Phosphate Distribution at the Ferric (hydr)oxide-Water Interface: Individual and Simultaneous Effects of Bicarbonate and Calcium Ions

Abstract Concentrations of bicarbonate (HCO3-) are typically high in natural waters and can enhance internal phosphate (PO4) loading from sediment to the overlying water. However, chemical constituents at the sediment-water interface controlling bicarbonate-induced PO4 mobility are poorly understood. This study evaluates the individual and simultaneous effects of HCO3- and calcium (Ca2+) ions on the mobility of PO4 at the ferric (hydr)oxides-water interface by conducting PO4 adsorption pH envelope and desorption experiments. Results from the individual PO4 adsorption pH envelope experiments show that HCO3- competes with PO4 for the surface binding sites and decreases PO4 adsorption to ferric (hydr)oxides. In contrast, Ca2+ exerts the opposite effect to HCO3- and enhances PO4 adsorption and these effects of HCO3- and Ca2+ are highly pH dependent and intensified under alkaline conditions. For instance, PO4 adsorption decreased by 30% and increased by 32% at pH 8 in the presence of 0.25 mM HCO3- and Ca2+, respectively, compared to the adsorption in 10 mM NaCl, which are 39 and 67% at pH 9. The PO4 desorption pH envelope experiments display a similar pattern as of adsorption pH envelope. However, the amount of adsorbed-PO4 desorbed at a given pH is lower than the amount expected from adsorption studies. This partial release of adsorbed-PO4 implies the immobilization of PO4 at the solid surface, potentially via the formation of stronger surface complexes. Therefore, the high concentrations of HCO3- may increase PO4 mobility in the aqueous phase by restricting the re-adsorption of sediment released-PO4 rather than by PO4 description from sediment mineral surfaces. However, this competitive effect of HCO3- may be minor in the system buffered by calcite dissolution.