Coordination nature of aluminum (oxy)hydroxides formed under the influence of tannic acid studied by X-ray absorption spectroscopy

The effect of biomolecules on the mechanisms of the formation and nature of the transformation products of Al (oxy)hydroxides at the atomic and molecular levels and the impact on their nano-scale surface chemistry remain to be uncovered. In this article, the coordination structure of Al in Al (oxy)hydroxides formed under the influence of tannic acid was studied with X-ray absorption near edge structure (XANES) spectroscopy. Al K-edge and L-edge spectra show that as the tannate/Al molar ratio (MR) was increased from 0, 0.001, 0.01 to 0.1, the coordination number of Al changed from the sixfold coordination to mixed six-, five-, and/or fourfold coordination in the structural network of the Al (oxy)hydroxides formed under the increasing perturbation of tannic acid. In O K-edge spectra, the intensity of the peak assigned to the π^∗ at 532.1 eV increased as the tannate/Al MR increased, with the spectrum of the Al precipitate formed at a tannate/Al MR of 0.1 being almost identical to that of tannic acid. These results indicate that tannate ligands are incorporated into the structural network of short-range ordered Al (oxy)hydroxides to perturb their structural configuration during the formation of Al precipitates under the influence of tannic acid. With increasing tannate/Al MR, the Al (oxy)hydroxides decreased in amount and developed structural defects and the Al-tannate precipitates increased in amount. The decrease in the coordination number of Al in the Al (oxy)hydroxides is attributed to steric and electronic factors which cause the change in Al-O bonding, because Al is complexed with tannate which has different functional groups and is much larger in size compared with OH and H₂O ligands. The surface reactivity of a metal-O bond is related to its covalency and coordination geometry. The findings obtained in the present study are, thus, of fundamental significance in understanding the structural and surface chemistry of Al (oxy)hydroxides and their impact on the transformation, transport, and fate of nutrients and pollutions in the environment.     

Adsorption of phenanthrene on Al (oxy)hydroxides formed under the influence of tannic acid

Although there has been substantial research done on adsorption of metals/metalloids by Al (oxy)hydroxides, little is known regarding the adsorption of polyaromatic hydrocarbons (PAHs) on Al (oxy)hydroxides, especially those formed in the presence of organic acid. This paper investigated the adsorption of phenanthrene on Al (oxy)hydroxides formed with initial tannate/Al molar ratios (MRs) 0, 0.001, 0.01, and 0.1 (referred to MR0, MR0.001, MR0.01, and MR0.1, respectively) through batch adsorption experiments and FTIR study. The results showed that Al (oxy)hydroxides were important sorbents for phenanthrene. The adsorption kinetic data were fitted well with the pseudo-second-order equation. According to a modified Freundlich model, the adsorption capacities of Al (oxy)hydroxides followed a descending order of MR0.1 > MR0 ≥ MR0.01 > MR0.001, which was inconsistent with the organic carbon content in the Al (oxy)hydroxides. Adsorption capacity correlated with the specific surface area, micropore area, and micropore diameter of Al (oxy)hydroxides, yet the relationships were not statistically significant (P > 0.05). FTIR results showed that physical interaction was essential in phenanthrene adsorption onto the Al (oxy)hydroxides, which could be explained by an entropy-driven process. Surface hydrophobicity of Al (oxy)hydroxides played a key role in phenanthrene adsorption. Additional π–π electron donor–acceptor interaction of phenanthrene (acting as electron donor) with aromatic ring of tannic acid (electron acceptor) could be also important in phenanthrene adsorption by high MR Al (oxy)hydroxides, yet it needs further study. The findings obtained in the present study are of fundamental significance in understanding the mechanism of immobilization of PAHs in low organic matter but oxide-rich soils.     

Effect of low-molecular-weight organic acids on the distribution of mobilized Al between soil solution and solid phase

Low-molecular-weight (LMW) organic acids occur widely in soils. Results in pure mineral systems and podzols suggest that LMW organic acids can promote the dissolution of Al from kaolinite, Al oxides and soils, but limited information is available concerning the role of these organic acids on Al mobilization in variable charge soils as yet. This paper deals with the effect of LMW organic acids on Al mobilization and mobilized Al distributed between the solution phase and exchangeable sites in two acidic variable charge soils. The results indicated that LMW organic acids accelerated Al mobilization through proton- and ligand- promoted reactions. The ability of different organic acids to mobilize Al followed the order: citric acid > oxalic acid > malonic acid > malic acid > tartaric acid > salicylic acid > lactic acid > maleic acid. This order was in general agreement with the magnitude of the stability constants of Al–organic acid complexes. The ratio of soluble Al to exchangeable Al also increased as the stability constants increased. These results showed that the organic acids with strong Al-complexation capacity were most effective in Al mobilization, whereas the weak organic acids promoted the retention of mobilized Al by the soil exchangeable sites. Increase in both organic acid concentration and solution pH promoted Al mobilization and also increased the ratio of soluble Al to exchangeable Al due to the increase in the concentration of the effective organic ligands, especially in the strong organic acid systems. These findings may have their practical significance for establishing more effective amelioration procedures for variable charge soils with increased acidity and higher mobility of Al.     

Kinetics and mechanisms of kaolinite dissolution: effects of organic ligands

The effect of protons, low molecular weight organic ligands, soil humic acid (HA), and stream water dissolved organic matter (DOM) on the rate of dissolution of kaolinite was examined. In acid solution (no ligands present) the rate of dissolution increased with increasing [H⁺] and the rate of Si dissolution was generally faster than Al. Low molecular weight organic ligands markedly increased the dissolution rates of both Al and Si in the following order: oxalate > malonate ≈ salicylate > o-phthalate. In the presence of organic ligands, the rate of Al dissolution was generally much greater than Si. Soil HA and stream water DOM did not promote the dissolution of kaolinite under the experimental conditions examined in this study.

The dissolution kinetics of Al were interpreted in terms of a surface complexation model and the rate equations described in terms of the concentrations of specific (i.e. inner sphere) surface complexes.     

Leaching of Trace Elements from Rocks under the Action of Organic Acids

Experiments were carried out on the leaching of trace elements (Li, Rb, Cs, Sr, Ba, V, Mn, Fe, Co, Ni, Cu, Tl, Y, La, Ce, Th, and U) from unmodified ashes of Karymsky volcano (the Kamchatka Peninsula) under the interactions with 0.01 M oxalic, salicylic, tartaric, citric, and acetic acids at various proportions of solid and liquid phases. Based on the data we obtained, it was concluded that the trace elements were mainly mobilized owing to the destruction of crystalline structures of rock-forming minerals, as well as to the reduction of Fe(III) and Mn(IV) oxide–hydroxides into soluble Fe(II) and Mn(II) compounds (in the case of oxalic acid). The formation of organic complexes increased the stability of metals in a solution and provided the attainment of higher concentrations of the dissolved forms compared to the case without organic ligands.     

低分子有机酸对聚合羟基铁铝-蒙脱石复合体吸附磷的影响

在一定条件下,利用钠基化蒙脱石(Na-Mt)合成聚合羟基铁铝-蒙脱石复合体(HyFeAl-Mt),M3+/Mt比为10mmol/g,Fe3+/Al3+摩尔比分别为0.1、0.2和0.5。研究了酸性和弱酸性条件下低分子有机酸(以柠檬酸、草酸和苹果酸为代表)及其添加顺序对HyFeAl-Mt复合体吸附磷的影响。结果表明,低分子有机酸对HyFeAl-Mt复合体吸附磷有抑制作用,且随着低分子有机酸浓度的增加愈加明显,酸性条件下的抑制作用比弱酸性条件下明显;低分子有机酸羧基个数越多、分子结构越大,对HyFeAl-Mt对吸附磷的影响越大;酸性环境中低分子有机酸与磷共存时,磷吸附曲线较好地符合Freundlich方程,且其对磷在HyFeAl-Mt0.2上的最大吸附量影响较小;磷与低分子有机酸的添加顺序不同也影响HyFeAl-Mt复合体对磷的吸附。     

A novel procedure to detect low molecular weight compounds released by alkaline ester cleavage from low maturity coals to assess its feedstock potential for deep microbial life

In recent years the widespread occurrence of microorganisms was demonstrated in deep marine and terrestrial sediments. With this discovery inevitably the question of the potential carbon and energy sources for this deep subsurface microbial life arises. In the current study a new method for the investigation of low molecular weight (LMW) organic acids linked to the kerogen matrix is presented. These LMW organic acids form a potential feedstock for deep microbial populations. Twelve coal samples of different maturity (vitrinite reflectance (R₀) of 0.28–0.80%) from several coal mines on the North and South Island of New Zealand (NZ) were examined to assess the amount of bound LMW organic acids. Formate, acetate and oxalate were detected in significant amounts whereas the amounts of these compounds decrease with increasing maturity of the coal sample. This decrease of LMW organic acids mainly correlates to the phase of diagenetic alteration of the organic matter characterized by the release of oxygen containing compounds. Concomitantly, it coincides with temperature conditions assumed to be still compatible with microbial life in the deep subsurface. First assessments of the feedstock potential and generation rates of LMW organic acids indicate that the NZ coals investigated exhibit the potential to feed deep terrestrial microbial life with appropriate substrates over geological time spans.     

Comparison of the Complexation of Cu and Cd by Humic or Fulvic Acids and by Ligands Observed in Lake Waters

The complexation of Cu and Cd by ligands observed in filtered unfractionated lakewaters is compared to the complexation by humic and fulvic acids. Complexation parameters (conditional stability constants and ligand concentrations) of Suwannee River fulvic acids (FA), purified peat humic acids (HA) and of ligands in lakewater samples have been determined using the same methods (ligand-exchange and CSV (cathodic stripping voltammetry) or ASV (anodic stripping voltammetry)), and the same titration ranges of Cu, Cd and organic carbon concentrations. The performance of the used techniques is first evaluated in FA and HA suspensions, and gives comparable results with the literature values for the same materials, according to published models (5-site model, NICA model) and parameters. Model calculations using the WHAM model for FA and HA (Tipping, 1994) are also presented. The comparison of titrations of FA and HA with Cu and Cd with those of lakewater samples indicates that stronger ligands than FA and HA are present at low concentrations in the lakewaters. Specific strong ligands occur in particular in eutrophic lake waters, whereas in a lake with higher metal concentrations and low biological productivity the ligands more closely match the fulvic acid characteristics.     

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.     

Unraveling the atomic structure of biogenic silica: evidence of the structural association of Al and Si in diatom frustules

We used X-ray absorption spectroscopy at the Al K-edge to investigate the atomic structure of biogenic silica and to assess the effect of Al on its crystal chemistry. Our study provides the first direct evidence for a structural association of Al and Si in biogenic silica. In samples of cultured diatoms, Al is present exclusively in fourfold coordination. The location and relative intensity of X-ray absorption near-edge structure (XANES) features suggests the structural insertion of tetrahedral Al inside the silica framework synthesized by the organism. In diatom samples collected in the marine environment, Al is present in mixed six- and fourfold coordination. The relative intensity of XANES structures indicates the coexistence of structural Al with a clay component, which most likely reflects sample contamination by adhering mineral particles. Extended X-ray absorption fine structure spectroscopy has been used to get Al-O distances in biogenic silica of cultured diatoms, confirming a tetrahedral coordination. Because of its effect on solubility and reaction kinetics of biogenic silica, the structural association between Al and biogenic silica at the stage of biosynthesis has consequences for the use of sedimentary biogenic silica as an indicator of past environmental conditions.     

X-ray absorption spectroscopy study on the effect of hydroxybenzoic acids on the formation and structure of ferrihydrite

Organic ligands are known to interfere with the polymerization of Fe(III), but the extent of interference has not been systematically studied as a function of structural ligand properties. This study examines how the number and position of phenol groups in hydroxybenzoic acids affect both ferrihydrite formation and its local (<5 Å) Fe coordination. To this end, acid Fe(III) nitrate solutions were neutralized up to pH 6.0 in the presence of 4-hydroxybenzoic acid (4HB), 2,4-dihydroxybenzoic acid (2,4DHB), and the hydroquinone 3,4-dihydroxybenzoic acid (3,4DHB). The initial molar ligand/Fe ratios ranged from 0 to 0.6. The precipitates were dialyzed, lyophilized, and subsequently studied by X-ray absorption spectroscopy and synchrotron X-ray diffraction. The solids contained up to 32 wt.% organic C (4HB ∼ 2,4DHB < 3,4DHB). Only precipitates formed in 3,4DHB solutions comprised considerable amounts of Fe(II) (Fe(II)/Fe_tot ≤ 6 mol%), implying the abiotic mineralization of the catechol-group bearing ligand during Fe(III) hydrolysis under oxic conditions. Hydroxybenzoic acids decreased ferrihydrite formation in the order 4HB ∼ 2,4DHB ? 3,4DHB, which documents that phenol group position rather than the number of phenol groups controls the ligand’s interaction with Fe(III). The coordination numbers of edge- and double corner-sharing Fe in the precipitates decreased by up to 100%. Linear combination fitting (LCF) of Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra revealed that this decrease was due to increasing amounts of organic Fe(III) complexes in the precipitates. Although EXAFS derived coordination numbers of Fe in ferrihydrite remained constant within error, all organic ligands decreased the coherently scattering domain (CSD) size of ferrihydrite as indicated by synchrotron X-ray diffraction analysis (4HB < 2,4DHB ? 3,4DHB). With decreasing particle size of ferrihydrite its Fe(O,OH)₆ octahedra became progressively distorted as evidenced by an increasing loss of centrosymmetry of the Fe sites. Pre-edge peak analysis of the Fe K-edge XANES spectra in conjunction with LCF results implied that ferrihydrite contains on an average 13 ± 3% tetrahedral Fe(III), which is in very good agreement with the revised single-phase structural model of ferrihydrite (Michel, F. M., Barron, V., Torrent, J., Morales, M. P. et al. (2010) Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism. Proc. Natl. Acad. Sci. USA107, 2787-2792). The results suggest that hydroxybenzoic acid moieties of natural organic matter (NOM) effectively suppress ferrihydrite precipitation as they kinetically control the availability of inorganic Fe(III) species for nucleation and/or polymerization reactions. As a consequence, NOM can trigger the formation of small ferrihydrite nanoparticles with increased structural strain. These factors may eventually enhance the biogeochemical reactivity of ferrihydrite formed in NOM-rich environments. This study highlights the role of hydroquinone structures of NOM for Fe complexation, polymerization, and redox speciation.     

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.     

Iron solid-phase differentiation along a redox gradient in basaltic soils

Iron compounds in soil are multifunctional, providing physical structure, ion sorption sites, catalytic reaction-centers, and a sink for respiratory electrons. Basaltic soils contain large quantities of iron that reside in different mineral and organic phases depending on their age and redox status. We investigated changes in soil iron concentration and its solid-phase speciation across a single-aged (400 ky) lava flow subjected to a gradient in precipitation (2200-4200 mm yr⁻¹) and hence redox history. With increasing rainfall and decreasing Eh, total Fe decreased from about 25% to <1% of the soil mass. Quantitative speciation of soil solid-phase iron was constrained by combining ⁵⁷Fe Mössbauer spectroscopy (MBS) at 295 and 4.2 K with powder X-ray diffraction, selective chemical extractions, and magnetic susceptibility measurements. This approach allowed us to partition iron into (1) nanoparticulate and microcrystalline Fe^III-(oxy)hydroxides, (2) microcrystalline and bulk Fe^III-oxides, (3) organic/silicate bound Fe^III, and (4) ferrous iron. The Fe^III-(oxy)hydroxide fraction dominated solid-phase Fe, exhibiting a crystallinity continuum based on magnetic ordering temperature. The continuum extended from well-ordered microcrystalline goethite through nanocrystalline Fe^III-(oxy)hydroxides to a nano Fe^III-(oxy)hydroxide phase of extremely low crystallinity. Magnetic susceptibility was correlated (R² = 0.77) with Fe^III-oxide concentration, consistent with a contribution of maghemite to the otherwise hematite dominated Fe-oxide fraction. The Fe^III-(oxy)hydroxide fraction of total Fe decreased with increasing rainfall and was replaced by corresponding increase in the organic/silicate Fe^III fraction. The crystallinity of the Fe^III-(oxy)hydroxides also decreased with increasing rainfall and leaching, with the most disordered members of the crystallinity continuum, the nano Fe^III-(oxy)hydroxides, gaining proportional abundance in the wetter sites. This finding runs counter to the conventional kinetic expectation of preferential removal of the most disordered minerals in a reductive dissolution-dominated environment. We suggest the persistence of highly disordered Fe phases reflects the dynamic redox conditions of these upland soils in which periods of anoxia are marked by high water-throughput and Fe²⁺(aq) removal, while periodic Fe oxidation events occur in the presence of high concentrations of organic matter. Our ⁵⁷Fe Mössbauer study shows basalt-derived nano-scale Fe^III phases are more disordered than current synthetic analogs and have nano-structural characteristics that are linked to their formation environment.     

Kinetics of brucite dissolution at 25°C in the presence of organic and inorganic ligands and divalent metals

Brucite (Mg(OH)₂) dissolution rate was measured at 25°C in a mixed-flow reactor at various pH (5 to 11) and ionic strengths (0.01 to 0.03 M) as a function of the concentration of 15 organic and 5 inorganic ligands and 8 divalent metals. At neutral and weakly alkaline pH, the dissolution is promoted by the addition of the following ligands ranked by decreasing effectiveness: EDTA ≥ H₂PO₄⁻ > catechol ≥ HCO₃⁻ > ascorbate > citrate > oxalate > acetate ∼ lactate and it is inhibited by boric acid. At pH >10.5, it decreases in the presence of PO₄³⁻, CO₃²⁻, F⁻, oxine, salicylate, lactate, acetate, 4-hydroxybenzoate, SO₄²⁻ and B(OH)₄⁻ with orthophosphate and borate being the strongest and the weakest inhibitor, respectively. Xylose (up to 0.1 M), glycine (up to 0.05 M), formate (up to 0.3 M) and fulvic and humic acids (up to 40 mg/L DOC) have no effect on brucite dissolution kinetics. Fluorine inhibits dissolution both in neutral and alkaline solutions. From F sorption experiments in batch and flow-through reactors and the analysis of reacted surfaces using X-ray Photoelectron Spectroscopy (XPS), it is shown that fluorine adsorption is followed by its incorporation in brucite lattice likely via isomorphic substitution with OH. The effect of eight divalent metals (Sr, Ba, Ca, Pb, Mn, Fe, Co and Ni) studied at pH 4.9 and 0.01 M concentration revealed brucite dissolution rates to be correlated with the water molecule exchange rates in the first hydration sphere of the corresponding cation.The effect of investigated ligands on brucite dissolution rate can be modelled within the framework of the surface coordination approach taking into account the adsorption of ligands on dissolution-active sites and the molecular structure of the surface complexes they form. The higher the value of the ligand sorption constant, the stronger will be its catalyzing or inhibiting effect. As for Fe and Al oxides, bi- or multidentate mononuclear surface complexes, that labilize Mg-O bonds and water coordination to Mg atoms at the surface, enhance brucite dissolution whereas bi- or polynuclear surface complexes tend to inhibit dissolution by bridging two or more metal centers and extending the cross-linking at the solid surface. Overall, results of this study demonstrate that very high concentrations of organic ligands (0.01-0.1 M) are necessary to enhance or inhibit brucite dissolution. As a result, the effect of extracellular organic products on the weathering rate of Mg-bearing minerals is expected to be weak.     

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).     

δC of low-molecular-weight organic acids generated by the hydrous pyrolysis of oil-prone source rocks

Low-molecular-weight (LMW) aqueous organic acids were generated from six oil-prone source rocks under hydrous-pyrolysis conditions. Differences in total organic carbon-normalized acid generation are a function of the initial thermal maturity of the source rock and the oxygen content of the kerogen (OI). Carbon-isotope analyses were used to identify potential generation mechanisms and other chemical reactions that might influence the occurrence of LMW organic acids. The generated LMW acids display increasing ¹³C content as a function of decreasing molecular weight and increasing thermal maturity. The magnitudes of observed isotope fractionations are source-rock dependent. These data are consistent with δ¹³C values of organic acids presented in a field study of the San Joaquin Basin and likely reflect the contributions from alkyl-carbons and carboxyl-carbons with distinct δ¹³C values. The data do not support any particular organic acid generation mechanism. The isotopic trends observed as a function of molecular weight, thermal maturity, and rock type are not supported by either generation mechanisms or destructive decarboxylation. It is therefore proposed that organic acids experience isotopic fractionation during generation consistent with a primary kinetic isotope effect and subsequently undergo an exchange reaction between the carboxyl carbon and dissolved inorganic carbon that significantly influences the carbon isotope composition observed for the entire molecule. Although generation and decarboxylation may influence the δ¹³C values of organic acids, in the hydrous pyrolysis system described, the nondestructive, pH-dependent exchange of carboxyl carbon with inorganic carbon appears to be the most important reaction mechanism controlling the δ¹³C values of the organic acids.     

Mechanism of aluminum release from variable charge soils induced by low-molecular-weight organic acids: Kinetic study

The kinetic curves of aluminum release from two variable charge soils and a kaolinite within 48 h can be divided into three stages: the first stage located within the initial 30 min, at which the release rate of Al was the fastest one and the released Al dominantly originated from exchangeable Al and amorphous Al pools. The Elovich equation fit the kinetics data at this stage fairly well. The moderate and the slow stages occurred within 0.5-2 and 2-48 h, respectively. During these two stages, the released Al was mainly attributed to Al oxides, poorly crystalline kaolinite and easily weathered hydrous mica. The different linear equations also fit the kinetics data at these two stages well. The rate of Al release decreased sharply with time during the fast stage, but the rate remained constant during the moderate and slow stages. In Ultisol, Al oxides were the more important pool for Al release than poorly crystalline kaolinite and easily weathered hydrous mica during the latter two stages. In Oxisol, poorly crystalline kaolinite was the more important Al pool. Compared to the control system, the presence of organic acids increased the rate and quantity of Al release from variable charge soils. The ability of organic acids to accelerate Al release followed the order: oxalic acid > citric acid > malic acid > lactic acid. This is generally in consistent with the magnitude of the stability constants of the Al-organic complexes. The release rate of Al also increased with the rise in concentration of organic acids.     

Uranium(VI) sorption to hematite in the presence of humic acid

A long-standing problem in aquatic geochemistry has been the incorporation of natural organic matter (NOM) into speciation models. The general effect of NOM on metal ion sorption by particles has been understood for some time, and significant progress has been made in elucidating some of the details of the role of NOM through the use of surrogate organic acids such as citric acid. However, a gap exists between the general observations that have been made of NOM behavior and the inclusion of NOM in surface chemical models for metal ion sorption. In this paper, we report on the results of a study on the sorption of U(VI) by hematite in the absence and presence of Suwannee river humic acid (HA) and over a range of other system conditions (e.g., pH, I). Essential HA characteristics (e.g., its acid/base, metal binding, and surface chemical properties) were “captured” by representing the HA as an assembly of monoprotic acids with assumed pK values and without explicit correction for electrostatic effects. The ternary system (hematite/HA/U(VI)) was simulated through the combination of the binary submodels (i.e., CO₃²⁻/hematite, U(VI)/HA, U(VI)/hematite, and HA/hematite) with model constants fixed at the values determined from simulations of the respective experimental systems. However, the “summed-binary” approach undersimulated experimental results, and the ternary system model required the postulation of two ternary surface (Type A) complexes composed of the uranyl ion, hematite surface sites, and the model ligands comprising the HA. Consideration of the HA in this manner permitted the simulation of HA effects on U(VI) sorption by hematite over a range of solution conditions using a general speciation model.     

Municipal compost-based mixture for acid mine drainage bioremediation: Metal retention mechanisms

An upflow packed column was operated to evaluate the potential of a mixture of municipal compost and calcite to promote sulphidogenesis in the remediation of a simulated mine water at high flows (>0.1 m d⁻¹). Results showed that the pH was neutralised and metals (Fe, Al, Zn, Cu) were significantly removed. Metal removal was attributed to the combined result of precipitation as metal (oxy)hydroxides and carbonates, co-precipitation with these (oxy)hydroxides and sorption onto the compost surface rather than to precipitation as metal sulphides. The two last mechanisms are especially significant for Zn, whose hydroxide is not expected to precipitate at pH 6–7. Before the saturation of compost sorption sites, 60% of the influent Zn was estimated to have been removed by co-precipitation with Fe- and Al-(oxy)hydroxide and 40% by sorption onto the municipal compost.     

Aluminium X-ray absorption Near Edge Structure in model compounds and Earth’s surface minerals

Aluminium K-edge X-ray absorption near edge spectra (XANES) of a suite of silicate and oxides minerals consist of electronic excitations occurring in the edge region, and multiple scattering resonances at higher energies. The main XANES feature for four-fold Al is at around 2 eV lower energy than the main XANES feature for six-fold Al. This provides a useful probe for coordination numbers in clay minerals, gels, glasses or material with unknown Al-coordination number. Six-fold aluminium yields a large variety of XANES features which can be correlated with octahedral point symmetry, number of aluminium sites and distribution of Al-O distances. These three parameters may act together, and the quantitative interpretation of XANES spectra is difficult. For a low point symmetry (1), variations are mainly related to the number of Al sites and distribution of Al-O distances: pyrophyllite, one Al site, is clearly distinguished from kaolinite and gibbsite presenting two Al sites. For a given number of Al-site (1), variations are controlled by changes in point symmetry, the number of XANES features being increased as point symmetry decreases. For a given point symmetry (1) and a given number of Al site (1), variations are related to second nearest neighbours (gibbsite versus kaolinite). The amplitude of the XANES feature at about 1566 eV is a useful probe for the assessment of Al^IV/Al^total ratios in 2/1 phyllosilicates. Al-K XANES has been performed on synthetic Al-bearing goethites which cannot be studied by ²⁷Al NMR. At low Al content, Al-K XANES is very different from that of α-AlOOH but at the highest level, XANES spectrum tends to that of diaspore. Al-K XAS is thus a promising tool for the structural study of poorly ordered materials such as clay minerals and natural alumino-silicate gels together with Al-subsituted Fe-oxyhydroxides. Received: 18 December 1996 / Revised, accepted: 2 June 1997