Pb(II) Binding To Ferrihydrite-Bacteria Composites: The Surface Complaxation Model And Molecular Mechanism

Iron（hydr）oxides、bacterials and humic substances are the major active constituents in soils. They not only control the adsorption and utilization of nutrients, but also affect the migration and transformation of pollutants. In addition, they are regarded as an important "scavenger" to heavy metal ions. Those active components often exist as a composites in soils. This study focused on HFO-bacteria complexs, combining a series of analytic technique such as isothermal adsorption, synchrotron radiation（XAFS）, micro-fluorescence（μ-SRF）, isothermal titration calorimetry（ITC）, to study the distribution of Pb（II） on the HFO-bacteria surface, to investigatethe corresponding molecular mechanism of Pb（II） binding onto HFO、bacterias and their composites, and to describe the adsorption behavior of Pb（II） onto HFO-Bacteria composites with surface complexation model（SCM）. The main results obtained are as follows:1. We precipitated ferrihydrite via rapid Fe（III） hydrolysis in the absence and presence of the non-Fe metabolizing 、 Gram-positive bacterium B.subtilis or Gram-negative bacterium S.marcescens. XRD results of both ferrihydrite and ferrihydrite composites showed two broad reflexes at d = 1.5 ? and 2.5 ?, characterized as 2-line ferrihydrite. SEM images of the ferrihydrite and ferrihydrite composites depicted the networks of coalesced aggregates that were comprised of smaller nanoparticle clusters coated on the surface of bacteria. IR spectra indicated that the structural hydroxy shifted to higher wavenumbers, and the surface hydroxy vibration gradually decreased.2. The potentiometric titration and isothermal adsorption experiments were used to establish surface complexation model in order to predict the adsoption behavior of Pb（II） onto HFO, bacteria and their composites. The 4-sites diffuse double layer model was used to describe acid-base titration curves of bacteria, where pK for each site was set as 2.5、4.7、6.6、9.0. Our modelling results showed that site densities were 0.7366、0.4882、0.5732、0.4628 mmol/g dry weight for B.subtilis and 0.4524、0.4163、0.6421、0.7753 mmol/g dry weight for S.marcescens. Pb（II） adsorption experiments gave an ―S‖ type adsorption curve. To be more specifically, Pb（II） adsorption was in a low level over the pH range of pH 2.5 4.0, while in the range of pH 4.0 6.0, Pb（II） adsorption increaseed sharply followed by a flat curve. Surface hydroxyl of HFO, carboxyl and phosphate group of B. subtilis surface seemed to be formed a 1:1 complex with Pb（II）, and the reaction constants of surface and structural hydroxyl HFO were 9.96 and 14.05 respectively, which were much larger than that of the bacterial surface groups. Linear combination model could well describe Pb（II） binding to HFO-bacteria complexation,and the HFO is the main contributor to HFO-bacteria composites.3. It’s the first time to use ITC to acquire the thermodynamic imformation of Pb（II） onto B.subtilis, S.marcescens, HFO. The result showed that the adsorption of Pb（II） onto B.subtilis, S.marcescens, HFO were exothermic process, and the enthalpy was over the range of-9.17 ^-59.87 kJ/mol. We also found thatthe enthalpy of HFO was 5 times higher than that of bacteria. In the adsorption of Pb（II） onto HFO-bacteria composites, Pb（II） was firsly adsorbedto the high affinity sites of HFO, and then bound to the surface of bacteria. The entropy of bacteria, HFO and their composites were 41.13¹93.47 J/mol/K, suggesting this adsorption behavior was an entropy-increasing process and were driven by both enthalpies and entropies.4. The use of microscopic spectroscopic techniques revealed the microscopic adsorption mechanism of Pb（II） onto bacteria、HFO and their composites. EXAFS results showed that Pb（II） formed a monodentate inner-sphere complexation with carboxyl and phosphate groups of bacteria surface, and a bidentate edge-sharing inner-sphere complexation with HFO. Liner Combination Fitting of HFO-Bacteria complexation in K-space indicated that HFO plays a major role of Pb（II） adsorption. For samples with the mass ratio of ferrihydrite to B.subtilis, at Fh B₄: 1 and FhB₁: 1, 94.1% and 74.6% of Pb（II） were complexed with ferrihydrite, and for samples with the mass ratio of ferrihydrite to S.marcescens at FhS₄: 1、FhS₁:1, the adsorbed proportion of Pb（II） onto ferrihydrite were 86.2% and 84.9%.5. Micro fluorescence spectra showed that the distribution of Pb, Fe, S were well linear correlate with each other, implying that bacteria were tightly bound to ferrihydrite. The fluorescence spectra of Pb enrichment region was selected to acquire XANES spectra, and the linear combination fitting of XANES spectra was conducted. Under the lower Pb concentration condition, Pb preferred binding to the bacteria, while under higher Pb concentration condition, Pb preferred binding to ferrihydrite.