original equation in the article: »FeOH + H3AsO4 <=> »Fe-H-AsO4<1-> + H<1+> + H2O with logK = 6.12 (weighted average equilibrium constant (intrinsic adsorption constants at As(T)/Fe = 0.053, 0.107 and 0.214)), pKa-values for H3AsO4 were 2.24, 6.76 and 11.6 (Allison et al. 1991, see article)