Insights into Pb(II) binding by Fe/Al hydroxide–microbe composite: XAFS spectroscopy and isothermal titration calorimetry study

Soil is a complex assemble of colloidal and particulate constituents, including minerals, organic matter and microbes. Minerals and organics are not separate entities; rather, they are constantly interacting with each other. Such mineral-organic interaction can greatly affect the sequestration of metallic elements at the soil-water interfaces. This study aims to explore the binding of Pb(II) in a model Fe/Al hydroxide-organo composite (Fe/Al-OM) made with Pseudomonas putida cells which, for the purposes of this study, can be considered to be an analogue of some natural soil microaggregates. We measure an enhanced Pb adsorption to the Fe/Al hydroxide-bacteria composite (Fe/Al-B) over pure Fe/Al hydroxide at pH < 5.5. We find that Fe/Al-B sorbs an intermediate amount of Pb between the end-member Fe/Al hydroxide and bacteria. We identify a bidentate edge-sharing Pb complex on the Fe/Al oxide surfaces with a Pb–Fe/Al bond distance of ~3.33 Å, as well as a monodentate carboxyl binding on the bacterial surfaces. We report that Pb binding is an exothermic, energetically favorable process accompanied by positive entropy. Moreover, Pb distribution shifts from the bacterial fraction to the mineral fraction in Fe/Al-B when pH increases from 4 to 6.5. Results from batch to spectroscopic and thermodynamic experiments will facilitate our understanding of Pb speciation, mobility and fate in soils and sediments.