Metal Ions Extraction with 2-Methyl-8-Hydroxy-Quinoline as Chelating Ligand in Supercritical Carbon Dioxide

Yang Haijian，Cai Zhuofu，Li Huanhuan，Zhao Lu，Tian Jia

(College of Chemistry and Materials Science，South-Central University for Nationalities，Wuhan，430074，China)

Metal waste contamination in environment is becoming a hot issue that we can not elude any more.Among the current technologies removing heavy metals from solid matrices or liquid samples，chelation combined with solvent extraction is one of the most widely used［1］.It does have high efficiency，however，there are also inherentlimitations in it.Now，supercritical fluid extraction(SFE)has become a better choice to conventional solvent extraction in many separation processes［2］.However carbon dioxide is not effective to extract metal ions from matrix directly，because the polar metal ions can be hardly dissolved in sc-CO2.Butwhen metalionsarechelated with ligands，the resultant metal complexes can be easily dissolved in sc-CO2，accordingly removed from the matrices［3］.And up to now，many chelating ligands，such as crowns，β-diketones，dithiocarbamates，phosphate，and amines have been proven to be useful for metal extraction in sc-CO2［4］.Therefore，this chelation-SFE has attracted worldwide interest and might be a promising technology for the treatment of metal-contamination waste materials.

It is known that quinoline is a strong chelating agent in coordination and can form stable complexes with almost all the heavy metals.Since we have tested the solubility of 2-methyl-8-hydroxyl-quinoline(MHQ)in sc-CO2，this paper studied the effects of time，temperature，pressure，and molar ratio of ligand to metal on the extraction efficiency of metal ions with MHQ.Then the extractions of 8 different metal ions were tested，the proposed extraction mechanism was analysed and the extraction constants were calculated in detail.

As we know，the critical system is a extrem system whose PH is kept around 3 under normal pressure and temperature，which makes it a highly acid system.Besides，a routine extraction lasts about 30 minutes，so in this case，compound will finally break up no matter how stable it is.In this work we didn＇t use any addictive while other articles introduced the toxic methanolas addictive.And the extraction equilibium constant was carefully calculated to further comprehend the extraction mechanism.

1 Experiment

1.1 Reagents and Apparatus

2-methyl-8-hydroxyl-quinoline，stock solutions of Cu2+，CO2+，Ni2+，Fe3+，Pb2+，Cd2+，Mn2+，Zn2+at 1000 μg mL－1(Acros Chem.Co.).CO2(99.99%purity，Wuhan Steel Co.).

CO2delivery pump(JASCO PU-CO2)，back pressure regulator(JASCO BP-1580-81)，Atomic absorption spectrophotometry (AAS，AA-6300，Shimadzu).

1.2 Extraction procedure

10 μL of metal ion solution from a stock solution(1.0 g/mol)was adsorbed on the 1 cm × 1 cm cellulose-based filter paper and the spiked papers were dried in an electric vacuum oven for 24 h at 373 K.Prior to extraction 15 μL ultra pure water was dropped into the vessel，then the spiked paper and pressurized CO2was introduced to the extraction vessel after stirring for 20 min.The extraction system was kept at a constant temperature by a temperature controller jacket with circulator.Afterextraction，the system was depressurized and cooled to room temperature.The spiked papers were digested with 5 mL HNO3(1 M)and analyzed with an AAS to measure the concentration of metal ions residue.The extraction efficiency was calculated according to the following equation［5］.

Where ciand cfare the initial and residual concentration of metal ions in the extraction vessel.

2 Results and discussion

2.1 Effect of time on extraction efficiency

5 different cases(10，20，30，60，90 min)were investigated to find out the effect of time on the extraction efficiency at 25 Mpa，323 K，and ligand to metal molar ratio of 50︰1 respectively.As shown in Fig.1，the extraction efficiencies increased steeply with the increasing of time before 30 min，later，longer time extraction resulted lower efficiency，due to decomposition of the complexes or the ligand during the prolonged exposure to the acidic environment of sc-CO2containing water.

Since the critical system is a extrem system whose PH is kept around 3 under normal pressure and temperature，which makes it a highly acid system，and a routine extraction lasts about 30 minutes，in this case the compound will finally break up no matter how stable it is.

2.2 Effect of pressure on extraction efficiency

The pressure has more influence on the extraction of metal ions from solid matrix in sc-CO2.As shown in Fig 2，the extraction efficiency increased with the increase of the CO2pressure.This might be due to the increase in the solubility of ligand and the complexes formed with metal ions，which resulted from a stronger solvent power by promoting the pressure［6］.

2.3 Effect of temperature on extraction efficiency

The varation of temperature during SFE affected the density of fluid，volatility of analytes and deadsorption of analytes from the solid matrix.With the increase of temperature，the saturated vapor pressure increased and the thermal motion of complexes at the actives sites of matrix intensified.Meanwhile，the density of the sc-CO2decreased with the increase of temperature，which resulted in a maximum value of the extraction efficiency.As shown in Fig 3，with the increase of temperature from 313 K to 343 K，the highest extraction efficiency was obtained at 323 K.

2.4 Effect of molar ratio of ligand to metal on extraction efficiency

The effect of ligand to metal ratio on extraction efficiency was investigated at 25 MPa，323 K，30 min，respectively.As shown in Fig.4，the extraction efficiency increased little after it reached 100.

2.5 Supercritical CO2extraction of mixed metal ions

According to the research on the extraction of 8 metal ions with MHQ，we calculated the extraction efficiency and extraction constants with ligand to metal molar ratio of 100 ∶1.As shown in Tab.1，the extraction efficiencies，except for Cu2+(87.29%)，were not very satisfactory，though the ligand formed complexes with metal ions，due to their low solubility or small equilibrium constants.

As the extraction efficiency for Cu2+was much higher than other metals，it can be assumed that MHQ might show some degree of selectivity for the extraction of Cu2+.To confirm this assumption，the extraction of mixed metal ions was tested at 25 MPa，323 K，30 min，ligand to metal molar ratio of 150︰1.As show in Fig.5，the extraction efficiencies for 8 metal ions were reduced compared to the single extraction cases.The extraction efficiency of Cu2+(58.88%)was still higher than the other metal ions，nevertheless，it did not show obvious selectivity compared to the single extraction efficiency of Cu2+(87.72%).

2.6 Analysis of extraction mechanism

Based on the measurements and our experience with these systems，the reaction of MHQ with metal ions in the solid phase can be represented as follows:

where jand r representthe stoichiometric coefficients，Mn+，B，(Mn+)rBj are metal ions，MHQ and chelations respectively.This reaction is characterized by an overall extraction constant，Kex，which is defined by Eq(3).

According to Tab.1，for the metal ions in the same extraction system，Kexincreased as the extraction efficiency increased.For the same chelating ligand，2-methyl-8-hydroxy-quinoline，the metal ions(Cu2+，Ni2+，Pd2+，Cd2+，Zn2+)and the chelating ligand formed complexes in a 1︰2 ratio，Kexof the chelating ligand increased as the extraction efficiency increased.The metal ions(Co2+，Fe3+，and Mn3+)，however，formed complexes with the chelating ligand at the ratio of 1︰3.It is can be seen that Kexwas not only related to the concentration of metal ions，but also to the value of r，j and n.Kexof these chelation increased with the increase of extraction efficiency under single metal ions extraction.

3 Conclusions

In conclusion，2-methyl-8-hydroxyl-quinoline(MHQ)has been proven to be an effective chelating ligand for extraction metal ions，especially for Cu2+from solid matrices in sc-CO2.The best extraction efficiency was 88.23%for Cu2+，obtained at 30 min，323 K，25 MPa，ligand to metal ratio of 200︰1.And the extraction constants Kexincreased with the increase of the extraction efficiency under the same system.

Acknowledgements

We are gratefulto NationalScience Natural Foundation of China(No.20607031)and the Beijing National Laboratory for Molecular Sciences(BNLMS)for the financial support and the valuable help of Prof.Han Buxing.

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