Physicochemical properties of adsorbents.
In this study, the adsorption of Copper Cu (II) and methyl Orange (MO) from aqueous solution, on Tamanrasset’s unmodified and modified Kaolinite clay which as low cost adsorbents, was studied using batch experiments. The adsorption study includes both equilibrium adsorption isotherms, kinetics and thermodynamics study. For the characterization of the adsorbent several properties are determined such as pH, the Specific Surface Area, the Point of Zero Charge and the Cation Exchange Capacity. Indeed, various parameters were investigated such as contact time, initial metal and dye concentration, mass of solid, pH of the solution and temperature. The adsorption process as batch study was investigated under the previews experimental parameters.
- Methyl Orange
The extensive use of chemicals in developing and developed countries over the last century increased the amount of dyes and heavy metals which is released into surface and underground water through discharges of wastewater produced from metallurgical, mining, chemical, research laboratories, printing paper and battery manufacturing industries [1, 2]. Copper (Cu (II)) is extremely toxic, not biodegradable and it accumulated in living organisms, and may thus pose a threat to human beings. In addition, Copper ions is considered as vital transition metal ion because of its necessity in biological activities of living organism, whereas at certain concentration, it causes serious damages to human health and environment [3, 4, 5].
Methyl orange (MO) is an azo soluble dye, shows low biodegradability and is soluble in water hence it is difficult to remove from aqueous solutions by common water purification methods. As other dyes MO is toxic and carcinogenic, posing serious hazards to humans and the environment .
Purification of water can be achieved by physicochemical and biological methods. The physicochemical methods include precipitation , membrane filtration , liquid–liquid extraction  reverse osmosis , electrolysis or ultrasonic electrolysis , electrodialysis , electrodeposition , ion exchange resins , incineration and electrokinetic method , flotation , flocculation , coagulation , photocatalysis , adsorption and biosorption . Biological process includes biodegradation or bioremediation , phycoremediation . Adsorption method is more effective, economic with high potential and low energy consumption specially it has the advantage of the utilization of abundant with low cost adsorbents.
Clay minerals in the soil play the role of a natural scavenger by removing and accumulating pollutants through ion exchange and adsorption process. Generally, these minerals are categorized into Montmorillonite, smectites, Illite and kaolinite . Kaolinite is a harmful charge clay mineral with a soft consistency. It made up of a silicon tetrahedral (T) sheet and an aluminum octahedral (O) sheet which called 1:1layer clay (2 sheets). Kaolinite has considered as an excellent adsorbent clay because of its high specific surface area, high exchange capacity, large potential for ion exchange, surface charges, charge density, chemical and mechanical stability, a variety of surface and structural properties, hydroxyl groups on the edge, silanol groups of crystalline defects or broken surfaces, and Lewis and Brönsted acidity [24, 25, 26, 27]. Kaolinite clay also has been widely accepted as low cost abundant adsorbent for the removal of copper and methyl orange from wastewater due to the surface structure and edges. Recently many studies used natural kaolinite , other scientists used purified kaolinite in other wise other researchers used organo kaolinite as adsorbents . However, kaolinite in its different geological origins may have variable chemical compositions and structural deformation. Elements such as Fe, Ti, Mn, Mg, and Cr, and impure phases, such as quartz, and Illite, are normally contained in natural kaolinite. Even, a small amount of these impurities may significantly affect the chemical properties of kaolinite. Those researches were carried out with physical or chemical modification of kaolinite in order to enhance the properties of kaolinite and to increase the adsorption capacity.
The aim of this paper is to access the ability of Tamanrasset’s kaolinite clay and its derivatives (purified, activated, pillared and modified) to adsorb Cu(II) and MO from aqueous solution. The effect of the contact time, temperature, mass of solid, solution pH and concentration of the adsorbate was studied. The kinetics and factors controlling the adsorption were also studied. Therefore, the physicochemical characteristics of materials were considered such as pH, the specific surface area, the point of zero charge and the cation exchange capacity.
2. Material and methods
2.1 The adsorbate and solution
The adsorbates in this study were copper and methyl orange prepared by dissolving CuCl2, 2H2O and MO into 1000 mL of deionized water to stock solution concentration of 1000 mg/L, the adjustment of pH in the solution was achieved by adding NaOH and HCl 0,1 M are from Sigma-Aldrich. The desirable experimental concentrations of solutions were prepared by diluting the stock solution with distilled water when necessary.
2.2 The adsorbents
2.2.1 Raw clay
The natural clay sample was obtained from Tamanrasset south of Algeria. It was prepared before use by sun drying, then ground into fin particles and sieved to a particle size of 300 μm after that, it was stored in a desiccator for late experimental use.
2.2.2 Purified clay
The clay was purified with the purpose of removing all crystalline phases and organic matter according to the procedure described by Robert and Tessier in order to obtain 2 micrometers clay fractions intercalated with sodium ions (Clay-Na) .
2.2.3 Activated clay
The chemical activated clay was carried out by adding 50 g of raw clay to 500 ml of sulfuric acid 1.5 M and refluxing at 110°C for 4 h. The resulting clay suspension was then rapidly quenched by adding 500 ml of ice water. After cooling the sample was washed several times with distilled water until neutral pH, then filtered, dried in oven and calcined at 500°C for 4 hours [31, 32].
2.2.4 Pillared clay
The pillaring solution was prepared by the slow addition of NaOH 0.225 M to a 0.5 M solution of AlCl3 at room temperature, until a molar ratio of OH/Al = 1.8 was reached. The pillaring of Clay-Na by polycation of aluminum is carried out according to the conventional procedure (cationic exchange with a heat treatment) .
In 500 ml Buchner, 10 g of purified clay was added to 250 ml of CTAB solution 0.02 M, the mixture was stirring at room temperature for 24 h. Then, the suspension was filtered and washed several times until the negative test of Br-. The CTAB-Clay was dried at 105°C for one hour, ground and stored in sterile glass box [34, 35].
2.3 Physicochemical characteristics of adsorbents
2.3.1 The pH of adsorbent
Ten grams of crushed kaolinite was stirred in 75 ml of deionized water in a beaker over the night then it filtered. The pH was measured by a glass electrode (pH METER HI2210) .
2.3.2 The specific surface area
Sear’s method was chosen to estimate the surface areas of clay adsorbents . 0.5 g of each clay was acidified with 0.1 M HCl to a pH 3–3,5. The volume was made up to 50 ml with distilled water after addition of 10 g of NaCl. The titration was carried out with standard 0.1 M NaOH buffer solution from pH 4 to pH 9. The volume, V, required to raise the pH from 4 to 9 was noted and the surface area was computed from the following equation:
2.3.3 The point of zero charge
2.3.4 The cation exchange capacity
The CEC of materials was calculated by methylene blue method .
2.4 Bath equilibrium studies
Bath experiments were performed in asset of 250 mL Erlenmeyer flasks that contain a volume of 100 mL in each flask of fixed initial concentrations of metal and dye solutions. The flasks were kept in a thermostated water bath (Wise bath® eed Back Control Digital Timer Function, Laboratory Instruments) shaker at a constant speed of 150 rpm for 360 min (6 h). The sample solutions were filtered at equilibrium to determine the residual concentrations. The amount of adsorbate adsorbed at the equilibrium condition, qe (mg/g), was calculated by the following Equations :
Were C0 and Ce are the initial and equilibrium metal and dye concentrations (mg/L), respectively. V is the volume of solution (L) and W is the mass of adsorbent used (g).
The concentration of Cu(II) before and after adsorption was determined by a flam atomic absorption spectrometry (Analyst 700 Perkin Elmer Atomic Absorption Spectrometer) and the concentration of MO was determined by UV–Visible spectrometry (M209T Spectronic Camspec).
2.5 Kinetics studies
The kinetic experiments, adsorption capacity of Cu (II) and MO at time t, qt (mg/g), was calculated as follows:
Were Ct (mg/L) is the concentration of copper/Methyl orange at any time t (min).
3. Results and discussion
3.1 Characterization of adsorbents
3.1.1 pH and the point of zero charge
The surface of kaolinite has a net positive surface charge at pH < PZC, whereas at pH > PZC, it has a negative surface charge. The PZC value or natural kaolinite was8.9 this value indicates that the adsorption of copper and methyl orange by untreated kaolinite will occur at pH ≥ 8.9.
3.1.2 Surface area
The specific surface area of 5 to 25 m2/g for the untreated kaolinite clay. The specific surface area of natural, purified, activated, pillared, and CTAB kaolinite are measured as 25.4, 36.7, 30.14, 42.1 and 43.15 m2/g, respectively. The specific surface area increased up to 30.1 for activated clay. Such the high value of specific area is not achieved in the present work by treatment with 1.5 M H2SO4 acid. The chemical modification opens up the edge of the platelets and as consequence, the surface area and the pore diameter increase too.
3.1.3 Cation exchange capacity
The kaolinite used in the present work had CEC of 5.92 meq/100 g as measured by methylene blue method. Kaolinite as other clay minerals contain both Brönsted and Lewis acid cites associated respectively with the interlamellar region and the edge sites.
The ions exchange capacity of kaolinite is attributed to the structural defects, broken bonds and structural hydroxyl transfers. Chemical modifications increase the total number of exchange sites. The results obtained are summarized in Table 1.
|Properties||pH||Point of Zero Charge||Specific Surface Area (m2/g)||Cation Exchange Capacity (meq/100 g)|
3.2 Bath studies
3.2.1 Effect of adsorbent dose
The adsorbent dose is an important parameter too because it determines the capacity of adsorbent for a given initial concentration of metal solution.
Figure 1a and b showed that the amount adsorbed qe decrease with the increase of adsorbent dosage. This is due to the increase in surface area and hence more available adsorption sites competing for the same number of initial ion concentration .
3.2.2 Effect of time contact and initial concentration
Equilibrium time is an important parameter in the studies of wastewater treatment. The adsorption of Cu2+ onto clay (0,08 g of raw, purified and pillared clay, 0.1 g of activated clay and 0.06 of modified CTAB clay) at various initial concentrations (5, 25, 50, and 100mg/L), and in the case of methyl orange the dosage of modified and unmodified clay was 0.1 g at various initial concentrations (5, 20, and 35 mg/L) was studied as a function of contact time in order to determine the necessary adsorption equilibrium time.
Figure 2a and b shows the effects of contact time and initial concentration on the adsorption of copper and methyl orange into raw clay. We notice that the adsorption is rapid at the initial stages during 10–50 minutes. This is due to the fast that there were a large number of vacant sites for metal on the external surface of clay particles. Then it’s gradually decreases with the progress of adsorption until the equilibrium is reached because of the active sites saturation of the adsorbent [43, 44]. As shown in Figure 2, the contact time for the Cu(II) and MO to reach equilibrium was 50 min and 60 min, respectively.
The Table 2 below shows the adsorption capacity of adsorbed at the equilibrium (qe) increased with an increase in the initial concentrations.
3.2.3 Effect of pH on the amount adsorbed
pH is the most important environmental factors influencing not only site dissociation, but also the solution chemistry and in the efficiency of adsorption , it effects both the dye structure and the surface on the adsorbent. As seen in Figure 3 the sorption capacity of copper increased whenever pH of the solution increases. While, the adsorption amount of methyl orange onto the clay increases as pH is lowered from 11 to 2, confirmed that the initial solution pH is a key adsorption parameter that strongly affects metal and dye adsorption because of the decreased positive charges on the adsorbent surface with an increased pH. The pH of the solution also affects the solubility and species of adsorbate, the adsorbents, and the degree of ionization of the adsorbate [46, 47, 48].
3.2.4 Effect of temperature on the amount of adsorption
Because temperature is an important parameter for adsorption process and bath adsorption studies were carried out at different temperatures and concentrations of Cu(II) ions and MO dye. The amount of sorption increasing with increasing temperature witch indicates that the nature of this adsorption is a chemical sorption . Whereas, the adsorption capacity of MO onto raw and activated kaolinite decrease whenever temperature increase, as shown if Figure 4, indicating possibilities of reversible adsorption process .
3.3 Kinetic studies
3.3.1 Pseudo-first order kinetic model
The pseudo first-order kinetic model is expressed as:
3.3.2 Pseudo-second order kinetic model
The pseudo second-order kinetic model is given as:
|Adsorbent||C0 (mg/L)||qe,exp (mg/g)||Pseudo-first order||Pseudo-second order|
|k1 (min−1)||qe,cal (mg/g)||R2||k2 (g/mg.min)||qe,cal (mg/g)||R2|
|Adsorbent||C0 (mg/L)||qe, exp (mg/g)||Pseudo-first order||Pseudo-second order|
|k1 (min−1)||qe, cal (mg/g)||R2||k2 (g/mg.min)||qe, cal (mg/g)||R2|
3.4 Adsorption isotherms
The adsorption isotherm of Cu2+ and MO was checked whether it fits the Langmuir and Freundlich isotherms (Table 5).
|qmax (mg/g)||KL (L/mg)||R2||KF (mg-11/n/gL1/n)||1/n||R2|
The Langmuir isotherm theory assumes that the adsorption is single-layer and takes place at homogenous sites specific to the adsorbent. The equation of Langmuir isotherm model is given as:
and the Freundlich isotherm assumes that the adsorption is multi-layer and that the surface of the adsorbent heterogeneous :
Where KL Langmuir isotherm constant (L/mg), n is the degree of non-linearity and KF is Freundlich isotherm constant (mg1–1/n L1/ng).
The data show that Langmuir model is more suitable to describe the adsorption reaction of copper and methyl orange on modified kaolinite and it is better fitted to Freundlich isotherm model onto natural clay with experimental data with higher R2 values. So it’s surface mono-layer adsorption and the adsorption sites are homogeneous.
3.5 The thermodynamic studies
The thermodynamic parameters
|ΔH° (KJ.mol−1)||ΔS° (KJ.mol−1)||ΔG°(KJ.mol−1)|
|303 K||313 K||323 K|
The thermodynamics and kinetic study of the removal of copper (II) and methyl orange from aqueous solution using natural clay and its composites has been investigated in this work. The adsorption process showed that purified and CTAB clays were effective in the uptake of copper and methyl orange from aqueous solutions until 34.12 mg/g and 15.78 mg/g, respectively. Whereas, activated and pillared kaolinite are not efficient adsorbent for the removal of Cu(II) and methyl orange, respectively; with amount of adsorption less than 2 mg/g. The amounts of Cu2+ and MO were found to vary with pH and the dose of adsorbent. The adsorption data conformed to Langmuir model, and its fitted to pseudo first order and pseudo second order. However, pseudo second order best described for the adsorption process. The determined negative free energy changes ΔG° and positive entropy ΔS° indicated the feasibility and spontaneous nature of the adsorption process. The negative value of enthalpy change ΔH° suggests that the adsorption process is exothermic, while, the interactions are entotermic accompanied by increase in entropy and Gibbs energy.