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Mitigation measures are needed to prevent large loads of phosphate originating in agriculture from reaching surface waters. Iron-coated sand (ICS) is a residual product from drinking water production. It has a high phosphate adsorption capacity and can be placed around tile drains, taking no extra space, which increases the farmers' acceptance. The main concern regarding the use of ICS filters below groundwater level is that limited oxygen supply and high organic matter concentrations may lead to the reduction and dissolution of iron (hydr)oxides present and the release of previously adsorbed phosphate. This study aimed to investigate phosphate adsorption on ICS at the onset of iron reduction. First, we investigated whether simultaneous metal reduction and phosphate adsorption were relevant at two field sites in the Netherlands that use ICS filters around tile drains. Second, the onset of microbially mediated reduction of ICS in drainage water was mimicked in complementary laboratory microcosm experiments by varying the intensity of reduction through controlling the oxygen availability and the concentration of degradable organic matter. After 3 yr, ICS filters in the field removed phosphorus under low redox conditions. Over 45 d, the microbial reduction of manganese and iron oxides did not lead to phosphate release, confirming field observations. Electron microscopy and X-ray absorption spectroscopy did not evince systematic structural or compositional changes; only under strongly reducing conditions did iron sulfides form in small percentages in the outer layer of the iron coating. Our results suggest that detrimental effects only become relevant after long periods of operation.

Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 μm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 μg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance., Environmental Science & Technology, 56 (19), ISSN:0013-936X, ISSN:1520-5851

Globally, surface water quality and ecosystem functioning are challenged by anthropogenic P inputs. While sterner legislation has led to lower external P loading, internal loading fed by legacy P accumulated in the sediment has become the controlling factor of surface water P concentrations in many European freshwater systems. Fe amendment is a treatment method to control internal P loading, but is not always successful on the long term. In Lake Terra Nova, a polymictic shallow peat lake in the Netherlands, treatment with FeCl3 only led to a temporary decrease in sedimentary P release. Two years after treatment seasonal peaks in surface water P concentrations started to appear and have been increasing in intensity for the past 8 years. Depth-resolved solid phase analysis by sequential Fe and P extractions was combined with bulk X-ray absorption spectroscopy (XAS) at the Fe K-edge and high-resolution micro-X-ray fluorescence spectrometry (µ-XRF) and µ-XAS. At spots with distinctively high Fe contents, pyrite and silicate-bound Fe are identified by microscopic and spectroscopic analyses. The spectroscopic data, however, also point to a finely dispersed Fe species in the sediment matrix which most likely corresponds to Fe complexed by OM in the surface sediment. The correlation of the distribution of P and Fe suggests that P is bound to these Fe-OM complexes. This interpretation is supported by the sequential extraction results which showed that the Fe treatment induced a shift in the dominant P pool from Ca-bound P to Fe- and OM-bound P. Overall, the results indicate that FeCl3 application caused a change in sediment P dynamics towards a highly redox sensitive system in which P bound to Fe-OM is released to the surface water during seasonally low bottom water oxygen concentrations. The results of this study therefore indicate that FeCl3 may not be the ideal additive for the remediation of internal P loading in peaty water bodies due to the high affinity of Fe to OM.

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.