The chemical speciation of iron (Fe) in oceans is influenced by ambient pH, dissolved oxygen, and the concentrations and strengths of the binding sites of dissolved organic matter (DOM). Here, we ...derived new nonideal competitive adsorption (NICA) constants for Fe(III) binding to marine DOM via pH-Fe titrations. We used the constants to calculate Fe(III) speciation and derive the apparent Fe(III) solubility (SFe(III)app) in the ambient water column across the Peruvian shelf and slope region. We define SFe(III)app as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM at a free Fe (Fe3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH)3(s)). A ca. twofold increase in SFe(III)app in the oxygen minimum zone (OMZ) compared to surface waters is predicted. The increase results from a one order of magnitude decrease in H+ concentration which impacts both Fe(III) hydroxide solubility and organic complexation. A correlation matrix suggests that changes in pH have a larger impact on SFe(III)app and Fe(III) speciation than DOM in this region. Using Fe(II) measurements, we calculated ambient DFe(III) and compared the value with the predicted SFe(III)app. The underlying distribution of ambient DFe(III) largely reflected the predicted SFe(III)app, indicating that decreased pH as a result of OMZ intensification and ocean acidification may increase SFe(III)app with potential impacts on surface DFe inventories.
Recently a dilute nitric acid extraction (0.43 M) was adopted by ISO (ISO-17586:2016) as standard for extraction of geochemically reactive elements in soil and soil like materials. Here we evaluate ...the performance of this extraction for a wide range of elements by mechanistic geochemical modeling. Model predictions indicate that the extraction recovers the reactive concentration quantitatively (>90%). However, at low ratios of element to reactive surfaces the extraction underestimates reactive Cu, Cr, As, and Mo, that is, elements with a particularly high affinity for organic matter or oxides. The 0.43 M HNO3 together with more dilute and concentrated acid extractions were evaluated by comparing model-predicted and measured dissolved concentrations in CaCl2 soil extracts, using the different extractions as alternative model-input. Mean errors of the predictions based on 0.43 M HNO3 are generally within a factor three, while Mo is underestimated and Co, Ni and Zn in soils with pH > 6 are overestimated, for which possible causes are discussed. Model predictions using 0.43 M HNO3 are superior to those using 0.1 M HNO3 or Aqua Regia that under- and overestimate the reactive element contents, respectively. Low concentrations of oxyanions in our data set and structural underestimation of their reactive concentrations warrant further investigation.
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.
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•P removal from drainage water by a pipe drain enveloped with Fe-coated sand was 93%.•Fe in the sand particle coatings was mainly present as siliceous ferrihydrite (Fh).•Reductive release of Fe(II) caused a gradual increase in effluent Fe over time.•Fe loss was insignificant compared to the amount of Fe wrapped around the drain.•Fh stability and its ability to retain P were not affected over time.
Rare earth elements (REE) naturally occur at trace levels in natural systems but, due to their increasing use in modern technologies, they are now released into the environment, and considered as ...emerging contaminants. Therefore, the development of numerical predictive models of their speciation in various physico-chemical conditions is required to predict their behavior, transport and potentially toxic effects on ecosystems. Because REE speciation is largely affected by natural organic matter, such as humic acids (HA), this study aimed at calibrating an advanced humic-ion binding model (Model VII) to allow predicting REE-HA binding in various pH conditions, ionic strength and REE/HA, as well as presence of competitor ions. First, REE complexation to monodentate O-containing ligands was evaluated using the Irving-Rossotti equation, which provided constraints for the optimization of REE-HA binding parameters for Model VII. Predictive capacities of Model VII were demonstrated by successfully modeling the effects of various cations (Al3+, Fe3+, Cu2+ and Ca2+) and carbonates on REE-HA binding. The large range of physico-chemical conditions for which Model VII is applicable suggest that the present model parameters might be used to more accurately predict the role played by NOM on REE speciation in very contrasting environments, such as in groundwaters, rivers, estuaries, seawater, soils or sediments. Therefore, this study provides a valuable numerical tool to predict the fate of REE in the environment.
Weak salt extracts can be used to assess the availability of trace metals for leaching and uptake by soil organisms and plants in soil. Before extraction, the International Organization for ...Standardization recommends to dry soils in an oven at a temperature of 40
°C. Effects of soil oven-drying on different fractions of dissolved organic matter (DOM) and the effect thereof on total concentrations and speciation of trace metals in weak salt extracts have, however, not been quantified yet. In this study, free metal concentrations and DOM speciation were determined in 2
mM Ca(NO
3)
2 extracts obtained from twelve field-contaminated soils in their field-moist state and after drying at 40
°C. Free metal concentrations were measured with the Donnan Membrane Technique. DOM was fractioned into humic acid (HA), fulvic acid (FA), and hydrophilic (Hy) compounds. Soil oven-drying led to significant increases in the concentrations of total DOM and total dissolved Cu and Ni. For the measurement of total dissolved Cu and Ni concentrations, it is, therefore, better to use field-moist soils. The release of Hy compounds was mainly responsible for the increase in DOM, which accounted for 64 to 77% of the increase in total dissolved organic carbon. Soil oven-drying left the free Cd
2+, Cr
3+, Cu
2+, Ni
2+, and Zn
2+ concentrations unchanged. Both field-moist and oven-dried soils can, therefore, be used for the measurement of free metal concentrations. Free Cd
2+, Cu
2+, Ni
2+, and Zn
2+ concentrations were predicted very well for both field-moist and oven-dried soils using ORCHESTRA, which includes the NICA-Donnan model. However, poor predictions were obtained for Cr
3+, for which better NICA parameters need to be derived.
► Soil oven-drying led to a significant increase in dissolved organic matter. ► Soil oven-drying significantly increased the total dissolved Cu and Ni concentrations. ► Measured free metal concentrations remained unchanged after soil oven-drying.
Phosphorus losses from agricultural soils is an important source of P in surface waters leading to surface water quality impairment. In addition to reducing P inputs, mitigation measures are needed ...to reduce P enrichment of surface waters. Because drainage of agricultural land by pipe drainage is an important pathway of P to surface waters, removing P from drainage water has a large potential to reduce P losses. In a field trial, we tested the performance of a pipe drain enveloped with Fe‐coated sand, a side product of the drinking water industry with a high ability to bind P, to remove P from the drainage water. The results of this trial, encompassing more than one hydrological season, are very encouraging because the efficiency of this mitigation measure to remove P amounted to 94%. During the trial, the pipe drains were below the groundwater level for a prolonged time. Nevertheless, no reduction of Fe(III) in the Fe‐coated sand occurred, which was most likely prevented by reduction of Mn oxides present in this material. The enveloped pipe drain was estimated to be able to lower the P concentration in the effluent to the desired water quality criterion for about 14 yr. Manganese oxides are expected to be depleted after 5 to 10 yr. The performance of the enveloped pipe drain, both in terms of its ability to remove P to a sufficiently low level and the stability of the Fe‐coated sand under submerged conditions in the long term, needs prolonged experimental research.
Here we evaluate the performance and limitations of two frequently used model-types to predict trace element solubility in soils: regression based “partition-relations” and thermodynamically based ...“multisurface models”, for a large set of elements. For this purpose partition-relations were derived for As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sb, Se, V, Zn. The multi-surface model included aqueous speciation, mineral equilibria, sorption to organic matter, Fe/Al-(hydr)oxides and clay. Both approaches were evaluated by their application to independent data for a wide variety of conditions. We conclude that Freundlich-based partition-relations are robust predictors for most cations and can be used for independent soils, but within the environmental conditions of the data used for their derivation. The multisurface model is shown to be able to successfully predict solution concentrations over a wide range of conditions. Predicted trends for oxy-anions agree well for both approaches but with larger (random) deviations than for cations.
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► Partition relations are robust predictors for metal cations between pH 3–8. ► Oxy-anions and DOC have a very similar pH dependent solubility. ► The DOC coefficient accounts for the pH dependence of oxy anion concentrations. ► Multisurface models can predict element solubility for a wide range of conditions.
Metal solubility can be predicted adequately both by empirical partition relations and process based multisurface models.
Use of Reactive Materials to Bind Phosphorus Chardon, Wim J.; Groenenberg, Jan E.; Temminghoff, Erwin J. M. ...
Journal of environmental quality,
20/May , Letnik:
41, Številka:
3
Journal Article
Recenzirano
Phosphorus (P) losses from agricultural soils have caused surface water quality impairment in many regions of the world, including The Netherlands. Due to the large amounts of P accumulated in Dutch ...soils, the generic fertilizer and manure policy will not be sufficient to reach in time the surface water quality standards of the European Water Framework Directive. Additional measures must be considered to further reduce P enrichment of surface waters. One option is to immobilize P in soils or manure or to trap P when it moves through the landscape by using reactive materials with a large capacity to retain P. We characterized and tested two byproducts of the process of purification of deep groundwater for drinking water that could be used as reactive materials: iron sludge and iron‐coated sand. Both materials contain low amounts of inorganic contaminants, which also have a low (bio)availability, and bound a large amount of P. We could describe sorption of P to the iron sludge in batch experiments well with the kinetic Freundlich equation (Q = a × tm × Cn). Kinetics had a large influence on P sorption in batch and column experiments and should be taken into account when iron‐containing materials are tested for their capability to immobilize or trap P. A negative aspect of the iron sludge is its low hydraulic conductivity; even when mixed with pure sand to a mixture containing 20% sludge, the conductivity was very low, and only 10% sludge may be needed before application is possible in filters or barriers for removing P from groundwater. Due to its much higher hydraulic conductivity, iron‐coated sand has greater potential for use under field conditions. Immobilizing P could be an option for using iron sludge as a reactive material.
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