Efficient Adsorption of Trivalent Rare Earth Elements Using Nano Copper Ferrite : Kinetics, Isotherms and Thermodynamics

Introduction

REEs consists of lanthanide series and scandium plus yttrium.They have 4f orbital as inner electronic configuration and outer shell has 5d^1-10 and 6s² electrons. This electronic configuration makes lanthanides more special for fast reactions. Their physical characteristic properties are almost close due to similar electronic configuration^1-3.They have difference in chemical properties because of lanthanide contraction of ionic radius and  electrostatic effect associated with increase of shielded nuclear charge through electrons. The trivalent form {M(III)}is the most thermodynamic stable form^4-6. Lanthanides have many applications in various fields such as super conductors, fertilizers, Lasers, catalysts and optical fibers. As they are unable to pass through cell membrane, they are non toxic.In the present work La(III) and Ce(III) are selected which are from light REEs family⁷.

Lanthanum is the 57^th element in the periodic table,it causes nervous system disorder in humans. Also the petroleum industry waste, which has lanthanum, dumped into waste waters.This causes severe damage to plants and animals⁸. The atomic number of cerium is 58 and the pure metal itself can ignite if scratched with knife. It is strong reducing agent and causes itching, sensitivity to heat and skin lesions are experienced⁹.The main interest in selecting La(III)and Ce(III)is that they are 80% global consumption and also present in industrial waste water.The main aim is to obtain a potential adsorbent for REE extraction from aqueous media.

Till date several methods are applied for the extraction of REE mainly using solvent extraction¹⁰, electro chemical precipitation¹¹, Ion exchange¹² and adsorption¹³. Among them adsorption is preferred  due to its reversibility, eco friendly nature, cheap, simplicity and hence applicable for the REE ions in aqueous samples¹⁴.However the investigation of REE from aqueous solution is limited. Adsorption is treated as efficient, quick and low cost procedure in the recovery of metals ¹⁵.

Nano spinel copper ferrites are used for the adsorption of REE from aqueous solution. As these spinel ferrites are famous for their thermal stability, chemical and magnetic properties.They are widely used as catalysts, magnetic fluids and microwave radiation isolators¹⁶. Nano spinel copper ferrite is prepared using co-precipitation method which is most widely used in the synthesis of ferrite nano materials. The nano particles are prepared by using the procedure explained Suryakala et al., and characterised as per analytical techniques¹⁷. These copper ferrites have tetragonal structure and are more stable at room temperature. Due to their thermal, chemical stabili ty and low toxicity these materials are used as adsorbents in the adsorption of above said  REEs¹⁸.

Experimental

All chemicals used are of analytical grade.The chemicals like Lanthanum nitrate, Cerium nitrate ,NaOH, HCl are purchased from Qualigen’s chemicals and glass ware. Solutions are prepared using deionised water (18.2M Ώ Cm). Solutions are prepared  just before the usage.  The nano spinel copper ferrite is synthesized using co precipitation method. In Coprecipitation the solutions are mixed directly without further time delay. It is an eco friendly procedure. Due to simplicity and high yield co-precipitation method is most widely used methods in the synthesis of magnetic nanoferrites. Co-precipitation is simple with high crystalline and textural properties.  

Synthesis of CuFe₂O₄:

Aqueous solution of iron nitrate Fe(NO₃)₃.9H₂O and copper nitrate Cu(NO₃)₃.6H₂O with stoichiomeric ratio Cu: Fe = 1:2 were stirred for one hour (for each 1 gram of nitrate, 3.3 ml of milli Q water are used. 1molar NaOH solution is added, for each

1 gram of nitrate, followed by further stirring for 15 minutes. The material is annealed for 2 hours at 7000 C, at ordinary atmosphere in the muffle furnace. This is further characterized using analytical techniques. Concentration of REE metal ion in both initial and with drawn samples are calculated through ICP-AES technique.

Adsorption Experiments

The equilibrium adsorption experiments can be performed individually at room temperature. The batch experiments are carried out to evaluate the adsorption process and REE mobilization. Every time a particular parameter is varied by fixing all other parameters at their optimum values and the effect of temperature, kinetics,isotherms and thermodynamic parameters are calculated. Also the parameters like effect of pH, nano spinel ferrite (adsorbent) dose, REE’s dose, contact time are optimized. However % of REE adsorbed can be calculated using the formula given as   

here O.D = optical density of REE .

Results and Discussion

Effect of pH

The pH is key factors during the adsorption process.The amount of REE’s adsorption onto nano copper ferrite was studied at pH=2 to 11. Results showed that when pH was higher than 7, the adsorption capacity was maximum. At lower pH, the adsorption capacity was not maximum, but the results are in agreement with the observations of other researcher’s group¹⁹. At lower pH values the protons are in competition with adsorbates (REEs) and nano copper ferrites retain very small amount of lanthanides, as a result the adsorption of La(III) and Ce(III) is very less. Also at when pH is increased adsorbate – adsorbent electrostatic repulsion decreases and adsorbed Lanthanides seems stable²⁰. A pH value of 7.8 is fixed as optimum for the entire study.  

Figure 1: pH effect on adsorption of REE on nano spinel copper ferrite Click here to View Figure

Effect of contact time

Adsorption capacity increases with time can be explained in 2 steps. That is in first 20 minutes a steep elevation in the sorption. Around 95% of the overall sorption takes place with in first 30 minutes of contact time. Proper reaction rate , diffusion mechanism are very significant in explaining the role of contact time²¹.

Figure 2: Contact time effect on adsorption of REE onto nano spinel copper ferrite. Click here to View Figure

When all the sites on porous material of the adsorbent reach saturation, the maximum adsorption is completed. Hence figure 2 explains the effect of contact time at optimum pH and at room temperature whsen the adsorbent dose is 0.5 g. More over in basic medium  REE ions are automatically precipitated as oxides, hydroxides,hence adsorption is significantly decreased²².

Adsorbent dosage ( nano spinel copper ferrite):

The nano spinel copper ferrite dosage is varied from 0.1 to 1gm under the optimum conditions of other parameters (pH= 7.8, contact time =30 min for both REEs and at room temperature). Relationship between nano spinel copper ferrite dosage and removal efficiency of the REE from their aqueous solutions is presented in figure 3. Nano spinel copper ferrite shows maximum removal efficiency of more than 90% ions at 500 mg. The rate of adsorption  increases with dose of adsorbent is due to availability of more adsorption sites with further growth of surface area. This reduced with increase in adsorbent dose. Overlapping of adsorption sites as a result of overcoming of copper ferrite particles²³. By increasing the initial adsorbent concentration in solution, the quantity of adsorbed lanthanides gradually increases. But increase starts to ease after 0.2 gm/L. Trend is probably because of the  adsorption sites saturation at higher concentrations. 

Figure 3: Effect of adsorbent dosage on adsorption of REE on nano spinel copper ferrite. Click here to View Figure

Dose of REEs

Effect of REEs dosage on adsorption can be done by preparing aqueous solution of lanthanum nitrate and cerium nitrate respectively. The range is from 5ppm to 30ppm. The dose of nano copper ferrite is 0.5 g throughout. The total volume is 30.0 ml. The temperature of the system is 300K with a pH of 7.8. The solution was centrifuged and the supernatent liquid concentration is observed using ICPAES, with time until the system approaches equilibrium²⁴.The same procedure is repeated until the error is minimized and attains reproducibility.The maximum adsorption was observed at 10 and 8 ppm respectively for the REE ions. This is because, establishment of saturated more ionic layer at lower concentration of metal ions, makes extra interaction near the adsorbent hindered in high concentration solution due to the interactions among REE cations existence on the copper ferrite medium. Also tendency for an ion to remain in Stern layer depends on the electro porosity of the metal ion²⁵.

Figure 4: Dosage of La(III) on adsorption  by nano spinel copper ferrite Click here to View Figure

Figure 5: Dosage of Ce (III) on adsorption by nano spinel copper ferrite. Click here to View Figure

Kinetic models

To compare the efficiency of nano spinel copper ferrite at various conditions and concentrations, to design and optimise an operating procedure , the analysis of equilibrium data is very important, because the adsorption is a mass transfer operation and is elaborated by a rate process. However to examine the characteristics of adsorption process, kinetic data, potential rate controlling data are analyzed using pseudo first order and pseudo second order.They can be represented as

k₁ is the rate constant for pseudo first order kinetics.

k₂ is the rate constant for pseudo second order kinetics

From reports the kinetics of the elements do follow pseudo second order  kinetics for both REEs. However R² for pseudo second order at different concentrations are above 0.99. Adsorption is chemisorption through sharing of electrons among copper ferrite and adsorbate²⁶. From this data it is suggested that adsorption rates are quick at lower concentrations for lanthanides. In other words time required for the equilibrium adsorption increases as RE ions concentration increases.

Table 1: parameters for PFO and PSO kinetics for the adsorption of REE on nano spinel copper ferrite.

PFO parameter	La(III)	Ce(III)	PSO parameter	La(III)	Ce(III)
Ci(mg/L)	10	8	Ci(mg/L)	10	8
K1 (min-1)	0.026	0.024	K2(g mg-1min-1)	2.45x 10-4	2.37x 10-4
qe (mg/g)	119.6	110	qe (mg/g)	116	112.4
R2	0.980	0.986	R2	0.996	0.996

 

Figure 6: Pseudo first order kinetics for the adsorption of REE on nano spinel copper ferrite. Click here to View Figure

Figure 7: Pseudo second order kinetics for the adsorption of REE on nano spinel copper ferrite. Click here to View Figure

Adsorption isotherm models

To establish equilibrium data of adsorption, the experimental data is studied with respect to Langmuir and Freundlich models. From the table 2,  the experimental data is fitted for Langmuir model and hence adsorption is mono layer²⁷. Due to the ionic radius closeness of La (III) = 1.051A⁰ and Ce (III) = 1.034A^0.  The q_m values for REEs is almost equal. As the Langmuir model assumes mono layer adsorption the data fits more into this. The linearised form is

q_max = adsorption  capacity (µg g^-1) and b is  energy of adsorption.

q_{e =} k_F C_e^1/n

k_F is adsorption capacity and “n” is adsorption intensity.

K_L and q_m are from linear fit of c_e/q_e vs c_e. Langmuir model is the most widely used and covers monolayer adsoption. This model is appropriate with specific parameters for adsorption of REEs on copper ferrites²⁸. Maximum mono layer adsorption capacities are found to be 333 and 345 mg/g for lanthanum and cerium respectively. However E values from 8 to 16 KJ/mol explain chemical adsorption.

Figure 8: Langmuir adsorption isotherms for  adsorption of REE by nano copper ferrite Click here to View Figure

Table 2: Parameters for Langmuir, Freundlich models for the adsorption of REE on nano spinel copper ferrite

Isotherm model	parameter	La (III)	Ce(III)
Langmuir	Q0  (mg/g)	333	345
	bL  (L/mg)	4.73 x10-2	2.09 x 10-2
	R2	0.997	0.989
Freundlich	KF  (mg/g)	20.6	9.82
	nF	1.78	1.40
	R2	0.918	0.920

 

Effect of temperature and thermodynamic parameters:

The thermodynamic studies are more important to understand the mechanism of adsorption for the practical applications. To study these  effects Gibb’s free energy and Van’t Hoff equation are followed.

ΔG⁰ = -RTlnK_L

_–ΔG⁰ is the change in Gibb’s  free energy , R  ideal gas constant (8.314J/mol/K), T  absolute temperature (K), K_L  Langmuir constant. ΔH⁰,ΔS⁰ are changes in enthalpy and entropy respectively. Effect is examined on the adsorption of REE by nano copper ferrite at the range of 300K 310K and 320K at pH 7.8 and constant time of 60 minutes.The concentration of La (III) and Ce(III) are 10 and 8 ppm respectively at the concentration of adsorbent is at 0.5g.

Both the REE are temperature dependent and at higher temperature the increase in the  dose of adsorbed lanthanides is because of the higher affinity of the available active sites. Positive values of enthalpy change (H⁰) decrease in the Gibb’s free energy change with rise of temperature encourages sorption mechanism is endothermic²⁹.

More over positive values of entropy change (ΔS⁰) means disorder of the system increases after metal sorption. Increase of entropy is due to release of water during the metal sorption of RE ions. This fact that ΔH⁰ is less than TΔS⁰. This explains that sorption process is through entropy rather than enthalpic changes³⁰.

Figure 9: Temperature effect on adsorption of REE by nano copper ferrite. Click here to View Figure

Figure 10: Thermodynamic parameters for the adsorption of REE by nano spinel copper ferrite. Click here to View Figure

Table 3: Thermodynamic parameters for La (III) and Ce(III) by adsorption of nano spinel copper ferrite

REE	ΔH0 (KJ mol-1)	ΔS0(J mol-1K)	T(K)	ΔG0 (KJ mol-1)	TΔS0(KJ mol-1)
La(III)	16.0	138	300	-24.8	40.8
			310	-25.6	41.2
			320	-27.1	42.8
Ce(III	11.0	126	300	-25.8	36.8
			310	-27.0	37.6
			320	-28.3	38.0

 

Conclusions

Nano spinel copper ferrite is applied as capable adsorbent in the removal of REE from aqueous solutions. Present investigation helps to understand the greater efficiency of nano spinel copper ferrite in terms of maximum adsorption capacity of 983 and 973 mg/g for La(III) and Ce(III) respectively. The optimum conditions for the parameters are obtained. The pH of 7.8 is opted for both the elements to reach the equilibrium in 30 minutes. Langmuir isotherm model is fitted well followed by pseudo second order kinetics. The system is thermodynamically spontaneous, endothermic and randomness increases with temperature. Hence to conclude the adsorption is economical, environmental benign procedure.

Acknowledgement

Authors thank GITAM authorities for providing necessary conditions to conduct the work.

Conflict of Interest

Authors have no conflict of interest regarding work and publication.

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