O Bonnaud

L Fesquet

DISTANCE LEARNING IS NOT AN ONLINE FACE-TO-FACE -OBSERVATIONS IN MICROELECTRONIC EDUCATION

Keywords: Distance learning during Covid crisis, microelectronics, knowledge and know-how in engineering, digital challenges, virtual teaching limitations

Due to the pandemic in early 2020, the French government, like many others, decided to temporarily close the universities and asked the faculties to entirely focus on online education or e-learning. In a world where digital contents exponentially increase, this pandemic has been an opportunity to develop distance e-learning. Many politicians found a unique opportunity to decrease or even eliminate face-toface teaching activities. In engineering, this experience has shown clear limits and weaknesses. This paper highlights several problems that raised in microelectronic education and good reasons to move back to face-to-face learning especially for the acquisition of know-how. Indeed, know-how is mandatory for engineers, masters and PhDs in science and technology and this will never be replaced by a virtual experience. Real practice is the necessary price to pay for meeting the technological challenges of the upcoming decade.

INTRODUCTION

Due to the pandemic in early 2020, the French government, like many others, decided to temporarily close the universities and asked the professors to entirely focus on online teaching and e-learning. While the online tools are useful and suitable for students temporarily away from their universities or living in countries with less educational opportunities, the limitations of these tools need to be analysed in depth. Online access to scientific publications, presentations or videos are interesting supports and useful complements to training, especially in the field of microelectronic engineering, which is the expertise field of the authors. Completely changing the educational paradigm from face-to-face to fully online teaching is another matter. This clearly leads to severe disappointments and disastrous results on the know-how acquisition, which is a strong requirement in engineering and, of course, in the field of microelectronics. This paper briefly presents the context of microelectronics, as well as the implementation of distance learning, which has been applied for several months. Due to this quite long experience, several key points are described and highlighted. The article concludes with an evaluation directly carried out by the students after the return of face-to-face classes and labworks on the platforms of the French national network for microelectronic education ("Coordination Nationale pour la Formation en Microélectronique").

MICROELECTRONICS: A HIGH PRIORITY ENGINEERING FIELD

The paper is set in the context of the higher education in microelectronics. This field is today a national priority due to the very dynamic microelectronic market and the strong needs of the semiconductor companies in terms of technologists and designers. The incredible multiplication of smart connected objects, deployed in the framework of the IoT (Internet of Things), has induced many developments in microelectronic circuits and systems covering a wide spectrum of applications, such as communication, health, security, energy, etc. [START_REF] Bonnaud | New Vision in Microelectronics Education: Smart e-Learning and Know-how, a Complementary Approach[END_REF]. At the same time, micro-sensors and -actuators are manufactured thanks to microelectronic technologies [START_REF] Bonnaud | New Approach for Sensors and Connecting Objects Involving Microelectronic Multidisciplinarity for a Wide Spectrum of Applications[END_REF] and highly contribute to the exponential growth of the connected objects [START_REF] Bryzek | Roadmap for the Trillion Sensor Universe[END_REF][START_REF] Bonnaud | The technological challenges of microelectronics for the next generations of connected sensors[END_REF]. As a result, the energy consumption of the internet is also exponentially increasing, roughly by a factor of 2 every 4 years [START_REF] Bonnaud | Innovative Strategy to Meet the Challenges of the Future Digital Society[END_REF]. Figure 1 shows this fast-paced evolution. Without any change, the electrical energy produced in the world will be entirely devoted to the Internet in 2030. This is of course not desirable. A part of the solution consists in reducing the electrical energy consumption of the electronic devices, circuits and systems by at least one order of magnitude over the next 20 years [START_REF] Bonnaud | Integration, communication and energy consumption: the challenges of higher education in electronics and microelectronics[END_REF]. The violet dashed curve on Figure 1 shows the energy evolution in that case; it seems negligible on the plot, but notice that the scale is logarithmic. The total energy save annually reaches 14,000 TWh. In other words, this corresponds to 14 times the energy of the global air traffic in 2019! Many governments have recognized this challenge and declared the electronic sector as a priority in their strategy.

Today, one of the main difficulties is the shortage of skilled workers in electronics and microelectronics [START_REF] Bonnaud | Skilled manpower shortage in microelectronics: a challenge for the French education microelectronics network[END_REF]. The solution consists in improving the training of technicians, masters, engineers and PhD students in order to provide them skills and know-how. This is the main motivation of the French national network, which provides a true know-how in microelectronics and nanotechnologies through practice thanks to shared and dedicated platforms at the national level [START_REF] Bonnaud | The new strategy based on Innovative Projects in Microelectronics and Nanotechnologies, Invited communication[END_REF].

SWITHCHING TO DISTANCE LEARNING

In order to improve the learning efficacy, several online tools have been devised to train the students. These are the so-called MOOCs [START_REF] Bonnaud | Tutorial on Microelectronics Technology[END_REF] and their associated tools [START_REF] Fox | From MOOCs to SPOCs[END_REF]. It also exists documents posted on the university digital platforms such as MOODLE, a learning management system [START_REF]Moodle definition[END_REF] and, more generally, the available online documents such as those found on the "Wikipedia" website, a free online encyclopedia [START_REF] Wikipedia | The Free Encyclopedia[END_REF]. Notice that these online tools are a good add-on to face-to-face education [START_REF] Bonnaud | MOOC and the Practice in electrical and information engineering: complementary approaches[END_REF]. Indeed, this gives to the students the ability to retrieve and sort information on the internet such as scientific articles. Moreover, the MOOCs offer a learning process, which goes from the knowledge acquisition to a self-evaluation, quickly giving to the students a feedback on their understanding.

Due to the pandemic and to the emergency situation, the learning activities have been modified in depth to comply with the governmental recommendations and rules, as shown in Figure 2. This figure shows the different education activities -including, lectures, tutorials, and labworks in microelectronics for designers and technologists -which has been impacted during the sanitary crisis. Most of these activities have been adapted to be operated in a remote way. This movement has been quickly done with many issues. Nevertheless, the academic community reacted in its majority by first reusing the existing online courses and dedicated tools. Then academics developed dedicated contents in order to facilitate distance learning. This quick shift from face-to-face to remote learning can be divided in four points:

-Reuse of MOOCs (Massive Open Online Courses), -Adaption of the contents and, sometimes, creation of dedicated documents for online courses, -Organization of online tutorials, -Online labwork practice (when it was possible).

In the sequel, we detail each point and describe their strengths and weaknesses.

MOOCs

The first author of this article has already experienced the creation of a MOOC dedicated to the learning of microelectronic technologies and entitled "Integrated Microelectronics Tutorial" [START_REF] Bonnaud | Tutorial on Microelectronics Technology[END_REF]. This MOOC is useful for preparing students to the cleanroom labworks of the French national network, GIP-CNFM [START_REF]Coordination Nationale pour la formation en Microélectronique and nanotechnologies[END_REF][START_REF] Bonnaud | GIP-CNFM: a French education network moving from microelectronics to nanotechnologies[END_REF], dedicated to higher education in microelectronics and nanotechnologies. This MOOC was created within the framework of the European association EAEEIE [START_REF] Eaeeie | European Association for Education in[END_REF], which was selected to manage successive European projects dedicated to the improvement of higher education in electrical and information engineering [START_REF] Martins | Towards the Harmonization of Electrical and Information Engineering Education in Europe[END_REF][START_REF] Thiriet | Results and dissemination of the EIE-Surveyor thematic network[END_REF]. With the help of students in the frame of a European mobility, this MOOC dedicated to microelectronics has been translated into several languages (Romanian, Portuguese, German, Finnish, English). This was not only a huge work but we faced issues such as the English version was not already completed before the first release of the initial French version! This implied new investment, which were not anticipated. Moreover, after 6 years, the software has so much evolved that the initial version was no longer available. In conclusion, MOOCs are only applicable if a permanent team is dedicated to their support and maintenance. This assumes that the institutions are able to lock a budget over a long period with the associated human resources. Recently, several academic institutions have created local pedagogical units in order to digitalize learning. Although there is a massive development of digital environments for the teachers and the students (Digital Working Environment), the creation and maintenance of such tools do not appear as an important priority. For instance, the European program called Horizon 2020 -EDUC-SHARE [START_REF]European program called Horizon[END_REF] does not mention this action in its eight priorities.

Content adaption for online courses

Face-to-face course are usually supported by a slideshow pointing out in a synthetic manner the main scientific or technical messages. The explanations and methodologies are given by the teacher, which come in complement to the slides. This approach offers a lively progression with frequent interactions with the students. Doing the same remotely through a virtual presentation (videoconference) deeply modifies the student and teacher behavior. Indeed, the students are more passive and reduce their interactions with the professors, while the teachers are frustrated to not see the audience reaction. This makes really difficult the little live adjustments done during the face-to-face lectures. Therefore, the slide content has to be much more developed and the teaching speed reduced in order to maintain minimal exchanges with the students during the videoconferences. As the slides tend to be often overloaded, the core content is much more difficult to catch by the audience. Another consequence is an extra preparation time for the teachers, which is unfortunately not rewarded by a better understanding and assimilation of the fundamentals by the students.

Organization of online tutorials

The tutorial sessions are a good way to assimilate the course fundamentals based on pedagogical examples. They allow students to acquire the concepts, to perform calculations and specially to master the methods and know-how. During these face-to-face sessions, the students lead the action with the support of the teacher who follows them and puts them back on track in case of mistakes. During these sessions, the students often exchange and provide a collective effort improving their understanding and promoting comments and questions, which are essential to develop the innovation spirit.

The online tutorials are often organized as homework, and the students usually receive their exercises before the session to save time and allow a short preparation. We are far from the panacea because the students are not supported during the tutorial preparation and most of them remain unable to start the exercises. This is clearly a strong limitation for such a practice. Moreover, the innate discussions and exchanges between students and teachers are usually lost by videoconferencing. In conclusion, the spirit of curiosity is lost even if some of the students prepare questions. The physical exchanges are crucial for the communication and, in remote situation, we have observed a wait-and-see attitude conducting students to simply retrieve the results provided by the teacher. All the dynamism of face-toface learning and the associated proactivity has disappeared.

-Online labwork practice

The training of technicians, engineers and doctors in a highly technical field requires the acquisition of knowledge and know-how. This latter is a major part of the skills and qualities for working in the industry and especially in microelectronics [START_REF] Bonnaud | The practice in microelectronics: a mandatory complement of the online courses in the context of digital society[END_REF]. In spite of incredible CAD tools for modeling and designing, microelectronic circuits and systems are so complex that companies always fabricate a physical prototype allowing the product validation and the adjustment of a huge number of parameters Therefore, specialist training requires extensive practical experience, which can only be obtained through laboratory work, projects, and supervised internships [START_REF] Bonnaud | Practice-oriented pedagogical strategy of the French microelectronics and nanotechnologies network[END_REF]. Switching from practical to virtual training leads to major issues..

The first problem is that many experiments cannot be remotely conducted. Indeed, the technical platforms are not intended to perform distance learning. Most of the testbenches, physical devices, circuits and instruments are not designed to be access through the internet. Only few experiments requiring a local operator can be remotely performed. This requires a perfect schedule and a careful preparation by the students' supervisors. This is for instance the case for the unique national circuit test system, a very expensive equipment located in Montpellier and remotely accessible by the French community [START_REF] Latorre | Integrated Circuits Testing: Remote Access to Test Equipment for Labs and Engineering[END_REF]. Nevertheless, this assumes that the students have already conducted several experiments on physical benches.

Circuit design seems a more favorable activity to be taught online. Indeed, remotely using dedicated software should not be a big issue. Nevertheless, this is not at all the case for several reasons:

-The first difficulty for the students is their ability to access remotely the microelectronic CAD platforms. Indeed, as these CAD tools are quite expensive, their location must remain at the University in order to avoid piracy. This implies to have a connection via a VPN and to install some software tools for redirecting the displays. In practice, many students are not enough familiar with Linux and such tools provoking dissatisfaction and time loss.

-The second issue is for the system administrators and the teachers that have to deliver instructions for remote connections. This task is not obvious because the personal setup of the student computers differs a lot. Indeed, some students own a PC under Linux, others under Windows and, finally, some others under Mac OS. Of course, the number of configurations is not limited to these three situations because the OS versions and releases have to be considered. This is clearly not a cakewalk for managing so many different configurations.

-Another problem is the complexity of microelectronic CAD tools, which deal with huge data libraries (typically several gigabytes) and generate a lot of files. This is particularly disturbing for an isolated student working at home. Indeed, discussing with his neighbor or asking a professor is not so easy even a videoconference is running. In this situation, the first student reaction is to understand by themselves what really happens (which is perfectly healthy), but, as the volume of data and the amount of files are large, it is particularly difficult (for a student) to sort the information.

-In addition, to these hurdles, getting started with microelectronic CAD tools requires to proceed several rigorous steps that a novice user may find very difficult. Indeed, design flows contain a lot of traps for the beginners and require an advanced understanding of the circuit and device models. The role of the teacher is then crucial for organizing the work on the computer, analyzing the physics or the code semantics when the design is made with a HDL (Hardware Design Language). A teacher is usually able to quickly guide the students in laboratory room, by responding to the questions and, more importantly, anticipating the difficulties thanks to a glance on the student group. The online teacher does not have a clear overview of the group and its difficulties. This makes delicate the student support and generates a considerable waste of time.

ONE YEAR OF DISTANCE LEARNING

Since March 2020, in France, the majority of theoretical courses, tutorials and even practical work at the university have been organized remotely by videoconference. Our study is based on observations made in microelectronic education but is surely applicable to other technological disciplines. Since the beginning of 2021 in engineering schools and master degrees, practical activities have been allowed. Thus, the students are back in the classroom for labworks and projects.

Benefiting from these face-to-face situations, a feedback from the students and their teachers has become possible. In the sequel, the proposed analysis mainly concerns engineer and master students in microelectronics, who are usually using of the French national network platforms. On these platforms, they learn the know-how for manufacturing microelectronic circuits, characterizing physical and electrical devices, designing integrated circuits or prototyping embedded systems. This leads to several remarks and comments.

On the remote activities:

-Being 8 hours per day in front of a computer screen with almost no time-out, isolated at home or in a small flat, is not ideal to keep the attention and, worst, favors a quick loss of focus.

-The students do not (or very marginally) interact with the teacher, as they do not feel his presence. Moreover, online asking questions is more difficult because most of the interactions with the teacher are lost, especially the gesture and body language.

On the pedagogy:

-Methodologies and scientific approaches require stimulating the students while remote learning favours their passivity.

-Understanding by the students is poor. They are not enough involved because they received a digital document. If they looked at the document before the lab, online labwork is less exciting and considered less difficult thanks to a larger support of the teacher team. Of course, this is just an illusion! -During the tutorial sessions, most of the students are waiting for the solution instead of seeking by themselves. This implies at the end a reduced autonomy and a lack of questions, which are however essential for the group dynamic.

On the role of the teacher:

-Teachers are spending a lot more time for preparing because everything should be written down as much as possible. This tends to lower the comments, which are useful for helping the understanding. In face-to-face situations, the teachers naturally limit the pre-written documents and displayed slides. This favors the curiosity and the discussions, for instance by giving analogies that are really useful in engineering or physics.

-In circuit design, labworks can be done online thanks to an important preparatory work for giving access to a large number of students while maintaining the network security. This implies a permanent and efficient technical team for managing the CAD tools and the local servers.

On the internet access:

-The excessive use of Internet on undersized networks leads to frequent communication breakdowns causing a loss of time at the level of the teacher, but also on the student side. Numerous sessions were cancelled due to an improper national data transfer; this limitation was available for online teaching of high schools as well! -Increasing the capacity of the servers and connections will solve the problem at the price of an additional work of the technical team, an increase of investment and a higher power consumption, which is not neutral for the carbon footprint.

After the resumption of a few face-to-face sessions in 2021, it appears a few salient points. Firstly, the permanent exchanges during the lab activities by the students with their classmates and teachers make their progression ten times faster than in a remote fashion. Secondly, face-to-face education tends to homogenize the student class and to reduce disparities. Therefore, the progress is collective and the know-how acquisition is much more efficient.

Finally, a survey of several groups of microelectronic engineering students showed that 90% of students confirm the low effectiveness of online training and report realizing the benefits of practical work on platforms with a teacher. Figure 3 summarizes all these points: the limitations of remote practices and the student feeling and analysis after the resumption of in-person activities. In engineering, permanent remote activities are clearly counter-productive!

Figure 1 .

 1 Figure 1. Evolution of the number of bytes transferred in the world and the related electrical energy consumption. Most of this energy is dissipated in electronic circuits and systems. The energy growth must be limited by improving the performance of electronics.

Figure 2 .

 2 Figure 2. Evolution of learning methods before and after the COVID pandemic. Almost all educational activities shifted to virtual.

Figure 3 :

 3 Figure 3: Limitations of the remote practice training and conclusion after resumption of few face-toface practical trainings.

ACKNOWLEDGEMENTS

The authors want to thank Lorraine Chagoya-Garzon, executive secretary of GIP-CNFM for her fruitful advice for the redaction of this paper.


Noémie Janot

Pascal E Reiller

Xing Zheng

Jean-Philippe Croué

Marc F Benedetti
email: benedetti@ipgp.fr

Characterization of humic acid reactivity modifications due to adsorption onto α-Al 2 O 3

Keywords: Adsorption, fractionation, DOC, mineral surface, organic matter

Adsorption of purified Aldrich humic acid (PAHA) onto α-Al 2 O 3 is studied by batch experiments at different pH, ionic strength and coverage ratios R (mg of PAHA by m² of mineral surface). After equilibration, samples are centrifuged and the concentration of PAHA in the supernatants is measured. The amount of adsorbed PAHA per m² of mineral surface is decreasing with increasing pH. At constant pH value, the amount of adsorbed PAHA increases with initial PAHA concentration until pH-dependent a constant value is reached. UV/Visible specific parameters such as specific absorbance SUVA 254 , ratio of absorbance values E 2 /E 3 and width of the electron-transfer absorbance band Δ ET are calculated for supernatant PAHA fractions of adsorption experiments at pH 6.8, to have an insight on the evolution of PAHA characteristics with varying coverage ratio. No modification is observed compared to original compound for R ≥ 20 mg PAHA /g. Below this ratio, aromaticity decreases with initial PAHA concentration. Size-exclusion chromatography -organic carbon detection measurements on these supernatants also show a preferential adsorption of more aromatic and higher size fractions. Spectrophotometric titrations were done to estimate changes of reactivity of supernatants from adsorption experiments made at pH ≈ 6.8 and different PAHA concentrations. Evolutions of UV/Visible spectra with varying pH were treated to obtain titration curves that are interpreted within the NICA-Donnan framework. Protonation parameters of non-sorbed PAHA fractions are compared to those obtained for the PAHA before contact with the oxide. The amount of lowaffinity type of sites and the value of their median affinity constant decrease after adsorption. From PAHA concentration in the supernatant and mass balance calculations, "titration curves" are obtained and fitted for the adsorbed fractions for the first time. These changes in reactivity to our opinion could explain the difficulty to model the behavior of ternary systems composed of pollutants/HS/mineral since additivity is not respected.

Introduction

The interactions between humic substances (HS) and mineral surfaces modify their fate and their interactions with contaminants in the environment. Due to HS heterogeneous nature, their composition and binding properties are modified during adsorption onto minerals [START_REF] Claret | Fractionation of Suwannee River Fulvic Acid and Aldrich Humic Acid on alpha-Al2O3: Spectroscopic Evidence[END_REF][START_REF] Davis | Adsorption of dissolved organics in lake water by aluminum-oxide -effect of molecular weight[END_REF][START_REF] Hur | Molecular weight fractionation of humic substances by dx[END_REF][START_REF] Janot | Using Spectrophotometric Titrations To Characterize Humic Acid Reactivity at Environmental Concentrations[END_REF][START_REF] Reiller | Sorption of Aldrich humic acid onto hematite: Insights into fractionation phenomena by electrospray ionization with quadrupole time-offlight mass spectrometry[END_REF][START_REF] Saito | Adsorption of humic acid on goethite: Isotherms, charge adjustments, and potential profiles[END_REF]. Interactions between HS and mineral surfaces are controlled by different processes: electrostatic and hydrophobic interactions, ligand-exchange between phenolic and carboxylic groups of humic substances and mineral surface sites. The adsorption of humic substances onto mineral surfaces varies with pH, ionic strength, and concentration ratios between organic matter and minerals [START_REF] Saito | Adsorption of humic acid on goethite: Isotherms, charge adjustments, and potential profiles[END_REF][START_REF] Schlautman | Adsorption of aquatic humic substances on colloidal-size aluminum-oxide particles -Influence of solution chemistry[END_REF][START_REF] Vermeer | Adsorption of humic acid to mineral particles. 1. Specific and electrostatic interactions[END_REF][START_REF] Weng | Adsorption of humic acids onto goethite: Effects of molar mass, pH and ionic strength[END_REF]), but also with humic substances' origin and the type of mineral surface [START_REF] Hur | Molecular weight fractionation of humic substances by dx[END_REF].

During adsorption, fractionation of organic matter aggregates occurs, leading to two pools of organic matter, a dissolved one and an adsorbed one, each one with different structural and chemical properties. Previous studies have shown the influence of this fractionation on the modification of structures and moieties on humic molecules. Generally, these studies analyze the dissolved organic fraction only, which stays in the supernatant after centrifugation of the mixture. The modification of UV/Visible parameters (such as SUVA, E 2 /E 3 , or ε 280 ) gives information on aromaticity of molecules: all studies show a preferential adsorption of molecules with higher aromaticity [START_REF] Chorover | Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces[END_REF][START_REF] Claret | Fractionation of Suwannee River Fulvic Acid and Aldrich Humic Acid on alpha-Al2O3: Spectroscopic Evidence[END_REF][START_REF] Gu | Adsorption and desorption of different organic-matter fractions on iron-oxide[END_REF][START_REF] Meier | Fractionation of aquatic natural organic matter upon sorption to goethite and kaolinite[END_REF][START_REF] Reiller | Sorption of Aldrich humic acid onto hematite: Insights into fractionation phenomena by electrospray ionization with quadrupole time-offlight mass spectrometry[END_REF][START_REF] Zhou | Size fractionation upon adsorption of fulvic acid on goethite: Equilibrium and kinetic studies[END_REF]. The apparent molecular weight (MW) of non-sorbed fraction has also been studied, using size-exclusion chromatography (SEC). Majority of studies have shown a preferential adsorption of higher MW fractions of various NOM extracts on goethite at pH between 4 and 7.5 [START_REF] Chorover | Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces[END_REF][START_REF] Meier | Fractionation of aquatic natural organic matter upon sorption to goethite and kaolinite[END_REF][START_REF] Zhou | Size fractionation upon adsorption of fulvic acid on goethite: Equilibrium and kinetic studies[END_REF]. However opposite results are reported, for instance [START_REF] Hur | Effects of pH and phosphate on the adsorptive fractionation of purified Aldrich humic acid on kaolinite and hematite[END_REF] have shown that the apparent MW of non-sorbed fractions of purified Aldrich humic acid onto hematite was pH dependent: below the point of zero charge (i.e. pH=8), high MW fractions are enriched in the solution and apparent lower MW at higher pH. This is consistent with the work of [START_REF] Reiller | Sorption of Aldrich humic acid onto hematite: Insights into fractionation phenomena by electrospray ionization with quadrupole time-offlight mass spectrometry[END_REF] which observed a preferential adsorption of low molecular weight (m/z) fraction using ESI-MS with the same system at pH 7. [START_REF] Kang | Humic acid fractionation upon sequential adsorption onto goethite[END_REF]

Materials and methods

Preparation of samples

Commercial Aldrich humic acid was purified (PAHA) according to [START_REF] Kim | Characterization of humic and fulvic-acids from Gorleben groundwater[END_REF] [START_REF] Claret | Fractionation of Suwannee River Fulvic Acid and Aldrich Humic Acid on alpha-Al2O3: Spectroscopic Evidence[END_REF][START_REF] Gu | Competitive adsorption, displacement, and transport of organic matter on iron oxide .1. Competitive adsorption[END_REF]. A non-contaminated set of supernatant solution has then been used as reference to choose the wavelengths of calibration in order to minimize the difference between UV and DOC measurements. For each wavelength between 220 and 600 nm, calibration curves were determined. Best results were found when using the mean of values obtained from wavelengths between 290 and 320 nm (see Figure S1 of the supplementary information (SI)).

LC-OCD measurements

The LC-OCD system (DOC-LABOR Dr. can then not be compared directly.

Titration experiments

A series of retention experiments at 10 g/L α- show a surface concentration up to 4.5 mg HA /m², depending on mineral, ionic strength, pH and initial coverage ratio [START_REF] Boily | Proton binding to humic acids and sorption of Pb(II) and humic acid to the corundum surface[END_REF][START_REF] Fairhurst | The influence of humic acid on europium-mineral interactions[END_REF][START_REF] Hur | Effects of pH and phosphate on the adsorptive fractionation of purified Aldrich humic acid on kaolinite and hematite[END_REF][START_REF] Murphy | The sorption of thorium (IV) and uranium (VI) to hematite in the presence of natural organic matter[END_REF][START_REF] Reiller | Retention behaviour of humic substances onto mineral surfaces and consequences upon thorium (IV) mobility: case of iron oxides[END_REF][START_REF] Schlautman | Adsorption of aquatic humic substances on colloidal-size aluminum-oxide particles -Influence of solution chemistry[END_REF][START_REF] Weng | Adsorption of humic acids onto goethite: Effects of molar mass, pH and ionic strength[END_REF] As previously observed for simple organic acids, the sorption of acid anions is linked to their protonation constant [START_REF] Evanko | Influence of structural features on sorption of NOM-analogue organic acids to goethite[END_REF]. From potentiometric titrations data of PAHA, the median intrinsic affinity parameters were found at logK̃H ,1 = 2.66 and logK̃H ,2 = 6.90 [START_REF] Janot | Using Spectrophotometric Titrations To Characterize Humic Acid Reactivity at Environmental Concentrations[END_REF], which gives maximum of affinity distribution around 5.0 and 9.2 at 0.01 M and 4.5 and 9.7 at 0.1 M, respectively. The distribution of affinity constants (K̃H ,i ) due to the PAHA heterogeneity induces a monotonous decrease of the sorption.

At low pH (< 5), inner-sphere complexation occurs between deprotonated carboxylic ligands of PAHA and positively charged surface groups of the oxide [START_REF] Weng | Adsorption of humic acids onto goethite: Effects of molar mass, pH and ionic strength[END_REF]. When pH increases, the net positive charge of alumina surface is decreasing and the more negatively charged PAHA is less attracted to it. The high point of zero charge of the surface, i.e. 9.6, also explains the significant amount of PAHA bound throughout the pH range studied.

Despite the low number of data at 0.01 M, adsorption of PAHA also seems to be slightly depend on ionic strength (see Figure 1), with amount of PAHA adsorbed decreasing with ionic strength above pH 5. [START_REF] Weng | Adsorption of humic acids onto goethite: Effects of molar mass, pH and ionic strength[END_REF] made the same observations for a HA/goethite system, in the same surface coverage conditions: 150 mg HA /L with 1 g/L of goethite with a 94 m²/g specific area, i.e. 1.60 mg HA /m² when we used 1.66 mg HA /m² in our experiment. The adsorption isotherms show an increase with increasing initial quantity, until a plateau is reached. This saturation threshold decreases with increasing pH, which is commonly found in the case of HS adsorption onto metal (hydr)oxides [START_REF] Filius | Adsorption of fulvic acid on goethite[END_REF][START_REF] Kang | Humic acid fractionation upon sequential adsorption onto goethite[END_REF][START_REF] Saito | Adsorption of humic acid on goethite: Isotherms, charge adjustments, and potential profiles[END_REF][START_REF] Schlautman | Adsorption of aquatic humic substances on colloidal-size aluminum-oxide particles -Influence of solution chemistry[END_REF][START_REF] Vermeer | Adsorption of humic acid to mineral particles. 1. Specific and electrostatic interactions[END_REF]. Saturation is reached for a lower R value at higher pH. 1.9 6.5

Characterization of supernatants

The three parameters shown in Table 1 are almost constant for 20 ≤ R ≤ 60 mg PAHA /g and comparable to the values of original PAHA at the same pH and ionic strength. For R < 20 mg PAHA /g α-Al2O3 , i.e. highest fractionation rates, SUVA 254 is decreasing.

Previous studies have shown that aromaticity is correlated to SUVA 254 as well as Δ ET [START_REF] Chin | Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances[END_REF][START_REF] Korshin | Influence of chlorination on chromophores and fluorophores in humic substances[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] and anti-correlated to E 2 /E 3 [START_REF] Peuravuori | Molecular size distribution and spectroscopic properties of aquatic humic substances[END_REF]. In our case, it means that PAHA fractions in solution for concentration ratios below 20 mg PAHA /g α-Al2O3 show a loss of 8/19 (2008) for the same system after adsorption at pH = 6.1, with a decrease observed for R < 10 mg PAHA /g α-Al2O3 (see Figure S2). The preferential adsorption of more aromatic HA fraction is in agreement with literature data [START_REF] Chorover | Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces[END_REF][START_REF] Claret | Fractionation of Suwannee River Fulvic Acid and Aldrich Humic Acid on alpha-Al2O3: Spectroscopic Evidence[END_REF][START_REF] Gu | Adsorption and desorption of different organic-matter fractions on iron-oxide[END_REF][START_REF] Meier | Fractionation of aquatic natural organic matter upon sorption to goethite and kaolinite[END_REF][START_REF] Reiller | Sorption of Aldrich humic acid onto hematite: Insights into fractionation phenomena by electrospray ionization with quadrupole time-offlight mass spectrometry[END_REF][START_REF] Zhou | Size fractionation upon adsorption of fulvic acid on goethite: Equilibrium and kinetic studies[END_REF].

LC-OCD measurements were performed on the same samples. Figure 3A shows the chromatograms obtained, with the organic carbon signal depending on elution time, larger aggregates being eluted faster. 3A and Table 2). This phenomenon implies that, compared to humics, these small organics are not as readily adsorbed onto the minerals under the present experimental conditions. Specially, as neutral organics are of very low UV absorbance [START_REF] Huber | Characterisation of aquatic humic and nonhumic matter with size-exclusion chromatographyorganic carbon detection -organic nitrogen detection (LC-OCD-OND)[END_REF], the increase of their relative content should reduce the SUVA of water samples, which is in agreement with observations (see Table 1 and Figure S2).

LC-OCD measurements were also performed on the supernatants of adsorption experiments at 0.01 M NaClO 4 and different pH values (macroscopic results shown in Figure 1).

Results are shown in Figure 3B and Table 2.

The analysis of fractionated DOC shows that when pH decreases and more PAHA is adsorbed onto the surface (Figure 1), the relative content of humics and building blocks (which are considered as break down products of humics)

is decreasing (see Table 2): these fractions are preferentially adsorbed onto the mineral.

Although the organic acids can also be removed, their relative content remains constant. Again, an ineffective removal of the neutrals increases their relative content in the residual DOC.

The LC-OCD analysis of these two series of supernatants shows that humics and humic-like substances (building blocks) have a similar behavior and are preferentially adsorbed onto the mineral surfaces in various conditions of pH, ionic strength and coverage ratio. For instance the building blocks could actually corresponds to the highly polar short-chain carboxylic acids identified by [START_REF] Ghosh | Colloidal Stability of Al2O3 Nanoparticles as Affected by Coating of Structurally Different Humic[END_REF] that could react with the surface mostly by ligand exchange of the carboxylic groups of the HA to yield innersphere complexes [START_REF] Kang | Humic acid fractionation upon sequential adsorption onto goethite[END_REF] and lateral interactions [START_REF] Ochs | Coordinative and hydrophobic interaction of humic substances with hydrophilic Al2O3 and hydrophobic mercury surfaces[END_REF].

Kinetic aspects of the process could also be important. For instance [START_REF] Van De Weerd | 042 desorption of a NOM mixture: effects of physical and chemical heterogeneity[END_REF] interpreted the data of [START_REF] Gu | Adsorption and desorption of natural organic-matter on iron-oxide -mechanisms and models[END_REF] from a kinetic point of view. The smaller molecules move faster and could thus be favored and slowly exchanged for the bigger ones. However, the ranges of molecular weights (from <3 kDa to >100 kDa) used in both papers rely on the determination by [START_REF] Mccarthy | Mobility of natural organic matter in a sandy aquifer[END_REF] based on ultrafiltration membranes calibrated with globular proteins. These calibrations give high molecular weight values because of the compact structure of proteins [START_REF] Chin | The abundance, distribution, and configuration of porewater organic colloids in recent sediments[END_REF][START_REF] Chin | Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances[END_REF]. As we did not perform the same molecular mass determinations, the direct comparison would then be tricky.

Spectrophotometric titrations

The differential absorbance spectra of all the supernatants were calculated as described by [START_REF] Janot | Using Spectrophotometric Titrations To Characterize Humic Acid Reactivity at Environmental Concentrations[END_REF]. Briefly, they are built from the absorbance spectra of PAHA solution recorded at different pH between 3 and 11, using the following equation:

Δ A pH (λ)= 1 l cell [ A pH (λ) DOC - A pH ref (λ ) DOC ref ]
where l cell is the cell length (cm), DOC and dx.doi.org/10.1016/j.watres.2011.11.042

Modeling

Data modeling was done within the NICA-Donnan formalism [START_REF] Kinniburgh | Ion binding to natural organic matter: competition, heterogeneity, stoichiometry and thermodynamic consistency[END_REF], optimizing the parameters with the software FIT [START_REF] Kinniburgh | FIT Non-linear Optimization Algorithm and User Manual[END_REF]). The number of data points in the spectrophotometric titration curves is much lower than in the potentiometric ones (80 vs 15). The parameters describing the charge vs. dx.doi.org/10.1016/j.watres.2011.11.042 The obtained fitting are satisfactory for all samples (r² > 0.98), but for 60 and 30 mg PAHA /g α-Al2O3 the best fit was obtained for m 1 values larger than 1, which is difficult to explain within the framework of the model and known titrations data of various humic substances from the literature [START_REF] Milne | Generic NICA-Donnan model parameters for proton binding by humic substances[END_REF]. Values of m 1 below 1 for these two ratios were optimized between neighboring values, since m 1 seems to increase when R decreases.

The obtained parameters are shown in Table S1 of SI and the corresponding fits are displayed on Figure 5.

The overall charge Q 1 + Q 2 decreases with the initial quantity of PAHA in the system, most of the reactive molecules being retained onto the mineral surface, their functional sites being directly fixed to the surface mostly via ligand exchange [START_REF] Ochs | Coordinative and hydrophobic interaction of humic substances with hydrophilic Al2O3 and hydrophobic mercury surfaces[END_REF][START_REF] Kang | Humic acid fractionation upon sequential adsorption onto goethite[END_REF]. evidenced by [START_REF] Ghosh | Colloidal Behavior of Aluminum Oxide Nanoparticles As Affected by pH and Natural Organic Matter[END_REF][START_REF] Ghosh | Colloidal Stability of Al2O3 Nanoparticles as Affected by Coating of Structurally Different Humic[END_REF]. There is no apparent evolution of the number of high proton-affinity-type (so-called phenolic) sites Q 2 , which seems to be constant above R = 20 mg PAHA /g α-Al2O3 and not significantly different from the original PAHA. However, it is lower for remaining PAHA fraction in solution at the two lowest ratios. This is consistent with the loss of aromaticity observed from the UV/Visible spectra for these supernatants (see Figure S2). The heterogeneity parameter m 1 increases with decreasing ratio.

This would mean that low proton-affinity-type sites of PAHA fraction in solution are more homogeneous than initial PAHA. When fractionation is more important, the apparent homogeneity of low affinity-type sites increases too.

The logK̃H ,1 values of supernatant fractions seem to slightly decrease with coverage ratio, which would mean a higher affinity for proton and a greater tendency for deprotonation. Previous works related to the adsorption of simple organic acids onto mineral oxides have shown that adsorption is maximal at the pH corresponding to the pK a of the acid (Gu et al. 1995, Schulthess and[START_REF] Schulthess | Competitive adsorption of aqueous carbonic and acetic-acids by an Aluminum-oxide[END_REF]. Some of these molecules, often considered as analogs of humic substances, show a maximum of adsorption below pH 4: it is the case of benzoic acid onto goethite [START_REF] Evanko | Influence of structural features on sorption of NOM-analogue organic acids to goethite[END_REF], phtalic acid onto hematite [START_REF] Gu | Adsorption and desorption of different organic-matter fractions on iron-oxide[END_REF] or salicylic acid onto δ-alumina [START_REF] Kraemer | Influence of pH and competitive adsorption on the kinetics of ligand-promoted dissolution of aluminum oxide[END_REF]. For these molecules, there is almost no more adsorption at pH higher than 6. By analogy, at the pH of our experiment (6.8), these molecules would be very weakly sorbed. The lower value of logK̃H ,1 for the humic molecules found in the supernatants regarding the original value may be related to the presence in these humic molecules of such organic moieties with very low pK a (3.0 for salicylic acid, 4.2 for benzoic acid and 2.9 for the first pK a of phtalic acid).

We can not conclude regarding the evolution of m 2 , since its values are too close from the original value. This is due to experimental pH In addition, it must be highlighted that these parameters are fitted from 15-points data sets, and that the correlation matrices show a strong correlation between Q 1 , m 2 and Q 2 , as shown in Table S2.

In

  ligand exchange of the carboxylic groups of the HA to yield inner-sphere complexes; and (3) hydrophobic interactions on the HA-goethite complex that can provide new 2/19 dx.doi.org/10.1016/j.watres.2011.11.042 hydrophobic sorption sites for high MWHA fractions with condensed aromatic or long chain aliphatic moieties. The proposition of a generic trend for size modification of HS molecules after adsorption onto oxides minerals is not straightforward. For Al 2 O 3 nanoparticles the influence of pH, which seems of great importance, and structural modifications of the HS fractions after adsorption onto a mineral oxide were evidenced[START_REF] Ghosh | Colloidal Behavior of Aluminum Oxide Nanoparticles As Affected by pH and Natural Organic Matter[END_REF][START_REF] Ghosh | Colloidal Stability of Al2O3 Nanoparticles as Affected by Coating of Structurally Different Humic[END_REF]. However, to our knowledge, no study has yet looked at the modification of reactivity of the produced organic fractions, which could explain the difficulty to model the behavior of ternary systems composed of pollutants/HS/mineral. The behavior of the pollutant in a ternary system cannot be described by linear additivity of binary systems, due to the interactions between natural organic matter and mineral surfaces, which modify the surface properties, aggregation, and metal ion complexation[START_REF] Christl | Interaction of copper and fulvic acid at the hematite-water interface[END_REF][START_REF] Robertson | Humic substances in the global environment and implication on human health[END_REF][START_REF] Vermeer | Metal ion adsorption to complexes of humic acid and metal oxides: Deviations from the additivity rule[END_REF]. It is actually difficult to access to the binding properties of the dissolved fraction of HS, due to its low concentration in solution after the adsorption experiments. Recent works(Dryer et al. 2008, Janot et al. 2010) have described a new spectrophotometric titration approach, which allows the assessment of the reactivity of HS in solution at relevant environmental concentration (i.e. <10 mg HS /L). The aim of this study is to use this spectrophotometric titration approach to study dissolved fractions of HS after adsorption experiment, in order to quantify modifications of their binding moieties due to the contact with the mineral surface. In this work the modifications of reactivity of Purified Aldrich Humic Acid (PAHA) fraction left in solution after adsorption onto α-Al 2 O 3 at different organic/mineral ratios are qualitatively characterized. HS adsorption onto oxides has mostly been studied with Fe oxides. α-Al 2 O 3 was chosen since aluminol surface sites of Al oxides are part of clay mineral reactive sites. In addition, α-Al 2 O3 has significant industrial applications (abrasive agent, insulator, and catalyst) that could lead to environmental exposure. The behavior of man-made nanoparticles exposed to a natural aquatic environment will be controlled by their interactions with natural organic matter which could dictate their bioavailability and toxicity to aquatic species. The quantification of PAHA adsorption onto α-Al 2 O 3 depending on pH and initial PAHA concentration was studied by batch experiments. An attempt to access reactivity of the adsorbed PAHA fraction is proposed.

Huber,

  Germany) is equipped with a size exclusion column TSK HW50S (Tosoh, Japan) and an online organic carbon detector. After the injection of a water sample, organic compounds in the sample are separated by the size exclusion unit according to their molecular size. Thereafter, the separated organics are oxidized sequentially and their carbon content is quantified by the online detector. On average, the ratio of the chromatogram DOC, i.e., total dx.doi.org/10.1016/j.watres.2011.11.042 DOC analyzed after SEC chromatography, to the DOC analyzed using a TOC analyzer (Shimadzu TOC-VCSH) is over 85%. Thus, the component analysis based on LC-OCD chromatograms is considered to be able to represent the fraction distribution in the water samples. For data acquisition and data processing, the customized software ChromCALC ® is used. According to the study of Huber et al. (2011) and with the assistant of the software, 4 major components from the chromatograms are identified in the present work: colloidal + macromolecular humics, building blocks, low molecular weight acids and neutrals. Before measurement, the system was calibrated according to the methods described by Huber et al. (2011). According to Shimadzu results, the water samples are diluted using mQ water to a DOC of 1 to 3 mg/L prior to LC-OCD measurement. Accordingly, PAHA stock solution was diluted 100 times and the supernatants from adsorption experiments were diluted with a factor of 3. Calibration of organic carbon measurements being different from the one used for adsorption quantification, results

  Al 2 O 3 and various initial PAHA concentrations(600, 300, 200, 120 and 70 mg/L), I = 0.1 mol/L (NaClO 4 ), pH = 6.8 ± 0.3, and 5 days of equilibration was also done. For each PAHA concentration, at least three replicates were made. The supernatants were mixed and the solution was diluted in 0.1 mol/L (NaClO 4 ) to obtain a 100-mL solution of 10 mg/L, which was then titrated between pH 3 and pH 11 using the spectrophotometric method described in[START_REF] Janot | Using Spectrophotometric Titrations To Characterize Humic Acid Reactivity at Environmental Concentrations[END_REF]. Spectrophotometric titrations allow access to the reactivity of a dilute solution of humic substances (HS) due to the variations in UV/Vis spectra of the solution with evolving pH. The pH was increased by addition of freshly prepared NaOH at 0.01 or 0.1 mol/L. Absorbance spectra were recorded in a 5 cm quartz cuvette at ca. 0.5 pH intervals using a Thermo Evolution 600 UV/Visible spectrophotometer between 190 and 600 nm. UV/Visible spectra were recorded at room temperature. PAHA concentrations in the supernatant were measured using Shimadzu TOC-VCSH analyzer. Quantification of PAHA adsorption onto α-Al 2 O 3 was made at different pH, for a coverage ratio R = 25 mg PAHA /g α-Al2O3 . Humic acid adsorption onto oxides reported in the literature

19 Figure 1 . 19 Figure 2 .

 191192 Figure 1. Influence of pH and ionic strength on PAHA adsorption onto α-Al 2 O 3 .Re-calculated data from[START_REF] Weng | Adsorption of humic acids onto goethite: Effects of molar mass, pH and ionic strength[END_REF] for HA adsorption onto goethite have been plotted for comparison.

Figure 3 .

 3 Figure 3. LC-OCD chromatograms of PAHA and of supernatants from adsorption experiments (A, top) at 0.1M and pH ≈ 6.8 (R in mg PAHA /gα-Al 2 O 3 ) and (B, bottom) at 0.01M and different pH (R ≈ 25 mg PAHA /gα-Al 2 O 3 ).

  Figure 4. The main evolution is the decrease of differential absorbance values with decreasing R value, at all the wavelength studied. The lack of differences in solution properties at high HS/surface ratio is accounted for by the small percentage of PAHA removed from the solution to the mineral surface (16%). In this situation big changes are not expected to be observed even if there is preferential adsorption of certain molecules. The effects are expected to become

Figure 4 . 19 Figure 5 .

 4195 Figure 4. Differential absorbance spectra of supernatants from retention experiments (pH ref ≈ 3.0) for (a) original PAHA and different organic/mineral ratios (b) R = 60 mg/g (c) R = 30 mg/g (d) R = 20 mg/g (e) R = 12 mg/g (f) R = 7 mg/g.

19 Figure 6 .

 196 Figure 6. NICA-Donnan parameters for the dissolved (blue) and adsorbed (green) fractions of PAHA, compared to the initial value (red line). Errors bars are equal to σ.

For

  the sake of clarity, the evolution of NICA-Donnan parameters with R and comparison to initial PAHA are plotted in Figure 6 (blue symbols). The progressive decrease of Q 1 with R shows a preferential adsorption of the molecules bearing the most reactive moieties. At experimental pH, i.e., 6are deprotonated and more easily retained onto the positively-charged alumina surface, which results in a non-sorbed PAHA fraction depleted in these moieties. It is also in agreement with the preferential sorption of highly polar short-chain carboxylic acids during humic acid sorption onto colloidal Al 2 O 3

6. 8

 8 under which the majority of phenolic-type sites are not deprotonated (logK̃H ,2 = 8.7 at I = 0.1 M). Thus only experiments made at very low R values would allow the more definitive conclusions on m 2 evolutions. Regarding the evolution of logK̃H ,2 it is again difficult to conclude since values seem higher in the nonsorbed PAHA molecules than in the original bulk PAHA.

Figure 7 .

 7 Figure 7.

Table 1 .

 1 SUVA 254 (Specific UV Absorbance at 254 nm) values are summarized in Table1, together with the ratio of absorbance values at 254 and 365 nm (E 2 /E 3 ) and the half-width of the electron-transfer band (Δ ET , in electron-volt) Parameters of UV absorbance spectra

	as described by Korshin et al. (1999). These
	values have been measured for the original
	PAHA solution as well as for the different
	supernatants.

UV/Visible measurements were made on the supernatants from adsorption isotherm at pH 6.8 and 10 g/L α-Al 2 O 3 . UV/Visible spectra of NOM samples can provide some structural information.

of the different samples (at pH = 6).

Table 2

 2

	shows

Table 2 .

 2 Percentage content of different fractions in water sample before and after adsorption as measured by LC-OCD. Definition for humics, building blocks, organic acids and neutrals can be found in[START_REF] Huber | Characterisation of aquatic humic and nonhumic matter with size-exclusion chromatographyorganic carbon detection -organic nitrogen detection (LC-OCD-OND)[END_REF].

	dx.doi.org/10.1016/j.watres.2011.11.042
	(see Figure

 dx.doi.org/10.1016/j.watres.2011.11.042adsorption. These changes in reactivity to our opinion could explain the difficulty to model the behavior of ternary systems composed of pollutants/HS/mineral since additivity is not respected. Model of ternary systems should explicitly take into account the different reactivity of both fractions.

Supplementary material

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.watres.2011.11.042.