Use of Bacteria To Stabilize Archaeological Iron

Abstract

Iron artifacts are common among the findings of archaeological excavations. The corrosion layer formed on these objects requires stabilization after their recovery, without which the destruction of the item due to physicochemical damage is likely. Current technologies for stabilizing the corrosion layer are lengthy and generate hazardous waste products. Therefore, there is a pressing need for an alternative method for stabilizing the corrosion layer on iron objects. The aim of this study was to evaluate an alternative conservation-restoration method using bacteria. For this, anaerobic iron reduction leading to the formation of stable iron minerals in the presence of chlorine was investigated for two strains of Desulfitobacterium hafniense (strains TCE1 and LBE). Iron reduction was observed for soluble Fe(III) phases as well as for akaganeite, the most troublesome iron compound in the corrosion layer of archaeological iron objects. In terms of biogenic mineral production, differential efficiencies were observed in assays performed on corroded iron coupons. Strain TCE1 produced a homogeneous layer of vivianite covering 80% of the corroded surface, while on the coupons treated with strain LBE, only 10% of the surface was covered by the same mineral. Finally, an attempt to reduce iron on archaeological objects was performed with strain TCE1, which led to the formation of both biogenic vivianite and magnetite on the surface of the artifacts. These results demonstrate the potential of this biological treatment for stabilizing archaeological iron as a promising alternative to traditional conservation-restoration methods.IMPORTANCE Since the Iron Age, iron has been a fundamental material for the building of objects used in everyday life. However, due to its reactivity, iron can be easily corroded, and the physical stability of the object built is at risk. This is particularly true for archaeological objects on which a potentially unstable corrosion layer is formed during the time the object is buried. After excavation, changes in environmental conditions (e.g., higher oxygen concentration or lower humidity) alter the stability of the corrosion layer and can lead to the total destruction of the object. In this study, we demonstrate the feasibility of an innovative treatment based on bacterial iron reduction and biogenic mineral formation to stabilize the corrosion layer and protect these objects.KEYWORDS akaganeite, biogenic minerals, Desulfitobacterium, vivianite, iron reduction I ron artifacts are among the most recurring items in archaeological findings. When buried, these objects develop a complex corrosion layer, the composition of which depends on the environmental conditions of the burial site (i.e., oxygen, humidity, salinity, pH, and presence of microorganisms). A stratified corrosion layer containing mainly hematite (Fe 2 O

Investigation of Biogenic Passivating Layers on Corroded Iron

“…Samples of 12.5 × 25 × 2-3 mm were obtained from a steel plate with a natural urban corrosion layer mainly composed of lepidocrocite and goethite [25,26]. The plate was exposed for about 10 years in an urban environment (Zürich, Switzerland).…”

“…This bacterium was selected as it can use a variety of electron acceptors, especially halogenated organic compounds and metals [29,30]. In addition, a previous study revealed that this strain was able to reduce Fe 3+ -citrate in the presence of 0.2% and 0.3% NaCl, and that it was more efficient in terms of production of iron phosphates on the surface of corroded iron coupons [25,26]. Bacterial pre-incubation was performed in the dark at 30 • C under agitation in a standard mineral medium under anoxic conditions in 500 mL serum bottles, until reaching an optical density (OD600) of 0.1−0.15, as previously described [31].…”

“…As part of our research topic, we investigated the potential of microbes for the stabilization of already corroded iron (archaeological objects and outdoor surfaces) by converting part of the reactive corrosion layer into more stable biogenic minerals ( Figure 1). In particular, we studied different bacterial species, Shewanella loihica, Desulfitobacterium hafniense and Aeromonas spp., for their ability to produce biogenic iron minerals on corroded steel coupons [24][25][26][27][28]. Hence, vivianite and siderite were produced by S. loihica on costal-exposed coupons, while D. hafniense and Aeromonas spp.…”

“…Hence, vivianite and siderite were produced by S. loihica on costal-exposed coupons, while D. hafniense and Aeromonas spp. induced the formation of vivianite and siderite on urban-exposed coupons [26][27][28]. In order to better understand the biomineralization process involved, corroded coupons exposed in an urban environment and treated with D. hafniense have been investigated through the present stratigraphic study.…”

Microbial biotechnology approaches to mitigating the deterioration of construction and heritage materials

“…Biocorrosion of iron and iron alloys (Schutz et al, 2015) Production of stable corrosion products via biogenic mineral precipitation (Cote et al, 2015;Comensoli et al, 2017)…”