Interaction between humic substances and metallic ions: a selectivity study of humic substances and their possible therapeutic application (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).
Comparative study of binding strengths of heavy metals with humic acid
Chemical Industry, 2013
The complexation of humic acid with certain heavy metal ions (Co(II), Ni(II), Cu(II), Zn(II) and Pb(II)) was investigated. The stability constants of humate complexes were determined by method which is based on distribution of metal ions between solution and resin in the presence and the absence of ligand, known as Schubert?s ion exchange method. Experiments were performed at 25 ?C, at pH 4.0 and ionic strength of 0.01 mol dm-3. It was found that the 1:1 complexes were formed between metal ions and humic acid. Obtained results of the stability constants, log ?mn, of complexes formed between the metal ions and humic acid follow the order Co(II) < Ni(II) < Cu(II) > Zn(II) which is the same like in the Irving-Williams series for the binding strength of divalent metal ion complexes. Stability constant of complex between Pb(II) ions and humic acid is greater than stability constants of other investigated metal-humate complexes. The investigation of interaction between heavy meta...
Trace metal–humate interactions. I. Experimental determination of conditional stability constants
Applied Geochemistry, 2000
The enhancement of mobility of radionuclides in the geosphere through complexation by humic substances is a source of uncertainty in performance assessment of radioactive waste repositories. Only very few data sets are available which are relevant for performance assessment of an underground repository for radioactive waste. Using the equilibrium dialysis-ligand exchange method developed at the Paul Scherrer Institut, conditional stability constants for the formation of complexes of Aldrich humic acid with Ca 2+ , NpO 2 + , Co 2+ , Ni 2+ , UO 2 2+ and Eu 3+ and complexes of Laurentian soil-and Suwannee River fulvic acid with Co 2+ , UO 2 2+ and Eu 3+ were measured. pH was varied between 5 and 10 and ionic strength between 0.02 and 0.2 M. The data are presented as equilibrium coecients that are free from any model assumptions. The equilibrium coecients increased in the order Ca 2+ Q NpO 2 + < Co 2+ < Ni 2+ < UO 2 2+ < Eu 3+. The quality of the data is assessed in an extended discussion of statistical and systematical errors, and by a critical`rereview' of the auxiliary stability constants used for the calculation of the equilibrium coecients. An approximate overall uncertainty of 0.5 log-units is estimated for the stability data reported. The conditional stability constants were found to increase markedly with increasing pH in the case of Co 2+ , UO 2 2+ and Eu 3+. For Ni 2+ , Ca 2+ and NpO 2 + this eect was less pronounced. For all metal ions tested, the in¯uence of ionic strength was of less importance, and the conditional stability constants did not show a signi®cant dependence on the type of humic substances investigated.
Journal of Applied Biomedicine
Humic substances as part of humus-soil organic matter-are compounds arising from the physical, chemical and microbiological transformation (humification) of biomolecules. They are important because they constitute the most ubiquitous source of non-living organic material that nature knows. Approximately 80% of the total carbon in terrestrial media and 60% of the carbon dissolved in aquatic media are made up of humic substances. Humic substances have important roles in soil fertility, and are considered to have primal relevance for the stabilization of soil aggregates. They can be divided into three components according to their solubility: humic acids, fulvic acids and humin. Humic acids are the most explored group of humic substances. Beyond their relevance for life these substances have industrial applications in the development of absorbents to be used at the sources of metal-poisoning. Being natural substances, their purification process is cheaper than the synthesis of any other sorbent and, moreover, due to their high operability, they absorb more than the absorbents used to date, such as active charcoals or clays. The specific properties of humic acid products enable their application in industry, agriculture, environmental and biomedicine.
Modeling the Interactions between Humics, Ions, and Mineral Surfaces †
Environmental Science & Technology, 2006
Model VI and the NICA-Donnan model are two successful models presently available for describing metal ion binding by humic materials. Both models deal with the complexity of the underlying processes (intrinsic heterogeneity, partial correlation between affinity distributions, variable stoichiometry, electrostatics) in a pragmatic way. The parameters of the NICA-Donnan model and their determination are discussed. The current interpretation of the ion-specific "non-ideality" parameters, n i , found in the NICA-Donnan model, emphasizes their role in determining the stoichiometry for the competitive binding of ions. The ratio n M /n H is reflected in the corresponding exchange ratios and the pH dependence of the metal ion binding. Experimental complications occur in testing models. Although Model VI and the NICA-Donnan models have much in common, there are differences that may become more apparent as the models are more widely tested. Recently ion binding to complexes of humics and oxides has been described by combining the NICA-Donnan model (ion binding to humics) with the CD-MUSIC model (ion binding to oxides). The impact of humic to oxide binding on both cation and anion binding is briefly discussed.
Characterization of Humic Substances by Functional Groups and Spectroscopic Methods
International Journal of Current Microbiology and Applied Sciences
Soil organic matter fractions are capable of forming complexes with metal ions. The ability of humic substances to form stable complexes with metal ions can be accounted for their high content of oxygen containing functional groups viz., carboxylic, phenolic aliphatic and alcoholic-OH groups. As these complex formation reactions between metal
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.
Chemical Characteristics of Humic Substances in Nature
Humic Substance [Working Title], 2021
Humic substances are the main constituents of natural organic matter, found in both aquatic and terrestrial environments. Humic substances are a complex, dispersed, and heterogeneous mixture of various organic compounds synthesized from organic matter residues, decomposed by microorganisms. Most scientists indicate that humic substances are as a supramolecular association of small heterogeneous molecules stabilized by weak intermolecular bonds. When these substances are presented in water intended for drinking or industrial use, it can have a significant impact on the treatability of this water and on the success of chemical disinfection processes, due to possible formation of organic compounds harmful to human health. Moreover, the humic substances can be used of several ways such as fertilizer to help in the development of plants, to improve soil erosion and to removal of organic compounds and metals from soils and waters. In addition, humic substances suggest an important role in...