apparent stability constant for I=0, for conversion to other ionic strengths, activity constants of all surface species are assumed equal to one; no electrical double layer correction to the mass laws for charged surface species; fit with U(VI) adsorption data [KD01] for pH values less than 6.0 and the thermodynamic set in table 2-1 of [DKP01]; termed model B3; model was needed for modeling systems with mixtures of quartz and ferrihydrite [DKJ01]