Leonardite Humic Acid Research Articles

Strength of Humic Acid Aggregates: Effects of Divalent Cations and Solution pH

Aggregation–dispersion, charging, and aggregate strength of Leonardite humic acid (LHA) were investigated in CaCl2 and MgCl2 solutions as a function of pH and ionic strength (I). The strength or the withstanding force of aggregates of humic substances (HSs) against breakage is important because this force influences the transport and distribution of pollutants and nutrients along with HSs through the change in the size of HS aggregates as a transport unit. We observed the dominancy of aggregation of LHA at high pH than at low pH in every case of CaCl2 and MgCl2 solutions. This observation suggests the higher binding efficiency of these divalent ions at high pH, though there was no obvious relation with electrophoretic mobility and aggregation of LHA. Further, we first revealed the numerical value of the strength of HS aggregates by using a simple experimental setup of aggregate breakup under laminar converging flow through a capillary tube. The obtained values of the strength of LHA aggregates were higher in the presence of CaCl2 solution than MgCl2 solution, and the strength increased with pH. The highest strengths of LHA aggregates in 30 mM (I) CaCl2 and MgCl2 solutions were around 5.8 and 2.4 nN, respectively, at pH around 9.

Isotherm and kinetic studies on the adsorption of humic acid molecular size fractions onto clay minerals

Humic acid (HA) can adsorb onto mineral surfaces, modifying the physicochemical properties of the mineral. Therefore, understanding the sorption behavior of HA onto mineral surfaces is of particular interest, since the fate and transport of many organic and inorganic contaminants are highly correlated to HA adsorbed onto clay surfaces. Due to the extreme heterogeneity of HA, the extracted IHSS Leonardite humic acid (LHA) used in this work was fractionated using an ultrafiltration technique (UF) into different molecular size fractions (Fr1, > 0.2 µm; Fr2, 0.2 µm–300,000 daltons; Fr3, 300,000–50,000 daltons; Fr4, 50,000–10,000 daltons; Fr5, 10,000–1000 daltons). Equilibrium and the kinetics of LHA and fraction adsorption onto kaolinite and montmorillonite were investigated. The results demonstrated that the maximum adsorption capacity of LHA, Fr1, Fr2, Fr3, Fr4, and Fr5 was 5.99, 13.69, 10.29, 7.02, 5.98, and 5.09 on kaolinite while it was 8.29, 22.62, 13.17, 8.91, 8.62, and 5.69 on montmorillonite, respectively. The adsorption equilibrium data showed that the adsorption behavior of LHA and its fractions could be described more practically by the Langmuir model than the Freundlich model. The rate of humic acid fraction adsorption onto clays increased with decreasing molecular size fraction and increasing carboxylic group content. Pseudo-first- and second-order models were used to assess the kinetic data and the rate constants. The results explained that LHA and its fractions adsorption on clay minerals conformed more to pseudo-second-order.

Effects of light irradiation on the complexes of cadmium and humic acids: The role of thiol groups

In natural environments, humic substances (HS) play a key role in the control of Cd stability by forming Cd-HS complexes. In this study, we investigated the influence of the photo-irradiation on the Cd levels remaining in Leonardite humic acid (HA) standard solution after chemical precipitation. The kinetic experiment showed that after the irradiation of ultraviolet light, especially UVC, Cd-HA complexes become more stable. The coexisting Cd in the HA solution under light irradiation seemed to enhance the stability of Cd-HA complexes. Both Fourier transform infrared and X-ray photoelectron spectrometric spectra indicated an increase in the thiol amounts in the HA samples treated by light irradiation. Therefore, we developed a fluorescent method to quantify thiol groups in HA with N-(1-pyrenyl) maleimide (NPM). The HA samples were initially spiked with known amounts of glutathione, the reference containing thiol groups, and then was titrated with various amounts of NPM. The measured thiol amounts in the HA treated by UV were found to significantly increase, and the increment was almost equivalent to those of Cd level in HA solution after chemical precipitation. Therefore, it was concluded that the increased thiol groups in HA induced by ultraviolet irradiation resulted in the formation of more stable Cd-HA complexes in solution.

Assessment of clay mineral selectivity for adsorption of aliphatic/aromatic humic acid fraction

Understanding the interactions of humic acids with clay minerals is important for modeling the geochemical fate and transport of nutrients and pollutants in soils and natural waters. This study assesses the selectivity of the clay minerals kaolinite and montmorillonite for the adsorption of the leonardite humic acid (LHA) fractions. The whole LHA was fractionated by ultrafiltration into five fractions of different molecular size (F1, >0.2 μm; F2, 0.2 μm–300,000 Da; F3, 300,000–50,000 Da; F4, 50,000–10,000 Da; F5, 10,000–1000 Da). Characterization by solid-state 13C nuclear magnetic resonance spectroscopy (NMR) indicated that the fraction >0.2 μm contains primarily aliphatic carbon, while the smaller molecules of the fraction <10,000 Da consists of predominantly aromatic carbon species. The adsorption characteristics of humic acid fractions onto kaolinite and montmorillonite at pH = 6 and 0.01 mol dm−3 NaCl ionic strength, were also evaluated by solid-state 13C NMR and UV–visible spectroscopy.UV–visible and 13C NMR spectra demonstrated preferential sorption by the clay minerals for different carbon types; kaolinite preferentially sorbed the aromatic carbon types, while montmorillonite preferentially sorbed the aliphatic carbon at the first point of LHA adsorption (low coverage). However, this preferential effect wasn't pronounced at the complete surface coverage (high coverage).

Water Vapor Binding on Organic Matter-Coated Minerals.

Atmospheric water vapor binding to soils is a key process driving water availability in unsaturated terrestrial environments. Using a representative hydrophilic iron oxyhydroxide, this study highlights key mechanisms through which water vapor (i) adsorbs and (ii) condenses at mineral surfaces coated with Leonardite humic acid (LHA). Microgravimetry and vibrational spectroscopy showed that liquid-like water forms in the three-dimensional array of mineral-bound LHA when present at total C/Fe ratios well exceeding ∼73 mg C per g Fe(26 C atoms/nm2). Below these loadings, minerals become even less hydrophilic than in the absence of LHA. This lowering in hydrophilicity is caused by the complexation of LHA water-binding sites to mineral surfaces, and possibly by conformational changes in LHA structure removing available condensation environments for water. An empirical relationship predicting the dependence of water adsorption densities on LHA loadings was developed from these results. Together with the molecular-level description provided in this work, this relationship should guide efforts in predicting water availability, and thereby occurrences of water-driven geochemical processes in terrestrial environments.

Interaction Kinetics of Sulfadiazine and N-Acetyl-sulfadiazine with Soil Humic Acid: ESR Investigations with Nitroxide Spin Label

The interaction of sulfadiazine (SDZ) and its main metabolite N-acetyl-SDZ (N-ac-SDZ) with model humic acid was investigated with stable paramagnetic nitroxide spin probes. Leonardite humic acid (LHA) was mixed with laccase to enhance the amount of reactive quinone groups of LHA and then incubated with nitroxide spin-labelled analogs of SDZ and N-ac-SDZ. The labeling at the pyrimidine moiety of SDZ leaves the aniline moiety susceptible to covalent binding to LHA, which is blocked by the N-acetylation. A broadened electron spin resonance (ESR) signal was observed for SDZ, which increased immediately after incubation and indicates strong restriction of the re-orientational motion of the spin probe, i.e., immobilization due covalent binding of the aniline moiety of SDZ to reactive quinone sites of LHA. A fast first-order reaction with a time constant of 17.6 ± 3.4 h of covalent binding was determined. The broadened ESR signal of N-ac-SDZ declined immediately after incubation with LHA and is caused by unspecific sorption to LHA, not by covalent binding. Short time constants of the bound and free SDZ were found for the reduction by the antioxidant sodium ascorbate demonstrating that SDZ and N-ac-SDZ are not physically entrapped by LHA.

Swelling and aggregation of Leonardite upon pH change and PbII binding: an AFM study

Environmental contextNatural organic materials, such as humic substances, play key roles in the binding and environmental fate of metals. We study the interaction of protons and metal ions with humic acids, and show changes to the mechanical properties of the particles and their capability to fix metal pollutants. The results will help refine current models of metal behaviour in the environment. AbstractThe swelling and aggregation of Leonardite humic acid, due to acid–base and PbII binding interactions, was studied through atomic force microscopy (AFM) tapping mode measurements and correlated with potentiometric experiments. These experiments allowed determination of parameters for the non-ideal competitive adsorption (NICA)-elastic polyelectrolyte network (EPN) model, which predicts size and electrostatic potential changes. AFM observations showed growth of agglomerates at low pH values. Height distribution analysis allowed discrimination of single particles from agglomerates. The size of individual particles increased slightly with pH increase. Agglomeration was evaluated through the dispersity, which increased at pH &lt; 5, concomitant with a decrease of the electrostatic repulsion and an increase of protonated carboxylic groups, thus the agglomeration is attributed to both factors. In the presence of PbII, agglomeration is observed to rise strongly with the increase in metal concentration, which is attributed to bridging of humic particles by PbII ions. The AFM ex situ results suggest consistency between NICA-EPN predictions and experimental behaviour.

Aggregation and charge reversal of humic substances in the presence of hydrophobic monovalent counter-ions: Effect of hydrophobicity of humic substances

To investigate the effect of hydrophobicity of humic substances (HSs) on their charging and aggregation, we studied the electrophoretic mobility and aggregation-dispersion of HSs in the presence of hydrophobic monovalent cations, namely, tetraphenylphosponium TPP+. The used HSs were standard Suwannee river fulvic acid (SRFA), Suwannee river humic acid (SRHA), and Leonardite humic acid (LHA) with different contents of aromatic carbons. All of the HSs in the presence of TPP+ showed charger reversal. The charge reversal pH or iso-electric point (IEP) of LHA was higher than that of SRFA and SRHA in every concentrations of TPPCl, demonstrating the strong hydrophobic interaction between HSs, especially LHA, and TPP+. We also found that the formation of large visible aggregates of all the HSs at lower pH in the presence of TPP+. Large HS aggregates were markedly formed for LHA, manifesting the existence of stronger hydrophobic attraction among LHAs with TPP+. The appearance of aggregates of all the HSs with TPP+ was confirmed by microscopic observation and the size determination by dynamic light scattering. The HS aggregates showed fractal structure. The values of fractal dimension Df of HS aggregates were 2–2.2 in quiescent conditions, indicating that the HS aggregates with TPP+ were formed via cluster-cluster aggregation with restructuring. The Df increased to 2.8–2.9 in stirring conditions, implying that the compact aggregates were formed through the continuous aggregate breakage and the regrowth between smaller aggregates and larger aggregates.

High-yield humic acid-based hard carbons as promising anode materials for sodium-ion batteries

A low-cost and sustainable anode material is essential for the future commercialization of sodium-ion batteries (SIBs). Among all proposed anode materials for SIBs, hard carbons are considered to hold the most promise. However, high cost and low carbon yield of precursors limit its industrialization process. Here the synthesis of a biomass-derived hard carbon from leonardite humic acid (LHA) through a facile process was reported. The obtained hard carbons with an amazing high carbon yield of 60.73% were evaluated as anode for SIBs. The LHA-based hard carbons exhibit a promising anode performance with a sodium storage capacity of 345 mAh g−1, an initial coulombic efficiency up to 73% and superior cyclic stability. Combined with the facile synthesis process and abundant resource, the LHA-based hard carbons may hold a promising future as anode materials for SIBs.

Molecular Dynamics Simulations of the Standard Leonardite Humic Acid: Microscopic Analysis of the Structure and Dynamics.

Humic substances (HS) are abundant in the environment and play an important role in a number of biogeochemical processes including microbial activity, soil aggregation, plant growth, the retention and release of nutrients, the environmental fate of pollutants, and carbon storage. They are flexible, relatively small molecules forming supramolecular structures through weak interactions. Despite the great importance of understanding their behavior at the atomic level, computational modeling, a premier high-resolution technique providing great level of detail, has been surprisingly little-employed to study humic substances. Here, we use the recently developed Vienna Soil Organic-Matter Modeler to create representative models of a real HS sample, the standard Leonardite humic acid. Molecular dynamics simulations were used to probe the structure and dynamics of the system at a range of hydration levels. The studied systems were characterized in terms of their physicochemical properties, including density, dielectric properties, hydrogen bonding, etc. Moreover, the strength of sorption was estimated for three small organic compounds: benzaldehyde, propan-2-ol, and acetone. Strikingly, the HS models were validated against experimental data showing a remarkable agreement with calculated properties. Finally, we make the equilibrated models of the standard Leonardite humic acid, together with corresponding force-field parameters, available at the Vienna Soil Organic-Matter Modeler.

Microsecond Molecular Dynamics Simulation of Methane Hydrate Formation in Humic-Acid-Amended Sodium Montmorillonite

Natural gas hydrate in marine sediments is a promising energy resource, while the atomistic level understanding of its formation on the organomineral complex remains limited. Microsecond molecular dynamics simulations were performed to investigate the methane hydrate growth in the sodium montmorillonite interlayer in the presence of natural sediment organic matter [leonardite humic acid (LHA)] at mass concentrations of 2 and 11%. The hydrate growth was characterized by the global and local four-body order parameter, surface distribution function, snapshots of molecular configurations, and face-saturated incomplete cage analysis. It clearly demonstrated the kinetic inhibition effects of LHA on hydrate formation on clay minerals, especially when the self-aggregation of LHA took place at a high concentration. Overall results highlighted the role of methane adsorption on LHA aggregates on the observed inhibition phenomenon, which changed the pathway of gas molecules by complex dynamic processes, such as aggre...

Experimental evidence of incomplete fluorescence quenching of pyrene bound to humic substances: implications for Koc measurements.

Fluorescence quenching (FQ) is extensively used for quantitative assessment of partition coefficients (KOC) of polycyclic aromatic hydrocarbons (PAHs) to natural organic materials - humic substances (HS). The presence of bound PAHs with incompletely quenched fluorescence would lead to underestimation of the KOC values measured by this technique. The goal of this work was to prove the validity of this assumption using an original experimental setup, which implied FQ measurements upon excitation into two distinct vibronically coupled electronic states. Pyrene was used as a fluorescent probe, and aquatic fulvic acid (SRFA) and leonardite humic acid (CHP) were used as the humic materials with low and high binding affinity for pyrene, respectively. Excitation of pyrene into the forbidden (S0-S1) and allowed (S0-S2) electronic states yielded two pairs of nonidentical FQ curves. This was indicative of incomplete quenching of the bound pyrene, and the divergence of the two FQ curves was much more pronounced for CHP as compared to SRFA. The two component model of fluorescence response formation was proposed to estimate the KOC values from the data obtained. The resulting pyrene KOC value for CHP (220 ± 20) g L(-1) was a factor 3 higher compared to the KOC value determined with the use of the Stern-Volmer formalism (68 ± 2) g L(-1). At the same time for aquatic FA the difference in FQ curves was almost negligible, which enables the use of the Stern-Volmer formalism for weakly interacting HS and PAHs.

Label Distribution in Tissues of Wheat Seedlings Cultivated with Tritium-Labeled Leonardite Humic Acid.

Humic substances (HS) play important roles in the biotic-abiotic interactions of the root plant and soil contributing to plant adaptation to external environments. However, their mode of action on plants remains largely unknown. In this study the HS distribution in tissues of wheat seedlings was examined using tritium-labeled humic acid (HA) derived from leonardite (a variety of lignites) and microautoradiography (MAR). Preferential accumulation of labeled products from tritiated HA was found in the roots as compared to the shoots, and endodermis was shown to be the major control point for radial transport of label into vascular system of plant. Tritium was also found in the stele and xylem tissues indicating that labeled products from tritiated HA could be transported to shoot tissues via the transpiration stream. Treatment with HA lead to an increase in the content of polar lipids of photosynthetic membranes. The observed accumulation of labeled HA products in root endodermis and positive impact on lipid synthesis are consistent with prior reported observations on physiological effects of HS on plants such as enhanced growth and development of lateral roots and improvement/repairs of the photosynthetic status of plants under stress conditions.

Kinetics of Rapid Covalent Bond Formation of Aniline with Humic Acid: ESR Investigations with Nitroxide Spin Labels

The bioavailability of many soil contaminants depends on their interaction with the soil organic matter. The paper presents a new approach of using stable paramagnetic spin labels for investigating the kinetics of covalent binding of specific xenobiotic functional groups with humic acids, a major organic matter fraction. Leonardite humic acid (LHA) was incubated with the nitroxide spin labels amino-TEMPO (4-amino-2,2,6,6-Tetramethylpiperidin-1-oxyl) and anilino-NO (2,5,5-Trimethyl-2-(3-aminophenyl)pyrrolidin-1-oxyl), respectively, which contain an aliphatic or aromatic functionality susceptible to interaction with LHA. Electron spin resonance spectra of LHA samples without and with the enzyme laccase were recorded at X-band frequency (9.43 GHz) at room temperature and neutral pH. Binding was detected by a pronounced broadening of the spectral lines after incubation of LHA for both spin labels. The development of a broad signal component in the spectrum of anilino-NO indicated the immobilization due to strong binding of the aniline group. The reorientational correlation time of bound anilino-NO is more than two orders of magnitude greater than that of the free label. The ratio of the amount of bound to the unbound species was used to determine the kinetics of the covalent bond formation. Reaction rate constants of 0.16 and 0.01 min−1 were determined corresponding to half-times of 4.3 and 69.3 min, respectively. Treatment of LHA with laccase enhanced the amount of the reacting anilino-NO species by a factor of 7.6, but left the reaction rate unaltered. Oxidative radical coupling was excluded using the spin trap agent n-tert-butyl-alpha-phenylnitrone.

Does As(III) interact with Fe(II), Fe(III) and organic matter through ternary complexes?

Up until now, only a small number of studies have been dedicated to the binding processes of As(III) with organic matter (OM) via ionic Fe(III) bridges; none was interested in Fe (II). Complexation isotherms were carried out with As(III), Fe(II) or Fe(III) and Leonardite humic acid (HA). Although PHREEQC/Model VI, implemented with OM thiol groups, reproduced the experimental datasets with Fe(III), the poor fit between the experimental and modeled Fe(II) data suggested another binding mechanism for As(III) to OM. PHREEQC/Model VI was modified to take various possible As(III)–Fe(II)–OM ternary complex conformations into account. The complexation of As(III) as a mononuclear bidentate complex to a bidentate Fe(II)–HA complex was evidenced. However, the model needed to be improved since the distribution of the bidentate sites appeared to be unrealistic with regards to the published XAS data. In the presence of Fe(III), As(III) was bound to thiol groups which are more competitive with regards to the low density of formed Fe(III)–HA complexes. Based on the new data and previously published results, we propose a general scheme describing the various As(III)–Fe–MO complexes that are able to form in Fe and OM-rich waters.

Adsorption of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) on a soil organic matter. A DFT M05 computational study

Adsorption of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) by soil organic matter considering the Leonardite Humic Acid (LHA) model at the M05/tzvp level of Density Functional Theory (DFT) applying cluster approximation has been investigated. Different orientations of CL-20 toward LHA surface were examined. It was found that deprotonation of LHA is required to obtain stable complexes with CL-20. Hydrogen bonds between CL-20 and deprotonated LHA were analyzed applying the atoms in molecules (AIM) theory. An attachment or removal of an electron with respect to the complex does not have significant effect on mutual orientation of the adsorbent in complexes. It was shown that adsorbed CL-20 does not undergo redox transformation and, therefore, adsorption on soil organic matter may be responsible for decrease of the degradation rate of CL-20 in soil.

Aggregation and stabilization of multiwalled carbon nanotubes in aqueous suspensions: influences of carboxymethyl cellulose, starch and humic acid

Aggregation and stability of multiwalled carbon nanotubes in aqueous solutions were investigated with two polysaccharide stabilizers (carboxymethyl cellulose and a water soluble starch) and a natural organic matter (leonardite humic acid).

Robust Method Using Online Steric Exclusion Chromatography-Ultraviolet-Inductively Coupled Plasma Mass Spectrometry To Investigate Nanoparticle Fate and Behavior in Environmental Samples.

The foundation of nanoscience is that the properties of materials change as a function of their physical dimensions, and nanotechnology exploits this premise by applying selected property modifications for a specific benefit. However, to investigate the fate and effect of the engineered nanoparticles on toxic metal (TM) mobility, the analytical limitations in a natural environment remain a critical problem to overcome. Recently, a new generation of size exclusion chromatography (SEC) columns developed with spherical silica is available for pore sizes between 5 and 400 nm, allowing the analysis of nanoparticles. In this study, these columns were applied to the analysis of metal-based nanoparticles in environmental and artificial samples. The new method allows quantitative measurements of the interactions among nanoparticles, organic matter, and metals. Moreover, because of the new nanoscale SEC, our method allows the study of these interactions for different size ranges of nanoparticles and weights of organic molecules with a precision of 1.2 × 10(-2) kDa. The method was successfully applied to the study of nanomagnetite spiked in complex matrixes, such as sewage sludge, groundwater, tap water, and different artificial samples containing Leonardite humic acid and different toxic metals (i.e., As, Pb, Th). Finally, our results showed that different types of interactions, such as adsorption, stabilization, and/or destabilization of nanomagnetite could be observed using this new method.

Spontaneous aggregation of humic acid observed with AFM at different pH

Atomic force microscopy in contact (AFM-C) mode was used to investigate the molecular dynamics of leonardite humic acid (HA) aggregate formed at different pH values. HA nanoparticles dispersed at pH values ranging from 2 to 12 were observed on a mica surface under dry conditions. The most clearly resolved and well-resulted AFM images of single particle were obtained at pH 5, where HA appeared as supramolecular particles with a conic shape and a hole in the centre. Those observations suggested that HA formed under these conditions exhibited a pseudo-amphiphilic nature, with secluded hydrophobic domains and polar subunits in direct contact with hydrophilic mica surface. Based on molecular simulation methods, a lignin–carbohydrate complex (LCC) model was proposed to explain the HA ring-like morphology. The LCC model optimized the parameters of β-O-4 linkages between 14 units of 1–4 phenyl propanoid, and resulted in an optimized structure comprising 45–50 linear helical molecules looped spirally around a central cavity. Those results added new insights on the adsorption mechanism of HA on polar surfaces as a function of pH, which was relevant from the point of view of natural aggregation in soil environment.

Molecular dynamic simulation of asphaltene co-aggregation with humic acid during oil spill

Humic acid in water and sediment plays a key role in the fate and transport of the spilled oil, but little is known about its influence on the aggregation of heavy oil asphaltenes which is adverse for remediation. Molecular dynamic simulation was performed to characterize the co-aggregation of asphaltenes (continental model and Violanthrone-79 model) with Leonardite humic acid (LHA) at the toluene–water interface and in bulk water, respectively, to simulate the transport of asphaltenes from oil to water. At the toluene–water interface, a LHA layer tended to form and bind to the water by hydrogen bonding which provided a surface for the accumulation of asphaltenes by parallel or T-shape stacking. After entering the bulk water, asphaltene aggregates stacked in parallel were tightly sequestrated inside the inner cavity of LHA aggregates following surface adsorption and structure deformation. Asphaltene aggregation in water was 2-fold higher than at the toluene–water interface. The presence of LHA increased the intensity of asphaltene aggregation by up to 83% in bulk water while relatively less influence was observed at the toluene–water interface. Overall results suggested that the co-aggregation of asphaltene with humic acid should be incorporated to the current oil spill models for better interpreting the overall environmental risks of oil spill.