Adsorption Removal of Phosphate from Water with an Fe-Zr Bimetallic Organic Framework

Hong-cheng CHEN; Yu-ming HUANG; Ping FENG

doi:10.13718/j.cnki.xdzk.2019.05.013

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An Fe-Zr bimetallic organic framework(MOF) was synthesized with the solvothermal method, and the adsorption characteristics of phosphate onto Fe-Zr MOF in aqueous solution were studied. The effects of solution pH and dosage of Fe-Zr MOF adsorbent on the removal of phosphate were investigated. The results showed that the maximum phosphate adsorption with Fe-Zr MOF was obtained at the neutral pH condition. Also, the adsorption of phosphate onto Fe-Zr MOF reached equilibrium at 24 h, and the pseudo-second-order kinetic model was well suitable for modeling phosphate adsorption by Fe-Zr MOF. The adsorption isotherm of phosphate fitted well with the Langmuir equation, with a maximum adsorption capacity of 285.7 mg/g. After ten regeneration cycles, the removal efficiency of phosphate remained as high as 80.6%, showing a good reusability of the Fe-Zr MOF adsorbent.

Corresponding author: Yu-ming HUANG ;

Received Date: 08/08/2018

Available Online: 20/05/2019

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Abstract: An Fe-Zr bimetallic organic framework(MOF) was synthesized with the solvothermal method, and the adsorption characteristics of phosphate onto Fe-Zr MOF in aqueous solution were studied. The effects of solution pH and dosage of Fe-Zr MOF adsorbent on the removal of phosphate were investigated. The results showed that the maximum phosphate adsorption with Fe-Zr MOF was obtained at the neutral pH condition. Also, the adsorption of phosphate onto Fe-Zr MOF reached equilibrium at 24 h, and the pseudo-second-order kinetic model was well suitable for modeling phosphate adsorption by Fe-Zr MOF. The adsorption isotherm of phosphate fitted well with the Langmuir equation, with a maximum adsorption capacity of 285.7 mg/g. After ten regeneration cycles, the removal efficiency of phosphate remained as high as 80.6%, showing a good reusability of the Fe-Zr MOF adsorbent.

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磷对能量转运和生物生长有重要作用，而且也是广泛使用的营养元素之一.然而，含磷产品的过量消费不可避免地会造成大量污染物，引起水体富营养化^[1].因此，研究简单、高效去除磷的方法有重要意义.吸附技术因其简单、高效及成本低已成为一种广泛使用的除磷方法^[2].金属有机框架(metal organic framework，MOF)是一类多孔材料，是由有机配体与无机金属离子或金属离子簇自组装而成的配位聚合物^[3]，具有高孔隙率、低密度及大比表面积等特点，可直接吸附水中的各种有机污染物^[4]，但鲜见其用于废水除磷的研究报道.本研究使用2-氨基对苯二甲酸作为配体，合成双金属Fe-Zr MOF，探讨其吸附去除水中磷酸盐的特性，研究结果可为除磷吸附剂的筛选奠定基础.

1. 实验方法与材料

1.1. 仪器及试剂

紫外-可见分光光度计(UV-2450，岛津苏州)；pH计(PHS-3D，上海精密科学仪器有限公司). FeCl₃·6H₂O，ZrCl₄及N，N´-二甲基甲酰胺(DMF)购自Sigma-Aldrich(上海，中国)；2-氨基对苯二甲酸NH₂BDC购自TCI化成工业发展有限公司(中国上海)；磷酸二氢钾、钼酸铵、抗坏血酸及酒石酸锑钾购自成都化学工业有限公司(成都，中国).其他化学试剂，如乙醇、HCl和NaOH均购自重庆市钛新化工有限公司(重庆，中国).

1.2. Fe-Zr MOF的合成

将1 mmol FeCl₃·6H₂O (0.271 g)、1 mmol ZrCl₄ (0.233 g)和2 mmol (0.362 g) NH₂BDC溶于40 mL DMF中，然后再加入0.2 g PVP，搅拌至完全溶解后，超声30 min，将混合溶液转移至反应釜中，加热至150 ℃反应24 h后，冷却至室温，用DMF和乙醇交替洗涤3次后，在90 ℃下干燥12 h，即得Fe-Zr MOF材料.分别按照n(Fe³⁺):n(NH₂BDC)=1:1，n(Zr⁴⁺):n(NH₂BDC)=1:1的比例将混合物溶解在20 mL DMF中(不添加PVP)，其余步骤同合成Fe-Zr MOF一样，即可得到Fe-MOF和Zr-MOF.

1.3. 实验方法

本实验采用恒温震荡摇床在25 ℃ 180 r/min进行吸附去除磷实验，pH值用0.1 mol/L NaOH或0.1 mol/L HCl进行调节.磷酸盐质量浓度用钼酸铵分光光度法进行测定^[4].吸附量按方程(1)计算：

式中q为吸附量(mg/g)；C₀为初始磷酸盐质量浓度(mg/L)；C_t为t时刻反应后磷酸盐的质量浓度(mg/L)；V为溶液体积(L)；m代表吸附剂的量(g).

吸附剂用量优化实验条件：Fe-Zr MOF吸附剂量从1 mg到40 mg，50 mL KH₂PO₄溶液(50 mg/L)，pH=7.0，温度为25 ℃，吸附时间为24 h.

pH值的优化实验条件：pH值优化范围从3.0到10.0，2 mg吸附剂，50 mL KH₂PO₄溶液(50 mg/L)，温度25 ℃，吸附时间24 h.

吸附动力学实验方法：将一系列50 mg/L磷酸盐溶液(50 mL)调至pH=7.0，加入2 mg Fe-Zr MOF，放入振荡器中反应30 min至30 h，将实验数据用准一级动力学和准二级动力学模型进行拟合^[5]：

式中q_e和q_t分别为平衡吸附量和t时刻的吸附量(mg/g)，k₁为准一级动力学常数(1/h)，k₂为准二级动力学常数(g/mg·h).

吸附等温线实验方法：将2 mg Fe-Zr MOF加入50 mL不同质量浓度(5~200 mg/L)的磷酸盐溶液(pH=7.0)中，然后在25 ℃以180 r/min摇动24 h，取上清液测定磷酸盐浓度，将实验数据用Langmuir(方程4)和Freundlich等温线(方程5)模型模拟^[6]：

式中C_e(mg/L)是磷酸盐的平衡质量浓度，q_e(mg/g)是平衡吸附量，q_max是最大吸附容量(mg/g)，K_L(L/mg)和K_F(mg/g)分别是Langmuir常数和Freundlich常数. n是Freundlich指数.

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Adsorption Removal of Phosphate from Water with an Fe-Zr Bimetallic Organic Framework

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Adsorption Removal of Phosphate from Water with an Fe-Zr Bimetallic Organic Framework

Corresponding author: Yu-ming HUANG ;