The Effect And Its Mechanism Of Humic Acid On The Algal Toxicity Of Nanoparticles

With the rapid development of nanotechnology, nanoparticles are increasingly released into the aqueous environment. More and more researches reported the toxicity of nanoparticles to aquatic organism with the mechanism however remaining to be argued. Furthermore, most of the nanotoxicity studies neglected the complexicity of natural water environment and the possible effects to the nanotoxicity. As a ubiquitous component of aquatic systems, natural organic matter (NOM) would interact with nanoparticles and alter their environmental behavior and thus their toxicity to aquatic organisms, which warrants more investigation. This thesis was therefore aimed to investigate the toxicity of several oxide nanoparticles to Chlorella sp., a species of unicellular green algae, and look into the influence and mechanism of humic acid (HA) on the nanotoxicity to the algae. Some valuable results are obtained.(1) The effects of nanoparticles on the algal growth varied with particle varieties and properties. Nano-TiO2 (anatase) and nano-ZnO were remarkably toxic to the algae, whereas 4 types of nano-SiO2, nano-Al2O3, and Nano-TiO2 (rutile) showed no significant inhibition to the algal growth.(2) The illumination could significantly influence the algal growth. However, the shadow effect of the nanoparticles was not the main cause for the algal toxicity of nano-TiO2 and nano-ZnO. Zn2+ ions was more toxic to the algae than nano-ZnO at low concentrations(< 50 mg/L), but the toxicity ranking of the two materials was just the reverse at higher concentrations. Nano-ZnO had higher algal toxicity and released less Zn2+ ions into the culture media than bulk-ZnO, which suggests that dissolved Zn2+ ions from nano-ZnO was not the only mechanism for the algal growth inhibition. Large aggregates of the nanoparticles entrapped and injured the algal cells by direct contact, which may contribute to the nanotoxicity.(3) Both dissolved and surface-bounded HA could noticeably mitigate the toxicity of nano-TiO2 to the algae. For example, the 96 h EC90 of nano-TiO2 was 10 mg/L, which was up to 154 mg/L in the presence of 5 mg/L of dissolved HA; 100 mg/L of nano-TiO2 had an inhibition at 96 h of 99.7% to the algal growth, however,100 mg/L of nano-TiO2 loaded with 10% HA (w:w) had an inhibition of only 79.1%.(4) Humic acid can adsorb onto the surfaces of nano-TiO2 by ligand exchange. Dissovled/bounded HA restricted the direct contact between nano-TiO2 and the algal cells via steric and/or electrostatic repulsion, which significantly decreased production of ROS and MDA of algal cells, lowered the oxidative stress and injury of algal cells, and thus mitigated the algal toxicity of nano-TiO2.