5 Redox Chemistry#
Introduction#
Earth’s atmosphere consists of about 20% molecular oxygen (\(\ce{O2 (g)}\)). This chemically reactive gas plays a crucial role in the metabolism of aerobic organisms as well as in various environmental processes that shape our world. The term “oxidation” was initially used to describe chemical reactions involving \(\ce{O2 (g)}\). Still, its definition has since evolved to encompass a broader and more significant category of reactions known as oxidation-reduction (or redox) reactions.
In this chapter, we explore how the loss or gain of electrons affects elements and their behavior in the environment. Below are some fundamental examples of redox reactions occurring on Earth through natural processes and human activities.
Acid Rain#
Due to their geological origin in ocean basins, coal from the Appalachian Mountains has a naturally higher sulfur content. This type of coal is also the most commonly consumed in the eastern half of the United States. When combusted, sulfur (\(\ce{S}\)) oxidizes, as shown in the reaction below, to form sulfur dioxide (\(\ce{SO2(g)}\)). A subsequent oxidation reaction converts \(\ce{SO2(g)}\) into sulfur trioxide (\(\ce{SO3(g)}\)), which then combines with water to form sulfuric acid (\(\ce{H2SO4(aq)}\)). Sulfuric acid is a strong acid that contributes to acid rain and the acidification of natural streams, leading to harmful impacts on the overall environment.
Acid Mine Drainage#
Metal-bearing sulfide minerals, such as iron pyrite (\(\ce{FeS2}\)), copper sulfide (\(\ce{Cu2S}\)), and lead sulfide (\(\ce{PbS}\)), are stable in reducing environments. However, these minerals can break down in oxidizing environments due to the oxidation of the sulfide anion (\(\ce{S^2-}\)) to \(\ce{S^6+}\), which is present in sulfate (\(\ce{SO4^2-}\)). This process can lead to the release of trace metals and arsenic into soils and aquatic systems.
A by-product of these reactions is the large-scale release of acids (\(\ce{H+}\) in the above reactions) into stream environments. Often, \(p\ce{H}\) drops to levels that cannot sustain aquatic life.
Arsenic in Bengal Basin#
Arsenic poisoning poses a significant health emergency in the Bengal Basin of the Indian subcontinent. The shallow aquifer materials in this region naturally contain thick deposits of arsenopyrite (\(\ce{FeAsS}\)) minerals. To combat high infant mortality rates in Bangladesh, the World Health Organization (WHO) recommended and funded the large-scale installation of groundwater wells, as groundwater typically has lower bacterial levels than surface water.
However, during the pumping of groundwater, the substantial drawdown of the water table led to the reduction of ferric iron (\(\ce{Fe^3+}\)) to ferrous iron (\(\ce{Fe^2+}\)) and the dissolution of iron hydroxides. This process releases arsenic into the water. As a result, aquifers that previously had dissolved arsenic levels well below drinking water standards (for example, 10 ppb in the USA) can now experience significant increases in dissolved arsenic, surpassing safe drinking water limits.
Chromium in Groundwater#
Groundwater contamination due to the unregulated dumping of hexavalent chromium (\(\ce{Cr(VI)}\) or \(\ce{Cr^6+}\)) waste by the electrical utility PG&E into unlined soil pits has created a health emergency in the community of Hinkley, California. The \(\ce{Cr}\) waste seeped into the aquifer, forming a large contamination plume that went undetected. It was only when the contaminated groundwater caused health issues within the local community that the problem became evident.
Since \(\ce{Cr(VI)}\) exists in water as oxyanions (either \(\ce{Cr2O7^2-}\) or \(\ce{CrO4^2-}\)), it does not readily bind (or sorb) to aquifer materials, allowing it to be easily transported through groundwater. Typically, treating this form of chromium requires its reduction to trivalent chromium (\(\ce{Cr^3+}\)), which is less toxic.
Widespread groundwater contamination risk from chromium | Stanford News
Excess nutrients in water#
Large-scale factory farming practices often rely on excessive use of fertilizers to boost crop and animal yields. However, a significant by-product of these practices is the discharge of high concentrations of nutrient elements such as nitrogen (\(\ce{N}\)) and phosphorus (\(\ce{P}\)) into streams via runoff. The accumulation of excess \(\ce{N}\) and \(\ce{P}\) in receiving bodies of water leads to an increase in algal populations, resulting in visible “blooms” characterized by green pigments floating on the water’s surface.
This process, known as eutrophication, occurs when algae decompose the available dissolved nitrogen and dissolved oxygen (\(\ce{O2}\), or DO) in the water. The aerobic microorganisms responsible for degrading the algae consume oxygen, which can lead to dangerously low DO levels that harm aquatic and marine life.
The discharge of excess fertilizers into streams has created “dead zones” and harmful algal blooms (HABs) in regions such as the Gulf of Mexico, the Great Lakes, and along the Atlantic coastline. In healthy aquatic environments, highly oxygenated surface waters in cool streams and lakes typically contain between \(\pu{7 mg L-1}\) and \(\pu{9 mg L-1}\) of DO. In contrast, wetlands and groundwater usually have naturally lower amounts of DO; groundwater generally contains DO levels between \(\pu{1 mg L-1}\) and \(\pu{6 mg L-1}\), while wetlands can experience a range of conditions from oxic to anoxic. It is important to note that DO levels below \(\pu{5-6 mg L-1}\) in aquatic environments can harm aquatic life.
Learning Goals#
Learning Goals
The main goals for this chapter are to:
identify oxidation and reduction processes and the role of electrons in redox processes
balance oxidation and reduction reactions
explain how electron activity is influenced in various environmental settings
show relationships between electron activity and \(p\ce{H}\) in a graphical format.