Characterization of the Fe(III)—fulvic acid reaction by Mössbauer spectroscopy

Fulvic acids have been isolated from a sandy loam (Countesswells series) and a clay soil (Tipperty series) and the products of their reaction with different amounts of iron over a range of pH from 0.5 to 11 analyzed by Mössbauer spectroscopy. Three distinct types of spectral component were detected at 77 K, a sextet from magnetically dilute Fe(III) and doublets from Fe(II) and Fe(III), the last arising from both organic complexes and poorly crystalline oxide species. In iron-fulvic acid mixtures the proportion of iron as Fe(II) increased as the pH was lowered from 5 to 1 by the addition of hydrochloric or nitric acid at all Fe to fulvic acid ratios (1:5 to 1:500). When the pH was lowered below 1 the amounts of Fe(II) decreased with the lower Fe to fulvic acid ratios, but increased with the higher ratios. The amounts of the Fe(III) component contributing to a doublet signal decreased with decreasing Fe:fulvic acid ratios. At low iron concentrations the iron appears to be strongly bound to the fulvic acid, but when the iron content is of the order of 1–2% uncomplexed Fe(III) species can be present. At pH > 2 these are hydrolysed ions which form poorly-crystalline oxides at higher pH. This was confirmed by analysis of spectra at 4.2 K. At pH < 2 free ions are present in solution. In solutions with high fulvic acid contents (greater than 100-fold excess) the reactions with iron are completely reversible, but in solutions with a lower proportion of fulvic acid to iron, where free ions are present, there is a lack of reversibility.     

Influence of humate-solute interactions on aqueous heavy metal ion levels

The extent to which heavy metal ions (Cu, Pb, Zn, Cd) are removed from aqueous solution by humic acid suspensions has been found to vary with solution pH, concentration of competing cations, nature of the organic material, and the complexing power of any ligands present. The amount adsorbed in acid media increased with pH until the threshold value required for partial dissolution of the solid, and formation of soluble metal humates, was exceeded The adsorption maximum pH, and the apparent capacity at lower values, varied with the substrate used and cation being sorbed. The affinity order sequence, as derived from adsorption isotherm studies, was Pb > Cu > Cd > Zn ? Ca > Mg. The retention of metal ion by the solid was reduced in the presence of ligands, with zero uptake occurring when the soluble complexes formed had a greater effective stability than those resulting from humic acid-metal ion interactions. With environmental systems having a high organic content, the humic acid component can play a dominant role in determining the final distribution of metal ions, and for predictive purposes, investigation of the effect of pH on uptake is more informative than measurement of adsorption isotherm parameters, since the observed trends reflect differences in functional group properties, relative affinities and solubility effects. The curves have minima which fall within the pH spreads encountered in natural systems, and small pH changes can cause significant variations in solution levels.     

Fulvic acid studies: evidence for a polycarboxylate co-ordination mode at soil pH

The reactions of two fulvic acid samples with Cu²⁺ and Cd²⁺ have been studied by potentiometric titration (pH and ion selective electrode). The metal-uptake curves for samples initially rendered ‘metal-free’ by a precipitation-solvent extraction technique resemble those for citric and malic acids at soil pH (4–6.5). The presence of aliphatic and aromatic components was established by ¹³C nmr spectroscopy; the presence of phenolic components was inferred from pH titrations. It is shown that a polycarboxylate component (simulated by citrate) would mask reactions of phenolic components with metals in weakly acidic and neutral media. The mode of coordination between fulvic acid and metals will vary with pH and with the metal: fulvic acid ratio.     

The effect of pH on the retention of Cu,Pb, Cd,and Zn by clay-fulvic acid mixtures

The addition of fulvic acid to clay suspensions (kaolinite, illite or montmorrillonite) resulted in increased uptake of Cu, Pb, Cd, and Zn ions over the pH range 3 to 6, due to the limited solubility of one of the metal-fulvate species formed. At higher pH values, residual metal ion was retained in solution, instead of precipitating as hydroxy species. The amount of total metal ion found in solution at equilibrium was determined by the quantity and type of clay added; the amount of organic acid present; and to a lesser extent, pH. The behavior of the clay-fulvic acid systems differed from that observed using other organic materials such as gelatine, tannic acid or a humic acid.     

Copper(II) complexes with refractory anionic surfactants found in sewage sludge

Electron spin resonance (ESR) spectrometry was used to study Cu(II) complexes with anionic surfactant compounds of the types that have been detected in sewage sludge or in the fulvic acid fraction of anaerobically-digested sewage sludge and found to resist biodegradation in the soil environment. The ESR spectra of frozen (77 °K), aqueous solutions of linear alkyl benzene sulfonates and fatty alcohol sulfate esters at Cu-ligand molar ratios ranging from 0.1 to 1 exhibited anisotropic patterns indicative of a d _x 2?y ² groundstate of Cu(II) bound into innersphere complexes with the ligands arranged in square planar coordination. Sulfonate-type surfactants, both in the acid and salt forms, appeared to complex Cu(II) more efficiently than ester sulfate-type surfactants. Conventional physical parameters calculated from the ESR spectra were consistent with fully oxygenated, 4 O-ligand binding sites for the Cu(II) ions and indicated the formation of similar unidentate or bidentate complexes between Cu(II) and the surfactants at any Cu/ligand molar ratio investigated. The spectral lineshapes and related parameters of the Cu(II)-anionic surfactant complexes, however, were not very similar to those observed previously for Cu(II)-sewage sludge FA complexes. It was concluded that anionic surfactants involved in Cu(II) complexation by sewage sludge or sewage sludge fulvic acid do not behave as isolated, independent ligands, but instead may participate as co-ligands with other O-containing functional groups and/or as moities incorporated into the fulvic acid structure.     

Effects of phthalic and salicylic acids on Cu(II) adsorption by variable charge soils

In the present study, the effect of two substituted benzoic acids on Cu(II) adsorption onto two variable charge soils was investigated, with the emphasis on the adsorption and desorption equilibrium of Cu(II). Results showed that the presence of organic acids induced an increase in Cu(II) adsorption onto the two soils. The extent of the effect was related to the initial concentrations of Cu(II) and organic acid, the system pH, and the nature of the soils. The effect of organic acids was greater for Oxisol than for Ultisol. Phthalic acid affected Cu(II) adsorption to a greater extent than salicylic acid did. The effect of organic acids varied with pH. The adsorption of Cu(II) induced by organic acids increased with increasing pH and reached a maximum value at approximately pH 4.5, and then decreased. It can be assumed that the main reason for the enhanced adsorption of Cu(II) is an increase in the negative surface charge caused by the specific adsorption of organic anions on soils because the desorption of Cu(II) adsorbed in organic acid systems was greater than that for the control. The desorption of Cu(II) absorbed in both control and organic acid systems also increased with increasing pH; it reached a maximum value at pH ∼5.25 for control and salicylic acid systems and at pH ∼5.1 for a phthalic acid system, then decreased. This interesting phenomenon was caused by the characteristics of the surface charge of variable charge soils.     

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.     

Fulvic acids: structure and metal binding

Equilibrium calculations have been carried out at 25°C and an ionic strength of 0.04 to determine the predominant metal binding sites on a well-characterized fulvic acid for magnesium(ll), calcium(ll), manganese(ll), iron(lll), copper(ll) and zinc(II). Statistical estimates of the concentration of 14 sites are obtained from a random molecular model of fulvic acid. Protonation and metal formation constants are estimated from those of simple model ligands and corrected to the above conditions. Results indicate the importance of phthalate sites for all metals except iron(III), salicylate sites for iron(lll) and acetylacetonate sites for copper(ll). Iron (III) ions are found to form ML₂ type complexes extensively. The affinity of the metal ions for fulvic acid as a whole is found to decrease in the order iron(III) > copper(II) > zinc(II) > manganese(Il) > calcium(II) > magnesium (II).     

Stability of ferric complexes with 3-hydroxyflavone (flavonol), 5,7-dihydroxyflavone (chrysin), and 3',4'-dihydroxyflavone

The acid dissociation and ferric stability constants for complexation by the flavonoids 3-hydroxyflavone (flavonol), 5,7-dihydroxyflavone (chrysin), and 3',4'-dihydroxyflavone in 50:50 (v/v) ethanol/water are determined by pH potentiometric and spectrophotometric titrations and the linear least-squares curve-fitting program Hyperquad. Over the entire range of pH and reagent concentrations spanning the titration experiments, the stoichiometry for iron-flavonoid complex formation was 1:1 for all three flavonoids examined. The three flavonoids were chosen for their hydroxy substitution pattern, with each possessing one of the three most commonly suggested sites for metal binding by the flavonoids. On the basis of the calculated stability constants, the intraflavonoid-binding site competition is illustrated as a function of pH via speciation curves. The curves indicate that the binding site comprised of the 3',4'-hydroxy substitutions, the catecholic site, is most influential for ferric complexation at the physiological pH of 7.4. The possibility for antioxidant activity by flavonoid chelation of ferric iron in the presence of other competitive physiological complexing agents is demonstrated through additional speciation calculations.     

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.     

Fluorescence analysis of copper(II) and zinc(II) binding behaviour of fulvic acids from pig slurry and amended soils

The evaluation of the mechanisms and extent of interaction of Cu(II) and Zn(II) with fulvic acids (FAs) is of importance for understanding the mobility and bioavailability of these metals in pig slurry (PS)‐amended soils. In this work, a fluorescence titration method and a single site model were used to characterize the interaction of Cu(II) and Zn(II) with FAs isolated from PS, from soils amended with 90 and 150 m³ ha⁻¹ year⁻¹ of PS for 7 years and from the corresponding unamended soil. The binding capacities of FAs and the strengths of metal ion‐FA complexes were larger for Cu(II) than for Zn(II). With respect to the unamended control soil FA, PS‐FA was characterized by smaller binding capacities and stability constants. The binding capacities and affinities of PS‐amended soil FA were intermediate between those of unamended soil FA and PS‐FA, and tended to decrease with increasing amounts of PS applied to soil, thus suggesting a partial incorporation of FA fractions of PS into native soil FA.     

Influence of pH on copper, lead and cadmium binding by an ombrotrophic peat

The effect of pH on the adsorption of copper (Cu), lead (Pb) and cadmium (Cd) by a peat soil was studied, and the results compared with those corresponding to cation binding by a dissolved peat humic acid (HA), and interpreted with a NICA–Donnan model. A potentiometric titration technique was used to determine the adsorption isotherms for H⁺, at different ionic strengths, and for Cu²⁺, Pb²⁺ and Cd²⁺ at different pH values, in a peat soil. The effect of the ionic strength on proton binding was similar for the soil (solid) organic matter and for dissolved HA. The adsorption isotherms for cation–peat and the binding curves cation–dissolved HA are almost parallel, although more cation was adsorbed per kg of C in the dissolved HA. The effect of pH on cation binding is similar for dissolved organic matter and for the organic soil. At low metal concentration the amount of adsorbed metal followed the order Cu²⁺ > Pb²⁺ > Cd²⁺. The cation-binding parameters obtained with the NICA–Donnan model allow excellent simulation of the effect of pH on the adsorption of Cu, Pb and Cd ions in the studied peat soil. The binding constants for the peat suspension were greater than the corresponding generic parameters for dissolved HA. Speciation calculations showed that for Cu and Pb, the most abundant fraction was the metal adsorbed on peat, whereas for Cd the most abundant fraction was dissolved metal.     

Competition from Cu(II), Zn(II) and Cd(II) in Pb(II) Binding to Suwannee River Fulvic Acid

This is a study of trace metal competition in the complexation of Pb(II) by well-characterized humic substances, namely Suwannee River Fulvic Acid (SRFA) in model solutions. It was found that Cu(II) seems to compete with Pb(II) for strong binding sites of SRFA when present at the same concentration as Pb(II). However, Cd(II) and Zn(II) did not seem to compete with Pb(II) for strong binding sites of SRFA. These two metals did compete with Pb(II) for the weaker binding sites of SRFA. Heterogeneity of SRFA was found to play a crucial role in metal–SRFA interactions. The environmental significance of this research for freshwater is that even at relatively low Pb(II) loadings, the metals associated with lead in minerals, e.g. Cu(II), may successfully compete with Pb(II) for the same binding sites of the naturally occurring organic complexants, with the result that some of the Pb(II) may exist as free Pb²⁺ ions, which has been reported to be one of the toxic forms of Pb in aquatic environment.     

Hydrolysis of naptalam and structurally related amides: inhibition by dissolved metal ions and metal (hydr)oxide surfaces.

In metal ion-free solutions, the secondary amide naptalam hydrolyzes more rapidly as the pH is decreased; intramolecular nucleophilic attack by a carboxylate side group is very likely involved. Millimolar levels of dissolved Cu(II) and Zn(II) inhibit hydrolysis between pH 3.6 and pH 6.5. Metal ion-naptalam complex formation is important since addition of the competitive ligand citrate lessens the inhibitory effect. The metal (hydr)oxide surfaces Al(2)O(3) and FeOOH inhibit naptalam hydrolysis to a lesser degree; inhibition is proportional to the extent of naptalam adsorbed. Secondary amides (propanil, salicylanilide, and N-1-naphthylacetamide) and tertiary amides (N-methyl-N-1-naphthylacetamide, furalaxyl, and N, N-diethylsalicylamide) that lack carboxylate side groups do not hydrolyze within 45 days of reaction, even when millimolar Cu(II) concentrations are present. Tertiary amides possessing carboxylate side groups (N,N-diethyl-3,6-difluorophthalamic acid and N, N-dimethylsuccinamic acid) do hydrolyze but are insensitive to the presence or absence of Cu(II). The inhibitory effect is believed to occur via metal coordination of (1) the carbonyl group of naptalam, which induces deprotonation of the amide group and makes the substrate less reactive toward nucleophilic attack; (2) the free carboxylate group of naptalam, which blocks intramolecular nucleophilic attack; or (3) a combination of the two.     

Modelling trace metal extractability and solubility in French forest soils by using soil properties

Soil/solution partitioning of trace metals (TM: Cd, Co, Cr, Cu, Ni, Sb, Pb and Zn) has been investigated in six French forest sites that have been subjected to TM atmospheric inputs. Soil profiles have been sampled and analysed for major soil properties, and CaCl₂‐extractable and total metal content. Metal concentrations (expressed on a molar basis) in soil (total), in CaCl₂ extracts and soil solution collected monthly from fresh soil by centrifugation, were in the order: Cr > Zn > Ni > Cu > Pb > Co > Sb > Cd , Zn > Cu > Pb = Ni > Co > Cd > Cr and Zn > Ni > Cu > Pb > Co > Cr > Cd > Sb , respectively. Metal extractability and solubility were predicted by using soil properties. Soil pH was the most significant property in predicting metal partitioning, but TM behaviour differed between acid and non‐acid soils. TM extractability was predicted significantly by soil pH for pH < 6, and by soil pH and Fe content for all soil conditions. Total metal concentration in soil solution was predicted well by soil pH and organic carbon content for Cd, Co, Cr, Ni and Zn, by Fe content for Cu, Cr, Ni, Pb and Sb and total soil metal content for Cu, Cr, Ni, Pb and Sb, with a better prediction for acidic conditions (pH < 6). At more alkaline pH conditions, solute concentrations of Cu, Cr, Sb and Pb were larger than predicted by the pH relationship, as a consequence of association with Fe colloids and complexing with dissolved organic carbon. Metal speciation in soil solutions determined by WHAM‐VI indicated that free metal ion (FMI) concentration was significantly related to soil pH for all pH conditions. The FMI concentrations of Cu and Zn were well predicted by pH alone, Pb by pH and Fe content and Cd, Co and Ni by soil pH and organic carbon content. Differences between soluble total metal and FMI concentrations were particularly large for pH < 6. This should be taken into account for risk and critical load assessment in the case of terrestrial ecosystems.     

Interactions of fulvic acid with aluminium and a proto‐imogolite sol: the contribution of E‐horizon eluates to podzolization

The podzolization process is examined in the light of measurements of the solubility characteristics of aluminium fulvate, the extent of dissolution of a proto‐imogolite sol by fulvic acid, the adsorption capacity of proto‐imogolite for fulvic acid and aluminium fulvate, and published evidence. Fulvic acid at 500 mg l^?1 acting on a proto‐imogolite (PI) preparation containing 0.95 mmol l^?1 Al as PI did not bring enough Al into solution at pH 4.5–5.0 over 4–15 months to cause significant precipitation of the fulvic acid. As allophanic Bs horizons of podzols typically have pH ≥ 4.8, fulvic acids entering them in drainage water cannot be quantitatively precipitated by dissolution of Al from the allophane. They are, however, strongly absorbed on the allophane, and this must be the mechanism that removes most of the fulvic acid at the top of the Bs horizon, and which contributes, along with colloidal humus and root decomposition, to the formation of a Bh horizon. We conclude that fulvic acid plays no active role in podzolization, but only recycles Al and Fe, that have been transferred by biological processes to the O horizon, back to the Bh horizon. The podzolization process, which leads to the formation of an allophanic Bs horizon underlying a progressively deepening E horizon, requires the dissolution of Al‐humate and allophanic precipitates at the Bh–Bs interface as well as progressive attack on the more readily weatherable minerals. Inorganic acids, particularly episodic fluxes of nitric acid, could play a major role in this, as well as attack by readily metabolized complexing acids such as oxalic and citric acids released by roots and fungi. In addition to throwing light on the podzolization process, the experimental results provide an explanation of the lower limit to C:Al ratios reported in natural waters, and a check on the applicability of the WHAM chemical equilibrium model to Al–fulvate–proto‐imogolite equilibria. In Ca‐containing fulvate solutions, Al‐fulvate begins to precipitate when C:Al falls below 50, which is also the limiting ratio observed in natural waters. WHAM calculations overestimate by 70–85% the amount of Al‐fulvate formed over 4 months at pH 4.5–5.0 in Ca‐containing fulvate–imogolite systems.     

三种滴灌肥对草莓生长、产量及品质的影响

为了研究滴施氨基酸、全生物有机肥和黄腐酸复合肥对草莓生长、产量及品质的影响.在克拉玛依白碱滩区设常规对照处理基础上,追施氨基酸、全生物有机肥和黄腐酸复合肥3种有机肥,共4个处理,3次重复,随机排列,分析3种滴灌肥与草莓特性之间的关系.结果表明:3种滴灌肥能够显著提高草莓株高、单株叶面积和全株生物量,成熟期分别提高18....     

Extraction of copper,lead, zinc or cadmium ions sorbed on calcium carbonate

The amount of sorbed metal ion released from CaC0₃ by 16 different extractants was found to vary with the chemical nature of the solution and the metal ion involved. In general, acid solutions dissolved a high proportion of both substrate and Cu, Ph, Cd coatings; complexing agents dissolved the same coatings but left most of the calcite; and competing cations (e.g. NH₄ ⁺, Ca²⁺) displaced primarily chemisorbed Cd and Cu. In Zn studies, little metal ion was retrieved by any extractant due to the limited solubility of the coatings formed at pH < 7.7. The diverse behavior observed in the sorption studies has been interpreted in terms of solubility and absorption equilibria. The pH of the CaC0₃ suspensions was high enough to precipitate all added Pb as hydroxy species, and excess Cu tended to precipitate at pH > 6.4 if one increased the soluble carbonate level (e.g. by adding acid). Unlike Cd and Cu, Zn was not chemisorbed; it formed sparingly soluble compounds such as ZnC0₃.2Zn(OH)₂, with excess coming out as Zn(OH)₂ at pH > 7.7. The significance of the results in respect to the mobility of metal ions in calcareous soils, and the evaluation of available levels, has been considered.     

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.     

CHEMICAL AND INFRARED SPECTROSCOPIC STUDIES OF FULVIC ACID FRACTIONS FROM A PODZOL

Infrared spectroscopy and chemical degradation indicated several fulvic acid fractions from an iron-humus podzol capable of complexing and translocating metals. Acidic polysaccharides, some of them resembling pectic acid, were isolated in appreciable quantities. These are able to bind metals through carboxyl groups, but peptide components present in the polysaccharide fraction may provide alternative metal complexing centres. The high yield of acid-extractable organic matter from the illuvial humus, together with its high content of carboxylic acid groups, make it likely that this fraction plays a major role in the translocation of metals. Phenolic compounds allegedly associated with this process were present in the fulvic acid solution in low amounts, and only in the extracts from the organic horizons.