Individual and Combined Effects of Bicarbonate and Calcium on Phosphate Adsorption to Ferric (Hydr)Oxides

Abstract

Elevated bicarbonate (HCO₃⁻) concentrations in natural waters can promote phosphate (PO₄) release from the bottom sediments to the overlying water. However, the chemical processes driving this bicarbonate-induced release at the sediment-water interface are not well understood. This study examines how HCO₃^- and calcium (Ca²⁺) affect PO₄ adsorption to ferric (hydr)oxide under static and dynamic pH conditions. Results show that HCO₃^- competes with PO₄ for the surface binding sites, reducing PO₄ adsorption under static pH conditions. In contrast, Ca²⁺ enhances PO₄ adsorption, particularly under alkaline conditions. For instance, at pH 8, the presence of 0.25 mM HCO₃^- reduced PO₄ adsorption, as reflected by a K_d value (the ratio of aqueous to solid-phase PO₄) that was 1.7 times higher than the K_d value of 0.39 observed in a solution containing only 10 mM NaCl. This competitive effect of HCO₃^- on PO₄ adsorption diminishes significantly when Ca²⁺ is simultaneously present. For example, the K_d value of PO₄ at pH 8 in the presence of HCO₃^- alone is 0.66, but decreases to 0.55, 0.37, and 0.24 with the addition of 0.05, 0.1, and 0.25mM Ca²⁺, respectively. A similar trend was observed in adsorption experiments conducted under dynamic pH conditions. However, the K_d value in dynamic pH experiments was approximately 84% lower than in static pH conditions, suggesting that PO₄ became immobilized on solid surfaces, likely due to the formation of stronger surface complexes over time. These results imply that high concentrations of HCO₃^- may increase PO₄ mobility in the water column by limiting PO₄ adsorption from the aqueous phase, rather than by enhancing its desorption from sediment surfaces.

J. Bangladesh Acad. Sci. 49(1); 137-146: June 2025