Thermodynamic Investigations On The Interactions Of Bacteria With Soil Clay Minerals

The interaction between bacteria and mineral is one of the most important processes in soil ecosystem.Understanding the mechanisms of bacterial sorption on mineral and the influence of soil minerals on the metabolic activity is of great environmental and geological significance.In this study,kaolinite,montmorillonite and goethite were selected as model minerals and Pseudomonas putida,Bacillus subtilis and Bacillus thuringiensis were used as model bacteria.The adsorption amount,adsorption heat and adsorption enthalpy of bacteria on clay minerals were obtained by equilibrium adsorption and isothermal titration calorimetry.Based on the surface thermodynamics and XDLVO theory,the free energy changes in sorption of bacteria on minerals were calculated.The metabolic activities of bacteria in the system with the presence of various concentrations of clay minerals were monitored.The effects of minerals on bacterial activity were evaluated,and the toxicities of cadmium on P.putida in the absence and presence of montmorillonite were assessed by microcalorimetric technique.The main results were outlined as follow:1.Isothermal titration calorimetry(ITC),with a combination of chemical adsorption-desorption,scanning electron microscopy(SEM) and Fourier transform infrared spectra(FTIR),were firstly employed to investigate the chemical characteristics and the thermodynamics of bacterial adsorption on kaolinite,montmorillonite and goethite.The greatest amount of bacteria was adsorbed by goethite(100%),followed by kaolinite(96.5%),and to a lesser extent by montmorillonite(61.5%).The maximal adsorption of P.putida on kaolinite and montmorillonite was found at pHs equivalent or close to the zero point charge of the two clay minerals,showing the important role of the non-electrostatic forces in the adsorption of bacteria.Hydrogen bonding was considered to be one of the main forces controlling the sorption of P.putida on kaolinite and montmorillonite,while the functional groups such as carboxyl,amino and hydroxyl group on surface of B.subtilis were involved in the adsorption on soil clay minerals. Electrostatic force and hydrogen bonding were the major forces governing the sorption of the bacteria on goethite.The adsorption of bacteria on soil clay minerals was exothermic process.The adsorption enthalpy of P.putida on minerals followed the order:goethite (-7.55×10^-8 mJ/cell)＞kaolinite(-5.83×10^-8 mJ/cell)＞montmorillonite(-4.15×10^-8 mJ/cell).The enthalpies decreased with increasing pH and ionic strength.For kaolinite and montmorillonite,the enthalpies increased with increasing temperature,suggesting that the adsorption of P.putida was driven by entropy,and compensated by enthalpy.For goethite,the enthalpies decreased with increasing temperature,showing that the adsorption was driven by enthalpy.The enthalpies evolved in the adsorption of B.subtilis on minerals were not correlated with pH,ionic strength and temperature,probably due to the fact that more biomolecules,functional groups and extracellular polymers are present on the surface of this gram positive bacterium.2.Based on surface thermodynamics and XDLVO theory,the free energy changes due to hydrophobic interactions(â–³G^H),the free energy changes originated from electrostatic force(â–³G^EL) and the total free energy changes for the adsorption of P.putida and B.subtilis on kaolinite and montmorillonite were obtained.The negative values of AGH and the positive values ofâ–³G^EL suggested that the hydrophobic force favored,and the electrostatic force unfavored the bacterial adsorption on kaolinite and montmorillonite. The absolute values ofâ–³G^H were much larger than those ofâ–³G^EL,indicating that the hydrophobic interactions play a more important role than electrostatic force in bacterial adsorption.The negative total free energy changes(â–³G) suggested that the sorption of bacteria on clay minerals is a thermodynamically spontaneous process.Greaterâ–³G values for bacterial adsorption on kaolinite than on montmorillonite indicated higher affinity of bacteria for kaolinite than for montmorillonite,which was in agreement with the results obtained from chemical adsorption and ITC studies.3.By using isothermal microcalorimetry,the impacts of soil clay minerals on the metabolic activity of bacteria at exponential and stationary stages were clarified.In the exponential growth stage,the growth rate constant(k) of P.putida increased by 18%,the time to reach the maximal heat power of bacterial growth was decreased by 50 min,and the metabolism activation energy was decreased by 2%in the presence of low amount(5 mg mL^-1) of kaolinite.Similarly,for B.subtilis,the growth rate constant increased by 8%, the peak time was decreased by 30 min,and the activation energy of bacterial metabolism was decreased by 0.2%in the presence of low amount of montmorillonite.It is assumed that the exponential growth of P.putida and B.subtilis could be stimulated by low amount of kaolinite and montmorillonite.However,the activity of B.subtilis was depressed by kaolinite,and the activity of P.putida could also be inhibited by montmorillonite.The activities of two bacteria studied were always inhibited by goethite as the presence of the mineral resulted in marked increase of metabolic activation energy of bacteria.In stationary stage,the activities of P.putida and B.subtilis were inhibited by all the minerals.For B.thuringiensis,the growth rate and total heat output were increased by the presence of the three minerals.The enhancing effect of mineral on the exponential growth was in the order of montmorillonite＞kaolinite＞goethite.The sporulation of B. thuringiensis was depressed by these minerals.The inhibitory effect followed the order: goethite＞kaolinite＞montmorillonite.4.The toxicities of cadmium on P.putida in the absence and presence of montmorillonite were evaluated based on thermokinetic parameter.The peak time,peak height and total heat output of P.putida were increased by the presence of montmorillonite.There was no significant difference for the half inhibitory concentrations (IC₅₀) of cadmium to P.putida with and without montmorillonite.The alleviating effect of montmorillonite on the toxicity of Cd²⁺ to P.putida attributed to the adsorption of Cd²⁺ by the clay mineral and the enhancing effect of the clay on the activity of bacteria.