Spectroscopic and conformational properties of size-fractions separated from a lignite humic acid

Mean Activity Coefficients of Humic Acids as Physicochemical Characteristics of Their Behavior in Water Environment

In this work, the dissociation of humic acids is investigated from the point of view of their mean activity coefficients. They are determined on the basis of two different concepts: sparingly soluble substance and multistep mechanism. It was found that the mean activity coefficients are generally higher, if the traditional concept is applied to the data, excepting the HA-A sample. Both the used concepts provide the mean activity coefficients dependent on the ionic strength, the amount of dissolved humic acids, and the types of electrolyte added in the studied suspensions. Their values based on the concept of a multistep mechanism and determined for humic acids in NaCl and NaI form a continuous curve and the individual character of their ions did not assert. It means that activity coefficients were affected only by the ionic strength and valence factor of the electrolytes irrespective of their chemical composition. The mean activity coefficients obtained for humic acids in HCl are lower in comparison with NaCl and NaI due to the common ion H⁺. Comparing the results obtained for individual humic acids, we can state that the results obtained for the HA-E sample are very different from those of other samples. It seems that its solubility is very high, but the majority of the dissolved particles remained in the molecular form and only a small amount of ions is formed.

Transport of arsenic loaded by ferric humate colloid in saturated porous media

The transport behavior of arsenic (As(V)) loaded by ferric humate (HA-Fe) colloid, denoted as HA-Fe/As(V), moving in a saturated quartz sand column, was tested in the laboratory under varying pH values, ionic strengths, and HA and Fe(III) content. The time-fractional advection-dispersion equation (fADE) model was then employed to analyze the observed migration of HA-Fe/As(V). Results showed that the stability of the HA-Fe colloid exhibited an upward trend with an increasing pH and HA content. An increasing HA content led to a decrease in the particle size of the HA-Fe colloid. However, the effect of Fe(III) concentration on colloidal particle size exhibited the opposite phenomenon. The ability of the HA-Fe colloid to load As(V) gradually increased with the increase of the Fe(III) concentration. During the co-transport of the HA-Fe/As(V) colloid, transport of As(V) was promoted with increasing pH, increasing HA and Fe(III) content, and decreasing ionic strength in the saturated porous medium. The transport behavior of As(V) can be well fitted by the fADE model. The model analysis revealed that sub-diffusion of As(V) was weakened in the HA-Fe/As(V) colloid with high HA content. Sub-diffusion of As(V) in the low pH colloid was stronger than that of the high-pH colloid, and the molecular diffusion and mechanical dispersion were more weakened in the high-pH colloid than that of the low-pH colloid. When observing varying ionic strengths, As(V) exhibited stronger sub-diffusion in the HA-Fe/As(V) colloid with a higher ionic strength. As for the Fe(III) content, transport of As(V) was mainly affected by sub-diffusion in the HA-Fe/As(V) colloid with a low Fe(III) content. These findings provided direct and necessary insights into the effects of the HA-Fe colloid on the migration of As(V) throughout saturated porous media under different hydrochemical conditions found in natural environments.

A comparative study on the structural features of humic acids extracted from lignites using comprehensive spectral analyses

The lignite reserves of Zhaotong and Mile in China are abundant and lignite utilizations are limited, however, humic acids (HAs) extracted from lignites play a significant role in many fields including agriculture, environmental protection and so on. Herein, the structures of HAs extracted from Zhaotong and Mile lignites (denoted as ZLHA and MLHA, respectively) were characterized and compared to each other using comprehensive spectral analyses. As a result, the UV-Vis spectrum analyses of HAs indicated that the molecular weight of MLHA is larger than that of ZLHA. Cross polarization magic angle spinning ¹³C NMR, which is rarely used to analyze the structures of HAs using fitting peaks, and FT-IR spectrum analyses indicated that both the aromaticity and the oxygen-containing group contents of ZLHA are higher than those of MLHA, and the HAs' aromaticity could be confirmed by the results of the X-ray diffraction patterns. Additionally, the main existing forms of the elements in the HAs were obtained from X-ray photoelectron spectrum analyses, which are not commonly used for HA analyses. In this work, the utilization of comprehensive spectral analyses was an effective method to study the structural features of ZLHA and MLHA and it could provide a basic reference for the applications of ZLHA and MLHA.

The structural features of humic acids extracted from Zhaotong and Mile lignites were analyzed using comprehensive spectral analyses including UV-vis, FTIR, CP/MAS ¹³C NMR, XRD and XPS.

Quantitative Structure-Activity Relationship of Humic-Like Biostimulants Derived From Agro-Industrial Byproducts and Energy Crops

Humic-like substances (HLSs) isolated by alkaline oxidative hydrolysis from lignin-rich agro-industrial residues have been shown to exert biostimulant activity toward maize (Zea mays L.) germination and early growth. The definition of a quantitative structure-activity relationship (QSAR) between HLS and their bioactivity could be useful to predict their biological properties and tailor plant biostimulants for specific agronomic and industrial uses. Here, we created several projection on latent structure (PLS) regression by using published analytical data on the molecular composition of lignin-derived HLS obtained by both ¹³C-CPMAS-NMR spectra directly on samples and ³¹P-NMR spectra after derivatization of hydroxyl functions with a P-containing reagent (2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane). These spectral data were used to model the effect of HLS on the elongation of primary root, lateral seminal roots, total root apparatus, and coleoptile of maize. The ¹³C-CPMAS-NMR data suggested that methoxyl and aromatic moieties positively affected plant growth, while the carboxyl/esterified functions showed a negative impact on the overall seedling development. Alkyl C seems to promote Col elongation while concomitantly reducing that of the root system. Additionally, ³¹P-NMR-derived spectra revealed that the elongation of roots and Col were enhanced by the occurrence of aliphatic hydroxyl groups, and guaiacyl and p-Hydroxyphenyl lignin monomers. The PLS models based on raw dataset from ¹³C-CPMAS-NMR spectra explained more than 74% of the variance for the length of lateral seminal roots, total root system and coleoptile, while other parameters derived from ¹³C-CPMAS-NMR spectra, namely the Hydrophobicity and Hydrophilicity of materials were necessary to explain 83% of the variance of the primary root length. The results from ³¹P-NMR spectra explained the observed biological variance by 90, 96, 96, and 93% for the length of primary root, lateral seminal roots, total root system and coleoptile, respectively. This work shows that different NMR spectroscopy techniques can be used to build up PLS models which can predict the bioactivity of lignin-derived HLS toward early growth of maize plants. The established QSAR may also be exploited to enhance by chemical techniques the bioactive properties of HLS and enhance their plant stimulation capacity.

Structural characteristics of humic acids derived from Chinese weathered coal under different oxidizing conditions

Humic acids derived from Chinese weathered coal were oxidized with hydrogen peroxide (H2O2) under various conditions, and their chemical composition and structure were examined. The raw material humic acids (HA) and oxidized humic acids (OHAs) were characterized by elemental analysis and ultraviolet visible (UV-Vis), Fourier transform infrared (FTIR), and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Our results show that aromatic functional groups accounted for more than 70% of the HA and OHAs and there were significant differences in their structures and compositions. Compared to the HA, the average H and N contents of the OHAs decreased by 5.15% and 2.52%, respectively, and the average O content of those of the OHAs increased by 5.30%. The hydrophobicity index (HI) of HA is higher than those of the OHAs. Importantly, in the hypothesis test between the properties and preparation conditions of humic acid using SPSS, the partial η2 of the temperature, hydrogen peroxide concentration, liquid-solid ratio, and time were 0.809, 0.771, 0.748 and 0.729, respectively; thus, among the preparation conditions, temperature is the most important factor affecting the humic acids properties.

Evaluating membrane fouling potentials of dissolved organic matter in brackish water

Isolating dissolved organic matter (DOM) is a preliminary step that improves the accuracy of its characterization. In this study, DOM in brackish water was clearly separated and evaluated by multiple characterization analyses. The sample was divided into three fractions by preparative high-performance liquid chromatography (preparative HPLC) according to molecular size. The homogeneity of each fraction was estimated by analytical size exclusion chromatography (SEC) and fluorescence excitation-emission matrix (FEEM). Pyrolysis gas chromatography-mass spectrometry (Py-GC/MS) and liquid chromatography-organic carbon detection (LC-OCD) were used to characterize the physicochemical properties of each fraction. Py-GC/MS revealed that Fraction 1 consisted of evenly distributed organic matter in order polysaccharides, proteins, polyhydroxy aromatics, lignins, and lipids. However, Fraction 2 was primarily composed of dominant lipids and low portion of proteins, and Fraction 3 was composed predominantly of lignins and lipids. The LC-OCD results showed that Fractions 1 and 2 had similar organic carbon (OC) compositions: a humic substance (ca. 37%), building blocks (ca. 10%), and neutrals (ca. 37%), whereas Fraction 3 contained a high proportion of neutrals (62%). In the fouling experiments, the distinct DOM characteristics in each fraction resulted in different declining flux behaviors, ranked as: Fraction 2 > Fraction 1 > Fraction 3.

May humic acids or mineral fertilisation mitigate arsenic mobility and availability to carrot plants (Daucus carota L.) in a volcanic soil polluted by As from irrigation water?

Carrot (Daucus carota L.) is a widely consumed root vegetable, whose growth and safety might be threatened by growing-medium arsenic (As) contamination. By this work, we evaluated the effects of humic acids from Leonardite and NPK mineral fertilisation on As mobility and availability to carrot plants grown for 60 days in a volcanic soil irrigated with As-contaminated water - representing the most common scenario occurring in As-affected Italian areas. As expected, the irrigation with As-contaminated water caused a serious toxic effect on plant growth and photosynthetic rate; the highest rate of As also inhibited soil enzymatic activity. In contrast, the organic and mineral fertilisation alleviated, at least partially, the toxicity of As, essentially by stimulating plant growth and promoting nutrient uptake. The mobility of As in the volcanic soil and thus its phytoavailability were differently affected by the organic and mineral fertilisers; the application of humic acids mitigated the availability of the contaminant, likely by its partial immobilisation on humic acid sorption sites - thus raising up the intrinsic anionic sorption capacity of the volcanic soil; the mineral fertilisation enhanced the mobility of As in soil, probably due to competition of P for the anionic sorption sites of the soil variable-charge minerals, very affine to available P. These findings hence suggest that a proper soil management of As-polluted volcanic soils and amendment by stable organic matter might mitigate the environmental risk of these soils, thus minimising the availability of As to biota.

Water-based fractionation of a commercial humic acid. Solid-state and colloidal characterization of the solubility fractions

BACKGROUND AND HYPOTHESIS

Humic acid (HA) is of considerable environmental significance, being a major component of soil, as well as being considered for application in other technological areas. However, its structure and colloidal properties continue to be the subject of debate, largely owing to its molecular complexity and association with other humic substances and mineral matter. As a class, HA is considered to comprise supramolecular assemblies of heterogeneous species, and herein we consider a simple route for the separation of some HA sub-fractions.

EXPERIMENTS

A commercial HA sample from Sigma-Aldrich has been fractionated into two soluble (S1, S2) and two insoluble (I1, I2) fractions by successive dissolution in deionized water at near-neutral pH. These sub-fractions have been characterized by solution and solid-state approaches.

FINDINGS

Using this simple approach, the HA has been shown to contain non-covalently bonded species with different polarity and water solubility. The soluble and insoluble fractions have very different chemical structures, as revealed particularly by their solid-state properties (¹³C NMR and IR spectroscopy, and TGA); in particular, S1 and S2 are characterized by higher carbonyl and aromatic contents, compared with I1 and I2. As shown by solution SAXS measurements and AFM, the soluble fractions behave as hydrophilic colloidal aggregates of at least 50nm diameter.

Assessment of trace heavy metals dynamics during the interaction of aqueous solutions with the artificial OECD soil: Evaluation of the effect of soil organic matter content and colloidal mobilization

A micro-contamination phenomenon was reproduced and studied at lab-scale, mimicking the irrigation of a standard artificial soil with a water solution containing three Heavy Metals (HMs) at trace concentration level. To assess the dynamics of micro-pollutants accumulation and migration trough the soil, the organic matter in the soil was varied, together with sodicity of the irrigation water. Accumulation of the investigated contaminants was observed mainly in the top layer (≤1 cm) of the irrigated soil. This was attributed to the high interaction capacity of the soil compared to the low HM concentrations in the water phase. HMs transport pattern was described assuming a multi-component mechanism including: i) the interaction of HMs with the colloidal phase of the soil; ii) the slow and constant release of small molecular weight ligands detaching from the soil immobile matrix; iii) the transportation of HMs through the soil by these low molecular weight chaperon molecules. The mobility was directly related to the soil organic matter (SOM), since higher amount of SOM correspond to a higher number of chaperon molecules. In the first centimetre of the soil the metals were mostly bound to the acid labile fraction. Very low mobilization was observed with increasing sodicity in the leaching water, since such conditions were unfavourable to the colloidal mobilization of SOM. This indicated that water/soil transfer of pollutant is not only related to the contaminant concentration in the irrigation water but also to the characteristics of the aqueous solution and to the physical-chemical properties of the soil.

Partitioning of humic acids between aqueous solution and hydrogel. 2. Impact of physicochemical conditions

The effects of the physicochemical features of aqueous medium on the mode of partitioning of humic acids (HAs) into a model biomimetic gel (alginate) and a synthetic polyacrylamide gel (PAAm) were explored. Experiments were performed under conditions of different pH and ionic strength as well as in the presence or absence of complexing divalent metal ions. The amount of HA penetrating the gel phase was determined by measuring its natural fluorescence by confocal laser scanning microscopy. In both gel types, the accumulation of HA was spatially heterogeneous, with a much higher concentration located within a thin film at the gel surface. The thickness of the surface film (ca. 15 μm) was similar for both types of gel and practically independent of pH, ionic strength, and the presence of complexing divalent metal ions. The extent of HA accumulation was found to be dependent on the composition of the medium and on the type of gel. Significantly more HA was accumulated in PAAm gel as compared to that in alginate gel. In general, more HA was accumulated at lower background salt concentration levels. The distribution of different types of HA species in the gel body was linked to their behavior in the medium and the differences in physicochemical conditions inside the two phases.

The physico-chemical properties and biostimulative activities of humic substances regenerated from lignite

The positive effect of humic acids on the growth of plant roots is well known, however, the mechanisms and role of their physical structure in these processes have not been fully explained yet. In this work, South-Moravian lignite was oxidized by means of nitric acid and hydrogen peroxide to produce a set of regenerated humic acids. The elemental composition, solid state stability and solution characteristics were determined and correlated in vitro with their biological activity. A modified hydroponic method was applied to determine the effects of their potassium salts on Zea mays seedlings roots with respect to the plant weight, root length, root division, and starch and protein content. The relations between the determined parameters were evaluated through Principal Component Analysis and Pearson's correlation coefficients. The results indicated that the most important factor determining the biological activity of South-Moravian lignite potassium humates is related to the nature of self-assemblies, while the chemical composition had no direct connection with the root growth of Zea mays seedlings. It was demonstrated a controlled processing that provided humic substances with different chemical and physicochemical properties and variable biological activity.

H-binding of size- and polarity-fractionated soil and lignite humic acids after removal of metal and ash components

A fractionation technique, combining dialysis removal of metal and ash components with hydrofluoric acid and pH 10 citrate buffer followed by chromatography of dialysis permeate on XAD-8 resin at decreasing pH values, has been applied to lignite humic acid (lignite-HA) and soil humic acid (soil-HA). H-binding data and non ideal competitive adsorption-Donnan model parameters were obtained for the HA fractions by theoretical analysis of H-binding data which reveal a significant increase of the carboxyl and the phenolic charge for the lignite-HA fractions vs. the parental lignite humic acid (LParentalHA). The fractionated lignite-HA material consisted mainly of permeate fractions, some of which were fulvic acid-like. The fractionated soil-HA material consisted mainly of large macromolecular structures that did not permeate the dialysis membrane during deashing. Chargeable groups had comparable concentrations in soil-HA fractions and parental soil humic acid (SParentalHA), indicating minimal interference of ash components with carboxyl and phenolic (and/or enolic) groups. Fractionation of HA, combined with theoretical analysis of H-binding, can distinguish the supramolecular vs. macromolecular nature of fractions within the same parental HA.

Evolution of natural organic matter by size exclusion chromatography during photocatalytic degradation by solvothermal-synthesized titanium dioxide

This study shows the effect of different titanium dioxides in transforming the structural properties of natural organic matter (NOM) during photocatalytic degradation with a solar UV light simulator. Titanium dioxide (TiO(2)) synthesized by the sol-gel method coupled with the solvothermal technique and Degussa P-25 TiO(2) were used. The evolution of NOM degradation was followed by size exclusion chromatography with dissolved organic carbon, ultraviolet and fluorescence detection (SEC-DOC, SEC-UV(254) and SEC-Fl(254/450)). For both catalysts, there was a preferential degradation of the larger molecules of NOM into medium and smaller molecular size fractions. However, the synthesized TiO(2) was found to be more efficient than Degussa P-25 TiO(2) for DOC removal, especially UV(254) absorption and Fl(254/450) removal.

Comparing the heterogeneity of copper-binding characteristics for two different-sized soil humic acid fractions using fluorescence quenching combined with 2D-COS

Heterogeneous distributions of copper-binding characteristics were compared for two ultrafiltered size fractions of a soil HA using fluorescence quenching combined with two-dimensional correlation spectroscopy (2D-COS). The apparent shapes of the original synchronous fluorescence spectra and the extent of the fluorescence quenching upon the addition of copper were similar for the two fractions. The stability constants calculated at their highest peaks were not significantly different. However, the 2D-COS results revealed that the fluorescence quenching behaviors were strongly affected by the associated wavelengths and the fraction's size. The spectral change preferentially occurred in the wavelength order of 467 nm → 451 nm → 357 nm for the 1–10 K fraction and of 376 nm → 464 nm for the >100 K fraction. The extent of the binding affinities exactly followed the sequential orders interpreted from the 2D-COS, and they exhibited the distinctive ranges of the logarithmic values from 5.86 to 4.91 and from 6.48 to 5.95 for the 1–10 K and the >100 K fractions, respectively. Our studies demonstrated that fluorescence quenching combined with 2D-COS could be successfully utilized to give insight into the chemical heterogeneity associated with metal-binding sites within the relatively homogeneous HA size fractions.

Biodegradation of Leonardite by an alkali-producing bacterial community and characterization of the degraded products

In this study, three bacterial communities were obtained from 12 Leonardite samples with the aim of identifying a clean, effective, and economic technique for the dissolution of Leonardite, a type of low-grade coal, in the production of humic acid (HA). The biodegradation ability and characteristics of the degraded products of the most effective bacterial community (MCSL-2), which degraded 50% of the Leonardite within 21 days, were further investigated. Analyses of elemental composition, (13)C NMR, and Fourier transform infrared revealed that the contents of C, O, and aliphatic carbon were similar in biodegraded humic acid (bHA) and chemically (alkali) extracted humic acid (cHA). However, the N and carboxyl carbon contents of bHA was higher than that of cHA. Furthermore, a positive correlation was identified between the degradation efficiency and the increasing pH of the culture medium, while increases of manganese peroxidase and esterase activities were also observed. These data demonstrated that both alkali production and enzyme reactions were involved in Leonardite solubilization by MCSL-2, although the former mechanism predominated. No fungus was observed by microscopy. Only four bacterial phylotypes were recognized, and Bacillus licheniformis-related bacteria were identified as the main group in MCSL-2 by analysis of amplified 16S rRNA genes, thus demonstrating that Leonardite degradation ability has a limited distribution in bacteria. Hormone-like bioactivities of bHA were also detected. In this study, a bacterial community capable of Leonardite degradation was identified and the products characterized. These data implicate the use of such bacteria for the exploitation of Leonardite as a biofertilizer.

Some theoretical and practical aspects in the separation of humic substances by combined liquid chromatography methods

Permanent need to understand nature, structure and properties of humic substances influences also separation methods that are in a wide scope used for fractionation, characterization and analysis of humic substances (HS). At the first glance techniques based on size-exclusion phenomena are the most useful and utilized for relating elution data to the molecular mass distribution of HS, however, with some limitations and exceptions, respectively, in the structural investigation of HS. The second most abundant separation mechanism is reversed-phase based on weak hydrophobic interactions beneficially combined with the step gradients inducing distinct features in rather featureless analytical signal of HS. Relatively great effort is invested to the developments of immobilized-metal affinity chromatography mimicking chelate-forming properties of HS as ligands in the environment. Surprisingly, relatively less attention is given to the ion-ion interactions based ion-exchange chromatography of HS. Chromatographic separation methods play also an important role in the examination of interactions of HS with pesticides. They allow us to determine binding constants and the other data necessary to predict the mobility of chemical pollutants in the environment. HS is frequently adversely acting in analytical procedures as interfering substance, so more detailed information is desired on manifestation of its numerous properties in analytical procedures. The article topic is covered by the review emphasizing advances in the field done in the period of last 10 years from 2000 till 2010.

Europium(III) complexed by HPSEC size-fractions of a vertisol humic acid: small differences evidenced by time-resolved luminescence spectroscopy

The size fractionation of a humic acid (HA) by high performance size exclusion chromatography (HPSEC) was used as a proxy for the filtration effect during HA transport through a porous medium with minimum specific chemical interactions. The modification of the Eu(III)-HA complexes' formation with the different size-fractions, as compared to the bulk HA, was studied in time-resolved luminescence spectroscopy (TRLS). Clear modifications in Eu(III)-HA complexes' structures were shown and related to the molecular characteristics of the separated size-fractions. The properties of most of size-fractions did not induce a major alteration of the affinity towards Eu(III). Only the most hydrophilic fractions eluted in the tail of the chromatographic peak, representing about 11% of total fractions-weight, gave some significantly different parameters. Using a simplistic complexation model, it was found that the available complexation sites decreased with the size reduction of humic fractions.

Humic substances induce lateral root formation and expression of the early auxin-responsive IAA19 gene and DR5 synthetic element in Arabidopsis

Humic substances (HS) have positive effects on plant physiology, but the molecular mechanisms underlying these events are only partially understood. HS exert auxin-like activity, but data supporting this hypothesis are under debate. To investigate the auxin-like activity of HS, we studied their biological effect on lateral root initiation in Arabidopsis thaliana. To this aim we characterised HS by means of DRIFT and (13)C CP/MAS NMR spectroscopy, and measured their endogenous content of IAA. We then utilised a combination of genetic and molecular approaches to unravel HS auxin activity in the initiation of lateral roots. The data obtained using specific inhibitors of auxin transport or action showed that HS induce lateral root formation mostly through their 'auxin activity'. These findings were further supported by the fact that HS used in this study activated the auxin synthetic reporter DR5::GUS and enhanced transcription of the early auxin responsive gene IAA19.

Nonlinear binding of phenanthrene to the extracted fulvic acid fraction in soil in comparison with other organic matter fractions and to the whole soil sample

Fractions of soil organic matter in a natural soil were extracted and sorption (or binding) characteristics of phenanthrene on each fraction and to the whole sample were investigated. The organic carbon normalized single point sorption (or binding) coefficient followed lipid > humin (HM) > humic acid (HA) > fulvic acid (FA) > whole soil sample, while the nonlinear exponent exhibited lipid > FA > HA > whole soil sample > HM. FA showed nonlinear binding of phenanthrene as it often does with other fractions. HM and HA contributed the majority of organic carbon in the soil. The calculated sorption coefficients of the whole soil were about two times greater than the measured values at different equilibrium phenanthrene concentrations. As for phenanthrene, the sorption capacity and nonlinearity of the physically mixed HA-HM mixtures were stronger as compared to the chemically reconstituted HA-HM composite. This was attributed to (besides the conditioning effect of the organic solvents) interactions between HA and HM and acid-base additions during fractionation.

Online high-performance size exclusion chromatography-nuclear magnetic resonance for the characterization of dissolved organic matter

The substantial heterogeneity of dissolved organic matter (DOM) inhibits detailed chromatographic analysis with conventional detectors as little structural information can be obtained in the presence of extensive coelution. Here we examine the direct hyphenation of high-performance size exclusion chromatography (HPSEC) with nuclear magnetic resonance (NMR) spectroscopy to determine how size-distinguished fractions differ in composition. The results support the applicability of using HPSEC to generate more homogeneous fractions of DOM prior to NMR analysis and demonstrate that structure is significantly altered with size. The largest fractions are enriched in carbohydrate- and aromatic-type structures. The midsized material is substantial and is representative of carboxyl-rich alicyclic molecules (CRAMs). The smallest material has strong signatures of material derived from linear terpenoids (MDLT). Both CRAMs and MDLT have been recently hypothesized as major components of DOM, and detection by HPSEC-NMR confirms their existence as unique and separable entities. This preliminary work focuses on NMR hyphenation to HPSEC due to widespread use of HPSEC to characterize DOM. Online hyphenation is useful not only for time-efficient analysis of DOM but also for that of other highly complex samples such as those found in many environmental analyses.

Chemical composition and bioactivity properties of size-fractions separated from a vermicompost humic acid

Preparative high performance size-exclusion chromatography (HPSEC) was applied to humic acids (HA) extracted from vermicompost in order to separate humic matter of different molecular dimension and evaluate the relationship between chemical properties of size-fractions (SF) and their effects on plant root growth. Molecular dimensions of components in humic SF was further achieved by diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY-NMR) based on diffusion coefficients (D), while carbon distribution was evaluated by solid state (CP/MAS) (13)C NMR. Seedlings of maize and Arabidopsis were treated with different concentrations of SF to evaluate root growth. Six different SF were obtained and their carbohydrate-like content and alkyl chain length decreased with decreasing molecular size. Progressive reduction of aromatic carbon was also observed with decreasing molecular size of separated fractions. Diffusion-ordered spectroscopy (DOSY) spectra showed that SF were composed of complex mixtures of aliphatic, aromatic and carbohydrates constituents that could be separated on the basis of their diffusion. All SF promoted root growth in Arabidopsis and maize seedlings but the effects differed according to molecular size and plant species. In Arabidopsis seedlings, the bulk HA and its SF revealed a classical large auxin-like exogenous response, i.e.: shortened the principal root axis and induced lateral roots, while the effects in maize corresponded to low auxin-like levels, as suggested by enhanced principal axis length and induction of lateral roots. The reduction of humic heterogeneity obtained in HPSEC separated size-fractions suggested that their physiological influence on root growth and architecture was less an effect of their size than their content of specific bioactive molecules. However, these molecules may be dynamically released from humic superstructures and exert their bioactivity when weaker is the humic conformational stability as that obtained in the separated size-fractions.

Effects of a humic acid and its size-fractions on the bacterial community of soil rhizosphere under maize (Zea mays L.)

The effects of a humic acid (HA) and its size-fractions on plants carbon deposition and the structure of microbial communities in the rhizosphere soil of maize (Zea mays L.) plants were studied. Experiments were conducted in rhizobox systems that separate an upper soil-plant compartment from a lower compartment, where roots are excluded from the rhizosphere soil by a nylon membrane. The upper rhizobox compartment received the humic additions, whereas, after roots development, the rhizosphere soil in the lower compartment was sampled and sliced into thin layers. The lux-marked biosensor Pseudomonas fluorescens 10586 pUCD607 biosensor showed a significant increase in the deposition of bioavailable sources of carbon in the rhizosphere of soils when treated with bulk HA, but no response was found for treatments with the separated size-fractions. PCR-DGGE molecular fingerprintings revealed that the structure of rhizosphere microbial communities was changed by all humic treatments and that the smaller and more bioavailable size-fractions were more easily degraded by microbial activity than the bulk HA. On the other hand, highly hydrophobic and strongly associated humic molecules in the bulk HA required additional plant rhizodeposition before their bio-transformation could occur. This work highlights the importance of applying advanced biological and biotechnological methods to notice changes occurring in plant rhizodeposition and rhizosphere microbial activity. Moreover, it suggests correlations between the molecular properties of humic matter and their effects on microbial communities in the rhizosphere as mediated by root exudation.

Relationship between photosensitizing and emission properties of peat humic acid fractions obtained by tangential ultrafiltration

Peat humic acid was fractionated by tangential ultrafiltration into six nominal molecular weight (NMW) fractions, HA5-10, HA10-20, HA20-50, HA50-100, HA100-300 and HA > 300, which were purified by dialysis using a 0.5 kDa membrane. The absorbing and emission properties of the separated fractions were compared and their ability to generate singlet oxygen under light excitation was evaluated, using furfuryl alcohol (FFA) as a singlet oxygen scavenger. The absorbance, the emission intensity, and the apparent first order rate constants of FFA loss were normalized per mole of organic carbon (a*, IF*, and k*, respectively). The fraction absorbance decreased with NMW, except for HA > 300 which was less absorbing than HA100-300. The low NMW fractions and the HA > 300 fraction generally showed lower k* and IF* values compared to the HA50-100 and HA100-300 fractions. A plot of k* versus IF* indicates that the first order rate constant of FFA photo-oxygenation increased with the intensity of fluorescence at 380, 430, and 500 nm (R2 = 0.77-0.84). This shows that the distribution of fluorescent centers among fractions paralleled that of photosensitizing centers. Plotting k* or IF* versus a* at365 nm reveals the apparent relative quantum efficiency of the different fractions. Higher values for low NMW fractions and HA50-100 are either due higher percentages of absorbing centers able to produce singlet oxygen or exhibit fluorescence or to lower quenching processes.

Molecular rigidity and diffusivity of Al3+ and Ca2+ humates as revealed by NMR spectroscopy

NMR techniques were applied to follow changes in molecular rigidity and diffusion of complexes formed between a humic acid (HA) and either aluminum or calcium ions, added in amounts ranging from 0.05% to 1% of HA carboxylic acidity. Spin-lattice relaxation time in the rotating frame (T1rho,(H)) and diffusion coefficients (D) of humic-metal complexes were obtained from 13C cross-polarization magic angle spinning (CP-MAS), and 1H-diffusion order spectroscopy (DOSY) spectra, respectively. Molecular rigidity of humic complexes increased significantly with metal addition throughout the full carbon spectral region, being more pronounced for triple-charged Al than for double-charged Ca. However, T1rho(H) values of spectral intervals suggested that molecular rigidity increase was generally in the following order: aliphatic C > aromatic/double bonds C > carboxyl C. Concomitantly, DOSY spectra showed that addition of both Al and Ca decreased substantially the diffusivity of humic alkyl components and increased that of aromatic and hydroxyalkyl components, thereby indicating that complexation induced a molecular-size increase in the former and a decrease in the latter. These results suggest that saturated and unsaturated long-chain alkanoic acids in HA were preferentially involved in metal complexation with Al and Ca, with consequentincrease of conformational rigidity and molecular size of humic hydrophobic domains. Conversely, more hydrophilic or mobile humic components appeared relatively less affected by the molecular and intermolecular rearrangements induced in HA by complexation with metals. Such NMR approach appears thus liable to evaluate the response to metal complexation of specific chemical entities present in the bulk HA and provides a further insight in the molecular architecture of humic-metal complexes.

Comparison of the heterogeneity within bulk sediment humic substances from a stream and reservoir via selected operational descriptors

The heterogeneity within a bulk humic substances (HS) of two sediment HS from different sources (stream and reservoir) was compared using selected operational descriptors for ultrafiltration (UF) fractions. The operational descriptors investigated included the specific UV absorbance (SUVA), the C/N ratio, the ratio of humic-like/fulvic-like fluorescence (HLF/FLF), the humification index (HIX), the molecular weight (MW) and the organic carbon normalized pyrene binding coefficient (K(oc)). The UF fractions exhibited a linear or a log-linear trend with respect to MW for most operational descriptors. The two sediment HS exhibited similar patterns in the fluorescence excitation-emission matrix (EEM). The trends among some operational descriptors of the UF fractions based on one HS source were not the same as those obtained from the other HS source. For the C/N ratio, HLF/FLF, and the pyrene binding reactivity, the UF fractions of the stream sediment HS had higher values than those of the reservoir sediment HS UF fractions. For SUVA and HIX, however, a single relationship was presented with MW irrespective of HS sources. This indicates that SUVA and HIX may be more related to MW of the sediment HS than to the chemical nature and/or the source. A common strong correlation between HLF/FLF and pyrene K(oc) was also observed for the UF fractions of the two HS, suggesting that HLF/FLF might serve as a good descriptor to predict the extent of pyrene binding independent of sediment source.

The role of natural purified humic acids in modifying mercury accessibility in water and soil

Contamination of soils with mercury can be a serious problem. It can be mobilized or stabilized by humic substances (HS) containing binding sites with reduced sulfur that can have different binding capacities for CH(3)Hg(+) and for Hg(2+). In this work we investigated the influence of different humic acids (HAs, extracted from lignite, compost, and forest soil) on mercury mobility and availability, both in a model solution and in soil samples from a mercury-polluted region. The technique of diffusive gradients in thin-films (DGT), which is capable of measuring: (i) free metal in solution; (ii) dissociated metal complexes previously mobilized by HA; (iii) mobilized metal-HA complexes that liberate metals by dissociation or by exchange reaction between the metal-HA complexes and the chelating groups on the resin-gel, was used in solutions and soils. The DGT measurements in solution, together with ultrafiltration, allowed estimation of the lability of Hg-HA complexes. Ultrafiltration results were also compared with predictions made by the windermere humic-aqueous model (WHAM). According to both these different approaches, Hg(2+) resulted nearly 100% complexed by HAs, whereas results from ultrafiltration showed that 32 to 72% of the CH(3)Hg(+) was bound to the HAs, with higher values for compost and lower values for forest and Aldrich HA. The DGT-measured mercury in soils was below 0.20 microg L(-1), irrespective of the extent of the contamination. Addition of HA increased the concentration of DGT-measured mercury in soil solution up to 100-fold in the contaminated soil and up to 30-fold in the control soil. The level of the increase also depended on the HA. The smallest increase (about 10 times) was found for lignite HA in both control and contaminated soils. The addition of forest HA gave the largest increases in DGT-measured mercury, in particular for the contaminated soil. Overall, the results demonstrated that DGT can be used for estimating the lability of mercury complexes in solution and for verifying enhanced mercury mobility when HA is added to contaminated soils.

Photoinductive activity of humic acid fractions with the presence of Fe(III): the role of aromaticity and oxygen groups involved in fractions

Relationship between the photoinductive activity and the properties of humic acids (HA) fractions were investigated with and without Fe(III). Three fractions were separated based on the molecular weight (M(w)) and were obtained following the order of M(w): F(A)>F(B)>F(C). Compared to F(A) and F(B), photodegradation of atrazine under simulated sunlight was much faster in solution containing F(C), whose structure was dominated by greater aromaticity, more oxygen groups and fluorophores. The interaction of HA fractions and Fe(III) was studied using fluorescence spectrometry and F(C) had the largest quenching constant. The capacity of electron transfer, estimated from the amount of photoformed Fe(II), was also highest for F(C). Thus, the Fe(III)-F(C) complex was efficient in phototransformation of atrazine in nearly neutral aqueous solutions. These results suggest that the aromaticity and oxygen groups content of HA exert great influence on the binding ability of metals and on the fate of pollutants in natural waters.