Competitive adsorption of magnesium and calcium with phosphate at the goethite water interface: Kinetics, equilibrium and CD-MUSIC modeling

In natural environments, competitive interactions of ions with charged particles often control bioavailability and mobility of ions. In these systems, chemical reactions are often in a semi-equilibrium state; thus, along with equilibrium data, kinetic data are of great importance in predicting ion bioavailability and mobility. Therefore, in this research, kinetics and equilibrium adsorption interactions of magnesium, calcium and phosphate on goethite are investigated as a function of time, pH, ion loading and ionic strength in single and binary ion systems. The thermodynamically valid model parameters, obtained from single ion systems by applying the CD-MUSIC model, are used to predict ion interactions in more complex systems, i.e. binary ion systems. Results indicate that the kinetics of phosphate and magnesium adsorption is pH-dependent and electrostatic attractive and repulsive forces dominantly control the reaction. Thus, adsorption of magnesium and calcium influences significantly both the amount of adsorbed phosphate and the reaction time, and vice versa. Therefore, the equilibration time for phosphate and magnesium adsorption on goethite is approximately ≤ 1 h when electrostatic forces are attractive and approximately 24 h when electrostatic forces are repulsive. Variation in ionic strength (0.01-0.3 M NaCl) has no significant effect on the magnesium and calcium adsorption, suggesting a large difference in the adsorption energy of Ca and Mg compared to Na, which is very likely due to the difference in their adsorption mechanism. Based on the model calculations, formation of two inner sphere surface complexes, as (FeOH)2Mg and (FeOH)2MgOH, is the dominant mechanism for magnesium adsorption. For calcium, however, a monodentate outer sphere surface complex dominates its adsorption at acid pH range and a monodentate inner sphere complex at basic pH range (FeOHCa). This difference results in a different amount of charge on electrostatic planes that influences strongly phosphate adsorption such that phosphate adsorption is significantly higher in Ca-PO4 systems than in Mg-PO4 systems.