Humic Substances Fractions Research Articles

Comparing optical versus chromatographic descriptors of dissolved organic matter (DOM) for tracking the non-point sources in rural watersheds

Identification of the major dissolved organic matter (DOM) sources in watersheds is critical for an efficient management to control the non-point sources associated with DOM and for an understanding of the local carbon cycle. In this study, several DOM indicators, which are derived from fluorescence spectroscopy and size exclusion chromatography, were compared for their applicability to track DOM sources in three forested-agricultural watersheds. Several potential source end-members were collected from upper catchments, which included fallen leaves, riparian reeds, riparian weeds, paddy soils, field soil, riverside soil, cow manure, swine manure, and poultry manure, and the values of their representative DOM descriptors were estimated along with those of the stream samples during high flow periods. The relative source contributions to streams were estimated via an end member mixing analysis using the paired descriptors that were selected based on the criteria in terms of the value ranges and the significant differences in stream samples between the high and low flow periods. Biological index (BIX) and the ratio of the terrestrial humic-like to the humic-like component were found to be optimum optical tracers, while the aromaticity and the C/N ratio of humic substance fraction were an applicable pair of chromatographic tracers. The optical tracers were superior to the latter because of more consistent and less variability in the assigned sources between the different streams in the same watersheds. The optical tracer-based estimations revealed that the major DOM sources to the high flow stream samples were soil (22.3%–58.5%) and manures (12.9%–51.3%). This comparative study provides new insights into the practical feasibility and the overlooked limitations of the common DOM descriptors for source discrimination.

Assessing Redox Properties of Natural Organic Matters with regard to Electron Exchange Capacity and Redox-Active Functional Groups

Redox processes in groundwater play an important role in bioavailability, toxicity, and mobility of redox-active elements and contaminants. A recent study has demonstrated that low-molecular-weight fraction (LMWF) of humic substances with great number of redox-active functional groups (RAFGs) exhibits great reducing capacity. However, whether LMWF of natural organic matter (NOM) exhibits high redox capacity still remains unclear. Therefore, this study extracted Pahokee peat NOM (PPNOM) and Leonardite NOM (LNOM) from soils, and then LMWFs in these NOMs were collected using a dialysis method. Electron exchange capacities (EEC) and RAFGs of LMWF NOMs at different Eh were analyzed using a novel electrochemical method and a three-dimensional excitation emission fluorescence (3DEEM) spectroscopy. We found that the reducing capacity in LMWF PPNOM was approximately 5-6 times higher than the bulk NOM, while only 7.8% LMWF PPNOM was accounted for in the bulk NOM. An increasing in EEC (EAC + EDC, where EAC is the electron accepting capacity and EDC is the electron donating capacity) of LMWF PPNOM and LNOM with Eh reduced from −0.49 V to −0.69 V. Additionally, an obvious increase in fluorescent intensities of quinone-like fluorophores before and after being reduced LMWF LNOM is responsible for high EAC of LMWF LNOM. These findings provide a better understanding of relationship between RAFGs Eh in LMWF of NOM, further helping in predicting and protection of groundwater environment and fate of transformation and transport for redox-active contaminants in groundwater.

Elevated nanofiltration performance via mussel-inspired co-deposition for sustainable resource extraction from landfill leachate concentrate

During the disposal of highly saline wastewaters, i.e., landfill leachate concentrate, a conceptual and paradigmatic transformation from contaminant elimination to sustainable resource extraction urges for novel selective membranes. Mussel-inspired modification through dopamine self-polymerization can be a competitive alternative to tailor the membrane surface properties for enhanced permselectivity to target compounds. Herein, co-deposition of polydopamine-polyethylenimine (PEI) complex onto loose nanofiltration (NF) membranes was conducted using persulfate as trigger, which resulted in a significant alteration in membrane surface properties. Particularly, the pore size of bio-inspired NF membranes decreased, along with a reduction in molecular weight cut-off (MWCO) from 640 to 298 Da after 120-min bio-inspired coating, which yielded an enhancement in rejection of humic substances from 95.1% to 98.9% in filtration of landfill leachate concentrate. Furthermore, the charge of membrane surface was tailored to be more positive due to incorporation of positively-charged PEI, which markedly undermined the electrostatic repulsion force for enhanced salt permeation. These bio-inspired NF membranes showed a superior selectivity between humic substances and salts for their potential fractionation from the landfill leachate concentrate. Through an integrated NF-based diafiltration procedure, the concentration of humic substances was remarkably enriched from 1779.4 to 17247.1 mg·L−1 with 96.0% recovery. Additionally, 99.5% desalination efficiency was achieved, resulting in a high purity (i.e., 98.3%) for humic substances as potential organic fertilizer application. These findings suggest that the bio-inspired co-deposition is an effective strategy to tailor the surface properties of NF membranes for efficient fractionation of humic substances and salts, in view of sustainable resource recovery from the landfill leachate concentrate.

Resistance of Soil Organic Matter in “Humic A Horizons” of the Mountainous Region in the State of Rio De Janeiro

Soils with high organic carbon content, such as those with A horizon, occur in different regions of Brazil. This study aimed to determine the quantitative and qualitative characteristics of organic matter regarding the humic A horizon under different land uses in Bom Jardim, in Rio de Janeiro State, Brazil. Samples from forest, pasture, Eucalyptus plantation, coffee cultivation, olericulture, and also passion fruit soils were sampled and analyzed. In October 2011, undisturbed samples from humic A horizons, were collected for soil density analysis. In addition, disturbed samples were collected for the determination of organic carbon (C-org) content, total carbon, humic substance fractions contents and natural abundance of (13)Carbon (delta C-13). Low C-org contents were observed in areas under olericulture, pasture and passion fruit cultivation. The fulvic acid fraction was higher than other fractions, regardless of land use. The small variability in delta C-13 indicated maintenance of original C-org and suggest high resistance of Soil Organic Matter in humic A horizons.

The Structure of Humic Substances in the Agricultural Genetically Modified Soils of the Kansk Forest-Steppe

In two long-arable areas that were intended for sowing rapeseed and false flax for oilseeds at LLC «Experimental Industrial Farm Solyanskoye» in the Kansk forest-steppe, the structure and humus substances reserves in agrochernozems were studied. The soil cover of key plot No. 1 with the wide-steeply-sloping terrain with weakly pronounced microrelief, which was intended for rape, was dominated by typical medium and strong clayey-illuvial agrochernozems. Key plot No. 2 intended for false flax was located on a smooth hillside of a wide ridge and was characterized by the complexity of the soil cover represented by a combination of clay-illuvial typical agrochernozems of various kinds, thick podzolized clay-illuvial agrochernozems, and thin cryogenic micellar agrochernozems. It has been shown that agrochernozems featured high content of humus in the layer both 0 – 20 and 0 – 40 cm thick. The content of humus carbon (Ctot) and that of carbon of the water-soluble organic matter (CH2O) had little spatial variability (CV = 1.3 – 11.7 %), unlike the carbon content of the alkali-soluble organic matter (C 0.1 n. NaOH), which in the studied agrochernozems had a very high degree of spatial variability (Cv = 18.7 – 66.1 %). Heterogeneity of the soil cover of the slope part of the plot was the factor that determined the average reduction in the content of the reserves of all fractions of humic substances, except for the water-soluble humus carbon. It has been found that the share of the reserves of stable humus carbon (Cstab) was 89 – 95 % of the total carbon reserves of humus, thus the share of CH2O and C0.1 n. NaOH decreased with increasing the complexity of the soil cover from 11 to 5 % of Ctotal. Fields with a pronounced meso- and microrelief and thin kinds of agrochernozems are recommended for sowing false flax, which is a crop that is less demanding to the soil conditions than rapeseed.

Microbially reducible extent of solid-phase humic substances is governed by their physico-chemical protection in soils: Evidence from electrochemical measurements

A significant fraction of humic substances (HS) in natural soils is protected due to the interactions with mineral surfaces or occlusion within aggregates and clay microstructures, which could render the solid-phase HS less spatially accessible to microorganisms and thus affect their extracellular electron transfer. However, more diverse and convincing evidence is needed to further verify the link between extracellular electron transfer of solid-phase HS and their physico-chemical protection mechanisms in soils. In this study, soil physical fractionation and mediated electrochemical measurements were combined for the first time to assess the microbially reducible extents of solid-phase HS in mineral soils. The results show that the solid-phase HS in soil contain several pools with different microbially reducible extents, due to the fact that different pools employ distinct protection mechanisms of soil organic matter against microbial attacks; the particulate HS exert the greatest microbially reducible extent, followed by the microaggregate occluded HS and non-aggregated silt associated HS, and non-aggregated clay associated HS exert the lowest microbially reducible extent. Our work provides a new evidence to validate the view that the extracellular electron transfer process of solid-phase HS in natural soils is largely controlled by their physico-chemical protection mechanisms.

Chemical and spectroscopic characterization of humic substances from sediment and riparian soil of a highly polluted urban river (Suquía River, Córdoba, Argentina)

The Suquía River, the largest urban river in Córdoba (Argentina), has been severely polluted for decades. Actions must be taken to restore its environmental quality by managing riparian zones for increased water-self purification. The current study aimed to characterize organic matter (OM) dynamics and humic substances (HS) spectrochemical properties along the lower-middle basin of the Suquía River. Riparian soil (0–20 cm) and sediment (0–10 cm) samples were collected from a reference location (S1) and four polluted sites (S2–S5) during a low-flow period. The contents of soil and sedimentary OM and HS fractions were analyzed by wet oxidation, as well as HS Fourier transform infrared (FT-IR) and ultraviolet–visible (UV–Vis) spectrochemical properties. The OM and HS fractions from riparian soil were high upstream of Córdoba City (S1 and S2, 50.2–50.4 g/kg OM) and within a 50 km downstream location (S5, 30.9 g/kg OM) owing to a surplus of fresh plant biomass-carbon (C) inputs. Highly heterogeneous sediment samples did not show any significant differences among sites (P > 0.05). The lowest values of the ratio of absorbances at 465 and 665 nm (E4/E6) (1.78) and the Δ log K (0.15) coefficient (a measure of HS maturity degree) were obtained downstream of Córdoba City, for both riparian soil and sediment, indicating that HS were enriched by more condensed aromatic structures within highly degraded portions of the river. All samples exhibited similar IR spectra, implying overlapping recalcitrant-C structures at the functional group level, but with different absorbance intensity. Data from the current study constitute a baseline for understanding the chemical nature of HS from sediment and riparian soil along the Suquía River and can be used as a reference for future studies tracking OM compositional changes over time.

Tungsten accumulation by wild plants growing on mining wastes and contaminated soils

The transfer of metal and metalloid trace elements (MTEs) from contaminated soil to grapevines is a major issue for grape consumption and for the associated health risks. Based on an isotopic approach, we shed light on the concept of MTE bioavailability. The bioavailable fractions are identified by using the Sr-isotope ratio as a proxy for MTEs. This allows us to differentiate three soil reservoirs: the ‘current available fraction’ in soil water, the ‘reserve available fraction’ stored in mineral phases of the soil fractions, and the ‘non-available fraction’. The reserve available fraction, representing 10 to 60% of bulk soil depending on the MTE, includes the exchangeable, carbonates, humic substance and oxides fractions. The 87Sr/86Sr isotopic signatures of grape berries and vine leaves show an additional source of MTEs, which is imported by foliar uptake and can contribute up to 10% of the MTEs in leaves. In addition, root-uptake and translocation rates show high accumulation rates of Co, Sn and Cu, and low ones for As, Sb, Zn and Cd. A daily intake between 1 and 3 kg of (dry grapes) would reach the benchmark dose level for a 0.5% (BMDL0.5). While such a daily intake of grapes is unreasonable, consumption of other local vegetables and fruit would contribute to the daily intake. Hence, a chronic arsenic exposure is of great concern for human health in mining areas. We outline the importance of geochemical tracers, such as Sr isotopes, when determining the transfer and translocation of MTEs in plants. Our method presents a high-precision evaluation of the bioavailability and bioaccumulation of MTEs, and a better understanding of these processes in plants, thus leading to a better assessment of the environmental risk on human health.

Examination of molecular space and feasible structures of bioactive components of humic substances by FTICR MS data mining in ChEMBL database

Humic substances (HS) are complex natural mixtures comprising a large variety of compounds produced during decomposition of decaying biomass. The molecular composition of HS is extremely diverse as it was demonstrated with the use of high resolution mass spectrometry. The building blocks of HS are mostly represented by plant-derived biomolecules (lignins, lipids, tannins, carbohydrates, etc.). As a result, HS show a wide spectrum of biological activity. Despite that, HS remain a ‘biological activity black-box’ due to unknown structures of constituents responsible for the interaction with molecular targets. In this study, we investigated the antiviral activity of eight HS fractions isolated from peat and coal, as well as of two synthetic humic-like materials. We determined molecular compositions of the corresponding samples using ultra-high resolution Fourier-transform ion cyclotron resonance mass-spectrometry (FTICR MS). Inhibitory activity of HS was studied with respect to reproduction of tick-borne encephalitis virus (TBEV), which is a representative of Flavivirus genus, and to a panel of enteroviruses (EVs). The samples of natural HS inhibited TBEV reproduction already at a concentration of 1 µg/mL, but they did not inhibit reproduction of EVs. We found that the total relative intensity of FTICR MS formulae within elemental composition range commonly attributed to flavonoid-like structures is correlating with the activity of the samples. In order to surmise on possible active structural components of HS, we mined formulae within FTICR MS assignments in the ChEMBL database. Out of 6502 formulae within FTICR MS assignments, 3852 were found in ChEMBL. There were more than 71 thousand compounds related to these formulae in ChEMBL. To support chemical relevance of these compounds to natural HS we applied the previously developed approach of selective isotopic exchange coupled to FTICR MS to obtain structural information on the individual components of HS. This enabled to propose compounds from ChEMBL, which corroborated the labeling data. The obtained results provide the first insight onto the possible structures, which comprise antiviral components of HS and, respectively, can be used for further disclosure of antiviral activity mechanism of HS.

Adsorption of Cu(II) on soil humin: batch and spectroscopy studies

Understanding the interactions of humic substances with metal ions is of great importance in forecasting the behaviors and fates of trace metals in the environment. Humin is the least understood humic substance fraction due to its close association with the mineral matrix in soil. In this study, humin was prepared from three zonal soils (i.e., dark brown soil, yellow brown soil, and latosol) in eastern China by demineralization using HF–HCl solution after removing alkaline-soluble humic substance fractions. The adsorption properties of Cu(II) onto both humin and humic acid were examined using batch and spectroscopy (SEM, EDS, FTIR, XPS, and XAS) techniques. With respect to humic acid, humin was more aliphatic and less polar. The adsorption kinetics of humin and humic acid were best fit with pseudo-second-order kinetics equation, and the adsorption isotherms were well depicted by the Langmuir model. However, the quantity of adsorbed Cu(II) on humin was inferior to that of Cu(II) adsorbed onto humic acid. After adsorption, the surface morphology of humin and humic acid changed. Carboxyl, hydroxyl, aromatic N, and primary amine N on the surfaces of the two humic substance fractions took part in Cu(II) adsorption, and Cu(II) was bound with O/N and C atoms via inner-sphere complexation. Our results suggested that humin and humic acid had different adsorption capacity but similar adsorption mechanisms toward Cu(II).

Evolution of Dissolved Organic Matter Along a Septic System Plume: Evidence of Sustained Biogeochemical Activity in the Groundwater Zone

AbstractDissolved organic matter (DOM) is an important energy source for biogeochemical reactions. However, DOM is often assumed to be recalcitrant in the groundwater zone due to extensive microbial processing in the overlying soil zone and long groundwater residence times. Consequently, further heterotrophic microbial processes proceed at inherently low rates. Septic systems provide an ideal opportunity to study the evolution of groundwater DOM due to the development of confined and easily identifiable plumes of known age. Here we use measures of DOM composition (size‐exclusion liquid chromatography, ultraviolet absorbance, and C:N) to quantify DOM evolution along an exceptionally well‐characterized septic‐impacted groundwater plume. Elevated concentrations (5 to 8 mg C/L) close to the tile bed decrease to values below 2 mg C/L with lower UV‐absorbing components normalized to overall DOM concentration (specific ultraviolet absorbance) along the six‐year‐old plume flow path. The humic substance fraction (HSF) comprises the largest percentage of DOM and decreases in concentration along the septic plume. HSF properties also change with travel time becoming lower in molecular weight, specific ultraviolet absorbance, and C:N. Denitrification continues along the length of the plume concomitant with changes in DOM and HSF composition. Thus, HSF, typically considered recalcitrant in subsurface environments, is actively cycled by microorganisms on multiyear time frames in accordance with the recent paradigm shift for soil organic matter evolution. Lastly, measures of DOM composition indicate changes in DOM that are not evident from measures of concentration alone.

Kinetic aspects of humic substances derived from macrophyte detritus decomposition under different nutrient conditions.

Autochthonous particulate organic carbon (POC) is an important precursor of humic substances (HS), and macrophytes represent the major source of POC in tropical aquatic ecosystems. Autochthonous HS influence the carbon supply, light regime, and primary production within freshwater systems. This study addresses the conversion of POC from two macrophyte species into HS and their mineralization under different nutrient conditions (oligotrophic to hypereutrophic). A first-order kinetic model was adopted to describe the conversion routes. The POC conversion rate to HS for detritus derived from Paspalum repens was similar under different nutrient conditions, but eutrophication decreased the kR (global coefficient reaction) for detritus from Pistia stratiotes due to its high detritus quality (C:N:P ratio). Fulvic acids were the main fraction of HS in both plants. The mineralization of humic acids from P. stratiotes was inhibited at higher nutrient availability, while eutrophication increased the mineralization of fulvic acids from P. repens. The main route of POC cycling is humification through fulvic acid formation (up to 40% of POC). The intrinsic characteristics of the source detritus were the main forcing functions that stimulated the cycling of HS. In tropical aquatic ecosystems, the degradation of autochthonous carbon decreased due to eutrophication, thus contributing to the diagenetic process in the long term.

Binding characteristics of heavy metals to humic acid before and after fractionation by ferrihydrite

The fractionation of humic substances (HS) at the mineral and water interface can change the constituents and reactivity of HS, but there is still a lack of the understanding of the effects of HS fractionation on the binding characteristics of heavy metals to HS. In this study, the binding characteristics of five heavy metals (Cd, Cu, Ni, Pb, and Zn) to humic acid (HA) before and after adsorption by ferrihydrite were investigated by employing two-dimensional correlation spectroscopy (2D COS) integrated with synchronous fluorescence and Fourier transform infrared (FTIR) spectroscopies. 2D COS analyses of the fluorescence results indicated that the susceptibility of the fluorescence of humic-like fraction to heavy metals significantly decreased after the adsorption of HA by ferrihydrite, which may be due to the fact that humic-like components were preferentially adsorbed by ferrihydrite. However, the fractionation processes did not alter the metal binding sequence and affinity to different HA components. 2D COS analyses of the FTIR results suggested that fractionation processes decreased the susceptibility of COO− groups to heavy metals, and changed the metal binding sequence to polysaccharides C-O and aryl groups, with the exception of Pb. Furthermore, model calculations showed that the binding ability of heavy metals to both humic-like and fulvic-like fractions decreased after the adsorption of HA by ferrihydrite. The results of this study contribute to predicting heavy metal behavior in the environment.

Accelerating effects of humin on sulfide-mediated azo dye reduction

As an important fraction of humic substances, humin has been found capable of stimulating bioreduction reactions. However, whether humin could promote abiotic reduction and the effects of coexisting soluble humic substance and insoluble mineral remained unsolved. In this study, a humin sample was isolated from a paddy soil. Cyclic voltammetry, electron paramagnetic resonance, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses of the humin indicated the existence of redox-active quinone moieties and other oxygen-containing groups. The humin could be reduced by sulfide and its presence stimulated the abiotic reduction of acid red 27 (AR27) and four other azo dyes by sulfide. In the presence of 100–1000 mg/L intact humin, the sulfide-mediated AR27 reduction efficiency in 7 d was enhanced from 56.3% to 92.5%. The stimulating behavior of intact humin was observed for 100–300 mg/L AR27 and increased with the increase of sulfide concentration (1.2–3.0 mM). Much higher stimulating effects were found with the presence of humin pre-reduced by sulfide. Moreover, for sulfide-mediated AR27 reduction, the coexistence of humin (500 mg/L) and humic acid (10–30 mg/L) or Wyoming sodium-montmorillonite (SWy-2, 1–4 g/L) led to better promotion activities than the presence of single component. And synergistic promotion of sulfide-mediated AR27 reduction was observed with coexisting humin and SWy-2 due to enhanced Fe(II) production. These findings extended our understanding of the influence of humin on reductive transformation of pollutants in the environment.

Characterization of wastewater from landfills - amount and type of humic substances extracted from leachate

This study aimed to characterize the humic substances (HS) extracted from landfills wastewater – leachates, with different ages of exploration. To reach the objective it was applied spectroscopy techniques, UV-Vis and FTIR spectra, as well as the ratio between the absorbance analysed. First, the HS were extracted, then fractionated in fulvic acids (FA) and humic acids (HA) and it was evaluated the phytotoxicity. HS content in the leachates were higher than the typical values found in the natural aquatic humic sources. It has been identified that the leachate HS, HA and FA aromatic fractions increased with the increase of the landfilling age, suggesting that the degree of humification increased with the landfilling age. All the HS showed a high aromaticity and humification degree. The HS extracts irrespective of their source presents similar structural composition. The functional groups found are in agreement with the literature: phenols, alcohols, carboxylic groups, aliphatic structures, among others. It was found that HA are mainly organic matter with a higher aromatic degree than FA. The HS and HA showed absence of phytotoxicity, testing by germination index, suggesting that may be used to produce liquid organic fertilizers.

Correction to "Facet-Mediated Adsorption and Molecular Fractionation of Humic Substances on Hematite Surfaces".

Correction to "Facet-Mediated Adsorption and Molecular Fractionation of Humic Substances on Hematite Surfaces".

Facet-Mediated Adsorption and Molecular Fractionation of Humic Substances on Hematite Surfaces.

Interactions between dissolved organic matter (DOM) and iron oxyhydroxides have important environmental and geochemical implications. The present study employed two hematite nanocrystals to investigate the adsorption and molecular fractionation of two typical humic substances (HSs) using electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). Hematite with a predominant exposure of {100} facets induced more pronounced adsorption and molecular fractionation of HSs than {001} facets, indicating that the interfacial adsorptive fractionation process of HSs was mediated by exposed facets of hematite. Further exploration of the surface OH groups of the two hematite nanocrystals confirms that the facet-mediated molecular fractionation of HSs was attributable to the abundance of singly iron-atom coordinated -OH sites on the hematite surfaces. Molecules with a high oxidation state and high aromaticity such as oxidized black carbon, polyphenol-like, and tannic-like compounds preferentially formed ligand-exchange complexes with singly coordinated -OH groups on the hematite surfaces, inducing the selective binding and molecular fractionation of HSs at the mineral-water interface. These results demonstrate that singly iron-atom coordinated -OH sites determine DOM adsorption and mediate molecular fractionation on hematite surfaces, and this contributes substantially to our understanding of the molecular mechanisms of iron oxyhydroxide-mediated molecular exchange of DOM in soils and/or sediments.

Innovative isotopic method to evaluate bioaccumulation of As and MTEs in Vitis vinifera

The transfer of metal and metalloid trace elements (MTEs) from contaminated soil to grapevines is a major issue for grape consumption and for the associated health risks. Based on an isotopic approach, we shed light on the concept of MTE bioavailability. The bioavailable fractions are identified by using the Sr-isotope ratio as a proxy for MTEs. This allows us to differentiate three soil reservoirs: the ‘current available fraction’ in soil water, the ‘reserve available fraction’ stored in mineral phases of the soil fractions, and the ‘non-available fraction’. The reserve available fraction, representing 10 to 60% of bulk soil depending on the MTE, includes the exchangeable, carbonates, humic substance and oxides fractions. The 87Sr/86Sr isotopic signatures of grape berries and vine leaves show an additional source of MTEs, which is imported by foliar uptake and can contribute up to 10% of the MTEs in leaves. In addition, root-uptake and translocation rates show high accumulation rates of Co, Sn and Cu, and low ones for As, Sb, Zn and Cd. A daily intake between 1 and 3 kg of (dry grapes) would reach the benchmark dose level for a 0.5% (BMDL0.5). While such a daily intake of grapes is unreasonable, consumption of other local vegetables and fruit would contribute to the daily intake. Hence, a chronic arsenic exposure is of great concern for human health in mining areas. We outline the importance of geochemical tracers, such as Sr isotopes, when determining the transfer and translocation of MTEs in plants. Our method presents a high-precision evaluation of the bioavailability and bioaccumulation of MTEs, and a better understanding of these processes in plants, thus leading to a better assessment of the environmental risk on human health.

Molecular insights into reversible redox sites in solid-phase humic substances as examined by electrochemical in situ FTIR and two-dimensional correlation spectroscopy

Solid-phase humic substances (also termed humins, HMs) are the largest fraction of humic substances (HSs) in soils and sediments, which are recently shown to be capable of mediating electron transfer in many key biogeochemical processes. However, the redox properties of HMs remains poorly investigated, likely due to their structural complexity and the lack of efficient methodology. Herein, an electrochemical in situ FITR spectroscopic (EC-FTIRS) technique coupled with two-dimensional correlation spectroscopy (2D-COS) was employed for the first time to study the redox process of complex HMs at a molecular level. In situ FTIR identified that the IR bands of quinone and phenolic moieties in the HMs were potential dependent, suggesting that they were the main redox sites in response to the redox transformation of the HMs. Meanwhile, In situ FTIR characterizations showed that the significant variations in IR bands positioned at ~1500 and ~1470 cm−1 in response to the applied potentials, providing evidence for the presence of quinone radical and dianion intermediates during the redox process. The 2D-COS analysis was used to further explore variations in infrared intensities as a function of the potentials applied to reduce or oxidize the HMs, which indicated the occurrence of the typical redox reactions of quinone in the HMs. These results improve our understanding on the redox mechanism of HMs at the molecular level and have significant implication for in-depth understanding of biogeochemical processes.

Application of Natural Humates for Remediating Contaminated Urban Soils and Stimulating Plant Growth

Long-term laboratory and microfield experiments have revealed the effective doses of humates for their use as detoxifiers of urban soils. The developed original technique allowed us to modify the commercial product Ekstra by fundamentally changing the ratio of amphiphilic fractions of humic substances. Field tests of new Ekstra growth showed its high efficacy as a plant growth stimulator and antistress preparation, which makes it possible to mitigate (or greatly reduce) the toxic effects of various pollutants on plants in contaminated areas.