Chapter IV, Section C, Item 2:  Acid Precipitation / Acid Drainage

SOx and NOx is not a Dr. Seuss rhyme. Rather it refers to various (x) oxides (O) of sulfur (S) and Nitrogen (N). When the sulfide or nitrogen is oxidized, they donate their electrons to oxygen gas. If water is also present, ultimately some of the water’s oxygen and electrons get incorporated into the sulfur-oxide or nitrogen-oxide compounds, leaving water’s hydrogen behind in the form of acid.

The Earth’s fossil fuels and most of its metal ores were originally formed in reducing environments, thus reduced metallic sulfide compounds, such as iron sulfide (pyrite), are usually incorporated into coal, oil, and metallic ore resources. When the sulfide is oxidized to SOx, either from combustion of coal in power plants or petroleum in automobiles, or from the smelting of metal sulfide ores, sulfuric acid forms from, and mixes with, moisture in the atmosphere to form acid precipitation. SOx can also be formed by ground water dissolving and oxidizing (leaching) metallic sulfide exposed in rock or mine tailings on the surface, in which case sulfuric acid is mixed in water runoff to form acid drainage. Acid precipitation and acid drainage problems are two separate environmental issues, but the main culprit, sulfide, and the result are the same: the acidification of soils and surface waters. Acid drainage problems are generally considered “point-source” pollution problems, localized to rock exposures such as coal or sulfide ore mine tailings, or even road cuts, but concentrations of acid can be extreme enough for acidification to extend well beyond the local waterway. Acid precipitation problems, given the abundance of tailpipes and smokestacks, are generally considered “non-point source” pollution problems, extending globally and slowly acidifying large quantities of surface and ground water.

In the case of acid precipitation, SOx is not alone. In the case of smog, ground-level ozone is not alone. NOx is also a culprit, an acid-forming and smog-forming gas that thus contributes to acid precipitation and air pollution. With combustion, atmospheric nitrogen becomes another notorious electron donor, oxidizing to NOx, and in the presence of atmospheric moisture, forms nitric acid in much the same way SOx forms sulfuric acid. The amount of NOx produced increases with increasing temperature of combustion. The high temperature combustion in 33 Indy race cars yields enough NOx emission for one Indianapolis 500 race day to be compared to the emissions of the Los Angeles freeways.

What’s the problem with the increasing acidity (decreasing pH) of water bodies? Aquatic life has a limited tolerance level for acidity. Eventually the acidity kills aquatic animal life and eventually even plant life. The acid dissolves the carbonate shells of microscopic plankton, destroying the base of the aquatic food chain. Acid also tends to mobilize metals, dissolving them into the water rather than leaving solid forms behind. Fossil fuels, particularly coal, contain toxic heavy metals, most notably arsenic, mercury, lead, and cadmium that are released as aerosols during combustion. The combination of metal-mobilizing acid precipitation can have a devastating impact on entire ecosystems, and may be the principal means of human exposure to heavy metals.

From the energy perspective, what does it take to neutralize acid? An electron donor, specifically an OH-base or other alkali compound such as bicarbonate mined from limestone. As with any non-point contamination with a broad geographic distribution, an ounce of prevention is worth a pound of cure. The preventative measure is to remove SOx and NOx from power plant smokestacks using scrubbers, and from petroleum using refining technologies. Both require energy by the second law, and thus cut directly into the profit of the energy provider. In the case of coal, co-generation technology has recently been developed where SOx from smokestack emission is reacted with lime, mined from limestone, to produce gypsum (hydrated calcium sulfate). The gypsum has economic value, used such products as wall-board, and thus has made the effort somewhat more profitable. Other issues with coal remain: the effects of mining, the need to reduce heavy metal aerosols, disposal of fly-ash, and the sequestering of CO2 as part of “clean” coal to address climate change. Climate change and the ensuing debate of “clean coal” have largely taken the attention away from the former issues.

Many old power plants still need to be upgraded to reduce acid forming emissions. The acid precipitation problem was clearly known to the scientific community by the mid-1970's, but was met with political stalling. The response of the Reagan administration toward acid precipitation mirrors the response of the Bush administrations toward climate change. Rather than dealing economically with the issue, to buy time, doubt was sown and research promoted, even if the issue had been resolved within the scientific community. The exact processes that turned SOx into sulphuric acid were not completely known, so experiments were conducted to prove the link. In the case of acid precipitation, the reason why acidity had a greater impact on New England and Canadian lakes than lakes in the midwest was also clearly known. Unlike the New England and Canadian lakes, the midwest lakes had a source of acid neutralizing bicarbonate in the bedrock and soil. But the knowledge did not stop experiments dividing lakes in half, acidifying one half while leaving the other as a scientific control. No surprise, the acidified halves died while the control halves were unchanged. The experiments did serve to quantify the effects of different levels of acidity. In the end, the acid precipitation problem was worse than previously thought, but years of delay had been added.