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Methodological Framework for the Assessment of Bioavailable Iron with Operationally Defined Fractions and Comparative Ecotoxicological Assessment
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Methodological Framework for the Assessment of Bioavailable Iron with Operationally Defined Fractions and Comparative Ecotoxicological Assessment

Emiliano Balsamo Crespo
Southern Cross University
Doctor of Philosophy (PhD), Southern Cross University
2025
DOI:
https://doi.org/10.25918/thesis.563
Appears in  Recent Southern Cross PhD Theses
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Balsamo Crespo E 2025 PhD33.73 MBDownloadView
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Abstract

Iron bioavailability pH2 extraction operationally defined bioavailable Fe Suspended solids Iron water quality guidelines Iron DGV Chronic toxicity Ecotoxicology Environmental Risk Assessment Environmental chemistry
Recent reviews of water quality guidelines values (WQGV) for iron (Fe) in Canada and Australia have advanced bioavailability-based criteria for aquatic protection. Review of laboratory and mesocosm data indicates chronic toxicity at Fe concentrations above its solubility limit, suggesting Fe colloids are bioavailable. However, total Fe in natural waters also includes potentially less bioavailable mineralised Fe oxides and oxyhydroxides within suspended solids (SS), unlike the freshly precipitated colloids used in experiments. This incompatibility between laboratory speciation and field conditions increases the chances of compliance errors, resulting in inefficient environmental policies. Accordingly, new guidelines recommend analytical methods targeting bioavailable Fe; however, it is necessary to first operationally define and validate the implementation of such criteria, which is the overarching goal of this thesis. To achieve this, analytical methods were tested to specifically recover freshly precipitated Fe in the presence of mineralised Fe species by comparing filterable Fe, total recoverable Fe, and room temperature nitric acid extractions at pH 2 and pH 4 for up to 16 hours. Furthermore, reproduction tests with Ceriodaphnia dubia exposed to freshly precipitated and well-mineralised Fe in the nano- and micrometre range, along with a SiO2 control were used in single and combined exposure treatments to predict toxicity as a function of total recoverable, filtered and pH2-extracted Fe; and SS fractions. While pH 4 extraction was deemed unsuitable due to poor recovery towards Fe ageing, extractions at pH 2 between 6 and 16 hours successfully defined a bioavailable Fe fraction (BFe) that recovered freshly precipitated species. This method remained effective for samples stored up to 14 days, preventing underestimations from sample ageing during transportation. Results demonstrated that clear negative effects were predicted by the SS fraction, including a particle size effect, but these effects were independent of the Fe content and less potent than the inhibition caused from exposure to more bioavailable fresh Fe precipitates. Due to interactive effects, it is advised that both BFe and SS fractions be included as predictor variables for better estimation of chronic toxicity. The pH 2 extraction is readily adoptable by commercial and regulatory stakeholders, minimises recovery of less bioavailable Fe from SS, and produces compatible toxicity estimations to those used in statistical models for WQGV derivation. Finally, environmental risk assessment requires further investigation regarding the interaction with SS and the protective effects of dissolved organic carbon to improve compliance efficiency with respect to the WQGV.

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