Ion binding to natural organic matter: General considerations and the NICA–Donnan model (original) (raw)
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A unifying model of cation binding by humic substances
Geochimica et Cosmochimica Acta, 1992
Model V describes the binding of ions by humic substances in terms of ~omplexation at discrete sites, modified by electrostatic attraction and/or repulsion, and also takes account of nonspecific binding due to counte~on accumulation. The model operates over wide ranges of pH (3-11) and ionic strength (0.00 1-1 M). Electrostatic effects on specific binding are described with an empirical relationship involving net humic charge and an electrostatic interaction factor. Accumulation of counterions is described by Donnan-type expressions. The model assumes the presence of eight proton-dissociating groups in the humic material, distinguished by intrinsic pK values. In general, the description of proton dissociation requires seven parameters, but for fulvic-type material only six are needed. The proton-dissociating groups may interact individually with other ions, or pairs of them may form bidentate sites. Binding at the monodentate and bidentate sites is characterized by intrinsic equilibrium constants for cation-proton exchange; there are two such constants (pK MHA andp&na) for each cation. Model parameters are derived from published data for fulvic-type material on proton dissociation (eight data sets) and metal binding (twenty-six data sets, eleven metals). In the case of proton dissociation, the greatest variability among samples is in site densities, while intrinsic dissociation constants and electrostatic interaction factors are relatively consistent. With parameters for proton di~~iation fixed, adjustments of pKMHn and p&or, permit reasonable fitting of metal binding data, including pH dependence. There are insu~cient available data to evaluate properly ionic strength and competition effects on metal binding, but correct trends are reproduced by the model. Values of pK MHA for metals indicate that binding strength increases in the order Mg*+ < Cazf < Mn2' < Cd*+ c Co*+ < Ni2+-Zn2+ < Pb2+ < Cu2+ < V02+. The strong. correlation between pKMHA and the analogous constant for lactic acid may be useful for estimating values of pKMhlA in cases where humic binding data are lacking. 'KMHAKMHB) ' (KIKz) 'GHANA 1 / (KlK41 'BLOB' ' (KlK6' '%HA%s~B) ' @iK8) '%HA%HB) ' (K2K3f %HA%HB) ' (K2K5) (%HA%HB) ' (K2K7) (%HA%HB) ' (K3K4) 'KMHAKMHB) ' (K3K6) (%HA%HB) ' (K3K8) (~~~HB~ ' (K4K5) '~HA~B) ' ('qK7) In the present study, values of d,,,, and dpir were taken to be 0.3 nm and 0.45 nm, respectively, and r was taken to be 0.8 nm (see MODEL DESCRIPTION).
Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model
Environmental Science & Technology, 1996
A Donnan-type model for nonspecific binding of electrolyte ions has been combined with the nonideal competitive adsorption (NICA) model for specific binding to produce a model for ion binding to humic substances. The model considers site heterogeneity, non-ideality, multicomponent competition, and electrostatic interactions. The NICA-Donnan model was fitted to data for H, Ca, Cd, Cu, and Pb binding by a purified peat humic acid. The model fits were good and covered a wide range of pH and free metal concentrations. The parameters from these single metal data sets were then used to predict the competitive effect of Ca on Cd and Cu binding at various pHs. These predictions agreed well with the experimental data although there were some small but systematic differences. The new NICA-Donnan model also predicted reasonably well the increase in Cd and Cu binding on changing from a 0.1 M KNO 3 background electrolyte to 0.01 M KNO 3 . A shortcoming of the model is that in some cases it significantly underestimated the H + /M 2+ exchange ratio, especially at high pH and for Cu binding.
Humic Ion-Binding Model VII: a revised parameterisation of cation-binding by humic substances
Environmental Chemistry, 2011
Environmental contextNatural organic matter exerts a powerful control on chemical conditions in waters and soils, affecting pH and influencing the biological availability, transport and retention of metals. To quantify the reactions, we collated a wealth of laboratory data covering 40 metals and acid–base reactions, and used them to parameterise the latest in a series of Humic Ion-Binding Models. Model VII is now available to interpret field data, and contribute to the prediction of environmental chemistry. Humic Ion-Binding Model VII aims to predict the competitive reactions of protons and metals with natural organic matter in soils and waters, based on laboratory results with isolated humic and fulvic acids (HA and FA). Model VII is simpler in its postulated multidentate metal binding sites than the previous Model VI. Three model parameters were eliminated by using a formal relationship between monodentate binding to strong- and weak-acid oxygen-containing ligands, and removing fa...
Humic matter and contaminants. General aspects and modeling metal ion binding
Pure and Applied Chemistry, 2000
Humic substances are soil and fresh-water components that play an important role in the binding and transport of both organic and inorganic contaminants. Transport of the contaminants due to ground-and fresh-water dynamics is directly related to the risks associated with contaminations. The mobility of soluble humic substances is related to their interaction with soil mineral particles. Some key references for the binding of organic and inorganic contaminants and for the binding of humics to mineral particles are presented. Humic substances also play a role in the analysis of the contaminants in natural waters and with remediation of water or soil polluted with pesticides, heavy metal ions, and radionuclides. These aspects are illustrated with some examples. The problems that are encountered with the modeling of the binding of contaminants to humics and of heavy metal ions in particular are illustrated by considering the nonideal competitive adsorption model (NICA) extended with electrostatic interactions. The NICA-Donnan model gives quite good results for the description of metal ion binding, as is illustrated for metal ion binding to purified peat humic acid (PPHA). Finally, some remarks are made with respect to the use of the NICA-Donnan model in general purpose speciation programs and of simplified versions of the model for predictions under restricted environmental conditions.
Analytica Chimica Acta, 1990
are characterized by a vanabIe ekctnc potential and by a vanety of bmdmg sites Ieadmg to chenucal heterogeneity Bmdmg of ions to these substances 1s Influenced by both factors A methodology based on acid-base tltratlons at several salt levels IS presented that allows for the assessment of an appropriate electrostatic double-layer model and the mtrmslc proton affuuty dlstnbutlon The double-layer model 1s used for the conversIon of pH to pHs for each data pomt, where Hs 1s the proton concentration m the diffuse Iayer near the bmdmg site. It 1s shown that with an appropnate double-layer model the proton bmdmg curves at different salt levels converge mto one '*master curve" when pIotted as a function of pHs. The mtnnslc proton affmlty dlstnbutlon can then be denved from the "master curve" using the LOGA method. A ngorous analysis of metal bmdmg to hurmc substances is complex and m practice 1s not feasible. Under two different (slmphfymg) assumptions, namely fuIIy coupled and uncoupled bmdmg, It 1s shown how mtnnslc metal ion affuuty dlstnbutlons can be obtamed Mode1 calcutatlons show that apparent metal ion affuuty dlstnbutlons do not resemble the mtnnslc metal Ion affinity dlstnbutlon.
Computers & Geosciences, 1994
WHAM (Windermere Humic Aqueous Model) is designed to calculate equilibrium chemical speciation in surface and ground waters, sediments, and soils. The model is suitable especially for problems where the chemical speciation is dominated by organic matter (humic substances). WHAM combines Humic Ion-Binding Model V with a simple inorganic speciation code for aqueous solutions. Precipitation of aluminum and iron oxides, cation-exchange on an idealized clay mineral, and adsorption-desorption reactions of fulvic acid also are taken into account. The importance of ion accumulation in the diffuse layers surrounding the humic molecules is emphasized. Model calculations are performed with a BASIC computer code running on a Personal Computer.
Journal of Colloid and Interface Science, 2001
To describe the binding of protons and metals to humic substances, the Stockholm Humic Model (SHM) has been developed. The model employs a discrete-site approach similar to that of Model V/VI, although it has another electrostatic submodel, based on the Basic Stern concept. An empirical set of equations has been introduced to account for the extra screening of charge inside the gel-like structures of the humic substances. Six adjustable parameters are needed to describe proton binding to humic or fulvic acids. To simulate metal binding, equilibrium constants are defined for mono-and bidentate coordination and an extra parameter, LK 2 , accounts for binding-site heterogeneity. It is shown that the SHM appears to be able to describe proton binding well; the quality of the fits is similar to those obtained by Model V/VI and the NICA-Donnan model. The SHM was capable of correctly describing metal binding and competitive interactions over a wide range of conditions, although the model performance was not very convincing concerning the ionic strength dependence of metal binding and stepwise proton-metal exchange stoichiometries. C 2001 Academic Press
Cation binding by acid-washed peat, interpreted with Humic Ion-Binding Model VI-FD
European Journal of Soil Science, 2004
A sample of ombrotrophic peat from Moor House in northern England was extensively extracted with dilute nitric acid (pH 1) to free it of bound cations. Suspensions of the acid-washed peat (5-30 g l À1 ), prepared with different concentrations of background electrolyte (NaCl and KCl), were used to conduct batch acid-base titrations. A strong dependence of proton release on ionic strength (I) was observed, the apparent acid dissociation constant (pK app ) being found to decrease by approximately 1.0 for each tenfold increase in I. This behaviour could not be explained satisfactorily with Humic Ion-Binding Model VI, a discrete-site/electrostatic model of cation binding by humic substances, parameterized with data from laboratory studies on isolated samples. More success was obtained by abandoning the impermeable-sphere electrostatic submodel used in Model VI, and instead assuming the peat to consist of aggregates with fixed internal volume, and with counterion accumulation described by the Donnan model, as proposed by Marinsky and colleagues. The fixed-volume Donnan model (Model VI-FD) could also approximately explain other reported results from acid-base titrations of peat, including the effects on the titrations of complexing cations (Al, Ca, Cu). Copper titrations of the Moor House sample were performed using an ion-selective electrode, with peat suspensions in the acid pH range, at two ionic strengths, and in the presence of Al and Ca. The measured concentrations of Cu 2þ were in the range 10 À13 À10 À5 M. Model VI-FD provided reasonable fits of the experimental data, after optimization of the intrinsic binding constant for Cu, the optimized value being close to the default value derived previously from data referring to isolated humic substances. The optimized constants for Al and Ca, derived from their competition effects, were also close to their default values. Additional experiments were performed in which the centrifugation-depletion method was used to measure the binding of a cocktail of metals (Al, Ni, Cu, Zn, Cd, Eu, Pb) at a single pH. The model correctly predicted strong binding of Al, Cu, Eu and Pb, and weaker binding of Ni, Zn and Cd. For the strongly binding metals, the dissolved forms were calculated to be mainly due to complexes with dissolved humic matter, whereas the free ions (Ni 2þ , Zn 2þ , Cd 2þ ) dominated for the weakly binding metals. Acid-washed soil appears to provide a valuable intermediate between isolated humic substances and untreated soil for the investigation of cation binding by natural organic matter in the natural environment.