Removal performance and mechanism of U(VI) in water by pore forming enhanced phosphorus loaded thermosensitive microspheres
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摘要: 为了解决海藻酸钠微球溶胀性差、吸附剂和被吸附物间的传质阻力大和干燥后吸附位点少的问题，采用自由基聚合和离子交联法制备了一种热响应互穿聚合物网络水凝胶微球，并对其进行“造孔+磷酸基团功能化”改性(简称P/PF@TR-IPN)。通过单因素试验研究了铀初始浓度、ZnO的含量、投加量、pH值、温度、干扰离子和吸附时间等对U(VI)吸附的影响，探究了其再生性能。在U(VI)初始浓度为10 mg∙L−1，pH值为4，P/PF@TR-IPN的投加量为0.4 g∙L−1，温度为25℃的条件下，6 h内P/PF@TR-IPN对U(VI)的去除率为94.8%，比造孔微球(PF@TR-IPN)和空白微球(TR-IPN)分别提高了18.5%和30.03%。随着温度从20℃增加到50℃，温敏微球P/PF@TR-IPN的溶胀率从6.98%降至5.14%。P/PF@TR-IPN的BET比表面积比TR-IPN增大了28.5倍。当pH值为4，温度为30℃和20℃时，P/PF@TR-IPN对U(VI)的最大吸附量分别为76.99 mg∙g−1和85.62 mg∙g−1。U(VI)的去除遵循单分子层化学吸附过程，拟二级动力学模型和Langmuir模型可以较好拟合P/PF@TR-IPN对U(VI)的吸附过程。试验表明，3D多孔结构和磷酸基团新活性吸附位点的形成是U(VI)去除效率增加的两个主要因素，P/PF@TR-IPN去除U(VI)的主要机制包括磷酸基团共沉淀的化学吸附作用和海藻酸钠含氧官能团的表面络合作用。Abstract: In order to solve the problems of poor swelling property of sodium alginate microspheres, large mass transfer resistance between adsorbent and adsorbate, and few adsorption sites after drying, a thermal-responsive interpenetrating polymer network hydrogel microspheres were prepared by free radical polymerization and ionic crosslinking, and modified by "pore forming+phosphate group functionalization" (P/PF@TR-IPN). The effects of initial uranium concentration, ZnO content, dosage, pH value, temperature, interfering ions and adsorption time on U(VI) adsorption were studied by single factor experiment, and its regeneration performance was explored. When the initial concentration of U(VI) is 10 mg∙L−1, the pH value is 4, the dosage of P/PF@TR-IPN is 0.4 g∙L−1, and the temperature is 25℃, the removal rate of U (VI) by P/PF@TR-IPN is 94.8% within 6 h, which is 18.5% and 30.03% higher than that of PF@TR-IPN and TR-IPN, respectively. With the increase of temperature from 20 ℃ to 50℃, the swelling rate of temperature-sensitive microspheres P/PF@TR-IPN decreases from 6.98% to 5.14%. The BET specific surface area of P/PF@TR-IPN is 28.5 times larger than that of TR-IPN. When pH value is 4 and the temperature is 30℃ and 20℃, the maximum adsorption capacity of P/PF@TR-IPN for U(VI) is 76.99 mg∙g−1 and 85.62 mg∙g−1, respectively. The removal of U(VI) follows the monolayer chemical adsorption process, and the pseudo-second-order kinetic model and Langmuir model can better fit the adsorption process of U(VI) by P/PF@TR-IPN. The results show that the 3D porous structure and the formation of new active adsorption sites of phosphate groups are the two main factors for the increase of U(VI) removal efficiency, the main mechanisms of U(VI) removal by P/PF@TR-IPN include the chemical adsorption of phosphate group coprecipitation and the surface complexation of sodium alginate oxygen-containing functional groups.
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Key words:
- thermal response /
- sodium alginate /
- pore forming /
- phosphorus /
- adsorption performance /
- uranium /
- hydrogel
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图 1 空白微球(TR-IPN)、造孔微球(PF@TR-IPN)、造孔+磷酸基团功能化改性(P/PF@TR-IPN)三种微球的球体 ((a), (d), (g))、外表面 ((b), (e), (h)) 和横截面 ((c), (f), (i)) 的SEM图像
Figure 1. SEM images of sphere ((a), (d), (g)), outer surface ((b), (e), (h)) and cross section ((c), (f), (i)) of blank (TR-IPN), pore forming (PF@TR-IPN) and "pore forming+phosphate group functionalization" (P/PF@TR-IPN)
图 2 P/PF@TR-IPN吸附U(VI)前 (a) 和后 (b) 的EDS图谱
Figure 2. P/PF@TR-IPN EDS spectrum before (a) and after (b) adsorption of U(VI)
图 3 NIPA、SA及P/PF@TR-IPN吸附前后的FTIR图谱
Figure 3. FTIR spectra of NIPA, SA and P/PF@TR-IPN before and after adsorption
图 4 (a) P/PF@TR-IPN吸附U(VI)前后的XPS图谱：(a) 全图谱；(b) U4f；(c) C1s；(d) O1s；(e) P2p
Figure 4. XPS spectra of P/PF@TR-IPN before and after adsorption of U(VI): (a) Full spectrum; (b) U4f; (c) C1s; (d) O1s; (e) P2p
图 5 (a) TR-IPN和P/PF@TR-IPN的N2吸脱附等温线；(b) P/PF@TR-IPN和TR-IPN的孔径分布
Figure 5. (a) TR-IPN and P/PF@TR-IPN N2 adsorption desorption isotherm; (b) Pore size distribution of P/PF@TR-IPN and TR-IPN
STP—Standard temperature and pressure; V—Actual adsorption amount of nitrogen on the surface of the sample; D—Sample hole diameter
图 6 ZnO的含量对铀吸附性能 (a) 和溶胀性能 (b) 的影响
Figure 6. Effect of ZnO content on uranium adsorption (a) and swelling (b)
图 7 (a) pH对铀吸附的影响；(b) 铀在不同pH下的存在形式；(c) P/PF@TR-IPN的Zeta电位变化图；(d) 不同投加量对吸附U(VI)的影响
Figure 7. (a) Effect of pH on uranium adsorption; (b) Existing form of uranium at different pH; (c) Zeta potential change diagram of P/PF@TR-IPN; (d) Effect of different dosage on adsorption of U(VI)
pHpzc—Point of zero charge
图 8 温度对P/PF@TR-IPN吸附性能 (a) 和溶胀性 (b) 的影响
Figure 8. Effect of temperature on adsorption (a) and swelling properties (b) of P/PF@TR-IPN
图 9 干扰离子 (a) 和离子强度 (b) 对P/PF@TR-IPN除铀性能的影响
Figure 9. Effect of interference ion (a) and ionic concentration (b) on uranium removal performance of P/PF@TR-IPN
图 10 (a) 吸附时间对P/PF@TR-IPN吸附不同浓度U(VI)的影响；(b) 拟一级动力学模型拟合曲线；(c) 拟二级动力学模型拟合曲线；(d) 颗粒内扩散模型拟合曲线
Figure 10. (a) Effect of adsorption time on the adsorption of different concentrations of U(VI) by P/PF@TR-IPN; (b) Fitting curves of pseudo-first-order kinetic model; (c) Fitting curves of quasi-second-order kinetic model; (d) Fitting curves of intra-particle diffusion model
qt—Adsorption capacity at t time; qe—Equilibrium adsorption capacity; t—Adsorption time
图 11 (a) P/PF@TR-IPN吸附铀的非线性等温模型拟合；(b) lnK0与1/T的关系图
Figure 11. (a) Nonlinear isothermal model fitting diagram of uranium adsorption by P/PF@TR-IPN; (b) Relationship between lnK0and 1/T
qe—Equilibrium adsorption capacity; Ce—Uranium concentration at adsorption equilibrium; K0—Langmuir coefficient related to the affinity of binding site; T—Temperature
图 12 不同循环次数的P/PF@TR-IPN吸附-解吸效率
Figure 12. Adsorption-desorption efficiency with different cycles of P/PF@TR-IPN
表 1 TR-IPN和P/PF@TR-IPN的孔隙特征参数
Table 1. Pore characteristic parameters of TR-IPN and P/PF@TR-IPN
Sample BET surface area/(m2·g−1) Average pore diameter/nm Pore volume/(cm3·g−1) TR-IPN 0.074 7.922 0.016 P/PF@TR-IPN 2.109 6.187 0.054 
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表 3 P/PF@TR-IPN吸附等温线模型的相关参数
Table 3. Relevant parameters of P/PF@TR-IPN adsorption isotherm model
T/K Langmuir model Freundlich model qmax/(mg∙g−1) KL/(L∙mg−1) R2 KF/(L∙mg−1) n R2 293 85.62 2.250 0.950 44.82 4.540 0.863 303 76.99 1.367 0.937 37.28 4.036 0.914 313 72.66 1.086 0.986 33.82 3.820 0.926 Notes: qmax—Adsorption capacity per unit mass of the adsorbent; KL—Langmuir coefficient related to the affinity of binding site;
KF, n—Constants that are related to the adsorption capacity and the adsorption intensity, respectively.
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表 2 P/PF@TR-IPN吸附铀的动力学模型参数
Table 2. Kinetic model parameters of uranium adsorption by P/PF@TR-IPN
C0/
(mg∙L−1)qe,exp/
(mg∙g−1)Pseudo-first-order model Pseudo-second-order model Intraparticle diffusion model K1/
min−1qe,cal/
(mg∙g−1)R2 K2/
min−1qe,cal/
(mg∙g−1)R2 Kd1f/
(mg∙(g∙
min0.5)−1)C1 R12 Kd2f/
(mg∙(g∙
min0.5)−1)C2 R22 Kd3f/
(mg∙(g∙
min0.5)−1)C3 R32 5 12.30 0.019 8.758 0.976 0.0049 12.658 1 1.457 −0.286 0.983 0.610 4.923 0.995 0.011 12.028 0.511 10 24.62 0.015 16.087 0.997 0.0024 25.329 1 2.869 −1.397 0.988 1.547 5.424 0.996 0.031 23.876 0.585 15 36.85 0.012 16.124 0.979 0.0021 37.538 0.999 4.707 −2.464 0.962 1.857 12.854 0.948 0.077 34.912 0.494
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表 4 不同吸附剂对U(VI)吸附效果的比较
Table 4. Comparison of adsorption effects of different adsorbents on U(VI)
Adsorbents qmax/(mg∙g−1) T/K pH Ref. Magnetic biochar 52.63 318 4 [37] Fe3O4@SiO2 52.00 298 6 [38] MSD 31.54 293 5 [39] MY@SiO2-PEI 173.99 303 6 [35] Sulfonated GO 45.05 293 2 [40] HAP-AC-Alginate 18.66 298 6 [41] P/PF@TR-IPN 85.62 293 4 This work Notes: MSD—Magnesium silicate/diatomite; MY@SiO2-PEI—Polyethyleneimine modified magnetic yeast composites; Sulfonated GO—Sulfonated graphene oxide; HAP-AC-Alginate—Nano-hydroxyapatite coated activated carbon impregnated alginate; P/PF@TR-IPN—Pore forming enhanced phosphorus loaded thermal-responsive interpenetrating polymer network hydrogel microspheres.
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表 5 P/PF@TR-IPN吸附铀的热力学参数
Table 5. Thermodynamic parameters of uranium adsorption by P/PF@TR-IPN
T/K lnK0 ΔG0/(kJ·mol−1) ΔH0/(kJ·mol−1) ΔS0/(J·(mol·K)−1) 293 6.17 −14.99 −40.86 −88.29 303 5.56 −14.11 313 5.10 −13.23 Notes: ΔG0—Standard free energy change; ΔH0—Standard enthalpy change; ΔS0—Standard entropy change.
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