Kinetic and Thermodynamic Study of the Dephosphation of Wastewater by Clay Materials from Côte d’Ivoire

Phosphorus is introduced into the aquatic environment from different point sources, mainly by domestic and industrial wastewater; contributing to the eutrophication of water bodies. The most common way for wastewater dephosphation is the injection of expensive chemicals into these bodies of water. Thus, the main objective of this work was to find an alternative to chemicals by using clay materials from Côte d’Ivoire to adsorb phosphate ions from aqueous solutions. The clay samples, taken from various regions were characterized by X-ray diffraction. They have a different mineralogical composition. The influence of various parameters such as the pH of the medium, contact time, the ion force, temperature, etc. on the adsorption was studied. Adsorption is influenced by the temperature, the pH of the medium, valence of the saturated cation and the clay composition. It is described by a kinetic model of the pseudo-second order. The salt of the medium has no significant effect on the process. The thermodynamic study revealed that adsorption is spontaneous, endothermic and that it is done by electrostatic means of physical nature.

nancial health, etc., but the various anthropic activities exercised by man on the natural environment considerably degrade the quality of the environment and water resources. Indeed, the demographic growth of urban cities, the establishment of industrial areas near waterways, the use of chemical fertilizers, the development of livestock have considerably increased the production of wastewater that is often enriched in organic matter and nutrients (phosphorus and nitrogen, in particular) necessary for plant growth. These waters, when discharged into nature without adequate treatment, cause an imbalance in aquatic ecosystems and eutrophication of lakes, rivers, streams and watercourses with low hydrodynamics [1] [2] [3] [4].
In fact, eutrophication is characterized by the anarchic appearance of aquatic plants (algae, macrophytes...) [5] and plankton when a bodie of water contains a high concentration of phosphate and nitrogen. These invasive aquatic plants considerably reduce the quantity of drinking water, prevent the practice of sports, tourism activities, fishing (industrial or artisanal), the production of drinking water [3] [5] [6], which has very significant repercussions on the human and socio-economic health of a municipality or a region. Today, eutrophication is a major environmental problem.
To solve this problem of eutrophication, manual and mechanical removal, the application of herbicides, biological attack and the use of financial and material resources are commonly used, but it turns out that these processes are not very effective and that aquatic plants reappear immediately. In addition, several studies have proposed preventive and curative methods to fight against eutrophication [2] [7] [8]. These studies point out that phosphorus is the limiting factor [1] [9] [10] [11] on which action must be taken to reduce the proliferation of aquatic plants, and consequently the limitation of the eutrophication phenomenon. The dephosphatation of wastewater is undoubtedly a means of controlling eutrophication. It can be done by physicochemical processes based on precipitation reactions with iron, aluminum or calcium based reagents, biological processes with bacteria and adsorption which has many advantages, and can be done with several types of materials including biomaterials.
In a policy of environmental protection, biomaterials, natural, abundant, inexpensive and with great capacities of phosphate adsorption, are increasingly used [12] [13] [14]. It is appropriate to complement these studies by valorizing other biomaterials as phosphate adsorbents. It is within this framework that our study is included, with the ambition to participate in an environmental protection policy to use clay materials from Côte d'Ivoire for wastewater dephosphation.
To achieve this goal, after having collected clay samples in various regions of Ivory Coast, we will determine their mineralogical characteristics, then study the influence of various parameters such as the contact time, the pH of the medium, the temperature, the ionic strength and the quantity of clay on the adsorption, finally to identify the mechanisms of adsorption brought into play and to make a comparative study of the effectiveness of the adsorbents.

Clay Samples
The study is carried out on three different clays of the soils of Ivory Coast. These are a sample from Korhogo noted KOR, a sample from Katiola noted KAT and a sample from Anyama noted ANY. These samples were previously characterized by X-ray diffraction using a BRUKER D8 ADVANCE apparatus operating at the wavelength λ = 1.7903 Å of cobalt.
The clay samples taken from the soil, are dried in the shade for several days, then they are crushed in an agate mortar. The resulting powder is sieved to 100 μm. Then, these well-crushed and sieved samples, underwent sedimentation to collect the clay fraction with diameter < 2 µm. Then, the collected clay fractions, were treated with saturating cations (Mg 2+ and/or Na + ). This treatment resulted in 6 clay solutions at 10 g•l −1 , designated ANY-Na; ANY-Mg; KAT-Na; KAT-Mg; KOR-Na and KOR-Mg that served as adsorbents.

Adsorption Procedure
The solution used as the phosphate source was obtained by dissolving potassium hydrogen phosphate KH 2 PO 4 in distilled water.

Mineralogical Analysis
The X-ray diffraction spectra obtained are presented in Figures 1-4: • The diffractograms in Figure 1 and Figure    • The diffractograms in Figure 3 and Figure 4 show that the KAT

Mineralogical Composition
The theoretical results of the mineralogical composition of the clay samples are given in Table 1.
The results show that sample ANY is richer in quartz (25.69%), illite (21.44%) and chlorite (23.54%) than the other two samples, however it contains the lowest content of smectite (12.50%). The highest smectite content (20.90%) is contained in sample KAT. As for sample KOR, it is relatively rich in goethite

Effect of Contact Time
The results obtained during the tests related to kinetics, are presented in Figure   5(a) and Figure 5(b).

Effect of pH on Phosphate Removal
The results obtained during the tests relating to the influence of the pH of the medium on the adsorption of phosphates, are presented on Figure 7(a) and  The results show that the quantity of phosphate adsorbed by the samples increases between pH = 2 and pH = 6 then a strong decrease is observed when the pH is higher than 8. This decrease is probably due to the charged surface of the clay losing the electrostatic attraction on the phosphates. Moreover, the evolution profile of the quantity of phosphates adsorbed from the samples ANY-Na, ANY-Mg, KAT-Mg and KOR-Mg, shows a plateau between pH 4 and pH 8. For samples KAT-Na and KOR-Na, the evolution profile shows a peak around pH 6. Overall, the adsorption capacity reaches a maximum value between pH 4 and 8.

Effect of Ion Force on Adsorption
The study of the effect of the salinity of the medium on the adsorption, is represented on Figure 8(a) and Figure 8

Effect of Temperature
The tests relating to the influence of the temperature on the adsorption of phosphates by the clay, gave the results represented on Figure 9(a) and Figure   9(b).
These results reveal that increasing the temperature increases the adsorption capacity of the clay. This suggests that the adsorption reaction is endothermic.

Kinetic Models of Adsorption
The kinetic study of adsorption allows us to examine the influence of contact time on retention. For this purpose, the pseudo first order (Equation (1)) and pseudo second order (Equation (2)) kinetic models were applied to describe the mechanism of phosphate adsorption kinetics by clay samples.
The parameters of the two kinetic models are grouped in Table 2.
According to the parameters reported in

Thermodynamic Study of the Adsorption
The thermodynamic parameters are determined from the following equations [17] [18]: The values of the thermodynamic parameters of the different clay supports used are grouped in Table 3.

Discussion
The In an acidic environment, the adsorption of hydronium ions H 3 O + creates additional positive charges on the clay surface, which could increase the electrostatic attraction of phosphate anions. The distribution curve in Figure 10 shows that the phosphate anions are progressively put into solution from pH 2. Thus, the phosphates bind to the charged surface of the clay as they are produced, resulting in an increase in the retention efficiency of the phosphates. A similar result was obtained by [12] [24] [26]. As for the reduction of the performance of retention of phosphorus when the pH is basic, would be related to several factors.  PO − arriving at the adsorbent surface [21]. In a basic environment, negative charges on the clay surface outnumber positive charges. Therefore, the pH of the medium has an effect on phosphate adsorption. However, the rate of phosphate adsorption is better around a neutral pH. This result corroborates those of [27] [28] [29] [30]. According to [27], the natural environment of pH varying between 5.5 and 7.5 is favorable for phosphate removal.
The study of the effect of temperature on adsorption showed that the reaction is endothermic in nature. This result is in agreement with those of [27] [32] [33].
Above 313 K, a decrease of the adsorption capacity was observed for some samples. This decrease could be due to the desorption of some phosphates. A similar result was obtained by [23] [24] during nitrate and phosphate removal by a modified bentonite. In Effect, a decrease in adsorption capacity was observed at temperatures above 303 K by [23] and 311 K by [24] respectively. The magnitude of the free energy ∆G is used to determine the type of adsorption process [13]. Thus, ∆G values greater than −20 kJ•mol −1 , as in our case, would correspond to an electrostatic interaction between the adsorption sites and the phosphate ions. This result explains well the observations of the influence of pH on adsorption. The values of ∆H, which are lower than 40 kJ•mol −1 , suggest that the adsorption is due to physisorption or physical adsorption and involves only relatively weak intermolecular forces such as Van Der Waals forces. However, this type of mechanism is characterized by its rapidity and reversibility by allowing an accumulation of layers (multilayer formation). Hence the rapid adsorption kinetics during the first hour of contact and the desorption observed beyond 313 K. Moreover, the increase in temperature could make the bonds involved in the process even weaker, which could lead to the detachment of the adsorbate from the surface and pass into solution.
The phase of increase of the dephosphation rates when the salt concentration is lower than 10 −2 mol•l −1 , shows that the presence of alkaline elements in solu- Alkalis not retained by the clay, could form screens that would prevent some phosphates from accessing the adsorbent surface so that further adsorption of phosphates would be less easy. This could explain the nearly constant rates when the salt concentration is greater than 10 −2 mol/L. However, the presence of Ca 2+ , Mg 2+ , etc. does not interfere with the adsorption process [34].
The performance of magnesian samples must be due to the binding intensity of cations mediating the binding of phosphate ions. This intensity depends on the valence and the degree of hydration of the cation [35]. In fact, weakly hydrated bivalent ions such as Mg 2+ , i.e. surrounded by a weak layer of water, are more energetically retained than strongly hydrated monovalent ions such as Na + [35].
Thus during saturation of clay samples, the amount of magnesium bound would be large compared to that of sodium. Therefore, magnesium bridges would be more numerous than sodium bridges. Moreover, according to Figure 10, the monovalent phosphoric ion 2 4 H PO − would be the predominant species in the pH range of our tests (pH between 6 and 7). Thus during the adsorption process, two 2 4 H PO − ions could be retained via a Mg 2+ ion, while via a Na + ion, only one 2 4 H PO − ion could bind. As a result, magnesian samples could remove more phosphates than sodium samples.  were carried out. The results of the kinetic showed that the adsorption process follows the model of pseudo-second order. The values of the thermodynamic parameters indicate that the reaction is a spontaneous, endothermic physisorption (or physical adsorption) and that a disorder occurs at the solid-liquid interface during the reaction. The adsorption of phosphates is influenced by the pH of the medium, the temperature, the valence of the saturating cation and its degree of hydration, the presence of iron oxide and the mineralogical composition of the clay. However, the salinity of the medium does not disturb the process.

Conclusions
In perspective, it would be pertinent: • to treat the natural effluents with the clay materials. Indeed, the natural water has a much more complex composition than the synthetic solutions which were used for this work. Thus a comparison could be made on the thermodynamic and kinetic plan and to check if there is similarity of the results; • to study the desorption of phosphates retained on the surface of the clay in order to consider a probable use of the sludge produced as fertilizer in agriculture since some samples showed a tendency to desorption.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.