Structural Characteristics Of Humic Acids Research Articles

Elemental Composition and Structural Features of Humic Acids from Floodplain Soils of the Selenga River Delta

The study of the elemental composition and structure of humic acids of floodplain soils of the delta of the Selenga River (Western Transbaikalia) was carried out. The research area belongs to the delta meadowmarsh and foreststeppe region, which is an area where deposits of sandy and silty fractions are actively carried out from the entire basin. Here, soils and vegetation are natural biofilters. Depending on the bioclimatic conditions, groundwater regimes, and sediment composition, soils are formed that differ in the composition of humus and humic acids (HA). The objects of research were alluvial (meadow, meadowmarsh, meadow saline) soils of the Selenga delta, according to the classification WRB – Fluvisols. The analysis of 13C-NMR spectra of HA preparations of the studied floodplain soils made it possible to identify ranges of chemical shifts belonging to carbon atoms of various functional groups and molecular fragments. NMR spectroscopy data showed that humic acids of alluvial meadow soil are more enriched in aromatic fragments than humic acids of meadowmarsh and meadow saline soils. A slightly higher degree of aromaticity in alluvial meadow and meadowmarsh soil is associated with environmental conditions and the component composition of the precursors of humification. An increase in the proportion of aliphatic structures and a decrease in the proportion of aromatic fragments in the composition of preparations of meadow salt marsh soil make it possible to judge the simplification of the structure of humic acid. Soil moisture in combination with salinization significantly reduces the rate of transformation of soil organic matter and leads to an increase in the proportion of non-oxidized aliphatic fragments. The use of NMR spectroscopy in the study of soils has significantly expanded the understanding of its composition and structure, deepened the understanding of the mechanisms of humification and transformation of soils of various genesis.

The Quantity and Quality of Humic Substances following Different Land Uses in Karst Peak-Cluster Depression in Guangxi, China

There were some ecological and environmental problems in limestone soil in the karst peak-cluster depression in Pingguo City of Guangxi, such as the destruction of soil structure, soil acidification and soil fertility decline, and these problems were closely related to soil organic matter. The soil in this site was classified as Cambisols. Therefore, this project took soil under five major land uses (grassland, afforestation, sugarcane field, corn field and pitaya field) in the karst area of Guangxi as the research object. The contents and molecular structure properties of humic acids, fulvic acids and humins in soils were studied by solid-state 13C nuclear magnetic resonance spectroscopy and elemental composition. From the perspective of the chemical structure of humic substances, the differences in the quantity and structural characteristics of humic acids, fulvic acids and humins in different land uses were revealed. The results showed that the organic carbon content of both afforestation (34.83 g kg−1) and natural restored grassland (31.67 g kg−1) were significantly higher than that of sugarcane field (17.60 g kg−1), corn field (16.35 g kg−1) and pitaya field (14.31 g kg−1) (p < 0.05). The contents of three humic fractions in grassland were relatively high, and the contents of three humic fractions in sugarcane field were relatively low (p < 0.05). The structural characteristics of humic substances showed that the protein components of the three humic fractions in the afforestation were high, indicating that the humic substances in the afforestation contained more unstable components. The Alkyl C/O-alkyl C and Hydrophobic C/hydrophilic C ratios of the three humic fractions of corn field were high, indicating high stability and maturity in humic substances in corn field.

EVALUATION OF DETOXIFYING ABILITY OF ORGANOMINERAL COMPLEXES BASED ON HUMIC ACIDS AND Al30-PILLARED MONTMORILLONITE IN RELATION TO OIL IN WATER

The detoxifying ability of humic-clay complexes obtained by sorption of humic acids of two types of peat on the original and modified forms of montmorillonite with respect to oil in water has been studied. The differences found in the values of the adsorption parameters of humic acids on clay minerals are due to the structural features of humic acids, as well as the textural properties of montmorillonite forms. Due to the layered-columnar structure and the presence of slit-like meso- and micropores, Al30-pillared montmorillonite is a more effective matrix for obtaining humic-clay complexes: the maximum adsorption reaches 73 mg/g. Humic acids immobilized on the mineral surface act on the ecotoxicant as a natural sorbent and slow down the migration of petroleum hydrocarbons. Humic-clay complexes do not have an inhibitory effect on the growth of the test object - duckweed small. Detoxification of oil in water using a humic-clay complex based on Al30-pillared montmorillonite is higher than that based on the original montmorillonite. The detoxification coefficient of the humic-clay complex based on Al30-pillared montmorillonite reaches 80%; at the same time, the toxic effect of oil in water decreases from 65% to 13%. The resulting humic-clay complexes act as stabilizers of the oil-water emulsion. They are capable of being spontaneously sorbed at the oil-water interface with the formation of a structural-mechanical barrier around oil droplets, which transfer oil into the water column with further intensive decomposition by microorganisms - oil destructors - due to the large contact surface. Oil is not released from mineral "capsules" and does not form a surface film, which allows the use of humic-clay complexes in the in situ purification process.

Effects of Corn Straw and Biochar Returning to Fields Every Other Year on the Structure of Soil Humic Acid

Returning straw and biochar to the fields can change the structure of humic acid in soil and affect the content of soil organic matter. However, returning straw and biochar to the fields in successive years also increases the insect pests of crops and causes nitrogen competition, thus increasing the soil burden. Therefore, to provide a theoretical basis for the study of the time effect of returning fields, three treatments were set in this paper to study the effects of straw and biochar returning every other year on the soil organic matter and humic acid (HA) structure. Elements, Fourier transform infrared spectroscopy (FTIR), thermogravimetric spectroscopy (TG), three-dimensional fluorescence spectroscopy, and solid state 13C-NMR spectroscopy were used to comprehensively investigate the structural characteristics of humic acid in soil. The results showed that after straw and biomass charcoal are returned to the field every other year, compared with 2018, after planting in 2020, the average content of soil organic carbon will increase by 29.49% and 36.14%, respectively. HA showed a phenomenon of increased aliphatic property and decreased aromaticity; aromatic carbon of HA decreased, while alkyl carbon and alkoxy carbon increased; the molecular structure of HA developed toward simplification. In the short term, the effect of biomass charcoal returning was more significant. This study can be used as a reference for the research on the time effect of returning straw and biomass carbon to the fields, and can also provide a basis for the research on the composition of humus after returning organic materials to the fields.

Investigating the Evolution of Structural Characteristics of Humic Acid Generated during the Continuous Anaerobic Digestion and Its Potential for Chromium Adsorption and Reduction

Humic acid (HA), as an important by-product, has been demonstrated to affect anaerobic digestion performance and subsequent land application of digestate via the batch anaerobic digestion process. However, the knowledge about the evolution of structure and function of HA during continuous anaerobic digestion (AD) is still unclear. Therefore, the current study examined the structural changes in HA produced during the continuous AD process and its metal-adsorption-reduction abilities. The results of three-dimensional fluorescence spectroscopy showed a general upsurge in humic-like components’ abundance (70–77%), with an increase in humification index (2.56–3.43). Likewise, the content of HA increased from 4.8 g L−1 to 6.9 g L−1 in the continuous AD process. The evolution of C-H, O-H, C=O, C=C, and C-O functional groups of HA was observed via the 2D COS FTIR analysis. Moreover, the concurrent dynamics of functional groups contributed to the higher adsorption (255.2 mg g−1) of Cr (VI) and reduction (60.3 mg g−1) of Cr (VI) to Cr (III) after 168 days of the continuous AD process. The findings of the current study not only advanced understanding of the evolution of HA during continuous anaerobic digestion and its metal remediation potential but also support further research toward developing an eco-friendly and innovative strategy for the remediation of heavy metals contaminated soils employing anaerobic digestate as an auxiliary agent.

Potential Hydrothermal-Humification of Vegetable Wastes by Steam Explosion and Structural Characteristics of Humified Fractions.

In this work, steam explosion (SE) was exploited as a potential hydrothermal-humification process of vegetable wastes to deconstruct their structure and accelerate their decomposition to prepare humified substances. Results indicated that the SE process led to the removal of hemicellulose, re-condensation of lignin, degradation of the cellulosic amorphous region, and the enhancement of thermal stability of broccoli wastes, which provided transformable substrates and a thermal-acidic reaction environment for humification. After SE treatment, total humic substances (HS), humic acids (HAs), and fulvic acids (FAs) contents of broccoli samples accounted for up to 198.3 g/kg, 42.3 g/kg, and 166.6 g/kg, and their purification were also facilitated. With the increment of SE severity, structural characteristics of HAs presented the loss of aliphatic compounds, carbohydrates, and carboxylic acids and the enrichment of aromatic structures and N-containing groups. Lignin substructures were proved to be the predominant aromatic structures and gluconoxylans were the main carbohydrates associated with lignin in HAs, both of their signals were enhanced by SE. Above results suggested that SE could promote the decomposition of easily biodegradable matters and further polycondensation, aromatization, and nitrogen-fixation reactions during humification, which were conducive to the formation of HAs.

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.

Effects of different corn straw amendments on humus composition and structural characteristics of humic acid in black soil

ABSTRACT Straw returning to the field has become a common way to utilize crop residues. This method does not only solve the problem of rational utilization of straw but also improves soil organic carbon (SOC) quality and quantity. So far, various methods of straw return exist, but the comparison of these methods has not been sufficiently studied, especially in the black soil of Northeast China. The purpose of this study is to evaluate the effects of different corn straw amendments applied under equal carbon (C) mass on soil humus composition and structural characteristics of humic acid (HA). The pot experiment was carried out by simulating corn straw return, which included four treatments: no corn straw applied (CK), corn straw (CS), humidified corn straw (HCS) and corn straw biochar (Bc). Soil HA was characterized by Fourier transform infrared spectroscopy (FTIR), elemental analyzer and thermogravimetric analysis (TG). The results showed that CS, HSC, and Bc treatments increased SOC content by 7.51%, 31.03%, and 33.65%, respectively, compared to CK. Bc treatment showed a greater degree of humification (79.16%) and increased the C content of HA by 10.87% and 7.37% compared with CS and HCS, respectively. The FTIR spectra and elemental results revealed that CS and HCS treatments increased the aliphaticity of the HA structure, while Bc increased the aromaticity of C in HA structure, rendering HA structure complicated. It can, therefore, be inferred that the application of biochar will improve humus compositions, SOC content and enhances the stability of HA structure.

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.

Influence of Land Use on the Structural Feature of Sedimentary Humic Acids in Rivers in Northwest Hokkaido, Japan.

Colloidal organic matter is an important factor in our understanding of the transport of trace metals by rivers to coastal areas and the characteristics of organic matter in the sediment, including the effect of humic substances on the transport of such metals. The structural features of humic acids (HAs) derived from surface sediments collected from the mouths of four rivers in northwest Hokkaido, Japan, were evaluated from the perspective of land use in their basins. The sediments in two rivers (referred to as NS and SK) were lacking in HAs, but the sediments of the others (referred to as SM and PO) contained relatively high levels of HAs. UV-vis spectroscopic characteristics suggested that the HAs in SM and PO contained components derived from humified materials. The PO and SM basins contain rice paddy fields, and TMAH-py-GC/MS analysis showed that the pyrograms of paddy soil HA had a similar character to those of PO-HA and SM-HA samples. Thus, the differences of the structural features can be attributed to the land use in river basins. The findings indicate that the properties of organic river sediments are heavily influenced by the type of land use in the river basins.

Evolution over years of structural characteristics of humic acids in Black Soil as a function of various fertilization treatments

Humic substances are the relatively stable and recalcitrant pool of soil organic carbon in the agricultural system. Humic acid (HA) is an active part of soil humic substances. However, little information exists on how structural characteristics of HA evolved with fertilization years in Black Soil. Soil samples were collected in 1997, 2002, 2008, and 2012 from the Long-term Fertilization Station of Black Soil. The methods of 13C-nuclear magnetic resonance spectroscopy, elemental composition analysis, and infrared spectroscopy were employed to analyze the structure of HA. Long-term fertilization had no significant (P > 0.05) effect on the ratio of O and S to C in HA. The ratio of H to C in HA increased with fertilization years after the application of organic manure (M) single or combined with chemical NPK fertilizers. The ratios of aliphatic C to aromatic C, and alkyl C to O-alkyl C in soil HA increased by 7.72% and 20.3%, respectively, in MNPK and M treatments, whereas NPK treatment declined the ratio of aliphatic C to aromatic C by 2.67% compared with CK treatment. The HA structure tends to become aliphatic and simplified with years of organic manure combined with NPK fertilizers. To save costs, organic manure was only applied once within the period of crop rotation and was combined with NPK every year to satisfy crop demands for soil nutrients.

Adsorption of Dissolved Organic Matter with Different Relative Molecular Masses on Inorganic Minerals and Its Influence on Carbamazepine Adsorption Behavior

In this work, sorption experiments were conducted to understand the adsorption characteristics of dissolved organic matter (DOM) of different relative molecular masses on minerals and the effects of their interaction with PPCPs on the adsorption process. This study chose carbamazepine (CBZ) as the target pollutant, quartz sand as the inorganic mineral, and commercial humic acid (HA) as the DOM. We studied the structural characteristics of HA with different relative molecular masses and the impact of their interaction with CBZ on their adsorption based on dialysis experiments and infrared spectroscopy. It was found that large molecular weight fractions of HA, which contain more hydroxyl, aromatic hydrocarbons, and aliphatic hydrocarbons, were mainly hydrophobic substances, while the smaller fractions, containing more alcohols and carboxyl groups, were mainly hydrophilic substances. The combination of HA and CBZ had two major mechanisms, the combination of amino groups of CBZ and polar functional groups in small molecular weight fractions of HA and hydrophobic interactions between hydrophobic groups and aliphatic or/aromatic compounds in large molecular weight HA. The interaction between CBZ and HA resulted in obvious differences in the adsorption of HA or CBZ. When CBZ does not exist, hydrophobic combinations occurred between HA through the hydrophobic components and quartz sand, which also experienced the exchange coordination reaction with the hydroxyl or carboxyl group of HA. In addition, HA adsorbed the aliphatic and hydrophobic macromolecular components. Quartz sand was used to adsorb the large hydrophobic molecules. When CBZ existed, it adsorbed the small molecular weight fractions of HA. The maximum HA adsorption capacity of quartz sand decreased. CBZ could be adsorbed on quartz sand by hydrophobic interaction, van Edward forces, and polarity interactions in the absence of HA. When adding HA, the adsorption increment of carbamazepine on quartz sand was due to the co-adsorption or accumulation of HA and CBZ after their combination.

Identifying structural characteristics of humic acid to static and dynamic fluorescence quenching of phenanthrene, 9-phenanthrol, and naphthalene

Fluorescence quenching is a sensitive and fast method to quantify the interactions between a fluorescent organic contaminant and a quencher, such as dissolved organic matter (DOM). Dynamic fluorescence quenching is resulted from molecular collision, not the real binding, and thus it complicates the binding data interpretation. On the other hand, static fluorescence quenching occurs for fluorescent contaminants of ground states, which decreases the concentration of freely dissolved contaminants. However, how a particular structure in DOM contributes to the static and dynamic fluorescence quenching of a fluorescent contaminant is still unclear, which has greatly hindered the application of fluorescence quenching technique. A humic acid (HA) extracted from sediment was chemically modified, i.e., bleaching, acid hydrolysis, and decarboxylation. HAs before and after these modifications were used in fluorescence quenching experiments for phenanthrene (PHE), 9-phenanthrol (PTR) and naphthalene (NAP). Different quenching mechanisms were observed for these chemicals depending on HA properties. For PHE and NAP, aromatic components showed static quenching, while carboxyl groups primarily showed dynamic quenching. Aromatic components and carbohydrates in HAs primarily bound (static quenching) rather than collided (dynamic quenching) with PTR. Carboxyl groups showed interactions with PTR through dynamic quenching only when carboxyl groups were on the benzene ring. Based on the results, we emphasized that dynamic quenching should be carefully excluded in fluorescence quenching studies. This line of study is important to establish a general relationship between DOM properties and static/dynamic quenching contributions.

Response of humic acid structure to soil tillage management as revealed by analytical pyrolysis

The effects on the structural features of humic acids (HA) from dryland farming soils under long term management practices have been approached by analytical pyrolysis (Curie-point pyrolysis-gas chromatography/mass spectrometry, Py-GC/MS). The field experiments (started in 1987) include conventional, minimum and no-tillage plots, as well as non cultivated plots. The HAs isolated from the various plots showed significant differences in their pyrolytic behavior, in particular regarding the total abundances of alkyl pyrolysis compounds (fatty acids, alkenes and alkanes). The occurrence of very short-chain fatty acids (C5–C11) in uncultivated plots could be indicative of constitutional alkyl structures in the relictual HA from undisturbed soil. The effect of soil tillage managements substantially increased total abundances of fatty acids in plots under conservation practices (mainly no-tillage).The HAs from uncultivated soils showed the greatest percentages of alkanes and alkenes. This was associated to the increased proportions of even C-numbered alkene homologues from C12 to C18, possibly related to the incorporation of microbial compounds during the humification process. High percentage of alkylbenzenes and catechols were also characteristic of the uncultivated plots. The increased proportions of methoxyphenols, in special of the syringyl (dimethoxyphenyl) type, in HAs from plots subjected to conventional tillage pointed out to humification processes based on progressive alteration of plant lignins. From the viewpoint of soil quality, the results suggest comparatively advanced transformation stages of the HA from uncultivated plots, which means that conservation tillage practices seems to lead to increasing soil C levels, at expenses of the accumulation of comparatively recent organic matter derived either from altered lignins and/or microbial biomass.

Characterization of humic acids from antarctic soils by nuclear magnetic resonance

The elemental composition and structural features of humic acids (HAs) from Antarctic soils (King George Island, Larsemann Hills, Lindsay Island) have been studied. It has been found that their elemental composition and molecular structure are intermediate between those of the HAs and fulvic acids (FAs) of Eurasian soils (from the average values). The degree of hydrophilicity of the studied HAs is comparable to that of FAs. The low content of aromatic moieties in the HAs is related to the absence or very low proportions of phenyl propane fragments in the sources of humus formation. It has been shown that the HAs from Antarctic soils compose a separate group of humic acids whose specific features are related to hard climatic conditions and specific features of humus formation sources.

ВЛИЯНИЕ СТРУКТУРНЫХ ХАРАКТЕРИСТИК ГУМИНОВЫХ КИСЛОТ НА ЭФФЕКТИВНОСТЬ ВЗАИМОДЕЙСТВИЯ С КАТИОНАМИ ПОЛИВАЛЕНТНЫХ МЕТАЛЛОВ

The paper considers the interaction of modified peat humic acids with salts of heavy metals under various pH. It was shown the change of structural characteristics of humic acids in the process of mechanical activation under reducing conditions. Regardless of the conditions mechanical activation slightly increases the degree of aromaticity due to possible condensation reactions. At mechanical activation of peat in the air without any reagents one observes oxidation of the peripheral carbohydrate fragments in the structure of humic acids. Modification in argon reduces the number of carboxyl and carbonyl groups, phenolic hydroxyls; oxidized aromatic fragments and "lignin" components in the composition of humic acids and naturally increases a proportion of alkyl substituentsIt is revealed that modified humic acids enriched oxygen-containing groups have the highest sorption capacity in relation to ions of copper and nickel. Probability and stability of the forming chelate complexes are determined by the composition and the number of oxygen-containing ionogenic groups and acid-base balance. Sorption capacity of the humic acids with respect to a nickel ion is lower than that to a copper ion.

Standardization of Humic Acids of Lowland Wood-Grass Peat from Tomsk Region

Humic acids (HA) of peat consist of a complicated mixture of high-molecular-weight and polyfunctional alicyclic, hydroaromatic, aromatic, and heterocyclic compounds substituted by normal and branched alkyl chains of various lengths containing both saturated and unsaturated bonds with various functional groups [1]. Such structural features of HA enable them to participate in a variety of redox reactions and in enzyme—substrate interactions. HA can affect osmotic pressure, form chelate complexes, etc. Furthermore, HA are heteropolymers of aryl-glyco-proteide nature that can provide a source of macromolecular organic structural fragments for biosyntheses occurring in living organisms. Colloidal dispersions of HA exhibit surfactant and surface-electrical properties. As a result, HA can reduce the surface tension and viscosity of solutions and protoplasm, exhibit ionophoric activity, and change the electrophysiological properties of cell membranes. Taken together, the aforementioned properties are responsible for the multi-faceted pharmacological properties of HA. Their antioxidant activity due to a whole series of structural features deserves special attention. Aromatic HA contain a large number of carboxylic and quinoid groups that are redox catalysts and are responsible for the antioxidant activity [2, 3]. This is consistent with the protective properties of HA under the influence of various hazardous environmental factors such as intoxication, hypoxic states, immunopathologies, and viral and microbial infections.

Genesis of structural features of humic acids from the sediments of a high productive coastal zone

Genesis of structural features of humic acids from the sediments of a high productive coastal zone

Structural alterations of humic acid fractions in a steel slag-compost fertilizer during fertilization. Analysis by pyrolysis/methylation-gas chromatography/mass spectrometry

Alterations in the structural features of humic acids (HAs) in a steel slag-compost fertilizer for supplying soluble Fe(II) to barren ground in coastal areas were monitored during a 6-month period of fertilization. HAs, the major organic fractions in the fertilizer, were extracted at different periods of time (2, 4 and 6 months) in the absence and presence of steel slag. To focus on the polymeric structure of the HA-backbone and the organic fractions, which are sorbed onto the HA-polymeric matrix (e.g., lipids), the extracted HA samples were analyzed by pyrolysis-gas chromatography/mass spectrometry under conditions where the sample is methylated with tetramethylammonium hydroxide (TMAH-py-GC/MS). The structural features analyzable by TMAH-py-GC/MS for the HA were dramatically altered in the presence of steel slag during the fertilization period. The major modification involved a decrease in phenolic moieties and a significant increase in the sulfur-containing pyrolysate compounds. These results indicate that the steel slag has a significant effect in altering the structure of HA in the fertilizer. TMAH-py-GC/MS analysis of the HAs indicated that the sulfur in the HAs was present in an organic form and not as elemental sulfur. Significant increases in the levels of biomarker fatty acids (iso- and anteiso-C15:0 and C17:0 carboxylic acids), which are related to the activity of sulfate reducing bacteria, suggests that the significant alterations in the HA structures in the presence of steel slag can be attributed to the sulfurization of organic matter in the fertilizer to form hydrogen polysulfides by the reduction of sulfate ions in seawater and/or steel slag via microbial processes.

Characterization of humic acids in sediments from dam reservoirs by pyrolysis-gas chromatography/mass spectrometry using tetramethylammonium hydroxide: Influence of the structural features of humic acids on iron(II) binding capacity

The structural features of humic acids (HAs) isolated from sediments on the bottom of dam reservoirs that can affect their binding capacities for Fe(II) were investigated by pyrolysis-gas chromatography/mass spectroscopy using tetramethylammonium hydroxide (TMAH-py-GC/MS). The binding capacities for Fe(II) increased with increasing O/C molar ratio, suggesting that the oxygen-containing functional group content plays a role in the binding of Fe(II). However, it was not possible to identify specific binding-sites for Fe(II) by TMAH-py-GC/MS analysis. Although C 16:1ω7, iso-C 15:0 and anteiso-C 17:0 fatty acids, which serve as molecular markers of anaerobic microbial activity, were detected in all of the HA samples, the contents of these acids were not correlated with binding capacities for Fe(II). However, the ratio of C 16:0 to C 16:1ω7 fatty acids, which is used as an index of anaerobic bacterial activity, increased with increasing Fe(II) binding capacities of the HAs. It thus appears likely that the activities of anaerobic bacteria on the bottom of dam reservoirs contribute to alterations in the structural features for HAs, and that this process results in increased binding capacities for Fe(II).