11.4.2: Acid mechanism

Mineral dissolution caused by microbial acid production is widespread. Microbial reactions that directly cause or induce acid generation are broadly distributed in natural environments. Moreover, the stability of many common rock-forming minerals, including aluminosilicates and carbonates, is sensitive to pH, reflecting the fact that they are salts composed of strong base cations and weak acid anions (Faure, 1991). As acid is produced, the weak acid anions of the minerals can protonate, which in turn can help push mineral dissolution reactions forward.

As an example, calcite \(\left(\text{CaCO}_{3}\right)\) dissolution can be written as: \[\text{CaCO}_{3} \longleftrightarrow \text{Ca}^{2+} + \text{CO}_{3}^{2-}\]

Carbonate \(\left(\text{CO}_{3}^{2-}\right)\) is an anion of carbonic acid \(\left(\text{H}_{2} \text{CO}_{3}\right)\), which is a weak acid and only starts to completely dissociate as a solution’s pH rises above 8. Most natural waters have near-neutral pH (e.g., pH 6 – 8), and under those conditions the carbonate ion largely protonates and forms bicarbonate: \[\text{CO}_{3}^{2-} + \text{H}^{+} \longleftrightarrow \text{HCO}_{3}^{-}\]

Combining reactions \(\PageIndex{1}\) and \(\PageIndex{2}\) gives the following overall reaction: \[\text{CaCO}_{3} + \text{H}^{+} \longleftrightarrow \text{Ca}^{2+} + \text{HCO}_{3}^{-}\]

Because hydrogen ions are included as a reactant in reaction \(\PageIndex{3}\), acid produced by microorganisms can push the reaction forward and cause mineral dissolution.

Many different microbial reactions generate acid. Major microbial sources of acid include carbonic acid and organic acids generated by organic matter degradation, nitric and nitrous acid produced by nitrifying microorganisms, and sulfuric acid produced by sulfur oxidizers. Furthermore, iron-oxidizing microorganisms can induce acid generation where the ferric iron they produce then participates in hydrolysis reactions (Section 12.1.3).

To place these reactions within an environmental context, we consider microbial acid generation in a soil linked with subsurface mineral weathering, as shown in Figure \(11.3\). In the conceptual diagram, products of soil respiration include carbon dioxide generated by plant respiration and microbial degradation of organic matter. Some portion of that carbon dioxide can be emitted to the atmosphere, and some can dissolve into water, forming carbonic acid (Reaction \(11.1.2.2\)), and then migrate into the subsurface with infiltrating water. A recent study at the Santa Catalina Mountains-Jemez River Basin Critical Zone Observatory, for example, found that two-thirds of the carbon dioxide from soil respiration was transported into the subsurface, where it drove mineral weathering reactions (Sanchez-Canete et al., 2018). Lastly, in addition to carbonic acid, organic matter degradation also generates organic acids such as oxalic, fumaric, and succinic acids, which also help drive mineral weathering.

Acid production by nitrification appears to be growing in significance as a result of human activities that add ammonium to the environment. Major anthropogenic sources of ammonium include sewage, livestock waste, and nitrogen fertilizer. Nitrogen fertilizer can take multiple forms, but commonly, ammonium-based fertilizers are applied to crop soils where microorganisms then convert the ammonium to nitrate via nitrification (Böhlke, 2002). Enormous amounts of nitrogen fertilizers are applied to crop soils globally each year to increase food production (Gruber and Galloway, 2008; Liu et al., 2010). One of the consequences is acidification of crop soils and the water passing through them into the subsurface (i.e., groundwater recharge) (Böhlke, 2002). As an example, Guo et al. (2010) demonstrated that the pH of crop soils across China have decreased significantly since the 1980’s. Acid produced by nitrification stimulates weathering in soils and the shallow subsurface and has been found to cause concentrations of numerous solutes to increase in groundwater and streams (Böhlke, 2002; Perrin et al., 2008).

Dissolved sulfide and sulfide minerals can be stable under anoxic conditions, but where they are exposed to oxygen or nitrate, then microorganisms can oxidize them and generate acid in the process, as noted previously (Section 11.4.1). Such exposure can be natural, but it is common in mining areas, where rocks and sediment are disturbed and those not containing sufficient densities of ore may be left exposed to oxidative weathering (Newsome and Falagán, 2021; Nordstrom et al., 2015). Sulfide within subsurface rocks and sediment can also be exposed to oxygen introduced through boreholes or water level fluctuations (Schreiber et al., 2000). Moreover, nitrogen fertilizer use can also stimulate weathering of subsurface sulfide by increasing fluxes of nitrate into anoxic zones of the subsurface (Xu et al., 2021).

---