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

• A pH-dependent improvement of PP removal by Ca2+ was observed on Fe(OH)3.
• The formation of FeOCa+ was responsible for PP removal at pH < 7.5.
• The removal amount of PP was increased at pH ⩾ 7.5 due to the precipitation of Ca2P2O7.

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 Ca2+ on the AFH was relative to the interaction of Ca2+ 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 [Ca2+] of 0, 25 and 50 mg/L, respectively. Moreover, this improvement of the PP removal in the presence of Ca2+ was pH-dependent. At pH < 7.5, the formation of surface complex 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 Ca2P2O7 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 Ca2P2O7 indicates extra Ca2+ 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.

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