Patent Application
20140323685
The present invention relates to wastewater treatment, and particularly to a cross-linked polyzwitterion/anion for the removal of strontium from aqueous solutions.
Radioactive wastes containing a variety of radionuclides severely pollute the environment. Fission product like strontium 89Sr and 90Sr with half-lives of 51 days and 29 years, respectively, can contaminate underlying layers of soil and groundwater. Strontium's resemblance to calcium enables its easy incorporation into bone, and by virtue of being a very strong β-emitter, it leads to the development of bone sarcoma and leukemia. It is, thus, quite necessary to remove strontium, one of the most hazardous elements, from wastewater.
The literature describes many sorbent materials to remove larger concentrations of strontium ions. However, it remains challenging to develop new materials for removing strontium ions at ppb-levels. Thus, a cross-linked polyzwitterion/anion for the removal of strontium from aqueous solutions solving the aforementioned problems is desired.
The cross-linked polyzwitterion/anion for the removal of strontium from aqueous solutions is a polyzwitterion having the following structure:
and the corresponding anion formed by treating the polyzwitterion with a base, e.g., sodium hydroxide. The cross-linked polyzwitterion/anion (CPZA) was prepared using Butler's cyclopolymerization protocol. The CPZA resin was found to have a very good adsorption capacity for Sr2+ ions at low concentrations. The relatively strong rapid initial adsorption of 83% Sr2+ ions was followed by slower adsorption of the remaining 17%, which was described by an intraparticle diffusion model. The adsorption followed the Lagergren second-order kinetic model, and Temkin as well as Freundlich isotherm models. The negative ΔGs and ΔH ensured the spontaneity and the exothermic nature of the adsorption process. The excellent adsorption and desorption efficiencies implied the efficacy of the resin in removing (as well as recovering) the metal ions from aqueous solution.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The cross-linked polyzwitterion/anion for the removal of strontium from aqueous solutions is a polyzwitterion having the following structure:
and the corresponding anion formed by treating the polyzwitterion with a base, e.g., sodium hydroxide, having the following structure:
The cross-linked polyzwitterion/anion (CPZA) was prepared using Butler's cyclopolymerization protocol. The CPZA resin was found to have a very good adsorption capacity for Sr2+ ions at low concentrations. The relatively strong rapid initial adsorption of 83% Sr2+ ions was followed by slower adsorption of the remaining 17%, which was described by an intraparticle diffusion model. The adsorption followed the Lagergren second-order kinetic model, and Temkin as well as Freundlich isotherm models. The negative ΔGs and ΔH ensured the spontaneity and the exothermic nature of the adsorption process. The excellent adsorption and desorption efficiencies implied the efficacy of the resin in removing (as well as recovering) the metal ions from aqueous solution.
Butler's cyclopolymerization protocol has been utilized to synthesize the current CPZA resin 5 (
Zwitterionic ester 1, which was prepared by reacting an equimolar mixture of N,N-diallyl-N-carboethoxymethylamine and 1,3-propanesultone in acetonitrile, and on acidic hydrolysis, afforded the zwitterionic acid 2 in excellent yield (95%), as shown in
A mixture of monomer 2 (92.5 mol %), cross-linker 3 (7.5 mol %), and SO2 (100 mol %) in DMSO solvent underwent cyclo-copolymerization in the presence of an initiator (AIBN) to give cross-linked polyzwitterion (CPZ 4) as a white solid. CPZ 4, upon treatment with excess NaOH, led to the formation of the present cross-linked polyzwitterion anion CPZA 5. To the best of our knowledge, this is the first cross-linked polymer via Butler's cyclopolymerization protocol that ensured the embedding of the zwitterionic as well as anionic motifs in each repeating unit. Elemental analysis of CPZ 4 confirmed the incorporation of monomer 2 and cross-linker 3 into CPZ 4 in an approximate molar ratio of 93:7, which is similar to the feed ratio.
The thermogravimetric analysis (TGA) curve of CPZA 5, shown in
The IR spectrum of zwitterionic monomer 2 indicates the presence of sulfonate group by its strong characteristic bands at 1197 cm−1 and 1040 cm−, while the absorption at 1728 cm−1 is attributed to C═O stretch of COOH. The corresponding absorptions for the CPZ 4 appear at 1212, 1041, and 1741 cm−1, respectively (
For the unloaded resin CPZA 5, the absorption due to C═O stretch of COOH was missing. Instead, the symmetric and antisymmetric stretching of COO− appeared at 1407 and 1631 cm−1 (
The effect of pH (in the range 2.5-9) on the uptake of Sr2+ was investigated at a fixed concentration (1 mg L−1) and time of 24 h at 23° C. Results of metal uptake at different pH are shown in
As shown in
where C0 and Cf are the initial and final concentrations of Sr2+ ions in mg L−1, respectively, W is the weight of the polymer in g, and V is the volume of the solution in L. Data presented are the average of three runs, and varied by less than 4% in all the cases studied.
The adsorption kinetics, which describes the relationship between adsorption capacity and adsorption time, is presented in
Lagergren first-order kinetics describes the adsorption process in a solid-liquid system based on the adsorption capacity of the solid, where it assumes that one metal ion is adsorbed onto one adsorption site on the surface of the adsorbent. The linear form of the model can be described in the following equation (Eq. 2):
where qe and qt (mg g−1) are the adsorption capacities at equilibrium and at time t (h), respectively, and k1 is the first-order rate constant. The k1 and qe were evaluated experimentally using the slope and intercept of the plots of log(qe−qt) versus t (
The linear Lagergren second-order kinetic model can be expressed by the following equation (3):
where k2 is second-order rate constant, and qt and qe are the respective adsorption capacities of the metal ions at a time t and at equilibrium e.
It is evident from
The adsorption activation energy can be deduced from the rate constants (k2) obtained from the Lagergren second-order kinetic model (see
where k2 is the second order rate constant (g mg−1 h), Ea the activation energy (kJ mol−1), R is the universal gas constant (8.314 J mol−1 K), and T is the solution temperature (K). A plot of ln k2 versus 1/T gives a linear plot, with slope −Ea/R (see
The mechanism of adsorption can be understood by determining the rate-limiting step, and this can be determined by using some adsorption diffusion models, which are always constructed on the basis of three consecutive steps, namely, (1) film diffusion (i.e., diffusion across the liquid film surrounding the adsorbent particles); (2) intraparticle diffusion (i.e., diffusion in the liquid contained in the pores and/or along the pore walls); and (3) mass action (i.e., fast physical adsorption and desorption between the adsorbate and active sites). The intraparticle diffusion model assumes that the metal ions are transported from the solution through an interface between the solution and the adsorbent (i.e., film diffusion), followed by a rate-limiting intraparticle diffusion step, which brings them into the pores of the particles in the adsorbent. The following equation expresses the relation of the adsorption capacity and time:
q
t
=x
i
+k
p
t
0.5 (5)
where qt is the adsorption capacity at time t, kp is the rate constant of intraparticle diffusion, and xi is related to boundary layer thickness. According to the Weber-Morris model, a straight line fit for the plot of qt versus t0.5 passing through the origin implies the intraparticle diffusion as the rate-limiting step. Since the initial linear plot did not pass through the origin, there is an intercept, which indicates that rapid adsorption occurs within a short period of time (see
The intercept xi of the linearized line was used to define the initial adsorption factor (Ri) as:
where xi is the initial adsorption amount and qe is the final adsorption amount at the longer time. The xi and qe values of 0.303 and 0.365 mg g−1, respectively, gave an R, value of 0.17, which means that the relatively strong rapid initial adsorption has already reached 83% before the first adsorption data was collected at a time of 5 min. The other 17% of adsorption is governed by intraparticle diffusion, as indicated by the linear relation with an excellent square of correlation coefficient (R2) of 0.9947 (see
As shown in
where Ce and qe are the concentrations of metal ion in the solution and resin, respectively, and Qm and b are the Langmuir constants. The Langmuir plot of Ce/qe versus Ce enables the calculation of Langmuir constants from the intercept and slope of the linear plot. The adsorption of Sr2+ by the CPZA resin did not fit the Langmuir isotherm model, so the relevant graph has not been displayed.
The Freundlich isotherm model, which describes the non-ideal adsorption occurring on a heterogeneous surface with uniform energy as well as multilayer adsorption, are expressed by the following equations (8) and (9):
where kf and n represent the Freundlich constants, which can be calculated from the slope and intercept of the linear plot of log qe versus log Ce, as presented in
The Temkin isotherm equation assumes that the heat of adsorption of all the molecules in a layer decreases linearly with coverage due to adsorbent-adsorbate interactions, and that the adsorption is characterized by a uniform distribution of the bonding energies up to some maximum binding energy. The Temkin isotherm has been used in the following form:
A linear form of the Temkin isotherm can be expressed as:
where R is the gas constant (8.314 J mol−1K−1), T is temperature (K), A is the equilibrium binding constant (L/g) corresponding to the maximum binding energy, and constant B=RT/b is related to the heat of adsorption. A plot of qe versus ln Ce (shown in
Adsorption experiments were also performed to obtain the thermodynamic parameters. The adsorption capacity decreases with the increase of temperature, thus suggesting the exothermic nature of the adsorption process and the weakening of bonds between Sr2+ and active sites of adsorbents at high temperatures (see the table in
As the temperature increases, the ΔG values become less negative, indicating that the adsorption process is less favorable at the higher temperatures. The negative ΔH of −14.5 kJ mol−1 indicates the exothermic nature of the adsorption process; similar exothermic behavior (ΔH=−5.9 and −5.2 kJ mol−1) has been noted in two different reports describing the adsorption of strontium on polyacrylonitrile/zeolite composite adsorbent and Dowex 50W-X Resins. The negative value of ΔS (−35.5 J mol−1K−1) is quite unexpected. However, it suggests the decrease in randomness at the solid/solution interface during the adsorption process as a result of replacement of Na+ with Sr2+.
Unloaded CPZ 4 and CPZA 5, as well as loaded resin 5, were investigated by scanning electron microscopy (SEM). Unloaded CPZA resins (60 mg) were immersed in Sr(NO3)2 solution (500 ppm in Sr2+, 20 mL) for 24 h at a pH of 3, filtered, and dried under vacuum until constant weight was achieved. Loaded and unloaded CPZA polymers were then sputter-coated for 2 min with a thin film of gold.
SEM and EDX images (
In the following examples, elemental analysis was carried out on a Perkin Elmer Elemental Analyzer Series II Model 2400. IR spectra were recorded on a Perkin Elmer 16F PC FTIR spectrometer. 1H and 13C spectra were measured on a JEOL LA 500 MHz spectrometer using HOD signal at δ4.65 and dioxane signal at 67.4 ppm as internal standards, respectively. Atomic absorption spectroscopy (AAS) analysis was performed using AAS model iCE 3000 series (Thermo Scientific). Scanning electron microscopy (SEM) images were taken by TESCAN LYRA 3 (Czech Republic) equipped with Oxford, Energy-Dispersive X-ray spectroscopy (EDX) detector model X-Max. Thermogravimetric analysis (TGA) was performed using a thermal analyzer (STA 429) manufactured by Netzsch (Germany). The polymer sample to be tested (usually ˜5 mg) was placed in a platinum crucible. Aluminum oxide (Al2O3; ˜4 mg) was placed in an identical platinum crucible as a reference sample. With the sample carrier system, which had two sets of 10% Pt—Pt/Rh thermocouples, the sample carrier was placed in the middle of the vertical furnace, which was programmed and controlled by a microprocessor temperature controller. The temperature was raised at a uniform rate of 10° C./min. The analyses were made over a temperature range of 20-800° C. in an air atmosphere flowing at a rate of 100 mL/min.
Azoisobutyronitrile (AIBN) from Fluka AG was purified by crystallization from a chloroform-ethanol mixture. Diallylamine from Fluka Chemie AG (Buchs, Switzerland) was used as received. Dimethylsulfoxide (DMSO) was dried over calcium hydride overnight and then distilled under reduced pressure at a boiling point of 64-65° C. (4 mmHg). All solvents used were of analytical grade. Considering the high toxicity of radioactive ions, aqueous solutions of non-radioactive Sr2+ ions were used in sorption experiments. For this purpose, Sr(NO3)2 was purchased from Fisher Scientific Company (New Jersey, USA).
The zwitterionic monomer N-carboethoxymethyl-3-(N,N-diallylamino) propanesulfonate (1) was prepared in 82% yield by reacting an equimolar mixture of N,N-diallyl-N-carboethoxymethylamine and 1,3-propanesultone in acetonitrile (130 cm3 for 0.13 mol amine) at 72° C. for 72 h, mp (methanol/acetone/diethyl ether) 162-163° C.
A solution of zwitterionic ester 1 (16.4 g, 53.7 mmol) in 6 M HCl (60 mL) was heated in a closed vessel at 80° C. for 27 h. The resulting solution was then freeze-dried to obtain 2 as a thick liquid, which was crystallized from a mixture of MeOH/CH3CN/diethylether as white crystals (14.2 g, 95%). (Found: C, 47.4; H, 7.0; N, 4.9; S, 11.3%. C11H19NO5S requires C, 47.64; H, 6.91; N, 5.05; S, 11.56%); νmax (KBr) 3500-2600 (br), 3015, 2980, 2930, 1728, 1642, 1479, 1400, 1324, 1197, 1166, 1081, 1040, 999, 972, 957, 941, 888, 801, 732, 698, 619, 596, 573, and 521 cm−1; δH (D2O) 2.07 (2H, quint, J=7.05 Hz), 2.77 (2H, t, J=6.7 Hz), 3.47 (2H, apparent t, 7.95 Hz), 3.98-4.02 (6H, m), 5.56 (4H, m), 5.84 (2H, m), (HOD: 4.65); δC (D2O) 18.36, 47.86, 56.82, 58.32, 62.68 (2C), 124.25 (2C), 130.42 (2C), 167.61 (dioxane: 67.40 ppm).
Monomer 3, a cross-linker, was prepared as described in Ali et al., J. Appl. Polym. Sci., 61 (1996), 1077-1085, the contents of which are hereby incorporated by reference in their entirety.
Sulfur dioxide (1.28 g, 20 mmol) was absorbed in a mixture of 2 (5.55 g, 20 mmol) and cross-linker 3 (1.62 mmol, 0.52 g) in DMSO (7.0 g). Initiator AIBN (140 mg) was then added, and the mixture stirred at 65° C. for 24 h. At the end of the elapsed time, the hardened solid mass was soaked in water for 24 h with frequent changing of water, and finally dried under vacuum at 55° C. to a constant weight (5.00 g, 68%). (Found: C, 39.2; H, 5.9; N, 4.2; S, 17.7%. monomer 2/SO2C11H19NO7S2 (92.5 mol %) and monomer 3/SO2, C16H28Cl2N2O2S (7.5 mol %) requires C, 39.56; H, 5.74; N, 4.34; S, 18.00%).
A mixture of CPZ 4 (4.6 g, 13.4 mmol) and sodium hydroxide (0.74 g, 18.5 mmol) in water (150 ml) was stirred for 24 h, after which it was poured into excess methanol. The resin CPZA 5 was filtered and dried under vacuum at 55° C. to a constant weight (4.7 g, 97%). TGA analysis showed initial decomposition around 250° C. (Found: C, 37.5; H, 5.4; N, 4.1; S, 16.7%. monomer 2/SO2C11H18NNaO7S2 (92.5 mol %) and monomer 3 (dihydroxide)/SO2C16H30N2O4S (7.5 mol %) requires C, 37.79; H, 5.27; N, 4.17; S, 17.02%).
The procedure for the adsorption experiments of the cross-linked polymer CPZA 5 for Sr2+ ions can be described briefly as follows. A mixture of CPZA (50 mg) in an aqueous Sr(NO3)2 (1 mg L−1) solution (20 mL) was stirred using a magnetic stir-bar at different pH for 24 h. The filtrate was analyzed by atomic absorption spectroscopy to determine the amount of Sr2+ that remained. The adsorption capacity (qSr
For adsorption kinetic studies, the resin sample was stirred in a 1 mg L−1 Sr(NO3)2 solution for different adsorption times at pH 3. The adsorption isotherm was constructed by changing the concentration of Sr(NO3)2 solution from 200 μL−1 (i.e. ppb) to 1000 μL−1 at 25° C. for 24 h. Based on the adsorption data from experiments carried out at different temperatures, the activation energy for the adsorption process and thermodynamic parameters ΔG, ΔH and ΔS for Sr2+ removal were calculated.
A desorption experiment was conducted by stirring 50 mg CPZA resin in aqueous Sr(NO3)2 (1 mg L−1) solution (20 mL) for 24 h. The loaded CPZA resin was then filtered, dried, and immersed in 0.1 M HNO3 for 24 h. After filtration, the amount of Sr2+ ions desorbed in the filtrate was determined. The efficiency of the desorption process was calculated by the ratio of desorbed amount of Sr2+ ions to the amount of adsorbed Sr2+ ions in the CPZA resin. The results indicated the percentage efficiency of the desorption process to be 88%.
Unloaded and loaded resins were investigated by FTIR spectroscopy. Unloaded resins (30 mg) were immersed in 0.1 M Sr(NO3)2 solution (10 mL) for 4 h at a pH of 3, filtered, and dried under vacuum until constant weight was achieved.
The cross-linked polyzwitterion/anion (CPZA) resin was prepared using Butler's cyclopolymerization protocol. The CPZA provided an opportunity to test the efficacy of a zwitterionic/anionic motif for the first time in the removal of a model metal ion in low concentrations. The resin was found to have a very good adsorption capacity for Sr2+ ions at low concentrations. The relatively strong rapid initial adsorption of 83% Sr2+ ions was followed by slower adsorption of the remaining 17%, which was described by an intraparticle diffusion model. The adsorption followed a Lagergren second-order kinetic model, and Temkin as well as Freundlich isotherm models. The negative ΔGs and ΔH ensured the spontaneity and the exothermic nature of the adsorption process. The excellent adsorption and desorption efficiencies implied the efficacy of the resin in removing (as well as recovering) the metal ions from aqueous solution.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
