04 COMPLEXATION AND CHELATION

Complexation and Chelation

  • Definition: Complexation refers to the process in which metal ions form complexes with ligands, whereas chelation is a specific type of complexation where a ligand bonds to a metal ion at two or more coordination sites.

Chemical and Biochemical Phenomena in Stratified Water Bodies

  • Gas Exchange:

    • Gases such as CO2 and O2 exchanged with the atmosphere.

    • Chemical equations indicate processes like photosynthesis and acid-base reactions:

      • For example: HCO3 + OH- <-> Ca2+ + CO3(2-)

  • Microbial Action:

    • Microbial processes lead to leaching and uptake of nutrients (e.g., CH2O). Against a background of sediment groundwater interactions.

Metals Dissolved in Water

  • Properties depend on the species of metals dissolved in bodies of water.

  • Speciation:

    • Crucial for environmental chemistry in waters.

Chemical Speciation

  • Definition: Determining individual concentrations of different chemical forms that make up the total concentration of an element.

Trace Metal Speciation

  • Involves determining:

    • Free ions

    • Complexes (labile and non-labile)

    • Colloids

    • Total dissolved concentrations

Ligands and Complexes

  • Hydrated Metal Ion Complexation:

    • Example: Cyanide can form complexes with iron, Fe(CN)2-, Fe(CN)3-, etc.

  • Coordination Compounds:

    • Comprised of complex ions and counter ions.

  • Ligands:

    • Entities that bond with metal ions and donate electrons.

Donor Atom and Coordination Number

  • Donor Atom: Atom in ligand attached to the metal (e.g., in complex [Ag(NH3)2]+, N serves as donor).

  • Coordination Number: Number of donor atoms around metal ion.

Classification of Ligands

  1. Monodentate Ligands:

    • Have one donor atom (e.g., ammonia, water, cyanide).

  2. Bidentate Ligands:

    • Have two donor atoms (e.g., ethylenediamine, oxalate).

  3. Polydentate Ligands:

    • Have multiple donor atoms (e.g., EDTA).

Chelation

  • Chelating Agents:

    • Can bind to metal ions at multiple sites, forming stable ring structures.

  • Stability:

    • Chelates are more stable than complexes composed of unidentate ligands.

    • Stability increases with the number of chelating sites.

Humic Substances as Complexing Agents

  • Humic substances are important natural complexing agents formed from vegetation decay.

  • Classification:

    • Humic acid

    • Fulvic acid

    • Humin

  • They strongly affect water properties via acid-base reactions and solubility effects.

Binding of Metal Ions by Humic Substances

  • Binding types include:

    • Chelation (e.g., between carboxyl and phenolic groups).

    • They notably bind iron and other metals, influencing their bioavailability and mobility.

Redox Chemistry in Natural Waters

  • Oxidation-Reduction Reactions:

    • Organic matter oxidation is a significant process (e.g., decomposition reactions).

    • Microbial activity can significantly alter oxygen and nutrient cycles.

  • Oxygen Demand:

    • Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are metrics of organic matter and contaminants.

Sediment Formation and Characteristics

  • Formed through physical, chemical, and biological processes.

  • Contain various organisms and pollutants that interact with the water column.

  • Pore Water:

    • Water found in spaces between sediment particles, crucial for nutrient processing.

Microbial Activities in Aquatic Systems

  • Microorganisms (bacteria, fungi, protozoa) are vital for nutrient cycles:

    • They decompose organic matter, influence gas exchange, and manage nutrient availability.

  • Types of Microorganisms:

    • Algae: Involved in photosynthesis and biomass production.

    • Fungi: Decompose organic matter and have a role in nutrient cycling.

    • Protozoa: Participate in food webs and can impact water quality.

Importance of Microbial Processes

  • Microbial metabolism plays a key role in carbon cycling:

    • Methanogenesis occurs in anoxic conditions, yielding methane as a byproduct.

    • Microbial decomposition of organic matter is crucial for nutrient release and energy production in aquatic ecosystems.