Interactions between montmorillonite and fulvic acid

The isotherms at 20°C for the adsorption of ¹⁴C-labelled fulvic acid from aqueous solutions by montmorillonite containing different exchangeable cations, have been determined. Below a concentration of 0.45 mg/ml and at near neutral pH, all samples give linear isotherms, the slope of which increases in the order Ba²⁺ < Ca²⁺ < Zn²⁺ < La³⁺ < Al³⁺ <Cu²⁺ < Fe³⁺. In the absence of interlayer expansion, the shape of the isotherms is interpreted in terms of solute entry into intercrystalline pores within a clay domain. The affinity of fulvic acid for the clay surface is related to the ionic potential or polarising power of the exchangeable cation. It is inferred that fulvic acid adsorbs by hydrogen bonding between an anionic group of the acid and a water molecule in the primary hydration shell of the saturating cation. Comparison of the data with those for the humic acid extracted from the same organic matter source, indicates that secondary interactions between adsorbed molecules or directly with the montmorillonite surface, contribute to the overall affinity. With samples saturated with Al³⁺, Fe³⁺, and Cu²⁺ ions, part of the adsorbed fulvic acid may also be attached to the clay by a complexation reaction involving the metals as such, or when they exist as polyhydroxy compounds at the mineral surface.     

Acid pyrophosphate extraction of soil fulvic acids

A simple three step method is described for isolation of soil fulvic acids in high yield. The complexing agent H₂P₂O₇^2? (at pH 2) is used to release soil-bound fulvic acids. Extraction of humic acids is minimal. Selective separation of the protonated fulvic acids from the ionic extractant is achieved on a non-ionic polyacrylate resin (Amberlite XAD-7); after washing the resin, fulvic acids were retrieved in >98% yield by adjusting the pH to 6.5. Two problems associated with the classical alkali extraction method are avoided: possible alkaline oxidation of phenolic components, and their oxidation by Fe³⁺ under the acidic conditions employed to precipitate humic acids. The product typically has an ash weight of <0.6% after one XAD treatment. The method has been applied to three soils and one IHSS peat sample.     

Formation of metal-humic acid complexes by titration and their characterization by differential thermal analysis and infrared spectroscopy

Humic acid (HA) extracted from a Eustis loamy sand (Psammentic Paleudult, Red Yellow Podzolic soil) was flocculated by titration with Al³⁺-, Fe³⁺-, Cu²⁺-, Zn²⁺-, Mn²⁺-, Ba²⁺-, Ca²⁺-, and Mn²⁺-chloride solutions, respectively, to determine possible development of metal-HA complexes, as reported by Flaig et al. (1975), and Tiurin and Kononova (1962). Titration was conducted with HA solutions with an initial pH 11.5 or 7.0. The results indicated that the cations used, except Mg²⁺, yielded insoluble complexes with HA, irrespective of initial pH. After titration, the pH of the metal-HA flocs was 6.0–7.0, which was expected in view of the presence of cation exchange and buffering capacity of HA compounds. More complex formation through electrovalent and covalent bonding by COO^? and phenolic OH groups of the HA molecule was only attained by the use of HA solutions with pH 11.5. On the other hand, less complex formation occurred by the use of HA solutions with an initial pH 7.0, through electrovalent bonding by COO^? groups. Differential thermal analysis (d.t.a.) curves of HA showed shifts in temperatures of the main decomposition peak as a result of flocculation with the different metals. Based on the type of the cations involved, the metal-humic acid flocs could be listed in the following decreasing order of thermal stability: Al³⁺ = Zn²⁺ = Mg²⁺ ≥ HA > Ca²⁺ > Ba²⁺ > Fe³⁺ > Cu²⁺ > Mn²⁺. A systematic relationship could not be found indicating that trivalent ions resulted in the formation of thermally less stable metal-humic acid flocs than divalent ions, as has been reported for HA-metal complexes. Physical mixtures of HA and metal hydroxides exhibited d.t.a. features resembling those of original (nontreated) HA, but not those of the HA-metal flocs.Infrared spectroscopy revealed increased absorption for COO^? vibrations at 1620 and 1400cm^?1 in spectrograms of metal-HA flocs compared to that of original humic acid, a phenomenon explained by many authors to be caused by bonding of the metal ions in hydrated form to the carboxyl or phenolic hydroxyl groups or both of the humic acid molecule. HA-flocs formed from solutions with an initial pH 11.5 had identical i.r. spectra compared with those formed from solutions with an initial pH 7.0.     

Influence of pH and organic substance on the adsorption of As(V) on geologic materials

The adsorption of As(V) on alumina, hematite, kaolin and quartz has been measured as a function of pH (2 to 10), and As concentrations (10^?4 to 10 ^?8 M; in the alumina and kaolin systems only). The effects of sulfate (0 to 80 mg L^?1) and fulvic acid (0 to 25 mg L^?1) were studied. The charge of the solid surface and the As speciation in solution (determined by pH) were the most important chemical parameters affecting the sorption behavior. At pH below PZC of the solid, there was a qualitative correlation between the adsorption and the anion exchange capacity of the solid. For hematite at low pH (below 5) there was a reduction of the adsorption possibly related to the formation of positively charged species. The presence of sulfate or fulvic acid reduced the adsorption.     

Influence of natural and synthetic humic substances on the activity of urease

The activity of a purified urease, obtained from Bacillus pasteurii, was inhibited by humic and fulvic acids obtained from an agricultural soil. Enzyme kinetic studies showed that the humic substances affected the affinity of the enzyme for its substrate (K_m) and the maximum velocity of the reaction (V_max). The V_max was inhibited to the same extent by both humic (HA) and fulvic (FA) acids, the precise effect depending on the pH and concentration of humic substance. At pH 4.0, HA concentrations of 25 pg cm^?3 and 10 μg cm^?3 inhibited the V_max by 38.5% and 20% respectively. HA and FA had similar effects on the K_m but in this case the lowering of the affinity of the enzyme for its substrate was not concentration dependent in the range 0–25 μg cm^?3 of humic substance. Typically, the affinity was decreased from a K_M of 50 mM in the control to 67 mM in the presence of HA and FA. The effects were not due primarily to the ash or N contents of the humic substances because de-ashed humic acid and synthetic model humic (made from catechol, guaiacol, pyrogallol, resorcinol and protocatechuic acid) and fulvic acid (made from polymaleic acid), containing virtually no ash or N, were equally as effective. The effect was not related to the phenolic monomers which, before polymerization, had no effect on urease activity.     

Stability constants of complexes of metals with humic and fulvic acids under non-acid-conditions

Stability constants of complexes of four divalent metal ions viz. Cu²⁺, Zn²⁺, Mn²⁺ and Ca²⁺ with humic (HA) and fulvic acids (FA) at pH values of 7 and 8 were determined. The log K (logarithm of the stability constant) ranged from 3.09 to 7.77 and from 2.22 to 5.98 for metal-humic and metal fulvic acid complexes, respectively. Sequentially, the order of stability constants were as follows: Cu> Ca> Mn> Zn and Cu> Ca> Zn> Mn for metal -HA and metal-FA complexes, respectively, indicating a higher degree of complexation with Cu metal ion.     

Effect of soil depth on acid properties of humic substances extracted from an ombrotrophic peat bog in northwest Spain

The most southerly ombrotrophic peat bogs in Europe are in Galicia (northwest Spain). The humic matter in them originates from chemical processes in anaerobic conditions. We studied the acid properties of fulvic acids and humic acids isolated from two peat horizons of an ombrotrophic peat bog by potentiometric titration. Solutions containing 25, 50 and 100 mg l⁻¹ of each humic substance were titrated at ionic strengths 0.005 m , 0.01 m and 0.1 m (with KNO₃ as the inert electrolyte). Charge curves were analysed with a Donnan model to determine the intrinsic proton binding parameters. The concentration of the humic substance affected the charge curves more significantly at pH exceeding 6, and tended to disappear at greater concentrations. The proton binding conditional constants decreased with increasing ionic strength, this effect being more significant in the carboxylic groups with less affinity for protons. The proton binding constant of the carboxyl groups in a fulvic acid was one order of magnitude less than the value for the corresponding humic acid, whereas for the phenolic groups the values for both fractions were similar. The total content of acid groups was approximately 2 mol kg⁻¹ greater in the fulvic fraction than in the humic fraction. Both humic fractions from the lower horizon contained more acid groups than those from the upper horizon, mainly because the content of carboxyl groups increases with soil depth. Therefore, the humic substances in the lower horizon of the peat will be more negatively charged, which will affect their solubility and the binding of metal ions.     

Catalytic effect of various metal ions on the methylation of mercury in the presence of humic substances

Methylation of Hg²⁺ (Hg(NO₃)₂) in the presence of fulvic acid (FA) and various metal ions has been studied. The concentrations of Hg²⁺ and FA ranged from 5 to 20 mg L^?1 and 171 to 285 mg L^?1 DOC, respectively. The pH range was 3 to 6.5. FA was isolated from an acid brown-water lake by XAD-8 polymeric adsorbent. Methylmercury production in the dark during 2 to 4 days incubation at 30 °C increased with increasing concentrations of Hg²⁺ ion and FA as well as with additions of metal ions (5 to 10 × 10^?5 mole L^?1 The observed catalytic activity of metal ions followed the order Fe³⁺ (Fe²⁺) > Cu²⁺ ≈ Mn²⁺, > Al³⁺. The production of methylmercury had a pH-optimum around 4 to 4.5 at the conditions tested.     

Behavior of phenolic substances in the decaying process of plants

The behavior of phenolic substances in the decaying process of rice straw, ladino clover, and fanen leaves of red oak under moist conditions, and also of rice straw under various conditions were compared in the laboratory. The amounts of phenolic substances, divided into either humic acid and fulvic acid fractions, or ether-extractable, butanol-extractable and organic solvent-unextractable fractions, and the amounts of individual phenolic acids were periodically determined during incubation for 150 days. The following results were obtained.

1) The amounts and behavior of phenolic substances in various fractions differed considerably among the plant materials. The total amount of phenolic substances was remarkably larger in red oak leaves than in the others during the whole period of incubation. The amounts of phenolics in the fulvic acid fraction changed to a larger extent than those in the humic acid fraction during the decaying process of plant materials. The changes in total amount of phenolics in decaying red oak leaves and ladino clover were mainly due to changes in the level of relatively hydrophilic phenolics in the fulvic acid fraction, but the changes in decaying rice straw were mainly due to changes in the level of relatively lipophilic phenolics.

2) Rice straw and ladino clover, especially the former, contained large amounts of p-coumaric and ferulic acids, but these decreased rapidly in the early stage of the decaying process. The amounts in red oak leaves were small, but did not decrease markedly during incubation.

3) The changes in amounts of phenolics in both humic acid and fulvic acid fractions in the decaying process of rice straw were largely influenced by temperature, moisture, and pH, but not to a large extent by C/N ratio and the presence of soil. At higher temperatures under moist conditions, phenolic substances disappeared rapidly. Also, acidification of the system inhibited the degradation process.     

Cation binding by acid-washed peat, interpreted with Humic Ion-Binding Model VI-FD

A sample of ombrotrophic peat from Moor House in northern England was extensively extracted with dilute nitric acid (pH 1) to free it of bound cations. Suspensions of the acid‐washed peat (5–30 g l^?1), prepared with different concentrations of background electrolyte (NaCl and KCl), were used to conduct batch acid–base titrations. A strong dependence of proton release on ionic strength (I) was observed, the apparent acid dissociation constant (pK_app) being found to decrease by approximately 1.0 for each tenfold increase in I. This behaviour could not be explained satisfactorily with Humic Ion‐Binding Model VI, a discrete‐site/electrostatic model of cation binding by humic substances, parameterized with data from laboratory studies on isolated samples. More success was obtained by abandoning the impermeable‐sphere electrostatic submodel used in Model VI, and instead assuming the peat to consist of aggregates with fixed internal volume, and with counterion accumulation described by the Donnan model, as proposed by Marinsky and colleagues. The fixed‐volume Donnan model (Model VI‐FD) could also approximately explain other reported results from acid–base titrations of peat, including the effects on the titrations of complexing cations (Al, Ca, Cu). Copper titrations of the Moor House sample were performed using an ion‐selective electrode, with peat suspensions in the acid pH range, at two ionic strengths, and in the presence of Al and Ca. The measured concentrations of Cu²⁺ were in the range 10^?13?10^?5 m . Model VI‐FD provided reasonable fits of the experimental data, after optimization of the intrinsic binding constant for Cu, the optimized value being close to the default value derived previously from data referring to isolated humic substances. The optimized constants for Al and Ca, derived from their competition effects, were also close to their default values. Additional experiments were performed in which the centrifugation‐depletion method was used to measure the binding of a cocktail of metals (Al, Ni, Cu, Zn, Cd, Eu, Pb) at a single pH. The model correctly predicted strong binding of Al, Cu, Eu and Pb, and weaker binding of Ni, Zn and Cd. For the strongly binding metals, the dissolved forms were calculated to be mainly due to complexes with dissolved humic matter, whereas the free ions (Ni²⁺, Zn²⁺, Cd²⁺) dominated for the weakly binding metals. Acid‐washed soil appears to provide a valuable intermediate between isolated humic substances and untreated soil for the investigation of cation binding by natural organic matter in the natural environment.     

The significance of solid‐state 13C NMR spectroscopy of whole soil in the characterization of humic matter

Abstract

Differences in characteristics of humic matter were investigated by solid‐state CP/MAS ¹³C NMR spectroscopy of whole (nontreated) materials and their extracted humic fractions. Samples used in the analysis were lignite, a commercial humate AG, and the Bh horizons of a Mascotte and a Lawnwood soil. Humic fractions were extracted by the 0.1 M NaOH or Na₄P₂O₇ (pH 9.8) method. The humic (HA) and fulvic acid (FA) obtained were weighed and analysed for total acidity, carboxyl and phenolic‐OH group contents. Whole lignite, humate AG and soil samples, and the HA and FA fractions were analyzed by solid state CP MAS ¹³C NMR and infrared spectroscopy. Carbon, H, and N contents were determined by chemical analysis. NMR spectra of the combined HA+FA extracts resembled the spectra of the whole materials. No additional signals were detected, indicating that alien compounds were not produced during the extraction. The best spectra were obtained with HA samples produced by the NaOH method. These spectra closely resembled those of the untreated materials. Spectral and chemical differences noticed between the HA (or FA) fractions were attributed more to differences in origin than to the extraction procedure. Aliphatic, aromatic and carboxyl groups were the major components of HA from lignite and humate AG. In contrast, HA from the two Haplaquods were characterized by four major components: the aliphatic, polysaccharide, aromatic, and carboxyl groups. Regardless of origin, all the HA fractions contained similar functional groups, as indicated by their close similarities in infrared spectra.     

Reactions of nucleic acid bases with inorganic soil constituents

The adsorption at pH's 4, 6 and 8 of adenine, guanine, cytosine, thymine and uracil on clays (montmorillonite, illite and kaolinite), Fe- and Al-oxides (goethite, hematite and gibbsite), a soil, and on a laboratory-prepared fulvic acid-montmorillonite complex was investigated. Portions of the clays and soil were saturated with H⁺, Fe³⁺ and Ca²⁺.Quantitatively, the extent of adsorption of nucleic acid bases by the clays was proportional to their exchange capacities, but the nature of the dominant cation had only minor effects. By contrast, the adsorption was strongly affected by pH, tending to decrease with increase in pH. Adsorption on goethite and gibbsite was lower than that on clays, while adsorption of nucleic acid bases on soils was slightly lower than that on oxides. The fulvic acid-montmorillonite complex adsorbed substantial, although smaller amounts of purines and pyrimidines, than did montmorillonite alone. The main adsorption mechanism at pH 4 appeared to be cation exchange whereas at pH 8 complex formation between the nucleic acid bases and cations on inorganic surfaces seemed to occur.The results of this and earlier work show that both inorganic and organic soil constituents adsorb nucleic acid bases. Which adsorption reaction predominates will depend on the clay and organic matter content and on the pH.     

Influence of P Deficiency on Major Secondary Metabolism in Flavonoids Synthesis Pathway of Chrysanthemum morifolium Ramat

The aim of this paper is to study the influence of P deficiency on secondary metabolites of flavonoid in Chrysanthemum morifolium Ramat. The regression equation between flavonoid and phenolic acid content was Y = ?294.46X + 150.66 (R² = 0.9205, P &lt; 0.01) in check and Y = 42.62X + 2.49 (R² = 0.9564, P &lt; 0.01) in P deficient treatment (Y, flavonoid content; X, phenolic content). There were two principal components to control the flavonoid major synthesis process, that was principal 1 [phenolic acid, phenylalanine ammonia-lyase (PAL), cinnamic acid and p-coumaric acid] (88.17%) and principal 2 [4-coumarate coenzyme A ligase (4CL), Cinnamate-4-hydroxylase (C4H)] (9.64%) domination under normal growth of C. moriloium. However, under P deficiency condition, the principal components were principal 1 (phenolic acid, cinnamic acid and p-coumaric acid and C4H) (81.46%) and principal 2 (PAL) (18.53%) domination, respectively. The influence of P deficiency on flavonoid major synthesis pathway was caused by the change of PAL and C4H activities.     

Chemical–structural information from solid-state 13C NMR studies of a suite of humic materials from a lower montane forest soil,Colorado, USA

Chemically and physically fractionated samples extracted from the surface horizon of a soil developed under a mix of coniferous and deciduous vegetation in southwestern Colorado were studied. ¹³C NMR data on this soil's organic matter and its HF(aq)-washed residue, as well as the classic acid/base-separated humic fractions (humic acid, fulvic acid, humin), were examined for chemical–structural detail, e.g., the various structural functionalities present (especially lipids, carbohydrates, aromatics, polypeptides and carbonyl/carboxyls). Among the humic fractions, it was found that the lipid concentrations are in the order humic acid>fulvic acid= humin; for carbohydrates the order is fulvic acid>humin>humic acid; for aromatic carbons the order is humic acid>humin>fulvic acid; for polypeptides it is humic acid>fulvic acid>humin and for carbonyl/carboxyl species it is humin>humic acid>fulvic acid, but the differences are small. ¹³C spin–lattice relaxation times indicate that at least two types of “domains” exist in each, corresponding to “higher” and “lower” concentrations of paramagnetic centers, e.g., Fe³⁺.     

Complexation of Cu2+ and Pb2+ by peat and humic acid

The binding of metal to humic substances is problematical. The approaches for studying metal binding to organic matter are briefly reviewed. Ion-selective electrodes (Cu²⁺ and Pb²⁺) were used to measure metal complexation by a whole peat and an extracted humic acid (HA) fraction. Scatchard plots and calculation of incremental formation constants were used to obtain values for the binding constants for the metals onto both peat and HA. Both the peat and the humic acid had a larger maximum binding capacity for Pb²⁺ than for Cu²⁺ (e.g. at pH = 5 HA gave 0·188 mmol Cu²⁺ g^?1 and 0·564 mmol Pb²⁺ g^?1: peat gave 0·111 mmol Cu²⁺ g^?1 and 0·391 mmol Pb²⁺ g^?1). Overall, the humic acid had a larger metal binding capacity, suggesting that extraction caused conformational or chemical changes. The binding constants (K₁) for Cu²⁺ increased with increasing pH in both peat and humic acid, and were larger in the peat at any given pH (e.g. at pH = 5 HA gave log K₁= 2·63, and peat gave log K¹= 4·47 for Cu²⁺). The values for Pb²⁺ showed little change with pH or between peat and humic acid (e.g. at pH = 5 HA gave log K¹= 3·03 and peat gave log K¹= 3·00 for Pb²⁺). In the peat, Cu²⁺ may be more able to bind in a 2:1 stoichiometric arrangement, resulting in greater stability but smaller binding capacity, whereas Pb²⁺ binds predominantly in a 1:1 arrangement, with more metal being bound less strongly. Whole peat is considered to be more appropriate than an extracted humic acid fraction for the study of heavy metal binding in organic soils, as this is the material with which metals introduced into an organic soil would interact under natural conditions.     

Determination of the free ion concentration of trace metals in soil solution using a soil column Donnan membrane technique

Accurate measurement of the free metal ion is difficult, especially for trace metals present in very small concentrations (less than micromolar) in natural systems. The recently developed Donnan membrane technique can measure the concentrations in solution in the presence of inorganic and organic complexing agents. We have developed this method further to make it applicable for analysing soil samples. The major development is the linkage of a soil column with the Donnan cell. The operational aspects of the method, including equilibrium time, buffering capacity and correction for differences in ionic strength, were investigated and optimized. The method was applied to determine concentrations of free Cu²⁺, Cd²⁺, Pb²⁺ and Zn²⁺ in the soil solution of 15 soil samples (pH 2.9–7.1, organic C < 2.9–137.4 g kg^?1, clay 0.2–51.6%). Compared with other speciation methods, the Donnan membrane technique has the advantage of allowing the measurement of several elements simultaneously and it minimizes the disturbance of substrate. The detection limit of the technique is about 10^?9 m . This method can be applied to study the biogeochemical behaviour of metals in soils, sediments and other solid materials.     

FULVIC ACID: I. PARTIAL FRACTIONATION1

Fulvic acid extracted from peat was partially fractionated on anion and cation exchange resins. In the latter case the fractionation is attributed to the presence in fulvic acid of components possessing amine groups; this was confirmed by chemical means and by infrared spectroscopic evidence. The infrared and ultraviolet spectra of fulvic acid and its various fractions are discussed and the occurrence of carboxyl, phenolic, and amine groups in fulvic acid and the fractions obtained is considered. It was further established that nitrous acid can destroy the amine groups present in fulvic acid.     

应用穆斯堡尔(Mössbauer)谱学方法研究Fe+++, Fe++与胡敏酸结合的性质

本文应用穆斯堡尔谱学方法,研究了在不同pH条件下Fe⁺⁺⁺,Fe⁺⁺与胡敏酸结合的性质。计算机拟合的pH3.0⁵⁷Fe-胡敏酸络合物泥浆的穆斯堡尔谱和参数表明,三对四极双峰(图1,AA',BB',CC')是合理的。在pH3.0 Fe⁺⁺⁺以高自旋态存在,它和胡敏酸的结合多于一种环境类型。在pH1.0的⁵⁷Fe-胡敏酸络合物中,观察到了Fe⁺⁺⁺的穆斯堡尔谱信号,但没有Fe⁺⁺的信号,在这个样品离心分离出的液体部分检测到了加入量的59.7%的铁,表明在pH1.0时相当数量的Fe⁺⁺⁺被胡敏酸还原成Fe⁺⁺,Fe⁺⁺并不与胡敏酸牢固结合。在30伏电析过的⁵⁷Fe-胡敏酸络合物样品中(pH2.8)出现了Fe⁺⁺⁺,Fe⁺⁺的穆斯堡尔谱,这一结果指示出,在电析过程中由于⁵⁷Fe⁺⁺⁺-胡敏酸络合物悬浮液pH的降低,一部分Fe⁺⁺⁺还原成Fe⁺⁺,在pH2.8相当数量的Fe⁺⁺与胡敏酸牢固结合。根据在80K记录的穆斯堡尔谱,在pH1—3的范围出现了磁有序成份,但在室温记录的穆斯堡尔谱没有磁分裂。     

Interactions between Essential Nutrients with Platinum Group Metals in Submerged Aquatic and Emergent Plants

Increasing environmental concentrations of platinum group metals (PGMs), in particular platinum (Pt), rhodium (Rh) and palladium (Pd), from catalytic converters has been reported worldwide. The impact of these three metals on the uptake and use of essential mineral nutrients was examined using two plant models: the submerged aquatic plant, Elodea canadensis, and the terrestrial emergent plant, Peltandra virginica. Plants were grown for 2 weeks in nutrient solutions with either Pt⁴⁺ at concentrations between 0.05 and 5 mg/L, or a 0.1 mg/L Pt⁴⁺, Rh³⁺, Pd²⁺ mixture. Some treatments received additional Ca²⁺, Zn²⁺, or humic acid (with varying pH) to study how these conditions affected PGM uptake. Metal concentration analyses were conducted using a graphite furnace atomic absorption spectrometer (GFAAS) or an inductively coupled plasma emission spectrometer (ICP). Growth response was assessed through total chlorophyll content. There was significant Pt accumulation in plant tissues, from 55 to 326 times the concentration in nutrient solution. At pH 8, the addition of humic acid doubled Pt accumulation in comparison to the control. Additional exogenous minerals did not significantly affect PGM uptake, nor did the uptake of PGMs interfere with the uptake of Ca, Fe or Cu. Synthesis of chlorophyll in new shoots was not affected by Pt accumulation; however, visible chlorosis was observed in older shoots at 5 ppm Pt. Roadside Daucus carota samples from four heavy traffic locations in Dutchess County (New York) were also assessed for PGM content. Pt, Pd and Rh concentrations averaged 14.6, 10.2, and 0.7 μg/g, respectively.