| Layered double hydroxides(LDHs),a class of two-dimensional layered anionic nanomaterials,have been widely used in many fields,such as optical catalysis,biomedicine,preparation of functional materials and treatment of environmental wastewater,especially as adsorbent materials in the field of environmental wastewater treatment have been incorporated into China's chemical industry standards(HG/T 5549-2019).With the increasing of production,transportation and application,LDHs will inevitably be discharged into the natural water environment through surface water,and it is easy to form a composite pollution system with pollutants such as organic dyes widely distributed in the water environment,which may cause serious pollution to the water body and endanger the health of aquatic organisms.However,the current research on LDHs is mainly focused on its application fields,the potential negative impact of LDHs and its composite pollution system on the water environment is unclear.Therefore,exploring the toxic effects and mechanisms of LDHs,organic dyes and their composite systems on aquatic organisms is helpful to comprehensively and truly evaluate the environmental behavior and biological effects of LDHs.In this study,organic three-dimensional layered double hydroxides(O3D-LDH)were successfully synthetized by the soft template method.The morphology,particle size,structure,surface charge and stability of O3D-LDH were characterized by the modern test methods such as scanning electron microscopy(SEM),transmission electron microscopic(TEM),X-ray diffraction(XRD),Zeta potential(Zeta)and particle size analyzer-dynamic light scattering(DLS),respectively.The model organism Chlorella vulgaris was selected as the tested organism,and the biological effect and toxicity mechanism of O3D-LDH,methyl orange and their composite system on Chlorella vulgaris were studied mainly from the three aspects of algae growth,photosynthesis and physical interactions.The main conclusions were as follows:(1)The synthetized O3D-LDH was proved by the SEM,TEM,XRD and other test methods,and the structural stability(Zeta potential:0?-80 mv)and uniform dispersion(particle size:1?10?m)of O3D-LDH were determined by using Zeta potential and DLS particle size analysis,respectively.(2)Effects of O3D-LDH on the growth of Chlorella vulgaris showed a typical dose-time-effect relationship,and the negative effects of the algae were enhanced with the increase of O3D-LDH concentration and exposure time.The EC50-48h,EC50-72h,and EC50-96h values of Chlorella vulgaris were 430.95,197.85 and 90.12mg·L-1,respectively.The toxicity mechanisms of O3D-LDH to Chlorella vulgaris were as follows:with the increase of O3D-LDH concentration,the agglomeration and co-deposition between O3D-LDH and Chlorella vulgaris were increased.On the one hand,O3D-LDH was adhered to the surface of the algae to form“shadow effect”,which reduced the light absorption of the algae in the photosynthetic process and inhibited the growth of the algae cells.On the other hand,when O3D-LDH was interacted with the algae cells,the lamellar structure of the material can separate and destroy the cell wall and chloroplast structure of the algae cells.The above“shadow effect”and“fragmentation effect”of O3D-LDH can induce a large amount of reactive oxygen species(ROS)in the chloroplast structure of the algal cells,resulting in oxidative damage,that is,oxidative stress,to the algae.At the same time,the self-protection mechanisms of the algae,such as chlorophyll a and carotenoid,were activated to maintain the oxidation balance by increasing the content.(3)Effects of methyl orange on the growth of Chlorella vulgaris(biomass and growth inhibition rate)showed a typical dose-effect relationship,the negative effects of the algae were enhanced with the increase of dye concentration,and methyl orange had a high toxicity to Chlorella vulgaris in short time.In addition,The EC50-48h,EC50-72h,EC50-96h values of Chlorella vulgaris were 65.67,55.43 and 52.97 mg·L-1,respectively.The toxicity mechanisms of methyl orange to Chlorella vulgaris were as follows:methyl orange can agglomerate with the algal cells to produce co-settlement.In this process,owing to high concentration,large chroma and poor transmittance of methyl orange,it can affect the light absorption in the photosynthetic process of the algae,and inhibit the formation of chlorophyll a to some extent.Meanwhile,a large amount of reactive oxygen species(ROS)can be induced in the chloroplast structure of the algal cells,which can cause oxidative damage and reduce the PS II activity in the capsule membrane of cytoplasm.Both photosynthetic pigment content(include chlorophyll b and carotenoid),and pigment ratios(include chlorophyll a/chlorophyll b and carotenoid/total chlorophyll)changed greatly.(4)The combined toxic effects of the composite system of O3D-LDH and methyl orange on Chlorella vulgaris were as follows:with the increase of O3D-LDH concentration,the joint toxicity of O3D-LDH and methyl orange on the biomass of Chlorella vulgaris changed from decrease to increase,while the growth inhibition rate of Chlorella vulgaris changed from increase to decrease.The combined toxicity showed synergistic effect in the low O3D-LDH concentration range below 50 mg·L-1and then antagonistic effect in the high O3D-LDH concentration range from 50 to500 mg·L-1.The toxicity mechanisms of the composite system of O3D-LDH and methyl orange on Chlorella vulgaris were as follows:both O3D-LDH and methyl orange can effectively bind to hemicellulose in algal cell wall and phospholipid bilayer on cell membrane through hydrophobic interaction,thus producing agglomeration and co-deposition.The synergistic effect showed in the low O3D-LDH concentration range(<50 mg·L-1)was caused by the enhanced adsorption of O3D-LDH by the algal cells.It was related to the enhanced hydrophobicity of the adsorbed products of O3D-LDH and methyl orange.The antagonistic effect showed in the high O3D-LDH concentration range(50?500 mg·L-1)was possibly related to their competition of surface charge site of the algal cells. |
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