Adsorption of Cr(Ⅵ) on porous sodium alginate/polyethyleneimine hydrogel beads and its mechanistic study
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摘要: 海藻酸钠(SA)是一种生物质材料，具有来源广泛、价格低廉的特性，被众多科研人员用于实验室研究，制备成吸附剂去除水溶液中的金属离子。但目前制备的大多数SA基吸附材料是实心水凝胶状，具有比表面积较低、吸附速率慢、吸附容量小的缺点。本研究以SA为基体，向其中添加碳酸钙和聚乙烯亚胺(PEI)，以戊二醛为交联剂，经冷冻干燥后制备出多孔的SA/PEI凝胶球，探究其对水溶液中Cr(Ⅵ)的吸附特性。通过改变实验条件，研究pH值、Cr(Ⅵ)初始浓度、吸附温度、吸附时间等对SA/PEI凝胶球吸附性能的影响；引入吸附动力学和热力学模型对吸附过程进行分析；采用FTIR、Zeta电位、SEM、XPS对SA/PEI凝胶球合成及吸附Cr(Ⅵ)机制进行综合分析。结果表明，SA/PEI凝胶球对Cr(Ⅵ)的去除率与初始浓度呈负相关；该吸附过程符合拟二级动力学和Langmuir等温吸附模型，且该吸附反应是自发的吸热过程，在温度为318.15 K、pH值为2时，Langmuir等温吸附拟合所得最大吸附量为262.83 mg/g。SA/PEI凝胶球对Cr(Ⅵ)的吸附机制主要为静电作用导致的物理吸附。Abstract: The sodium alginate (SA) is a biomass material which is abundant and can be easily acquired. It is currently used by many scientific researchers in laboratory research to prepare adsorbents to remove metal ions from aqueous solutions. However, SA based adsorbents generally exist as hydrogels, which are low in specific surface areas, slow in adsorption rates and have low adsorption capacities. In this study, calcium carbonate and polyethyleneimine (PEI) were added to SA matrix, and glutaraldehyde was used as a crosslinking agent to prepare porous SA/PEI beads via freeze-drying. The adsorption characteristics of synthesized adsorbent for Cr(Ⅵ) in aqueous solution were studied. The adsorption behaviors of Cr(Ⅵ) ions were evaluated by varying the experimental conditions including pH values, initial metal ion concentrations, adsorption temperature and adsorption time. Adsorption kinetics and thermodynamic models were applied to analyze the adsorption process. Characterization methods, including FTIR, Zeta potential, SEM, and XPS were comprehensively used to analyze the synthesis mechanism of SA/PEI beads and the mechanism of Cr(Ⅵ) adsorption. The results show that the removal rate of Cr(Ⅵ) by SA/PEI beads is negatively related to the initial concentration; the adsorption process conforms to the pseudo-second-order kinetics and Langmuir isotherm adsorption model, and the adsorption reaction is a spontaneous endothermic process. When the temperature is 318.15 K and the pH value is 2, the Langmuir isotherm adsorption fitting shows that the maximum adsorption capacity is 262.83 mg/g. The adsorption mechanism of SA/PEI beads on Cr(Ⅵ) is mainly physical adsorption dominated by electrostatic interactions.
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Key words:
- sodium alginate /
- hydrogel /
- adsorption mechanism /
- Cr(Ⅵ) /
- adsorbent /
- chemical modification
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图 1 海藻酸钠/聚乙烯亚胺(SA/PEI)凝胶球的主要制备步骤
Figure 1. Major fabrication steps of sodium alginate/polyethyleneimine (SA/PEI) hydrogel beads
图 2 pH值对SA/PEI凝胶球吸附Cr(Ⅵ)的影响
Figure 2. Effect of pH value on Cr(Ⅵ) uptake by SA/PEI hydrogel beads
图 3 200 mg/L Cr(Ⅵ)在溶液中Cr(Ⅵ)离子形态的平衡分布
Figure 3. Equilibrium distribution of Cr(Ⅵ) species in aqueous with a total Cr(Ⅵ) concentration of 200 mg/L
图 4 SA/PEI凝胶球在不同pH值下的Zeta电位
Figure 4. Zeta potential of SA/PEI hydrogel beads under various pH values
图 5 温度和Cr(Ⅵ)浓度对SA/PEI凝胶球的Cr(Ⅵ)去除率的影响
Figure 5. Effects of temperature and Cr(Ⅵ) concentration on Cr(Ⅵ) removal rate by SA/PEI hydrogel beads
图 6 不同浓度Cr(Ⅵ)溶液中Cr2O72−和HCrO4−的含量
Figure 6. Percentage contents of Cr2O72− and HCrO4− in different concentrations of Cr(Ⅵ) aqueous
图 7 吸附时间对SA/PEI凝胶球吸附Cr(Ⅵ)吸附量的影响
Figure 7. Effect of adsorption time on adsorption capacity of Cr(Ⅵ) by SA/PEI hydrogel beads
图 9 SA/PEI凝胶球对Cr(Ⅵ)的Langmuir和Freundlich等温吸附模型拟合
Figure 9. Fitting of Langmuir and Freundlich isotherm adsorption models for adsorption of Cr(Ⅵ) by SA/PEI hydrogel beads
图 10 SA和SA/PEI凝胶球吸附Cr(Ⅵ)前后的FTIR图谱
Figure 10. FTIR spectra of SA and SA/PEI hydrogel beads before and after Cr(Ⅵ) adsorption
图 11 SA/PEI凝胶球吸附Cr(Ⅵ)前((a)~(c))和后((d)~(f))的SEM图像及EDS图谱
Figure 11. SEM images and EDS spectra of SA/PEI hydrogel beads before ((a)−(c)) and after ((d)−(f)) Cr(Ⅵ) adsorption
图 12 SA/PEI凝胶球吸附Cr(Ⅵ)前后的XPS图谱
Figure 12. XPS spectra of SA/PEI hydrogel beads before and after adsorption of Cr(Ⅵ)
表 1 各类试剂及其性能
Table 1. Various reagents and properties
Reagent Property Sodium alginate Viscosity: 200−500 mPa.s Polyethyleneimine Molecular weight Mw=600 Glutaraldehyde Purity 25%−28% CaCO3 Analytical reagent 1,5-Diphenylcarbohydrazide Analytical reagent K2Cr2O7 Analytical reagent CaCl2 Analytical reagent 
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表 2 SA/PEI凝胶球吸附Cr(Ⅵ)的热力学参数
Table 2. Thermodynamic parameters of Cr(Ⅵ) adsorbed by SA/PEI hydrogel beads
T/K $\ln {K_{\rm{c}}}$ $\Delta G$/(kJ·mol−1) $\Delta H$/(kJ·mol−1) $\Delta S$/(J(mol·K)−1) 298.15 1.83 −4.54 17.52 74.58 308.15 1.98 −5.07 318.15 2.45 −6.48 Notes: T—Absolute temperature; Kc—Equilibrium constant; ΔG—Gibbs free energy; ΔH—Enthalpy change; ΔS—Entropy change.
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表 3 SA/PEI凝胶球吸附Cr(Ⅵ)的动力学模型拟合参数
Table 3. Kinetic parameters of pseudo-first/second-order models for Cr(Ⅵ) adsorption by SA/PEI hydrogel beads
C0/(mg·L−1) Pseudo-first-order kinetic model Pseudo-second-order kinetic model K1/(mg(g·h−1)−1) qe/(mg·g−1) R2 K2/(mg(g·h−1)−1) qe/(mg·g−1) R2 100 0.5829 91.3769 0.9900 0.0101 99.0000 0.9473 200 0.5031 167.3778 0.9602 0.0036 191.4030 0.9433 Notes: C0—Initial concentration of Cr(Ⅵ); K1, K2—Rate constants for pseudo-first-order and pseudo-second-order equations, respectively; qe—Amounts of metal adsorbed at equilibrium; R2—Goodness-of-fitting.
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表 4 SA/PEI凝胶球去除Cr(Ⅵ)的Langmuir和Freundlich等温线模型参数
Table 4. Parameters of Langmuir and Freundlich isotherms on Cr(Ⅵ) removal by SA/PEI hydrogel beads
T/K Langmuir isotherm Freundlich isotherm KL/(L·mg−1) qm/(mg·g−1) R2 KF/(L·mg−1) n R2 298.15 0.1113 233.31 0.9587 71.24 4.2827 0.9337 308.15 0.1286 242.07 0.9679 74.67 4.1841 0.9438 318.15 0.1837 262.83 0.9826 99.09 4.8828 0.9077 Notes: KL—Langmuir isotherm constant; KF, n—Two Freundlich isotherm constants; qm—Maximum metal uptake capacity.
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表 5 SA/PEI凝胶球与已报道的SA基吸附剂去除Cr(Ⅵ)性能比较
Table 5. Comparison of adsorption capacity of Cr(Ⅵ) removal by SA/PEI hydrogel beads and others reported SA based adsorbents
Adsorbent Adsorption capacity/(mg·g−1) Reference SA-polyaniline nanofibers 75.82 [16] TEPA functionalized alginate beads 77.00 [29] Nanoscale zerovalent iron/biochar/Ca-alginate beads 86.40 [30] Nano zero-valent iron/carbon/alginate composite gel 35.25 [31] Magnetic nano-hydroxyapatite encapsulated alginate beads 29.14 [32] Fe nanoparticles embedded graphene oxide alginate beads 33.90 [33] SA/PEI 262.83 This work
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