Efficient and controllable phosphate removal on hydrocalumite by multi-step treatment based on pH-dependent precipitation

The phosphate removal on hydrocalumite (Ca2Al-Cl-layered double hydroxide, Ca2Al-Cl-LDH) was evaluated by PHREEQC program, which indicated that a pH-dependent dissolution–reprecipitation process was responsible for the phosphate elimination. The evaluation results also suppose that in the solution with high phosphate concentration, the formation of CaHPO4·2H2O (DCPD) and AlPO4 at pH < 7.0 attributed to more phosphate removed than that in the conventional treatment where only hydroxyapatite (HAP) was precipitated. This hypothesis was confirmed by XRD characterization and the removal efficiency of phosphate in batch experiments. Accordingly, the multi-step treatment process (MST) was conducted for the removal phosphate in real P-containing wastewater, in which DCPD and AlPO4 were formed at low pH and HAP at high pH. This resulted in the increasing amount of phosphate removal to 9.26 mmol/g Ca2Al-Cl-LDH due to the high consuming efficiency of Ca and Al in the whole process. The other benefit of MST was 12.7% of mass reduction of the as-obtained sludge, compared to the conventional one-step treatment. Our work proposed a potential strategy for the improvement of phosphate removal by Ca2Al-Cl-LDH.

► PHREEQC model indicated P removal on Ca2Al-Cl-LDH dependent on initial [P] and pH.
► High P removal amount was observed at pH 5.0 as CaHPO4·2H2O and AlPO4 formed.
► Multi-step treatment improved P removal on Ca2Al-Cl-LDH in the real P wastewater.