In the past two decades, the technology of supercritical CO2 fluid extraction (CO2-SFE) has developed quite rapidly in the extraction of active ingredients of natural products.
In environmental science research, considerable progress has also been made in the extraction of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organic phosphorus and organochlorine pesticides.
At present, the feasibility of supercritical CO2 fluid extraction of heavy metals by more and more attention.
Traditional heavy metal solvent extraction, pretreatment time-consuming and labor-intensive, may also produce errors, the use of a large number of organic solvents, if not properly handled, will cause environmental pollution.
The supercritical CO2 fluid (SCF-CO2), good selectivity, the process is simple, fast extraction, low energy consumption, post-treatment is simple, with the advantages of solvent extraction.
SCF-CO2 generally can not directly extract positively charged heavy metal ions, but it is effective for the extraction of electroneutral heavy metal complexes, which makes it possible to extract heavy metals by SCF-CO2.
Supercritical CO2 Fluid Extraction Principle Overview
The supercritical fluid refers to a fluid that is above the critical temperature Tc and the critical pressure Pc. At the critical point, the fluid has a high gas diffusion coefficient and low viscosity, but also has a similar liquid density and good solubility.
The logarithm of its solubility within a certain range of fluid density logarithm of a linear relationship, it can be controlled by T, P, change its density, thereby changing the solubility of substances for selective extraction.
Common supercritical fluids are CO2, NH3, C2H4, C3H8, H2O, etc., and because the critical temperature of CO2 is 304K, the critical pressure is 7.4MPa, extraction conditions are more gentle, its chemical properties are stable, can be recovered after extraction, will not result in Solvent residues, known as “green solvents,” become the most widely used supercritical fluid.
Principle of Supercritical CO2 Fluid Extraction of Heavy Metals
Heavy metal ions with a positive charge, has a strong polarity, making van der Waals forces between heavy metal ions and SCF-CO2 is very weak, difficult to extract directly.
The general approach is to choose a negatively charged metal complexing agent that neutralizes the positive charge of the metal ion. Due to the derivative effect, the polarity of the resulting neutral complex has been greatly reduced. In addition, the polar modifier is added to enhance its solubility in SCF-CO2 for extraction.
In order to understand the extraction efficiency of SCF-CO2, Laitz et al, Liu and Lopez-Avilia and Wang and Marshall reported that the concentrations of Cu2 +, Co2 + Cd2 +, Zn2 +, Mn2 +, Pb2 +, Ni2 + and metalloid As3 + were adsorbed on adsorbents such as silica gel and blank sand, and then dithiocarbamate (DDC) was used as a complexing agent, combined with CH3OH modifier, extraction.
The results show that the extraction effect is roughly equivalent to the traditional solvent extraction method.
In addition, Lin and Wai , Laitz and Tachikawa  et al. Used mixtures of tributyl phosphate (TBP) and thenoyl difluoroacetone (TTA) Metal Ln3 +, radioactive elements UO22 + and Th4 +, experiments show that, SCF-CO2 extraction method not only saves time, but also significantly increased metal recovery.
Extraction of the general method and equipment flow
SCF-CO2 extraction of heavy metals general method is divided into two kinds, one is the first static extraction with SCF-CO2 excess complexing agent for a period of time, the fluid carrying a complexing agent and then dynamic extraction of heavy metal ions in the sample, the extract after decompression, Heavy metal complexes and fluid phase separation.
Another method is to make heavy metal ions and excess complexing agent fully cooperate, and then use SCF-CO2 dynamic extraction of heavy metal ions. Compared with the two methods, the latter has the advantages of simple operation, but the extraction effect is not as good as the former.
SCF-CO2 extraction of heavy metal equipment is generally made of high pressure stainless steel, but Liu and Avilia that the use of polymer materials PEEK as equipment materials to avoid corrosion of the steel with the compound, but the material pressure resistance , Heat resistance is not as good as steel, is still in the trial phase.
On-line analysis of extracts
In recent years, due to the high sensitivity of large instruments, fast and stable analysis of characteristics, has become the mainstream of trace metal analysis, especially metal organic analysis. SCF-CO2 extraction of heavy metal complexes obtained, can be easily on-line analysis. Common methods include spectral analysis, chromatographic analysis, spectral – chromatographic analysis.
Spectral analysis is a classical method of trace and trace heavy metals. At present, spectrophotometry, AAS, AES and ICP-MS are the most widely used methods. For example, Laintz and Wai  analyzed Li-FDDC complexes with UV-visible spectrophotometer. Wang and Marshall  used AAS to analyze the complexes of As, Cd, Cu, Mn, Pb and Zn. Lin and Wai  analyzed TBP (tributyl phosphate) complexes of Cr, Fe and Ni by ICP-MS, and Laintz and Tachikawa  determined the lanthanide-based TBP complexes by ICP- Good analytical data.
Chromatography is one of the main methods of metal organic analysis. However, due to the generally low vapor pressure of the metal complex, if the temperature is increased to increase the volatility of the components, the pyrolysis of the components in the analysis is easily caused by the GC. Although HPLC can avoid this problem, the complex adsorbs easily on the stationary phase . The mobile phase of supercritical fluid chromatography (SFC) is a fluid that is above critical temperature and pressure and is involved in solute partitioning. It combines both the high diffusion coefficient of gas and the ability to dissolve the sample. Both SFC and HPLC Length, suitable for analysis of metal complexes . MehdiAhraf-Khorassani [18,19] and others, the use of CH3OH-modified CO2 fluid as the mobile phase, C18 as the stationary phase at 50-100 ℃, 20-50Mpa conditions, the successful separation and determination of ferrocene, heavy metals – Acetophenone complexes, and Ni, V porphyrin compounds.
Chromatography – Spectrometry
The method first uses chromatography to separate and enrich the metal complex, and then measure the spectrum. For example, GC-AES can be used in combination with metals and organic compounds measuring 10-9, such as alkylmercury, organic arsenic, tetraalkyl lead and the like. Liu and Lopez-Avilia  first separated the volatile metal-FDDC complexes from each other by gas chromatography, and then scanned them with Atomic Emission Detection.
Factors that affect the extraction
Choice of compound and its effect on extraction
SCF-CO2 extraction of heavy metals is the key to find a suitable compound, the compound should have a small polarity, solubility in SCF-CO2, good stability, and good selectivity for different heavy metals and so on. Currently SCF-CO2 extraction of heavy metals commonly used with the following types of compounds: dithiocarbamate complexing agent, organic phosphorus complexing agent, β-diketone complexing agent, amine complexing agent, crown ether, porphyrin.
Such complexes with more than 40 kinds of metal and non-metal to form a stable complex, and not very polar. Laitz and Wai found that the solubility of heavy metal M FDDC complexes in SCF-CO2 increased 2-3 orders of magnitude after introducing fluorine substituents. However, Wang and Marshall  showed that the solubility of M-DDC in SCF-CO2 increased with the order of butyl, ethyl and pyrrole. This is because as the carbon chain grows, the polarity of the complex decreases and the solubility in SCF-CO2 also increases.
Organic phosphorus compounding agent
Due to its good selectivity for uranium and thorium in highly acidic media, and its stable radioactivity, organophosphorus complexing agents have become the most effective extractants for lanthanide and actinide metal.
Yoshihiro et al. studied the solubility of different organophosphorus complexing agents in SCF-CO2 and found that the solubility of the five organophosphorus complexing agents decreased according to their order of increasing molecular weight.
This is consistent with the poor solubility of SCF-CO2 for high molecular weight compounds.
For heavy metal SCF-CO2 extraction, in addition to selecting a single extractant, but also can choose a variety of complexing agent synergistic extraction. When Lin and Wai extracted lanthanide metals, TBP and β-diketones (such as HFA, TTA and FOD) were combined to achieve an extremely high extraction rate of 92% -98%.
Effect of SCF-CO2 density
The density of SCF-CO2 is one of the important factors affecting the extraction rate of heavy metals.
The larger the density of SCF-CO2 is, the larger the solubility c of the metal complex in the fluid is and the higher the extraction rate is.
The relationship between ρ and c is:
LnC = mLnρ + K where m is a coefficient greater than zero; K is a constant, which is related to the chemical properties of the extractant and solute
According to the experimental results of Laintz and Wai, Cu2 + is hardly extracted below the critical density (the density at the critical point of CO2 is 0.47 g / cm3), and once it is higher than the critical density , The extraction rate of Cu2 + increased rapidly with the density of SCF-CO2, and the growth rate slowed down after the density reached a certain value.
Pressure and temperature
To change the density of SCF-CO2, you must change the pressure and temperature. The density of SCF-CO2 increases with increasing pressure and decreases with increasing temperature. The results show that under lower pressure, the effect of increasing temperature and decreasing SCF-CO2 density on the extraction rate of heavy metals is more prominent than that of increasing temperature, which is called “temperature negative effect stage”.
At higher pressures, the effect of increasing temperature on the heavy metal extraction is more pronounced than decreasing the density of the CO2 fluid, which is reflected in an increase in the extraction rate, which is called the “temperature positive effect phase.” Therefore, we must also consider the pressure factor, depending on the temperature and the density of SCF-CO2 what kind of factors on the heavy metal extraction rate changes play a leading role, so as to select the appropriate temperature and pressure.
With SCF-CO2 extraction of heavy metals, the temperature requirements slightly higher, usually 50-60oC, the pressure is 10-35Mpa.
Influence of cosolvent
SCF-CO2 used as an extractant is itself a non-polar solvent.
Due to the generally polar nature of heavy metal complexes, the solubility in SCF-CO2 is somewhat lower. In this case, it is necessary to use polar extractants such as NH3 (but NH3 is relatively difficult to recover) or to use polar modifiers to improve the solubility of heavy metal complexes in SCF-CO2. Modifier addition, but also reduce the operating temperature and pressure, shorten the extraction time. Suitable modifier, the molecular structure should have both lipophilic groups, but also pro-CO2 groups.
Commonly used cosolvents
At present more commonly used modifiers are methanol, acetone, ethanol, ethyl acetate, etc., of which methanol is the most widely used. Experiments by Liu and Avilia show that the extraction rate of Cu2 +, Co2 +, Cd2 + and Zn2 + can be significantly increased by adding 0.5% methanol to SCF-CO2.
Modifier modification can be explained by intermolecular interactions. The extract and modifier solvation between the association, enhance the intermolecular force. For the heavy metal extraction in environmental samples, due to the complex matrix, the modifier also plays a role of competing for the active sites of the substrate with the material to be extracted, so that the binding force between the extracted material and the substrate is weakened so as to be extracted more easily, Selective extraction.
As a kind of modifier, the derivatization reagent can reduce the polarity of the extracted material and is used for the extraction of phenols and ionic compounds. For example, Cai et al. and others used Grignard reagent (RMgX) as a derivatizing agent, Organotin compounds into neutral R4Sn, and then extracted. Johanson et al. successfully extracted methylmercury in sediments using butylmagnesium chloride as a derivatizing agent.
It is important to point out that modifiers have a limited role in improving the solubility of supercritical fluids and in the entrapment of extraction systems, leading to an increase in coextractions and possibly interfering with analytical assays. Therefore, the amount of modifier to be small, generally not more than 5% mol.
Effect of acidity
Acidity can affect the stability of heavy metal complexes.
For metal coordination reaction, acidity increases, resulting in acid effect, the stability of the complex decreases; acidity decreases, metal ions hydrolysis, complex dissociation.
For the heavy metal complex-SCF-CO2 equilibrium system, the mechanism by which acidity affects the extraction rate of heavy metal complexes is not yet clear due to the lack of necessary thermodynamic equilibrium parameters.
However, it is certain that acidity will affect the form and solubility of free-form heavy metals in SCF-CO2.
Existing State of Heavy Metals and the Influence of Medium
The existence of heavy metals and the presence of medium on the extraction efficiency have a greater impact. For the laboratory to add sample extraction, adsorbed on the filter paper, cellulose, silica gel and other media on the metal ions, because of the simple form, easier with the combination of agents, easy to extract.
However, the environmental samples, biological products complex matrix, heavy metals usually exist in multiple phases, such as humus in soil, fatty foods lipid matrix, are likely to have varying degrees of cooperation with heavy metals, these natural complexes tend to There are stable coordination key, only to convert it into easily released ionic form, with the combination of agents, can be extracted. In addition, the presence of heavy metals also has an effect on the extraction.
In general, the extraction of solid-phase samples is relatively easy, for liquid samples, in order to improve gas-liquid contact conditions, the extractor is usually filled or countercurrent extraction.