Effect And Mechanism Of Natural Organic Matter On Combined Toxicity Of Nano-TiO₂ And Organochlorines Pollutants To Algae

While nano titanium dioxide(TiO₂)is widely produced and used,its potential environmental impact has attracted more and more attention.Organic pollutants(OCs)are ubiquitous in water,easy to accumulate in organisms,and most of them are highly toxic.Natural water contains a large amount of natural organic matter(NOM),which will interact with OCs in the water and affect the fate of OCs.At the same time,nano-TiO₂will interact with OCs and NOM in the water after entering the water environment,resulting in compound toxicity to aquatic organisms.Therefore,it is necessary to study the effect and mechanism of nano-TiO₂and three OCs(atrazine,pentachlorobenzene(Pe CB)and 3,3',4,4'-Tetrachlorinated biphenyl(PCB-77))on the combined toxicity of Chlorella from the cellular,subcellular and molecular levels.It can provide a scientific basis for the ecological risk assessment of composite pollution of nano materials and organic pollutants in water.Through the analysis of the experimental results,the following conclusions are obtained:(1)NOM can reduce the toxicity of nano-TiO₂ to Chlorella vulgaris,reduce the integrity of Chlorella vulgaris cell wall and cell membrane,make it difficult for atrazine and Pe CB to enter algae cells,and then reduce the enrichment of atrazine and Pe CB by Chlorella vulgaris.NOM increased the enrichment of PCB-77 in composite exposure,because nano-TiO₂ significantly reduced the concentration reduction rate of PCB-77,coupled with the desorption lag of PCB-77 enriched on Chlorella.The toxicity of pollutants to organisms is related to the enrichment of pollutants by organisms.The higher the enrichment,the greater the toxicity of pollutants to organisms.NOM reduced the enrichment of nano-TiO₂,atrazine and Pe CB by algal cells in atrazine group and Pe CB group,and correspondingly reduced the compound toxicity of the two groups;The toxicity of nano PCB-77 to algal cells increased accordingly.The compound toxicity of nano-TiO₂ and atrazine is synergistic;It is additive with Pe CB;It is antagonistic to PCB-77;After adding NOM,nano-TiO₂and atrazine had antagonistic effect at low concentration and synergistic effect at high concentration;It is additive with Pe CB;It has antagonistic effect with PCB-77.(2)TEM was used to observe the changes of submicroscopic structure of algal cells.The main reason for the appearance of electron dense body and vacuole structure in Chlorella cells exposed with or without NOM was caused by nano-TiO₂.Infrared difference spectrum analysis showed that the sugar,phospholipid and protein structures of Chlorella vulgaris were changed in varying degrees under the single and combined exposure of atrazine,PCB-77 and nano-TiO₂ in the presence or absence of NOM.(3)When nano-TiO₂ and OCs were exposed in the presence or absence of NOM,the addition of NOM inhibited the circulation of TCA in PCB-77 group and the photosynthesis of Chlorella,thus hindered the formation of ATP,and made NOM increase the toxicity of Chlorella.When pollutants are adsorbed on the cell surface or destroy the cell membrane and cell wall into algal cells,algal cells will trigger some protective reactions to maintain normal cell functions,including antioxidant defense system,osmotic pressure regulation,cell membrane fluidity and enhanced algal cell division to inhibit the toxicity of pollutants to algal cells.When these protective reactions cannot resist the attack of pollutants,there will be such destructive reactions as damage to algal cell membrane,damage to macromolecules such as DNA or protein,unclear organelle structure of algal cells,separation of plasma wall and disturbance of energy metabolism caused by damage to photosynthesis and cell respiration,which will eventually lead to cytotoxicity and apoptosis...