logK for Protolysis reaktion and background electrolyte complexation from [WDA00], site-type not clearly defined, assumed to be singly, R^2 = 0.99, residual mean of sum of squares = 6.820E-11; original equation in the study: 2 »Al-OH<0.5-> + H3AsO3 = [(»Al-OH)2-(OH)2-As(OH)] with logK = 2.888 (AsO3<3-> + 3H<1+> = H3AsO3, logK = 42.65 (Appelo and Postma, 1994))