Influence of natural organic acids on the Mg/Ca ratio in the bottom sediments of highly mineralized lakes

Thermodynamic modeling of equilibria in the system water–rock–organic acids was used to study the influence of organic acids on Ca and Mg redistribution between a solution and a solid phase in connection with the use of calcites of variable composition Ca_xMg_1–xCO₃ as indicators of paleoclimatic environments. In the thermodynamic model, high-molecular humic substances (fulvic + humic acids) were represented by a set of independent metal-binding centers. Therefore, their number was preset based on the given density of proton- or metal-binding sites. The numerical implementation of several geochemical situations involving the dissolution/deposition of calcites with different Mg contents showed that the main effect of fulvic and humic acids is the acidification of solutions and the reduction of carbonate stability. Although humic substances can play an important role in fixing Ca and Mg and removing them from solution, their actual concentrations in natural media (<<1 g/L) do not cause significant changes in the composition of Ca_xMg_1–xCO₃ phases. On the other hand, there is quantitative evidence that variations in the Mg/Ca ratio in a solution and a solid phase are significantly influenced by the evaporative concentration of Mg-oversaturated solutions, alkalization/acidification during their evolution, or CO₂ content variations owing to changes in climate and lake activity.     

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.     

1H NMR spectra of humic and fulvic acids and their peracetic oxidation products

¹H NMR spectra of humic (HA) and fulvic (FA) acids and their oxidative degradation products are reported. The HA shows the presence of -(CH₂)_n - CH₃ (n > 6) chemical fragments belonging to n-alkanes and/or n-fatty acids physically adsorbed onto the macromolecule structure. These fragments are absent in the FA fraction. Both humic fractions reveal the presence of similar amounts of aromatic protons which partly undergo exchange phenomena. The importance of this experimental observation is discussed. Oxidative degradation seems to cause partial cleavage of aromatic rings, more pronounced in the FA than in the HA. The degraded FA shows a higher total acidity and a higher phenolic OH content than the degraded HA. Both degraded fractions display some sharp singlet signals at 1.9 and 3.9 ppm arising from protons belonging to repetitive chemical fragments probably formed during the oxidation reaction. Tentative assignments of these signals are given. A general analysis of the HA and FA degraded spectra seems to indicate that the chemical fragments which undergo peracetic oxidation are substantially similar. The extent of oxidation of the two humic fractions is different. The HA degradation products reveal the presence of oligomeric structures, whereas the degraded FA appears less resistant to the oxidizing agent.     

Organic chemical characterization of sediment-trap particulates from San Nicolas,Santa Barbara,Santa Monica and San Pedro Basins,California

Particulate matter was collected during September–October, 1977, in particle traps suspended 30–60 m above the floor of San Nicolas, Santa Barbara, Santa Monica and San Pedro Basins, off the coast of southern California. The trap particulates were analyzed for C₁₅–C₃₅ hydrocarbons using gas chromatography (GC) and GC-mass spectrometry. Kerogens and humic acids were characterized by H/C, N/C, δ¹³C, δ¹⁵N and δ³⁴S ratios, and by electron-spin resonance. Hydrocarbons arising from fresh and weathered petroleum, marine autochthonous and terrestrial sources were identified. The rates of petroleum deposition during the collection period followed the order: San Nicolas Basin < Santa Barbara Basin ~ Santa Monica Basin < San Pedro Basin, with the largest amount of weathered petroleum being deposited in San Pedro Basin. The rates of petroleum deposition are correlated more strongly with human activities such as shipping, and the discharge of municipal and industrial wastes, than with natural submarine oil seepage. Analyses of kerogens and humic acids indicate that the majority of the organic matter in the trap particulates is of marine origin. The water column overlying Santa Barbara Basin appears to have the highest marine productivity of the four basins studied.     

Composition,habitats, seasonal changes and productivity of macroalgae in Grays Harbor Estuary,Washington

Twenty-nine taxa of macroalgae were collected from the Grays Harbor Estuary, Washington, from 17 April 1980 to 4 June 1981. Outer (oceanic) sites contained higher numbers of species than sites located in the inner portion of the estuary. Macroalgae were found in several habitats including attached to boulders, logs, tree roots, other algae, and angiosperms, as mats in sand, and drift.Fucus distichus ssp.edentatus andEnteromorpha intestinalis occurred at the greatest number of sites and were found throughout the year. The standing stock of the perennialFucus remained relatively consant, while that of anE. intestinalis andBlidingia minima var.subsalsa complex showed a significant peak between late spring and early summer as well as a winter minimum. The occurrence of most other taxa was highly seasonal. Net productivity rates for the most abundant macroalgal taxa were moderate to high relative to rates published for algae in other North American estuaries. It is concluded that, although inconspicuous, macroalgae may represent an important contributor of organic carbon to the Grays Harbor estuarine system.     

Tightly bound aliphatic acids in Lake Biwa sediments: Their origin and stability

Tightly bound geolipids were separated from a 200 m sediment core of Lake Biwa by a second saponification of sediments from which unbound and bound lipids had been extracted. Tightly bound fatty acids, β-hydroxy acids, ω-hydroxy acids and α,ω-dicarboxylic acids were released; their concentrations ranged from 4.7–31.5, 5.6–60.5, 3.4–9.5 and 0.2–5.4 μg/g dry sediment, respectively. These geolipids were considered to be incorporated in humic substances.Tightly bound fatty acids showed a unimodal distriution with a peak at C₁₆, suggesting that they originate from algae and bacteria and that they are more stable than unbound and bound fatty acids in the sediments. Most of the total β-hydroxy acids, which probably originate from bacteria, were found in the tightly bound fraction. This suggests that a large portion of tightly bound geolipids are formed in the water column and in surface sediments as a result of microbial alteration dead algae. Each class of tightly bound lipids showed higher concentrations between depths of 3 and 15 m in the sediments, where primary production was thought to be enhanced in the past.     

Effects of age on the chemical structure of paleosol humic acids and fulvic acids

Humic acids and fulvic acids were extracted from six paleosols in Southern Italy. Humic acids (HAs) constituted between 96.5 and 99.2% of the total extracts; the remaining materials consisted of fulvic acids (FAs). Radiocarbon ages of the HAs ranged from about 6,000 to close to 29,000 years B.P., δ¹³C values averaged $\text{?25.6 ± 0.3‰}$ The HAs were characterized by chemical (elemental and functional group analyses) and spectroscopic (IR, ESR, ¹³CNMR, E₄/E₆ ratios) methods. FAs were characterized by chemical methods, E₄/E₆ ratios and IR spectra.The chemical and spectroscopic analyses showed practically no differences in the chemical structure and composition of the six HAs and FAs, so that age appeared to have little effect on these parameters. The paleosols were found to be closed systems with low polysaccharide and protein contents, thus providing unfavorable substrates for microbial activity. The preservation of the humic materials in the paleosols may have been due to low biological activity and/or to retention by amorphous minerals. The HAs did not appear to be affected by temperatures higher than 170–200°C over the 23,000 year period which we observed.     

Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 2: 2D NMR spectroscopy

Carboxyl groups are abundant in natural organic molecules (NOM) and play a major role in their reactivity. The structural environments of carboxyl groups in IHSS soil and river humic samples were investigated using 2D NMR (heteronuclear and homonuclear correlation) spectroscopy. Based on the ¹H-¹³C heteronuclear multiple-bond correlation (HMBC) spectroscopy results, the carboxyl environments in NOM were categorized as Type I (unsubstituted and alkyl-substituted aliphatic/alicyclic), Type II (functionalized carbon substituted), Type IIIa, b (heteroatom and olefin substituted), and Type IVa, b (5-membered heterocyclic aromatic and 6-membered aromatic). The most intense signal in the HMBC spectra comes from the Type I carboxyl groups, including the ²J_CH and ³J_CH couplings of unsubstituted aliphatic and alicyclic acids, though this spectral region also includes the ³J_CH couplings of Type II and III structures. Type II and III carboxyls have small but detectable ²J_CH correlations in all NOM samples except for the Suwannee River humic acid. Signals from carboxyls bonded to 5-membered aromatic heterocyclic fragments (Type IVa) are observed in the soil HA and Suwannee River FA, while correlations to 6-membered aromatics (Type IVb) are only observed in Suwannee River HA. In general, aromatic carboxylic acids may be present at concentrations lower than previously imagined in these samples. Vibrational spectroscopy results for these NOM samples, described in an accompanying paper [Hay M. B. and Myneni S. C. B. (2007) Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 1: Infrared spectroscopy. Geochim. Cosmochim. Acta (in press)], suggest that Type II and Type III carboxylic acids with α substituents (e.g., -OH, -OR, or -CO₂H) constitute the majority of carboxyl structures in all humic substances examined. Furoic and salicylic acid structures (Type IV) are also feasible fragments, albeit as minor constituents. The vibrational spectroscopy results also suggest that much of the “Type I” signal observed in the HMBC spectrum is due to carboxylic acid esters and possibly α-substituted alicyclic acids.     

Effect of fulvic acids on sorption of U(VI), Zn,Yb, I and Se(IV) onto oxides of aluminum,iron and silicon

The sorption of Yb³⁺, UO²⁺₂, Zn²⁺, I⁻ and SeO²⁻₃ onto Al₂O₃, Fe₂O₃ and SiO₂ were determined by a batch technique in the presence and absence of fulvic acids. The effects of fulvic acid on sorption were compared. The existing general consensus, that humic substances tend to enhance metal cation sorption at low pH, reduce metal cation sorption at high pH and reduce inorganic anion sorption between pH values 3 to 10, was generally shown to be true. However, in this work many exceptions to the general consensus were found. The study indicated that the effect of humic substances on sorption of inorganic cations or anions depends not only on pH, but also on the nature of the oxide, the nature of humic substance, fractionation of the humic substance by sorption, the relative strength of complexes of both soluble and sorbed humic substances, the extent of surface coverage by humic substance, the initial concentration of humic substance and the inorganic electrolyte composition.     

Molecular weight distribution,analytical and spectroscopic characterization of humic fractions sequentially isolated by organic solvents from a brown coal humic acid

A sequential fractionation procedure employing a series of selected mild organic solvents of different polarity has been applied for the isolation of chemically different organic fractions from a brown coal humic acid. Elemental composition, molecular weight distribution, i.r. and electron spin resonance analysis were carried out on the isolated humic fractions. They were characterized by: (a) a low polydispersity, (b) a decreasing aliphatic and increasing aromatic character along the series, (c) very different molecular weight which significantly correlated with E₄/E₆ ratios (particle aggregation and molecular association) and free radical concentrations (chemical and biochemical activity). Significant correlations were found between physico-chemical parameters of the isolated humic fractions, i.e. M_n, M_w, E₄/E₆ ratios, spins/g contents and the dielectric constants of the solvents used. This suggested the efficiency of the applied procedure in isolating chemically different organic fractions from the bulk, original humic acid.     

Effects of heat treatment on fluorescence properties of humic substances from sandy soil in arid land and their Hg(II) binding behaviors

The topsoil temperature in arid areas of Xinjiang, China can be up to about 80°C in summer. This may significantly affect the chemical properties of soil humic substances. However, the effects of high temperature on characteristics of soil humic substances and their complexation with toxic metals are still poorly known. In the present study, binding of Hg(II) to unheated soil humic substances and heated soil humic substances from sandy soils was comparatively investigated using three-dimensional excitation?Cemission matrix (EEM) fluorescence spectroscopy. Two fluorescent peaks (peak I at Ex/Em?=?365?C370/470?C474?nm; peak II at Ex/Em?=?270?C275/468?C472?nm) identified as humic-like fluorescence were observed in the EEM spectra of humic substances. Both peaks were clearly quenched by Hg(II), indicating the strong interaction of humic-like substances with Hg(II), and showed blue shifts after heat treatment. Heat treatment caused an increase of the fraction of accessible fluorophore (f _a), binding sites number (n) and effective quenching constants (logK _a), indicating that more binding sites in humic substances could bind Hg(II) and form more stable humic substances?CHg(II) complexes after heat treatment. However, a decrease of binding constants (logK _b) suggested that heat treatment would reduce the binding capacity of each binding site of humic substances to Hg(II). This study implies the transport of Hg(II) may be affected by high temperature in the arid zone due to the modification of the physicochemical properties of humic substances in soil.     

The effect of organic matter and oxygen on the degradation of bacterial membrane lipids in marine sediments

The biodegradation of purified radiolabelled membrane lipids from a methanogenic bacterium and a pseudomonad were investigated in mangrove, beach and high marsh marine sediments under aerobic and anaerobic conditions. The effect of organic matter on the amount and rate of degradation was also examined by supplementing beach sediments with humic acids. In aerobic sediments, CO₂ was the major product of lipid degradation while under anaerobic conditions both CO₂ and CH₄ were major end products and the overall rates were reduced (up to 40%) relative to aerobic conditions. Total bacterial numbers increased during all incubations with the largest increases occurring in anaerobic sediments supplemented with humic acids. No lipid degradation occurred in aerobic or anaerobic sediments treated with formaldehyde or autoclaving. In low organic beach sediments, the ester-linked phospholipid of the pseudomonad was degraded much more rapidly than the diphytanyl glycerol diether of the methanogen with 69% of the phospholipid degraded in 96 hours versus only 4% of the methanogen lipid. Lipid degradation in both aerobic and anaerobic sediments was highly correlated to organic matter content with increasing amounts of organic matter inhibiting degradation. Long incubations (75 days) of the diphytanyl glycerol ether resulted in 51% degraded to CO₂ in low (0.5%) organic mangrove sediments while only 9% was mineralized in high (10.8%) organic marsh sediments. Physicochemical sorption of membrane lipids to the organic matrix is proposed as a mechanism which protects membrane lipids from microbial attack and degradation.     

Analysis of methylated humic acids from fossil fuels by size exclusion chromatography and NMR

Molecular Mass (MM) distributions in humic acids from peat and two lignites have been investigated by high performance size-exclusion chromatography (SEC) after methylation and found to be in the range approximately 300 to more than 5000, with number average NM(M_n) near 800. Values of M_n determined by SEC were in good agreement with those from vapour pressure osmotery. High field¹³C NMR spectra of the methylated humic acids are extremely well resolved, and indicate that the samples are significantly different, despite their similar elemental compositions and MM distributions.     

The stability constant of the UO22+ -humic acid complex

Between pH 3.5 and 7, humic acids form a water-soluble uranyl-humic acid complex with the uranyl ion, UO₂²⁺ (log β₁ = 7.8 ± 0.4 at μ = 0.1). The stability constant of the complex is practically independent of the ionic strength (μ). Experimental results can be explained by the formation of a complex in which every uranyl ion is simultaneously bonded to one phenolic oxygen and one dissociated carboxyl group. The bonding through the phenolic group is considerably stronger than that through carboxylic groups.     

Nonaqueous titration of functional groups in humic acid

Humic acid was titrated by sodium methoxide in dimethylsulfoxide using platinum-calomel electrode systems. Adding benzoic acid and phenol as internal standards to humic acid yielded two inflections. The titer at the first inflection point was equivalent to the carboxyl groups whose pK_a (H₂O) values were less than 7. The difference between the titers at the two inflection points was equivalent to the phenolic hydroxyl groups whose pK_a (H₂O) values were 7–10. Calculated results for the carboxyl and phenolic hydroxyl groups in humic acid obtained by the nonaqueous titration method agreed closely with those obtained by conventional methods.     

Pyrolysis-GC-MS and NMR studies of dissolved seawater humic substances and isolates of a marine diatom

¹³C- and ¹H-NMR spectra were obtained for humic substances isolated from a coastal marine environment and also for the intracellular and extracellular extracts of a marine diatom. Phaeodactylum tricornutum Bohlin. Highly branched alkyl chains constitute a large proportion of the structure of the marine humic material, whereas aromatic components are less important. Carbohydrate-type materials, possibly uronic acids, are also present in appreciable amounts. Furans (derived from carbohydrates). pyrroles and nitriles (derived from proteins, nucleic acids and/or tetrapyrroles) and phenols and methylphenols (non-lignin derived) are pyrolysis products derived from pyrolysis-gas chromatography-mass spectrometry of the extracts. The results indicate the similarities in chemical structure of P. tricornutum exudate and dissolved marine humic material.     

过硫酸钠氧化-流动注射化学发光法在地下水腐植酸含量测定中的应用

在碱性条件下过硫酸钠能氧化腐植酸发生化学发光反应。本研究以过硫酸钠-腐植酸化学发光体系为基础,建立了腐植酸的过硫酸钠氧化-流动注射化学发光测定方法,同时对测定方法的负高压及增益、泵速、过硫酸钠浓度、氢氧化钠浓度等影响因素进行了优化实验。方法的线性范围为0.1～500 mg/L(相关系数为0.9985),检出限为0.076 mg/L,对浓度为0.5 mg/L的腐植酸进行11次平行测定,相对标准偏差(RSD)为3.47%。利用该方法对5种不同地区地下水中的腐植酸进行测定,样品的加标回收率在98.33%～107.50%之间。该方法无需分离,简单易行,对实际样品测定结果满意。     

Proton-binding study of standard and reference fulvic acids, humic acids, and natural organic matter

The acid-base properties of 14 standard and reference materials from the International Humic Substances Society (IHSS) were investigated by potentiometric titration. Titrations were conducted in 0.1 M NaCl under a nitrogen atmosphere, averaging 30 min from start to finish. Concentrations of carboxyl groups and phenolic groups were estimated directly from titration curves. Titration data were also fit to a modified Henderson-Hasselbalch model for two classes of proton-binding sites to obtain “best fit” parameters that describe proton-binding curves for the samples. The model was chosen for its simplicity, its ease of implementation in computer spreadsheets, and its excellent ability to describe the shapes of the titration curves. The carboxyl contents of the IHSS samples are in the general order: terrestrial fulvic acids > aquatic fulvic acids > Suwannee River natural organic matter (NOM) > aquatic humic acids > terrestrial humic acids. Overall, fulvic acids and humic acids have similar phenolic contents; however, all of the aquatically derived samples have higher phenolic contents than the terrestrially derived samples. The acid-base properties of reference Suwannee River NOM are surprisingly similar to those of standard Suwannee River humic acid. Results from titrations in this study were compared with other published results from both direct and indirect titrations. Typically, carboxyl contents for the IHSS samples were in agreement with the results from both methods of titration. Phenolic contents for the IHSS samples were comparable to those determined by direct titrations, but were significantly less than estimates of phenolic content that were based on indirect titrations with Ba(OH)₂ and Ca(OAc)₂. The average phenolic-to-carboxylic ratio of the IHSS samples is approximately 1:4. Models that assume a 1:2 ratio of phenolic-to-carboxylic groups may overestimate the relative contribution of phenolic groups to the acid-base chemistry of humic substances.     

Electron paramagnetic resonance characteristics of the humic acids from a podzol

A study has been made of solid and solution electron paramagnetic resonance (EPR) spectra of humic acids from different horizons in a podzolic soil. Hyperfine splitting was observed in the solution spectra of humic acids from all horizons and depended on the strength of alkali and the period of dissolution. The upper organic horizons L, F and O₁ contained humic acids with some spectral characteristics in common with lignin. Humic acid from the lower horizons showed different spectra. At least 5 different radical signals were present.     

Solubility and migration ability of rhodium in natural conditions: model experimental data

The contents of dissolved rhodium species in the near-neutral environments have been studied for the first time and data on the interaction of Rh with organic matters of natural waters and its sorption behavior during contact with the components of geochemical barriers were obtained. The solubility method was used to analyze the behavior of rhodium hydroxide in the Rh(OH)_x–H₂O and Rh(OH)_x–H₂O–FA (fulvic acids) systems. The possible contents of inorganic species of rhodium and its compounds with humic organic ligands were determined within the pH range typical of surface waters. The solubility of rhodium shows a twoorder- of magnitude increase in the presence of humic matters (FA). The sorption interaction of the soluble rhodium species with the main components of geochemical barriers such as iron oxyhydroxides (III), (including fulvic-acid modified ones), alumosilicates, and precipitates of humic acids in contact with natural waters was studied. It was revealed that rhodium has the high affinity to all studied materials; its species are sorbed by ferrihydrite within several hours. It is suggested that rhodium is mainly transferred as colloid with suspended particulate matters of waters and then is accumulated in bottom sediments. The differences revealed in the sorption behavior of Pt(IV), Pd(II) and Rh(III) may be used to predict the distribution of the considered platinum group elements between the components of ecosystems.