RATES
Ferrihydrite
 -start
 1 A0 = 54000 * m0 # initial surface area, 600 m2/g
 2 SS = (1 - SR("Goethite")/1e3) # 1000 * more soluble than goethite
 10 if SS < 0 then goto 30 # dissolve only
 20 moles = A0 * (m/m0)^2.3 * 10^-9 * act("H+")^0.45 * SS
 30 save moles * time
 -end
Goethite
 -start
 1 A0 = 5400 * m0 # initial surface area, 60 m2/g
 2 SS = (1 - SR("Goethite")) # SI = 0 for goethite
 10 if SS < 0 then goto 30
 20 moles = A0 * (m/m0) * 10^-11 * act("H+")^0.45 * SS
 30 save moles * time
 -end

SOLUTION 1
 pH 1.5; pe 10 O2(g) -0.01
 Fe 1. Goethite 1
 S 1. charge

EQUILIBRIUM_PHASES 1; Pyrite

KINETICS 1
 Ferrihydrite; -m0 0.025; -form FeOOH
 Goethite; -m0 0.1
 -step 0 2*2e6 12*5e6 3e7
INCREMENTAL_REACTIONS true

USER_GRAPH
 -head time Ferrihydrite Goethite pH SI_go
 -axis_titles Years mmol/L pH
 -axis_scale x_axis 0 3 0.5; -axis_scale y_axis 0 0.1; -axis_scale sy_axis 0 7.5 1.5
 -init false
 -start
 10 graph_x total_time/3.15e7
 20 graph_y kin("Ferrihydrite"), kin("Goethite")
 30 graph_sy -la("H+"), SI("Goethite")
 -end
END