Pierre Delage

Trung-Tinh Le

Anh-Minh Tang

Yu-Jun Cui

Xiang-Ling Li

Suction effects in deep Boom clay block samples

Keywords: Clays, Laboratory tests, Radioactive waste disposal, Sampling, Suction

. The various experimental data gathered in this study finally evidenced a relatively good state of conservation of the block sample used.

Introduction

Various investigations have been carried out on Boom clay, a stiff clay from Belgium, in the context of research into deep nuclear waste disposal (SAFIR 2, 2001). Investigations are linked to research conducted at the Mol Underground Research Laboratory (URL), excavated in a layer of Boom clay at a depth of 223 m by SCK-CEN, the Belgian organisation for nuclear studies, near the city of Mol.

Most research has been carried out on triaxial specimens that were trimmed from blocks extracted during excavation sequences in the URL. Various questions arose concerning the quality of the specimens and the best procedure to adopt prior to running triaxial tests [START_REF] Winter | Specimen preparation technique for a very stiff clay: A technical note[END_REF][START_REF] Coll | Endommagement des roches argileuses et perméabilité induite au voisinage d'ouvrages souterrains[END_REF]. Special concern was related to the great depth at which the blocks were extracted, to the corresponding stress release and to the resulting suction [START_REF] Skempton | Horizontal stresses in an over-consolidated Eocene clay[END_REF][START_REF] Skempton | The behaviour of saturated clays during sampling and testing[END_REF][START_REF] Doran | Estimation of in-situ stresses using anisotropic elasticity and suction measurements[END_REF], obviously larger than in common geotechnical engineering practice. The swelling observed in Boom clay specimens when releasing suction by putting them in contact with saturated porous stones in the triaxial cell also provided some concerns because of possible alteration of the natural initial microstructure [START_REF] Sultan | Etude du comportement thermo-mécanique de l'argile de Boom: expériences et modélisation[END_REF] with some consequence for the mechanical properties of the clay, and more particularly the overconsolidation ratio.

In this paper, suction effects in deep Boom clay block samples are investigated through the characterisation of the water retention and of the swelling properties of the clay.

The relationship between suction and stress changes during loading and unloading sequences are also examined by running oedometer tests with suction measurements. Finally, the effect of suction release under an isotropic stress close to the estimated suction is investigated.

Material and experimental methods

The study was carried out on intact Boom clay specimens extracted at a depth of 223 m in the URL of Mol, Belgium. The Boom clay formation belongs to the Rupelian geological period in the Tertiary sub-era, which dated from 36 to 30 million years before present. Its thickness is around 102 m in the Mol area (Figure 1) and the layer is gently dipping (± 1°) towards the North-North-East [START_REF] Mertens | The origin and development of joints in the Boom clay formation (Rupelian) in Belgium[END_REF]. As commented by [START_REF] Horseman | Geotechnical characterisation of Boom clay in relation to disposal of radioactive waste[END_REF] cited in [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF], the clay is geologically lightly overconsolidated but the yield stress of Boom clay may be larger than the preconsolidation pressure due to mechanisms such as creep and diagenesis, resultanig in an "overconsolidation" ratio R OC = 2.4.

Table 1 presents the mineralogical composition of Boom clay, taken from different works [START_REF] Decleer | Mineralogical characteristics of the Rupelian Boom clay[END_REF][START_REF] Al Mukhtar | The fabric of a clay soil under controlled mechanical and hydraulical stress states[END_REF]SAFIR 2, 2001). The clay fraction is dominant (50% to 60% < 2 µm) but some differences exist in the smectite fraction between and SAFIR 2 on the other hand (17%). Other differences are observed in the kaolinite and illite fractions. These differences can be due to the natural variability of Boom clay. They show however that some confirmation about the mineralogical composition of Boom clay is still needed. The geotechnical properties of Boom clay are shown in Table 2. In-situ water content measurements were made on excavated blocks during the excavation in the URL (connecting gallery, excavated between 23 January and 23 April 2002). Average values between 24.5% and 25.5% were obtained at that time. Excavated blocks were immediately vacuum packaged in reinforced aluminium foil and thermo-welded. Blocks were stored in a room with temperature varying between 15 and 20°C and an average relative humidity of 45%. The laboratory water content determination that was made during this study (in 2005) gave smaller values of between 20.2 and 21.6%. The difference in water content between the two measurements is related to some drying that may have occurred during the sample storage between 2002 and 2005. As compared to the initial suction at the time when the sample was excavated, this drying obviously corresponds to an increase in suction that will be further commented based on the water retention properties of the sample.

The water retention properties were determined on both rectangular clay samples (30x30x10 mm approximately) and cylindrical oedometer samples (d = 70 mm, h = 20 mm).

Starting from initial water contents close to 21%, the rectangular clay samples were dried at controlled values of suction by using the vapour equilibrium method (see for instance [START_REF] Delage | The relationship between suction and swelling properties in a heavily compacted saturated clay[END_REF] with 5 saline solutions: CuSO 4 (s = 2.8 MPa); K 2 SO 4 (s = 4.2 MPa); KNO 3 (s = 8.5 MPa); NaCl (s = 37.8 MPa) and MgCl 2 (s = 152.8 MPa). Along the drying path, three specimens were used at each suction level to determine the water content at equilibrium.

Along the wetting path, the three oedometer specimens were smoothly wetted by putting them in contact with humid filter papers and the resulting suction was afterwards measured by using a tensiometer. The volume changes of the oedometer samples were determined with a precision calliper. The volume changes of the rectangular specimens were determined by hydrostatic weighing after having immersed the samples in a non aromatic hydrocarbon liquid called Kerdane.

An oedometer cell equipped with a high suction tensiometer (capable of measuring to 1500 kPa) was used to measure suction changes during oedometer compression, as suggested by [START_REF] Dineen | A new approach to osmotically controlled oedometer testing[END_REF] (Figure 2). The principle of the high suction tensiometer was first reported by [START_REF] Ridley | A new instrument for the measurement of soil moisture suction[END_REF]. The tensiometer used in this work was built at ENPC-CERMES. As seen in the figure, the tensiometer is located within the base of the oedometer cell in contact with the lower face of the sample, whereas the upper face of the sample is in contact with a porous stone that is fixed to the piston.

Finally, an isotropic compression test was carried out in a high-pressure triaxial cell (see [START_REF] Delage | On the thermal consolidation of Boom clay[END_REF] on a standard triaxial specimen (d = 38 mm, h = 76 mm). The confining pressure and the back-pressure were applied by two GDS volume/pressure controllers. During the test, the sample volume changes were measured by monitoring the volume of water in the cell through the pressure/volume controller used for the confining pressure. As seen further on, this was necessary for i) controlling the volume changes during compression sequence with dry porous stones (i.e. with no water permitted to infiltrate inside the sample) and ii) controlling the swelling when water was allowed to hydrate the sample through the porous stones. In high pressure metal cells, this method appeared to give satisfactory results [START_REF] Delage | On the thermal consolidation of Boom clay[END_REF].

Experimental results

Water retention curve and swelling behaviour

Figure 3 shows some aspects of the water retention properties of Boom clay presented in terms of changes in water content (w) and of degree of saturation (S r ) as a function of the logarithm of suction (log s). As described previously, starting from initial water contents of 20.2 -21.6%, three points were obtained along a wetting path (with measured suctions equal to 180, 280 and 600 kPa respectively) and five suctions were used along a drying path (with imposed suctions equal to 2. 8, 4.2, 8.5, 37.8 and 152.8 MPa and with three specimens at each suction level). The data obtained along the drying path show a good compatibility between the various points obtained under the same suction, both in terms of water content and degree of saturation.

The S r -log s plot shows that the two points obtained along the drying path at the two low suctions (2.8 and 4.2 MPa) indicate that the samples remained saturated. Desaturation starts above 4.2 MPa and the degree of saturation at a 8.5 MPa suction is 90%. As shown in the figure, the air entry value of Boom clay can be estimated at approximately 5 MPa. At the highest suction (152.8 MPa), the degree of saturation is equal to 31%.

Along the wetting path, the curve shows that, curiously, the degree of saturation of the oedometer samples decreases below 100%, with values lying between 90 and 100%. Since the volume change measurements carried out by hydrostatic weighing on the samples subjected to suctions of 2.8 and 4.5 MPa showed that the sample remained saturated, it is believed that the initial state of the sample close to w = 21% was saturated (unfortunately, no determination of the degree of saturation was made in the initial state). Hence, the samples subjected to suctions lower than the initial one that reached higher values of water content (22.5, 29 and 29.5%) should not desaturate. The value of degree of saturation reported in the figure along the wetting path hence appear to show some discrepancy due to the lack of precision of the precision calliper volume measurements, as compared to hydrostatic weighing. Under the hypothesis of saturated state, the increase in water content obtained along the wetting path corresponds to some swelling that will be further investigated later on. Conversely, up to the air entry value pressure (5 MPa) drying occurs with some shrinkage under a saturated state.

The curve follows the main drying path at suction higher than 5 MPa when the sample starts desaturating. At a suction as high as 152.8 MPa, Boom clay is able to retain 5% water content.

Referring to Table 1, this could indicate a high smectite content, probably closer to 33% than 17%.

Water retention data obtained by [START_REF] Bernier | Suction-controlled experiments on Boom clay[END_REF] and [START_REF] Romero | Water permeability, water retention and microstructure of unsaturated compacted Boom clay[END_REF] on compacted Boom clay samples at a dry unit mass of 1.7 Mg/m 3 are also represented for comparison. The data show that the curve of [START_REF] Romero | Water permeability, water retention and microstructure of unsaturated compacted Boom clay[END_REF] is parallel, with less water being retained by the compacted sample at same suction. The curve of [START_REF] Bernier | Suction-controlled experiments on Boom clay[END_REF] is similar to that of [START_REF] Romero | Water permeability, water retention and microstructure of unsaturated compacted Boom clay[END_REF] at high suction.

Figure 4 shows the volume changes with respect to suction that correspond to the drying and wetting paths of Figure 3. A significant swelling of 18% is observed when suction is reduced to 180 kPa. A shrinkage of 15% is observed at a suction of 152.8 MPa. The slope that characterises swelling (average slope

It is difficult to provide a value of the initial suction of the sample just after extraction

based on the data from the curve of Figure 3. The drying that reduced the water content from an average in-situ value close to 25% down to an average value of 21% after the storage period obviously increased the sample suction.The suction given by the figure at a water content close to the in-situ water content (25%) is equal to 400 kPa.

As quoted by various authors including [START_REF] Skempton | The behaviour of saturated clays during sampling and testing[END_REF], [START_REF] Bishop | The influence of pore water tension on the strength of a clay[END_REF] and [START_REF] Doran | Estimation of in-situ stresses using anisotropic elasticity and suction measurements[END_REF], there is a relation between the suction of a saturated sample and the state of stress at the depth at which the sample has been extracted. In the case of the "perfect sampling" [START_REF] Bishop | The influence of pore water tension on the strength of a clay[END_REF]) of an isotropic elastic sample, the suction of the sample is equal to the mean effective stress supported by the sample prior to extraction. [START_REF] Doran | Estimation of in-situ stresses using anisotropic elasticity and suction measurements[END_REF] showed however that this was no longer true in anisotropic elastic soils even after "perfect sampling" due to coupling of volumetric strain with deviator stress during unloading.

Due to the depth at which it has been sampled (223 m) and to the high corresponding in situ vertical effective stress (estimated at 2.45 MPa with an average soil unit mass ρ = 2.1 Mg/m 3 at a depth of 223 m), Boom clay can certainly be considered as a cross anisotropic material. This is also suggested by the scanning electron microscope observations carried out by [START_REF] Dehandschutter | Britlle fractures and ductile shear bands in argillaceous sediments: inferences from Oligocen Boom Clay (Belgium)[END_REF] in which bedding planes were clearly observed (note also that the Boom clay layer is gently dipping (±1°) towards the North-North-East -see Mertens et al. (2004) and Figure 1).

Based on in-situ stresses estimations, [START_REF] Horseman | Triaxial experiments on Boom clay[END_REF] calculated for Boom clay a K 0 value equal to 0.8. An equivalent effective mean stress (p') can be derived from the vertical load (σ' v ) as follows:

MPa 12 . 2 ' 87 . 0 ) ' 2 ' ( 3 1 ' p v h v = = + = σ σ σ [1]
This value is significantly higher than the suction of 400 kPa that could be estimated at w = 25% on Figure 3. The 400 kPa value seems to be too small to be explained by possible effects related to non perfect sampling and to cross anisotropy and a deeper examination of the water retention properties (Figure 3) appears to be necessary. In this regard, Figure 5 presents a zoom of the water retention curve in which an approximate estimated initial state has also been plotted by adopting the mean effective stress calculated above (2.21 MPa) as an approximate suction at a water content of 25%. Based on this hypothesis, the drying process that occurred during storage can be interpreted. The figure shows that the position of the initial point is reasonably compatible with the drying curve of the water retention curve. Note that, given the shape of the curve, this is also true for suction values down to 1 MPa. Also, the shape of the wetting path determined by the three points obtained at low suction in the oedometer illustrates an hysteresis effect typical of clays, as observed for instance by [START_REF] Croney | The movement and distribution of water in soils[END_REF] on a "heavy clay soil" (w p = 26%, w L = 78%) that had been subjected to a suction cycle starting from its initial "undisturbed" state. In other words, at the same water content, the suction reached by a sample after a drying/wetting cycle can be significantly lower than the initial suction.

An estimation of the sample suction with a value of water content close to 21% can now be made by considering the estimated drying path of Figure 5. Accounting for the uncertainty concerning the initial state, the Figure shows that this value should be between 2 and 3 MPa. This range would also have been obtained by starting from an initial value of 1 MPa.

Oedometer tests

The three samples used for determining the wetting path were then used for oedometer testing, with initial suction values equal to 180, 280 and 600 kPa respectively (Figure 3). The results of the oedometer compression test with suction measurement carried out with an initial suction of 280 kPa are presented as a function of time in Figure 6 Figure 6b shows that each loading step induces an instantaneous settlement followed by an equilibration stage. Interestingly, instantaneous coupled peaks in positive pressure are observed at each loading step (Figure 6c), even when starting from an initial negative pore pressure. For vertical stresses smaller than 800 kPa, measured pore water pressures come back and stabilise at negative values. The preservation of a suction state within the sample means that no water has been expelled from the sample, resulting in a constant water content condition.

The transition from negative to positive pressure was observed when loads greater than 800 kPa were applied. Water pressure stabilised at 0 kPa after the 800 and 1600 kPa loading stages. This shows that the suction/pressure gauge worked properly in the positive pressure range, describing a standard positive pore pressure dissipation process and a decrease in water content of the sample with water expelled in the porous stone located above the sample. Similarly, instantaneous coupled pressure decreases are observed during unloading step, with suction as high as 700 kPa reached when passing from 1600 kPa to 800 kPa vertical stress. Note that this instantaneous suction value is close to the load release (800 kPa) which is reasonably compatible with saturated "inverse" consolidation.

The results of the three oedometer tests are presented in Figure 7 in diagrams giving the changes in vertical stress with respect to suction changes, once equilibrium has been reached. These diagrams show how suction is reduced by loading and then increased by unloading, starting from three different initial suctions.

During compression, a linear relation between changes in suction and vertical load is observed with slopes ds/dσ v that vary between -0.45 and -0.56. Compression paths also show that zero suction is reached at smaller loads when starting from smaller initial suctions, s i : (a) σ vn = 1200 kPa for s i = 600 kPa; (b) σ vn = 600 kPa for s i = 280 kPa; (c) σ vn = 450 kPa for s i = 180 kPa. The slopes obtained during unloading were higher and varied from -0.59 to -0.80. The variability between the different slopes measured in Figure 7 is related to the different initial void ratios of the samples, given in Figure 4. Both samples hydrated at smaller initial suction (180 and 280 kPa) exhibited around 18% swelling with similar void ratios respectively equal to 0.86 and 0.87. For both samples, the slopes in the unloading stage (-0.74 at 180 kPa and -0.80 at 280 kPa) are larger than in the loading stage (-0.45 at 180 kPa and -0.53 at 280 kPa) with slopes slightly smaller at lower suction. The 600 kPa suction sample that exhibited a significantly smaller swelling (4%, leading to a void ratio of 0.64) has the highest value of slope during the loading phase (-0.56) and the lowest in the unloading phase (-0.59). The variability of the slopes in the loading phase (between -0.56 and -0.45) is less than in the unloading phase (between -0.59 and -0.80).

The range of the slopes determined during the unloading sequences (between -0.59 and -0.80) gives an idea of the changes in suction that occur in Boom clay samples when releasing the vertical stress. They can be compared by the value of s/σ' v obtained by [START_REF] Bishop | The influence of pore water tension on the strength of a clay[END_REF] for natural soils, in which s is the sample suction and σ' v the in-situ vertical effective stress. Based on the sample suction obtained here (between 2 and 3 MPa), values of s/σ' v between 0.94 and 1.42 are obtained. These too high values show the effects of drying.

The resulting uncertainty makes it difficult to use for a precise determination of suction changes induced by stress release.

Suction release under isotropic stress

While wetting Boom clay sample at low suctions under zero stress, a swelling of around 18% due to suction release has been observed (Figure 4). This swelling is consistent with the 11-14% swelling found by [START_REF] Coll | Endommagement des roches argileuses et perméabilité induite au voisinage d'ouvrages souterrains[END_REF] under a low effective pressure in the triaxial apparatus. This saturation procedure and the resulting swelling is likely to alter the initial state of the soil and to lead to unexpected low values of the yield stress, as shown by Horseman et al. (1993) and [START_REF] Sultan | Etude du comportement thermo-mécanique de l'argile de Boom: expériences et modélisation[END_REF]. After [START_REF] Graham | Stress release, undrained storage, and reconsolidation in simulated underwater clay[END_REF], during saturation under the in-situ state of stress, suction reduces to zero and no further swelling should occur, allowing to put the sample in contact with water and to apply a back pressure in a standard manner with no significant modification of the initial microstructure.

A test in which suction was released under a constant isotropic stress was carried out on a sample having an initial water content of 21.6% (Figure 8). In order to minimise the volume change during soaking, an isotropic stress close to the estimated initial sample suction was chosen. Based on the range of suctions estimated from the water retention curve (2 -3 MPa), it was decided to adopt an isotropic stress of 2.5 MPa. This value is larger than the insitu mean effective stress estimated previously (2.12 MPa). The sample was isotropically compressed from 0.1 MPa to 2.5 MPa while keeping the porous stones dry. The porous stones were then soaked and a low back pressure (50 kPa) was applied to the soil sample while measuring volume changes. The confining pressure and the back pressure were afterwards simultaneously increased in order to keep the effective stress equal to 2.5 MPa, the final back pressure being equal to 1 MPa. Finally, the soil sample was compressed up to an effective pressure of 10 MPa with a constant low pressure change rate (0.5 kPa/min). [START_REF] Sultan | Temperature effects on the volume change behaviour of Boom clay[END_REF] showed that this rate was slow enough to ensure drained conditions in Boom clay. The corresponding experimental data are shown in Figure 8. Under 2.5 MPa, a slight swelling is observed during saturation (ε v =1.7%), that corresponds to an increase in void ratio from 0.582 to 0.608. This swelling is much smaller than that (14%) obtained after saturation at low confining pressure by [START_REF] Coll | Endommagement des roches argileuses et perméabilité induite au voisinage d'ouvrages souterrains[END_REF]. Furthermore, the yield stress (p y ') obtained in Figure 8 is close to 5 MPa; giving an overconsolidation ratio R OC = 2.1, in good agreement with the data of [START_REF] Horseman | Triaxial experiments on Boom clay[END_REF]: R OC = 2.4) and Coll (2005: R OC = 2.2). Note however that, as commented by [START_REF] Horseman | Geotechnical characterisation of Boom clay in relation to disposal of radioactive waste[END_REF] cited in [START_REF] Burland | On the compressibility and shear strength of natural clays[END_REF] based on geological evidence, the yield stress of Boom clay may be larger than the preconsolidation pressure due to mechanisms such as creep and diagenesis.

By saturating the sample under low effective pressure [START_REF] Sultan | Etude du comportement thermo-mécanique de l'argile de Boom: expériences et modélisation[END_REF] obtained an underestimate value p y ' = 0.4 MPa. This showed that swelling may alter the natural microstructure of a swelling clay and erase the memory of the stress history by reducing significantly the overconsolidation ratio.

The data of the test of Figure 8 confirm, as suggested by [START_REF] Graham | Stress release, undrained storage, and reconsolidation in simulated underwater clay[END_REF], that some precautions have to be taken before releasing the suction of a natural sample. Note however that the slight swelling observed during the test indicate that the initial suction might be higher than 2.5 MPa.

Conclusions

Suction effects were investigated in intact Boom clay block samples extracted at great depth (223 m) in the underground research laboratory of Mol (Belgium). Suction effects in deep intact samples are considered to be significant because of the relationship between suction and the in-situ stress state of the sample, as shown by various authors [START_REF] Skempton | Horizontal stresses in an over-consolidated Eocene clay[END_REF][START_REF] Skempton | The behaviour of saturated clays during sampling and testing[END_REF][START_REF] Doran | Estimation of in-situ stresses using anisotropic elasticity and suction measurements[END_REF]. Suction effects were investigated through the characterisation of the water retention and of the swelling properties of intact Boom clay. Some drying that occurred during the storage period between the block extraction and the present experimental investigation was interpreted in terms of hysteresis effects. The sample suction was estimated to be between 2 and 3 MPa. The swelling-shrinkage behaviour under changes in suction was investigated and an air entry value of 5 MPa was determined.

The relationship between suction and stress changes during loading and unloading sequences was also examined by running oedometer tests with suction measurements. Slopes ds/dσ v between -0.59 and -0.80 were obtained in the unloading phases, to compare to the range 0.35 -0.75 of values of s/σ' v values proposed by [START_REF] Bishop | The influence of pore water tension on the strength of a clay[END_REF] for natural soils.

The s/σ' v values obtained with the block sample used in this work was in the range 0.94 -1.42. These too high values showed the negative effects of drying.

The effect of suction release under an isotropic stress close to the estimated suction (2.5 MPa) was finally investigated. A slight swelling (1.7%) was observed, and a further compression sequence showed that a satisfactory overconsolidation ratio (2.1) was obtained.

This result confirmed the importance of taking some precautions before putting a swelling soil in contact with water prior to triaxial testing, as suggested by [START_REF] Graham | Stress release, undrained storage, and reconsolidation in simulated underwater clay[END_REF].

These data confirmed the importance of suction and suction release effects, particularly in deep swelling samples of stiff clay. Obviously, some of the conclusions drawn here should be confirmed or improved by running similar tests on fresh samples just after extraction, in order to get rid of drying effects.

List of tables

Table 1. Mineralogical composition of Boom clay according to [START_REF] Decleer | Mineralogical characteristics of the Rupelian Boom clay[END_REF][START_REF] Al Mukhtar | The fabric of a clay soil under controlled mechanical and hydraulical stress states[END_REF] and Water content (w)

List of Notations

w [START_REF] Romero | Water permeability, water retention and microstructure of unsaturated compacted Boom clay[END_REF] w [START_REF] Bernier | Suction-controlled experiments on Boom clay[END_REF] Degree

  et al. (1997) found a similar trend on compacted Boom clay specimens subjected to change in suction under a small vertical load in the oedometer.

  as follows: (a) loading sequence (standard oedometer step loading with loading stages generally close to 24h); (b) vertical displacement; (c) pore water pressure changes, starting from an initial value of -280 kPa.

Figure 4 .

 4 Figure 1. Boom clay formation in the North of Belgium. The Underground Research Laboratory is located near the city of Mol. The clay formation is gently dipping (±1°) towards the North North-East (Mertens et al., 2003). ............................................................................................................................................................ 21 Figure 2. Oedometer cell equipped with a tensiometer at the basis. ..................................................................... 21 Figure 3. Water retention properties of Boom clay sample in terms of changes in water content and degree of saturation versus suction (wetting and drying paths followed respectively starting from a 21% initial water content). ........................................................................................................................................................ 22 Figure 4. Volume changes of Boom clay samples under suction changes during drying and wetting.................. 22 Figure 5. Hysteresis effects in the lower suction range......................................................................................... 23 Figure 6. Oedometer compression test with suction measurement. Vertical stress (a), vertical displacement (b) and suction changes (c) are given as a function of elapsed time (initial suction 280 kPa)............................ 24 Figure 7. Suction variation under vertical stress change during oedometer compression for three tests with initial suction equal to 600, 280 and 180 kPa respectively...................................................................................... 25 Figure 8. Isotropic compression test. .................................................................................................................... 26

Figure 1 .

 1 Figure 1. Boom clay formation in the North of Belgium. The Underground Research Laboratory is located near the city of Mol. The clay formation is gently dipping (±1°) towards the North North-East[START_REF] Mertens | The origin and development of joints in the Boom clay formation (Rupelian) in Belgium[END_REF].

Figure 2 .

 2 Figure 2. Oedometer cell equipped with a tensiometer at the basis.

Figure 3 .

 3 Figure3. Water retention properties of Boom clay sample in terms of changes in water content and degree of saturation versus suction (wetting and drying paths followed respectively starting from a 21% initial water content).

Figure 4 .

 4 Figure 4. Volume changes of Boom clay samples under suction changes during drying and wetting.

Figure 5 .Figure 7 .

 57 Figure 5. Hysteresis effects in the lower suction range

  SAFIR 2 (2001). ..................................................................................................................................... 20 Table2. Geotechnical properties of Boom clay according to[START_REF] Belanteur | Étude des comportements mécanique, thermo-mécanique et hydro-mécanique des argiles gonflantes et non gonflantes fortement compactées[END_REF] andDehandschutter et al. (2005)............................................................................................................................................................ 20 Figure 1. Boom clay formation in the North of Belgium. The Underground Research Laboratory is located near the city of Mol. The clay formation is gently dipping (±1°) towards the North North-East (Mertens et al., 2003). ............................................................................................................................................................ 21 Figure 2. Oedometer cell equipped with a tensiometer at the basis. ..................................................................... 21 Figure 3. Water retention properties of Boom clay sample in terms of changes in water content and degree of saturation versus suction (wetting and drying paths followed respectively starting from a 21% initial water content). ........................................................................................................................................................ 22

	List of figures

Table 1 . Mineralogical composition of Boom clay according to Decleer et al. (1983), Al Mukhtar et al. (1996) and SAFIR 2 (2001).

 1

	d	sample diameter
	e	void ratio
	h	sample height
	K 0 coefficient of earth pressure at rest
	p c ' preconsolidation pressure
	R OC overconsolidation ratio
	s	suction
	s i	initial suction
	S r	degree of saturation
	w	water content
	w i	initial water content
	ε V volumetric deformation
	σ h horizontal stress
	σ v vertical stress
	σ' v vertical effective stress
	σ vn vertical stress when suction decreases to 0

Table 2 . Geotechnical properties of Boom clay according to Belanteur et al. (1997) and Dehandschutter et al. (2005).

 2


Manon Kania

Mathieu Gautier
email: mathieu.gautier@insa-lyon.fr

Denise Blanc

Maria Lupsea-Toader

Laurent Merlot

Maria- Chiara Quaresima

Rémy Gourdon

Kania Manon

Gautier Mathieu

Blanc Denise

Lupsea-Toader Maria

Quaresima Merlot Laurent

Gourdon Rémy

Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland

Keywords: French Constructed Wetlands, Sludge deposit, Trace elements release, Acid and Basic Neutralization Capacity, Organic matter, Sequential extraction

Highlights

 Leaching of inorganic elements into aqueous solution was very low at neutral pHs.

 The high organic content of the sample influenced the leaching of several trace metals.

 Copper was the trace metal most associated to organic matter in the sample.

 Copper pH-dependent leaching followed the leaching pattern of organic components.

 Leaching of organic compounds and associated trace metals was increased at alkaline pHs.

Introduction

Constructed wetlands are considered among the technologies best adapted to domestic wastewater or storm water treatment from small communities. In France, the development of Vertical Flow Constructed Wetlands (VFCWs) has been considerable since the end of the 90s.

French VFCWs have proved to be efficient for the treatment of carbon and nitrogen [START_REF] Kim | Performance evaluation of partially saturated vertical-flow constructed wetland with trickling filter and chemical precipitation for domestic and winery wastewaters treatment[END_REF][START_REF] Molle | How to treat raw sewage with constructed wetlands: An overview of the French systems[END_REF][START_REF] Morvannou | Treatment performances of French constructed wetlands: results from a database collected over the last 30 years[END_REF]. For phosphorus removal, additional treatment stages are necessary to guarantee low level release [START_REF] Molle | Traitement des eaux usées domestiques par marais artificiels: etat de l'art et performances des filtres plantés de roseaux en France[END_REF][START_REF] Kim | Performance evaluation of partially saturated vertical-flow constructed wetland with trickling filter and chemical precipitation for domestic and winery wastewaters treatment[END_REF]. One of the solutions is to precipitate dissolved phosphates by injecting FeCl3 before the percolation of the influent through the filter stages. The precipitates thus formed, mixed with the suspended solids from the influent, progressively accumulate at the surface of the French VFCW in the form of a sludge layer [START_REF] Chazarenc | Influence of surface layer on hydrology and biology of gravel bed vertical flow constructed wetlands[END_REF][START_REF] Molle | Traitement des eaux usées domestiques par marais artificiels: etat de l'art et performances des filtres plantés de roseaux en France[END_REF]. Several studies have already been published on sludge characteristics and their evolutions in reed bed treatment systems [START_REF] Caicedo | Comparison of heavy metal content in two sludge drying reed beds of different age[END_REF][START_REF] Matamoros | Musk fragrances, DEHP and heavy metals in a 20 years old sludge treatment reed bed system[END_REF][START_REF] Nielsen | Sludge reed bed facilities: operation and problems[END_REF][START_REF] Nielsen | Sludge quality after 10-20 years of treatment in reed bed systems[END_REF]. However, there is still a lack of specific studies focused on surface sludge deposits from French VFCWs.

Besides phosphorous and nitrogen issues, a diversity of metallic pollutants are transported in wastewater streams [START_REF] Vriens | Quantification of Element Fluxes in Wastewaters: A Nationwide Survey in Switzerland[END_REF] and may accumulate in this layer via (co)-precipitation, complexation or adsorption mechanisms onto particulate organic matter (OM) and/or mineral phases such as clays, Fe-, Mn-or Al-oxides and oxy-hydroxides. These retention mechanisms have positive effects when considering the treatment performance of a French VFCW [START_REF] Gill | Long term heavy metal removal by a constructed wetland treating rainfall runoff from a motorway[END_REF]. However over the years of operation of the French VFCW, the sludge layer may undergo various biophysicochemical changes (pH; Eh), which may influence the fate of trace metals and/or organic matter [START_REF] Grybos | Increasing pH drives organic matter solubilization from wetland soils under reducing conditions[END_REF], leading to potential environmental risks. For example, acidification of the sludge layer may arise from (i) exposure to acidic rainfalls, with pH reported to range from 4.4 to 5.7 in eastern France [START_REF] Sanusi | Chemical composition of rainwater in Eastern France Atmospheric environment[END_REF] or 2.89 in some parts of China [START_REF] Du | Effect of acid rain pH on leaching behavior of cement stabilized lead-contaminated soil[END_REF], (ii) temporary or seasonal contribution of acidic wastewater, or (iii) malfunctioning of the system leading to flooding of the filters and acidogenic biodegradation of organic matter [START_REF] Kim | pH and Eh effects on phosphorus fate in constructed wetland's sludge surface deposit[END_REF].

In addition, the thickening of the sludge layer over the years of operation may progressively reduce the permeability of the systems. It is commonly considered that surface sludge layer should be dredged from the filters after 10 to 15 years of operation [START_REF] Chazarenc | Influence of surface layer on hydrology and biology of gravel bed vertical flow constructed wetlands[END_REF]. Dredged sludge may be used for land application, provided the associated potential environmental risks are acceptable. European and French regulations have set threshold limits for trace metals contents (Cd, Cr, Cu, Hg, Ni, Pb, Zn) in sludge and in the receiving soil. Total contents is not however the key factor in the potential environmental risk of trace metals release from sludge. Mobility, which is related to their chemical speciation and the physico-chemical local conditions, has a major influence on their potential release to the water phase [START_REF] Yang | Comprehensive risk assessment of heavy metals in lake sediment from public parks in Shanghai[END_REF]. A proper assessment of the potential risks of sludge deposits in land application should therefore include the evaluation of the potential mobility of the contaminants, and the influence of environmental conditions such as acidity or redox potential on their leaching potential [START_REF] Chatain | Determining the experimental leachability of copper, lead, and zinc in a harbor sediment and modeling[END_REF][START_REF] Gonzalez | Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash[END_REF]. Although a large number of studies have been published on the subject in the field of solid waste or contaminated soils management, to our knowledge, no specific study has yet been released on the speciation and mobility of inorganic contaminants present in French VFCW sludge deposits.

This study was therefore conducted to experimentally assess the release of major inorganic elements (Al, Ca, Fe, Si and K) and a selection of trace metals (Cr, Cu, Ni, Zn, Cd and Pb) from surface sludge deposits sampled from a French VFCW sewage treatment plant.

The selected plant was already extensively studied by [START_REF] Kim | Physical-chemical characterization of sludge and granular materials from a vertical low constructed wetland for municipal wastewater treatment[END_REF][START_REF] Kim | Effect of aging on phosphorus speciation in surface deposit of a vertical flow constructed wetland[END_REF] and Kania et al. (2018a andb, 2019). In this purpose, a methodology was developed based on waste management protocols [START_REF] Chatain | Determining the experimental leachability of copper, lead, and zinc in a harbor sediment and modeling[END_REF][START_REF] Gonzalez | Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash[END_REF]. Batch pH-dependent leaching tests were conducted following acid and base neutralization capacity (ANC-BNC) European standard CENTS/TS 14429, in order to investigate the influence of pH conditions on the leaching behavior of the material. This method was complemented by sequential extractions of major and trace elements [START_REF] Chatain | Effect of experimentally induced reducing conditions on the mobility of arsenic from a mining soil[END_REF][START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF] to assess their distribution within the sludge deposits mineralogical phases.

Materials and methods

Origin of the sludge deposits sample

A representative sample of surface deposits was collected from the sludge layer of a full-scale French VFCW sewage treatment plant located in Vercia (France). The plant was being in operation for 8 years at the sampling date. Previous studies described in details the design of the plant and its performance [START_REF] Kim | Performance evaluation of partially saturated vertical-flow constructed wetland with trickling filter and chemical precipitation for domestic and winery wastewaters treatment[END_REF]. The plant was designed and operated according to AZOE-NP ® process, which combines an aerobic trickling filter and ferric chloride injection prior to the classical 2 stages of filters. It was designed for 1100 population equivalent and received mostly domestic wastewaters from the Vercia town, and some winery effluents during autumn.

Surface deposits were sampled using a power shovel to extract the entire depth of the sludge layer at 8 spots of the surface of the first filter. The samples were then treated on site to remove gravels and reeds rhizomes manually, mixed and carefully homogenized by quartering with shovels before being transported to the laboratory within no ≥3 h. There, an aliquote fraction was freeze-dried and then crushed at 1 mm. The powder obtained was carefully homogenized and stored at 4°C until they were used.

Previous studies (Kania et al. 2018a, b, c) conducted on 14 different French VFCW plants, including the one selected here, revealed remarkably similar characteristics of the surface deposits taken from plants of 2-3 years of age and more. This group of samples revealed very strong analogies in terms of organic matter composition and physical properties, and was designated as the "mature deposits" typology. The surface deposits sample selected in the present study was representative of this group.

Chemical composition

The Organic matter (OM) content was determined by combustion at 550 °C until constant mass of a given precise mass of the surface deposits sample (AFNOR -NF EN 15169, 2007). The results was considered to represent the Particulate Organic Matter (POM) since the soluble organic matter was negligible (Kania et al., 2018c). Elemental analyses were performed by alkaline fusion (LiBO2) followed by acid digestion (HNO3) and subsequent analyses of the solutions by inductively coupled plasma atomic emission spectrometry (ICP-AES) (IRIS Advantage ERS, Thermo Scientific). The following elements were analyzed : silicon (Si), aluminum (Al), iron (Fe), manganese (Mn), magnesium (Mg), calcium (Ca), sodium (Na), potassium (K), titanium (Ti) and phosphorus (P). Other trace metals (chromium (Cr), copper (Cu), nickel (Ni), zinc (Zn), cadmium (Cd) and lead (Pb)) were analyzed using a Sciex Perkin Elmer ELAN 5000a. ICP-mass spectrometer.

Carbon and sulfur contents were determined with a HORIBA EMIA 320V2 analyzer. 1 g of dry powdered samples was calcined at 1400 °C under a flow of O2, followed by quantification of carbon and sulfur oxides by infrared absorption.

Sequential chemical extraction

A sequential chemical extraction protocol adapted from [START_REF] Chatain | Effect of experimentally induced reducing conditions on the mobility of arsenic from a mining soil[END_REF][START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF] was carried out in triplicates to determine the distribution of metals in the sludge, with respect to five fractions, namely: (F1) exchangeable fraction, (F2) extractible fraction bound to carbonates, (F3) reducible fraction (bound to Fe-Mn oxides), (F4) oxidizable fraction (bound to organic matter), (F5) and residual fraction. The so-called "reducible" and "oxidizable" fractions corresponded to phases that can be dissolved respectively by reduction (oxides and oxi-hydroxides) or oxidation (sulfides or organic matter). A dry mass of 1 g of sample was extracted in 5 successive stages according to the following protocol:  Oxidizable fraction F4 (bound to organic matter): the solid residue collected from the previous step was mixed with 3 mL of 0.02 M HNO3 and 5 mL of 30 % H2O2 adjusted to pH 2 with HNO3, and the mixture was heated at 85 ± 2 °C for 2 h with occasional agitation. A second 3 mL aliquot of 30 % H2O2, adjusted to pH 2 with HNO3, was then added and the sample was heated again at 85 ± 2 °C for 3 h with intermittent agitation.

After cooling, 5 mL of 3.2 M NH 4 OAc in 20 % (v/v) HNO 3 was added and the sample was diluted to 20 mL and agitated continuously for 30 min. The addition of NH4OAc is designed to prevent adsorption of extracted metals onto the oxidized sample.

 Residual fraction F5: this fraction was calculated as the difference between the cumulated extracted fractions and the total content of each element.

Between each successive extraction, the suspensions were centrifuged at 10 000 rpm for 30 min. After filtration at 0.45 µm, the supernatant was analyzed for trace elements, and the residue was further washed with 20 mL of deionized water and centrifuged for 30 min. Then the solid residue was further extracted with 20 mL of water, whereas the supernatant was discarded. Major and minor and trace elements were then analyzed in the respective solutions by ICP-AES (Ultima 2; Horiba Jobin Yvon SAS).

Experimental leaching protocol

The ANC-BNC leaching protocol was based on the CENTS/TS 14429 European standard, which is used in the domain of waste management. 5 g of dry sludge deposits sample and 50 mL of leachant (liquid-to-solid ratio = 10 g/mL) were mixed in a 250 mL centrifuge tube (polypropylene copolymer, Nalgene ® ). All assays were triplicated. The following aqueous leachants were used to evaluate the effect of pH on the leaching behavior:

 deionized water,  aqueous solutions of hydrochloric acid at different concentrations,  aqueous solutions of potassium hydroxide at different concentrations.

Blanks were also carried out in triplicates under the same conditions as the assays, but without the addition of sample. All the tubes (tests and blanks) were placed for 48 h in a rotary shaker set at a speed of 9 rpm. The contact time was defined according to the European standard (CENTS/TS 14429) and already used for ANC-BNC test conducted on sediment with high content of OM [START_REF] Chatain | Determining the experimental leachability of copper, lead, and zinc in a harbor sediment and modeling[END_REF]. pH was then measured in all suspensions, and the tubes were centrifuged at 10000 rpm for 10 min. The supernatants were filtered at 0.45 µm through Sartorius cellulose acetate filters and stored at 4 °C before analyses. The remaining sludge material was collected from each tube, freeze-dried and stored at 4 °C for further analyses.

Leachates analyses

The leachates were analyzed for Dissolved Total Carbon (DTC) and Dissolved Inorganic Carbon (DIC) using a Shimadzu 505A Carbon analyzer. Dissolved Organic Carbon (DOC) was calculated as the difference between DTC and DIC. Total concentration of major (Al, Ca, Fe, Si and K) and trace elements (Cr, Cu, Ni, Zn, Cd and Pb) were measured in the solutions by ICP-AES (Ultima 2; Horiba Jobin Yvon SAS). Orthophosphate (PO4 3-) ions were analyzed by ion chromatography (Dionex, Model C25).

Ultraviolet-visible (UV-VIS) absorbances were measured at 254, 465, 665 nm using a Shimadzu UV-2450 spectrophotometer. SUVA (specific ultraviolet absorbance) and A1 indexes were then calculated according to Eqs. ( 1 (2) SUVA index was considered as an indicator of the humification of the dissolved organic compounds based on its correlation with the degree of aromaticity [START_REF] Weishaar | Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon[END_REF], according to Eq. ( 3): Aromaticity = 6.52 SUVA+3.63

(3) A1 was proposed as a humification index [START_REF] Orlov | Electron absorption spectra of humic substances[END_REF]. Both humic and fulvic substances absorb at 465 nm, whereas only the more condensed humic substances absorb at 665 nm [START_REF] Rutkowska | Effect of crop rotation and nitrogen fertilization on the quality and quantity of soil organic matter. Soil processes and current trends in quality assessment (Book)[END_REF]. It is usually considered that A 1 ratios below 5 are characteristics of humified samples [START_REF] Zbytniewski | Characterization of natural organic matter (NOM) derived from sewage sludge compost. Part 1: chemical and spectroscopic properties[END_REF].

Results and discussion

Composition of sludge deposits

The elemental composition of the sludge deposit is shown in Table 1. The material was predominantly organic with a POM content of 62.0 ±2.5 % w/w dry matter (DM). Silicon, calcium and iron contents were found to be the major inorganic elements, at concentrations of 5.7, 3.8 and 3.5% w/w DM respectively. The high iron content was attributed to the implementation of ferric chloride injection on the treatment plant, used to remove phosphorus.

Contents in trace metals Zn, Cu, Pb, Cr, Ni and Cd were respectively 1219, 623, 64.2, 57.4, 30.5 and 1.7 mg/kg of DM. These concentrations were all below the French regulatory limits for sludge application on agricultural lands. Except for Cu which was analyzed at higher concentrations here, very similar contents in trace metals were reported in sludge from reed bed systems [START_REF] Caicedo | Comparison of heavy metal content in two sludge drying reed beds of different age[END_REF][START_REF] Matamoros | Musk fragrances, DEHP and heavy metals in a 20 years old sludge treatment reed bed system[END_REF][START_REF] Nielsen | Sludge quality after 10-20 years of treatment in reed bed systems[END_REF]. The trace metals contents were also similar to the concentrations classically reported in activated sludge, ranging from 1 to 10 mg/kg of DM for Cd, 10-100 for Cr, Ni and Pb, and 100-1000 for Zn and Cu [START_REF] Lachassagne | Devenir de micropolluants présents dans les boues d'épuration, du traitement à l'épandage agricole : Application aux micropolluants métalliques (Cd, Cu) et organiques (médicaments) issus du traitement biologique conventionnel d'effluents urbains ou hospitaliers (Becoming of micropollutants in sewage sludge, from treatment to agricultural application: Application to metallic (Cd, Cu) and organic micropollutants (drugs) resulting from the conventional biological treatment of urban or hospital effluents)[END_REF]. The relatively high contents in Zn and Cu observed here may be attributed to anthropic origins such as leaching of roof construction materials for zinc and grape crops pesticide treatment for copper.

Distribution of major and trace elements

The results from the sequential extractions are shown in Fig. 1. The major elements Si, Al and to a smaller extent Fe were found to be predominantly associated to the residual fraction (from 59 to 98 w/w % of DM). This observation indicated that these elements were present mostly as constituents of stable mineral phases, thereby suggesting their low mobility. S was mostly associated to the oxidizable fraction whereas Ca was more evenly distributed in the different fractions. Si, Al, Fe and S were almost not present in the exchangeable fraction (F1), except Ca, probably in the form of calcite. Fe was significantly present in the oxidizable fraction F4 (34 w/w % of DM), probably because of the flocculation of soluble or colloidal organic constituents by FeCl3 used to precipitate phosphates. The presence of calcite in the same surface deposits sample was already shown [START_REF] Kim | Physical-chemical characterization of sludge and granular materials from a vertical low constructed wetland for municipal wastewater treatment[END_REF] and in waste materials with lower organic contents, such as bottom ash or soils [START_REF] Gonzalez | Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash[END_REF][START_REF] Samourgiannidis | Comparison of Two Sequential Extraction Methods and the DTPA Method for the Extraction of Micronutrients from Acidic Soils[END_REF]. it can be seen that 52% were associated to Fe-Mn oxides, 21% to the oxidizable fraction, suggesting possible association to organic matter [START_REF] Bruland | Complexation of cadmium by natural organic ligands in the central North Pacific[END_REF], and 27% to the residual fraction. Cr was mostly associated to the residual fraction (72%) and to a lesser extent to the oxidizable fraction (25%), probably associated to organic matter. Cr(VI) is known to be easily reduced to Cr(III) by OM [START_REF] Di Luca | Metal retention and distribution in the sediment of a constructed wetland for industrial wastewater treatment[END_REF]. Therefore Cr(III), which is less soluble than Cr(VI), was probably largely predominant due to the high OM content in sludge deposits. The association of chromium with organic matter has already been reported in organic material such as tannery sludge [START_REF] Chen | Adsorption behavior comparison of trivalent and hexavalent chromium on biochar derived from municipal sludge[END_REF]. On the contrary in mineral materials, Cr was reported to be mainly associated to the residual fraction, as reported in bottom ash [START_REF] Gonzalez | Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash[END_REF].

Cu and Pb were found mainly within the oxidizable fraction (72 and 65% respectively), probably sorbed to organic matter. The observed high affinity of these elements with OM confirmed previous studies on harbor sediment [START_REF] Chatain | Determining the experimental leachability of copper, lead, and zinc in a harbor sediment and modeling[END_REF] and activated sludges [START_REF] Fuentes | Simple and sequential extractions of heavy metals from different sewage sludges[END_REF]. The formation of stable complexes between copper and organic ligands was reported by several authors [START_REF] Da Silva | Heavy metal distribution in recent sediments of the Tietê-Pinheiros river system in Sao Paulo state, Brazil[END_REF][START_REF] Lu | Partitioning of copper onto suspended particulate matter in river waters[END_REF]. Most of the remaining copper and lead was found in the residual fraction (24% and 32%, respectively) suggesting an association to stable mineral phases such as calco-alumino-silicate phases [START_REF] Fuentes | Simple and sequential extractions of heavy metals from different sewage sludges[END_REF][START_REF] Gonzalez | Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash[END_REF].

Ni and Zn were distributed in all the fractions except F1 (exchangeable fraction). Almost 50% of Ni total content was found in the residual fraction, as already reported in activated sludge by [START_REF] Fuentes | Simple and sequential extractions of heavy metals from different sewage sludges[END_REF]. The rest of Ni content was equally associated to organic matter (26%) and Fe-Mn oxides (24%), as already reported by [START_REF] Jenne | Controls on Mn, Fe, Co, Ni Cu and Zn concentrations in soils and water: the significant role of hydrous Mn and Fe oxides[END_REF] in soils. Previous studies by other authors reported Zn predominance in the carbonates-bound fraction in sediments, activated sludges or bottom ash [START_REF] Di Luca | Metal retention and distribution in the sediment of a constructed wetland for industrial wastewater treatment[END_REF][START_REF] Fuentes | Simple and sequential extractions of heavy metals from different sewage sludges[END_REF][START_REF] Gonzalez | Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash[END_REF].

The results were very consistent with the partitioning of these metals in the sediments from a constructed wetland treating rainfall runoff from a motorway [START_REF] Gill | Long term heavy metal removal by a constructed wetland treating rainfall runoff from a motorway[END_REF]. However, in comparison to these sediments, the trace metals in sludge deposit from French VFCW treating domestic wastewater were observed to be less associated with Fe-Mn oxides and more associated with organic matter. [START_REF] Wiśniowska | The effect of selected acidic or alkaline chemical agents amendment on leachability of selected heavy metals from sewage sludge[END_REF] recently reported data on the distribution of Pb, Cd and Ni in digested sewage sludges. They distinguished mobile (fractions 1 and 2) and not mobile fractions (fractions 3, 4 and 5). The mobile forms of Pb, Cd and Ni in sludge deposit from constructed wetlands were found in much smaller quantity (≤4%) than in digested sewage sludge (in the range 25-42%).

Leaching behavior of the sludge deposits and pH effect

Sludge deposits buffering capacity

Fig. 2 shows the pH variation of sludge deposits suspensions in de-ionized water spiked with concentrated acidic (positive scale) or alkaline (negative scale) aqueous solutions. Results obtained in water alone showed that the "natural" pH was 6.4. The set of experimental tests altogether revealed a low alkaline or acidic neutralization capacity of the sludge deposits, with BNCpH 12 = -3.5 mol eq/kg and ANCpH 2 = 3.5 mol eq/kg. A different behavior was reported for more inorganic materials with higher carbonates contents. For example, [START_REF] Chatain | Determining the experimental leachability of copper, lead, and zinc in a harbor sediment and modeling[END_REF] reported on marine sediments a high ANCpH 2 = 7.5 mol eq/kg with a strong buffering capacity at pH 6.0, due to the dissolution of carbonate compounds. More generally, mineral materials such as ashes [START_REF] Chen | Characterization of sewage sludge ash and technical and environmental evaluation for potential utilization as secondary raw materials in construction[END_REF][START_REF] Gonzalez | Multi-Analytical Approach and Geochemical Modeling for Mineral Trace Element Speciation in MSWI Bottom-Ash[END_REF] or cementitious materials [START_REF] Peyronnard | Study of mineralogy and leaching behavior of stabilized/solidified sludge using differential acid neutralization analysis[END_REF] were reported to exhibit one or several strong buffering capacities along the pH scale. The low alkaline and acidic neutralization capacity observed for the sludge deposits sample studied here was attributed to their high organic content (62.0 %, Table 1). Similar observations were reported by other authors for organic sediments or soil [START_REF] Fang | Comparative characterization of sewage sludge compost and soil: Heavy metal leaching characteristics[END_REF][START_REF] Martin-Torre | Metal release from contaminated estuarine sediment under pH changes in the marine environment[END_REF].

Leaching of major elements

Figs. 3 and4 plot the concentrations of the different elements analyzed in the leachates as a function of pH. The observed release was controlled by several processes, notably: (i) the form and distribution of the considered element within the sludge particles, (ii) the accessibility and solubility of the respective bearing phases and their sensitivity to pH, (iii) the solubility of the considered element depending upon the pH and ionic composition of the aqueous phase. Fig. 3 shows that Al and Fe leaching behaviors were very similar, following an inversed bell-shaped curve typical of their amphoteric properties. Their release was very low between pH 4 and 7 (<0.02%) and increased at higher and lower pH, in good agreement with their association to the residual fraction discussed above. In a similar manner, Si, which has also shown to be mostly associated to the residual fraction such as silicate, was solubilized marginally (less than 2% of its total content) and almost regardless of pH. Ca leaching was also low and relatively independent from pH between pH 6 and 10. Leaching slightly increased at pH above 10 whereas a much stronger increase was observed at pH below 6, reaching close to 100% of release. This observation, combined with the results from fractionation discussed above, which showed that Ca was predominantly associated with carbonates in fraction F2, suggested that the dissolution of calcite was the key mechanism controlling the leaching of Ca.

The complete dissolution of calcite under acidic conditions was already reported by other authors [START_REF] Drapeau | Spatial and Temporal Stability of Major and Trace Element Leaching in Urban Stormwater Sediments[END_REF]. On the contrary, a higher fraction of sulfur was leached under alkaline condition (maximum release of 43%). Other published studies dealing with the leaching behavior of mineral waste such as bottom ash from municipal solid waste incineration on the contrary reported no leaching of sulfates at alkaline pH [START_REF] Gonzalez | Geochemical modeling of major and trace elements leaching behavior from MSWI bottom ash[END_REF]. The different behavior observed here may therefore be attributed to the high organic contents of sludge deposits although further investigations would be needed to elucidate the mechanisms.

The leaching profiles of total phosphorus (P Tot ) and orthophosphate ions (PO 4 3-) followed an inversed bell curve as observed for Al and Fe. Leaching was minimal between pH 6 and 8 and increased under more acidic or alkaline conditions, reaching releases close to 34% of phosphorus total content at pH 11.8 and above 49 % at pH 1.5. Under acidic conditions, it was observed (Fig. 3) that the leaching of PTot and PO4 3-were similar, indicating that PO4 3-was the dominant phosphorus species in solution. Under alkaline conditions, the leachates contained more PTot than orthophosphates, highlighting the presence of one or several other form(s) of phosphorus such as organic phosphorus. In previous studies conducted on sludge deposits sampled from the same wetland, [START_REF] Kim | pH and Eh effects on phosphorus fate in constructed wetland's sludge surface deposit[END_REF] showed that under alkaline conditions, phosphorus was predominantly leached as inorganic forms bound mainly to Fe and to a smaller extend to Ca and Al.

Leaching of trace metals

Fig. 4 shows the results obtained for the leaching of trace metals versus pH. In most cases, leaching was very low around neutrality but increased drastically under more acidic pH, and for some elements also under alkaline conditions. Among the selected trace metals, Zn was present at the highest concentration in the sludge deposit sample. Its inversed bell-shaped pHdependent leaching curve was in agreement with its amphoteric properties. The fractions of Zn total content leached under either acidic or alkaline conditions were very high (close to 100% at pH 1.5). Based on the results from fractionation tests (Fig Cr, Ni and Cd were present in the sludge deposits at much lower concentrations than Zn, and were therefore released at lower concentrations into the aqueous solutions. Cr leaching was below its analytical limit of quantification between pH 3 and 8. It was released in more significant quantities under more acidic or more alkaline conditions, in agreement with its amphoteric properties. However the fraction of Cr leached remained low as compared to the other trace metals (around 12% of its total content at pH 1.5 and 3% at pH 11.8), which was attributed to its association to the stable residual fraction, as mentioned above (Fig. 1.b). Cd and Ni showed very similar profiles with a bell-shaped pH-dependent leaching curve. This observation was surprising since unlike Al, Fe and Zn discussed above, Cd and Ni are not known to exhibit amphoteric properties. They were therefore not expected to leach significantly under alkaline conditions. Fig. 1 showed however that a high proportion of Cd (ca. 50%), and to a smaller extend Ni (ca. 20%), was associated to Fe-Mn oxides (fraction F3) in the sludge deposit sample. Sequential extractions showed that about 20% of both Cd and Ni were also associated to the oxidizable fraction (organic matter). The strong leaching of Cd and high pH values may therefore be explained by several mechanisms such as their desorption from Fe-Mn oxides, the dissolution of the Fe-oxides bearing phases, and the leaching of organic ligands from the particulate organic matter.

In a similar manner, Fig. 4 showed that Cu, although not amphoteric either, was leached more extensively under alkaline conditions, with close to 64% of its total content leached at pH 11.8. vs 54.0% at pH 1.5. Since the fractionation results showed that around 80% of Cu total content was associated to OM (Fig. 1.b), this observation was attributed to the affinity of copper to OM as will be discussed in the following section. Finally Pb was also highly associated to OM as shown in Fig. 1.b, but its concentration was low and therefore its concentration in the leachates were close to the limits of solubilities.

Influence of organic matter on trace metals leaching from sludge deposits

Leaching behavior of organic matter

Fig. 5.a shows the release of dissolved total carbon (DTC) and dissolved organic carbon (DOC) vs. pH. Almost all carbon measured in the leachates was organic. DOC leaching observed at natural pH (6.4) was less than 4% of the total content in organic carbon. Decreasing pH had little effect on DOC release. On the contrary, alkaline conditions showed a strong increase in DOC release. Up to around 50% of the dissolved organic carbon was leached at pH 11.8. Under these very alkaline conditions, dissolved inorganic carbon was measured at relatively small concentrations in the leachates. Biopolymers such as lignin, humic and fulvic acids are known to be partially hydrolyzed or solubilized under alkaline conditions [START_REF] Grybos | Increasing pH drives organic matter solubilization from wetland soils under reducing conditions[END_REF]. The observed release of DOC may therefore be explained by these phenomena affecting the particulate organic matter (POM) in contact with aqueous alkaline solutions [START_REF] Mikutta | Review: Organic matter removal from soils using hydrogen peroxide, sodium hypochlorite, and disodium peroxodisulfate[END_REF].

Characterization of dissolved organic matter

The aromaticity and humification of the organic species released during the ANC-BNC test was assessed using indexes derived from UV-VIS absorbance measurements. Figure 5.b

shows that the values of SUVA measured at alkaline pH values were over three, indicating that the leached organic species exhibited a high level of humification [START_REF] Chin | Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances[END_REF]. SUVA

was found to increase strongly with increasing pH, following in a proportional manner the increase of DOC release. This observation and the evolution of the aromaticity ratio, reaching 37% at pH 12.3 (Figure 5.b), suggested that the organic species leached in this pH range were increasingly humified. On the contrary, under acidic conditions, the aromaticity ratio of the DOC was in general below 10%, suggesting the release of poorly humified compounds. [START_REF] Chin | Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances[END_REF] reported that low molecular weight microbial metabolites would generally exhibit an aromaticity index below 15%.

The evolution of ratio A1 in the leachates is shown in Fig. 5.b. Between pH 2 and 6, A1 values were above 5, indicating a low level of humification. The ratio decreased at pH > 6, down to ca. 1 at pH around 8 to 11 revealing a strong aromaticity of the organic compounds released in this range of alkaline pHs.

Relationships between DOC and trace metals leaching behaviors

Sequential chemical extractions (Fig. 1.b) revealed that the trace metals most bound to OM were Cu and Pb. However, it was also observed that significant proportions of the other metals (around 20%) were associated to the organic matter. The leaching of OM may therefore strongly influence the respective leaching behavior of all trace metals studied here.

Fig. 6 compares the leaching profiles of the selected elements obtained in the present study from the organic sludge deposit to the profiles of the same elements collected from published studies conducted on bottom ash from municipal solid waste incineration [START_REF] Gonzalez | Geochemical modeling of major and trace elements leaching behavior from MSWI bottom ash[END_REF], which are mostly inorganic waste (only 2.1% of organic matter content). For all considered elements except Cr, it can be seen that the extent of leaching at alkaline pHs was much stronger from the organic sludge deposits than from the inorganic waste, especially for copper. Lead was present in the sludge deposits at much lower concentrations than Cu and was therefore released at much smaller concentrations with only few data above the quantification limit (Fig. 4). However, Fig. 6 shows that the release of lead at alkaline pH was stronger from the organic sludge deposits than from the inorganic bottom ash (Fig. 6). The amplification of leaching under alkaline conditions was attributed to the association the considered elements to the particulate organic matter (POM) of the sludge deposit sample. Alkaline pHs induced a release of organic constituents from the POM [START_REF] Davis | Complexation of trace metals by adsorbed natural organic matter[END_REF][START_REF] Lu | Characterization of copper complexation with natural dissolved organic matter (DOM)-link to acidic moieties of DOM and competition by Ca and Mg[END_REF], probably through hydrolysis phenomena, resulting in the leaching of associated metals, particularly copper [START_REF] Nelson | Effects of Road Salts on Heavy Metal Mobility in Two Eastern Washington Soils[END_REF]. This conclusion was confirmed by the humified nature of organic species leached at high pH values, confirming conclusions from previously published studies [START_REF] Hernandez | Detection of copper(II) and zinc(II) binding to humic acids from pig slurry and amended soils by fluorescence spectroscopy[END_REF][START_REF] Plaza | Molecular and quantitative analysis of metal ion binding to humic acids from sewage sludge and sludge-amended soils by fluorescence spectroscopy[END_REF][START_REF] Plaza | Copper(II) complexation of humic acids from the first generation of EUROSOILS by total luminescence spectroscopy[END_REF]. [START_REF] Qi | Sorption of Cu by organic matter from the decomposition of rice straw[END_REF] showed previously that the humification of organic matter increased its sorption capacity of copper. It was therefore concluded that the leaching of trace metals from the sludge deposits, especially copper, was influenced by their association with the organic fraction in the sludge deposits structure.

Conclusions

A representative sample of sludge deposits taken from an 8-year old French VFCW was used to investigate the leaching behavior of its major elements and a selection of trace metals.

The physical properties and organic matter composition of the selected sample was previously shown to be representative of a typology of surface deposits from French VFCW of more than 2-3 years of age [START_REF] Kania | Comparative characterization of surface sludge deposits from fourteen French Vertical Flow Constructed Wetlands sewage treatment plants using biological, chemical and thermal indices[END_REF]Kania et al. 2018 a and b). The results obtained here firstly showed that only marginal proportions of the total contents in major elements and trace metals were leached from the surface deposits into the aqueous solution when the sample was contacted with water at the close-to-neutral pH obtained at thermodynamic equilibrium (pH 6.4). Acid and Base Neutralization Capacities tests inspired from waste leaching protocols proved to be relevant to assess the effects of pH on leaching. A pH-dependent leaching profile was observed for all monitored elements. The metals exhibiting a strong leaching under alkaline pH were either amphoteric elements (such as Zn, Al, Fe) and / or associated to organic matter within the sludge deposits. This was particularly the case for copper, which was released more extensively under alkaline than acidic conditions, following a similar pH-dependent profile as the sludge organic matter itself. The leaching pattern of copper was explained by the fact that most of copper (more than 70% of its total content) was associated in the sample with organic matter as shown by sequential extractions of trace metals. The sequential extractions showed that the other trace metals were distributed in several fractions, including significant proportions associated to organic matter. Their leaching was therefore influenced in part by the leaching of organic ligands at alkaline pHs. The major elements were mostly found in the residual, stable fraction, and were therefore poorly leached either under acidic or alkaline conditions.



  Exchangeable fraction F1: extraction at room temperature for 1 h with 8 mL of a molar magnesium chloride solution (1 M MgCl, pH 7) under continuous agitation;  Extractible fraction F2 (bound to carbonates): extraction with 8 mL of 1 M NaOAc solution adjusted to pH 5 with acetic acid (HOAc), under continuous agitation;  Reducible fraction F3 (bound to Fe-Mn oxides): extraction with 20 mL of 0.04 M NH2OH.HCl (hydroxylamine hydrochloride solution), in 25 % (v/v) HOAc, performed at 96 ± 3 °C with occasional agitation and for the time needed for complete dissolution of the free iron oxides;

Fig. 1 .

 1 Fig.1. Distribution of (a) major elements and (b) trace elements in sludge deposits as evaluated by sequential chemical extraction. The results obtained for trace elements distribution are shown in Fig. 1.b. Regarding Cd,

Fig. 2 .

 2 Fig. 2. Acid / alkaline titration curve of the sludge deposits sample.

Fig. 3 .

 3 Fig. 3. Leaching of major elements and S and P from sludge deposits sample at equilibrium with water at room temperature and different pH values. The leached fraction (black dots) and total contents (red lines) are related to the left axis of the graphs. The axis on the right side of the graphs is related to the grey dashes showing the results as % released of total content.

  . 1.b), the leaching of Zn under acidic conditions was explained by the dissolution of carbonates. However, other mechanisms were possible. McBride et al. (1979) suggested that Zn release under acidic pH would be also affected by the fraction of Zn sorbed onto Fe or Al oxides.

Fig. 4 .

 4 Fig. 4. Leaching of target trace elements from sludge deposits sample at equilibrium with water at room temperature and different pH values. The leached fraction (black dots) and total contents (red lines) are related to the left axis of the graphs. The axis on the right side of the graphs is related to the grey dashes showing the results as % released of total content.

Fig. 5 .

 5 Fig. 5. Leaching of organic matter from sludge deposit sample at equilibrium with water at room temperature and different pH values. L/S ratio = 10 L/kg; contact time: 48h (a) cumulated release of dissolved total and organic carbon and (b) A1 (A465/A665) index of dissolved organics; SUVA (L.mg - 1 .m -1 ) and aromaticity of dissolved organics.

Fig. 6 .

 6 Fig. 6. Leaching profiles of selected trace metals from bottom ash (blue dots, from Gonzalez et al., 2014) and sludge deposit (black dots, this study).

Table 1

 1 Organic Matter (OM) content and elemental composition of sludge deposit sample.

	OM and	Concentrations	Trace metals	Concentrations
	major elements	(% w/w DM)		(mg/kg DM)
	OM	62.0	Zn	1219
	CTot	33.0	Cu	623
	NTot	3.0	Pb	64.2
	STot	0.7	Cr	57.4
	Si	5.7	Ni	30.5
	Ca	3.8	Cd	1.7
	Fe	3.5
	P	2.0
	Al	2.1
	K	0.4
	Mg	0.3
	Ti	0.2
	Na	0.1

Acknowledgments

The authors wish to thank Nathalie Dumont and David Lebouil of DEEP for chemical analyses (ICP-AES), the LEHNA (Ion chromatography) and the SARM (Service of Analyses of Rocks and Minerals) for their contributions to chemical analyses. They also thank the French ANRT (National Agency for Technological Research) and the French company SCIRPE for their financial and scientific support.