Functional Groups Of Humic Acid Research Articles

XAFS studies of cobalt(II) binding by solid peat and soil-derived humic acids and plant-derived humic acid-like substances

This work has examined cobalt(II) binding by a variety of solid humic acids (HAs) isolated from peat, plant and soil sources at temperatures down to 60 K. The results confirm that X-ray absorption near-edge spectroscopy (XANES) measurements cannot distinguish between aquo and carboxylato ligands in the inner coordination sphere of Co(II). However, between 1 and 2 inner-sphere carboxylato ligands can be detected in all the peat, plant and soil-derived HA samples by extended X-ray absorption fine structure (EXAFS) measurements, indicating inner-sphere coordination of HA-bound Co(II). The precision of C(carboxylate) detection is limited by the extent and quality of the data and the contribution from inner-sphere O to the Fourier transformed peaks used to detect carbon. Putative chelate ring formation is consistent with a relatively negative entropy change in step A, the stronger Co(II) binding step by HA functional groups, and could relate to ‘non-exchangeable’ metal binding by HSs.

Boric acid adsorption on humic acids: Ab initio calculation of structures, stabilities, 11B NMR and 11B, 10B isotopic fractionations of surface complexes

Boric acid, B(OH) 3, forms complexes in aqueous solution with a number of bidentate O-containing ligands, HL −, where H 2L is C 2O 4H 2 (oxalic acid), C 3O 4H 4 (malonic acid), C 2H 6O 2 (ethylene glycol), C 6H 6O 2 (catechol), C 10H 8O 2 (dioxynaphthalene) and C 2O 3H 4 (glycolic acid). McElligott and Byrne [McElligott, S., Byrne, R.H., 1998. Interaction of B(OH) 3 0 and HCO 3 - in seawater: Formation of B(OH) 2 CO 3 - . Aquat. Geochem. 3, 345–356.] have also found B(OH) 3 to form an aqueous complex with HCO 3 - 1 . Recently Lemarchand et al. [Lemarchand, E., Schott, J., Gaillardeet, J., 2005. Boron isotopic fractionation related to boron sorption on humic acid and the structure of surface complexes formed. Geochim. Cosmochim. Acta 69, 3519–3533] have studied the formation of surface complexes of B(OH) 3 on humic acid, determining 11B NMR shifts and fitted values of formation constants, and 11B, 10B isotope fractionations for a number of surface complexation models. Their work helps to clarify both the nature of the interaction of boric acid with the functional groups in humic acid and the nature of some of these coordinating sites on the humic acid. The determination of isotope fractionations may be seen as a form of vibrational spectroscopy, using the fractionating element as a local probe of the vibrational spectrum. We have calculated quantum mechanically the structures, stabilities, vibrational spectra, 11B NMR spectra and 11B, 10B isotope fractionations of a number of complexes B(OH) 2L − formed by reactions of the type: B ( OH ) 3 + HL - ⇒ B ( OH ) 2 L - + H 2 O using a 6-311G(d,p) basis set and the B3LYP method for determination of structures, vibrational frequencies and isotopic fractionations, the highly accurate Complete Basis Set-QB3 method for calculating the free energies and the GIAO HF method with a 6-311+G(2d,p) basis for the NMR shieldings. The calculations indicate that oxalic acid, malonic acid, catechol and glycolic acid all form stable complexes (Δ G < 0 for Reaction (1)),which are deshielded (less negative δ) vs. the (C 2H 5) 2OBF 3 reference by 3.6, 1.5, 6.5 and 5.4 ppm, respectively, and which are isotopically lighter than B(OH) 4 - (more negative δ 11B) by 3‰, 2‰, 5‰ and 2‰, respectively. The calculated 11B NMR shifts match well literature values and with the results of Lemarchand et al. (2005), while the calculated isotopic fractionations are also consistent with their results, but show much smaller deviations from B(OH) 4 - t than indicated by these authors. This is a consequence of the use by Lemarchand et al. (2005) of a value of 1.0194 [Kakihana, H., Kotake, M., Satoh, S., Nomura, M., Okamoto, M., 1977. Fundamental studies on the ion-exchange separation of boron isotopes. Bull. Chem. Soc. Jpn. 50, 158–163.] for the 11B, 10B isotopic exchange equilibrium constant of the B(OH) 3, B(OH) 4 - pair which is obsolete and should be replaced by the new purely experimentally determined value of 1.0285 for 0.63 molar ionic strength [Byrne, R.H., Yao, W., Klochko, K., Kaufman, A.J., Tossell, J.A., 2006. Experimental evaluation of the isotopic exchange equilibrium 10 B(OH) 3 + 11 B(OH) 4 - = 11 B(OH) 3 + 10 B(OH) 4 - in aqueous solution. Deep-Sea Res. 1 (53), 684–688.] or 1.0308 for pure water [Klochko, K., Kaufman, A.J., Yao, W., Byrne, R.H., Tossell, J.A., 2006. Experimental measurement of boron isotope fractionation in seawater. Earth Planet Sci. Lett. 248, 276–285]. Given this correction the B(OH) 2L − complexes are observed to be isotopically lighter than B(OH) 4 - by only a few ‰. Changes in 11B NMR shift and 11B, 10B isotope fractionations for the B(OH) 2L − complexes, compared to B(OH) 4 - , are found to be correlated to some extent with distortions of the O–B–O angles from tetrahedral values and/or with B–O bond strength sums. Similar free energies for the corner-sharing and 4-ring isomers of B(OH) 2 CO 3 - suggest a mechanism for creation of both BIII and BIV environments when B is incorporated into calcite.

Interaction of trace elements in acid mine drainage solution with humic acid

The release of metal ions from a coal mining tailing area, Lamphun, Northern Thailand, is studied by leaching tests. Considerable amounts of Mn, Fe, Al, Ni and Co are dissolved in both simulated rain water (pH 4) and 10 mg L −1 humic acid (HA) solution (Aldrich humic acid, pH 7). Due to the presence of oxidizing pyrite and sulfide minerals, the pH in both leachates decreases down to ∼3 combined with high sulfate concentrations typical to acid mine drainage (AMD) water composition. Interaction of the acidic leachates upon mixing with ground- and surface water containing natural organic matter is simulated by subsequent dilution (1:100; 1:200; 1:300; 1:500) with a 10 mg L −1 HA solution (ionic strength: 10 −3 mol L −1). Combining asymmetric flow field-flow fractionation (AsFlFFF) with UV/Vis and ICP-MS detection allows for the investigation of metal ion interaction with HA colloid and colloid size evolution. Formation of colloid aggregates is observed by filtration and AsFlFFF depending on the degree of the dilution. While the average HA size is initially found to be ⩽ 2 nm , metal–HA complexes are always found to be larger. Such observation is attributed to a metal induced HA agglomeration, which is found even at low coverage of HA functional groups with metal ions. Increasing the metal ion to HA ratio, the HA bound metal ions and the HA entities are growing in size from <3 to >450 nm. At high metal ion to HA ratios, precipitation of FeOOH phases and HA agglomeration due to colloid charge neutralization by complete saturation of HA complexing sites are responsible for the fact that most of Fe and Al precipitate and are found in a size fraction > 450 nm . In the more diluted solutions, HA is more relevant as a carrier for metal ion mobilization.

O-Alkylation of a lignite humic acid by phase-transfer catalysis

A mild phase-transfer catalytic reaction has been conducted to O-alkylate the acidic functions of a lignite humic acid (HA), using tetrabutylammonium hydroxide as the phase-transfer catalyst. The HA acidic functional groups were made to react, in tetrahydrofuran, by nucleophilic substitution with several alkyl halides-methyl iodide, and ethyl, propyl, and butyl, and benzyl bromide. The occurrence of the O-alkylation reaction was assessed by elemental analysis and 1H NMR, CPMAS 13C NMR, and FTIR spectroscopy. Bonding of alkyl groups increased the carbon and hydrogen content and the H/C ratios of all the humic reaction products. Increased nitrogen in the reaction products suggested incomplete removal of the phase-transfer catalyst after purification of the alkylated HA. 1H NMR and CPMAS 13C NMR spectra of alkylated products provided evidence of the successful occurrence of the alkylation reactions. Infrared spectra confirmed the NMR results, revealing the characteristic absorption of newly formed alkyl and aryl ethers and esters in the alkylated products and C-H stretching in the aromatic ring of the benzylated derivative. These findings indicate that humic matter can be successfully alkylated with several different alkylating groups by catalysed phase-transfer reaction. This O-alkylation reaction has the advantage of being mild, versatile, and high-yielding compared with traditional methylation reactions applied to HA. The possibility of introducing different alkyl groups into the HA by a mild phase-transfer reaction may become useful by enabling improved fractionation of humic supramolecular associations and further understanding of the molecular nature of humic substances.

Mechanisms of alkaline buffering by peat and quantitative estimation of its buffering capacity

The alkaline buffering capacity of humic substances in persistent organic materials such as peats has been utilized for remediation of alkaline soils. The purposes of the present study were to reveal the mechanism of alkaline buffering and to quantify the alkaline buffering capacity of humic substances contained in three peat samples obtained from Hokkaido, Canada, and Sri Lanka. Acidic functional groups in humic acid and fulvic acid extracted from the three natural peats were quantified by alkalimetric titration and pK distribution analyses of the titration curves. The results indicate that the inherent degree of humification of the organic matter can be identified from the pK distribution, which showed that the humic substances in peats contained diprotic acids. The present study also showed that the ratios of amounts of deprotonation to organic carbon content, as well as the organic carbon content, were needed for quantification of the pH buffering capacity. Moreover, the study showed that the composition of the peat from Sri Lanka was different from those of other peats and that it is a effective organic material for buffering under alkaline conditions.

Humic substance-enhanced ultrafiltration for removal of cobalt

It is well known that the membrane separation process combined with surfactant micelle (micellar-enhanced ultrafiltration) or polyelectrolyte (polyelectrolyte-enhanced ultrafiltration) can remove heavy metal ions or radionuclides effectively. However, the complexing agent, surfactant or polyelectrolyte remained in effluent is a serious disadvantage of these methods. In this study, humic substances (HS) were used as complexing agents instead of synthetic chemicals. The HS are sorts of natural organic matters and their functional groups such as carboxyl and phenyl groups can bind with the cation and form complexes. The effects of HS concentration and pH on the removal of cobalt were investigated. At the HS concentration of 3 g/L and pH of 6, over 95% of cobalt was removed by regenerated cellulose membrane with molecular weight cut-off (MWCO) of 3000. As the HS concentration increased, the removal of cobalt was also enhanced because of the increase in binding sites (functional groups). The removal of cobalt increased from 72.5% to 97.5% as pH increased from 4 to 8 at the HS concentration of 3 g/L. It resulted from the more deprotonation of functional groups in humic acid at higher pH.

Role of Humic Acid and Quinone Model Compounds in Bromate Reduction by Zerovalent Iron

Experiments were conducted to examine the role of humic acid and quinone model compounds in bromate reduction by Fe(0). The reactivity of Fe(0) toward bromate declined by a factor of 1.3-2.0 in the presence of humic acid. Evidence was obtained that the quick complexation of humic acid with iron species and its adsorption passivated the iron surface and decreased the rate of bromate reduction by Fe(0). On the other hand, in the long run, the reduced functional groups present in humic acid were observed to regenerate Fe(II) and reduce bromate abiotically. Compared with the case of humic acid only, the simultaneous presence of Fe(II) and humic acid significantly increased the bromate removal rate. Fe(III)/Fe(II) acted as a catalyst in the oxidation of humic acid by bromate. Anthraquinone-2,6-disulfonate (AQDS) and lawsone did not cause any significant effect on the bromate reduction rate by Fe(0). However, the redox reactivity of lawsone in the presence of Fe(III) was evident, while AQDS did not show any under the tested conditions. The difference was attributable to the presence/ absence of reducing functional groups in the model compounds. The electron spin resonance further demonstrated that the redox functional groups in humic acid are most likely quinone-phenol moieties. Although the bromate reduction rate by regenerated Fe(II) is a few times slower than that by Fe(0), the reactive Fe(II) can be, alternatively, reductively formed to maintain iron surface activation and bromate reduction to prolong the lifetime of the zerovalent iron.

Adsorption of lignite-derived humic acids on coal-based mesoporous activated carbons

The adsorption by a coal-based mesoporous activated carbon of humic acids (HAs) isolated from two Polish lignites was studied. For comparison, a commercial Aldrich humic acid was also included into this study. The differences in chemical structure and functional groups of HAs were determined by elemental analysis and infrared spectroscopy DRIFT. Two activated carbons used differed in terms of mesopore volume, mesopore size distribution, and chemical properties of the surface. The kinetics of adsorption of HAs have been discussed using three kinetic models, i.e., the first-order Lagergren model, the pseudo-second-order model, and the intraparticle diffusion model. It was found that the adsorption of HAs from alkaline solution on mesoporous activated carbon proceeds according to the pseudo-second-order model. The correlation coefficients were close to 1. The intraparticle diffusion of HA molecules within the carbon particle was identified to be the rate-limiting step. Comparing the two activated carbons, the carbon with a higher volume of pores with widths of 10–50 nm showed a greater removal efficiency of HA. An increase in the Freundlich adsorption capacity with decreasing carbon content of HA was observed. Among the HAs studied, S-HA shows characteristics indicating the highest contribution of small-size fraction. The S-HA was removed by both activated carbons to the highest extent. The effect of pH solution on the adsorption of HA was examined over the range pH 5.4–12.2. It was found that the extent of adsorption decreased with decreasing pH of the solution.

Long-term effects of amendment with municipal solid waste compost on the elemental and acidic functional group composition and pH-buffer capacity of soil humic acids

The main objective of this research was the evaluation of cumulative and residual, long-term (9 years) effects of amendment with municipal solid waste compost (MWC) used at rates of 20 and 80 t ha −1 on the chemical, structural and functional properties of soil humic acids (HAs). For this propose, HAs were isolated from the MWC sample, the unamended control soil and MWC-amended soils by conventional procedures, and analysed for elemental and acidic functional group composition, and by differential scanning potentiometry to determine their pH-buffer capacity and average apparent dissociation constants. With respect to the control soil HA, the MWC-HAs had a greater content of N and smaller contents of O and acidic groups, smaller molecular size and a limited humification degree. In the residual-effect experiment, the application of MWC affected only partially the elemental composition of soil HA, whereas an increase was measured for the acidic functional group contents, especially at the larger amendment rate. In the cumulative-effect experiment, the repeated application of MWC affected markedly both the elemental and acidic functional composition of soil HA, thus suggesting the occurrence of a major incorporation of structural units of MWC-HA into amended soil HAs. Up to six apparent average dissociation constants were calculated for the HAs studied, which corresponded to the various acidic functional groups in HAs. The MWC-amended soil HAs from residual treatments exhibited a buffer capacity and a number of apparent dissociation constants ( K app) greater than those of HAs from the control soil and the cumulative experiment soils. The buffer capacity of HAs from the cumulative treatments increased below pH 7 and decreased above this pH, with respect to that of the control soil HA. No significant differences were observed as a function of the amendment rate used.

A study of coagulation mechanisms of polyferric sulfate reacting with humic acid using a fluorescence-quenching method

A fluorescence-quenching method is developed to assess the effect of pH on the coagulation mechanism of humic acids (HA) reacting with metal ions. A polyferric sulfate (PFS) synthesized in our laboratory is adopted as the coagulant to simplify the hydrolysis process and increase the experimental precision. The following results are discovered. When the concentration of PFS increases from 2 to 10 mg/L, the effective pH range of HA removal changes from 4.0–5.0 to 4.0–7.5. At increased coagulation pH, the ferric ions may still react with HA but unable to neutralize the surface charge completely. The residual concentrations of HA measured by fluorescence spectrophotometer are lower than those by TOC, as a consequence of the fluorescence-quenching effect. This demonstrates that the coagulation of HA by PFS at low pH is mainly due to charge-neutralization. The adsorption of the HA on the pre-formed iron hydroxide flocs is accompanied by the dissociation of Fe ions from the floc structure until the equilibrium has been reached, which is evidenced by the presence of the Fe-HA complexes in the solution during adsorption experiment. This is quite different from the characteristics of flocs formed by PFS associated with HA in the coagulation. Within the pH range investigated, the complex-formation and the hydrolysis are the two competitive reactions happened between the hydroxide ions in solution and the functional groups of HA. Therefore, the removal of HA is not caused by adsorbing onto the iron hydroxide resulted from PFS hydrolysis, but through the complex-formation between the PFS and the HA.

Sorption of cadmium on humic acid: Mechanistic and kinetic studies with atomic force microscopy and X-ray absorption fine structure spectroscopy

Humic acids (HA) constitute a major fraction of the organic matter in soils and sediments. Little is known about the kinetics and mechanisms of Cd sorption by HAs, especially those pertaining to the surface features and structure of the Cd-humate complexes. We investigated Cd sorption by HA using the conventional batch method, N2-BET method, atomic force microscopy (AFM), and extended X-ray absorption fine structure (EXAFS) spectroscopy to gain a better understanding of the kinetics and mechanisms. Cadmium sorption by HA can be described by the parabolic diffusion equation. The increase in the apparent diffusion coefficient with an increase in the initial Cd concentration was less pronounced at higher initial Cd concentrations, apparently due to an enhancement in aggregation of the Cd-humate complexes. The HA before and after Cd sorption at Cd concentrations ≤ 10–6 M in different reaction times was spheroidal in shape. The Cd-humate complexes formed at 10–4 and 10–3 M Cd gradually changed to resemble the shape of a bean with increasing reaction time, thereby substantially decreasing their specific surface. Cadmium ions raveled the spheroidal HA particles into a bean-like structure by coordinating to six O atoms from the functional groups of HA, with a Cd-O distance of 2.297(2) Å. The Cd-O bond strength decreased with increasing initial Cd concentration. Key words: Cadmium, humate complexes, conformation, atomic force microscopy, EXAFS

Use of amino-carbonyl reaction and chemiluminescence detection to the flow-injection determination of some amino acids

Some amino acids were found to react with carbonyl functional groups of humic acid. Products of this reaction give strong chemiluminescence during their oxidation with N-bromosuccinimide (NBS) in alkaline solution. Humic acids from different sources produced similiar signal magnitude. This effect was employed to the flow-injection determination of glycine and arginine in pharmaceutical formulations with considerable selectivity against different amino compounds. The proposed method is fast and simple. Detection limit is 0.20 and 0.25 mg·l −1 for glycine and arginine respectively, and 115 samples per h can be determined.

Effect of phenolic acids in humic acid on the degradation of pentachlorophenol by the photo‐Fenton reaction

At pH = 5.0, the degradation of pentachlorophenol (PCP) by the photo‐Fenton reaction was facilitated in the presence of humic acid (HA). To identify the functional groups in HA that contributed to this facilitation, the degradation kinetics of PCP were investigated for six types of phenolic acids. The pseudo‐first‐order rate constants of PCP degradation (K 3) increased with those for the photoreduction of Fe(III) (k 1 ).This indicates that the photoreduction of Fe(III) is an important step in the degradation of PCP by the photo‐Fenton reaction. Moreover, the K 3values for gallic, protocatechuic and caffeic acids were larger than those for syringic, vanillic and ferulic acids. This shows that the phenolic acids in HA, which contain neighboring hydroxyl groups, are major contributors to the facilitation of PCP degradation.

Sorption on Humic Acids as a Basis for the Mechanism of Primary Accumulation of Gold and Platinum Group Elements in Black Shales

In order to asses the contribution of sorption by complexation to the concentration of noble metals at early stages of the formation of their deposits in black shales, the sorption of Au(III), Pt(IV), Rd(II), Rh(III), Ru(IV), and Os(IV) ions was studied on ash-free preparations of humic acids (HA) separated from peat of the Tver region and marine sediment samples taken on the Peruvian shelf. Data on the nature and protolytic characteristics of oxygen-containing HA functional groups were obtained. It has been shown that carboxyl groups and phenol oxygroups, which ensure the HA complexation with ions of noble metals, are present in the HA structure. The dissociation constant values for HA carboxyl groups (pKa) and the distribution function of these groups in their pKa values have been established. It has been revealed that the pKa value for both of the HA groups varies within two orders of magnitude: the average value is equal to 6.1 for HA from peat and 7.0 for HA from marine sediments. A fairly high and similar for both of the HA groups sorption capacity with respect to Au(III), Rd(II), Rh(III), Ru(IV), and Os(IV) ions was established in model experiments. It is equal to 320–350 mg g–1 for Au, 100–110 mg g–1 for Pd, 11–12 mg g–1 for Rh, 16–19 mg g–1 for Ru, and 23 mg g–1 for Os. The study of platinum(IV) sorption revealed that humic acids from peat and marine sediments do not virtually sorb Pt(IV), and this observation is important for understanding genetic features of the formation of noble metal deposits in black shales. Based on sorption isotherms for Au(III), Pd(II), Rh(III), and Ru(IV), the conditional affinity constant values for HA sorption centers with respect to ions of these metals were calculated by the method of quantitative physicochemical analysis. These values prove that complex compounds forming at the HA surface possess a high strength: the logβ values for the Au(III)–HA, Pd(II)–HA, Rh(III)–HA, and Ru(IV)–HA compounds are equal to 6.0, 5.0, 3.2, and 3.5, respectively.

Charge Adjustments upon Adsorption of a Weak Polyelectrolyte to a Mineral Oxide: The Hematite–Humic Acid System

The proton adsorption to a mixture of purified Aldrich humic acid (PAHA) and hematite is investigated. Basic insight into the charge adjustment process is obtained by using a self-consistent-field lattice theory for polyelectrolyte adsorption. The calculations indicate that upon adsorption the component with the highest initial charge density tends to induce charges on the other component. The number of induced charges can show a maximum when the surface charge and the charge of the segments in direct contact with the surface roughly balance each other. Experimentally, the humic acid–hematite system is investigated by proton titrations. The alterations in charge density caused by adsorption of PAHA to hematite are investigated by comparing the proton adsorption on the individual samples with that on their mixtures. Upon adsorption a part of the functional groups of humic acid forms complexes with some of the surface sites of hematite. This interaction reduces the proton binding to the humic acid at relatively low pH and it promotes the proton adsorption on the oxide surface at relatively high pH.

Effect of humic acid on the sorption of trifluralin by soils

This study was undertaken to develop a method for the extraction of trifluralin (α,α,α, trifluoro‐2,6 ‐ dinitro‐N,N ‐ dipropyl‐p‐toluidine) from soil and humic acid. The adsorption‐desorption constants were evaluated through Freundlich isotherms. The studies were carried out with a Podzolic soil from Pindorama, SP, with the humic acid (HA) extracted from the same soil, with the same soil free of organic matter by calcination, and finally with the same soil fortified with 2% of humic acid. The results indicated that the quantity of organic matter was a key factor for the adsorption of the herbicide trifluralin in soil. The order followed by trifluralin with respect to its adsorption was: humic acid > soil spiked with 2% of humic acid > soil > calcinated soil. These results, in addition to the infrared studies, indicated that the functional groups in humic acid, namely carboxylic and phenolic, were involved in the interaction with trifluralin.

Effect of calcium ion on the fouling of nanofilter by humic acid in drinking water production

The influence of the charge of humic acid (HA) on the fouling of nanofilters was investigated as a function of pH and concentration of calcium ions. The charge of the humic acid as well as the zeta-potential of the membrane surface were measured to elucidate the mechanism of humic acid deposition on the membrane surface. The negative charge of humic acid as well as the negative zeta-potential of the membrane surface fouled with humic acid increased with higher pH. As a result the further deposition of humic acid on the membrane surface was expected to decrease with higher pH because of larger repulsive forces. With moderate calcium ion concentration, however, the adsorption of humic acid onto the membrane surface decreased until neutral pH and then increased again. It was attributed to calcium ion bridging between two free functional groups of humic acids. Calcium ion rejection decreased in the presence of humic acid deposited on membrane surface. This could be explained by the effect of humic acid on the net charge of the membrane. The addition of calcium chelating agent, EDTA, improved flux, especially at alkaline conditions. The diagram of Ca–EDTA vs pH was used to explain the flux improvement.

Tight metal binding by humic acids and its role in biomineralization †

Analytical and thermodynamic data, EPR, FTIR, solution 1H and solid-state 13C cross polarization magic angle spinning NMR and solid-state extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra have been recorded for purified humic acids (HAs) isolated from a German peat (GHA), an Irish peat (IHA), an unpolluted New Hampshire bog soil (NHA) and their tightly bound copper(II), iron(III) and manganese(II) forms. Brief water washing of partly or fully metal-loaded HAs leaves ‘tightly’ bound metal in the isolated freeze-dried solids. Most of this metal is removed by washing with 0.1 M HCl, indicating acidic HA functional groups as principal metal binding sites. The number of nearest-neighbour atoms co-ordinated to tightly bound CuII (four), FeIII (six, probably with distorted geometry) and MnII (six, undistorted) in solid GHA, IHA and NHA were determined by XANES and EXAFS spectroscopy with reference standards. Isotherms measured at 20.0 °C and pH 2.4–3.2 with [M]total = 0.18–25.8 mM for tight, reversible Cu2+(aq), Fe3+(aq), and Mn2+(aq) binding by solid IHA and NHA fit the Langmuir model and give the pH-independent stoichiometric site capacities νi and equilibrium constants Ki for metal binding at specific HA sites i = A, B and C. Tight binding sites A, B and C of IHA are occupied by CuII, sites A and B by FeIII and site A by MnII, while only identical metal binding site A in NHA is tight enough to resist metal removal by brief water washing. A new helical HA molecular model based on the empirical formula C36H30N2O15·xH2O visualizes metal binding and the likely roles of HAs in biomineralization. Site A is suggested to be carboxylate, mixed ligands probably constitute site B, and site C is tentatively assigned as the interior of the HA helix. Binding free energies and EPR evidence suggest that Cu2+(aq), Fe3+(aq) and Mn2+(aq) rapidly transfer between specific HA binding sites. This affects rates of metal release and transfer to minerals.

Effect of Ionic Strength on Complexing Equilibrium Between Copper (II) and Humic Acid

Abstract The effect of ionic strength on copper(II) complexing ability of humic acid (HA) was investigated. The conditional stability constants and the copper(II) complexing capacities were evaluated at the ionic strength of 0.005 - 0.1. The copper(II) complexing capacities of HA increased with ionic strength. By extrapolating the conditional stability constants to ionic strength 0, the thermodynamic stability constant was evaluated. The free ligand concentration in the solution containing copper(II) and humic acid was calculated by using the thermodynamic stability constant obtained. The free ligand concentration also increased with ionic strength. The increase of copper(II) complexing ability of HA with ionic strength was discussed from the viewpoints of the dissociation of the acidic functional groups in HA and the conformational change of HA.

Trace metals in humic acids from recent Skagerrak sediments

Humic acids isolated from a sediment core from the eastern Skagerrak have been characterized by elementary analysis and IR analysis. Heavy metal concentrations in these isolates show a general trend to decrease with depth. IR-spectroscopic analysis indicates that functional groups of humic acids from deeper sediment layers differ from those of the uppermost samples. Higher contents of Ti and Cr and to a lesser extent of Zn, Cu, Mn, and V in the uppermost layer reflect an increase in supply of these elements in the youngest depositional history. While bulk sediments did not show significant enrichment humic acids are enriched in heavy metals.