Multi-step removal mechanism of pyrophosphate using CaFe-layered double hydroxide at high pH

• Pyrophosphate (PP) was highly removed by CaFe-LDH in multi-steps at pH 12.
• PP removal at low PP concentration was due to Ca2P2O7·2H2O precipitation.
• CaP2O7 2− incorporated LDH was predominant in PP removal at high initial PP concentration.
• Structure of CaP2O7 2− incorporated LDH was different from that of pristine LDH.

Pyrophosphate (PP) is frequently used as a ligand in the electroplating process at high pH. This research investigated PP removal over Ca2Fe-layered double hydroxide (LDH) at pH 12. The maximum amount of PP removal was up to 7.4 mmol (as P) at the LDH dosage of 1.0 g/L. As represented by the concentration of total phosphate ([TP]), the isotherm with increasing initial PP reveals that PP removal over LDH underwent multi-processes: (1) at initial [TP] < 3.2 mmol/L, PP removal was attributed to Ca2P2O7·2H2O formation while at initial [TP] = 3.2–6.9 mmol/L PP removal kept at 3.2 mmol/g due to the competitive formation of CaP2O72− instead of Ca2P2O7·2H2O; (2) At [TP] = 6.9 –12.0 mmol/L, formed CaP2O72− is incorporated into the LDH framework and reconstruct the layered structure to remove PP together with Ca2P2O7·2H2O precipitation; (3) At [TP] > 12.0 mmol/L, CaP2O72 − incorporation becomes predominant in PP removal. Interestingly, in the structure of CaP2O7 2−-incorporated LDH, CaP2O7 2− is intercalated with Ca2 + being supposedly incorporated into the LDH hydroxide vacant site. Therefore, the result provides a possible way to remove a large PP amount using Ca2Fe-LDH at high pH.