Advances in Supercritical CO2 Fluid Extraction of Heavy Metals
ABSTRACT: The combination of supercritical CO2 fluid and metal complexing technology opens up a new way for heavy metal extraction. In this paper, the research status of supercritical CO2 fluid extraction of heavy metals is introduced, the factors affecting the extraction are summarized, and the future development trend is prospected, so that readers can have a better understanding of the research progress of this technology. It is believed that with the further study of supercritical CO2 fluid extraction of heavy metals, it will have a broader application prospect.
Supercritical CO2-SFE (supercritical CO2-SFE) technology has developed rapidly in extracting effective components from natural products in the past two decades. In the field of environmental science, considerable progress has been made in the extraction of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organophosphorus, organochlorine pesticides and other organic pollutants. At present, more and more attention has been paid to the feasibility study of supercritical CO2 fluid extraction of heavy metals.
Traditional solvent extraction of heavy metals is time-consuming and laborious, and may cause errors. If a large number of organic solvents are used improperly, they will pollute the environment. Supercritical CO2 fluid (SCF-CO2) has the advantages of good selectivity, simple process, high extraction speed, low energy consumption and simple post-treatment, which are not available in solvent extraction [4,5].
Generally, SCF-CO2 can not directly extract positively charged heavy metal ions, but it is effective for extracting electrically neutral heavy metal complexes, which makes it possible for SCF-CO2 to extract heavy metals . On the basis of summarizing the basic characteristics of SCF-CO2, the principle and method of extracting heavy metals by SCF-CO2 will be discussed emphatically, and the status of on-line analysis of heavy metal extracts will be introduced. Finally, the factors affecting the extraction of heavy metals by SCF-CO2 were summarized.
II. Status Quo
1. Summary of Supercritical CO2 Fluid Extraction Principle
Supercritical fluid refers to the fluid above the critical temperature Tc and the critical pressure Pc. At the critical point, fluids have the same high diffusion coefficient and low viscosity as gases, and have the same density and good solubility as liquids. The logarithm of solubility is linearly related to the logarithm of fluid density in a certain range. Therefore, selective extraction can be carried out by controlling T, P and changing their density.
Common supercritical fluids are CO2, NH3, C2H4, C3H8, H2O and so on. Because the critical temperature of CO2 is 304K and the critical pressure is 7.4MPa, the extraction conditions are mild and the chemical properties are stable. After extraction, the supercritical fluids can be recovered without causing solvent residues. They are called “green solvents” and become the most widely used supercritical fluids. [7,8]
2. Principle of Supercritical CO2 Fluid Extraction of Heavy Metals
The van der Waals force between heavy metal ions and SCF-CO2 is very weak, and it is difficult to extract directly . The general method is to select metal complexes with negative charges and neutralize the positive charges of metal ions. The polarity of the neutral complexes has been greatly reduced due to the coordination derivative effect. The solubility of the neutral complexes in SCF-CO2 has been enhanced by the combination of polar modifier and extraction .
In order to understand the extraction efficiency of SCF-CO2, Laitz et al. [4,11], Liu and Lopez-avilia , Wang and Marshall [10,12], adsorbed heavy metals such as Cu2+, Co2+, Cd2+, Zn2+, Mn2+, Pb2+, Ni2+, As3+ on silica gel and blank sand, and then used dithiocarbamate (DDC) as complexing agent and CH3OH modifier. SCF-CO2 extraction was carried out. The results showed that the extraction effect was similar to that of traditional solvent extraction. In addition, Lin and Wai [13, 15], Laitz and Tachikawa  used the mixture of tributyl phosphate (TBP) and thiophene formyl difluoroacetone (TTA) as the complexing agent to extract lanthanide metals Ln3+, radioactive elements UO2 2+ and Th4+ with SCF-CO2. Experiments showed that the SCF-CO2 extraction method not only saved time, but also significantly improved the metal recovery rate.
3. General methods and equipment flow of extraction
There are two general methods for extracting heavy metals by SCF-CO2. One is to extract excess complexes with SCF-CO2 statically for a period of time, and then extract heavy metal ions dynamically with the complexes. After decompression, the extracts are separated from the fluids. In another method, heavy metal ions are extracted dynamically by SCF-CO2 after completing with excess complexes. Compared with the two methods, the latter one has the advantage of simple operation, but the extraction effect is not as good as the former.
SCF-CO2 extraction equipment for heavy metals is generally made of stainless steel with high pressure resistance. However, Liu and Avilia  believe that PEEK, a polymer material, can be used as equipment material to avoid the corrosion of steel by mixing agent. However, the pressure resistance and heat resistance of this material are not as good as steel, and it is still in the trial stage.
4. On-line analysis and determination of extracts
In recent years, the analysis of trace metals, especially organometallic compounds, has become the mainstream because of the high sensitivity, fast and stable analysis characteristics of large-scale instruments. The heavy metal complexes extracted by SCF-CO2 can be conveniently analyzed on-line. Common methods include spectral analysis, chromatographic analysis, spectral-chromatographic analysis, etc.
(1) Spectrometric analysis
Spectral analysis is a classical method for trace and trace heavy metals. At present, spectrophotometry, AAS, AES and ICP-MS are widely used. For example, Laintz and Wai  used ultraviolet-visible spectrophotometer to analyze Li-FDDC complexes, Wang and Marshall  used AAS to analyze As, Cd, Cu, Mn, Pb, Zn complexes. Lin and Wai  used ICP-MS to analyze TBP (tributyl phosphate) complexes of Cr, Fe and Ni, and Laintz and Tachikawa  used ICP-AES to determine TBP complexes of lanthanide metals. Good analytical data were obtained.
(2) Chromatographic analysis
Chromatography is one of the main methods for the analysis of organometallic compounds. However, because the vapor pressure of metal complexes is generally low, if the volatility of components is increased by increasing temperature, it is easy to cause pyrolysis of components in GC analysis. Although this problem can be avoided by HPLC, the complex is easily adsorbed on the stationary phase . The mobile phase of supercritical chromatography (SFC) is a fluid above the critical temperature and pressure, which participates in the distribution of solutes. It has not only high diffusion coefficient of gas, but also strong ability to dissolve samples. It combines the advantages of GC and HPLC, and is suitable for the analysis of metal complexes . Mehdi Ahraf-Khorassani [18,19] et al. successfully separated and determined ferrocene, heavy metal-acetone complex, porphyrin compounds of Ni and V using CH3OH modified CO2 fluid as mobile phase and C18 as stationary phase at 50-100 C and 20-50 Mpa.
In this method, the metal complexes were separated and enriched by chromatography, and then determined by spectroscopy. For example, GC-AES can measure 10-9 grade organometallic compounds, such as alkyl mercury, organic arsenic, tetraalkyl lead, etc. Liu and Lopez-avilia  separated metal-FDDC complexes with certain volatility from each other by gas chromatography, and then scanned by Atomic Emission Detection.
III. Factors Affecting Extraction
1. Selection of Complexes and Their Effects on Extraction
The key to extracting heavy metals by SCF-CO2 is to find suitable complexes, which have the characteristics of low polarity, high solubility, good stability and good selectivity for different heavy metals in SCF-CO2. At present, the commonly used complexes for SCF-CO2 extraction of heavy metals can be divided into the following categories: dithiocarbamate complexes, organophosphorus complexes, beta-dione complexes, amine complexes, crown ethers, porphyrins and so on [5, 9, 10, 11, 12].
(1) Dithiocarbamates (DDC)
These complexes can form stable complexes with more than forty metals and non-metals, and their polarity is small. Laitz and Wai  found that the solubility of FDDC complexes of heavy metal M in SCF-CO2 increased by 2-3 orders of magnitude after introducing fluorine substituents. Wang and M arshall  showed that the solubility of M-DDC in SCF-CO2 increased in the order of butyl, ethyl and pyrrole. This is because the polarity of the complexes decreases with the increase of the carbon chain and the solubility in SCF-CO2 increases accordingly.
(2) Organic phosphorus complexes
Because uranium and thorium have good selectivity and stable radioactivity in high acidic medium, organophosphorus complexes have become the most effective extractants for lanthanide and actinide metals [13, 14, 15]. Yoshihiro et al.  Through the solubility study of different organophosphorus complexes in SCF-CO2, it was found that the solubility of five organophosphorus complexes decreased in the order of their molecular weight increasing. This is consistent with the poor solubility of SCF-CO2 to compounds with high molecular weight.
For SCF-CO2 extraction of heavy metals, not only a single extractant, but also a variety of complexes can be selected to extract the heavy metals synergistically. When Lin and Wai  extracted lanthanide metals, TBP and beta-dione (such as HFA, TTA, FOD) were combined to achieve a very high extraction rate of 92%-98%.
2. Effect of SCF-CO2 Density
The density of SCF-CO2 is one of the important factors affecting the extraction rate of heavy metals. The higher the density of SCF-CO2 is, the higher the solubility C of metal complexes in fluids is, and the higher the extraction rate is. The relationship between Rho and C is as follows:
In LnC = mLnP + K formula, the coefficients of M are greater than zero, and K is constant, which is related to the chemical properties of extractants and solutes.
From the experimental results of Laintz and Wai , it can be seen that under the condition of lower critical density (0.47g/cm3), Cu2+ is hardly extracted. Once higher than the critical density, the extraction rate of Cu2+ increases rapidly with the density of SCF-CO2, and the growth trend slows down when the density reaches a certain value.
To change the density of SCF-CO2, pressure and temperature must be changed. The density of SCF-CO2 increases with increasing pressure and decreases with increasing temperature. The results show that under lower pressure, higher temperature and lower density of SCF-CO2 have more significant effect on extraction rate of heavy metals than higher temperature, which is called “negative temperature effect stage”. Under higher pressure, the effect of higher temperature on heavy metal extraction is more obvious than that of lower density of CO2 fluid, which is reflected in the increase of extraction rate, called “positive temperature effect stage”. Therefore, the pressure factor must be considered at the same time, depending on the temperature and the density of SCF-CO2 which factors play a leading role in the change of extraction rate of heavy metals, so as to select the appropriate temperature and pressure. Extraction of heavy metals by SCF-CO2 requires a slightly higher temperature, usually 50-60oC, and a pressure of 10-35Mpa.
3. The influence of modifier
SCF-CO2, as an extractant, is a non-polar solvent. Because of the polarity of heavy metal complexes, the solubility in SCF-CO2 is smaller. Polar extractants such as NH3 (but recovery of NH3 is relatively difficult ) or polar modifiers are needed to improve the solubility of heavy metal complexes in SCF-CO2. The addition of modifier can also reduce the operating temperature and pressure and shorten the extraction time. Suitable modifiers should have both lipophilic and CO2-affinity groups in their molecular structure.
At present, the commonly used modifiers are methanol, acetone, ethanol, ethyl acetate and so on, among which methanol is the most widely used. The experiments of Liu and Avilia  showed that the extraction rate of Cu2+, Co2+, Cd2+, and Zn2+ could be significantly improved by adding 0.5% methanol to SCF-CO2.
The modification of modifiers can be explained by the interaction between molecules. The solvent association between the extract and the modifier enhances the intermolecular force. For the extraction of heavy metals from environmental samples, due to the complexity of the matrix, the modifier also plays a role in competing with the extract for the active point of the matrix, which weakens the bonding force between the extract and the matrix, thus making it easier to be extracted, and at the same time increasing the selectivity of extraction.
As a class of modifiers, derivatization reagents can reduce the polarity of the extracts, and are mostly used for the extraction of phenols and ionic compounds. For example, Cai et al.  used Grignard reagent (RMgX) as derivative, converted organic tin compounds into neutral R4Sn, and then extracted. Johanson et al.  succeeded in extracting methylmercury from sediments using butyl magnesium chloride as derivative.
It should be pointed out that the role of modifier is limited. It can improve the solubility of supercritical fluid, at the same time, it will weaken the capture effect of extraction system, resulting in the increase of co-extractants, and may interfere with the analysis and determination. Therefore, the dosage of modifier should be small, generally not more than 5% mol.
4. Effect of acidity
Acidity affects the stability of heavy metal complexes. For metal complexes, acidity increases, acid effect occurs, and stability of complexes decreases; acidity decreases, metal ions hydrolyze and complexes dissociate. For the SCF-CO2 equilibrium system of heavy metal complexes, the mechanism of acidity affecting the extraction rate of heavy metal complexes is still unclear due to the lack of necessary thermodynamic equilibrium parameters. But it is certain that acidity will affect the form and solubility of free heavy metals in SCF-CO2.
5. The Existence Morphology of Heavy Metals and the Effect of Medium
The existence form of heavy metals and the existing medium have great influence on the extraction efficiency. For laboratory sample extraction, metal ions adsorbed on filter paper, cellulose, silica gel and other media are easy to bind with complexes and extract because of their simple morphology. However, the matrix of environmental samples and biological products is complex, and heavy metals usually exist in various phases, such as humus in soil and fat matrix in lipid food. These natural complexes often have stable coordination bonds. Only when they are converted into ionic forms which are easy to release and combined with complexes, can they be carried out. Extraction.
In addition, the medium in which heavy metals exist also influences the extraction. Generally speaking, the extraction of solid phase samples is easier than that of liquid phase samples. In order to improve the gas-liquid contact, the extractor usually needs to add fillers or take countercurrent extraction.
It should be noted that the current research on SCF-CO2 extraction of heavy metals is still in the pre-experimental stage of adding metal ions with known concentrations. However, due to the high efficiency, high speed, good selectivity and solvent-free secondary pollution of heavy metals, SCF-CO2 extraction is expected to become an effective means of pretreatment and analysis of heavy metals in environmental samples.
Although there is still a lack of economic data on large-scale treatment of environmental heavy metal pollution by using SCF-CO2 extraction technology, with the development of industrial-grade SC-CO2 fluid extraction technology, SCF-CO2 extraction technology can save energy, time and effort compared with traditional liquid-liquid extraction and biodegradation. Advantages will gradually emerge. At present, the technology has been used abroad to treat alkyl lead and methyl mercury pollution in soil and sediment [21,22].
Successful extraction of some lanthanide actinides from SCF-CO2 will open up a new way for separation of rare earth elements and treatment of nuclear waste. This technology can also be used to extract precious metals Pd and Pt. In the petrochemical industry, SFE removes heavy metals from petroleum products, such as the lead removal from gasoline, which meets the environmental protection requirements of unleaded gasoline.
For Cd, Pb, As, Cd, Hg, Cu in food, oil, fruit, vegetable, animal viscera and other biological products, as well as Pb  in cosmetics, SCF-CO2 extraction can not only avoid solvent residues, but also maintain the original color, fragrance and nutritional activity of biological products, and conform to the current trend of greening living goods.
As for the extraction of heavy metals by SCF-CO2, we believe that the following two aspects should be considered in the future: first, the study of the method itself, through the selection of suitable complexes and modifiers, more thermodynamic and kinetic data can be obtained, and the theoretical model of supercritical fluid extraction of heavy metals can be established; second, the study of samples, from the perspective of chemical form and matrix of heavy metals in samples, should be studied. The interaction between fluid, modifier and matrix , only in this way can it be possible to avoid blindness and make the supercritical CO2 fluid extraction technology of heavy metals more widely used in scientific research and production.
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