PNG 550
Reactive Transport in the Subsurface

3.5 Homework Assignment 2 (Optional)

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Homework Question 2 (Optional) (total 30 bonus points for students who do the work and develop a neat solution)

Set up a batch reactor model for AsO43- sorption on Fe(OH)3 given initial conditions and parameters in Tables 6. Run the simulation to understand how sorbed concentrations are affected by different parameters and geochemical conditions.

  1. Effects of initial pH: Compare the surface concentration of AsO43-, PO43- and Na+ at the initial pH at 2, 4, 6, 7, 8 and 10, respectively. Plot the concentrations of the three ions as a function of pH. Describe the differences and explain your observations.
  2. Effects of Na+ concentration: compare the surface concentrations of AsO43-, PO43- and Na+ by setting the Na+ conc. at 10-4, 10-3, 10-2, 10-1 and 100 mol/L, respectively. Plot the sorbed concentration of the three ions as a function of Na+ conc. Describe the difference and explain your observations.
  3. Effect of equilibrium constants: compare the surface concentrations of AsO43-, PO43- and Na+ by setting the LogKapp for $A s 0_{4}^{3-}$ surface complexation reaction at 15.6, 16.6, 17.6, respectively. Plot the sorption amount of the three ions as a function of LogKapp. Describe the observations and explain why.
Table 6. Initial conditions, surface complexation reactions and constants (Riese, 1982; Zeng et al., 2008)
Initial conditions Value
Temperature 25oC
pH 7.0
AsO4--- 0.005 mol/L
Na+ 0.001 mol/L
Fe+++ 0
PO4--- 0.001 mol/L
Site density $\equiv \mathrm{FeOH}$ $1.0 \times 10^{-6} \mathrm{~mol} / \mathrm{m}^{2}$
Specific surface area 50 m2/g
Fe(OH)3 volume fraction 0.2
Reactions Log K
$\equiv\mathrm{FeOH}+\mathrm{H}^+\Leftrightarrow\ \equiv\mathrm{FeOH}_2^+$ 5.10
$\equiv \mathrm{FeOH} \Leftrightarrow \quad \equiv \mathrm{FeO}^{-}+\mathrm{H}^{+}$ -10.70
$\equiv\mathrm{FeOH}+\mathrm{Na}^+\Leftrightarrow\ \equiv\mathrm{FeO}-\mathrm{Na}+\mathrm{H}^+$ -9.00
$\equiv \mathrm{FeOH}+\mathrm{AsO}_{4}^{3-}+\mathrm{H}^{+} \Leftrightarrow\left(\equiv \mathrm{FeOAsO}_{3}\right)^{2-}+\mathrm{H}_{2} \mathrm{O}$ 16.6
$\equiv \mathrm{FeOH}+\mathrm{PO}_{4}^{3-}+\mathrm{H}^{+} \Leftrightarrow\left(\equiv \mathrm{FeOPO}_{3}\right)^{2-}+\mathrm{H}_{2} \mathrm{O}$ 16.9