Effect of humic matter on metal adsorption onto clay materials: Testing the linear additive model (original) (raw)
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Effect of humic matter on metal adsorption onto clay materials
GeCAS, 2008
Migration of contaminants with low affinity for the aqueous phase is essentially governed by interaction with mobile carriers such as humic colloids. Their impact is, however, not sufficiently described by interaction constants alone since the humic carriers themselves are subject to a solidliquid distribution that depends on geochemical parameters. In our study, co-adsorption of the REE terbium (as an analogue of trivalent actinides) and humic acid onto three clay materials (illite, montmorillonite, Opalinus clay) was investigated as a function of pH. 160 Tb(III) and 131 I-labelled humic acid were employed as radiotracers, allowing experiments at very low concentrations to mimic probable conditions in the far-field of a nuclear waste repository. Humate complexation of Tb was examined by anion and cation exchange techniques, also considering competitive effects of metals leached from the clay materials. The results revealed that desorption of metals from clay barriers, occurring in consequence of acidification processes, is generally counteracted in the presence of humic matter. For all clay materials under study, adsorption of Tb was found to be enhanced in neutral and acidic systems with humic acid, which is explained by additional adsorption of humic-bound Tb. A commonly used composite approach (linear additive model) was tested for suitability in reconstructing the solid-liquid distribution of Tb in ternary systems (Tb/humic acid/clay) on the basis of data determined for binary subsystems. The model can qualitatively explain the influence of humic acid as a function of pH, but it failed to reproduce our experimental data quantitatively. It appears that the elementary processes (metal adsorption, metal-humate complexation, humic acid adsorption) cannot be considered to be independent of each other. Possible reasons are discussed.
Environmental Science & Technology, 1999
The adsorption of cadmium ions to a mixture of Aldrich humic acid and hematite is investigated. The actual adsorption to the humic acid-hematite complex is compared with the sum of the cadmium ion adsorptivities to each of the isolated components. It is shown that the sum of the cadmium ion adsorptivities is not equal to the adsorption to the complex. In general, the adsorption of a specific metal ion to the complex can be understood and qualitatively predicted using the adsorptivities to each of the pure components and taking into account the effect of the pH on the interaction between humic acid and iron oxide on the metal ion adsorption. Due to the interaction between the negatively charged humic acid and the positively charged iron oxide, the adsorption of metal ions on the mineral oxide in the complex will increase as compared to that on the isolated oxide, whereas the adsorption to the humic acid will decrease as compared to that on the isolated humic acid. As a result, the overall adsorption of a specific metal ion to the complex will be smaller than predicted by the additivity rule when this metal ion has a more pronounced affinity for the humic acid than for the mineral oxide, whereas it will be larger than predicted by the additivity rule when the metal ion has a higher affinity for the oxide than for the humic acid.
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).
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
Geochimica et Cosmochimica Acta, 2014
The mobility of contaminants in the subsurface hydrosphere can be governed by their interaction with aquatic humic substances, which may act as carriers. For modelling migration processes, retardation of humic molecules at mineral surfaces must be considered. There is, however, a lack of clarity concerning the reversibility of adsorption of these natural polyelectrolytes. In this work, evidence was provided that a dynamic adsorption equilibrium exists. For this purpose, adsorption of humic substances (purified Aldrich humic acid and an aquatic fulvic acid) onto kaolinite was examined in tracer exchange studies by means of 14 C-labelled humic material. In addition, the kinetics of adsorption and desorption were investigated in batch experiments. Attaining the equilibrium state of adsorption took considerably longer for the humic acid than for the fulvic acid (24 h and 4 h, respectively). In desorption experiments, initiated by diluting the supernatant at constant pH, no net release was observed for both substances within a time frame of 4 weeks. However, when introducing radiolabelled humic or fulvic acid as a tracer into pre-equilibrated adsorption systems in the state of surface saturation, quantitative exchange was found to take place. This indicates that adsorption of humic matter is in fact a reversible process, albeit an exchange time of more than 4 weeks was required for both humic materials. Models for humic-bound contaminant transport (presuming dynamic equilibria) are thus applicable under appropriate conditions.
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.
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.
Geochimica et Cosmochimica Acta, 2013
A number of geodisposal concepts for intermediate level radioactive waste involve 15 geological emplacement within cementitious repositories. Such facilities, once rehydrated 16 with groundwater, will create high pH environments due to aqueous phase reaction of the 17 cements. This work focuses on the interactions of several important long-lived radionuclide 18 cations with dissolved organic matter (DOM) constituents (humic and fulvic acids) under high 19 pH conditions. We also sought to test the comprehensive speciation model WHAM/Humic 20 Ion Binding Model VII for these specific conditions. Results for Th demonstrate high fractions 21 present as organic complexes at all pH values. Binding of neptunyl to DOM shows a 22 maximum over the pH range expected within an evolving repository. Uranyl exhibits 23 decreasing binding with pH, however, the majority of metal in solution is present as organic 24 complexes under the lower pH conditions investigated (10-10.5). We have updated the 25 WHAM/Model VII binding values for UO 2 2+ , and have for the first time added NpO 2 + values to 26 the database. These updates now allow application of the model for more complex mixtures 27 across the entire repository pH range. Calculations for three simulated cement interstitial 28 waters (representing different degradation phases) suggest U(VI) and Np(V) are not likely to 29 be significantly bound to DOM under these conditions. 30
The role of metal complexation in the solubility and stability of humic acid
Materials Science and Engineering: C, 1996
Humic acid (HA) obtained from a variety of sources displays somewhat varied chemical composition and residual metal content. In spite of extensive purifcation procedures as much as 0.5% metal (by weight) can remain in the humic acid. In some cases, particularly for Cu 2+, the metal binding appears site specific, while in others the cupric ion is less strongly bound. Furthermore, the iron content of the samples appears to be dependent on the source and purification procedure. Quantitative removal or exchange of the residual metal is expected to alter the stability and solubility properties of the HA. The treatment of highly purfied HA with Chelex® resin has afforded a material in which the amount of the so-called residual metal has been reduced by 40% or more. This reduction in metal content appears to affect solution aggregation phenomena and consequently the rate of flocculation. Metal removal depends on pre-treatment conditions of the HA sample and is highly pH and temperature dependent. These observations suggest that chemical transformations of HA at high pH and moderate temperature may affect the equilibrium state between the bound metal in HA and the metal-Chelex® complex. The apparently selective removal of metal suggests a. specific role of the metal.
Humic acid-divalent cation interactions
Thermochimica Acta, 2003
The adsorption behavior of divalent cations M 2+ (Cu, Ni, Co and Zn) with commercial humic acid (HAAl) and also with an extracted fraction of peat soil (HAPs) was followed in aqueous solution. The series of adsorption isotherms were fitted to a modified Langmuir equation. The maximum number of moles adsorbed gave: 0.55 ± 0.02, 0.66 ± 0.02, 0.54 ± 0.02, 0.40 ± 0.02 mmol per gram for HAAl and 0.63 ± 0.03, 0.61 ± 0.06, 0.55 ± 0.02, 0.54 ± 0.03 mmol/g for solid HAPs, for copper, nickel, zinc and cobalt, respectively. The same interaction followed calorimetrically gave endothermic values: 2.4 ± 1.0, 8.4 ± 0.9, 18.3 ± 0.9, 10.6 ± 0.9 kJ mol −1 and 18.4 ± 1.2, 15.9 ± 1.4, 15.4 ± 1.2, 15.0 ± 1.2 kJ mol −1 for HAAl and HAPs, respectively, for the same sequence. Because all Gibbs free energies were negative. Complexation must be accompanied by an increase in entropy.