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<p>Perfluorosulphonate Nafion cation exchange ionomer membrane (du Pont de Nemours and Co) is a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer with ion rich clusters arranged in a reversed micelle structure. The permselective nature of membrane allows separation and is affected by various experimental parameters (pH, solution composition). Pretreatment of Nafion using a prescribed method (developed by DuPont) was essential prior to its use. Selective separation is generally achieved by using masking agents to reduce interferences. Therefore, it became crucial to understand the effects of both pretreatment protocols and presence of complexing agents. Orthophenanthroline was used as a complexing ligand both in solution as well as loaded within the pretreated membranes. Water sorption of pretreated membranes incorporated with metal ions, ligands and metal–ligand complex has been studied. The water sorption of the pretreated impregnated membrane follows the order metal ions &gt; metal – Oph &gt; Oph. The use of various models (Peleg, pseudo first order and pseudo second order) showed that the water uptake is physisorption in nature. The water molecules are present in a single layer bound to the surface as water of hydration of the cations. The initial rate of sorption is found to be highest for hot acid pretreated Nafion. The presence of orthophenanthroline in solution showed a greater decrease in Cu uptake as compared to Zn. However, impregnation of the ligand within the membrane showed a preferential uptake of Cu. Both the observations could be attributed to the stronger complexation of Cu with ligand. Kinetic and mechanism modelling of data indicates that metal ion uptake is chemisorption in nature and follows both intraparticle and film diffusion mechanisms. Simulated binary mixtures of Cu and Zn in different ratios were used for separation using the impregnated membranes. It was seen that a preferential uptake of Cu was achieved. Under the conditions of study (pH = 5), the ligand was not leached out of the membranes. The removal of ligand and its metal complex could be achieved using 0.1&#xa0;M acid. UV Visible spectroscopy of loaded membranes confirmed the change in water cluster as well as the presence of both orthophenanthroline and its metal ion complex within the water clusters. The difference in spectra of copper complexes in solution and membrane confirm the presence of disparate structures in membrane phase (probable dimer formation). DFT calculations carried out to understand the process reveal that Cu can form both 1:1 and 1:2 complexes to nearly same extent while Zn preferably forms the 1:2 complex. The binding energy values of the 1:2 metal complexes (Cu 44.8&#xa0;kcal/mol and Zn 32.1&#xa0;kcal/mol) indicate Cu forms a stronger complex as compared to Zn.</p>

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Metal and water sorption characteristics of Nafion membranes: role of pretreatment and complexing agents

  • 期刊论文

  • 发表日期：

    2026年3月12日

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  • Jayshree Ramkumar,
  • S. Chandramouleeswaran,
  • Naman K. Bharti,
  • Mahesh Sundararajan

摘要

Perfluorosulphonate Nafion cation exchange ionomer membrane (du Pont de Nemours and Co) is a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer with ion rich clusters arranged in a reversed micelle structure. The permselective nature of membrane allows separation and is affected by various experimental parameters (pH, solution composition). Pretreatment of Nafion using a prescribed method (developed by DuPont) was essential prior to its use. Selective separation is generally achieved by using masking agents to reduce interferences. Therefore, it became crucial to understand the effects of both pretreatment protocols and presence of complexing agents. Orthophenanthroline was used as a complexing ligand both in solution as well as loaded within the pretreated membranes. Water sorption of pretreated membranes incorporated with metal ions, ligands and metal–ligand complex has been studied. The water sorption of the pretreated impregnated membrane follows the order metal ions > metal – Oph > Oph. The use of various models (Peleg, pseudo first order and pseudo second order) showed that the water uptake is physisorption in nature. The water molecules are present in a single layer bound to the surface as water of hydration of the cations. The initial rate of sorption is found to be highest for hot acid pretreated Nafion. The presence of orthophenanthroline in solution showed a greater decrease in Cu uptake as compared to Zn. However, impregnation of the ligand within the membrane showed a preferential uptake of Cu. Both the observations could be attributed to the stronger complexation of Cu with ligand. Kinetic and mechanism modelling of data indicates that metal ion uptake is chemisorption in nature and follows both intraparticle and film diffusion mechanisms. Simulated binary mixtures of Cu and Zn in different ratios were used for separation using the impregnated membranes. It was seen that a preferential uptake of Cu was achieved. Under the conditions of study (pH = 5), the ligand was not leached out of the membranes. The removal of ligand and its metal complex could be achieved using 0.1 M acid. UV Visible spectroscopy of loaded membranes confirmed the change in water cluster as well as the presence of both orthophenanthroline and its metal ion complex within the water clusters. The difference in spectra of copper complexes in solution and membrane confirm the presence of disparate structures in membrane phase (probable dimer formation). DFT calculations carried out to understand the process reveal that Cu can form both 1:1 and 1:2 complexes to nearly same extent while Zn preferably forms the 1:2 complex. The binding energy values of the 1:2 metal complexes (Cu 44.8 kcal/mol and Zn 32.1 kcal/mol) indicate Cu forms a stronger complex as compared to Zn.