Advances in Heavy Metal Extraction Using Organophosphorus Compounds: A Comprehensive Review
Abstract
:1. Introduction
2. Main Families of OPCs and Their Applications
2.1. Classification of Organophosphorus Compounds
2.2. Properties of the Main Organophosphorus Compounds
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- Acid Dissociation Constant (pKa) and Metal Extraction Efficiency: The acid dissociation constant (pKa) of the organophosphorus acids plays a significant role in metal extraction procedures. The more negative the pKa value of the organophosphorus acid, the more acidic it will be, leading to better proton release and assisting metal complexation. This consequently leads to higher metal distribution coefficient (D) and lower pH1/2 value (the pH value for 50% extraction of the metal). Further, the hydrophobic character of the produced metal complex greatly controls extraction effectiveness; the more hydrophobic the complexes, the more favorably they will distribute into the organic phase, hence the increased metal distribution coefficient.
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- Extractant Hydrophobicity and Partitioning Behavior: Hydrophobicity of the extractant itself is also a critical consideration in determining metal extraction. The greater the partition coefficient (Kp) of the extractant between organic and aqueous phases, the more hydrophobic it becomes, which allows for greater transfer of the metal complex to the organic phase. Such enhanced hydrophobicity is typically related to a lower pH1/2 value because the extractant is able to work well even at lower acidic pH. Additionally, the metal–ligand complex stability constant (β) gives an idea about how much the ligand favors the metal ion; the higher the β value, the tighter the bonding, thereby increasing the metal distribution coefficient and lowering the pH1/2 value.
3. Influence of the Chemical Nature of Organophosphorus Additives on Metal Extraction Performance
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- Extraction Efficiency Across Metals:
- ○
- Scandium (III) exists mostly as a hydroxide species under pH 5.5, i.e., (Sc(OH)3) or Sc(OH)4−, depending on solution conditions. In our case (media rich in chloride), Sc(OCl)3 can also form, but this is not as common unless excessive chloride concentration is encountered.
- ○
- Based on HSAB theory, scandium (III) is a “hard acid” due to its small ionic radius, high charge density, and preference for hard donor ligands such as oxygen. This classification agrees with its strong bonding with oxygen donor ligands, such as those found in TBP (−(P(=O)(OR)3). Given the above explanation, we now provide a comparative analysis of TBP and TOPO extraction behavior:
- •
- TBP (Tributyl Phosphate) is an oxygen donor ligand, and it reacts strongly with hard acids like scandium (III). The capacity to form stable Sc(III)-TBP complexes justifies the description of scandium (III) as a hard acid. This is consistent with HSAB theory.
- •
- TOPO (Tri-n-Octylphosphine Oxide) contains both a soft phosphorus donor and a hard oxygen donor. While TOPO can coordinate with borderline as well as hard acids, its coordination with scandium (III) is primarily through the oxygen donor, again confirming scandium (III)’s hard acid status. However, the softer phosphorus donor in TOPO introduces some ambiguity, which should be demonstrated experimentally.
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- Cyanex® 272 and Cyanex® 301 achieve near-complete extraction (>98%) for several metals, including Fe, Ga, Cu, Co, and Zn, under optimized conditions. This underscores their versatility and effectiveness as cationic exchangers.
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- Sodium diethyl dithiophosphate achieves a Cd extraction yield of 93.1%, demonstrating its suitability for soft Lewis acids like cadmium.
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- pH: OPCs function best at acidic pH ranges (pH 2–4, for instance), where metal ions are present as free cations or weakly solvated ions. For example, Cyanex® 301 functions optimally in terms of extraction yields for Cu, Zn, and Co at pH 2, while Cyanex® 272 optimally functions for Fe and Ga at the same pH.
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- Contact Time and Temperature: Shorter contact times (e.g., 10 min) and moderate temperatures (e.g., 25 °C) can provide high extraction efficiencies, as seen with Cyanex® 301 and 302.
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- Diluent Effects: Kerosene and toluene are low-polarity, general-purpose diluents with high efficacy for stabilizing organic phase complexes. The diluent choice could influence extraction kinetics and phase separation efficiency.
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- Selective Extraction: Cyanex® 301 is highly selective for Cu compared to other metals in sulfate solutions, which renders it useful for hydrometallurgical applications in mixed-metal streams. Sodium diethyl dithiophosphate selectively extracts Cd in acidic medium, even with trivalent iron present.
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- Competing Ions: Competitive ions (e.g., protons, sulfate, chloride) may reduce the efficiency of extraction by competing for coordination sites or metal-extractant compound instability.
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- Strengths:
- Highly selective and tunable owing to structural change.
- Ability to blend with a broad range of extractive media from acidic to close to neutral solutions.
- Developed scalability for industrial application.
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- Limitation:
- Reduced extraction efficiency for soft acids when oxygen donor ligands are employed.
- Sensitivity of process parameters such as pH, temperature, and diluent polarity.
- Environmental concerns over the disposal of spent extractants and organic solvents.
3.1. Thermodynamic Fundamentals of Metal-OPC Extraction
Factors Influencing Thermodynamics
Molecular Structure of Extractants
Temperature Dependence
Solvent Effects
Nature of the Metal Ion
Kinetic Factors
3.2. Case Study: Copper Extraction with Cyanex 301
- Enthalpy Change (ΔH): The reaction is exothermic (ΔH < 0), which indicates intense bonding between copper ions and the extractant.
- Entropy Change (ΔS): The negative change in entropy indicates decreased randomness with the formation of a complex.
- Gibbs Free Energy (ΔG): The negative value confirms spontaneity in the extraction process.
3.3. Influence of the Atom Directly Bound to Phosphorus
3.4. Influence of the Nature of the Attached Group (Substituents R)
4. Conclusions
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- Influence of the atom directly linked to phosphorus on the extraction,
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- Influence of the alkyl chain of the OPC on the extraction of metals,
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- Influence of the presence of oxygen in the alkyl chain on the extraction of metals.
5. Future Perspectives
Author Contributions
Funding
Conflicts of Interest
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OPC | Structure | Formula | Examples | |
---|---|---|---|---|
(i) Trivalent: PIII | ||||
Phosphines | PR3 | Allyldiphenylphosphine Benzyldiphenylphosphine | ||
Phosphinites | R2P(OR)1 | Methyldiphenylphosphinite | ||
Phosphonites | R1P(OR)2 | Diethyl methylphosphonite | ||
Phosphites | P(OR)3 | Triethyl phosphite | ||
Phosphorous acid ester | P(OH)3 | Phosphorous acid | ||
Phosphoneous acid | RP(OH)2 | Methylphosphonous acid | ||
Phosphineous acid | R2POH | Bis(p-tolyl) phosphinous acid | ||
(ii) Pentavalent: Pv | ||||
Phosphine oxide | R3P(O) | TOPO Cyanex 923 | ||
Phosphinic acid | R2P(O)OH | Cyanex 272 Glufosinate | ||
Phosphonic acid | RP(O)(OH)2 | (Aminomethyl) phosphonic acid | ||
Phosphoric acid Esters | (OR)2P(O)OH | Diisobutyl phsphoric acid D2EHPA EHPA | ||
Organophosphates: OP | Phosphates | (O)P(OR)3 | TBP Dichlorvos Trimethyl phosphate | |
Phosphonates | RP(O)(OR)2 | TMP Diisopropyl methyl-phosphonate | ||
Phosphinates | R2P(O)(OR) | Aluminium diethyl-phosphinate | ||
Amido-OP | Phosphoramidates | (OR)OPN(R)2 | Fenamiphos | |
Phosphoramidothioates | (OR)2OPN(R)2 | Isofenphos | ||
Or | ||||
OR(SR)OPN(R)2 | Methamidophos | |||
Thio-Organophosphates | Monothiophosphinic acid | R2P(S)OH | Cyanex 302 | |
Dithiophosphinic acid | R2P(S)SH | Cyanex 301 | ||
Dithiophosphonic Acid | RP(S)(SH)2 | Bis(2,4,4trimethylpentyl) dithiophosphonic acid | ||
Phosphorothioates (S=) | SP(OR)3 | Parathion; Diazinon Bromophos; Fenthion | ||
Phosphorothioates (S-substituted) | (RS)P(O)(OR)2 | Demeton-S-methyl Echothiophate | ||
Dithiophosphates | (OR)2PS(S,H) | Dibutyl dithiophosphate Danafloat™ 068 AERO 3501® | ||
Dithiophosphonates | RP(S)(OR) | Bis[4-ethoxyphenyl-O-alkyl] Dithiophosphonates | ||
Dithiophosphinates | R2P(S)(SR) | AEROPHINE® 3418A | ||
Phosphonothioates (S=) | RP(S)(OR)2 | Leptophos | ||
Phosphonothioates (S-substituted) | RP(O)(SR) | VX | ||
Phosphorodithioates | (RS)PS(OR)2 (RO)PO(SR)2 | Malathion Methidathion Azinphos-ethyl Azinphos-methyl Dimethoate Disulfoton | ||
Phosphorotrithioates | P(O)(SR)3 | Tribufos |
Extractant | Process | Experiences | Diluent | Media [Sol] | Metal | %Ext | Ref. | |
---|---|---|---|---|---|---|---|---|
TBP (Tri-n-butyl phosphate) | Solvent Extraction | [Cd] = 5 mol/L [HClO] = 6.1 M | Kerosene | H3PO4 | Cd | 100 | [66] | |
[Sc] = 32.0 g/L | pH = 5.5 | - | HCl | Sc | 90 | [67] | ||
Ionquest® 801/PC88A (2-ethylhexyl-2-ethyl hexyl phosphonic acid) | Solvent Extraction | [PC88A] = 0.5 M O/A = 1.0 | pH = 2.3 | Kerosene | Non motioned | Mn | 89.6 | [68] |
pH = 2.3 | Cu | 91.5 | ||||||
pH = 2.7 | Co | 95.4 | ||||||
pH = 4.1 | Ni | 81 | ||||||
Cyanex® 921/TOPO (Trioctyl phosphine oxide) | Solvent extraction and liq/liq extraction | The extraction tests were carried out in various media: HNO3, HCl, H2SO4, and H3PO4. | pH = 1 T = 30 °C | Toluene | HNO3 | U | 99 | [69] |
HCl | 99 | |||||||
H2SO4 | 35.5 | |||||||
H3PO4 | 27.2 | |||||||
D2EHPA/BAYSOLVEX® (Di-(2-ethylhexyl) phosphoric acid). | Kerosene | HNO3 | 38.5 | |||||
Toluene | HNO3 | 98 | ||||||
H3PO4 | 93.1 | |||||||
Kerosene | HNO3, | 53.9 | ||||||
Cyanex® 923 | Liq–liq extraction | [HCl] = 0.5 M | O/A = 1.5 t = 10 min T = 25 °C | Kerosene | HCl | Sc | 99 | [70] |
Th | 97 | |||||||
[H2SO4] = 2 M | H2SO4 | Sc | 96 | |||||
Ti | 50 | |||||||
Zr | 99 | |||||||
Cyanex® 925 | [HCl] = 0.5 M | HCl | Sc | 80 | ||||
Th | 90 | |||||||
[H2SO4] = 2 M | H2SO4 | Sc | 60 | |||||
Ti | 50 | |||||||
Zr | 91 | |||||||
Cyanex® 272 Bis (2,4,4-trimethyl pentyl) phosphinic acid | Solvent Extraction | [Cyanex272]m = 100 g/L | pH = 2 | Kerosene | H2SO4 | Ti | 100 | [71] |
Fe | 100 | |||||||
Ga | 98 | |||||||
Cyanex® 301 Di-isooctyl dithiophosphinic acid | Solvent Extraction and Ion exchange | [Cyanex]301,302 = 0.25 M Tcontact = 10 min T = 25 °C pH = 2 | Toluene | H2SO4 | Cu | 100 | [72] | |
Zn | 95 | |||||||
Co | 100 | |||||||
Fe | 100 | |||||||
Cyanex ® 302 (Diisooctylthio-phosphinic acid) | Cu | 100 | ||||||
Zn | 63 | |||||||
Co | 9 | |||||||
Fe | 40 | |||||||
Sodium diethyl dithiophosphate | Flotation | [Collector]/[Cd2+] = 3 With trivalent iron reduction | - | H3PO4 (29% P2O5) | Cd | 93.1 | [73,74,75] | |
Dithiophosphate di(sec-butyl) | Precipitation | [P2O5]m = 350 mg/L [Extractant]m = 2.5 kg/t P2O5 | - | H3PO4 | Cd | 55 | [76] | |
Di(4-methylhexyle) Dithiophosphinate | Precipitation | [P2O5]m = 350 mg/L [Extractant]m = 2.5 kg/tP2O5 | - | H3PO4 | Cd | 50 | [76] | |
Diisobutyl dithiophosphinate | Precipitation | %P2O5m = 30% | Kerosene | H3PO4 | Cd | 98 | [77,78] | |
Sodium diisobutyl dithiophosphinate | Precipitation | [Cd]initial = 18 mg/L | pH = 5 | - | H3PO4 | Cd | 100 | [78] |
Extractant Class | Example: Carbon Chain Length | Heavy Metal Target | Effect of Carbon Number on Extraction Yield | Key Observations | Extraction Medium | Reference |
---|---|---|---|---|---|---|
Phosphoric Acid Esters | Diethyl Phosphate (C2) | Cadmium (Cd2+) | Short chains exhibit poor hydrophobicity, limiting transfer to organic phases; low extraction yields. | Limited solubility in organic solvents due to short alkyl chains; ineffective for efficient heavy metal extraction. | 1 M HCl | [83,95] |
Di-(2-ethylhexyl) Phosphate (C8) | Cadmium (Cd2+) | Increased hydrophobicity enhances extraction yield by promoting partitioning into the organic phase. | Optimal balance between hydrophobicity and solubility; excellent performance for Cd2+ extraction. | 2 M H2SO4 | [83,96] | |
H3PO4 | ||||||
Dioctyl Phosphate (C16) | Cobalt (Co2+) | Very long chains improve hydrophobicity but may introduce steric hindrance or aggregation issues. | High extraction efficiency for Co2+ but operational challenges such as viscosity increase or reduced selectivity for smaller ions. | 3 M HCl | [89] | |
Diisooctyl Phosphate (C18) | Mercury (Hg2+) | Extremely long chains enhance hydrophobicity but may reduce compatibility with certain organic solvents. | Excellent extraction efficiency for Hg2+; however, excessively long chains may lead to aggregation or precipitation in the organic phase. | 1 M HNO3 | [97] | |
Phosphonic Acids | Methylphosphonic Acid (C1) | Mercury (Hg2+) | Minimal hydrophobicity limits extraction efficiency; remains largely in aqueous phase. | Short chains are ineffective for transferring metal complexes into the organic phase. | 0.5 M HCl | [98] |
Octylphosphonic Acid (C8) | Mercury (Hg2+) | Enhanced hydrophobicity improves extraction yield by stabilizing ion pairs and transferring them into organic solvents. | Significant improvement in Hg2+ extraction efficiency compared to shorter chains; optimal chain length balances hydrophobicity and solubility. | 2 M H2SO4 | [99] | |
Dodecylphosphonic Acid (C12) | Nickel (Ni2+) | Longer chains further improve extraction yield but may cause viscosity issues during solvent extraction. | Excellent extraction efficiency for Ni2+; however, excessively long chains may lead to aggregation or precipitation in the organic phase. | - | [100] | |
Tetradecylphosphonic Acid (C14) | Lead (Pb2+) | Very long chains enhance hydrophobicity but may reduce selectivity for smaller metal ions. | High extraction efficiency for Pb2+ but potential operational challenges such as increased viscosity or reduced solubility in organic solvents. | 1 M HNO3 | [101] | |
Phosphine Oxides | Trimethyl Phosphine Oxide (C3) | Zinc (Zn2+) | Low hydrophobicity results in poor extraction yields due to limited partitioning into organic solvents. | Short chains fail to effectively stabilize metal complexes in the organic phase. | Neutral (pH 7) | [100] |
Tributyl Phosphine Oxide (C12) | Zinc (Zn2+) | Increased hydrophobicity significantly enhances extraction yield by promoting organic phase partitioning. | Long chains provide robust extraction efficiency for Zn2+; optimal chain length ensures high hydrophobicity without excessive steric hindrance. | 1 M HCl | [66] | |
Trioctyl Phosphine Oxide (C24) | Lead (Pb2+) | Extremely long chains enhance hydrophobicity but may reduce selectivity and cause operational challenges. | High extraction efficiency for Pb2+ but potential issues with viscosity and reduced compatibility with certain organic solvents. | 2 M H2SO4 | [99,102] | |
Triphenyl Phosphine Oxide (C18) | Chromium (Cr3+) | Aromatic chains improve extraction yield for larger metal ions due to π–π interactions and enhanced hydrophobicity. | Excellent extraction efficiency for Cr3+; aromatic chains show higher selectivity for larger metal ions compared to aliphatic chains. | 1 M HCl | [66] | |
Phosphonates | Ethylphosphonate (C2) | Copper (Cu2+) | Moderate hydrophobicity leads to moderate extraction efficiency; limited by short chain length. | Short chains dissolve more in water reducing their effectiveness for Cu2+ extraction. | 0.5 M H2SO4 | [35,68] |
Hexylphosphonate (C6) | Copper (Cu2+) | Enhanced hydrophobicity improves extraction yield by favoring organic phase partitioning. | Long chains significantly increase Cu2+ extraction efficiency while maintaining good solubility in organic solvents. | 1 M HCl | [103,104] | |
Octylphosphonate (C8) | Chromium (Cr3+) | Very long chains improve hydrophobicity but may introduce steric hindrance for larger metal complexes. | Excellent extraction efficiency for Cr3+; however excessively long chains may reduce selectivity for certain metal ions. | 2 M HCl | [105] | |
Thiophosphoric Acids | Diethyl Thiophosphate (C2) | Arsenic (As3+) | Short chains exhibit poor hydrophobicity, limiting transfer to organic phases; low extraction yields. | Limited solubility in organic solvents due to short alkyl chains; ineffective for efficient heavy metal extraction. | 1 M HCl | [73,93,106] |
Di-(2-ethylhexyl) Thiophosphate (C8) | Arsenic (As3+) | Increased hydrophobicity enhances extraction yield by promoting partitioning into the organic phase. | Optimal balance between hydrophobicity and solubility; excellent performance for As3+ extraction. | 2 M H2SO4 | [106] |
Extractant | Environment WPA | Dose | Extraction Yield % | Ref. |
---|---|---|---|---|
P2O5 mg/L * | kg/t P2O5 ** | |||
Di(4-methyl-2-pentyl) dithiophosphate | 245 | 1.75 | 65 | [76] |
Diceryl dithiophosphate | 350 | 2.5 | 45 | |
Di-isobutyl dithiophosphate | 350 | 2.5 | 10 | |
Di(sec-butyl) dithiophosphate | 350 | 2.5 | 55 | |
Dinonylphenyl dithiophosphate | 350 | 2.5 | 40 | |
Diisobutyl dithiophosphinate | 245 | 1.75 | 55 | |
2-éthylhexyl dithiophosphinate | 350 | 2.5 | 50 |
Extractant | pKa | pH | Extraction Yield (%) | Structure | Description |
---|---|---|---|---|---|
D2EHPA (Di-(2-ethylhexyl) phosphoric acid) | 2.76 | 4 | 96 | The extraction yield (%) of Co (II) from the chloride acid by the cationic exchangers diluted in kerosene as a function of equilibrium pH. T = 25 °C O/A = 1 [chloride] = 1 M | |
EHPA Bis(1,3-di-2-ethylhexyloxy propan-2-yl)phosphoric acid | 4.02 | 4 | 60 | ||
UPA Bis(undecan-6-yl) phosphoric acid | 2.85 | 4 | 70 | ||
Cyanex ®272 | 3.72 | 4 | 35 | ||
Ionquest ® 801 | 3.33 | 3.8 | 80 |
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Essakhraoui, M.; Boukhair, A.; Bentiss, F.; Mazouz, H.; Beniazza, R.; Haneklaus, N. Advances in Heavy Metal Extraction Using Organophosphorus Compounds: A Comprehensive Review. Metals 2025, 15, 524. https://doi.org/10.3390/met15050524
Essakhraoui M, Boukhair A, Bentiss F, Mazouz H, Beniazza R, Haneklaus N. Advances in Heavy Metal Extraction Using Organophosphorus Compounds: A Comprehensive Review. Metals. 2025; 15(5):524. https://doi.org/10.3390/met15050524
Chicago/Turabian StyleEssakhraoui, Meriem, Aziz Boukhair, Fouad Bentiss, Hamid Mazouz, Redouane Beniazza, and Nils Haneklaus. 2025. "Advances in Heavy Metal Extraction Using Organophosphorus Compounds: A Comprehensive Review" Metals 15, no. 5: 524. https://doi.org/10.3390/met15050524
APA StyleEssakhraoui, M., Boukhair, A., Bentiss, F., Mazouz, H., Beniazza, R., & Haneklaus, N. (2025). Advances in Heavy Metal Extraction Using Organophosphorus Compounds: A Comprehensive Review. Metals, 15(5), 524. https://doi.org/10.3390/met15050524