The Molecular Mechanisms Of Heavy Metals Interactions With The Bacteria-soil Active Particles Micro-Interfaces

The mobility, speciation, transport and bioavailability of toxic metallic cations in the soil environment depend largely on their interactions (adsorption, complexation and redox) with inorganic and organic surfaces, principally microorganisms, minerals and their composites, respectively. Therefore, the interactions of heavy metals and soil components are always the hot spots studied by environmental chemistry and soil science investigators. In our studies, the common bacteria, extracellular polymeric substances (EPS) and minerals were used as model materials. The binding characteristics of Cu(Ⅱ) and Cd(Ⅱ) by the individual components (bacteria, EPS, minerals) and binary composites (bacteria-mineral composites, EPS-mineral composites) were investigated using a combination of chemical modifications, batch adsorption experiments, isothermal titration calorimetry (ITC), Fourier transform infrared spectroscopy (FTIR), X-ray absorption fine structure (XAFS) spectroscopy. The main results were listed as following:1. The effect of functional groups on Cu(Ⅱ) and Cd(Ⅱ) adsorption on bacteria (Bacillus thuringiensis, Escherichia coli, Cyanobacterium Spirulina platensis) were studied. The order of Cu(Ⅱ) and Cd(Ⅱ) adsorption capacity was S. platensis> B. thuringiensis> E. coli. Esterified cells resulted in the reduction in the binding of Cu(Ⅱ) and Cd(Ⅱ) on S. platensis, B. thuringiensisand E. coli by 45.6%-55.5%,72.3%-75.3% and 8.2%-22.3%, respectively, which demonstrated that the carboxyl groups on S. platensis and B. thuringiensis surfaces play a more important role in the binding of metal ions than that on E. coli. Potentiometric titration results provide the further evidences that the carboxyl groups on B. thuringiensis surfaces was the major ligands responsible for the binding of Cu(Ⅱ) and Cd(Ⅱ). As for E. coli, it is likely that the phosphate groups were important for the binding of metal ions. A percentage of 53.7% and 72.7% of adsorbed Cu(Ⅱ) on S. platensis surfaces was desorbed by NH4NO3 and EDTA, respectively. The percent desorption of Cd(Ⅱ) by NH4NO3 and EDTA was 58.0% and 80.7%. This results indicated that Ion exchange and complexation are the dominating mechanisms for Cu(Ⅱ) and Cd(Ⅱ) adsorption on S. platensis surfaces. XAFS analysis provided further evidence for the inner-sphere complexation of Cu by carboxyl ligands and showed that Cu was complexed by two 5-membered chelate rings on S. platensis surface.2. The binding characteristics of Cu(Ⅱ) by the soluble EPS of Bacillus subtilis were investigated using a combination of XAFS, ITC and batch adsorption experiments. The results showed that the adsorption kinetics was rapid and 95% of biosorption capacity was achieved in the first 10 min of contact and then attained adsorption equilibrium in 60 min. The adsorption capacity and rate decreased with the increase of ion strength. The kinetics experiments demonstrated that Second-order equation, Intra-particle diffusion and Elovich equation all provide the good fit to the experimental data. The fitting results indicated that a chemical reaction mechanism and intra-particle diffusion in the first time play an important role in Cu(Ⅱ) adsorption. Furthermore, XAFS analysis provided further evidence on the inner-sphere complexation of Cu by carboxyl ligands for the adsorption of Cu(II) on EPS (B. subtilis) and B. subtilis surface. Cu(Ⅱ) is complexed by one or two 5-membered chelate rings on B. subtilis surface and is complexed by one 5-membered chelate on EPS (B. subtilis). The larger adsorption heat was observed for the incteraction between Cu(Ⅱ) and the high-affinity sites on the EPS. Moreover, the high-affinity sites on the EPS were the major place for the formation of 5-membered chelate rings.3. The role of bound EPS in Cu(Ⅱ) and Cd(Ⅱ) adsorption by Bacillus subtilis and Pseudomonas putida was investigated. The total site concentrations on untreated B. subtilis and P. putida surface were 2.89×10-3 and 1.85×10-3mol g-1 and decreased by 62.3%and 38.9%, respectively, after EPS molecules were removed by CER, suggesting that the removing of EPS from bacterial cells can significantly reduce the site concentrations on bacterial surfaces. As compared with untreated bacteria, Cd(Ⅱ) adsorption decreased by 51.4% and 9.7%, respectively on EPS-free B. subtilis and P. putida. Cu(Ⅱ) adsorption decreased by 37.8% and 25.4%, respectively on EPS-free B. subtilis and P. putida. These results indicated that the absence of EPS on bacteria may significantly reduce the concentration of binding sites and Cu(Ⅱ) adsorption capacity, especially for Gram-positive B. subtilis. Surface complexation modeling of titration data showed the similar pKa values of functional groups (carboxyl, phosphate and hydroxyl) between untreated and EPS-free bacteria. A three site non-electrostatic surface complexation modeling of titration data showed the similar pKa values of functional groups (carboxyl, phosphate and hydroxyl) between untreated and EPS-free bacteria. FTIR spectra also showed that no significant difference in peak positions was observed between untreated and EPS-free bacteria and carboxyl and phosphate groups were responsible for Cd adsorption on bacterial cells. Our study suggested that generalized model could be used to quantify the bacteria-metal adsorption behavior in geologic systems.4. Impact of bacteria and mineral types on the surface sites and adsorption behaviors were revealed in this study. As for individual components, the order of Cu(Ⅱ) adsorption capacity was B. thuringiensis (28.15 mg g-1)> P. putida (20.70 mg g-1) montmorillonite (14.30 mg g-1)> goethite (9.43 mg g-1)> kaolinite (3.78 mg g-1). The B. thuringiensis- and P. putida-montmorillonite mixture have more adsorption sites (1.94%～6.20%) and bound 16.4%-30.6%% larger amount of Cu(Ⅱ) than that predicted by their individual components. However, the bacteria-goethite composites have less adsorption sites (6.26%) and bound 19.6% less amount of Cu(Ⅱ) than that predicted by their individual components that predicted by their individual components. The different changes in the concentration of surface sites between montmorillonite- and goethite-bacteria composites suggest that the strength of interaction between bacteria and minerals affects the concentration of reactive sites on their composite surfaces. Our results demonstrated that the interaction of montmorillonite with bacteria increased the reactive sites and resulted in greater adsorption of Cu(Ⅱ) on their composites, while goethite-bacteria composite decreased surface sites and adsorption capacity for Cu(Ⅱ). XAFS analysis showed that the adsorption of Cu(Ⅱ) on bacteria and their composites with minerals was an endothermic reaction, while that on minerals was exothermic. The enthalpy changes (△Hads) from endothermic (6.14 kJ mol-1) to slightly exothermic (-0.78 kJ mol-1) suggested that Cu(Ⅱ) is complexed with the anionic oxygen ligands on the surface of bacteria-mineral composites. Large entropies (32.96-58.89 J mol-1 K-1) of Cu(Ⅱ) adsorption onto bacteria-mineral composites demonstrated the formation of inner-sphere complexes in the presence of bacteria. The thermodynamic data obtained in this study are the first to investigate the binding mechanism in terms of calorimetric determinations, which implied that Cu(Ⅱ) mainly bound to the carboxyl and phosphoryl groups as inner-sphere complexes on bacteria and mineral-bacteria composites.5. The adsorption of Cu(Ⅱ) by EPS extracted from Pseudomonas putida, minerals and their composites were investigated. The EPS-montmorillonite mixture have 5.2% more adsorption sites and bound 13.9% larger amount of Cu(Ⅱ) than that predicted by their individual components, However, the bacteria-goethite composites have 8.5% less adsorption sites and bound 19.1% less amount of Cu(Ⅱ) than that predicted by their individual components that predicted by their individual components. Our results presented that the interaction of montmorillonite with EPS increased the reactive sites and resulted in greater adsorption of Cu(Ⅱ) on their composites, while goethite-EPS composite decreased surface sites and adsorption capacity for Cu(Ⅱ). The△Hads values of the adsorption of Cu(Ⅱ) on EPS and mineral-EPS composites were in the range of 19.34～24.11 kJ mol-1. The measured△Sads values for Cu(Ⅱ) adsorption on EPS and mineral-EPS composites (99.53～121.98 J mol-1 K-1) indicated that Cu(Ⅱ) mainly interacts with carboxyl and phosphoryl groups as inner-sphere complexes on EPS molecules or their composites with minerals. The thermodynamic data obtained in this study are the first to investigate the binding mechanism in terms of calorimetric determinations, which implied that Cu(II) mainly interacts with carboxyl and phosphoryl groups as inner-sphere complexes on EPS molecules or their composites with minerals.6. The interactive molecular of goethite with extracellular polymeric substances (EPS) isolated from P. putida was investigated. The adsorption isotherms of EPS on goethite conformed to the Langmuir equation and the amount of EPS-C,-N and-P adsorbed followed the order:EPS-C (27.57 mg g-1)> EPS-N (10.27 mg g-1)> EPS-P (6.32 mg g-1). However, the adsorption energy constant (K) and distribution coefficient (Kd) of EPS on goethite were in the sequence of EPS-P> EPS-N> EPS-C, indicating that P-containing moieties was adsorbed strongly and preferentially than EPS-N and EPS-C. Emergence of the new band and the disappearance stretching vibration of PO2 are consistent with inner-sphere complexation of EPS phosphate groups (deriving principally from phosphodiesters of nucleic acids and proteins) at goethite surface hydroxyls. XAFS studies demonstrated that phosphate can form monodentate or bidentate inner-sphere complexes with goethite surface sites and the structure of complexes is sensitive to changes in pH. The two different inner-sphere structure may occur for the adhesion of EPS to a-FeOOH surface.1) The phosphate groups of EPS can form a monodentate inner-sphere complex, where one oxygen of the anion binds directly the Fe atom of a FeOH1/2-group, releasing the attached OH-.2) A bidentate inner-sphere complex, where two oxygens of the anion bind two Fe atoms of two adjacent FeOH1/2-groups. The phosphate groups of EPS. Solution pH is an important factor affecting the complexes structures due to the influence of pH on the deprotonation or protonation of the phosphate groups of EPS.