Humic Acid Molecules Research Articles

Proton Binding to Humic Acids: Electrostatic and Intrinsic Interactions

The proton adsorption behavior of eight humic acids (HA) and a fulvic acid (FA) was studied as a function of pH and KNO3 concentration. The emphasis is on the comparison of the different humics with respect to their ion binding properties and on the comparison of two different models to describe the electrostatic interactions: the Donnan model and the impermeable sphere (IS) model. Viscosimetric data were used to estimate the hydrodynamic volumes and radii of the HA molecules. These data were incorporated in the electrostatic models and calculations could be carried out without any adjustable parameter. The Donnan model in combination with hydrodynamic volumes obtained by viscometry cannot adequately describe the electrostatic effects related to changes of the electrolyte concentration. This model leads to good prediction of the HA behavior if unrealistically large volumes are used for fulvics and unrealistically large volume-salt concentration dependencies are used for humics. The IS model can successfully reproduce experimental proton adsorption data with physically realistic radii. The good performance of the IS model and the poor performance of the Donnan model is directly related to the fact that the hydrodynamic volumes of the molecules are too small to allow for charge compensation within the molecular limits. The combination of viscometry with the IS model leads to a consistent description of the electrostatic in humics and to a consistent way of positioning the master curves. Therefore, the electrostatic potentials and the intrinsic affinity distributions of the different samples can be compared on an equal basis. The similarities in the intrinsic affinity distributions give faith for the possibility to develop a generic model to describe the ion binding to humics.

Experimental study on the unintentional abiotic methylation of inorganic mercury during analysis: Part 2: Controlled laboratory experiments to elucidate the mechanism and critical discussion of the species specific isotope addition correction method

The chemical behaviour of inorganic mercury in complex soil and sediment samples, possibly responsible for the methylation of mercury, was further investigated. Following the previous methylation experiments with the mentioned problematic matrices the possible pathways of inorganic mercury methylation aided by humic acid were discussed on a chemical basis. The HPLC-ICP-MS system described in the first part was used as a valuable tool for the investigation of the abiotic mercury methylation with the aid of enriched mercury isotopes. Experiments with different carboxylic acids, which are the major functional groups in a high molecular weight ? humic acid molecules have shown that they are good methyl donors for Hg 2+. Organic acids with α-methyl groups showed the highest methylation efficiency. Furthermore thermal experiments have shown that the artifact formation potential of a humic rich sediment sample was lowered by heat treatment and nearly removed by treatment at 200°C–300°C within 3 h. Sandy sediments showed an artifact formation potential as well. In this case a heat treatment at 700°C for 3 h was necessary to remove the abiotic methylation potential. The application of the Species-Specific Isotope Addition method for sediments was discussed critically. Consideration of the methylation due to a humic acid molecule showed that the isotope and the ambient inorganic mercury could undergo different amounts of methylation. Therefore the actual value may deviate from the value obtained from the SSIA method. This means also that it could be possible that the SSIA method could detect only a small amount of an actually considerably higher existing artefact. To be certain about the comparability of the spiked 200Hg 2+ tracer and the ambient inorganic mercury, the exact mechanisms leading to the measured artefact which are responsible and up to now no unidentified have to be clarified.

L3C NMR spectra and elemental composition of fractions with different particle sizes from an andosol humic acid

Elemental composition and solution 13C NMR spectra for six humic acid fractions with different particle sizes (i.e. 3K, 10K, 30K, l00K, 300K, and 500K) which were prepared from a humic acid in an Umbric Andosol by successive gel permeation chromatography were determined. Elemental composition of the particle size fractions clearly differed from one another. Contents of nitrogen and hydrogen increased with the increase of the particle size. Small and middle particle size fractions (3K to l00K) showed low contents of hydrogen and nitrogen corresponding to those in typical Andosol humic acids, while the contents in the large fractions (300K and 500K) were distributed in the ranges of those in the other humic acids. As the particle size increased, the HIC ratio increased whereas the O/H decreased. In the HIC versus OIC diagram, the large particle size fractions (300K and 500K) were observed in the area around humic acids with a low humification degree. Variation of the chemical structural properties with particle size differences was assumed, based on the analysis of the HIC versus OIH and HIC versus OIC diagrams. The changes of the carbon species with particle size differences were examined based on the 13C NMR spectra. As the particle size increased, the aliphatic C content increased whereas the carboxylic and aromatic C content decreased and, therefore, the aromaticity decreased. In addition, a negative correlation (r= -0.977) was found to be significant at 0.1% level between the HIC ratios and aromaticity. The peak strength around 30 ppm assigned to chains of methylene group increased remarkably with the increase of the particle size. It was suggested that the changes of the content of the aliphatic carbon with the decrease of the particle size may depend on the changes of the aliphatic chain length. In conclusion, these findings suggest that the Andosol humic acid molecule may be composed of humic acid components with long chains of aliphatic groups for the larger particle size fractions, and of their homologous series through the reduction of the length of the aliphatic long chain for the smaller particle size fractions. These assumptions were compatible with those made in a series of studies previously reported.

Scanning force microscopy of spin‐coated humic acid

We report on investigations of spin-coated peat humic acid solutions by scanning force microscopy allowing, for the first time, direct imaging of individual single humic acid molecules with a minimum diameter of 1.5–3.5 nm. The measured height of the molecules deposited onto a mica substrate increases with decreasing pH value of the solution. This behaviour can be explained by means of the random coil model if one takes the existence of charge carriers in organic macromolecules into consideration. With increasing humic acid concentration the molecules arrange into a few monolayer-thick agglomerates formed during the process of film deposition. In unfiltered solutions and in 1000 nm filtrates we find additional particles well distinct from the above-mentioned molecules in size and properties. They possess an equivalent spherical diameter of 70–160 nm and behave inert when the ambient conditions are changed. Copyright © 1999 John Wiley & Sons, Ltd.

Effects of solution environment on humic acid fouling during microfiltration

Microfiltration is increasingly used in drinking water treatment for the removal of particles, turbidity, and microorganisms to meet more stringent water quality requirements. Fouling by natural organic matter (e.g., humic and fulvic acids) is a critical factor in microfiltration processes. Experiments were performed to evaluate the effects of solution pH, salt concentration, and ionic composition on the extent and mechanisms of humic acid fouling during microfiltration. Fouling was more severe at high salt concentration due to the tighter packing of the charged humic acids within the fouling layer caused by the increased electrostatic shielding. The effects of solution pH were more complex and likely reflect alterations in humic acid charge, hydrophobicity, and extent of aggregation. Divalent cations such as calcium significantly increased humic acid fouling and aggregation through the formation of intra- or intermolecular bridges between the negatively charged humic acid molecules. These results provide important insights into the physical and chemical interactions governing humic acid fouling during microfiltration.

Scanning force microscopy of spin-coated humic acid

We report on investigations of spin-coated peat humic acid solutions by scanning force microscopy allowing, for the first time, direct imaging of individual single humic acid molecules with a minimum diameter of 1.5-3.5 nm. The measured height of the molecules deposited onto a mica substrate increases with decreasing pH value of the solution. This behaviour can be explained by means of the random coil model if one takes the existence of charge carriers in organic macromolecules into consideration. With increasing humic acid concentration the molecules arrange into a few monolayer-thick agglomerates formed during the process of film deposition. In unfiltered solutions and in 1000 nm filtrates we find additional particles well distinct from the above-mentioned molecules in size and properties. They possess an equivalent spherical diameter of 70-160 nm and behave inert when the ambient conditions are changed.

Volume and structure of humic acids studied by viscometry: pH and electrolyte concentration effects

Viscometry was used to evaluate the effects of pH and supporting electrolyte concentration on the intrinsic viscosities of eight humic acids and one fulvic acid. Two synthetic poly(acrylic acid) (PAA) samples of different molecular weight were also studied for comparison. Humic and fulvic acid molecules behave as flexible entities that can swell or shrink in response to changes in pH and ionic strength. An increase in the solution pH leads to the development of negative charges in the molecules with the consequent electrostatic repulsion between ionized groups and molecular swelling. Increasing the ionic strength increases the screening of charges and leads to molecular shrinkage. The pH dependence decreases with increasing electrolyte concentration and at 10 −1 M electrolyte the intrinsic viscosity is almost pH independent. The general behavior of PAAs is similar to that of the humics, though the effects of pH and electrolyte concentration are much larger for the PAAs. The degree of hydration of the humics differs for different samples. There are compact samples with low water content and swelling properties whereas other humics are more hydrated and flexible. All the studied humics have an internal structure that limits the expansion of the molecules when the electrolyte concentration is decreased. The latter is in accordance with the low values of the Mark–Houwink coefficient, α, of humics.

Degradation of Soil Humic Extract by Wood- and Soil-Associated Fungi, Bacteria, and Commercial Enzymes.

> Abstract An alkaline humic extract (HE) of a black calcareous forest mull was exposed to 36 fungal and 9 eubacterial isolates in liquid standing culture. At 21 d in fungi, and 4 d in bacteria, the groups of wood-degrading basidiomycetes, terricolous basidiomycetes, ectomycorrhizal fungi, soil-borne microfungi, and eubacteria had reduced the absorbance (A340) of HE media by 57, 28, 19, 26 and 5%, respectively. Gel permeation chromatography revealed that the large humic acid molecules were more readily degraded than the smaller fulvic acid molecules and served as a sole source of carbon and energy. The more active HE degraders reduced the overall molecular weight of humic and fulvic acids by 0.25 to 0.47 kDa. They also reduced the chemical reactivity of HE to tetrazotized o-dianisidine, indicating the degradation of hydroxylated aromatic molecules (which are responsible for this reaction). Decreases in absorbance, molecular weight, and reactivity were caused by fungal manganese peroxidase, horseradish peroxidase, beta-glucosidase, and abiotic oxidants such as H2O2 and Mn(III) acetate. It is concluded that fungi, some of which are propagated in contaminated soils to control xenobiotics, metabolize HE compounds enzymatically. They use enzymes which are also involved in the degradation of soil xenobiotics. Because of reductions in the molecular weight of HE, which is a potential carrier of heavy metal ions and xenobiotics, solubility and motility of humic substances in soil and surface waters are increased.

Differential thermal analysis of sedimentary humic acids in the light of their origin

Results of studies on thermal decomposition using differential thermal analysis (DTA) of humic acids (HAs) extracted from the Szczecin Lagoon bottom sediments as dependent on their origin are presented in this paper. The results show that elemental composition and parameters of thermal decomposition can be promising parameters for classification of sedimentary HAs. Three types of sediments were distinguished according to their origin: allochthonous (mainly anthropogenous), terrestrial, and aquatic. Significant correlations were found between the HA elemental composition and the parameters of their thermal decomposition. The results show that nitrogen content in HAs is inversely proportional to aromaticity of HA molecules and to their heat of combustion.

Adsorption of Humic Acids to Mineral Particles. 2. Polydispersity Effects with Polyelectrolyte Adsorption

The adsorption of humic acid to mineral particles can be characterized by specific and electrostatic interactions and by adsorption fractionation. In this paper we focus on adsorption fractionation and the influence of the polydispersity (with respect to size) of humic acids on the adsorption process and the shape of the isotherms at different pH values. In the previous paper the effects of electrostatic interactions were described. The adsorption from a mixture of two humic substances, purified Aldrich humic acid (PAHA) and Laurentian fulvic acid, on nonporous hematite is studied. It is shown that, in time, the large humic acid molecules replace the readily adsorbed, small, fulvic acid molecules. Preferential adsorption of large humic molecules over small ones also affects the shape of the adsorption isotherm of a single humic acid. After a high affinity start, the adsorbed amount of PAHA gradually increases with increasing humic acid concentration. This is mainly due to an increasing adsorption fraction...

Adsorption of Humic Acid to Mineral Particles. 1. Specific and Electrostatic Interactions

The adsorption of humic acid to mineral particles can be characterized by specific and electrostatic interactions and by polydispersity effects. In this paper we focus on the adsorbed amount and discuss the importance of specific and electrostatic interactions. The adsorption of purified Aldrich humic acid onto hematite has been measured as a function of pH and salt concentration as well as in the presence of two cadmium concentrations. Besides the adsorbed amount, the thickness of the adsorbed layer has also been studied. The experimental results are discussed in relation to the polyelectrolyte behavior of the humic acid. At high pH and low salt concentration the adsorption is low and the humic acid molecules are adsorbed relatively flat on the surface. At low pH and high salt concentration a substantial fraction of the adsorbed humic acid is not in direct contact with the surface, which results in a relatively high adsorbed amount and a large layer thickness. PAHA adsorption at not too low pH leads to a...

Metal-humic interactions: A random structural modelling approach

The binding of the metal cations copper and calcium to peat humics and ground water (aquatic) humic and fulvic acids has been studied using ion selective electrochemistry under a variety of experimental conditions (pH, ionic strength, etc.). Competition between the metals for sites on humic acid molecules was also studied. The Random molecular model developed by Murray and Linder (1983, 1984) has been adapted in an attempt to simulate humate-metal binding behaviour. The Murray and Linder model uses a random structural approach to identify the likely metal binding sites present in each humic sample. The model contains no adjustable parameters, but instead relies on experimentally derived variables, namely, percentage carbon and hydrogen plus carboxylate and phenol concentrations. The model constructs molecules of humic acids which conform to these measurements and then searches the structures for a series of predefined metal binding sites. After generating a large number of similar structures, all of which conform to the experimental data, average concentrations of the binding sites are calculated. In the present work, the model has been extended to include nitrogen donor atoms and to search for nitrogen containing metal binding sites. In order to predict metal speciation, a version of the geochemical speciation code PHREEQE was devised in which the electrostatic effects produced by the large humic charge were accounted for. Tests on the model showed that the binding strengths predicted by the computer were consistently less than those observed by experiment, probably due to underestimation of the binding site complexity.

Atomic force microscopy of humic acid

Structural analysis of humic acid extracted from a Cecil soil was conducted by atomic force microscopy (AFM) of monolayer thin films of humic acid deposited on mica strips using the Langmuir‐Blodgett deposition technique. The humic acid specimens display a fibre‐like morphology showing prominent hexagonal structures of benzene rings at a molecular level of resolution. The side chains attached to these ring structures were identified by infrared spectroscopy to be carboxyl and carbonyl groups as indicated by absorption bands at 1625 and 1720 cm‐1, respectively. The presence of C‐H alkyl chains were detected by bands at 2917 and 2850 cm‐1. This AFM observation provides the first direct evidence supporting the theoretical description of humic acid molecules consisting of benzene rings with various sizes of alkyl chains.

Binding Effects on Humic-Mediated Photoreaction: Intrahumic Dechlorination of Mirex in Water

A kinetic model was developed describing the effects of hydrophobic partitioning to humics on second-order humic-mediated photoreactions in aqueous solution. Model development and evaluation were in terms of the humic-mediated dechlorination of the hydrophobic chlorocarbon, mirex. Mirex dechlorination in irradiated (λ ≥ 290 nm) Aldrich humic acid (HA) solution was examined as a function of humic acid and scavenger concentration. Kinetic models in which HA solution is described as a homogeneous or single-phase solution were not able to describe the effects of HA and scavenger concentrations on mirex dechlorination rates. However, these effects were successfully described by including partitioning in the models to characterize the bound phase and dissolved phase mirex reactions separately. The resulting model uses homogeneous phase kinetics to describe reaction in the dissolved phase. Bound phase reaction is described as occurring within individual HA molecules with the distribution of scavengers within HA molecules following a Poisson distribution. Used to assess the relative reactivities of the bound and dissolved phases, the model indicated that the reaction of mirex in HA solution is confined to the humic phase. The general predic tions and utility of the model are evaluated both in terms of experimental data and pollutant photodegradation.

Structural information obtained from spectral analysis (UV-VIS, IK, 1H NMR) of particle size fractions in two humic acids

Abstract Humic acids (HAs) extracted from an Umbric Andosol (UA) and a Dystric Cambisol (DC) were fractionated into 7 fractions by successive gel permeation chrontatography (GPC) and their fractions were studied by spectral analysis. With increasing particle sizes, the absorption of methylene groups and amide of peptides increased in the IE spectra of both HAs and that of carboxylic groups decreased in the IR spectra of UA, the HA obtained from an Umbric Andosol. Based on the 1? NMR data in UA, there was a significantly negative linear correlation (−0.950, p< 0.001) between Har (δ 6.0-9.0 ppm) and HC-O (δ 3.0-4.3 ppm). Such a negative linear correlation (−0.868, p<0.01) was also observed between Har and Hβ (δ 0.9-1.6 ppm). In addition, with increasing particle sizes, the percentage of aromatic protons decreased (from 30.6 to 4.2%). These results suggest that the increase in HA particle sizes depends on the length of aliphatic groups and the bond of methoxyl groups, and HAs were composed of HA molecules of homologous series. On the other hand, in DC, HA obtained from Dystric Cambisol, no significant relationships were observed between each type of protons and aromatic protons. It is suggested that a mixture of non-humic substances smeared the spectral data of DC.

Viscosimetric and chromatographic studies of soil humic acids

The aim of this study was to obtain information on molecular parameters of humic acids (HA) extracted from soils under various fertilization treatments. HAs were extracted from a grey-brown podzolic soil and chernozem under two long-term static field experiments. The fertilization variants were control (without fertilization) and manuring. Viscosimetric and gel permeation chromatography determinations were conducted in NaOH (pH = 10.5) and NaCl (pH = 7.0) solutions. A few models were used for calculations of the shapes of the HA molecules. The results confirmed the properties of HAs typical for high-molecular polyelectrolytes. The size of their molecules depended on the pH and the concentration of HAs in solution. As pH increased, both their mean molecular weight and spheric parameters increased. Fertilization modified molecular parameters of HAs. HAs extracted from the soils under mineral fertilization had molecules of similar parameters. HAs from manured soils had lower mean molecular weight associated with a supply of fresh organic matter.

Group Contribution Method for Predicting Equilibria of Nonionic Organic Compounds between Soil Organic Matter and Water

Soil organic matter (SOM) is known to be responsible for nonionic organic compound sorption in water-saturated soil, but the equilibria and kinetics of this process are not fully understood. Since the kinetics depend on the equilibrium driving force, understanding the equilibrium solubility of organic compounds in SOM is particularly important. This work describes a method of predicting SOM-water partition coefficients for nonionic organic compounds based on the UNIFAC-type models. Partition coefficients are calculated from the predicted activity coefficients of organic compounds in water and in SOM phases. A model humic acid molecule is used to represent SOM as a polymeric phase, and the ELBRO-FV model is applied to predict SOM-solute activity coefficients. This method resulted in partition coefficients that were within 1 order of magnitude of experimental values for most compounds studied. Chlorinated compounds do not seem to be modeled well by this method, and predictions may be in error by an order of magnitude. Using data rather than UNIFAC predictions for the water phase activity coefficients improved the predictions to within a factor of 3 of the experimental values for all compounds where activity data was available.

Adsorption and transformation of salt marsh related humic acids by quartz and clay minerals

Humic acids isolated from living and dead Spartina alterniflora plants and the marsh mud of a Georgia salt marsh estuary were adsorbed up to 12% on quartz particles, 85% on kaolinite and 90% on montmorillonite. The results of, for example, elemental analyses (up to 13.5% higher C content in humic acids remained after interaction with clay minerals) and Fourier transform infrared (FTIR) spectra indicate that aliphatic structures of the heterogenous humic acid molecules were preferentially adsorbed. However, since the ash contents of humic acids remaining in solution rose to as high as 45%, this fraction also apparently reacts with clay minerals. Up to 13.7% and 4.8% of humic acids were re-extracted from kaolinite and montmorillonite, respectively, using 0.1 N H 2SO 4. A mixture of 0.1 M NaOH + 0.1 M Na 4P 2O 7, enhanced the yields of 34.6% for kaolinite and 68.4% for montmorillonite. These numbers indicate that appreciable amounts of humic acids were transformed into humins which cannot be extracted by alkali from the mineral matrix. All re-extracted humic acids were rich in ash, which was composed of the clay mineral used. Although we found previously that the salt marsh related humic acids can be decomposed by microorganisms, their adsorption on mineral surfaces, especially on clay minerals, may contribute to their geochemical stability in the salt marsh environment.

Radioactive labelling and characterisation of humic materials

Humic acids can radiolabelled either by chemical techniques, either with iodine or with carbon-14. This paper discusses the advantages and disadvantages of the different labelling techniques for chemical radiolabelling of humic materials as well as the optimum conditions for the labelling. Various techniques have been applied in order to characterize the labelled material to elucidate possible changes of the inherent characteristics of the humic acids introduced during the labelling process. These techniques comprise ultrafiltration, e.g. size fractionation, metal complexing ability, UV absorbance and fluorescence, and DEAE cellulose abstraction. It has been demonstrated that with respect to these features, the radiolabelling techniques do not alter the humic acid molecules.

Solubilized coal as a substrate for biodesulfurization

Problem areas in a multi-step coal desulfurization process were investigated. The process involves mild hot air oxidation, followed by alkaline extraction to separate the resultant humic acid molecules from the insoluble pyrite and ash, and microbial removal of organic sulfur from the solubilized coal molecules. A computer-controlled and -monitored solubilization reactor for humic acid production was constructed. Studies designed to evaluate the physical/chemical nature and solubility characteristics of the humic acids were undertaken, with the ultimate aim of ensuring accessibility of sulfur atoms buried in the interior of these large hydrocarbon macromolecules. One per cent Tween-80 enhanced the solubility of humic acids four-fold. Radiotracer labelled humic acids containing 10.8 × 10 −11 Bq mg −1 of 125I were produced. These labelled humic acids will be used to develop an accurate and dependable assay for microbial organic desulfurization.