Adsorption kinetics of 2-chloroacétophénone and 4-bromoacétophénone thiosemicarbazone on Oueme Valley clays ( R . Benin )

Laboratoire National de Pharmacognosie, Centre Beninois de la Recherche Scientifique et Technique (CBRST), BP 06 Oganla, Porto-Novo, Benin. Laboratory of Physic and Synthesis Organic Chemistry (LaCOPS), University of Abomey-Calavi, Faculty of Sciences and Technics (FAST),BP: 4521 Cotonou, Benin. Laboratory of Physical Chemistry, University of Abomey-Calavi (UAC), Faculty of Sciences and Technics (FAST), Cotonou, Benin. Louvain Drug Research Institute (LDRI), School of Pharmacy, Université catholique de Louvain, B1 7203 Avenue Emmanuel Mounier 72, B-1200 Brussels, Belgium.

Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Tabe 1. Physicochemical characteristics of synthesized 2-CAT and 4-BAT.Eluent CH 2 Cl 2 / EtOAc 2/1 (Fatondji, 2011) natural material with various therapeutic applications (antibacterial, antiseptic).
In Benin, this natural resource still awaiting exploitation is available in the Oueme valley and is scarcely used by local population to cure ulcers, muscular lesions, and persistent diarrhea.In order to incorporate these thiosemicarbazones into clays as excipients to improve their therapeutic efficacy, the clay must be endowed with good capacities of sorption and desorption of these organic compounds.The aim of this paper is to follow up the evolution of the clay adsorption of 2chloroacetophenone thiosemicarbazone (2-CAT) and 4bromoacetophénone thiosemicarbazone (4-BAT) in ethanol.
White, yellow, black and red consumables clay, were taken from the Oueme valley, in the township Dangbo and Aguegues (southern of Benin).A portion of each sample was dried in an oven at 105 ± 2°C, powdered, sieved to 180 μm and stored for the sorption study.The second parts were kept in plastic boxes at room temperature for toxicity studies, because freezing may reduce their sensitivity (Caquet., 1994).

Characterization of clays
Water and organic material content, acid-base nature, specific surface area and cation exchange capacity of each sample of clays were determined.

Water content
Water content was determined by Audigié et al. (1978) method.

Organic material content
Clay's organic material content is measured according to NF EN 12879 norm.

pH measurements
The pH was measured following the method proposed by Aïssata (2009).

Cation-exchange capacity (C.E.C.)
Cation-exchange capacity (CEC) is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, available for exchange with the soil solution.It is expressed as milliequivalent of hydrogen per 100 g of dry soil (m eq /100 g), C.E.C was assessed according to Metson (1956) for clays below 180 μm and following the AFNOR NF X 31-130 norm.

Specific surface
The specific area in m 2 /g is assessed by methylene blue colorimetry according to Hicham El (2006).

Cytotoxicity test on shrimp larvae
The test is performed against Artemia salina Leach by the method of Michael et al. (1956) resumed by Vanhaecke et al. (1981) and Sleet and Brendel (1983) and proposed in the literature as a simple bio -assay method for assessment of preliminary toxicity of natural active products (Solis et al., 2001).The eggs of A. salina were incubated in sea water until hatching of young larvae (48 h).Then, series of solutions of each tested clay sample at varying and progressive concentrations were prepared.A defined number of larvae, (sixteen) were introduced into each solution.All solutions and control solution containing no active substance were left under stirring for 24 h.Counting under a microscope the number of dead larvae in each solution was used to evaluate the toxicity of the solution.In the case where there was death in the control medium, the data was corrected by Abbott's formula: Data (dose-response) are transformed by logarithm and the LC 50 were determined by linear regression (Hafner et al., 1977).Tests were carried out in triplicate.

Sorption technique
The initial concentrations of thiosemicarbazone were: 0.0625, 0.125, 0.25 and 0.5 mg/ml.0.02 g of clay (< 180 μm) is mixed with 2 ml of an ethanolic solution of thiosemicarbazone.The suspensions are stirred for 10 h at room temperature.The supernatant is assayed by high performance thin layer chromatography (HPTLC) after centrifugation to determine the  residual thiosemicarbazone concentration.

Physicochemical parameters of the clays
As can be seen in Table 2, the studied clays are rather hydrophilic with a content of water ranging between 33.38 and 59.95%.The white (59.95%) and black clays (54.73%) are notably more hydrophilic than the red (33.38%) and yellow clays (36.70).Clay's CEC values are found in the range 16.01 and 23.79 m eq / 100 g.These figures are satisfactory since Morel (1996) found for kaolinite values between 5 and 15 m eq /100 g, for illite and chlorite values between 10 and 40 m eq /100 g.These clays exhibit a weak adsorption capacity in accordance with their low specific area (2.29 -4.82 m 2 .g - ).The white clay exhibits the highest specific surface.Clay's pH figures are all below 7, attesting to their acidic character.Yellow, red and black clays are found clearly more acidic (3.5 ≤ pH ≤ 5), than the white one (6.04).

Larval toxicity
The shrimp larvae are sensitive to clays.We noticed a gradual increase in the number of dead larvae gradually as the extract concentration increased (Figure 1).The LC50 (Table 3) compared to the values of table drawn by   Mireille (1995) sparkling (Table 4) shows that these clays are not toxic.Thus, consumables clays of the valley can be used without toxicological risk.In view of these results of LC 50 figures (Table 5) compared with those established by Mireille (1995) demonstrate the lack of toxicity of these clays.

Adsorption kinetics of 2-CAT and 4-BAT in ethanol
According to Figures 2 and 3, adsorption kinetics of 2-CAT and 4-BAT on white clay shows that there is no apparent equilibration time for adsorption of thiosemicarbazones by this clay and that adsorption is reversible.After a fast initial phase, adsorption is maximal after 2 h.Desorption follows the same kinetic behaviour and reaches a plateau after 2 h.Thiosemicarbazone's adsorption is considerably influenced by its initial concentration in ethanol.Maximal sorption goes up to 63% for 4-BAT and 89% for 2-CAT after 2 h equilibration.

Sorption isotherm of 2-CAT and 4-BAT
A sorption isotherm (also adsorption isotherm) describes the equilibrium of the sorption of a material at a surface (more general at a surface boundary) at constant temperature.It represents the amount of material bound at the surface (called the sorbate) as a function of the material present in the solution.In this study, sorption isotherms were determined at room temperature after 2 h equilibration and were found of the concave S-shape type, suggesting a low affinity of the white clay (Figures 4 and 5 and Table 5) for thiosemicarbazone derivatives according to Giles et al. (1974).Modeling of these isotherms give intensity parameters which fit with those computed following to the models of Langmuir and Freundlich (R 2 > 0.8).

Conclusion
Selected clays originating from Oueme valley of Benin are not toxic.They are highly hydrophilic, rich in organic materials and are of acidic nature.They exhibit low specific area and CEC comparative analysis of their characteristics shows that the white clay, less acidic and hydrophilic shows the best adsorption capacity.Adsorption isotherms suggest a low affinity between the white clays and thiosemicarbazones and can be rationalized following the models of Freundlich and Langmuir.
Adsorption kinetics demonstrates that adsorption is reversible and optimal after 2 h.

Figure 1 .
Figure 1.Dose-response curves illustrating the sensitivity of shrimp larvae to clay species.

Figure 2 .
Figure 2. Kinetics test of adsorption 4-BAT on the white clay in ethanol medium.

Figure 3 .Figure 4 .
Figure 3. Kinetics test of adsorption 2-CAT on the white clay in ethanol medium.

Figure 5 .
Figure 5. Adsorption isotherms of 2-CAT on the white clay in ethanol medium at 25 ± 2°C. .

Table 3 .
LC50 value of the various supplies clays.