Iron addition for long-term phosphate fixation
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Eutrophic conditions due to high phosphate loading represents a significant water quality issue in the current and coming decade. Iron has therefore been suggested as a solution to bind phosphate despite its redox sensitivity. Iron’s redox sensitivity causes Fe based P fixation to occur through two different processes in oxic sediments and anoxic sediments. In oxic sediment Fe(III)(s) adsorbs PO43-(aq) to its surface where under suboxic conditions, Fe(III) will be reduced to Fe(II) and releasing PO43-(aq). Vertical upwards Fe(II)(aq) transport and lead to Fe oxidation and ferrous wheel functioning while downwards transport to anoxic zones may lead to authigenic vivianite mineralization under favorable conditions. Conditions unfavorable to ferrous wheel functioning and vivianite mineralization are known to be affected by SO42-(aq) and OM but the long-term understanding of shallow lake P fixation as a result of iron addition is lacking. Here the porewaters and sediments of 4 iron treated shallow lakes and 3 untreated shallow reference lakes have been investigated. Surface and porewater analyses were carried out through spectrophotometric measurements, ICP-OES, and IC from which saturation indices were calculated. Sediments were analyzed through CN, CS, TGA, and XRF-P. Quantification of treatment iron was investigated as an issue related to treatment duration and viability. Results reveal Fe(s) recovery that is at least 87% and increased P(s) and S(s) retention compared to reference lakes. Controls on recovery are suggested to be related to treatment method iron application heterogeneity and groundwater fluxes. Redox zonation in treated lakes has been affected by iron addition. Elevated Fe(II)(aq) concentrations at deeper depths have indicated that iron reduction is occurring over a larger depth interval. This has resulted in the simultaneous removal of Fe(II)(aq) and SO42-(aq) with depth. This indicates an effect on iron redox cycling and conditions for authigenic vivianite mineralization. By affecting the redox zonation Fe(II)(aq) may be hindered from diffusing upwards by FeSx mineralization in the presence of S2-(aq) limiting iron within the ferrous wheel. Additionally, authigenic vivianite mineralization may be favored if reactive Fe(III) is reduced in the absence of S2-(aq). Sampled lakes indicate several favorable sediment and porewater layers where authigenic vivianite may be formed and ferrous wheel function is occurring. This suggests that iron addition can be an effective method of long-term phosphate fixation.