Recent Progress in Heavy Metal Extraction by Supercritical CO2

Review pubs.acs.org/IECR

Recent Progress in Heavy Metal Extraction by Supercritical CO2 Fluids Fengying Lin, Dagang Liu,* Sonakshi Maiti Das, Nana Prempeh, Yan Hua, and Jiangang Lu* Department of Chemistry, Nanjing University of Information Science and Technology, Nanjing, 210044, China ABSTRACT: SFE-CO2 is a novel, promising, environmentally benign and inexpensive extracting method developed over the past few years to overcome environmental problems encountered due to the use of conventional solvents. One component (the extractant) is separated from another (the matrix) using SCF-CO2 as the extracting solvents. This method is widely used for extracting heavy metals from environmental contaminant, bioactive compounds like antioxidants, plant medicine and natural products or remediating heavy metal contaminated soil by using good binding activity of SCF-CO2 to heavy metal ions. The major advantages of the use of SCF-CO2 as a solvent are its superior mass transfer properties, easy recycling and lack of secondary waste formation. We try to focus on the recent advances in SCF-CO2 extraction technology for heavy metal extraction. In this review, the mechanism, procedure, and application of heavy metal extraction by SCF-CO2 are summarized in a comprehensive manner and the factors affecting the extraction efficiency are analyzed. We try to provide some meaningful information about heavy metal extraction by SCF-CO2 and make it a preferable option in heavy metal treatments.

1. INTRODUCTION Heavy metals, including Pb, Cd, Cr, Fe, Cu, Zn, Ni and W, etc., are a kind of inorganic pollutant with high toxicity and potential detriment that have specific gravity greater than or equal to 5.0. Some of these heavy metals are essential trace elements, such as Cu, Fe, Zn, while some have relative low toxicity, such as Ni and Cr.1−4 However, when accumulated to a certain amount or undergoing a change of valence in the human body, they will become very toxic; as examples, a high concentration of Cu2+ can cause hepatic and renal damage, gastrointestinal irritation, capillary damage,5,6 and Cr3+ becomes a strong carcinogen when oxidized to Cr6+.7 In addition, Pb, Hg, Cd, As, which are quasi-metals and nonessential in the human body,8 will be poisonous even if they are taken in trace.9−12 Along with rapid economic development, environmental pollution and ecological destruction have become increasingly worse, causing a major impact on human health and survival. The potentially harmful heavy metal elements have a serious contribution to the environmental pollution in soil, air, and water as well as in biome.13,14 Heavy metals can be absorbed by plants and soil, entering the food chain, and thus causing serious damage to the human health. Furthermore, the security of natural products plant medicine has also been threatened whereby tests have shown that these plants may contain traces of heavy metal ions that tend to bioaccumulate in a biological organism over time, compared to the chemical’s concentration in the environment.15,16 Compounds accumulate in living things any time that they are taken up and stored faster than they are broken down (metabolized) or excreted. Heavy metal pollution is refractory, easily accumulative, highly toxic, latent and protracted, and has irreversible characteristics; therefore heavy metal pollution management has received more and more attention from agricultural, ecological, and environmental science related fields in both the domestic and the international sector.17,18 Some effective methods such as chemical precipitation, barrier separation, adsorption, reverse osmosis, ion-exchange, © 2014 American Chemical Society

and electrochemical technologies have been applied to extract heavy metals. Barrier separation is a process whereby the macromolecules of the solution are intercepted by a barrier’s screen mesh and separated with small molecules and solution.19 The process uses the diversity of the select permeability with the barrier and the mixture of components, taking the concentration gradient of solute on either side of the barrier as mass transfer power. The barrier of this method requires cleaning and maintenance often because the effective separation of material that depends on the barrier causes pollution of the barrier and the attenuation of flux.20 The development of composite ion-exchange membranes for the separation of specific cations is the latest initiative to overcome the limitations observed with organic as well as inorganic membranes.21−23 The properties of resins for ion exchange are affected by the chemical structure and kinds of functional groups of the resins. There is a great variability of the chemical structures of these ion exchange resins based on polystyrene, polyacrylic, and other matrices, which are modified by monofunctional (sulfonic, carboxylic, phosphonic, phenolic etc.), bifunctional (hydroxamic and amidoxime), or polyfunctional (diphosphonic, sulfonic and carboxylic acid) groups.24,25 Such ion exchangers have advantages such as large ion exchange capacity, good chemical, thermal, and mechanical resistances. Solvent-extraction is based on the principle that compounds are transferred from a solvent to another solvent, taking into account the difference of solubility or partition coefficient of two kinds of solvents. The time and energy wasted for pretreatment limits the application of solvent extraction. In addition, there is a chemical action between solutes and extracting agents in solvent-extraction; so the metal compounds Received: Revised: Accepted: Published: 1866

October 22, 2013 January 3, 2014 January 12, 2014 January 12, 2014 dx.doi.org/10.1021/ie4035708 | Ind. Eng. Chem. Res. 2014, 53, 1866−1877

Industrial & Engineering Chemistry Research

Review

the critical point, the dissolvability of SCF is very sensitive to temperature and pressure. Furthermore, by reducing pressure and/or increasing temperature, SCF turns into normal gas and is recycled, and the extracted materials are separated and collected for purification.53 Therefore, SFE always takes advantage of the increasing dissolution ability of SCF with changing pressure and temperature and has become one of attractive eco-friendly alternatives to conventional solvent extraction for the extraction of heavy metals from polluted samples54−56 because of its high diffusivity and low viscosity to extract heavy metal ions. This study is aimed at reviewing existing knowledge to provide meaningful information about SFE-CO2 and further demonstrate how effectively it can be applied in heavy metal treatment.

usually participate in various chemical conversions, such as complex formation, hydrolysis, dissociation, etc. After such a type of possible chemical conversion, the concentration ratio of metal ion with solvent can change in the organic and in the water phase.26 These methods often require large quantities of organic solvents, which are harmful to health and can cause serious environmental problems.27 Adsorption has been one of the most widely studied techniques for heavy metal removal and offers several advantages such as low cost, ease of use, high removal capacity, and flexibility in design and operation.28 A diverse range of sorbents such as cation-exchange resin or chelating resin,29 clay,30 yeast,31 and mesoporous materials32 have been successfully applied for heavy metal removal. Mesoporous resins, which are a synthetic or natural kind of porous polymer adsorbents without ion exchange groups but have a good network structure and high specific surface area, exhibit satisfactory results for removal of different heavy metals.33,34 The adsorption properties of a porous resin are realized through surface adsorption, surface electrical properties, or the formation of hydrogen bonds.35 This technique selectively adsorbs organic substances from aqueous solution by physical adsorption to achieve the purpose of separation and purification. Macroporous adsorption resin has high physical and chemical stability, large specific surface area, large absorption capacity, and other advantages like reusability.36,37 Recently, the hydrophilic natural polymers such as chitin, chitosan, cellulose, alginate, and soy protein, which have properties such as high efficiency, reusability, and thermal stability, have attracted scientific attention to serve the purpose of heavy metal extraction.38,39 These polymers are of great interest due to their ability of incorporating different chelating groups such as amine, hydroxyl, and carboxylic groups into the polymeric networks.40 The mechanism for this type of biosorption is associated with chemisorptions, complexation, ion-exchange, microprecipitation, and surface adsorption, etc.41,42 In our foregoing work, mechanical defibrillated chitin nanofibers exhibited fast and efficient removal of heavy metal ions in the water system;43 and soy protein was heat-denatured and transformed into soy protein hollow microspheres which were proven to be a good biosorbent because of its high chelation ability.44 In addition, flocculation and microorganism methods are two common ways to treat heavy metal pollution. Microorganisms were used to reduce the toxicity of the heavy metals in sludge and sewage; for example, the Sphaerotilus natans landed on the Fe3O4 nanoparticles and were used as complex biological adsorbent to remove heavy metal ions in wastewater.45−47 In the 1970s, a new type of chemical technology known as SCF caught the attention of researchers. SCF has many advantages, such as good selectivity, facile, fast, and easily recycled process, low surface tension, inert, low energy consumption, simple post-treatment, low cost, and nontoxicity, etc.48 Over the past 20 years, SFE technology has developed rapidly in extracting effective components of natural products and becoming a new technology that extracts the active components from a natural plant, which, in the future, especially would include natural medicine. In the researches of environmental pollution, SFE has achieved good results in organic pollutants, such as extraction of polycyclic aromatic hydrocarbons, polychlorinated biphenyls, the organic phosphorus, chlorine pesticide, etc.49−52 By controlling pressure and temperature, some special components are extracted according to size, boiling point, polarity, and molecular weight because, in

2. PRINCIPLE AND PROCESS OF THE HEAVY METAL EXTRACTION BY SFE-CO2 2.1. Properties of SCF-CO2. SCFs have properties that fall between those of normal gases and liquids. Above the critical T and P, the extraction agent will go into the supercritical state which is different from the gas, liquid, and solid states. Several specific gases are commonly used as SCFs, such as CO2, C2H6, NH3, and H2O, etc. Table 1 lists some examples of substances Table 1. Critical Points of Some Commonly Used Solvents57,58 gases carbon dioxide ethane methane ethylene propane nitrous oxide propylene xenon water ammonia benzene toluene methanol ethanol acetone

critical temperature (K)

critical pressure (MPa)

critical density (g/mL)

304.17

7.38

0.448

305.34 190.55 282.35 369.85 309.15 365.0 289.70 647.3 405.6 562.1 591.7 512.6 513.9 508.1

4.87 4.59 5.04 4.24 7.28 4.62 5.87 22.05 11.28 4.89 4.11 8.09 6.14 4.70

0.203 0.2 0.218 0.217 0.45 0.232 0.118 0.322 0.325 0.302 0.292 0.272 0.276 0.278

used as supercritical solvents and their corresponding T and P. For most of the solvents high pressure is required to achieve SCF state. Therefore, T and P at critical points are always taken as judgments for selecting the experimental conditions. CO2 is always being used as the SFE agent.57,58 Figure 1 shows the phase diagram of CO2.59 The Pc and Tc of CO2 are 72.8 atm and 31.1 °C, which represent the highest temperature and pressure at which the substance can exist as a vapor and liquid in equilibrium. Above critical temperatures and pressures, CO2 behave as a SCF which can effuse through and dissolve solids. For the generation of SCF-CO2 the advantageous temperature is 31.1 °C which is very close to ambient temperature. Marr and Gamse calculated density behavior of CO2 with changing pressure which is depicted in Figure 2.60 The variation of CO2 densities with pressure at different temperature provides a useful optimization process of SFE-CO2 in the case of heavy metal extraction.60 1867

dx.doi.org/10.1021/ie4035708 | Ind. Eng. Chem. Res. 2014, 53, 1866−1877


Review

Figure 1. The phase diagram of CO2.59 Figure 3. Schematic diagram of the designed supercritical extraction apparatus.62 CO2 supply vessel (A), cooler (B), CO2 pump (C), three heaters (D, F, H), extraction vessels/cells (E1, E2, and E3), two separators (G1, G2), condenser (I), CO2 storage vessel (L).

E2. In E3, the pressure is reduced to 59.2 atm, and the CO2 is regenerated into gas; at last, CO2 leaves the second separator (evaporator) in a gaseous state, by the heat supplied by the heater H. The gaseous CO2 solvent is then conveyed to the condenser where it becomes liquid and is finally reused and recycled. During SFE-CO2 processing, static and dynamic modes are always combined to achieve high efficiency. One method is that the fluid carrying complexing agent dynamically extracts heavy metal ions from the sample after using SCF-CO2 in static mode to extract the excessive complexing agent, and then the heavy metal complex separates with the fluid phase after decompression. Another method is that the heavy metal ions completely cooperate with the excessive complexing agents and then are extracted by the dynamic SCF-CO2. On the basis of the results from the two above-mentioned methods, it is found that the latter has the advantage of simple operation, but it is not as effective as the former one;63,64 for example, Takeshita et al.65 established a highly efficient system with joint static and dynamic extraction modes. In the extraction cells, the static extraction was carried out with stirring rate at 350 rpm for 15 min. After that SCF-CO2 and AA was continuously supplied by the high-pressure pump for dynamic extraction. In addition, the spectrophotometric cell containing a free piston for measurement of solubility of complexing agents was devised to monitor the dynamic incorporation process of metal ions into the SCF-CO2 extraction system.66 It is worth noting that during the extraction, the cost analysis should be always done according to the equation put forward by Perrut.67 The relative cost of a supercritical plant scales as (V*Q)1/4, where V is the column volume and Q is the flow rate. ASPEN and ICARUS are softwares normally used for process design and cost analysis, respectively. However, the cost analysis is completely dependent on the specific industrial application, process parameters, and plant design.67 This extraction technique is also applicable to the direct dissolution of spent nuclear fuels into SCF-CO 2 for reprocessing, which is called “SuperDirex” in Japan.68 Radioactive contaminants such as plutonium and americium can be removed from soil by SFE using SCF-CO2, because of the fear that solvent degradation occurs due to hydrolysis and radiolysis during uranium extraction. Meanwhile, industry applications of SCF-CO2 at a large scale have been developed for heavy metal

Figure 2. Density behavior of CO2 calculated by the equation of Bender.60

2.2. Device and Process of SFE-CO2. Today, more than 100 SFE plants are going on worldwide for widespread application in various fields.61 But among the SFEs, extraction by SCF-CO2 has received the most attention because of its numerous advantages, broad range of application, and ease of handling. The device applied in SFE for extraction is mainly composed of four parts: extraction kettle, separate kettle, pumps, and heat exchanger. There are two types of industrialized SFE equipment, solid state and liquid state SFE-CO2, based on the state of extracts. Additionally, the solid state extraction can be divided into an intermittent extraction system and a semicontinuous extraction system.62 Figure 3 displays the industrial SFE equipment with a yearly capacity of about 3000 ton proposed by Luca Fiori, generally similar to many other commonly used extraction facilities.62 Three extraction vessels/cells namely E1, E2, and E3 are equipped for efficient extraction, in which E3 is responsible for CO2 depressurization, discharge of exhausted substrate, and reloading new substrate and pressurization of CO2. Besides that, the system consists of one CO2 pump (C), three heaters (D, F, H), two separators (G1, G2), one condenser (I), one CO2 storage vessel (L), one cooler (B), and one CO2 supply vessel (A). During the process of SFE, precooled, liquid-state CO2 successively flows through the CO2 pump where compression is done under the required pressure. After it is preheated in a supercritical state, CO2 is absolutely pumped into extraction vessels under high pressure. Partial extraction takes place at E1, and then the extraction is fully completed in 1868



Review

extraction from natural food and pharmaceutical products. Environmental remediation like soil decontamination by CO2 extraction is attractive since soil can be disposed easily after treatment. However, it is noted that only organic pollutants of low polarity can be easily removed, even if some progresses were disclosed on chelatant use.69 But it is improbable that heavy metals are subjected to supercritical fluid extraction in reasonable technical and economical conditions. 2.3. Advantages of CO2 as a SCF. CO2 is the most common SCF solvent, and has gained a wide range of acceptance for use as an extractant due to some advantages as follows: low critical temperature and pressure low cost wide availability nonflammability environmental friendliness as it can be recycled and reused high diffusivity low viscosity and surface tension superior mass transfer properties compared to organic solvent largely a byproduct of industrial processes or brewing SCF has many characteristics, such as its high density (2 orders of magnitude larger than other liquids), similar viscosity with gas, varied dielectric constant dramatically changing with pressure, and higher mass transfer rate. Besides the abovementioned virtues, CO2 has other advantages over other SCF, such as larger critical density, a greater solubility for most solutes, noncombustible, good chemical stability, nontoxic, odorless, safe, inexpensive, etc. Solvency characteristics of SCFCO2 can be varied with small changes in temperature and pressure. Therefore, CO2 is the most widely used fluid in SFE.63 At present, more and more attention is being paid to the feasibility study on the SFE of heavy metals. SFE has not only successfully been proven to extract heavy metals and other ionic substances by combining complex extraction technology; it has also provided high extraction efficiency, selectivity, low surface tension, and a lower risk of solvent secondary pollution. Thus, SFE-CO2 has become the new and effective way of treating heavy metal pollutants.63

Figure 4. Removal efficiencies for some metals from contaminated sludges using various extraction technologies: (a) removal efficiencies for Al, Cd, Cr, Cu and Fe; (b) removal efficiencies for Pb, Mn, Hg, Ni and Zn.55

the cost is reduced in the biobleaching process, but constant monitoring of temperature and aeration of the system renders it disadvantageous. The efficiency of removal was inferior by the electroreclamation process compared to that of the chemical treatment, bioleaching, and SFE.

4. FACTORS AFFECTING EXTRACTION EFFICIENCY OF HEAVY METALS BY SFE-CO2 As mentioned above, SFE-CO2 has high sensitivity and accuracy, diffusivity, and is a low surface tension method with good selectivity, environmental friendliness, and repeatability when compared to other methods. On the other hand, it still has some defects like generation of many byproducts and also high dependence on chelating additives. Therefore, it is necessary to overcome some defects to obtain good results at low environmental and economical cost. Some parameters affecting the extraction yields should be described and analyzed in order to get extraction efficiency as high as possible. 4.1. Extraction Pressure. Extraction pressure is one of the important factors in the supercritical extraction process. According to the ideal gas state equation: (1) PV = nRT n can be thought as equal to ρV/M. so the equation can be formulated as: P = ρRT /M (2)

3. REMOVAL EFFICIENCY OF HEAVY METALS BY SFE-CO2 Different experiments for the extraction of heavy metals from different sources by using SCF-CO2 have been conducted by several researchers. The efficiency of SCF-CO2 as an extractant can be evaluated from these valuable researches. The broad range of efficiencies for the removal of the several metals from sludge by the different extraction technologies was reviewed by Babel and del Mundo Decera, and the dependence of the removal efficiencies on the extraction technology is presented in Figure 4.55 It is revealed that the SFE process has a high removal efficiency for Pb, Hg, Zn, Cu, and Cd ions depending on the ligands used. The chemical extraction method also produced high removal efficiency of heavy metals like Pb, Ni, Zn, and Cd, Cr, but the chemical extraction process required more quantities of inorganic acid to extract the heavy metals and thus required large quantities of lime to neutralize the sludge for ultimate disposal in the environment.55 Therefore, this process is expensive as well as environmentally unattractive. In comparison to that of the chemical process, about 80% of

When temperature and molar mass are constant, pressure is directly proportional to the density, that is, the density behavior 1869



Review

Table 2. List of Chelating Agents58,85 metal Th Ur Cu Zn In(III) Ga(III) Nd(III), Eu(III) Pd, Pt, Rh Cr, Cu, As Zn, Pb, Mn, Cd, Cu, V, Sb, Ni, Mo, Cr, and Co Zn, Cu, Pb, Cd, and Co

matrix

extraction condition

tissue paper residues radioactive metal wastes copper foil water acidic aqueous solution acidic aqueous solution acidic aqueous solution spent automobile catalytic converters treated wood fly ash

200 bar, 60 °C 200 bar, 40 °C 240 bar, 60 °C 83−138 bar, 40 °C 138 bar, 70 °C 205 bar, 70 °C 200 bar, 50 °C 300 bar, 40−80 °C

organophosphorus reagents (TBP, TOPO, TPP, TPPO, TBPO) surfactant (NP-4) dialkyldithiocarbamate lithium salts Cyanex302 AcAcH, TTAH, PySH, and piperidinyldithiocarbamic acid AcAcH, TTAH, and PySH Oxa-diamides TBP ligands

chelating agents

200 bar, 60 °C 200 bar, 40 °C

Organophosphorus Cyanex 302 Cyanex 302, TBP, D2EHPA

sand and fly ash

80−200 bar, 45 °C

Cyanex 302, D2EHPA, D2EHTPA, diisooctylphosphinic acid, NaDDC, Aliquat 336, Cyanex 923

accelerated by increasing temperature. The third aspect was beneficial for extraction whereas the two former aspects exhibited disadvantages. Zhou et al.76 pointed out that the temperature at the range of 50 and 60 °C were a corresponding optional extraction temperature for Pb and As extraction from calcium lactate, respectively. However, the extraction yield of Hg changed slightly in the experimental temperature that ranged from 30 to 60 °C. Vincent et al.77 had studied the extraction of neodymium ion from its oxide using TTA-TBP-methanolin/SCF-CO2. They showed that an increase in temperature from 50 to 65 °C lowered the conversion into adduct. This was supported by the fact that due to the higher solubility of TBP in SCF-CO2 at 50 °C, than that at 65 °C, more TBP was exposed to the already formed hydrated chelate of Nd-TTA, thus leading to a higher conversion. However, Zhu et al.78 obtained the opposite results when they extracted Nd from Nd2O3 with the TBP-HNO3 complex in SCF-CO2; that was, the higher temperature resulted in higher extraction efficiency by fixing pressure at 21 MPa. 4.3. Extraction Time. In general, extraction time is proportional to the yield in the initial process of extraction. After a certain extraction time, the extraction yield reached equilibrium.79 As mentioned above, SFE-CO2 extraction generally includes dynamic extraction and static extraction processes, in which the static extraction process can make the complexing agent swell the matrix and fully react to form metal complexes, and in dynamic extraction, SCF-CO2 binding metal complexes are released from the matrix. Yuan et al.80 investigated the effect of extraction time on Cu, Cd, Pb, released from green tea using static combined with dynamic extraction processes. During the static extraction process, the yield of Pb and Cd increased with time until it reached the maximum value at 30 min, after that the yield decreased slightly over time. The extraction yield of Cu showed a different trend, from the beginning to 30 min, the yield basically remained unchanged, and then a decreased yield was exhibited. This was as a result of the instability of Cu-TBP/HNO3 complex in SCFCO2. However, in the process of dynamic extraction the yields of three metals were proportional to time in 30 min; after that, Cu showed an increased trend, while Pb and Cd displayed a decreased trend. 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.

of CO2 follows with increasing pressure. The density of SCFCO2 increases with an increase of pressure; meanwhile, the solubility increases which can also be observed in Figure 2. Seifried and Guidard measured the solubility of Cu(TTA)2 in SCF-CO2 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 pressure71 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. They have also reported that about 510 μg/g of Cr was extracted from the wood sample at 30.4 MPa, in the first 15 min, which was equivalent to about 26 μg of Cr in the 50 mg wood sample. Another interesting study of the extraction of uranium oxides from solid waste was conducted by Meguro et al.72 The efficiencies of the complexation and the dissolution processes were estimated by applying different pressures. They have observed a negative pressure effect above 20 MPa. They assumed that the pressure effect was due to a decrease of the reactivity of the HNO3-TBP complex by an increase of an extent of the solvation of the HNO3-TBP complex by the CO2 molecules in the supercritical CO2 phase of higher pressure or higher density. Tai et al. also presented some data in their work dealing with the effect of changing pressure on extraction efficiency during the extraction of Zn2+ by using SCF-CO2.73 They concluded that the increased pressure retarded the masstransfer rate for the zinc ion extraction from aqueous solutions. Similarly, Cui et al.74 found that extraction efficiency of Cu or Pb decreased with the increased pressure. The optional pressure was 20.0 MPa. It can be concluded that the extraction efficiency increased with increasing pressure. But the increase of pressure can cause a lowering of the diffusion capacity and mass transfer coefficient of carbon dioxide leading to a reduction of extraction effects. Therefore, it is necessary to select a suitable operation pressure.36,64 4.2. Extraction Temperature. Temperature effect on extraction can be demonstrated in the following three ways.75 First, the density of the SCF-CO2 and the solubility of complexing agent and metal complexes are reduced as temperature rises. Second, volatility of complexing agent, modifier, and metal complex increased with the increasing temperature. Third, the complex and coordination reaction was 1870



Review

Therefore, determination of static and dynamic extraction time should be integrated into the device power consumption to yield an extraction equilibrium.81 4.4. Complexing Agent. It is known that carbon dioxide is a kind of nonpolar SCF, but heavy metal ions have strong polarity, thus leading to difficulty in extracting heavy metal ions from polar substances directly. In the extraction process, metal ions are difficult to dissolve into the CO2 phase because of minimal interaction between the SCF and metal ions. Two methods are mainly adopted to improve the feasibility and efficiency of extraction. First, suitable complexing agents with positive or negative charges were introduced to coordinate heavy metal ions. It is worth noting that the coordinating complex should be stable and easily dissolved in the SCF-CO2. Second, modifiers like ethanol, methanol, etc., were added in the SCF-CO2 to improve the polarity. Several research works have been conducted for extraction of heavy metals with the aim at process optimization. High solubilities of the chelating agents and their metal complexes in pure or modified SC−CO2, fast chelation kinetics, and higher complexing specificity are the key regulators for selective extraction of a metal ion or a group of metal ions. 4.4.1. Structure of Complexing Agents. Commonly used complexing agents for extracting heavy metals are dithiocarbamic acid ligand, organophosphorus salts, AcAcH, amine, crown ether, porphyrin, etc.82−84 Table 2 presents a list of ligands and appropriate conditions for heavy metal removal by SCFCO2,58,85 and the structure of the different complexing agents is shown in Figure 5.86 The most frequently used chelating agents for heavy metal extractions are Cyanex 302, Aliquat 336, D2EHPA, D2EHTPA, etc. Ligands can be divided into four broad groups according to their nature by Kersch et al.87 acid ligands (e.g., Cyanex 302, D2EHPA etc) acid chelating ligands (e.g., NaDDC,88 DMA, and MMA89 anion exchangers (e.g., Aliquat 336) solvating ligands (e.g., Cyanex 923) As mentioned above, one of the most important things is to generate stable metal complexes for extraction. These complexing agents are soluble, stable, a little polar, nontoxic, and harmless, inexpensive, and easy to get, as well as having a good selectivity for heavy metals.79 4.4.2. Solubility of Complexing Agents. One of the key influencing factors in SCF-CO2 extraction is the solubility of metal complexes in SFE. The solubility of metal chelates in SCF-CO2 depends on the structure of the complexing agents and the selection of metal ions. Wang et al. studied the solubility of Et2NH2DDC in SCF-CO2 with ethanol as the cosolvent. They have reported that the solubility of solute i can be expressed as90

yi =

⎡ P Pisat(T )φî sat(T ) exp⎣⎢∫ sat Pi

( ) dP⎤⎦⎥ Vis RT

φî F P

(3)

Takeshita and Sato91 provided a very simple way to estimate the solubility of naphthalene in SCF-CO2. This can be expressed as y2 =

(W2/M 2) {(p − pw )V1/RT } + (W2/M 2)

Figure 5. Structure of different complexing agents.86

One very interesting work done by Kachi et al.92 indicates that the Raman spectral shifts could be useful tools to measure the

(4) 1871



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reached equilibrium. It is worth noting that coordination effects of the complexing agent on the heavy metal ions were different. Zhou et al.76 discovered SFE-CO2 extraction efficiency of heavy metals from calcium lactate. Extraction effects of Pb and Hg were fairly good with the addition of a complexing agent from 2% to 8%, but the extraction efficiency of As decreased slightly. Above all, the selection of complexing agent should thoroughly optimize the structure, dosage, concentration, solubility, and stability, etc. However, in the extraction process, not all of the metal complex could be extracted because the extraction efficiency is defined by SCF solvent, metal complex characteristics, metal complexes, matrix binding status, etc. Therefore, we should consider not only the nature of the complexing agent, but also the generation of metal complex solubility in SCF when choosing the suitable complexing agent.63 After all, a good complexing agent should have the following properties:97 high solubility of complexes and ligand high selectivity of metal transfer ability to undergo reversible complexation reaction for stripping high mass transfer properties maximum safety to extracted system 4.5. Modifying Agent. SCF-CO2 is a nonpolar solvent, and heavy metal complexes generally should have a strong polarity. Therefore, to enhance the polarity of these metal complexes there is a need to add a small amount of modifier, thereby effectively improving the extraction ability. The modifiers are always volatile and miscible and with extracted substances and supercritical components, swelling, and wetting heavy metals from the matrix, thus improving transfer matter fluency. On the other hand, the modifiers change the critical point of the mixed solvents correspondingly, to be closer to the extraction temperature.81 By this way, the operating pressure is lowered, and the extraction time is shortened. Nowadays the commonly used modifiers are methanol, ethanol, acetone, and ethylacetate, etc. 4.5.1. Alcohol. Vincent et al. pointed out that the rate of extraction is enhanced with the addition of methanol.77 That was followed by the rapid reduction of extraction after 1.67 h indicating the loss of ligands-TTA and TBP from the system due to the increased solubility of these ligands in the SCFCO2−methanol system. Yuan et al80 extracted Cu, Pb, and Cd from green tea, using TBP/HNO3 as a complexing agent and methanol, ethanol, water, and acetone as modifiers, respectively. Ion extraction yields of Cu, Cd, Pb in the case of ethanol as a modifier were 74.5%, 62.1%, and 62.7%, respectively. Therefore, ethanol was an optional modifier. Cui et al.63 used Et2NH2DDC as the complexing agent and ethanol as the modifier for SFE-CO2 extraction of heavy metals from Chinese herbal medicine. The modifier ethanol solution containing 1% Et2NH2DDC had an impact on the extraction. The weight of extracted Cu was dependent on the dosage of modifying agent, while the amount of modifier had little effect on As, Hg, and Pb. 4.5.2. Water. The presence of water as a polar modifier can greatly increase the extraction efficiency.98−101 Wai et al.98 took LiFDDC as a complexing agent for extracting Hg2+ from cellulose filter paper. The extraction yield was not more than 12% from the original matrix because Hg2+ could be adsorbed by the cellulose matrix so that the SCF had difficulty competing with the uptake of Hg2+ by cellulose. When a small amount of

CO2-philicity of chelating agents in SCF-CO2. They have studied with AcAcHs (AA, TFA, and HFA) and UO2(HFA)2DMSO. Their research work revealed that the Raman spectral bands of CO2 in SCF-CO2 containing AcAcHs were shifted to a lower wavenumber (red-shifts) compared with those of neat SCF-CO2. This Raman spectrum was found to be useful as the measure for the strength of Lewis acid−base interactions formed between CO2 and solutes in SCF-CO2. The degree of solubility of metal complexes highly depended on the character of the hydrocarbon or fluorocarbon shell surrounding the central metal atom.93 A very comprehensive study to assess the solubility of metal complexes containing 49 metals and 15 free ligands in SCF-CO2 was conducted by Smart et al.94 A fluorine-substituted ligand system exhibited the most suitable configuration for high SCF solubility. But the limitation for using fluorine-substituted ligands at large scale was their high cost. Hydrocarbon-based ligands, aliphaticsubstituted systems showed high solubility and can compromise the cost. Phenyl-substituted ligands showed the lowest solubility and are less-interesting for high scale applications. From their study, it was observed that the maximum metal complex solubilities were in the range of 30−60 g/L. Dithiocarbamate is one of the most commonly used complexing agents, which can form complexes with more than 40 metal ions, and the solubility of its complex is even higher in SCFCO2 than in normal fluids.95 Zhou et al.76 used tetrabutylammonium bromide and DDC (mass ratio 1:1) to generate the Cu−DDC complex for synergic solvent extraction. When the content of the complexing agent was 2%, a better extraction effect was achieved; however, M(FDDC)2 in SCF-CO2 always exhibited higher solubility than other nonfluorinated analogues.63 Laintz et al. investigated the solubility of two complexes coordinated by the same metal ion, M-DDC and M-FDDC in SCF-CO2.64 Experimental results showed that MFDDC had a 2−3 times higher solubility than M-DDC in SCFCO2 because the fluoride substituent is an electron-withdrawing group that makes charge distribution of the whole metal complex more uniform. By this way the polarity metal complexes were weakened, leading to increasing the solubility in nonpolar SCF-CO2. Meanwhile, when DDC was substituted by alkyl with carbon chain length (C1∼C5), the solubility of complex agents increased with increasing number of alkyl carbon number. The fact is as the alkyl substituent chain length increased, the polarity of complex agents was decreased. However, a further increase in the carbon chain length, did not reduce the solubility anymore because the continuous increment of molar cohesive energy and molar volume are not significant.91 For example, two alkyl substituent dithiocarbamate salts [(C4H9)4N][SC(S)N(C4H9)2] and [(C4H9)4N][SC(S)N2H5)2], have different polarity. The former is less polar and has solubility of 23.24 μg/(min/mL) in the SCF-CO2 under 45 °C and 17.0 MPa, whereas the later has a solubility of 2.91 μg/(min/mL) under the same conditions.63 Therefore, the low polar complexing agent will benefit for elevating solubility in SCF-CO2 and enhancing extraction effects. 4.4.3. Dosage of Complexing Agent. The dosage of complexing agent has a great impact on the extraction yield by SFE-CO2. Murphy and Erkey investigated the effect of complexing agent TFA and FOD on the extraction of Cu2+ in aqueous solution and found the extraction yield linearly increased with the increases of excessive yield up to 10-fold higher yield.96 However, when the excessive yield was out of linear range and rose above 40 times, the extraction yield 1872



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water (10 μL) was dipped into the matrix, the extraction yield could be elevated up to 84%. The increased efficiency might be due to the enhanced interaction between Hg2+ and the solvent modifier. 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 Cr3+ and Cr6+ by using LiFDDC at 20.3 MPa and 60 °C in which SCF-CO2 was the medium for extraction. They found that extraction efficiency was higher for a wet sample in comparison to that of the dry sample. Water contained in the matrix sample can also negatively influence the extraction effects. The study for extracting La3+, Eu3+, Lu3+, and uranyl [(UO2)2+] ions in SCF-CO2 in which FOD performed as a chelating agent was reported by Lin et al.100 An extraction efficiency of less than 20% was obtained for La3+, Eu3+, and Lu3+ from a wet matrix. Furthermore, when water content exceeds a few percent it can affect the mechanical performance of SFE by causing restrictor plugging, and also can affect the extraction process itself. 4.5.3. Other Modifiers. Chloroform has also proven to be a good modifier. Roggeman et al.102 evaluated the comparison of solubilities of metal-chelate complex [iron tris(pentane-2,4dionate) -Fe(acac)3], in SCF-CO2 and in modified SCF-CO2 with chloroform as a modifier. Their spectroscopic analysis, modeling and solubility analysis from experimental data showed a huge increase in solubility in SCF-CO2 with 3 mol % chloroform compared to that of only SCF-CO2. Additionally, the spectroscopic measurements presented local solvation around the metal chelate complex in SCF-CO2/3 mol % chloroform mixture. Wenclawiak et al.89 showed in their experiments that DMA, MMA, and their thioglycolic acid methylester derivatives can be extracted with extraction efficiencies greater than 90% at 400 atm. In addition, ketone modifiers like HFA, TTA, TFA, and AA, fluorinated AcAcHs, and methanolic acetylacetone did not produce high positive changes in extraction efficiency.103−105 Johansson et al.106 evaluated three different modifiers (methanol, water, and acetone) to extract alkyllead from both sediment and urban dust. The optimum conditions were reported as 80 °C, 446 atm, and 10% methanol, which led to the recovery of 96, 106, and 80%, of trimethyllead, triethyllead, and diethyllead, respectively. However, if the content of modifier was too high, it interfered with analysis and the selectivity of SCF-CO2 was reduced. Normally, the modifier amount is generally no more than 5% (volume ratio). 4.6. pH. Systemic pH has a great impact on the stability, form, and coordination number of a heavy metal complex. If the pH value did not meet the conditions, the complexing agent could be ionized to form complexation. When there is a pH value decrease, the complex cannot keep its good stability because of the acidity effect. On the contrary, an increment of pH would lead to hydrolysis of metal ions and dissociation of the complex. Vincent et al.77 used SFE to extract Nd ions by complex from neodymium oxide. The yields of Nd were 0.26, 0.16, 0.2 at pH values of 2.78, 2.81, and 2.84, respectively, which indicated the fact that a higher yield was produced at lower pH value. Hanrahan et al.107 designed a unique extraction process which utilized a cheap, commercially available Triton X-100 as a complexing agent that formed reversed micelles in SCF-CO2. The extraction efficiency was determined at different pH values that were maintained by buffer. The results are given in Figure 6. They found that pH had an effect on the affinity of

the metal ion to the ligand, the solubility of the Triton-metal complex, and the formation of reversed micelles.108

Figure 6. The effect of pH on the extraction efficiencies for various metal ions in a biphasic water−CO2 system.107

The pH would also affect the forms and solubility of free heavy metals in the SCF-CO2 fluid. However, due to the lack of the necessary thermodynamic equilibrium parameters, the mechanism of pH on the SFE was still not clear. In addition, low or high pH would cause equipment corrosion at different levels. 4.7. Heavy Metal Species and Medium. Heavy metal species exhibited different extraction efficiency because of their different affinity to chelating agents. Their medium also affects the extraction efficiency, because the species were easily combined with complexing agent during the extraction. The heavy metals were easily absorbed on filter paper, cellulose, silica gel, and other medium. Sometimes environmental samples like biological products and food ingredients had a variety of phase states; for example, food lipids in a fat matrix are stable to coordinate heavy metal ions. Hence it is difficult to extract heavy metal from these matrices except when the metal has transformed into integrated ions to react with complexing agents.63 In addition, extraction in a solid phase sample was relatively easier than that in a liquid phase sample because extra filler or countercurrent extraction should be adopted to improve the contact area of gas−liquid phase and extraction efficiency.108 It is reported that Cu2+ from the silica surface to supercritical CO2 has a rapid transfer within the first few minutes of contact followed by a gradual increase to approach a saturation value in the fluid phase.48 Lin et al.109 studied the extraction efficiencies of thorium and uranium ions from solid and liquid materials. The results of extracting uranyl ions from sand were 72%, 15%, and 94% by neat supercritical CO2 containing TTA, TBP, and mixed TTA+TBP, compared to 38%, 5%, and 70% from aqueous solutions. The results of extracting Th(IV) ions from sand were 74%, 10%, and 93%, while the results were 70%, 6%, and 87% from aqueous solutions. 4.8. Surfactants. Microemulsions are thermodynamically stable aggregates of amphiphilic surfactants with water as cores. Wang et al.110 conducted experiments of metal extraction from 1873



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about the effect of these parameters on the SCF-CO2 extraction method we can tailor the process as per our requirements or goals. At the same time, focusing on the extraction of heavy metal ions and not removing favorable components should also be given considerable attention.

solid by using this technique of microemulsions in SCF-CO2. Figure 7 shows the structure of microemulsions/surfactants in

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 2558731090. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to National Natural Science Foundation of China (No. 51103073 & 21277073), Natural Science Foundation of Jiangsu Province (No. BK2011828), Scientific Research Foundation for the Returned Overseas Chinese Scholars, and Qing Lan Project and Six Talented Peak Program of Jiangsu Province and the Priority Academic Program Development of Jiangsu Higher Education Institutions for financial support.

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Figure 7. Water-in-CO2 microemulsion (reverse micelle).111

SCF-CO2. Compared with conventional washing or acid leaching processes, which require large quantities of water, the CO2 microemulsion technique could greatly minimize liquid waste generation. Usually grams of metals can be extracted with microliters of water. Microemulsion entraps polar metal ions in water cores and disperses in SCF-CO2, exhibiting an excellent medium for the separation of metals from solid matrices. Moreover, it can reduce the use of huge amount of chelating agents, modifiers, etc. The reverse micelles means that the hydrophilic groups of the amphoteric surfactants, such as, sodium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate spontaneously gathered inward together in nonpolar organic solvents. The surfactant formed tiny droplets which have a spatial scale in a nanoscale pool of colloid, and is a self-organized and arranged-ordered structure with thermodynamic stability. Ding et al. took advantage of the reverse micelles technology to optimize and improve the effects of extracting traces of heavy metal ions in calcium lactate by SFE. The effect of reverse micelles in SFE was better because the reverse micelles formed complexation to the metals, weakened the force between the complexing agent and the media, and enhanced the solubility in SCF-CO2.111 However, it is very difficult to form effective reverse micelles.

5. CONCLUSIONS This paper summarizes the principle and characteristics of SFECO2, and also made a comparison to other commonly used methods. This method has wide applications in the field of environmental removal of toxic heavy metals from traditional Chinese medicine, food processing, spent nuclear fuels, and soil, etc. However, the influencing parameters in the extraction process such as amount and concentration of chelating agents, temperature, pressure, extraction time, pH, etc., should be comprehensively understood. At present, the core problem of the supercritical CO2 extraction technology is to find efficient, nontoxic, nonpolluting complexing agents, which not only have high solubility in the SCF-CO2, but also have strong a complexing ability and better selectivity. From the discussion 1874

SYMBOLS AND ACRONYMS SFE-CO2 = supercritical CO2 fluid extraction SCF-CO2 = supercritical CO2 fluids SCF = supercritical fluid SFE = supercritical fluid extraction T = temperature P = pressure AA = acetylacetone V = volume ρ = density TTA = thenoyltrifluoroacetonate TBP = tri-n-butyl phosphate AcAcH = β-diketone TTAH = fluorinated-diketone PySH = thiopyridine Cyanex = 302 diisooctyl-thiophospinic acid Aliquat = 336 sodium diethylthiocarbamate D2EHPA = bis(2-ethylhexyl)phosphoric acid D2EHTPA = bis(2-ethylhexyl)monothiophosphoric acid DDC = diethyldithiocarbamate DMA = dimethylarsenic acid MMA = monomethylarsenic acid Cyanex = 923 trialkyl phosphine oxide Et2NH2DDC = diethylammonium diethyldithiocarbamate yi = mole fractions of solute Psat i (T) = pressure of saturated solution at temperature T φ̂ sat i (T) = fugacity coefficient of solute in saturated solution at temperature T Vsi = molar volume of solute in solid phase R = universal gas constant (8.314 Pa·m3/(mol/K)) φ̂ Fi = fugacity coefficient of solute in SCF phase W2 = the weight of the trapped naphthalene Pw = the vapor pressure of water V1 = the gas volume measured by the gas meter at temperature T M2 = the molar mass of naphthalene TFA = trifluoroacetylacetone HFA = hexafluoroacetylacetone UO2(HFA)2DMSO = uranyl β-diketonato complex dx.doi.org/10.1021/ie4035708 | Ind. Eng. Chem. Res. 2014, 53, 1866−1877


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DMSO = dimethyl sulfoxide M(FDDC)2 = bis(trifluoroethyl) dithiocarbamate/metal complexes FDDC = bis(trifluoroethyl)dithiocarbamate FOD = 2,2-dimethyl-6,6,7,7,8,8,8-heptafluorine-3,5-symplectic diketone Triton X-100 = hydrocarbon surfactant

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Recent Progress in Heavy Metal Extraction by Supercritical CO2

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