Interaction Mechanisms Of Different Coated Hematite With Shewanella And Polystyrene Nanoplastics

As key solid-phase components in soils,minerals mediate a variety of geochemical processes and are important for soil structure and ecosystem stability.In particular,iron oxides,which are widespread in the soil environment,are rich in reactive functional groups on their surfaces and play a crucial role in controlling soil interfacial reactions.Due to their specific physicochemical properties and complex composition,biocolloids（mainly consisting of microorganisms and their secreted metabolites）and engineered colloids released into the environment by human activities are widely present in soils and participate in a variety of geochemical reactions at the interface with iron oxides,including the cycling of key elements and their influence on the fate of pollutants.However,the microscopic mechanisms of colloidal interaction at the iron oxides interface are still poorly studied,especially in terms of the dynamics of the colloidal response at the iron oxides interface.Therefore,in this study,Shewanella oneidensis MR-1 and its secreted extracellular polymer（EPS）were selected as the representative biocolloids in the environment,polystyrene nanoplastic（PSNP）as the representative exogenous engineered colloid,and hematite as the representative iron oxide,using a combination of dissipative quartz crystal micro The dynamic adsorption processes and molecular mechanisms of biocolloids and engineered colloids on the surface of iron oxides were investigated using a combination of dissipative quartz crystal microbalance（QCM-D）,laser scanning confocal microscopy（CLSM）and two-dimensional Fourier transform infrared correlation spectroscopy（2D-FTIR-COS）interfacial characterisation techniques.The main results of this paper are as follows.（1）The mechanism of the influence of hematite surface encapsulation on the adsorption process of MR-1 was clarified.the results of QCM-D revealed that MR-1formed a rigid and stable cellular adsorption layer on the pure hematite surface,and both HA and SiO₂ encapsulation reduced the density of cells on the hematite surface during the initial adsorption phase and inhibited the stability of cell attachment to the hematite surface at a later stage.2D-FTIR-COS analysis showed that the polysaccharide fraction of the cells dominated the initial adsorption on the hematite-HA and hematite-SiO₂ surfaces.In terms of the long-term adsorption process,the HA coating hindered the adsorption of amide groups and promoted the binding of the polysaccharide fraction to hematite.（2）The effect of HA and SiO₂ coating on the electron transfer between Shewanella oneidensis MR-1and hematite was elucidated.The results of cyclic voltammetric curves（CV）showed that the peak anodic current density of the bare hematite electrode was 0.61±0.08μA/cm²,which was higher than that of hematite-HA（0.44±0.09μA/cm²）and hematite-SiO₂（0.18±0.04μA/cm²）,indicating that HA and SiO₂ encapsulation inhibited the electron transfer between Shewanella oneidensis MR-1 and hematite.The HA coating was found to increase the electrochemically active surface area（EASA）and conductivity of hematite by electrochemical characterisation,but the electron transfer resistance of hematite-HA and hematite-SiO₂ with Shewanella oneidensis was 5.6 and 11.5 times higher than bare hematite,respectively,as determined by electrochemical impedance spectroscopy.The electron transfer pathway between hematite-HA and Shewanella oneidensis MR-1 is probably transferred through the HA coating on the surface of hematite and bypasses the hematite core,while the insulating SiO₂ almost prevents electron transfer between hematite and Shewanella oneidensis MR-1.In conjunction with our previous studies on the adsorption process that found that the hematite surface coating inhibited the stability and degree of tightness of cell adsorption on the hematite surface,hematite-HA inhibited interfacial extracellular electron transfer mainly by affecting the degree of tightness of adsorption on the Shewanella oneidensis MR-1 cells and the adsorption of protein components.（3）The different adsorption mechanisms of loosely bound EPS（LB-EPS）and tightly bound EPS（TB-EPS）on hematite were revealed.the QCM-D results showed that LB-EPS formed a soft and thick adsorption layer on the hematite surface through rapid initial deposition and gradually transformed into a rigid and tight structure,whereas TB-EPS showed overall less a tight adsorption layer was formed from the initial stage.EDLVO analysis indicated that the rapid adsorption of LB-EPS was attributed to electrostatic attraction.The molecular mechanism of adsorption was further revealed by 2D-FTIR-COS.The carbon molecules of polysaccharides in LB-EPS dominated the initial interfacial interactions,while phosphate groups and protein components played the main stabilising role in the subsequent long-term adsorption.proteins in TB-EPS contributed significantly to the initial adsorption,facilitating the formation of tight adsorption layers.（4）The strength and mode of action of polystyrene nanoplastics（PSNP）in the dynamic deposition at different encapsulated hematite interfaces was elucidated.We fitted a model based on the frequency and dissipation signal values obtained from QCM-D monitoring of the dynamic deposition of PSNP and found that the deposition rate and amount of PSNP deposited at low ionic strength were suppressed at hematite-HA and hematite-SiO₂ surfaces.At 1 mmol/L Na Cl concentration,PSNP was deposited on the humic acid and silica coated hematite surfaces in a dominant manner with viscoelastic contact,which was detrimental to the stability of the adsorption layer,and shifted to a free oscillatory deposition mode with increasing ionic strength,enhancing the interaction between PSNP and the coated hematite.On the pure hematite surface,on the other hand,PSNP is always deposited in a free oscillatory manner at 1-400 mmol/L Na Cl concentrations,and the high adsorption strength of this interfacial interaction is not conducive to particle migration across the hematite interface.