Shuquan Chang*,
Heliang Fu,
Xian Wu,
Chengcheng Liu,
Zheng Li,
Yaodong Dai and
Haiqian Zhang
Jiangsu Engineering Laboratory of Nuclear Energy Equipment Materials, College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China. E-mail: chsq@nuaa.edu.cn; Tel: +86-25-52112903
First published on 29th October 2018
In this work, compressible Prussian blue/polyurethane sponges (PB@PUS) for selective removal of cesium ions were prepared via an in situ radiation chemical route. The characterization results indicate that uniform PB nanoparticles were successfully synthesized and well dispersed on the porous skeleton of sponge. Batch and fixed-bed column experiments were detailedly conducted to investigate their adsorption performances. Batch adsorption experiments reveal that PB@PUS exhibited good selective removal property for cesium ions in a wide range of pH, whose maximal adsorption capacity and removal efficiency reached 68.6 mg g−1 and 99%, respectively. The adsorption processes could be described by the Langmuir isotherm adsorption model and pseudo-second-order adsorption kinetic model. The fixed-bed column experiments show that the breakthrough and exhaustion time obviously increased with the decrease of flow rate and initial cesium ions concentration. The breakthrough curves could be well fitted by the Thomas model and Yoon–Nelson model. The theoretical saturated adsorption capacity of PB@PUS-3 calculated from the Thomas model was 68.2 mg g−1. The as-prepared samples were light, stable and compressible, which can be applied in radioactive wastewater treatment.
Many physico-chemical methods have been developed to treat radioactive wastewater, including evaporative concentration, solvent extraction, membrane separation, electrodialysis, ion exchange and adsorption etc.4–6 Among these methods, adsorption is simple, efficient and economical, which has been widely applied in radioactive wastewater treatment.3 A variety of adsorbents, such as zeolites, clays, permutite, activated carbon, carbon nanotubes, chitosan, alginate, cellulose and their composites, have been employed to remove cesium ions from wastewater.7–12 Transition metal ferrocyanides such as iron ferrocyanide (also known as Prussian blue, PB) have special lattice structure and can exchange metal ions with cesium ions.13 They have been widely applied in the adsorption of cesium ions from radioactive wastewater due to their low cost, non-toxicity, good stability and high selectivity.14–19 A commercial product CsTreat®, which is a kind of transition metal hexacyanoferrate ion exchanger, has been widely applied in nuclear power plants for radioactive cesium separation.20,21 The fixed-bed column is a kind of simple and efficient equipment for continuous adsorption, which has been widely used in wastewater treatment. However, traditional PB powders are difficult to be applied in fixed-bed columns due to the following reasons: (1) they are easy to be released from the columns and result in secondary contamination; (2) they cannot be completely separated from the treated solution using either filtration or centrifugation; (3) their contact area will be reduced because of the aggregation; (4) they cannot ensure the smooth flow of solution among adsorbents. In order to solve these problems, PB/GO/PVA–alginate hydrogel beads, potassium copper hexacyanoferrate/cellulose hydrogel, PB immobilized magnetic hydrogel, PB/cellulose fiber, PB/diatomite/carbon nanotubes spongiform, PB/chitosan/rayon fibers, porous three-dimensional graphene foam/PB composite, PB/PVA composite nanofiber, granulated copper hexacyanoferrate porous networks have been successfully fabricated and applied to remove cesium from wastewater.22–32 Modifying PB particles in bulk materials with porous structures can obviously improve their adsorption performances, which also need to be further developed according to the actual demand. In previous studies, batch adsorption experiments were usually carried out to characterize the adsorption properties of as prepared PB composites. However, their adsorption behaviour in practical application cannot be properly reflected by batch experiments.
In this work, Prussian blue/polyurethane sponge complex adsorption materials for selective removal of cesium ions were proposed and prepared via an in situ radiation chemical route (Fig. 1). Their morphology and structures were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Fourier transform infrared spectrometer (FT-IR). Batch experiments and fixed-bed column experiments were carried out to investigate their adsorption performances, including adsorption capacity, cesium removal efficiency, adsorption isotherms, adsorption kinetics, breakthrough curves and the influences of pH, co-existing ions, cesium ions concentration, adsorbent amount, flow rate etc. The fabrication strategy and adsorption mechanism were also discussed in detail. Compared with traditional PB composites, the as prepared PB/polyurethane sponge composites had obvious porous structures, high adsorption capacity and good compressibility, which could be conveniently stored, efficiently used and easily post-treated in wastewater treatment.
![]() | (1) |
![]() | (2) |
In order to calculate their adsorption isotherms, 0.2 g adsorbent (pure PUS, PB@PUS-1, PB@PUS-2 or PB@PUS-3) was put into 25 mL Cs+ solution at a series of initial concentrations (5, 100, 200, 400, 600, 800, 1000, 1500, 2000 mg L−1) and kept for 3 h at 25 °C. The pH of solution was 7. Then, adsorbents were separated from solution using tweezers. After that, Cs+ concentration in the solution was tested.
In kinetics experiments, 0.2 g adsorbent was put into 25 mL 1000 mg L−1 Cs+ solution and kept for different time (5 min, 15 min, 30 min, 45 min, 1 h, 1.5 h, 2.5 h, 4 h, 5 h and 6 h) at 25 °C. After the adsorbent was separated, the Cs+ concentration in the solution was tested.
Different experiments were performed to investigate the influences of pH, adsorbent amount and competing ions in Cs+ solution. The pH of Cs+ solution was adjusted to 3, 5, 7, 9 and 11 with 0.1 M HCl and NaOH. The amount of adsorbents in the total volume of Cs+ solution was set to 2, 4, 6, 8 and 10 mg mL−1. In order to study their selective adsorption behaviour, adsorbents were put in the mixed solution containing Cs+ and competing ions (Na+, K+, Mg2+ or Ca2+) with the same concentration (1000 mg L−1). Unless specified otherwise, the adsorption conditions are the same as that mentioned above (Cs+ concentration: 1000 mg L−1; adsorbent amount: 8 mg mL−1; contact time: 3 h; temperature: 25 °C; pH: 7).
Three independent experiments were carried for each experiment. Data represent “mean ± SD (significant difference)”.
The loading behaviour of samples into the fixed-bed column was expressed in terms of the normalized concentration Ct/C0 (where C0 and Ct are the inlet Cs+ concentration and outlet Cs+ concentration at time t, respectively) as a function of time (t) for a given bed height, giving a breakthrough curve. When the column was exhaust, the total effluent volume (Veff, mL) was calculated from the following equation:
Veff = νttotal | (3) |
For a given feed concentration and flow rate, the area under the breakthrough curve can be obtained by integrating the adsorbed concentration (C0 − Ct) versus t (min) plot. The total adsorption amount qtotal (mg) was obtained from the following equation:
![]() | (4) |
The equilibrium Cs+ maximum capacity (qs, mg g−1) of the column was calculated as the following:
![]() | (5) |
Total amount of cesium ions entering column (mtotal, g) was calculated from the following equation:
![]() | (6) |
The Cs+ removal percentage (Rtotal, %) for the fixed-bed column at saturation was calculated as the following:
![]() | (7) |
The breakthrough adsorption capacity (qb, mg g−1) at the time of breakthrough (tb, min) was determined using the same computational method as that of qs.
The X-ray diffraction patterns of pure PU and PB/PU sponges fabricated on different conditions were shown in Fig. 3A. Compared with pure PU sponges, PB/PU sponges have several new diffraction peaks at around 17.5°, 24.8°, 35.4°, 39.7°, 43.8° and 51.0°, which correspond to the (200), (220), (400), (420), (422) and (440) planes of Prussian blue (JCPDS 73-0687).33 The FT-IR spectra of samples are shown in Fig. 3B. The peaks at around 3278 cm−1, 2973 and 2869 cm−1, 1720 cm−1, 1638 cm−1, 1538 cm−1, 1224 cm−1, 1101 cm−1 are separately ascribed to O–H and N–H stretching, C–H stretching absorption bands, CO stretching, O–H bending vibration, C–H stretching vibrations, C–N stretching vibrations, C–O–C stretching vibrations.34 Compared with pure PU sponge, a strong peak at around 2083 cm−1 appears in PB/PU sponges, which is the characteristic peak of the C
N stretching vibration in Prussian blue.35 XRD and FT-IR results reveal that the nanoparticles on the surface of porous skeletons are Prussian blue. They also indicate that the structures of PU sponges are not obviously damaged under irradiation.
![]() | (8) |
Qe = KfCe1/n | (9) |
The adsorption isotherms and parameters fitting with the Langmuir and Freundlich models are given in Fig. 4 and Table 1.
Adsorbent | Langmuir model | Freundlich model | ||||
---|---|---|---|---|---|---|
Qm (mg g−1) | KL (L mg−1) | R2 | Kf (mg1−1/n L−1/n g−1) | 1/n | R2 | |
Pure PUS | 2.49 | 0.00726 | 0.994 | 0.225 | 0.340 | 0.986 |
PB@PUS-1 | 34.9 | 0.00143 | 0.999 | 0.508 | 0.528 | 0.984 |
PB@PUS-2 | 49.2 | 0.00182 | 0.999 | 0.969 | 0.498 | 0.976 |
PB@PUS-3 | 68.6 | 0.00221 | 0.998 | 1.64 | 0.481 | 0.965 |
Between the two isotherm models, the Langmuir model fits the adsorption data better than the Freundlich model, which can be confirmed by the correlation coefficients. Above results indicate that adsorption sites in two samples are uniform and belong to single layer adsorption. According to the Langmuir model, the maximal adsorption capacity of PB@PUS-3 is 68.6 mg g−1, which is almost twenty eight times compared to that of pure PU sponge (2.49 mg g−1). The excellent adsorption property of PB/PU sponge is attributed to the synergetic effects between PB and PU sponge. PB nanoparticles have special lattice and can exchange its potassium ions with cesium ions, which provide numerous specific adsorption sites for cesium ions. PU skeletons of sponge prevent the aggregation of PB nanoparticles, increase the specific surface area and provide enough channels for the solution, which increase the contact opportunity between PB and cesium ions.
In order to ascertain the adsorption equilibrium time for Cs+, the adsorption capacity of samples under different adsorption time was tested and shown in Fig. 5. The adsorption process can be divided into two stages. In the first stage, the adsorption processes increase rapidly. Its adsorption capacity can reach 17.5 mg g−1 within 30 min. Then, adsorption gradually increases and reaches equilibrium in 2.5 h. After that, there is no significant adsorption for Cs+. To describe the adsorption kinetics, the pseudo-first-order adsorption kinetic model (eqn (10)) and pseudo-second-order adsorption kinetic model (eqn (11)) were employed to fit the experimental data.
qt = qe(1 − e−k1t) | (10) |
![]() | (11) |
![]() | ||
Fig. 5 Adsorption kinetic curve of PB@PUS-2 sample. Data represent “mean ± SD” from three independent experiments. |
The adsorption kinetic parameters fitting with pseudo-first-order and pseudo-second-order models are shown in Table 2 and Fig. 5. The results reveal that the pseudo-second-order model (R2 = 0.999) fits the adsorption data better than pseudo-first-order model (R2 = 0.976). Adsorption processes of PB/PU sponge might be controlled by chemical interactions, which include the ions exchange between Cs+ and K+ in PB.37
Adsorbent | Pseudo-first-order equation | Pseudo-second-order equation | ||||
---|---|---|---|---|---|---|
qe (mg g−1) | k1 (min−1) | R2 | qe (mg g−1) | k2 (g mg−1 min−1) | R2 | |
PB@PUS-2 | 21.0 | 4.62 | 0.976 | 22.5 | 0.322 | 0.999 |
The Cs+ removal efficiency is related to the amount of adsorbent in the solution. Thus, they were tested under different adsorbent amount and given in Fig. 6A. The Cs+ removal efficiency is obviously raised as the increase of adsorbent amount. When adsorbent amount is 6 mg mL−1, the Cs+ removal efficiency can reach 96%. When the amount of adsorbent is 8 mg mL−1, the Cs+ removal efficiency is more than 99%. The pH of solution is considered as an important factor to affect the Cs+ adsorption performances. Herein, the Cs+ removal efficiency of PB@PUS-2 at different pH was tested and shown in Fig. 6B. When pH is 7the Cs+ removal efficiency is the highest and reaches 99.9%. The Cs+ removal efficiency of PB@PUS-2 at pH 5–11 is more than 93%, which verifies that it can be applied in a wide range of pH values. The effect of common co-existing ions in seawater (K+, Na+, Ca2+ and Mg2+) was investigated and shown in Fig. 6C. The results reveal that co-existing metal ions can affect the adsorption capacity of PB@PUS-2 to some extent. The Cs+ removal efficiencies in the presence of Mg2+, Ca2+, Na+ and K+ are 97.0%, 91.3%, 91.1% and 85.7%, respectively. The influence of competing cations may be related to the ionic radius, hydrated ionic radius, charge-radius and electronegativity. In general, PB@PUS-2 sample has highly selective adsorption ability for Cs+. As is shown in the inset of Fig. 5, PB-induced blue colour and adsorbent fragments-induced murky did not appear during the sorption process. After adsorption, adsorbents were removed from the solution. The remaining solution was near colourless and transparent. There was no obvious suspended matter and precipitation. Neither PB particles nor PU fragments was released from bulk sponge during adsorption and separation processes, which indicate that PB/PU sponge has good stability.
![]() | ||
Fig. 6 Cs+ removal efficiency of PB@PUS-2 sample under different adsorbent amount (A), pH (B) and competing cations (C). Data represent “mean ± SD” from three independent experiments. |
Adsorbent | Experimental conditions | Experimental parameters of breakthrough curves | |||||||
---|---|---|---|---|---|---|---|---|---|
C0 (mg L−1) | v (mL min−1) | H (cm) | tb (h) | ts (h) | Veff (mL) | qb (mg g−1) | qs (mg g−1) | Rtotal (%) | |
a C0 = influent concentration (mg L−1), v = flow rate (mL min−1), H = bed height (cm), tb = breakthrough time (h), ts = saturation time (h), Veff = effluent volume (mL), qb = adsorption at breakthrough (mg g−1), qs = adsorption at saturation (mg g−1), Rtotal = total cesium removal at saturation (%). | |||||||||
PB@PUS-1 | 10 | 3 | 10 | 9 | 32 | 5760 | 15.4 | 32.2 | 55.9 |
PB@PUS-2 | 10 | 3 | 10 | 18 | 47 | 8460 | 30.8 | 51.8 | 61.2 |
PB@PUS-3 | 10 | 3 | 10 | 28.5 | 52 | 9360 | 48.6 | 67.0 | 71.6 |
PB@PUS-2 | 20 | 3 | 10 | 8 | 24 | 4320 | 27.4 | 49.2 | 57.0 |
PB@PUS-2 | 30 | 3 | 10 | 5 | 17 | 3060 | 25.1 | 50.6 | 55.2 |
PB@PUS-2 | 10 | 4 | 10 | 12.5 | 35 | 8400 | 28.5 | 50.3 | 59.9 |
PB@PUS-2 | 10 | 5 | 10 | 9.5 | 27 | 8100 | 26.8 | 48.9 | 58.1 |
The breakthrough curves can be used to predict the efficiency of fixed-bed column system. Therefore, breakthrough curves under different conditions were analyzed and fitted using the Thomas model and Yoon–Nelson mode.38 The Thomas model is one of the most widely used breakthrough models for predicting breakthrough curves, which assumes Langmuir kinetics for adsorption and desorption with no axial dispersion. The Yoon–Nelson model is based on the assumption that the decrease in the probability of each adsorbate to be adsorbed is proportional to the probability of its adsorption and breakthrough on the adsorbent.39 The Thomas model and Yoon–Nelson model can be expressed as the eqn (12) and (13), respectively.
![]() | (12) |
![]() | (13) |
The curves and parameters fitting with Thomas and Yoon–Nelson models are given in Fig. 7 and Table 4. The correlation coefficients of Thomas and Yoon–Nelson models are high, which indicate that they are able to describe the dynamic behaviour in the fixed-bed column. As the initial cesium concentration increases, the value of kTH decreases because the driving force for adsorption is the concentration difference of cesium ions between adsorbents and solution. The value of kTH increases with the increase of the flow rate. According to the Thomas model, the theoretical saturated adsorption capacities of three samples are 33.7 mg g−1 (PB@PUS-1), 52.0 mg g−1 (PB@PUS-2) and 68.2 mg g−1 (PB@PUS-3), respectively. They are consistent with the results of static adsorption experiments.
Adsorbent | C0 (mg L−1) | v (mL min−1) | H (cm) | Thomas model | Yoon–Nelson model | |||
---|---|---|---|---|---|---|---|---|
kTH (mL mg−1 min−1) | qe (mg g−1) | kYN (h−1) | τ (h) | R2 | ||||
PB@PUS-1 | 10 | 3 | 10 | 0.496 | 33.7 | 0.298 | 18.7 | 0.997 |
PB@PUS-2 | 10 | 3 | 10 | 0.451 | 52.0 | 0.271 | 28.9 | 0.996 |
PB@PUS-3 | 10 | 3 | 10 | 0.514 | 68.2 | 0.308 | 37.9 | 0.998 |
PB@PUS-2 | 20 | 3 | 10 | 0.391 | 51.7 | 0.469 | 14.4 | 0.998 |
PB@PUS-2 | 30 | 3 | 10 | 0.341 | 52.2 | 0.613 | 9.66 | 0.998 |
PB@PUS-2 | 10 | 4 | 10 | 0.550 | 51.5 | 0.331 | 21.4 | 0.999 |
PB@PUS-2 | 10 | 5 | 10 | 0.757 | 52.4 | 0.454 | 17.5 | 0.998 |
This journal is © The Royal Society of Chemistry 2018 |