Use of La-, Ce-, Y-, Fe- bentonites for Removing Phosphate Ions from Aqueous Media

Clays play an important role in the environment. By removing cations and anions either through ion exchange, adsorption and precipitation, or all these combined, they can act as natural decontaminating agents of numerous pollutants. In this study, four modified bentonites (La-, Ce-, Y-, Fe-bentonite) were prepared and characterized, and their phosphate sorption capabilities were measured in batch experiments. Equilibrium times were also examined. The activation energy of the sorption process was calculated. The La- , Ce- and Y-bentonite can bind similar amount of phosphate ions, while iron-bentonite can bind only half of it compared to La-, Ce- and Y-bentonite.

Many information exists about La-modified bentonite Phoslock, invented by CSIRO (Commonwealth Scientific and Industrial Research Institution) in Australia in the mid-1990s [10]. Beside the La-modified bentonite, more economical alternatives, such as yttrium (III), cerium (III) and iron (III) ions modified bentonites were also investigated.
The cation exchange capacity (CEC) was determined by ammonium acetate method [20]. The CEC value of Ca-bentonite is 81 meqv/100g.
For iron (III)-bentonite, FeCl 3 (50 g) was solved in acetone while cooling bath was used because of significant solvation heat. After the whole of FeCl 3 was dissolved, and a stoichiometric amount of Ca-bentonite (based on calcium and iron (III) ions) was added to the system. The product was placed on a magnetic stirrer for 1 hour, in order to facilitate homogenization and it was filtered. Then the bentonite was washed with acetone and air dried at room temperature [21].
The K α -lines of the lanthanide elements can be excited with 185 MBq 241 Am radioactive source.

Kinetic studies of phosphate on modified bentonites
Phosphate sorption experiments were conducted using a batch technique. The modified bentonite (100 mg) was added to 20 cm 3 of KH 2 PO 4 (5x10 -4 mol/dm 3 ). During this measurement the rate of sorption of phosphate ion was measured on La(III)-, Ce(III)-, Y(III)-and Fe(III)-bentonite after 10-300 min shaking. All samples were placed in a Bühler KS15A shaking bath at the desired temperature (1, 25, 40 °C). For cerium and yttrium the soprtion was faster, so the reaction time was at 2-120 min. After the sorption samples were filtered through a 0.45 μm pore size syringe filter, the concentration in the supernatant (c e ) was measured by the molybdate blue spectrophotometric method using a Cary 100 UV-VIS spectrophotometer at 660 nm [22]. pH was measured with Clean PH200 pH meter, and Clean Cs1030 combined glass electrode. The amount of the sorbed phosphate was calculated in the following way: the difference of the initial and the measured phosphate concentration (after the given time) was divided by the amount of bentonite sorbent. In addition to the photometric measurements, the kinetics of the phosphate sorption was calculated using the software called Scientist. The first-order kinetic model Eq. (1) is: where t is the time, A is the phosphate concentration in bentonite at any time, A e is the maximum amount of sorbed phosphate on bentonite, k is the first-order rate constant.

Equilibrium studies of phosphate on modified bentonites
The sorption isotherms were determined by batch equilibration of 100 mg of each bentonite sample with 20 cm 3 of aqueous phosphate solutions of varied initial concentrations (ranging from 0.005 to 0.0005 mol/dm 3 ). All samples were placed in a Bühler KS15A shaking bath at the desired temperature. In order to determine the effect of temperature on sorption, isotherms were established at 1, 25, 40 °C. After the equilibrium time (1h) defined in the kinetic experiments the samples were filtered through a 0.45 μm pore size syringe filter and then the concentration in the supernatant (c e ) was measured with spectrophotometer. The amount of the sorbed phosphate was calculated in the following way: the difference of the initial and the measured phosphate concentration (after the given time) was divided by the amount of bentonite sorbent (a e ). The Langmuir equation (2) is commonly used to describe phosphate equilibrium sorption on soils, phyllosilicate minerals and metal oxides, and can be expressed mathematically as follows: Where a e is the amount adsorbed at equilibrium (mol/g), c e is the equilibrium solution concentration (mg/l), z is the maximum sorption capacity (mg/g) and K is solubility [17].

Desorption experiments of phosphate on modified bentonites
These experiments aimed to determine the quantity of phosphate which can undergo desorption. The modified bentonite (100 mg) was added to 20 cm 3 of KH 2 PO 4 (5x10 -4 mol/dm 3 ). The samples were placed in a Bühler KS15A shaking bath at 25 °C. After the sorption samples were filtered through a 0.45 μm filter and then the concentration in the supernatant (c e ) was measured by the molybdate blue spectrophotometric method using a Cary 100 UV-VIS spectrophotometer at 660 nm. The modified bentonites saturated with phosphate (100 mg) were added to 20 cm 3 of distilled water. All samples were placed in a Bühler KS15A shaking bath at 25 °C. After the equilibrium time the samples were filtered through a 0.45 μm pore size syringe filter and then c e was measured with spectrophotometer. REE and Fe concentrations measured by X-ray fluorescence analysis are summerized in Table 1. The REE content is given in mol/g. The bounded REE's are given in percentage compared to CEC. This means that the REE concentration was higher than expected from CEC.

Sorption kinetics of phosphate on modified bentonites
The sorption kinetic data of phosphate on the four bentonites are presented as a function of contact times (Figs. 1-4). The sorption of phosphate on the four bentonites increases with time.  In order to estimate the activation energy (Ea) of the phosphate sorption, the kinetic data were analyzed by the Arrhenius equation (3). The activation energy (Ea) was calculated from the slope of the ln (k) versus 1/T plots.
where E a is the activation energy (kJ/mol), R is the gas constant (8.314 J/K mol), T is the temperature (K), and k 0 is Arrhenius constant [23]. The calculated Ea values are presented in Table 2 and in Table 3, which show that these values were higher than the corresponding value for other adsorbents. For example, Ea value for the adsorption of phosphate onto dolomite is 5.44 kJ/mol [24]. Similar E a results were obtained for La-bentonite, according to Hagseresht [10] the E a value was higher than expected. In our opinion, these activation energy values cannot be too high because the typical range of the diffusion activation energy of heterogeneous reaction is in the range of 8-33 kJ/mol. The diffusion of the relatively large phosphate ion in the interlayer space of montmorillonite is thought to be the rate determining step. Our previous studies [25] revealed that the iron (III) ions sorbed on bentonite produce iron oxide hydroxide compounds in the interlayer spaces. Moreover, the sorbed amount of iron (III) exceeds the cation exchange capacity suggesting that these compounds can be present on the outer surfaces. The iron (III) oxide hydroxide compounds can sorb the phosphate more easily than interlayer rare earth cations.

Isotherms of phosphate on modified bentonites
The isotherm of phosphate on Y-bentonites is presented in Fig. 5. Similar figures were obtained for the other modified bentonites. The corresponding isotherm parameters were obtained by plotting ce/a (g/dm 3 ) versus ce (mol/dm 3 ), where a e is the amount adsorbed at equilibrium (mol/g), c e is the equilibrium solution concentration (mg/l). The sorption of phosphate on modified bentonites reaches equilibrium at about 1 h.
Under the same experimental conditions, La-, Ce-and Y-bentonites can bind similar amount of phosphate ions. Iron (III) bentonite, despite its high numbers of active sites binded only half much phosphate as La-, Ce-, and Y-bentonite. The high solubility of the bentonite-phosphate-iron(III) complex inhibits the phosphate sorption in some degree. The number of active sites (Table 4) on La-, Ce-and Y-bentonite is nearly equal.
The solubility of the sorption complex (rare earth ionphosphate-bentonite) ( Table 4) decreases as the temperature increases. The tendencies of active sites and solubility vs time are similar to rare earth bentonites, showing the exothermic phosphate sorption reaction. The number of active sites is the highest at 1 °C and is in a good agreement with the rare earth concentration of the modified bentonite, proving the REEPO 4 (rare earth ion-phosphate) surface precipitation mechanism. This suggests an exothermic surface precipitation process. It is interesting to note that the solubility of some phosphate precipitates changes just at the same way [26].

Desorption of phosphate on modified bentonites
Based on the measurements, a negligible amount of phosphate was dissolved during desorption.

Conclusion
Four modified bentonites (La, Ce, Y, and Fe(III)) were applied to remove phosphate ions from aqueous solution. The kinetics, equilibrium time, sorption isotherms were examined.
The activation energy (E a ) of phosphate sorption on rare earth bentonites varied between 22-37 kJ/mol. Phosphate ions sorb in the interlayer space as REEPO 4 . The activation energy of phosphate sorption on iron (III) modified bentonite is less than that of rare earth bentonite, namely 10 kJ/mol.
Under the same experimental conditions, La-, Ce-and Y-bentonites can bind similar amount of phosphate ions, while iron only half much phosphate as La-, Ce-, and Y-bentonite.
A negligible amount of phosphate was dissolved during desorption. Examination about enhancement and re-use of the sorbents will be carried out.
These results show that modified clays could play an important role in the treatment of eutrophication processes in the aquatic environment. Ce-oxide is 2.5 times cheaper than La-oxide. Fe(III)-chloride is 25 times cheaper than La-oxide. Preparation of these two modified bentonite is more economical.