Extraction of Chromium(III) and Chromium(VI) Species from Solid Matrices Using Green Solvent Supercritical Carbon Dioxide
Determination and removal of heavy metals from natural samples are the essential requirements in a variety of environmental investigations. 1 Chromium speciation has attracted great attention in scientific communities due to the toxic nature of Cr(VI) relative to Cr(III). 2 Hexavalent chromium is considered to be extremely toxic and carcinogenic. In aqueous solution, trivalent chromium is difficult to extract due to its strong hydration by water molecules. This strong hydration prohibits the
... on prohibits the access of chelating agents to the metal centers in conventional solvent extraction. 3,4 With a selective chelating agent and under specific conditions, Cr(VI) and Cr(III) can be separated from aqueous solutions by solvent extraction. 5 For example, EPA methods 218.3 and 218.4 indicate that Cr(VI) can be extracted by ammonium pyrrolidinedithiocarbamate (APDC) into isobutyl methyl ketone (IBMK) at pH 2.8. 6 In the EPA's methods, Cr(III) is initially oxidized to Cr(VI) by KMnO4, followed by extraction with APDC. The amount of Cr(III) is calculated based upon the difference of total Cr minus Cr(VI) removed. Studies also indicate that Cr(VI) reacts with ammonium pyrrolidinedithiocarbamate ligand to form two products, Cr(PDC)2(OPDC) and Cr(PDC)3. The former involves coordination with two normal dithiocarbamate ligands and one dithioperoxycarbamate ligand. The latter was formed with three PDC ligands. 5 The complexes formed by Cr(VI) and Cr(III) individually with sodium diethyldithiocarbamate (NaDDC) give different chemical structures. For example, Tande et al. 3 and Hope et al. 8 reported that Cr(III) forms a Cr(DDC)3 complex with NaDDC, whereas Cr(VI) is first reduced by the ligand and then forms two different complexes Cr(DDC)3 and Cr(DDC)2(ODDC), where ODDC represents an oxygen atom inserted between the metal ion and one of the sulfur atoms. 3,8 These two complexes formed by Cr(VI) with NaDDC can be separated by HPLC. 3, 9 Extraction of Cr(III) by APDC occurs at higher pH but appears to have a very low efficiency at room temperature. The nonextractability of Cr(III) by solvent extraction is ascribed to the fact that Cr(III) is strongly hydrated and the displacement of coordinated water molecules by dithiocarbamate ligand is kinetically difficult. 3,7 The complexes formed by Cr(VI) or Cr(III) with NaDDC have different chemical structures. Of particular interest to analytical chemists is the possibility of applying in situ chelation/SFE technique for the extraction metal species Cr(VI) and Cr(III) from solid materials. Only a few papers have been reported regarding the investigation of Cr extraction with ScCO2. Supercritical fluid extraction has become an attractive alternative to conventional solvent extraction from environmental samples. There are several advantages from an environmental standpoint, rooted in the characteristics of carbon dioxide being nontoxic, nonflammable, and having the potential for recycling.    The in situ chelation/SFE technology presented here shows advantages over conventional solvent extraction methods, in which it greatly minimizes the organic solvent generation in decontamination processes. Carbon dioxide is an excellent solvent for SFE due to its moderate critical constants (Temp. = 31˚C and Pc = 7.4 MPa), inertness, and tunable solvating strength. Solutes can often be separated from the supercritical fluid by simply venting the benign CO2 into the atmosphere, leaving pure solutes behind. Moreover, supercritical CO2 can easily penetrate and diffuse into small pores of solid matrices and effectively remove organic and inorganic species. The following factors are important for effective extraction of metal species using ScCO2: the solubility of the chelating agent, the solubility and stability of the metal complex, the pressure, the temperature, and the chemical forms of metal species and 1337 ANALYTICAL SCIENCES NOVEMBER Supercritical fluid extraction (SFE) provides an environmentally green technique to decontaminate chromium species from solid matrices using supercritical fluid carbon dioxide (ScCO2). Methanol and a small amount of water were found to significantly improve the extraction efficiency. The fluorinated chelating agent lithium bis(trifluoroethyl)dithiocarbamate (LiFDDC) was effective in removing Cr ions in methanol-modified CO2 via in situ chelation/SFE technique. This paper indicates that the extraction efficiencies of Cr(III) and Cr(VI) from solid matrices can be greatly increased to more than 92% in the presence of a small amount of water, using 5% methanol-modified CO2 containing LiFDDC as an extractant. Chromium species in a wood waste sample in the form of chromated copper arsenate (CCA) can be extracted, but the extraction efficiency is not as good as expected, possibly due to the complications of the chemistry of Cr species in different oxidation states and to matrix effects.