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.

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.

Selenium Uptake and Methylation by the Microalga Chlamydomonas reinhardtii.

Biogenic selenium (Se) emissions play a major role in the biogeochemical cycle of this essential micronutrient. Microalgae may be responsible for a large portion of these emissions via production of methylated Se compounds that volatilize into the atmosphere. However, the biochemical mechanisms underlying Se methylation in microalgae are poorly understood. Here, we study Se methylation by Chlamydomonas reinhardtii, a model freshwater alga, as a function of uptake and intracellular Se concentrations and present a biochemical model that quantitatively describes Se uptake and methylation. Both selenite and selenate, two major inorganic forms of Se, are readily internalized by C. reinhardtii, but selenite is accumulated around ten times more efficiently than selenate due to different membrane transporters. With either selenite or selenate as substrates, Se methylation was highly efficient (up to 89% of intracellular Se) and directly coupled to intracellular Se levels (R(2) > 0.92) over an intracellular concentration range exceeding an order of magnitude. At intracellular concentrations exceeding 10 mM, intracellular zerovalent Se was formed. The relationship between uptake, intracellular accumulation, and methylation was used by the biochemical model to successfully predict measured concentrations of methylated Se in natural waters. Therefore, biological Se methylation by microalgae could significantly contribute to environmental Se cycling.

Characterization of colloidal Fe from soils using field-flow fractionation and Fe K-edge X-ray absorption spectroscopy.

Colloids may facilitate the transport of trace elements and nutrients like phosphate in soil. In this study, we characterized soil colloids (<0.45 µm), extracted from four agricultural soils by Na-bicarbonate and Na-pyrophosphate, by two complementary analytical techniques; asymmetric flow field-flow fractionation (AF4) and X-ray absorption spectroscopy (XAS). The combined results from AF4 and XAS show that colloidal Fe is present as (i) free Fe-(hydr)oxide nanoparticles, (ii) Fe-(hydr)oxides associated with clay minerals, and (iii) Fe in clay minerals. Free Fe-(hydr)oxide nanoparticles, which can be as small as 2-5 nm, are extracted with Na-pyrophosphate but not with Na-bicarbonate, except for one soil. In contrast, Fe-(hydr)oxides associated with clay minerals are dispersed by both extractants. XAS results show that the speciation of Fe in the colloidal fractions closely resembles the speciation of Fe in the bulk soil, indicating that dispersion of colloidal Fe from the studied soils was rather unselective. In one Fe-rich soil, colloidal Fe was dominantly dispersed in the form of free Fe-(hydr)oxide nanoparticles. In the other three soils, dispersed Fe-(hydr)oxides were dominantly associated with clay minerals, suggesting that their dispersion as free nanoparticles was inhibited by strong attachment. However, in these soils, Fe-(hydr)oxides can be dispersed as oxide-clay associations and may as such facilitate the transport of trace elements.

Influence of arsenate adsorption to ferrihydrite, goethite, and boehmite on the kinetics of arsenate reduction by Shewanella putrefaciens strain CN-32.

The kinetics of As(V) reduction by Shewanella putrefaciens strain CN-32 was investigated in suspensions of 0.2, 2, or 20 g L(-1) ferrihydrite, goethite, or boehmite at low As (10 µM) and lactate (25 µM) concentrations. Experimental data were compared with model predictions based on independently determined sorption isotherms and rates of As(V) desorption, As(III) adsorption, and microbial reduction of dissolved As(V), respectively. The low lactate concentration was chosen to prevent significant Fe(III) reduction, but still allowing complete As(V) reduction. Reduction of dissolved As(V) followed first-order kinetics with a 3 h half-life of As(V). Addition of mineral sorbents resulted in pronounced decreases in reduction rates (32-1540 h As(V) half-life). The magnitude of this effect increased with increasing sorbent concentration and sorption capacity (goethite < boehmite < ferrihydrite). The model consistently underestimated the concentrations of dissolved As(V) and the rates of microbial As(V) reduction after addition of S. putrefaciens (∼5 × 10(9) cells mL(-1)), suggesting that attachment of S. putrefaciens cells to oxide mineral surfaces promoted As(V) desorption and thereby facilitated As(V) reduction. The interplay between As(V) sorption to mineral surfaces and bacterially induced desorption may thus be critical in controlling the kinetics of As reduction and release in reducing soils and sediments.

Catalyzed oxidation of arsenic(III) by hydrogen peroxide on the surface of ferrihydrite: an in situ ATR FTIR study.

Knowledge of arsenic redox kinetics is crucial for understanding the impact and fate of As in the environment and for optimizing As removal from drinking water. Rapid oxidation of As(III) adsorbed to ferrihydrite (FH) in the presence of hydrogen peroxide (H2O2) might be expected for two reasons. First, the adsorbed As(III) is assumed to be oxidized more readily than the undissociated species in solution. Second, catalyzed decomposition of H2O2 on the FH surface might also lead to As(III) oxidation. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy was used to monitor the oxidation of adsorbed As(III) on the FH surface in situ. No As(III) oxidation within minutes to hours was observed prior to H2O2 addition. Initial pseudo-first-order oxidation rate coefficients for adsorbed As(III), determined at H2O2 concentrations between 8.4 microM and 8.4 mM and pH values from 4 to 8, increased with the H2O2 concentration according to the equation log k(ox) (min(-1)) = 0.17 + 0.50 log [H2O] (mol/L), n = 21, r2 = 0.87. Only a weak pH dependence of log k(ox) was observed (approximately 0.04 logarithm unit increase per pH unit). ATR-FTIR experiments with As(III) adsorbed onto amorphous aluminum hydroxide showed that Fe was necessary to induce As(III) oxidation by catalytic H2O2 decomposition. Supplementary As(III) oxidation experiments in FH suspensions qualitatively confirmed the findings from the in situ ATR-FTIR experiments. Our results indicate that the catalyzed oxidation of As(III) by H2O2 on the surface of iron (hydr)oxides might be a relevant reaction pathway in environmental systems such as surface waters, as well as in engineered systems for As removal from water.