What are Chelates?
Chelates are organic compounds that form stable complexes with metal ions by coordinating through two or more atoms. The chelating agents contain donor atoms like nitrogen, oxygen, or sulfur that can form coordinated covalent bonds with the metal ion. Common chelating agents include EDTA, DTPA, DOTA, and their derivatives.
Properties of Chelates
Chelation Mechanism
The chelation process involves the formation of a ring structure with the metal ion coordinated to the donor atoms of the chelating agent. This results in an entropically favored and highly stable complex due to the chelate effect. The stability of the chelate complex depends on factors like the chelate ring size, the number of donor atoms, and the nature of the metal ion.
Stability and Selectivity
Chelates exhibit high thermodynamic stability and kinetic inertness, making them resistant to dissociation or substitution reactions. The stability constants (log K) of chelates can range from 10 to 30, depending on the metal ion and chelating agent. Chelating agents can be designed to selectively bind specific metal ions by tuning the donor atoms and ring size.
Chelate Examples
Some of the most widely used chelating agents are ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA), amino tris(methylenephosphonic acid) (ATMP), ethylenediamine tetramethylene phosphonic acid (EDTMP), 1-hydroxyethane 1,1-diphosphonic acid (HEDP), ethylenediaminedisuccinic acid (EDDS), iminodisuccinic acid (IDS), chitosan, hydroxamic acid, oxalic acid, galactaric acid, metaphosphoric acid, phytic acid, citric acid, fumaric acid, malic acid, and maltol.
Uses & Benefits of Chelates
Applications in Various Industries
- Water Treatment: Chelating agents like EDTA and DTPA are used to remove heavy metal ions and prevent scale formation in water treatment systems.
- Oil and Gas Industry: They are added to stimulation acids and scale removal formulations to control iron precipitation and remove scale deposits in oil wells and pipelines.
- Cleaning and Household Products: Chelating agents enhance the performance of cleaners, detergents, and personal care products by binding to metal ions that can cause staining or interfere with active ingredients.
- Pharmaceuticals: Certain chelating agents like EDTA, DMSA, and deferoxamine are used as antidotes for heavy metal poisoning by facilitating the excretion of toxic metal ions from the body.
- Agriculture: Water-soluble metal complexes of chelating agents like EDTA are used as micronutrient fertilizers to enhance plant growth.
Benefits and Advantages
- Prevent precipitation and scale formation by sequestering metal ions.
- Enhance the stability and shelf-life of products like hydrogen peroxide and bleaches.
- Facilitate the removal of toxic heavy metals from the body in cases of poisoning.
- Improve the bioavailability and delivery of essential micronutrients in agriculture.
- Provide water-soluble forms of metal ions resistant to inactivation by anions like phosphates.
Application Case
Product/Project | Technical Outcomes | Application Scenarios |
---|---|---|
Chelated Micronutrient Fertilizers | Chelating agents enhance the bioavailability and uptake of essential micronutrients like iron, zinc, and manganese in plants, leading to improved crop yields and quality. | Agricultural applications, particularly in soils with high pH or calcareous conditions where micronutrient deficiencies are common. |
Chelated Contrast Agents | Chelating agents form stable complexes with gadolinium or other metal ions used in contrast agents, reducing their toxicity and improving their stability and solubility for better imaging quality. | Medical imaging techniques like magnetic resonance imaging (MRI) and X-ray imaging, where chelated contrast agents enhance the visibility of specific tissues or organs. |
Chelated Pulp Bleaching Agents | Chelating agents like EDTA and DTPA are used in pulp bleaching processes to sequester metal ions that can catalyze the decomposition of hydrogen peroxide, leading to more efficient and environmentally friendly bleaching. | Paper and pulp industry, where chelated bleaching agents improve the brightness and quality of the final product while reducing the environmental impact. |
Chelated Metal Catalysts | Chelating agents can stabilize and solubilize metal catalysts, enhancing their activity, selectivity, and recyclability in various organic synthesis reactions, leading to improved yields and reduced waste. | Chemical industry, particularly in the production of fine chemicals, pharmaceuticals, and specialty polymers, where chelated metal catalysts offer improved efficiency and sustainability. |
Chelated Radiopharmaceuticals | Chelating agents form stable complexes with radioactive metal ions like technetium-99m or gallium-68, enabling their use as diagnostic or therapeutic radiopharmaceuticals with improved pharmacokinetics and reduced toxicity. | Nuclear medicine applications, including diagnostic imaging techniques like single-photon emission computed tomography (SPECT) and positron emission tomography (PET), as well as targeted radionuclide therapy. |
Latest innovations of Chelates
Novel Chelating Agent Structures
Tripodal chelating agents based on tris(2-aminoethyl)amine (tren), tris(3-aminopropyl)amine, or nitrilotriacetic acid (NTA) platforms have been extensively studied. These ligands form strong complexes with Fe3+ and other trivalent metals, with binding constants ranging from 1028 to 1033, provided there are 5-6 atoms connecting the platform and the hydroxamate functional group. New chelating agents like NODHA, NOTHA, and NODHA-PY constructed on 1,4,7-triazacyclononane (TACN) with hydroxamic acid or pyridine moieties have also been developed for zirconium-89 (89Zr) positron emission tomography (PET) imaging applications.
Improved Chelating Performance
Recent advances focus on enhancing chelating ability, stability, and selectivity. A chelating agent containing EDTA, NTA, iron ion stabilizers, dichloroethane, ethanol, sodium hydroxide, carbon disulfide, and adjusters exhibits stable performance, long lifetime, high compatibility with acidizing systems, and effectively inhibits precipitation. Biodegradable polyacidic chelate-based breaker fluid systems have been engineered for improved biodegradation, reduced toxicity, smaller environmental footprint, and efficient filtercake removal in oil and gas wells.
Environmentally Friendly Chelating Agents
To address environmental concerns, safer alternatives to conventional chelating agents like EDTA and phosphonates have been developed. These include chelating products containing biosurfactants, which replace or reduce the use of chemical chelating agents and descaling agents, making them environmentally and operationally friendly. Additionally, chelating agents based on 4-amino-3-phenylbutyric acid hydrochloride, DL-3,4-dihydroxyphenyl glycol, 4,4′-dichlorodiphenyl sulfone, and nano-SiO2 modified polyacrylic resin have been proposed for desulfurization wastewater treatment in coal-fired power plants.
Emerging Applications
Chelating agents find diverse applications beyond traditional areas like metalworking, oil and gas production, and water treatment. For instance, they are used in fermentation media to remove minerals and affect the behavior and growth of microorganisms, potentially increasing the production of compounds of interest. Chelating agents are also employed in the preparation of highly luminescent and stable chelates for time-resolved fluorometric assays, diagnostics, research, and high-throughput screening.
In summary, the latest innovations in chelating agents focus on developing novel structures, improving performance, addressing environmental concerns, and exploring emerging applications, driven by the need for more effective, selective, stable, and sustainable chelating solutions across various industries.
Technical challenges
Novel Chelating Agent Structures | Developing novel chelating agent structures based on tripodal platforms like tris(2-aminoethyl)amine (tren), tris(3-aminopropyl)amine, or nitrilotriacetic acid (nta) with hydroxamate functional groups for enhanced binding to metal ions like Fe3+. |
Improved Chelating Performance | Enhancing the chelating ability, stability, and selectivity of chelating agents through structural modifications, such as incorporating EDTA, NTA, iron ion stabilizers, and adjusters to improve performance, compatibility, and lifetime. |
Chelating Agents for Zirconium-89 PET Imaging | Designing and evaluating novel chelating agents like NODHA, NOTHA, and NODHA-PY constructed on 1,4,7-triazacyclononane (TACN) with hydroxamic acid or pyridine moieties for zirconium-89 (89Zr) positron emission tomography (PET) imaging applications. |
Biodegradable Polyacidic Chelate-Based Breaker Fluids | Developing biodegradable polyacidic chelate-based breaker fluid systems for improved biodegradation, reduced toxicity, and smaller environmental footprint in oil and gas well treatments. |
Chelating Agents for Fermentation Media | Utilizing chelating agents like R1—CH(COOX1)—N(CH2COOX1)2 (I) in fermentation media to remove minerals, affect microorganism behavior and growth, and increase production of compounds of interest. |
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