Traitement de lixiviats stabilisés de décharge par des membranes de nanofiltration

Le terme "lixiviat" ou "jus de décharge", désigne l'eau qui a percolé à travers les déchets en se chargeant de polluants. Ces effluents pollués doivent être traités.En raison des exigences croissantes des normes de rejet et de la stabilisation des lixiviats au cours du temps, de nouvelles techniques ont fait leur apparition dans ce domaine. La technologie de l'osmose inverse s'est développée dans de nombreux pays européens. Cependant cette technique sélective et coûteuse, se justifie seulement quand les normes sont drastiques. C'est pourquoi la nanofiltration pourrait constituer une solution intermédiaire.L'objectif de ce travail est de contribuer à mieux maîtriser cette technique pour l'élimination de la Demande Chimique en Oxygène (DCO) récalcitrante, subsistant après les traitements biologiques classiques.Nous avons évalué les performances de trois membranes (organiques et minérales) pour l'abaissement de la charge organique, en fonction des conditions hydrodynamiques (vitesse et pression).Chacune de ces membranes possède un comportement spécifique vis à vis de ces lixiviats stabilisés (adsorption, polarisation de concentration, obstruction des pores).L'influence d'une coagulation préalable sur les performances d'épuration a également été examinée pour l'une des membranes.Cette étude constitue une étape préliminaire au dimensionnement d'une installation.Landfill leachate is the name given to water that has passed through solid waste and contains organic and mineral contaminants. Therefore this effluent must be treated before discharge to the environment. Because of new norms and the stabilization of leachates with time, new treatment methods have been designed. Thus, reverse osmosis is used in many European countries. But the use of reverse osmosis is only justified when norms are severe, because the treatment is highly selective (salt rejection >99%) and very expensive. In other cases, nanofiltration may be an interesting alternative.The purpose of the present work is to propose a process for recalcitrant organic matter in order to optimize the technique. Thus, three membranes (organic and mineral) have been used to evaluate their ability to decrease the Chemical Oxygen Demand (COD) of the leachate. This study helps to determine the size of the device. First, physical parameters were investigated. Each time, the hydraulic regime was turbulent (Re > 2500). Higher permeation fluxes were obtained with organic membranes than with the mineral one (80 L·h-1·m-2 compared to 25 L·h-1·m-2) under the same experimental conditions (10 bar and 3.4 m·s-1). Tangential flow rates higher than 2.5 m·s-1 do not influence COD retention; at lower flow rates polarisation concentration may occur. The removal of COD is achieved in the three cases. At 10 bar an acceptable value of less than 120 mg O2·L-1 (norm) is obtained. The inorganic membrane (Tech-Sep) gives the best results (COD rejection: 70 % at 10 bar).Membranes behave differently toward landfill. The organic membrane MP 20 (Weizmann membrane, cut-off 450 Dalton (Da), polyacrylonitrile) shows low adsorption with landfill leachate. The organic membrane MP 31 (Weizmann membrane, cut off 450 Da, polysulfone) gave a high COD retention ratio; the values for irreversible fouling and static adsorption are of the same order ofmagnitude; a strong membrane-foulant interaction must occur, which improves membrane selectivity. The mineral membrane N01A (Tech-Sep membrane, cut off 1000 Da, zirconium oxide), like MP-31, gives high static adsorption with leachate and irreversible fouling as well. The latter phenomenon can be explained by the obstruction of membrane pores by leachate particles, the size of which is near the membrane cut-off point. Fouling and static adsorption contribute to the increase in the membrane rejection rate. We studied coagulation as a pretreatment to improve performances of the N01A membrane. Experiments have been carried out with Jar-Test and FeCl3-like coagulants. The optimal amount of coagulant was 1.4 g Fe·L-1; 60% COD reduction was achieved. The results obtained with the N01A membrane are improved: reduction of COD rises from 78% to 92 %, concentration polarisation is lower, and therefore the flux increases up to 53 L·h-1·m-2. This value still remains lower than the organic membrane fluxes (respectively 80 L·h-1·m-2 for organic membranes and 25 L·h-1·m-2 for N01A). However, coagulation may not be the appropriate pretreatment because the fouling index of the supernatant after coagulation was similar to that of the raw leachate. Permeability measurements after treatment show that internal fouling is still important (25%). In fact, coagulation does not remove molecules with molecular weights around 500 Daltons, and consequently these particles still obstruct the membrane pores. The phenomenon limits the performance (flux) of this membrane

Influence de l'adsorption d'alginates sur les propriétés de membranes organiques d'ultra et de microfiltration

Les applications potentielles des cultures de microalgues et cyanobactéries en dépollution d'effluents dans des photobioréacteurs à membrane souffrent de performances limitées par un colmatage de l'élément filtrant dû en grande partie aux exopolysaccharides sécrétés par ces micro-organismes. Cette étude du laboratoire quantifie les effets de l'adsorption de ces polysaccharides sur des membranes organiques d'ultra et microfiltration tangentielle de matériaux et charges de surface différents. L'alginate de sodium est utilisé comme adsorbat modèle. Les membranes propres sont d'abord testées par une mesure de flux à l'eau pure, puis mises en contact avec une solution d'alginate durant un temps choisi. Le flux à l'eau pure des membranes après adsorption est ensuite à nouveau mesuré.La réduction relative du rayon de pore (ZEMAN, 1983) met en évidence l'effet de la mouillabilité et des charges superficielles. L'étude comparée de membranes d'ultra et microfiltration montre que cette réduction relative du rayon de pore augmente avec le seuil de coupure ou le diamètre de pore. L'effet de la concentration révèle aussi que la résistance hydraulique d'adsorption (MATTHIASSON, 1983) à l'équilibre évolue selon l'isotherme de LANGMUIR. Le modèle cinétique traduisant l'évolution de la résistance d'une membrane d'ultrafiltration proposé par BAKLOUTI et al.(1984), amélioré par AIMAR et al. (1988) puis discuté par RUIZ-BEVIÁ et al. (1997), est complété par un nouvel exposant agissant sur le facteur temps.La comparaison des résistances à l'écoulement de membranes de microfiltration avec celle d'une membrane d'ultrafiltration hydrophile neutre permet de dégager des critères de choix pour l'optimisation du fonctionnement d'un photobioréacteur à membrane utilisable en dépollution d'effluents.Potential applications of microalgae and cyanobacteria for treatment of wastewater effluents using membrane-photobioreactors suffer from limited performance due to fouling effects, mainly attributable to exocellular polysaccharides secreted by these micro-organisms. A membrane photobioreactor is defined as a process associating the culture of photosynthetic micro-organisms with a continuous separation by membrane filtration of the biomass and the water treated. The goal of the present laboratory-scale study was to quantify polysaccharide adsorption effects on organic membranes (ultra and microfiltration) characterised by different materials and surface charges. Sodium alginate was used as the "model adsorbate".Seven plane organic membranes were tested. The influence of membrane cut-off (or of pore diameters) as well as that of the material polyethersuphone (PES), polyacrylonitrile (PAN), polyvinilidene fluoride (PVDF) and of its properties (hydrophobicity, surface charges, …) were assessed. The study consisted of two parts :1. the first part was concerned with the kinetics of alginate adsorption and the influence contact time and solute concentrations on the reduction of pore diameter (ZEMAN, 1983) or on the increase of hydraulic resistance (MATTHIASSON, 1983);2. the second part dealt with adsorption equilibrium (formulations of LANGMUIR and FREUNDLICH).The study constituted the first step of a research program aimed at developing membrane photobioreactors for the treatment of specific industrial effluents. The fluid used to test the membranes was quality II pure water (ISO 3696 norm). Tangential velocities were set to 2.5 m.s-1, corresponding to a Reynolds number of 2500. To represent exopolysaccharides, we used alginic acid at concentrations of 1, 10 and 50 g, neutralised with sodium hydroxide at pH 9. New (or clean) membranes were first characterised through pure water flux measurements. J0, the flux of pure water for a new membrane, was obtained (flowrate / unit of surface area), and then the membrane was kept in contact, for a definite duration, with the alginate solution. After adsorption and rinsing, the pure water flux was measured again. Ja, the pure water flux, was measured through the membrane after adsorption.Adsorption model at equilibrium:The effect of adsorption is quantified under the form of the relative pore size reduction as described by ZEMAN (1983) and included in the relation : ∆r / r=1 - (Ja / Jo)1/4. A variation of this quantification is that of the MATTHIASSON model (1983) applied to the pure water flux, based on DARCY's law expressing the relative value of the hydraulic resistance of the adsorbed layer Ra in relation to the intrinsic resistance of the membrane Rm : Ra / Rm=(Jo / Ja) - 1.To express adsorption phenomena at the solid/liquid interface of membranes, we used LANGMUIR's law together with MATTHIASSON's experimental observation (1983): the relative resistance Ra / Rm due to adsorbed compounds is proportional to the mass "x" of solute adsorbed per unit of membrane surface area, x=Kx.Ra. If one assumes that the mass m of a homogeneous plane membrane per unit of membrane surface area is proportional to its adsorbing surface area Ω per unit of membrane surface area (m=Km.Ω), and if one combines the flux equations expressed by DARCY's and POISEUILLE's laws, then the result is m=K'm.Rm in a homogeneous membrane. Substituting x and m in LANGMUIR's law results in the equilibrium model Rae / Rm=(Jo / Ja) - 1=a.c / (1 + bc) in which c=concentration of adsorbing solute; a and b are coefficients; and Rae is the resistance due to compounds adsorbed at equilibrium. Kinetic model: To show the evolution of membrane resistance with time, we suggest the introduction of an empirical exponent j over the time parameter in the AIMAR et al. model (1988).Results: The effect of changing the alginate concentration reveals that the hydraulic resistance of adsorption, at equilibrium, (MATTHIASSON, 1983) evolves according to LANGMUIR's isotherm. The relative decrease of pore radius ∆r / r in the presence of l g.l-1 of sodium alginate shows that a quasi-plateau is obtained after two hours using the most hydrophobic membrane. The curves ∆r / r=f (t) for five membranes made of different materials, monitored during the transition phase before the plateau with common 1 g.l-1 concentrations, reveal similar adsorption behaviour, characterised by the limiting common value ∆r / r=0.06 ± 0.005. However, the uncharged hydrophilic membrane PAN 3038 stands out owing to a much lower ∆r / r value of 0.09. This peculiar behaviour can also be observed in the influence of the alginate concentration: hydrophobic and charged hydrophilic membranes display a saturation effect with ∆r / r little affected by the increase of alginate concentration, whereas the uncharged hydrophilic membrane PAN 3038 displays a ∆r / r value three to six times lower with great sensitivity to concentration effects at concentrations below 10 g.l-1. The model Rae / Rm=(Jo / Ja) - 1=a.c / (1 + bc) is in agreement with the experimental results obtained with hydrophobic and hydrophilic membranes. The proposed kinetic model shows that time dependence of R (t) does not seem to be linked to the nature of membranes. However, compared with concentration, R (c) is very sensitive to the nature of membranes. A comparative study of ultra and microfiltration membranes shows that the reduction in ∆r / r values increases with molecular weight cut-off (or pore diameter).Criteria for the choice of membranes: A comparative study of three polyacrylonitrile membranes reveals that membrane 3038 PAN (neutral) displays a very interesting, peculiar behaviour: its adsorption, expressed by ∆r / r or Rae/(Rae+Rm) is four to six times weaker than that of the other two. The surface charge of membranes seems to influence the intensity of adsorption in a significant way. Wetability also has a strong influence on adsorption. The sum of resistances Rae + Rm of ultrafiltration membrane 3038 PAN is only four times as great as those of hydrophobic microfiltration membranes. Experimentation already showed that, in the presence of microparticles, interactions between the layer of adsorbed alginate and microparticles will increase the likelihood of fouling of microfiltration membranes, decreasing their resistance down to the level of very little adsorbing ultrafiltration membrane IRIS 3038 (ROSSIGNOL et al., 1999).A culture system of marine microalgae in a membrane photobioreactor using ultrafiltration membrane IRIS 3038 PAN displayed a stable permeation flux during 6 weeks and easy regeneration, which meant adsorption was almost nil. The ability of some microalgae to assimilate ammonia nitrogen, nitrates and phosphates contained in waste water with excellent efficiencies (e.g., Phormidium bohneri: SYLVESTRE et al., 1996) allows one to consider using membrane photobioreactors in the treatment of home or industrial effluents. Other microalgae such as Chlorella salina (GARNHAM et al., 1992) are capable of fixing large amounts of heavy metals (Co, Mn, Zn, etc…); grown in membrane photobioreactors, they could depollute industrial effluents

Réduction de la DCO dure des lisiers de porc et lixiviats par nanofiltration

Malgré un traitement biologique préalable, les lisiers et les lixiviats de décharge ont en commun de conserver une Demande Chimique en Oxygène (DCO) résiduelle supérieure à 500 mg O2.l-1 : valeur 4 à 5 fois trop élevée pour un rejet direct dans le milieu naturel. La nanofiltration pourrait constituer une solution comme traitement de finition. Dans le cadre de cette étude expérimentale, deux membranes de nanofiltration sont mises en œuvre à l'échelle pilote afin de comparer leur efficacité pour réduire la DCO non biodégradable des deux effluents précités. Dans un premier temps, l'étude menée à concentration constante, montre que les performances (flux de perméation et DCO dans le perméat) dépendent principalement du couple membrane - effluent. Dans le cas du lisier, la couche de colmatants formée à la surface de la membrane a un caractère compressible et peu structuré ce qui entraîne une plus grande sensibilité aux variations de conditions hydrodynamiques. Dans le cas des lixiviats, la couche formée est moins dépendante des conditions opératoires. Après avoir sélectionné les meilleures conditions opératoires pour réduire la DCO des deux effluents étudiés, les essais de nanofiltration sont ensuite menés en mode "concentration" en fixant la pression à 15 bar et la vitesse de recirculation à 1,5 m.s-1. L'obtention d'un facteur de réduction volumique de 4 entraîne, d'une part, une diminution plus accentuée des flux de perméation dans le cas du lisier que dans celui du lixiviat et, d'autre part, une augmentation plus importante de la DCO du perméat pour le lisier. La valeur de la DCO devient alors supérieure, en fin de concentration, à celle requise pour un rejet en milieu naturel (120 mg O2.l-1).Pig manure and landfill leachate cannot be treated only by conventional biological treatment because a "refractory" COD persists, superior to 500 mg O2.l-1 : four times too high for a direct discharge in the environment. Nanofiltration, an intermediate process between reverse osmosis and ultrafiltration, may be an interesting alternative as a final treatment. In nanofiltration, lower pressure can be used and fluxes are higher than for reverse osmosis. The present study compared the treatability of pig manure and landfill leachate after biological treatment using a pilot-scale nanofiltration plant. Performances were evaluated in terms of permeate COD and permeate flux versus operating conditions (applied pressure, crossflow velocity and recovery rate). Two tubular organic nanofiltration membranes with 450 diameter cut-offs were used for pilot-scale testing: MPT-20 (polyacrylonitrile) and MPT-31 (polysulfone). Preliminary experiments carried out at constant concentrations show that performance (permeation flux and permeate COD) depends mainly on the nanofiltration membrane/effluent coupling. Permeate fluxes obtained with the MPT-20 membrane were higher than those obtained with the MPT-31. The increased crossflow velocity produced a particularly marked flux increase for pig manure. Moreover, the flux obtained with pig manure decreased at pressures superior to 15 bars whereas for the landfill leachate it became constant regardless of the pressure applied. COD retention was better in the case of pig manure and increased with pressure. On the other hand, high crossflow velocity helped reduce the COD retention, particularly for pig manure. The difference stems mainly from the foulant layer on the membrane surface. This layer is compressible and not organised; in the case of pig manure, it may explain the influence of hydrodynamic parameters: crossflow velocity favours the back migration of potential foulant such as colloids from the membrane surface to the bulk liquid phase. This may explain an increased mass transfer and consequent reduction of COD retention at high tangential velocities. Moreover, higher pressure generates a dense layer, which leads to a reduction of mass transfer. The influence of operating conditions was less important for the leachate, as the foulant layer may be more organised and have better cohesion.In the second part of this study, the nanofiltration pilot plant was operated in concentration mode in order to evaluate the influence of recovery rate on flux and retention. Since COD retention is better with the MPT-31 membrane, the latter was used for concentration experiments. The applied pressure was fixed at 15 bar and crossflow velocity at 1.5 m.s-1. Both effluents were concentrated with a volume reduction factor of 4. However this reduction of retentate volume led to both a drop in permeation flux and a rise of permeate COD, to a value above to the environmental norm of 120 mg O2.l-1

Techniques à membranes appliquées à l'élimination des matières en suspension dans un circuit semi-fermé d'aquaculture

Les piscicultures en circuits semi-fermés sont confrontées au problème de l'élimination permanente des matières en suspension (M.E.S.) et des substances azotées. Les procédés conventionnels utilisés pour retenir les M.E.S. (décantation, hydrocyclones, filtres mécaniques à tambour rotatif, filtration gravitaire) ne donnent par entière satisfaction. Par contre, la filtration sur membranes permet d'arrêter en totalité les particules en suspension et les bactéries.On montre d'abord que les teneurs en M.E.S. et leurs répartitions granulométriques mesurées sur des échantillons prélevés en bassins d'aquaculture varient avec la taille des poissons et l'heure du prélèvement et on met en évidence la présence en nombre important de particules submicroniques.Différents essais de filtration sur membranes sont ensuite présentés :- d'une part, avec des membranes d'ultrafiltration capillaires à peau interne utilisées en potabilisation des eaux : on examine l'influence des paramètres hydrodynamiques (pression transmembranaire, vitesse de recirculation) afin de rechercher les conditions optimales de fonctionnement. Le flux de perméat ne dépasse pas dans le meilleur des cas 100 l.h-1.m-2.- d'autre part, avec des membranes de microfiltration organiques planes en fluorure de polyvinylidène (PVDF) et tubulaires en céramique. Les flux obtenus avec les membranes organiques sont de l'ordre de 250 l.h-1.m-2Dans tous les cas, la rétention des M.E.S. est totale.Cependant l'estimation de l'investissement et des coûts de fonctionnement pour une pisciculture en circuit fermé de taille industrielle conduit à des prix trop élevés pour que l'utilisation des membranes dans ce domaine soit à ce jour économiquement envisageable.A problem confronting semi-closed circuit aquaculture is the need for continuous elimination of suspended matter (SM) and nitrogenous substances. Conventional processes used to retain SM (settling tanks, hydrocyclones, rotating-drum mechanical filters, gravity filtration) are not entirely satisfactory. However, membrane filtration has recently been shown to allow removal of suspended particles and bacteria. The present study evaluates the performance of different ultrafiltration and microfiltration membranes for water processing in a semi-closed aquaculture system. A brief economic analysis of treatment costs is proposed based on the results.The marine aquafarm studied produces about 5 tons of turbot per year with a plant volume of about 100 m3. The water processing line is fitted with a rotating-drum mechanical filter that stops the largest particles and ejects 1 m3 h-1 of loaded water into the surrounding environment. Another 2 m3 h-1 are cleared out by overflowing the pumping pit. These volumes are renewed at a rate of 3% per hour by pumping saltwater from an underground source. Crossflow filtration was performed on rejections from both the mechanical filter and pumping pit overflow. SM contents and granulometric distributions determined by laser diffractometry were found to vary with sample source and withdrawal time, and size of fish in the pens. A comparison of granulometric distributions in volume percent and numerical percent underscores the presence of a great number (> 98 %) of submicron particles.To limit the risk of mechanical-pore fouling due to blockage by particles, organic membranes in the form of internal-skin capillaries (pore diameters of about 10 to 20 nm) were initially employed. These membranes, used in drinking water production, are relatively inexpensive. The experimental device was fitted with an interchangeable volumetric pump (with gears or monoscrew). Adjustable parameters were transmembrane pressure and circulation velocity within the module. Analysis of the influence of these hydrodynamic parameters revealed that pressures higher than 1 bar were unnecessary, as beyond this point permeate flux no longer increased. Optimal flux did not exceed 100 L h-1 m-2 with the gear pump. Replacing the latter with a monoscrew pump improved permeate flux up to 70 %.Tests were also performed with flat microfiltration organic membranes of polyvinylidene fluoride (PVDF) with pore diameters ranging from 0.1 to 8 µm. The flux obtained with these membranes was roughly 250 L h-1 m-2 and presented little variation with varying pore diameter. Comparative tests carried out on tubular membranes showed lower fluxes than those obtained with organic membranes which, considering their much higher cost, makes them less attractive in this context. The use of membranes in aquafarming is without precedent. An economic analysis of the practice was carried out based on financial assessments of processing of surface waters into drinking water, for which outputs to be treated and SM contents were of the same order of magnitude. With operating costs from 0.35 to 0.95 FF per cubic meter of filtered water, expected investment for a fishfarm producing 100 tons of fish a year is currently 3 to 4 times too great to consider economically profitable the use of membranes for water treatment in closed-circuit aquafarming

Model for the on-site matrix elements of the tight-binding hamiltonian of a strained crystal: Application to silicon, germanium and their alloys

We discuss a model for the on-site matrix elements of the sp3d5s* tight-binding hamiltonian of a strained diamond or zinc-blende crystal or nanostructure. This model features on-site, off-diagonal couplings between the s, p and d orbitals, and is able to reproduce the effects of arbitrary strains on the band energies and effective masses in the full Brillouin zone. It introduces only a few additional parameters and is free from any ambiguities that might arise from the definition of the macroscopic strains as a function of the atomic positions. We apply this model to silicon, germanium and their alloys as an illustration. In particular, we make a detailed comparison of tight-binding and ab initio data on strained Si, Ge and SiGe.Comment: Submitted to Phys. Rev.

How reproducible is the acoustical characterization of porous media?

There is a considerable number of research publications on the characterization of porous media that is carried out in accordance with ISO 10534-2 (International Standards Organization, Geneva, Switzerland, 2001) and/or ISO 9053 (International Standards Organization, Geneva, Switzerland, 1991). According to the Web of Science(TM) (last accessed 22 September 2016) there were 339 publications in the Journal of the Acoustical Society of America alone which deal with the acoustics of porous media. However, the reproducibility of these characterization procedures is not well understood. This paper deals with the reproducibility of some standard characterization procedures for acoustic porous materials. The paper is an extension of the work published by Horoshenkov, Khan, Bécot, Jaouen, Sgard, Renault, Amirouche, Pompoli, Prodi, Bonfiglio, Pispola, Asdrubali, Hübelt, Atalla, Amédin, Lauriks, and Boeckx [J. Acoust. Soc. Am. 122(1), 345-353 (2007)]. In this paper, independent laboratory measurements were performed on the same material specimens so that the naturally occurring inhomogeneity in materials was controlled. It also presented the reproducibility data for the characteristic impedance, complex wavenumber, and for some related pore structure properties. This work can be helpful to better understand the tolerances of these material characterization procedures so improvements can be developed to reduce experimental errors and improve the reproducibility between laboratories

A simple equivalent plate model for dynamic bending stiffness of three-layer sandwich panels with shearing core

Equivalent or condensed plate models are being used in various industries to reduce the computation time in finite element modelling. Out of the available equivalent plate models, the model developed by J.L.Guyader in 1978 exhibits high agreement with Lamb wave theory but it requires some time for implementation. Therefore, in this paper, a simple model is proposed to quickly compute the dynamic equivalent parameters of a three-layer sandwich panel. Although the model is formulated from only four parameters, which could be easily computed via the asymptotic and transition behaviours of the sandwich panel, it is shown to be able to capture the equivalent dynamic response for the entire frequency range

Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst

Electrocatalysts for oxygen reduction are a critical component that may dramatically enhance the performance of fuel cells and metal-air batteries, which may provide the power for future electric vehicles. Here we report a novel bio-inspired composite electrocatalyst, iron phthalocyanine with an axial ligand anchored on single-walled carbon nanotubes, demonstrating higher electrocatalytic activity for oxygen reduction than the state-of-the-art Pt/C catalyst as well as exceptional durability during cycling in alkaline media. Theoretical calculations suggest that the rehybridization of Fe 3d orbitals with the ligand orbitals coordinated from the axial direction results in a significant change in electronic and geometric structure, which greatly increases the rate of oxygen reduction reaction. Our results demonstrate a new strategy to rationally design inexpensive and durable electrochemical oxygen reduction catalysts for metal-air batteries and fuel cells.close34

Robustness of the charge-ordered phases in IrTe2{\mathrm{IrTe}}_{2} against photoexcitation

We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe2, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occurring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches time scales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low- temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved