Analysis of the variable charge of two organic soils by means of the NICA‐Donnan model

Abstract

We have tested to see if the generic set of NICA-Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back-titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non-ideal, competitive-adsorption NICA-Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H þ affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, V D , varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log K i int ) andthe heterogeneity of the site (m i ), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by 13 C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.

Proton binding on untreated peat and acid-washed peat

“…Samples were collected from the upper horizon (0-100 cm) of an ombrotrophic peat located in Galicia (NW Spain), in the same geographic area where we have obtained samples for studies of other organic soils (Vasiliadis et al, 2007;Villaverde et al, 2009). The soil and site characteristics are shown in Table 1.…”

“…The method used for acid-base titrations is described elsewhere (Vasiliadis et al, 2007). Briefly, 50 mL of a solution of KNO 3 at the concentration required to control the ionic strength were added to 0.1 g of peat, in a thermostatted cell.…”

“…In some cases, untreated organic soil samples have been used to determine the proton binding parameters (Ge and Hendershot, 2004;Gustafsson and Kleja, 2005;Vasiliadis et al, 2007;Villaverde et al, 2009), whereas other authors have used samples free of inorganic cations and anions (except for H + ), obtained by acid-washing the organic soil (Cooke et al, 2007(Cooke et al, , 2008Smith et al, 2004). The observed differences between the untreated and acid-washed organic soil titration curves were attributed to the presence of cations other than H + , but no attempt was made to establish whether the parameters that describe the acidbase properties of the acid-washed organic soil could be used to describe the acid-base properties of the untreated organic soil.…”

Interactions Between Ionic Pesticides and Model Systems for Soil Fractions

“…Because of this, modelling of ion binding to humic substances requires a model that accounts for the heterogeneity of functional groups in terms of their affinity for protons, and the non-specific electrostatic interactions. The Donnan approach (Kinniburgh et al, 1999) has been used extensively and successfully in the last decade to describe ionic interactions with humic acids (Tipping, 2002;Companys et al, 2007;Vidali et al, 2009), and more recently with organic soils (Vasiliadis et al, 2007;Villaverde et al, 2009). It assumes that the surface charge, Q, is neutralized by counter-ions within a gel phase known as the Donnan volume, V D. This requires the use of the basic equation of electrical charge balance:…”

“…Some authors (Smith et al, 2001) have proposed a modification of the Donnan model for solid SOM consisting of the use of fixed Donnan volume that is not dependent on the ionic strength. In order to interpret the contribution of the electrostatic effect on the PQ-peat interaction, we considered a value of 0.1 for parameter b (equation 3), obtained in a study of proton binding in peat soils of similar origin (Vasiliadis et al, 2007). With the aim of comparing the affinity of both SOM samples for the cationic pesticide, and taking into account the difference in the amount adsorbed, the binding parameters were optimized, which enabled interpretation of the adsorption of PQ on the peat soil within a wide range of conditions (pH and ionic strength).…”