Formation and aging of Fe(III) and Ca precipitates in exfiltrating anoxic groundwater and effects on phosphate retention

<p>The oxidation of dissolved Fe(II) upon exfiltration of anoxic groundwaters into oxic surface waters leads to precipitation of poorly crystalline Fe(III)-solids that strongly bind phosphate (PO₄) and thereby can attenuate eutrophication. Fresh Fe(III)-precipitates may transform into more crystalline phases over time, which may lead to the release of initially co-precipitated PO₄. The formation and transformation of Fe(III)-precipitates in natural waters is strongly affected by other solutes (Ca, Mg, PO₄, silicic acid (SiO₄)) that interfere with Fe(III) precipitation and transformation, and thereby also affect PO₄ binding. Furthermore, in Ca-containing waters, the repartitioning of PO₄ released from Fe(III)-precipitates into Ca-carbonates or &#8211;phosphates, could limit PO₄ release.</p> <p>For better understanding the fate of PO₄ in aquatic environments, there is a need for a mechanistic understanding of coupled Fe(III)- and Ca-precipitate formation and transformation processes induced by groundwater exfiltration, and their effects on PO₄ sequestration. In this laboratory study, we examined the effects of Ca, Mg, and SiO₄ on the formation and transformation of Fe(III)- and Ca-precipitates in bicarbonate-buffered aqueous solutions upon Fe(III)-precipitate formation by Fe (II) oxidation in the presence of PO₄, over aging periods up to 100 d. Changes in precipitate structures were probed with spectroscopic and microscopic techniques and linked to changes in the retention or release of PO₄ over time.</p> <p>The results show that especially Ca and SiO₄ contribute to effective PO₄ retention via multiple interdependent processes, and thereby strongly attenuate PO₄ release over extended periods of time.</p> <p>&#160;</p> <p>REFERENCES</p> <p>Senn, A.-C.; Kaegi, R.; Hug, S. J.; Hering, J. G.; Mangold, S.; Voegelin, A., <em>Composition and structure of Fe(III)-precipitates formed by Fe(II) oxidation in near-neutral water: Interdependent effects of phosphate, silicate and Ca.</em> Geochim. Cosmochim. Acta 2015, 162, 220&#8211;246.</p> <p>Senn, A.-C.; Kaegi, R.; Hug, S. J.; Hering, J. G.; Mangold, S.; Voegelin, A., <em>Effect of aging on the structure and phosphate retention of Fe(III)-precipitates formed by Fe(II) oxidation in water.</em> Geochim. Cosmochim. Acta 2017, 202, 341&#8211;360.</p> <p>&#160;</p>