Remediation of 137Cs radionuclide in Nuclear Waste Effluents by Polymer Composite: Adsorption

kinetics, isotherms and radiation hardness studies

M.A. Olatunji1, M.U. Khandaker1, H.N.M.E. Mahmud2

1Radiation Lab, Department of Physics, University of Malaya,

2Polymer Lab, Department of Chemistry, University of Malaya,

50603 Kuala Lumpur, Malaysia

6th Asia-Pacific Symposium on Radiochemistry 2017 Sunday 17 September – 22 Friday 2017

ICC Jeju. Jeju Island, Korea

• Nuclear technologies appear to be a significant breakthrough towards achieving a better quality of life.

• However, the increasing use of nuclear materials produces huge amount of radioactive wastes, and creates a significant cause of pollution to water bodies across the globe.

• The old practice of keeping radioactive wastes until their decay to stable forms is not very convenient approach.

• Therefore, removal of radioactive materials from waste effluent is the only viable approach to prevent the damaging effect of radiation exposures to living beings

Introduction

Why studied the removal of radioactive 137Cs?

A considerable amount of 137Cs is dispersed into the environment as waste effluents from the use of radioisotopes in medicine, industry and also as a fission product.

Because of its acute toxicity, non-biodegradability and relatively long half-life, 137Cs creates serious problems once ingested into the human body via various routes.

Therefore, it is important to remove 137Cs from the aqueous nuclear wastes before its disposal to the environment.

Current removal techniques

• Many methods such as chemical precipitation, osmosis, adsorption etc. can be used for their removal.

• Adsorption is the most efficient due to its effectiveness

in treating low level radioactive waste solution, low-cost, and the simplicity of operation.

• Many adsorbents are available but conducting polymers such as polypyrrole, polyaniline etc. have assumed a significant role, due to low-cost, abundance, facile synthesis and presence of reactive functional groups i.e. O, S and N donor atoms.

Work done under this study

• Synthesis, Characterization and Investigation of surfactant-doped polypyrrole/activated carbon (PPy/SDBS/AC) composite as an adsorbent for the removal of 137Cs radioisotope.

• Evaluation of the effect of various parameters such as contact time, adsorbent dosage, initial ion concentration and temperature on the adsorption.

• Adsorption kinetics, isotherms and behaviour of adsorbents under gamma irradiation were also studied here in.

Materials and Method: Synthesis of PPy/SDBS/AC composite

Characterization of AC and as prepared composite-FESEM

• FESEM images: PPy shows its characteristic globule-like structure, while a stronger compactness and more macroporous structural surface like honey-comb were observed on the PPy/SDBS/AC when compared with AC, which shows that PPy was formed on the AC particles and bound them together.

Pure AC

PPy/SDBS/AC Pure PPy

• Figure represents the nitrogen adsorption-desorption isotherm for PPy and PPy/SDBS/AC composite. It shows N2 adsorption by PPy is low while PPy/SDBS/AC shows higher adsorption at low relative pressure. This indicates the presence of micropores in the PPy/SDBS/AC composite. The insets represent pore size distribution which also show to be suitable for the Cs ion adsorption as most metal ions pore sizes are less than this range.

• BET analyses showed that the specific surface area increased from 10.57 m2/g for PPy to

793 m2/g for PPy/SDBS/AC composite. The binding of the AC microparticles by the polymer increased the surface area of the polymer composite.

Characterization of as prepared composite-BET

Pure PPy Pure PPy/SDBS/AC

• Figure represents the X-ray diffraction patterns for PPy and PPy/SDBS/AC composite. • XRD pattern of PPy reveals that there was not any detectable crystalline phase. The

amorphous peak centered at 2θ = 26.12o is as a result of the scattering from polymer chains at the inter-planar spacing. After modifying with AC, amorphous peak shifted to 2θ = 24o and another weak peak at 2θ = 44.11o which confirms the successful formation of PPy within AC particles with the aid of sulfonate dopant ions.

Characterization of as prepared composite-XRD

Batch adsorption experiments 0.2 g of PPy or PPy/SDBS/AC

composite

50 mL radioactive solution (20-

100mg/l)

Mixture shaking for 6 h at 20 OC and at 200

rpm

Filtrate separated by filtration and

radioactivity measured

Removal efficiency calculated

Measurements and Evaluation of various parameters

Ao- initial radioactivity Ae- final radioactivity N- net count T- counting time εγ- detection efficiency Iγ- fraction of γ-ray per disintegration V- volume of solution

C- initial Cs ion concentration m- mass of PPy or PPy/SDBS/AC

qt- adsorption capacity at time ‘t’ qe- adsorption capacity at equilibrium k1- pseudo first order constant k2- pseudo second order constant

Adsorption kinetic studies

Activity calculations

qe- adsorption capacity at equilibrium Ce- equilibrium/ final concentration qm- maximum monolayer adsorption capacity KF and n- Freundlich constants, related to adsorption capacity and adsorption intensity, respectively

Adsorption isotherm studies

Measurements and Evaluation of various parameters

R- universal gas constant (8.314 Jmol/K) ∆Ho- enthalpy change ∆So- entropy change ∆Go- free Gibbs energy change T- absolute temperature (293-323 K)

Adsorption thermodynamics studies

Results and Discussion

Results-Effect of PPy/SDBS/AC dosage on removal efficiency

• An increase in dosage from 0.05 to 0.15 g leads to a rapid increase in removal efficiency from 54.1 % to 93.4 % for PPy/SDBS/AC but steadily from 15 % to 40.6 % for PPy. The lower removal efficiency by PPy is due to its poor water dispersion. Equilibrium adsorption was reached with 0.2 g with 97 % removal by composite and 42 % by PPy.

• However, a further increase in dosage above 0.2 g has an insignificant effect on removal efficiency, due to the decrease in Co ion concentration in the solution with respect to higher amount of polymer composite.

Results-Effect of contact time on removal efficiency

Figure shows that an increase in the contact time up to 120 min increases the interaction of Cs with the adsorbent surface due to strong electrostatic interaction between the sorbent surface and metal ions as well as more available sorption sites. Further increase in time slows the saturation of adsorption at 200 min when no significant adsorption was noticed, because the sorbent surface has been occupied in case of PPy and/ or reduction in the adsorbate ions in the aqueous phase in case of PPy/SDBS/AC.

Results-Effect of contact time on removal efficiency

Figure shows the fitting of kinetic data to the pseudo-first-order and pseudo-second-order models

Kinetic models Parameter PPy PPy/SDBS/AC

Adsorption capacity, qexp (mg/g) 2.40 4.83 PFO qcal (mg/g) 2.39 4.79

Constant, k1 (/min) 0.03 0.027 R2 0.924 0.983 χ2 0.03 0.029

PSO qcal (mg/g) 2.61 5.27 k2 (g/mg/min) 0.02 0.008 R2 0.912 0.979

χ2 0.03 0.035

Results-Table represents the adsorption kinetic parameters.

• As shown, the pseudo first order can best describe the adsorption data, due to the close relationship between the kinetic data and theoretical results as well as the higher R2 and lower χ2 compared to that of PSO.

• This implies that the adsorption of Cs by the polymer composite occurs due to physisorption involving the weak intermolecular forces (such as Van der Waals).

• The consequent of this interaction is that the adsorbed Cs can be recovered from the sorbent without loosing its identity by desorption.

Results-Effect of initial concentration on removal efficiency

Figure shows an increase in adsorption capacity due to the fact that as Cs ion concentrations increased in solution, effective interactions between Cs ions and surface functionalities of the adsorbents increased. On the other hand, increase in initial concentration shows a decrease in the removal efficiency due to reduction in available sorption sites at higher Cs ion concentration. PPy surface was saturated with 65 mg/L Cs concentration.

PPy PPy/SDBS/AC

Results-Table represents adsorption isotherm model parameters

Langmuir isotherm Freundlich isotherm

Adsorption capacity, qm (mg/g) 23.01 kF (L/mg) 6.67 b (L/mg) 0.27 n 2.75

R2 0.990 R2 0.948

χ2 0.43 χ2 2.15

• Langmuir model fits well.

• This indicates complete monolayer coverage of Cs onto sorbent surface and also a homogeneous distribution of active sites on the adsorbent, because the Langmuir equation assumes that the surface is homogeneous.

Langmuir isotherm Freundlich isotherm

Adsorption capacity, qm (mg/g) 6.40 kF (L/mg) 1.23 b (L/mg) 0.06 n 2.98

R2 0.956 R2 0.818

χ2 0.07 χ2 0.30

• Separation factor, RL estimated lies within 0.04-0.47 and also n < 10 in both cases. This shows adsorption process of Cs by PPy and PPy/SDBS/AC is favourable.

PPy

PPy/SDBS/AC

Results-Effect of Temperature on removal efficiency

Table represents the dependency of adsorption process on temperature. As shown, increase in temperature results to steady rise in removal efficiency and also shows that binding affinity of sorbent surface to Cs ion increases. In both cases, the thermodynamic parameters reveal the process is feasible, endothermic and spontaneous in nature. The value of ∆H0 < 40 kJ indicates physical sorption, supporting the fitting of kinetic data to the pseudo-first-order model.

Temp (K)

R (%) Kd (L/g) ∆G0 (kJmol-1)

∆H0 (kJmol-1)

∆S0 (Jmol-1)

PPy 293 42 0.21 3.84 4.46 2.11 303 44 0.22 3.82 313 45.3 0.23 3.80 323 46.8 0.24 3.77

PPy/SDBS/AC 293 95 4.78 -3.76 36.5 137 303 96.75 7.47 -5.13 313 98 12.3 -6.51 323 99 19 -7.88

Effect of gamma irradiation on the surface properties of PPy and PPy/SDBS/AC sample

• gamma irradiation shows practically no significant change in the surface functional groups of pure PPy

• Gamma irradiation results to self-organized structure formation in PPy/SDBS/AC • Surface functional group shifts is noticed in PPy, while PPy/SDBS/AC shows formation

of new surface groups • Further studies still ongoing.

(a)

C-H symmetric & asymmetric in CH2 & CH3 aliphatic

O-H stretch in pyrroles

C=C

C-H in-plane and out-of-

plane deformations

C-O stretch

(b)

C-H symmetric & asymmetric in CH2 & CH3 aliphatic

O-H stretch in pyrroles

C=C ring stretch

SiO2 symmetric and asymmetric stretch in sulfonates

C-H in-plane and out-of-

plane deformations

C-O stretch

Conclusions • We have successfully synthesized SDBS-doped polypyrrole/activated carbon

(PPy/SDBS/AC) composite via oxidative polymerization

• The as-prepared polymer composite was investigated for adsorption of radioactive cesium with equilibrium adsorption being achieved at 240 min contact time, 0.2 g sorbent dosage, and within 20-100 mg/L initial Cs concentration.

• Adsorption data was best reproduced by the pseudo-first-order kinetic model, which suggests possible recovery of adsorbed Cs without loosing its identity by desorption.

• The maximum monolayer adsorption capacity estimated by the Langmuir isotherm model was 23 mg/g using PPy/SDBS/AC higher than 6.40 mg/g obtained by using PPy, revealing that the surface of the composite is capable of adsorption of 137Cs from aqueous phase.

• The adsorption process showed to be endothermic in nature, spontaneous and feasible within the temperature range of 293 – 323 K.

• The effect of gamma radiation within 200 kGy revealed an improve ordered system in the composite while the pure polypyrrole remain unchanged after irradiation. The study on the effect of gamma radiation of the adsorbents on the sorption capacity is currently ongoing.

• The study suggests that the as-prepared adsorbent can be given consideration in practical nuclear effluents treatment.

Thanks for your kind attention!

Acknowledgements

The author (M. A. Olatunji) is grateful to the University of Malaya for providing a partial support to present this work in the APSORC17.