N-15 NMR spectra of naturally abundant nitrogen in soil and aquatic natural organic matter samples of the International Humic Substances Society

The naturally abundant nitrogen in soil and aquatic NOM samples from the International Humic Substances Society has been characterized by solid state CP/MAS ¹⁵N NMR. Soil samples include humic and fulvic acids from the Elliot soil, Minnesota Waskish peat and Florida Pahokee peat, as well as the Summit Hill soil humic acid and the Leonardite humic acid. Aquatic samples include Suwannee River humic, fulvic and reverse osmosis isolates, Nordic humic and fulvic acids and Pony Lake fulvic acid. Additionally, Nordic and Suwannee River XAD-4 acids and Suwannee River hydrophobic neutral fractions were analyzed. Similar to literature reports, amide/aminoquinone nitrogens comprised the major peaks in the solid state spectra of the soil humic and fulvic acids, along with heterocyclic and amino sugar/terminal amino acid nitrogens. Spectra of aquatic samples, including the XAD-4 acids, contain resolved heterocyclic nitrogen peaks in addition to the amide nitrogens. The spectrum of the nitrogen enriched, microbially derived Pony Lake, Antarctica fulvic acid, appeared to contain resonances in the region of pyrazine, imine and/or pyridine nitrogens, which have not been observed previously in soil or aquatic humic substances by ¹⁵N NMR. Liquid state ¹⁵N NMR experiments were also recorded on the Elliot soil humic acid and Pony Lake fulvic acid, both to examine the feasibility of the techniques, and to determine whether improvements in resolution over the solid state could be realized. For both samples, polarization transfer (DEPT) and indirect detection (¹H–¹⁵N gHSQC) spectra revealed greater resolution among nitrogens directly bonded to protons. The amide/aminoquinone nitrogens could also be observed by direct detection experiments.     

Interaction of Cr(III) with the humus acids of soil,water, and bottom sediments

The interaction of Cr(III) with humus acids [fulvic (FA) and humic (HA) acids] was studied in the systems Cr(III)-FA, Cr(III)-HA, and Cr(III)-FA-HA. Chromium(III) reacts with FA extracted from the headwaters of the Moscow River and from the Krapivenka River (a tributary of Lake Seliger) to form a highly soluble high-molecular-weight Cr(III)-hydroxofulvate complexes with \(\bar \beta _{11} \) = 1.93 × 10⁶ and 5.70 × 10⁶, respectively. Humic acids extracted from peat in the Tver area and the sapropel of Lake Seliger behave as complexing sorbents with conditional affinity constants logβ = 3.78 and 3.23 for Cr(III) sorption at HA sites in the peat and sapropel, respectively. In the three-component system Cr(III)-FA-HA, the Cr(III) distribution coefficient between solution and precipitate is controlled by the pH value and FA content in the solution and decreases by 1–1.5 orders of magnitude with increasing fulvic acid content.     

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.     

Impact of natural organic matter on H2O2-mediated oxidation of Fe(II) in a simulated freshwater system

The oxidation of Fe(II) by H₂O₂ has been studied in the presence of Suwannee River fulvic acid, a standard form of natural organic matter, by adding inorganic Fe(II) to solutions containing both H₂O₂ and fulvic acid and monitoring the total Fe(II) concentration using a luminol chemiluminescence method. At pH 8.4 and in the absence of competing metals, Suwannee River fulvic acid significantly retards the rate of Fe(II) oxidation due to gradual formation of a species that is oxidized more slowly than inorganic Fe(II) by both O₂ and H₂O₂. It is suggested that rapid formation of a weak Fe(II)-fulvic acid complex that is not readily oxidized by H₂O₂ is the cause of the reduction in the initial oxidation rate, and that the subsequent further reduction in oxidation rate is a result of the formation of a second type of Fe(II)-fulvic acid complex that is resistant to both O₂ and H₂O₂ oxidation. A kinetic model has been developed that supports this conceptual model. The results demonstrate that, under certain conditions, natural organic matter may stabilize Fe(II) in the presence of elevated H₂O₂ concentrations, significantly increasing the lifetime of ferrous iron and reducing the flux of hydroxyl radicals produced through this oxidation pathway.     

The reaction of ozone with isolated aquatic fulvic acid

Aquatic fulvic acid, isolated from a North Carolina bay lake, was reacted with ozone, an alternative oxidant in drinking water treatment. Ozonated samples were acidified, extracted with ether, dried, concentrated, and methylated prior to GC-MS analysis. Identified reaation products include mono-, di- and tribasic aliphatic acids and benzene tricarboxylic acids. The products with the highest relative concentrations were succinic and malonic acid, although all products were at relatively low concentration levels. Many of the products identified in this study have also been seen among the reaction products of fulvic acid and other oxidants, such as Cl₂, CIO₂ and KMnO₄.     

Permanganate oxidation products of algal humic-like substances

Algal humic-like substances were oxidized with KMnO₄. The oxidation products were analyzed by GC-MS. Normal and branched fatty acids, dicarboxylic and benzene carboxylic acids were identified. Aliphatic acids were by far the most prominent compounds. Similarities between humic and fulvic acids of both Fucus and Laminaria algae were apparent. Fucus humic acids, however, showed a more aromatic structure than their Laminaria counterpart. The contribution of algae to marine sedimentary humus is discussed.     

Lanthanide-humic substances complexation. I. Experimental evidence for a lanthanide contraction effect

The interaction of the lanthanides (Ln) with humic substances (HS) was investigated with a novel chemical speciation tool, Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry (CE-ICP-MS). By using an EDTA-ligand competition method, a bi-modal species distribution of LnEDTA and LnHS is attained, separated by CE, and detected online by sector field ICP-MS. We quantified the binding of all 14 rare earth elements (REEs), Sc and Y with Suwannee river fulvic acid, Leonardite coal humic acid, and Elliot soil humic acid under environmental conditions (pH 6-9, 0.001-0.1 mol L⁻¹ NaNO₃, 1-1000 nmol L⁻¹ Ln, 10-20 mg L⁻¹ HS). Conditional binding constants for REE-HS interaction (K_c) ranged from 8.9 < log K_c < 16.5 under all experimental conditions, and display a lanthanide contraction effect, Δ_LK_c: a gradual increase in K_c from La to Lu by 2-3 orders of magnitude as a function of decreasing ionic radius. HS polyelectrolyte effects cause K_c to increase with increasing pH and decreasing ionic strength. Δ_LK_c increases significantly with increasing pH, and likely with decreasing ionic strength. Based on a strong correlation between Δ_LK_c values and denticity for organic acids, we suggest that HS form a range of tri- to tetra-dentate complexes under environmental conditions. These results confirm HS to be a strong complexing agent for Ln, and show rigorous experimental evidence for potential REE fractionation by HS complexation.     

Molecular modeling of Al and benzene interactions with Suwannee fulvic acid

The effects that Al³⁺ and benzene interactions exhibit on a model fulvic acid were investigated. Energy minimizations of the structures mimicking the interactions of Al³⁺-Suwannee fulvic acid (SFA), benzene-SFA and Al³⁺-benzene-SFA were run with a solvation sphere of 60 H₂O molecules with the semi-empirical methods PM3 and PM5. The semi-empirical method PM3 was run with Gaussian 98 and CAChe Workstation Pro 6.1.1 to compare the results of the energy minimization algorithms in the two programs. PM5 calculations were run with CAChe Workstation Pro 6.1.1. Molecular dynamics (MD) simulations were run in Cerius² (Accelrys Inc., San Diego, CA) using the Universal Force Field (UFF) 1.02 [Rappé A. K., Casewit C., Colwell K., Goddard W., and Skiff W. (1992) UFF, a full periodic-table force-field for molecular mechanics and molecular dynamics simulations. J. Am. Chem. Soc.114(25), 10024-10035] and COMPASS force field [Sun H. (1998) COMPASS: an ab initio force-field optimized for condensed-phase applications—overview with details on alkane and benzene compounds. J. Phys. Chem. B102, 7338-7364]. Single point calculations were run on the minimized structures at the B3LYP/6-31G(d) level to obtain more accurate estimates of the energy on the minimized structures derived from the PM3, PM5, and UFF methods and to normalize energies to the same reference state. This methodology was used as the standard of comparison for all the models to assess whether or not a given configuration was reasonably stable.The PM3/G03 energy minimizations predicted the lowest B3LYP/6-31G(d) potential energies of the methodologies examined in this study. Thus, this method is considered the most reliable of those tested. The PM3/G03 method predicted that there would be aromatic-aromatic interactions between benzene and SFA. The presence of Al³⁺ was predicted not to interfere with aromatic-aromatic interactions between benzene and SFA, but benzene may influence the location of metal complexation to SFA.     

Sorption of Cu and Pb to kaolinite-fulvic acid colloids: Assessment of sorbent interactions

The sorption of Cu(II) and Pb(II) to kaolinite-fulvic acid colloids was investigated by potentiometric titrations. To assess the possible interactions between kaolinite and fulvic acid during metal sorption, experimental sorption isotherms were compared with predictions based on a linear additivity model (LAM). Suspensions of 5 g L⁻¹ kaolinite and 0.03 g L⁻¹ fulvic acid in 0.01 M NaNO₃ were titrated with Cu and Pb solutions, respectively. The suspension pH was kept constant at pH 4, 6, or 8. The free ion activities of Cu²⁺ and Pb²⁺ were monitored in the titration vessel using ion selective electrodes. Total dissolved concentrations of metals (by ICP-MS) and fulvic acid (by UV-absorption) were determined in samples taken after each titration step. The amounts of metals sorbed to the solid phase, comprised of kaolinite plus surface-bound fulvic acid, were calculated by difference. Compared to pure kaolinite, addition of fulvic acid to the clay strongly increased metal sorption to the solid phase. This effect was more pronounced at pH 4 and 6 than at pH 8, because more fulvic acid was sorbed to the kaolinite surface under acidic conditions. Addition of Pb enhanced the sorption of fulvic acid onto kaolinite at pH 6 and 8, but not at pH 4. Addition of Cu had no effect on the sorption of fulvic acid onto kaolinite. In the LAM, metal sorption to the kaolinite surface was predicted by a two-site, 1-pK basic Stern model and metal sorption to the fulvic acid was calculated with the NICA-Donnan model, respectively. The LAM provided good predictions of Cu sorption to the kaolinite-fulvic acid colloids over the entire range in pH and free Cu²⁺ ion activity (10⁻¹² to 10⁻⁵). The sorption of Pb was slightly underestimated by the LAM under most conditions. A fractionation of the fulvic acid during sorption to kaolinite was observed, but this could not explain the observed deviations of the LAM predictions from the experimental Pb sorption isotherms.     

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.     

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.     

Solubilization of hydrocarbons by the dissolved organic matter in sea water

Water samples from Narragansett Bay and the Providence River, and fulvic acid/ saline water solutions were examined for their ability to solubilize n-alkane (n-C₁₆ and n-C₂₀), isoprenoid (pristane) and aromatic (phenanthrene and anthracene) hydrocarbons and dibutyl phthalate. Removal of the dissolved organic matter (D.O.M.) from the natural samples by activated charcoal and by ultra-violet oxidation resulted in a 50–99 per cent decrease in the amounts of n-alkanes and isoprenoid hydrocarbons solubilized. This decrease was directly related to the amount of D.O.M. removed. The solubilities of the aromatic hydrocarbons were unaffected by the D.O.M. Fulvic acid from a marine sediment, surface active organic material isolated at a chloroform/sea water interface, organic material extracted from a marine sediment by sea water, and organic matter contributed by a municipal sewage effluent, promote n-alkane solubility when added to NaCl solutions and re-enhance solubility when added to organic depleted sea water. The solubility of No. 2 fuel oil increased 2.5 times in the presence of fulvic acid (3.7 mg C/l.) with most of the increase seen in the alkane and isoprenoid components.N-Alkane solubility increases in fulvic acid/saline water solutions with increasing pH and reaches a maximum with respect to ionic strength at I = 0.3. There is evidence to suggest that the mode of solubilization of the hydrocarbons is by incorporation into micelles formed by intermolecular association of the surface active humic-type monomers. The presence of ionic species is a prerequisite for micelle formation.     

Effects of pH and Ca competition on complexation of cadmium by fulvic acids and by natural organic ligands from a river and a lake

The technique of competitive ligand-exchange/anodic stripping voltammetry (CLE-SV) was used to investigate effects of pH and Ca concentration on cadmium complexation by fulvic acid (FA), as well as Cd speciation in two different freshwaters, a hardwater Lake Greifen and a softwater River Wyre. Binding of Cd to Suwannee river FA (10 mg/l) was determined at different pH (7–8.5) and in the presence of various concentration of Ca²⁺ (0–2 mmol/l). The results from one-ligand discrete models were compared to simulations by the WHAM VI model. In Lake Greifen, the determined dissolved [Cd²⁺] ranged from 10⁻¹³ to 10⁻¹² mol/l, and the conditional stability constant with natural ligands was log K _CdL about 9.5–10.5 (pH 8.6–8.8) with ligand concentrations of 1.2–7.8 × 10⁻⁶ mol/g C. In the softwater River Wyre, dissolved [Cd²⁺] ranged from 4 × 10⁻¹² to 1 × 10⁻¹¹ mol/l, and the ligands were weaker (log K _CdL 8.9–9.8, pH 8.0) with lower ligand concentrations (0.9–2.3 × 10⁻⁶ mol/g C). The titration curves of FA samples were close to the simulated curves by the WHAM VI model at pH 8.0–8.5, but deviated more from the model at lower pH, indicating that the results determined with CLE-SV for Cd-FA complexation are relevant to the data base in the model. Calculation of the Ca competition for Cd binding by FA showed a competition effect of similar strength as the measured one, but indicated a systematic difference between measured and modeled data at pH 7.5. Using the WHAM model for comparison with FA, the complexation of Cd by the River Wyre ligands was close to that of FA, whereas stronger complexation was observed in the Lake Greifen water. These differences may originate from different ligand composition in the lake and the river.     

Sorption of cesium and iodide ions onto KENTEX-bentonite

The sorption of cesium and iodide ions onto KENTEX-bentonite was investigated using batch test and in-diffusion test methods. The cesium ions were highly sorbed on the bentonite, and the experimental data fit the Freundlich isotherm well. The distribution coefficient, K _d, of the cesium ions was variably affected by the chemical conditions of the solution (initial ion concentration, pH, salinity) and temperature. An increasing pH of solution increased the K _d. However, there were different K _d values that decrease with an increase in the initial ion concentration, salinity, and temperature. The iodide ions, on the contrary, were negligibly sorptive. The K _d values obtained from the in-diffusion tests were quite lower than those from the batch tests, which could be explained by changes in the pore water chemistry and surface area available for sorption.     

Ligand extraction of rare earth elements from aquifer sediments: Implications for rare earth element complexation with organic matter in natural waters

The ability of organic matter as well as carbonate ions to extract rare earth elements (REEs) from sandy sediments of a Coastal Plain aquifer was investigated for unpurified organic matter from different sources (i.e., Mississippi River natural organic matter, Aldrich humic acid, Nordic aquatic fulvic acid, Suwannee River fulvic acid, and Suwannee River natural organic matter) and for extraction solutions containing weak (i.e., CH₃COO⁻) or strong (i.e., ) ligands. The experimental results indicate that, in the absence of strong REE complexing ligands in solution, the amount of REEs released from the sand is small and the fractionation pattern of the released REEs appears to be controlled by the surface stability constants for REE sorption with Fe(III) oxides/oxyhydroxides. In the presence of strong solution complexing ligands, however, the amount and the fractionation pattern of the released REEs reflect the strength and variation of the stability constants of the dominant aqueous REE species across the REE series. The varying amount of REEs extracted by the different organic matter employed in the experiments indicates that organic matter from different sources has different complexing capacity for REEs. However, the fractionation pattern of REEs extracted by the various organic matter used in our experiments is remarkable consistent, being independent of the source and the concentration of organic matter used, as well as solution pH. Because natural aquifer sand and unpurified organic matter were used in our experiments, our experimental conditions are more broadly similar to natural systems than many previous laboratory experiments of REE-humic complexation that employed purified humic substances. Our results suggest that the REE loading effect on REE-humic complexation is negligible in natural waters as more abundant metal cations (e.g., Fe, Al) out-compete REEs for strong binding sites on organic matter. More specifically, our results indicate that REE complexation with organic matter in natural waters is dominated by REE binding to weak sites on dissolved organic matter, which subsequently leads to a middle REE (MREE: Sm-Ho)-enriched fractionation pattern. The experiments also indicate that carbonate ions may effectively compete with fulvic acid in binding with dissolved REEs, but cannot out compete humic acids for REEs. Therefore, in natural waters where low molecular weight (LMW) dissolved organic carbon (DOC) is the predominant form of DOC (e.g., lower Mississippi River water), REEs occur as “truly” dissolved species by complexing with carbonate ions as well as FA, resulting in heavy REE (HREE: Er-Lu)-enriched shale-normalized fractionation patterns. Whereas, in natural terrestrial waters where REE speciation is dominated by organic complexes with high molecular weight DOC (e.g., “colloidal” HA), only MREE-enriched fractionation patterns will be observed because the more abundant, weak sites preferentially complex MREEs relative to HREEs and light REEs (LREEs: La-Nd).     

Alkaline permanganate oxidation of kerogens from Cretaceous black shales thermally altered by diabase intrusions and laboratory simulations

Potassium permanganate oxidative degradations were conducted for kerogens isolated from Cretaceous black shales (DSDP Leg 41, Site 368), thermally altered during the Miocene by diabase intrusions and from unaltered samples heated under laboratory conditions (250–500°C).Degradation products of less altered kerogens are dominated by normal C₄–C₁₅ α,ω-dicarboxylic acids, with lesser amounts of n-C₁₆ and n-C₁₈ monocarboxylic acids, and benzene mono-to-tetracarboxylic acids. On the other hand, thermally altered kerogens show benzene di-to-tetracarboxylic acids as dominant degradation products, with lesser or no amounts (variable depending on the degree of thermal alteration) of α,ω-dicarboxylic acids. Essentially no differences between the oxidative degradation products of naturally- and artificially-altered kerogens are observed.As a result of this study, five indices of aromatization (total aromatic acids/kerogen; apparent aromaticity; benzenetetracarboxylic acids/total aromatic acids; benzene-1,2-dicarboxylic acid/benzenedicarboxylic acids; benzene-1,2,3-tricarboxylic acid/benzenetricarboxylic acids) and two indices of aliphatic character (Total aliphatic acids/kerogen; Aliphaticity) are proposed to characterize the degree of thermal alteration of kerogens.Furthermore, a good correlation is observed between apparent aromaticity estimated by the present KMnO₄ oxidation method and that from the ¹³C NMR method (DENNIS et al., 1982).     

Deprotonation energies of a model fulvic acid. I. Carboxylic acid groups

A model Suwannee fulvic acid (SFA [Leenheer, J.A., 1994. In: Baker, L.A. (Ed.), Chemistry of Dissolved Organic Matter in Rivers, Lakes and Reservoirs. Advances in Chemistry Series, vol. 237. American Chemical Society]) was energy minimized in various deprotonation states using semi-empirical methods. The structures were minimized in the isolated SFA phase and SFA with 60 water molecules to mimic the first solvation sphere. The relative energies of deprotonation were calculated at four carboxylic acid sites with Hartree-Fock (HF/6-31G(d)) and density functional theory (B3LYP/6-31G(d)) methods. Comparisons were made between the theoretical methods and states of solvation. Isolated and solvated models resulted in different relative deprotonation orders. The energy changes calculated for removing a H⁺ from a given carboxylic acid group as a function of overall model molecule charge are large enough to explain the large variations of carboxyl group pK_as in dissolved natural organic matter. Analysis of the SFA structure as a function of molecular charge is also discussed.     

Colloid formation in groundwater: effect of phosphate,manganese, silicate and dissolved organic matter on the dynamic heterogeneous oxidation of ferrous iron

Subsurface aeration is the in situ oxidation of Fe from groundwater that is used to make drinking water potable. When subsurface aeration is applied to an anaerobic groundwater system with pH>7, Fe(II) is oxidised heterogeneously. The heterogeneous oxidation of Fe(II) can result in the in situ formation of Fe colloids. To study this, the effect of substances commonly found in groundwater (e.g. PO₄, Mn, silicate and fulvic acid) on the heterogeneous oxidation process was measured. The heterogeneous oxidation of Fe(II) becomes retarded when PO₄, Mn, silicate or fulvic acid is present in the groundwater in addition to Fe(II). Phosphate and fulvic acid retarded the oxidation process most. The heterogeneous oxidation was described using a model with a homogeneous (k₁) and an autocatalytic oxidation rate constant (k′₂). From the modelling it followed that the homogeneous oxidation rate constant was not affected or even slightly elevated whereas the autocatalytic oxidation rate constant decreased remarkably by the addition of PO₄, Mn, silicate or fulvic acid. From speciation calculations it followed that the decreased availability of the Fe(II) species can only explain a small part of the retarded autocatalytic oxidation process. Therefore exploratory calculations were performed to gain insight into whether the adsorption of PO₄ or fulvic acid could explain the retarded autocatalytic oxidation. These calculations showed that the adsorption of fulvic acid could explain the retarded autocatalytic oxidation process. In contrast the adsorption of PO₄ only partly explained the retarded autocatalytic oxidation process. In terms of colloid formation this study shows that the heterogeneous oxidation of Fe(II) in presence of PO₄, Mn, silicate or fulvic acid leads to the formation of Fe colloids.     

Branched-chain and cyclopropanoid acids in a recent sediment

From the recent sediments of a small productive lake in the English Lake District 10-methyl and cis-9, 10-methylene C₁₆ acids and also 10-methyl, cis-9, 10-, and cis-11, 12-methylene C₁₈ acids have been identified as constituents of the branched/cyclic acid fraction.