Functional Groups Of Humic Acid Research Articles

Comprehensive insight into the transformation mechanism of Cd fractionation in the components of paddy soils under cysteine leaching

Cadmium (Cd) contamination in paddy soils poses a serious threat to agricultural production and human health. In this study, cysteine was selected as the leaching agent for remediating Cd-contaminated paddy soils because of its effective, low cost and ecofriendly characteristics. Microcosmic simulation experiment was using to investigate the fate of Cd and elucidate the transformation mechanism of Cd in the key components of paddy soil under cysteine leaching. Results showed that the total Cd removal rate of paddy soils reached 46.8–87.4% under the optimal conditions. This was mainly attributed to the decrease of exchangeable fraction (F1), carbonate bound fraction (F2), iron/manganese-bound fraction (F3) and organic matter bound fraction (F4) of Cd. After cysteine leaching, soil fertility was promoted due to the increase in nitrate nitrogen (NN), ammonia nitrogen (AN), available phosphorus (AP) and organic matter (OM). pH, CaCO3, available potassium (AK), NN, AP, amorphous iron oxides (Feo) were the main contributors to Cd fractionation transformation. The desorption of Cd from ferrihydrite (Fh) and goethite (Gt) surfaces is hypothesized to stem from dual mechanisms: the dissolution due to the low solution pH and the reducibility of cysteine, which facilitated the conversion of Fh and Gt to hematite (Hm). And the -SH complexation also ascribed to the activation of Cd bound by Fe-OH in Fh and Gt and oxygen-containing functional groups in humic acid (Ha). For chlorite (Ch), the primary driver for Cd desorption was identified as an ion-exchange process, whereby electrostatically adsorbed Cd2+ were replaced by H+ produced by cysteine protonation.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Environmental colloid behaviors of humic acid - Cadmium nanoparticles in aquatic environments

Humic acid (HA), a principal constituent of natural organic matter (NOM), manifests ubiquitously across diverse ecosystems and can significantly influence the environmental behaviors of Cd(II) in aquatic systems. Previous studies on NOM-Cd(II) interactions have primarily focused on the immobilization of Cd(II) solids, but little is known about the colloidal stability of organically complexed Cd(II) particles in the environment. In this study, we investigated the formation of HA-Cd(II) colloids and quantified their aggregation, stability, and transport behaviors in a saturated porous media representative of typical subsurface conditions. Results from batch experiments indicated that the relative quantity of HA-Cd(II) colloids increased with increasing C/Cd molar ratio and that the carboxyl functional groups of HA dominated the stability of HA-Cd(II) colloids. The results of correlation analysis between particle size, critical aggregation concentration (CCC), and zeta potential indicated that both Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO interactions contributed to the enhanced colloidal stability of HA-Cd(II) colloids. Column results further confirmed that the stable HA-Cd(II) colloid can transport fast in a saturated media composed of clean sand. Together, this study provides new knowledge of the colloidal behaviors of NOM-Cd(II) nanoparticles, which is important for better understanding the ultimate cycling of Cd(II) in aquatic systems.

Pristine cobalt humic acid xerogels embedded with ultrafine cobalt sulfide for enhanced and stable lithium-ion storage

The electrochemical performance of pristine metal–organic xerogels as anodes in lithium-ion batteries is reported for the first time. We propose a novel synthesis approach for the in situ generation of highly dispersed, ultrafine cobalt sulfide nanoparticles on humic acid gels (CoSHA). The CoS nanoparticles in CoSHA have an average diameter of approximately 3 nm. CoSHA electrodes demonstrate enhanced lithium storage capacity, delivering a capacity of 662 mAh g−1 at 0.1 A g−1. They also show stable long-term cycling performance, with no capacity decay after 900 cycles at 1.0 A g−1. Furthermore, our experiments indicate that the improved lithium-ion adsorption results from the oxygen-containing functional groups in humic acid and the ultrafine CoS active sites. This study offers a practical methodology for synthesizing ultrafine metal sulfides and new insights into using pristine gel-based electrodes for energy storage and conversion.

Coagulation performance and mechanism analysis of humic acid by using covalently bonded coagulants: effect of pH and matching mechanism of humic acid functional groups.

Conventional inorganic coagulants (Al, Fe) and Al/Fe-based covalently bonded flocculants (CAFMs) had different hydrolysis species at different pHs, which subsequently led to differences in their binding sites and complexation ability with humic acid (HA). Studying the binding sites and interactions between CAFMs, AlCl3 (Al), and FeCl3 (Fe) hydrolysates and HA molecules is critical to understanding the coagulation mechanism. The results found that CAFM 0.6, Al, and AlCl3 combined FeCl3 (Al/Fe) removed more than 90% of HA at pH 6, and CAFMs showed higher HA removal rate than that of Al, Fe, and Al/Fe under the same reaction conditions. The flocs of CAFMs contained abundant -NH2/OH as well as the large particle size, compact structure, and excellent settling performance. The hydrolyzed species of Al and Fe were predominantly Alb and Feb at pH 6, but the hydrolyzed species of CAFMs were primarily (Al + Fe)c. Moreover, the hydrolyzed species of Al and Al/Fe were found to complex with HA functional groups such as -COOH, C = O, C-H/C-C, C = C, and C-OH to form ligand bonds, while the hydrolyzed species (Al + Fe)c of CAFMs could deeply interact with HA functional groups including C-O, -COOH, C = O, C-H/C-C, C = C, and C-OH by the adsorption and sweeping.

Synergistic effect triggered by Fe2O3 and oxygen-induced hydroxyl radical enhances formation of amino-phenolic humic-like substance

Metal oxides play a promising role in the transformation of polyphenols and amino acids involved in naturally occurring humification. The objective of this study was to explore the synergistic interactions between Fe2O3 and O2 in the formation of humic substances under a controlled O2 atmosphere (0%, 21% and 40% O2 levels). The results indicate that an O2 level of 21% with Fe2O3 was optimal for humic acid (HA) production. Hydroxyl radicals (∙OH) formed and promoted the formation of HA in the presence of O2, and O2 improved the enhancing capacity of Fe2O3 by oxidizing Fe(II) to Fe(III). Moreover, the combination of these processes resulted in a synergistic improvement in humification. The evolution of functional groups in HA suggested that O2 promoted the formation of oxygen-containing groups such as lipids, and Fe2O3 was conducive to the formation of dark-coloured polymers during the darkening process of humification. Furthermore, the O2 level of 40% inhibited the formation of HA by reducing the transformation from Fe(III) to Fe(II). The XRD results showed few changes in the composition of Fe2O3 before and after humification, which indicated that Fe2O3 was a catalyst and an oxidant. The heterospectral UV–Vis/FTIR results suggested that ∙OH attacked phenolic rings to form the aromatic ring skeleton of HA and benefit the ring-opening copolymerization of humic precursors. In addition, structural equation modelling demonstrated that dissolved Fe was the key parameter affecting the HA yield. These findings provide new insights into the synergism of O2-mediated ∙OH production associated with metal oxide-facilitated humification.

Antimony(V) removal from water within ferrihydrite transformation: Influence of humic acid

Antimony (Sb), a widely distributed heavy metal pollutant, poses a formidable threat to human health and the delicate ecological environment. Anthropogenic activities such as mining, industrial processes and the widespread use of Sb-containing products aggravate the toxicity and potential risks of Sb to the environment. The mobility and fate of Sb(V) in the water environment is affected by ferrihydrite (Fhy) transformation and humic acid (HA). However, the key role of HA on Sb(V) removal in the transformation of iron minerals remains unclear. In this study, the transformation of Fhy into goethite was significantly inhibited as a result of HA structural coating, while active functional groups of HA enhanced the combination with iron minerals and Sb(V) under acidic conditions. Residual Sb(V) concentration in water was less than 5 µg/L, which meets the United States Environmental Protection Agency (USEPA) recommended Sb concentration of 6 µg/L. Under alkaline conditions, the dissociation of HA provided more negative charges for the surface of transformed hematite and magnetite. Due to the stronger electrostatic repulsion and steric effects of HA, further removal of Sb(V) was limited. This work provided multiple secondary mineralization pathways of Fhy under the impact of HA and mechanistic insights into the removal process of Sb(V), which could contribute meaningful information regarding the interaction between Sb(V) and iron oxyhydroxides.

Adsorption of Congo Red onto Humic Acid Isolated from Peat Soil Gambut Regency, South Kalimantan

Humic acid is one of the green materials for wastewater treatment including for removal dyes as an adsorbent. Humic acid was isolated from peat soil, Gambut Regency, South Kalimantan following International Humic Substances Society method, and used to adsorb Congo red. The adsorption process was carried out in a batch system and the effect of pH, contact time, and adsorption capacity of Congo red on humic acid were studied. The adsorbent characterization by using FTIR. The results showed that the adsorption of Congo red on humic acid occurred at the optimum pH of pH 6, the optimum contact time was 120 minutes. The adsorption capacity of Congo red onto humic acid of 33.33 mg/g and follows the Langmuir model with R2 = 0.9926. The characterization of humic acid functional groups before and after adsorption of Congo red showed that the signal at 1,712.79 cm-1 were shifted to 1,705.07 cm-1 and 1,273.02 cm-1 were shifted to 1,265.30 cm-1. These suggested that the mechanism interaction was the electrostatic interaction between -NH3+ functional group of Congo red group and -COO- functional group of humic acid. It means that humic acid isolated from peat soil could be used as an adsorbent for the removal Congo red.

Effects of UV-irradiation on Cd[formula omitted] and Pb[formula omitted] binding to humic/fulvic acids: Methodological guidance to eliminating the interference of dissolved organic matter on SWASV detection of heavy metals in soil extracts

Humic acid (HA) and fulvic acid (FA) ubiquitous in the environment will interfere with the electrochemical signals of Cd2+ and Pb2+, thereby decreasing the detection accuracy of square-wave anodic stripping voltammetry (SWASV) for Cd2+ and Pb2+ in soil and sediment extracts. However, insight into the mechanism of HA and FA interference is lacking. This study investigated the interference mechanism of HA and FA on Cd2+ and Pb2+ SWASV currents from multiple perspectives and provided methodological guidance for eliminating the HA and FA interference. Our results indicated that the main interfering factor was the complexation of active functional groups in HA and FA, which converted free-state heavy metal (HM) ions into HA- and FA-complexed HMs (HA-HMs and FA-HMs) undetectable by SWASV. More O-containing groups in FA caused its more severe interference on Cd2+ and Pb2+ than HA. After 60 min of ultraviolet (UV) photolysis, the SWASV current recovery rates of Cd2+ and Pb2+ in HA-HMs almost reached 100%, while FA-HMs achieved the same goal after only 24 min of UV photolysis. The photo-degradation behaviors revealed that the mechanism of interference elimination was that UV photolysis destroyed the molecular structure and active groups of HA and FA, thereby releasing free-state Cd2+ and Pb2+ from HA-HMs and FA-HMs. This study also verified the above-obtained conclusions using real samples. The finding of this study is that advanced oxidation processes can eliminate the interference of HA and FA, thereby improving the detection accuracy of Cd2+ and Pb2+ in soil and sediment extracts by SWASV.

Humic Acids Preparation, Characterization, and Their Potential Adsorption Capacity for Aflatoxin B1 in an In Vitro Poultry Digestive Model.

Vermicompost was used for humic acid (HA) preparation, and the adsorption of aflatoxin B1 (AFB1) was investigated. Two forms of HA were evaluated, natural HA and sodium-free HA (SFHA). As a reference, a non-commercial zeolitic material was employed. The adsorbents were characterized by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), energy-dispersive X-ray spectroscopy (EDS), zeta potential (ζ-potential), scanning electron microscopy (SEM), and point of zero charge (pHpzc). The adsorbent capacity of the materials when added to an AFB1-contaminated diet (100 µg AFB1/kg) was evaluated using an in vitro model that simulates the digestive tract of chickens. Characterization results revealed the primary functional groups in HA and SFHA were carboxyl and phenol. Furthermore, adsorbents have a highly negative ζ-potential at the three simulated pH values. Therefore, it appears the main influencing factors for AFB1 adsorption are electrostatic interactions and hydrogen bonding. Moreover, the bioavailability of AFB1 in the intestinal section was dramatically decreased when sorbents were added to the diet (0.2%, w/w). The highest AFB1 adsorption percentages using HA and SFHA were 97.6% and 99.7%, respectively. The zeolitic material had a considerable adsorption (81.5%). From these results, it can be concluded that HA and SFHA from vermicompost could be used as potential adsorbents to remove AFB1 from contaminated feeds.

Varying the hydrophobicity of humic matter by a phase-transfer-catalyzed O-alkylation reaction

An O-alkylation reaction catalyzed by tetrabutylammonium hydroxide (TBAH) as a phase-transfer agent was applied to a humic acid (HA) to modify its hydrophobic properties. The carboxyl and hydroxyl functional groups of HA acted as nucleophiles in substitution reactions (Sn2) with methyl iodide, pentyl bromide and benzyl bromide added in amounts equimolar to 20, 60 and 80% of HA total nucleophilic sites. The occurrence of O-alkylation was shown by DRIFT spectrometry, NMR spectroscopy, High Performance Size Exclusion Chromatography (HPSEC) and elemental analysis of reaction products. DRIFT spectra showed changes in C–H stretching and bending regions following the insertion of methyl and pentyl groups, while the incorporation of benzyl groups revealed the characteristics aromatic C–H stretching bands. Both liquid- and solid-state NMR spectra revealed characteristic signals for alkyl/aryl esters and ethers. HPSEC chromatograms of alkylated materials invariably displayed an increase in hydrodynamic volume in respect to the original HA, thereby suggesting that the enhanced hydrophobicity conveyed further associations among humic molecules. Analytical, HPSEC and spectroscopic results suggest that benzylation was the most effective reaction at all percentages of HA total nucleophilicity, followed, in the order, by pentylation and methylation, The benzylation reaction was used to improve reaction and work-up conditions and show that HA could be efficiently alkylated also with substantial reduction of TBAH amount, with no THF addition, increase of reaction time and of washing cycles to remove catalyst impurities. These findings indicate that the hydrophobicity of humic substances can be modulated through a mild O-alkylation reaction under a phase-transfer catalysis according to the extent of exposed HA nucleophilic sites. Such a structural modification of humic matter may have multiple chemical, environmental and biological applications.

Reduction capacity of humic acid and its association with the evolution of redox structures during composting

The reducing capacity (RC) of compost-derived humic acid (HA) is related to the type and number of redox-active functional moieties in its structure and has a considerable environmental influence on its geochemical redox cycle. Composting treatment can affect the redox-active fractions of organic substances through microbial transformation and degradation. However, the relationship between the RC of compost-derived HA and its fluorescence component and infrared spectra remains unclear. In this study, we assessed the response of the organic reducing capacity (ORC) and inorganic reducing capacity (IRC) of compost-derived HA to the stabilization of organic solid waste materials by analyzing the redox-active functional groups of HA extracted at different composting times. The results demonstrated that the RC of compost-derived HA continuously increased during composting because of the formation of fulvic- and humic-like fluorescent components, which consist of amide, phenolic hydroxyl, quinone, and aromatic groups. Adsorption occurred between HA and FeCit by aliphatic and out-of-plane aromatic CH, which released free hydrogen and increased the Fe-binding site; consequently, ORC was obviously higher than IRC. The results of this study could provide an understanding of the transformation of the fluorescent substances and functional groups that affect redox properties during composting; therefore, this study has considerable significance for exploring the application of compost products.

Effect of humic acid on bioreduction of facet-dependent hematite by Shewanella putrefaciens CN-32

Interfacial reactions between iron (Fe) (hydr)oxide surfaces and the activity of bacteria during dissimilatory Fe reduction affect extracellular electron transfer. The presence of organic matter (OM) and exposed facets of Fe (hydr)oxides influence this process. However, the underlying interfacial mechanism of facet-dependent hematite and its toxicity toward microbes during bioreduction in the presence of OM remains unknown. Herein, humic acid (HA), as typical OM, was selected to investigate its effect on the bioreduction of hematite {100} and {001}. When HA concentration was increased from 0 to 500 mg L−1, the bioreduction rates increased from 0.02 h−1 to 0.04 h−1 for hematite {100} and from 0.026 h−1 to 0.05 h−1 for hematite {001}. Since hematite {001} owned lower resistance than hematite {100} irrespective of the HA concentration, and hematite {100} was less favorable for reduction. Microscopy-based analysis showed that more hematite {001} nanoparticles adhered to the cell surface and were bound more closely to the bacteria. Moreover, less cell damage was observed in the HA-hematite {001} treatments. As the reaction progressed, some bacterial cells died or were inactivated; confocal laser scanning microscopy showed that bacterial survival was higher in the HA-hematite {001} treatments than in the HA-hematite {100} treatments after bioreduction. Spectroscopic analysis revealed that facet-dependent binding was primarily realized by surface complexation of carboxyl functional groups with structural Fe atoms, and that the binding order of HA functional groups and hematite was affected by the exposed facets. The exposed facets of hematite could influence the electrochemical properties and activity of bacteria, as well as the binding of bacteria and Fe oxides in the presence of OM, thereby governing the extracellular electron transfer and concomitant bioreduction of Fe (hydr)oxides. These results provide new insights into the interfacial reactions between OM and facet-dependent Fe oxides in anoxic, OM-rich soil and sediment environments.

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.

Humic acid induced indirect photolysis of polybrominated diphenyl ethers under visible light irradiation

Polybrominated diphenyl ethers (PBDEs), as typical persistent organic pollutants (POPs) have been world widely detected in environmental media and biological tissues. Compared with the extensive study on direct photolysis of PBDEs in the ultraviolet region (UV), indirect photodegradation of PBDEs under visible light triggered by the co-existing substances in the environment has been largely ignored. Herein, the indirect photolysis and debromination of organic brominated pollutants decabromodiphenyl ether (BDE209) induced by humic acid (HA) have achieved both in homogeneous and heterogeneous system under visible light irradiation. It shows a new transformation pathway of PBDEs in environment that plays an important role on their ultimate environmental fate. Different from direct photolysis of PBDEs in UV light, the indirect photolysis of PBDEs by HA under visible light irradiation experiences the electrons transfer from HA to PBDEs induced by a halogen-bond between HA and PBDEs. HA contributes to the activation of C-Br bond via formation of halogen bond, and promotes electron transfer under visible light irradiation, leading to meta-C-Br cleavage preferentially. To evaluate the effects of various functional groups in HA, eleven low-molecular-weight organic compounds (LMWOs) containing reactive oxygen terminal are selected to study the indirect photolysis of BDE209. Combined with the density functional theory (DFT) calculation results, it is shown that carboxyl group is proposed to be the key functional group capable of providing oxygen terminals to form halogen bonds. This work provides novel and valuable insights into the photolysis and debromination of PBDEs with co-existing HA in the environment under sunlight.

Coagulation behavior of polyaluminum-titanium chloride composite coagulant with humic acid: A mechanism analysis

The hydrolysate species of metal-based coagulants and the binding sites of humic acid (HA) are highly dependent on the pH conditions. Exploring the binding sites and modes between coagulant hydrolysates and HA molecules is critical to understanding the coagulation mechanism. In this paper, the binding behavior between HA molecules and the hydrolysates of a polyaluminum-titanium chloride composite coagulant (PATC) was investigated under different pH conditions by semi-quantitative FTIR and XPS. It was found that oligomeric and mesopolymeric hydrolysates were the dominant species under acid conditions, which could complex with the hydroxyl and carboxyl groups of HA by forming COAl/Ti coordinate bonds. However, large amounts of H+ could compete with Al3+ and weaken the removal efficiency of HA. With the increase of pH, the hydrolysis process of the PATC and the deprotonation of HA were simultaneously underway. Under weakly acid conditions, the complexation of the aluminum hydrolysates with carboxyl groups was improved due to the gradually diminishing competition of H+ and the enhanced charge neutralization of the further polymerized hydrolysates. Consequently, the maximum UV254 removal by adding PATC was observed at pH 6. Under alkaline conditions, the sweeping effect of amorphous hydroxide dominated the HA removals, which was accompanied by the surface complexation of Al/Ti nuclear with carboxyl groups as well as the hydrogen bonds between hydroxyl and hydroxide. This study provides a new clue for the interaction mechanisms between the hydrolysates of composite coagulants and the dominant functional groups of HA at various pH conditions.

VUV-H2O2 photolysis as a pretreatment method for improving the SWASV detection accuracies of Cd2+ and Pb2+ in soil extracts

The detection accuracy of square-wave anodic stripping voltammetry (SWASV) for Cd2+ and Pb2+ in soil extracts is interfered with by soluble humic acid (HA). The HA interference originated from the complexation between functional groups of HA with heavy metals (HMs) that decreased the SWASV signals of Cd2+ and Pb2+. The advanced oxidation process combination of vacuum ultraviolet (VUV) and H2O2 (VUV-H2O2) was selected as an appropriate pretreatment method to breakdown HA-HM complexes and restore SWASV accuracy. Key parameters, including photolysis time and H2O2 concentration, were systematically optimized to efficiently eliminate HA interference. The results showed that after HA-HMs underwent photolysis for 15 min under 200 mg/L H2O2, the SWASV signals of Cd2+ and Pb2+ were almost restored, and the SWASV detection accuracies of Cd2+ and Pb2+ were approximately on par with those in pure HMs. Then, the degradation behaviors of HA in the VUV-H2O2 photolysis system were investigated. The results showed that the ultraviolet absorbance, dissolved organic carbon content, and fluorescence intensity of HA-HMs gradually decreased with prolonged photolysis, suggesting that the functional groups of HA were degraded and that the complexing ability of HA to HMs was inhibited. More importantly, the SWASV detection accuracy of Cd2+ and Pb2+ in real soil extract solutions was improved by VUV-H2O2 photolysis, verifying its practicality. The findings of this study further promote the application of electrochemical analysis in soil heavy metal detection.

Bioaugmentation mechanism on humic acid formation during composting of food waste

In this study, microbes were added to food waste compost in order to investigate the bioaugmentation mechanism of Humic acid (HA) formation. Thermogravimetric analysis, structural equation model, Fourier transform infrared spectroscopy and statistical analysis were utilized to explain the bioaugmentation mechanism. The results showed that bioaugmentation increased humification rate and degree. Bioaugmentation not only promoted the formation of aromatic structures and CC bonds but also brought different change orders of functional groups in HA. The HA obtained in bioaugmentation group (BA, 7.51 g/kg) was significantly higher compared to the control group (CK, 2.37 g/kg). Similarly, the HA/FA of BA (1.90) was also higher than that of CK (0.62), and peaked at 2.34 on day 40. The polyphenol humification pathway played a major role regardless of the addition of inoculant. However, the exogenous microbes promoted protein and carbohydrate degradation in the initial stage, and the abundance of precursors (amino acids and reducing sugars) enhanced both Maillard and polyphenol humification pathways. When polyphenol was insufficient in later stage, bioaugmentation mainly embodied in the strengthening of Maillard humification pathway. This finding benefited the practice of directional humification process of food waste composting.

Stability of humic acid carbon under conservation agriculture practices

Recently conservation agriculture (CA) using the principles of zero tillage, crop residue management and suitable crop rotation has gained the momentum and quite popular among the farmers. Researchers throughout the globe have shown the potential of CA in enriching soil organic carbon storage in various soil types and cropping systems. But there is very limited study on the effect of CA on soil humic acid content and their stability and abundance of functional groups. An experiment was conducted in an Alfisols with four scenarios (designated as Sc) namely conventional farmers practice (Sc1) of transplanted rice (TPR) followed by conventional tilled broadcasted wheat (CT- wheat) with residue removal, transplanted rice (TPR) followed by zero tillage (ZT) wheat and mung bean with partial residue retention (Sc2), direct seeded rice (DSR) followed by ZT- wheat and mung bean with full residue retention (Sc3), ZT-maize followed by ZT-wheat and mung bean (Sc4) at the research farm of ICAR-Central Soil Salinity Research Institute, Karnal, Haryana, India. The objectives of this study were to assess the humic acid (HA) content and stability of soil carbon through desorption study and to characterize the functional groups of HA by Fourier Transform Infrared (FTIR) spectroscopy under different scenarios. HA was extracted from soil samples collected from 0 to 15 and 15–30 cm depth after rice harvesting in 2018 after nine years of the experiment. Results showed that Sc4 recorded significantly highest HA content at 0–15 depth (4.23 g/kg) and 15–30 cm soil depth (2.08 g/kg). Lower E4/E6 ratio revealed higher stability and humification of humic acid carbon under CA based scenarios. Lowest desorption rate constant (0.20/day) was observed under Sc4 indicating higher organic carbon stability in soil. Higher labile carbon and nitrogen was observed in soils under Sc3 as revealed through desorption study. FTIR spectroscopy results confirmed the prevalence of higher number of functional groups (OCO, CO or CHO) in HA under CA based scenarios with an order of Sc4 >Sc3 >Sc2 >Sc1. Therefore, CA is not only efficient in enriching the organic carbon in soil but also in more stable complex of HA with soil particles having long term implications in soil carbon sequestration for future posterity.

Research on the Adsorption of Heavy Metal Ions from Model Solutions by Humic Acids Isolated from Sapropel

The article presents the results of research on adsorption of Cu2+, Ni2+ and Pb2+ ions from model solutions with concentrations of 2–25 mg/dm3 under static conditions using humic acids isolated from the sapropel of Lake Lipovo, Novgorod region as an adsorbent. Adsorption equilibrium in the model solution-adsorbent systems occurs in 2 hours. The kinetic curves of adsorption of Cu2+, Ni2+ and Pb2+ ions from model solutions by humic acids are described with a pseudo-second order kinetic model; consequently, the limiting stage of the adsorption process is the chemical interaction between heavy metal ions and functional groups of humic acids in a 1:1 ratio. The kinetic constants of the adsorption of heavy metal ions by humic acids have been calculated. It was found that the adsorption of Cu2+, Ni2+ and Pb2+ ions occurs on the surface of the sorbent; the Langmuir equation is most acceptable for describing the process. The constants of adsorption equilibrium in the model solution-sorbent systems are calculated according to the Langmuir equation. The study of the adsorption of heavy metal ions on humic acids isolated from the sapropel of Lake Lipovo, Novgorod region, has shown the perspective of using sorbents for purifying industrial wastewaters.