Uranium is one of the most seriously threatening heavy metals because of its high toxicity and some radioactivity. Excessive amounts of uranium have found their ways into the environment through the activities associated with the nuclear industry. Uranium contamination poses a threat in some surface and groundwaters. There is a need for controlling the heavy metal especially uranium emissions into theenvironment. Conventional methods for removing heavy metals from industrial effluents (e.g. precipitation and sludge separation, chemical oxidation or reduction, ion exchange, reverse osmosis, membrane separation, electrochemical treatment and evaporation) are often ineffective and costly when applied to dilute and very dilute effluents. Recently, biological removal processes has been attracting considerable attention for removing heavy metals from aqueous wastes and screening for microorganisms having higher potential for removing heavy metals from wastes has been promising so far. Although the fact that marine algae are capable of biosorbing radio nuclides such as radium, thorium and uranium has been known for a long time, the biosorption of uranium by Cystoseira indica algae, a brown algae biomass which found vastly in Iran, has not been investigated. The aim of this paper is to determine the effect of the basic parameters such as pH, contact time and initial metal concentration on the uranium biosorption by Cystoseira indica algae and to compare between sorption on protonated, Ca-pretreated and non-pretreated algae biomass in a batch system.

Kinetics studies show that uranium ions adsorption rate by Cystoseira indica algae is high at the beginning but plateau values are reached in about 3 hrs. This result is typical for biosorption of metals involving no energy-mediated reactions, where metal removed from solution is due to purely physico/chemical interactions between the biomass and metal in solution. The fast dynamic behaviour of uranium uptake suggests a minimal residence time (or contact time) for a process based on these preparations. In order to modelling the sorption rate of uranium on Cystoseira indica algae suspension in a well-agitated batch system, pseudo-second order rate equation was applied. Assuming the sorption capacity of uranium on the biomass is proportional to the number of active sites occupied on the sorbent, then the pseudo-second order equation is given by:

         t =0   ,    q_t=0              [1]

Where k is the equilibrium rate constant of pseudo-second order sorption kinetics (g/mg x min), q_t the amount of sorbate on sorbent at time t (mg/g), and q_eq the equilibrium uptake (mg/g). Equation [1] can be integrated and rearranged in to:

                                                [2]

The values of the parameters, q_eq and k are calculated from the slops and the intercepts of the straight lines tabulated in Table 1. The values of the theoretical q_eq for both sorbents are in good agreement with those found experimentally. Also the correlation coefficients for both sorbents are greater than0.999. These results indicate that the sorption of uranium on the sorbents investigated in this study follows a pseudo-second order kinetics.

Earlier studies on heavy metal biosorption have shows that pH is an important parameter effecting the biosorption process. The effect of initial pH on uranium ion biosorption capacity of Cystoseira indica algae was studied at 350 mg/l initial uranium concentration and at 30ºC. Experimental results show the biosorption of uranium for both protonated and non-pretreated algae increased with pH up to 4 and then declined with further increase in pH. The maximum equilibrium uptake value was found at pH 4.

The metal ion binding in biosorption could be attributed to several mechanisms such as ion exchange, complexation, electrostatic attraction and microprecipitation. For algae biomass, ion exchange has been considered as a main mechanism responsible for metal sequestering. The ion exchange mechanism for uranyl ions binding to the biomass is complicated by the fact that the uranium cation UO₂²⁺ is hydrolyzed in aqueous solutions within the range of the sorption system pH. Portioning of the hydrolysed uranium species depends on the solution pH and on the total uranium concentration in the solution. In the range of acidic to near neutral pH values, four major hydrolysed complex ions, UO₂²⁺, (UO₂)₂(OH)₂²⁺, UO₂OH⁺, (UO₂)₃(OH)⁵⁺ and a dissolved solid schoepite (4UO₃.H₂O), a hydrous uranium oxide, exist in the solution (Baes and Mesmer, 1976). The hydrolysis equilibrium constants are pK = 5.8 for UO₂OH⁺, pK = 5.62 for (UO₂)₂(OH)₂²⁺ and pK = 15.63 for (UO₂)₃(OH)₅⁺. At pH 4 and uranium concentration 350mg/l all hydrolyzed ions exist in the solution. According to Collins and Stotzky, the hydrolyzed species can obviously be sorbed better than the free hydrated ions. Particularly the monovavalent, compared with the divalent hydrolyzed ions, have even higher affinity to the biomass in ion exchange with protons because they could replace single protons on separate binding sites in the biomass.

The percentage of UO₂²⁺ in the solution increases with decreasing the pH of system. The lower pH suppresses the enhancement of uranium biosorption occurring normally because of the hydrolyzed ions. When the pH becomes low enough, for example at pH 2.6, the divalent free UO₂²⁺ becomes the dominant ion form in the solution for a wide uranium concentration range from 0.3 to 1000 mg/l. In addition, since the UO₂²⁺ is divalent, it can only replace two protons on the adjacent binding sites of the biomass but cannot react with those sites which are farther apart from each other. In other words, at low pH some binding sites are not available to the divalent UO₂²⁺. On the other hand, the non-ion dissolved solid schoepite starts appearing in the solution when the pH is too high. The uranium sorption may be hindered by the decrease in ion concentration in this situation.                    

The initial concentration provides an important driving force to overcome all mass transfer resistance of uranium between the aqueous and solid phases. Experimental results show at low initial uranium concentration, the equilibrium sorption capacity of the all sorbents is directly proportional to the initial uranium concentration in the solution and in the high initial uranium concentration range, the equilibrium sorption capacity of the all forms of the algae biomass go to a constant value. Also the results show that the pre-treatment of the algae biomass considerably influences the equilibrium sorption capacity of the sorbent and the Ca-pretreated Cystoseira indica algae is better than protonated and non-pretreated Cystoseira indica algae and the protonated algae is better than non-pretreated algae in the all range of initial uranium concentration. The pre-treatment process enhances biomass surface ions. Since the biosorption of uranium is largely ion exchange process, the processed samples of the biomass have higher sorption capacity in comparison with raw. Also the Ca-treatment is more effective than the protonation, because the Ca²⁺ ions are divalent and can replace with two monovalent uranium ions. Proton, nevertheless, can replace with only one. In this paper, the characteristics of the surface groups on Cystoseira indica algae were not determined. Acidic sites, carboxylic and phosphatic types or amino-type sites are more likely to take part in ion exchange. It is supposed to be determined by potentiometric titration.

The equilibrium established between adsorbed metal ion on the biosorbent and unadsorbed metal ion in solution can be represented by adsorption isotherms. Two isotherm equations havebeen tested in the present study, namely, the Langmuir, Freundlich.

The Langmuir and Freundlich adsorption parameters with the correlation coefficients (R²) for the biosorption of uranium on the biosorbent are listed in Table 2. High regression correlation coefficients (>0.9887) for the Langmuir model show that the adsorption process of uranium by Cystoseira indica algae can be well defined by this model. As can be seen from Table 2, the Freundlich isotherm is only suitable for describing the biosorption equilibrium of uranium by Ca-pretreated and non-pretreated algae and it can not fit reasonably the biosorption equilibrium of uranium by protonated algae. While the Freundlich model does not describe the saturation behaviour of the biosorbent, Q⁰, the Langmuir constant represents the monolayer saturation of equilibrium. The maximum capacity, Q⁰ determined from the Langmuir isotherm defines the total capacity of the biosorbent for uranium. From Table 2 the maximum adsorption capacity for uranium on the Ca-pretreated, protonated and non-pretreated Cystoseira indica algae is 454.5, 322.58 and 224.67 mg/g, respectively.

The study indicated that the biomass of the Cystoseira indica algae particularly its Ca-pretreated form could be used as an efficient biosorbent material for the treatment of uranium ions-bearing waste water streams. The kinetics of adsorption by this biomass was rapid with 90% of the total adsorption occurring within first 30 min. Also the uranium biosorption on the biomass was found to follow pseudo-second order kinetics. The adsorption capacities were solution pH dependent and maximum adsorption capacities of Ca-pretreated, protonated and non-pretreated Cystoseira indica algae was determined to be 454.5, 322.58, 224.67 mg/g respectively at a solution pH of 4. The Langmuir and Freundlich adsorption models were used for the mathematical description ofthe biosorption equilibrium of uranium ions to Cystoseira indica algae andthe obtained results showed that the adsorption equilibrium data fitted very well tothe Langmuir model in the studied concentrationrange for all investigated biomass forms. Assumingthe batch biosorption as a single-staged equilibrium operation, the separation process can bemathematically defined using these isotherm constants to estimate the residual concentration ofmetal ions or amount of biosorbent for desiredpurification.

BAES, C.F. and MESMER, R.E. The hydrolysis of cations. Wiley-Interscience, John Wiley and sons, New York 1976. 512 p.