Adsorption Of Model Extracellular Polymeric Substance Molecules By Clay Minerals

Extracellular polymeric substance(EPS)is the general term for the hydrated polymer secreted by microorganisms during the growth and reproduction process,and EPS widely exists outside of microorganism cells and inside of microorganism aggregates.The adsorption of EPS and minerals can affect a series of biogeochemical reaction processes,such as colonization of bacteria,formation of aggregates,biological leaching,and extracellular electron transfer.This article takes bovine serum albumin and carboxymethyl dextran as the model proteins and polysaccharides of EPS,and selects the common clay minerals(montmorillonite,kaolinite and goethite)in the soil as the experimental materials.The modern analytical techniques including atomic force microscopy(AFM)and isothermal microcalorimetry are used to study the adsorption reaction of extracellular proteins and polysaccharides on the surface of minerals,aiming to predict the behavior of EPS-minerals in the adsorption process,and provide a basis for the improvement of the theory of microbial and soil mineral adsorption.The main results are as follows:(1)Based on atomic force microscopy technology,the force between different bovine serum albumin/carboxymethyl dextran and minerals have been obtained.The results showed that as the ionic strengths increased,the adhesion of bovine serum albumin/carboxymethyl dextran to minerals showed a trend of first increasing and then decreasing.When the ionic concentration is 10 m M,the adhesion force between bovine serum albumin and montmorillonite,kaolinite and goethite reaches the maximum,which is 374±6 p N,224±27 p N and 151±43 p N respectively;The maximum adhesion force between dextran and montmorillonite,kaolinite and goethite is 170±32 p N,141±13 p N and 168±27 p N.The increasing of p H(3.9-8.4)will change the force between the model molecule and montmorillonite/kaolinite.When the p H is 3.9,the adhesion force is the largest.The adhesion force of bovine serum albumin to montmorillonite and kaolinite is 353±9 p N,274±79 p N and 215±32 p N,respectively;and the force between dextran and montmorillonite and kaolinite is 276±7 p N,167±47 p N and 334±21 p N.According to the results of atomic force microscopy,the order of adhesion force between bovine serum albumin and minerals is: montmorillonite>kaolinite>goethite.The surface hydrophobicity of montmorillonite is stronger than that of goethite and kaolinite,indicating that the hydrophobic interaction force can be used to explain the result.Under acidic and neutral conditions,adhesion force between goethite and carboxymethyl dextran is stronger than others.At this time,the surface of goethite is positively charged and there is electrostatic repulsion between goethite and dextran,and montmorillonite/kaolinite is negatively charged and the force between them is electrostatic attraction.It is speculated that the non-electrostatic force other than hydrophobic force causes extracellular polysaccharides to be more likely to interact with Goethite.(2)Comparing adsorption heat between clay minerals and model molecules,it shows that the result of bovine serum albumin and montmorillonite(48.3 ?J)is the largest,which followed by kaolinite(35.3 ?J),and the smallest is goethite(31.0 ?J).It suggested that after the bovine serum albumin adsorbed on the montmorillonite the energy of the system is lower,and the formed complex is more stable.The order of heat of adsorption between carboxymethyl dextran and minerals is: goethite(26.7?J)> kaolinite(24.8 ?J)> montmorillonite(7.7 ?J).Goethite-dextran complex is the stable existence.