Sorption of selected radionuclides from liquid radioactive waste by sorbents of biological origin: The alkaline earth alginates

Introduction of the so-called green chemistry in 1990 had a great impact on all other fields of experimental science, including radiochemistry and nuclear technology. The observed development of radiochemical separation methods requires the intense use of solvents and reagents that form large amounts of unnecessary sludge. Thus, several environmentally friendly extractants, sorbents and solvents have recently been proposed. However, for economic reasons, the replacement of chemicals used in existing separation methods and radioactive waste disposal technologies with the novel, green reagents is still slow.

One of the most effective methods of eliminating radioactive metals from wastewater is biosorption, a process that uses materials of biological origin (biosorbents). Low costs, easy regeneration of the biosorbent and the possibility of metal recovery are among the major advantages of biosorption compared with conventional treatment [1, 2]. Biosorption is a physico-chemical process consisting of absorption, adsorption, ion exchange, surface complexation and sometimes precipitation, although not all of these processes are always in use simultaneously. Biosorbents can be organic or inorganic materials and are necessarily water-insoluble. They can be either living or dead biomass

According to the definition provided in the Regulation of the Polish Minister of Economy in 2006, biomass is “solid or liquid substances of plant or animal origin that are biodegradable, derived from products, waste and residues from agricultural and forestry production, as well as from industry processing their products, as well as the part of the remaining waste that is biodegradable” [3].

Alginates are among the most common biosorbents currently used for radioactive metal-bearing aqueous streams. They are biopolymers of alginic acid, extracted from different types of algae, mainly green but also brown organisms. Calcium alginate has already been found to exhibit a relatively high uptake rate and distribution coefficient of ⁹⁰Sr²⁺ [6] and ²⁴¹Am³⁺ ions [4, 7]. However, separation of the metal-loaded sorbent from the purified solution is often a problem, and the use of magnetic alginate sorbents has thus received significant attention in recent years as a means of solving this technical problem [4].

Large amounts of radioactive aqueous waste that contains long-life, low-active radionuclides, such as ⁶⁰Co (half-life (t_1/2) of 15.3 years), ⁹⁰Sr (30.2 years) and ¹³⁷Cs (28.8 years) are mainly produced by nuclear power plant (NPP) installations. Additionally, nuclear medicine and scientific laboratories generate liquid waste that may also contain other radionuclides: ²⁴¹Am (t_1/2 = 432.2 years), ¹⁹²Ir (73.8 days), ¹²⁵I (59.6 days), ¹⁰³Pd (17.0 days) or ¹⁰⁶Ru (1.02 years). The composition of the novel wastes varies unpredictably and changes during storage due to the decay of the radionuclides. Moreover, data on the radionuclide content of liquid wastes are not usually published in commonly available sources. Solutions should be concentrated and solidified before final disposal [8]. Such radionuclides are also of great concern especially when a NPP fails and pollutes the environment, as well as the potable water (e.g., the Chernobyl or Fukushima catastrophes) [9].

The main objective of this work was to study the alginate spherical gels formed by three alkaline earth metals (calcium, strontium and barium) as potential sorbents in the processing of radioactive liquid wastes, either to remove the radionuclides from the contaminated aqueous solutions or to protect the environment from their release. Specifically, the sorption of ²⁴¹Am³⁺, ⁸⁵Sr²⁺ and ¹³⁷Cs⁺ from diluted aqueous solutions was studied. The radionuclides studied are the three most abundant radionuclides in Polish radioactive waste [10].

Analytical grade chemicals (Sigma-Aldrich or Avantor (earlier, POCh), both Polish branches) and deionized water were used. The radiotracers (¹³⁷Cs, ⁸⁵Sr and ²⁴¹Am) were purchased as acidic aqueous solutions from Polatom (the National Centre for Nuclear Research, Świerk, Poland).

Working solutions that contained all the studied radionuclides (ca. 10⁻⁶ M, each) were made gravimetrically by adding the necessary amounts of standard solutions in 10⁻³ M nitric acid and were examined for radiochemical purity by γ-spectrometry.

Spherical Ca²⁺, Sr²⁺ and Ba²⁺ alginate sorbents were prepared according to the procedure already described [6]: 0.5 M aqueous solution of the alkaline earth chloride (one, in any synthesis) was dropped into a continuously stirred 2% w/w aqueous solution of sodium alginate. The reaction was carried out at ambient temperature. The sorbent, obtained in the form of granules of ca. 3 mm diameter, was kept in the mother liquid for a further 24 h. The sorbent was then transferred into a solution containing 0.01 M KCl and 0.001 M CaCl₂ and kept for a maximum of two weeks for the experiments. Before starting any experiment, the sorbent was carefully washed with bi-distilled water.

The adsorption properties of the alginates towards micro amounts (10⁻⁸–10⁻⁹ mol·dm⁻³) of four radionuclides of Cs⁺, Sr²⁺ and Am³⁺ were investigated in batch experiments. A weighed amount of the sorbent (100 mg) was agitated (speed: 400 rpm; ambient temperature of ca. 21 ± 1°C) with 3 mL of any solution containing all the radionuclides studied. After the required phase contact time, samples were withdrawn from the shaker and left for the phases' separation. The supernatant was then confectioned into 1 mL portions and the equilibrium radionuclide concentrations were determined by measuring their radiation activity concentrations. All radioactivity measurements were performed in the well-type geometry. Each experiment was performed in triplicate, and the radiation activity of each sample was also measured three times.

Values of the distribution ratio (K_d), i.e. the normalized ratio of the radioactivity concentration in the equilibrium solid phase to the initial radioactivity concentration in the purified solution, were calculated using Eq. (1): (1) Kd=AsolidA0⋅Vm=A0−AeqA0⋅Vm {K_d} = {{{A_{{\rm{solid}}}}} \over {{A_0}}} \cdot {V \over m} = {{{A_0} - {A_{{\rm{eq}}}}} \over {{A_0}}} \cdot {V \over m} where: A₀ and A_eq denote the specific activity of the initial and equilibrium solutions, respectively, (cpm·mL⁻¹); A_solid denotes the radiation activity of the absorbed radionuclide (cpm·mg⁻¹), calculated as the difference between the values measured for the initial and equilibrium solutions; V is the volume of the solution (mL); and m is the mass of the sorbent (mg).

In extension to the sorption studies, the desorption of radionuclides Cs⁺, Sr²⁺ and Am³⁺ was also examined from all spherical alginate sorbents loaded with a mixture of radionuclides. For this purpose, water, 0.01 M HCl, 0.01 M NaNO₃, 0.01 M NaHCO₃ and 0.01 M Na₂SO₄ solutions were used as the desorbing media. All experiments were carried out under the following conditions: –

shaking time: 4 h,

In this work, the sorption of micro concentrations (10⁻⁸–10⁻⁹ M) of Cs⁺, Sr²⁺ and Am³⁺ by the alginate sorbents was studied to check their ability to decontaminate aqueous solutions and store radionuclides in the solid phase. Three alkaline earth cations (calcium, strontium and barium) were analysed as gelling agents for sodium alginate.

For establishing the potential decontamination procedure, the main parameters tested were the effect of contact time on the sorption of the radionuclides, the effect of pH, the effect of the mass of metal sorbed and the efficiency of desorption of the radionuclides from the metal loaded sorbent.

Effect of contact time on sorption of the radionuclides

Results for the rate of Am³⁺ adsorption as a function of agitating time for all alginate sorbents are given in Fig. 1a. For the calcium alginate, after a shaking time of 120 min, no more metal was adsorbed. In the case of strontium and barium alginates, a plateau of Am³⁺ uptake is reached within 180 min. As seen in Fig. 1b, the adsorption plateau of Cs⁺, Sr²⁺ and Am³⁺ by calcium alginate (pH 5) is reached at about the same time, at 120 min. The values of the distribution ratio are 2.1 × 10⁴, 5.4 × 10³ and 5.5 × 10², respectively.

Fig. 1

Values of the americium distribution ratio (K_d) were found to be 2.1 × 10⁴, 1.1 × 10⁴ and 1.6 × 10⁴, respectively. This means that about 98% of Am³⁺ may be removed by sorption with calcium alginate, while the uptakes of 80% and 70% were obtained for strontium- and barium alginates, respectively. Furthermore, it was found that Sr²⁺ radionuclide may be removed from aqueous solutions using the calcium, strontium and barium alginate with an efficiency of 86%, 85% and 81%, respectively. In turn, only about 5–10% of Cs⁺ ions are removed by any alginate.

Effect of pH on the sorption of micro amounts of the radionuclides

The acidity of the decontaminated solution is among the most significant factors determining the solution chemistry of metals, i.e. pH-dependent metal hydrolysis and speciation, complexation or redox reactions influence the potency of the sorption of metals. In addition, acidity reflects the behaviour of the adsorption sites of the sorbent (dissociation, protonation, etc.).

The dependence of the distribution ratio values of Am³⁺ on the initial pH of the solution is shown in Fig. 2a. The most favourable pH value was found to be around 5–6. One can imagine that, at sufficiently low pH values, the binding sites of the adsorbent also interact with hydronium ions, which compete with metal ions and decrease the metal ion interaction with the binding sites of the sorbent. As the pH increases, the charge of the binding sites becomes more negative, which promotes the adsorption of the metals. After pH 5–6, the amount of different americium hydroxy- and carbonate forms starts to play a significant role (see Fig. 3), which decreases the sorption efficiency. In our study, at around pH 5, an interplay of the different forms of americium cations, dissociation of the carboxyl groups of the polyuronic chain and concurrent sorption of the hydronium ions creates the most favourable conditions for metal sorption. The same can be stated for the sorption of Sr²⁺ and Cs⁺ ions.

Fig. 2

Fig. 3

Figure 2b shows that, independent of the studied metal, the most favourable conditions for sorption by the alginates is an aqueous solution pH of around 5–6.

Comparison of the radionuclide sorption in the optimum conditions

Comparison of the spherical calcium, strontium and barium alginates as potential sorbents for the removal of Cs⁺, Sr²⁺ and Am³⁺ radionuclides from the dilute (10⁻⁸–10⁻⁹ M) aqueous solutions is presented in Fig. 4. It can be seen that logarithms of the partition constants (log K_d) between the solid and liquid phases for Am³⁺ and Sr²⁺ cations vary between 3 and 4.5. In turn, the Cs⁺ cation is much less sorbed, and the K_d values are one or two orders lower. Such poor sorption of monovalent cations may probably be a result of the solubility of the alkali metal alginates in water.

Fig. 4

The values of the partition coefficients shown in Fig. 4 for each radionuclide do not significantly depend on the alginate gelling metal (calcium, strontium or barium). Such weak dependence has already been confirmed for the magnetic alginates [3].

The biosorption of Cs⁺, Sr²⁺ and Am³⁺ ions by three alkaline earth alginates was studied in a batch system as a function of the initial pH of the aqueous solution, contact time of the phases and the radionuclide content in the purified solution. The pH slightly affects the uptake of metal ions; however, the optimum initial pH of the solution was found to be 5–6 for all systems studied. The optimum equilibration time of the solution was established as 120 min with calcium alginate and 180 min with strontium and barium alginates. The Freundlich isotherm was found to be the best fit for the obtained sorption results, which means that the surface of the sorbent is heterogeneous, due to irregularly distributed mannuronic and glucuronic units. Only the latter are involved in the metal bonding process. The great dilution of the radionuclides (10⁻⁸–10⁻⁹ M) suggests a monolayer coverage of the surface.

Different desorbing solutions were tested for the alginate sorbents loaded with the radionuclides, and no systematics were found in the desorption of the radionuclides.