Humic Acid Aggregates Research Articles

The competition of humic acid aggregation and adsorption on clay particles and its role in retarding heavy metal ions

Humic acid (HA) is of great importance in controlling the fate of heavy metals (HMs), however, the pivotal influence of HA aggregation within the HA-clay-HM ternary system on retarding HM mobility remains obscure. This study performed molecular dynamics simulations to delve into the consequences of HA aggregation on the environmental behavior of Cd2+ and Pb2+ (0.1–0.6 M) in the co-existence of illite particles. HA can readily aggregate into clusters, adhering to the illite surface or freely dispersing in the solution. These HA clusters significantly modulate HM mobility, contingent upon their location, arrangement, and interaction with illite. Consequently, HA exhibited a pronounced retardation effect on HM migration, stemming from the competition between HA aggregation and its adsorption on illite. Additionally, the retardation effect of HA aggregation was more obvious for Cd2+ (as compared to Pb2+), owing to its stronger interaction with the functional groups of HA. These findings contribute to the development of potential HA-based strategies for remediation of heavy metal-contaminated sites.

Insight into the molecular transformation pathways of humic acid in the co-composting of bagasse and cow manure after adding compound microorganisms

The inoculation of compound microorganisms effectively processes lignocellulosic biomass and promotes the formation of humic acid (HA). However, the basic mechanism of HA formation in compost remains unclear. In this study, we comprehensively characterized the transformation of HA molecules during co-composting. The addition of compound microorganisms enhanced the degradation of proteinoids and lignocellulose in the dissolved organic matter (OM), thereby accelerating the humification of bagasse cow dung co-compost. Elemental and UV–Vis spectral analyses showed that inoculation with the compound microorganisms enhanced the oxidation level and aromatization of the HA structure. Furthermore, two-dimensional Fourier-transform infrared spectroscopy correlation analysis suggested that the addition of compound microorganisms accelerated the decomposition and oxidation of OM. Accordingly, premature aggregation of HA was avoided, and the side chains bonded to HA were more abundant, promoting the evolution of HA to higher-order structures. By influencing the degree of aromaticity, molecular weight, and the C2 fluorescence peak of HA, the inoculation of compound microorganisms promoted HA synthesis during bagasse composting. These results provide an important basis for revealing the mechanism by which compound microorganisms are inoculated to promote HA formation at the molecular level and serve as a reference for the production of high-quality and high-maturity compost.

Facilitated transport of toluene and naphthalene with humic acid in high- and low-permeability systems: Role of ionic strength and cationic type

The occurrence of colloids on pollutants transport in groundwater has attracted more attention. However, the research on the regulation mechanism of colloids on combined pollutants transport in heterogeneous aquifers is limited. In this study, a series of tank experiments were conducted to systematically investigate the effects of ionic strength, and cation type on humic acid (HA) facilitated transport of toluene (TOL), and naphthalene (NAP) in high- and low-permeability systems. The results showed that HA facilitated pollutants transport in low Na+ solution. In Ca2+ solution, the presence of HA hindered pollutants transport, and the inhibition increased with the increase of ionic strength. Both in Na+ solution and low Ca2+ solution, the influence of heterogeneous structure on pollutant transport played a dominant role, and TOL and NAP had a greater transport potential in the high permeability zone (HPZ) due to the preferential flow. Whereas, deposition of HA aggregates, and electrostatic attractive interaction had negative effects on transport than groundwater flow in high Ca2+ solution. Pollutants were prone to accumulate at the bottom of the HPZ, and the top of the low permeability zone (LPZ). These new findings provide insights into the mechanism of colloids influence on the pollutants transport in heterogenous aquifer.

Heterogeneous Aggregation of Humic Acids Studied by Small-Angle Neutron and X-ray Scattering.

Aggregation of humic acids (HAs) was studied by small-angle neutron and X-ray scattering techniques. The combination of these techniques enables us to examine the aggregation structures of HA particles. Two HAs with distinctive compositions were examined: a commercial HA (PAHA) and a HA extracted from deep sedimentary groundwater (HHA). While macroscopic coagulation tests showed that these HAs were stable in solutions except for HHA at pH < 6, small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) revealed that they formed aggregates with sizes exceeding the sub-micrometer length scale. The SAXS curves of PAHA remarkably varied with pD = log aD+, where aD+ stands for the activity of deuterium ions, whereas the SANS curves did not. With the help of theoretical fittings, it was revealed that PAHA aggregates consisted of two domains: poorly hydrated cores and well-hydrated proton-rich shells. The cores were (dis)aggregated with pD inside the aggregates of the shells. The SANS and SAXS curves of HHA resembled each other, and their intensities at low q, where q stands for the scattering vector, increased with a decrease of pD, indicating the formation of homogeneous aggregates within the spatial resolutions of SANS and SAXS. This study revealed that distinctive aggregation behaviors exist in humic substances with nm-scale heterogeneous structures like PAHA, which is important for their roles in the fate of contaminants or nutrients in aqueous environments.

Deciphering the specific interaction of humic acid with divalent cations at the nanoscale

Humic acid (HA), a representative class of natural organic matter (NOM), can interact with a variety of divalent cations to form the metal-HA complexes, which is of vital significance in both the natural and engineered environments. To decipher the specific HA-divalent cation interaction mechanism, the intermolecular forces and adhesion energies between two HA surfaces in aqueous environment with varying divalent cations were directly quantified at the nanoscale using atomic force microscope (AFM). In the presence of Ca2+, the considerably increased HA aggregation and HA-Ca(II)-HA adhesion were detected with elevated Ca2+ concentration mainly due to the enhanced bridging effect, while the excessive amounts of Ca2+ could significantly suppress the aggregation and adhesion behavior. The adhesion energy was also found to be dependent on other aqueous conditions (e.g., pH, cation type), with the highest adhesion energy measured for Cu2+ (∼1.642 mJ m−2), approximately 2 ∼ 3-fold higher than the case of Zn2+ (∼0.950 mJ m−2), Ca2+ (∼0.829 mJ m−2) and Mg2+ (∼0.583 mJ m−2). Such robust adhesion for Cu2+ could stem from the synergistic bonding of Cu2+ with both the oxygen and amine sites of HA, which was evidently different from the single metal–oxygen bonding for Zn2+, Ca2+ and Mg2+ shown by X-ray photoelectron spectroscopy (XPS). Based on density functional theory (DFT) simulation, HA molecules could bridge with all these four types of divalent cations mainly by the deprotonated catechol and phthalic acid (PA) structures in the bidentate coordination mode, while the deprotonated aniline could only bond with Cu2+, ultimately leading to the highest Cu(II)-HA binding energy among the studied divalent cations. This work provides insights into the quantitative understanding of HA-divalent cation interaction mechanisms, shedding light on developing effective strategies to modulate the aggregation, transport and fate of NOM in aqueous environments as well as engineering applications.

Molecular Insights into the Interaction Mechanism Underlying the Aggregation of Humic Acid and Its Adsorption on Clay Minerals.

Humic acid (HA) is ubiquitous in both terrestrial and aquatic environments, and understanding the molecular interaction mechanisms underlying its aggregation and adsorption is of vital significance. However, the intermolecular interactions of HA-HA and HA-clay mineral systems in complex aqueous environments remain elusive. Herein, the interactions of HA with various model surfaces (i.e., HA, mica, and talc) were quantitatively measured in aqueous media at the nanoscale using an atomic force microscope. The HA-HA interaction was found to be purely repulsive during surface approach, consistent with free energy calculation; during retraction, pH-dependent adhesion was observed due to the protonation/deprotonation of HA that influences the formation of hydrogen bonds. Different from the mica case, hydrophobic interaction was detected for the HA-talc system at pH 5.8, contributing to the stronger HA-talc adhesion, as also evidenced by adsorption results. Notably, HA-mica adhesion strongly depended on the loading force and contact time, most likely because of the short-range and time-dependent interfacial hydrogen bonding interaction under confinement, as compared to the dominant hydrophobic interaction for the HA-talc case. This study provides quantitative insights into the fundamental molecular interaction mechanisms underlying the aggregation of HA and its adsorption on clay minerals of varying hydrophobicity in environmental processes.

Interaction between microplastics and humic acid and its effect on their properties as revealed by molecular dynamics simulations

The characteristics and fates of microplastics (MPs) and humic acid (HA) in the environment are significantly influenced by their interactions. Thus, the influence of the MP–HA interaction on their dynamic characteristics was explored. Upon MP–HA interaction, the number of hydrogen bonds established in the HA domains decreased significantly, and the water molecules bridging the hydrogen bonds shifted to the exterior regions of the MP–HA aggregates. The distribution intensity of Ca2+ located at ∼0.21 nm around HA deceased, indicating that the coordination of Ca2+ with the carboxyl on HA was impaired in the presence of MPs. Additionally, the Ca2+–HA electrostatic interaction was suppressed because of the steric hindrance of the MPs. However, the MP–HA interaction improved the distribution of water molecules and metal cations around the MPs. The diffusion coefficient of HA decreased from 0.34 × 10−5 cm2/s to 0.20–0.28 × 10−5 cm2/s in the presence of MPs, implying that the diffusion of HA was retarded. The diffusion coefficients of polyethylene and polystyrene increased from 0.29 × 10−5 cm2/s and 0.18 × 10−5 cm2/s to 0.32 × 10−5 cm2/s and 0.22 × 10−5 cm2/s, respectively, indicating that the interaction with HA accelerated the migration of polyethylene and polystyrene. These findings highlight the potential environmental hazards posed by MPs in aquatic environments.

Mechanism study of synergistic effect of organic and inorganic foulants in membrane distillation

In this paper, we investigated the mechanism of membrane fouling during the membrane distillation (MD) process by using humic acid (HA) and calcium carbonate (CaCO3) as the model organic and inorganic foulants respectively. The aggregation induced crystallization theory was introduced to analyze the fouling process. Field emission scanning electron microscopy (FESEM) images and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and variation of foulants on membrane surfaces that treated different feeds at different concentration factors. The interface free energies of the membranes surfaces were calculated based on contact angles and surface tensions between the testing solutions and the membrane surfaces. The results indicated that the aggregations of HA onto the hydrophobic polyvinylidene fluoride (PVDF) membrane surface altered membrane surface properties, which reduced critical nucleus radius (R⁎) and facilitated the heterogeneous nucleation of CaCO3. Furthermore, the presence of calcium ion enhanced the aggregation of HA, which further promoted the crystallization of CaCO3.

Aggregation Kinetics and Mechanism of Humic Acid Reaction with Cs+ and Co2+ Metal Ions Using Batch Techniques

Humic substances have a potential role in the fate and transport of toxic metal ions in the environment due to their colloidal characteristics and abundant surface functional groups. Batch techniques (DLS, EPM, FT-IR and fluorescence EEM) were developed to assess the aggregation mechanisms of humic acid (HA) reacting with Cs+ or Co2+ electrolyte ions in this work. The kinetic experimental results indicated that a much lower Co2+ ion concentration (0.03–1.50 mmol/L) induced rapid aggregation of HA compared to that of Cs+ (3.0–15 mmol/L), and the divalent Co2+ ion was far more effective in enhancing HA aggregation than monovalent Cs+. The aggregation kinetics of HA were also found to be pH-dependent, and a much lower pH condition (pH 5.0) caused more rapid aggregation (the largest hydrodynamic diameter of ~3000 nm) compared to those at pH 7.0 (the largest hydrodynamic diameter of ~2000 nm). Positively charged metal ions in the solution can lower the electrostatic repulsive force between HA molecules through charge neutralization, thus leading to the rapid aggregation of HA aggregates. Furthermore, the carboxylic and phenolic groups on the HA surface were also involved in the aggregation reaction to form inner complexes and accelerate the aggregation process.

Humic Acids Aggregates as Microheterogeneous Reaction Media: Alkaline Hydrolysis Reactions

The influence of humic aggregates in a water solution upon the chemical stability under basic conditions of different substrates was reviewed. The kinetic behavior of each substrate was modeled in terms of a micellar pseudophase model.

Understanding the Effect of pH on the Solubility and Aggregation Extent of Humic Acid in Solution by Combining Simulation and the Experiment.

Molecular dynamics (MD) simulations were performed to investigate the dynamics of humic acid (HA) in an aqueous solution and the influence of pH, temperature, and HA concentration. The HA model employed in MD simulations was chosen and validated using experimental chemical composition data and Fourier transform infrared (FTIR) spectra. The simulations showed that the HA molecule has a strong propensity to adopt a compact conformation in water independent of pH, while the aggregation of HA was found to be pH-dependent. At high pH, the ionized HAs assembled into a thread-like structure, maximizing contact with water. At low pH, the neutral HAs formed a droplet-like aggregate, minimizing contact with the solvent. The simulation results are consistent with experimental data from dynamic light scattering (DLS) measurements and transmission electron microscopy (TEM) imaging. This work provides new insight into the folding and aggregation of HA as a function of pH and a molecular-level understanding of the relationship between the acidity and the structure, solubility, and aggregation of HA, with direct implications for HA-based remediation strategies of contaminated sites.

Interaction between humic acid and silica in reverse osmosis membrane fouling process: A spectroscopic and molecular dynamics insight

Combined organic and inorganic fouling is a primary barrier constraining the performance of reverse osmosis (RO) membrane. In this work, we conducted a systematic study focusing on the synergetic fouling effects of silica and humic acid (HA) in RO process, and found the critical silica concentration where the fouling pattern changed qualitatively. When the silica concentration was lower than 6 mM at a typical HA concentration of 50 mg·L−1, no severe fouling was observed, while silica reaching this critical point could cause severe synergetic fouling with HA. Concentrated silica above the critical point acted as the prior foulant with marginal fouling effect caused by HA. A variety of solutions and surface-based characterizations were performed to elucidate the synergistic fouling responsibility for silica and HA. Our study suggests that the carboxylic groups from HA formed hydrogen bonds with silica hydrate, inducing silica adsorption onto HA aggregates at low silica particle concentrations. The HA network was bridged together to form large foulants due to the silica-silica interaction above the silica critical concentration. These mechanisms were further confirmed by molecular dynamics simulations. This study provides an in-depth insight into the combined organic-inorganic fouling and can serve as a guideline to optimize feed conditions in order to mitigate fouling of RO in wastewater reusing industry.

HUMIC ACIDS COMPOSITION OF ARABLE SOD-PODZOLIC SOIL AFTER LONG-TERM APPLICATION OF TRADITIONAL AND UNCONVENTIONAL ORGANIC FERTILIZERS

The elemental composition and structure of humic acids (HA) of arable sodpodzolic soil (Eutric Albic Retisols (Abruptic, Loamic, Cutanic) was studied inPerm Agricultural Research Institute – division of PFRC. Application of traditional and unconventional organic fertilizers was fulfilled in long-term stationary experiment. The carbon content in HA of sod-podzolic soil varied from 30.7 to 34.6; hydrogen – 28.9-35.5, oxygen – 21.1-27.9, nitrogen – 1.9-2.2%. The Н/С ratio for all treatments was &gt;1, the structure of the supramolecular associations of humic acids is predominantly aliphatic. Long-term use of manure, sewage sludge (SS) and their combination with mineral fertilizers led to the enrichment of humic acids with nitrogen. The maximum degree of HA oxidation was observed with the use of cattle manure. The FTIR spectra of humic acids had absorption bands of carboxyl, hydroxyl, methyl, methylene, methoxyl and other groups in a wide wavelength range. At 1720 cm-1, an absorption band was recorded, which had a high intensity in the control variant and was due to oscillations of the &gt;C=O group of carboxylic acids. With an increase in the load of the anthropogenic factor on the soil (application of organic and mineral fertilizers), a decrease in its intensity is observed. The structure of supramolecular HA aggregates of the control variant, with the introduction of NPK and unconventional organic fertilizer – SS, is characterized by a higher content of aromatic fragments, as evidenced by a clear existence of the absorption band at 1628 cm–1. Cattle manure application promoted the formation of humic acids with a branched aliphatic structure.

Calcium-enhanced retention of humic substances by carbon nanotube membranes: Mechanisms and implication

Efficient removal of humic substances (HS) in natural water is of importance to drinking water treatment. In this study, the adsorption and retention of HS by multi-walled carbon nanotube (MWCNT) membranes were systematically investigated by dynamic filtration of synthetic and natural surface water. In the absence of Ca(II), HS were dominated by small, dissolved species albeit the varying pH. Accordingly, HS retention by the MWCNT layers only ranged from 15%-65% at the end of the filtration. In contrast, the presence of Ca(II) in the feed water partially transformed HS molecules into colloidal aggregates as found by light scattering analyses. Furthermore, molecular dynamics (MD) simulation results reveal that Ca(II) complexation with -COO- on MWCNT and humic acid (HA) not only leads to HA aggregation in the feed solution, but also promotes HA adsorption onto carboxylated MWCNT. The modeling results are consistent with the high retention of HS by the carboxylated MWCNT membrane, i.e., >90% for the synthetic model water and >85% for the natural water, at a moderate calcium concentration range of 0.5–2.0 mM. Considering the widespread presence of calcium in natural water, these findings suggested that carboxylated MWCNT has a potential for effective adsorptive filtration of HS in drinking water.

Pre-aggregation of Al13 in optimizing coagulation for removal of humic acid

The effective removal of humic acid (HA) by coagulation has been extensively investigated for water treatments. However, the limitations of pH variation and excessive residual aluminum issues were still factors needed to be considered. In this study, to investigate the coagulation mechanism for removing HA by Al13 and optimize Al13 operation for removing HA, Al13 and preformed Al13 aggregates (Al13agg) were applied to remove HA at different pH conditions. The results showed that preformed Al13agg exhibited superior HA removal performance than Al13 due to its wide pH range and low residual Al level. During coagulation, Al13 and Al13agg interacted with HA in their original status, but the DSlope325-375 difference implied that the complexation capacity between HA and Al13agg was stronger than Al13. The new peaks of HPSEC representing larger molecular weight substances were formed under acidic and neutral conditions, which indicated that HA firstly aggregated into larger complexed molecules by interacting with Al13 or its hydrolysates and was subsequently removed by forming large flocs which was completely different from Al13agg situation. Therefore, the different coagulation mechanisms played the roles in HA removal for Al13 and Al13agg which were studied in this paper. It was believed that the complexation and charge neutralization effects dominated coagulation process for Al13 while sweep flocculation and adsorption coagulation were main driving force for Al13agg in HA removing. This work provides significant understanding of HA removal by Al13 and Al13agg coagulation, which can help to design and optimize the high efficiency coagulant based on Al polycations.

Molecular Dynamics Simulation of the Interaction between Common Metal Ions and Humic Acids

Humic acids (HAs) have important environmental and geochemical effects on soil, water environments and sediment. HAs strongly complex some metal ions, which affects the migration of metal ions and the colloidal aggregation of HA. Here, the complexation of Ca2+ and Mg2+ with HA in aqueous solution under neutral conditions has been systematically studied by molecular dynamics (MD) simulation. The results show that the aggregation of HA is caused by the complexation of HA and metal ions, mainly due to the intermolecular bridging between Ca2+, Mg2+ and COO− groups. Monodentate and bidentate coordinations have been found between Ca2+ and COO− groups of different HA molecules in the same simulation system. Mg2+ only has a monodentate coordination with COO− group.

Strong improvement of nanofiltration performance on micropollutant removal and reduction of membrane fouling by hydrolyzed-aluminum nanoparticles

Increasing attention has been focused on the removal of micropollutants from contaminated drinking source water. However, low rejection efficiency and membrane fouling still inhibit further application of nanofiltration membrane in this field. Interesting results were found that the residual hydrolyzed-aluminum nanoparticles from supernatant after coagulation and sedimentation strongly improved the nanofiltration performance for micropollutant removal. A simulated raw water containing humic acids, micropollutants and kaolinite clay was employed to investigate the factors of water matrix affecting the nanoparticle-enhanced nanofiltration for micropollutant removal. Results of experiments showed that these hydrolyzed-aluminum nanoparticles easily induced the aggregation of bisphenol-A (BPA) and humic acids in the supernatant. The enhancement of BPA removal was mainly attributed to the repelling interaction between the Al-BPA-DOC complexity and in situ-modified membrane surface during nanofiltration.‘This in situ surface modification by the hydrolyzed-aluminum nanoparticles improved membrane hydrophilicity, roughness and positively-charging capacity. For the treatment of River Songhua water spiked with micropollutant, the percentage removal of BPA was improved to be 88.5%, much more than the case of single nanofiltration without coagulation (60.7%). Meanwhile, the membrane fouling was reduced by 2.13 times than the case of single nanofiltration without the dynamically deposited-layer of nanoparticles. This in situ modification of nanofiltration membrane by hydrolyzed-aluminum nanoparticles achieved excellent removal efficiency for micropollutants from River Songhua water background.

Coagulation mechanisms of humic acid in metal ions solution under different pH conditions: A molecular dynamics simulation

Humic acid (HA) exerts a variety of significant environmental and geochemical influences on the soils, sediments and aqueous environments. The interaction with metal ions induces strong HA-metal complexation, thus effecting the transport of the toxic metals as well as the colloidal aggregation of HA. In the present work, we systematically report and analyze the aggregation mechanisms of HAs in solutions filled with different heavy cations (Ag+, Cd2+ and Cr3+) or common metal ions (Na+, Ca2+ and Al3+) under neutral and low pH conditions by using molecular dynamic simulations. We aim to explore the effects of pH, metal ions type and other possible weak interactions on the aggregation capabilities of HA. Scrutiny of the simulation results showed that the aggregation of HAs under neutral condition was driven by the HA-metal complexation which combined the effects of electrostatic attraction and inter-molecular bridging between cations and COO– groups. Larger extent of aggregation was found in heavy metal ions compared with the common ones. On the other hand, under low pH condition, due to the protonation states of carboxyl and phenolic group, the aggregation of HAs was stabilized mainly by weak forces, such as hydrogen bonds between different functional groups. In addition, other weak interactions such as the hydrophilic and hydrophobic effects, the cation-π interactions have also been proposed to be progressive effects on the coagulation behavior. Our computational studies give supplement to the experimental observation and provide insights into the intrinsic mechanisms of the aggregation behavior of HAs and their complexation with metal ions. Such computational modelling supplied a highly effective tool for qualitatively evaluating their roles in environmental remediation.

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.

Effect of co-existing Co2+ ions on the aggregation of humic acid in aquatic environment: Aggregation kinetics, dynamic properties and fluorescence spectroscopic study

The fate and transport of humic substances in the aquatic environments depend significantly on their interactions with co-existing ions. Herein, we employed dynamic light scattering (DLS) measurement, molecular dynamic (MD) simulation and fluorescence spectrometry to investigate the aggregation of humic acid (HA) in the presence of Co2+ ions. The aggregation kinetics was depicted by hydrodynamic diameter (<Dh>) and the attachment efficiency (α) of HA aggregates. α increases gradually in the reaction-limited (slow) regime due to the decrease of the double layer repulsion, and the energy barrier is eliminated to a certain extent in the diffusion-limited reaction while α close to unity. The complexation between functional groups (i.e. carboxylic and phenolic groups) of HA and Co2+ ions contributes significantly to the aggregation process of HA. MD simulation and density functional theory (DFT) calculation demonstrate that the aggregation process of HA can be promoted by Co2+ through several inter- or intra-molecular interactions between HA and the Co2+ ions. The results provide a pathway for insight into the interactions between HA and metal ions, which is important for deeply understanding the environmental behaviors of HA in natural aqueous systems.