Performance and mechanism of the amine-modified silica aerogel for the removal of Cu(II)
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摘要: 为有效去除液相中重金属Cu(II)，以正硅酸乙酯为原料，3-氨丙基三乙氧基硅烷为氨基化试剂，通过共缩聚法合成氨基改性SiO2气凝胶(NG)。系统考察pH、离子强度、时间、温度等因素对NG去除Cu(II)的影响，结合吸附动力学模型、吸附等温模型、吸附热力学、位点能量分布理论分析其吸附机制。研究结果表明，pH在3.00~6.00条件下，Cu(II)吸附量随pH升高而增大。离子强度由0 mol/L增至0.08 mol/L时，Cu(II) 吸附量受抑制作用呈逐渐降低趋势，FTIR分析显示NG与Cu(II)主要形成外层络合物。NG吸附Cu(II)时间在8 h内基本达到平衡，其吸附主要经过边界层扩散、颗粒内扩散与化学吸附等过程，且该吸附过程最符合准二级动力学模型与Freundlich模型。温度升高有利于促进吸附反应发生，Cu(II)最大吸附量达到130.45 mg/g，其吸附过程属吸热、熵增加的自发反应。位点能量分布显示随吸附反应进行，Cu(II)优先占据NG上高能量吸附位点，再占据低能量吸附位点，NG吸附Cu(II)的主要机制是外层络合与静电作用。Abstract: In order to remove Cu(II) from the liquid phase efficiently, the amine-modified silica aerogel (NG) was prepared by co-condensation method using tetraethyl orthosilicate as raw material and 3-aminopropyltriethoxysilane as amino agent. The effects of pH, ionic strength, time, temperature and other factors on the removal of Cu(II) by NG were systematically investigated. The adsorption mechanism of Cu(II) on the NG was analyzed by combining adsorption kinetics model, adsorption isotherm model, adsorption thermodynamics and site energy distribution theory. The results demonstrate that the adsorption capacity of Cu(II) increase with pH value from 3.00 ~ 6.00, and the adsorption is inhibited by the addition of ionic strength at the range of 0 ~ 0.08 mol/L. The outer-sphere complexes formed by Cu(II) and NG are confirmed by using FTIR analysis. Furthermore, the adsorption equilibrium is achieved within 8 h, and the adsorption process mainly go through boundary layer diffusion, intra-particle diffusion and chemisorption. The adsorption process is best fitted with the pseudo-second-order model and Freundlich model. The increase of temperature is beneficial to promote the adsorption reaction of Cu(II), and the maximum adsorption capacity reaches to 130.45 mg/g. The adsorption process is endothermic and entropy increasing spontaneous reaction. The energy distribution show that Cu(II) is preferentially adsorbed on the high-energy adsorption sites on NG and then occupied low-energy adsorption sites. Overall, the adsorption mechanism is mainly attributed to the electrostatic interaction and the outer-sphere complexation．
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Key words:
- aerogel /
- adsorption /
- Cu(II) /
- kinetics /
- model /
- site energy distribution
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图 1 氨基改性SiO2气凝胶(NG)的SEM图像和实物照(a)、表面接触角(b)、 XRD图(c)、N2吸附-脱附等温线与孔径分布曲线(d)
Figure 1. SEM and physical image of amine-modified silica aerogel (NG) (a), contact angle of the surface (b), XRD pattern (c), Nitrogen absorption-desorption isotherms and pore size distribution (d)
图 2 pH对NG吸附Cu(II)的影响(ρ0=20 mg/L, I=0 mol, T=298 K,m=0.02 g, V=50 mL) (a)、水溶液中Cu(II)分布形态(b)
Figure 2. Effect of pH on absorption of Cu(II) by NG(ρ0=20 mg/L, I=0 mol, T=298 K, m=0.02 g, V=50 mL) (a), Speciation of Cu(II) in the system (b)
图 3 离子强度对NG吸附Cu(II)的影响(ρ0=20 mg/L, pH=5.15, T=298 K, m=0.02 g, V=50 mL) (a)、NG吸附Cu(II)前后FTIR图谱(ρ0=20 mg/L, pH=5.15, I=0 mol, T=298 K, m=0.02 g, V=50 mL) (b)
Figure 3. Effect of ionic strength on absorption of Cu(II) by NG (ρ0=20 mg/L, pH=5.15, T=298 K, m=0.02 g, V=50 mL) (a), FTIR spectra of amine-modified silica aerogel before and after Cu(II) adsorption(ρ0=20 mg/L, pH=5.15, I=0 mol, T=298 K, m=0.02 g, V=50 mL) (b)
图 4 NG吸附Cu(II)的动力学曲线(ρ0=20 mg/L, pH=5.15, I=0 mol, T=298 K, m=0.02 g, V=50 mL) (a)、颗粒内扩散模型拟合曲线(b)
Figure 4. Kinetic curves of Cu(II) adsorption by NG(ρ0=20 mg/L, pH=5.15, I=0 mol, T=298 K, m=0.02 g, V=50 mL) (a), Plot of intra-particle diffusion (b)
图 5 不同等温模型对NG吸附Cu(II)的非线性拟合
Figure 5. Nonlinear fitting curves of Cu(II) absorption onto NG under different models
图 6 NG吸附Cu(II)的表面溶质与溶剂之间吸附能量差(a)、位点能量分布(b)
Figure 6. (a)Adsorption energy difference between surface solute and solvent on NG for Cu(II) (a), Site energy distribution (b)
图 7 XPS全谱(a)、Cu2 p精细谱(b)、N1 s精细谱(c)、C1 s精细谱(d)、O1 s精细谱(e)、Si2 p精细谱(f)Fig.7 XPS survey spectrum (a), Cu2 p fine spectrum (b), N1 s fine spectrum (c), C1 s fine spectrum (d), O1 s fine spectrum (e), Si2 p fine spectrum (f)
表 1 NG吸附Cu(II)的动力学模型参数
Table 1. Kinetic model parameters for Cu(II) adsorption onto NG
Kinetic model Parameter Result Pseudo-first order model qe,c/(mg·g−1) 40.6704 k1/(min−1) 0.0217 R2 0.9053 Pseudo-second order model qe,c/(mg·g−1) 45.1501 k2/(g·mg−1·min−1) 0.0006 R2 0.9653 kp1; kp2/(mg·g−1·min−0.5) 6.1698; 1.1281 Intra-partical diffusion model C1; C2 −7.7112; 17.6597 R12; R22 0.9760; 0.9538 Notes：qe,c$ - $Adsorbed amount of Cu(II) at a given time and the equilibrium concentration; k1, k2 and kp$ - $Rate constans for the pseudo-first order, pseudo-second order and intraparticle diffusion, respectively; R2$ - $Coefficient of determination; C$ - $Dsorption constant. 
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表 2 NG吸附Cu(II)的等温模型参数
Table 2. Isotherm parameters of Cu(II) adsorption onto NG at varying temperatures
Model and parameter 298 K 308 K 318 K Langmuir nonlinear fit ${q_{{\text{e}},{\text{c}}}}{\text{ = }}\dfrac{{7.39{\rho _{\text{e}}}}}{{1 + 0.05533{\rho _{\text{e}}}}}$ $ {q_{{\text{e}},{\text{c}}}}{\text{ = }}\dfrac{{14.15{\rho _{\text{e}}}}}{{1 + 0.10868{\rho _{\text{e}}}}} $ ${q_{{\text{e}},{\text{c}}}}{\text{ = }}\dfrac{{16.43{\rho _{\text{e}}}}}{{1 + 0.11819{\rho _{\text{e}}}}}$ R2
Freundlich nonlinear fit
R2
Sips nonlinear fit
R2
Temkin nonlinear fit0.9913
${q_{{\text{e}},{\text{c}}}}{\text{ = 29}}{\text{.55}}\rho _{\text{e}}^{0.2986}$
0.9980
${q_{{\text{e}},{\text{c}}}}{\text{ = }}\dfrac{{29.55{\rho _{\text{e}}}^{0.2987}}}{{1 + 0.00004{\rho _{\text{e}}}^{0.2987}}}$
0.9976
${q_{{\text{e}},{\text{c}}}}{\text{ = }} - 3.86 + 25.67\ln {\rho _{\text{e}}}$0.9966
${q_{{\text{e}},{\text{c}}}}{\text{ = 47}}{\text{.55}}\rho _{\text{e}}^{0.2080}$
0.9995
${q_{{\text{e}},{\text{c}}}}{\text{ = }}\dfrac{{49.26{\rho _{\text{e}}}^{0.2525}}}{{1 + 0.08598{\rho _{\text{e}}}^{0.2525}}}$
0.9994
${q_{{\text{e}},{\text{c}}}}{\text{ = 27}}{\text{.97}} + 20.49\ln {\rho _{\text{e}}}$0.9958
${q_{{\text{e}},{\text{c}}}}{\text{ = 54}}{\text{.45}}\rho _{\text{e}}^{0.1942}$
0.9997
${q_{{\text{e}},{\text{c}}}}{\text{ = }}\dfrac{{54.45{\rho _{\text{e}}}^{0.1942}}}{{1 + 0.00006{\rho _{\text{e}}}^{0.1942}}}$
0.9996
${q_{{\text{e}},{\text{c}}}}{\text{ = }}35.97 + 20.74\ln {\rho _{\text{e}}}$R2 0.9952 0.9992 0.9990
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表 3 不同种类气凝胶对Cu(II)吸附效果
Table 3. Cu(II) adsorption by different aerogels
Number Sorbent Reaction condition Adsorption capacity/(mg·g-1) Reference 1 Carbon aerogel pH=7.00, T=298 K, t=10 min 86.27 [29] 2 MnFe2O4-Cellulose aerogel pH=6.00, T=298 K, t=100 min 63.30 [30] 3 Graphene oxide aerogel pH=6.30, T=313 K, t=30 min 29.59 [31] 4 Porous alginate aerogel bead pH=4.50，T=298 K，t=950 min 126.82 [32] 5 Graphene oxide/carboxymethyl chitosan aerogel pH=5.00, T=303 K，t=600 min 95.37 [33] 6 Amine-modified sillica aerogel pH=5.00, T=318 K, t=720 min 130.45 Present study
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表 4 NG吸附Cu(II)的热力学参数
Table 4. Thermodynamic parameters for the adsorption of Cu(II) onto NG
T KC ΔG/(kJ·mol−1) ΔH/(kJ·mol−1) ΔS/(J·(mol·K)−1) 298 29550 −25.50 24.21 167.27 308 47550 −27.58 318 54450 −28.83 Notes：T—Temperature; ΔG—Gibbs free energy; ΔH—Change in enthalpy during adsorption; ΔS$ - $Adsorption process entropy change.
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