Chapter 7 Adsorption thermodynamics and recovery of uranium 99
Chapter 7. Adsorption thermodynamics and recovery of uranium from aqueous solutions by Spatoglossum 7.1. Materials 7.1.1. Preparation of sorbent Biomass of brown alga S. asperum was collected from the seashores of Malvan (Maharashtra, India). The after collection was washed thoroughly with tap water. This was followed by washing three times with the deionised water and finally by glass distilled water in order to get a clean that is free from silt, sand, diatoms and other epiphytic organisms. Biomass after cleaning was dried at an ambient temperature of 25 ± 2 ºC, and stored as whole at room temperature. Some portion of the was powdered using the mortar and pestle. The particle size that could pass through the sieve of 500 µm but was retained by the 250 µm sieve size was used for the experiment. 7.1.2. Chemicals UO 2 (NO 3 ) 2.6H 2 O (Merck, Germany) was used to prepare the uranium solution. The ph of the uranium solution was adjusted to required values by using Na 2 CO 3 or HNO 3. 7.2. Methods All experiments were performed using powdered having particle size between 250-500 µm, and the whole. The optimum ph for the process being 5.5 and equilibrium attained in 3 hours, all the experiments were performed at ph 5.5, and the time given to attain equilibrium was 3 hours. The Estimation of uranium (VI) was done by Arsenazo (III) method (Savvin, 1961). Briefly 0.5 ml sample was mixed 100
with 0.1 ml of oxalic acid (4%) and 0.1 ml of Arsenazo (III) (0.05%) and diluted with hydrochloric acid (4 M) to a total volume of 2.5 ml before analyzing at a wavelength of 650 nm. The data presented in the result represents the average of triplicate readings ± standard deviation. The data presented in the result represents the average of the triplicate readings ± standard error. The statistical analysis was done for Analysis of variance (One way ANOVA and Tukey s significance test) OriginPro 7.5 software. The values having P<0.05 were considered as significantly different. For each of the experiments, solutions with out were used as controls. The biosorption equilibrium of uranium per unit algal (mg of U/g dry weight of algal ) was calculated using following expression q e = (C 0 C)V /W (7.1) where C 0 and C are the concentrations of uranium (mg/l) in the solution before and after the biosorption respectively. V is the volume of uranium solution used in liters and W is the amount of used in grams. 7.2.1. Isotherm modeling and thermodynamics of uranium sorption Adsorption curve data were determined by repeatedly contacting the with freshly prepared uranium solution. Briefly, 50 ml, 100 mg/l of uranium solution was brought in contact with 25 mg of algal. After the equilibrium was achieved, C f1 and q e1 were calculated. The was filtered and again brought in contact with freshly prepared 100 mg/l uranium solution. The C f = (C f1 + C f2 ) and q e = (q e1 + q e2 ) were calculated after achieving the equilibrium. The process was repeated till the did not show further removal of uranium. This data was fitted to linearized Langmuir and Freundlich adsorption isotherms (Langmuir, 1918; Freundlich, 1907). 101
Linearized form of Langmuir isotherm can be represented as C e /q e = [(1/q max )*(1/b)] + C e /q max (7.2) Where q max is the maximum metal uptake (mg/g) and b the ratio of adsorption / desorption rates related to energy of adsorption. The linearised form of Freundlich equations is ln q e = ln K f + 1/n * (ln C e ) (7.3) where q e is the equilibrium metal uptake capacity (mg/g), and C e is the residual uranium concentration in the solution (mg/l). The constant K f represents Freundlich constant and it is a measure of adsorption capacity and 1/n the intensity of adsorption. Gibbs free energy change ( Gº) was calculated using the following equation Gº = - RT ln b (7.4) Where Gº is the Gibbs free energy (kj/mol), R the universal gas constant (8.314 x 10-3 kj/mol K), T the absolute temperature (K), and b is the constant from Langmuir isotherm when the concentration of uranium solution was in mol/l. Gibbs free energy is related to enthalpy and entropy change (Nelson and Cox, 2005) Gº = Hº -T ºS (7.5) Using equation (7.4) we can say - RT ln b = Hº -T Sº (7.6) or log b = ( Sº/(2.303 R)) ( Hº/ (2.303 R T)) (7.7) where Hº is the heat of of adsorption or enthalpy change (kj/mol), and Sº is the entropy change (kj/(molk)). 7.2.2. Uranium elution and regeneration of 102
7.2.2.1. Metal loading 25 mg of (Powdered and whole) was brought in contact with 50 ml of uranium solution having an initial concentration of 100 mg/l at ph 5.5. After shaking at 150 rpm in an environmental control shaker at a temperature of 25 C for 3 hours, the solutions were filtered and the C f was estimated. 7.2.2.2. Desorption This loaded with the uranium, was brought in contact with 5 ml, 0.1 M of various desorbing agents at 25 C. After 3 hours of shaking at 150 rpm, the amount of uranium in the desorbing solution was estimated. 7.3. Results and discussions 7.3.1. Isotherm modeling and thermodynamics of uranium sorption Langmuir and Freundlich isotherm models were evaluated to examine the sorption of uranium at 288, 298 and 308 ºK. The adsorption parameters calculated are presented in Table 7.1.The r 2 values indicated that Langmuir model would describe the system better than Freundlich isotherm, for both types of and at all the temperatures studied. The maximum metal loading capacity (q max ) and Langmuir constant (b) was calculated from the slope and intercept of the plot C e /q e versus C e (equation 7.2). K f and n were calculated from the intercept and slope of the plot ln q e versus ln C e (equation 7.3). With the increase in temperature q max increased. As the sorption data fitted to the Langmuir model, it suggested a monolayer uptake of the metal on homogeneous surface, having uniform energies of adsorption for all the binding sites with out any interaction between the adsorbed ions. 103
Table 7.1. Parameters of the Langmuir adsorption equation and Freundlich isothermal equation at 288, 298, and 308 ºK. V = 50 ml, W = 25 mg, Temperature = 25 C, Shaking = 150 rpm, Contact time = 3 hours. Temp (ºK) Biomass type q max (mg/g) Langmuir parameters Freundlich parameters b r 2 K f n r 2 288 Powdered 555.5 0.0351 0.999 3.5014 3.0959 0.904 Whole 666.6 0.0545 0.997 3.5014 3.5014 0.879 298 Powdered 588.2 0.0432 0.999 3.5014 3.2541 0.883 Whole 666.6 0.0666 0.998 3.5014 3.9292 0.956 308 Powdered 588.2 0.0428 0.998 3.5014 3.0883 0.878 Whole 833.3 0.1428 0.997 3.5014 3.6324 0.952 Using the equation (7.4), Gº was calculated at 288 ºK, 298 ºK, and 308 ºK. The results are presented in Table 7.2. The negative values of Gº for both types of at all the temperatures studied suggested the feasibility and spontaneous nature of the process. Gº values in the range of -20 kj/mol are attributed with the electrostatic interactions between the charged molecules (Zafar et al., 2007). The values of Gº obtained in our study thus suggested a physicochemical mechanism involved in the sorption process. Values of enthalpy and entropy change for both types of were calculated from the slope and intercept of the plot log b vs 1/T (equation 6) and are presented in Table 7.2. The ºH values for both types of were -33.85 and -31.84 kj/mol for powdered and whole. The negative values for the enthalpy change indicated that the sorption process was of exothermic nature, 104
and indicated that the adsorption involved electrostatic attractions, chemisorption, and complexation mechanisms. Table 7.2. Thermodynamics parameters of uranium biosorption by S. asperum. Biomass type ºG (kj/mol) ºH ºS 288 ºK 298 ºK 308 ºK (kj/mol) (J/mol) Powdered -20.11-23.06-23.70-33.85 195 Whole -22.82-24.10-26.78-31.84 181 When the ºH values are in the range of - 29 kj/mol, the adsorption is mainly due to electrostatic attractions, and chemisorption, and, when the value is in the range of -60 kj/mol, the mechanism of adsorption also involves the complexation in addition to these interactions (Mungasavalli et al., 2007). The values of ºS for powdered and whole were 195 and 181 J/mol respectively. Positive ºS values indicated increase in randomness during the adsorption reaction, and high affinity of S. asperum to uranium. 7.3.2. Uranium elution and regeneration of Uranium elution and regeneration of is an important aspect of bioremediation technology. HCl, HNO 3, Na 2 CO 3, NaHCO 3, Citrate, and Di-sodium EDTA (0.1 M) were investigated for this purpose. The values of q ads, q des and %Recovery are presented in Table 7.3. Among the tested desorbing agents citrate was selected for desorption of uranium from the. % Recovery of uranium was higher by citrate as compared to HCl and HNO 3. While investigating Na 2 CO 3 and NaHCO 3 for desorption, disintegration in was observed. The reason for the disintegration of by Na 2 CO 3 and NaHCO 3 could be due to the damage 105
caused to the cell wall and cell wall proteins. Di-sodium EDTA was not selected because of less %removal of uranium. Table 7.3. Desorption of uranium from loaded by various desorbents. Desorbing agent q ads q des %Recovery Powdered Whole Powdered Whole Powdered Whole 0.1 M HCl 175.6 ± 6 191.8 ± 4 151.9 ± 5 149.9 ± 6 86.5 ± 3 78.1 ± 3 0.1 M HNO3 0.1M Na2CO3 0.1 M NaHCO3 0.1 M Citrate 0.1M di- sodium- EDTA 175.6 ± 5 195.1 ± 3 147.3 ± 4 158.2 ± 5 83.8 ± 2 81.3 ± 2 175.8 ± 4 188.3 ± 6 166.1 ± 5 164.4 ± 3 94.5 ± 3 87.3 ± 3 176.8 ± 5 190.1 ± 5 160.6 ± 6 153.4 ± 2 90.8 ± 3 80.73 ± 2 174.3 ± 3 189.2 ± 4 157.4 ± 5 162.6 ± 2 90.3 ± 3 86.3 ± 3 175.1 ± 3 191.1 ± 4 49.5 ± 7 97.15 ± 5 28.3 ± 4 50.9 ± 3 Citrate is reported to form binuclear complexes with uranium. It acts like a chelating agent (Francis et al., 1992), and being an organic compound there are less chances for the damage to. Thus, indicating the possibility of the reuse of. 7.3.4. Sorption/desorption cyclic studies For a cost effective sorption process, repeated use of should be possible. This investigation was done in order to find out the efficiency of S. asperum for its repeated use in removal and recovery of uranium. Metal loading and the desorption 106
experiments were done as explained in sections 2.5.1 and 2.5.2 respectively. 0.1 M citrate was used as the eluting agent. Same was investigated for adsorption and subsequent desorption, for five consecutive cycles. The results are presented in Fig. 7.1 and 7.2. The performance of for its repeated use was good. Fig 7.1. q ads and q des values for V cycles of repeated use of powdered. Adsorption q (mg/g) 180 160 140 120 100 80 60 40 20 0 I II III Number of cycles IV V Desorption Desorption Adsorption Fig 7.2. q ads and q des values for V cycles of repeated use of whole. q (mg/g) 200 180 160 140 120 100 80 60 40 20 0 I II III Number of cycles IV Adsorption Desorption Desorption Adsorption V 107
In the five cycles of sorption and desorption studied, the uranium uptake capacity of decreased from 175 to 138, and 194 to 124 mg/g, for powdered and whole, respectively. Percent recovery of uranium by 0.1 M citrate also decreased from 90 to 81, and 87 to 78, for powdered and whole respectively. Fig 7.3. Percent recovery of uranium in different cycles of sorption/desorption by powdered and whole. Percent removal 92 90 88 86 84 82 80 78 76 74 72 I II III Number of cycle IV powdered whole V After every cycle of desorption a decreased uranium uptake by the, and a decreased % removal of uranium was observed. The percent recovery of uranium from whole was less as compared to the powdered. The reason could be because of the involvement of more strong interactions between uranyl ions and the whole. 7.4. Conclusions 108
The results obtained show that temperature highly affected the metal loading capacity of the biosorbent. Biosorption increased with the increase in temperature upto 55 C. Thus, making the use of S. asperum possible as a biosorbent for uranium across a range of temperature. Adsorption equilibrium data fitted better to Langmuir isotherm model, and, thus, suggested a monolayer uptake of the metal on homogeneous surface, having uniform energies of adsorption for all the binding sites with out any interaction between the adsorbed ions. Isotherm constants increased with increase in temperature. Evaluation of thermodynamic constants ( ºG, ºH, ºS) indicated spontaneity, feasibility and exothermic nature of the sorption reaction, and, furthermore sorption process progressed with the increase in entropy. Uranium from the metal loaded could be eluted with 90% efficiency by using 0.1 N citrate as a desorbing agent. The repeated use of for the sorption of uranium was checked for five consecutive cycles, and the results indicated the possible repeated use of. 109