Overview Data Input Data Output DIC Charge Balance Physico-Chem Params Reactions Redox Mineral Phases

Dissolution and Precipitation

The mineral table — as shown in the right screenshot — is part of aqion’s main input panel. It opens/closes with the checkbox Minerals.

Once the mineral table is open, you can enter two parameters for each mineral:

• the amount of mineral in mmol/L (as an initial value for mineral dissolution)
• the saturation index (SI)

To enter these values, please double-click on the row of the corresponding mineral.

The right screenshot shows an example where gypsum is added to pure water (with an initial amount of 20 mmol/L).

Typical applications are:

• Dissolution of Calcite (in an open/closed CO₂ system)
• Solubility of Gypsum

Alphabetical Sorting. You can sort the mineral phases by one-click on the gray table header:

• click on Mineral — sorting by the name of the mineral
• click on Formula — sorting by the chemical formula of the mineral

The list of all solid phases used in aqion is presented here (about 100 minerals).

Minerals in Chemical Equilibrium

The database contains about 250 minerals. However, the table in the screenshot above only lists the minerals that are actually included in the equilibrium calculations.

It is only a small subset from the entire ensemble of the available minerals (this subset is marked with a bullet (•) in the tables here). The preselected minerals are characterized by relatively fast dissolution and/or precipitation kinetics. These include calcite, gypsum, hydroxides and other amorphous phases.

The user can modify the selection of “active mineral phases”. For this purpose please use the Settings in the upper menu bar, which opens the config panel.

Mineral Precipitation

Two conditions should be fulfilled for the precipitation of a given mineral:

1. the mineral must be listed as an “active mineral phase” (to enter the equilibrium calculation)
2. saturation index: SI > 0

The mineral will only precipitate if the solution is supersaturated (SI > 0), otherwise nothing will happen.

The amount of the precipitated mineral in mmol/L is determined by thermodynamics. In equilibrium calculations, the precipitation stops when exact saturation is achieved (SI = 0).

Mineral Dissolution

Three conditions should be fulfilled for the dissolution of a given mineral:

1. the mineral must be listed as an “active mineral phase” (to enter the equilibrium calculation)
2. a non-zero initial amount of the mineral in mmol/L
3. the solution should be undersaturated with respect to the mineral phase (SI < 0)

The dissolution stops when either saturation is achieved (SI = 0) or when the initial inventory is exhausted.

In some cases we are interested in the composition and pH of a water after a definite amount of mineral is dissolved (irrespective of whether or not thermodynamics permit this). To answer this question we refer to the so-called forced mineral dissolution.

Venn Diagram

In equilibrium calculations we must distinguish between three subsets of mineral phases (from the largest to the smallest):

• the minerals available in the thermodynamic database
• the (user-selected) minerals that enter the equilibrium calculation
• the precipitated minerals

The logical relationship between these subsets is shown in the Venn diagram:

In natural waters, a very small number of minerals precipitate (if at all), because the solution is undersaturated with regard to most of them. Moreover, the Gibb’s phase rule is strictly observed.