The influence of groundwater chemistry on arsenic concentrations and speciation in a quartz sand and gravel aquifera)

APPENDIX 2 PARAMETER VALUES USED IN SURFACE COMPLEXATION MODEL CALCULATIONS IN FIG. 4

Parameter	Value
Ionic Strength	0.003
m²/l^a	1240
Surface species equilibrium expressions and log K values^b
>FeOH =>FeO^- + H⁺	-8.93
>FeOH + H⁺=> FeO H₂⁺	7.29
>FeOH+ H₃AsO₃ = >FeH₂AsO₃ + H₂O	5.41
>FeOH+As O₄^3- + 3H⁺ = >FeH₂AsO₄ + H₂O	29.31
>FeOH+As O₄^3- + 2H⁺ = >FeHAs O₄^- + H₂O	23.51
>FeOH+As O₄^3- = >FeAs O₄^3-	10.58
>FeOH+P O₄^3- + 3H⁺ = >FeH₂PO₄ + H₂O	31.29
>FeOH+P O₄^3- + 2H⁺ = >FeHP O₄^-+ H₂O	25.39
>FeOH+P O₄^3- + H⁺ = >FeP O₄² + H₂O	17.72
Solution species equilibrium expressions and log K values^b
H⁺+H₂As O₃^- = H₃AsO₃	9.29
H⁺+As O₄^3- = HAs O₄²	11.50
2H⁺+As O₄^3- = H₂As O₄^-	18.46
3H⁺+As O₄^3- = H₃AsO₄	20.70
H⁺+P O₄^3- = HP O₄²	12.35
2H⁺ = P O₄^3- =H₂P O₄^-	19.55
3H⁺+P O₄^3- = H₃PO₄	21.70

^aSurface area-to-volume ratio.
^bAll log K values from Dzombak and Morel (Ref. 58). Activity coefficients for aqueous species calculated using Davies equation.

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