Design of highly CO2-soluble chelating agents .2. Effect of chelate structure and process parameters on extraction efficiency

In our previous papers, we described a strategy for design, synthesis, and evaluation of a group of highly CO2-soluble chelating agents that can be used to effectively extract a variety of metals from various matrices into pure CO2 at room temperature and pressures below 13.8 MPa. Conventional chelating agents are functionalized with so-called ''CO2-philic'' tails, namely, fluoroalkyl, fluoroether, or silicone-containing materials, which allows solubilities greater than 100 mM at pressures less than 13.8 MPa. Here we have studied the effect of several process variables, including pressure, nature of the extraction matrix, presence of interfering ions, multiple-stage reactions, molar ratio of chelating agent to metal, and also time of extraction on the extraction efficiency of these chelating agents. Results show that once the minimum pressure for solubilizing the chelating agent is reached, increasing the pressure has little or no effect on the efficiency of extraction. Further, the low pH of a mixture of water and high-pressure CO2 hinders the extraction efficiency of some chelating agents significantly, while exercising little apparent effect on others. In addition, the extraction efficiency of the chelating agent can be improved using a multiple-stage extraction scheme. Highly selective chelating agents can be designed that will extract the target metal even in the presence of a 10-fold excess of interfering ions.