pH-Dependent improvement of pyrophosphate removal on amorphous ferric hydroxide by incorporating Ca2+

Abstract The interaction of calcium and pyrophosphate (PP) on the surface of amorphous ferric hydroxide (AFH) was investigated at pH 3.5–11.0. Zeta potential analysis of solid samples after PP removal indicates that the removal of PP by Ca 2+ on the AFH was relative to the interaction of Ca 2+ with different species of PP. The reduction of PP removal amount with the pH increasing was observed in all cases at the initial PP concentration ranging from 5 to 50 mg/L. The lowest PP removal amount at the initial PP concentration of 25 mg/L was 5.0, 11.1 and 21.7 mg/g with the initial [Ca 2+ ] of 0, 25 and 50 mg/L, respectively. Moreover, this improvement of the PP removal in the presence of Ca 2+ was pH-dependent. At pH  FeOCa + was supposed, which is expected to absorb and remove PP. This resulted in a Ca/P ratio ranging from 2.5 to 25.6 in the solid sample. In contrast, the precipitation of Ca 2 P 2 O 7 was responsible for the increasing of PP removal amount in basic solution, which resulted in the reduction of Ca/P in the solid sample to 2.0 at pH ⩾ 7.5. The Ca/P molar ratio higher than 1.0 in Ca 2 P 2 O 7 indicates extra Ca 2+ adsorbed in the sample during the precipitation. Therefore, our results suggest an improved treatment of PP wastewater through the adsorption on AFH with a small Ca addition at low pH.