship to work such as love of nature and passion for animals, which have been inhibited by more intensive practices?

Table 1 . Sample characteristics in March 2018.

 1

				Type of livestock system	Direct selling (Yes/N o)	Year of enrolment in the network	Number of times attended a network meeting
	AJPA	Maine et Loire	60-65	Beef cattle	Yes	2014	9
	SJJR	Maine et Loire	60-65	Beef cattle	No	2013	5
	LB	Maine et Loire	50-55	Cattle and sheep meat	Yes	2015	3
	CG	Maine et Loire	40-45	Beef cattle	No	2015	6
	FL	Loire Atlantique 30-35	Sheep meat	Yes	2016	4
	TR	Loire Atlantique 30-35	Sheep meat	Yes	2013	3
	EFP	Haute Loire	45-50	Goats, cattle	Yes	2014	3
	CV	Haute Loire	40-45	Sheep meat	Yes	2014	5
	ET-CL	Loire	30-35	Sheep, cattle, goats	Yes	2017	2
	JMW	Hérault	50-55	Beef cattle	Yes	2014	3

March 29 th -April 1 rst , 2021 Clermont

-Ferrand (France)

All quotes in italics are drawn from our corpus.

March 29 th -April 1 rst , 2021Clermont-Ferrand (France)


C Banc

M Gautier

Gautier, M. * D Blanc-Biscarat

M Lupsea-Toader

R Marsac

R Gourdon

Influence of pH on the release of colloidal and dissolved organic matter from Vertical Flow Constructed Wetland surface sludge deposits

Keywords: Lixiviation, Organic carbon, Molecular weight, Ultrafiltration, Aromaticity, pH variations BOD: Biochemical Oxygen Demand LOC: Large Organic Colloids

The very rapid development of vertical flow constructed wetlands (VFCW) in Europe and throughout the world has induced a growing concern on the conditions of management of their surface sludge deposits (SD). The leaching of organic components from SD was explored in this study with respect to their potential impact on biochemical oxygen demand (BOD) and their role as complexing agents in the release of trace contaminants. The aromaticity and molecular size of leached organic compounds was investigated over a large range of pHs. Experimental results showed that the pH of the leaching aqueous solution influenced the concentration and characteristics of leached OC. Around the natural pH of SD aqueous suspensions (6 to 8), truly dissolved organics (<3 kDa) exhibited high aromatic content. Under alkaline conditions (pH >8), large organic colloids (>30 kDa) were found predominant, exhibiting a low aromaticity growing with pH. Under acidic conditions (pH<6), hydrophilic truly dissolved organics were leached. Comparison of concentration, aromaticity and molecular size of OC leached from 14-year and 7-year SD suggested that ageing of SD reduced their sensitivity to pH conditions and may improve their agronomic properties.

Introduction

In porous media such as soil, sediment and sludge, the partition of organic molecules between solid and dissolved states controls the fate of many trace contaminants [START_REF] Pédrot | Double pH control on humic substance-borne trace elements distribution in soil waters as inferred from ultrafiltration[END_REF][2][START_REF] Sauvé | Solid-Solution Partitioning of Metals in Contaminated Soils: Dependence on pH, Total Metal Burden, and Organic Matter[END_REF]. The presence of complexing groups in solid organic matter (SOM) contributes to the retention of trace metals within the solid fraction. The solubilization of organic molecules through SOM hydrolysis thereby induces the release and transport of associated pollutants [START_REF] Kikuchi | Correlations between aromaticity of dissolved organic matter and trace metal concentrations in natural and effluent waters: A case study in the Sagami River Basin, Japan[END_REF][START_REF] Pearson | Predicting Copper Speciation in Estuarine Waters-Is Dissolved Organic Carbon a Good Proxy for the Presence of Organic Ligands?[END_REF] which may alter the quality of the receiving water bodies.

A large variety of organic molecules may be leached in the dissolved form. Depending of the SOM of origin, these molecules may exhibit different molecular sizes and chemical compositions, particularly with regards to their functional groups (aromatic rings, carboxylic acids, N-containing groups, etc.). Molecular fractionation using ultrafiltration [START_REF] El-Sayed | Assessment of clay mineral selectivity for adsorption of aliphatic/aromatic humic acid fraction[END_REF][START_REF] Pokrovsky | Organic and organo-mineral colloids in discontinuous permafrost zone[END_REF] and UV-visible spectroscopy [START_REF] Peacock | UV-visible absorbance spectroscopy as a proxy for peatland dissolved organic carbon (DOC) quantity and quality: considerations on wavelength and absorbance degradation[END_REF][START_REF] Wang | Coagulation Kinetics of Humic Aggregates in Mono-and Di-Valent Electrolyte Solutions[END_REF] have been used extensively to characterize the molecular weight (MW) and aromaticity of DOM. SUVA 254 , defined as the ratio between absorbance at a wavelength of 254 nm and dissolved organic carbon concentration, is a well-known indicator of DOM aromaticity. It has also been reported to be a good indicator of DOM capacity to sorb trace metals in organic rich water [START_REF] Kikuchi | Correlations between aromaticity of dissolved organic matter and trace metal concentrations in natural and effluent waters: A case study in the Sagami River Basin, Japan[END_REF][START_REF] Yan | Relationship between the characterization of natural colloids and metal elements in surface waters[END_REF] or organic-rich soil solutions [START_REF] Araújo | Association between extracted copper and dissolved organic matter in dairy-manure amended soils[END_REF]. Moreover, the chemical speciation of associated trace metals has been shown to be largely affected by the molecular weight of organic ligands in natural water samples [START_REF] Pokrovsky | Organic and organo-mineral colloids in discontinuous permafrost zone[END_REF][START_REF] Gangloff | Factors controlling the chemical composition of colloidal and dissolved fractions in soil solutions and the mobility of trace elements in soils[END_REF]. Finally, the aromaticity of DOM, estimated by SUVA 254 or other parameters, has also been shown to play a role in biodegradabily as shown by the anti-correlation observed between the biological oxygen demand BOD and the aromaticity of dissolved organic compounds [START_REF] Marschner | Controls of bioavailability and biodegradability of dissolved organic matter in soils[END_REF].

The release of DOM from SOM may occur by different mechanisms, such as the hydrolysis of macromolecular structures [START_REF] Durce | Alteration of the molecular-size-distribution of Boom Clay dissolved organic matter induced by Na+ and Ca2+[END_REF][START_REF] Piccolo | THE SUPRAMOLECULAR STRUCTURE OF HUMIC SUBSTANCES[END_REF] or the desorption of organic molecules attached to solid mineral phases [START_REF] Chen | The role of organo-clay associations in limiting organic matter decay: Insights from the Dajiuhu peat soil, central China[END_REF][START_REF] Gao | Organic matter coatings of soil minerals affect adsorptive interactions with phenolic and amino acids[END_REF]. The extent to which each mechanism may contribute to the overall leaching phenomenon is dictated by the physico-chemical characteristics of the solid matter of origin [18] and the environmental conditions of the medium. pH conditions appears to be a key factor of influence [START_REF] Pédrot | Double pH control on humic substance-borne trace elements distribution in soil waters as inferred from ultrafiltration[END_REF][START_REF] Grybos | Increasing pH drives organic matter solubilization from wetland soils under reducing conditions[END_REF].

Several studies have reported that dissolved organics with high MW and aromaticity were preferentially aggregated by solid mineral phases [18,[START_REF] Xu | Molecular weight-dependent adsorption fractionation of natural organic matter on ferrihydrite colloids in aquatic environment[END_REF], resulting in a poor solubilization of these molecules.

Alkaline pHs were reported to destabilize organo-mineral associations via the deprotonation of the ligands in both phases inducing anionic electrostatic repulsion [START_REF] Avena | Desorption of Humic Acids from an Iron Oxide Surface[END_REF][START_REF] Avena | Disaggregation Kinetics of a Peat Humic Acid: Mechanism and pH Effects[END_REF]. Pédrot et al. [START_REF] Pédrot | Dynamic structure of humic substances: Rare earth elements as a fingerprint[END_REF] reported that low pHs could also induces the disaggregation of supramolecular assemblies of humic substances, leading to the release of low molecular weight organic molecules.

Constructed wetlands are engineered nature-based systems designed to treat domestic wastewaters and similar effluents [START_REF] Vymazal | Is removal of organics and suspended solids in horizontal sub-surface flow constructed wetlands sustainable for twenty and more years?[END_REF]. So-called "Classical French" vertical flow constructed wetlands (VFCWs) are based on the percolation of unsettled wastewaters through 2 successive stages of filters filled with gravels or sand and planted with reeds. This process induces the formation of a sludge deposit (SD) layer at the surface of the first-stage filter which has been shown to play a major role in the retention and degradation of a large variety of contaminants during regular conditions of operation [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF][START_REF] Kania | Analytical indicators to characterize Particulate Organic Matter (POM) and its evolution in French Vertical Flow Constructed Wetlands (VFCWs)[END_REF].

Organic matter in SD is mostly present in the solid form and its concentration ranges between 40% and 90% of dry mass [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF]. It originates predominantly from anthropic sources (wastewaters), but may also be autochtonous (e.g. biofilms, plants and microbial cells fractions; [START_REF] García | Contaminant Removal Processes in Subsurface-Flow Constructed Wetlands: A Review[END_REF]) or allochtonous (e.g. terrestrial organic residues from the catchment area). The surface sludge layer is the siege of numerous reactions which induce the evolution of organic matter [START_REF] Kania | Analytical indicators to characterize Particulate Organic Matter (POM) and its evolution in French Vertical Flow Constructed Wetlands (VFCWs)[END_REF][START_REF] Kim | pH and Eh effects on phosphorus fate in constructed wetland's sludge surface deposit[END_REF] that could in turn influence the quantity and the chemical nature of the DOM. Some studies have already monitored the concentration of DOM in VFCW systems, from inflow to outflow [START_REF] Barber | Nature and Transformation of Dissolved Organic Matter in Treatment Wetlands[END_REF][START_REF] Clark | Optical properties of chromophoric dissolved organic matter (CDOM) and dissolved organic carbon (DOC) levels in constructed water treatment wetland systems in southern California, USA[END_REF][START_REF] Park | Transformation of dissolved organic matter in a constructed wetland: A molecular-level composition analysis using pyrolysis-gas chromatography mass spectrometry[END_REF]. Kania et al. [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF] showed that SDs were sources of DOM and reported a pH-dependent leaching pattern. However, despite the significant scientific and operational implications this observation may have during the operation of VFCW systems or for the use of the sludge outside the system, little is known on the influence of pH on the fluxes, molecular weight and other characteristics of DOM leached from the SDs. Yet, by controlling the quantities, nature, and molecular size of DOM, the pH conditions could strongly influence the associated release of contaminants from the SDs [START_REF] Pédrot | Double pH control on humic substance-borne trace elements distribution in soil waters as inferred from ultrafiltration[END_REF][START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF].

During the operation of a wastewater treatment plant unit, the SD layer is usually submitted to pHs ranging between 6 and 9 [START_REF] Park | Transformation of dissolved organic matter in a constructed wetland: A molecular-level composition analysis using pyrolysis-gas chromatography mass spectrometry[END_REF][START_REF] Yu | Dynamic control of disinfection for wastewater reuse applying ORP/pH monitoring and artificial neural networks[END_REF]; However, more acidic conditions (pH below 6) can be observed with winery effluents for example [START_REF] Rodríguez-Chueca | Treatment of winery wastewater by sulphate radicals: HSO5-/transition metal/UV-A LEDs[END_REF]. A better description of the leaching of organic compounds from the SDs may also provide useful information regarding the final use of the sludge material in land applications where it would be submitted to pHs ranging from 5 to 9 [START_REF] Gao | Spectral characteristics of dissolved organic matter in various agricultural soils throughout China[END_REF].

The general objective of this study was thus to generate scientific knowledge on the release of organic colloids from the SOM of VFCW surface sludge deposits, and more specifically on the effects of pH on the quantities and physical-chemical characteristics of the leached organic compounds. Previous work from our group compared the characteristics of 14 SDs samples from different origins and identified 2 typologies of samples according to their age, namely the older SDs group beyond 3 years of age, and the younger SDs group of 1 year or less [START_REF] Kania | Analytical indicators to characterize Particulate Organic Matter (POM) and its evolution in French Vertical Flow Constructed Wetlands (VFCWs)[END_REF]. The present study focused on sludge deposits of the older SDs group (beyond 3 years of age) which was considered as representative of a steady state mode of operation of the VFCW unit. The work was therefore done using SD samples taken from a same plant at different ages above 3 years, respectively 7 and 14 years, in order to investigate the influence of ageing on the leaching properties. SD samples were leached under controlled conditions over a wide pH range to generate information which would not be accessible in field studies. The aqueous extracts were fractionated by ultrafiltration and then analyzed for DOC concentration. Each ultrafiltrate was then further analyzed to evaluate the chemical nature of the organic compounds.

Materials and methods

Sampling

SD were sampled from the Vertical Flow Constructed Wetland (VFCW) treatment plant of Vercia (FRANCE) already described in details by Kim et al.[35]. This plant is designed according to the AZOE-NP patented process (EP1857419A1). It is considered as a "French system" because it receives unsettled wastewater. It differs from the so-called "classical French system" by some additional operations, in particular the implementation of a trickling filter before the first filter stage and the presence of a watersaturated zone in the lower part of the first stage filter, providing anoxic conditions to allow partial denitrification. SD samples were taken from the surface sludge layer of the first-stage filter which had been left in place since the first day of operation of the treatment plant following the protocol reported by Kania et al. [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF]. The 7-year samples were taken by Kania et al. [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF], freeze-dried for conservation and stored at 4 °C The 14-year samples were taken specifically for the present study following the same protocol. All samplings were done in winter time for the good accessibility to the sludge layer due to the absence of the reeds. Several kg of SD were collected over the whole depth of the sludge layer (>15 cm) from 8 different spots at the surface of the first-stage filter. All samples were sorted manually on site to remove reeds rhizomes and gravels, and subsequently mixed together by shoveling. A representative sample of a few kg was then prepared by quartering, refrigerated and transported to the laboratory within a maximum of 4 hours. Then, aliquot fractions were prepared, crushed and sieved at 1 mm, freeze-dried and stored at 4 °C for further analyses and assays.

Analysis of particulate matter

Organic carbon contents of the two SD samples were determined by a carbon sulfur analyzer (EMIA-320V2, Horiba Scientific Co., Ltd) previously decarbonized by hot digestion with diluted HCl. The precision of the analysis for carbon is ±2 mg/kg. The freeze-dried SD samples were analyzed in duplicates for humic-like, fulvic-like and humin-like components, following a protocol adapted from Serra-Wittling et al. [START_REF] Serra-Wittling | Impact of Composting Type on Composts Organic Matter Characteristics[END_REF]. To obtain humic and fulviclike content, a dry mass of 1g of SD was suspended and mixed for 2 hours in 50 mL of a 0.1 M sodium hydroxide aqueous solution. The suspension was then centrifuged at 10 000 g for 15 min and filtered at 0.45 µm using Sartorius acetate cellulose filters. The solutions, containing humic-and fulvic-like substances, were analyzed for OC using a Shimadzu TOC-L analyzer with an uncertainty of ± 5%. The residual solids contained all the SD constituents which were not extracted under the alkaline conditions used. Their OC content, representing the humin-like fraction, was calculated as the difference between the total organic carbon (TOC) content of the initial SD sample and the OC content of the filtered solution. To obtain fulvic-like content, SD solutions were acidified to pH 1.5 with 1 M sulfuric acid aqueous solution and kept overnight at +4°C to allow precipitation of humic-like substances. Fulvic-like substances were collected in the solutions by centrifugation at 6000 g for 15 min followed by filtration at 0.45μm. The filtered solutions were analyzed for OC concentration.

The SD samples were also analyzed by thermogravimetry (TGA) coupled to differential scanning calorimetry (DSC) using a METTLER TOLEDO TGA /DSC analyzer. Analyses were done in duplicates.

Aliquote sub-samples of 20 mg dry mass (DM) were heated from 35 to 900 °C at a rate of 10 °C.min -1 under an airflow of 50 NmL.min -1 . Blanks were run under the same conditions without any samples.

Mass losses obtained from TGA and heat fluxes obtained from DSC were monitored as a function of the temperature. The data were used to calculate R TGA and R DSC indexes as indicators of the stability of the organic matter as described previously [START_REF] Kania | Analytical indicators to characterize Particulate Organic Matter (POM) and its evolution in French Vertical Flow Constructed Wetlands (VFCWs)[END_REF]. These indexes were respectively calculated as the ratios between the mass loss (R TGA ) and the heat release (R DSC ) recorded between 400 and 600°C to those recorded between 200 and 400 °C. They were shown to increase with the contents in complex organic compounds such as lignin, poly aromatic structures of high MW and O-containing functional groups in the solid organic matter SOM [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF].

pH-dependent leaching test

A batch leaching test, based on the CENTS/TS 14429 European standard, was used to evaluate the effect of pH on the release of organic components from the sludge deposits (SD) samples. The assays were performed in Nalgene® polypropylene centrifuge tubes, at a solid-to-liquid ratio of 100 g of SD dry matter per liter of leaching solution. Prior to the tests, all tubes were soaked in 1M HCl aqueous solution overnight and then rinsed with deionized water. All assays were duplicated.

The SD samples were contacted with: (i) deionized water for natural pH conditions (blank), (ii) 0.022, 0.08, 0.13, 0.25, 0.35 M nitric acid aqueous solutions for acidic conditions and (iii) 0.02, 0.03, 0.06, 0.1, 0.15, 0.20 M potassium hydroxide aqueous solution for alkaline conditions. The number of moles of protons (acidic solutions) or hydroxide ions (alkaline solutions) in each leaching solution was calculated as the product of HNO 3 or KOH concentrations by the volumes of solution used in each assay.

Altogether, 5 acidic and 6 alkaline leaching solutions were used, in addition to the blank at natural pH.

All assays and blank were stirred at room temperature in a rotary shaker set at 9 rpm until an apparent steady state was reached, which was observed within 48 hours in preliminary experiments.

Fractionation of leached compounds

The suspensions obtained from the batch leaching experiments described above with the two SD samples at the different pHs tested were centrifuged at 4000 rpm (2400 g) and the supernatants were filtered at 0.45 µm using Sartorius cellulose acetate filters to separate SOM from truly dissolved or colloidal forms.

The solutions filtered at 0.45µm were analyzed for OC concentration and the results expressed as corresponding to the colloidal and truly dissolved compounds leached. They were then further Then, ultracentrifugation tubes equipped with Sartorius Vivaspin cellulose acetate ultrafiltration membranes of 30 kDa, 10 kDa and 3 kDa were used to fractionate the soluble components according to their MW. Centrifuge ultrafiltrations were performed at 3000g for 30 min using a swinging bucket rotor.

Ultracentrifugation cells were previously rinsed with a 0.1M NaOH solution followed by de-ionized ultrapure water to guarantee initial concentrations of dissolved organic carbon (DOC) below 0.1 mg.L - 1 in the ultrafiltrate.

Analyses of suspensions and solutions

The suspensions obtained from the leaching experiments were analyzed for pH and conductivity before being filtered. A Consort C3020 meter equipped with Bioblock 11706358 and Consort SK10T electrodes was used for pH and conductivity.

Organic carbon (OC) concentrations were measured in solid, colloidal and truly dissolved fractions using a Shimadzu TOC-V CSH analyzer previously calibrated with a standard solution of potassium hydrogen phthalate. The uncertainty of the analysis was estimated to ± 5%. OC concentrations in blanks at natural pH (leaching in de-ionized ultrapure water) were found to be systematically below 0.2 mgC.L -1 .

The colloidal and truly dissolved fractions were further analyzed for UV-visible absorbance using a Perkin-Elmer Lambda 25 spectrophotometer to record UV-visible spectra from 800 to 220 nm with 0.5 nm increments. Analyses were done in duplicates at room temperature in 1-cm light-path quartz cells.

The ratio of the absorbances measured at 250 and 365 nm (referred to as E 2 :E 3 ) was calculated as an indicator of the size of the UV-absorbing organic molecules present in the solutions [START_REF] Peacock | UV-visible absorbance spectroscopy as a proxy for peatland dissolved organic carbon (DOC) quantity and quality: considerations on wavelength and absorbance degradation[END_REF]. It was considered that an increase of the ratio revealed a decrease of the average molecular size of the UVabsorbing molecules in solution [START_REF] Helms | Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter[END_REF]. The specific ultraviolet absorbance at 254 nm (SUVA 254 , L.mgOC - 1 .m -1 ), defined as the ratio of the absorbance at 254 nm to the organic carbon concentration was used as an indicator of the degree of aromaticity of the molecules [START_REF] Helms | Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter[END_REF][START_REF] Weishaar | Evaluation of Specific Ultraviolet Absorbance as an Indicator of the Chemical Composition and Reactivity of Dissolved Organic Carbon[END_REF]. Values of SUVA 254 above 4 and below 2 were considered to reveal highly aromatic and poorly aromatic molecules, respectively [START_REF] Park | Transformation of dissolved organic matter in a constructed wetland: A molecular-level composition analysis using pyrolysis-gas chromatography mass spectrometry[END_REF].

Results and discussion

Comparison of the SDs physicochemical properties

General characteristics of the SDs

Table 1-Analyses of surface sludge deposits (SD) from Vercia VFCW wastewater treatment plant sampled after 7 and 14 years of operation Sample TOC (1) (g TOC/g DM) Humic-like (2) (% of TOC) Fulvic-like (2) (% of TOC) Humin-like (2) (% of TOC) R DSC R TGA N tot (2) (% of DM) P tot (2) (% of DM) (1) Uncertainty of ± 2%;

(2) Uncertainty of ± 5%

The results of the characterization tests of our samples including the total amount of organic carbon (TOC), the extractions of humic and fulvic-like compounds and thermogravimetric indexes are shown in Table 1. TOC contents in both SD samples (0.17 and 0.23 g TOC/g DM, for 14-years and 7-years SD respectively) were far above the concentrations reported in literature for natural systems like natural wetlands sediments in which TOC rarely exceeds 10% [START_REF] Guénet | Evidence of organic matter control on As oxidation by iron oxides in riparian wetlands[END_REF][START_REF] Yin | Effects of soil moisture on carbon mineralization in floodplain wetlands with different flooding frequencies[END_REF]. TOC content in SD samples are closer to the 30-45% of TOC content generally observed in the first peatland soil profiles of temperate climate [START_REF] Chen | The role of organo-clay associations in limiting organic matter decay: Insights from the Dajiuhu peat soil, central China[END_REF][START_REF] Hong | Adsorption of organic matter on clay minerals in the Dajiuhu peat soil chronosequence, South China[END_REF]. The rest of the sludge deposits' matter is composed of inorganic matter such as Feoxyhydroxydes, carbonates, silica and clays [START_REF] Kim | Effect of Aging on Phosphorus Speciation in Surface Deposit of a Vertical Flow Constructed Wetland[END_REF]. The lower TOC content in 14-year, compared to 7year SD, was assigned to the mineralization of organic matter which was reported in several studies [START_REF] Kim | Effect of Aging on Phosphorus Speciation in Surface Deposit of a Vertical Flow Constructed Wetland[END_REF][START_REF] Sellami | Maturity assessment of composted olive mill wastes using UV spectra and humification parameters[END_REF].

About 70 to 75% of the TOC was present in the humin-like fraction in both samples (Table 1). Fulvicand humic-like compounds were found to account for approximately 31% and 25% of the TOC content in the 7-year and 14-year samples, respectively. Previous studies from literature showed that the humin fraction's OC exhibited a higher concentration in aromatic and polyhydroxy aromatic compounds than the humic and fulvic-like fraction [START_REF] Weishaar | Evaluation of Specific Ultraviolet Absorbance as an Indicator of the Chemical Composition and Reactivity of Dissolved Organic Carbon[END_REF][START_REF] Yao | Labile and recalcitrant components of organic matter of a Mollisol changed with land use and plant litter management: An advanced 13C NMR study[END_REF]. This observation was confirmed here by TGA-DSC analyses that revealed higher values of R TGA and R DSC indexes for the 14 years old sludge deposit (Table 1). These results indicate that with aging, the sludge deposits' concentrates more and more aromatic and Ocontaining functional groups despite the continuous input of low aromatic organic matter from wastewater [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF]. (negative values, alkaline solutions) in each leaching solution, expressed per g of SD dry matter. The pH buffering capacity of solid samples such as soils, sediments or composts is largely influenced by the density of functional groups of OM [START_REF] Nelson | Soil pH buffering capacity: a descriptive function and its application to some acidic tropical soils[END_REF][START_REF] Plaza | Acid-Base Properties of Humic and Fulvic Acids Formed during Composting[END_REF]. Plaza et al. [START_REF] Plaza | Acid-Base Properties of Humic and Fulvic Acids Formed during Composting[END_REF] evidenced that OM enrichment in carboxyl and phenolic groups, occurring with composting process of various sludge residue, increase their acid buffering capacity. The buffer capacity of the two SDs were similar over most of the pH range but, with respect to 7-year SD, the 14-year SD shows a greater buffering capacity for acidic conditions (pH~5).

Acid-base buffering capacity of SDs

Thus, this was probably due to the enrichment of the 14-year SD in aromatic and O-containing functional groups as observed in the previous section. This change in acid-base proprieties occurring with aging could improve the SD's resistance to acidic rain or acidic agricultural soils. Fig. 2 shows the concentration of organic carbon measured at equilibrium in the 7-year and 14-year SD leachates after filtration at 0.45 µm ( ). The lowest extraction was observed at neutral pH (i.e

[𝑂𝐶] 0.45 µm pH 6.5), where only 0.4% and 1.6% of the TOC content was leached for 14-year and 7-year SD, respectively. These leaching values are similar to those reported in literature for agricultural soils with pH close to 7 and much lower TOC contents [START_REF] Corvasce | Characterization of water extractable organic matter in a deep soil profile[END_REF][START_REF] Fang | Comparative characterization of sewage sludge compost and soil: Heavy metal leaching characteristics[END_REF]. Under these circum-neutral pH conditions, the conformational structures of the organic matter and organo-mineral aggregates are less impacted than under more acidic conditions [START_REF] Pédrot | Dynamic structure of humic substances: Rare earth elements as a fingerprint[END_REF] resulting in a lower leaching.

For both samples, leaching of organic compounds increased below pH 5 and above pH 7. Leaching at neutral to acidic pH affected only a very small proportion (less than 5%) of TOC present in the samples:

the highest release under acidic conditions was obtained at pH 2.2 with 323 mg/L for the 14-year and 830 mg/L of OC for the 7-year SD, corresponding to 1.8% and 4.8% of the TOC respectively. The increased release of OC observed, for both SD samples, under acidic conditions is the net effect of several phenomena [18]. For example, under acidic conditions, protonation of hydroxyl groups and the development of a positive surface charge at mineral surfaces favored the adsorption of negatively charged organic molecules, thereby limiting the transfer of OC into the aqueous solution [18,[START_REF] Tipping | Humic Ion-Binding Model VII: a revised parameterisation of cation-binding by humic substances[END_REF]. But, the increase of protons competition under acidic conditions reduce cation bridging of organic molecules and could induce the release of OC [18,[START_REF] Guggenberger | Dissolved organic carbon control in acid forest soils of the Fichtelgebirge (Germany) as revealed by distribution patterns and structural composition analyses[END_REF]. The sum of these phenomenon produces a net increase of the OC leached concentration observed under acidic conditions as compared to circum-neutral pH conditions.

The effect of pH on the leaching of soluble compounds was much stronger over the alkaline range (Fig. 2). For the 14-year SD, leaching was increased by a factor 4 from pH 6.5 to 8.0 (from 65 mg/L to more than 300 mg/L of OC), and by a factor 20 from pH 8 to 12 (6000 mg/L of OC at pH 12, Fig. 2). At pH 12, more than 33% of the overall TOC content of the sludge deposits was leached into the solution. For the 7-year SD, the maximum release, obtained at pH 12, was around 8000 mg/L, corresponding to 48% of the overall TOC content. The increasing solubilization of organic molecules with increasing pH could be attributed to the deprotonation of SD's mineral surfaces (e.g. Fe-oxyhydroxydes, clays…) occurring with increasing pH conditions which would disrupt organo-mineral association [START_REF] Avena | Desorption of Humic Acids from an Iron Oxide Surface[END_REF][START_REF] Avena | Kinetics of Humic Acid Adsorption at Solid-Water Interfaces[END_REF] and lead to the solubilization of organic molecules associated to these mineral surfaces.

The chemical structure of organic functional groups was also reported as a factor controlling the solubilization of organic molecules [START_REF] Matiasek | The chemical fingerprint of solubilized organic matter from eroded soils and sediments[END_REF]. This factor may explain the differences observed in the leaching behavior of the 7-year and 14-year samples (Fig. 2), which cannot be attributed only to their different TOC contents. Over the entire pH range, released OC concentrations were higher for the 7year SD but the largest gap was observed close to neutral pH (i.e pH 6.5) where 14-year SD released about 4 times less OC than the 7-year SD. This result emphasized that the 7-year SD contained more easily leachable organic molecules than the 14-year sample. This observation was attributed to the higher aromaticity of the 14-year sample and its higher content in oxygenated functional groups (Table 1), which increased the proportion of OC trapped in organo-mineral aggregates [START_REF] Bhattacharyya | Iron speciation in peats: Chemical and spectroscopic evidence for the co-occurrence of ferric and ferrous iron in organic complexes and mineral precipitates[END_REF] and thus OC stability.

pH-dependent characteristics of soluble organic compounds

Figure 3a shows the SUVA 254 values of the soluble organic compounds leached over the tested pH range for the 7-year and 14-year after filtration at 0.45 µm. At neutral pH (i.e. pH 6.5), SUVA values around 1 L.mgOC -1 .m -1 were recorded for both 7-year and 14-year SD, suggesting a low aromaticity and high proportion of proteins, lipids and carbohydrates. Aromatic compounds were reported to have a higher affinity with mineral phases [START_REF] Gao | Organic matter coatings of soil minerals affect adsorptive interactions with phenolic and amino acids[END_REF][START_REF] Kramer | Long-term carbon storage through retention of dissolved aromatic acids by reactive particles in soil[END_REF] and were therefore probably sorbed under these pH conditions. The SUVA measured here were similar to those obtained by other authors on natural wetlands [START_REF] Wang | Concentration dynamics and biodegradability of dissolved organic matter in wetland soils subjected to experimental warming[END_REF],

sediments [START_REF] He | Differences in spectroscopic characteristics between dissolved and particulate organic matters in sediments: Insight into distribution behavior of sediment organic matter[END_REF] and various agricultural soils with pH varying from pH 8 to pH 5 [START_REF] Gao | Spectral characteristics of dissolved organic matter in various agricultural soils throughout China[END_REF]. Under alkaline conditions, an increase of SUVA 254 values was observed, up to 2.5 L.mgOC -1 .m -1 (14years SD) and 3.5 L.mgOC -1 .m -1 (7-years SD) at pH 12, suggesting the release of more aromatic organic compounds. The higher aromaticity of the compounds leached at increasingly alkaline pHs was attributed to the desorption of fulvic and humic-like compounds. Above pH 8 or so, SUVA 254 values were higher for the 7-year SD than the 14-year sample (Fig. 3a) despite the aromatic content deduced from table 1, which were lower in the 7-year SD. This observation could be attributed to a stronger stability in the 14-year SD as compared to the 7-year sample of the complexes between organic compounds and clay [START_REF] El-Sayed | Assessment of clay mineral selectivity for adsorption of aliphatic/aromatic humic acid fraction[END_REF] or metal-oxyhydroxides [START_REF] Kaiser | The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils[END_REF] within the SD particles. Under very acidic conditions, the SUVA of the leached compounds was slightly higher than under natural pH conditions (around 1.5 and 0.5-1.0 respectively). This result may be explained by the possible dissolution of carrier mineral phases [START_REF] Kögel-Knabner | Organo-mineral associations in temperate soils: Integrating biology, mineralogy, and organic matter chemistry[END_REF] at very acidic pH leading to the release of associated organic compounds.

Figure 3b shows the evolution of E2:E3 values of the soluble organic compounds leached over the tested pH range for the 7-year and 14-year after filtration at 0.45 µm. For both samples, higher E2:E3 are observed below pH 7 compared to pH above 7. This result suggests that the SD release smaller UVabsorbing organic compounds at acidic pHs. In addition, the constant increase in E2:E3 values from pH 7 to pH 2 suggests a gradual decrease in the size of the UV-absorbing released molecules as the solutions become more acidic. At pH values below 5, the 14-year SD soluble organic compounds induced a higher proportion of small size organic components assimilated to fulvic acids. Between pH 7 and pH 12, for both samples, E2:E3 remains stable in the range of 4.5 to 3.5. These values were similar to those obtained for pore water of peatland that fluctuate around 4 [START_REF] Peacock | UV-visible absorbance spectroscopy as a proxy for peatland dissolved organic carbon (DOC) quantity and quality: considerations on wavelength and absorbance degradation[END_REF][START_REF] Strack | Changes in dissolved organic carbon quality in soils and discharge 10years after peatland restoration[END_REF] and for agricultural soil solution [START_REF] Tiefenbacher | Short-term effects of fertilization on dissolved organic matter (DOM) in soil leachate, Soils and biogeochemical cycling[END_REF]. Finally, the evolution of SD characteristics with ageing seemed to affect only partially the molecular size of UVabsorbing leached organic molecules as shown by the similar E2:E3 between the two SDs.

Ultrafiltration of the leached organic compounds from the 14-year SD

Assessment of the molecular size of emitted organics compound at various pH from SD

The OC concentration measured in each filtrates (0.45 and 0.22 µm) and ultrafiltrates (30, 10 and 3 kDa) depending on pH conditions is presented in Figure 4a. The OC concentrations were very similar in the 0.45 µm and 0.22 µm filtrates regardless of pH conditions, indicating that only a very small quantity of compounds were leached in the size range comprised between 0.22 and 0.45 µm. The same was observed for 30 kDa and 10 kDa filtrates over the whole pH range, indicating that very few molecules are comprised between 10 and 30 kDa. At acidic pH, the comparison of the different curves indicated that a large proportion of organic molecules exhibited a molecular size below 3 kDa. At neutral and alkaline pH's, it was observed that the OC <0.45µm curve differed from the others indicating a large proportion of molecules with a molecular size higher than 30 kDa. The pH-dependent molecular size of organic compounds released from the 14-year SD can be simplified by grouping the results within different molecular size classes [START_REF] Durce | Alteration of the molecular-size-distribution of Boom Clay dissolved organic matter induced by Na+ and Ca2+[END_REF][START_REF] Xu | Dynamic molecular size transformation of aquatic colloidal organic matter as a function of pH and cations[END_REF]. The size of the leached organic compounds was therefore described here in a simplified manner according to the following classes: (i)

Large Organic Colloid (LOC) between 30 kDa and 0.45 µm, (ii) Small Organic Colloid (SOC) between 3 kDa and 10 kDa and (iii) "truly" Dissolved Organic Carbon (tDOC), smaller than 3 kDa. All designations and acronyms, along with the equations used for the calculations, are gathered in supplementary data in Table S1.

The relative abundance of leached organic compounds in each molecular-size fraction, expressed in % of total released OC concentration, is shown in Figure 4b. Over the whole pH range, the percentage of small colloids remained relatively constant (between 5 and 20%), but the proportions of LOC and truly dissolved compounds varied considerably with the pH. Under acidic conditions, truly dissolved compounds were predominantly leached (about 60% of overall leached OC), whereas large colloids were predominant above pH 7. This observation was in agreement with the increase of E 2 :E 3 measured on the OC <0.45µm , observed from pH 6.5 to 2 (Fig. 3 b.) confirmed that the size of the leached organic compounds was smaller as the pH was decreased. The size distribution of leached organic compounds remained relatively stable below pH 6 (tDOC predominant) and above pH 8 (LOC predominant).

However, the E2:E3 suggests a progressive decrease in the MW of leached organic compounds with acidification, which cannot be detected with the 3 kDa cut-off. Thus, the proportion of molecules <3 kDa remains constant but within this class, the released molecules are increasingly smaller with acidification. The strongest variations in the size distributions were observed between pH 6 and 8 that is, naturally occurring pH conditions. An increase of one pH unit (from pH 6 to pH 7) was found to divide by 2.5 the proportion of truly dissolved compounds (from more than 50% to less than 20% of overall of leached OC). Mineral phases preferentially sorb high MW OC with higher content in aromatic and O-containing polar groups [START_REF] Xu | Molecular weight-dependent adsorption fractionation of natural organic matter on ferrihydrite colloids in aquatic environment[END_REF][START_REF] Specht | Characterization of NOM adsorption to clay minerals by size exclusion chromatography[END_REF], thus the desorption of OC from these phases occurring with alkalization of the solution conditions could both explain the increasing of OC MW and the relative aromaticity of released OC described in further details in the following paragraph. The preferential solubilization of small MW organic matter under acidic conditions have been previously observed on soils batch experiments [START_REF] Pédrot | Double pH control on humic substance-borne trace elements distribution in soil waters as inferred from ultrafiltration[END_REF] and in acidic soil solutions [2]. This phenomenon could be explained by both pH and conductivity of the SD solutions. Under acidic pH conditions, organic molecules with higher net surface charge are preferentially solubilized [START_REF] Tipping | The distribution of humic substances between the solid and aqueous phases of acid organic soils; a description based on humic heterogeneity and charge-dependent sorption equilibria[END_REF]. Fulvic acids, which MW is smaller than humic acids, have stronger total charge per gram of carbon and higher proportion in weak proton affinity sites compare to humic acids [START_REF] Weng | Adsorption of Humic Substances on Goethite: Comparison between Humic Acids and Fulvic Acids[END_REF]. These proprieties of small organic molecules assimilated to fulvic acids could explain the release of tDOC from the SD submitted to acidic conditions. Besides pH conditions, the size of the released OC could be affected by the conductivity of the solution which is described in Figure 5. The dissociation of metal oxides and the important release of multivalent cation showed via ICP-MS analysis by Kania et al. [START_REF] Kania | Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland[END_REF] on the 7-year SD sample could explain the significant increase in conductivity observed under acidic conditions for the 14-year SD. This phenomenon occurring with the acidification of the solutions may promote the aggregation of large macromolecules (>20 kDa) into solid OM >0.45 µm [START_REF] Durce | Alteration of the molecular-size-distribution of Boom Clay dissolved organic matter induced by Na+ and Ca2+[END_REF][START_REF] Sillanpää | Removal of natural organic matter in drinking water treatment by coagulation: A comprehensive review[END_REF].

Smaller molecules were not affected resulting in an increase of the proportion of dissolved compounds with the acidification.

Aromaticity of the leached organic compounds in the different size ranges and effect of pH

Figure 6 shows the SUVA 254 values of the leached compounds in each size range at the different pHs of equilibrium. Over the entire pH range, the SUVA of large and small colloids (LOC and SOC) were very similar, increasing with the pH from 0.4 at pH 6.5 to 3 L.mgOC -1 .m -1 at pH 12. The SUVA 254 was however always below 3, suggesting a high proportion in hydrophilic molecules. Several factors may explain the relative low aromaticity measured for large and small colloidal fractions, such as the preferential sequestration of aromatic compounds by minerals [START_REF] Specht | Characterization of NOM adsorption to clay minerals by size exclusion chromatography[END_REF] present in the solid phases. Moreover, it has been shown that weak acids groups and aliphatic structure can be linked via hydrogen, hydrophobic or cation bonding to aromatics rings of the large and small colloidal fraction [START_REF] Piccolo | THE SUPRAMOLECULAR STRUCTURE OF HUMIC SUBSTANCES[END_REF][START_REF] Kipton | Solubility and fractionation of humic acid; effect of pH and ionic medium[END_REF] thus decreasing the aromaticity of the supramolecular assemblies observed in these size fraction. (Bars represent the analytical uncertainty).

The SUVA 254 of the tDOC fraction remained close to 1 L.mgOC -1 .m -1 over the acidic and alkaline pH ranges, but fluctuated greatly around the neutral pH (6.5) from 1.4 L.mgOC -1 .m -1 at pH 6 to 7 L.mgOC -1 .m -1 at pH 8. These high SUVA 254 values suggested that truly dissolved organic matter contained a high proportion of aromatic molecules like phenols, phenolic acids and flavonoids [START_REF] Klotzbücher | Processes controlling the production of aromatic water-soluble organic matter during litter decomposition[END_REF]. This near-neutral pH range is also the range with the lowest conductivity (Fig. 5) and the smallest amount of tDOC released (Fig. 6). The important tDOC aromaticity could be reasonably explained by the low conductivity suggesting low cations release and thus low quantity of cationic bridging between low MW aromatic structure and other low MW hydrophilic compounds decreasing the aromaticity such as polysaccharides.

The major results of this section (i.e transition between a release of LOC versus a release of tDOC and the release of highly aromatic truly dissolved molecules) are obtained in a pH range ranging from 5.5 to 8. These results were even more important as the sensitivity of SD is manifested in a critical pH range that SD may encounter during regular conditions of operation with environmental implications described in the following section.

Environmental significance

The results presented in this study contribute to a better description of the mechanisms of retention and release of organic compounds during the contact between aqueous solutions and sludge deposits solid organic matter. The experimental conditions used allow potential applications to various conditions that the sludge may encounter either in the VCFW during its operation or outside during its reuse in agriculture for example. The pH range tested in this study by far encompasses the possible situations that may be encountered in practice. Results show the effects of pH on the quantities, nature and molecular size of leached organics, and provide indications on the retention or co-transport of pollutants with organic carriers and the potential associated environmental impacts.

SD layer play a major role in the performance of VFCW systems [START_REF] Molle | French vertical flow constructed wetlands: a need of a better understanding of the role of the deposit layer[END_REF]. By reducing the release and transfer of OC and associated contaminants from the SD solid organic matter, ageing may improve the performance in carbon removal. However, for the same reason, SD ageing may be unfavorable to denitrification which requires sufficient quantities of bioavailable OM [START_REF] Grießmeier | Influence of the Potential Carbon Sources for Field Denitrification Beds on Their Microbial Diversity and the Fate of Carbon and Nitrate[END_REF].

At the end of their operational life, SD may be reused as soil applications to increase or restore organic matter content of the receiving soils [START_REF] Senesi | A comparative survey of recent results on humic-like fractions in organic amendments and effects on native soil humic substances[END_REF]. This study suggests the importance of the SD ageing process to improve the environmental safety and agronomic performance of this practice. The main benefits of soil application of organic amendments such as compost [START_REF] Matiasek | The chemical fingerprint of solubilized organic matter from eroded soils and sediments[END_REF][START_REF] Wang | Concentration dynamics and biodegradability of dissolved organic matter in wetland soils subjected to experimental warming[END_REF] or related materials is in the formation of soil organo-mineral aggregates which reduce the erosion phenomena and contribute to the improvement of the agronomic properties of the soil such as its porous structure and water retention capacity. However, application of readily biodegradable matter (as in younger SDs) would cause a series of undesired phenomena such as oxygen depletion due to its rapid aerobic mineralization. Improving the agronomic properties of the soil while reducing the potential environmental impacts is achievable by applying stable forms of organic matter [START_REF] Senesi | A comparative survey of recent results on humic-like fractions in organic amendments and effects on native soil humic substances[END_REF] such as in older SDs. 14-year sludge deposits were also shown in this study to reduce the release of soluble organic compounds regardless of pH conditions, thereby limiting carbon loss in the receiving soil and increasing the residence time of solid organic matter. Under regular operating conditions, the deposits in the surface sludge layer of VFCWs would be in contact with close to neutral to slightly alkaline solution (Fig. 7). This study showed that small pH variations in this range would strongly affect the leaching of organic matter, both quantitatively and qualitatively (size and aromaticity). In this pH range, the colloidal fraction was found to contain mostly hydrophilic compounds. The low SUVA 254 values [START_REF] Hansen | Optical properties of dissolved organic matter (DOM): Effects of biological and photolytic degradation[END_REF] as well as, in a less extent, the molecular weight [START_REF] Amon | Bacterial utilization of different size classes of dissolved organic matter[END_REF] of this colloidal pool are indicators of a high biodegradability of these molecules. Indeed, molecular weight and SUVA 254 are positively and negatively, correlated to the biodegradability of natural organic molecules, respectively. Thus, the leaching of this colloidal fraction could negatively influence water quality indicator such as the Biochemical Oxygen Demand (BOD) of the treated water. The "truly" dissolved fractions containing a high proportion of aromatic compounds could have strong interactions with the micropollutants possibly present in the SD. However, the high aromaticity, and consequently the probable low biodegradability of these molecules, limits their contribution to the BOD of the leachates.

Under alkaline conditions, the leached organics were predominantly in the form of large colloids, containing aromatic compounds. These pH conditions would increase the interactions between the leached organic molecules and trace metals thus enhancing the mobility of such pollutants from the SD to the receiving environment.

Under conditions of flooding for example, anoxic to anaerobic conditions may cause the installation of reductive conditions associated with possible acidification due to the formation of volatile fatty acids [START_REF] Robles-Martínez | Effect of baling on the behaviour of domestic wastes: laboratory study on the role of pH in biodegradation[END_REF]. SD would then leach poorly aromatics molecules of small molecular size (<3 kDa), probably composed by low-molecular weight organic acids, carbohydrates and small amino-acids, having the capacities to sorb heavy metals contaminants [2] and would be more bio-available [START_REF] Marschner | Controls of bioavailability and biodegradability of dissolved organic matter in soils[END_REF].

Conclusion

The nature of the organic compounds leached from the SD under a wide range of pH was approached here through their size distribution using (ultra)filtration and their aromaticity using UV-visible spectroscopy indicators. Under acidic conditions, leached organics were predominantly (more than 50%) in the form of truly dissolved compounds tDOC made of poorly aromatic molecules. Under alkaline conditions however, tDOC was less than 20% of leached organics which were associated to large colloids for more than 70%. The release of OC trapped in solid organo-mineral association increased the overall aromaticity and molecular size of the leachates. This study clearly demonstrated important changes in the nature of the leaching molecules in the natural range of pH (i.e. pH 5 to pH 8). These results showed that a small variation in pH could induce strong effects on the characteristics of the released organic matter. The organic components in the large colloids were mostly carbohydrates, proteins, and aliphatic molecules, whereas the truly dissolved components were predominantly aromatic molecules.

The transformation of the SD occurring with ageing increased their acid-base buffering capacity, reduced the amount and aromaticity of the leached organic molecules without changing the molecular size of UV-absorbing molecules. These effects of ageing may be seen as improving the capacity of the SD surface layer to retain pollutants during the operation of a VFCW unit, and also the suitability for land applications of sludge deposits dredged from French VFCW.

[

  𝑂𝐶] 0.45 µm filtered at 0.22 µm on Sartorius microporous cellulose acetate.

Figure 1 .

 1 Figure 1. Acid and basic titration curves of the 14-year and 7-year SD. The error bars, comprised

3. 2 .Figure 2 .

 22 Figure 2. Effect of pH on the leaching of OC from 7-year and 14-year solid deposits sampled from

Figure 3 .

 3 Figure 3. Effect of pH on a) the SUVA and b) E2:E3 ratio of the soluble organic compounds

Figure 4 :

 4 Figure 4: Effect of pH on a) the size distribution of the leached organic compounds and b) the

Figure 5 :

 5 Figure 5: Conductivity of the 14-year SD sample recorded at equilibrium in the batch leaching

Figure 6 .

 6 Figure 6. SUVA 254 values of leached organic compounds as a function of pH and MW fractions.

Figure 7 .

 7 Figure 7. Conceptual solid-solution partition of organic matter and evolution of the quality of the

Acknowledgments

This work was supported by the C-FACTOR project funded by ANR (project number ANR-18-CE01-0008). Through the support of the GeOHeLiS analytical platform of Rennes University, this publication is also supported by the European Union through the European Regional Development Fund (FEDER), the French ministry of Higher Education and Research, the French Region of Brittany and Rennes Metropole.

This work was realized within the Graduate School H2O'Lyon (ANR-17-EURE-0018) and Université de Lyon (UdL), as part of the programme "Investissements d'Avenir " run by Agence Nationale de la Recherche (ANR).

The authors would like to thank the "Bonus Qualité Recherche" of INSA Lyon for funding part of this research project.