# Environmental Chemistry¶

• The primary consideration is the oxidation state of any species within the environment
• 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

## 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

## 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

## 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