Benjamin Sawadogo

Ousmane Bamba

Dominique Chardon
email: dominique.chardon@ird.fr

Landform-regolith mapping in the West African context

Keywords: Pediment, Weathering profile, Duricrust, Detrital duricrust, Mineral exploration

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Regolith cover is often an obstacle to mineral exploration in the tropics because of poor or uneven outcrop conditions of the geological substrate. Exploration relies on sampling of surface material (soil, lag, duricrust, or even termite mounds or vegetation) to detect geochemical anomalies resulting from the geochemical dispersion of a primary bedrock mineralization through the regolith. However, interpretation of a surface geochemical anomaly depends on the nature and the history of the regolith.

The regolith may occur in situ on the rocks from which it derives by weathering or may have been transported and re-weathered. The main challenges for geochemical exploration are therefore to (i) choose the appropriate spatial density, depth and technique of sampling, (ii) identify the right regolith material to sample and (iii) interpret geochemical anomalies [START_REF] Anand | A guide for mineral exploration through the regolith in the Yilgarn Craton, Western Australia[END_REF]. Landform-regolith mapping is therefore a crucial prerequisite to mineral exploration in the tropics. Several mapping protocols have been developed in the last decades [START_REF] Butt | The development of regolith exploration geochemistry in the tropics and sub-tropics[END_REF], the most popular being derived from, or comparable to, the RED model [START_REF] Anand | Classification and atlas of regolith-landform mapping units: Exploration perspectives for the Yilgarn Craton[END_REF][START_REF] Anand | A guide for mineral exploration through the regolith in the Yilgarn Craton, Western Australia[END_REF]. That model categorizes the landscape into 3 regolith-landform regimes: relict (R, for lateritic residuum), erosional (E, for erosional plains and steep slopes) and depositional (D, sediments masking the weathering profile or the bedrock). RED-type models were applied (or slightly adapted) in sub Saharan West Africa (e.g., [START_REF] Zeegers | Seasonally humid tropical terrains (savannas)[END_REF][START_REF] Arhin | Regolith mapping of deeply weathered terrain in savannah regions of the Birimian Lawra Greenstone Belt, Ghana[END_REF][START_REF] Butt | The development of regolith exploration geochemistry in the tropics and sub-tropics[END_REF]. While implicitly or explicitly referring to RED-type models, most authors and exploration geologists categorize the ubiquitous Al-Fe duricrust cover of West Africa as lateritic residuum (i.e., duricrusts are the ultimate residual term of weathering, topping a weathering profile derived from the underlying bedrock) (e.g., [START_REF] Boeglin | Géochimie, degrésd'évolutionetlithodépendance des cuirasses ferrugineuses de la région de Gaoua au Burkina Faso[END_REF][START_REF] Roquin | Element distribution patterns in laterites of southern Mali: consequence for geochemical prospecting and mineral exploration[END_REF][START_REF] Zeegers | Seasonally humid tropical terrains (savannas)[END_REF][START_REF] Bowell | Geochemical exploration for gold in tropical soils.Four contrasting case studies from West Africa[END_REF][START_REF] Tardy | Petrology of laterites and tropical soils[END_REF][START_REF] Freyssinet | Ore-forming processes related to lateritic weathering[END_REF]; see [START_REF] Butt | Relief inversion in the geomorphological evolution of sub-Saharan West Africa[END_REF].

However, most (>80%) part of West Africa is occupied by pediments capped by detrital Fe-duricrusts or unconsolidated detrital sediments [START_REF] Chardon | West African lateritic pediments: Landform-regolith evolution processes and mineral exploration pitfalls[END_REF]; see also [START_REF] Bolster | Regolith mapping: Is it really necessary?[END_REF][START_REF] Butt | Relief inversion in the geomorphological evolution of sub-Saharan West Africa[END_REF]. These pediments are inherited from 3 Neogene paleolandscape stages of the well documented regional morphoclimatic sequence (Fig. 1). The ubiquity of paleo-and active pediment systems in West Africa therefore prompts the reevaluation of landform-regolith mapping protocols.

Here we present an integrative landform-regolith mapping chart for West Africa based on the case presented in this paper. We aim at providing geomorphologists, geologists, geochemists and the mineral exploration community with field documentation, identification criteria and illustrations of the typical landforms and regolith. The chart accounts for the specificities of the remnants of the regional morphoclimatic sequence (Fig. 1) -particularly the residual or detrital character of the Fe duricrusts -and the remaining, non-duricrusted portions of the landscape. We show how the resulting landform-regolith map is converted into a soil prospectivity map, which is used for (i) defining sampling strategies, (ii) interpreting potential geochemical anomalies and (iii) targeting lateral extensions of known deposits. The mapping protocol is shown to be applicable to wetter contexts and, more generally, to other tropical shield provinces.

Context and method

Our study is based on a regolith mapping project of part of the Goren greenstone belt in North-Central Burkina Faso, in the semi-arid Soudanian climatic zone (Fig. 2). The study area is typical of the greenstone-hosted gold deposits of the Eburnean orogen of the southern West African craton (e.g., [START_REF] Milési | Early Proterozoic ore deposits and tectonics of the Birimian orogenic belt, West Africa[END_REF]; Fig. 2). The belt is the richest in Burkina Faso in terms of gold indices and prospectivity [START_REF] Castaing | Carte géologiqueetminière du Burkina Faso à 1/1 000 000[END_REF]. The mapping protocol is illustrated on a typical 7 x 6 km extract (Figs. 3 and4) of a larger mapped area (26 x 15 km) that allowed for a comprehensive inventory of the spatial diversity of the regolith covers. The geological substrate comprises a typical Birimian greenstone association of low-grade schistose volcanosediments (sandstone-pelite associations interlayered with mafic and felsic volcanics) and metabasic rocks (gabbro, diorite and andesite) (Fig. 3).

For landform-regolith mapping, we used a 1:10 000 scale, 1-m contour base map derived from a smoothed Lidar digital elevation model (e.g., Figs. 3 and4).

Mapping was undertaken along a number of itineraries throughout the landscape (Fig. 3) complemented by photointerpretation that allowed refinement of various map units.

Field mapping consisted mostly of (1) landscape interpretations and establishment of landscape chronologies, (2) description and identification of the duricrust-capped remnants of the West African morphoclimatic sequence (Fig. 1), (3) description and classification of the geomorphology and regolith cover of the erosional landforms separating the duricrust-capped remnants and (4) description and classification of the material being actively remobilized by mechanical erosion and transport.

Through comprehensive field investigation the definition of 20 landformregolith association map units (section 3; Fig. 4) was established. These units may be grouped into 4 main categories on the basis of the nature/degree of genetic link between the material exposed at the surface and the underlying geological substrate (section 4). This categorization has implication on the intensity of expression of a geochemical anomaly through the regolith that is related to the underlying bedrock mineralization, the occurrence of "transported" anomalies in the regolith or the potential occurrences of bedrock anomalies masked by the regolith.

Landform-regolith associations

Upper paleolandsurfaces

1. 1. Bauxitic (African) paleolandsurface (1BxD)

It is preserved as 120-to 150-m high, duricrust-capped plateaus that are flat or slightly convex, with less than 25 m of elevation amplitude for a single plateau (Fig. 6a and6b).

In the study area, the largest plateau is 1 km long and 40 m wide (Fig. 4). The weathering profile underlying the bauxitic duricrust is 50-to 100-m thick (thickest on mafic rocks). It is observed on the steep sides of plateaus, whose edge consists of a scarp made in the duricrust. The profile consists mainly of a kaolinite rich saprolite underlying a 5 to 10 m thick duricrust. Saprolite is absent on quartzite and silicified volcanosediments, so that the duricrust rests directly on saprock. Duricrusts are of two main types: bauxites, that are massive or breccia-like (Fig. 7a to 7d), and Fe bauxites, that are nodular or pisolitic (Fig. 7e and7f). Breccia-like bauxites are more common on volcanosedimentary substrate and massive bauxites are equally found on mafic rocks and volcanosediments. Fe bauxites are degraded bauxites enriched in iron (e.g., [START_REF] Boulangé | Relation between lateritic bauxitization and evolution of landscape[END_REF][START_REF] Boulangé | Les formations bauxitiques latéritiques de Côte d'Ivoire : les faciès, leur transformation, leur distribution et l'évolution du modelé[END_REF] and mostly found on the lowermost portions of wide plateaus. (2InD,2InC,2InR) Relics of that paleolandsurface occur as two duricrust-capped geomorphological types.

1. 2. Intermediate paleolandsurface

The most common type (1) is smooth hilly slopes or ridges sloping away from bauxitic plateaus (Fig. 4, Fig. 5 and Fig. 6b). They are stepped under the bauxitic plateaus (most of the time 50 m under the plateau edge) and, in some cases, attached to it in such a way that there is continuity between the Bauxitic and Intermediate duricrusts. The downslope topographic profile of the relics or their surface envelope is convexoconcave to convex. Relics have 50 to 500 m of across-slope width, with a slope ranging from 5 to 20 % and an elevation amplitude of 20 to 75 m. Type 2 relicts are 50-to 500m-wide isolated plateaus that dominate the lowland by 15 to 60 m (Fig. 6c). Plateaus are inclined (4 to 10 % slope), with an individual elevation amplitude of 10 to 30 m. The disposition and the distribution and slope of type 2 plateaus indicate that the Intermediate paleolandsurface radiated as wide piedmonts around the Bauxite-bearing topographic massifs and that type 1 relics represent upslope portions of these piedmonts. The hilly morphology of type 1 relics appears to result from the collapse of the paleolandsurface by mass wasting after its abandonment by relief inversion. The Intermediate weathering profile comprises a 10 to 50 m thick saprolite horizon, a carapace (that can be more than 10 m thick) and a topping duricrust (5-10 m thick for type 1 and 2-5 m thick for type 2 geomorphological occurrences).

Relics of the Intermediate paleolandsurface are mapped as 3 landform-regolith associations. The first one (2InD) is the original duricrust-capped landsurface. The duricrust found on type 1 relics is an Al-Fe duricrust, often dismantled into meter-size blocks (Fig. 8a). This duricrust consists of a mix of Fe oxyhydroxides hosting Al-Fe rich relicts(Fig. 8b) or a pack of nodules / pisoliths(Fig. 8c) recalling that of the Fe bauxites (Fig. 7e).The nodular / pisolitic Al-Fe duricrust appears to be the late result of the weathering process that started by the reworking of the massive or breccia-like bauxite into nodular or pisolitic Fe bauxite. Besides, that type of Intermediate Al-Fe duricrust has been shown to derive from the ferruginization of an evolved bauxitic duricrust [START_REF] Boulangé | Relation between lateritic bauxitization and evolution of landscape[END_REF]. This is further supported, in the present case, by the common occurrences of bauxites embedded in the Al-Fe Intermediate duricrust that have not been completely "digested" by the ferruginization process (Fig. 8d). Therefore, the duricrust covering the type 1 Intermediate ridges could be mistaken for a duricrust of the Bauxitic paleolandsurface without a careful geomorphological investigation. The duricrust capping isolated Intermediate plateaus (type 2) is a dense, dark-violet Fe duricrust (Figs. 8e and8f). It is formed by the coalescence of Fe oxyhydroxide nodules and tubes, with a coherent interstitial matrix (less than 10% of the volume) made up mostly of Fe oxyhydroxides and kaolinite.

The second landform-regolith association related to the Intermediate paleolandsurface (2InC) corresponds to type 1 or 2 geomorphic occurrences that expose the carapace layer instead of the duricrust cap. This configuration results from the stripping of the duricrust or its non-development. The carapace (Fig. 9) is characteristically reddish or dark-red and friable and contains goethitic, -and, to a lesser extent, hematitic -concretions 1 to 30 cm in size. The concretions are pedorelictual and are angular, sub-angular, nodular, tubular or platy; others are cutanes.

Concretions are dark-violet and some preserve bedrock fabrics. The carapace matrix is a pulverulent ochre-orange mix of clays and Fe oxyhydroxide nodules (Fig. 9). The carapace with low matrix content (Fig. 9b) should not be mistaken for the nodular Intermediate Fe duricrust (Fig. 8e) or that hosting scattered heterometric nodules (Fig. 9c) for a conglomeratic duricrust (see section 3. 2).

The third and last landform-regolith association related to the Intermediate paleolandsurface (2InR) results from the erosion / collapse of type 1 or 2 landscape relics and concomitant dismantling of their topping duricrust. The typical result is a residual hill covered with dispersed blocks or gravel of Intermediate duricrust. The saprolite and/or carapace layers of the weathering profile can crop out on the slopes or the crestal region of the residual hill.

2.Paleo-glacis systems

Relics of the High and Middle glacis systems are preeminent in the landscape, as it is the case throughout West Africa and particularly on granite-greenstone terrains (e.g., [START_REF] Grimaud | Neogene cratonic erosion fluxes and landform evolution processes from regional regolith mapping (Burkina Faso, West Africa)[END_REF]. These systems used to form the piedmonts of the massifs preserving relics of the upper paleolandsurfaces (Figs. 4 and5).

2. 1. Geomorphology

High glacis relics are inclined (4% mean slope) and very slightly concave plateaus (Figs.

10a to 10c), with upslope and downslope heights of 10-40 m and 5-15 m, respectively.

They have a downslope width ranging from 50 to 700 m and rarely exceed 1km in across-slope length. Relics are not connected to the massifs anymore for a peripheral hollow (Fig. 5) formed after abandonment of the High Glacis. The duricrust capping the High glacis is a 2 to 4 m thick Fe duricrust that marks the edge (scarp) of plateaus and tops a weathering profile less than 30 m thick (Fig. 11a).

Middle glacis relics occupy the largest area of all the duricrust-capped paleolandsurfaces (Fig. 4). They have lower slopes (1-3%) than those of the High glacis and form low-lying plateaus (< 5 m high) with downslope widths of up to 2 km (Fig. 10d). The middle glacis may form a continuous surface with the High glacis marked by a very subtle break of slope and (especially) a change in the thickness and nature of the capping duricrust (Fig. 10c). A comparable continuity is also very commonly observed between the Middle glacis and the Low glacis / active pediment (Fig. 5). The weathering profile under the middle glacis is thin (<3 m), with a carapace-Fe duricrust layer rarely more than 1 m thick (Fig. 11b). The Fe duricrust forms a scarplet at plateau edges.

2. 2. Landform-regolith associations of the High and Middle glacis systems

The typical landform-regolith association of the High glacis (3HgD) is a detrital Fe duricrust formed by cemented clastic material. Two common types are distinguished: (i) coarse conglomeratic (Fig. 11c), preferentially found on upper slopes of the relics and close to the high topographic massifs and (ii) finer-grained (sand-gravel granulometry; Fig. 11d) in distal parts of High glacis (i.e., closer to the main drains at the time the glacis was functional). In the first type, pebbles are angular, sub-angular, or rounded.

They are mostly bauxitic and Intermediate duricrusts, vein quartz, transported Fe concretions or, more rarely, bedrock. The matrix is a mix of clay, silt, sand and gravel made of reworked saprolite, duricrust clasts, Fe oxyhydroxide nodules and clasts. In the finer-grained type, the nature of the gravel is comparable to that of the coarse conglomeratic type and the matrix does not exceed the sand granulometry. On initial field observations, the detrital nature of the fine-grained duricrust may be overlooked, with fateful implication as to whether the duricrust material formed in-situ or was transported before cementation. The duricrust-capped landform-regolith association for the Middle glacis (4MgD) compares to that of the High glacis (3HgD) although the coarse conglomeratic type is found only in the uppermost portions of some glacis relics.

The Middle glacis Fe duricrust is detrital and has a gravel -sand granulometry. It has vacuoles and is more porous than that of the High glacis (Figs 11e and11f).

Areas of preserved High or Middle glacis exposing the carapace instead of the duricrust have been ascribed to a distinctive association (3HgC and 4MgC, respectively).

The High glacis carapace is scoriaceous and highly porous, light-colored (pinkish to yellowish; Fig. 12 Finally, similarly to the Intermediate paleolandsurface, some glacis plateaus have been partly eroded, leaving residual hills that are covered with scattered blocks of the original duricrust. These blocks directly overly the carapace, mottled clay or saprolite horizon of the weathering profile. The residual hills correspond to associations3HgR and 4MgR, for the High and Middle glacis, respectively. These associations are ambiguous as to whether they represent bedrock weathered in situ or transported material. Indeed, exhumed horizons of the weathering profile may still be fine-grained clayey to sandy detrital sediments.

2. Low Glacis system (5LgD)

A specificity of the Low glacis system is that is has remained functional in the area in the form of a regional active pediment. It is why its geomorphology and regolith shall be described in detail in section 3.4. It is however worth mentioning here that upslope portions of the Low glacis are in many locations connected to lower portions of the Middle glacis to form a single surface as a result of polygenic evolution (e.g., Figs. 4 and5). Also, along portions of some streams, the sedimentary cover of the glacis is locally removed so that the roof of the Low glacis Fe duricrust (5LgD) is exposed. The map areal extend of that association is therefore very limited. But it may underlay the sedimentary blanket of the active pediment over significant areas, at least along major drains. The Low glacis Fe duricrust is detrital and mostly conglomeratic, generally thin (<0.5 m) and weakly cemented compared to that of the Middle glacis. More generally in West Africa, the Low glacis may beat best capped by a carapace or may not even be indurated at all [START_REF] Grandin | Aplanissementscuirassésetenrichissement des gisements de manganèsedansquelquesrégionsd'Afrique de l'Ouest[END_REF].

Erosional reliefs (Br, Sa, Wp, LtS)

A considerable portion of the reliefs is not protected by a duricrust and is currently undergoing erosion. That erosion contributes to further exhume bedrock and weathering profile material that constitute detrital sediments feeding the active pediment. Erosional terrains group 4 landform-regolith associations.

The first association (Br, for Bedrock Slope) refers to hilly bedrock terrains that can reach 100 m in height (Fig. 13a). They form massifs elongate in the direction of the dominant NE structural grain of the rocks, with the structural fabric (schistosity) locally expressed as aligned ridges in the high-resolution topography. Massifs of volcanosediments are generally flat-topped, as opposed to hilly for metabasites. Crests of the massifs can bear scattered blocks of Bauxitic or Intermediate duricrust and very rarely remnants of saprolite. Some slopes of the massifs are incised by ravines and gullies partially filled up with bedrock and duricrust clasts.

Association Sa designates all the erosional terrains exposing a saprolite. By definition, saprolite derives from weathering of the bedrock. It is very fined-grained (clay-silt granulometry), pulverulent, and kaolinite rich. The saprolite is generally darkyellowish to reddish on mafic rocks, often with whitish spots as ghosts of plagioclase crystals. On volcanosediments, it ranges in color from whitish to intense yellow. The saprolite commonly preserves ghosts of the schistosity planes and may enclose oriented platy relics of early ferruginized bedrock. The last association of erosional terrains is that of lithosoils (LtS). It consists of hilly terrains with in a thin (<50 cm) unconsolidated layer resting atop the bedrock. This layer derives from the disaggregation of the bedrock and is mostly made of the underlying bedrock clasts, ranging in size from silt to gravel (Figs. 13e and13f). This association generally occupies smooth reliefs at the foot of the main topographic massifs and above the active pediment (Fig. 4). These terrains are generally grasscovered and therefore provide less sediment to the active pediment than the other erosional terrains. Lithosoils are dark-grey on mafic rocks and propitious for agriculture.

They are generally lightly ochre on volcanosediments (Figs. 13e and13f).

4. Active pediment (ApS, ApT, ApF)

The active pediment occupies the largest area. It is a regional surface made of quasiplanar facets connecting the foot of all reliefs to the current main river drains over distances commonly exceeding 10 km (outside the map area of Fig. 4). Its uppermost slopes do not exceed 3 % and its mean slopes are less than 1 %. Over an overwhelming part of its surface, the pediment is covered by a detrital sedimentary layer (association ApS, Fig. 14a), which tends to thicken downslope (generally up to several meters in thickness, except near the main river drains where channels more than 10-m deep can be filled-up by sediments). The sedimentary layer -at least its uppermost surface -is reworked at each rainy season and may therefore be considered as transiting on the pediment. Coarse material exposed at the surface of the pediment mostly consists of debris of Al-Fe and Fe duricrust, Fe nodules (Fig. 14b) and vein quartz (more rarely bedrock), large pebbles being restricted to uppermost slopes near reliefs. There, conglomerates can locally be weakly cemented by Fe oxyhydroxides (Fig. 14c). Downslope, the pediment surface has two main lithologies: a light-colored mix of clay, silt and sand, deriving from weathering profiles (Fig. 14a) or a pellicular ferruginous gravel resting on that same light-colored material (Fig. 14b). Sections exposed in stream cuts show that alluvial gravel layer(s) underlay the fine-grained material (Figs. 14d and14e), bearing in mind that the Low glacis detrital Fe duricrust may floor the sedimentary cover over significant areas. Weathering of the sedimentary cover is attested at depth by the development of smectite (greenish staining, Figs. 14d and14e) or carbonate nodulation (white spots). Importantly, the fine-grained sedimentary cover of the active pediment should not be mistaken for a saprolite preserved on its bedrock.

At various locations on the active pediment, stream channels are filled-up along some of their portions by sediments (mostly fine-grained), forming alluvial terraces that may not maintain for more than a few rainy seasons. Although an integral part of the active pediment surface, these sedimentary infillings are distinct and ascribed a specific landform-regolith association (ApT for Active pediment Terrace).

Several alluvial fans fed by the badlands on sides of the reliefs have been mapped and are attributed to a landform-regolith association (ApF). These fans constitute transient piedmont accumulations of mostly fine-grained sediments (essentially reworked saprolite) and duricrust pebbles that will ultimately make their way on the active pediment surface. The fans can locally reach 10 m in thickness. They are often exploited for placer gold.

Terrain classification, implications for mineral exploration

1. Lateritic residuum with potential supergene enrichment

That category of regolith comprises the associations related to the upper paleolandsurfaces and the upper part of their weathering profile preserved by the paleo-glacis (blue on Fig. 15). These associations expose the carapace horizon (2InC,2InR,3HgOc,4MgOc) or the duricrust (1BxD, 2InD) topping a thick weathering profile resulting from protracted (>20 My) periods of intense weathering [START_REF] Valeton | Bauxites and associated terrestrial sediments in Nigeria and their position in the bauxite belts of Africa[END_REF][START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. They may therefore host anomalies forming typical mushroom-shaped halos rooted in the bedrock (e.g., [START_REF] Freyssinet | Ore-forming processes related to lateritic weathering[END_REF]; Fig. 16b).These terrains are therefore suitable for geochemical sampling. However, elements known to be mobile in supergene environments such as As, Ag, Zn, Cu, Ni, Co or U (e.g., [START_REF] Tardy | Petrology of laterites and tropical soils[END_REF]would be preferentially leached in the residuum (mottled clay, carapace and duricrust) of the weathering profile (these elements could still be anomalous at low concentrations, possibly reflecting an underlying mineralization). Conversely, the residuum is more likely to concentrate non-mobile metals (e.g., Al, Zr, Ti, Sb, W) than the underlying saprolite because it has undergone volume loss and because these elements are typically hosted by minerals such as quartz, zircon, rutile or anatase that can survive most intense weathering (e.g., [START_REF] Valeton | Element concentration and formation of ore deposits by weathering[END_REF][START_REF] Freyssinet | Ore-forming processes related to lateritic weathering[END_REF][START_REF] Salama | A regolith geochemical anomaly formed by accumulation and corrosion of resistant minerals from Scuddles VMS primary ore, Western Australia[END_REF].

Weathering profiles of the upper paleolandsurfaces are known for their supergene mineral deposits in Al, Fe and Mn (e.g., [START_REF] Grandin | Aplanissementscuirassésetenrichissement des gisements de manganèsedansquelquesrégionsd'Afrique de l'Ouest[END_REF][START_REF] Valeton | Element concentration and formation of ore deposits by weathering[END_REF]. Upper paleolandsurface remnants are preserved mostly on greenstone belts and dolerites sills, which host most known primary bedrock concentrations in West Africa (e.g., Au, Zn, Pb, Cu, Ni; [START_REF] Milési | Early Proterozoic ore deposits and tectonics of the Birimian orogenic belt, West Africa[END_REF]. The potential for supergene concentration in these weathering profiles is therefore high, but still largely unexplored. Of particular interest are metals such as Ti, Zr, V, Nb, Cr, Sn, W, Th, Hf and rare earth elements [START_REF] Boulangé | Les formations bauxitiques latéritiques de Côte d'Ivoire : les faciès, leur transformation, leur distribution et l'évolution du modelé[END_REF][START_REF] Braun | Mobilization and redistribution of REEs and thorium in a syenitic lateritic profile: A mass balance study[END_REF][START_REF] Tardy | Petrology of laterites and tropical soils[END_REF][START_REF] Butt | Evolution of regoliths and landscapes in deeply weathered terrain-implications for geochemical exploration[END_REF][START_REF] Sidibé | Petrography, mineralogy, geochemistry and genesis of the Balaya bauxite deposits in Kindia region, Maritime Guinea, West Africa[END_REF] that would be pre-concentrated in the bedrock.

2. In-situ regolith and bedrock

This category groups erosional terrains (Br,Sa,Wp,LtS;Figs. 15) etched down to the bedrock or covered with unconsolidated regolith that is connected to the geological substrate from which it derives (green on Fig. 15). They are mostly restricted to the slopes and interfluves of the main topographic massifs. They are suitable for geochemical sampling because they should express anomalies connected with the bedrock. Identification and mapping of these terrains become crucial away from the main topographic massifs because they represent the only access to in-situ regolith in the low land. In these low land contexts, they are found only on the sides of paleoglacis relict plateaus (Sa, WtS; Figs. 4 and15). Low-lying composite paleo-pediment landscapes being the norm rather than the exception in West Africa [START_REF] Chardon | West African lateritic pediments: Landform-regolith evolution processes and mineral exploration pitfalls[END_REF], particular attention should be given to this landform-regolith category.

Dispersion halos are generally narrow or nearly absent in the saprolite or the bedrock [START_REF] Freyssinet | Ore-forming processes related to lateritic weathering[END_REF][START_REF] Bamba | Morphopédologieetanomaliegéochimique[END_REF][START_REF] Anand | A guide for mineral exploration through the regolith in the Yilgarn Craton, Western Australia[END_REF]See Fig;16), which form an overwhelming part of these landform-regolith associations. This therefore requires a dense sampling grid -typically 50 m -to make sure a mineralization is not missed.

3. Transported regolith

3. 1. Exploration challenges

This category comprises the detrital Fe duricrusts of the paleo-glacis (3HgD, 4MgD) and the sedimentary cover of the active pediment (ApS, ApF, ApT). That domain represents by far the widest and most continuous of the study area (Red on Fig. 15). It is also the case over most of Central and Northern Burkina Faso and more generally over the northern Soudanian zone and the Sahelian zone (Fig. 2). This category of regolith masks the weathering profile and the bedrock. This means that mineral deposits may be hidden under these covers (Fig. 16a). Mineralized material can be transported on the pediment or the paleo-glacis, part of that material being trapped in the pediment sedimentary cover (loose or duricrusted) to form a "transported" anomaly (e.g., Fig. 16d and16e). Upward gaseous transfers and insect or plant root activity can bring mineralized material to the surface of the sedimentary cover [START_REF] Anand | A review of metal transfer mechanisms through transported cover with emphasis on the vadose zone within the Australian regolith[END_REF][START_REF] Salama | Geochemical exploration for supergene copper oxide deposits, Mount Isa Inlier, NW Queensland, Australia[END_REF]. But the expression of an anomalous halo rooted in the bedrock through a duricrusted pediment sedimentary cover, although possible, is unlikely.

Transported regolith can host secondary metal concentration such as alluvial placers, as exemplified by gold on active pediments in Burkina Faso [START_REF] Ouangrawa | Evolution des particulesd'or en milieu lateritiquesoudano-sahelien : Alluvions et colluvionsdansl'environnement d'un gîtefilonien, Piela, Burkina Faso[END_REF][START_REF] Bamba | Évolutiongéodynamiqueexterneet implication sur le devenir de l'or en contexte sub-sahélien[END_REF]. Paleo placers should be considered in the detrital Fe duricrust of the paleo-glacis (3HgD, 4MgD). Besides, gold and base metal deposits have been discovered in detrital duricrusts in other lateritic environments [START_REF] Anand | Gold and pathfinder elements in ferricrete gold deposits of the Yilgarn Craton of Western Australia: A review with new concepts[END_REF][START_REF] Salama | Supergene gold in silcrete as a vector to the Scuddles volcanic massive sulfides, Western Australia[END_REF]. Solute transport of metals (e.g. Cu, Mg, U, Ni, Au, REE; [START_REF] Boulangé | Rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at Passa Quatro, Minais Gerais, Brazil[END_REF][START_REF] Sillitoe | Supergene oxidized and enriched porphyry copper and related deposits[END_REF][START_REF] Reich | Geological and economic significance of supergene metal deposits[END_REF][START_REF] Xueqiu | Geochemical challenges of diverse regolith-covered terrains for mineral exploration in China[END_REF] down the slope of pediments can also lead to exotic deposits, mostly in pediment sedimentary cover. These deposits would produce dispersion halos comparable to those formed by mechanical transport on pediments (e.g., Fig. 16d and16e).

3. 2. Sampling strategy

Classical soil sampling at 40-50 cm depth would most likely not attain the in-situ regolith, as the transported cover is systematically thicker. By definition, paleo-glacis duricrusts, which are all detrital, are not suitable for aiming at the regolith sitting on its parental bedrock. Sampling leaves or the bark of deeply rooted trees such as Acacia or Eucalyptus or termite mounds would be recommended (e.g., [START_REF] Gleeson | Gold exploration in Niger using soils and termitaria[END_REF][START_REF] Anand | Geochemical dispersion through transported cover in regolith-dominated terrains-toward an understanding of process[END_REF][START_REF] Salama | Geochemical exploration for supergene copper oxide deposits, Mount Isa Inlier, NW Queensland, Australia[END_REF][START_REF] Arhin | Gold in Plant: a biogeochemical approach in detecting gold anomalies undercover-a case study at Pelangio Gold Project at Mamfo area of BrongAhafo[END_REF]. But systematic sampling by aircore or auger is the most appropriate to attain the saprolite underlying the transported cover. In doing so, the orientation and spacing out of the boreholes should be adapted to the potential width of the prospected mineralized bodies as the dispersion halo may be narrow or even absent in the saprolite (Fig. 16). Typically, narrow (<20 m) Au-bearing quartz dikes or REE-mineralized pegmatite dikes would easily be missed if samples are more than 100 m apart. In such a case a 50-m spacing would be required. Aircore or auger sampling of the saprolite requires rigorous description of the material drilled before reaching the saprolite, as well as diagnostic identification criteria of the saprolite itself (heterogeneity, texture, occurrence of pedorelics, lithorelics, lithostructures or relict minerals, see description of association Sa, section 3.3). These tests should help prevent mistaking the saprolite seating on its parental bedrock for fine-grained transported material. Once the in-situ character of the regolith is attested, one should favor sampling of residuum material instead of saprolite (see section 4.1).

3. 3. Mechanical dispersion on glacis / pediment and "transported" anomalies

Paleo-glacis relics and active pediments are not necessarily obstacles to exploration.

They can also keep tracks of a mineralization exhumed in their upslope geomorphic environment. Indeed, glacis / pediments are erosional and transportation slopes. Their detrital overburden therefore can bear erosional products of mineralized bodies and even placers (Fig. 16). Mechanical dispersion of these products on glacis / pediments should produce very wide halo stretched down slope, over hundreds of meters to several kilometers [START_REF] Sanfo | Gold: a useful tracer in subsahelian laterites[END_REF][START_REF] Parisot | Dynamique de l'oretd'autresminérauxlourdsdans un profild'altérationcuirassé du Burkina Faso, Afrique de l'Ouest. Intérêt pour l'interprétation de la mise en place des matériauxconstituant les cuirasses de haut glacis[END_REF][START_REF] Ouangrawa | Dispersion mécanique de l'ordans les matériaux de surface :exemple du site aurifère de Piéla (Burkina-Faso)[END_REF][START_REF] Ouangrawa | Evolution des particulesd'or en milieu lateritiquesoudano-sahelien : Alluvions et colluvionsdansl'environnement d'un gîtefilonien, Piela, Burkina Faso[END_REF][START_REF] Bamba | Morphopédologieetanomaliegéochimique[END_REF][START_REF] Bamba | Évolutiongéodynamiqueexterneet implication sur le devenir de l'or en contexte sub-sahélien[END_REF]. Identification of such anomalies on the active pediment may be used to track its primary (endogenous) source upslope thanks to a topographic map at the appropriate resolution. The same approach may be used on paleo-glacis systems, providing the landscape at the time the considered glacis was functional is reconstituted [START_REF] Chardon | West African lateritic pediments: Landform-regolith evolution processes and mineral exploration pitfalls[END_REF].

Other regolith

The last category groups Paleo-glacis relicts exposing their carapace layer (3HgC, 4MgC) and the residual hills of paleo-glacis systems (3HgR, 4MgR) that also generally expose the carapace (orange on Fig. 15). That category has the smallest areal extent. In this category, determination of the in-situ or transported character of the regolith is not straightforward. Indeed, a glacis carapace formed by the weathering of a fine-grained transported cover may not be distinguished from one that evolved from saprolite resting on its parental bedrock. Caution is therefore required in sampling and interpreting surface geochemical results, as such material can display an anomaly rooted in the bedrock, a transported anomaly or can mask a bedrock anomaly. Only auger or aircore drilling of the deep regolith proved to seat on its bedrock can avoid errors in interpreting the geochemistry. The same care should apply to the description of the drilled material down to the bedrock as in the case of the transported regolith (section 4.3.2).

Exploring for the lateral extension of an existing ore body

The prospectivity map (Fig. 15) may be used to look for the suspected continuity of a steep planar ore body. In the present case, the NE trend of steeply dipping mineralized structures is outlined by the pattern of artisanal pits aligned on the structures (Fig. 15).

The pits are restricted to the green domain (in situ regolith and bedrock; Fig. 15), as access to the mineralization is facilitated there by the absence of duricrust and partial or complete ablation of the in-situ regolith. Linear extrapolation of the mineralized trends will allow identifying the categories of regolith susceptible to overly a lateral extension of the mineralization. Given the expected behavior of a deeply rooted anomaly upward through each category of regolith (Fig. 16), one can check whether a lateral extension of the mineralization is expressed in the regolith. Outside the green domain (in situ regolith and bedrock; Fig. 15), an extension would typically not be noticed by surface sampling in the red domain (transported regolith) and potentially the orange domain (other regolith) (Fig. 15). In the green domain, soil sampling on the extrapolated structural trend would be sufficient. In the red and orange domains, trenches across the extrapolated trend, auger drilling or inclined aircore drilling would be needed to attain the regolith preserved on its parental bedrock. That approach should be complemented by sampling of the weathering profile exposed on the talus of paleo-glacis relict plateaus (Wp; Fig. 4, green in Fig. 15) along the extrapolated trace of the mineralization. Finally, the potential intersection of a mineralized trend with a blue domain (lateritic residuum; Fig. 15) would lead to suspect an enrichment in the residuum in that domain if the considered element is non-mobile in the supergene environment.

Concluding remarks

In this paper we have proposed a landform-regolith mapping protocol adapted to the West African morphoclimatic context. That protocol particularly takes into account the detrital nature of the Fe duricrusts capping the ubiquitous relics of the paleo-pediment systems and considers the detrital sediments covering the functional pediment that may be mistaken for weathering material preserved in-situ. The landform-regolith mapping chart is declined into a prospectivity chart, which is used to recommend adapted exploration strategies. The mapping protocol further allows for the definition of exploration guides for supergene mineral concentrations.

The mapping chart is pertinent for greenstone terrains of the Soudanian and Sahelian morphoclimatic zones (typically savanna environment and drier; Fig. 1). It can also be applied to the vast low-relief areas of composite paleo-glacis landscapes lacking relics of the upper paleo land surfaces, which encompasses the various geological substrates of the sub region. In wetter (southern) West Africa (Guinean and Forest zones, Fig. 1), the Low glacis did not develop. Field reconnaissance indicates that away from topographic massifs having kept tracks of the upper paleo land surfaces, the Middle glacis is rarely duricrusted. It is capped by a carapace or, more commonly, it is not weathered beyond the mottled clay stage. In these terrains, a careful study of landforms and associated regolith should lead to the recognition / use of the associations defined in the present work.

The ubiquity of paleo-and active lateritic pediments in West Africa where Fe duricrusts had all long been considered as residual by exploration geologists urges reevaluation of landform-regolith mapping practices, dispersion models and exploration strategies. The mapping protocol developed for the West African context may prove useful elsewhere on the African continent and more generally on other tropical shields of the world. If, as in West Africa, lateritic pediments may have been overlooked in other shield terrains of the world, the renewal of the methods to access bedrock resources would be required, because they may still be underestimated. [START_REF] Chardon | West African lateritic pediments: Landform-regolith evolution processes and mineral exploration pitfalls[END_REF]. Abandonment ages of the paleolandsurfaces are after [START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39Ar-40Ar dating of supergene K-Mn oxides[END_REF]and [START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

  ). If present, Fe nodules are millimeter or centimeter in size and bedrock structures can persist in the carapace, by contrast with the Intermediate carapace. The carapace of the Middle glacis compares to that of the High glacis but Fe concretions are scarce and most of the time invisible macroscopically. In absence of bedrock structures, whether the glacis carapace was formed by weathering of the underlying bedrock is difficult to assess. It may derive from cementation of fine-grained detrital material. Areas of preserved High or Middle glacis exposing the Intermediate carapace layer are not uncommon and have been ascribed specific associations (3HgOc and 4MgOc, respectively). These associations show that the lower parts of the intermediate landscape have been only very weakly eroded by the High and Middle glacis, which removed the Intermediate duricrust without stripping the rest of the underlying Intermediate profile. Although they correspond to glacis surfaces, these associations result from weathering of the underlying bedrock.

  Steep slopes and talus of duricrust-capped plateaus constitute a specific landform-regolith association (WpS for Weathering Profile Slope). These slopes have exhumed various layers of weathering profiles (saprolite, mottled clay, carapace) that are often covered by screes of duricrust (Figs.13b to 13d). The slopes are vegetated to various degrees as stream reappearances are common under the duricrust at the edge of plateaus. Badlands are common in these terrains (Figs.13d), providing direct access to the weathering profile in incisions. Importantly, these terrains provide the only access to weathering profiles underneath detrital Fe duricrusts of the High and Middle glacis.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Climatic zonation of West Africa with location of the study area in the

Figure 3 .

 3 Figure 3.Geology and topography of the study area with the main field stations,

Figure 4 .

 4 Figure 4.Landform-regolith map and chart.The contours are 1 m. Associations 3HgOc,

Figure 5 .

 5 Figure 5.Synthetic cross-section showing the main landform-regolith associations in

Figure 6 .

 6 Figure 6.Geomorphology of upper paleolandsurfaces. (a) Bauxitic plateau. The plateau

Figure 7 .

 7 Figure 7.Main petrographic types of duricrustscapping the bauxitic paleolandsurface

Figure 8 .

 8 Figure 8.Main types of Intermediate duricrust (2InD). (a)-(d) are Al-Fe duricrustsfrom

Figure 9 .

 9 Figure 9. Main types of carapace of the Intermediate weathering profile. (a) Nodular,

Figure 10 .

 10 Figure 10.Geomorphology of paleopediment systems. (a) High glacis forming the

Figure 11 .

 11 Figure 11. Regolith types of the paleo-pediment systems. (a) Upper weathering profile

Figure 12 .

 12 Figure 12. (a) Typical sandy vacuolar carapace of the High glacis. Before exposure,

Figure 13 .

 13 Figure 13.Erosional terrains. (a) Hilly bedrock landscape (metabasic rocks). (b) Saprolite

Figure 14 .

 14 Figure 14.Geomorphology and regolith of the active pediment. (a) General view of the

Figure 15 .

 15 Figure 15. Soil prospectivity map deduced from the landform-regolith map of Figure

Figure 16 .

 16 Figure 16.Idealized geochemical dispersion halos in West African granite-greenstone

Acknowledgments

This work was supported by the IRD (JEAI FasoLith). The contour map was provided by NordGold. We thank John Learn and particularly Claire Floriet for making this work and its publication possible. The two referees are thanked for their helpful comments.

Sawadogo et al


Rafik Balti

Nourhène Zayoud

Florence Hubert

Lucie Beaulieu

Anthony Massé
email: anthony.masse@univ-nantes.fr

Fractionation of Arthrospira platensis (Spirulina) water soluble proteins by membrane diafiltration

Keywords: Spirulina, biorefinery, ultrafiltration, protein, phycocyanin

Membrane diafiltration process was developed in order to fractionate the water soluble proteins of the supernatant obtained after Spirulina bead milling disruption and centrifugation. The obtained crude extract was filtered with 50 kDa, 150 kDa, 300 kDa or 0.2 µm membranes.

Phycocyanin purified extracts were produced after filtration on the 300 kDa or 0.2 µm membranes. The filtration on 0.2 µm membrane allowed high recovery (more than 89 %) of Phycocyanin A and C into the retentate and permeate as well as separation of both molecules (depletion of C-PC into the retentate relatively to the A-PC; enrichment into the permeate).

Blue colour permeates, poor in chlorophyll, were successfully produced. Concentrated solutions with more than 80% (dry weight basis) of proteins were obtained. The most severe fouling occurred with the 300 kDa membrane leading to low permeate fluxes (65 L.h -1 .m -2 ). A fractionation of proteins by the size was possible for the 50, 150 and 300 kDa diafiltrations.

Introduction

Microalgae are one of the most promising renewable feedstocks of healthy food ingredients and functional food products. Due to their richness of highly valuable bioactive compounds, they can be used to enhance the nutritional value of food and, thus, to favorably affect human health by improving the well-being and quality of life, but also curtailing disease and illness risks [START_REF] Matos | Microalgae as a healthy ingredient for functional food: A review Food Funct[END_REF].

Among them, Arthrospira platensis (Spirulina), a blue-green alga, is a photosynthetic filamentous cyanobacterium often considered for biomass production due to its high cell growth rate, facility of harvesting and potential market as a human food supplement and animal feed.

The microalga A. platensis possesses high protein content, unique composition of fatty acids, pigments (phycocyanin and β-carotene), and high carbohydrate content [START_REF] Mohapatra | Cyanobacteria: an emerging source for drug discovery[END_REF]. Besides, this microalga contains 46-71% (w/w) (dry weight basis) of protein and has interesting nutritional properties due to its content in essential amino acids, vitamins (particularly vitamin B12) as well as minerals [START_REF] Fox | Spiruline -Technique, pratique et promesse[END_REF][START_REF] Becker | Micro-algae as a source of protein[END_REF][START_REF] Benelhadj | Effect of pH on the functional properties of Arthrospira (Spirulina) platensis protein isolate[END_REF].

Given its interesting characteristics, the cultivation of Spirulina could be a solution to improve human health, especially nutrition in Third World countries and also to develop industrial crops.

This cyanobacterium currently seems one of the best solutions for the simple production of a dietary supplement of high quality and, as a result, large companies have embarked on the culture of this organism on an industrial scale. In addition to proven nutritional properties, Spirulina is now enjoying a revival of interest from the international scientific community because of its possible use as a source of pharmaceuticals, and nutraceuticals compounds.

Indeed, the potential of this microalga seems to be important and this mainly thanks to its two phycobiliproteins, C-phycocyanin (C-PC) and allophycocyanin (A-PC), acting as a component of light-collecting complexes in the cyanobacteria [START_REF] Fernández-Rojas | Nutraceutical properties of phycocyanin[END_REF][START_REF] Cuellar-Bermudez | Extraction and purification of high-value metabolites from microalgae: essential lipids, astaxanthin and phycobiliproteins[END_REF][START_REF] Kannaujiya | Chapter 10 -Phycobiliproteins and their commercial significance[END_REF]. C-PC is the major photosynthetic pigment compared to A-PC [START_REF] Sotiroudis | Health aspects of Spirulina (Arthrospira) microalga food supplement[END_REF]. C-PC has demonstrated a great interest for both nutraceutical and health food industries (natural blue colorant), as well as in biomedical research and clinical diagnostics (fluorescent probes) [START_REF] Mishra | Effect of preservatives for food grade C-PC from Spirulina platensis[END_REF][START_REF] Sekar | Phycobiliproteins as a commodity: trends in applied research, patents and commercialization[END_REF][START_REF] Pandey | Biotechnological applications of cyanobacterial phycobiliproteins[END_REF]. Furthermore, it has also been highlighted that this pigment has positive effects on impaired physiological conditions such as antioxidation, antiinflammation, antitumor, and immunity enhancements [START_REF] Liu | Medical Application of Spirulina platensis Derived C-Phycocyanin[END_REF].

Thus, the whole Spirulina can be used but its refining, that is to say the extraction and purification of molecules from this bio-resource bring real added value. Currently, microalgal soluble proteins are recovered using chemical methods such as pH shifting [START_REF] Benelhadj | Effect of pH on the functional properties of Arthrospira (Spirulina) platensis protein isolate[END_REF][START_REF] Ba | Haematococcus pluvialis soluble proteins: extraction, characterization, concentration/fractionation and emulsifying properties[END_REF], three phases partitioning [START_REF] Waghmare | Concentration and characterization of microalgae proteins from Chlorella pyrenoidosa[END_REF] or aqueous two-phase extraction [START_REF] Desai | Extraction and stability of selected proteins in ionic liquid based aqueous two phase systems[END_REF]. Unfortunately, these environmentally unfriendly techniques use a large amount of solvents and chemicals and can lead to the extraction of low-purity proteins with high salt content, denaturation and poor functional properties, or to be incompatible with industrial-scale applications. In this context, membrane filtration technologies, especially ultrafiltration, are presented as advantageous methods to purify and concentrate components of microalgal biomass with mild operating and chemicalfree conditions [START_REF] Balti | Concentration and purification of Porphyridium cruentum exopolysaccharides by membrane filtration at various cross-flow velocities[END_REF][START_REF] Balti | Prepurification by membrane filtration of paralytic shellfish toxins from Alexandrium minutum dinoflagellate[END_REF][START_REF] Zaouk | Filterability of exopolysaccharides solutions from the red microalga Porphyridium cruentum by tangential filtration on a polymeric membrane[END_REF]. The membrane filtration is notably useful for the concentration of large volumes and can be easily automatized and scaled up to an industrial level [START_REF] Bourseau | Recovery of aroma compounds from seafood cooking juices by membrane processes[END_REF]. Also, this reduces the possible denaturation, deactivation and/or degradation of interest biological molecules. To date, several works have employed this technique on the separation and concentration of microalgal proteins, notably from Chlorella vulgaris [START_REF] Ursu | Extraction, fractionation and functional properties of proteins from the microalgae Chlorella vulgaris[END_REF], Haematococcus pluvialis [START_REF] Ba | Haematococcus pluvialis soluble proteins: extraction, characterization, concentration/fractionation and emulsifying properties[END_REF], Tetraselmis suecica [START_REF] Safi | A two-stage ultrafiltration process for separating multiple components of Tetraselmis suecica after cell disruption[END_REF] and Nannochloropsis gaditana [START_REF] Campos | Developing a filtration process to obtain an enriched microalgal protein fraction[END_REF][START_REF] Safi | Biorefinery of microalgal soluble proteins by sequential processing and membrane filtration[END_REF]. Concerning Spirulina, a water-soluble protein-pigment complex such as phycocyanin can be recovered by membrane filtration after cell disruption. [START_REF] Herrera | Recovery of c-phycocyanin from the cyanobacterium Spirulina maxima[END_REF] [START_REF] Herrera | Recovery of c-phycocyanin from the cyanobacterium Spirulina maxima[END_REF] used ultrafiltration membrane with a molecular weight cut-off of 50 kDa to concentrate up to a volume reduction factor of 1.9 a Spirulina maxima's extract. This methodology made it possible to obtain food grade phycocyanin. [START_REF] Jaouen | Clarification and concentration with membrane technology of a phycocyanin solution extracted from Spirulina platensis[END_REF] [START_REF] Jaouen | Clarification and concentration with membrane technology of a phycocyanin solution extracted from Spirulina platensis[END_REF] obtained 96 % of phycocyanin retention during the filtration on a 100 kDa tubular membrane of an extract resulting from ultrasonic breakage of Spirulina platensis. More recently, [START_REF] Chaiklahan | Separation and purification of phycocyanin from Spirulina sp. using a membrane process[END_REF] [START_REF] Chaiklahan | Separation and purification of phycocyanin from Spirulina sp. using a membrane process[END_REF] obtained 99 % of phycocyanin retention during the filtration of a Spirulina sp. extract, on a membrane with a molecular weight cut-off equal to 50 kDa. These works show that membrane filtration can be used for an almost complete retention of phycocyanin. An original work would be to show that it is possible to use membranes for fractionation/separation of proteins of Spirulina. In this last case, a part of proteins should pass through the membranes while the other part should be retained. The fractionation could in particular make it possible to produce different protein extracts of different size classes: each class could have different bio-activities. Nevertheless, severe membrane fouling can induce a non-selective separation and low permeate fluxes.

Proteins would be difficult to recover in case of membrane adsorption, protein-protein or protein-polysaccharide interactions in the bulk solution and/or membrane [START_REF] Wan | Fractionation of Proteins Using Ultrafiltration : Developments and Challenges[END_REF]. Moreover, filtration performance will depend on the membrane chosen (material, pore diameters), feed protein concentration, physico-chemical characteristics of solutions to be filtered (pH, ionic strength) as well as operating conditions of filtration (pressure, cross-flow velocity, temperature, etc). Thus, only an experimental study can account for extending of membrane fouling and the feasibility to fractionate proteins coming from crushed Spirulina.

Thus, the aim of this study was to evaluate the feasibility to use diafiltration for protein fractionation of Spirulina (A. platensis). Ultrafiltration in diafiltration mode was used to selectively let pass proteins through the membranes and produce different protein extracts more or less purified, notably in salt and chlorophyll. Various molecular weight cut-offs of membranes were used. Membrane fouling extend was evaluated in relationship with the selectivity of operations as well as hydraulic performances.

Material and methods

Food grade dried Spirulina (A. platensis) powder was purchased from Bio Gatrana Society (Sidi Bouzid, Tunisia). Solutions of sodium hydroxide and nitric acid were prepared from analytical grade pellets and 70% of nitric acid respectively. These solutions were used for the cleaning of membranes. All other chemicals used are of analytical grade and were purchased from Sigma Aldrich Chimie (Lyon, France).

Spirulina protein extraction by bead milling

Spirulina suspensions were prepared in Milli-Q® water and used as feed for the disruption experiment. The suspensions contained 20 grams of dried Spirulina per liter of water. Cell disruption was conducted in a Dyno®-mill multi-lab from Willy A Bachofen AG (Muttenz, Switzerland) containing 0.5-0.75 mm glass beads at 80% filling percentage. Experiments were run in continuous mode in 3 passes. Samples were pumped from an agitated feed tank to a 600 mL grinding chamber containing the grinding medium at a flow rate of 150 mL/min and a rotation speed of 10 m.s -1 . The system was operated in batch recirculation mode, with a constant agitation speed of 2986 rpm. At the end of the first passage the suspension was recovered in a second tank and transferred to the first agitated tank to be pumped again. The temperature of the suspension was maintained at ∼10 °C with a cold glycerol solution circulating in the cooling jacket of the grinding chamber. After cell disruption the suspensions were centrifuged using a Sorvall RC6 Plus centrifuge (Thermo Fisher Scientific Inc, Illkirch-Graffenstaden, France) at 12,000 x g for 30 min at 5 °C. The supernatant was stored at -20 °C before thawing at ambient temperature for at least 12 h, just before the membrane filtration experiments.

Filtration experiments

A diafiltration process was conducted using an experimental set-up previously described by [START_REF] Guilbaud | Influence of operating conditions on direct nanofiltration of greywaters: applications to laundry water recycling aboard ships[END_REF] [START_REF] Guilbaud | Influence of operating conditions on direct nanofiltration of greywaters: applications to laundry water recycling aboard ships[END_REF], equipped with a 0.032 m² tubular ceramic membranes (TAMI Industries, Inside CeRAM, Cell 60, 7 channels) with different molecular weight cut offs (50 kDa, 150 kDa, 300 kDa) or pore size diameters (0.2 µm) at a fixed trans-membrane pressure (TMP) of 4 bar and with a cross-flow velocity of 5 m.s -1 . All filtrations were carried out at 20 °C, in batch mode. The TMP and cross-flow velocity were kept constant during experiments.

The supernatant obtained after cell disruption then centrifugation (2 L) was diafiltrated with MilliQ water having a pH and a conductivity respectively equal to 6.5 and 0.12 mS.cm -1 . A constant volume of the solution (2 L) was maintained within the feed tank during the diafiltrations. Five diavolumes were added before stopping filtration (addition of 10 L of MilliQ water). During this step, retentate and permeate samples were taken for future analysis, at the beginning and at the end as well as in the course of the filtration. The permeate is weighted during the filtration using an electronic balance with the accuracy of 0.01 g. Thus, the permeate flux expressed in L.h -1 .m -2 was evaluated from the permeate mass according to equation 1:

J = Mpermeate / (ρ . t . A) (1)
where Mpermeate (kg) is the permeate mass, ρ (kg.L -1 ) is the volume mass of the permeate, t (h) is the permeation time and A (m 2 ) is the membrane area.

After each diafiltration experiment, the membrane was chemically cleaned according to the manufacturer recommendations. Firstly, the membrane was cleaned with sodium hydroxide solution (pH = 12 -Sigma-Aldrich S8045) at 60 °C for 30 minutes in closed loop (recirculation of permeate and retentate into the feed tank). Then, the pilot plant was rinced with MilliQ water until neutral pH at 20 °C in the open loop (discharge of the permeate). Afterward, the membrane was cleaned using a nitric acid solution (pH = 1.2 -Sigma-Aldrich 695041) at 50 °C for 15 minutes in closed loop. Finally, the membrane was rinced by MilliQ water until neutral pH.

This procedure was also used for the membrane deconditioning. More than 90 % of the water fluxes were recovered after the cleaning.

The mean retention rate (R) of components was calculated as described in equation 2: R = (CR -CP) / CR x 100 [START_REF] Mohapatra | Cyanobacteria: an emerging source for drug discovery[END_REF] CR and CP refer to the concentration within the retentate and permeate respectively.

The recovery rates within the retentate (YR) and the permeate (YP) were calculated following After each diafiltration, 9.5 L of its permeate were introduced into a tank in order to be concentrated on a 5 kDa tubular ceramic membrane (TAMI Industries, Inside CeRAM, Cell 60, 7 channels, 0.032 m²). All concentrations were carried out at 20 °C, in batch mode, at a TMP and cross-flow velocity equal to respectively 6 bar and 5 m.s -1 .

Thus, for the rest of the article, the "crude extract" will refer to the supernatant obtained after the centrifugation of the microalga grinding solution. The crude extracts will be diafiltered. The "raw permeate of diafiltration" will refer to the permeate of diafiltration without further treatment. The "mean raw permeate" refer to the whole permeate recovered at the end of a diafiltration process. The "concentrated permeate of diafiltration" will refer to the permeate recovered after diafiltration and concentrated on a 5 kDa membrane. It corresponds to the final retentate of a concentration experiment.

Soluble protein quantification

Protein content was quantified by Lowry's method, using a Sigma-Aldrich kit (BCA1-1KT) [START_REF] Lowry | Protein measurement with the folin phenol reagent[END_REF]. Basically, in alkaline conditions, proteins react with copper to form a cupric complex which is then reduced in cuprous ions, giving a purple color to the solution. First, a solution was prepared by adding 50 mL of BCA solution (Sigma-Aldrich, B9643, bicinchoninic acid, sodium tartrate and sodium bicarbonate in NaOH at 0.1 N, pH = 11.25) and 1 mL of copper sulfate (Sigma-Aldrich, C2284, CuSO4.5H2O, 40 g.L -1 ). Then, 100 μL of the sample was added to 2 mL of test solutions in a 15 mL polypropylene falcon tube. The blank was made with Milli-Q® water. The tubes were homogenized for 5 seconds (vortex at 3000 rpm) before being incubated at 37 °C for 30 minutes. The calibration curve was carried out using Bovine Serum Albumin (BSA) (Sigma-Aldrich, P0914). Finally, the absorbance at 562 nm was measured using V-630 spectrophotometer (Jasco, Bouguenais, France). Each sample was analysed in triplicate.

Phycocyanin analysis

The concentrations of A-PC and C-PC were measured using a UV-VIS spectrophotometer (Jasco, Bouguenais, France) at the wavelengths of 620 and 652 nm. The total amount of C-PC and A-PC were calculated using equations ( 5) and ( 6) [START_REF] Chaiklahan | Separation and purification of phycocyanin from Spirulina sp. using a membrane process[END_REF][START_REF] Bennett | Complementary chromatic adaptation in a filamentous blue green alga[END_REF]:

CC-PC = [A620 -0.474 (A652)] / 5.34 (5)
CAPC = [A652 -0.208 (A620)] / 5.09

Where CA-PC is the Allophycocyanin concentration (g.L -1 ), CC-PC is the C-phycocyanin concentration (g.L -1 ), A620 is the optical density of the sample at 620 nm and A652 is the optical density of the sample at 652 nm.

Chlorophyll analysis

A total of 200 μL of supernatant was mixed with 1,3 mL of pure methanol and then incubated in the dark for 1 h at 45 °C. Subsequently, the samples were centrifuged at 10,000 g for 10 min at 20 °C. The organic phase (methanol) containing the pigments was recovered, and the chlorophyll concentration (mg.L -1 ) was determined by using the equation ( 7) according to [START_REF] Ritchie | Consistent sets of spectrophotometric chlorophyll equations for acetone, methanol and ethanol solvents[END_REF] [START_REF] Ritchie | Consistent sets of spectrophotometric chlorophyll equations for acetone, methanol and ethanol solvents[END_REF]:

Cchlorophyll = (9.3443 A652) + (4.3481 A665) (7)
2.6. Dry matter and ash content determination All recovered fractions (permeates and retentates) were freeze-dried (Christ Alpha 1-2 LD Plus, France) then placed in pre-weighed crucible and dried at 105 °C then 550 °C until constant mass. Thus, the dry matter and ash were determined respectively. The values presented are the average of three measurements.

Total carbon and total nitrogen measurements

Total Carbon and Total Nitrogen were determined with an automated dry combustion method (Dumas method) using an elemental analyser (vario MICRO cube, Elementar Analysensysteme, Hanau, Germany).

Molecular weight distribution determination

Molecular weight distribution of the Spirulina extracts was performed by gel permeation chromatography on a Superdex™ Peptide 10/300 GL column (GE Healthcare, Baie-D'Urfé, Qc, Canada) using a FPLC system (Akta Avant, GE Healthcare, Baie-D'Urfé, Qc, Canada).

The mobile phase consisted of 50 mM sodium phosphate buffer containing 150 mM of NaCl at pH 7.0. The column was calibrated according to the manufacturer's guidelines with proteins of known molecular weight as reference samples. Ribonuclease A (1 g.L -1 ), aprotinin (1 g.L -1 ), vitamin B12 (0.1 g.L -1 ) and bovine serum albumin (1 g.L -1 ) (Sigma-Aldrich, Oakville, ON, Canada) were mixed together and injected (0.5 mL) within the column for the calibration.

Briefly, the Spirulina fraction was solubilized at 1 mg.mL -1 with the mobile phase and then injected (0.5 mL) within the column. Samples were eluted (isocratic) at a flow rate of 0.8 mL.min -1 . Proteins were detected by monitoring the absorbance at 214 nm. UNICORNTM 7.0 software was used for data collection and results analysis. The samples were analysed in duplicate. The samples freeze dried (Christ Alpha 1-2 LD Plus, France) then analysed using a FPLC method were:

-The final retentate of diafiltration experiments -The final retentate of concentration experiments. This sample corresponds to the "concentrated permeate of diafiltration".

Description of separation of compounds

Several parameters were calculated in order to evaluate the degree of separation between a first compound (1) and a second one [START_REF] Mohapatra | Cyanobacteria: an emerging source for drug discovery[END_REF]. The extent of separation (ξ) links the permeate (YP) and retentate (YR) recovery rates following the equation ( 8) [START_REF] Rony | The extent of separation : a universal separation index[END_REF] :

ξ1/2 = abs (YR.1 . YP.2 -YP.1 . YR.2) (8)
Since the highest value of the recovery rate is 1 and the smallest value is 0, ξ varies between 1 and 0. The perfect separation will induce an extent separation equal to 1.

The separation factor (α) was defined as the ratio of recovery rates. The separation factor related to the retentate is defined as the ratio of the retentate recovery rates (YR) as shown in the equation ( 9) [START_REF] Sandell | Meaning of the Term "Separation Factor[END_REF].

αR.1/2 = YR.1 / YR.2 (9)
The separation factor related to the permeate is defined as the ratio of the permeate recovery rates (YP) using the equation [START_REF] Mishra | Effect of preservatives for food grade C-PC from Spirulina platensis[END_REF].

αP.1/2 = YP.1 / YP.2 [START_REF] Mishra | Effect of preservatives for food grade C-PC from Spirulina platensis[END_REF] Finally, sugar or nitrogen contents of dry matter were calculated within the concentrated permeate of diafiltration.

Statistical analysis

All experiments were conducted in triplicate. Statistical analyses were carried out using the professional edition of the Statistical Package for the Social Sciences (SPSS) version 16.0 software package (SPSS Inc. Chicago, IL, USA) and expressed as means ± standard deviation (SD). ANOVA test was carried out and a p-value lower than 0.05 (p<0.05) was considered significant.

Results and discussion

Biochemical characterization and mass balance at the in/outlet of diafiltrations

The pH and conductivity of the crude extract were respectively equal to 7.27 and 3.95 mS.cm -1 .

The mass volume and dynamic viscosity at 20°C were equal to 1001 kg.m -3 and 1.6.10 -3 Pa.s, respectively. The nitrogen and carbon represented 10.9 and 46.0 % of the dry matter weight.

The total protein, C-PC, A-PC and chlorophyll-a concentrations into the crude extract were respectively equal to 11.78, 2.99, 3.06 and 0.030 g.L -1 (Table 1). The presence of chlorophyll induced the green colour of samples. Ash contents of dry matter was initially equal to 9.3 % and decreased up to 2-3 % into all final retentates after diafiltrations (Data not shown).

Desalination of retentates by diafiltration may probably participate in this drop. In the course of diafiltrations, a transmission of proteins through the membranes occurred and particularly through the 0.2 µm membrane since the protein concentration of the permeate was the highest (0.68 g.L -1 -Table 1). Thus, 29 % of proteins were recovered within this mean raw permeate.

On the contrary, few proteins passed through the 50 kDa membrane. Thus, the final retentate of 50 kDa diafiltration remained the most protein concentrated (9.99 g.L -1 ) while its permeate was the lowest protein concentrated (0.19 g.L -1 ). Almost the same protein concentrations (7.50 and 7.27 g.L -1 ) were found, respectively, into the final retentate of the diafiltrations on 150 or 300 kDa membranes. Unfortunately, 8 to 28 % of proteins were lost (not recovered within retentate or permeate) during diafiltrations (Table 2). A part of proteins would stay into the pilot plant, that is to say traped on the membrane or piping surface (participation to the fouling), inside the pump, sensors, etc. The highest recovery rate was obtained during the diafiltration with the 50 kDa membrane (92 %) whereas the lowest (72 %) was obtained in the case of the diafiltration with the 300 kDa membrane. As mentioned later, the 300 kDa membrane seemed the most prone to the fouling since the drop of permeate fluxes was the highest. Consequently, the unrecovered proteins could participate to the fouling of this membrane and increase the resistance formed by the layer near the membrane interface.

Each diafiltration induced good rejection of chlorophyll-a (more than 99%). Thus, the blue colour has been strengthened in all the permeates and especially those from 0.2 μm as shown in the photos of table 3. Thus, permeates could be used as blue dye in food industry. Whatever the diafiltration, 87 to 88 % of chlorophyll-a were recovered, mainly within the retentates (Table 2). Global recovery rates showed no significant difference (p≥0.05) between all membranes used. The 0.2 µm membrane showed a better performance in terms of recovery of C-PC and A-PC than the three other membranes (Table 2). This diafiltration process seems interesting for the recovery and pre-purification of Spirulina phycobiliproteins because it induced little loss of these biomolecules. This diafiltration step appears also more interesting because it allowed the fractionation of C-PC from A-PC. Indeed, the separation factor (αC-PC/A-PC) was the lowest for the mean retentate of 0.2 µm (0.65) while it was the highest for its permeate (2.43) (Table 4).

In other terms there was a depletion of C-PC into the 0.2 µm retentate relatively to the A-PC and an enrichment in the 0.2 µm permeate. The same conclusion was made by the extent separation parameter (ξC-PC/A-PC) which was equal to 0.001, 0.012, 0.041 and 0.220 for respectively diafiltrations on the 50, 150, 300 kDa and 0.2 µm membranes.

The dry matter of the concentrated permeate of 0.2 µm diafiltration was protein rich (approximately 80 % of dry matters); the content of sugars into this dry matter (1.5 %) was the highest but it remained low (Table 4). By taking a ratio of 6.27 between protein and nitrogen, it would seem that more than 80 % of this dry matter (from the concentrated permeate of 0.2 µm diafiltration) would be composed of proteins [START_REF] Safi | Influence of microalgae cell wall characteristics on protein extractability and determination of nitrogen-to-protein conversion factors[END_REF]. It should be noted that a difference of composition exists between mean raw permeate of diafiltration and the concentrated permeate.

Indeed, during the concentration step, a part of compounds can be loose because they can foul the membrane but also because a transmission through the membrane occurs notably salts; desalination takes place during the concentration step.

Finally, the diafiltrations allowed to produce extracts where the proportion of phycocyanin among proteins was increased. Thus, antioxidant activity of the dry matter would be increased comparatively to the crude extract. Indeed, the 0.2 µm diafiltration permeate was enriched in C-PC since the separation factor (αC-PC/Protein) was equal to 1.38 against maximum 1.03 for other ones (Table 4). Retentate of 300 kDa was the most enriched in C-PC with a separation factor (αC-PC/Protein) equal to 1.25 versus maximum 1.02 for others. On the other hand, the 300 kDa permeate and 0.2 µm retentate were A-PC purified since the separation factors (αA-PC/Protein) were the highest that means respectively equal to 1.29 and 1.56.

Molecular weight distributions of concentrated permeates were obtained by size exclusion chromatography in FPLC mode (Fig. 1). Chromatogram profiles were different depending on the membrane used for the diafiltration. Indeed, the shape of curves related to the diafiltration steps with the 50, 150 and 300 kDa membranes were almost similar but relatively different from that obtained with the 0.2 µm membrane. Except for the diafiltration with the 0.2 µm membrane, molecular weight distributions of retentates and permeates were more tight compared to the clarified crude extract. Steric exclusion chromatograms have shown that diafiltrations on 50, 150 and 300 kDa make it possible to almost completely exhaust the retentates in proteins smaller than 0.9 kDa while permeates were almost exclusively composed of proteins of less than 19 kDa (Fig. 1). Thus, molecular-weight fractionation seems to occur during the diafiltration on the 50, 150 and 300 kDa membranes. On the other hand, the diafiltration with the 0.2 µm membrane induced different results. Indeed, proteins larger than 19 kDa would pass through the membrane and the mass distribution remains widespread. UVvisible spectra directly performed on the final retentates of diafiltrations confirmed that the 0.2 µm membrane behaves differently from other membranes (Table 3). Indeed, part of the compounds which absorb between 500 and 670 nm disappears from the final 0.2 µm retentate while no loss seems to occur for the other retentates.

3.2. Performance monitoring in the course of filtrations

Evaluation of the hydraulic performances

All diafiltration experiments were achieved after 287 minutes (Table 1). The diafiltration process using the four membranes (50, 150, 300 kDa and 0.

Evaluation of the selectivity performances

The conductivities of retentates were also monitored in the course of each diafiltration (Fig. 3).

Initially, it was equal to 3.9 mS.cm -1 for the crude extract. The retentate conductivities decreased as diafiltration progressed and finally reached around 157 to 244 µS.cm -1 . During the diafiltrations, pH remained relatively constant between 7.1 and 7.4 (Fig. 3). Almost similar retentate conductivity was reached at the end of the 50 and 300 kDa diafiltrations as well as 150 kDa and 0.2 µm ones. Diafiltrations on 150 kDa and 0.2 µm as well as 50 and 300 kDa membranes seem again to behave very closely. Charged compounds could preferentially be retained by the latter two membranes.

The protein normalized concentrations and retentions were determined throughout diafiltrations (Fig. 4). Protein normalized concentrations of retentates ranged from 48 to 90 %. Whatever the diafiltration, the protein concentrations of retentate were the highest at the beginning of the filtration and then decreased; at the beginning, a part of proteins could clog the membrane and/or pass through the membrane. Interestingly, it should be noted that the rejection rates increased while diafiltrations progressed may be because small compounds were already passed through the membrane or because the fouling of the membrane induced a more impermeable barrier than the membrane itself. It could be interesting to carry out backwashing during diafiltration in order to know if the selectivity performances could be maintained in the course of diafiltrations and consequently if a larger portion of proteins could pass through the membrane. The diafiltration with the membrane having the lowest molecular weight cut-off (50 kDa) induced the highest protein concentrations into the retentate while the permeate was the least concentrated. Consequently, the mean rejection rate was the highest (97.3 %). The highest drop of protein concentration into the retentate concerned the diafiltration on the 0.2 µm. This operating condition would allow let pass the highest quantity of proteins through the membrane. As shown into the figure 4, the protein concentrations into the retentates of all diafiltrations seemed to more or less stabilize after 3 diavolumes. To pursue the diafiltration beyond this level could not be judicious since few proteins would pass through the membranes.

In the same way, A-PC was also monitored during diafiltrations (Fig. 5). Rejection rates increased in the course of diafiltrations. A-PC concentrations were globally the lowest for the 300 kDa retentate. As reminder the permeate fluxes were also the lowest for this membrane (Fig. 2). Consequently, these compounds could participate to the fouling of the membrane that's why they are found in small quantities in the retentate; they could be in significant quantity on the surface of the membrane. After a sharp decrease of A-PC concentration in the 300 kDa retentate, a slight increase would occur. This increase could be due to a release of A-PC foulants into the retentate.

Normalized concentrations (CR/CR0) of C-PC in the retentate were ranged from 50-66, 55-77, 55-87, 47-60 %, for respectively the 50, 150, 300 kDa and 0.2 µm membranes (Fig. 6). The 0.2 µm permeate was always the most C-PC rich which induces the lowest retention rate. This membrane seems very interesting in order to fractionate proteins and more specifically to isolate the A-PC into the retentate. Unfortunately, the passage of C-PC through the membrane decreased during the diafiltration. It may be interesting to carry out backwashing to maintain the membrane in the initial state of filtration in order to keep the C-PC rejection rates as low as possible. Finally, in view of the results of the transmission rates of C-PC, A-PC, proteins through the membranes and notably 0.2 µm membrane, one can ask whether it is economically sustainable to continue the diafiltrations beyond 3 diavolumes because from this, protein, A-PC and C-PC transmission is low.

Conclusion

This study demonstrated the potentiality of membrane diafiltration process for the fractionation of water-soluble proteins from A. platensis after cell disruption by bead milling. Diafiltration with the 0.2 µm membrane proved to be extremely useful and satisfactory alternative in obtaining phycobiliproteins (C-PC and A-PC) enriched fractions free from chlorophyll. Protein recovery and production of various water soluble protein fractions with different sizes, purity and concentration levels was obtained. Several concentrated filtrates could be valorised in food formulation, notably as natural colorant. Nevertheless, future studies will be led in order to demonstrate that several fractions have specific bioactivities and technofunctional properties.

  equations 3 and 4, respectively. YR = (CRend . VRend) / (CR0 . VR0) x 100 (3) YP = (CPend . VPend) / (CR0 . VR0) x 100 (4) CR0 and VR0 refer to the concentration and volume of the crude extract to be filtered respectively. CRend and CPend refer to the concentrations within the retentate and the whole permeate obtained at the end of diafiltrations. VRend and VPend refer to all the volumes of retentate (2 L) and permeate recovered at the end of diafiltrations (10 L). Total recovery rate (YT) corresponds to the retentate plus the permeate recovery rates.

  2 µm) was stopped with the same manner after 5 diavolumes. The recovered volume of permeate was monitored in the course of diafiltrations (Fig.2.a). It was a quasi-linear function of time for all filtrations. So, no drastic change of production capacity occurred during operations. The 0.2 µm membrane appears as the most permeable since its mean permeate flux was equal to 119 L.h -1 .m -2 ; 70, 105 and 65 L.h - 1 .m -2 for respectively the 50, 150 and 300 kDa membranes. Interestingly, the mean permeate flux of the 300 kDa membrane was the lowest and close to the 50 kDa one whereas the molecular weight cut-off of membranes is relatively different. In fact, the polarisation concentration and/or the fouling of the membrane could control the filtration behaviour and induce almost similar permeate fluxes.The permeate volumes obtained during the concentration step on a 5 kDa membrane were also monitored (Fig.2.b). All concentration operations were stopped after the same volume reduction factor. The permeate volumes were not proportional to the time since the slopes of the curves decreased showing the presence of a fouling and/or a change in the polarisation concentration from the beginning to the end of the concentration. Interestingly, the mean permeate fluxes were the highest (61 -63 L.h -1 .m -2 ) for the concentration step of the permeates coming from the 50 kDa and 300 kDa diafiltrations. These diafiltration membranes would allow producing the permeate with the lowest fouling potential of the 5 kDa membrane. On the other hand, the mean permeate fluxes ranged between 44 and 48 L.h -1 .m -2 during the concentration step of the permeates obtained after diafiltration on 150 kDa and 0.2 µm membranes.

Acknowledgments 18 This study was financially supported by the "PHC Utique" program of the French Ministry of Foreign Affairs and Ministry of Higher Education and Research and the Tunisian Ministry of Higher Education and Scientific Research in the CMCU project number 19G1124. Credit authorship contribution statement R. Balti : Conceptualization, Methodology, Validation, Investigation, Formal analysis, Writing -original Draft, N. Zayoud : Investigation, F. Hubert : Investigation, L. Beaulieu : Investigation, Writing, Anthony Massé : Conceptualization, Methodology, Resources, Investigation, Writing, Supervision

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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