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.     

Arsenic redox transformation by humic substances and Fe minerals

The toxicity and mobility of the redox-active metalloid As strongly depends on its oxidation state, with As(III) (arsenite) being more toxic and mobile than As(V) (arsenate). It is, therefore, necessary to know the biogeochemical processes potentially influencing As redox state to understand and predict its environmental behavior. The first part of this presentation will discuss the quantification of As redox changes by pH-neutral mineral suspensions of goethite [α-Fe^IIIOOH] amended with Fe(II) using wet-chemical and synchrotron X-ray absorption (XANES) analysis (Amstaetter et al., 2010). First, it was found that goethite itself did not oxidize As(III). Second, in contrast to thermodynamic predictions, Fe(II)–goethite systems did not reduce As(V). However, surprisingly, rapid oxidation of As(III) to As(V) was observed in Fe(II)–goethite systems. Iron speciation and mineral analysis by Mössbauer spectroscopy showed rapid formation of ⁵⁷Fe–goethite after ⁵⁷Fe(II) addition and the formation of a so far unidentified additional Fe(II) phase. No other Fe(III) phase could be detected by Mössbauer spectroscopy, EXAFS, scanning electron microscopy, X-ray diffraction or high-resolution transmission electron microscopy. This suggests that reactive Fe(III) species form as an intermediate Fe(III) phase upon Fe(II) addition and electron transfer into bulk goethite but before crystallization of the newly formed Fe(III) as goethite.The second part of the presentation will show that semiquinone radicals produced during microbial or chemical reduction of a humic substance model quinone (AQDS, 9,10-anthraquinone-2,6-disulfonic acid) can react with As and change its redox state (Jiang et al., 2009). The results of these experiments showed that these semiquinone radicals are strong oxidants and oxidize arsenite to arsenate, thus decreasing As toxicity and mobility. The oxidation of As(III) depended strongly on pH. More arsenite (up to 67.3%) was oxidized at pH 11 compared to pH 7 (12.6% oxidation) and pH 3 (0.5% oxidation). In addition to As(III) oxidation by semiquinone radicals, hydroquinones that were also produced during quinone reduction, reduced As(V) to As(III) at neutral and acidic pH values (less than 12%) but not at alkaline pH. In an attempt to understand the observed redox reactions between As and reduced/oxidized quinones present in humic substances, the radical content in reduced AQDS solutions was quantified and Eh-pH diagrams were constructed. Both the radical quantification and the Eh-pH diagram allowed explaining the observed redox reactions between the reduced AQDS solutions and the As.In summary these studies indicate that in the simultaneous presence of Fe(III) oxyhydroxides, Fe(II), and humic substances as commonly observed in environments inhabited by Fe-reducing microorganisms, As(III) oxidation can occur. This potentially explains the presence of As(V) in reduced groundwater aquifers.     

Selenite retention by nanocrystalline magnetite: Role of adsorption, reduction and dissolution/co-precipitation processes

We studied selenite () retention by magnetite () using both surface complexation modeling and X-ray absorption spectroscopy (XAS) to characterize the processes of adsorption, reduction, and dissolution/co-precipitation. The experimental sorption results for magnetite were compared to those of goethite (Fe^IIIOOH) under similar conditions. Selenite sorption was investigated under both oxic and anoxic conditions and as a function of pH, ionic strength, solid-to-liquid ratio and Se concentration. Sorption onto both oxides was independent of ionic strength and decreased as pH increased, as expected for anion sorption; however, the shape of the sorption edges was different. The goethite sorption data could be modeled assuming the formation of an inner-sphere complex with iron oxide surface sites (SOH). In contrast, the magnetite sorption data at low pH could be modeled only when the dissolution of magnetite, the formation of aqueous iron-selenite species, and the subsequent surface complexation of these species were implemented. The precipitation of ferric selenite was the predominant retention process at higher selenite concentrations (>1 × 10⁻⁴ M) and pH < 5, which was in agreement with the XAS results. Sorption behavior onto magnetite was similar under oxic and anoxic conditions. Under anoxic conditions, we did not observe the reduction of selenite. Possible reasons for the absence of reduction are discussed. In conclusion, we show that under acidic reaction conditions, selenite retention by magnetite is largely influenced by dissolution and co-precipitation processes.     

The relationship between molecular composition and fluorescence properties of humic substances

The quantity and quality of dissolved organic matters have been widely characterized by fluorescence spectroscopy, yet the relationship between the fluorescence properties of dissolved organic matters and its molecular composition remains poorly described in the literature. Here, we measured the fluorescence excitation–emission matrix of 17 well-characterized humic substance standards to determine a range of fluorescence parameters, including classical fluorescence indices (e.g., fluorescence index, biological index and humification index) and parameters derived from parallel factor analysis (e.g., component contribution). Relationships between humic substance’s fluorescence and compositional parameters were then statistically examined using canonical correspondence and simple correlation analyses. The canonical correspondence analysis generally suggested that most fluorescence parameters determined here are highly associated with the amount of aliphatic and aromatic compounds in humic substances. However, the correlation analysis between single molecular and fluorescence parameters indicated that the fluorescence properties of humic substances including the parallel factor analysis component contribution also significantly correlate well with several aspects of the molecular composition of humic substances, such as elemental composition, carbon species, acidic functional group and iron complexation. Overall, our results suggest that measurement of humic substance’s fluorescence is beneficial in understanding the molecular composition and environmental functions of dissolved organic matters in natural and engineered waters.     

Interactions of mercury with different molecular weight fractions of humic substances in aquatic systems

Interactions of mercury (Hg) with different molecular weight fractions of humic substances (HS) play an important role in controlling distribution, diffusion, speciation, and bioavailability of Hg in natural systems. This study suggests that Hg prefers to associate with higher molecular weight fractions of HS and this association particularly predominates at low pH and high ionic strengths of the medium. The concentrations of aggregated HS (with higher molecular weight) become high at lower pH (acidic condition) and high ionic strength. Molecular weight of HS gradually decreases with the increasing pH (basic condition) and low ionic strength of the medium. The disaggregation property of HS which involves the release of monomers from the surface of the aggregates produces HSs of different intermediate molecular weight with different Hg complexing capacity. Distribution of Hg in different molecular weight fractions of HS is dependent on aggregational and disaggregational properties of HS in aquatic medium. Association of Hg with high molecular weight fraction of HS may alter distribution and bioavailability of Hg in a system as the bioreactivity of organic matter decrease along a continuum of size in aquatic medium.     

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.     

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.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5 mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25-30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215-393 mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe²⁺, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.     

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.     

The lignin component of humic substances: Distribution among soil and sedimentary humic,fulvic, and base-insoluble fractions

Vanillyl, syringyl and cinnamyl phenols occur as CuO oxidation products of humic, fulvic and base-insoluble residual fractions from soils, peat and nearshore marine sediments. However, none of these lignin-derived phenols were released by CuO oxidation of deepsea sediment or its base-extractable organic fractions. Lignin analysis indicated that peat and coastal marine sediments contained significantly higher levels of recognizable vascular plant carbon (20–50%) than soils and offshore marine sediments (0–10%).Although accounting for less than 20% of the total sedimentary (bulk) lignin, lignin components of humic acid fractions compositionally and quantitatively resembled the corresponding bulk samples and baseinsoluble residues. Recognizable lignin, presumably present as intact phenylpropanoid units, accounted for up to 5% of the carbon in peat and coastal humic acids but less than 1% in soil humic acids. Fulvic acid fractions uniformly yielded less lignin-derived phenols in mixtures that were depleted in syringyl and cinnamyl phenols relative to the corresponding humic acid fractions.Within the vanillyl and syringyl families the relative distribution of acidic and aldehydic phenols is a sensitive measure of the degree of oxidative alteration of the lignin component The high acid/aldehyde ratios and the low phenol yields of soils and their humic fractions compared to peat and coastal sediments indicate extensive degradation of the lignin source material. Likewise, the progressively higher acid/aldehyde ratios and lower phenol yields along the sequence: plant tissues (plant debris)-humic acids-fulvic acids suggest that this pattern represents the diagenetic sequence for the aerobic degradation of lignin biopolymers.     

Characterization of groundwater humic substances: influence of sedimentary organic carbon

A total of 35 groundwaters from 4 different aquifer systems in Germany are investigated for their physico-chemical properties, dissolved organic C (DOC) and humic and fulvic acids. Humic substances are isolated and characterized for their elemental composition, UV/Vis and fluorescence spectroscopic properties, size distribution by gel permeation chromatography (GPC) and ¹⁴C content. For isolation of sufficient quantities of humic substances a mobile sampling system is developed based on a combination of reverse osmosis (RO) and XAD–8 adsorption chromatography. One of the aquifer systems (Gorleben) covers a wide range of hydrogeochemical conditions, whereas the other 3 aquifer systems (Munich, Franconian Albvorland and Fuhrberg) have less diverse properties. One specific feature of the Gorleben aquifer system is the presence of a very high DOC, which, in contrast to other aquifer systems, contains considerable amounts of aquatic humic acid. This is attributed to the release of aquatic humic substances originating from sedimentary organic C (SOC) that is abundant in Gorleben sediments. The results show that aquatic humic substances from different aquifer systems have dissimilar properties which differ from one another. Systematic differences are found among humic substances from different regions of the Gorleben aquifer system. Such differences are considered to be caused by the mixing of humic substances from the SOC. However, exact quantification of such mixing appears difficult because overlapping effects of different geochemical processes feigning a dissolution of SOC cannot be excluded.     

Determination of the conditional interaction constant between colloidal technetium(IV) and Gorleben humic substances

Varying pertechnetate (Tc(VII)) doses were reduced to Tc(IV) in the presence and absence of Gorleben humic substances with the aid of magnetite, a reducing Fe(II)-containing surface. In absence of humic substances dissolved Tc(IV) concentrations are over-saturated with respect to the known TcO₂ · nH₂O solubility and increase with increasing Tc(VII) dose due to the formation of a range of mononuclear to colloidal Tc(IV) species. In presence of dissolved humic substances, the Tc solubility is enhanced due to the additional interaction of dissolved Tc(IV) species with humic substances. Both in the absence and the presence of dissolved humic substances a sorption mechanism controls the distribution of the range of mononuclear to colloidal Tc(IV) species between the solid and the liquid phase. The proposed reaction mechanism between Tc(IV) and HS is represented by Σ[TcO(OH)₂]_n+HS = [TcO(OH)₂]_n − HS in which Σ[TcO(OH)₂]_n stands for the sum of monomeric and polynuclear (colloidal) Tc(IV) species present in the equilibrium solution. A log K-value of 2.9 (±0.3) was quantified from a modified Schubert approach which is based on the competition of HS and magnetite for all dissolved Tc(IV) species and was found independent of Tc–HS loading, Tc–magnetite loading and pH.     

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.     

Organic matter-metal interactions in Recent sediments: the role of humic substances

Atomic emission spectrographic analysis of the trace inorganic constituents of marine humic substances gave the following range of concentrations: Si, 200 ppm to > 2%; Al, 400 ppm to ~ 1%; Fe, 600–3000 ppm; Ca, 600 ppm to > 2%; Mg, 20–6000 ppm; Na, 600 ppm to > 2%; Ag, < 6–600 ppm; B, < 60–1000 ppm; Cu, 600–4000 ppm; Mn, 8–100 ppm; Mo, <20–3000 ppm; Ni, 100–1000 ppm; Pb, < 40–600 ppm; Sn, 40–600 ppm; Ti, < 20–2500 ppm; V, 20–200 ppm; Zn, 350–4500 ppm; Zr, < 60–500 ppm.Humic substances contain a sizeable portion of the Cu, Mo and Zn found in sediments, but are less important for Ni, Co and Pb, and are insignificant for the Mn, V and Fe content. The metals are mostly introduced into the humates during their diagenetic formation in sediment by dissolution of metals from various mineralogical phases. A precursor of the sedimentary humates, the polymeric organic material dissolved in interstitial water, contains most of the Cu and Zn, about half of the Ni, Fe and Co, and very little of the Mn found in interstitial water. Comparison of the data on humates with that obtained by H₂O₂ treatment of sediments indicates that Cu, Zn and possibly most of the Mo are associated with organic matter, but that Ni and Co are associated with sulfides.     

Redox and nonredox reactions of magnetite and hematite in rocks

Redox and nonredox reactions causing pseudomorphic replacement of hematite by magnetite and magnetite by hematite are compared.Pseudomorphic replacements resulting from redox reactions are known as martitization [replacement of magnetite by hematite due to oxidation; reaction (1)] and mushketovitization [replacement of hematite by magnetite due to reduction; reaction (2)]. These two replacements cause characteristic ore textures and volume changes (reaction (1): increase of 1.66%; reaction (2): decrease of 1.64%). These small volume changes are the reason that martitization and mushketovitization are widespread in many rocks under condition, however, that oxidizing or reducing fluids (solutions) are present.The same initial and end products may also be involved in nonredox reactions. Reaction (3) is the replacement of hematite by magnetite due to simple addition of Fe²⁺ atoms under basic conditions. This reaction causes an increase of the volume of 47.6%. Reaction (4), causing a volume decrease of 32.2%, is the replacement of magnetite by hematite due to leaching of Fe²⁺ atoms under acidic conditions. From these volume changes it is concluded that reaction (4) may occur in many rock types, whereas reaction (3) is restricted to unlithified sediments only. However, ore textures caused by nonredox reactions are unknown and therefore their occurrence in rocks is hypothetical.     

Groundwater chemistry and redox processes: Depth dependent arsenic release mechanism

Patchy occurrences of elevated As are often encountered in groundwater from the shallow aquifers (<50 m) of the Bengal Delta Plain (BDP). A clear understanding of various biogeochemical processes, responsible for As mobilization, is very important to explain this patchy occurrence and thus to mitigate the problem. The present study deals with the periodical monitoring of groundwater quality of five nested piezometeric wells between December 2008 and July 2009 to investigate the temporal changes in groundwater chemistry vis-a-vis the prevalent redox processes in the aquifer. Geochemical modeling has been carried out to identify key phases present in groundwater. A correlation study among different aqueous redox parameters has also been performed to evaluate prevailing redox processes in the aquifer. The long term monitoring of hydrochemical parameters in the multilevel wells together with hydrogeochemical equilibrium modeling has shown more subtle differences in the geochemical environment of the aquifer, which control the occurrence of high dissolved As in BDP groundwater. The groundwater is generally of Ca-HCO₃ type. The dissolved As concentration in groundwater exceeded both WHO and National drinking water standard (Bureau of Indian Standards; BIS, 10 μg L⁻¹) throughout the sampling period. The speciation of As and Fe indicate persistent reducing conditions within the aquifer [As(III): 87-97% of As_T and Fe(II): 76-96% of Fe_T]. The concentration of major aqueous solutes is relatively high in the shallow aquifer (wells A and B) and gradually decreases with increasing depth in most cases. The calculation of SI indicates that groundwater in the shallow aquifer is also relatively more saturated with carbonate minerals. This suggests that carbonate mineral dissolution is possibly influencing the groundwater chemistry and thereby controlling the mobilization of As in the monitored shallow aquifer. Hydrogeochemical investigation further suggests that Fe and/or Mn oxyhydroxide reduction is the principal process of As release in groundwater from deeper screened piezometric wells. The positive correlations of U and V with As, Fe and Mn indicate redox processes responsible for mobilization of As in the deeper screened piezometric wells are possibly microbially mediated. Thus, the study advocates that mobilization of As is depth dependent and concentrations of As in groundwater depends on single/combined release mechanisms.     

Long-living free radicals study of stepwise pyrolyzed melanoidins and humic substances

ESR measurements of stepwise-pyrolyzed melanoidins and humic substances (at various temperatures, mesh size, and pH values) furnished the following information: the melanoidin structure stabilizes the long-living free radicals in a manner similar to humic substances; the g and Ng values of melanoidins are similar to those of the humic substances, the cleavage of CC and CX (X = heteroatom) bonds increases the Ng value. Thermogravimetric curves, weight loss by stepwise pyrolysis, and ¹³C-CP/MAS NMR were found to be in good correlation with ESR data regarding the structural features of melanoidins and humic substances.