Assessing redox properties of standard humic substances

Redox properties of humic substances (HS) control important biogeochemical processes. Thus, accurate estimation of redox properties of HS is essential. However, there is no general consensus regarding the best available measurement method of HS redox properties. In this study, we compared several common HS redox property measurement methods using anthraquinone-2,6-disulfonate (AQDS) as model compound, and standard Elliot soil humic acid (1S102H, ESHA), reference Pahokee peat (1R103H, PPHA), and Suwannee River natural organic matter (1R101N, SRNOM) as representative HS. We found that the H₂/Pd reduction method followed by incubation with ferric citrate (FeCit) reagent was incomplete, and the H₂/Pd reduction method followed by incubation with potassium ferricyanide (K₃Fe(CN)₆) was insensitive. Stannous chloride (SnCl₂) reduction followed by titration of excess stannous (Sn²⁺) by potassium dichromate (K₂Cr₂O₇) was found to be most accurate. These findings will help in future investigations on detailed characterizations of functional groups of HS responsible for oxidation/reduction reactions.     

Interactions between magnetite and humic substances: redox reactions and dissolution processes

Humic substances (HS) are redox-active compounds that are ubiquitous in the environment and can serve as electron shuttles during microbial Fe(III) reduction thus reducing a variety of Fe(III) minerals. However, not much is known about redox reactions between HS and the mixed-valent mineral magnetite (Fe₃O₄) that can potentially lead to changes in Fe(II)/Fe(III) stoichiometry and even dissolve the magnetite. To address this knowledge gap, we incubated non-reduced (native) and reduced HS with four types of magnetite that varied in particle size and solid-phase Fe(II)/Fe(III) stoichiometry. We followed dissolved and solid-phase Fe(II) and Fe(III) concentrations over time to quantify redox reactions between HS and magnetite. Magnetite redox reactions and dissolution processes with HS varied depending on the initial magnetite and HS properties. The interaction between biogenic magnetite and reduced HS resulted in dissolution of the solid magnetite mineral, as well as an overall reduction of the magnetite. In contrast, a slight oxidation and no dissolution was observed when native and reduced HS interacted with 500 nm magnetite. This variability in the solubility and electron accepting and donating capacity of the different types of magnetite is likely an effect of differences in their reduction potential that is correlated to the magnetite Fe(II)/Fe(III) stoichiometry, particle size, and crystallinity. Our study suggests that redox-active HS play an important role for Fe redox speciation within minerals such as magnetite and thereby influence the reactivity of these Fe minerals and their role in biogeochemical Fe cycling. Furthermore, such processes are also likely to have an effect on the fate of other elements bound to the surface of Fe minerals.

Open image in new window

     

Biogeochemical transformation of Fe minerals in a petroleum-contaminated aquifer

The Bemidji aquifer in Minnesota, USA is a well-studied site of subsurface petroleum contamination. The site contains an anoxic groundwater plume where soluble petroleum constituents serve as an energy source for a region of methanogenesis near the source and bacterial Fe(III) reduction further down gradient. Methanogenesis apparently begins when bioavailable Fe(III) is exhausted within the sediment. Past studies indicate that Geobacter species and Geothrix fermentens-like organisms are the primary dissimilatory Fe-reducing bacteria at this site. The Fe mineralogy of the pristine aquifer sediments and samples from the methanogenic (source) and Fe(III) reducing zones were characterized in this study to identify microbiologic changes to Fe valence and mineral distribution, and to identify whether new biogenic mineral phases had formed. Methods applied included X-ray diffraction; X-ray fluorescence (XRF); and chemical extraction; optical, transmission, and scanning electron microscopy; and Mössbauer spectroscopy.All of the sediments were low in total Fe content (≈ 1%) and exhibited complex Fe-mineralogy. The bulk pristine sediment and its sand, silt, and clay-sized fractions were studied in detail. The pristine sediments contained Fe(II) and Fe(III) mineral phases. Ferrous iron represented approximately 50% of Fe_TOT. The relative Fe(II) concentration increased in the sand fraction, and its primary mineralogic residence was clinochlore with minor concentrations found as a ferroan calcite grain cement in carbonate lithic fragments. Fe(III) existed in silicates (epidote, clinochlore, muscovite) and Fe(III) oxides of detrital and authigenic origin. The detrital Fe(III) oxides included hematite and goethite in the form of mm-sized nodular concretions and smaller-sized dispersed crystallites, and euhedral magnetite grains. Authigenic Fe(III) oxides increased in concentration with decreasing particle size through the silt and clay fraction. Chemical extraction and Mössbauer analysis indicated that this was a ferrihydrite like-phase. Quantitative mineralogic and Fe(II/III) ratio comparisons between the pristine and contaminated sediments were not possible because of textural differences. However, comparisons between the texturally-similar source (where bioavailable Fe(III) had been exhausted) and Fe(III) reducing zone sediments (where bioavailable Fe(III) remained) indicated that dispersed detrital, crystalline Fe(III) oxides and a portion of the authigenic, poorly crystalline Fe(III) oxide fraction had been depleted from the source zone sediment by microbiologic activity. Little or no effect of microbiologic activity was observed on silicate Fe(III). The presence of residual “ferrihydrite” in the most bioreduced, anoxic plume sediment (source) implied that a portion of the authigenic Fe(III) oxides were biologically inaccessible in weathered, lithic fragment interiors. Little evidence was found for the modern biogenesis of authigenic ferrous-containing mineral phases, perhaps with the exception of thin siderite or ferroan calcite surface precipitates on carbonate lithic fragments within source zone sediments.     

Effects of humic substances on Fe(II) sorption onto aluminum oxide and clay

We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)–Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)–Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)–Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)–Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)–Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments.     

Aggregation of aquatic humic substances

Humic substances (HS) were isolated from Penwhirn Reservoir (PR) and Esthwaite Water (EW) and their removal from solution by centrifugation was studied as a function of pH, humic concentration and molecular weight, and CaCl₂ concentration. Large amounts (up to 50%) of PR HS could be removed but only small amounts (? 3%) of EW HS. At pH ? 5 removal of PR HS by Ca²⁺ can be explained satisfactorily in terms of decreases in humic solubilities induced by complexation with the cation. However, removal induced by protonation of the PR HS is unusual in that it decreases with increasing humic concentration.The results suggest that PR HS comprise a range of molecules differing in solubility, with the high-molecular-weight (40,000) components being the least soluble. The EW HS consist of molecules of weight-average molecular weight 5000 and resemble similarly sized PR HS in that they remain unaggregated in solution even when highly complexed with Ca²⁺.     

The adsorption of aquatic humic substances by iron oxides

The interactions of humic substances from Esthwaite Water with hydrous iron oxides (α-FeOOH, α-Fe₂O₃, amorphous Fe-gel) have been examined by measuring adsorption isotherms and by microelectrophoresis. In Na⁺-Cl^?-HCO₃^?at I = 0.002 M (medium I) the extent of adsorption decreases with increasing pH. The results are consistent with a mechanism involving ligand exchange of humic anionic groups with H₂O and OH^?of surface Fe-OH₂⁺and Fe-OH groups respectively, with an increasing degree of protonation of the adsorbed humics as the adsorption density increases at constant pH.At pH 7 in a medium containing Mg²⁺, Ca²⁺ and SO₄^2?, at their Esthwaite Water concentrations and at I= 0.002 M (medium II) the adsorption capacity of goethite (α-FeOOH) is approximately twice that in medium I. Electrophoresis experiments show that the extra capacity is associated with coadsorption of Mg²⁺ and/or Ca²⁺ ions.When the iron oxides are added to samples of Esthwaite Water itself they become negatively charged and plots of electrophoretic mobility against pH for the natural water are identical to those in medium II plus humics.     

Phosphorus in marine and non-marine humic substances

The phosphorus content of marine humic acids (HA) is in the range of 0.1–0.2%. The C/P ratios of the HA are 300 to 400. Marine fulvic acids (FA) contain 0.4–0.8% P and have C/P ratios of 80 to 100. High molecular weight organic matter dissolved in pore waters (DOM) contains 0.5% P and has C/P of 90. The data suggest that during the formation sequence: Plankton → DOM → FA → HA → Kerogen, phosphorus is lost, mainly in the FA → HA (and possibly also in the HA → Kerogen) step. Diagenesis of sedimentary humic acids is accompanied by loss of phosphorus (as well as of nitrogen) to form HA with C/P ratios of 1000.Soil humic substances resemble marine humates in P content (0.3%) and soil FA's are about three to fivefold enriched in P relative to HA. C/P ratios are lower in soil HA (ca. 200) as compared with marine HA. Humic acids from diagenetic products such as peat and lignite are highly depleted in P. Rough calculations indicate that humate bound P may account for 20–50% of the organic phosphorus reservoir in sediments. The chemical speciation of this P is unknown, but lack of correlation with ash, Fe, Ca or Al content (in marine humates, at least) indicates that it is organically bound.     

The luminescent humic substances in speleothems

Calcite speleothems deposited in caves by seepage waters derived from the overlying soils of the epikarst are typically coarsely crystalline, colored various shades of brown through orange and white, and strongly luminescent. For most speleothems, the color and luminescence is due to humic substances incorporated in the calcite crystals. For a quantitative assessment of the luminescent properties of these materials, the organic content of the dissolved speleothems was separated by gel chromatography and spectra measured as a function of molecular weight. The most intense luminescence was obtained from molecular weight fractions in the range of 4000–6000 daltons, a range characteristic of fulvic acids. In addition, there was considerable variation in detail from sample to sample that would merit further investigation.     

Molecular size of aquatic humic substances

Aquatic humic substances, which account for 30 to 50% of the organic carbon in water, are a principal component of aquatic organic matter. The molecular size of aquatic humic substances, determined by small-angle X-ray scattering, varies from 4.7 to 33 Å in their radius of gyration, corresponding to a molecular weight range of 500 to greater than 10,000. The aquatic fulvic acid fraction contains substances with molecular weights ranging from 500 to 2000 and is monodisperse, whereas the aquatic humic acid fraction contains substances with molecular weights ranging from 1000 to greater than 10,000 and is generally polydisperse.     

Al(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation

Published experimental data for Al(III) and Fe(III) binding by fulvic and humic acids can be explained approximately by the Humic Ion-Binding Model VI. The model is based on conventional equilibrium reactions involving protons, metal aquo ions and their first hydrolysis products, and binding sites ranging from abundant ones of low affinity, to rare ones of high affinity, common to all metals. The model can also account for laboratory competition data involving Al(III), Fe(III) and trace elements, supporting the assumption of common binding sites. Field speciation data (116 examples) for Al in acid-to-neutral waters can be accounted for, assuming that 60-70 % (depending upon competition by iron, and the chosen fulvic acid : humic acid ratio) of the dissolved organic carbon (DOC) is due to humic substances, the rest being considered inert with respect to ion binding. After adjustment of the model parameter characterizing binding affinity within acceptable limits, and with the assumption of equilibrium with a relatively soluble form of Fe(OH)₃, the model can simulate the results of studies of two freshwater samples, in which concentrations of organically complexed Fe were estimated by kinetic analysis.The model was used to examine the pH dependence of Al and Fe binding by dissolved organic matter (DOM) in freshwaters, by simulating the titration with Ca(OH)₂ of an initially acid solution, in equilibrium with solid-phase Al(OH)₃ and Fe(OH)₃. For the conditions considered, Al, which is present at higher free concentrations than Fe(III), competes significantly for the binding of Fe(III), whereas Fe(III) has little effect on Al binding. The principal form of Al simulated to be bound at low pH is Al³⁺, AlOH²⁺ being dominant at pH >6; the principal bound form of Fe(III) is FeOH²⁺ at all pH values in the range 4-9. Simulations suggest that, in freshwaters, both Al and Fe(III) compete significantly with trace metals (Cu, Zn) for binding by natural organic matter over a wide pH range (4-9). The competition effects are especially strong for a high-affinity trace metal such as Cu, present at low total concentrations (∼1 nM). As a result of these competition effects, high-affinity sites in humic matter may be less important for trace metal binding in the field than they are in laboratory systems involving humic matter that has been treated to remove associated metals.     

Thermal characterization of natural and synthetic humic substances

Thermogravimetric technique was used for the characterization of natural (humic) and synthetic (melanoidins) substances. The influence of pH on the thermal stability of humic substances was studied. A similarity in thermal behaviour of natural humic substances and of melanoidins (prepared from an excess of sugar) and the unique thermal properties of melanoidins (prepared from basic amino acids) was observed. Thermal behavior of natural and synthetic substances was compared with model compounds of sugar, peptide and kerogen types.     

Rapid photo-oxidation of Mn(II) mediated by humic substances

The oxidation of Mn(II) by O₂ to Mn(III) or Mn(IV) is thermodynamically favored under the pH and pO₂ conditions present in most near surface waters, but the kinetics of this reaction are extremely slow. This work investigated whether reactive oxygen species, produced through illumination of humic substances, could oxidize Mn at an environmentally relavent rate. The simulated sunlight illumination of a solution containing 200 μM Mn(II) and 5 mg/L Aldrich humic acid buffered at pH 8.1 produced ∼19 μM of oxidized Mn (MnO_x where x is between one and two) after 45 minutes. The major oxidants reponsible for this reaction appear to be photoproduced superoxide radical anion, O₂⁻, and singlet molecular oxygen, ¹O₂. The dependencies of MnO_x formation on Mn(II), humic acid, and H⁺ concentration were characterized. A kinetic model based largely on published rate constants was established and fit to the experimental data. As expected, analysis of the model indicates that the key reaction rate controlling MnO_x production is the rate of decomposition of a MnO₂⁺ complex formed from the reaction of Mn(II) with O₂⁻. This rate is strongly dependent on the Mn(II) complexing ligands in solution. The MnO_x production in the seawater sample taken from Bodega Bay, USA and spiked with 200 μM Mn(II) was well reproduced by the model. Extrapolations from the model imply that Mn photo-oxidation should be a significant reaction in typical surface seawaters. Calculated rates, 5.8 to 55 pM h⁻¹, are comparable to reported rates of biological Mn oxidation, 0.07 to 89 pM h⁻¹. Four fresh water samples that were spiked with 200 μM Mn(II) also showed significant MnO_x production. Based on these results, it appears that Mn photo-oxidation could constitute a significant, and apparently unrecognized geochemical pathway in natural waters.     

The adsorption of aquatic humic substances by two oxides of manganese

A study was made of the adsorption of humic substances (HS) by Mn₃O₄ and by oxide B, a preparation with the β-MnOOH diffraction pattern but having a manganese oxidation state of 3.4. The interactions follow trends found for other oxides. Thus in 0.01 mol dm^?3 NaCl adsorption decreases with increasing pH, while it is enhanced by Ca²⁺. The HS adsorb more strongly to the oxide with the higher zero point of charge (Mn₃O₄), while the effect of Ca²⁺ is greater for oxide B.Microelectrophoretic measurements show that the oxide particles take on the electrokinetic characteristics of the adsorbed HS. However it was found that the magnitude of the mobility depends on the underlying oxide surface and on the source of the HS. The electrokinetic properties of the two oxides dispersed in surface water samples of Esthwaite Water, Cumbria, England, can be accounted for by the adsorbed HS together with coadsorbed Ca²⁺.     

Unique properties of humic substances from sapropel

Doklady Earth Sciences - Sapropel from inland Russian water reservoirs is becoming a popular raw material for medicinal purposes, production of sorbents, organomineral fertilizers, and food...     

The effect of silica and natural organic matter on the Fe(II)-catalysed transformation and reactivity of Fe(III) minerals

The Fe(II)-catalysed transformation of synthetic schwertmannite, ferrihydrite, jarosite and lepidocrocite to more stable, crystalline Fe(III) oxyhydroxides is prevented by high, natural concentrations of Si and natural organic matter (NOM). Adsorption isotherms demonstrate that Si adsorbs to the iron minerals investigated and that increasing amounts of adsorbed Si results in a decrease in isotope exchange between aqueous Fe(II) and the Fe(III) mineral. This suggests that the adsorption of Si inhibits the direct adsorption of Fe(II) onto the mineral surface, providing an explanation for the inhibitory effect of Si on the Fe(II)-catalysed transformation of Fe(III) minerals. During the synthesis of lepidocrocite and ferrihydrite, the presence of equimolar concentrations of Si and Fe resulted in the formation of 2-line ferrihydrite containing co-precipitated Si in both cases. Isotope exchange experiments conducted with this freeze-dried Si co-precipitated ferrihydrite species (Si-ferrihydrite) demonstrated that the rate and extent of isotope exchange between aqueous Fe(II) and solid ⁵⁵Fe(III) was very similar to that of 2-line ferrihydrite formed in the absence of Si and which had not been allowed to dry. In contrast to un-dried ferrihydrite formed in the absence of Si, Si-ferrihydrite did not transform into a more crystalline Fe(III) mineral phase over the 7-day period of investigation. Reductive dissolution studies using ascorbic acid demonstrated that both dried Si-ferrihydrite and un-dried 2-line ferrihydrite were very reactive, suggesting these species may be major contributors to the rapid release of dissolved iron following flooding and the onset of conditions conducive to reductive dissolution in acid sulphate soil environments.     

Methylation patterns of aquatic humic substances determined by13C NMR spectroscopy

¹³C NMR spectroscopy is used to examine the hydroxyl group functionality of a series of humic and fulvic acids from different aquatic environments. Samples first are methylated with¹³C-labeled diazomethane. The NMR spectra of the diazomethylated samples allow one to distinguish between methyl esters of car☐ylic acids, methyl ethers of phenolic hydroxyls, and methyl ethers of phenolic hydroxyls adjacent to two substituents. Samples are then permethylated with¹³C-labeled methyl iodide/NaH.¹³C NMR spectra of permethylated samples show that a significant fraction of the hydroxyl groups is not methylated with diazomethane alone. In these spectra methyl ethers of carbohydrate and aliphatic hydroxyls overlap with methyl ethers of phenolic hydroxyls. Side reactions of the methyltion procedure including carbon methylation in the CH₃I/NaH procedure, are also examined. Humic and fulvic acids from bog, swamp, groundwater, and lake waters showssome differences in their distribution of hydroxyl groups, mainly in the concentrations of phenolic hydroxyls, which may be attributed to their different biogeochemical origins.     

Sedimentary humic acid and fulvic acid as surface active substances

Surface tension of sedimentary fulvic acid (FA) and humic acid (HA) with molecular weight from < 10,000 to > 300,000 was measured at 5°C and 25°C, over a wide range of concentrations (0.114-107.4 g/l) at pH 8. HA was in the form of sodium humate. Surface tension decreases with an increase in HA and FA concentration and both HA and FA were found to be surface active materials with FA exhibiting the lowest surface tension (31 dynes/cm).Plots of surface tension vs. log concentration gave two straight lines with a break at a certain concentration similar to surfactants. From the concentration at the break point, aggregation concentration (AGC) was determined. For HA with molecular weight above 10,000, the AGC decreased with an increase in molecular weight. The more hydrophobic the HA, the greater was the tendency to form aggregates. Surface excess (surface concentration) was determined (2.3 × 10^?10?5.5 × 10^?10mol/cn²) from the slope of the plot of surface tension vs. log concentration for concentrations lower than the AGC. Adsorption of HA into the surface layer increased with increasing molecular weight of HA.     

天然黏土矿物和腐殖酸对纳米乳化油吸持的实验研究

为探讨天然黏土矿物及有机质对纳米乳化油在多孔介质中迁移滞留的影响，本文选取高岭石和蒙脱石这两种黏土矿物以及有机质的典型代表腐殖酸，开展了单一矿物、有机质及有机矿质复合物对纳米乳化油的吸持批实验研究，并运用比表面积全分析、扫描电镜(SEM)、傅里叶红外光谱(FTIR)、X射线衍射(XRD)等技术手段探讨了吸持机理。实验结果表明，介质对纳米乳化油的吸持均符合Freundlich模型；单一矿物及腐殖酸对纳米乳化油的吸持能力表现为：蒙脱石>腐殖酸>高岭石，有机矿质复合样品的吸持能力表现为：蒙脱石-腐殖酸>高岭石-腐殖酸，且均大于其对应的单一样品，出现了“1+1>2”的现象，表明介质组成越复杂，对纳米乳化油的吸持滞留程度越大。进一步分析证实，纳米乳化油主要通过氢键和疏水作用吸持在矿物和腐殖酸表面，表面结构性质是高岭石和蒙脱石吸持过程中的主导因素，因此蒙脱石具有更强的吸持能力，而腐殖酸的吸持主要通过颗粒间聚集作用来实现；对于复合样品，吸持主要通过氢键、配体交换和疏水作用结合来实现。腐殖酸与矿物的复合会增加吸持位点并且增强矿物表面疏水性，从而促进吸持。腐殖酸与纳米乳化油的共吸...     

Sources of sedimentary humic substances: vascular plant debris

A modern Washington continental shelf sediment was fractionated densimetrically using either an organic solvent, CBrCl₃, or aqueous ZnCl₂. The resulting low density materials (<2.06 g/ml) account for only 1% of the sediment mass but contain 25% of the sedimentary organic carbon and 53% of the lignin. The C/N ratios (30–40) and lignin phenol yields ($\text{Λ = 8}$) and compositions indicate that the low density materials are essentially pure vascular plant debris which is slightly enriched in woody (versus nonwoody) tissues compared to the bulk sediment. The low density materials yield approximately one-third of their organic carbon as humic substances and contribute 23% and 14% of the total sedimentary humic and fulvic acids, respectively. Assuming that the lignin remaining in the sedimentary fraction is also contained in plant fragments that yield similar levels of humic substances, then 50% and 30% of the total humic and fulvic acids, respectively, arise directly from plant debris.Base-extraction of fresh and naturally degraded vascular plant materials reveals that significant levels of humic and fulvic acids are obtained using classical extraction techniques. Approximately 1–2% of the carbon from fresh woods and 10–25% from leaves and bark were isolated as humic acids and 2–4 times those levels as fulvic acids. A highly degraded hardwood yielded up to 44% of its carbon as humic and fulvic acids. The humic acids from fresh plants are generally enriched in lignin components relative to carbohydrates and recognizable biochemicals account for up to 50% of the total carbon. Humic and fulvic acids extracted directly from sedimentary plant debris could be responsible for a major fraction of the biochemical component of humic substances.