Formation and transformation of Fe(III)- and Ca-precipitates in aqueous solutions and effects on phosphate retention over time

Abstract

Exfiltration of anoxic phosphate-rich groundwater into surface water leads to the oxidation of dissolved Fe(II) and the formation of Fe(III)-precipitates that can retain phosphate (PO4) and thereby attenuate eutrophication. Fresh Fe(III)-precipitates transform into more stable phases over time, and retained PO4 may be released again. In parallel, CO2 outgassing can promote the formation of Ca-phosphates or -carbonates that also sequester PO4. In laboratory experiments, we studied the effects of Mg, Ca, silicate (SiO4) and PO4 on these processes. Fresh Fe(III)-precipitates were formed in bicarbonate-buffered aqueous solutions at pH ∼ 7.0 via the oxidation of 0.5 mM Fe(II) in the presence of 0.15 or 0.025 mM PO4, at Mg or Ca concentrations of 0, 0.4, 1.2 or 4 mM and in the absence or presence of 0.5 mM SiO4. After CO2 outgassing, the suspensions were allowed to age for 100 d at pH ∼ 8. Changes in the composition and structure of Fe(III)- and Ca-precipitates over time were probed with spectroscopic and microscopic techniques and were linked to variations in the retention of PO4. The oxidation of Fe(II) led to effective PO4 removal via the formation of Fe(III)-precipitates that consisted of amorphous (Ca-)Fe(III)-phosphate ((Ca)FeP), ferrihydrite (Fh) and, in SiO4-free treatments, lepidocrocite (Lp). During aging, FeP and Fh that had formed in the absence of Mg, Ca and SiO4 rapidly and nearly completely transformed into Lp. Via effects on molecular- and nanoscale precipitate structure, Mg slowed down FeP transformation into Fh, stabilized Fh, and decreased the crystallinity of Lp (in SiO4-free suspensions), Ca stabilized CaFeP against transformation into Fh, and SiO4 stabilized Fh and (Ca)FeP. Core/shell CaFeP/Fh particles formed in electrolytes that contained Ca and SiO4 hardly transformed within 100 d. Calcite only formed at low dissolved PO4 concentrations and, by incorporation of PO4, contributed to PO4 retention. Higher levels of dissolved PO4 inhibited calcite formation but could induce Ca-phosphate precipitation. Differences in precipitate formation and transformation pathways and kinetics were reflected in the extents of PO4 release over the 100-d aging period, ranging from rapid release of 77% of the total PO4 in the treatment without Mg, Ca and SiO4 at 0.15 mM total PO4 to slow release of only 0.1% of the total PO4 at initial concentrations of 4 mM Ca, 0.5 mM SiO4, and 0.025 mM PO4. In summary, this study reveals the conditions and the extents and timescales over which Fe(III)- and Ca-precipitates form and transform and how these processes affect PO4 immobilization in near-neutral natural waters. The detailed new insights into the coupling between Fe(III)- and Ca-precipitate formation and into the interdependent effects of Mg, Ca, SiO4 and PO4 are not only relevant with respect to PO4 but also with respect to the cycling of trace elements in natural and engineered systems.