Industrialization and urbanization are responsible for the dispersion of anthropogenic heavy metals in atmospheric air, soils and aquatic environments. As a consequence of industrial activities, air emissions were found to contain large quantities of heavy-metals bound to particulate matter and with adverse effects to humans and environmental health [1]. Thus, the potential associations between heavy-metals exposure and decreased pulmonary function in adults were demonstrated [2,3]. Heavy metals exposure is also a serious threat to brain function, resulting in neurodegenerative diseases such as Amyotrophic Lateral Sclerosis, Alzheimer, and Parkinson’s disease [4,5]. Note further that Renal and Urological diseases were associated with environmental heavy metal exposure [6]. Lack of access to potable water becomes a major problem in the world. In particular, the situation is quite critical in some developing countries. Nevertheless, most of water-treatment techniques are not economically viable due to their high cost and difficult maintenance. New methods using efficient, cheap and environmentally friendly adsorbents (and being naturally present in the concerned country), are therefore needed to be developed for the removal of heavy metals from wastewater.  Also, the preparation of the adsorbent and its use has to be simple in order to accommodate rural conditions in developing countries. Nowadays, many researchers have brought great attention on employing the natural and low-cost zeolites as adsorbents and ion exchangers for heavy metals removals [7-12], softening [13, 14] and/or desalination [15-17]. These compounds are hydrated aluminosilicate minerals with the following chemical formula [18-20]:

Where M represents the cationic species bound to the zeolitic framework and n its valence; x and y correspond to the number of “AlO₄” and“SiO₄” tetrahedra per crystallographic unit cell, respectively; and z the number of water molecules per unit cell. A “positive charges” deficit in the zeolitic framework results from the substitution of Si⁴⁺ ions by Al³⁺ ions during the zeolitization process. Zeolites possess pores and cavities with molecular dimensions through which water molecules and monovalent and/or divalent cations (such as: Na⁺, K⁺ or Ca²⁺) can diffuse inside their lattice. These innocuous cations which permit to balance the net negative charge of the zeolite, are known to be capable to exchange with toxic cationic metals in wastewaters. The raw material used in the present work was a brick containing metakaolinite. This brick was made by craftsmen in Central African Republic (CAR) by heating kaolinite-rich soils at 550-650°C for about one week in charcoal ovens. In the lab, the brick was modified by a simple chemical/thermal treatment with sodium hydroxide. The chemical transformation of brick metakaolinite led to zeolites with silicon/ aluminium ratios close to 1, a value which corresponds to a minimum y/x ratio in formula (1) according to the well-known Lonwenstein’s rule [21]. The zeolitic framework then contained a maximum number of negatively charged sites. The present study focused on assessing the adsorption capacity of NaOH-activated brick in the removal of some heavy metals (Cd²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) from aqueous solutions in batch experiments. The dicationic metals Cd²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ were selected in this work because of a growing concern about their contamination in lakes, rivers and/or ground-waters in developing countries. In addition, these metals present a scientific interest concerning fundamental environment studies due to their different physico-chemical characteristics (crystal/ Stokes/ hydrated radius and electronegativity according to Pauling Scale) and their variable (mononuclear and/or poly-nuclear hydroxyl) complexation in water, depending upon the medium pH. In order to describe how cationic metal interacts with alkali brick and obtain comprehensive understanding of the nature of interaction, modelling the experimentally measured data with Langmuir, Freundlich and Dubinin-Radushkevich (D-R) equations was conducted.  Thermodynamic and physical properties of metal ions (ion radius, ionic potential and hydration free energy) were examined as influencing factors able to interpret/explain the relative tendency of cationic metals to absorb on to alkali brick. The viscosity of hydrated metal solution and the entropy of hydration of metal cation (which relates to the ordering of water molecules around the charged ions) were also surveyed and their implication to the adsorption process was appraised.

Adsorbents Preparation

The raw brick used in this study came from Bangui region in Central African Republic. Before use as an adsorbent, several physical / chemical treatments were carried out on the brick. First, it was broken into grains and sieved with sizes ranging from 0.7 to 1.0 mm. Second, brick pellets were treated in our laboratory under the following alkali conditions: : 10 g of Bangui brick reacted in 40 mL of a diluted NaOH solution (0.6 mol.L^-1) at room temperature for one night under slow shaking at a speed of 120 rpm. This procedure was afterwards followed by a fixed-temperature increase of the mixture at 90°C for a constant reaction time of six days. The recovered grains were afterwards rinsed several times with MilliQ water and dried at 90°C for 24 hours.

Chemicals

All chemicals employed in the experiments were analytical grades. Sodium hydroxide; Cd (NO₃)₂.4H₂O; Co (NO₃)₂.6H₂O; FeCl₂.4H₂O; Mn (NO₃)₂.4H₂O; Ni (NO₃)_2.6H₂O; Pb (NO₃)₂; and Zn (NO₃)₂.4H₂O were supplied by DISLAB (France).

ICP-AES Analyses

During batch studies, recovered solutions were analyzed for element contents using ICP-AES (Inductively Coupled Plasma - Atomic Emission Spectroscopy; model Varian Pro Axial View).

Isotherm Study

To study the adsorption behaviour of cationic metal ions on alkaline brick, the experiments for adsorption isotherms were performed at room temperature and under constant stirring condition. Isotherm study was carried out on brick grains with 0.7 -1.0 mm sizes. These experiments were performed in ten 100ml-flasks –each one containing 1 g of brick pellets— in which were added 50 ml of a metal cation solution with a concentration ranging from 5.71x10^-4 to 1.46x10^-3 mol.L^-1. These flasks were placed on a mechanical (orbital) shaker (Model: IKA Labortechnik KS 250 basic) and gently shaked at a speed of 120 rpm. Adsorption-isotherm experiments lasted one night although a reaction time of 4 hours at a temperature of 17°C ± 1°C was sufficient for the system to attain thermodynamic equilibrium. Afterwards, suspensions were filtered and recovered solutions were analyzed in order to determine iron concentrations using an ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy; Model: Varian Pro axial view) spectrometer. The metal uptake and percentage removal were calculated using the relationships:

Where C_o (mg.L^-1) and C_e (mg.L^-1) are the initial and equilibrium concentrations of divalent metal, respectively; V (L) is the volume of the aqueous solution used in batch experiments; and m (g) is the mass of alkaline brick added in the solution.

Electron Microscopy Analysis

Micrographies of representative specimens of modified brick were recorded by using an environmental scanning electron microscope (ESEM, Quanta 200 FEI). Elemental analysis was performed using ESEM/EDS (ESEM, model: QUANTA–200–FEI, equipped with an Energy Dispersive X-Ray Spectrometer EDS X flash 3001 and monitored by QUANTA–400 software elaborated by Bruker). Different surface areas ranging from 0.5 to 3.5 mm² were targeted on modified-brick grains and examined by ESEM.

Characterization of Adsorbent Material

X-ray diffraction__ X-ray diffractogram of raw brick (not shown here) displays a broad signal ascribed to badly crystallized metakaolinite in addition to sharp and intense peaks of quartz and others at lower intensities attributed to illite and rutile [22]. After treating this brick with sodium hydroxide at 90°C for 6 days, metakaolinite was transformed into NaA (LTA) and NaP zeolites. The crystalline structures of alkali brick were previously studied by X-ray diffraction [23]. Briefly, the distinguishing peaks of zeolite NaA (LTA) at 2θ = 7.2°, 12.5°, 16.1°, 30.0° and 30.8° could be imputed to lattice plans of (200), (222), (420), (644 and 820) and (822 and 660), respectively [24]. The distinguishing peaks of zeolite NaP at 2θ = 12.5°, 17.7°, 21.7°, 28.1°, and 33.4° could be imputed to lattice plans of (101), (200 and 002), (211, 112 and 121), (310, 301, and 103) and (132, 213, 312, and 321), respectively [24]. ESEM analysis__ The ESEM image of raw brick displays mainly quartz crystals associated with aluminosilicate aggregates (clays) with surface roughness and cracks (Figure 1).

Figure 1: ESEM images of raw brick (1A) and alkali (zeolitized) brick (1B).

After the chemical treatment of the brick with NaOH, the ESEM image of alkali brick shows new crystalline micro-specimens with either cubic or spherical shapes (Figure 2). The sizes of the cubic crystals range from 7 to 10 µm, and these cubic specimens were found to be comparable with those observed previously for the A-type zeolite by the SEM technique [25,26]. As for the spherical shape specimens with their sizes varying from 4 to 8 μm, they were identified morphologically as NaP zeolite when compared to that reported in the literature [27-30].  The yellowish colour of cubic and spherical particles indicated the abundance of sodium in the zeolitic frameworks present at brick surfaces. Using ESM/EDS, the atomic percentages of sodium in NaA and NaP were found to vary from 8.3 to 9.9. The analyzed Na % on brick zeolites could be assimilated to the “x” indice in chemical formula (1) or the number of “AlO₄” tetrahedra per crystallographic unit cell.

Figure 2: Langmuir isotherms and Feundlich isotherms for Cd(II), Co(II), Fe(II), Mn(II), Ni(II), Pb(II) and Zn(II) adsorption onto zeolitized brick at room temperature.

Adsorption Isotherms

Langmuir Isotherm Model: According to this model, adsorption takes place in a totally homogeneous adsorption surface in which the active sites are identical and energetically equivalent, and in addition no interactions between the adsorbate ions occur. The mathematical expression used for the Langmuir isotherm model [31] is given by:

where Q_e (mgg^-1) is the amount of heavy metal ions (Cd²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) adsorbed on alkali brick pellets at equilibrium; C_e (mgL^-1) is the concentration of heavy metal ions in the solution at equilibrium; Q_max (mgg^-1) is the maximum amount of cationic metals adsorbed on to the adsorbent; and K_L (Lmg^-1) represents the Langmuir constant of adsorption. By plotting 1/Q_e against 1/C_e, straight lines were obtained (Figure 2). From the intercepts and the slopes of these plots, Q_max and K_L values were determined. All the Langmuir-model parameters are listed in (Table 1).

Table1: Langmuir and Freundlich isotherm parameters for Cd (II), Co (II), Fe (II), Mn (II), Ni (II), Pb (II) and Zn (II) adsorptions onto zeolitized brick at room temperature.

The correlation coefficients for Langmuir model (0.9045 ≤ R² ≤ 0.9982) were found to be slightly higher than those of the Freundlich model (0.9065 ≤ R² ≤ 0.9937). Adsorption isotherm data then fitted somewhat better Langmuir model than Freundlich model, and thereby the involved process should be more considered as a monolayer adsorption one.

To illustrate the intrinsic properties of the Langmuir model, the dimensionless parameter, R_L, was evaluated from the relationship:

Where C_o (mgL^-1) is the initial metal concentration. The R_L values calculated for heavy metal cations ranged all from 0 to 1 (see Table 1), suggesting that the adsorption processes of metal ions by alkali brick were favourable.

Freundlich Isotherm Model: This model is an empirical isotherm which corresponds better to the description of a highly heterogeneous surface. Its mathematical expression is given by (see ref. 32):

Where K_F (mg.g^-1) is the Freundlich constant as an indicator of the adsorption capacity; and 1/n_F represents the adsorption intensity. By plotting ln(Q_e) against ln(C_e), the Freundlich model parameters”K_F“ and “n_F” could be assessed from the slope and intercept of the plot (Figure 2). These parameters are listed in (Table 1). The values of n_F, which represents the favourability of the adsorption, were found to be higher than 1.0, revealing that the adsorption of Cd²⁺, Cu²⁺, Fe²⁺, Ni²⁺ and Pb²⁺ ions on to alkali-brick pellets was favourable.

Dubinin-Radushkevich (D-R) Isotherm Model: In order to assess the nature of bonding between heavy metal cations and alkali brick, D-R isotherm was applied to our system. The linear form of the D-R isotherm is given by (see ref. 33):

Where Q_D-R (mg.g^-1) is the maximum mass of the adsorbed species per unit mass of adsorbent; β (mol².J^-2) is the D-R isotherm constant which is related to the mean free energy of adsorption per mole of the adsorbate; and ε is the Polanyi potential which is defined as [33]:

Where C_e (g.g^-1) is the equilibrium concentration of the adsorbate in the solution; R is the gas constant (8.314 J.K^-1.mol^-1); and T is the absolute temperature (K). By plotting LnQ_e against ε², straight lines were obtained (Figure 3) with relatively good correlation coefficients (0.9002 ≤ R² ≤ 0.9783). From the slopes and intercepts of these plots, D-R parameters were calculated and listed in (Table 2). The change in free energy was evaluated from the β value by using the following equation [33]:

Table2: Dubinin-Radushkevich isotherm parameters for Cd (II), Co (II), Fe (II), Mn (II), Ni (II), Pb (II) and Zn (II) adsorptions onto zeolitized brick at room temperature.

It was generally admitted that for a physical adsorption mechanism the ED-R value is ≤ 8 kJ.mol^-1, whereas for an ion exchange and chemical reaction the D-R energy  is about 8-16 kJ.mol^-1 [7]. The E_D-R values which are listed in Table 2 range from 13.128 to 18.432 kJ.mol^-1, indicating the involvement of an ion-exchange mechanism.

Figure 3: Dubinin-Radushkevich isotherms for Cd(II), Co(II), Fe(II), Mn(II), Ni(II), Pb(II) and Zn(II) adsorption onto zeolitized brick at room temperature.

By plotting the molar amount of Na⁺ released in the reaction solution during batch or kinetics experiments against the molar amount of heavy metal adsorbed on to alkali brick, straight lines were obtained (Figure 4), and the slopes corresponding to the molar ratios [Na⁺]_released/[Me²⁺]_adsorbed were found to be close to 2. This finding assumed the occurrence of an interfacial reaction between (sodic) negatively charged sites (>Al?O^- and >Si?O^-) on the brick surface and hydrated metal cations in the solution according to the ion-exchange reaction:

2(>S O^-Na⁺) + Me²⁺  →  (>SO)₂Me + 2Na⁺     -------- (10)

Figure 4: Molar amount of Na+ released in the solution (mol.L-1) against the molar amount of heavy metal adsorbed onto alkali brick (mol.L-1) during batch or kinetics experiments.

Determining Factors for Adsorption of Heavy Metal Ions on Alkali Brick

By fitting the isotherm with the Langmuir model,  the maximum adsorption capacities (Q_max) of Cd²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ were determined as: 85.38, 44.70, 67.87, 62.48, 29.41, 101.82 and 67.03 µmolg^-1, respectively. The Q_max  value which revealed the amount of heavy metal adsorbed on to alkali brick pellets, followed the sequence: Ni²⁺ < Co²⁺ < Mn²⁺ < Zn²⁺ ≈ Fe²⁺ < Cd²⁺ < Pb²⁺. This order is comparable with that found for instance by Choi and his co-workers [34]: Cd²⁺ < Pb²⁺, by using Mg-modified zeolite as adsorbent. However, it differs from that found for instance by Visa [35] and Qiu and his co-workers [10]: Cd²⁺ < Ni²⁺ < Pb²⁺, when using zeolite materials derived from fly ashes. In previous works [7,10,36-38], the authors often ascertained that the observed differences in adsorption performance of zeolitic materials toward heavy metal cations were related to the thermodynamic and physical characteristics of these ions. In order to explain the relative tendency of metal cations to adsorb on to alkali-brick pellets from aqueous solutions, we compiled literature data on thermodynamic and physical properties of these metal ions and, in what follows we attempted to correlate them with experimental results. Heavy metal radii  (Figure 5A) displays the change in crystal radius (R_cryst.), Stokes radius (R_St.) and hydrated radius (R_hyd.) in relation to the maximum adsorption capacity (Q_max) which were determined for heavy metal cations from the Langmuir isotherm model. As can be seen in this figure, the Q_max values of Cd²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ exhibit no clear correlation with crystal radius, Stokes radius or hydrated radius. Based on metal radii and the resulting size order of these ions in relation to Q_max values, one can conclude that the size of metal cation alone would not be a determining factor in the adsorption process studied here. Ionic potential__ this parameter is an indicator of the electrostatic properties of an ion when interacting with other ions either of similar charge (yielding electrostatic repulsion forces) or of opposite charge (yielding electrostatic attraction forces). Ionic potential can be assimilated to the ratio of ionic charge (Z) to crystal radius (R_cryst.) either in relation to Z/R_cryst. or Z²/R_cryst.. As presented in (Figure 5B), the maximum adsorption capacity (Q_max) obtained for each selected heavy metal cation plotted against the ratios Z/R_cryst. and Z²/R_cryst. exhibits a relatively good correlation (R² = 0.8541 and 0.7775, respectively), indicating the implication of electrostatic forces in the adsorption mechanism.

Figure 5: Maximum adsorption capacity, Qmax (from Langmuir model) for cationic metal (Cd (II), Co (II), Fe (II), Mn (II), Ni (II), Pb (II) and Zn (II)) plotted against its crystal radius (Rcryst.), Stokes radius (RSt.) and hydrated radius (Rhyd.) __5A.  Maximum adsorption capacity (Qmax) obtained for  Cd(II), Co(II), Fe(II), Mn(II), Ni(II), Pb(II) and Zn(II) plotted against their ratios Z/Rcryst.. And Z2/Rcryst. (Where Z represents the charge of the cationic metal (+2); and Rcryst. its crystal radius__5B.

This phenomenon is confirmed by the abundance of (sodic) negatively charged sites on zeolite surfaces in case the molar Al/Si ratio or x/y ratio in formula (1) becomes close to 1. nHydration free energy__ The association between a heavy metal cation and water molecules leads to the formation of a hydration shell and the release of (exothermic) hydration free energy, “ΔG_hyd.”. (Figure 6) compares the variation of maximum adsorption capacity of divalent metal ions (Q_max calculated by the application of the Langmuir isotherm model to our system) in relation to their free energy of hydration. For that, we used different values of “ΔG_hyd.” reported by Marcus [39, 40], Noyes Fawcett and Kepp [41-43]. When compared to Marcus, Fawett and Kepp values, maximum adsorption capacity (Q_max) exhibits relatively good correlations with hydration free energy (with correlation coefficients, R², ranging from 0.8405 to 0.8785), whereas better correlations were found between Q_max and Noyes values (R² = 0.9575), however, with less points on the curve (or by taking into account a lower number of heavy metals).  These findings revealed that ?after averaging all the values of hydration free energy (“ΔG_hyd.”) for each metal reported by  [43]? metal cations with smaller “exothermic” hydration free energy like Pb²⁺ and Cd²⁺ ions [-1472 ± 46 kJmol^-1 for Pb (II) and -1799 ± 44  kJmol^-1 for Cd(II)] have softer hydration shells than Fe²⁺ ion (-1895 ± 55 kJmol^-1), Cu²⁺ ion (-2055 ± 45 kJmol^-1) or Ni²⁺ ion (-2004 ± 84 kJmol^-1). Hence, this thermodynamic behaviour of hydrated lead (II) and cadmium (II) enabled an easier rearrangement and loss of the water molecules of hydration around charged ions than for the studied metals at the brick surface before diffusing inside zeolitic pores/channels.

Figure 6: Maximum adsorption capacity, Qmax (from Langmuir model) for Cd(II), Co(II), Fe(II), Mn(II), Ni(II), Pb(II) and Zn(II) plotted against their free energy of hydration which was reported by Marcus, Noye,and Fawcett kepp [39-43].

Influence of the Solution Fluidity and Entropy of Metal Hydration over the Maximum Adsorption Capacity

Jones-Dole viscosity coefficients (B) of ions in solution bring some relevant information about ions solvation and its effect on the arrangement / ordering of water molecules around the charged ions. As a consequence, according to Jenkins and Marcus [44] some potentially important relations / dependences exist between B-coefficients and other ion-additive properties such as: Gibbs free energies and entropies of ions hydration / solvation. Thus, by plotting the viscosity B-coefficient against both the Gibbs free energy (“ΔG_hyd.”; see Jenkins and Marcus [44]) and the entropy of hydration (“Δ_trimS_hydr.” which represents the entropy change for the immobilization of the water molecules around the metallic ion; see Marcus and Loewenschuss [45]) for the following cationic metals: Cd²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺, we obtained in this work some strong correlations between the B-coefficient and the thermodynamic parameters:  ΔG_hyd. And “Δ_trimS_hydr (Figure 7). In what follows, we tried to analyze the Jones-Dole coefficient (B) and entropy of hydration (Δ_trimS_hydr.)  In relation to the ability of cationic metals to adsorb on to alkaline brick. Medium fluidity__ it was previously showed that, as water molecules are arranged around ions to generate hydration shells, water tends to become more viscous (Jenkins and Marcus, [44].

Figure 7: Viscosity B-coefficient plotted against the Gibbs free energy (“ΔGhyd.”; see Jenkins and Marcus, 1995)     7A, and the entropy of hydration (“ΔtrimShydr.”; see Marcus and Loewenschuss, 1984) for the following cationic metals: Cd2+, Co2+, Fe2+, Mn2+, Ni2+, Pb2+ and Zn2+  7B.

In order to appraise the importance of the viscosity of hydrated metal solution (η) in the adsorption process studied, the medium viscosity was referred to that of the pure water (η_o) by using the Jones-Dole expression [44]:

Where A and B are ion specific constants; and c is the solute concentration. The Jones-Dole viscosity coefficient (B) was considered here as a potential indicator of the degree of water structures around the ion, and thereby, of the bonding strength of hydration shells covering the cationic metal. Taking into account this consideration, we assumed in the present work that, in the case of this adsorption system, the mobility of the ions through zeolitic pores/channels should depend as well on the fluidity of the medium near the brick surface. As fluidity is defined as the reciprocal of viscosity (1/η), we then used the reciprocal of the Jones-Dole viscosity coefficient (1/B) as a potential indicator of the fluidity of the liquid in the aim to explain the ability of metal ions to diffuse through NaA and NaP pores of brick zeolites. By plotting the maximum adsorption capacity (Q_max) of alkali brick towards heavy metals against the 1/B ‘fluidity’ parameter (Figure 8A), it was shown that the fluidity of the ions solution affected the cationic adsorption performance. The Q_max value increased with an increase of the medium fluidity and hence this latter

Where A and B are ion specific constants; and c is the solute concentration. The Jones-Dole viscosity coefficient (B) was considered here as a potential indicator of the degree ofwater structures around the ion, and thereby, of the bonding strength of hydration shells covering the cationic metal. Taking into account this consideration, we assumed in the present work that, in the case of this adsorption system, the mobility of the ions through zeolitic pores/channels should depend as well on the fluidity of the medium near the brick surface. As fluidity is defined as the reciprocal of viscosity (1/η), we then used the reciprocal of the Jones-Dole viscosity coefficient (1/B) as a potential indicator of the fluidity of the liquid in the aim to explain the ability of metal ions to diffuse through NaA and NaP pores of brick zeolites. By plotting the maximum adsorption capacity (Qmax) of alkali brick towards heavy metals against the 1/B ‘fluidity’ parameter (Figure 8A), it was shown that the fluidity of the ions solution affected the cationic adsorption performance. The Qmax value increased with an increase of the medium fluidity and hence this latter contributed favourably to adsorption yield. Entropy of metal hydration__There is also some correlation between the “ΔtrimShydr.” entropy change for the immobilization of the water molecules around the metallic ion and the Qmaxcapacity (Figure 8B), reflecting the further dependence of the Qmaxvalue with the degree of cohesion of the water molecules within the hydration shell of the cationic ion. This investigation revealed clearly that a Qmaxincrease was intimately related to a higher ΔtrimShydr. Entropy change as a result of a weaker packing density of water molecules or a ‘softer’ hydration shell (which was evidenced mostly in the case of the Pb2+ions adsorption). Indeed, the cationic metal with the easiest ionic passage (i.e., Pb2+) had the highest hydration free energy (an averaged value: ΔGhydr.)= -1513 kJ.mol-1; see Kepp [43] and the biggest entropy change for the immobilization of the water molecules around the metallic ion (ΔtrimShydr. = -151 J.K-1.mol-1; see Marcus and Loewenschuss [45] when compared to the values measured for the other metals studied here.

Figure 8: Maximum adsorption capacity (Qmax from Langmuir model) of alkali brick towards heavy metals against the reciprocal of the Jones-Dole viscosity coefficient, 1/B (8A) and the “ΔtrimShydr.” entropy change for the immobilization of the water molecules around the metallic ion (8B).

Comparison of the Uptake Capacity of Alkali Brick with Literature Data

The Qmax values obtained for alkali brick are somewhat lower than those generally found in the literature with different zeolites or mesoporous materials (Table 3). However, this material still remains an interesting adsorbent for producing potable water in rural regions of Central African Republic because of its easy accessibility, making and maintenance. Furthermore, after metal saturation adsorbent regeneration can easily be undertaken by a concentrated solution of sodium chloride followed by a diluted solution of sodium hydroxide [46]. The presence of sand (60-65 wt %) in the brick contributes strongly to assure a good permeability and to facilitate the flow of water through brick beds during adsorption column experiments [23].

Table 3: Comparison of the uptake capacity of alkali brick towards Cd (II), Co (II), Fe (II), Mn (II), Ni (II), Pb (II) and Zn (II) with that of some zeolites or mesoporous materials in previous studies. (nat.): natural; (mod.): modified.

A composite of zeolites (NaA and NaP) and sand was prepared from a brick containing metakaolinite by means of a chemical activation with sodium hydroxide. This material was applied for the removal of heavy metal cations from aqueous solutions in batch adsorption experiments. From isotherm analysis, experimental data were well described by Langmuir and D-R adsorption models, suggesting the involvement of a monolayer adsorption and an ion exchange mechanism. The maximum adsorption capacity (Q_max) determined from Langmuir isotherm model followed the sequence: Ni²⁺ < Co²⁺ < Mn²⁺ < Zn²⁺ ≈ Fe²⁺ < Cd²⁺ < Pb²⁺. The observed differences in Q_max values were found to be related to two ion characteristics: (i) one physical, the ionic potential as an indicator of the electrostatic properties of the cationic metal; and (ii) another one thermodynamic, the hydration free energy of the hydrated metal as an indicator of the loss of water molecules in its hydration shell, thus enabling the transfer of the resulting “dehydrated” cation through pores/channels of NaA and NaP zeolites. This transfer led finally to an interfacial sodium/metal exchange, as evidenced by chemical analyses of solutions recovered from batch experiments. The Q_max value was also found to be intimately related to: (i) the medium fluidity; and (ii) the degree of cohesion/ordering (i.e. entropy of metal hydration) of water molecules within the hydration shell of the cationic ion. Thus, the ionic passage through the meso/macro-pores of NaA and NaP zeolites at brick surfaces was improved by a decrease of the viscous effects due to inter-ionic interactions and an increase of the entropy of hydration (Δ_trimS_hydr.) of metal cation.

Nicole Poumaye: Conceptualization, Methodology, Software, Validation, Formal analysis.

Oscar Allahdin: Conceptualization, Methodology, Software, Validation, Formal analysis.

Ludovic Lesven: Conceptualization, Methodology, Supervision.

Michel Wartel: Conceptualization, Methodology, Supervision.

Abdel Boughriet: Conceptualization, Methodology, Writing.

Declaration of Competing Interest

The authors declare no competing financial interests.

Acknowledgments

These scientific works were undertaken successfully owing to the cooperation between the University of Lille (France) and the University of Bangui (Central African Republic). This collaboration (being still under way) and the Grant-in-Aid to Nicole Poumaye for her Doctoral-Thesis preparation have been financially supported by the Embassy of France at Bangui. The authors gratefully thank David Dumoulin (Chemical Engineer) and Véronique Alaimo (Chemical Technicians) for helping us usefully in certain delicate chemical and nalytical/spectroscopic analyses.

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