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 (PO43-) removal by a green alga Chlorella vulgaris were investigated. We found that PO43- removal efficiency was accelerated with high concentrations of nano-ZnO (0.04-0.15mM) but reduced with low concentrations of nano-ZnO (0.005-0.04mM) compared to the control (without nano-ZnO), suggesting that PO43- 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 PO43- was interacted with nano-ZnO or the dissolved Zn2+ from nano-ZnO. Furthermore, we confirmed this interaction induced the formation of Zn3(PO4)2 crystallites sedimentation by employing X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS), which finally accelerates the removal of PO43-.

Antagonistic effect of nano-ZnO and cetyltrimethyl ammonium chloride on the growth of Chlorella vulgaris: Dissolution and accumulation of nano-ZnO.

The interaction of nanoparticles with coexisting chemicals affects the fate and transport of nanoparticles, as well as their combined effects on aquatic organisms. Here, we evaluated the joint effect of ZnO nanoparticle (nano-ZnO) and cetyltrimethyl ammonium chloride (CTAC) on the growth of Chlorella vulgaris and explored the possible mechanism. Results showed that an antagonistic effect of nano-ZnO and CTAC (0.1, 0.2 and 0.3 mg L-1) was found because CTAC stop nano-ZnO being broken down into solution zinc ions (Zn2+). In the presence of CTAC, the zinc (including nano-ZnO and released Zn2+) showed a higher adsorption on bound extracellular polymeric substances (B-EPS) but lower accumulation in the algal cells. Moreover, we directly demonstrated that nano-ZnO was adsorbed on the algal B-EPS and entered into the algal cells by transmission electron microscope coupled with energy dispersive X-ray (TEM-EDX). Hence, these results suggested that the combined system of nano-ZnO and CTAC exhibited an antagonistic effect due to the inhibition of CTAC on dissolution of nano-ZnO and accumulation of the zinc in the algal cells.

Role of extracellular polymeric substances from Chlorella vulgaris in the removal of ammonium and orthophosphate under the stress of cadmium.

The interactions between the soluble extracellular polymeric substances (S-EPS), bound EPS (B-EPS) of algae and heavy metal, would affect the removal of ammonium (NH4(+)-N) and orthophosphate (PO4(3-)-P) from wastewater by algae-based techniques. This study investigated the role of Cd(2+)-mediated EPS from Chlorella vulgaris on NH4(+)-N and PO4(3-)-P removal. The results showed that the removal efficiencies of NH4(+)-N and PO4(3-)-P still separately remained 62.6% and 64.9% under 1.0mg/L Cd(2+), compared to those without Cd(2+), mainly attributing to enhanced S-EPS and B-EPS contents of the algae. The increased of PS (polysaccharides) and PN (proteins, e.g., tryptophan-like and tyrosine-like components) led to accelerated interactions of Cd(2+) with PS and PN in EPS fractions, especially for B-EPS, due to a higher detected distribution of Cd(2+) (e.g., about 55.4% in B-EPS). Thus, algae-based techniques are stable treatment methods for wastewater in which NH4(+)-N and PO4(3-)-P coexist with heavy metals.