Interaction between europium ions and selected size fractions of humic acids (original) (raw)
Related papers
Radiochimica Acta, 1985
The complexation behavior of humic acid towards Eu3+ ions in 0.1 M ionic strength (NaClO4) aqueous solution has been investigated in the pH range 5.0 to 6.0 by an ultrafiltration technique. Two classes of coordination sites (log K1 = 7 .9 +/- 0.2 l mol-1, log K2 = 6.0 t 0.1 I mol-1) are found. The overall complexation capacity increases from 0.78 to 1.14 mmol g-t while the fraction of this capacity due to the strongly complexing sites goes from 33% at pH 5.0 to 82% at pH 6.0. The complexation strength of both classes of sites does not vary with pH. Radioecological implications to the environmental behavior of trivalent lanthanides and actinides are briefly discussed.
A comparative study of proton and alkaline earth metal binding by humic substances
Analytica Chimica Acta, 1994
Humic substances (HS) were extracted from the soil and water of an upland catchment and characterised with respect to molecular weight, C, H, 0, N and S contents and ash content. Three samples were obtained: peat htmric acid, aquatic humic acid and aquatic fulvic acid. The weight average molecular weights were found to increase from 2400 for aquatic fulvic acid, to 6300 for aquatic humic acid and to 16500 for peat humic acid. Acid-base titrations were performed to assess the proton binding behaviour at different ionic strengths in NaCl and in the presence of important cation competitors (calcium and magnesium). The results from the titrations were fitted to a recently developed discrete-site/electrostatic model of cation-humic binding. Model parameters showed systematic trends with molecular weight and good agreement with parameters derived from the analysis of literature data. In the order peat humic, aquatic humic and aquatic fulvic acid, the milli-equivalents of carboxylic acid groups per gram of HS (n,) increased. In the same order both the negative log,, of the mid-range intrinsic proton dissociation constant for the strong acid groups (pK,) and the empirical constant, P, which accounts for electrostatic effects, decreased.
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.
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...
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).
Analytical …, 2003
The effect of metal ions (Co, Cu, Ni, Pb, Zn) on the molecular weight distribution of humic substances (HSs) obtained from compost is studied. We believe this is the first of this type of study applied in this way to humic substances. Size exclusion chromatography is coupled with two on-line detection systems (spectrophotometric and ICPMS) to study the binding of metal ions by humic substances leached from compost. ICPMS provided highly specific, sensitive, and multielement analytical information that enabled obtaining direct experimental evidence for the participation of metal ions in molecular size distributions of humic compounds. The compost extract or its high molecular weight fraction (>5000) was put in contact with EDTA or citrate ions, thereby competing with HSs for binding metals. The experiments were carried out by varying the pH maintained by Tris-HCl or CAPS buffer (pH 8.0 and 10.3) and keeping the ionic strength constant. The elution profile of humic substances using UV/ visible detection was compared with those from ICPMS detection of Co, Cu, Ni, Pb, and Zn in the same chromatographic runs. The results obtained suggested that both bridging between small molecules and complexation/ chelation by individual molecules are involved in metal ion binding to humic substances. The use of ICPMS to study the role of metal ions in aggregation/disassociation of humic substances proposed in this work is promising. Coupling element-specific detection with SEC or other separation systems allows better understanding of the mobility and bioaccessibility of elemental species in the environment and further elucidation of the dissolved humic structure.
Geoderma, 2003
For the first time a method is presented to study the distribution of metals between humic and fulvic fractions in natural soils. Total organometallic complexes are separated by extraction with 0.1 M pyrophosphate. After acidification to pH 1.5 with H 2 SO 4 the fraction associated with humic acids is separated by precipitation and the remaining solution contains the complexes associated with fulvic complexes. Concentrations of 11 elements (Al, Fe, Ca, Mg, K, Mn and the trace elements Cu, Pb, Zn, Cd, and Cr) were determined in the pyrophosphate and the fulvic acid extracts, and the fraction bound to humic acids was estimated by difference. Typical results from application of the method to andosols, cambisols, and podzols are presented. Fulvic acid is generally the main humic fraction reacting with metals, except for Ca where around 70% of the organic bound pool is found in the humic fraction. The corresponding figure is of the order of 30 -40% for Al, Fe, Mn and Cr and even less for Pb, Cu, Zn, and Cd. For some elements the humic/fulvic distribution appears to be different between soil types. Some factors that may disturb the results obtained by the present method are discussed. D
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1998
The dynamic aggregation properties of humic matter and the influence of intramolecular electrostatic interaction lead to an increase of polydispersity at pH > 5 in two humic samples studied ("Purified Peat Humic Acid", PPHA and "Laurentian Fulvic acid", LFA). The smaller entities formed at pH increasing above 5 are responsible for the increase of the diffusion coefficient as "seen" by voltammetry. The voltammetric results show that the diffusion coefficient of the humic complex increases from 5 x 10-12 m 2 s-1 (pH<5) up to 2xl0-11m2s-~ (pH=6) for Cd/PPHA and from 6×10-11m2s-1 (pH < or =5) up to 1.2 x 10-1° m 2 s-1 (pH=5.5) for Pb/LFA. Systematic shifts in voltammetric results with changing ligand concentrations often attributed to adsorption of humic matter on the electrode and/or heterogeneity effects, may also be due to incorrect diffusion coefficient values used in speciation calculations. The aim of this paper is to show that the diffusion coefficient of metal-humic complexes is a function of pH. It is necessary to take this into account to obtain more accurate speciation data from voltammetric results and interpret the physicochemical background of the metal humic interaction.