4.5 Mineral Dissolution and Precipitation

Groundwater is typically in simultaneous contact with mixed assemblages of different solid minerals. At shallow depths, infiltration of dilute undersaturated meteoric water typically promotes mineral dissolution. The dissolution of minerals not only contributes to the acquisition of solutes by groundwater but also fosters the development of secondary porosity and increased hydraulic conductivity. This is especially pronounced in karst systems that occur in areas with carbonate bedrock. With increasing depth and residence time underground, groundwater gradually approaches equilibrium with respect to the minerals that are present. At the same time, chemical and microbiological reactions may cause groundwater to become over- or undersaturated and bring about mineral precipitation or dissolution, respectively. In contrast to dissolution reactions, mineral precipitation processes in groundwater systems promote the formation of coatings on mineral grain and fracture aperture surfaces. This can lead to cementation and closure of pore throats with a coincident decrease in porosity and hydraulic conductivity.

Mineral dissolution processes are classified as either congruent or incongruent reactions. Congruent dissolution refers to minerals that dissolve completely into their constituent ions, whereas incongruent dissolution applies to minerals that partially dissolve and leave behind a residual solid weathering product. Both types of mineral dissolution processes tend to consume protons as reactants, which forces the release of sorbed cations into solution to conserve electroneutrality.

The most common source of protons in mineral dissolution reactions is carbonic acid, which is generated from the degradation of organic matter by heterotrophic microbial activity. Other inorganic and organic acids are produced by microorganisms as well. These include sulfuric acid from the oxidation of sulfide minerals, as well as a wide variety of carboxylic acids such as acetic acid and oxalic acid. Among these, organic acids are known to contribute to ligand-promoted mineral dissolution reactions by complexing released cations. In these reactions, the base (L) of an acid (HL) is the metal-complexing ligand for the metal cation (Mez+) as shown in Equation 33.

Mez+ + HL = H+ + MeLz-1 (33)

In ligand-promoted dissolution, the formation of the metal-ligand complex essentially removes the free metal cation (product of the dissolution reaction) from solution. This causes a shift away from equilibrium and sustains further dissolution in accordance with Le Chatelier’s principle. The same shift in the equilibrium solubility of a mineral occurs when a change in oxidation state of a dissolution product occurs. For example, the oxidation of Fe2+ to Fe3+ from the dissolution of Fe(II)-bearing silicate minerals by lithotrophic Fe(II)-oxidizing bacteria promotes the dissolution reaction (Shelobolina et al., 2012; Shirokova et al., 2016).

Of the various microorganisms active under anaerobic conditions, dissimilatory iron and manganese reducers are notable as active agents in the reductive dissolution of oxide minerals. The hydrous iron and manganese oxides utilized by these microorganisms as source electron acceptors for anaerobic respiration typically occur as thin coatings on other mineral grains, as well as particulate organic materials. Dissolution of these coatings by microbial reduction under low oxygen conditions frequently results in gleyic (gray-blue-green) color characteristics, such as those evident in hand samples of borehole cuttings and cores from Mn(IV)- and Fe(III)-reduction zones in groundwater systems (Figure 17).

Photograph showing An excavation exposing gray-blue-green gleyic discoloration produced at the base of a forested slope in response to downwards infiltration of water and microbial reduction of red-brown iron oxide minerals under anaerobic conditions.

Figure 17  An excavation exposing gray-blue-green gleyic discoloration produced at the base of a forested slope (bottom profile) in response to downwards infiltration of water and microbial reduction of red-brown iron oxide minerals (upper two profiles) under anaerobic conditions. The distance along the excavation is approximately 2.0 m and the depth of the lower gleyed profile is about 0.6 m. (Reproduced under the terms of Creative Commons Attribution 3.0 license from Schwarz et al., 2018).

Microorganisms contribute to the precipitation of a wide variety of minerals including oxides, phosphates, carbonates, sulfides, and silicates as shown in Figure 18 (Fortin et al., 1997; Ferris et al., 2000). The mechanisms of microbial mineral precipitation are diverse, but generally involve two distinct phases: nucleation and crystal growth. Nucleation is the most critical stage for mineral precipitation and occurs either homogeneously or heterogeneously. In homogeneous reactions, mineral nuclei are formed by the random collision of ions in solution. Conversely, heterogeneous nucleation involves the formation of crystal nuclei on the surfaces of homologous (similar crystallographic mineral surfaces) or foreign solids such as microbial cells. Of these two nucleation processes, heterogenous nucleation dominates in groundwater systems. Once a stable nucleus has formed, crystal growth can spontaneously proceed provided that the concentrations of ions in solution continue to exceed the solubility product of the solid mineral phase (the solution must be oversaturated).

A thin-section transmission electron micrograph showing heterogenous nucleation and precipitation of a solid mineral phase on a bacterial cell attached to a mineral grain.

Figure 18  A thin-section transmission electron micrograph showing heterogenous nucleation and precipitation of a solid mineral phase (indicated by arrows) on a bacterial cell attached to a mineral grain (M). Scale bar = 360 nm.

The Gibbs energy for crystal nucleation is constrained by the bulk free energy of the solution (ΔGbulk) and the interfacial free energy of the corresponding solid phase (ΔGinterface) as shown in Equation 34.

ΔGr = ΔGbulk + ΔGinterface (34)

The bulk free energy term is a function of the degree to which a solution is oversaturated (Equation 21) as expressed by Equation 35,

\displaystyle \Delta G_{bulk}=-RT\textup{ln}\frac{IAP}{K_{so}} \displaystyle =-2.303RT\textup{log}_{10}\frac{IAP}{K_{so}} \displaystyle =-2.303RT(SI) (35)

whereas the interfacial free energy depends on interfacial surface tension of the mineral phase (γ) and molar surface area of the nucleus in contact with water (Acw) as described by Equation 36.

ΔGinterface = Acwγ (36)

The interfacial free energy term represents the work that must be done to create a new mineral surface. Together, these relationships provide a useful model to better understand microbial contributions to mineral precipitation.

Microbial activity will often trigger a change in solution chemistry that leads to oversaturation and a higher SI value. For example, bacterial Fe(II) oxidation often gives rise to dissolved Fe3+ concentrations that far exceed the solubility of iron oxides (Emerson et al., 2010; Edwards et al., 2018). This alone can induce mineral precipitation by lowering the bulk free energy term for both homogeneous and heterogeneous nucleation reactions. However, chemically reactive sites on microbial cells that facilitate ion sorption at nucleation sites will tend to reduce the mean interfacial surface energy of the solid phase and decrease the surface of the nucleus in contact with the bulk solution. The expected result is a reduction in the overall interfacial free energy, which is conducive to heterogeneous nucleation and precipitation.


Groundwater Microbiology Copyright © 2021 by F. Grant Ferris, Natalie Szponar, and Brock A. Edwards. All Rights Reserved.