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Environmental Chemistry

  • The primary consideration is the oxidation state of any species within the environment
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  • The figure to the right show a few contaminants and their oxidated and reduces states
    • These are ranked according to their anaerobic/aerobic nature

Electrochemistry

  • Defined as the study of the interchange of chemical and electrical energy
  • It is primarily focused around redox reactions
    • The use of electricity to drive chemical reaction
    • The use of chemistry to produce electricity
  • Redox couples are always written as reduction reactions
    • +ve for reduction, -ve for oxidation
  • Standard state reduction potentials \(E^\circ\) represent the “ease” with which the reaction will occur
    • Are always related back to the standard hydrogen electrode (\(E^\circ=0.000V\))
    • Are measured at standard state (1M, 1atm.)
      • Since environmental systems are also pH dependent, and this process will produce protic/aprotic species, the pH will change, changing the reduction potential
  • Most weathering processes are acid/base rather than redox reactions
  • Many redox processes within natural systems are biologically mediated

Catalysis

Heterogeneous - catalyse from a different phase - surface catalysis Homogeneous - catalyse from the same phase - solute catalysis

Redox thermochemistry, the energy difference between product and reactant is \(E^\circ\)

  • Redox reactions determine the speciation (what form the element exist in) of many of the environmental element
    • This also determine whether or not the compound is a solute or particulate matter

In natural bodies of water, the level of oxygen mixing determines the reduction state of the body of water

  • The more mixed the body of water, the higher the reduction potential (more oxidised) over the depth.
  • In more stagnant environments, the oxygen is pulled from the water, creating a more reduced environment as the depth increases

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Pourbaix Diagrams

  • Show the speciation of the specific component, based on the environmental conditions
    • pH is shown horizontally, with a vertical line showing a proton transfer
    • Reduction potential is shown vertically with horizontal lines representing electron transfer

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Microbial Mediation

  • Since microbes can use compounds other than oxygen for energy production, they can often oxidise other species such as iron, nitrogen and phosphorous

    • They can’t carry out difficult redox reactions, but instead catalyse simple ones
    • This plays a large part in the equilibrium of a natural system
    • As oxygen levels deplete in the environment, different energy sources are used, with successively lower reduction potentials

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There are a few reactions of note, namely

Denitrification - removal of nitrate

* \(\ce{2.5C + 2NO− + 2H+ -> N + 2.5CO + 2HO}\) * Useful for wastewater treatment

Ferric iron reduction

  • \(\ce{C_{org} + 4Fe(OH)3 + 8H+ <=> CO2 + 4Fe^{2+} + 10H2O}\)

  • More important in groundwater systems than in open bodies

    • In open bodies of water, the iron will ppt out as rust and will not be particularly available
    • Can then be bound to \(\ce{CO3^{2−}}\) or \(\ce{FeS/FeS2}\) depending on the further reduction process
    • Is often found as soluble \(\ce{Fe(H2O)6^{2+}}\) in anoxic sediments
    • When exposed to oxygen, Fe(II) is typically quickly oxidised to Fe(III)

Sulphate reduction

  • \(\ce{SO^{2−} + 2C_{org} + 2H2O <=> H2S + 2HCO3−}\)
  • If the pH is less than 7, \(\ce{HS−}\) will form and exchange with \(\ce{FeS}\)

Fermentation reactions

  • Oxidise organic matter to form \(\ce{CO2}\)
  • Can often ferment organic matter to produce \(\ce{CH4}\) as well as \(\ce{CO2}\)

Bioremediation

  • The process of using bacteria to fix issues
  • Commonly used to reduce soluble \(U^{6+}\) to \(U^{4+}\)
  • Too much bacterial build up can prevent nutrients from dispersing evenly through the media to be remediated