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

Abstract

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 –phosphates, could limit PO4 release.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.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. REFERENCESSenn, A.-C.; Kaegi, R.; Hug, S. J.; Hering, J. G.; Mangold, S.; Voegelin, A., Composition and structure of Fe(III)-precipitates formed by Fe(II) oxidation in near-neutral water: Interdependent effects of phosphate, silicate and Ca. Geochim. Cosmochim. Acta 2015, 162, 220–246.Senn, A.-C.; Kaegi, R.; Hug, S. J.; Hering, J. G.; Mangold, S.; Voegelin, A., Effect of aging on the structure and phosphate retention of Fe(III)-precipitates formed by Fe(II) oxidation in water. Geochim. Cosmochim. Acta 2017, 202, 341–360.