# 5 Redox Chemistry ## Introduction Earth’s atmosphere contains about $\pu{20 \%}$ molecular oxygen, $\ce{O2 (g)}$, a chemically reactive gas that plays an essential role in the metabolism of aerobic organisms and in many environmental processes that shape the world. The term oxidation was originally used to describe chemical reactions involving $\ce{O2 (g)}$, but its meaning has evolved to refer to a broad and important reaction class known as _oxidation-reduction (redox) reactions_. In this chapter, we focus on how loss or gain of electrons can impact elements and their behavior in the environment. Below are some basic examples of redox reactions that occur on Earth, both naturally and due to human actions. ### Acid Rain Coal from the Appalachian Mountains have naturally higher sulfur content, due to their geological origin in the ocean basins. This coal is also the most common type of coal consumed in most of the eastern half of the US. When this coal is combusted, $\ce{S}$ is oxidized as shown in the reaction below to form $\ce{SO2(g)}$. Subsequent oxidation reaction turns $\ce{SO2(g)}$ into $\ce{SO3(g)}$, which leads to formation of sulfuric acid ($\ce{H2SO4(aq)}$) by combining with water. Sulfuric acid is a strong acid and leads to acid rain as well as acidification of natural streams, leading to deleterious impacts on overall environments. $$ \begin{align*} \ce{ S(s) + O2(g) &-> SO2(g\\ 2 SO2(g) + O2(g) &-> 2 SO3(g) \\ SO3(g) + H2O(l) &-> H2SO4(aq) \\ H2SO4(aq) &-> H+(aq) + HSO4^-(aq) & } \end{align*} $$ ### Acid Mine Drainage Metal-bearing sulfide minerals such as iron, copper, and lead pyrites ($\ce{FeS2}$, $\ce{Cu2S}$, and $\ce{PbS}$), are stable in reducing environments but break down in oxidizing environments driven by oxidation of the sulfide anion ($\ce{S^2-}$) to $\ce{S^6+}$ (present in $\ce{SO4^2-}$), can result in release of trace metals and arsenic to soils and aquatic systems. $$ \begin{align*} \ce{ FeS2 + 14 Fe^3+ + 8 H2O &-> 16 H+ + 15 Fe^2+ + 2 SO4^2-\\ PbS + 8 Fe^3+ + 4 H2O &-> 8 H+ + SO4^2- + Pb^2+ + 8 Fe^2+ } \end{align*} $$ 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 any aquatic life. [Mine Drainage | U.S. Geological Survey (usgs.gov)](https://www.usgs.gov/mission-areas/water-resources/science/mine-drainage) ### Arsenic in Bengal Basin Arsenic poisoning and the health emergency in Bengal Basin of the Indian subcontinent. The shallow aquifer materials in the Bengal Basic naturally contain thick deposits of arsenopyrite ($\ce{FeAsS}$) minerals. To address high infant mortality rates in Bangladesh, the WHO recommended and funded a large-scale installation of groundwater wells as groundwater naturally contained low bacteria compared to stream water. During pumping of groundwater, the extensive drawdown of the water table allowed reduction of ferric ($\ce{Fe^3+}$) to ferrous ($\ce{Fe^2+}$) iron and dissolution of iron hydroxides, which in turn released arsenic into solution. Aquifers where dissolved arsenic was once well below drinking-water standards (e.g., $\pu{10 ppb}$ in the USA) can experience substantial increases of dissolved arsenic that exceed safe drinking water standards. [Arsenic Contamination In Groundwater In Bangladesh: An Environmental And Social Disaster | IWA Publishing](https://www.iwapublishing.com/news/arsenic-contamination-groundwater-bangladesh-environmental-and-social-disaster)
### Chromium in Groundwater Groundwater contamination during unregulated dumping of $\ce{Cr(VI)}$ ($\ce{Cr^6+}$) waste by a large electrical utility, PG&E, into unlined soil pits created a health emergency in the community of Hinkley, CA. The $\ce{Cr}$ waste seeped into the aquifer and created a large plume that went undetected until the contaminated well water created many health problems in the local community. Since $\ce{Cr(VI)}$ is present in water as an oxyanion ($\ce{Cr2O7^2-}$ or $\ce{CrO4^2-}$), it is not "attracted" (sorbed) to aquifer materials, this form of Cr can easily be transported in groundwater environments. Typically, treatment of this form requires that $\ce{Cr^6+}$ to be reduced to $\ce{Cr^3+}$, which is less toxic. [Widespread groundwater contamination risk from chromium | Stanford News](https://news.stanford.edu/2018/07/23/widespread-groundwater-contamination-risk-chromium/)
### Excess nutrients in water Large-scale factory-farming practices use excessive fertilizers to increase crop and animal yield. A by-product of these practices is the discharge of large concentrations of nutrient elements such as $\ce{N}$ and $\ce{P}$ in stream runoff. The excess $\ce{N}$ and $\ce{P}$ in the final receiving bodies of water result in an increase in algal populations in water. Excess algae create "blooms" that can be seen in the form of green pigments that float on water. Eutrophication, where degradation of algae in water bodies reduces available dissolved $\ce{N}$ and $\ce{O2(aq)}$ (DO) in the water. Aerobic microbial organisms responsible for degradation (consumption) of algae also consume during this process. Ultimately, DO reduces to a dangerously low level that is detrimental to aquatic and marine life. Excess fertilizer discharge in stream runoff have created "dead zones" and harmful algal blooms (HAB) in the Gulf of Mexico, the Great Lakes, and along the Atlantic coastal areas. Highly oxygenated surface waters in cool streams and lakes typically contain $\pu{7-9 mg L-1}$ of DO. Wetlands and groundwater contain naturally lower amounts of DO than do surface waters (e.g., groundwater generally contain DO from $\pu{1-6 mg L-1}$ while wetlands can vary between oxic and anoxic conditions). In aquatic environments, DO levels below $\pu{5-6 mg L-1}$ are detrimental to aquatic life.
## Learning Goals ```{admonition} Learning Goals The main goals for this chapter are to: 1. identify oxidation and reduction processes and the role of electrons in redox processes 2. balance oxidation and reduction reactions 3. explain how electron activity is influenced in various environmental settings 4. show relationships between electron activity and $p\ce{H}$ in a graphical format. ``` ## References 1. [Ch. 16 Electrochemistry - Chemistry: Atoms First | OpenStax](https://openstax.org/books/chemistry-atoms-first/pages/16-introduction)