Selective removal of ZnO nanoparticles from water using 3-aminopropyltriethoxysilane (APTES) and monitoring by meso-tetra(carboxyphenyl)porphyrin (TCPP)

Abstract Metal oxide nanoparticles (MONPs) such as TiO2 and ZnO have been engineered for various industrial manufacturing processes with an annual global production exceeding millions of tonnes. Among MONPs, environmental exposure to ZnO nanoparticles may cause a significant toxicity due to bioaccumulation in the human body. Therefore, detection and removal of ZnO nanoparticles in the aqueous environment is highly desired. In this work, 3-aminopropyltriethoxysilane (APTES) was chosen to treat aqueous suspensions containing ZnO and other metal oxide nanoparticles. APTES first formed a thin surface layer on ZnO nanoparticles and continued in branching polymerization to reach out towards other APTES-ZnO nanoparticles. Eventually, a web-like sediment settled down onto the bottom. A high removal efficiency over 99% has attained for ZnO nanoparticles in aqueous suspension. The sedimentation was selective towards ZnO nanoparticles, due to the amino group of ATPES preferentially interacting with ZnO to form a sturdy condensation coagulate. Other MONPs (such as Al2O3, CaO, CeO2, CuO and TiO2) did not form any web-like sediment with APTES but could co-precipitate with the ZnO nanoparticles. Meso-tetra(carboxyphenyl)porphyrin (TCPP) could be added as an indicator to co-precipitate with the sediment, with its color changing from red purple to bright green during the formation of APTES-TCPP-ZnO sediment.