Regulation Of Nanoparticle Environmental Behaviors By Biofilms And Its Application In Paddy Fields

With the rapid development of nanotechnology,more and more nanoparticles(NPs)have been introduced into the agricultural soil environment,which has attracted great attention for its risks in agriculture and ecosystem.NPs can be released into paddy soils through multiple pathways,which not only cause hazards to soil ecology and plant growth,but also may threaten human health and food security.Biofilm is a highly structured microbial aggregate with a complex chemical composition and spatial structure,which mainly grows at the soil-water interface.And the interaction between biofilms and NPs will further determine the environmental behaviors and effects of NPs throughout the whole paddy ecosystem.Therefore,in order to reveal the actual environmental behavior of NPs in paddy fields,a comprehensive evaluation on the influence of biofilms on the agroecological risk of NPs is conducted.In this study,environmental behaviors of typical metal oxide nanoparticles of cerium oxide nanoparticles(CNPs)were studied by adsorption aggregation experiments,migration experiments and pot experiments.Electron microscopy,dynamic light scattering,and DLVO theory were used to investigate the mechanisms by which biofilms affect environmental behaviors of NPs.Furthermore,the migration law of NPs in the paddy soil-biofilm-plant system was clarified,indicating the great potential of biofilms in regulating the risks of NPs in paddy field.The main results were as follows:(1)Heteroaggregation mechanisms between nanoparticles and soil minerals and the effects of extracellular polymers(EPS)on their aggregation are revealed.The variety in the heteroaggregation between CNPs with different soil minerals is mainly induced by the difference in their surface properties,such as surface charge,specific surface areas and surface complexation.The heteroaggregation of CNPs with goethite and hematite mainly relies on electrostatic attraction,while its aggregation with kaolinite and montmorillonite mainly relies on van der Waals forces and surface interactions.EPS increased the electrostatic repulsion between the nanoparticles and soil minerals and then reduced their aggregation and sedimentation.When EPS was present,the OD₁ values of co-settling for CNPs-kaolinite,CNPs-montmorillonite,CNPs-goethite,and CNPs-hematite decreased by 47.5%,56.4%,94.8%,and 76.2%,respectively.EPS reduces heteroaggregation mainly by enhancing the electrostatic repulsion between colloidal particles by changing the surface potential of CNPs and SMPs.(2)The effects of biofilms and soil minerals on the transport of NPs and the underlying mechanisms were explored.In the clean quartz sand medium,the nanoparticles of the type,concentration and size can affect their transport in a saturated porous medium.and the traditional DLVO theory could well explain the transport of nanoparticles in the saturated porous medium without coating.Both biofilms and soil minerals can inhibit the transport of nanoparticles in saturated porous medium,which mainly affects the transport of nanoparticles through adsorption and heteroaggregation effects on nanoparticles,and non-DLVO interactions as well as the surface characteristics of the coatings contribute greatly to enhancing nanoparticle deposition.(3)The rules of migration and accumulation of NPs in the biofilm-soil-plant system of paddy fields were revealed.The NPs mainly accumulated in the surface soil,and only a small portion of them were able to migrate to the deep soil and were taken up by rice plants.Biofilms can accumulate large amounts of NPs through adsorption and sedimentation effects.Meanwhile,soil minerals can reduce the migration of NPs into the deep soil by retain most of the NPs in the surface soil through heteroaggregation and retarding on the dispersion and transport processes of NPs in paddy soil.The accumulation efficiency of NPs in plants was significantly greater than that of common particles,but the size of NPs has less influence on the migration of NPs in soil and plants.Compared with other tissues of rice,NPs are more easily accumulated in the grain,and the presence of biofilm can reduce the accumulation of NPs by rice to some extent.