Supercritical CO2 Extraction of Heavy Metals Cu, Zn and Cd from Aqueous Solution using D ithizone as Chelating Agent

Corresponding Author: Jeton Halili Department of Chemistry, Faculty of Mathematical and Natural Sciences, University of Prishtina, Prishtina, Kosovo Email: jeton.halili@uni-pr.edu Abstract: Metal ions in liquid samples can be extracted by supercritical CO2, containing a suitable ligand. This paper presents the first experimental data for the extraction of three heavy metals Cu, Zn and Cd from aqueous samples in the presence of Dithizone as chelating agent. Extractions by supercritical CO2 have been performed at 120 bars with 50°C used dynamic technique. After adding the CO2 on the aqueous metal solution, the pressure and temperature were set and extraction was performed in three periodical times; 30, 60 and 120 min at constant temperature and pressure with a flow rate of CO2 2 mL min −1 . After the extraction, the CO2 was released slowly through a restrictor. Methanol was used as a modifier (v/v) 10% and the pH value was set (10). After the extraction process, the remaining aqueous solution in the extractor was analyzed for its metal content by Atomic Absorption Spectrophotometer (AAS), determining the recovery of the metal by CO2. During dynamic extraction noticed increased of extraction in function of time for all three metals; Cu (61.12-73.52%, RSD = 2.694, n = 3); Zn (78.33-87.00%, RSD = 1.243, n = 3) and Cd (70.60-82.40%, RSD = 2.227, n = 3).


Introduction
Chelating combined with solvent extraction is one of the most widely used techniques for preconcentration and separation of metal ions from aqueous samples for analytical purposes Chein (1991). In many cases, solvent extraction requires the use of toxic organic liquids, generating environmental problems. The technology of Supercritical Fluid Extraction (SFE), offers the opportunity to efficiently extract both relatively non-polar analytes as well as ionic materials (such as metal ions) that can be mobilized with the addition of complexing agents Kronholm (2007). The supercritical fluid extraction SFE method is becoming popular alternative technique for the extraction of a wide range organo-metallic and inorganic analytes Wang and Marshall (1995). The high diffusivity, low viscosity and variable solvent strength as a function of density (P, T) are some of the attractive features of supercritical fluids Gere et al. (1983). Their main advantage lays in the possibility of separating and drying the product by simple expansion, while the gas can be recovered, recycled and reused without the need of purification steps. The environmental benefits of using SFE in industrial processes, such as low energy consumption during operation, show their potential of replacing the far more environmentally damaging conventional organic solvents. Carbon dioxide is the most commonly used solvent as SFE, due primarily to its low critical parameters (Tc = 31.1°C, Pc = 7.38 MPa), being inexpensive, non-toxic, non-flammable, readily available and recycling capability (Seied et al., 2004;Shamsipur et al., 2001;Takeshita et al., 2000). Direct extraction of metal ions by supercritical CO 2 is inefficient, because of the charge neutralization requirement and the weak solute-solvent interactions Cui et al. (2000). One approach of SFE of metals is by converting charged metal ions into neutral metal chelates using organic chelating agents. Carbon dioxide is a non-polar gas and is suitable for solubility of nonpolar substances. In order to increase the solubility of polar species in SC-CO 2 , the polarity of supercritical fluid must be increased via utilization of co-solvents such as methanol, ethanol (Niehaus et al., 1997;Tessy et al., 2009;Ghoreishi et al., 2009). Extraction of metal ions from an aqueous phase into CO 2 is influenced by the presence of carbonic acid which lowers the pH of the aqueous phase to approximately 3. While some metals are extracted efficiently at pH ~ 3 (Toews et al., 1995;Niemeyer and Bright, 1998) other metal ions, cannot be efficiently extracted at such an acidic pH.
Diverse experiments for the extraction of heavy metals from different sources by using SC-CO2 have been conducted by several researchers. Laintz et al. (1992) demonstrated Cu extraction from liquid and solid samples using SC-CO 2 in the presence of ligand bi's (trifluoroethyl) dithiocarbamate (FDDC). A variety of chelate agents, including dithiocarbamates, β-diketone, reagents organophosphorus compound, macrocyclic compounds and surfactants, are used for metal complexity in SC-CO 2 .
The main objective of this research was to develop SC-CO 2 extraction procedure for metal ions in liquid samples and to elucidate further the parameters linked to the SFE extraction process. Complex metal ions were determined in the presence of Dithizone. The main goal was to optimize the extraction factors to give better results in qualitative and quantitative scope, without neglecting the use of statistical and instrumental analytical methods to achieve fast quantitative verification.

Materials and Methods
Reagent ICP standard solution of Copper, Zinc, Lead and Cadmium (Fluka) were obtained. Dithizone (MERCK) was used as chelating agent. Methanol extra pure (Merck) was used as reagent of reaction and collecting agent. The reagents used were all analytical grade and of ultra-pure quality. De-ionized water is used in all experiments.

Analytical Techniques
Analytical techniques were used to monitor the outcome of experiments conducted during this study.

Atomic Absorption Spectrometry (AAS)
Flame Atomic absorption spectrometer (Solaar M6 TJA Solutions) equipped with cathode lamp "hollow" were used for determining the concentration of metals such as Cd Cu and Zn. The burner system air-acetylene is used. Flow rate of nebulizer, 5ml min −1 . Wavelengths used are: Cd: 228.8 nm (0.5 nm), Cu: 324.8 nm (0.5 nm) and Zn: 213.9 nm (0.5 nm).
Calibration of the instrument was made using 1000 mg/L standard solution of these metals. All concentrations of these metals are expressed in parts per million (ppm).

Apparatuses
All experiments were carried out in a supercritical fluid extraction equipment at the Technical University of Graz, Institute of Chemistry. To carry out the objectives of this study, the supercritical extraction system shown in Fig. 1 was used. All extractions were performed using a special stainless steel cylindrical extractor vessel (50 mL). CO 2 pump, bath circulation, oven and vessels were used from company "Applied Separation". Tubes, valves and accessories were obtained from company "Nova Swiss". For the analysis of the metal ions extracted, an Atomic absorption spectrometry was used.

Procedure
During our research, we used dynamic techniques of supercritical CO 2 extraction. The extraction were performed in three time periods; 30 min, 60 min and 120 min with the flow rate 2 mL min −1 The dynamic extraction procedure is described below.
From the stock standard solution was prepared 20 mL of 10 mg L −1 concentration metal solution of Cu, Zn and Cd. The sample was placed in extraction cell and chelating agent (Dithizone) was added. Extractions were performed in presence of modifier Methanol 10% in basic pH value (pH = 10). Carbon dioxide was supplied from a supply tank, maintained at a pressure of approximately 53 bar. The extraction was carried under supercritical condition temperature 50°C, pressure 120 bar. The solute was extracted at a constant temperature and pressure of CO 2 in different periods of time; 30 min, 60 min and 120 min with flow rate 2 mL min −1 . Then the solute was removed from the extraction vessel and the vessel was cleaned with the izopropanol, to collect any solute that had precipitated within the vessel during depressurization. When the extraction was completed, solution was removed from the extraction vessel and was digested for 20 min in a microwave with HNO 3 (65%) at a temperature of 180°C. The subsequent solutions were analyzed with AAS (atomic absorption spectrometry). The extraction efficiencies were calculated based on the amount of the metal ion in the aqua sample before and after the extraction.

Results and Discussion
The results of our research are presented in tables and charts as follows.
The use of supercritical fluid for extraction of various substances found wide applications recently. This method is now used to determine the total contents of (organo-metallic) complex compounds. Recently SFE technique was used by Liu et al. (1994). In preliminary experiments, organo-metallic mercury compounds were extracted with super-critical fluid using CO 2 as solvent and 5% to 10% methanol as modifier and the extraction rate was 64-89%. Authors Chein et al, (1993) during their research concluded that metal ions can be extracted with SFE using CO 2 in the presence of suitable ligands. They have reported the application of SFE for the extraction of Cu 2+ , Cd 2+ , Zn 2+ and Hg 2+ in the presence of ligand lithium bis (trifluoroethyl) dithiocarbamate (LiFDDC). Negatively charged ligand (FDDC-) reacts with metal ions to form neutral chelate complexes which are soluble in CO 2 .
In the scientific literature to our knowledge there were no data concerning the use of Dithizone as a ligand during the extraction of heavy metals with supercritical CO 2 . However scientists have studied chemical properties of Dithizone. Fischer and Leopoldi, (1937) postulates that all metal dithizonates can exist in socalled keto-forms, in which the hydrogen atom of one phenylimino-group is replaced by metal (as II). Such complexes form preferentially in acid or neutral solution, but in basic solutions or with a deficiency of dithizone, they pass over into "enol" complexes (as I or III) derived from the "thiol" form of the reagent by the loss of two hydrogen atoms (Fig. 3). Irving et al. (1949) have studied dithizone as an acid and they have found the first dissociation constant. They also concluded that the extraction of metal-dithizone complex is influenced by pH value.
The extraction of metal-dithizone complexes during our research is done in pH values (pH = 10). Based on the results in Table 2, during dynamic extraction is noticed increasing of extraction in function of time. This increase is linear in terms of pressure 120 bar and temperature 50°C (Fig. 2). Our results agree with the results of other authors regarding the effect of time. According to literature data Osamu et al. (2012) the solubility of TBP-HNO 3 complex in supercritical CO 2 gradually increased as a function of time from 20min to 80 min and the recovery fraction for M TBP-HNO 3 complex was (20 min = 1.8*10 −5 ; 80 min = 7.6*10 −4 ). This is also reflected in our case for the metal-dithizone complexes.
Researchers Fengying et al. (2014) have studied the effect of pressure and temperature on the extraction efficiency. Undoubtedly pressure plays a very important role in this process. Based on the ideal gas law: n -is the amount of substance and can be defined as equal to ρV/M and the equation can be formulated as: When the temperature and molar mass are constant, the pressure is directly proportional to the density. SCF-CO 2 density increases with pressure. Authors Seifried and Guigard, (2003) have measured the solubility of Cu (TTA) 2 in SCF-CO 2 in different pressures and have found that the solubility ranges in values; 0.99*10 −6 , 2.45* 10 −6 , 3.62*10 −6 and 3.66*10 −6 mol/mol at pressure 102 bar, 129 bar, 147 bar and 157 bar at the same temperature 40°C. During our study we have measured the efficiency of extraction in fixed pressure (120 bar) ( Table 1).
Temperature is another important factor in the process of extraction heavy metals with super-critical CO 2 . In thermodynamic terms, when the temperature increases, the saturated vapor-pressure increases correspondingly, the solutes dissolve in the supercritical CO 2 easily, but on the other hand the increase in temperature reduces the density of CO 2 which means also reduces CO 2 dissolving ability. According to the author mentioned above, the most efficient extraction temperature was obtained at 60°C. During our experiment the extraction temperature was obtained 50°C (Table 1).

Conclusion
Dithizone was used as chelating agent in this experiment of extraction of Cu, Zn and Cd from aqua samples. The extraction efficiency, which is expressed as % extraction at three different operation time (30 min, 60 min and 120 min), varies from (61.12 to 73.52 for Cu%; 78.33 to 87.00 for Zn%; 70.60 to 82.40 for Cd%) in fixed value of pressure and temperature (P = 120 bar and T = 50°C).