4.4 Behavior of Bacteria as Geochemically Reactive Solids

The solid phase reactivity of microbial cells is related to the presence of acid functional groups, for example carboxyl and phosphoryl substituents, in the structural polymers of their cell walls, external sheaths, and extracellular polysaccharides (Kulczycki et al., 2002, 2005; Kennedy et al., 2011; Hao et al., 2013). Deprotonation of these various functional groups contributes to the development of surface charge on cells and provides discrete complexation sites for the electrostatic sorption of dissolved counter ions (Martinez and Ferris 2001; Smith and Ferris, 2001; Sokolov et al., 2001; Phoenix et al., 2007). These reactions are especially important for the solid phase partitioning of dissolved ions that are present in groundwater at a concentration below that imposed by the solubility of any mineral in which they might occur.

A general competitive sorption reaction between a proton (H+) and a metal cation (Mez+) at a protonated surface carboxyl binding site on a microbial cell (B·COOH) can be written as shown in Equation 27.

B·COOH + Mez+ = B·COOMez-1 + H+ (27)

The release of protons and sorption of metal cations is described by apparent rate constant (Kapp) and pH-conditional (KpH) sorption constant (Equation 28).

\displaystyle K_{pH}=\frac{K_{app}}{[\textup{H}^{+}]}=\frac{[\textup{B}\cdot \textup{COOMe}^{\textup{z}-1}]}{[\textup{B}\cdot \textup{COOMe}][\textup{Me}^{\textup{z+}}]} (28)

The equilibrium mass action expression emphasizes that ion sorption by microorganisms depends not only on proton (pH) and dissolved metal cation concentrations but also on the number of reactive chemical groups per cell.

The pH dependence of ion sorption reactions in an important intrinsic feature of solid phase sorbents, including microorganisms. For those metals that predominantly exist as cations in solution, sorption is significantly enhanced as pH increases and surface groups deprotonate. Conversely, oxyanions of metals and metalloids sorb better at low pH values as surface groups become protonated. The chemical properties of the sorbate ions will also influence sorption behavior. A particularly important factor is ionic potential, or the ratio of electric charge to radius of an ion. Sorption of smaller, highly charged ions is favored over larger, weakly charged ions. A change in ionic charge arising from aqueous complexation reactions has a similar effect as sorption strength tends to decrease when ions are complexed.

A plot that describes the amount of a dissolved chemical species that is sorbed by a sorbent as a function of increasing concentration, measured at constant temperature, is called a sorption isotherm. Three main types of sorption isotherms have been used to quantify sorption reactions involving bacteria and other sorbent solids. Considering a metal cation as the sorbate, the first type of sorption isotherm is represented by a linear distribution (partition) coefficient (Kd) as illustrated by Equation 29.

[B·Mez-1] = Kd[Mez+] (29)

A major limitation of the linear distribution isotherm (Figure 15a) is that it does not consider the finite number of sorption sites that exist on a solid phase sorbent. Instead of constantly increasing with increasing sorbate concentrations, the amount that is sorbed will tend to progressively decrease as sorption sites are occupied. This can lead to an overestimation of the amount of a chemical species that is sorbed, particularly at high sorbate concentrations. The deviation from linear behavior is captured to some extent in the empirical Freundlich sorption isotherm that is shown as Equation 30.

[B·Mez-1] = KF[Mez+]1/n (30)


KF = Freundlich sorption coefficient (L3/M)
1/n = exponent of non-linearity (Figure 15b)

When n = 1, the Freundlich relationship reduces to the linear distribution isotherm. The Freundlich isotherm is limited in that it does not include an explicit account for the number of available sorption sites, nor does it allow for variations in pH and ionic strength. This makes it difficult to compare interpretations of sorption data between locations.

Graphs comparing the amount sorbed as a function of increasing equilibrium dissolved sorbate concentration predicted by the a) linear distribution and b) Freundlich sorption isotherms.

Figure 15  Comparison of the amount sorbed as a function of increasing equilibrium dissolved sorbate concentration predicted by the a) linear distribution and b) Freundlich sorption isotherms.

The Langmuir sorption isotherm is the third type of sorption isotherm (Figure 16). Derived from mass action and mass balance considerations, the Langmuir isotherm is extensively used in environmental and microbial geochemistry. From the total number of available sorption sites (BTotal), the number of unoccupied sites is expressed by Equation 31.

[B·COOH] = BTotal – [B·Mez-1] (31)

Substitution of Equation 31 into the mass action expression (Equation 27) and rearrangement results in Equation 32.

Graph illustrating the Langmuir sorption isotherm.

Figure 16  Illustration of the Langmuir sorption isotherm. a) The amount sorbed asymptotically increases with equilibrium dissolved sorbate concentration until all of the sorption sites are filled and saturated. b) If the dissolved sorbate concentration exceeds the solubility limit of a mineral phase (Ksp), surface precipitation may occur and result in an apparent increase in the amount sorbed. c) If the initial surface precipitates are small and more soluble than larger mature crystals, dissolved concentrations may increase beyond the Ksp limit before decreasing as surface mineral precipitates grow in size.

\displaystyle [\textup{B}\cdot \textup{Me}^{\textup{z}-1}]=\frac{B_{Total}K_{pH}[\textup{Me}^{\textup{z+}}]}{(1+K_{pH}[\textup{Me}^{\textup{z+}}])} \displaystyle =\frac{B_{Total}K_{app}[\textup{Me}^{\textup{z+}}][\textup{H}^{+}]^{-1}}{(1+K_{app}B_{Total}K_{app}[\textup{Me}^{\textup{z+}}][\textup{H}^{+}]^{-1})} (32)

A key feature of the Langmuir sorption isotherm is that, at high concentrations of sorbate ions (and high pH in the case of cation sorption), the amount sorbed from solution asymptotically approaches (saturates) the total number of available sorption sites. Once all of the sorption sites are filled, no further sorption will occur. If dissolved ion concentrations continue to increase, the solution may eventually become oversaturated with respect to a mineral phase of some kind. This can trigger heterogeneous nucleation and surface precipitation of minerals on sorbents such as bacterial cells. The transition between sorption and surface precipitation is representative of a reactive continuum of solid phase partitioning reactions for dissolved ions in both pristine and contaminated systems (Warren and Ferris, 1998).


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