Removal of trace heavy metals from a natural medicine material by supercritical CO₂ chelating extraction

Removal of arsenic, mercury, lead, and copper from a natural medicine material by supercritical carbon dioxide containing a chelating ligand, diethylammonium diethyldithiocarbamate (Et₂NH₂DDC), was studied. The contents of As, Hg, Pb, and Cu in the natural medicine material, which was a mixed powder of six plants, were < 2, 0.2, 2, and 10 μg/g, respectively. It was shown that Et₂NH₂DDC was an effective ligand for the extraction of the heavy metals. The effects of pressure, temperature, modifier, and time on the extraction were systematically investigated. The results showed that there were the extraction efficiency-based optimal temperatures of arsenic, lead, and copper with the order T_m,Pb < T_m,As < T_m,Cu, while the extraction efficiency of mercury decreased as the temperature increased. When the pressure or the amount of the modifier, ethanol in supercritical CO₂, was increased, the extraction efficiencies of arsenic, lead, and mercury were not enhanced noticeably, while the extraction efficiency of copper increased markedly. Except for lead, the extraction efficiencies of As, Hg, and Cu increased as the extraction time extended. With the suitable pretreatment, the natural medicine material in which the content of arsenic was 4 times more than the limit of the U.S. Food and Drug Administration (FDA) could meet the standard after the extraction with Et₂NH₂DDC in supercritical ethanol-modified carbon dioxide.