Adsorption And Aggregation Mechanisms Between Bacterial Extracellular Polymeric Substances And Soil Minerals And Nanoparticles

As the bioorganic matter that ubiquity in the environment,extracellular polymeric substances?EPS?are hydrated biopolymers continuously produced during growth and metabolism of the soil heterotrophic biomass.Interactions between EPS and inorganic colloids can affect a broad variety of geochemical processes,such as microbial attachment and biofilm formation,particle aggregation and deposition,mineral dissolution,bioleaching and biomineralization.Based on these,in this study,we selected the natural soil minerals?e.g.montmorillonite,kaolinite and goethite?,manufactured nanoparticles?NPs??e.g.titanium dioxide?and EPS that secreted by familiar soil bacteria?Bacillus subtilis?as the experimental materials,and focused on the environmental effect of EPS.The adsorption and binding mechanism of EPS on soilmineralswerestudiedusingbatchadsorptionexperiments,Derjaguin-Landau-Verwey-Overbeek?DLVO?theory,laser scanning confocal microscope?CLSM?,X-ray photoelecgon spectroscopy?XPS?,Attenuated total reflection-Fourier transform infrared spectroscopy?ATR-FTIR?,Raman spectroscopy and Near edge X-ray absorption fine structure?NEXAFS?.In addition,we also investigate the effect of EPS on the stability of natural and manufactured NPs under different pH,ionic strength and ion valence.As follows are the main results:?1?We combined CLSM and batch adsorption experiments to clarify the adsorption mechanism of EPS on montmorillonite,kaolinite and goethite.Firstly,we found the preferential adsorption of protein in EPS on the surface of montmorillonite and kaolinite,whereas nucleic acid on goethite,in the form of imaging;Secondly,the surface properties and structure of EPS can be changed by the increasing pH,making the mass fraction of EPS on soil minerals decrease with increased solution pH;Finally,through classic DLVO theory,we know that electrostatic force play an important role in the interaction between EPS and three soil minerals,and the chemical interaction also play an important role in the adsorption of EPS on goethite at different solution pH.?2?We applied ATR-FTIR/2D-CoS to investigate the adsorption of EPS on soil minerals in situ for the first time and investigate the effect of ionic strength on the adsorption mechanisms of EPS on soil minerals.The results show that the conversion of the secondary structure of proteins from a random coil conformation to aggregated strands,?-helical and turns,leading to increased adsorption of EPS-proteins and explaining the overall increasing adsorption of EPS-proteins on goethite surfaces with an increasing concentration of electrolyte,and this results are consistent with that obtained by C 1s NEXAFS.Results obtained from ATR-FTIR integrated with 2D-CoS allow us to construct a comprehensive overview detailing the molecular level EPS-goethite interaction process,take the adsorption rate of EPS bands with the goethite surface in 0.005mol/L NaCl solution as an example:carboxylate C=O,polysaccharide C-O(1396cm^-1,1132 cm^-1)?Amide I C=O(1650 cm^-1)?Amide II C-N(1540 cm^-1)?nucleic acid P-O,P=O(1035 cm^-1,1086 cm^-1)?Amide III CH₂(1455 cm^-1).?3?The effect of EPS on the stability of natural NPs?goethite?under different environmental conditions?pH,ionic strength and ionic valence?had been studied comprehensive.Results show that increasing concentration of EPS can promote NP aggregation only when they are positively charged,in other case,the addition of EPS can inhibit NP aggregation.In NaCl,NaNO₃ and Na₂SO₄ solution,the aggregation rate of goethite NPs increases with increasing electrolyte concentration,but the addition of EPS inhibits NP aggregation.While in Na₃PO₄solution,the aggregation rate of goethite NPs increases at low Na₃PO₄concentration,but decreases as Na₃PO₄ concentrations increases;at high Na₃PO₄however,charge screening by sodium counter ions plays an important role in increasing NP aggregation rate.?4?Aggregation kinetics of goethite-EPS colloids,as influenced by three metal cations,Cu?II?,Pb?II?and Ca?II?,was investigated by time-resolved dynamic light scattering?DLS?.The potential of the three cations to enhance aggregation of the goethite-EPS colloids increased in the order Cu<Pb<Ca,which is ascribe to the bridging effect by Ca?II?is stronger than the surface adsorption and exchange interaction by Pb?II?and Cu?II?;after the addition of three concentrations of EPS,the ability of the three cations to enhance aggregation of the goethite-EPS colloids increased in the order Ca<Cu<Pb,the aggregation ability of heavy metal ion?complexation effect?are stronger than calcium ion?non-specificity cation exchange?.?5?The influence of EPS on the aggregation behavior of TiO₂ NPs in the presence of NaCl and CaCl₂ have been investigated.Results indicate that EPS has a strong influence on the aggregation process and therefore the transport of TiO₂ NPs in natural environments.In the presence of NaCl and low concentration CaCl₂,EPS can facilitate the stabilization of TiO₂ NPs effectively,with extracellular protein playing an important role in this process.However,at high CaCl₂ concentration,the presence of EPS can enhance the aggregation rate,which is ascribed to the aggregation of the dissolved expolysaccharide macromolecules through intermolecular bridging,which in turn links the TiO₂ NPs and aggregates together,resulting in enhanced aggregate growth.The effect of EPS on the stability of manufactured NPs?titanium dioxide?under different environmental conditions?pH,ionic strength and ionic valence?had also been studied.Results show that increasing concentration of EPS can promote NP aggregation only when they are positively charged,in other case,the addition of EPS can inhibit NP aggregation.