Synchrotron X-ray studies of metal-ion partitioning at biofilm (organic film)/metal-oxide/water interfaces [electronic resource]
Keywords: Geological and Environmental Sciences.

Heavy metal contaminants pose a widespread environmental problem due to their toxicity, persistence, and non-biodegradable nature. The fate and transport of metal ions in natural and anthropogenically perturbed environments are largely controlled by sorption and complexation reactions with mineral surfaces in soil and sediments. However, biofilms and natural organic matter (NOM) are ubiquitous in natural environments, and often partially coat fine-grained minerals such as common Fe- and Al-(oxy)hydroxides. These coatings contain abundant reactive functional groups, and are expected to induce significant modifications to the properties of mineral surfaces and to compete for metal-ion sorption with the mineral surface. Despite significant research efforts on metal-ion interactions with minerals, bacterial surfaces and NOM, the potential effects of organic coatings on mineral reactivity and their influence on metal-ion sorption are largely unverified experimentally due to the complexity of the systems and the lack of appropriate analytical tools. This knowledge gap has been further complicated by the fact that multiple metal ions are often present together in natural systems and may interfere with each other in sorption reactions. Therefore, information about metal-ion partitioning between mineral surfaces and organic coatings at complex environmental interfaces, including multi-metal-ion and multi-component systems, is required for better understanding metal cycling under realistic environmental conditions. In this research, the partitioning of selected metal ions such as Pb(II) and Zn(II) between organic coatings and underlying highly polished, oriented single-crystal [alpha]-Fe2O3 and [alpha]-Al2O3 substrates was systematically measured in biofilm (NOM)/metal-oxide/water systems using synchrotron-based long-period x-ray standing wave fluorescence yield (LP-XSW-FY) spectroscopy. A multi-layer biofilm (S. oneidensis MR-1), a highly reactive organic compound (PAA), and a soil-based humic acid (Elliott soil humic acid) were used as model organic coatings. These three organic coatings vary significantly in chemical reactivity (i.e. the number and types of reactive functional groups and their metal-binding affinities), and represent several common types of organic coatings that exist in nature. The result of this research provides valuable insights about the effects of common types of organic coatings on metal-ion sorption and mineral reactivity. We found that metal-ion distributions at complex biofilm (NOM)/metal-oxide/water interfaces are governed by the intrinsic properties of both the thin-film coatings and the underlying metal-oxide substrates, including the nature of binding sites, their metal-binding affinities, and the surface charges of the organic coatings and metal-oxide surfaces, as well as extrinsic parameters such as solution composition, the presence of competing ions, and reaction time. By using three types of organic coatings with dramatically different chemical reactivities, we confirmed a general trend occurring at biofilm (NOM)/metal-oxide/water interfaces: the presence of an organic coating does not change the intrinsic order of reactivity of metal-oxide surfaces. In addition, this research explored important new aspects that should be considered when evaluating the fate and transport of metal ions in the environment by studying competitive sorption in multi-metal-ion and multi-component systems. In both biofilm/metal-oxide and PAA/metal-oxide systems, we found that single-metal-ion partitioning results cannot be used to predict mixed-metal-ion partitioning behavior, suggesting that caution should be taken when applying knowledge obtained in single-metal-ion systems to more complex multi-metal-ion and multi-substrate systems.