Charlotte Jacquemot
email: jacquemot@lscp.ehess.fr

Emmanuel Dupoux

Christophe Pallier

Anne-Catherine Bachoud-Lévi

Comprehending spoken words without hearing phonemes

Introduction

Several models posit that two levels are involved in spoken word comprehension: the sublexical level and the lexical level (Marslen-Wilson, 1984;[START_REF] Mcclelland | The TRACE model of speech perception[END_REF][START_REF] Mehler | Cognitive models of speech processing: psycholinguistic and computational perspectives[END_REF][START_REF] Norris Dg | Shortlist: a connectionist model of continuous speech recognition[END_REF]. The sub-lexical level is responsible for the transformation of the continuous acoustic signal into phonological entities (phonemes, syllables or features).

The lexical level consists in the selection of the lexical form of the word and the retrieval of the semantic information.

Several cases of patients with a lexical impairment without sub-lexical deficit have been documented (Blumstein et al., 1977a;[START_REF] Kohn | Word-meaning deafness: A phonological-semantic dissociation[END_REF][START_REF] Franklin | A distinctive case of word meaning deafness[END_REF]. Such patients have no problem with phonemic identification or discrimination tasks, but are impaired in word recognition. Interestingly, the reverse pattern of sub-lexical impairment without major comprehension deficits has rarely been reported (see one exception in Blumstein et al., 1977b). This is expected in models where the sub-lexical and lexical levels are sequentially organized. Indeed, any deficit at the sub-lexical level should mandatorily translate into corresponding problems in lexical identification.

In this paper, we describe a patient who presents a strong dissociation between performance on sub-lexical and lexical tasks in the unexpected direction. While he was extremely poor in a sub-lexical discrimination task, he was only mildly impaired in lexical tasks. The patient had a global aphasia resulting from a left parieto-temporal ischemia. Tested with the Boston Diagnostic Aphasia Examination, he showed impairment in oral comprehension, and strong deficits in naming and repetition. Here, we focus on his speech comprehension deficit and in particular on the relatively spared lexical level compared to the drastic impairment of the sublexical level.

Severely impaired sub-lexical processing

Preliminary testing revealed that the patient had strong difficulties in identifying phonemes.

Of course, phoneme identification involves metalinguistic knowledge, and we tested sublexical processing in a discrimination task.

AX discrimination task

Sixty pairs of words and 60 pairs of nonwords were constructed with a C 1 VC 2 VC 3 structure (C for consonant and V for vowel) differing or not by a single consonant (C 1 , C 2 or C 3 ). Pairs were presented over headphones and the patient had to indicate whether or not the two stimuli were identical. Pairs were either spoken by the same speaker or by two speakers.

We found that the subject was at chance (one speaker: 53% correct, χ2(1)=.03, P>.1; two speakers: 52% correct, χ2(1)=0.0, P>.1). There was no significant effect of lexical status: word and nonword condition did not differ from chance level (words: 55% correct, χ2(1)=.13, P>.1; nonwords: 50% correct, χ2(1)=0, P>.1).

Partially spared lexical processing

The patient was severely impaired in the phonemic discrimination task. This ought to predict a large impairment in lexical processing. We tested this prediction in a lexical decision task.

Lexical decision

Words (N=64) and nonwords (N=32) differing by only one phoneme from words, were selected. Words belong to two categories (concrete N=48 and abstract N=16) and were matched for frequency [START_REF] Content | Brulex: Une base de données lexicales informatisées pour le français écrit et parlé[END_REF]. Stimuli were presented over headphones and the patient had to indicate whether it was a word or a nonword.

The patient was impaired in this task (68% correct) but results were significantly above chance level (χ2(1)=7.81, P=.005). Results for high frequency words were significantly better than for low frequency words (70.3% correct versus 53.1% correct respectively; F(1,60)=4.4; P=.041). A concreteness effect was also found, showing an advantage for concrete words 1 (69.8% correct for concrete words, 37.5% correct for abstract words; F(1,60)=11.6 P=.001). The patient's results suggest a non-homogeneous impairment at the lexical level, with a more severe deficit for low frequency and abstract words. Yet, the patient performed better overall in the auditory lexical decision than in the phonemic discrimination task, especially for the spared part of the lexicon (81.3% correct for high frequency concrete words). How could such a good performance be obtained? One possibility is that the patient succeeded in guessing what the input word was even with a very imprecise phonological input pattern.

Well preserved phonological details in the spared part of the lexicon

We tested the precision of the phonological representation using two tasks (auditory wordwritten word matching and auditory word-picture matching) where we directly compared the performance on a target word to a control one-phoneme different item (word or nonword). In the two experiments, we only analyzed a subset of the data containing high frequency concrete words, as suggested by the lexical decision experiment.

Auditory-written word matching

In the auditory word-written word matching task, the patient listened to a word and had to select the corresponding one among a choice of three written items. There were the target and two distractors -a word and a nonword -phonologically related to the auditory item and differing by one phoneme from the target. The patient performed 90.1% correct (N=44), which is significantly above chance level (χ2(1)=38.1, P<10 -5 ) 2 .

Auditory word-picture matching

In the auditory word-picture matching task, the patient was presented a picture and an auditory stimulus, and had to decide whether they matched. The auditory stimulus could be the correct name of the picture or a phonological distractor (word or nonword) that differed by one phoneme from the correct name. The patient performed quite well in this task 3 (89.8% correct, N=48, χ2(1)=97, P<10 -5 ).

The patient performed better with the two matching tasks than expected from the phonemic judgment results. If only a vague phonological pattern of the word was available at the lexical level, we would predict difficulties in selecting the target word and confusions with the phonological distractors. The accurate results suggest that a detailed phonological representation has been used.

Discussion

The behavior of our patient is paradoxical. On the one hand, he seems to be incapable of discriminating phonemes in a standard AX task. On the other hand, when tested with a subset of the lexicon (high frequency concrete nouns), he seems to have a nearly intact ability to use phonological representation (Fig. 1). We discuss two possible accounts.

The first account challenges the claim that sub-lexical processing is not necessary for lexical access. According to direct access and exemplar models, words are stored directly as acoustic patterns, without the need for any preliminary phonological processing (see [START_REF] Klatt | Speech perception: a model of acoustic-phonetic analysis and lexical access[END_REF][START_REF] Db | Some thoughts on "Normalization[END_REF]. In such models, the patient could be impaired in phonological processing, while the direct route between the acoustic and lexical levels would allow intact word comprehension (at least, insofar as the lexicon itself is unimpaired). Provided one believes in the independent plausibility of such direct access models, there still is one difficulty to account for: Our patient was impaired in the AX discrimination task to an equal extent whether there was a change in speaker or not. Importantly, in the same speaker condition, exactly the same acoustic token was presented when the two stimuli were identical. In other words, the AX task could have been performed at the acoustic level. Does the chance performance in this condition mean that the patient also has an impaired acoustic level?

The second approach, which we favor, proposes that the patient has no deficit in sub-lexical processing, but rather, a general deficit in reading-out the information encoded at the acoustic and sub-lexical levels. A same-different AX task requires the patient's ability to consciously compare two representations and decide whether they are identical or not. We propose that the patient has impaired conscious access to the phonological details of speech input, while retaining the ability to process them for the purpose of lexical access. One could frame this proposal within the Shortlist model (Norris 1994), whereby the phonological representation used for performing detection and judgment tasks is separate from the one used for bottom-up word recognition. Under such an analysis, our patient could be compared to the cases of blindsight patients, who have access to some high level properties of the stimuli without being aware of low level ones (Farah 1994).

* *

Footnotes:

1 We found a finer grained dissociation within concrete items, with a superiority of living over non-living items. The details of the semantic aspect are described in Dupoux, Jacquemot and Bachoud-Lévi (in preparation).

2 When tested with a dictation and a reading aloud tasks, the patient was severely impaired.

This suggests that grapheme-to-phoneme and phoneme-to-grapheme routes are severed and cannot explain the accurate performance of the subject in the auditory-written word matching task. We also checked that written word recognition was intact with a written version of the matching task. The patient had to select among three written words (the target item plus a phonological and a semantic distractors) the one matching with a test picture. His performance was flawless. This suggests that the patient had no problem to comprehend written words (nor to recognize pictures).

3 When tested in picture naming the performance was very poor, suggesting that the relatively good performance in auditory word-picture matching task was not due to covert picture naming and internal word monitoring.

Figure 1 :

 1 Figure 1: Summary of performance of the patient on the AX discrimination, auditory-written

Acknowledgements:

This work was supported by a Cognitique PhD scholarship awarded to C. Jacquemot, a Contrat PRA (AP/HP-CNRS) awarded to A.C. Bachoud-Levi, a BioMed grant N° PSS*1046, a grant from the Groupement d'Intérêt Scientifique Sciences de la Cognition (99N35/0008), a grant from the ACI Cognitique, as well as support from an ACI Blanche awarded to Christophe Pallier.


R Choumane Ab

S Peulon

Electrodeposited birnessite thin film: An efficient eco-friendly sorbent for removing heavy metals from water

Keywords: Layered manganese oxides, Phyllomanganate, Electrodeposition, Heavy metals, Depollution, Wastewater treatment

Sorption is an efficient process for transferring heavy metals from polluted water to solid phase. Birnessite, which is a non-toxic layered manganese oxide, is particularly interesting for this objective due to its high sorption capacities towards heavy metals. In this paper, pure and nanostructured birnessite was synthesized as adherent thin films onto a solid substrate by electrochemistry with different characteristics for high sorption (e.g. mass, Average Oxidation State). The sorption of Pb (II) and Cd (II) by these films was studied in terms of pH, ionic strength, sorption isotherm and kinetic order. The birnessite thin film with the highest AOS after synthesis (3.76) presents significant adsorption capacity for Pb (II) (538 mg-Pb/g) and Cd (II) (254 mg-Cd/g). A total decontamination at room temperature without filtration and energy input was obtained on model solutions and real effluents by increasing only the number of samples or the surface of electrodeposited birnessite on one sample. These birnessite thin films appear as very interesting eco-friendly sorbents for eliminating lead and cadmium from polluted aquatic media due to their easy synthesis and optimization process, and the possible reuses by efficient desorption, in very low-cost conditions.

Introduction

One of the most common soil and groundwater contaminations is due to heavy metals, more particularly by lead and cadmium. These high toxic elements are dangerous for animals and humans in particular that can damage the nervous system and may be carcinogen [START_REF] Yang | Effect of pH, ionic strength and temperature on sorption of Pb (II) on NKF-6 zeolite by batch technique[END_REF][START_REF] Lee | Sorption characteristics of lead cations on microporous organo-birnessite[END_REF]. These two metals released from numerous industrial products such as dyes, batteries, metallurgy. Without proper treatment, they can be eventually discharged in the environment, which would be particularly dramatic because they are no (bio)degradable and tend to accumulate in all aquatic and terrestrial environments [START_REF] Lee | Sorption characteristics of lead cations on microporous organo-birnessite[END_REF][START_REF] Mastouri | II) transfer across a liquid-liquid microinterface facilitated by a complex formation with 8-Hydroxyquiniline; Application to quantification of Pb(II), Cd(II) and Zn(II) alone or in mixture in effluents[END_REF].

There are various technologies for removing heavy metals from aquatic media such as filtration, surface complexation, chemical precipitation, ion exchange and sorption [START_REF] Wang | Effect of pH and aging time on the Kinetic dissociation of 243 Am (III) from humic acide coated gamma-Al2O3 a chelating resin exchange study[END_REF][START_REF] Trivunac | Removal of heavy metal ions from water by complecation-assisted ultrafiltration[END_REF][START_REF] Wang | Using of chelating resin to study the kinetic desorption of Eu (III) from humic acid-Al2O3 colloid surfaces[END_REF][START_REF] Dabrowski | Selective removal of the heavy metal ions from waters and industrial wastewaters by ionexchange methode[END_REF].

Among these methods, sorption is considered as one of the most important processes transferring heavy metals from aquatic to solid phase, because of its efficiency and economics [START_REF] Zhao | Pb 2+ adsorption on birnessite affected by Zn 2+ and Mn 2+ pretratements[END_REF][START_REF] Zhao | XAFS studies on surface coordination of Pb 2+ on birnessite with different average oxidation states[END_REF][START_REF] Chowdhury | Synthesis and characterization of radiation grafted films for removal of arsenic and some heavy metals from contaminated water[END_REF]. Numerous adsorbents present high properties of lead, cadmium and other heavy metals sorption such as diatomite [START_REF] Sheng | Adsorption of Pb (II) on diatomite as effected via aqueous solution chemistry and temperature[END_REF], carbon nanotubes [START_REF] Li | Adsorption thermodynamic, kinetic and desorption studies of Pb 2+ on carbon nanotubes[END_REF], zeolite [START_REF] Yang | Effect of pH, ionic strength and temperature on sorption of Pb (II) on NKF-6 zeolite by batch technique[END_REF], bentonite [START_REF] Wang | Influence of pH, soil humic/fulvid acid, ionic strength, foreign ions and addition sequences on adsorption of Pb (II) onto GMZ bentonite[END_REF], manganese oxides [START_REF] Wang | Removal of lead (II) from aqueous solution by adsorption onto manganese oxide-coated carbon nanotubes[END_REF], and birnessite in particular [START_REF] Zhao | XAFS studies on surface coordination of Pb 2+ on birnessite with different average oxidation states[END_REF][START_REF] Wang | Sorption behavior of heavy metals on birnessite: Relationship with its Mn average oxidation state and implications for types of sorption sites[END_REF][START_REF] Zhao | Relation of lead adsorption on birnessite with different average oxidation states of manganese and release of Mn 2+ /H + /K +[END_REF].

Birnessite, which is a non-toxic lamellar manganese oxide, possesses high adsorption capacities and abilities to remove spontaneously trace metals present in environment due to its very open structure [START_REF]Manganese oxide minerals: Crystal structures and economic and environmental signifiance[END_REF][START_REF] Tebo | Biogenic manganese oxides: Properties and mecanisms of formation[END_REF][START_REF] Zhang | A novel nanostructured Fe-Ti-Mn composite oxide for highly efficient arsenic removal: Preparation and performance evaluation[END_REF]. Apart from its excellent heavy metal uptake capacity, it is also interesting because of eco-friendly nature and low costs for its synthesis [START_REF] Wei | Manganese oxide-based materials as electrochemical supercapacitor electrodes[END_REF][START_REF] Zhao | Incorpration of Manganese Dioxide within Ultraporous Activated Graphene for High-Performance Electrochemical Capacitors[END_REF]. There are a lot studies in this field but still need many research opportunities [START_REF] Md | Manganese oxides and their application to metal ion and contaminant removal from wastewater[END_REF]. For example, the required filtration steps after the synthesis of birnessite powder and its use after sorption reaction can be very energetic steps, the recycle and reuse of material after adsorption, and obviously the increasing of the efficient for sorption, etc... Adding, there is no pilot or industrial scale investigations published at this time and there is need more potential research [START_REF] Md | Manganese oxides and their application to metal ion and contaminant removal from wastewater[END_REF].

In the literature, the studies concerning lead and cadmium sorption were performed with birnessite powders in classical batch experiments. In this paper, the original methodology proposed is to synthesize birnessite as a thin film deposited onto a solid conductive substrate by electrochemistry, and to use them for developing innovative wastewater treatments. Indeed, such samples present many advantages for these potential applications:

(i)
Electrochemistry is a very suitable method for depositing adherent thin films, well crystallized and homogeneous, in very soft, fast and low cost conditions, with a good control of the mass, thickness and intrinsic properties for birnessite [START_REF]Advances in Electrochemistry and Electrochemical Engineering[END_REF][START_REF] Peulon | Iron compounds electrodeposited onto a transparent semiconductor: Synthesis and characterisation by UV-vis spectroscopy[END_REF][START_REF] Larabi-Gruet | Studies on electrodeposition from Mn (II)species of thin layers of birnessite onto transparent semiconducteur[END_REF][START_REF] Zaied | Studies of N-demethylation oxidative and degradation of methylene blue by thin layers of birnessite electrodeposited onto SnO2[END_REF][START_REF] Ithurbide | Electrophoretic deposition of siderite thin layers: Influence of electrode potential and deposition time[END_REF][START_REF] Ndjeri | Degradation of glyphosate and AMPA (amino methylphosphonic acid) solutions by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF] Pensel | Efficient electrochemical treatment based on electrodeposited thin films of birnessite for mineralisation of AMPA (aminomethylphosphonic acid) in very soft conditions[END_REF][START_REF] Zaied | Spontaneous oxidative degradation of indigo caramine by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF]Synthèses et caractérisations des oxydes de manganèse déposés en couches minces par voie électrochimique : Application à la dépollution des effluents industriels chargés en colorants organiques[END_REF], but also for other materials such as ZnO [START_REF] Peulon | Mechanistic Study of Cathodic Elecfrodeposition of Zinc Oxide and Zinc Hydroxychioride Films from Oxygenated Aqueous Zinc Chloride Solutions[END_REF][START_REF] Wanotayan | Materials Science in Semiconductor ProcessingMaterials Science in Semiconductor ProcessingMaterials Science in Semiconductor Processing[END_REF], iron compounds [START_REF] Peulon | Iron compounds electrodeposited onto a transparent semiconductor: Synthesis and characterisation by UV-vis spectroscopy[END_REF], or titanium dioxide [START_REF] Chigane | Preparation of titanium dioxide thin films by indirect-electrodeposition[END_REF];

(ii) The thin films avoid filtration after their synthesis and also after their interaction with the contaminated media, which can be very energetic steps and can complicate the decontamination processes;

(iii) The possibility of desorption and reuse of the same sample since the birnessite remains in the form of adherent film onto the solid substrate;

(iv) The possibility to work on large scale simply by increasing the number of samples or the surface of one sample. Indeed, electrochemistry is already a method largely used at large scales in various surface treatment industries (e.g. aeronautic…).

For that, the main objectives of this work were in first to optimize electrodeposited birnessite thin films with the optimal mass of material in the aim to create efficient sorbents for Pb (II) and Cd (II) ions removal from aquatic media. Average Oxidation States (AOS) of Mn is also one of the most important parameters: the sorption of heavy metals increases with the oxidation state of Mn [START_REF] Zhao | XAFS studies on surface coordination of Pb 2+ on birnessite with different average oxidation states[END_REF][START_REF] Wang | Sorption behavior of heavy metals on birnessite: Relationship with its Mn average oxidation state and implications for types of sorption sites[END_REF][START_REF] Zhao | Relation of lead adsorption on birnessite with different average oxidation states of manganese and release of Mn 2+ /H + /K +[END_REF]. In second part, different parameters that can influence the sorption capacity of Pb (II) and Cd (II) ions were studied for the best material. In addition, the possibilities of desorption and reuse were studied for the optimized thin film. At the end, the selected material as thin film was tested for treatment of real effluents on larger scales. 5% HNO3, certified by using High Performance ICP-OES, Alfa Aesar) were used without further purification. All solutions were prepared by using Milli-Q water.

Experimental

Electrodeposited birnessite thin films

The electrodeposition of birnessite thin films was carried out in a classical electrochemical set-up with three electrodes as described elsewhere [START_REF] Peulon | Iron compounds electrodeposited onto a transparent semiconductor: Synthesis and characterisation by UV-vis spectroscopy[END_REF][START_REF] Zaied | Studies of N-demethylation oxidative and degradation of methylene blue by thin layers of birnessite electrodeposited onto SnO2[END_REF][START_REF] Ithurbide | Electrophoretic deposition of siderite thin layers: Influence of electrode potential and deposition time[END_REF][START_REF] Ndjeri | Degradation of glyphosate and AMPA (amino methylphosphonic acid) solutions by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF] Pensel | Efficient electrochemical treatment based on electrodeposited thin films of birnessite for mineralisation of AMPA (aminomethylphosphonic acid) in very soft conditions[END_REF]. The reference electrode was a Mercurous Sulfate Electrode (MSE, E = 0.6513 V/ENH). The working electrode was a glass plate covered with tin dioxide SnO2:F (FTO Fluorine-doped Tin Oxide, Solems); the surface in contact with the solution was delimited by an adhesive mask (S = 3 cm 2 ). The counter electrode was a platinum wire wound spirally for having a surface equal to the working electrode. All electrochemical syntheses were performed at room temperature using a microautolab potentiostat/galvanostat system controlled by a computer using the GPES software.

The solution was put in the classical electrochemical cell ([MnSO4] = 1.6 10 -3 M in

[Na2SO4] = 0.4 M, free pH close to 5.6), and was gently stirred continuously during all synthesis. The electrodeposition was performed by oxidation of Mn (II) ions, present in the solution as soluble species, into a Mn (III)/Mn (IV) solid deposited directly onto the conductive substrate as an adherent thin film [START_REF]Manganese oxide minerals: Crystal structures and economic and environmental signifiance[END_REF][START_REF] Nakayama | Electrochemical synthesis of birnessite-type layered manganese oxides for rechargeable lithium batteries[END_REF]. The oxidation reaction was performed by chronoamperometry (constant potential) at fixed electric charges [START_REF] Zaied | Studies of N-demethylation oxidative and degradation of methylene blue by thin layers of birnessite electrodeposited onto SnO2[END_REF][START_REF] Ndjeri | Degradation of glyphosate and AMPA (amino methylphosphonic acid) solutions by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF] Pensel | Efficient electrochemical treatment based on electrodeposited thin films of birnessite for mineralisation of AMPA (aminomethylphosphonic acid) in very soft conditions[END_REF][START_REF] Zaied | Spontaneous oxidative degradation of indigo caramine by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF]Synthèses et caractérisations des oxydes de manganèse déposés en couches minces par voie électrochimique : Application à la dépollution des effluents industriels chargés en colorants organiques[END_REF].

After electrodeposition, the thin films were rinsed with Milli-Q water and dried in ambient air during some hours. After that, the mass of the birnessite thin film was stable and determined by weighing with a precision balance (Sartorius ME and SE Series, Electronic Analytical Balances and Semi-Microbalances). Each measurement was repeated several times to estimate the repeatability and the reproducibility of the electrodeposition.

Average Oxidation States (AOS) of manganese is an important parameter for heavy metals sorption. For that, the AOS of each sample was determined by potentiometric titration in three steps [START_REF]Caracterisation structural de la birnessite : Influence du protocole de synthèse[END_REF]. The thin film of birnessite electrodeposited on substrate was completely dissolved in the Mohr's salt acidified solution under ultrasounds and then titrated with a potassium permanganate solution (1.25 10 -3 M) in order to determine the AOS by this potentiometric method [START_REF]Caracterisation structural de la birnessite : Influence du protocole de synthèse[END_REF]. Each measure was repeated several times to estimate the repeatability and reproducibility.

Effects of pH, ionic species and temperature on sorption capacities

One birnessite thin film was introduced into a cell containing 50 mg.L -1 of lead (Pb(II))

or cadmium (Cd(II)) solution prepared from the standard solution with pure milli-Q water. All experiments were performed at 298 K under gently stirring. Preliminary studies show that 24 hours was enough to reach the sorption equilibrium.

The pH of the solution was adjusted with NaOH (1M) for studying its influence on sorption: the pH ranges varied from 1.7 (free) to 5.2 for Pb (II) or 1.8 (free) to 6.1 for Cd (II) solution respectively. The adsorption was not performed beyond these pH values due to the possible precipitation of Pb(OH)2 or Cd(OH)2.

The effect of ionic species was studied by adding NaNO3 at different concentrations from 0.01 to 1M. The thermodynamic study was performed at temperature varying from 283 to 313 K in a thermostatic oven. These two studies were done at the fixed optimal pH (4.2).

No filtration step was required because the birnessite thin film stayed always very adherent on the solid support. Besides, the sorption of Pb (II) or Cd (II) onto the substrate alone (SnO2:F) was negligible (≈ 0.1%).

Desorption studies

Different parameters can be influenced the possibility to desorb heavy metal from birnessite sample such as the chemical medium and the pH value. For that, desorption of Pb (II)

and Cd (II) from birnessite thin film was performed either in Milli-Q water or NaNO3 (1M) solution at different pH values from 2.5 to 6.2, by adding NaOH (1M) or H2SO4 (1M) if necessary. All experiments were carried out at 298 K under gently stirring for 24h.

Data processing

The concentration of Pb (II) or Cd (II) present in the solution was determined by using the specific Spectroquant® test (1.09717.0001, 0.01 -5.00 mg.L -1 Pb Spectroquant® and 1.01745.0001, 0.01-0.50 mg.L -1 Cd Spectroquant®; absorbance measurements at wavelength 520 nm by a spectrophotometer for Pb(II) and Cd(II)). The amount of heavy metal adsorbed onto the electrodeposited thin film of birnessite was calculated by the difference between the initial amount and equilibrium one using Eq. 1:

n M(II) adsorbed =n M(II) initial -n M(II) equilibrium at t Eq.1
With n in mol, and t, corresponding to a fixed duration

The sorption capacity was calculated using Eq.2:

Qe = (𝐶0-𝐶𝑒)𝑉 𝑚 Eq.2
With Qe, the sorption capacity (mg.g -1 ); C0, the initial concentration of Pb (II) or Cd (II) (mg.L -1 ); Ce, the concentration at the equilibrium time (mg.L -1 ); V, the volume of the solution (L); m, the mass of the electrodeposited birnessite thin film onto the solid substrate (g).

The amount of Pb (II) or Cd (II) adsorbed at equilibrium was studied at different temperatures (298-318K) to determine the thermodynamic parameters for the adsorption process, from Eq.3, Eq.4 and Eq.5:

Kd = 𝑄𝑒 𝐶𝑒 Eq.3 ln 𝐾𝑑 = ΔS 𝑅 + ΔH 𝑅𝑇 Eq.4 ΔG = ΔH-TΔS Eq.5
With Kd, the distribution coefficient for the adsorption; ΔS, the entropy; ΔH, the enthalpie; ΔG, the Gibbs energy; T, the temperature (K); R the gas constant (J mol -1 K -1 ).

The desorption of Pb (II) or Cd (II) from birnessite thin film was calculated as follows,

Eq.6:

M (II)desorbed (%) = 𝑚𝑀(𝐼𝐼)𝑑 𝑚𝑀(𝐼𝐼)𝑎 * 100 Eq.6
With mM(II)d and mM(II)a (mg), the amounts of M(II) (Pb (II) or Cd (II)) desorbed and adsorbed onto birnessite thin film respectively.

Characterizations of birnessite thin film by GIXRD, SEM and EDS analysis

Before and after sorption, the crystal structure of the thin films was characterized by Grazing Incidence X-ray Diffraction GIXRD (λCu=1.540600 Å). The nanostructures of materials were observed by Scanning Electron Microscopy (SEM) using a JEOL JSM-6100.

These observations were supplemented by Energy Dispersive Spectroscopy analyzes (EDS).

The distribution of the heavy metals on the sample after sorption experiments was analyzed by elementary cartography. All characterizations were carried out directly without any preparation step since the material was sufficient conductive.

Results and discussion

Optimization of electrodeposited birnessite thin films

In our previous studies, we have demonstrated that pure and very homogeneous of wellcrystallized H-birnessite (Mn7O13.5H2O) can be electrodeposited in aqueous solutions (NaSO4 as electrolyte) by chronoamperometry at fixed electrical charges [START_REF] Larabi-Gruet | Studies on electrodeposition from Mn (II)species of thin layers of birnessite onto transparent semiconducteur[END_REF][START_REF] Zaied | Studies of N-demethylation oxidative and degradation of methylene blue by thin layers of birnessite electrodeposited onto SnO2[END_REF][START_REF] Ndjeri | Degradation of glyphosate and AMPA (amino methylphosphonic acid) solutions by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF] Pensel | Efficient electrochemical treatment based on electrodeposited thin films of birnessite for mineralisation of AMPA (aminomethylphosphonic acid) in very soft conditions[END_REF][START_REF] Zaied | Spontaneous oxidative degradation of indigo caramine by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF]Synthèses et caractérisations des oxydes de manganèse déposés en couches minces par voie électrochimique : Application à la dépollution des effluents industriels chargés en colorants organiques[END_REF]. The general principle of the electrodeposition is based on the oxidation of Mn (II) ions soluble present in the solution into a direct Mn (III) / Mn (IV) solid deposited onto a conductive substrate as an adherent thin film, according to Eq. 7, [START_REF] Larabi-Gruet | Studies on electrodeposition from Mn (II)species of thin layers of birnessite onto transparent semiconducteur[END_REF].

7 Mn (II) + 13 H2O Mn7O13.5H2O + 26 H + + 12 e - Eq. 7

These electrodeposited thin films are very efficient for the degradation of organic pollutants by spontaneous redox reactions on their surface [START_REF] Zaied | Studies of N-demethylation oxidative and degradation of methylene blue by thin layers of birnessite electrodeposited onto SnO2[END_REF][START_REF] Ndjeri | Degradation of glyphosate and AMPA (amino methylphosphonic acid) solutions by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF] Pensel | Efficient electrochemical treatment based on electrodeposited thin films of birnessite for mineralisation of AMPA (aminomethylphosphonic acid) in very soft conditions[END_REF][START_REF] Zaied | Spontaneous oxidative degradation of indigo caramine by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF]Synthèses et caractérisations des oxydes de manganèse déposés en couches minces par voie électrochimique : Application à la dépollution des effluents industriels chargés en colorants organiques[END_REF]. The optimal samples for these applications have been synthetized by chronoamperometry (E imposed = 0.6 V) with 1.8

Coulombs, as fixed electric charge, onto a FTO surface equal to S = 3 cm² (0.6 C/cm²) [START_REF] Ndjeri | Degradation of glyphosate and AMPA (amino methylphosphonic acid) solutions by thin films of birnessite electrodeposited onto SnO2[END_REF][START_REF] Pensel | Efficient electrochemical treatment based on electrodeposited thin films of birnessite for mineralisation of AMPA (aminomethylphosphonic acid) in very soft conditions[END_REF].

These experimental conditions were used for the synthesis of sample named HB.1 in this study. The mass of H-birnessite electrodeposited was equal to 1.17 10 -3 g, Table 1, in good agreement with the expected theoretical mass calculated according to the Faraday's law (Eq.8)

for this value of Q, Figure 1:

m = 𝑄 𝑛.𝐹 . M Eq. 8
with m, the theoretical mass; Q, the electric charge (Coulombs); n, the number of electrons required for the formation of one mole of H-birnessite; F, Faraday constant (96485 C.mol -1 ); M, molar mass of H-birnessite (Mn7O13.5H2O), [START_REF]Synthèses et caractérisations des oxydes de manganèse déposés en couches minces par voie électrochimique : Application à la dépollution des effluents industriels chargés en colorants organiques[END_REF].

The thickness of the electrodeposited material on the solid substrate is directly proportional to the mass, and can be calculated according to the Eq. 9, [START_REF]Synthèses et caractérisations des oxydes de manganèse déposés en couches minces par voie électrochimique : Application à la dépollution des effluents industriels chargés en colorants organiques[END_REF][START_REF] Birnessite | a new manganese oxide mineral from Aberdeenshire[END_REF], Table 1.

e =

𝑚/𝑠 ρ Eq.9

With e, the thickness of the electrodeposited thin film (cm); m, the mass of the thin film (g); s, the surface of the film (3 cm 2 ) and ρ, the density of birnessite (3 g.cm -3 ).

The Mn Average Oxidation States determined for this sample is equal to 3.60, Table 1, a value comparable to the natural birnessite found in soils [START_REF] Zhao | XAFS studies on surface coordination of Pb 2+ on birnessite with different average oxidation states[END_REF][START_REF]Advances in Electrochemistry and Electrochemical Engineering[END_REF][START_REF] Manceau | In-situ X-rayabsorption study of the mechanism of Cr (III) oxidation at the Mn oxide/water interface[END_REF]. Previous characterizations of this type of sample (HB.1) by different techniques such as XRD, XPS and XANES/EXAFS spectroscopy confirm systematically the formation of a very well crystallized and pure H-birnessite [START_REF] Phd | Films électrodéposés d'oxydes de manganèse : application à la dégradation des polluants émergents dans l'environnement[END_REF][START_REF]In situ XANES measurements during electrodeposition of thin film: Example of birnessite, a promising material for environmental application[END_REF].

The objective of the present paper is to develop H-birnessite thin films with high sorption properties towards heavy metals such as Pb (II) and Cd (II) ions. For that, it was necessary to found the best compromise between the optimal amount of the sorbent and a high value of AOS. For reaching these objectives, the fixed electric charged was decreased for the same value of fixed potential.

Table 1 resumes the experimental conditions and the values of the Mn Average

Oxidation States of each sample determined by the potentiometric method described in the experimental section. The AOS varies from 3.60 (HB.1) to 3.76 (HB.3). For each condition, current measured is well stable and constant during all the duration confirming a systematic good electronic conduction of the electrodeposited material. After synthesis, all samples are homogenous and adherent, with experimental masses directly proportional to the value of the fixed electric charge, and close to the expected theoretical masses, confirming that the faradic yield and the precipitation yield are close to 100 % even for low values of Q, Figure 1.

Moreover, recent ex situ XAS measurements have confirmed that independently of the amount of matter (Q fixed varied from 0.06 C/cm² to 0.6 C/cm²), the structure of H-Birnessite stayed identical for this value of potential [START_REF] Phd | Films électrodéposés d'oxydes de manganèse : application à la dégradation des polluants émergents dans l'environnement[END_REF][START_REF]In situ XANES measurements during electrodeposition of thin film: Example of birnessite, a promising material for environmental application[END_REF]. These experimental conditions for electrodeposition of birnessite thin films seem interesting for the fixed objectives: good adherence, well reproducibility and rapidity of the synthesis (<< 1 hour).

Sorption studies

Effect of the mass and AOS of electrodeposited material on sorption capacities

The differences between these three types of samples are the mass of material and the AOS of Mn. Therefore, it was important to determine the best material with the higher capacity of Pb (II) and Cd (II) sorption. For that, one sample of each type was introduced into a solution containing lead or cadmium (50 mg.L -1 ), with the pH adjusted at 4.2 by adding NaOH (1M).

The sorption studies were performed at 298 K under stirring during 24 h. Figures 2 A and2B shows that HB.3 sample with the highest value of AOS (3.76) and the lowest mass of electrodeposited material (0.33 10 -3 g) presents better adsorption capacities for Pb (II) and Cd (II) ions. Therefore, only this type of sample was used for determining the kinetic and the influence of interaction conditions.

Effect of pH and ionic species on sorption reaction

Birnessite has a Point of Zero Charge (PZC) close to 2-3 [START_REF] Beak | Linking solid phase speciation of Pb sequestered to birnessite to oral Ob bioaccessibilit: Implications fot soil remediation[END_REF][START_REF] Della Puppa | Adsorption of copper, cadmium, lead and zinc onto a synthetic manganese oxide[END_REF]. The point of zero charge corresponds to the pH value when the surface is neutral. Below this pH, the birnessite surface has a positive surface density and hence a repulsion towards cationic species. Above this pH, the birnessite surface has a negative surface density and so an attraction for cationic species. That is why this study of the pH was important. Moreover, pH has a direct effect on the chemistry of lead and cadmium, (e.g. formation of hydroxides complexes) [START_REF]Mise en oeuvre controlée des réactions dans les operations de chimie en solution[END_REF].

These experimental studies concerning the influence of pH were performed by using one sample of HB.3 put into a solution containing one heavy metal (50 mg.L -1 ), presenting different pH values from 1.7 (free) to 5.2 by adding NaOH (1M) for Pb (II), and from 1.86 (free) to 6.12 by adding NaOH (1M) for Cd (II). In these ranges of pH, Pb(II) and Cd(II) are present as cationic species, [START_REF]Analytical Applications of EDTA and Related Compounds[END_REF]. Figure 3 shows that better adsorption capacities were obtained for pH superior to PZC as expected. The value of pH equal to 4.2 seems interesting for obtaining significant sorption and was chosen in the rest of this study for Pb (II) and Cd (II) studies. It was noticed that the pH of the solution after Pb (II) or Cd (II) sorption has a little decrease, which can be attributed to exchange between heavy metal and H + that exists onto the birnessite sites.

The influence of ionic species was studied during the interaction of one sample of HB.3

with lead or cadmium solutions (50 mg.L -1 ) at pH = 4.2 by adding 0.01, 0.1 and 1 M of NaNO3 salt. The sorption capacities show that the presence of Na + ion has no significant effect on the adsorption capacity of HB.3 sample even at high concentration (not shown). This means that the sorption of Pb (II) or Cd (II) onto birnessite thin film is still efficient in presence of competitive cations and would not be affected in the case of effluents charged with salt.

Sorption kinetics

Two kinetic orders are proposed generally in the literature for data interpretation concerning birnessite in powder form [START_REF] Ho | Pseudo-second order model for sorption processes[END_REF]: a pseudo-first order (Eq.10) and a pseudo-second order (Eq.11):

Ln (qeqt) = Ln (qe)k.t

Eq. 10

With qe and qt, the amounts of Pb (II) or Cd (II) adsorbed by the solid (e.g. birnessite thin films) (mg/g) at equilibrium and at time t (min), and k, the rate constant of the pseudo-first order model (min -1 ).

t/qt = 1/k'qe 2 + t/qe Eq. 11

With k', the rate constant of the pseudo-second order model (mg.g -1 .min -1 ).

Kinetic experiments sorption were investigated by using one sample of the bestselected material (HB.3). Figure 4 shows that the pseudo-second order of Pb (II) and Cd (II) adsorption by the birnessite thin film HB.3 (R 2 =0.9996 and 0.9939 respectively) is well adapted. The rate constants k' are equal to 3.43 10 -5 mg.g -1 .min -1 for Pb (II) and 1.76 10 -5 mg.g - 1 .min -1 for Cd (II) sorption. The difference in the values indicates a better affinity of Pb (II) for HB.3 sample than Cd (II). These results confirm the mechanism of chemisorption in good agreement with the studies of the literature based on birnessite powders, [START_REF] Lee | Sorption characteristics of lead cations on microporous organo-birnessite[END_REF][START_REF] Della Puppa | Adsorption of copper, cadmium, lead and zinc onto a synthetic manganese oxide[END_REF].

Sorption isotherms

The shape of the adsorption isotherm reveals the nature of the interactions during adsorption processes. Among the eighteen types of isotherms listed in the Giles classification [START_REF]Mise en oeuvre controlée des réactions dans les operations de chimie en solution[END_REF], there are two typical forms for the adsorption in solution: Langmuir and sigmoid [START_REF]Chapitre Twelve -Porous Texture and surface characterization from Liquide-Solid Interactions: Immersion Calorimetry and adsorption from solution, adsorption bt carbons[END_REF].

Sorption isotherm experiments were performed for evaluating Pb (II) and Cd (II) sorption capacities on HB.3 sample. with powders of birnessite or also others manganese oxides, Table 2. Moreover, the synthesis of such HB.3 samples is fast (≈ 12 minutes), performed in very soft conditions without steps of filtration neither after the synthesis nor after the interaction with heavy metals, which are great advantages for future applications at very low costs.

Characterization of materials by GIXRD, SEM and EDS analysis.

GIXRD measurements of HB.3 sample after synthesis confirm the formation of pure and crystallized H-birnessite (Mn7O13.5 H2O) by the presence of two characteristic peaks at 12.2° and 36.4° according to JCPDS 23-1239 card, Figure 6. The other peaks correspond to the SnO2:F substrate according to JCPDS 41-1445 card. The HB.3 samples containing the maximal amount of adsorbed heavy metal (Pb (II) or Cd (II)) were also characterized by GIXRD. After adsorption of Pb (II) ions (HB.3-Pb), the first peak at 12.2° is absent and the second peak at 36.4° is widened. This can be due to a slight loss of crystallinity of the sample after sorption of lead. However, after Cd (II) sorption (HB.3-Cd), the characteristic peak of the H-birnessite at 12.2° is still present, whereas the second characteristic peak of the H-birnessite at 36.4° becomes also wider, certainly due to the same reasons, Figure 6.

SEM characterization of HB.3 before sorption reports that the sample is very homogeneous and nanostructured with the characteristic sheets of birnessite [START_REF]Manganese oxide minerals: Crystal structures and economic and environmental signifiance[END_REF]. The crystal sizes vary from 70 nm to 80 nm in thickness, and from 1.0 μm to 1.2 μm in length, and entropy (ΔS), using Eq. 3, 4 and 5. These values are summarized in Table 3.

Negative values of ΔG indicate that the adsorption process by the HB.3 birnessite thin film is spontaneous. The degree of spontaneity of the reaction enhances with temperature attributed to an increase of the sites number in the active surface available on the adsorbent, and to the decrease in the thickness of the layer surrounding the adsorbent with temperature [START_REF] Eren | Synthesis, structural characterisation and Pb (II) adsorption behavior of K-and Hbirnessite samples[END_REF][START_REF] Aarfane | Kinetic and thermodynamic study of the adsorption of Red195 and Metylene blue dyes on fly ash and bottom ash in aqueous medium[END_REF].

Positive values of ΔH indicate the endothermic behaviour of the adsorption reaction.

This fact is proved by the increased of Pb (II) and Cd (II) sorption with temperature. Positive values of ΔH suggest also that a large amount of heat is consumed to transfer Pb (II) and Cd (II) ions from the aqueous solution to the solid phase.

Positive values of ΔS reflect the affinity of birnessite samples for Pb (II) or Cd (II). The cations must release of their water hydration before their interaction with the surface. Such release of water from Pb (II) or Cd (II) cations results in positive values of ΔS. It is further noted that the value of ΔS, in the case of cadmium, is lower than that for lead, traducing a greater affinity of the HB.3 film for Pb (II) than for Cd (II). This result can be globally explained in the following way: the exchange of Pb (II) or Cd (II) with H + requires the breaking of binding of OH groups on the birnessite surface, which is an endothermic process. In addition, the sorption of Pb (II) or Cd (II) requires a diffusion process, which is an endothermic process that is why the increasing in temperature favours the sorption of Pb (II) or Cd (II) in the birnessite structure [START_REF] Eren | Synthesis, structural characterisation and Pb (II) adsorption behavior of K-and Hbirnessite samples[END_REF][START_REF] Aarfane | Kinetic and thermodynamic study of the adsorption of Red195 and Metylene blue dyes on fly ash and bottom ash in aqueous medium[END_REF].

Desorption studies

The desorption of Pb (II) or Cd (II) present in HB.

Application: Interests of this sorbent for number up and/or scale up?

A study was carried out by increasing the number of birnessite thin films (HB.3) put into an aqueous solution containing 6.5 μmol of lead (26.91 mg.L -1 ). The initial pH was adjusted

Conclusion

Adherent birnessite thin films were synthesized by a fast electrochemical method at room temperature and at low costs. The better type HB.3 thin film presents higher Pb (II) and Cd (II) sorption capacity compared to equivalent powder birnessite in term of AOS. This high sorption capacity compared to powders may be explained by the nanostructuration of the surface and/or the very low quantities of the solid (0.33 mg). Moreover, these thin films are very interesting for obtaining a total decontamination of solutions containing Pb(II) without filtration steps by increasing only the number of samples or the surface of the electrodeposited birnessite on one sample introduced into the solution. In addition, desorption of heavy metals is possible in very simple conditions leading to possible reuses with similar efficiency.

Due to their simple conditions of synthesis and optimization process, these ecofriendly materials are very encouraging for efficient Pb (II) and Cd (II) removal from polluted aquatic media at very low-costs.

Tables:

Figures 5 A

 5 and 5B prove that the adsorption isotherm is the sigmoid type for Pb (II) and Cd (II) sorption by the birnessite thin film HB.3. The two observed concavities could be explained by the presence of two different types of sorption sites[START_REF] Giles | A general treatment and classification of the solute adsorption isotherm I[END_REF][START_REF]Chapitre Twelve -Porous Texture and surface characterization from Liquide-Solid Interactions: Immersion Calorimetry and adsorption from solution, adsorption bt carbons[END_REF]. The maximal adsorption capacity by one thin film of birnessite HB.3 can be determined from the isotherm plateau. The values are equal to (538 ± 20) mg.g -1 for Pb (II) and (254 ± 11) mg.g -1 for Cd (II) ions respectively. These values indicate that one HB.3 birnessite thin film possesses great sorption capacities in comparison to results presented in the literature

Figure 7 . 7 . 3 - 8 . 9 . 3 . 4 Figure 11 ,

 773893411 Figure 11, it was possible to deduce the thermodynamic parameters relating to the adsorption

  3 samples (HB.3-Pb and HB.3-Cd respectively) was carried out at 298K for 24 hours with stirring, either in milli-Q water or in a NaNO3 solution (1 M), with pH values ranging from 2.5 to 6.2 by adding H2SO4 (1M) or NaOH (1M) if necessary. These experimental choices (NaNO3 and acidic pH) are based on our experimental results and on the intrinsic properties of birnessite: the pH can play on the charge of the surface (PZC) and the presence of sodium ions and H + can favour mainly their insertion at the expense of heavy metal. The maximal desorption of heavy metals from HB.3 samples whether the heavy metal (Pb (II) or Cd (II)) was obtained at pH = 2.5, in good agreement with the PZC of birnessite, and in a NaNO3 solution. In these conditions, the desorption after 24h was estimated to ≈ 70 % in the case of Pb (II), and ≈ 50 % in the case of Cd (II), Figures 12 A and 12 B. In addition, the reuse of the same sample of HB.3 was tested for four times with adsorption / desorption cycle, and the adsorption efficiency remains similar for these four cycles in both cases (Pb (II) or Cd (II)).

at 4 .Figure 13 .

 413 2 and the solution was stirring for 24 hours at 298 K, The amount of Pb(II) adsorbed increases with the number of samples to reach a total decontamination of the solution after 24 hours at room temperature by simple contact of 12 samples of HB.3 without any filtration steps. An equivalent solution of Pb (II) was treated with only one sample of HB.3 electrodeposited onto a total surface of 36 cm 2 , an equivalent surface of 12 HB.3 samples. A total elimination of Pb (II) of the solution was also obtained after 24 hours at 298 K. Similar tests were carried out with success on two real samples containing lead and cadmium coming from CSTB (Centre Scientifique et Technique du Bâtiment -Nantes France) and wastewater treatment plant, confirming the good ability to eliminate these heavy metals even in real wastewaters.

Figure 1 :BFigure 2 :Figure 3 :

 123 Figure 1: Relation between the mass of electrodeposited materials (g) (experimental and theoretical one) and the electric charge Q (C) at E imposed = 0.6 V

Figure 4 : 3 )Figure 6 :Figure 7 :Figure 8 :Figure 9 :Figure 11 : 3 Figure 13 :

 43678911313 Figure 4: Kinetic models for the sorption of Pb (II) and Cd (II) by one birnessite thin film (HB.3)

Table 1 :

 1 Conditions for synthesis of each type of birnessite thin film electrodeposited at E = 0.6 V onto SnO2:F solid substrate, the experimental mass, the thickness of electrodeposited thin film (e) and the Average Oxidation States (AOS)

	Sample Q (C)	t (s)	Experimental	e	AOS	Mn	Mn
				mass	x10 -3	(± 0,05)	IV	III
				x10 -3 (g)	(cm)		(%)	(%)
	HB.1	1.80	≈2400	1.17	0.130	3.60	60	40
	HB.2	0.90	≈1200	0.60	0.066	3.66	66	34
	HB.3	0.45	≈700	0.33	0.037	3.76	76	24

Table 2 :

 2 Adsorption capacity and experimental parameters for removing Pb (II) and Cd (II) by using manganese oxides (Birnessite, Todorokite, Cryptomelane, Hausmannite) in powder form (literature) in comparison to electrodeposited birnessite thin films (HB.3) (this study)

	Sorbents	Adsorption conditions	Adsorption capacity
		t (h) pH T(K)

Table 3 : Thermodynamic parameters of Pb (II) and Cd (II) adsorption by using electrodeposited birnessite thin film (HB.3)

 3

	Metal ions	ΔH (KJ.mol -1 )	ΔS (J.mol -1 .K -1 )	ΔG (KJ.mol -1 )
				283K 298K 313K
	Pb (II)	67.54	251.63	-3.66	-7.44 -11.22
	Cd (II)	19.68	86.01	-4.66	-5.95 -7.24

Acknowledgements

Thanks go to V. Ji for XRD characterization (ICMMO -CNRS -UPSud), R. Pires for SEM observations, EDS and elementary cartography analyses (ICMPE -CNRS -UMR 7182), N. Legarrec and A. Couzinet for providing the real samples from CSTB (Nantes-France), and A.

Masy for providing the real samples from wastewater treatment plant (Evry-France).


Juho Karhu
email: juho.karhu@helsinki.fi

Hadrien Philip

Alexei Baranov

Roland Teissier

Tuomas Hieta

Sub-ppb detection of benzene using cantilever-enhanced photoacoustic spectroscopy with a long-wavelength infrared quantum cascade laser

We report a novel photoacoustic spectrometer for trace gas sensing of benzene. A quantum cascade laser emitting at the wavelength 14.8 µm is used as the light source in the spectroscopic detection. This wavelength region contains the strongest vibrational band of benzene, which is free of spectral overlap from common trace gases, making it a strong candidate for sensitive benzene detection. Cantilever-enhanced photoacoustic spectroscopy is used for detection. This simple and robust measurement setup can reach a benzene detection limit below 1 ppb.

Cantilever-enhanced photoacoustic spectroscopy (CEPAS) is used in sensitive trace gas detection, with detection limits demonstrated at parts-per-trillion (ppt) level and below [1,2], and it has been applied to measurements of various volatile organic compounds (VOC) [3,4]. Of the many VOC, there is substantial interest in measuring benzene (C 6 H 6 ). It is used widely in industry but it is a known carcinogen and worker exposure is tightly regulated [5]. Benzene is also a component in gasoline, and a major exposure path to the general public is from traffic and driving [6]. Many of the most sensitive benzene detection methods rely on gathering a sample and sending it for offline analysis in a laboratory, although sensitive sensors are also being developed [7]. Optical detection methods, based on spectroscopy, would provide an opportunity for development of sensitive portable instruments, which could be used in measurements on the field or set up for long term autonomous data collection.

Most of the optical benzene trace gas detectors focus on the strong infrared band located at about 3 µm [8]. Detection limits in the low parts-per-billion (ppb) range have been reached, for example, using a difference frequency generation (DFG) light source [9] and using an optical parametric oscillator together with CEPAS [3]. However, the benzene absorption band at 3 µm has severe spectral overlap with many other VOCs, most importantly toluene and xylene isomers, which are typical interferents in benzene measurements. For example, in the prior CEPAS study for benzene detection, the detection limit increased by an order of magnitude when this interference was properly accounted for [3].

In this letter, we describe a novel CEPAS spectrometer, which takes advantage of the strongest benzene infrared band, ν 4 , located at about 14.8 µm. This band is accessed using a longwavelength quantum cascade laser (QCL). The ν 4 band has been applied before in benzene sensing, using a DFG source and a cryogenically cooled diode laser, but the reported detection limits have been in the part-per-million (ppm) range [10,11]. A detection limit in the low ppb range was reached with a Fouriertransform infrared spectrometer, but only after sample preconcentration of up to 40 min [12]. In addition to being the strongest vibrational transition of benzene, the ν 4 -band is also significantly narrower than the other benzene infrared bands, and does not overlap with the spectra of typical interfering species. There is, however, spectral interference from a nearby CO 2 transition. Nevertheless, the spectrometer described in this letter is able to reach a detection limit in the range of 1 ppb and below in the presence of varying ambient CO 2 concentration. Figure 1 shows spectra of benzene and CO 2 around the target wavelength range, simulated using the SPECTRA online tool from data available in the GEISA database [13].

A schematic picture of the simple measurement setup is depicted in Fig. 2. The used light source was a continuous-wave QCL, developed at the University of Montpellier. The used QCL is based on the InAs and AlSb materials, which demonstrated the first and only semiconductor lasers operating in a continuous wave at room temperature in the long wavelength mid-infrared [14]. A device with a distributed feedback grating [15] was fabricated in order to obtain single mode emission at the frequency of the ν 4 band of benzene. The QCL operated on a water-cooled thermoelectric cooler. The accessible tuning range was from 671.5 cm -1 to 675 cm -1 and the output power was up to 5 mW. The operational temperature used in the experiments was about 250 K. The laser beam passed through a 9 cm long photoacoustic cell with a cantilever microphone detector. The inner diameter of the cell was 4 mm. The QCL beam was focused at about 20 cm away from the output window so that the beam diameter was smaller than the cell inner diameter, over a distance of 10 cm. The cell windows were anti-reflection coated ZnSe windows. The reflectivity of the coating was specified only up to 12 µm, but the transmission was measured to be above 80 % over the whole measurement wavelength range. The optical power after the photoacoustic cell was measured with a thermopile detector, to normalize the photoacoustic response. The gas pressure inside the cell was 200 mbar and the cell was stabilized to the temperature of 50 • C. Benzene gas samples were generated by mixing compressed air with gas containing 52 ppm of benzene in nitrogen balance. When a sample clean of benzene was required for reference measurements, the compressed air was filtered through active carbon. Fig. 2. Schematic picture of the measurement setup. The laser beam from a QCL emitting at 14.8 µm is sent through a photoacoustic cell containing the sample gas and a cantilever microphone with optical readout. A thermopile detector (TP) is used to measure the power of the laser after the photoacoustic cell. The data recording and the laser control is done using a LabVIEW program running on a PC.

Benzene was detected from the Q-branch of the ν 4 band at about 673.973 cm -1 in wavenumber. This spectral region is mostly clear from obvious interferents, except there is a CO 2 line at 674.075 cm -1 , which is partially overlapping with the benzene Q-branch. To reduce the interference from the CO 2 transition, the detection was done using wavelength modulation absorption spectroscopy (WMAS), with a modulation frequency of 60 Hz.

The laser wavelength was tuned by the laser current. The wavelength axis was referenced using the CO 2 line at 674.075 cm -1 and the benzene Q-branch. The signal is demodulated at the second harmonic of the modulation frequency. The WMAS spectrum near the benzene band is shown in Fig. 3 for benzene and CO 2 . In the benzene concentration measurements, ten points were first measured on top of the CO 2 transition in order to align the wavelength axis, as well as to record the photoacoustic signal, which corresponds to the CO 2 concentration, since there is significant absorption from the tail of the CO 2 line over the benzene Q-branch. The laser current was then shifted by a fixed offset value to measure the spectral response at the wavelength of the benzene Q-branch. Total of 23 wavelength steps were recorded around the center of the benzene absorption feature, over a wavenumber range of about 0.04 cm -1 . Each measurement point was averaged for about 1.8 s. A single scan, which consisted of the gas exchange, the CO 2 line measurement and the scan over the benzene Q-branch, took about 70 s. In terms of the signal level, typical CO 2 concentration in air results in a signal level that corresponds to about 90 ppb of benzene at the peak of the benzene signal (Fig. 4). The CO 2 signal is nearly flat over much of the benzene peak, making it relatively easy to distinguish the spectra of the two components. The CO 2 contribution to the benzene signal was compensated using a CO 2 reference spectrum, which had been measured from benzene-free compressed air. This reference spectrum contained the signal at the CO 2 line, as well as the CO 2 contribution over the benzene Q-branch. For individual scans, the CO 2 reference spectrum was scaled so that the signal at the CO 2 line was equal to that measured during the scan. This scaling accounts for any changes in the ambient CO 2 concentration. The scaled reference spectrum was then subtracted from the benzene spectrum measured during the scan, in order to eliminate the CO 2 contribution. A reference benzene spectrum was recorded with a gas sample containing 204 ppb of benzene. Benzene concentration was calculated from the spectrum measured during a scan with a linear least-squares fit of the reference benzene spectrum, with the assumption that the photoacoustic signal depends linearly on the concentration. The linearity of the spectrometer was tested by repeating the measurement at four benzene levels. Each level was recorded for about 30 minutes. The resulting calibration curve is shown in Fig. 5 and it displays excellent linearity.

The stability of the measurement system was tested by recording benzene-free compressed air over a time period of about 60 hours. Effectively, this means fitting the benzene reference spectrum into the residual spectrum after the CO 2 contribution has been subtracted from the measured spectrum. The results show good long-term stability in benzene-free measurement and indicates that the CO 2 spectral compensation is effective. The zero level stays nearly constant, but small drifting below ppb level can be detected. Figure 6 shows Allan deviation of the zero level measurement, together with the time trace. The Allan deviation reaches a minimum with a noise equivalent concentration of about 0.15 ppb with about 200 min of averaging. Another long time trace of 15 hours was also measured with a constant benzene level of 26 ppb and the results are shown in Fig. 7. The noise was slightly higher in this measurement, but it reaches a similar level with long averaging times. The normalized noise equivalent absorption (NNEA) is calculated as the product NNEA = σ α P √ t, where σ α is the noise equivalent absorption coefficient, P is the optical power and t is the averaging time. To estimate NNEA, we scaled the maximum absorption from the benzene Q-branch, as simulated with SPECTRA for 26 ppb of benzene at 200 mbar [13], with the signalto-noise ratio at 100 min averaging time, according to Fig. 7. The optical power within the CEPAS cell, taking into the account the window transmission, was about 2.4 mW in this measurement. This corresponds to NNEA of about 2.8 × 10 -9 cm -1 WHz -1/2 . For best CEPAS measurements, the NNEA is typically in the order of 10 -10 cm -1 WHz -1/2 [1, 2]. The higher NNEA here is mostly due to the fact that the WMAS modulation amplitude was chosen as a compromise between maximizing the benzene signal and minimizing the interference from the CO 2 transition. Maximizing the WMAS signal with benzene alone would result in about six times stronger signal. It should also be noted that the averaging time used in the NNEA calculation included the CO 2 line measurements and gas exchange cycles.

In conclusion, we have demonstrated a simple but sensitive spectroscopic detector for benzene, based on CEPAS and a continuous-wave QCL emitting at 14.8 µm. The measurement setup reaches a minimum noise equivalent benzene concentration of 150 ppt. If the detection limit is taken as three times the standard deviation, this converts to 450 ppt, which is, to our knowledge, lowest reported in the literature for optical benzene sensors. Furthermore, the benzene ν 4 band used here in the spectroscopic measurements is free of spectral overlap from many common interferents, such as toluene and xylenes, which solves a typical shortcoming of benzene detectors based on infrared spectroscopy.

Funding. Jenny and Antti Wihuri Foundation; "Investment for the Future" program (Equipex EXTRA, ANR-11-EQPX-0016).

Disclosures. TH: Gasera Ltd. (E).

Fig. 1 .

 1 Fig. 1. Simulated spectra of pure benzene (black) and CO 2 (grey) at 200 mbar. The CO 2 spectrum has been multiplied by 10 for easier comparison.

Fig. 3 .

 3 Fig. 3. Photoacoustic wavelength modulation spectrum of 200 ppb of benzene in nitrogen (black) and CO 2 in air (grey). The CO 2 spectrum has been scaled by dividing the signal by 10, for easier comparison.

Fig. 4 .

 4 Fig. 4. Spectrum of 200 ppb of benzene in air near the benzene absorption peak. The figure also depicts spectra for ambient CO 2 and 200 ppb of benzene in N 2 , to show the relative contributions from the two components.

Fig. 5 .

 5 Fig. 5. Photoacoustic signal of benzene measured at different concentrations and a linear fit to the concentration data. Each concentration was measured for about 30 minutes. The reported concentration is the mean value and the error bars are statistical errors from each set of concentration data (3σ). The nominal concentration is calculated from the flow ratios of the mass flow controllers used to dilute the benzene sample.

Fig. 6 .

 6 Fig. 6. Allan deviation of a benzene zero level measurement (above) and the time trace used to calculate the deviation (below).

Fig. 7 .

 7 Fig. 7. Allan deviation of a benzene concentration measurement with a 26 ppb sample of benzene (above) and the time trace used to calculate the deviation (below).