SURFACE COMPLEXATION OF ZN ON FERRIHYDRITE

The surfaces of oxides carry a charge that depends on pH and composition of the solution. The charge enhances sorption of counter-ions (with a charge opposite to the surface), and repels co-ions (with the same charge as the surface). Many heavy metals bind so strongly that they even sorb on oppositely charged surfaces. Sorption on charged surfaces is calculated with surface complexation models. For example, the sorption of Zn2+ on ferrihydrite (hydrous ferric oxide, or 2-line FeOOH) follows the reaction:
     Zn2+ + Hfo_sOH ↔ Hfo_sOZn+ + H+
The K value for the reaction depends on the charge of the surface.
The inclusion of charge effects in surface complexation contrasts with ion exchange, which is a charge-neutral reaction. Generally, ion exchange can explain most of sorption behavior of cations in aquifers. However, surface complexation models must be invoked for some heavy metals like Pb2+ and Cu2+ which sorb very strongly, and for oxyanions such as H2AsO4-.

The surface complexes are a function of the various solute ions which compete for the surface sites, the charge of the surface, and the ionic strength. PHREEQC contains the surface complexation model of Dzombak and Morel, which assembles sorption data on ferrihydrite.
Often, a sorption edge is presented in the literature, which depicts the % sorbed as function of the pH.

The PHREEQC input file zn_sor.phr calculates the sorption edge of Zn2+ on 1 mM ferrihydrite in 0.1 M NaNO3.
SOLUTION 1 is prepared with 0.1 M Na+ and a concentration of NO3- that depends on pH. To obtain pH = 13, the Na concentration is increased to 0.15 M. To the solution, 88 mg pre-equilibrated ferrihydrite is added (keyword SURFACE and identifier -equil). Next, 1 µM Zn(NO3)2 is added (keyword REACTION). In USER_GRAPH, the remaining solute Zn concentration is subtracted from total added Zn, and plotted as % sorbed.
The solutions, surfaces, and reactions for the other pH's are written to the selected output file Zn_1um.prn with USER_PUNCH. Subsequently, the file is read in with keyword INCLUDE$, and processed.
(If you don't want to program, you can write out the file completely, as is done for the next series of experiments.)

The calculations are repeated for 10 µM Zn(NO3)2. The acid branch of the sorption edge for the higher concentration is shifted to higher pH and is flatter because sorption of high Zn concentrations also occurs on weak sites. At increasing pH various solute complexes such as Zn(OH)42- compete with the surface sites and reduce sorption to near-zero at pH = 13.

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