Uranium biosorption by Spatoglossum asperum J. Agardh:

Chapter 6 Uranium biosorption by Spatoglossum asperum J. Agardh: 76

Chapter 6. Uranium biosorption by Spatoglossum asperum J. Agardh: Characterization and equilibrium studies. 6.1. Materials 6.1.1. Collection of biomass Biomass of brown alga S. asperum was collected from the sea shores of Malvan (Maharashtra, India). The biomass 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 biomass 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 biomass at room temperature. Some portion of the biomass 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. 6.1.2. 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. 6.2. Methods Unless otherwise indicated, for all experiments 25 mg of dry biomass was introduced into 50 ml of uranium solution in 150 ml conical flasks. After 3 hours of shaking at 150 rpm and 30 C, the supernatant was separated by centrifugation (10,000 rpm for 10 minutes) and used for estimating the dissolved uranium concentration. Estimation 77

of uranium (VI) was done by Arsenazo (III) method (Savvin, 1961). The data presented in the result represents the average of 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. All experiments were performed using powdered biomass having particle size between 250-500 µm, and, whole biomass, at ph 5.5. For each of the experiments, solutions without biomass were used as controls. The biosorption equilibrium of uranium per unit algal biomass (mg of U / g dry weight of algal biomass) was calculated using following expression q e = (C 0 C)V /W (6.1) 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 biomass used in grams. 6.2.1. Effect of ph on the sorption of uranium The effect of ph on the biosorption of uranium was studied using initial uranium concentrations of 20 mg/l, 50 mg/l, and and100 mg/l. The residual uranium concentration was estimated from the samples withdrawn after 2 hours of contact time. The range of ph studied was 2 to 10. Uranium solution was continuously stirred while adjusting the ph until a constant required reading was observed. 6.2.2. Effect of contact time on the sorption of uranium Effect of contact time was investigated using an initial uranium concentration of 100 mg/l. The initial ph was adjusted to 5.5. Periodically 0.5 ml of sample was 78

withdrawn, centrifuged at 10,000 rpm for 10 minutes and the dissolved uranium concentration was estimated. 6.2.3. Protonation of sorbent Biomass (Whole and powdered biomass) was protonated by using three different protonating agents viz 0.1 M HCl, 0.1 M HNO 3, and 0.1 M H 2 SO 4. For protonation biomass was brought in contact with protonating agent at a concentration of 10 g biomass/l. After 4 hours of contact time, at 25 ºC and a shaking of 150 rpm, biomass was filtered and washed thoroughly by distilled water, till a constant ph of 5.5 was achieved. Biomass was then dried at an ambient temperature of 37 C. 25 mg of this protonated biomass was then brought in contact of 50 ml of uranium solution having an initial concentration of 300 mg/l. 6.2.4. Effect of ions on the sorption of uranium Uranium sorption in presence of various cations and anions was investigated in bimetallic combination. Equimolar concentrations of test ion (420 µm) and uranium (420 µm equivalent to 100 mg/l) were prepared. 50 mg of biomass was brought in contact with 150 ml of bimetallic solution having ph 5.5. The cations used were Na +, K +, Pb 2+, Cd 2+, Ag 2+, NH 4, and Mg 2+ (all nitrate salts), and the anions used for the study were CO 2-3, NO - 3, SO 2-4, Acetate, and Citrate. 6.2.3. Effect of temperature on the sorption of uranium Temperature effects were investigated for five different temperatures 15 ºC ± 3ºC, 25 ºC ± 3ºC, 35 ºC ± 3ºC, 45 ºC ± 3ºC, and 55 ºC ± 3 ºC. Initial uranium concentration used was 500 mg/l and initial ph was 5.5. 79

6.2.4. Effect of initial metal ion concentration on the sorption of uranium Experiment was performed for five different initial uranium concentrations ranging from 20 mg/l to 600 mg/l at an initial ph value of 5.5. 6.3. Results and Discussions 6.3.1 Effect of ph on the sorption of uranium Effect of ph on the biosorption of uranium was studied using initial uranium concentrations of 20 mg/l, 50 mg/l, and 100 mg/l. Range of ph studied was 2 to 10. The ph of aqueous solution is an important controlling factor in a sorption process. It influences both, the speciation of uranium in aqueous solution, and binding sites present on surface of biomass (Kalin et al., 2005). For initial uranium concentration of 20 mg/l using both the types of biomass, a decreased uranium uptake was observed at ph 2. Percent removal of uranium was more than 95 across the ph range 3-9, and decreased to 80 at ph 10, Fig 6.1(a&b). Such a ph independent uranium removal has not been reported for any of the biosorbents. 80

Fig. 6.1(a). Effect of ph on uranium removal by powdered biomass. V = 50 ml, W = 25 mg, agitation speed = 150 rpm, contact time = 2 hours. 120 100 20 mg/l 50 mg/l 100 mg/l Percent removal of U 80 60 40 20 0 0 4 8 12 ph Fig. 6.1(b). Effect of ph on uranium removal by whole biomass. V = 50 ml, W = 25 mg, agitation speed = 150 rpm, contact time = 2 hours. 120 100 20 mg/l 50 mg/l 100 mg/l Percent removal of U 80 60 40 20 0 0 4 8 12 ph 81

The ph independent uranium removal by S. asperum from dilute wastes gives an additional advantage for use of this biomass as biosorbent. To the best of our knowledge this is first report on a uranium biosorbent exhibiting a ph independent uranium removal from dilute aqueous wastes. For 50 mg/l initial uranium concentration, and while using powdered biomass we observed more than 88% removal of uranium across the ph range 4-10, and 82% of uranium removal was observed for whole biomass across a ph range 3-8. Percent removal of uranium versus ph for the initial uranium concentration of 100 mg/l resulted in a bell shaped curve with optimum uranium removal (>96%) at ph 5-6. The reason for the decreased sorption at ph 2 could be because at this ph there is a very high concentration of H + and H 3 O +, which compete with other ions for binding sites present on surface of biomass. The sorption of uranium at different ph values has been attributed to the binding of uranium to ionized functional groups present on surface of biomass. Different functional groups present on the biomass remain in ionized form at or near their pk values. Ligands like carboxyl, amino, and phosphate having the pk values in the range of 3-5 would be involved in uranium sorption in the ph range 3 to 5. Removal of uranium from ph 6-10 could be due to presence of ligands having pk values between 6 and 13 (hydroxyl, Imidazole, Sulphide, Amino and Imino) (Davis et al., 2003: Kalin et al., 2005). Carbonates form uranyl-carbonate complexes above ph 6 and reduce bioavailability of uranium. It is not carbonate that competes with uranyl ions for surface binding sites on the biomass. Rather, aqueous carbonate competes with surface binding sites for uranyl ions (Wazne et al., 2006). In our study we used Na 2 CO 3 for adjusting ph of solution, and also effect of 82

atmospheric CO 2 comes into picture. Above ph 6, due to the presence of these carbonates, formation of mixed carbonato-hydroxo-u (VI) complex ion (UO 2 ) 2 CO 3 (OH) 3 - takes place. This mixed complex ion (UO 2 ) 2 CO 3 (OH) 3 - predominates but coexists with significant amounts of (UO 2 ) 3 (OH) + 5, (UO 2 ) 4 (OH) + 7, (UO 2 ) 3 (OH) 2-8 and UO 2 (CO 3 ) 4-3 complex ions in the solution in the ph range 7-8. Above ph 8 all U (VI) complex ions in the solution are transformed to UO 2 (CO 3 ) 4-3 complex ion (Krestou and Panias, 2004). As already discussed, that, the affinity to cell wall ligands for anionic metal species is low. Thus formation of these carbonato-hydroxo-u (VI) complex ions reduces the availability of U (VI) for biosorption. This could be the reason that various researchers could not observe U (VI) sorption at high ph values. But in this study percent removal of uranium observed above ph 6 using an initial uranium concentration of 100 mg/l by powdered biomass was 96 ± 0.2 at ph 6, 94.5 ± 0.14 at ph 7, 85.3 ± 0.46 at ph 8, 70.3 ± 0.7 at ph 9, and 30.1 ± 1.1 at ph 10. For whole biomass percent removal of uranium was 97.7 ± 0.3 at ph 6, 89.8 ± 0.3 at ph 7, 63.3 ± 0.4 at ph 8, 55.6 ± 0.6 at ph 9, and 23 ± 1.4 at ph 10.Wazne et al., (2006) reported the removal of U (VI) in presence of carbonates from aqueous solutions at ph>6 while using TiO 2 as sorbent and an initial uranium concentration of 1 mg/l. The advantage of S. asperum over TiO 2 as a sorbent for uranium is due to its natural availability in nature, which cuts down its cost of synthesis as compared to TiO 2. For the best of our knowledge there are no reports on any sorbent from biological origin that has efficiently removed uranium from aqueous solutions in presence of carbonates, as we are reporting in this study. 6.3.2. Effect of contact time on the sorption of uranium 83

Sorption kinetics was investigated at an initial uranium concentration of 100 mg/l. The effect of contact time on sorption of uranium brings out two important physicochemical aspects viz. kinetics and equilibria of the process. Kinetics describes the metal ion uptake rate. The fast rate of uptake is considered a good characteristic of sorbent. The state of equilibrium achieved in a sorption process helps in determining the distribution of metal ions in solid and liquid phases, and capacity of sorbent for sorption. Plot of q e versus time is shown in Fig. 6.2. The metal removal was rapid with more than 65% and 49% of total biosorption taking place in 30 minutes using powdered and whole biomass respectively. Equilibrium could be achieved in two hours of contact time. Amount of uranium biosorbed at the time of equilibrium was 185 ± 1.5 and 190 ± 1.3 mg/g for powdered and whole biomass respectively. Fig. 6.2. Biosorption kinetics for powdered and whole biomass. V = 50 ml, W =25 mg, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm, C 0 = 100 mg/l. 200 180 160 140 q (mg/g) 120 100 80 60 whole biomass powdered biomass 40 20 0 0 120 240 time (min) 84

After the equilibrium was attained, q e remained constant (studied for 24 hours, data not shown). The higher rate of biosorption in the initial stage of the biosorption could be due the electrostatic interactions between the metal ions and the surface ligands on the algal biomass. Binding sites or ligands present on the surface of biomass bind to uranyl ions as soon as they come in contact with each other. As time progresses availability of these binding sites reduces, thus reducing rate of biosorption (Bhat et al., 2008). The fast rate of sorption in powdered biomass as compared to whole biomass could be due to increased surface area (caused because of powdering of the biomass) and thus possibly increasing the availability of surface ligands, which in turn would have led to faster initial rate as compared to the whole biomass. 6.3.3. Effect of protonation on the sorption of uranium The aim of protonation of the sorbent was to eliminate metal ions like Na +, K +, Ca 2+, and Mg + 2 etc, which bind to the acid functional groups of alga in sea water. At ph<2, H + out compete other ions and strip off them from the ligands (Kalin et al., 2005). S. asperum is a brown marine alga. In general 40% of dry weight of brown algae is constituted by alginates. Alginates in brown algae are rich in carboxylic groups. The adsorption capacity of algae has been directly related to the presence of these sites on alginate. In the chemical alterations implied by the protonation, the proton displaces light metal ions from these binding sites (Davis et al., 2003). Thus making a binding site available, that would have earlier been occupied by a light metal ion. The results of the protonation of dried biomass S. asperum are shown in Table 6.1. 85

A slight increase in q e was observed in protonated powdered biomass, and a slight decrease was observed while using protonated whole biomass. Difference in q e for protonated and non protonated biomass (whole and powdered biomass) was not significant. The reason for these observations could be due to higher affinity of uranium towards binding sites as compared to the light metal ions. Therefore, for all the subsequent experiments biomass in non-protonated form was investigated. This gives an additional advantage for the use of S. asperum as sorbent. Because, it makes the process more cost effective, and also eliminates the chances of chemical pollution caused by the use of protonating agents. Table 6.1. Effect of protonation of biomass on the biosorption of uranium. V = 50 ml, W = 25 mg, C 0 = 300 mg/l, Temperature = 25 C, Shaking = 150 rpm, Contact time = 3 hours. Protonating agent q e (mg/g) Powdered biomass Whole biomass Control (Non protonated 390.3 ± 5 403.1 ± 7 biomass) 0.1 M HCl 395.4 ± 4 395.2 ± 5 0.1 M HNO 3 392.3 ± 6 394.2 ± 7 0.1 M H 2 SO 4 400.6 ± 3 386.9 ± 5 6.3.4. Effect of ions on the sorption of uranium Various metal ions remain present in waste waters containing uranium. In mixtures, these metal ions compete with uranium for the binding sites, based on the molecular size, shape, and the configuration of binding site, and, thus decrease the removal of uranium by the sorption process (Kalin et al., 2005). The search for the sorbents of uranium, whose efficiency is not affected by a broad range of cations and anions, has 86

been an active field of research in uranium biosorption. Uranium sorption by S. asperum was investigated in presence of equimolar concentration of various ions. The results are presented in Table 6.2. Among the cations investigated, Pb 2+ resulted in a decreased (50 %) removal of uranium, and among the anions tested, citrate showed antagonism in uranium sorption. Citrate ions form multidentate complexes with variety of toxic metal ions and radionuclides. The formation of binuclear complex between citrate and uranium would have reduced its availability for binding to biomass (Francis et al., 1992). Various researchers have reported the effect of ions on uranium sorption by different biosorbents. Table 6.2. Effect of various cations and anions on the biosorption of uranium at ph 5.5. V = 150 ml, W = 50 mg, Contact time = 3 hours, Temperature = 30 C, Shaking = 150 rpm. Cations q e (mg/g) Control 252 ± 5 Na + 251 ± 6 K + 254 ± 4 Mg 2+ 256 ± 5 Cd 2+ 240 ± 6 Ag 2+ 255 ± 4 + NH 3 256 ± 5 Pb 2+ 136 ± 5 Anions 2- CO 3 258 ± 4 - NO 3 258 ± 5 2- SO 4 253 ± 6 Acetate 256 ± 5 Citrate 119 ± 5 87

6.3.5. Effect of temperature on the sorption of uranium Temperature effects were investigated for five different temperatures 15 ºC ± 3ºC, 25 ºC± 3ºC, 35 ºC ± 3ºC, 45 ºC ± 3ºC, and 55 ºC ± 3 ºC at an initial uranium concentration of 500 mg/l. Uranium biosorption increased with increase in temperature as shown in Fig 6.3. When temperature was increased from 15 ºC to 55 ºC, q e increased from 437.5 ± 3.5 to 459 ± 3.5 and 432.1 ± 2.5 to 440 ± 2.7 mg/g for whole and powdered biomasses respectively. Different reasons for temperaturedependent variation in metal sorption have been suggested, such as, the increased biosorption at higher temperatures could be due to availability of some new binding sites at higher temperatures, and/or due to higher affinity of binding sites to metal ions (Goyal et al., 2003). A high metal sorption at higher temperature is an indication of chemisorption that is the involvement of strong interactions between uranyl ions and binding sites present on biomass. As at higher temperatures weak interactions like hydrogen bonding and Vander-waal s interactions are broken and do not contribute to sorption of metal ions (Bhat et al., 2008). At all the temperatures studied, whole biomass showed more uranium uptake than powdered biomass. This could be due to non-availability of some binding sites caused by the grinding of biomass. Final ph of the solutions increased. The final ph of the solutions using powdered biomass was 4.5, 4.7, 5.2, 5.5, and 5.9, and for whole biomass it increased to 4.4, 4.5, 5.0, 5.6, and 5.9, when the temperature was 15 ºC ± 3ºC, 25 ºC ± 3ºC, 35 ºC ± 3ºC, 45 ºC ± 3ºC, and 55 ºC ± 3 ºC respectively. The reason for the decrease in ph at 15 ºC ± 3ºC, 25 ºC ± 3ºC and 35 ºC ± 3ºC could be due to exchange of 88

hydrogen ions from biomass for uranyl ions, showing ion exchange as the principle of metal biosorption at these temperatures. Fig.6.3. Effect of temperature on uranium biosorption. V = 50 ml, W = 25 mg, ph = 5.5, agitation speed = 150 rpm, C 0 = 500 mg/l. 480 w hole biomass pow dered biomass 460 q e (mg/g) 440 420 400 15 25 35 45 55 Temperature ( ο C) 6.3.6. Effect of initial uranium concentration on the sorption of uranium Experiment was performed for six different initial uranium concentrations ranging from 20 mg/l to 600 mg/l. The results are shown in Fig. 6.4. The highest value of q e was observed to be 554.4 ± 17.4 and 643.06 ± 17.6 mg/g, at C e of 323.35 and 281.4 mg/l for powdered and whole biomass respectively. Amount of uranium biosorbed increased at a fast rate from the initial uranium concentration of 20 mg/l to 400 mg/l. With the concomitant increase in uranium concentration from 20 600 mg/l number of uranyl ions increased. At low uranium concentration, saturation of binding sites 89

present on biomass could not be achieved, as the number of uranyl ions was low as compared to number of binding sites. Fig. 6.4. Effect of initial metal ion concentration on uranium biosorption. V = 50 ml, W = 25 mg, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm 700 600 Whole biomass Pow dered biomass 500 q, mg/g 400 300 200 100 0 0 100 200 300 400 500 600 700 800 [U], ppm Increase in concentration of uranium resulted in the increase of uranyl ions, thus increasing q e, till saturation of biomass was achieved. Once the binding sites present on the biomass got saturated with uranium, availability of binding sites present on biomass for uranium decreased. When initial metal ions concentration is high, metal ion sorbed is also high. This is due to efficient use of sorptive capacity of sorbent, because of a higher concentration gradient pressure (Saxena et al., 2006). Biosorption distribution coefficient was also determined. Biosorption distribution coefficient K is defined as the ratio of equilibrium concentration of metal ion in solid and aqueous phase (K = q e /C e ), and has the unit ml/g dry weight of biomass. High distribution coefficient is considered as a characteristic of good biosorbent (Bhainsa and D Souza, 1999). The powdered and whole biomasses exhibited 90

maximum K values of 60,005 and 1,95,341 ml/g at C e of 0.657 and 0.164 mg/l, respectively (Fig. 6.5). Fig 6.5. Biosorption distribution coefficient versus residual uranium concentration. V = 50 ml, W =25 mg, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm. 250000 w hole biomass pow dered biomass 200000 K (ml/g) 150000 100000 50000 0 0 50 100 150 200 250 300 350 C f (mg/l) Adsorbents having a distribution coefficient as small as 10 ml/g are being utilized by various industries for separation processes (Akhtar et al., 2007). Bhainsa and D Souza (1999) reported a K value of 10000 ml/g for uranium sorption by Asperigillus fumigtus, and in another study Akhtar et al. (2007) repored a K value of 80000 ml/g for uranium sorpion using biomass of Trichoderma harzianum. The biosorption distribution coefficient achieved in this study was higher than that achieved in above mentioned reports, and is an indication of high affinity between uranyl ions and S. asperum, and a possible use of this biomass for commercial application in separation techniques. 91

6.3.7. Kinetic modeling of uranium sorption Kinetics of uranium uptake was modeled using the pseudo-first order and pseudosecond order Lagergren equation. The integrated form of pseudo-first order reaction of Lagergren for sorption can be expressed as log(q e -q t ) = log(q e ) - (k 1 /2.303) t (6.2) where, q t is the amount of metal sorbed (mg/g dry weight) time t (min) and k 1 is the rate constant of pseudo-first order sorption (min -1 ). The integrated linear form of the pseudo-second order rate of Lagergren (Ho and McKay, 1999) can be expressed as: t/q = 1/ (k 2. q 2 e ) + (1/ q e ) t (6.3) where k 2 (g mg -1 min -1 ) is the rate constant for the pseudo-second order sorption. The observed and experimental values for q e, and r 2 for pseudo-first order, and pseudo-second order kinetics are shown in the Table 6.3. The observed experimental q e values were close to the values of q e (for both powdered and whole biomass) obtained from the slope of the linear plot Fig. 6.6(b) of (t/q t Vs t) for the pseudosecond order rate kinetics as shown in the Table 6.3. Therefore, the pseudo-second order rate kinetics model best described the experimental data. The initial rate of uptake (h) was calculated from the expression h = k 2. q 2 e. It was found to be more than double for the powdered biomass as compared with whole biomass, thus confirming our observations of faster initial rate kinetics in powdered biomass than the whole biomass. 92

Fig. 6.6(a). Pseudo-first order plot [log ( q e -q t ) Vs t], for powdered and whole biomass.v = 50 ml, W = 25 mg, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm, C 0 = 100 mg/l. 2.5 2 w hole biomass pow dered biomass log (q e -q t ) 1.5 1 0.5 0 0 50 100 150 200 250 300 t (min) Fig. 6.6(b). Pseudo-second order plot (t/q t Vs t), for powdered and whole biomass. V = 50 ml, W = 25 mg, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm, C 0 = 100 mg/l. 1.4 1.2 w hole biomass pow dered biomass 1 0.8 t/q t 0.6 0.4 0.2 0 0 50 100 150 200 250 300 t (min) 93

Table 6.3. Pseudo-first order, pseudo-second order, and experimental values for whole and powdered biomass. V = 50 ml, W = 25 mg, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm, initial uranium concentration = 100 mg/l. Biomass Experimental q e (mg -1 gm) Pseudo-first order k 1 q e r 2 min -1 mg -1 gm Pseudo-second order k 2 q e r 2 min -1 mg -1 gm Initial rate (h) mg/g.min Whole biomass Powdered biomass 194.4 0.012 164.4 0.97 1.56*10-4 204 0.96 6.49 192.6 0.010 150.1 0.97 3.4*10-4 200 0.99 13.6 6.3.8. Equilibrium modeling for uranium sorption Adsorption curve data were fitted to linearized Langmuir and Freundlich adsorption isotherms (Langmuir, 1918; Freundlich, 1907). The Langmuir isotherm is a means to interpret hyperbolic adsorption data. It is based on equation used in Michaelis-Menten enzyme kinetics, and describes the adsorption of metal ions to a finite number of ligand sites in a single layer on the cell surface. Linearised form of Langmuir isotherm can be represented as C e /q e = [(1/q max )*(1/b)] + C e /q max (6.4) 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 ) (6.5) where K f represents Freundlich constant and is a measure of adsorption capacity, and 1/n the intensity of adsorption. The biosorption of uranium (VI) by S. asperum 94

was well described by Langmuir isotherm Fig 6.7(a) for both the types of biomasses (whole biomass and powdered biomass). Langmuir isotherm displayed r 2 >0.99 for both the powdered and whole biomass. q max was calculated from the slope of Langmuir isotherm (eq 4) Fig 6.7(a), and its values were 588 and 666 mg/g for powdered and whole biomass respectively. This suggests that biosorption of uranium onto the S. asperum biomass was limited to monolayer and interaction was only between metal ion and biomass binding site, and not among the metal ions (Langmuir, 1918). The value of b was calculated from the intercept of the plot of ln q e Vs ln C e Fig. 7(b) from the equation no (5) and was found out to be 5.84 * 10-2 and 8.87 * 10-2 l/mg for powdered and whole biomass respectively. Uranium sorption data was also fitted to Freundlich isotherm. The r 2 value obtained from the plot (Fig. 6.7(b).) for whole biomass was 0.87 and 0.93 for powdered biomass. K f and n were calculated from the intercept and slope of equation (2). n was 2.5 for powered biomass and 2.8 for whole biomass. As the value obtained for n lies in the range 1 < n < 10, it suggested a beneficial adsorption (Freundlich, 1907). 1/n values for both powdered and whole biomass are less than 1, which is suggestive of heterogeneous surface possession by the biomass having identical adsorption energy in all sites. 95

Fig 6.7(a). Langmuir isotherm for powdered and whole biomass. V = 50 ml, W = 25 mg, C 0 =50 600 mg/l, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm 0.7 0.6 w hole biomass pow dered biomass 0.5 C e /q e (g/l) 0.4 0.3 0.2 0.1 0 0 50 100 150 200 250 300 350 C e (mg/l) Fig 6.7(b). Freundlich isotherm for whole biomass and powdered biomass. V = 50 ml, W = 25 mg, C 0 = 50 600 mg/l, temperature = 25 ºC, ph = 5.5, agitation speed = 150 rpm. 7 pow dered biomass w hole biomass 6 5 4 ln q e 3 2 1 0 0 1 2 3 4 5 6 ln C e 96

Table 6.4. Values obtained from Freundlich and Langmuir isotherms for whole and powdered biomass. Langmuir isotherm Freundlich isotherm Type of biomass q max (mg/gm) b r 2 K f n r 2 Whole biomass 666.6 0.008876 0.99 44640 2.8 0.87 Powdered biomass 588 0.0584 0.99 19520 2.5 0.93 4.4. Conclusion There are number of biosorbents for uranium which have been already reported. But, to the best of our knowledge none has proved to be an ideal biosorbent. For metal uptake biosorbents having a metal loading capacity >15% are considered as good biosorbents. Search for sorbents from biological origin having higher metal loading capacities has been an active field of research. In the same context from our lab we have earlier reported a fungal biomass of Asperigillus fumigatus having a metal loading capacity of 423 mg/g (Bhainsa and D Souza, 1999), and a bacterial biomass of Pseudomonas spp. having a metal loading capacity of 541 mg/g (Sar and D Souza, 2001). Akhtar et al. (2007) reported another fungal biomass Trichoderma harzianum having a metal loading capacity of 612 mg/g. In comparison to the above mentioned biomasses, the biosorbent (S. asperum) used in our study has several advantages as described below: Natural occurrence of S. asperum on the sea coasts makes it an easily available biomass.being photosynthetic it is a cheap source for biosorbent material. 97

A ph independent uranium removal across ph range of 3 to 9 at a low initial metal concentration by S. asperum is a desired characteristic for an ideal biosorbent. This is the first study reporting a ph independent uranium sorption across such a wide range of ph. An efficient uranium sorption by S. asperum across the temperature range of 15-55 C, indicates the feasibility of sorption process at varying temperatures. High biosorption distribution coefficient of 195341 ml/g, elucidated a high affinity of S. asperum towards uranium. A high q max is characteristic of a good biosorbent. In this study we could achieve a high q max (666 mg/g) as compared to the reported uranium biosorbents used in dead form. The better performance of S. asperum in form of whole biomass as compared with powdered biomass eliminates the possibilities of excessive bed compression caused by the use of powdered biomass in the fixed bed reactors. Biosorption characterization revealed S. asperum to be a promising and ideal biosorbent for the removal of uranium from the dilute aqueous solutions (having varying ph and temperatures). In subsequent studies sorption process involving the use of S. asperum for the removal of uranium, the feasibility and spontaneity of the reaction, and, the reusability of the biomass was checked by studying the thermodynamics and desorption of the process. 98