Fast and efficient cesium removal from simulated radioactive liquid waste by an isotope dilution–precipitate flotation process

Chemical Engineering Journal 275 (2015) 342–350

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Fast and efficient cesium removal from simulated radioactive liquid waste by an isotope dilution–precipitate flotation process Mohamed A. Soliman a, Ghada M. Rashad b, Mamdoh R. Mahmoud b,⇑ a b

Egypt Second Research Reactor, Atomic Energy Authority, P.O. Box 13759, Cairo, Egypt Nuclear Chemistry Department, Hot Laboratories Center, Atomic Energy Authority, P.O. Box 13759, Cairo, Egypt

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Removal of Cs-137 by an isotope

dilution–precipitate flotation was studied.  Sodium tetraphenylborate precipitating agent was used with SLS and CPC surfactants.  Lower remaining Cs-137 activity values than the permissible value are obtained.  CsTPB precipitate was efficiently separated within half a minute.  The removal mechanism of cesium by SLS and CPC was proposed at different pH values.

a r t i c l e

i n f o

Article history: Received 10 February 2015 Received in revised form 28 March 2015 Accepted 30 March 2015 Available online 15 April 2015 Keywords: Cesium Precipitation Flotation Removal Radioactive liquid waste

a b s t r a c t The performance of an isotope dilution–precipitate flotation process for Cs-137 removal from low-level liquid radioactive waste is evaluated in this study using sodium tetraphenylborate (NaTPB) as the precipitating agent. In absence of Fe(III), neither the anionic sodium lauryl sulfate (SLS) nor the cationic cetylpyridinium chloride (CPC) could float the CsTPB precipitate over the studied pH range of 2.5–11.9. It is proposed that iron oxide coated CsTPB particles are formed upon addition of Fe(III) solution and their floatability is greatly dependent on the solution pH and the Fe(III) content. At the optimum conditions, remaining Cs-137 activity values <1.3 nCi/L (SLS, pH 6.8) and <2.3 nCi/L (CPC, pH 10.2) are achieved from radioactive wastewater within half a minute which are below its discharge limits. The mechanism of flotation is proposed and cesium removal is suggested to proceed via flotation of iron oxide coated CsTPB. Flotation technique is compared to other conventional solid–liquid separation processes (decantation, centrifugation and filtration) for separation of the iron oxide coated CsTPB particles. The results showed that the best separation was attained by flotation process. The data obtained by the present combined process, precipitation followed by flotation, for Cs-137 from radioactive wastewater are also compared to those reported by other treatment methods. Based on the data obtained, the combined process has great potential as a radioactive wastewater treatment technology. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Due to its long half-life (T1/2 = 30.17 years), high fission yield (6.5%), high solubility/mobility and high radiotoxicity, the Cs-137 ⇑ Corresponding author. Tel.: +20 1221925641. E-mail address: [email protected] (M.R. Mahmoud). http://dx.doi.org/10.1016/j.cej.2015.03.136 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

radionuclides-contaminated radioactive liquid wastes must be effectively treated prior its release into the environment. Chemical precipitation, ion exchange, membrane separation solvent extraction and adsorption are the usual methods used for the treatment of radioactive liquid wastes [1–4]. Among them, chemical precipitation method is a well-established method for the removal of radionuclides from radioactive wastewaters and is

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M.A. Soliman et al. / Chemical Engineering Journal 275 (2015) 342–350

in regular use [5]. Unfortunately, precipitation of alkali metals is mostly difficult since their salts usually are very water soluble [6]. Moreover, this method is inappropriate for treatment of wastewaters having very low concentrations of the radionuclide. Herein, an isotope dilution-precipitation process has been used in this work. This process is based on the utilization of stable cesium (Cs-133) as the carrier and sodium tetraphenylborate (NaTPB) as the precipitating agent. When coexisted with Cs-137 in the aqueous solution, the stable cesium will increase the total cesium concentration and thus the solubility product for cesium tetraphenylborate (CsTPB), Ksp = 1.76  109 mol2/L2, can be exceeded [7]. Numerous previous studies reported that NaTPB forms poorly soluble CsTPB with cesium in aqueous solutions [1,6–9] and thus providing a promising method for removal of radiocesium from radioactive liquid wastes. Other precipitating agents are also reported in literature for cesium [6]. Nevertheless, CsTPB had the lowest Ksp of all those cited. Based on this finding, NaTPB was used in this study as a precipitating agent for cesium. For precipitation of Cs-137 from nuclear wastes, much more precipitating agent than necessary is required. This is because not only CsTPB is formed, but also other alkali metal salts (e.g. KTPB). The radioactivity of the nuclear waste, especially the high level radioactive one, causes radiolytic decomposition of TPB and primarily environmentally harmful benzene is formed [10]. Therefore, the solid phase (CsTPB) should be separated as soon as formed, instead of storage, to avoid such decomposition. To achieve this goal, a fast and efficient solid–liquid separation process should be used. Generally, separation of the precipitate during chemical precipitation treatment is often problematic. This is because the traditional solid–liquid separation processes (decantation, centrifugation, evaporation and membrane filtration) facing many challenges during their application particularly when dealing with large volumes of wastewaters. Consequently, foam separation (or simply flotation) has been evaluated in this study as an alternative technique. Precipitate flotation is a foam separation process that involves the precipitation of the ionic species to be removed, using a suitable precipitating agent, prior to its flotation with an adequate surfactant (collector). The interaction between the collector and the precipitate produces hydrophobic particles which then become able to attach to the air bubbles, introduced into the suspension, and is then removed in the foam phase [11]. In this work, NaTPB is used as a precipitating agent for cesium and two types of surfactants, namely, anionic sodium lauryl sulphate (SLS) and cationic cetylpyridinium chloride (CPC), are used as collectors. Besides its prosperous use for the treatment of water and wastewater [12,13], flotation has been also broadly applied as a solid–liquid separation technique [11,14–16]. Though, no publications are reported on the removal of cesium radionuclides from aqueous solutions by precipitate flotation process, suggested in the present work, using the NaTPB precipitating agent. The aim of the present work was to determine the adequate conditions for the removal of Cs-137 from low-level liquid radioactive waste (LLLRW) by precipitate flotation. To achieve this purpose, the effects of the solution pH, collector type and concentration and stable cesium concentration were studied. The employed process, flotation, was also compared to the conventional solid–liquid separation processes for Cs-137 removal from LLLRW. 2. Experimental 2.1. Flotation apparatus and reagents The flotation system used in this work was previously described in detail [14]. In essence, it is consisted of a pure nitrogen cylinder

connected to a flotation column through a fine pressure reduction unit, manometer and filtered flasks containing baryta solution and water. The flotation column, in which the flotation experiments were carried out, is supported over a filter flask and is made of a G4 sintered glass disc 4 cm in diameter fused to a Pyrex glass column about 40 cm in height, drawn at the bottom in the form of a funnel. A tap is sealed to the flotation column at a distance of 0.8 cm above the fritted disk to enable sample withdrawing from the bulk solution for radioactivity and pH measurement. Nitrogen gas was introduced through the fritted disk at a rate of 25 cm3/min. Sodium tetraphenylborate, NaTPB, was obtained from Merck. Fresh solution of the desired concentration prepared with NaTPB was kept in a dark bottle to avoid photochemical decomposition. Stable cesium chloride having a purity of 99.99% and ferric nitrate nonahydrate were obtained from Sigma–Aldrich. The radioactive cesium, Cs-137, was purchased from the Amersham Radiochemical Center and was used for solution spiking. The collectors, sodium lauryl sulfate (SLS) and cetylpyridinium chloride (CPC), were obtained from Merck. The collector solutions were freshly prepared daily. Absolute ethanol, provided by Merck, was used as a solvent for CPC as well as a frother in order to decrease the bubble size and increase the foam stabilization. Because SLS is not soluble in absolute ethanol, it was dissolved in 50% (v/v) solution of ethanol and water to allow the simultaneous addition of the collector and the frother. During all the experimental work, ethanol was added at a dose rate of 1 mL per 250 mL aqueous suspension. Adjusting the solution pH was done with reagent grade sodium hydroxide and hydrochloric acid. The chemical composition of the synthetic low-level liquid radioactive waste (LLLRW) tested in this work is represented in Table 1. 2.2. Operational procedure, analysis and data presentation In the present work, the experimental procedure for cesium removal involves two steps: (i) precipitation and (ii) flotation. For the first step, precipitation, NaTPB was added to a solution of stable cesium spiked with Cs-137 and CsTPB precipitate was immediately formed according to the following reaction:

CsCl þ NaTPB ! CsTPB # þNaCl

ð1Þ

For all experiments, the initial Cs-137 activity was 30 nCi/L and NaTPB was added to produce a 1:1 M ratio of cesium to tetraphenylborate ion. The initial stable cesium concentrations were varying from 5  105 to 7.5  104 mol/L. After NaTPB addition, the suspensions were stirred at 200 rpm for 5 min using a magnetic stirrer. Ferric nitrate solution, of the desired concentration, was then added and the pH of the mixture (Cs-TPB/Fe(III)) was adjusted to the desired value. Except for studying the effect

Table 1 Characteristics of LLLRW. Characteristics

Value

Total dissolved solids pH Chloride Sulfate Phosphate Carbonate Oxalate Potassium Sodium Magnesium Calcium Iron

9.35 6.85 4 58 0.3 158.5 0.028 16.5 34 18 32 0.1

All units are in mg/L except pH.

M.A. Soliman et al. / Chemical Engineering Journal 275 (2015) 342–350

Original volume of the suspension VR ¼ Volume of the collapsed foam

ð2Þ

DF ¼

Initial cesium concentration Its residual concentration in the liquid phase

ð3Þ

ER ¼

Cesium concentration in the collapsed foam Its concentration in the liquid phase

ð4Þ

In order to well interpret the data obtained in the present work, the solid phase separated by decantation/filtration and dried at 65 °C for 24 h was characterized by X-ray diffraction (XRD) using a Philips PW1830 diffractometer with Cu Ka as the incident radiation. The d-values corresponding to the 2h values were compared with the data available in X-ray data file.

Cs; NaTPB Cs; NaTPB; 0.00001 M Fe(III) Cs; NaTPB; 0.0005 M Fe(III)

35

Remaining Cs-137 Activity (nCi/L)

of Fe(III) concentration (0–0.001 mol/L), Fe(III) concentration was maintained at 1  104 mol/L. The suspension was aged under stirring for 5 min. The collector solution (either SLS or CPC) of the required concentration was then added. For the second step, flotation, the suspension was transferred to the flotation cell where air bubbles were passed. At the termination of each experiment, a sample was withdrawn from the liquid phase of the flotation column through the sampling tap. Before sample withdrawal, about 5 mL of the solution was drained from the sampling tap. The collected withdrawal samples were analyzed for the remaining Cs137 activity. Between experiments, the flotation column was thoroughly cleaned with 1 N HNO3 followed by four rinses with distilled water. Experiments for cesium removal from LLLRW were carried out using 7.5  104 mol/L stable cesium, different NaTPB concentrations (7.5  104 and 1  103 M) and different collector concentrations (5  105 and 1  104 mol/L SLS; 1  104 and 2.5  104 mol/L CPC). Flotation experiments of cesium from the synthetic radioactive waste were carried out in triplicate under identical conditions. The radioactivity of cesium before and after flotation was measured radiometrically. The flotation results of cesium are presented as remaining Cs-137 radioactivity, volume reduction (VR), decontamination factor (DF) and enrichment ratio (ER), where:

Cs; 0.0005M Fe(III) Cs; NaTPB; 0.0001 M Fe(III)

30 25 20 15 10 5 0 2

4

6

8

10

12

pH Fig. 1. Effect of pH on removal of 30 nCi/L Cs-137 using 2.5  104 mol/L SLS. Stable cesium conc. = 3  104 mol/L; (Cs-133 + Cs-137):NaTPB = 1:1.

30

Remaining Cs-137 Activity (nCi/L)

344

25

20

15

10

Cs; NaTPB Cs; 0.0005 M Fe(III) Cs; NaTPB; 0.00001M Fe(III) Cs; NaTPB; 0.0001 M Fe(III) Cs; NaTPB; 0.0005 M Fe(III)

5 2

4

6

8

10

12

pH

3. Results and discussion

Fig. 2. Effect of pH on removal of 30 nCi/L Cs-137 using 2.5  104 mol/L CPC. Stable cesium conc. = 3  104 mol/L; (Cs-133 + Cs-137):NaTPB = 1:1.

3.1. Effect of operational parameters 3.1.1. Effect of the solution pH and collector type It is recognized that the pH of the solution plays an important role in flotation processes. Depending on the solution pH, various interfacial properties and reaction routes may be found [17]. The effect of the solution pH on the floatability of CsTPB precipitate was studied using SLS and CPC at various Fe(III) concentrations (0–5  104 mol/L). The results obtained are shown in Figs. 1 and 2. At the studied pH range 2.5–11.9, it can be observed from these figures that the remaining Cs-137 activity is significantly affected by the solution pH, the collector type and the Fe(III) concentration. For the two collectors, neither precipitate flotation using NaTPB as a precipitating agent (Cs/NaTPB system) nor adsorbing colloid flotation using Fe(III) precipitate as carrier (Cs/Fe system) succeeded to float cesium from aqueous solutions. On the other hand, the CsTPB precipitate is efficiently floated in presence of Fe(III) ion and the remaining Cs-137 activity, either by SLS or CPC, is dependent on the initial concentration of Fe(III). At 5  105 mol/L Fe(III), inappreciable removals are obtained for cesium (remaining Cs-137 activity 25 nCi/L) by SLS and CPC (Cs/NaTPB/5  105 mol/L Fe system, Figs. 1 and 2). At higher Fe(III) concentrations, the remaining Cs-137 activity at the studied pH range is dependent on the

collector type. For the anionic SLS collector (Fig. 1), remaining Cs-137 activity values of about 2.5 nCi/L are achieved in the pH range of 5.1–7.6 at 1  104 mol/L Fe(III), while values of about 4 nCi/L are attained at 5  104 mol/L Fe(III) in the pH range of 3.5–8.9. At pH values higher than 8 (for 1  104 mol/L Fe) and 9 (for 5  104 mol/L Fe) the flotation efficiency of CsTPB is reduced as indicated by increasing the remaining Cs-137 activity. For the cationic CPC collector (Fig. 2), almost no removals are obtained for Cs-137 at pH values lower than 7, while significantly removed at pH values higher than 8. Remaining Cs-137 activity values of about 4.6 are obtained in the pH ranges of 9.3–10.5 and 10.2– 11.9 at 1  104 and 5  104 mol/L Fe(III), respectively. Based on the obtained data in Figs. 1 and 2, it can be deduced that the CsTPB precipitate is only floated in presence of Fe(III) ions by SLS or CPC. When coexisted with CsTPB precipitate, Fe(III) formed an iron oxide coat on this precipitate and the resulted product is proposed as iron oxide coated CsTPB. As shown by Figs. 1 and 2, the iron oxide coated CsTPB particles are efficiently floated with the anionic SLS collector at pH values lower than 8, while with the cationic CPC collector at pH values higher than 8. These findings suggest that the iron oxide coat is positively charged at pH

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3.1.2. Effect of collector concentration It is known that the presence of surfactant in a froth flotation operation plays an important role in both promoting and stabilizing the foam [18]. Two types of common surfactants, namely, anionic SLS and cationic CPC, at various concentrations were examined as flotation collectors for the floatability of the iron oxide coated CsTPB particles at pH values of 6.5 and 10.2, respectively. This is to find the most suitable concentration of SLS and CPC for flotation of the formed particles and hence removal of cesium from aqueous solution. The results obtained are represented in Fig. 3. From the data presented in Fig. 3, it can be noted that the floatability of the iron oxide coated CsTPB particles is significantly enhanced by increasing the collector concentration and subsequently plateaued. For the anionic SLS collector, the remaining Cs-137 activity decreased from 16.51 nCi/L to 4.28 nCi/L as the initial collector concentration increased from 1.5  105 mol/L to 3  105 mol/L. On the other hand, increasing the CPC concentration from 1.5  105 mol/L to 7.5  105 mol/L decreased the remaining Cs-137 activity from 29.64 nCi/L to 5.82 nCi/L. Collector concentrations higher than 3  105 mol/L (for SLS) and 7.5  105 mol/L (for CPC) slightly enhanced the removal efficiency of cesium. Lower remaining Cs-137 activity values of 3.61 nCi/L and 4.47 nCi/L are obtained using SLS and CPC, respectively, at collector concentration of 2.5  104 mol/L. This is in conformity with literature that flotation efficiency increases with increasing collector concentration [14,16,19]. Generally, precipitate flotation involves the precipitation of the ionic species prior to its flotation with the addition of a suitable collector. According to Sang-June and Son [20], the collector concentration must be sufficient to produce stable and persistent foam maintaining the precipitate on the top of the solution and preventing redispersion thereof. This can explain the unfloatability of the iron oxide coated CsTPB particles at CPC concentrations 63  105 mol/L, where no removals are recorded for Cs-137 as shown by Fig. 3. In the aqueous solution, the anionic SLS surfactant + (C12H15OSO3Na) could be ionized into C12H15OSO 3 and Na , while the cationic CPC surfactant (C19H42NBr) ionized into C19H42N+ and Br. When coexisted with the iron oxide coated CsTPB

SLS; pH = 6.5 CPC; pH = 10.2

Remaining Cs-137 Activity (nCi/L)

30

25

20

15

10

5

particles, the polar head groups of the ionized SLS and CPC could be electrostatically adsorbed onto the surface particles at pH values of 6.5 and 10.2, respectively, with the nonpolar end pointing toward the aqueous phase. Such attachment produced hydrophobic particles that could be floated on air bubbles to the surface of the solution and thus Cs-137 is removed by either SLS or CPC. The decrease in the removal efficiency of Cs-137 at the lower studied collector concentrations may be due to inadequate collector/precipitate ratio. By increasing the concentration of SLS and CPC, either more iron oxide coated CsTPB particles could be combined to the collector heads or more collector heads could be combined to the particles. This could result in either formation of more hydrophobic particles or enhancement in the hydrophobicity of the particles. Thus, floatability of the iron oxide coated CsTPB is increased as shown by Fig. 3, where the remaining Cs-137 value decrease with the increase of the initial collector concentration. According to Medina et al. [11] and Matis and Mavros [17], a collector concentration lower than the stoichiometric ratio was sufficient for precipitate flotation. This is because the surfactant reacts, in this case, only with the precipitate’s surface. This finding can explain the low requirement of the collector for efficient Cs137 flotation (3  105 mol/L SLS and 7.5  105 mol/L CPC, Fig. 3). 3.1.3. Effect of stable cesium concentration The effect of stable cesium concentration on the removal efficiency of cesium by the isotope dilution–precipitate flotation process at pH values of about 6.5 and 10 using SLS and CPC, respectively, is shown in Fig. 4. As can be seen from this figure, the final Cs-137 activity decreased with increasing the stable cesium concentration. Remaining Cs-137 activity values of 1.5 and 2.1 nCi/L are achieved at 7.5  104 mol/L using SLS and CPC, respectively, which are below its Derived Concentration Standards (DCS) value of 3 nCi/L developed by the U.S. Department of Energy [21]. An initial stable cesium concentration of 7.5  104 mol/L was used in the subsequent experiments. In order to evaluate the performance of the removal process developed in the present work for cesium, the final Cs-137 activity values experimentally obtained with SLS and CPC are compared with those theoretically calculated. Based on the initial concentration of cesium (Cs-133 and Cs-137), the initial concentration of NaTPB and the solubility product of CsTPB precipitate (Ksp = 1.76  109 mol2/ L2), the theoretical final Cs-137 activity can be calculated [7].

25

20

15

10

5

0 0.00000

0 1.52E-5

3.05E-5

6.10E-5

1.22E-4

2.44E-4

SLS CPC Theoritical

30

Remaining Cs-137 Activity (nCi/L)

values below 8, while negatively charged at pH values above 8. Accordingly, the subsequent experiments were conducted at pH values of about 6.5 and 10 using SLS and CPC, respectively.

0.00015

0.00030

0.00045

0.00060

0.00075

Stable Cesium Concentration (mol/L)

Collector Concentration (mol/L) Fig. 3. Effect of collector type and concentration on the remaining Cs-137 activity. Initial Cs-137 activity = 30 nCi/L; Stable cesium conc. = 3  104 mol/L; (Cs133 + Cs-137):NaTPB = 1:1; Fe(III) conc. = 1.0  104 mol/L.

Fig. 4. Effect of stable cesium concentration on the remaining Cs-137 activity using SLS (at pH 6.5) and CPC (at pH 10). Comparison of the experimental data to the theoretical values. Initial Cs-137 activity = 30 nCi/L; (Cs-133 + Cs-137):NaTPB = 1:1; Fe(III) conc. = 1.0  104 mol/L; collector conc. = 2.5  104 mol/L.

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According to Rogers et al. [7], the theoretical final Cs-137 activity was calculated via determination the amount of Cs-137 precipitated using the following equation:

Cs137 precipitated ¼

!

Cs137 i Cs137 i

þ

Cs133 i

  Cstotal  Cstotal i f

ð5Þ

where Cs137 and Cs133 are the initial concentration of Cs-137 and i i stable cesium in the aqueous solution, respectively. Cstotal and i Cstotal are the initial and final concentration of both isotopes (Csf can be determined theoreti133 + Cs-137), respectively. The Cstotal f cally by using the solubility product as follows:

  K sp ¼ Cstotal ðTPBf Þ f

ð6Þ

where TPBf is the final concentration of tetraphenylborate ion. For this precipitation reaction with both ions being monovalent, every mole of cesium consumed one mole of TPB. Consequently, the TPBf can be determined by subtracting the final total cesium concentration from the initial TPB concentration and Eq. (6) becomes:

   K sp ¼ Cstotal TPBi  Cstotal f f

ð7Þ

By assuming that TPBi is much larger than Cstotal , the final total f ¼ K sp =TPBi and thus Eq. (5) becomes: cesium is expressed as Cstotal f

Cs

137

precipitated ¼

!

Cs137 i Cs137 i

þ

Cs133 i

Cs137 þ Cs133  i i

K sp TPBi

 ð8Þ

The final Cs-137 concentration can then be determined by the following equation:

Cs137 ¼ Cs137  f i

Cs137 i Cs137 þ Cs133 i i

!

Cs137 þ Cs133  i i

K sp TPBi

 ð9Þ

Comparing the calculated values of Cs137 with those experimenf tally obtained using SLS and CPC, it can be seen that the experimental data agrees well with the theoretical values particularly at stable cesium concentrations P2  104 mol/L. At stable cesium concentrations lower than 2  104 mol/L, the values of Cs137 f experimentally obtained using SLS and CPC are deviated from the theoretical values. As shown above (Eq. (7)), the theoretical Cs137 f values were calculated by assuming that the initial TPB concentration is much larger than the final cesium concentration. At the lower studied stable cesium concentrations, this assumption is not acceptable, which can interpret the deviation of the experimental values from the calculated data. The slight decrease in the remaining Cs-137 activity obtained with SLS than the theoretically calculated values at stable cesium concentrations higher than 2  104 mol/L may be due to ion flotation of some unprecipitated cesium ions. 3.2. Application to LLLRW To emphasize the efficiency of the isotope dilution–precipitate flotation process proposed in the present work as a new method for radiocesium removal from aqueous solutions, it was necessary to test the potential application of this process for removal of cesium from radioactive liquid wastes. Therefore, the following section is directed and devoted toward the removal of Cs-137 from synthetic aqueous solution similar to the low-level liquid radioactive waste (LLLRW) generated at the Nuclear Research Center at Inshas, Egypt. From the waste management point of view, the efficiency of a decontamination process depends on its capability to segregate radioactive waste into: (i) a concentrate containing most

of the radioactive nuclei whose volume is as small as possible, and (ii) a relatively large volume of decontaminated effluent appropriate for direct discharge to the environment. Thus, the ultimate objective of a developed decontamination process is to achieve both high volume reduction (VR) and decontamination factor (DF). Experiments from LLLRW were conducted at the optimized conditions without adjusting the pH of the wastewater for SLS, while for CPC the pH was adjusted to about 10.5. The results obtained are shown in Fig. 5. For the two collectors, SLS and CPC, it can be seen from Fig. 5 that the flotation efficiency of cesium is strongly dependent on both the precipitant and collector concentrations. Under the optimum conditions, remaining Cs-137 activity values of 1.27 nCi/L (for SLS, column C) and 2.29 nCi/L (for CPC, column F) are achieved, which are below the reported DCS value. Besides, high decontamination factors (DF = 25 (for SLS) and 13 (for CPC)), enrichment ratios (ER = 2550 (for SLS) and 2000 (for CPC)) and volume reduction values (VR = 110 (for SLS) and 165 (for CPC)) are also attained. It is known that NaTPB has the ability to precipitate not only cesium cations, but also the other alkali metals, ammonia, and other monovalent cations like silver and thallium [6,7]. Of the wastewater constituents (Table 1), potassium is the only cation that can be precipitated by NaTPB forming KTPB precipitate. Thus, the increase in the remaining Cs-137 activity at the lower studied NaTPB concentration for SLS and CPC can be attributed to inadequate NaTPB/(Cs+ + K+) molar ratio. As shown in Fig. 5, the required amount of NaTPB for efficient removal of cesium from LLLRW is less than the sum of the concentrations of cesium and potassium cations. It is reported that CsTPB has a solubility of 2.8  105 mol/L, while the KTPB solubility is 1.45  104 mol/L [6]. This implies that cesium and potassium cannot be completely precipitated by NaTPB. Therefore, NaTPB below the stoichiometric molar ratio of cesium and potassium was sufficient to obtain remaining Cs-137 activity values below the DCS value. Although the NaTPB concentration increased from 7.5  104 to 1  103 mol/L, the flotation efficiency of cesium (remaining Cs-137 activity, DF, and ER) is negatively affected at 5  105 mol/L SLS while slightly enhanced at 1  104 mol/L CPC. These findings can be attributed to inadequate collector/precipitate ratio. At 1  103 mol/L NaTPB, increasing the SLS and CPC concentrations to 1  104 and 2.5  104 mol/L, respectively, significantly enhanced the flotation efficiency of cesium), which supported the above explanation. Fig. 6 shows the digital camera photographs for the studied system, iron-oxide coated CsTPB, before and after flotation from LLLRW. As shown by this figure, the iron-oxide coated CsTPB is efficiently floated within half a minute. This figure reflects the effectiveness of foam separation technique for separating the formed particles, iron oxide coated CsTPB, and thus its applicability for remediation of radioactive wastewaters. 3.3. Flotation mechanism of cesium The data obtained in the present work showed that neither the anionic SLS collector nor the cationic one, CPC, had the ability to float the CsTPB precipitate. In presence of Fe(III) solution, the CsTPB particles are coated with iron oxide and the overall product, iron oxide coated CsTPB, is successfully floated by SLS and CPC at pH values below and above 8, respectively. Fig. 7 shows the powder X-ray diffraction patterns of iron oxide coated CsTPB at pH 6.5 and 10. The XRD curves of this figure exhibit strong diffraction peaks at the d-values of 4.19, 2.74, and 2.46 (indicating the presence of goethite, a-FeOOH) and at 3.76, 2.59, 2.39, and 1.68 (indicating the presence of hematite, a-Fe2O3) [22]. Because the coat of CsTPB is a mixture of goethite and hematite, it is referred to ‘‘iron oxide’’ for the sake of simplicity and the

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M.A. Soliman et al. / Chemical Engineering Journal 275 (2015) 342–350

SLS CPC

SLS CPC

25

10 20

8 15

6

DF

Remaining Cs-137 Activity (nCi/L)

12

10

4

5

2

0

0

(A)

(B)

(C)

(D)

(E)

(A)

(F)

(B)

(C)

(D)

(E)

(F)

3000

500

SLS CPC

SLS CPC 2500

400

2000

VR

ER

300 1500

200 1000

100

500

0

0

(A)

(B)

(C)

(D)

(E)

(F)

(A)

(B)

(C)

(D)

(E)

(F)

Fig. 5. Remaining Cs-137 activity, decontamination factor (DF), volume reduction (VR) and enrichment ratio (ER) for flotation of 30 nCi/L Cs-137 from LLLRW using SLS and CPC at pH values of 6.85 and 10.2, respectively. (A) 7.5  104 mol/L NaTPB; 5.0  105 mol/L SLS. (B) 1.0  103 mol/L NaTPB; 5.0  105 mol/L SLS. (C) 1.0  103 mol/L NaTPB; 1.0  104 mol/L SLS. (D) 7.5  104 mol/L NaTPB; 1.0  104 mol/L CPC. (E) 1.0  103 mol/L NaTPB; 1.0  104 mol/L CPC. (F) 1.0  103 mol/L NaTPB; 2.5  104 mol/L CPC.

Fig. 6. Digital camera photographs for the flotation cell before (A) and after 0.5 min (B) flotation of iron oxide coated CsTPB.

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Fig. 7. XRD patterns of iron oxide coated CTPB at (A) pH 6.5 and (B) pH 10.

formed product is denoted as ‘‘iron oxide coated CsTPB’’. It is reported that the points of zero charge of goethite and hematite are at pH values of 8 and 8.5, respectively [23]. The pHpzc (pH of point of zero charge) is the important index to find the status of balance between positive and negative charges in the surface of the mineral [24]. If the pH of the solution is below the pHpzc, the surface is net positively charged, while the surface is negatively charged if the pH of the solution is greater than the pHpzc [24], as expressed in Eqs. (10) and (11): pH
BFeOH þ Hþ ! BFeOHþ2 pH>pHpzc

BFeOH ! BFeO þ Hþ

ð10Þ

above 8 (pH > pHpzc, Eq. (11). This negatively charged surface electrostatically facilitated the attachment of the cationic CPC collector (Fig. 8, path b) and thus the particles of iron oxide coated CsTPB are floated at pH >8 using CPC. Based on the previous discussion, a schematic representation for cesium removal is proposed and presented in Fig. 8. According to this figure, the removal mechanism of cesium consists of four stages: (i) precipitation of cesium by NaTPB and the formation of CsTPB particles, (ii) coating the CsTPB particles with iron oxide via addition of Fe(III) solution, (iii) electrostatic attachment of the collector to the produced iron oxide coated CsTPB particles, and (vi) flotation of the collector/iron oxide coated CsTPB product.

ð11Þ

According to Eq. (10), the iron oxide shell of CsTPB is positively charged at pH <8 and thus the efficient removals obtained for cesium can be attributed to the successful flotation of iron oxide coated CsTPB as a result of electrostatic attraction with the anionic SLS collector (Fig. 8, path a). Correspondingly, the surface of iron oxide coated CsTPB particles had a negative charge at pH values

3.4. Comparison with other solid–liquid separation processes In this section, the iron oxide coated CsTPB particles were separated from LLLRW by other conventional separation processes (decantation, centrifugation and membrane filtration) and the obtained results are compared to those obtained by flotation as

Fig. 8. Schematic representation of collector attachment to the iron oxide coated CsTPB.

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Remaining Cs-137 activity (nCi/L)

5

Decantation after 1h Centerifugation at 5000 rpm Filtration through 0.45µm filter Flotation at 15 cm3/min with SLS Flotation at 15 cm3/min with CPC

4

3

2

VR values shown in Fig. 5, (iv) lower energy requirement, and (v) applicability for treatment of large volumes of wastewaters. Based on the data obtained in Fig. 9 and the above discussion, the flotation process can be considered the most efficient treatment process for cesium removal from LLLRW. Accordingly, it can be concluded that the isotope dilution–precipitate flotation process, followed in the present study, can be effectively applied for treatment of radioactive liquid wastes contaminated with Cs137 radionuclides. 3.5. Comparison with other reported processes

1

0

Solid-liquid separation processes Fig. 9. Removal of cesium from LLLRW by various solid–liquid separation processes.

shown in Fig. 9. This figure shows that the remaining Cs-137 activity values obtained by flotation (1.21 nCi/L at pH 6.83 with SLS and 2.17 nCi/L at pH 10.23 with CPC) and membrane filtration (2.45 nCi/L through 0.45 lm filter) are below the developed DCS value. On the other hand, decantation after 1 h and centrifugation at 5000 rpm resulted in remaining activity values of 5.32 and 3.93 nCi/L, respectively, which are slightly greater than the developed DCS value. Although the traditional solid–liquid separation processes show relatively satisfactory results, they have some disadvantages. Decantation process produced sludge, iron oxide coated CsTPB, with high water content (only 80% of the initial feeding solution, 200 mL, was decanted). Additionally, the iron oxide coated CsTPB precipitate settle slowly. For the membrane filtration process, the filter was periodically backwashed to dislodge solids and restore its capacity. This backwash process generated a large volume of residuals. Also, this process required frequent replacement of the filter. Furthermore, all these processes have high operational cost and are inappropriate, from the technical point of view, for treating large volumes of wastewaters. All the drawbacks of the conventional solid–liquid separation processes applied in this work have been previously reported in literature [16]. On the other hand, the flotation process was not only exhibited the higher removal efficiency for cesium, but also has many advantages. These advantages include: (i) simplicity of the apparatus, (ii) faster solid–liquid separation process since the ultimate cesium removal was achieved within half a minute, (iii) production of more concentrated sludge occupying smaller volumes as indicated by the high

In order to justify the viability of the proposed method as an effective treatment process for cesium, the decontamination factor (DF) obtained in the present study from LLLRW has been compared with some other values reported previously in the literature. Table 2 compares the DFs obtained for Cs-137 from LLLRW by precipitate flotation process using SLS (pH 6.5) and CPC (pH 10.2) with those reported in the literature using other processes. From this comparison, it is evident that the processes reported by Kent et al. [25] and Chmielewski and Harasimowicz [26] exhibited DFs of 35 and 32–45, respectively, which are higher than those obtained in this work (DFs = 13 and 25 at pH 10.2 and 6.5, respectively). However, the isotope dilution–precipitate flotation method introduced in the present study shows satisfactory and comparable values of DF and can be considered better than those reported in Table 2. From the comments column represented in Table 2, it can be seen that the comparative treatment processes require gravitational sedimentation, centrifugation and/or membrane filtration for separating the solid phase. Undoubtedly, such requirements increase not only the cost of the treatment process, but also the treatment time. In the present work, flotation process has been efficiently utilized for separating the solid phase, where satisfactory DFs for cesium are obtained within half a minute (Table 2) and the remaining Cs-137 activity values (1.2 and 2.3 nCi/L at pH 6.5 and 10.2, respectively) are below the DCS value. This is our belief that the isotope dilution–precipitate flotation process considered in the present investigation can be potent and good alternative to other treatment processes for radiocesium removal from radioactive liquid wastes, as can be obviously seen from Table 2. 4. Conclusions Removal of Cs-137 from LLLRW by an isotope dilution precipitate-flotation process was investigated. The CsTPB precipitate was failed to float either by SLS or CPC collectors, while was succeeded via addition of Fe(III) solution and the flotation efficiency was dependent on the solution pH, collector type and concentration, and Fe(III) and stable cesium concentrations. For SLS, the

Table 2 Comparison of decontamination factors of various treatment processes for removal of cesium. Compound

Comments

DF

References

Transition-metal hexacyanoferrate(II) + organic polymer Transition-metal hexacyanoferrate(II) Zeolites Polyethyleneimine + chitosan + copper cyanoferrate Transition-metal hexacyanoferrates Transition-metal hexacyanoferrates Sodium diuranate Sodium diuranate Ferric hydroxide Zeolites Sodium tetraphenylborate + ferric nitrate Sodium tetraphenylborate + ferric nitrate

40 min mixing, gravitational sedimentation Filtration using 0.1 lm membrane Alkaline pH, 24 h contact, centrifugation at 12,000 rpm pH 9.5–10.5, complexation and ultrafiltration pH 11.5, process wastewater, scavenging precipitation, filtration pH 13, LLLRW, centrifugation at 5000 rpm, filtration with 0.2 lm membrane pH 6–10, Floc gravity, overnight settling pH 11, Floc gravity, overnight settling pH 6–11, floc gravity, overnight settling Gravitational sedimentation LLLRW, pH 6.5, precipitate flotation using SLS, 0.5 min flotation LLLRW, pH 10.2, precipitate flotation using CPC, 0.5 min flotation

4.1 >22 10 and 23 32–45 12 35 1.4 1.9 1.4 10 25 13

[27] [27] [28] [26] [25] [25] [29] [29] [29] [30] Present study Present study

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efficient removals of Cs-137 were attained in the pH range of 5.1– 7.6 and 3.5–9 at Fe(III) concentrations of 1  104 and 5  104 mol/L, respectively. Remaining Cs-137 activity values <5 nCi/L were obtained using CPC in the pH range of 9.3–10.5 and 10.2– 11.8 at Fe(III) concentrations of 1  104 and 5  104 mol/L, respectively. The effect of collector concentration revealed that the remaining Cs-137 activity was strongly affected particularly at collector concentrations <3  105 mol/L (for SLS) and 7.5  105 mol/L (CPC). Comparing the calculated values of the remaining Cs-137 activity with those experimentally obtained by the employed treatment process showed their agreement especially at stable cesium concentrations >2  104 mol/L. Application of the present process for Cs-137 removal from LLLRW resulted in remaining Cs-137 activity values of 1.27 and 2.29, DFs of 25 and 13, ERs of 2250 and 2000, and VRs of 110 and 165 for SLS and CPC, respectively. Flotation, decantation, centrifugation and membrane filtration were compared for separation of the iron oxide coated CsTPB particles. Of these technologies, flotation exhibited the best results. Moreover, the DF values obtained for Cs-137 from LLLRW were compared with those reported using other treatment processes and it is concluded that the present process can be effectively applied for Cs-137 removal from radioactive liquid wastes.

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