Effect of polyethylene glycol on fluoride removal performance of hydroxyapatite
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摘要: 为改善羟基磷灰石(HAP)合成过程易团聚导致其除氟效率不高的不足,本文应用清洁简便、绿色环保的电化学合成法,将亲水性强、分散性优异的非离子型表面活性剂聚乙二醇(PEG)加入到制备HAP的混合支持电解液中,在铜片作工作电极的表面制备出新型HAP复合材料(PEG/HAP),并与纯HAP的晶体结构、孔径、比表面积、表面形貌、元素占比和官能团对比,以揭示PEG/HAP除氟效率高于HAP的内在机制。结果发现,PEG/HAP与HAP有相同的晶面结构特征蜂、元素和化学键,但PEG/HAP的各元素含量占比、羟基和磷酸根离子官能团的吸收峰位和吸收强度与HAP 有一定差异;PEG使HAP从短棒状的表面形貌变成有利于交换和吸附氟离子的多孔和孔隙结构,其平均孔径由16.58 nm减小到11.93 nm,比表面积从24.29 m²/g增加到29.83 m²/g;虽然PEG/ HAP与HAP的吸附类型均为IV型的H3滞后环,二者介孔分布范围一致,但PEG/ HAP的微孔和介孔数量明显高于HAP。尽管两种材料对氟离子的吸附反应均显示熵增、吸热和自发过程特征,吸附等温模型均符合Langmuir-Freundlich,但PEG/HAP的颗粒内扩散速率常数略大于HAP,PEG/HAP的吸附氟离子容量(9.56 mg/g)高于HAP (8.36 mg/g);且去除氟离子的循环再生次数从HAP的4次增加到PEG/HAP的6次。此外,PEG的存在并没有影响制备条件参数如支持电解液pH值对HAP吸附氟离子容量的影响趋势,但却使HAP吸附氟离子容量增加。共存阴离子如Cl−、NO3 −、SO4 2−、CO3 2−均不干扰PEG/HAP和HAP对氟离子的吸附。Abstract: The removal efficiency of hydroxyapatite (HAP ) to fluoride was low due to its easy agglomeration during the synthesis process. Based on this, a clean, simple, green and environmentally fiendly electrochemcial synthesis method was applied to improve the fluoride removal efficiency. Polyethylene glycol (PEG), a non ionic surfactant with strong hydrophilicity and excellent dispersion, was added to the mixed support electrolyte for preparing HAP. A new type of HAP composite (PEG/HAP) was prepared on the surface of copper sheet as the working electrode. By contrast to HAP, the crystal structure, pore size, specific surface area, surface morphology, elemental proportion, and functional groups of PEG/HAP were analyzed to reveal the instrinsic mechnism of the higher fluoride removal efficiency of PEG/HAP than HAP. The results show that PEG/HAP and HAP have the same crystal plane structure characteristics, elements and chemical bonds, while the proportion of various elements as well as the absorption peaks and intensity of hydroxyl and phosphate ion functional groups in PEG/HAP have certain differences compared to HAP. PEG transformed HAP from a short rod-shaped surface morphology to a porous and porous structure that facilitated the exchange and adsorption of fluoride ions. Average pore size of PEG/HAP decreased from 16.58 nm of HAP to 11.93 nm, and its specific surface area increased from 24.29 m2/g OF HAP to 29.83 m2/g. Although the adsorption types of PEG/HAP and HAP were both IV type H3 hysteresis loop, and their mesoporous distribution ranges were consistent, the number of micropores and mesopores in PEG/HAP were significantly higher than those in HAP. Although the adsorption reactions of both materials for fluoride ions exhibited entropy increase, endothermic, and spontaneous process characteristics with the adsorption isotherm model conforming to Langmuir-Freundlich, the intra particle diffusion rate constant of PEG/HAP was slightly higher than HAP. Therefore, the maximum adsorption capacity of PEG/HAP for fluoride ions can reach 9.56 mg/g, which was higher than that of HAP of 8.36 mg/g. Compared with 4 times of recycle regeneration for removing fluoride ions of HAP, PEG/HAP can arrived at 6. In addition, the presence of PEG did not affect the change trend of preparation parameters such as electrolyte pH on the adsorption capacity of HAP for fluoride ions. However, PEG increased the adsorption capacity of HAP for fluoride ions. All coexisiting ions such as Cl−, NO3 −, SO4 2−, and CO3 2− did not interfere with the adsorption of fluoride ions for PEG/HAP and HAP.
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Key words:
- polyethylene glycol /
- hydroxyapatite /
- electrochemical synthesis /
- defluoridation /
- adsorptivity
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图 9 PEG/HAP和HAP吸附F−的lnKd与1/T 线性曲线(F−浓度:(a) 1.5 mg/L;(b) 2.0 mg/L;(c) 4.0 mg/L;(d) 6.0 mg/L;(e) 8.0mg/L;(f) 10.0 mg/L)
Figure 9. Linear curves of lnKd vs 1/T of PEG/HAP and HAP adsorption F− (F− concentration: (a) 1.5 mg/L; (b) 2.0 mg/L; (c) 4.0 mg/L; (d) 6.0 mg/L; (e) 8.0mg/L; (f) 10.0 mg/L)
Kd—Partition coefficient of adsorption process; T—Absolute temperature of adsorption process
表 1 HAP和PEG/HAP的孔结构参数
Table 1. Pore structure parameters of HAP and PEG/HAP
Material Parameters BET surface area/(m2·g−1) Average pore diameter/nm HAP 24.29 16.58 PEG/HAP 29.83 11.93 表 2 HAP和PEG/HAP元素占比
Table 2. Atomic ratio of HAP and PEG/HAP
Material C/at% O/at% P/at% Ca/at% HAP 14.18 59.29 12.47 14.06 PEG/HAP 17.43 56.22 11.23 15.12 表 3 PEG/HAP和HAP吸附F−的热力学参数
Table 3. Thermodynamic parameters of PEG/HAP and HAP adsorption F−
F− initial concention/(mg·L−1) T/K $\Delta {G}^{\ominus } $/(kJ·mol−1) $\Delta {S}^{\ominus } $/(J·K−1·mol−1) $\Delta {H}^{\ominus } $/(kJ·mol−1) PEG/HAP HAP PEG/HAP HAP PEG/HAP HAP 1.5 288.15 −20.6064 −20.8255 183.9473 108.6640 32.3980 10.4860 298.15 −22.4459 −21.9122 308.15 −24.2854 −22.9988 318.15 −26.1248 −24.0855 328.15 −27.9643 −25.1721 2 288.15 −20.6560 −20.8680 164.4675 112.0162 26.7353 11.4095 298.15 −22.3007 −21.9881 308.15 −23.9454 −23.1083 318.15 −25.5900 −24.2285 328.15 −27.2347 −25.3486 4 288.15 −18.1882 −17.3630 164.1350 169.6522 29.1073 31.5223 298.15 −19.8296 −19.0595 308.15 −21.4709 −20.7560 318.15 −23.1123 −22.4525 328.15 −24.7536 −24.1491 6 288.15 −16.6281 −15.7314 174.6855 167.8098 33.7075 32.6230 298.15 −18.3750 −17.4095 308.15 −20.1218 −19.0876 318.15 −21.8687 −20.7657 328.15 −23.6155 −22.4438 8 288.15 −15.8648 −14.9546 135.6928 151.0870 23.2351 28.5811 298.15 −17.2217 −16.4655 308.15 −18.5786 −17.9764 318.15 −19.9356 −19.4872 328.15 −21.2925 −20.9981 10 288.15 −15.2550 −14.6329 128.7755 129.4490 21.8517 22.6678 298.15 −16.5427 −15.9274 308.15 −17.8305 −17.2219 318.15 −19.1182 −18.5164 328.15 −20.4060 −19.8109 Notes: $\Delta {G}^{\ominus } $, $\Delta {S}^{\ominus } $ and $\Delta {H}^{\ominus } $—Standard Gibbs function change value, entropy change, and enthalpy change, respectively. 表 4 PEG/HAP与HAP的Langmuir-Freundlich模型拟合参数
Table 4. Fitting parameters of Langmuir-Freundlich of PEG/HAP and HAP
Model T/K qm/(mg·g−1) b n R2 PEG/HAP HAP PEG/HAP HAP PEG/HAP HAP PEG/HAP HAP Langmuir-Freundlich 288.15 4.6058 3.4550 3.0896 3.0662 1.2522 3.8708 0.9701 0.9615 298.15 5.8948 4.3757 3.2896 27.3055 1.2079 2.8634 0.9858 0.9694 308.15 6.8707 5.6192 2.8382 4.3399 0.8620 1.3814 0.9914 0.9818 318.15 8.7941 7.2896 2.7633 3.3643 0.9144 1.4300 0.9757 0.9960 328.15 10.2706 8.9194 2.9674 2.7605 0.9658 1.3416 0.9822 0.9941 Notes: b—Adsorption constant; n—Non-uniformity coefficient; R2—Correlation coefficient. 表 5 PEG/HAP动力学方程拟合参数
Table 5. Fitting parameters of kinetics equation for PEG/HAP
Equation Parameters R2 Pseudo first-order model k1/min−1 Qe/(mg·g−1) 0.8953 0.0171 3.4672 Pseudo second-order model k2 /(g·mg−1·min−1) Qe/(mg·g−1) 0.9854 0.0050 4.1203 Elovich A/(mg·g−1·min−1) B/(g·mg−1) 0.9663 0.2526 1.2902 Morrist intraparticle diffusion model k3/(g·g−1·min−0.5) C 0.9876 0.1774 0.6684 Notes: k1—Adsorption rate constant of the quasi first order kinetic model; k2—Adsorption rate constant of the quasi second order kinetic model; B—Desorption coefficient; A—Constant adsorption rate; k3—Internal diffusion rate constant; C—Boundary-layer thickness; Qe—Equilibrium adsorption capacity. 表 6 HAP不同动力学模型的拟合参数
Table 6. Fitting parameters of kinetics equation for HAP
Equation Parameters R2 Pseudo first-order model k1/min−1 Qe/(mg·g−1) 0.9186 0.0136 3.1689 Pseudo econd-order model k2/(g·mg−1·min−1) Qe/(mg·g−1) 0.9770 0.0043 3.8226 Elovich A/(mg·g−1·min−1) B/(g·mg−1) 0.9595 0.1740 1.3443 Morrist intraparticle diffusion model k3/ (g·g−1·min−0.5) C 0.9915 0.1712 0.3834 表 7 HAP和PEG/HAP吸附容量
Table 7. Adsorption capacity of HAP and PEG/HAP
Material q/(mg·g−1) 1st 2nd 3rd 4th 5th HAP 2.36 2.38 2.32 2.34 2.36 PEG/HAP 2.68 2.70 2.70 2.66 2.64 表 8 HAP和PEG/HAP的氟离子去除率
Table 8. Defluoridation efficient of HAP and PEG/HAP
Material Defluoridation efficient/% 1st 2nd 3rd 4th 5th HAP 59.0 59.5 58.0 58.5 59.0 PEG/HAP 67.0 67.5 67.5 66.5 66.0 表 9 HAP和PEG/HAP循环再生次数
Table 9. Recycling regeneration times of HAP and PEG/HAP
Material CF−/(mg·L−1) 1st 2nd 3rd 4th 5th 6th HAP 0.65 0.75 0.86 0.94 1.12 — PEG/HAP 0.42 0.51 0.64 0.73 0.86 0.94 Note: CF−—Remain concentration of F− in model waste water. -
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