Removal of Strontium, Cobalt, and Cesium from the Mixed Aqueous Solution Using Arthrobacter nicotiane Cells

The removal of strontium, cobalt, and cesium ions from the aqueous solution using biomass was investigated. Arthrobacter nicotianae known for its high capacity to remove cationic metal ions was used for the removal of these metal ions. Several factors affected the removal of these metal ions using A. nicotianae including solution pH, concentration of metal ions, and cell amount. The amount of each metal ion removed increased with the rising pH (1 - 5) of the solution. Meanwhile, the ion uptake per gram of dry biomass (mmol metal ion/g dry wt. cells) increased with increasing metal ion concentration. However, removal efficiency (%) decreased. The metal ion uptake (mmol metal ion/g dry wt. cells) was found to fit the Langmuir isotherm. Conversely, increasing the biomass content enhanced total percentage of metal ions removed (%); however, the amount of each metal ion removed (μmol metal ion/g dry wt. cells) decreased. The removal of strontium using A. nicotianae occurred rapidly, approaching equilibrium within approximately 5 min. The amounts of cobalt and strontium removed were higher than that of cesium removed. The maximum amount of cobalt, strontium and cesium is estimated to be 179, 175, and 56.1 μmol/g dry wt. of cells, respectively.

• Strontium Removal
• Cobalt Removal
• Cesium Removal
• Arthrobacter nicotiananae

1. Introduction

Radioactive elements such as cesium, iodine, strontium, plutonium, barium, cobalt, lanthanum, yttrium, and tellurium were detected around the Fukushima Daiichi nuclear power plant, Japan, which was damaged by a magnitude 9.0 earthquake and the subsequent tsunami.

The removal of radioactive elements from contaminated sources is a significant area of research in environmental control. Special attention has been given to studying microorganisms that remove containing bacteria [1] - [4] , actinomycetes [5] - [7] , fungi [5] [8] - [11] , and yeasts [4] [12] .

We investigated the removal and recovery of uranium from aqueous systems using microorganisms isolated from uranium mines [13] . Several bacterial strains with extremely high uranium removal capacities were identified suggesting that the microbial biomass could serve as an effective adsorbing agent for the removal and recovery of uranium and heavy metals present in aqueous systems surrounding the Fukushima Daiichi nuclear power plant.

We screened various species and strains of bacteria, actinomycetes, fungi, and yeasts to monitor the efficiency of uranium adsorption [14] . The basic features affecting uranium adsorption such as coexisting cations and anions, cell amounts, and the adsorption kinetics, were evaluated, with Arthrobacter nicotianae cells exhibiting the highest uranium uptake. Similarly, Additionally, the study examined the removal of another actinoid element, thorium ions, which may be present along with uranium in refining effluents [15] . Similarly, cadmium [16] and all of rare earths [17] [18] were also removed the highest amount of each metal ion using A. nicotianae cells in the all of microorganisms examined.

This paper discusses the use of biomass for the removal of strontium (Sr), cobalt (Co), and cesium (Cs) from the mixed solutions in water.

2. Materials and Methods

2.1. Culture of Microorganisms

The microorganisms were grown in a medium containing 3 g/L meat extracts, 5 g/L peptone, and 5 g/L NaCl in deionized water. The cultures of microorganisms, maintained on agar slants, were grown in 300 mL of the medium in a 500-ml flask with continuous shaking (120 rpm) at 30˚C. To get a sufficient number of resting microorganisms after separation from the growth medium, the cultures were grown for 72 h. The cells were collected by centrifugation, washed thoroughly with deionized water, and used for subsequent removal experiments.

2.2. Effect of pH on Metal Removal Using A. nicotianae Cells

Metals were supplied as nitrates. Effect of pH on metals removal using A. nicotianae cells was examined as followed. pH of the solution was adjusted to a desired value (1.0 - 5.0) using 0.1 M HNO₃. Resting cells (15 mg dry wt. basis) were suspended in 100 mL solutions containing 35 μM of each metal and incubated for 20 h at 30˚C. Microorganisms were then collected by filtration through a nitrocellulose membrane filter (pore size 0.2 μm). Control studies confirmed that free metals ions were not adsorbed onto the filter.

The amount of each metal removed using the cells was determined by measuring the difference between the initial and final metal content in the filtrate using an atomic absorption analysis quantometer (AA-6300, Shimadzu Corporation, Kyoto).

2.3. Dependence of Cesium, Cobalt, and Strontium Removal on External Metal Concentrations Using A. nicotianae Cells

Dependence of external metal concentrations on metals removal using A. nicotianae cells was examined as followed. Resting cells (15 mg dry wt. basis) were suspended in 100 mL solution (pH 5) containing metals (20 - 200 μM) for 20 h at 30 ˚C. The amount of each metal remaining in the cell-free filtrate was measured, as described above.

2.4. Dependence of Cesium, Cobalt, and Strontium Removal on Cell Amounts Using A. nicotianae Cell

Dependence of the cell amounts on metals removal using A. nicotianae cells was examined as followed. Resting cells (5 - 80 mg dry wt. basis) were suspended in 100 mL solution (pH 5) containing 130 μM of metal for 20 h at 30˚C. In this section the solution containing relatively high metal concentration (130 μM) because of using wide range of cell amounts, however, the solution containing 35 μM of metal because of a relatively low resting cells amount (constantly 15 mg) in the pH dependence experiment. Metals remaining in the cell-free filtrate were measured as described above.

3. Results and Discussion

3.1. Effect of pH on the Removal of Cobalt, Strontium and Cesium

The effect of pH on the removal of Sr, Co, and Cs containing the same concentration of each metal using A. nicotianae was examined. Metal removal was examined in solutions with varying pH (pH 1 - 5). Strontium hydroxide was precipitated at pH 6. As shown in Figure 1 , the removal efficiency of all metals increased with increasing pH of the solution. Removal efficiencies of Co and Sr pH 5.0 were 96% and 98% respectively, however, that of Cs was 27%. Accordingly, the removal of Co and Sr is easier than that of Cs. Additionally, approximately 40% of Sr was removed at pH 2, whereas Co and Cs were not removed under similar conditions. Approximately 34% of Co was removed at pH 3; however, Cs removal remained minimal under these conditions. Therefore, these metals can be effectively separated using A. nicotianae by adjusting the pH.

3.2. Effect of External Metal Concentration on the Removal of Cobalt, Strontium and Cesium

The effect of the external metal concentration on the removal of Co, Sr, and Cs was then examined. As illustrated in Figure 2(a) , the amount of each metal removed (μmol/g dry wt. cells) increased with increasing external metal concentration, whereas the total percentage of metal removed (%) decreased. Under these experimental conditions, Co (100%) and Sr (94.0%) were mostly removed from a solution containing 20 μM of each metal. However, the amount of removed total cesium (%) was 29.4% from the solution containing 20 μM of both metals. The maximum amounts of Co, Sr, and Cs removed were 215, 194, and 81 μmol/g dry wt. cells, respectively. Accordingly, the relative degree of each metal removed was observed to be Co > Sr >> Cs, indicating that A. nicotianae cells can remove Co and Sr more readily than Cs.

The relationships between the residual Co, Sr, and Cs concentrations in the solution and the amount of each metal removed are illustrated in Figure 2(b) . The figure clearly indicates that the removal of these metals using A. nicotianae cells obeyed the Langmuir isotherm, and Q = Q_maxK_LC_e/(1 + K_LC_e), where Q represents the amount of metal removed (μmol metal/g dry wt. cells), C_e is the residual metal in solution (μmol metal/L), and K_L is a Langmuir constant. The maximum amount of Co, Sr, and Cs removed (Q_max) and Langmuir constant (K_L) shown in Table 1 . Langmuir isotherm is indicated that the relative degree of maximum removal follows the order for Co = Sr > Cs.

Figure 1

Figure 1. Effect of pH on the removal of Sr, Co, and Cs using A. nicotianae cells.

Figure 2

(a) (b) Figure 2. (a) Effect of metal concentration on the removal of Co, Sr, or Cs using A. nicotianae cells. (b) Langmuir isotherm of each metal removal using A. nicotianae cells.

Figure 2

(a) (b) Figure 2. (a) Effect of metal concentration on the removal of Co, Sr, or Cs using A. nicotianae cells. (b) Langmuir isotherm of each metal removal using A. nicotianae cells.

Figure 2

(a) (b) Figure 2. (a) Effect of metal concentration on the removal of Co, Sr, or Cs using A. nicotianae cells. (b) Langmuir isotherm of each metal removal using A. nicotianae cells.

Table 1

Table 1. Maximum amounts of removed metal ions and Langmuir constant.

	Metal Ion
	Co	Sr	Cs
Qmax [μmol/g dry wt. cells]	179	175	56.1
KL [L/μmol]	−3.85	−0.632	4.58

3.3. Effect of Cell Amount on the Removal of Cobalt, Strontium and Cesium

The effect of A. nicotianae cell amount on the removal of Co, Sr, and Cs was examined the results are illustrated in Figure 3 . The total amount of each metal removed increased with rising cell amount, whereas the relative amount of each metal removed by the cells (μmol metal/g dry wt. cells) reduced. Under these experimental conditions, approximately 90% of Co and Sr were removed using over 78 mg of the dry wt. basis of the cells. However, the amount of total removed cesium was only 10% using 78 mg of the A. nicotianae cells used. The maximum amounts of Co, Sr, and Cs removed were 277, 227, and 176 μmol/g dry weight, respectively. cells using 5.2 mg dry wt. cells, respectively.

Figure 3

Figure 3. Effect of cell amounts on the removal of Co, Sr, and Cs using A. nicotianae cells.

4. Conclusions

The removal of Co, Sr, and Cs, metals detected around the Fukushima Daiichi nuclear power plant from a solution containing mixed metal ions using A. nicotianae was demonstrated and investigated.

The removal efficiency of the metal increases as the pH of the solution rises. The relative degree of metal removal by A. nicotianae cells was observed to be Co, Sr >> Cs.

The metal removal efficiency (μmol/g dry wt. cells) increased with increasing metal concentration in the solution. The relative degree of metal removal by A. nicotianae cells was Co = Sr > Cs.

The total percentage of metals removed increased with increasing cell number. and the relative degree of metal removal by A. nicotianae cells was Sr > Co > Cs.

In this paper, the main purpose is removal of radioactive ions such as Co. Sr, Cs from contaminated sources. Therefore, desorption of adsorbed metals and the reusability of the biomass was not examined. However, these metal ions are also rare metals, desorption and separation of adsorbed metals are now investigating. The most of adsorbed metals can be desorbed using immobilized microorganisms by column system. Desorption and separation of the metals adsorbed will be reported in the next paper. Additionally, the removal of Co, Sr, and Cs were almost adsorption, because of most of removed metals were easily desorbed using immobilized microorganisms by column system.

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