Interaction between microbes, iron and chlorine for the development of biotechnological approaches to stabilize corroded iron

Iron objects suffer inexorable oxidation and without any human intervention they would be completely damaged. This phenomenon occurs for iron surfaces, outdoor exposed structures as well as for archaeological iron objects. Several methods are currently available for the stabilisation of this type of metallic substrate, however, none of them is completely efficient, and several relay on the use of hazardous compounds. In addition, especially for outdoor iron and pipelines structure a permanent protective treatment does not exist. After few years these corroded surfaces have to be re-treated and in some cases replaced. This causes substantial maintenance costs having an important economic impact on our society. Regarding archaeological iron objects, an additional issue has to be considered. In fact, each object consists of a unique testimony of our past that should be preserved and studied. An archaeological object is usually unique and if the conservation interventions fail, all the information that the object could have revealed will be lost. Scientists agree with the fact that until now an efficient and durable stabilisation treatment for corroded iron does not exist. As a consequence, there is a pressing need to investigate new approaches. To this purpose, the present thesis investigated the potential of microorganisms (bacteria and fungi) for the development of stabilisation methods for corroded iron. Since one of the main issues for this metal is chlorine, this study examined two different strategies of chlorine removal and conversion of the unstable iron compounds into more stable biogenic minerals. The first approach was an indirect chlorine extraction, consisting on the microbial removal of iron ions present in chlorinated corrosion compounds. For this purpose, microbial biogenic minerals production and fungal iron adsorption were investigated. In particular, exploiting biogenic minerals production of the strains TCE1 and LBE of the anaerobic bacterium Desulfitobacterium hafniense, it was possible to convert a part of the corrosion layer of corroded iron coupons, as well as of archaeological iron nails, into biogenic vivianite and magnetite. In addition, this study allowed definitely to assess that fungi are not the best candidates to develop stabilisation methods for corroded iron based on biogenic minerals production. In fact, even though Beauveria bassiana produced some biogenic crystals their amount was not sufficient for a precise characterisation, and none of the factors tested stimulated a higher production. Nevertheless, interesting results were obtained for fungal iron uptake. Indeed, iron uptake of the fungus Alternaria sp. was successfully used for a biocleaning of corroded iron coupons. In addition, another biotechnological application exploiting fungal iron uptake was investigated. In this study the ability of bacteria to use iron chelated in fungal dead biomass as a bioavailable source of iron was proved for Pseudomonas fluorescens. This could then be exploited to improve iron bioavailability, as well as availability of organic carbon in soil for other microbes and maybe also plants. A second approach regarding a direct method for the removal of chlorine was also studied. Uptake of potassium and chlorine was proved for B. bassiana that produced aggregates containing these elements onto its biomass when exposed to FeCl2. However this ability could not be further exploited, as chlorine uptake was not the main resistance mechanism used by this fungus against chlorine, and an efficient uptake of this ion was not measured. Finally, aiming to remove chlorine from corroded iron, volatiles organic compounds production was studied. Preliminary results showed that NaCl stimulates the production of particular compounds not present in absence of this substance. Overall it can be affirmed that this study allow to assess that microorganisms are a valuable alternative for the stabilisation of corroded iron. Bacteria could be employed to stabilize the corrosion layer by producing stable biogenic minerals, while fungi could be used for biocleaning of corroded iron.