Selective Removal of ZnO Nanoparticles from Water Using 3-aminopropyltriethoxysilane (APTES) and Monitoring by Meso-tetra(carboxyphenyl)porphyrin (TCPP)

Metal oxide nanoparticles (MONPs) such as TiO 2 and ZnO have been engineered for various industrial manufacturing processes. Among MONPs, environmental exposure to ZnO nanoparticles may cause health concerns about their significant toxicity. This leads to a desire of efficient methods for their removal in the aqueous environment with facile detection. 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 link up with adjacent APTES-ZnO nanoparticles. Eventually, a web-like sediment settled down onto the bottom and was confirmed to be APTES-ZnO conjugation by FT-IR. A high removal efficiency over 99 % w of ZnO nanoparticles in aqueous suspension was attained using 2.0 % by volume of APTES. The sedimentation was selective towards ZnO nanoparticles due to electrostatic interaction with ZnO to form a sturdy condensation coagulate. Other MONPs (Al 2 O 3 , CaO, CeO 2 , CuO and TiO 2 ) 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. A filter paper or membrane placed at the bottom greatly facilitated the disposal of all sediments at a low cost.