The Efficiency of Strontium-90 Desorption Using Iron (III) Solutions in the Decontamination Process of Radioactive Soils

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1 Journal o Ecological Engineering Received: Accepted: Published: Volume 9, Issue 2, March 28, pages The Eiciency o Strontium-9 Desorption Using Iron (III) Solutions in the Decontamination Process o Radioactive Soils Olga Cheremisina, Vasiliy Sergeev *, Varvara Alabusheva, Alexander Fedorov, Alexandra Iliyna Department o Physical Chemistry, Saint Petersburg Mining University, Saint-Petersburg, 2 Line 2, 996, Russian Federation * Corresponding author s sergeev.spmi@yandex.ru ABSTRACT The paper presents the investigation on the estimated eiciency o iron (III) chloride solutions in the decontamination process o radioactive soils with 9 Sr, according to kinetic and thermodynamic characteristics o the desorption process. The speciic 9 Sr radioactivity o soil samples was (3.9±.3) 4 Bq g. The adsorption isotherms o Sr 2+ and Fe 3+ are described with the Langmuir equation. The values o Gibbs energy G 298 = kj mol- and equilibrium ion exchange constant K eq = 6.5 conirm the hypothesis o strontium removal rom soils with iron (III) cations. The eectiveness o the method is substantiated by experimental and calculated results o this study samples o radioactive soils are deactivated in 9% ater 9.5 hours, whereas the kinetic constant is 6.77 s -. The suggested method o soil cleanup with.2 M Fe 3+ solutions is optimal and complies with the environmental requirements. Keywords: decontamination o radionuclides, thermodynamic and kinetics investigations, desorption o strontium-9, iron (III) chloride. INTRODUCTION Radioactive isotopes o heavy metals are considered to be particularly dangerous matter which is produced in the course o anthropogenic activities, industrial accidents and nuclear tests. Strontium-9 isotope is mainly ixed on the surace o clay minerals [Bobovnikova 99]. Radionuclides undergo physicochemical reactions on the border between solid and liquid phases in soils, where strontium is ound in dierent orms. There are cations o radionuclides in the soil solution and cations which are ixed in solid (non-exchangeable). Fixation o exchangeable orm and remobilization o ixed radionuclides are caused by cations diusion in solid particles [Bulgakov 998, Bondarenko and Kononenko 2]. The majority o water-soluble 9 Sr (76%) occurs in cation orms [Martyushov et al. 995]. The authors Ningping and Mason [2] propose strontium-ree ground water or Sr(II) removal rom silica colloids and Ca-montmorillonites. The desorption rom montmorillonites is a time-consuming and ineective process the amount o recovered strontium is 6.5% ater 9 days. Desorption in the study [Martyushov et al. 995] was executed with dierent concentrations o HCl and HNO 3. The most eective outcome was achieved with. M solutions 83% and 82% or HCl and HNO 3, respectively. However, strong acids are too expensive and they orm acidic soils which are not applicable. The review o the literature on the 9 Sr desorption process indicates that there is a need to detect an eluent or radioactive soils recultivation which is sae and provides eective desorption parameters. It was proposed that ion exchange o strontium with high complex ormation properties will promote the extraction o radionuclides rom soils. Cations o Fe 3+ are characterized by a high charge and a small radius. Due to the high ionic potential, which is equal to z/r, Fe 3+ displaces other cations rom mineral surace, including 37 Cs and 9 Sr. 49

2 Thereore, it was suggested to apply iron (III) chloride solutions with ammonium chloride (or ph stabilization) or soils decontamination. The aim o this investigation was to estimate the eiciency o iron (III) chloride solutions in the decontamination process o radioactive soils with 9 Sr according to kinetic and thermodynamic characteristics o the desorption process. MATERIALS AND METODS The process o sorption was perormed on samples using water solutions with the speciic radioactivity o 5 Ci dm -3. The process was carried out with the limited value method. The sample weight was brought into contact with the 9 Sr solution, interblended roundly with a churn over two weeks in order to obtain a stable solution. The solid phase was separated rom the liquid phase and washed with distilled water. The speciic 9 Sr radioactivity o soil samples was (3.9±.3) 4 Bq g -. The 9 Sr desorption was applied dynamically. It was chosen to apply.2 mol dm -3 iron (III) chloride as an eluent solution. The process was held in a 5 cm height glass column with the cross-sectional area o.7 cm 2 and the interstitial volume o 3 cm 2. The underside o the column was illed with glass iber and a Sr-adsorbed soil sample o 2 g weight above it. The desorption agent was carried through the solid structure with the 9 4 m s - rapidity. Ater 3 cm 3 o the eluent passed the column, 5 and ml portions were collected and their 9 Sr radioactivity was analyzed. I there are both 9 Sr and 37 Cs radionuclides in soils, it is necessary to supplement ammonium chloride as a cesium carrier. Real contaminated soil patterns were collected in the area which is 5 away rom the 4th plant unit o the Chernobyl Electric Power Station. RESULTS AND DISCUSSION The adsorption isotherms o Sr2+ and Fe3+ are described with the Langmuir equation and shown in Figure, where is equilibrium concentration o the cations, g the activity actor and DS/L is the distribution constant between the solid and liquid phases. Figure 2 shows the linear dependence between the inverse o sorption and concentration. The equations are Г =.75 С and Г = or strontium and iron (III), С respectively. This result is in accordance with the study [Chirkst et al. 994, Chirkst et al. 23], where eectiveness o the Langmuir equation is described or cesium cations sorption on a variety o minerals. The absolute term o the Langmuir equation is equal to the inverse o the limit value o sorption, which is.34 mol kg - or Sr 2+ and.24 mol kg - or Fe 3+. Slope ratios were used to determine the adsorption thermodynamic equilibrium ratios o strontium and iron on clay minerals which are 294 and 73, respectively. The distribution coeicient o strontium between the solid and liquid phases depends on strontium (2+) concentration in equilibrium solution and coverage ratio o the adsorbent: Figure. The sorption isotherms o strontium (a) and iron (3+) cations (b) on clay minerals 5

3 Table. Characteristics o Sr 2+ and Fe 3+ distribution between the solution and clay samples Sr 2+ Fe 3+ С ꝏ 4, mol dm γ Sr D S/L С ꝏ 4, mol dm γ Fe D S/L Figure 2. Linear orm o the sorption isotherm or strontium (2+) (a) and iron (b) on clay minerals Г К DD Т ж = γγ Sr 2+( + Кс ) () However, limit dilution brings about the value o D S/L = Γ K =. which is the same as the experimental result (Table ). For the Sr 2+ aq Sr 2+ адс equilibrium, which characterizes the sorption ability o a strontium cation, the value o Gibbs energy is ΔΔΔΔ 298 = RTlnDD SS LL = 5.7 kj mol. This value illustrates rising o the sorption ability in response to hydroxonium cations transition with strontium cations due to increase o the charge. According to the study [Thermodynamic constants 98], under the condition o ΔΔ GG 298(Sr 2+ aq) = kkkk mol, the value o Gibbs energy or adsorbed cation ormation is ΔΔ GG 298 = kj mol. The ollowing equations are deined with the distribution coeicient o Fe 3+ and the Langmuir constant: Fe 3+ aq Fe 3+ ads (2) Fe(OH) 2+ aq Fe(OH) 2+ ads (3) Fe(OH) 2 + aq Fe(OH) 2 + ads (4) The main orms o iron (3+) in solutions with the value o ph 3. are taken into account with corresponding thermodynamic characteristics. The sorption o iron (III) polymeric orms is neglected. The summary o the equations (5)-(7) describes the adsorption process o iron (III) with m, n and p coeicients: nn mm = [Fe(OH)2 [Fe 3+ = KK h, ] [HH (5) 5

4 pp [Fe(OH) 2+] = nn [Fe(OH) 2 = KK h,2 [HH (6) mm + nn + pp = (7) Hydrolysis constants o iron (3+) are K h, = and [9]. Hydroxonium cations concentration is ound due to ph = 3.. Consequently, m =.24. n =.2 and p =.776. The summary equation o the iron (III) adsorption process is:.24 Fe 3+ aq +.2 Fe(OH) 2+ aq Fe(OH) 2 + aq.24 Fe 3+ ads +.2Fe(OH) 2+ ads Fe(OH) 2 + ads (8) Using the summary distribution coeicient, it is possible to determine the Gibbs energy or the reaction (8): Δ (8) G 298 = Δ RTln D S/L = 8.2±.3 kj mol -. In order to calculate the equilibrium constants or (2)-(4), it is necessary to make a series o operations. Δ (8)G 298 = m Δ (2)G n Δ (3)G p Δ (4)G 298. (9) Then: D= D m D 2 n D 3p, () where: D is the experimental summarized distribution coeicient o iron (III), D n partial distribution coeicients o dierent iron orms or the equations (2)-(4), and their values are: DD = DD = [Fe3+ ads] [Fe 3+ aq] DD 2 = [Fe(OH)2+ ads] [Fe(OH) 2+ aq] DD 3 = [Fe(OH) + 2 ads ] [Fe(OH) 2+ ads] + [Fe(OH) + 2ads ] С () (2) (3) (4) The sorption o Fe 3+ ions is neglected in (4) due to their small concentration. Calculating () (4), the values o D 2 and D 3 are: DD 3 = DD CC [Fe(OH)2+ ads] = DD CC KK 2 [Fe(OH) 2+ aq] (5) The values D = 27, [Fe(OH) 2+aq ] =.776C, =.2CC are substituted into the equation (5). Calculating together the equations () and (4), where the exchange coupling.5 constant is approximately (and D D 2, D 2 2 D 3 и D D 2 D 3 ), the distribution constants are D =2, D 2 =6 and D 3 =2. Then Δ (2) G = kj mol -, Δ (3) G =.2 kj 298 mol- and Δ (4) G = 7.4 kj 298 mol-. Table 2 shows the calculated Gibbs energy o adsorbed iron (3+) cations ormation and sorption equilibrium. Furthermore, there are three exchange reactions: 3Fe 3+ aq + 2Sr 2+ ads 3Fe 3+ ads + 2Sr 2+ aq Δ (6)G 298 = -3.2 kj equiv - ; К (6) = Fe(OH) 2+ aq + 2Sr 2+ ads 2Fe(OH) 2+ ads + 2Sr 2+ aq Δ (7)G 298 = kj equiv - ; К (7) = 2.5. Fe(OH) 2aq + + 2Sr 2+ ads Fe(OH) + 2ads + 2Sr 2+ aq Δ (8)G 298 = kj equiv - ; К (8) = 6.3. (6) (7) (8) The Gibbs energy or the summary ion exchange reaction with strontium is: D S G 298 =.72D (6) G D (7) G D (8) G 298 = kj mol-. Consequently, the total ion exchange equilibrium constant is 6.5. It means that strontium cations will expel rom clay minerals ater a leaching operation with iron (III) solutions. Iron (III) chloride solutions were used or strontium desorption rom clay minerals. According to the experimental data (Figure 3), the kinetic coeicient o 9 Sr sorption on modelling samples is s - through the Wigner-Polanyi equation. The initial speciic 9 Sr radioac- 52

5 Table 2. Thermodynamic characteristics o iron and strontium cations adsorption on clay minerals Reaction z+ D G 298 ( Kat aq ), kj mol - [47] z+ D G 298 ( Kat ads ), D G 298, kj mol - kj mol - Fe 3+ aq Fe3+ ads Fe(OH) 2+ aq Fe(OH)2+ ads Fe(OH) Fe(OH) aq 2 ads Sr 2+ aq Sr2+ ads K eq Figure 3. Desorption kinetics o 9 Sr rom modelling samples tivity o soils is (3.9±.3) 4 Bq g -. As a result, the time required to achieve 9% desorption is approximately 2.2 hours. Ater the process o ablution, the value o initial speciic 9 Sr radioactivity changed rom Bq g to.87 2 Bq g -. As a consequence, the extent o desorption is 75±5%. The kinetic constant or natural soil samples is 6.77 с - and the period o 9% desorption is 9.5 hours. Accordingly, the kinetic constant o real samples sorption is 5 times bigger than modelling samples. CONCLUSIONS The values o Gibbs energy G 298 =-4.65 kj mol - and equilibrium ion exchange constant K eq = 6.5 conirm the hypothesis o strontium removal rom soils with iron (III) cations. The eectiveness o this method is substantiated by experimental and calculated results o this study samples o radioactive soils are deactivated in 9% ater 9.5 hours, the kinetic constant is 6.77 s -. The suggested method o soil cleanup with.2 M Fe 3+ solutions is optimal and complies with environmental requirements. REFERENCES. Bobovnikova Ts.I., Virchenko E.P., Konoplev A.V. 99. Chemical deportments o long-lived radionuclides and their transormation in the soils o the Chernobyl accident area. Soil Science,, Bondarenko G.N., Kononenko L.V. 2. Kinetics o 9Sr and 37Cs deportments transormation in soils. Mineralogy Journal, 3, Bulgakov A.A Prediction methods o 9Sr and 37Cs distribution in natural soil-water systems. Extended abstract o dissertation. Vernadsky Institute o Geochemistry and Analytical Chemistry o Russian Academу o Sciences. 4. Chirkst D.E., Chaliyan K.N., Chaliyan A.G Thermodynamics study o radioactive 37Cs soils deactivation ater the Chernobyl accident. Radiochemistry, 36 (5), Chirkst D.E., Cheremisina O.V., Ivanov M.V. 23. Thermodynamics study o iron (III) cations sorption on clay materials. JAC, 76(6), Martyushov V.V., Spirin D.A., Bazilev V.V., Fedorova T.A The orms o radionuclides in the soils o East-Ural Radioactive Trace, Ecology, 2, Ningping L., Mason C.F. 2. Sorption-desorption behavior o strontium-85 onto montmorillonite and silica colloids. Applied Geochemistry, 6(4), Rohwedder J.R., Cadore S., Abate G., Grassi M.T. 24. Montmorillonite and vermiculite as solid phases or the preconcentration o trace elements in natural waters: Adsorption and desorption studies o As, Ba, Cu, Cd, Co, Cr, Mn, Ni, Pb, Sr, V, and Zn. Applied Clay Science, 99, Thermodynamic constants o materials: the book o reerences. 98. Edited by Glushko V.P. USSR,. 53