Accelerated effects of nano-ZnO on phosphorus removal by Chlorella vulgaris: Formation of zinc phosphate crystallites

Nanoparticles have been reported to induce toxicity to aquatic organisms, however, their potential impacts on phosphorus removal from wastewater by algae are unclear. In this study, the effects of nanoparticle ZnO (nano-ZnO) on phosphate (PO 4 3− ) removal by a green alga Chlorella vulgaris were investigated. We found that PO 4 3− removal efficiency was accelerated with high concentrations of nano-ZnO (0.04–0.15   mM) but reduced with low concentrations of nano-ZnO (0.005–0.04   mM) compared to the control (without nano-ZnO), suggesting that PO 4 3− removal efficiency by C. vulgaris was related to nano-ZnO concentrations. Moreover, we observed changes of nano-ZnO morphology and detected element P on the surface of nano-ZnO by using transmission electronic microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDX), indicating that PO 4 3− was interacted with nano-ZnO or the dissolved Zn 2+ from nano-ZnO. Furthermore, we confirmed this interaction induced the formation of Zn 3 (PO 4 ) 2 crystallites sedimentation by employing X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS), which finally accelerates the removal of PO 4 3− .