Bioprecipitation as a Bioremediation Strategy for Environmental Cleanup 23

2.3 Bioprecipitation

2.3.1 Overview: Principles and Applications

2.3.1.1 Chemical Precipitation

Precipitation is an intricate phenomenon involving thermodynamic and kinetic processes. The

process is governed by its thermodynamic properties, i.e., supersaturation state. A solution is

considered supersaturated when the solute and solvent are no longer in an equilibrium (saturated)

state (Karpiński and Bałdyga 2019, Lewis 2017). Equation 2.1 shows the supersaturation calculation,

whereby the supersaturation (σ) is calculated from the actual molar chemical potential (μ), the

molecule in equilibrium state (μeq), the universal gas constant (R) and the absolute temperature (T).

Based on the equation:

- The solution is in equilibrium when ∆μ (∆μ = μ – μeq) is equal to 0

- If ∆μ > 0 the solution is supersaturated, spontaneous precipitation will occur

- If ∆μ < 0 the solution is below the saturated state, spontaneous dissolution will occur (Karpiński

and Bałdyga 2019).

Supersaturation (Davey and Garside 2000, Karpiński and Bałdyga 2019)

(μ – μeq)

σ =

Eq. 2.1

RT

Phase diagrams can demonstrate the liquid or solid phase of a compound of interest (Karpiński

and Bałdyga 2019). The thermodynamic component of precipitation can be better understood by the

Gibbs phase rule, which identifies the number of possible phases and the degree of freedom of the

multiphase system in equilibrium (Faghri and Zhang 2006). The Gibbs phase rule both identifies

and inhibits solid precipitates formed from a solution (Yong et al. 2014).

Furthermore, the thermodynamic potential is described by Gibbs free energy. It explains the

maximum energy transfer of a closed system. The change in Gibbs free energy (∆G) can be used

to define primary homogenous nucleation. Equation 2.2 shows the calculation, where the number

of molecules (N), the reaction affinity (Φ), the crystal surface area (A) and the surface tension (σ)

are used to explain the work to produce crystals during precipitation of a supersaturated solution.

From Eq. 2.1, one can consider R as the molar chemical potential change or the Gibbs free energy

(Karpiński and Bałdyga 2019).

Gibbs Free Energy Change (Karpiński and Bałdyga 2019, Nielsen 1964)

∆G = –NΦ + Aσ

Eq. 2.2

While supersaturation state is the driving force of precipitation, the kinetic process illustrates

the rate of precipitation. Nucleation, crystal growth and agglomeration represent the primary kinetic

process involved in precipitation (Lewis 2017), these terms are further defined in Table 2.2. Again

∆μ

from Eq. 2.1, the term

represents the rate of nucleation and crystal growth (Karpiński and

Bałdyga 2019).

RT

Precipitation can be applied to soil, groundwater, surface water and wastewater treatment. For

soil and groundwater remediation, metal(loid)s can chemically precipitate out of the pore water

(Yong et al. 2014). These precipitates can adsorb onto soil particle surfaces (Yong et al. 2014) or can

form a cement matrix clogging the pore spaces (Mitchell and Soga 2005). For water and wastewater