SEARCH · PubMed Health

Results for “Humic Substances”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,441 records · Page 80Linked to original sources

PubMed2006

Complexation of copper(ll) in organic soils and in dissolved organic matter--EXAFS evidence for chelate ring structures.

Associations with functional groups of natural organic matter (NOM) are of great importance for bioavailability, toxicity, and mobility of trace metals in soils and waters. In this study, the coordination chemistry of copper, Cu(ll), in organic soils and dissolved organic matter (DOM) from soils and streams was investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. In both soil organic matter (SOM) and DOM (990-11 000 microg Cu g(-1) dryweight, pH 2.8-6.3), Cu(ll) was coordinated by 4 oxygen/ nitrogen (O/N) atoms at a distance of 1.92-1.95 angstroms in the first coordination shell. These four atoms are positioned in the equatorial plane of a Jahn-Teller distorted octahedron. In samples with a pH of 4.8-6.3, a second coordination shell with 2.0-3.8 C atoms was located at a distance of 2.76-2.86 angstroms. A significant improvement (19-39%) of the fit was obtained by including a third coordination shell with 2.0-3.8 O/C atoms involved in single scattering at an average distance of 3.69 angstroms and multiple scattering at an average distance of 4.19 angstroms. Our results provide evidence for inner-sphere complexation of Cu(ll) in NOM and suggest that Cu(ll) is complexed by either one or two five-membered chelate rings involving possible combinations of amino, carboxyl, or carbonyl functional groups. Ion activity measurements showed that less than 0.2% of total Cu was in the form of free Cu2+ in our samples at pH 4.8-6.3.

Copper↗

PubMed2006

Effects of surface sorption on microbial degradation of glyphosate.

Sorption may affect the bioavailability and biodegradation of pesticides in soils. The aim of this study was to test the effect of surface sorption on microbial utilization of the herbicide glyphosate as a source of phosphorus, nitrogen, or carbon. We added goethite to a humus soil to manipulate the soil's glyphosate sorption capacity. The addition of glyphosate generally either decreased microbial CO2 production or produced no effect. Additions of glyphosate, in combination with glucose and N, did not change the respiration rate in comparison with the same treatment but without glyphosate. In contrast, glyphosate additions combined with glucose and P decreased microbial growth, whereas the combination with goethite counteracted the negative effect. The different treatments were examined using attenuated total reflectance Fourier transform (ATR-FTIR) spectroscopy; the results suggest that glyphosate was de-carboxylated in the sorbed state. Stimulating microbial growth by the addition of glucose and nitrogen resulted in further oxidation of glyphosate and only phosphate was detectable on the goethite surface after 13 days incubation. Our results show that sorbed glyphosate is microbially degradable, and it retards microbial activity. This study emphasizes the importance of combining quantitative measurements with a molecular-level examination, to better understand biogeochemical processes.

Biodegradation, Environmental↗

PubMed2006

Influence of sediment components on the immobilization of Zn during microbial Fe-(hydr)oxide reduction.

The fate of Zn and other sorbed heavy metals during microbial reduction of iron oxides is different when comparing synthetic Fe-(hydr)oxides and natural sediments undergoing a similar degree of iron reduction. Batch experiments with the iron-reducing organism Shewanella putrefaciens were conducted to examine the effects of an aqueous complexant (nitrilotriacetic acid or NTA), two solid-phase complexants (kaolinite and montmorillonite), an electron carrier (anthraquinone disulfonic acid or AQDS), and a humic acid on the speciation of Zn during microbial reduction of synthetic goethite. Compared to systems containing only goethite and Zn, microbial Fe(III) reduction in the presence of clay resulted in up to a 50% reduction in Zn immobilization (insoluble in a 2 h 0.5 M HCl extraction) without affecting Fe(II) production. NTA (3 mM) increased Fe(II) production 2-fold and resulted in recovery of nearly 75% of Zn in the aqueous fraction. AQDS (50 microM) resulted in a 12.5% decrease in Fe(II) production and a 44% reduction in Zn immobilization. Humic acid additions resulted in up to a 25% decrease in Fe(II) production and 51% decrease in Zn immobilization. The results suggest that all the components examined here as either complexing agents or electron shuttles reduce the degree of Zn immobilization by limiting the availability of Zn for incorporation into newly formed biogenic minerals. These results have implications for the remediation of heavy metals in a variety of natural sediments.

Anthraquinones↗

PubMed2006

Effects of natural water ions and humic acid on catalytic nitrate reduction kinetics using an alumina supported Pd-Cu catalyst.

Catalytic nitrate reduction was evaluated for the purpose of drinking water treatment. Common anions present in natural waters and humic acid were evaluated for their effects on NO3(-) hydrogenation over a bimetallic supported catalyst (Pd-Cu/gamma-Al2O3). Groundwater samples, with and without powder activated carbon (PAC) pretreatment, were also evaluated. In the absence of inhibitors the NO3- reduction rate was 2.4 x 10(-01) L/min g cat. However, the addition of constituents (SO4(2-), SO3(2-), HS-, CI-, HCO3-, OH-, and humic acid) on the order of representative concentrations for drinking water decreased the NO3- reduction rate. Sulfite, sulfide, and elevated chloride decreased the NO3- reduction rate by over 2 orders of magnitude. Preferential adsorption of Cl- inhibited NO3- reduction to a greater extent than NO2- reduction. Partial regeneration of catalysts exposed to SO3(2-) was achieved by using a dilute hypochlorite solution, however Cu dissolution occurred. Dissolved constituents in the groundwater sample decreased the NO3- reduction rate to 3.7 x 10(-03) L/min g cat and increased ammonia production. Removal of dissolved organic matter from the groundwater using PAC increased the NO3- reduction rate to 5.06 x 10(-02) L/min g cat and decreased ammonia production. Elemental analyses of catalysts exposed to the natural groundwater suggest that mineral precipitation may also contribute to catalyst fouling.

Aluminum↗

PubMed2006

Impact of natural organic matter on monochloramine reduction by granular activated carbon: the role of porosity and electrostatic surface properties.

Steady-state monochloramine reduction in fixed-bed reactors (FBRs) was quantified on five types of granular activated carbon (GAC) using two background waters-one natural source water (LAW) containing 2.5-3.5 mg/L organic carbon and one synthetic organic-free water (NW). While more monochloramine was reduced at steady-state using NW compared to LAW for each GAC and empty-bed contact time studied, the differences in removal varied considerably among the GACs tested. Physical characterization of the GACs suggested that the degree of interference caused by natural organic matter (NOM) increased with increasing GAC surface area contained within pores greater than 2 nm in width. Acid/base and electrostatic properties of the GACs were not found to be significant in terms of NOM uptake, which indicated that size exclusion effects of the GAC pores overwhelmed the impact of the GAC surface chemistry. Therefore, selection of GAC to limit the impact of NOM on monochloramine reduction in FBRs should be based on pore size distribution alone, with the impact of NOM decreasing with decreasing mesoporosity and macroporosity.

Carbon↗

PubMed2006

Sorption equilibrium of a wide spectrum of organic vapors in Leonardite humic acid: experimental setup and experimental data.

The environmental fate of volatile and semivolatile organic compounds is determined by their partitioning between air and soil constituents, in particular soil organic matter (SOM). While there are many studies on the partitioning of nonpolar compounds between water and SOM, data on sorption of polar compounds and data for sorption from the gas phase are rather limited. In this study, Leonardite humic acid/air partition coefficients for 188 polar and nonpolar organic compounds at temperatures between 5 and 75 degrees C and relative humidities between < 0.01% and 98% have been determined using a dynamic flow-through technique. To the best of our knowledge, this is by far the largest and most diverse and consistent data set for sorption into humic material published so far. The major results are as follows: the relative humidity affected the experimental partition coefficients by up to a factor of 3; polar compounds generally sorbed more strongly than nonpolar compounds due to H-bonding (electron donor/ acceptor interactions) with the humic acid; no glass transitions in the range of 5-75 degrees C that would be relevant with respect to the sorption behavior of hydrated Leonardite humic acid were observed; our experimental data agree well with experimental partition coefficients from various literature sources.

Chromatography, Gas↗

PubMed2006

Fulvic acids as transition state of organic matter: indications from high resolution mass spectrometry.

Fulvic acids are one of the largest classes of dissolved organic matter, but they are poorly defined and of unclear origin. Three fulvic acid isolates of different origin were analyzed by size-exclusion chromatography coupled to electrospray ionization-Fourier-transform ion cyclotron resonance (FTICR)-mass spectrometry, and molecular formulas for 700-1900 species in these isolates were derived. All three isolates show the same pattern in the elemental composition of their molecules and a large congruence in the molecular sets. It is proposed that the elemental and structural regularity of fulvic acid molecules does not indicate one common precursor material and formation process, but that this regularity is due to both the strong reworking of source materials in the environment and the valency of the three elements (C, H, O) from which most fulvic acid molecules are formed. Potential molecular formulas of fulvic acids were predicted for a mass range of 60 amu based on a few presumptions. A good agreement was found between the predicted and the detected molecular formulas, and it is concluded that (poly-)carboxylic acids with very limited number of hydroxy groups are the major compound class in fulvic acid isolates. It appears that fulvic acids are metastable molecules that characterize a state of transition of diverse precursor compounds during their oxidation.

Benzopyrans↗

PubMed2006

Effect of process variables and natural organic matter on removal of microcystin-LR by PAC-UF.

The release of cyanobacterial toxins, such as microcystin-LR, in drinking water supplies is of increasing concern. In this study, we investigated the use of ultrafiltration (UF) combined with adsorption on powdered activated carbon (PAC) for the removal of microcystin-LR from drinking water. Process variables examined included PAC type, PAC dosage, membrane characteristics (material and pore size), and the presence of natural organic matter (NOM). Due to greater mesopore volume, wood-based activated carbon was up to 4-times more effective at removing microcystin-LR than coconut-based carbon, depending on contact time. Cellulose acetate (CA) membranes with a molecular weight cutoff (MWCO) of 20,000 Da did not reject or adsorb microcystin-LR. Membranes composed of polyethersulfone (PES) of similar pore size, on the other hand, adsorbed microcystin-LR presumably through hydrophobic interactions. A PES membrane with a MWCO of 5000 Da sorbed microcystin-LR, and also rejected 8.4% of the toxin through a size exclusion mechanism. When PAC was coupled to UF using PES membranes, greater removal of microcystin-LR occurred compared to when CA membranes were used due to sorption of the toxin to the PES membrane surface. The presence of Suwannee River fulvic acid (SRFA) reduced microcystin-LR removal by PAC-UF, primarily due to competition between SRFA and microcystin-LR for sites on the PAC surface.

Bacterial Toxins↗

PubMed2006

Modeling the formation of N-nitrosodimethylamine (NDMA) from the reaction of natural organic matter (NOM) with monochloramine.

This paper presents mechanistic studies on the formation of NDMA, a newly identified chloramination disinfection byproduct, from reactions of monochloramine with natural organic matter. A kinetic model was developed to validate proposed reactions and to predict NDMA formation in chloraminated water during the time frame of 1-5 days. This involved incorporating NDMA formation reactions into an established comprehensive model describing the oxidation of humic-type natural organic matter by monochloramine. A rate-limiting step involving the oxidation of NOM is theorized to control the rate of NDMA formation which is assumed to be proportional to the rate of NOM oxidized by monochloramine. The applicability of the model to describe NDMA formation in the presence of three NOM sources over a wide range in water quality (i.e., pH, DOC, and ammonia concentrations) was evaluated. Results show that with accurate measurement of monochloramine demand for a specific supply, NDMA formation could be modeled over an extended range of experimental conditions by considering a single NOM source-specific value of thetaNDMA, a stoichiometric coefficient relating the amount of NDMA produced to the amount of NOM oxidized, and several kinetic parameters describing NOM oxidation. Furthermore, the oxidation of NOM is the rate-limiting step governing NDMA formation. This suggests that NDMA formation over a 1-5 day time frame may be estimated from information on the chloramine or free chlorine demand of the NOM and the source-specific linear relationship between this demand and NDMA formation. Although the proposed model has not yet been validated for shorter time periods that may better characterize the residence time in some distribution systems, the improved understanding of the important reactions governing NDMA formation and the resulting model should benefit the water treatment industry as a tool in developing strategies that minimize NDMA formation.

Ammonia↗

PubMed2006

Water-soluble oligomer formation from acid-catalyzed reactions of levoglucosan in proxies of atmospheric aqueous aerosols.

Herein is reported the first laboratory observation of the oligomerization of levoglucosan studied under atmospherically relevant conditions. Oligomers up to 1458 Da (9-mer) were measured by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. A rational mechanism is proposed based on both the acid-catalyzed cationic ring-opening of levoglucosan and nucleophilic attack of ROH from levoglucosan on the hemi-acetal carbon to produce pyranose oligomers through the formation of glycosidic bonds. Oligomer formation is further supported by attenuated total reflectance Fourier transform infrared spectroscopy. Levoglucosan is a viable tracer for biomass burning aerosols, and the observed products may serve as secondary tracers for these types of aerosols, possibly providing additional information to facilitate source apportionment and better understand atmospheric processing of the aerosol parcel. Also, the processes supported here may contribute to the saccharide character of humic-like substances, which are proposed to be formed through the atmospheric processing of biomass burning aerosols.

Aerosols↗

PubMed2006

Nature and abundance of organic radicals in natural organic matter: effect of pH and irradiation.

Dissolved natural organic matter (NOM) plays an essential role in freshwater geochemical and biochemical processes. A major property, its redox behavior, can be attributed to the chinone building blocks, which can form stable radicals. However, electron paramagnetic resonance (EPR) data indicating free radicals on solid NOM are sparse. Here we present EPR spectra of 23 NOM from European surface waters isolated by reverse osmosis. The organic radical concentrations of NOM ranged from 5 x 10(15) to 1.84 x 10(17) spins g(-1), and g values ranged from 2.0031 to 2.0045. Number and type of organic radicals in solid NOM are significantly influenced by the pH of raw water. EPR experiments indicate the presence of semiquinone-type radicals in coexistence with carbon-centered "aromatic" radicals, with the semiquinone-type radicals dominating at alkaline pH. Basically these processes are reversible. Organic radical concentrations in NOM adjusted to pH 6.5 before freeze-drying correlate with iron and aluminum contents. UV- and VIS-irradiation of solid NOM can lead to more than a 10-fold increase of the concentration of organic radicals. These radicals were long-lived and had the same g value as the original radical. Similar effects were not observed with isolated humic and fulvic acids, demonstrating the limited reflection of environmental properties of organic carbon by the classical isolation procedure.

Benzopyrans↗

PubMed2006

Zinc coordination to multiple ligand atoms in organic-rich surface soils.

We report on the solid-phase speciation of naturally occurring Zn in metalliferous organic-matter-rich surface soils. Synchrotron-based studies were used to probe elemental distribution and associations in soil particles (micro-XRF) together with the mineralogy (micro-XRD) and Zn bonding environment (Zn-micro-XANES) at the micrometer-scale level. The average bonding environment of Zn was also probed for bulk soils using XANES. We found the distribution of elements within soil particles to be heterogeneous; however, some elements are consistently co-located. While conventional XRD analyses of whole soils did not identify any Zn mineral phase, synchrotron-based-micro-XRD analyses indicated that sphalerite (ZnS) is present in a particle from a wetland soil (soil labeled G3). Linear combination fit (LCF) analyses of XANES spectra collected for bulk soils (Zn-XANES) and microm-regions (Zn-micro-XANES) within soil particles suggest Zn bonds to oxygen-, nitrogen-, and sulfur-functional groups in these sulfur-, nitrogen-, and zinc-rich organic surface soils. The XANES spectra of all bulk soils and of all microm-regions except for the wetland soil (G3), where ZnS was the most significant constituent, were best fitted by the Zn-arginine reference compound and therefore seems to indicate Zn bonding to nitrogen. Thus, these results provide compelling evidence of the formation of highly covalent Zn-organic bonds in the organic-rich surface soils that were studied. This may explain in part why metal partition coefficients (Kd) are generally higher in organic soils, and why the toxic thresholds for total metal concentrations are higher in organic than in mineral soils.

Electron Spin Resonance Spectroscopy↗

PubMed2006

Arsenite and arsenate binding to dissolved humic acids: influence of pH, type of humic acid, and aluminum.

The fate of arsenic in the aquatic environment is influenced by dissolved natural organic matter (DOM). Using an equilibrium dialysis method, conditional distribution coefficients (Dom) for As(III) and As(V) binding onto two commercial humic acids were determined at environmentally relevant As/dissolved organic carbon (DOC) ratios and as a function of pH. At all pH values, As(V) was more strongly bound than As(III). Maximum binding was observed around pH 7, which is consistent with H+ competition for binding sites at low pH values and OH- competition for the arsenic center at high pH. For both oxidation states, Dom values increased with decreasing As/DOC ratios. Dom values were fitted as a function of the As/DOC ratio for As(III) and As(V). Compared to the aquatic humic acid, the terrestrial humic acid had a higher affinity for arsenic binding with 1.5-3 times higher Dom values under the same conditions. Al3+ in excess to arsenic successfully competed for strong binding sites at low As/DOC ratios. Under environmentally relevant conditions, about 10% of total As(V) may be bound to DOM, whereas >10% of As(III) is bound to DOM at very low As/DOC ratios only. Binding of arsenic to DOM should be considered in natural systems.

Aluminum↗

PubMed2006

Oligomerization of humic phenolic monomers by oxidative coupling under biomimetic catalysis.

Three humic phenolic monomers, catechol (CAT), caffeic acid (CAFF), and p-coumaric acid (COUM), were subjected to oxidative coupling catalyzed by biomimetic water-soluble iron-porphyrin (Fe(TDCPPS)CI) in either separate or mixed solution, and the reaction products were characterized by gas chromatography-mass spectrometry (GC-MS) and electrospray-mass spectrometry (ESI-MS). The GC-MS analysis proved the formation of C-C and C-O dimers, whereas the ESI-MS/MS analysis also suggested trimerization for all the monomers and tetramerization for CAT. On the basis of mass spectra, molecular structures were assigned to the observed oligomers. In the phenolic separate solutions, dimers represented about 65%, 44%, and 30% of reaction products for CAT, CAFF, and COUM, respectively, whereas trimers were from 4 to 5%. A relevant part of the products were unidentified oligomers and several degradation compounds, mostly aromatic aldehydes and alcohols and aromatic or aliphatic carboxylic acids. When all three humic phenolic monomers underwent the catalyzed coupling reaction in one mixed solution, 14% of the reaction products were identified as C-C dimers of CAT. Although no other C-O dimers of CAT, nor any dimers of COUM and CAFF, could be identified, some other structurally unknown oligomers were present among the reaction products of the mixed solution. However, no oligomers larger than tetramers were formed in either separate or mixed solutions. This work indicates the essential role of biomimetic metal-porphyrins in catalyzing the oxidative coupling of humic phenolic monomers in aqueous media, thereby promoting the polymerization of natural organic matter.

Biomimetics↗

PubMed2006

The impact of particle-bound cadmium on bioavailability and bioaccumulation: A pragmatic approach.

Studying the bioavailability of sediment-bound contaminants is complicated by many reasons, such as the variable composition of the particles, their temporal variations, the low levels of contaminant concentrations, their partitioning between diverse aqueous and particulate phases, and the variety of uptake routes that may involved with the biota. Therefore, simple and innovative methodologies should be tested as analogues for natural sediments. Among them, a diverse selection of artificial particles with well-defined surface properties, in the presence and absence of commercially available humic acids, has been proposed and used to investigate the bioavailability of several organic pollutants. For this work, this model was applied to investigate the uptake and accumulation of cadmium by the freshwater oligochaete Lumbriculus variegatus. The results showed that the uptake of the metal depended on the free dissolved Cd(II) species, while the contribution from the particles was negligible. Thus, the extent of cadmium bioaccumulated from each test system could be predicted as a function of the rate of absorption of the free dissolved Cd(II) species. These species were calculated either from the particle-water partition coefficients, or by using the MINEQL+ computer program. In general, the estimated accumulation levels were in good agreement with the experimental results.

Animals↗

PubMed2000

Foliar application of selenite and selenate to potato (Solanum tuberosum): effect of a ligand agent on selenium content of tubers.

The effect of a foliar spray of selenium on potatoes was investigated for 2 years. Amounts of 0, 50, and 150 g of Se ha(-)(1) were applied both as sodium selenate and as sodium selenite in water, either pure or with the addition of 0.15% of soluble leonardite as a source of humic acids (pH 7). Tuber selenium concentration increased with the application levels, both with sodium selenate and with sodium selenite, when only aqueous solutions were used. When humic acids were added, the tuber selenium level rose more markedly after the application of sodium selenate as compared to the case of the aqueous solutions; however, in the case of sodium selenite, the level showed a large increase only after the application of 50 g of Se ha(-)(1). Kinetics showed that humic acids raised the selenate availability, but no differences were found in the distribution of selenium in the tuber fractions. Foliar application of selenium with humic acids was proven to be a good way to increase the selenium content of potatoes, but the assimilation process of selenium was simpler with selenate than with selenite.

Humic Substances↗

PubMed2002

Adsorption of imidazolinone herbicides on smectite-humic acid and smectite-ferrihydrite associations.

Adsorption of imazapyr (IMZ), imazethapyr (IMZT), and imazaquin (IMZQ) was studied on two smectite-humic acid and two smectite-ferrihydrite binary systems prepared by treating a Wyoming smectite with a humic acid extracted from soil (4 and 8% w/w of the smectite) and with just-precipitated synthetic ferrihydrite (8 and 16% w/w of the smectite). Adsorption of the three herbicides on the smectite was not measurable at pH >4.5, presumably because of negative charges on the surface of the smectite. Adsorption on the smectite-humic acid systems was also not measurable, presumably because of negative charges on the surface, despite the high affinity of the three herbicides for humic acid, the adsorption order of which was IMZ < IMZT << IMZQ. Adsorption decreased in the order IMZ < IMZT < IMZQ on the smectite-ferrihydrite systems and IMZQ < IMZT < IMZ on ferrihydrite, although here the differences were small. These results show that even though pure smectite cannot adsorb herbicides, it modifies the adsorption capacity of ferrihydrite. The mutual interaction of active phases such as humic acid, ferrihydrite, and smectite alters the characteristics of the resulting surface and hence the adsorption process. Investigations of herbicide adsorption have been seen to produce more reliable results if conducted on polyphasic systems rather than on single soil components.

Adsorption↗

PubMed2002

Studies on bound (14)C-chlorsulfuron residues in soil.

The cause for phytotoxicity of bound residues of chlorsulfuron (2-chloro-N-[[4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl]benzenesulfonamide) to rotational crops is unknown. This study was conducted to determine the formation of nonextractable (bound) residues of chlorsulfuron in soil, and the distribution of bound residues in different organic matter fractions. The results showed that over 150 days, the extractable fraction of (14)C-residues decreased to 25.1% of applied chlorsulfuron, while bound residues concurrently increased to 47.1%. The distribution of (14)C-bound residues in soil organic matter fractions followed an order of humic acid (HA) < humin < fulvic acid (FA). Although the most bound residues were detected in the FA fraction, the amount associated with the humin fraction increased with time. After soil treatment by autoclaving, it was found that bound (14)C-chlorsulfuron residues became available again in the soil. One of the released products was 2-amino-4-hydroxyl-6-methyl-1,3,5-triazine (identified by GC-MS), which is a degradation product of chlorsulfuron.

Benzopyrans↗