Understanding Competitive Adsorption of NTMP and Silica on Ferric Hydroxide

Ferric hydroxide adsorbents are commonly used to remove phosphonate antiscalants from membrane concentrate solutions. This research investigated competitive adsorption of nitrilotris(methylenephosphonic acid) (NTMP) and silica on ferric hydroxide. Equilibrium adsorption isotherms were measured and column breakthrough and regeneration experiments were performed for an NTMP adsorbate in simulated membrane concentrate solutions with and without dissolved orthosilicic acid. Quantum chemistry simulations using density functional theory (DFT) were used to evaluate the thermodynamic favorability of possible adsorption mechanisms for NTMP and silica. Column breakthrough experiments showed that 86 mg/L of dissolved silica decreased adsorption of NTMP by 36% after 3,685 bed volumes. DFT modeling indicated that NTMP and silica adsorption on ferric hydroxide may occur through both mono- and bidentate complex formation. NTMP adsorption through a bidentate binuclear complex was 12 kcal/mol more energetically favorable than monodentate complexation. Dimerization reactions between adsorbed silica and dissolved silica greatly increased silica adsorption for dissolved silica concentrations greater than 60 mg/L. DFT modeling and experimental results also indicated that dissolved NTMP may react with adsorbed silica through condensation reactions. Although adsorbed silica decreased the total uptake of NTMP, it increased the efficiency of regeneration by 0.10 M NaOH solutions. This is consistent with adsorbed silica decreasing formation of bidentate binuclear NTMP complexes.