Showing papers on "Humic acid published in 1999"

X-ray absorption spectroscopic evidence for the complexation of HG(II) by reduced sulfur in soil humic substances

Abstract: Analysis of Hg(II) complexed by a soil humic acid (HA) using synchrotron-based X-ray absorption spectroscopy (XAS) revealed the importance of reduces sulfur functional groups (thiol (R-SH) and disulfide (R-SS-R)/disulfane (R-SSH)) in humic substances in the complexation of Hg(II). A two-coordinate binding environment with one oxygen atom and one sulfur atom at distances of 2.02 and 2.38 {angstrom}, respectively, was found in the first coordination shell of Hg(II) complexed by humic acid. Model calculations show that a second coordination sphere could contain one carbon atom and a second sulfur atom at 2.78 and 2.93 {angstrom}, respectively. This suggests that in addition to thiol S, disulfide/disulfane S may be involved with the complexation of Hg(II) in soil organic matter. The appearance of carbon atom in the second coordination shell suggests that one O-containing ligand such as carboxyl and phenol ligands rather than H{sub 2}O molecule is bound to the Hg(II). The involvement of oxygen ligand in addition to the reduced S ligands in the complexation of Hg(II) is due to the low density of reduced S ligands in humic substances. The XAS results from this experiment provided direct molecular level evidence for the preference of reduced S functional groups over oxygen ligandsmore » by Hg(II) in the complexation with humic substances.« less

Role of hydrophobic components of soil organic matter in soil aggregate stability

Abstract: Little is known about the effects of the individual components of organic matter (OM) on aggregate stability (AS) We hypothesized that AS of a Typic Haplustalf from which native OM was either removed or retained would be affected by incubation periods and application rates of a hydrophilic polysaccharide gum (G) and a hydrophobic stearic acid (S) with or without pretreatment with a hydrophobic humic acid (HA) Removal of OM reduced AS of unmodified soil by 40 and 20% after soil incubation for 7 and 40 d, respectively In both soil samples, AS was best at the highest rate of G (50 g kg 1 ) Its effect was better on Soil A (where OM was removed) than Soil B (where OM was retained) but diminished rapidly during 40 d At this rate, G increased AS by 750% in Soil A and by 335% in Soil B compared with no addition With S, aggregate stability increased more with time in Soil B than in Soil A Its maximum effect was also at the highest application rate (50 g kg -1 ), where AS increased 100% on Soil A and 131% on Soil B At the highest rate (02 g kg -1 ), HA increased AS by 73% on Soil B and 27% on Soil A The effect of HA alone did not vary with time Soil pretreatment with HA before addition of G reduced significantly both the state of aggregation and AS of both soils The reverse occurred when HA was applied before S After 40 d, S+HA increased AS in Soil B by 34%, whereas G and G+HA decreased AS by 14 and 4%, respectively We found that soil AS was improved and maintained with time more by hydrophobic than by hydrophilic components of organic matter Long-lasting aggregate stability of soils can be thus achieved by addition of hydrophobic humic material with hydrophobic organic wastes

Relationships between structure and binding affinity of humic substances for polycyclic aromatic hydrocarbons: relevance of molecular descriptors

Abstract: Partition coefficients for the binding affinities of pyrene, fluoranthene, and anthracene to 26 different humic materials were determined by fluorescence quenching. Sources included isolated humic acids, fulvic acids, and combined humic and fulvic fractions from soil, peat, and freshwater as well as Aldrich humic acid. Each of the humic materials was characterized by elemental composition, ultraviolet absorbance at 280 nm, molecular weight, and for 19 samples, composition of main structural fragments determined by 13C solution-state NMR. The magnitude of the Koc values correlated strongly with the independent descriptors of aromaticity of humic materials, including atomic H/C ratio, absorptivity at 280 nm, and three interdependent 13C NMR descriptors (CAr-H,R, ΣCAr, ΣCAr/ΣCAlk). Statistical comparison of humic sources grouped by the origin revealed that binding affinities were best predicted by the 13C NMR descriptors, with a slight prevalence of ΣCAr/ΣCAlk ratio, while molecular weight was the poorest pr...

Organic Waste Amendments Effect on Zinc Fractions of Two Soils

Abstract: Organic soil amendments can ameliorate metal toxicity to plants by redistributing metals to less available fractions. The objective of this study was to determine the effects of organic amendments on Zn distribution among soil fractions. Two soils were amended with five organic waste materials (some of which contained Zn) or commercial humic acid with and without 400 mg kg{sup {minus}1} Zn, incubated, and fractionated using a sequential extraction technique. Where no Zn was added most of the metals were in the residual fraction. Commercial compost, poultry litter, and industrial sewage sludge increased Zn in the exchangeable (EXC), organic (OM), and manganese oxide (MnOx) fractions due to Zn in the materials. Spent mushroom compost (SMC) redistributed Zn from the EXC fraction to the MnOx fraction for the coarse-textured soil. Where Zn was added, most of the metal was in the EXC and OM fractions. The SMC and humic acid lowered Zn in the EXC fraction and increased Zn in the other fractions. Effects of the organic materials on Zn in soil fractions were more evident for the sandy soil dominated by quartz in the clay than for the finer-textured soil dominated by kaolinite in the clay-size fraction. It was concluded thatmore » organic materials high in Zn can increase Zn in the EXC, OM, and MnOx fractions where the soil is not contaminated and others such as SMC and HA can lower the potential availability of Zn in contaminated soils by redistributing it from the EXC to less soluble fractions.« less

Effect of kinetics of complexation by humic acid on toxicity of copper to Ceriodaphnia dubia

Abstract: The rate of reaction of trace metal ions is an important consideration when studying the chemistry of trace metals in natural waters. The application of speciation models to natural water systems requires knowledge of kinetics if reactions are slow. Most bioassay and toxicity tests conducted in static and flow-through systems have not taken reaction kinetics into account. Therefore, results from these studies may overestimate the toxicity in the receiving waters. In the present study, the kinetics of the interaction of Cu(II) with humic acid (HA) and its influence on the toxicity of copper to Ceriodaphnia dubia were investigated by both chemical kinetic studies using a copper ion selective electrode and bioassay tests using a continuous flow-through bioassay system. A two-ligand site with first-order rate constants model gave a very good description of experimental kinetic data of the change of free Cu2+ concentration. Average k1 was 1.85/h and average k2 was 0.094/h. Bioassay tests indicated that different reaction times of copper with HA solution produced different toxic effects to organisms. We determined the hydrodynamic characteristics of the bioassay chambers to better describe the exposure of the organisms to free Cu2+. The bioassays supported the free ion activity model that the bioavailability and therefore toxicity of copper was directly correlated to the free Cu2+ concentration rather than to the total copper concentration. It was further shown that conventional chemical kinetics can be used to predict the toxicity of copper in these bioassays. This study supports the importance of considering reaction kinetics when studying the chemistry of trace metals in natural waters.

Atomic Force Microscopy of Humic Substances: Effects of pH and Ionic Strength

Abstract: Tapping mode atomic force microscopy was employed to determine the shape, size, and properties of the Suwannee River humic acid and a U.K. Geological Survey peat humic acid sorbed to mica. Humic acid macromolecules are shown to have a globular shape with an adsorbed height between 0.5 and 2 nm for a concentration of 10 mg L-1, pH > 5, and I < 500 mM NaCl. Aggregation of the humic macromolecules was not systematically observed at low pH and ionic strength. Instead aggregation may have been to be related to the hydrophobicity of the sample: no aggregates were observed for the relatively hydrophilic Suwannee River humic acid between pH 3 and pH 10 (I = 5−50 mM), while aggregates of peat humic acid were observed at low pH (pH < 5) and high ionic strength (I ∼ 500 mM). Calculated and measured hydrodynamic diameters determined under the same conditions were found to be systematically slightly larger than AFM heights, in good agreement with expectations given the different nature of the two measurements.

Metal Ion Adsorption to Complexes of Humic Acid and Metal Oxides: Deviations from the Additivity Rule

Abstract: 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 s...

Effect of Solution Chemistry on the Extent of Binding of Phenanthrene by a Soil Humic Acid: A Comparison of Dissolved and Clay Bound Humic

Abstract: The effect of pH, ionic strength, and cation in solution on the binding of phenanthrene by a soil humic acid in the aqueous phase was determined using fluorescence quenching. The phenanthrene binding coefficient with the dissolved soil humic, K{sub oc}, decreased with increasing ionic strength and solution cation valence. At low values of ionic strength, K{sub oc} values for this soil humic acid increased with increasing pH. For this humic sample, the experimental results were consistent with a conformational model of the humic substance in aqueous solution where, depending on solution conditions, some parts of the humic structure may be more open to allow increased PAH access to attachment sites. After sorption onto clays, supernatant solutions of the unadsorbed humic fraction yielded lower K{sub oc} values than the original bulk humic acid, suggesting that the humic substance was fractionating during its sorption onto the clays. Additionally, the extent of phenanthrene binding with the adsorbed humic fraction was lower than the results determined for the bulk humic acid prior to adsorption. The conformation of the humic substance when sorbed onto the inorganic surface appears to be affecting the level of phenanthrene binding by the humic acid.

Kinetics of humic acid adsorption at solid-water interfaces

Abstract: The adsorption kinetics of purified Aldrich humic acid (PAHA) onto hydrophilic (Fe2O3 and Al2O3) and hydrophobic (polystyrene and silanized SiO2) surfaces are studied by reflectometry. The initial rate of adsorption depends on the rate of transport and the rate of attachment. Attachment on hydrophilic surfaces is relatively fast at low pH where surface and HA attract each other electrostatically. Moreover, carboxylic and phenolic groups are exposed to the outside of the HA molecules, and these groups form complexes with surface hydroxyl groups. Due to the high attachment rate the process is transport-limited. At high pH, where surface and HA repel each other electrostatically, attachment is slow, and the adsorption rate is attachment-limited. At hydrophobic surfaces attachment of HA takes place through hydrophobic attraction. Hydrophobic groups are hidden in the inner part of HA molecules, and structural rearrangements are required before attachment can occur. The slow attachment leads to an attachment-li...

Effect of Humic Constituents on the Transformation of Chlorinated Phenols and Anilines in the Presence of Oxidoreductive Enzymes or Birnessite

Abstract: Chlorinated phenols and anilines are transformed and detoxified in soil through oxidative coupling reactions mediated by enzymes or metal oxides. The reactions may be influenced by humic constituents, such as syringaldehyde or catechol, that originate from lignin decomposition and are also subject to oxidative coupling. In this study, the effect of humic constituents on xenobiotic transformation was evaluated in vitro based on the determination of unreacted chlorophenols and chloroanilines. In experiments with peroxidase, laccase, and birnessite (δ-MnO2), the transformation of most chlorophenols was considerably enhanced by the addition of syringaldehyde. Less enhancement was observed using 4-hydroxybenzoic acid, and the addition of catechol resulted in a reduction of most transformations. The opposite was observed in experiments with tyrosinase, in which case catechol caused considerable enhancement of chlorophenol transformation. The varying effect of catechol can be explained by different transformatio...

Conformational Modeling of a New Building Block of Humic Acid: Approaches to the Lowest Energy Conformer

Abstract: Multiple structural models of humic acid (HA) building blocks have been reported. In this work, the modeling is based on two structural motifs: (i) the Steelink structure and (ii) a new humic acid [TNB] building block, which incorporates more fully the results of experimental data and retro-biosynthetic analyses. Both have significant conformational freedom complicated by their stereochemistry. A molecular modeling approach for the analysis of complex molecules with significant conformational freedom is described as it relates to the newly proposed humic acid building block. A potential energy surface for various conformers of the low-energy stereoisomer has been generated. Included in this discussion is the relationship of the stereochemistry to conformation and secondary structure.

Degradation of soil humic extract by wood- and soil-associated fungi, bacteria, and commercial enzymes

Abstract: > 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.

Influence of dissolved organic matter on the toxicity of copper to Ceriodaphnia dubia: Effect of complexation kinetics

Abstract: The reaction kinetics of copper interaction with dissolved organic matter (DOM) in water were studied in order to determine the effect of equilibration period on the toxicity of copper to aquatic organisms. The changes in physical and chemical forms of the copper during four reaction times were examined in four completely mixed reactors in series; the bioavailability of the copper as a function of these new forms was then determined with a flow-through bioassay system, using Ceriodaphnia dubia as a test organism. This study showed that the toxicity of copper to C. dubia decreased with increasing copper-DOM reaction time, which demonstrated that the copper reaction rate with dissolved organic components in the test water was slow. The toxicity of copper to C. dubia was closely related to the measured free-copper concentration (CU{sup 2+}) rather than to the total copper concentrations, a fact that supports the free ion activity model. The authors found that the LC50 of copper for C. dubia increased linearly with increasing total available binding sites. Although a similar trend was observed in both natural DOM and commercial humic acid, their results indicated that for a given copper-organic carbon ratio, copper binds more strongly to humic acid thanmore » to the natural DOM. This difference may be attributed to the greater copper binding affinity of humic acid (greater than that of other metal-binding organic fractions present in DOM, i.e., fulvic acid).« less