Laterite is a red or brown rock formed by weathering in tropical climates. In the broadest sense, it refers to all loose or indurated materials, rich in iron hydroxides or aluminum hydroxides, constituting soils, superficial horizons and deep horizons of weathering profiles. Laterites are found mainly in the intertropics. They cover almost 40% of the world’s continents. Lateritic soils are lean, leached and depleted in silica and fertilizing nutrients. Oxidized iron gives the red color of a laterite. Iron minerals such as goethite (FeOOH); siderite (Fe(CO)₃); ankerite (Ca(Fe,Mg)(CO)₃)₂); chlorite [(Mg,Fe)₅Al (Si₃Al)O₁₀ (OH)₂] and biotite K (Fe,Mg)₃ AlSi₃O₁₀ (OH)₂ can constitute metallotects hosting metalliferous concentrations (Al, Fe, Mn, Ni, Cu, Co, Cr, Ti, Au, Ag, Pt), some of which are of economic interest to countries in the intertropical zone [22] .

The laterite used in this study was taken from the Sabodala mining complex (Massawa). Tailings were collected from the mine’s first tailings storage pond. The lateritic armour was finely ground in an agate mortar and sieved to 80 µm ( Figure 1 ). The fine fraction was used for the various laboratory experiments. Mineralogical and chemical characterization of laterites and tailings was determined by XRD and ICP-OES.

Major minerals were identified with room temperature bulk-rock X-Ray Diffraction (XRD) analyses conducted at the “Institut des Matériaux de Nantes-IMN”, using a Brüker “D8 Advance” powder diffractometer operated in Bragg-Brentano geometry with a Cu anode sealed X-ray tube and a focusing Ge (1 1 1) primary monochromator (selecting the Cu Kα1 radiation; λ = 1.540598 Å). The XRD patterns were obtained on bulk powders between 3.5 and 70˚ 2Θ with steps of 0.0157˚ and a counting time per step of 0.8 s.

Alkaline fusion method was used to dissolve laterite samples and rock/soils reference materials. 100 mg of sample powder was stirred with 300 mg of lithium metaborate flux in a graphite crucible. The mixture was then heated at 1050˚C for 15 minutes and cooled to room temperature before being dissolved into a beaker filled with 100 mg of 5% HNO₃. Major and trace elements were analysed by ICP–OES (iCAP-6300, Thermo). The external calibration curves were constructed using the reference materials BIR-1, JA-1, AGV-2, Mica-Fe, FER-2 and the recommended values from Govindaraju K. [23] for BIR-1, JA-1, AGV-2 for Mica-FER and FeR-2. The boron derived from the 300 mg lithium metaborate flux was used as internal standard to monitor and correct for any instrumental drift.

MCA tests are based on a methodology developed by Cruz et al. [24] modified by Villeneuve [25] . These are also kinetic tests involving wetting-drying cycles [20] [23] . To study the environmental behaviour of mine tailings and the Reactive Barrier effect of laterite, we will perform two kinetic tests in MCA ( Figure 2 ). The first, known as the control test, consists of placing 100 g of mine tailings on 1.2 µm filter paper in a Büchner. In the second Büchner, the filter paper is topped with a 5 cm thick layer of laterite, and 100 g of residue is placed on top. Each Büchner is placed on a stand surmounting a 250 ml vacuum Erlenmeyer flask to collect the leachate ( Figure 2 ). The device is brought to room temperature. Leaching is carried out twice a week (every 3 and 4 days alternately). During each leach, 100 ml of deionized water is introduced into each MCA and remains in contact with the residues for a period of 3 hours. The water is then removed from the cell and collected in the Erlenmeyer flask. Electrochemical parameters pH, redox potential and conductivity are measured after 3 hours of rinsing. Part of the leachate is filtered and acidified for metals analysis by ICP-OES.

The retention rate of contaminants by laterite is calculated using the following relationship:

$\text{Retention rate}=\frac{Mc-Ml}{Mc}\times 100$ (1)

1) $Mc$ = Cumulative leached mass (mg) of the control MCA.

2) $Ml$ = Cumulative leached mass (mg) of the MCA with laterite.

The results of chemical analysis of laterites by ICP-OES are presented in Table 1 and Table 2 . The iron oxide level of the laterite studied represents 37.2%. This material is richer in ferric oxides than that from Dano (Burkina-Faso), i.e., 20.4% [27] . However, they are less affluent than Ouagadougou (Burkina-Faso), i.e. 42.31% [28] .

X-ray powder diffraction mineralogical analysis of laterites is shown in Figure 3 . Mineralogical analysis by X-ray diffraction shows that the laterites studied are composed of quartz (SiO₂), kaolinite (Al₂Si₂O₅(OH)₄), hematite (Fe₂O₃) and goethite (FeO(OH)). This composition is similar to that of Burkina-Fasso laterites [27] .

Chemical analysis of the tailings analyzed yields results presented in Table 3 . The arsenic, cadmium, nickel and antimony contents are 420.16, 5.41, 55.33 and 70.72, respectively. These levels are higher than the geochemical background [29] . Mineralogical analysis has shown that the tailings consist of carbonates and silicates. Carbonates are composed of calcite, CaCO₃, and ferrodolomite, CaMg_0.6Fe_0.4(CO₃)₂. Silicates include quartz (SiO₂), albite (NaAlSi₃O₈), muscovite (Kal₃Si₃O₁₀(OH)₂) and chlorite (Mg, Fe)₆(Si,Al)₄O₁₀(OH)₈ ( Table 4 ).

Samples were subjected to several rinses, in each of which physico-chemical parameters were measured to assess the rate of sulphide oxidation and carbonate mineral neutralization [25] [30] . The electrochemical parameters (pH and electrical conductivities) of the leachates are presented in Table 5 .

1. pH

Leachate pH levels vary little overall, and fluctuate around the “natural” pH, i.e. from 6.91 to 7.81 for the geochemical study MCA known as the control MCA. For the laterite beneficiation study MCA, leachate pH values decrease and oscillate between 6.75 and 7.39 ( Figure 4 ). This decrease is due to the presence of acid-generating iron (III) in the lateritic layer. At the start of the experiment, the pH of the leachates is slightly acidic, rising until the thirteenth day of leaching. This rise in pH is due to the dissolution of carbonates such as calcite, CaCO₃ and ferrodolomite (CaMg_0.6Fe_0.4(CO₃)₂). The drop in pH at day 17 shows dolomite exhaustion [31] . By the sixth wash (day 20), carbonate remobilization justifies the rise in pH. This remobilization is due to the dissolution of calcite, (CaCO₃), showing pH peaks. In the seventh wash, pH levels vary slightly but remain alkaline as the silicates dissolve. A comparative study of the leachates shows that the lateritic layer results in a slight decrease in pH. This lateritic influence has no environmental consequences, as pH values oscillate in the 6.5 - 9 range.

2. Electrical conductivity

The electrical conductivities of leachates are shown in Figure 5 . At the start of the leaching process, we observed a high electrical conductivity (12,150 µs/cm) of the leach water for the control MCA compared with that with a lateritic layer (5600 µs/cm). This is due to pre-oxidation of the materials, as explained by Mayer et al. [32] . This is due to pre-oxidation of the materials, as explained by Mayer et al. This stability is due to a lack of release of ions retained by the lateritic mass, thus showing chemosorption as the mechanism for retaining the ions responsible for electrical conductivity. This variation in electrical conductivity shows that laterite can act as a reactive barrier for ions present in mine tailings.

At the first wash, a high sulphate content was observed in leachates from the control MCA (3500 mg/l) compared to the MCA with lateritic layer (1202 mg/l) ( Figure 6 ). This is due to pre-oxidation of sulphides in mine tailings, as Mayer et al. [32] indeed explain. It’s interesting to note that pre-oxidation of a grain can modify the reactivity of sulphides, so the contact between our samples and water plus oxygen removed the oxidized surface part of the particles, taking 20 days to arrive at the unoxidised core of the mineral (sulphide passivation). The comparative study shows that the lateritic layer leads to a lowering of sulphate levels. The downward trend in sulphate levels in the leachate is due to the depletion of sulphates in the tailings and the non-release of sulphates retained in the laterite layer. The cumulative mass of sulphate leached from the control MCA is 412.6 mg greater than the 237.5 mg from the MCA with lateritic layer. The retention rate of the lateritic layer is 42.43%. The lateritic layer enables sulphates to be retained without being released.

The time trend in cumulative arsenic leaching from MCAs has not reached a plateau ( Table 6 , Figure 7 ). The absence of a plateau shows that mobilization kinetics are slow. Arsenic release from mine tailings into MCAs is low. In the control MCA, the cumulative mass of arsenic in the leachate is equal to 0.38 mg. That of the MCA containing laterite is equal to 0.17 mg. These leached masses are very low compared with the initial mass of arsenic in each mining residue, which is equal to 42.01 mg. The mobility factor for arsenic is low [22] .

Arsenic is considered to be a rather immobile element because most of this metalloid is bound to the residual mineral fraction. The lateritic layer acts as a reactive barrier to arsenic, reducing the leached mass from 0.38 g to 0.17 g. In terms of the results of the environmental study, the results of the kinetic tests confirmed that arsenic concentrations are much lower in the MCA with a lateritic layer. This result shows that laterite can be used to manage arsenic discharges generated by CND.

The time trend curves for the cumulative quantities of antimony leached from the test leachates are shown in Figure 8 . The control curve increases until day 20, before reaching a plateau at 0.07 mg. The curve for the cumulative quantities leached from the laterite-containing MCA shows three phases. A first phase with zero content up to the fourth leach. A second phase of increasing levels up to the seventh wash cycle, with a maximum of 0.03 mg. After this stage, the curve reaches a plateau at 0.03 mg. The initial antimony mass in each tailing is 7.84 mg. The masses of antimony leached into the MCAs are low (0.07 mg for control MCA and 0.03 mg for MCA with lateritic layer). As with arsenic, the distribution of antimony in mine tailings indicates that this element was rather immobile [33] [34] . The presence of steps in the time curves of cumulative antimony leaching masses shows that all the mobile fraction is leached. For the control MCA, 0.89% of the antimony represents the mobile fraction. In the case of a lateritic layer, 0.38% passes into the leachate. This difference shows that 0.51% was retained by the laterite barrier. Lateritic soil is an excellent sorbent for metalloids, removing 55.26% arsenic and 57.14% antimony ( Table 7 ).