Recycled Iron Phosphates: A New Phosphorus Fertilizer for Paddy Rice.

The potential of recycled iron phosphates (FePs), e.g., vivianites (Fe3(PO4)2·8H2O) and Fe(III)-rich phosphorus (P) adsorbent materials, as phosphorus fertilizer is limited by the strong interaction between Fe and P. In this study, the efficiency of FePs as P fertilizer was explored by applying them as granules or powder in flooded strongly P-fixing soils (acid and calcareous), thereby taking advantage of increased P release induced by reductive dissolution of P-bearing Fe(III) minerals. First, no P diffusion from granular FeP fertilizers into flooded soils was detectable by the diffusive gradient in thin films (DGT) technique and microfocused X-ray fluorescence (µ-XRF) analysis of thin soil sections, in contrast to detectable P diffusion away from granules of soluble triple superphosphate (TSP) fertilizer. On the contrary, powdered FePs demonstrated an excellent increase in extractable P (1 mM CaCl2) in a 120-day incubation experiment in flooded soils. Second, a pot experiment was performed with rice (Oryza sativa) grown in flooded acid and calcareous soils. The fertilizer value of FePs was remarkable when dosed as powder, as it was even up to 3-fold higher than TSP in the acid soil and similar to TSP in the calcareous soil. The beneficial effect of FeP over TSP in the acid soil is attributed to the slow release of P from FePs, which allows to partly overcome P fixation. The promising results of FePs as P fertilizer applied as powders in flooded soils debunk the generally accepted idea that FePs are poor sources of P while demonstrating the importance of the timing of FeP fertilizer application.

Phosphorus adsorption on iron-coated sand under reducing conditions.

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.

Spatiotemporal Mineral Phase Evolution and Arsenic Retention in Microfluidic Models of Zerovalent Iron-Based Water Treatment.

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.

Thermodynamic controls on rates of iron oxide reduction by extracellular electron shuttles.

Anaerobic microbial respiration in suboxic and anoxic environments often involves particulate ferric iron (oxyhydr-)oxides as terminal electron acceptors. To ensure efficient respiration, a widespread strategy among iron-reducing microorganisms is the use of extracellular electron shuttles (EES) that transfer two electrons from the microbial cell to the iron oxide surface. Yet, a fundamental understanding of how EES-oxide redox thermodynamics affect rates of iron oxide reduction remains elusive. Attempts to rationalize these rates for different EES, solution pH, and iron oxides on the basis of the underlying reaction free energy of the two-electron transfer were unsuccessful. Here, we demonstrate that broadly varying reduction rates determined in this work for different iron oxides and EES at varying solution chemistry as well as previously published data can be reconciled when these rates are instead related to the free energy of the less exergonic (or even endergonic) first of the two electron transfers from the fully, two-electron reduced EES to ferric iron oxide. We show how free energy relationships aid in identifying controls on microbial iron oxide reduction by EES, thereby advancing a more fundamental understanding of anaerobic respiration using iron oxides.

Use of iron-coated sand for removing soluble phosphorus from drainage water.

Mitigation measures are needed for reducing chronic dissolved phosphorus (P) losses from agricultural soils with a legacy of excessive P inputs to surface waters. Since pipe drains are an important pathway for P transport from agricultural soils to surface waters in flat areas, removing P from drainage water can be an effective measure. During a 4.5 year-field experiment, we tested the performance of a pipe drain enveloped with Fe-coated sand for removing soluble P from drainage water. Iron-coated sand is a by-product of the drinking water industry and has a high ability to bind P. The P concentration in the effluent from the enveloped pipe drain remained at a very low level over the entire monitoring period, with a removal percentage amounting to 93% for total P. During the field experiment, the enveloped pipe drain was below the groundwater level for a prolonged time. Nevertheless, no reduction of Fe(III) in the Fe-coated sand occurred during the first two years, most likely due to preferential reduction of Mn oxides present in the coatings of the sand particles, as reflected in elevated effluent Mn concentrations. Thereafter, reductive dissolution of Fe oxides in the coatings caused a gradual increase in the Fe concentration in the enveloped pipe drain effluent over time. Concomitantly, the dissolved Mn concentration decreased, most probably due to the depletion in easily accessible Mn oxides in the Fe-coated sand. The Fe in the Fe-coated sand was identified as silicate-containing ferrihydrite (Fh). The submerged conditions of the enveloped pipe drain neither affected the stability of Fh in the Fe-coated sand nor the ability of this measure to capture P from drainage water. Enveloping pipe drains with Fe-coated sand is an effective method for reducing dissolved P inputs from agricultural soils to surface waters and holds great promise for implementation in practice.

Arsenic and Other Geogenic Contaminants in Groundwater - A Global Challenge.

Groundwater is a much safer and more dependable source of drinking water than surface water. However, natural (geogenic) hazardous elements can contaminate groundwater and lead to severe health problems in consumers. Arsenic concentrations exceeding the WHO drinking water guideline of 10 µg/L globally affect over 220 million people and can cause arsenicosis (skin lesions and cancers). Fluoride, while preventing caries at low concentrations, has detrimental effects when above the WHO drinking water guideline of 1.5 mg/L and puts several hundred million people at risk of dental and skeletal fluorosis. In this article, we report on the geochemistry and occurrence of arsenic and fluoride in groundwater and on the development of global and regional risk maps that help alert governments and water providers to take appropriate mitigation measures for the provision of safe drinking water. We then summarize research on the removal of arsenic and fluoride from drinking water, focusing on adapted technologies for water treatment. Finally, we discuss the applicability of various measures in a larger context and future challenges in reaching the goal of access to safe drinking water for all.

Decreases in Iron Oxide Reducibility during Microbial Reductive Dissolution and Transformation of Ferrihydrite.

Ferrous iron formed during microbial ferric iron reduction induces phase transformations of poorly crystalline into more crystalline and thermodynamically more stable iron (oxyhydr)oxides. Yet, characterizing the resulting decreases in the reactivity of the remaining oxide ferric iron toward reduction (i.e., its reducibility) has been challenging. Here, we used the reduction of six-line ferrihydrite by Shewanella oneidensis MR-1 as a model system to demonstrate that mediated electrochemical reduction (MER) allows directly following decreases in oxide ferric iron reducibility during the transformation of ferrihydrite into goethite and magnetite which we characterized by X-ray diffraction analysis and transmission electron microscopy imaging. Ferrihydrite was fully reducible in MER at both pHMER of 5.0 and 7.5. Decreases in iron oxide reducibility associated with ferrihydrite transformation into magnetite were accessible at both pHMER because the formed magnetite was not reducible under either of these conditions. Conversely, decreases in iron oxide reducibility associated with goethite formation were apparent only at the highest tested pHMER of 7.5 and thus the thermodynamically least favorable conditions for iron oxide reductive dissolution. The unique capability to adjust the thermodynamic boundary conditions in MER to the specific reducibilities of individual iron (oxyhydr)oxides makes this electrochemical approach broadly applicable for studying changes in iron oxide reducibility in heterogeneous environmental samples such as soils and sediments.

Electrochemical Analysis of Changes in Iron Oxide Reducibility during Abiotic Ferrihydrite Transformation into Goethite and Magnetite.

Electron transfer to ferric iron in (oxyhydr-)oxides (hereafter iron oxides) is a critical step in many processes that are central to the biogeochemical cycling of elements and to pollutant dynamics. Understanding these processes requires analytical approaches that allow for characterizing the reactivity of iron oxides toward reduction under controlled thermodynamic boundary conditions. Here, we used mediated electrochemical reduction (MER) to follow changes in iron oxide reduction extents and rates during abiotic ferrous iron-induced transformation of six-line ferrihydrite. Transformation experiments (10 mM ferrihydrite-FeIII) were conducted over a range of solution conditions (pHtrans = 6.50 to 7.50 at 5 mM Fe2+ and for pHtrans = 7.00 also at 1 mM Fe2+) that resulted in the transformation of ferrihydrite into thermodynamically more stable goethite or magnetite. The changes in iron oxide mineralogy during the transformations were quantified using X-ray diffraction analysis. MER measurements on iron oxide suspension aliquots collected during the transformations were performed over a range of pHMER at constant applied reduction potential. The extents and rates of iron oxide reduction in MER decreased with decreasing reaction driving force resulting from both increasing pHMER and increasing transformation of ferrihydrite into thermodynamically more stable iron oxides. We show that the decreases in iron oxide reduction extents and rates during ferrihydrite transformations can be linked to the concurrent changes in iron oxide mineralogy.

Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions.

Iron (oxyhydr-)oxide reduction has been extensively studied because of its importance in pollutant redox dynamics and biogeochemical processes. Yet, experimental studies linking oxide reduction kinetics to thermodynamics remain scarce. Here, we used mediated electrochemical reduction (MER) to directly quantify the extents and rates of ferrihydrite, goethite, and hematite reduction over a range of negative reaction free energies, ΔrG, that were obtained by systematically varying pH (5.0 to 8.0), applied reduction potentials (-0.53 to -0.17 V vs SHE), and Fe2+ concentrations (up to 40 µM). Ferrihydrite reduction was complete and fast at all tested ΔrG values, consistent with its comparatively low thermodynamic stability. Reduction of the thermodynamically more stable goethite and hematite changed from complete and fast to incomplete and slow as ΔrG values became less negative. Reductions at intermediate ΔrG values showed negative linear correlations between the natural logarithm of the reduction rate constants and ΔrG. These correlations imply that thermodynamics controlled goethite and hematite reduction rates. Beyond allowing to study iron oxide reduction under defined thermodynamic conditions, MER can also be used to capture changes in iron oxide reducibility during phase transformations, as shown for Fe2+-facilitated transformation of ferrihydrite to goethite.

Enhanced As(III) oxidation and removal by combined use of zero valent iron and hydrogen peroxide in aerated waters at neutral pH values.

The oxidation and removal of As(III) by commercially available micro-scale zero-valent iron (mZVI) was studied in aerated synthetic groundwater with initially 6.7 µM As(III) at neutral pH values. Batch experiments were performed to investigate the influence of ZVI and H2O2 concentrations on As(III) oxidation and removal. Oxidation and removal kinetics was significantly increased by increasing ZVI concentration or by adding H2O2 in micromolar concentrations slightly higher than that of initial As(III). Observed half-lifes for arsenic removal without added H2O2 were 81-17 min at ZVI concentrations of 0.15-2.5 g/L, respectively. X-ray absorption spectroscopy (XAS) confirmed that almost all As(III) was converted to As(V) after 2 h of reaction in the pH range 5-9. Addition of 9.6 µM H2O2 to 0.15 g/L ZVI suspensions diminished half-lifes for arsenic removal from 81 to 32 min and for As(III) oxidation from 77 to 8 min, i.e., by approximately a factor of 10. The increased rate of As(III) oxidation is attributable to enhanced formation of oxidants by the Fenton reaction with higher initial concentrations of H2O2. In practice, results of this study suggest that addition of small amounts (<1 mg/L) of H2O2 in various forms (e.g. stable and widely available Na-percarbonate) to water prior to treatment could significantly enhance As(III) oxidation and removal with ZVI.

Redox-controlled changes in cadmium solubility and solid-phase speciation in a paddy soil as affected by reducible sulfate and copper.

The solubility of Cd in contaminated paddy soils controls Cd uptake by rice, which is an important food safety issue. We investigated the solution and solid-phase dynamics of Cd in a paddy soil spiked with ∼20 mg kg(-1) Cd during 40 days of soil reduction followed by 28 days of soil reoxidation as a function of the amounts of sulfate available for microbial reduction and of Cu that competes with Cd for precipitation with biogenic sulfide. At an excess of sulfate over (Cd + Cu), dissolved Cd decreased during sulfate reduction and Cd was transformed into a poorly soluble phase identified as Cd-sulfide using Cd K-edge X-ray absorption spectroscopy (XAS). The extent of Cd-sulfide precipitation decreased with decreasing sulfate and increasing Cu contents, even if sulfate exceeded Cd. When both Cu and Cd exceeded sulfate, dissolved and mobilizable Cd remained elevated after 40 days of soil reduction. During soil reoxidation, Cd-sulfide was readily transformed back into more soluble species. Our data suggest that Cd-sulfide formation in flooded paddy soil may be limited when the amounts of Cd and other chalcophile metals significantly exceed reducible sulfate Therefore, in multimetal contaminated paddy soils with low sulfate contents, Cd may remain labile during soil flooding, which enhances the risk for Cd transfer into rice.

Arsenic removal with composite iron matrix filters in Bangladesh: a field and laboratory study.

The main arsenic mitigation measures in Bangladesh, well-switching and deep tube wells, have reduced As exposure, but water treatment is important where As-free water is not available. Zero-valent iron (ZVI) based SONO household filters, developed in Bangladesh, remove As by corrosion of locally available inexpensive surplus iron and sand filtration in two buckets. We investigated As removal in SONO filters in the field and laboratory, covering a range of typical groundwater concentrations (in mg/L) of As (0.14-0.96), Fe (0-17), P (0-4.4), Ca (45-162), and Mn (0-2.8). Depending on influent Fe(II) concentrations, 20-80% As was removed in the top sand layer, but As removal to safe levels occurred in the ZVI-layer of the first bucket. Residual As, Fe, and Mn were removed after re-aeration in the sand of the second bucket. New and over 8-year-old filters removed As to <50 µg/L and mostly to <10 µg/L and Mn to <0.2 mg/L. Vertical concentration profiles revealed formation of Fe(II) by corrosion of Fe(0) with O2 and incorporation of As into forming amorphous Fe phases in the composite iron matrix (CIM) of newer filters and predominantly magnetite in older filters. As mass balances indicated that users filtered less than reported volumes of water, pointing to the need for more educational efforts. All tested SONO filters provided safe drinking water without replacement for up to over 8 years of use.

Temperature dependence and coupling of iron and arsenic reduction and release during flooding of a contaminated soil.

Arsenic (As) in soils and sediments is commonly mobilized when anoxic conditions promote microbial iron (Fe) and As reduction. Recent laboratory studies and field observations have suggested a decoupling between Fe and As reduction and release, but the links between these processes are still not well understood. In microcosm experiments, we monitored the formation of Fe(II) and As(III) in the porewater and in the soil solid-phase during flooding of a contaminated floodplain soil at temperatures of 23, 14, and 5 degrees C. At all temperatures, flooding induced the development of anoxic conditions and caused increasing concentrations of dissolved Fe(II) and As(III). Decreasing the temperature from 23 to 14 and 5 degrees C strongly slowed down soil reduction and Fe and As release. Speciation of As in the soil solid-phase by X-ray absorption spectroscopy (XAS) and extraction of the Fe(II) that has formed by reductive Fe(III) (hydr)oxide dissolution revealed that less than 3.9% of all As(III) and less than 3.2% of all Fe(II) formed during 52 days of flooding at 23 degrees C were released into the porewater, although 91% of the initially ascorbate-extractable Fe and 66% of the total As were reduced. The amount of total As(III) formed during soil reduction was linearly correlated to the amount of total Fe(II) formed, indicating that the rate of As(V) reduction was controlled by the rate of microbial Fe(III) (hydr)oxide reduction.