AsPhfo.ppi

SPECIATION                                                 # A non-standard As predominance plot: x = PO4, y = CO2
  calculationType                      ht1
  calculationMethod                    1
  mainSpecies                          As
  xmin                                 0.01                # PO4 on x-axis - start with a little P (finite P) to ensure closure of polygons
  xmax                                 400
  ymin                                 -3.5                # log (P_CO2(g)) on y-axis
  ymax                                 -0.5
  resolution                           100                 # 500 for finer plotting of triple points

PLOT
  plotTitle                            "As sorbed by Hfo with calcite, PO<sub>4</sub> & CO<sub>2</sub>"
  xtitle                               " PO<sub>4</sub> / (\mmol/kgw)"
  ytitle                               "<i>P</i><sub>CO2</sub> / atm"

CHEMISTRY

include ht1.inc                                            # standard 'hunt and track' file

SOLUTION 1
  -units umol/kgw
  As     1
  P      <x_axis>

SURFACE 1
  Hfo_w  Fe(OH)3(a) eq 0.2 5.34e4
SURFACE_SPECIES
  Hfo_wOH + CO3-2 + H+ = Hfo_wCO3- + H2O
  log_k  12.56
  Hfo_wOH + CO3-2 + 2H+= Hfo_wHCO3 + H2O
  log_k   20.62

EQUILIBRIUM_PHASES 1
  CO2(g) <y_axis> 10
  Calcite                                                  # forces equilibrium with calcite
  Fe(OH)3(a) 0 1e-3                                        # adds a little ferrihydrite
  O2(g)  -0.68 10
END