04 - Redox¶

Rainwater from Central Oklahoma, evaporated to a twentieth of its volume, and what the redox chemistry does while that happens.

Two things are being shown. Evaporation as a reaction – removing water rather than adding a reactant – and what happens to a solution that contains both an oxidised and a reduced form of nitrogen when it is asked to come to redox equilibrium.

Evaporation by removing water¶

TITLE Example 4a.--Rainwater evaporation

SOLUTION 1  Precipitation from Central Oklahoma

        units           mg/L

        pH              4.5   # estimated

        temp            25.0

        Ca              .384

        Mg              .043

        Na              .141

        K               .036

        Cl              .236

        C(4)            .1      CO2(g)  -3.5

        S(6)            1.3

        N(-3)           .208

        N(5)            .237

REACTION 1

        H2O     -1.0

        52.73 moles

SAVE solution 2

END

TITLE Example 4b.--Factor of 20 more solution

MIX

        2       20.

SAVE solution 3

END

There is no “evaporate” keyword. Evaporation is written as a reaction with a negative coefficient:

REACTION 1
        H2O     -1.0
        52.73 moles

– 52.73 moles of water taken out, which is the amount that concentrates the solution twentyfold. The second simulation then uses MIX with a factor of 20 to scale the result back to a kilogram of water, so that the concentrations are reported in the usual units.

This is worth seeing because it is how PHREEQC handles most processes that are not obviously a reaction. Anything expressible as adding or removing a quantity of something is a REACTION, and the sign is the whole difference.

The redox part¶

The rainwater analysis carries nitrogen in two oxidation states at once – N(-3) as ammonium, 0.208 mg/l, and N(5) as nitrate, 0.237 mg/l. In the real rainwater they coexist; thermodynamically they should not, because ammonium and nitrate are not at equilibrium with each other in an oxidising solution.

When PHREEQC is asked for an equilibrium distribution it resolves this: the two are brought to the same redox potential, which in practice means the reduced form is largely oxidised. The nitrogen total is conserved, and how it is divided between the states changes.

This is the general caution about redox in geochemical models. A measured analysis records what was there; an equilibrium model reports what would be there if the couples had come to equilibrium, which in natural waters they very often have not. The difference is not an error in either – it is what the two are for. 01 - Speciation calculation shows the other side of it, where particular elements are told which couple governs them.

The result¶

table_ex4¶
Solution number	Mass of water (kg)	Cl, micromole	Cl, micromole/kgw	Nitrate [N(5)], micromole/kgw	Dissolved nitrogen [N(0)], micromole/kgw	Ammonium [N(-3)], micromole/kgw	Calcite saturation index	Dolomite saturation index	Gypsum saturation index
1	1	6.656718505788	6.656718505788	16.92051974431	0	14.85007640007	-9.207140224891	-19.02456064954	-5.379168511906
1	0.05001680422005	6.656718505789766	133.0896407636	160.1200489895	475.0783936074	0	-9.36850322028	-19.35516674588	-2.933461616629
1	1.000336084401	133.13437011579535	133.0896407636	160.1200489895	475.0783936074	0	-9.36850322028	-19.35516674588	-2.933461616629

The table gives the evaporated composition and the redox-equilibrated distribution.

Source¶

Parkhurst, D. L. and Appelo, C. A. J. (2013). Description of input and examples for PHREEQC version 3. U.S. Geological Survey Techniques and Methods, book 6, chapter A43. This is Example 4 of that manual, using a precipitation analysis from Central Oklahoma.