Extraction of Chromium(III) and Chromium(VI) Species from Solid Matrices Using Green Solvent Supercritical Carbon Dioxide

Abstract

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 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. 5For 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. 10-12The in situ chelation/SFE technology presented here shows advantages over conventional solvent extraction method...

Recent Progress in Heavy Metal Extraction by Supercritical CO2 Fluids

“…Seifried and Guidard measured the solubility of Cu(TTA) 2 in SCF-CO 2 and found that solubilities were 0.99 × 10 −6 , 2.45 × 10 −6 , 3.32 × 10 −6 and 3.66 × 10 −6 mol/mol at pressures of 10.2, 12.9, 14.7, and 15.7 MPa, respectively, and temperature of 40 °C. 70 Wang and Chui evaluated the variation of extraction efficiencies with different pressure 71 and showed that the extractions of Cu were much higher than those of Cr and As. The extraction efficiency of Cu at 30.4 MPa was 3-fold higher than that at 10.1 MPa, indicating a high dependence on pressure.…”

“…Considering the time or cost saving, either static or dynamic extraction was set as 30 min. Wang et al 71 stated that the extraction efficiencies were about 87% at 40 min and 90% at 60 min dynamic time, hence, 20 min static extraction followed by 45 min dynamic extraction was chosen as standard extraction procedure. Therefore, determination of static and dynamic extraction time should be integrated into the device power consumption to yield an extraction equilibrium.…”

“…It is noteworthy that most environmental samples contain at least some water, which may either help or hinder the extraction process. Wang et al 71 successfully extracted Cr 3+ and Cr 6+ by using LiFDDC at 20.3 MPa and 60 °C in which SCF-CO 2 was the medium for extraction. They found that extraction efficiency was higher for a wet sample in comparison to that of the dry sample.…”

Non-chromatographic atomic spectrometric methods in speciation analysis: A review

“…[ 95] inorganic Hg species were extracted with the SeF 2 , by varying the operating parameters and it was possible to selectively extract methyl, phenyl and inorganic Hg from different matrices [98]. Species of chromium were also identified using the SFE [99,100]. Supercritical fluids modified by the addition of complexing agents have been utilized in the extraction of metal ions from various solid and liquid matrices [101].…”

Chemical Speciation of Chromium(III) and (VI) Using Phosphonium Cation Impregnated Polyurethane Foams Prior to Their Spectrometric Determination

“…clay minerals, gelatin, biosorbent, and active carbon, synthetic polymers, C18-bonded silica, silica gel immobilized with Zr(IV) and Zr(VI) phosphate have been used for preconcentration and/or separation of trace and ultra trace amounts of toxic metal ions from complex matrices. [8][9][10][11][12][13][14][15] Determination and chemical speciation of Cr(III) and Cr(VI) at trace levels in welding fumes and in drinking water samples using strong anion ion-exchange, 16 neutral alumina, 17 mini-columns packed with resin immobilized with 8-hydroxyquinoline, 18 activated carbon, 19 leaching procedure with sodium carbonate 5 and drawn and modified lingo cellulosic materials 20,21 have been reported. However, some of these SPE methods [10][11][12][13][14][15] are too expensive or unselective; they require careful experimental conditions, are time consuming and are not compatible with the detection limit of Cr(VI) in various matrices.…”