Extraction of heavy metals from aqueous solution using a dense gas process

This study demonstrates the potential of modified dense gases for the extraction of heavy metals such as copper and zinc from aqueous solutions. Carbon dioxide modified with the complexing agents tributyl phosphate (TBP) and di-(2-ethylhexyl)phosphoric acid (D2EHPA) was used as an organic phase to extract these metals from aqueous nitrate solutions. The CO2 extraction efficiency was determined at pressures above 100 bar and temperatures in the ranges of 25 degreesC to 45 degreesC. The extraction efficiency of modified carbon dioxide for zinc and copper was not significantly changed when the operating temperature and pressure changed from 25 degreesC to 40 degreesC and 120 to 200 bar, respectively. The effect of variables such as concentrations of metal ion and pH on the CO2 extraction efficiency was also examined. It was found that the CO2 extraction efficiency was dramatically dependant on the pH of the aqueous solutions. Increasing the pH of the aqueous solution from 1 to 7 lead to a 4 fold increase in zinc extraction efficiency. The CO2 extraction efficiency was 96% when the copper nitrate concentration decreased to 500 ppm.