Extraction of cadmium from dilute solution using supported liquid membrane

Area of Science:

• Environmental Chemistry
• Separation Science
• Materials Science

Background:

• Cadmium (Cd) is a toxic heavy metal posing significant environmental and health risks.
• Effective methods for cadmium removal from aqueous solutions are crucial for environmental remediation.
• Supported Liquid Membranes (SLMs) offer a promising technology for selective ion separation.

Purpose of the Study:

• To evaluate the extraction efficiency of three phosphoric acid derivatives for cadmium removal using SLMs.
• To elucidate the mechanism of cadmium extraction, including ion exchange and complex formation.
• To investigate the influence of various operational parameters on cadmium flux through the membrane.

Main Methods:

• Utilized three phosphoric acid derivatives: Di-(2-ethylhexyl) phosphoric acid (D2EHPA), 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A), and bis-(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272).
• Performed equilibrium studies to determine the stoichiometry of cadmium extraction and the release of H+ ions.
• Measured diffusion constants of cadmium-extractant complexes within the membrane phase.
• Systematically varied parameters including feed solution pH, extractant concentration, flow rate, strip solution concentration, and cadmium concentration to assess their impact on cadmium flux (J(Cd)).

Main Results:

• Equilibrium studies revealed a 1:2 stoichiometry for cadmium to extractant complexation, with the release of two H+ ions per Cd2+ ion.
• Diffusion constants for Cd-D2EHPA, Cd-PC-88A, and Cd-Cyanex 272 complexes were determined as 2.53 x 10(-9), 5.435 x 10(-9), and 11.22 x 10(-9) m(2)/s, respectively.
• Maximum cadmium extraction percentages were achieved at pH 7.5 with specific extractant concentrations (600 mol/m(3) for D2EHPA and PC-88A; 800 mol/m(3) for Cyanex 272).
• The extraction efficiency followed the order: D2EHPA > PC-88A > Cyanex 272.

Conclusions:

• Phosphoric acid derivatives are effective extractants for cadmium removal in SLM systems.
• The extraction mechanism involves ion exchange and complex formation, influenced by solution pH and extractant concentration.
• D2EHPA demonstrates superior performance for cadmium extraction compared to PC-88A and Cyanex 272 in the studied SLM configuration.