The reuse of nano-TiO2 under different concentration of CO32– using coagulation process and its photocatalytic ability in treatment of methyl orange

Photocatalysis is highly efficient for the treatment of dyeing wastewater. However, the wastewater still contains a large amount of nano-TiO 2 after photocatalysis. In order to reduce the cost of nano-TiO 2 , different coagulants were used to improve the solid/liquid separation efficiency. At the same time, the reuse of flocs has raised increasing attention after coagulation. In this study, we report on the recycling of flocs for the production of AlCl 3 and TiO 2 formed flocs (AT 1 ) and Al 13 and TiO 2 formed flocs (AT 2 ) through a simple calcination process. Results indicated the flocs formation processes under different concentration of CO 3 2– were Al species distribution and coagulant dosage dependent. Without HA molecules, the reaction between Al-based coagulants and CO 3 2– are the controlling steps in the coagulation process, and Al a and Al b species had different reaction processes. The influence of HA molecules was also studied, and the HA-Al aggregates which could bridge the TiO 2 nanoparticles , and thus induce sweep flocculation . Consequently, highly enhanced aggregation rate, due to the heteroaggregation between these aggregates and TiO 2 was observed. After calcination, the AT 2 -10 and AT 2 -20 (CO 3 2– : 10, 20 mmol/L) with petal-like structures (mesoporous), relatively high specific surface areas, i.e. 83.98 m 2 /g and 78.38 m 2 /g and high photocatalytic degradation, i.e. 72.79%, 67.78% (compared to nano-TiO 2 , AT 1 -10 and AT 1 -20), were obtained. Additionally, oxygen vacancy (OV) was generated on the surface of the recycled AT 2 -10 and AT 2 -20, which display strong photocatalytic performance for the degradation of MO greater than 65%, under ultraviolet (UV) irradiation and without any sacrificial reagents. This study proposes a new perspective on recycling coagulated flocs for practical application in wastewater treatment .