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Sorption of Nb(V) on pyrolusite (β-MnO2): Effect of pH, humic acid, ionic strength, equilibration time and temperature
Applied Radiation and Isotopes 154 (2019) 108887
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Sorption of Nb(V) on pyrolusite (β-MnO2): Effect of pH, humic acid, ionic strength, equilibration time and temperature
T
Madhusudan Ghosha,b, P.S. Remya Devia, K.K. Swaina,b,∗ a b
Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, India
H I GH L IG H T S
investigation of environmental parameters: sorption of Nb(V) on pyrolusite. • Thorough acid enhances sorption in acidic pH whereas sorption declined in basic pH. • Humic of sorption mechanism using ionic strength and temperature dependence study. • Elucidation • Sorption follows pseudo first order kinetics in pH 1and pseudo second order in pH 9.
A R T I C LE I N FO
A B S T R A C T
Keywords: Radionuclides Sorption Niobium Humic acid Ionic strength
The sorption of Nb(V) on pyrolusite has been studied and the effect of pH, ionic strength, humic acid, temperature and equilibration time were also investigated in a series of batch equilibrium experiments. The sorption was found to be affected by solution pH, ionic strength and humic acid. The sorption was high in neutral/near neutral pH (~96 %) but lower sorption was observed both in acidic (~55 %) and basic (~85 %) media. Sorption was decreased in acidic pH with increase of ionic strength and reverse effect was seen in basic pH although the effect is less prominent. Presence of humic acid causes enhancement of sorption in acidic pH whereas sorption declined in basic pH. The sorption process is endothermic in acid medium and exothermic in basic medium. In acid medium the sorption is entropy driven process. Kinetics of the sorption study was found to follow pseudo first order in acidic pH whereas pseudo second order in basic pH.
1. Introduction The migration behaviour of potentially released radioisotopes in environments has to be known for trustworthy long term assessment of nuclear waste repository. Repositories for nuclear waste should meet the norm that radionuclides from repository should not come into the human habitation in thousand years. The migration of the radionuclides depend on solubility of radionuclides in underground waters, complexation with anions present in aqueous stream, sorption on components of engineered and geochemical barriers (rocks and minerals) and the colloid formation (pseudo or true). Among all of these parameters, sorption of radionuclides at the water rock interface retards the migration in natural system. Thus, the factors that administer the radionuclides sorption on mineral surfaces have to be studied (Khasanova et al., 2007). A lot of work has been carried out to understand the sorption of radionuclide on several minerals which includes hydrous oxides of Fe ∗
(Waite et al., 1994; Sanchez et al., 1985; Powell et al., 2005), Mn (Means et al., 1978), Si and Al (Sylwestera et al., 2000), clay (Hartmann et al., 2008). According to most of the existing literature, tri- and tetravalent actinides causes no hazard, as they have high tendency to get sorbed onto the components used for their isolation and host rocks and owing to their extremely low solubility (Khasanova et al., 2007). However, current studies suggest the possibility of colloidal migration of these radionuclides in groundwater (McCarthy and Zachara, 1989; Clarke et al., 1996; Cooper et al., 1995). 239Pu was found to migrate distances upto 1.3 km in 30 years with smectite colloid at the test site in Nevada (Kersting et al., 1999). Penrose et al. (1990) have studied the behavior of Pu and Am in the Mortandad canyon (Los Alamos National Laboratory) and observed that colloid particles have facilitated the migration of these radionuclides to a distance of 3.39 km from the source. The reports indicate colloids dispersed in groundwater act as an auxiliary phase and enhance the transport of radio-nuclides. A three phase model consists of stationary phase, mobile phase and colloids
Corresponding author. Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India. E-mail address: [email protected] (K.K. Swain).
https://doi.org/10.1016/j.apradiso.2019.108887 Received 12 April 2019; Received in revised form 2 August 2019; Accepted 4 September 2019 Available online 05 September 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 154 (2019) 108887
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manganese oxide.
(intrinsic or pseudo) dispersed in mobile phase were introduced and able to explain satisfactorily the migration of radionuclides (McCarthy and Zachara, 1989; Kersting et al., 1999; Honeymann, 1999). So in this context the sorption study of radionuclides on naturally occurring colloids (pseudo) in aqueous stream is important. The most common natural organic matters present along with inorganic anions, is humic acid (HA). Presence of HA in ground water may alter the sorption as well as migration behaviour of radionuclides by complexing with the radionuclides or by changing the oxidation state of the radionuclides. It is also reported that HA itself get sorbed onto colloids and affect the sorption of radionuclides on colloids (Buckau, 2005; Xiaoli et al., 2007). The discharged Zr-Nb pressure tubes from Pressurized Heavy Water Reactor (PHWR) are presently stored in deep tile holes with concrete caps. In present study, we are dealing with 94Nb, a long lived radioisotope (t1/2 = 20300 y), which generates from 93Nb isotope during neutron irradiation of Zr-Nb pressure tube or cladding material. Here we report the sorption phenomenon of Nb(V) on laboratory synthesized pyrolusite and the detailed investigation regarding the effect of different physico-chemical parameters like pH, ionic strength, equilibration time, humic acid and temperature on sorption.
2.3.5. Zeta potential & particle size Particle size and Zeta potential of the oxide were measured using the LitesizerTM500, Anton-Paar. Zeta potential measurement is based on the electrophoretic light scattering (ELS), related to the speed of the particle under electric field which directly proportional to the surface charge of the particle or zeta potential. Particle size was measured by Dynamic Light Scattering (DLS) where the measurement of random motion of the particle is measured which directly associated with the particle size. 2.4. Batch sorption experiment 2.4.1. Binary sorption (HA - manganese oxide) Before studying the sorption of Niobium on manganese oxide, study on the sorption behavior of humic acid on manganese oxide, is required. For that, we have prepared a set of purified humic acid solution of concentration 0.5–15 mg L−1 in 0.05 M NaClO4 medium and UV–Vis spectrophotometric measurement (V 530, Jasco) was carried out to obtain the calibration plot. Afterwards another set of humic acid solutions of known concentration (15 mg L−1) were prepared in the same medium mentioned above and 50 mg of manganese dioxide was added to each of the solutions. pH of the solution adjusted between 1 to 12 using dilute NaOH/ HClO4 and measured using pH meter (Oakton, pH700, USA) calibrated using buffer solutions of pH 4, 7 and 9. Then solutions are allowed to equilibrate for 48 h and centrifugation carried out for 1 h at 4000 rpm. The concentrations of humic acid in supernatants were determined using UV–Vis spectrophotometer and hence the sorption of HA on manganese dioxide.
2. Materials & methods 2.1. Chemical reagents Sodium salt of humic acid (Sigma Aldrich), Sodium hydroxide (AR, Thomas baker), Sodium perchlorate (Sigma Aldrich), AR grade Manganese nitrate (S D Fine Chem), Extra pure AR Acetone (Sisco Research laboratory) and AR grade Perchloric acid (Otto Chemie Pvt. Ltd.) were used in the present study. 2.2. Preparation of pure pyrolusite (β-MnO2)
2.4.2. Binary sorption (Niobium - manganese oxide) Sorption of Niobium on manganese oxide was carried out using 94 Nb radiotracer of concentration 2.2 × 10−7 M. 50 mg of manganese oxide was taken separately in 60 mL polypropylene containers (Tarson) containing 25 mL solution having ionic strength 0.05 M NaClO4 and 100 μL of the tracer solution added to each of the set and pH adjustment carried out. The experiments were performed under ambient condition at 25 °C and rest of the experimental parameters was same as mentioned above. The activities in initial solutions and in supernatants (solution after centrifugation) were measured using High Purity Germanium (HPGe) detector (Canberra, France) coupled with 8 k channel analyzer (Relative efficiency: 30%; Resolution: 1.9 keV at 1332 keV of 60Co). The activity corresponds to 702.7 keV gamma ray of 94 Nb utilized to get the % sorption using the following equation below
Manganous nitrate solution was evaporated on a hot plate near to dryness which resulted in the formation of blackish thick solution. The blackish solution was then transferred into a silica crucible and heated in muffle furnace at 180 °C for 48 h. Then the product was ground, washed with de-ionized water and then heated for another 24h at 102 °C in an oven (Mckenzi, 1971). 2.3. Characterization 2.3.1. X-ray diffraction (XRD) X-ray diffraction pattern of the synthesized manganese oxide was recorded using powder X-ray diffractometer (rotating anode, Rigaku, Japan) utilizing CuKα (λ = 1.5406 and 1.5444 Å) radiation. The angle range was 10–70o between which diffraction data were collected with a step width of 0.02o and time 5 s.
% Sorption =
(Ai − Af ) X 100 Ai
where Ai and Af are the activity of supernatant after phase separation.
2.3.2. Fourier transform infra red (FTIR) study FTIR of the sample was carried out using a Tensor II FTIR system from Bruker. The spectrum of the sample KBr pellet was recorded in transmission mode.
(1) 94
Nb in initial solution and in the
2.4.3. Ternary sorption (Niobium - manganese oxide-HA) Ternary sorption experiment is similar to that of the process described in the previous section and the only difference is the sorption experiment carried out in presence of 5 mg L−1 humic acid. To understand the effect of humic acid on sorption in better means, study also carried out with variation of humic acid concentration (5–200 mg L−1) both in acid and basic medium.
2.3.3. Surface area measurement Specific surface area of oxide was measured using single point BET (Brunauer–Emmett–Teller) surface area analyzer (BARC make). The method is based on the sorption and desorption of nitrogen gas on sample at liquid nitrogen and room temperature respectively and consequitive determination using thermal conductivity detector.
2.5. Effect of ionic strength, temperature and equilibration time
2.3.4. Scanning electron microscopy (SEM) Scanning Electron microscope coupled with Energy dispersive XRay spectrometry (SEM-EDS, Instrument Model No. VEGA MV 2300T) was used to obtain the surface morphology of the synthesized
The effect of ionic strength on the sorption of Nb(V) on manganese dioxide were performed in the ionic strength range of 0.01 M–1.0 M NaClO4. Similarly the effect of equilibration time on sorption was studied for time intervals of 5 min, 1 h, 5 h, 24 h, 48 h and 72 h to 2
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Fig. 1. XRD pattern of manganese dioxide. Fig. 3. SEM image of synthesized manganese dioxide.
understand the kinetics of the sorption process. The temperature effect was also performed by carrying out sorption experiment at 5 °C, 25 °C, 50 °C and 75 °C. The distribution ratio (kd) was obtained using equation (2). All the above three experiment were performed both in acidic (pH 1) and basic medium (pH 9).
kd =
(Ai − Af ) V X Af m
(2)
Where m is the mass of pyrolusite in mg and V is the total volume of the solution in mL. 3. Result and discussion 3.1. Characterization of MnO2 The XRD pattern of the laboratory synthesized MnO2 is shown in Fig. 1. The XRD peaks are well matched with the ICDD diffraction pattern of pyrolusite. FTIR spectrum Fig. 2 data is in accordance with the literature reported data of pyrolusite (Pyrolusite R040153). The specific surface area of the oxide is about 9 m2g−1.The hydrodynamic particle size is in the range 410-650 nm. SEM image given in Fig. 3 shows the surface morphology of manganese dioxide. The zeta potential of MnO2 measured in the pH range of 1–12 and it is observed that the oxide has point of zero charge is about pH4 (Cristiano et al., 2011). Above pH 4, zeta potential is negative and the magnitude increase with increase of pH. At pH 1, the negative zeta potential of HA may be because of acidic carboxylic group dissociation. Relatively steeper change in zeta potential of HA at pH 4–5 and pH 8–9, may be attributed to
Fig. 4. Zeta potential of manganese dioxide, humic acid and mixed system.
dissociation of carboxylic (–COOH) and phenolic (–OH) group respectively. The zeta potential of the manganese dioxide-HA combined system also measured and found to lie between the two individual systems Fig. 4 but the PZC shifted slightly towards lower pH. 3.2. Bionary sorption (Humic acid-manganese dioxide) The sorption of HA on pyrolusite in the pH range 1–12 in 0.05 M NaClO4 medium is shown in Fig. 5. At low pHs as the surface of hydrous oxide is protonated i.e holds positive charges and hence enhancing the sorption of HA and this explains the quantitative sorption of HA on pyrolusite (90%). The surface charge of pyrolusite decreases with increasing pH, then after pH 4 (Point of Zero Charge) it becomes negative, and therefore HA sorption decreases, as HA bears negative surface charge over the whole pH range studied and this observation in accordance with literature (Fairhurst et al., 1995). Excluding the electrostatic attraction/repulsion factor, several mechanisms of sorption were reported in literature. This includes ligand switch over phenomenon between active sites on the surface of hydrous oxide and the functional groups of HA, hydrophobicity, solubility and humic material, HA conformational changes, HA concentrations and mineral surface area. 3.3. Binary sorption (Niobium - manganese oxide) The sorption of Nb on pyrolusite with and without HA is shown in Fig. 6. Generally, two factors namely electrostatic interaction and
Fig. 2. FTIR spectra of manganese dioxide. 3
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Fig. 7. Nb Sorption variation with HA concentration at pH1 and at pH9.
Fig. 5. Sorption of humic acid on MnO2 with variation of pH.
3.4. Ternary sorption (Niobium - manganese oxide-HA) The change in sorption behaviour of Nb on pyrolusite in presence of HA is shown in Fig. 6. That presence of humic acid increase the sorption in acidic pH but reverse effect was observed in basic pH. HA is found to have insignificant effect on sorption in neutral pH region. Similar type observation was seen (Fig. 7) when we carried out the sorption study at pH 1 and pH 9 with varying humic acid concentration (10–200 mg L−1). The effect becomes more severe with increase concentration of humic acid. At low pH, metal ions are feebly bound whereas humic substances are firmly sorbed by mineral surface. Hence mineral bound humic acid facilitate the sorption of Nb(V) on pyrolusite surface. However, in basic pH owing to its higher solubility, preferred to stay in solution rather than on hydrated MnO2 surface and dissolved humic acid undergo complexation using –COOH group, causing increase in concentration of Nb(V) in solution. The role of humic acid perceive here in agreement with several literature where it is stated that metal cations are weakly bound to mineral surface but raises in presence of HA (Benjamin and Leckie, 1981; Wang et al., 2006) but contradictory literature also available pointing that sorbed HA on surface reduces the available sites for metal ion and hence reduced sorption of metal ion in presence of humic acid in acidic pH (Murphy et al., 1994). At pH 1, it is observed that as the HA concentration exceeding 100 mg L−1, HA starts agglomerate and itself form a new colloidal phase which was not observed basic medium. This indicates at 100 mg L−1 HA concentration, the surface of pyrolusite is totally covered by humic acid and this surface coated pyrolusite causing the higher sorption and the mean while HA colloid also acting as sorbing phase for Nb(V) and thus the steeper enhancement of sorption in presence of high concentration of humic acid by humic acid.
Fig. 6. Sorption of Nb(V) on MnO2 in presence and absence of humic acid.
surface complexation control the sorption of radionuclides on colloids (Lang et al., 2013; Stumm and Morgan, 1996; Lieser, 1995; Silva and Nitsche, 1995). Electrostatic interaction depends upon the charge on colloid surface, and the species of Nb exist in pH range studied. The principal chemical form in acidic solution is NbO2+, while in slightly acidic to slightly basic solutions neutral species like HNbO3 or NbO (OH)3 exist. Above pH 9 the anionic species like NbO2(OH)2– or NbO (OH)4− or Nb(OH)6- are predominant (Lehto and Hou, 2010; Anderson et al., 1979). In the pH range 1–3, the sorption is low as both the pyrolusite and Nb species bear positive charge and whatever the sorption observed may be due to physisorption or ion exchange phenomenon between surface –OH group of pyrolusite and the positively charged Nb species. Again the same electrostatic repulsion higher pH (9–12) between the pyrolusite surface and the species of Nb explains it's lower sorption on pyrolusite. If we see more carefully, it is clear that the sorption above pH 9 is higher compared to the sorption in the pH range 1–3. This indicates physisorption alone is not responsible for Nb sorption pyrolusite above pH 9, some other mechanism also involved via which Nb get sorbed onto pyrolusite surface. However, it is very difficult to explain the very high sorption value in the pH range 5–9 where the surface charge on pyrolusite in negative and Nb predominantly exist as neutral species. The very high sorption in this pH range can be explained by the surface complexation between the hydroxyl group of neutral Nb species like HNbO3 or NbO(OH)3 and the surface hydroxyl group (-OH) of pyrolusite.
3.5. Effect of ionic strength Fig. 8 shows the sorption of Nb(V) on pyrolusite as a function of ionic strength. At pH1, increase of ionic strength resulted in decrease the sorption value where as mild enhancement of sorption was observed at pH 9. This ionic strength dependent sorption study can be utilized to differentiate between inner sphere and outersphere complexes. According to Hayes et al. (1988) variation of ionic strength can affect the sorption in case of outersphere complexation because (i) the solution electrolyte can engage itself with the nonspecific sorption of the ions on present binding sites of the surface. (ii) Ionic strength influence the interfacial potential and hence affect the activity of the adsorbing species as the solution electrolyte and the outer sphere complexes of adsorbing species are positioned at same plane in the triple layer model. In case of Inner-sphere complexation, the adsorbing species remain much closer to the surface as compared to background 4
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Fig. 8. Effect of ionic strength on sorption at pH1 and at pH9. Fig. 9. Logkd vs 1/T plot at pH1 and pH9 (relative uncertainty in kd: 4–8 %).
electrolyte; hence ionic strength can't affect this type of complexation. This infers that the possibility of physisorption or ion exchange or outersphere mechanism in acidic pH (Xiaoping et al., 2009). In basic pH, if we see the ionic strength dependent sorption data, it is clear that the effect of ionic strength much less prominent or nearly insensitive compared to in acidic pH. This observation indicates that at pH 9, some other phenomenon mainly responsible for sorption which possibly may be chemisorption.
Table 2 Thermodynamic parameters obtained from Logkd vs 1/T plot at pH1 and pH9.
Temperature is a critical parameter in sorption study, as it can directly affects the distribution ratio (kd) of the ion and finally kinetics of the sorption process. The estimation of thermodynamic parameters assists in getting insight into the adsorption mechanism of metal ions on various sorbents (Sheng et al., 2010; Zhao et al., 2010; Zhang et al., 2011). Similar approach is adopted in the present study to interpret the sorption mechanism of Nb on manganese dioxide. The temperature dependent variation of distribution ratio both acidic and basic pH is given in Table 1. This phenomenon can be utilized in Van't Hoff equation to deduce the thermodynamic parameters of the sorption process.
ΔS ΔH − 2.303R 2.303RT
(3)
The values of enthalpy, ΔH , and entropy, ΔS , are calculated from the slopes and intercepts of the log(Kd) vs.1/T plot (Fig. 9). Then The Gibbs free energy, ΔGo, of the sorption process is calculated from the equation: o
o
ΔG = ΔH-TΔS
ΔGo(kJM−1)
ΔHo(kJM−1)
ΔSo(JK−1M−1)
1.2 9
−10.9 −31.5
13.4 −29.84
88.42 1.9
explanation of the endothermic process is that Nb(V) ions are highly solvated in aqueous medium and before going to sorbed onto colloid surface, rupture of the hydration sphere occurred to some extent, and the process requires energy. It is also assumed that the energy required for the dehydration exceed the exothermicity of Nb(V) ions fixing on pyrolusite surface and make the whole process endothermic. The Gibbs free energy change (ΔG) is negative, indicates the process is spontaneous under experimental conditions. The positive values of entropy change (ΔS) reflect the increase of randomness at the solid/liquid interface during the sorption process and obviously the partial removal of water molecule due to the ion exchange process between the adsorbate and surface functional groups is the responsible factor (Yan-Hui et al., 2005). Slightly different observation of reduction in Kd at 75 °C was observed (Table 1) suggesting weak interaction between the Nb(V) and pyrolusite colloids. The decrease in Kd is exactly identical with the phenomenon of decreased physisorption at higher temperature. At pH 9, ΔHo is negative; specify the sorption process in exothermic. With increase of pH, the hydration sphere around Nb(V) get weaker as the oxo-hydroxy species dominates. This causes the reduction in energy required for rupture of very weak hydration sphere and at the same instant the energy liberated because of the chemical bond formation between Nb(V) hydroxy species and surface hydroxyl group of pyrolusite overcompensated and make the process exothermic. Although there are no hard and fast rule regarding the ΔHo values to interpret the sorption nature, the enthalpy of sorption values ranging from 2.1 to 20.9 kJ mol−1 (0.5–5 kcal mol−1) corresponds to physical sorption process and the values in the range of 20.9–418.4 kJ mol−1 (5.0 and 100 kcal mol−1) associated with the charge sharing or
3.6. Effect of temperature
log(K d ) =
pH
(4)
Where R is the universal gas constant (8.314Jmol−1K−1) and T is absolute temperature (K). The thermodynamic parameters (Table 2) help in understanding the mechanism concerning the adsorptive interaction of Nb(V) with Pyrolusite. The positive ΔHo value indicates the adsorption process is endothermic in acidic pH. One possible Table 1 kd values with associated uncertainty at pH1 and pH9 at different temperature. Temperature (K)
1/T (K−1)
kd (Lkg−1) (pH1)
Uncertainty
kd (Lkg−1) (pH 9)
Uncertainty
278 298 323 348
0.0035971 0.0033557 0.0030959 0.0028735
685 981 1210 1069
33 38 47 40
7065 3648 2008 1120
595 219 96 46
5
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Fig. 10. Pseudo first and second order fitting of kinetic data at (a) pH1 and (b) pH 9.
3.7. Effect of equilibration time
formation of coordinate/covalent bond between metal surfaces and metal ions (chemisorption) (Smith, 1981; Ozcan et al., 2005; Unl and Ersoz, 2006). The enthalpy change associated with sorption process at pH1 is significantly low (13.4 kJ mol−1), indicates the physisorption mechanism in acidic pH. At pH 9, the high enthalpy change (−29.8 kJmol1 ) indicating the chemisorption mechanism and analogous study was reported on thorium and uranium sorption on carbon nanotubes and natural zeolite respectively (Wang et al., 2011; Aytas et al., 2004). The error bars stand for calculated uncertainties resulted from (i) counting statistics (ii) volumetric operations and (iii) weighing with decreasing contributions, in the order mentioned. The standard error propagation method was employed to get the final uncertainties associated with the measured parameters. The uncertainty equation for % sorption (Equation (5)) and kd (Equation (6)) were derived based on equations (1) and (2).
U(% Sorption)
2 2 ⎡ UAo − Af ⎞ ⎛ U Ao ⎞ ⎤ = (% Sorption) ⎢ ⎜⎛ ⎟ + ⎥ A − Af ⎠ ⎝ Ao ⎠ ⎦ ⎣⎝ o ⎜
2
The kinetic modeling of the time dependent sorption data was studied using Lagergren's pseudo first order (Lagergren, 1898) and Ho's pseudo second order model (Ho, 1995). The Lagergren equation based on pseudo-first-order adsorption kinetics is expressed as follows:
dq = k1 (qe − qt ) dt
(7)
Where, qe is the amount of adsorbate adsorbed at equilibrium (μg/ mg), qt is the adsorbed adsorbate amount at time t (μg/mg) and k1 is the pseudo-first-order rate constant (h−1). The equation simplifies to
Log (q e − qt) = Logq e −
k1t 2.303
(8)
Ho's pseudo second order model expressed as
dq = k2 (qe − qt )2 dt
(9)
⎟
After integration from t = 0 to t = t, the equation simplifies to
(5)
t 1 t = + qt qe k2q 2e
2
UA UV 2 U 2 ⎡ UAo − Af ⎞ ⎞ + ⎜⎛ f ⎟⎞ + ⎛ W ⎞ ⎤ Ukd = kd ⎢ ⎜⎛ ⎟ + ⎛ ⎥ A − A V A ⎝ ⎠ ⎝W ⎠ ⎦ f⎠ ⎝ f ⎠ ⎣⎝ o
(10)
The observations of time dependent Nb(V) sorption on pyrolusite both in acidic (pH1) and basic mediua (pH9) are shown in Fig. 10. In acidic pH the kinetic data best fitted when we follow pseudo first order kinetics (regression coefficient: 0.9812) rather than pseudo second order model (regression coefficient: 0.9586). This indicates in acid medium, the sorption of Nb(V) on pyrolusite follows pseudo first order
(6)
where U represents the uncertainty in the respective parameter. As the pH changed from 1 to 12, the relative uncertainties for % sorption and kd varied 3–6 % and 4–8 % respectively. 6
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Interaction of hydrophobic organic compounds with mineral-bound humic substances. Environ. Sci. Technol. 28, 1291–1299. Ozcan, A., Ozcan, A.S., Tunali, S., Akar, T., Kiran, I., 2005. Determination of the equilibrium, kinetic and thermodynamic parameters of adsorption of Cu(II) ions onto seeds of Capsicum annuum. J. Hazard Mater. B124, 200–208. Penrose, W.R., Polzer, W.L., Essington, E.H., Nelson, D.M., Orlandilini, K.A., 1990. Mobility of plutonium and Americium through shallow aquifer in a semiarld region. Environ. Sci. Technol. 24 (2), 228–234. Powell, B.A., Fjeld, R.A., Kaplan, D.I., Coates, J.T., Steven, M.S., 2005. Pu(V)O2+ adsorption and reduction by Hematite and Goethite. Environ. Sci. Technol. 39 (7), 2107–2114. Pyrolusite R040153 - RRUFF Database: Raman Spectra, X-Ray Diffraction and Chemistry Data for Minerals. Sanchez, A.L., Murray, J.W., Sibley, T.H., 1985. The adsorption of Plutonium IV and V on goethite. Geochem. Cosmochim. Acta 49, 2297–2307. Sheng, G., Hu, J., Jin, H., Yang, S., Ren, X., Li, J., Chen, Y., Wang, X., 2010. Effect of humic acid, fulvic acid, pH, ionic strength and temperature on 63Ni(II) sorption to MnO2. Radiochim. Acta 98, 291–299. Silva, R.J., Nitsche, H., 1995. Actinide environmental chemistry. Radiochim. Acta 70 (71), 377–396. Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry. Wiley, New York. Smith, J.M., 1981. Chemical Engineering Kinetics, third ed. McGraw-Hill, New York. Sylwestera, E.R., Hudsona, E.A., Allen, P.G., 2000. The structure of uranium (VI) sorption complexes on silica, alumina, and montmorillonite. Geochem. Cosmochim. Acta 64 (14), 2431–2438. Unl, N., Ersoz, M., 2006. Adsorption characteristics of heavy metal ions onto a low cost
Fig. 11. Triple layer model for the sorption of Nb(V) on pyrolusite.
kinetics and the sorption occurring predominantly via physisorption or the outersphere complexation predominant. But the kinetic sorption data for basic medium is well fitted with pseudo second order model (regression coefficient: 0.9998) rather than pseudo first order model (regression coefficient: 0.7779). This infers that chemisorption is the predominant mechanism of sorption at pH 9. Based on the ionic strength, temperature dependent and kinetic sorption data, the probable mechanism of sorption of Nb(V) on pyrolusite is shown in schematic manner in Fig. 11. In acidic pH, the sorption is mainly due to physisorption or ion exchange type phenomenon whereas in neutral and basic medium, there is utmost chance for the formation of covalent bond between Nb and pyrolusite surface. We have tried to model the Nb sorption on manganese dioxide using FITEQEL software. However the inclusion of Niobium species in the model could not be done especially at low pH because of the lack of data regarding the hydrolysis constant of NbO2+ species which is supposed to predominate at the lower pH conditions and the dissociation constant of the surface hydroxyl groups of manganese dioxide. 4. Conclusion The knowledge that can be gathered from the above sorption study are (1) The sorption of Nb(V) on pyrolusite is quantitative in neutral to near neutral pH region. (2) In low pH mineral bound humic acid enhances the sorption of Nb(V) and in higher pH dissolved humic acid complexes with Nb(V) and reduced the sorption. (3) As the sorption strong depend on ionic strength, the sorption follow outer sphere complexation in low pH but mild dependent in basic pH reinforce the probability of innersphere complexation (4) The positive entropy change of the sorption process in acidic pH make the endothermic sorption process spontaneous. (5) Pseudo first order fitting in low pH suggests the high probability of physisorption whereas pseudo second order kinetic fitting proposes the probability of chemisorption mechanism at high pH. Declaration There is no conflict of interest regarding the manuscript. Acknowledgements Authors thank Dr. H Pal, Associate Director (A), Chemistry Group and Head, Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India for his support. 7
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Isot. 65, 375–381. Xiaoping, S., Suowei, W., Lei, C., Maolian, Z., Yunhui, D., 2009. Effect of pH, ionic strength and temperature on the sorption of radionickel on Na-montmorillonite. Appl. Radiat. Isot. 67, 1007–1012. Yan-Hui, Li, Zechao, Di, Jun, D., Dehai, Wu, Zhaokun, L., Yanqiu, Z., 2005. Adsorption thermodynamic, kinetic and desorption studies of Pb2+ on carbon nanotubes. Water Res. 39, 605–609. Zhang, L., Zhang, H., Ge, Z., Yu, X., 2011. Impact of environmental conditions on the sorption behavior of radionuclide 60Co(II) on MnO2. J. Radioanal. Nucl. Chem. 288, 537–546. Zhao, D., Yang, X., Zhang, H., Chen, C., Wang, X., 2010. Effect of environmental conditions on Pb(II) adsorption on β-MnO2. Chem. Eng. J. 164 (1), 49–55.
bio-polymeric sorbent from aqueous solutions. J. Hazard Mater. B136, 272–280. Waite, T.D., Davis, J.A., Payne, T.E., Waychunas, D.A., Xu, N., 1994. U(VI) adsorption on ferrihydrite: application of a surface complexation model. Geochem. Cosmochim. Acta 58 (24), 5465–5478. Wang, X., Xu, D., Chen, L., Tan, X., Zhou, X., Ren, A., Chen, Ch, 2006. Sorption and complexation of Eu(III) on alumina: effects of pH, ionic strength, humic acid and chelating resin on kinetic dissociation study. Appl. Radiat. Isot. 64, 414–421. Wang, M., Tao, X., Song, X., 2011. Effect of pH, ionic strength and temperature on sorption characteristics of Th(IV) on oxidized multiwalled carbon nanotubes. J. Radioanal. Nucl. Chem. 288, 859–865. Xiaoli, T., Xiangke, W., Changlun, C., Aihua, S., 2007. Effect of soil humic and fulvic acids, pH and ionic strength on Th(IV) sorption to TiO2 nanoparticles. Appl. Radiat.
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Sorption of Nb(V) on pyrolusite (β-MnO2): Effect of pH, humic acid, ionic strength, equilibration time and temperature
T
Madhusudan Ghosha,b, P.S. Remya Devia, K.K. Swaina,b,∗ a b
Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, India
H I GH L IG H T S
investigation of environmental parameters: sorption of Nb(V) on pyrolusite. • Thorough acid enhances sorption in acidic pH whereas sorption declined in basic pH. • Humic of sorption mechanism using ionic strength and temperature dependence study. • Elucidation • Sorption follows pseudo first order kinetics in pH 1and pseudo second order in pH 9.
A R T I C LE I N FO
A B S T R A C T
Keywords: Radionuclides Sorption Niobium Humic acid Ionic strength
The sorption of Nb(V) on pyrolusite has been studied and the effect of pH, ionic strength, humic acid, temperature and equilibration time were also investigated in a series of batch equilibrium experiments. The sorption was found to be affected by solution pH, ionic strength and humic acid. The sorption was high in neutral/near neutral pH (~96 %) but lower sorption was observed both in acidic (~55 %) and basic (~85 %) media. Sorption was decreased in acidic pH with increase of ionic strength and reverse effect was seen in basic pH although the effect is less prominent. Presence of humic acid causes enhancement of sorption in acidic pH whereas sorption declined in basic pH. The sorption process is endothermic in acid medium and exothermic in basic medium. In acid medium the sorption is entropy driven process. Kinetics of the sorption study was found to follow pseudo first order in acidic pH whereas pseudo second order in basic pH.
1. Introduction The migration behaviour of potentially released radioisotopes in environments has to be known for trustworthy long term assessment of nuclear waste repository. Repositories for nuclear waste should meet the norm that radionuclides from repository should not come into the human habitation in thousand years. The migration of the radionuclides depend on solubility of radionuclides in underground waters, complexation with anions present in aqueous stream, sorption on components of engineered and geochemical barriers (rocks and minerals) and the colloid formation (pseudo or true). Among all of these parameters, sorption of radionuclides at the water rock interface retards the migration in natural system. Thus, the factors that administer the radionuclides sorption on mineral surfaces have to be studied (Khasanova et al., 2007). A lot of work has been carried out to understand the sorption of radionuclide on several minerals which includes hydrous oxides of Fe ∗
(Waite et al., 1994; Sanchez et al., 1985; Powell et al., 2005), Mn (Means et al., 1978), Si and Al (Sylwestera et al., 2000), clay (Hartmann et al., 2008). According to most of the existing literature, tri- and tetravalent actinides causes no hazard, as they have high tendency to get sorbed onto the components used for their isolation and host rocks and owing to their extremely low solubility (Khasanova et al., 2007). However, current studies suggest the possibility of colloidal migration of these radionuclides in groundwater (McCarthy and Zachara, 1989; Clarke et al., 1996; Cooper et al., 1995). 239Pu was found to migrate distances upto 1.3 km in 30 years with smectite colloid at the test site in Nevada (Kersting et al., 1999). Penrose et al. (1990) have studied the behavior of Pu and Am in the Mortandad canyon (Los Alamos National Laboratory) and observed that colloid particles have facilitated the migration of these radionuclides to a distance of 3.39 km from the source. The reports indicate colloids dispersed in groundwater act as an auxiliary phase and enhance the transport of radio-nuclides. A three phase model consists of stationary phase, mobile phase and colloids
Corresponding author. Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India. E-mail address: [email protected] (K.K. Swain).
https://doi.org/10.1016/j.apradiso.2019.108887 Received 12 April 2019; Received in revised form 2 August 2019; Accepted 4 September 2019 Available online 05 September 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
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manganese oxide.
(intrinsic or pseudo) dispersed in mobile phase were introduced and able to explain satisfactorily the migration of radionuclides (McCarthy and Zachara, 1989; Kersting et al., 1999; Honeymann, 1999). So in this context the sorption study of radionuclides on naturally occurring colloids (pseudo) in aqueous stream is important. The most common natural organic matters present along with inorganic anions, is humic acid (HA). Presence of HA in ground water may alter the sorption as well as migration behaviour of radionuclides by complexing with the radionuclides or by changing the oxidation state of the radionuclides. It is also reported that HA itself get sorbed onto colloids and affect the sorption of radionuclides on colloids (Buckau, 2005; Xiaoli et al., 2007). The discharged Zr-Nb pressure tubes from Pressurized Heavy Water Reactor (PHWR) are presently stored in deep tile holes with concrete caps. In present study, we are dealing with 94Nb, a long lived radioisotope (t1/2 = 20300 y), which generates from 93Nb isotope during neutron irradiation of Zr-Nb pressure tube or cladding material. Here we report the sorption phenomenon of Nb(V) on laboratory synthesized pyrolusite and the detailed investigation regarding the effect of different physico-chemical parameters like pH, ionic strength, equilibration time, humic acid and temperature on sorption.
2.3.5. Zeta potential & particle size Particle size and Zeta potential of the oxide were measured using the LitesizerTM500, Anton-Paar. Zeta potential measurement is based on the electrophoretic light scattering (ELS), related to the speed of the particle under electric field which directly proportional to the surface charge of the particle or zeta potential. Particle size was measured by Dynamic Light Scattering (DLS) where the measurement of random motion of the particle is measured which directly associated with the particle size. 2.4. Batch sorption experiment 2.4.1. Binary sorption (HA - manganese oxide) Before studying the sorption of Niobium on manganese oxide, study on the sorption behavior of humic acid on manganese oxide, is required. For that, we have prepared a set of purified humic acid solution of concentration 0.5–15 mg L−1 in 0.05 M NaClO4 medium and UV–Vis spectrophotometric measurement (V 530, Jasco) was carried out to obtain the calibration plot. Afterwards another set of humic acid solutions of known concentration (15 mg L−1) were prepared in the same medium mentioned above and 50 mg of manganese dioxide was added to each of the solutions. pH of the solution adjusted between 1 to 12 using dilute NaOH/ HClO4 and measured using pH meter (Oakton, pH700, USA) calibrated using buffer solutions of pH 4, 7 and 9. Then solutions are allowed to equilibrate for 48 h and centrifugation carried out for 1 h at 4000 rpm. The concentrations of humic acid in supernatants were determined using UV–Vis spectrophotometer and hence the sorption of HA on manganese dioxide.
2. Materials & methods 2.1. Chemical reagents Sodium salt of humic acid (Sigma Aldrich), Sodium hydroxide (AR, Thomas baker), Sodium perchlorate (Sigma Aldrich), AR grade Manganese nitrate (S D Fine Chem), Extra pure AR Acetone (Sisco Research laboratory) and AR grade Perchloric acid (Otto Chemie Pvt. Ltd.) were used in the present study. 2.2. Preparation of pure pyrolusite (β-MnO2)
2.4.2. Binary sorption (Niobium - manganese oxide) Sorption of Niobium on manganese oxide was carried out using 94 Nb radiotracer of concentration 2.2 × 10−7 M. 50 mg of manganese oxide was taken separately in 60 mL polypropylene containers (Tarson) containing 25 mL solution having ionic strength 0.05 M NaClO4 and 100 μL of the tracer solution added to each of the set and pH adjustment carried out. The experiments were performed under ambient condition at 25 °C and rest of the experimental parameters was same as mentioned above. The activities in initial solutions and in supernatants (solution after centrifugation) were measured using High Purity Germanium (HPGe) detector (Canberra, France) coupled with 8 k channel analyzer (Relative efficiency: 30%; Resolution: 1.9 keV at 1332 keV of 60Co). The activity corresponds to 702.7 keV gamma ray of 94 Nb utilized to get the % sorption using the following equation below
Manganous nitrate solution was evaporated on a hot plate near to dryness which resulted in the formation of blackish thick solution. The blackish solution was then transferred into a silica crucible and heated in muffle furnace at 180 °C for 48 h. Then the product was ground, washed with de-ionized water and then heated for another 24h at 102 °C in an oven (Mckenzi, 1971). 2.3. Characterization 2.3.1. X-ray diffraction (XRD) X-ray diffraction pattern of the synthesized manganese oxide was recorded using powder X-ray diffractometer (rotating anode, Rigaku, Japan) utilizing CuKα (λ = 1.5406 and 1.5444 Å) radiation. The angle range was 10–70o between which diffraction data were collected with a step width of 0.02o and time 5 s.
% Sorption =
(Ai − Af ) X 100 Ai
where Ai and Af are the activity of supernatant after phase separation.
2.3.2. Fourier transform infra red (FTIR) study FTIR of the sample was carried out using a Tensor II FTIR system from Bruker. The spectrum of the sample KBr pellet was recorded in transmission mode.
(1) 94
Nb in initial solution and in the
2.4.3. Ternary sorption (Niobium - manganese oxide-HA) Ternary sorption experiment is similar to that of the process described in the previous section and the only difference is the sorption experiment carried out in presence of 5 mg L−1 humic acid. To understand the effect of humic acid on sorption in better means, study also carried out with variation of humic acid concentration (5–200 mg L−1) both in acid and basic medium.
2.3.3. Surface area measurement Specific surface area of oxide was measured using single point BET (Brunauer–Emmett–Teller) surface area analyzer (BARC make). The method is based on the sorption and desorption of nitrogen gas on sample at liquid nitrogen and room temperature respectively and consequitive determination using thermal conductivity detector.
2.5. Effect of ionic strength, temperature and equilibration time
2.3.4. Scanning electron microscopy (SEM) Scanning Electron microscope coupled with Energy dispersive XRay spectrometry (SEM-EDS, Instrument Model No. VEGA MV 2300T) was used to obtain the surface morphology of the synthesized
The effect of ionic strength on the sorption of Nb(V) on manganese dioxide were performed in the ionic strength range of 0.01 M–1.0 M NaClO4. Similarly the effect of equilibration time on sorption was studied for time intervals of 5 min, 1 h, 5 h, 24 h, 48 h and 72 h to 2
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Fig. 1. XRD pattern of manganese dioxide. Fig. 3. SEM image of synthesized manganese dioxide.
understand the kinetics of the sorption process. The temperature effect was also performed by carrying out sorption experiment at 5 °C, 25 °C, 50 °C and 75 °C. The distribution ratio (kd) was obtained using equation (2). All the above three experiment were performed both in acidic (pH 1) and basic medium (pH 9).
kd =
(Ai − Af ) V X Af m
(2)
Where m is the mass of pyrolusite in mg and V is the total volume of the solution in mL. 3. Result and discussion 3.1. Characterization of MnO2 The XRD pattern of the laboratory synthesized MnO2 is shown in Fig. 1. The XRD peaks are well matched with the ICDD diffraction pattern of pyrolusite. FTIR spectrum Fig. 2 data is in accordance with the literature reported data of pyrolusite (Pyrolusite R040153). The specific surface area of the oxide is about 9 m2g−1.The hydrodynamic particle size is in the range 410-650 nm. SEM image given in Fig. 3 shows the surface morphology of manganese dioxide. The zeta potential of MnO2 measured in the pH range of 1–12 and it is observed that the oxide has point of zero charge is about pH4 (Cristiano et al., 2011). Above pH 4, zeta potential is negative and the magnitude increase with increase of pH. At pH 1, the negative zeta potential of HA may be because of acidic carboxylic group dissociation. Relatively steeper change in zeta potential of HA at pH 4–5 and pH 8–9, may be attributed to
Fig. 4. Zeta potential of manganese dioxide, humic acid and mixed system.
dissociation of carboxylic (–COOH) and phenolic (–OH) group respectively. The zeta potential of the manganese dioxide-HA combined system also measured and found to lie between the two individual systems Fig. 4 but the PZC shifted slightly towards lower pH. 3.2. Bionary sorption (Humic acid-manganese dioxide) The sorption of HA on pyrolusite in the pH range 1–12 in 0.05 M NaClO4 medium is shown in Fig. 5. At low pHs as the surface of hydrous oxide is protonated i.e holds positive charges and hence enhancing the sorption of HA and this explains the quantitative sorption of HA on pyrolusite (90%). The surface charge of pyrolusite decreases with increasing pH, then after pH 4 (Point of Zero Charge) it becomes negative, and therefore HA sorption decreases, as HA bears negative surface charge over the whole pH range studied and this observation in accordance with literature (Fairhurst et al., 1995). Excluding the electrostatic attraction/repulsion factor, several mechanisms of sorption were reported in literature. This includes ligand switch over phenomenon between active sites on the surface of hydrous oxide and the functional groups of HA, hydrophobicity, solubility and humic material, HA conformational changes, HA concentrations and mineral surface area. 3.3. Binary sorption (Niobium - manganese oxide) The sorption of Nb on pyrolusite with and without HA is shown in Fig. 6. Generally, two factors namely electrostatic interaction and
Fig. 2. FTIR spectra of manganese dioxide. 3
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Fig. 7. Nb Sorption variation with HA concentration at pH1 and at pH9.
Fig. 5. Sorption of humic acid on MnO2 with variation of pH.
3.4. Ternary sorption (Niobium - manganese oxide-HA) The change in sorption behaviour of Nb on pyrolusite in presence of HA is shown in Fig. 6. That presence of humic acid increase the sorption in acidic pH but reverse effect was observed in basic pH. HA is found to have insignificant effect on sorption in neutral pH region. Similar type observation was seen (Fig. 7) when we carried out the sorption study at pH 1 and pH 9 with varying humic acid concentration (10–200 mg L−1). The effect becomes more severe with increase concentration of humic acid. At low pH, metal ions are feebly bound whereas humic substances are firmly sorbed by mineral surface. Hence mineral bound humic acid facilitate the sorption of Nb(V) on pyrolusite surface. However, in basic pH owing to its higher solubility, preferred to stay in solution rather than on hydrated MnO2 surface and dissolved humic acid undergo complexation using –COOH group, causing increase in concentration of Nb(V) in solution. The role of humic acid perceive here in agreement with several literature where it is stated that metal cations are weakly bound to mineral surface but raises in presence of HA (Benjamin and Leckie, 1981; Wang et al., 2006) but contradictory literature also available pointing that sorbed HA on surface reduces the available sites for metal ion and hence reduced sorption of metal ion in presence of humic acid in acidic pH (Murphy et al., 1994). At pH 1, it is observed that as the HA concentration exceeding 100 mg L−1, HA starts agglomerate and itself form a new colloidal phase which was not observed basic medium. This indicates at 100 mg L−1 HA concentration, the surface of pyrolusite is totally covered by humic acid and this surface coated pyrolusite causing the higher sorption and the mean while HA colloid also acting as sorbing phase for Nb(V) and thus the steeper enhancement of sorption in presence of high concentration of humic acid by humic acid.
Fig. 6. Sorption of Nb(V) on MnO2 in presence and absence of humic acid.
surface complexation control the sorption of radionuclides on colloids (Lang et al., 2013; Stumm and Morgan, 1996; Lieser, 1995; Silva and Nitsche, 1995). Electrostatic interaction depends upon the charge on colloid surface, and the species of Nb exist in pH range studied. The principal chemical form in acidic solution is NbO2+, while in slightly acidic to slightly basic solutions neutral species like HNbO3 or NbO (OH)3 exist. Above pH 9 the anionic species like NbO2(OH)2– or NbO (OH)4− or Nb(OH)6- are predominant (Lehto and Hou, 2010; Anderson et al., 1979). In the pH range 1–3, the sorption is low as both the pyrolusite and Nb species bear positive charge and whatever the sorption observed may be due to physisorption or ion exchange phenomenon between surface –OH group of pyrolusite and the positively charged Nb species. Again the same electrostatic repulsion higher pH (9–12) between the pyrolusite surface and the species of Nb explains it's lower sorption on pyrolusite. If we see more carefully, it is clear that the sorption above pH 9 is higher compared to the sorption in the pH range 1–3. This indicates physisorption alone is not responsible for Nb sorption pyrolusite above pH 9, some other mechanism also involved via which Nb get sorbed onto pyrolusite surface. However, it is very difficult to explain the very high sorption value in the pH range 5–9 where the surface charge on pyrolusite in negative and Nb predominantly exist as neutral species. The very high sorption in this pH range can be explained by the surface complexation between the hydroxyl group of neutral Nb species like HNbO3 or NbO(OH)3 and the surface hydroxyl group (-OH) of pyrolusite.
3.5. Effect of ionic strength Fig. 8 shows the sorption of Nb(V) on pyrolusite as a function of ionic strength. At pH1, increase of ionic strength resulted in decrease the sorption value where as mild enhancement of sorption was observed at pH 9. This ionic strength dependent sorption study can be utilized to differentiate between inner sphere and outersphere complexes. According to Hayes et al. (1988) variation of ionic strength can affect the sorption in case of outersphere complexation because (i) the solution electrolyte can engage itself with the nonspecific sorption of the ions on present binding sites of the surface. (ii) Ionic strength influence the interfacial potential and hence affect the activity of the adsorbing species as the solution electrolyte and the outer sphere complexes of adsorbing species are positioned at same plane in the triple layer model. In case of Inner-sphere complexation, the adsorbing species remain much closer to the surface as compared to background 4
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Fig. 8. Effect of ionic strength on sorption at pH1 and at pH9. Fig. 9. Logkd vs 1/T plot at pH1 and pH9 (relative uncertainty in kd: 4–8 %).
electrolyte; hence ionic strength can't affect this type of complexation. This infers that the possibility of physisorption or ion exchange or outersphere mechanism in acidic pH (Xiaoping et al., 2009). In basic pH, if we see the ionic strength dependent sorption data, it is clear that the effect of ionic strength much less prominent or nearly insensitive compared to in acidic pH. This observation indicates that at pH 9, some other phenomenon mainly responsible for sorption which possibly may be chemisorption.
Table 2 Thermodynamic parameters obtained from Logkd vs 1/T plot at pH1 and pH9.
Temperature is a critical parameter in sorption study, as it can directly affects the distribution ratio (kd) of the ion and finally kinetics of the sorption process. The estimation of thermodynamic parameters assists in getting insight into the adsorption mechanism of metal ions on various sorbents (Sheng et al., 2010; Zhao et al., 2010; Zhang et al., 2011). Similar approach is adopted in the present study to interpret the sorption mechanism of Nb on manganese dioxide. The temperature dependent variation of distribution ratio both acidic and basic pH is given in Table 1. This phenomenon can be utilized in Van't Hoff equation to deduce the thermodynamic parameters of the sorption process.
ΔS ΔH − 2.303R 2.303RT
(3)
The values of enthalpy, ΔH , and entropy, ΔS , are calculated from the slopes and intercepts of the log(Kd) vs.1/T plot (Fig. 9). Then The Gibbs free energy, ΔGo, of the sorption process is calculated from the equation: o
o
ΔG = ΔH-TΔS
ΔGo(kJM−1)
ΔHo(kJM−1)
ΔSo(JK−1M−1)
1.2 9
−10.9 −31.5
13.4 −29.84
88.42 1.9
explanation of the endothermic process is that Nb(V) ions are highly solvated in aqueous medium and before going to sorbed onto colloid surface, rupture of the hydration sphere occurred to some extent, and the process requires energy. It is also assumed that the energy required for the dehydration exceed the exothermicity of Nb(V) ions fixing on pyrolusite surface and make the whole process endothermic. The Gibbs free energy change (ΔG) is negative, indicates the process is spontaneous under experimental conditions. The positive values of entropy change (ΔS) reflect the increase of randomness at the solid/liquid interface during the sorption process and obviously the partial removal of water molecule due to the ion exchange process between the adsorbate and surface functional groups is the responsible factor (Yan-Hui et al., 2005). Slightly different observation of reduction in Kd at 75 °C was observed (Table 1) suggesting weak interaction between the Nb(V) and pyrolusite colloids. The decrease in Kd is exactly identical with the phenomenon of decreased physisorption at higher temperature. At pH 9, ΔHo is negative; specify the sorption process in exothermic. With increase of pH, the hydration sphere around Nb(V) get weaker as the oxo-hydroxy species dominates. This causes the reduction in energy required for rupture of very weak hydration sphere and at the same instant the energy liberated because of the chemical bond formation between Nb(V) hydroxy species and surface hydroxyl group of pyrolusite overcompensated and make the process exothermic. Although there are no hard and fast rule regarding the ΔHo values to interpret the sorption nature, the enthalpy of sorption values ranging from 2.1 to 20.9 kJ mol−1 (0.5–5 kcal mol−1) corresponds to physical sorption process and the values in the range of 20.9–418.4 kJ mol−1 (5.0 and 100 kcal mol−1) associated with the charge sharing or
3.6. Effect of temperature
log(K d ) =
pH
(4)
Where R is the universal gas constant (8.314Jmol−1K−1) and T is absolute temperature (K). The thermodynamic parameters (Table 2) help in understanding the mechanism concerning the adsorptive interaction of Nb(V) with Pyrolusite. The positive ΔHo value indicates the adsorption process is endothermic in acidic pH. One possible Table 1 kd values with associated uncertainty at pH1 and pH9 at different temperature. Temperature (K)
1/T (K−1)
kd (Lkg−1) (pH1)
Uncertainty
kd (Lkg−1) (pH 9)
Uncertainty
278 298 323 348
0.0035971 0.0033557 0.0030959 0.0028735
685 981 1210 1069
33 38 47 40
7065 3648 2008 1120
595 219 96 46
5
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Fig. 10. Pseudo first and second order fitting of kinetic data at (a) pH1 and (b) pH 9.
3.7. Effect of equilibration time
formation of coordinate/covalent bond between metal surfaces and metal ions (chemisorption) (Smith, 1981; Ozcan et al., 2005; Unl and Ersoz, 2006). The enthalpy change associated with sorption process at pH1 is significantly low (13.4 kJ mol−1), indicates the physisorption mechanism in acidic pH. At pH 9, the high enthalpy change (−29.8 kJmol1 ) indicating the chemisorption mechanism and analogous study was reported on thorium and uranium sorption on carbon nanotubes and natural zeolite respectively (Wang et al., 2011; Aytas et al., 2004). The error bars stand for calculated uncertainties resulted from (i) counting statistics (ii) volumetric operations and (iii) weighing with decreasing contributions, in the order mentioned. The standard error propagation method was employed to get the final uncertainties associated with the measured parameters. The uncertainty equation for % sorption (Equation (5)) and kd (Equation (6)) were derived based on equations (1) and (2).
U(% Sorption)
2 2 ⎡ UAo − Af ⎞ ⎛ U Ao ⎞ ⎤ = (% Sorption) ⎢ ⎜⎛ ⎟ + ⎥ A − Af ⎠ ⎝ Ao ⎠ ⎦ ⎣⎝ o ⎜
2
The kinetic modeling of the time dependent sorption data was studied using Lagergren's pseudo first order (Lagergren, 1898) and Ho's pseudo second order model (Ho, 1995). The Lagergren equation based on pseudo-first-order adsorption kinetics is expressed as follows:
dq = k1 (qe − qt ) dt
(7)
Where, qe is the amount of adsorbate adsorbed at equilibrium (μg/ mg), qt is the adsorbed adsorbate amount at time t (μg/mg) and k1 is the pseudo-first-order rate constant (h−1). The equation simplifies to
Log (q e − qt) = Logq e −
k1t 2.303
(8)
Ho's pseudo second order model expressed as
dq = k2 (qe − qt )2 dt
(9)
⎟
After integration from t = 0 to t = t, the equation simplifies to
(5)
t 1 t = + qt qe k2q 2e
2
UA UV 2 U 2 ⎡ UAo − Af ⎞ ⎞ + ⎜⎛ f ⎟⎞ + ⎛ W ⎞ ⎤ Ukd = kd ⎢ ⎜⎛ ⎟ + ⎛ ⎥ A − A V A ⎝ ⎠ ⎝W ⎠ ⎦ f⎠ ⎝ f ⎠ ⎣⎝ o
(10)
The observations of time dependent Nb(V) sorption on pyrolusite both in acidic (pH1) and basic mediua (pH9) are shown in Fig. 10. In acidic pH the kinetic data best fitted when we follow pseudo first order kinetics (regression coefficient: 0.9812) rather than pseudo second order model (regression coefficient: 0.9586). This indicates in acid medium, the sorption of Nb(V) on pyrolusite follows pseudo first order
(6)
where U represents the uncertainty in the respective parameter. As the pH changed from 1 to 12, the relative uncertainties for % sorption and kd varied 3–6 % and 4–8 % respectively. 6
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Fig. 11. Triple layer model for the sorption of Nb(V) on pyrolusite.
kinetics and the sorption occurring predominantly via physisorption or the outersphere complexation predominant. But the kinetic sorption data for basic medium is well fitted with pseudo second order model (regression coefficient: 0.9998) rather than pseudo first order model (regression coefficient: 0.7779). This infers that chemisorption is the predominant mechanism of sorption at pH 9. Based on the ionic strength, temperature dependent and kinetic sorption data, the probable mechanism of sorption of Nb(V) on pyrolusite is shown in schematic manner in Fig. 11. In acidic pH, the sorption is mainly due to physisorption or ion exchange type phenomenon whereas in neutral and basic medium, there is utmost chance for the formation of covalent bond between Nb and pyrolusite surface. We have tried to model the Nb sorption on manganese dioxide using FITEQEL software. However the inclusion of Niobium species in the model could not be done especially at low pH because of the lack of data regarding the hydrolysis constant of NbO2+ species which is supposed to predominate at the lower pH conditions and the dissociation constant of the surface hydroxyl groups of manganese dioxide. 4. Conclusion The knowledge that can be gathered from the above sorption study are (1) The sorption of Nb(V) on pyrolusite is quantitative in neutral to near neutral pH region. (2) In low pH mineral bound humic acid enhances the sorption of Nb(V) and in higher pH dissolved humic acid complexes with Nb(V) and reduced the sorption. (3) As the sorption strong depend on ionic strength, the sorption follow outer sphere complexation in low pH but mild dependent in basic pH reinforce the probability of innersphere complexation (4) The positive entropy change of the sorption process in acidic pH make the endothermic sorption process spontaneous. (5) Pseudo first order fitting in low pH suggests the high probability of physisorption whereas pseudo second order kinetic fitting proposes the probability of chemisorption mechanism at high pH. Declaration There is no conflict of interest regarding the manuscript. Acknowledgements Authors thank Dr. H Pal, Associate Director (A), Chemistry Group and Head, Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India for his support. 7
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