Effects of gamma(γ)-irradiation on the physicochemical properties and bioavailability of iron oxyhydroxides coprecipitated with varying concentrations of Na-alginate

This study investigated the impacts of γ-radiation at a final dose of 25 kGy on the physicochemical properties of a range of wet iron oxyhydroxides (2-line ferrihydrite, lepidocrocite, and goethite) synthesized in the presence of varying concentrations of the polysaccharide Na-alginate (starting solutions containing C/Fe ratios of 0, 0.5, 1.0, and 1.5). The degree of impact to the minerals was examined by chemical extractions, and various analytical techniques including XRD, FTIR-ATR, Mössbauer spectroscopy, and N2 adsorption-desorption isotherms, as well as by assessing their bioavailability towards the model Fe(III) reducing bacteria Shewanella putrefaciens CN32. Across all the coprecipitates studied, it was found that γ-irradiation led to a substantial dissolution of Fe and the concomitant release of Fe(II) and alginate into solution. Despite this observation, the bulk mineralogy and crystallinity of the studied iron oxyhydroxides, as determined by XRD and FTIR-ATR, did not appear to change. However, analyses via Mössbauer spectroscopy (77 and 5 K) revealed that the crystallinity of the 2-line ferrihydrites increased post-irradiation. Moreover, among the minerals studied, the specific surface area and porosity decreased for only the post-irradiated 2-line ferrihydrite coprecipitates with a C/Fe ratio of 0.5 or 1.0. The bioreduction rates of the studied minerals and their irradiated counterparts did not significantly differ, whereas the extent of bioreduction of post-irradiated 2-line ferrihydrite coprecipitates (C/Fe 0.5, 1.0, and 1.5) exhibited a significant increase of up to 28%. In contrast, the extent of Fe reduction for select post-irradiated lepidocrocite (C/Fe 0.5) and goethite (C/Fe 1.5) coprecipitates was slightly higher than determined for their corresponding native controls. The observed differences in bioavailability between the native and irradiated coprecipitates were attributed to irradiation induced alteration of particle aggregation and coagulation as determined by particle size analyses and visual observations. In summary, the findings suggest that even a low total dose of 25 kGy, γ-radiation can lead to significant physicochemical changes in coprecipitates with relatively low organic matter content. Therefore, future research designed to investigate organic matter-Fe composite systems in natural samples should be cognizant of the potential effects of sterilization through γ-irradiation. These effects could potentially result in inaccurate over- or under- estimations of the bioavailability of Fe or organic matter, as well as the possible adsorption capacity of organic matter-Fe coprecipitates for contaminants.