Interaction Mechanisms Of Extraceller Polymeric Substances From Bacteria With Soil Soilds Interface And Their Environmental Effects

Growth and metabolism of bacteria are accompanied by the production of extracellular polymeric substances (EPS), which are complex mixtures of various biomacromolecules. The interactions of EPS with soil solid interface have significant effects on many geochemical processes such as biofilm formation, the migration of bacterial cells, mineral dissolution, biomineralization and forming of soil. The important role of EPS in heavy metals accumulation in soil or aquatic environments has also received growing concern. The adsorption and binding mechanism of EPS from bacteria on Brown soil particles or minerals such as montmorillonite, kaolinite and goethite were studied using batch adsorption experiments, Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, isothermal titration calorimetry (ITC), scanning electron microscopy (TEM), fourier transform infrared spectrumscopy (FTIR) and X-ray Absorption Fine Structure (XAFS). Biochemical properties of EPS from six bacteria and their correlation with adsorption on goethite and adsorption for Cu(Ⅱ) were statistically investigated. The major results were summarized as follows:(1) EPS from Bacillus subtilis are composed dominantly of polysaccharides (437.6±11.8mg g-1) and proteins (249.1±3.3mg g-1), with nucleic acids (9.9±0.2mg g-1) as minor constituents. EPS contain three functional groups which correspond to carboxyl (pKa=4.58±0.33), phosphoryl (pKa=6.49±0.18) and amino or hydroxyl (pKa=9.11±0.41). The adsorption of EPS-C,-N and-P on montmorillonite, kaolinite and goethite conform to Langmuir equation quite well. The amount of EPS-C adsorption on montmorillonite was4.8and8.9-fold greater than that on goethite and kaolinite, respectively. EPS-N adsorption on montmorillonite was also far greater than that on goethite (8.9-fold) and kaolinite (16.2-fold). However, EPS-P adsorbed by goethite was about5-times greater than that by montmorillonite and kaolinite. Therefore, EPS-C and-N constituents (from proteins) are mainly adsorbed by montmorillonite and EPS-P constituents (from nucleic acids) are predominantly adsorbed by goethite. Goethite shows the highest affinity to EPS among the examined clay minerals and iron oxide. An increase in the concentration of cations and/or a decrease in the pH favored EPS adsorption on minerals. Combining with the FTIR result, hydrogen bonding and the electrostatic interaction are the main forces governing the adsorption of EPS on clay minerals. Besides these interaction forces, chemical bonding interactions (ligand exchange) also contribute to selective fractionation of EPS adsorption on goethite.(2) The binding characteristics of EPS from Pseudomonas putida on clay mineral and oxide were studied using a combination of DLVO theory, ITC, SEM, FTIR, XAFS and batch adsorption experiments. Extracted EPS from P. putida consist of polysaccharides (532.2±7.0mg g-1), proteins (152.2±1.8mg g-1) and nucleic acids (9.2±0.2mg g-1). A marked decrease in the adsorption of EPS on montmorillonite, kaolinite and goethite was observed with the increase of pH from3.0to9.0. This phenomenon conforms to DLVO theory and showed that electrostatic forces dominate the adsorption of EPS on mineral surfaces. Hydrogen bond, hydrophobic and covalent may also participate in the adsorption of EPS on montmorillonite (pH≥3.0), kaolinite (pH≥5.0) and goethite (pH≥9.0). SEM showed more firm structrue for EPS-goethite complex than EPS-montmorillonite and EPS-kaolinite complexes at pH7.0, which was in accordance with the higher K value of goethite of adsorption EPS-C,-N and-P than that of montmorillonite and kaolinite. Adsorption of EPS on minerals surfaces were exothermic (AHad=-0.02～-12.34J g-1), and governed by combination of forces such as hydrogen bond, hydrophobic and covalent interaction in addition to electrostatic force. The hydrogen bond between EPS and clay minerals were confirmed by FTIR, as well as P-OFe coordination bond between EPS and goethite. XAFS showed that phosphate groups of EPS can manly form monodentate inner-sphere complexes with Fe centers on the goethite surface at low pH, while gradually changed to the bidentate inner-sphere complexes as pH rising.(3) The adsorption of EPS from P. putida on soil particles could be quantitatively described with Langmuir isotherm equation. EPS are mainly adsorbed by fine soil particles, however organic matter hinder the adsorption of EPS on soil particles. EPS-N moieties mainly from proteins are adsorbed preferentially on clay, and EPS-C moieties predominantly from polysaccharides are adsorbed selectively by silt and sand. The mass fraction of EPS-C, and-N adsorbed on soil particles increased in the presence of0-30mM Na+or Ca2+and remained constant or increased slightly with continuously increasing ionic concentration at pH6.0. In contrast, the adsorption of EPS-P on soil particles was on the decline with the increase of NaCl and CaCl2. DLVO theory can explain the effects of size fraction and organic matter on the absorption of EPS, also predict the adsorption of EPS-C and-N trend with ionic strength. However this theory failed to explain the influence of ionic concentration on adsorption of EPS-P, indicated that the binding mechanism of EPS-P on soil particles was different to EPS-C and-N.(4) The biochemical properties of EPS extracted from different bacteria were characterized in order to charigy the key factors controlling EPS adsorption on minerals and heavy metal binding. Polysaccharides and protein accounted for65%-86%of EPS composition, and the ratios of polysaccharides and protein were0.14-6.81. The total function group contents were from7.61±0.39mmol g-1to34.78±0.38mmol g-1, with an average of23.5±0.66mmol g-1. Mw values of six EPS were from2.19×104g mol"1to3.51×105g mol-1. The equivalent diameter of EPS was in the range of525.9to1701nm, with negatively charged surface. Adsorption of EPS on goethite can be well described by Langmuir model. The adsorbed amount of EPS from S. suis on goethite was far greater than the other EPS, and showed a different trend of adsorption, indicating that polymeric interaction promoted multilayer adsorption of EPS from S. suis on goethite. Adsorption of EPS on goetihte showed a significant correlation with the macromolecular composition of EPS, especially the content of protein. Adsorption of Cu(Ⅱ) by EPS differed markedly, and Cu(II) adsorption capacity was up to830.6mg g-1. The equivalent diameter of EPS was the key factor affecting adsorption of Cu(Ⅱ).