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Strong relationships between Extractable Organic N (EON) and N mineralization suggest that they can be used to assess the N mineralization potential of soils. EON is often used to proxy Dissolved Organic Nitrogen (DON), which is assumed to play an intermediate role in N mineralization. We tested whether EON relates to N mineralization and how this relation is affected by the extraction methodology used. We synthesized results from 171 papers on correlation coefficients between EON and N mineralization using meta-analysis. EON was positively related to N mineralization, and its size explained 14-65% of the variation in N mineralization. Best results were obtained with hot CaCl2 and acid KMnO4 as extractants. EON extracted with alkali solutions explained less of the variation in N mineralization than the other soil tests. There was no direct relationship between the intensity of extraction and the performance of EON as predictor for N mineralization. When EON levels are used to make fertilizer-N recommendations, the extraction methodology used to develop the above described relationship is closely followed.

Determination of the isotopic signature of dissolved organic nitrogen (DON) is important to assess its dynamics in terrestrial ecosystems. Analysis of [sup.15]N-DON, however, has been hindered by the lack of simple, reliable, and established methods. We evaluate three off-line techniques for measuring the 15N signature of DON in the presence of inorganic N using a persulfate digestion followed by microdiffusion. The [sup.15]N-DON signature is calculated from the difference between total dissolved [sup.15]N ([sup.15]N-TDN) and inorganic [sup.15]N. We quantified the [sup.15]N recovery and signature of DON, N[H.sub.4.sup.+], and N[O.sub.3.sup.-] in a series of inorganic N/DON mixtures (with a TDN concentration of 10 mg N [L.sup.-1]) for three lab protocols. Phenylalanine was used as a model compound for DON. The best lab protocol determined the concentration of inorganic N and TDN prior to diffusion using improved spectrophotometric techniques. An accuracy of 88% for 15N-DON should be routinely possible; coefficient of variation was < 2.9%. Hence, reliable [sup.15]N-DON values are obtained over an DON concentration range of 2.3-10 mg [L.sup.-1]. High levels of DON could influence the accuracy of [sup.15]N-N[O.sub.3.sup.-] mainly at DON:N[O.sub.3.sup.-] ratios above 0.4. Evaluation of alternative N[O.sub.3] measurements is still necessary. Our method is applicable for soil solution samples and soil extracts and has no risk of cross-contamination. Potential applications are large, in particular for [sup.15]N tracer studies, and will increase our insight in DON behavior in soils. 10.1021/ac1018183

Donnan membrane technique (DMT) is developed and tested for determination of free anion concentrations. Time needed to reach the Donnan membrane equilibrium depends on type of ions and the background. The Donnan membrane equilibrium is reached in 1 day for [Cl.sup.-], 1-2 days for N[O.sub.3.sup.-], 1-4 days for S[O.sub.4.sup.2] and Se[O.sub.4.sup.2-], and 1-14 days for [H.sub.2]P[O.sub.4.sup.-] in a background of 2-200 mM KCl or [K.sub.2]S[O.sub.4]. The strongest effect of ionic strength on equilibrium time is found for [H.sub.2]P[O.sub.4], followed by S[O.sub.4.sup.2-] and Se[O.sub.4.sup.2-], and then by [Cl.sup.-] and N[O.sub.3.sup.-]. The negatively charged organic particles of fulvic and humic acids do not pass the membrane. Two approaches for the measurement of different anion species of the same element, such as Se[O.sub.4.sup.-2] and HSe[O.sub.3.sup.-], using DMT are proposed and tested. These two approaches are based on transport kinetics or response to ionic strength difference. A transport model that was developed previously for cation DMT is applied in this work to analyze the rate-limiting step in the anion DMT. In the absence of mobile/labile complexes, transport tends to be controlled by diffusion in solution at a low ionic strength, whereas at a higher ionic strength, diffusion in the membrane starts to control the transport. 10.1021/ac9029339

Among speciation techniques that are able to measure free metal ion concentrations, the Donnan membrane technique (DMT) has the advantage that it can measure many different free metal ion concentrations simultaneously in a multicomponent sample. Even though the DMT has been applied to several systems, like surface waters, soil solutions, and manure slurry, basic features and calibrations with model calculations of the laboratory and field DMT have not been done sufficiently yet. Therefore, we tested the application of the DMT on metal complexation with several synthetic and natural ligands and the applicability of the dynamic mode of the DMT. The results show that there is a high agreement between the calculated and measured free metal ion concentrations in solutions containing synthetic (nitriloacetic acid, diglycolic acid) and natural organic ligands (fulvic acid, humic acid) at various pH values. Both the laboratory DMT and the field DMT give very similar results. In a solution containing labile ligands, equilibrium time is smaller than in a donor solution containing inert ligands or no ligands. Moreover, when labile ligands are present in the donor solution, a dynamic procedure can be used to decrease equilibrium time. This procedure cannot be applied when no ligands or only inert ligands are present.

Addition of ion complexation ligands in the acceptor solution in the Donnan membrane technique (DMT) can lower its detection limit for free metal ion concentration in natural samples. In this paper, the influence of added ligands on the transport behavior of trace ions in DMT was studied using numerical and analytical models and experimental tests. The results show that addition of ligands in the acceptor can significantly influence the time to reach the Donnan membrane equilibrium. Depending on several factors, the flux can be controlled by the diffusion in the stagnant solution film at the solution-membrane interface, by the diffusion in the membrane, or by both. The conditions under which the diffusion in the solution film or in the membrane becomes the rate-limiting step are discussed and approximate analytical solutions for some special cases are presented. Very low concentrations of free metal ion can be measured using the ligand complexation DMT. Depending on the degree of complexation in the sample, the measurement can be based on either the Donnan membrane equilibrium (when the complexation degree is low) or the kinetic interpretation of the ion transport (when the complexation degree is high).

Free metal ions in aqueous and terrestrial systems strongly influence bioavailability and toxicity. Most analytical techniques determine the total metal concentration, including the metal ions bound by dissolved organic matter. Ion activity can be measured with ion-specific electrodes (ISEs) for some metals, but an electrode for Zn is not commercially available. As a result, very few data are available on Zn binding by natural dissolved organic matter. The aim of this study is to determine free Zn concentrations in purified humic acid solutions using the recently developed Donnan membrane technique. However, several analytical aspects of the Donnan membrane technique had to be clarified before reliable data could be composed. Cd was chosen for validation. This study shows that free Cd concentrations as measured by the Donnan membrane technique agreed well with Cd ISE measurements. It is also shown that the Donnan membrane technique could be used at high pH. The Donnan membrane technique provided consistent results in a range of p[[Cd.sup.2+]] = 3-9 and p[[Zn.sup.2+]] = 3-8 at pH 4, 6, and 8. Metal speciation in humic acid solutions was also calculated with the consistent NICA-Donnan model using generic parameters. The model could excellently describe the experimental data without adjusting any of the parameters ([R.sup.2.sub.Cd] = 0.971, [R.sup.2.sub.Zn] = 0.988).

The mobility of copper in copper-contaminated soils is sensitive to pH and calcium concentration in a medium where dissolved organic material is present. The experiment was conducted to enable the modeling of copper mobility under the non-ideal competitive adsorption (NICA) model. This would enable researchers to predict copper concentrations using soil copper content, pH and solid/dissolved organic materials concentration as parameters.

The complexation of heavy metals with dissolved organic matter (DOM) in the environment influences the solubility and mobility of these metals. In this paper, we measured the complexation of Cu, Cd, Zn, Ni, and Pb with DOM in the soil solution at pH 3.7-6.1 using a Donnan membrane technique. The results show that the DOM-complexed species is generally more significant for Cu and Pb than for Cd, Zn, and Ni. The ability of two advanced models for ion binding to humic substances, e.g., model VI and NICADonnan, in the simulation of metal binding to natural DOM was assessed by comparing the model predictions with the measurements. Using the default parameters of fulvic and humic acid, the predicted concentrations of free metal ions from the solution speciation calculation using the two models are mostly within 1 order of magnitude difference from the measured concentrations, except for Ni and Pb in a few samples. Furthermore, the solid-solution partitioning of the metals was simulated using a multisurface model, in which metal binding to soil organic matter, dissolved organic matter, clay, and iron hydroxides was accounted for using adsorption and cation exchange models (NICA-Donnan, Donnan, DDL, CD-MUSIC). The model estimation of the dissolved concentration of the metals is mostly within 1 order of magnitude difference from those measured except for Ni in some samples and Pb. The solubility of the metals depends mainly on the metal loading over soil sorbents, pH, and the concentration of inorganic ligands and DOM in the soil solution. [ABSTRACT FROM AUTHOR]

Investigates the contribution of sorption surfaces to the control of heavy metals activity in sandy soil. Description of soil; Importance of soil organic matter; Significance of acidification in solution concentration.