Fulvic Acid Complexes Research Articles

Impact of natural organic matter on H 2O 2-mediated oxidation of Fe(II) in a simulated freshwater system

The oxidation of Fe(II) by H 2O 2 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 H 2O 2 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 O 2 and H 2O 2. It is suggested that rapid formation of a weak Fe(II)–fulvic acid complex that is not readily oxidized by H 2O 2 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 O 2 and H 2O 2 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 H 2O 2 concentrations, significantly increasing the lifetime of ferrous iron and reducing the flux of hydroxyl radicals produced through this oxidation pathway.

Fourier transform ion cyclotron resonance mass spectral characterization of metal‐humic binding

The interaction between metals and naturally occurring humic substances and the thereby induced issues of bioavailability and hydrogeochemical turnover of metal ions in natural waters have been the subject of intense study for decades. Traditional bulk techniques to investigate metal-humic binding (e.g. potentiometry and inductively coupled plasma mass spectrometry (ICP-MS)) can provide quantitative results for the relative abundance and distribution of metal species in humic samples and/or overall binding constants. The shortcoming of these bulk techniques is the absence of structural detail. Ultra-high-resolution mass spectrometry, currently the only technique demonstrated to resolve individual humic ions, is not generally employed to provide the missing qualitative information primarily because the identification of metal complexes within the already complex mixtures of humic substances is non-trivial and time-consuming to the extent of eliminating any possibility for real-time manipulation of chelated analytes. Here, it is demonstrated that with tailored selection of the metal ion, it is possible to visually identify large numbers of metal-humic complexes ( approximately 500 for Be2+, approximately 1100 for Mn2+, and approximately 1500 for Cr3+) in real-time as the spectra are being acquired. Metal ions are chosen so that they form primarily even-m/z complexes with humic ions. These even-m/z complexes stand out in the spectrum and can readily be characterized based on molecular formulae, which here revealed for example that Suwannee River fulvic acid (SRFA) complexes encompassed primarily highly oxygenated fulvic acids of relatively low double-bond equivalence. Facile, real-time identification of even-m/z metal-humic complexes additionally allows for the specific selection of metal-humic complexes for MS(n) analysis and in-trap ion-neutral reactions enabling investigation of metal-humic complex structure. MS/MS data were collected to demonstrate the potential of the technique as well as highlight some of the remaining challenges.

フルボ酸第二鉄錯体とシデロフォアＤｅｓｆｅｒｒｉｏｘａｍｉｎｅ Ｂのリガンド交換速度論モデルの構築

As a ligand exchange kinetic model of an Fe(III)-fulvic acid complex and a siderophore, desferrioxamine B (DFOB) was developed. The model includes two pathways of Fe(III)-DFOB formation, (1) a disjunctive pathway where DFOB binds to unchelated Fe(III) following the dissociation of Fe(III) from Fe(III)-fulvic acid and (2) an adjunctive pathway where Fe(III)-DFOB is formed via the direct association of DFOB with an Fe(III)-fulvic acid complex. Iron binding sites in fulvic acid were classified into two ligand classes on the basis of binding strength. Fitting the kinetic model to experimental observation with a spectrophotometer, the rate constant of ligand exchange for a strong binding site [k1 (M-1 · s-1)] was determined to be k1 = 4.7 × 103 / [FA]T + 5.5 × 102, where [FA]T (mg · l-1) denotes the total fulvic acid concentration. The rate constant for a weak binding site [k2 (M-1 · s-1)] was estimated to be greater than 1 × 105. The model calculation provided profiles of ligand exchange rate and the amount of Fe(III) bound to each ligand class (relative ratio to total Fe), which are functions of fulvic acid concentration and both of fulvic acid and iron concentrations, respectively.

Superoxide-mediated Fe(II) formation from organically complexed Fe(III) in coastal waters

Fe(III) complexed by organic ligands (Fe(III)L) is the primary form of dissolved Fe in marine and coastal environments. Superoxide, typically produced in biological and photochemical processes, is one of the reducing agents that contributes to transformation of Fe(III)L to bioavailable, free dissolved Fe(II) (Fe(II)′). In this work, the kinetics of superoxide-mediated Fe(II)′ formation from Fe(III)L in a simulated coastal water system were investigated and a comprehensive kinetic model was developed using citrate and fulvic acid as exemplar Fe-binding ligands. To simulate a coastal environment in laboratory experiments, Fe(III)L samples with various ligand/Fe ratios were incubated for 5 min to 1 week in seawater medium. At each ratio and incubation time, the rate of superoxide-mediated Fe(II)′ formation was determined in the presence of the strong Fe(II) binding ligand ferrozine by spectrophotometrically measuring the ferrous–ferrozine complex generated at a constant concentration of superoxide. The Fe(II)′ formation rate generally decreased with incubation time, as Fe(III)L gradually dissociated to form less reactive Fe(III) oxyhydroxide. However, when the ligand/Fe ratio was sufficiently high, the dissociation of Fe(III)L (and subsequent Fe precipitation) was suppressed and Fe(II)′ was formed at a higher rate. The rate of Fe(II)′ produced during the experiment was explained by the kinetic model. The model confirmed that both the ligand/Fe ratio and incubation time have a significant effect on the pathway via which Fe(II)′ is formed from Fe(III)–fulvic acid complexes.

In vitro toxicology assessment of cadmium bioavailability on primary porcine oviductal epithelial cells

This study assessed the in vitro effects of cadmium ion (Cd 2+) and cadmium–fulvic acid complexes (Cd 2+–FA) on porcine oviductal epithelial cells (POEC) using transmission electron microscopy (TEM). Fresh POEC were cultured for 24 h and then exposed for 3 h in the tested solutions. Absorbed cadmium was analyzed by graphite furnace atomic absorption spectrophotometer (AAS). Dissociation and exchange rate constants were determined using the competing ligand, Chelex-100 and predicted cadmium species were calculated using the MINTEQA2 program. The results showed that the dissociation rate constant of Cd 2+–FA was equal to 1.023 × 10 −3 s −1 which was slower than the exchange rate at 2.062 × 10 −3 s −1. Nevertheless, the absorbed concentrations of Cd 2+ and Cd 2+–FA of POEC were similar and were 47 ± 6.25 μg L −1 and 54 ± 3.61 μg L −1, respectively. Although both levels of absorptions were not significantly different ( t-test p = 0.168 at α 0.01), their morphological effects as examined by TEM were substantially different with the effects being most marked with Cd 2+ > Cd 2+–FA > FA. Aggregations around nuclei and nuclear membranes were observed with FA treatment whilst Cd 2+–FA treatment produced more cytoplasmic damage. Cd 2+ treatment caused nuclear deformities. In conclusion, FA appears to penetrate the cells but was less likely to enter the nucleus. It also reduced the toxicity of Cd 2+ as the nuclei from the Cd 2+–FA treatment appeared normal. Nevertheless, some Cd 2+ could still enter the nucleus. This might be because there was still 67.8% Cd 2+ left unbound from the Cd 2+–FA treatment as calculated from the MINTEQA2 program compared to 99.1% Cd 2+ with Cd 2+ treatment, thus underlining the inherent toxicity of soluble cadmium ion.

Development and application of the diffusive gradients in thin films technique for the measurement of total dissolved inorganic arsenic in waters

The diffusive gradients in thin films (DGT) technique, utilizing an iron-hydroxide adsorbent, has been investigated for the in situ accumulation of total dissolved inorganic As in natural waters. Diffusion coefficients of the inorganic As V and As III species in the polyacrylamide gel were measured using a diffusion cell and DGT devices and a variety of factors that may affect the adsorption of the As species to the iron-hydroxide adsorbent, or the diffusion of the individual As species, were investigated. Under conditions commonly encountered in environmental samples, solution pH and the presence of anions, cations, fulvic acid, Fe III–fulvic acid complexes and colloidal iron-hydroxide were demonstrated not to affect uptake of dissolved As. To evaluate DGT as a method for accumulation and pre-concentration of total dissolved inorganic As in natural waters, DGT was applied to two well waters and a river water that was spiked with As. For each sample, the concentration obtained with use of DGT followed by measurement by hydride generation atomic absorption spectrometry with a Pd modifier (HG-AAS) was compared with the concentration of As measured directly by HG-AAS. The results confirmed that DGT is a reliable method for pre-concentration of total dissolved As.

Transformation dynamics and reactivity of dissolved and colloidal iron in coastal waters

We have investigated the chemical forms, reactivities and transformation kinetics of Fe(III) species present in coastal water with ion exchange and filtration methods. To simulate coastal water system, a mixture of ferric iron and fulvic acid was added to filtered seawater and incubated for a minute to a week. At each incubation time, the seawater sample was acidified with hydrochloric acid and then applied to anion exchange resin (AER) to separate negatively charged species (such as fulvic acid, its complexes with iron and iron oxyhydroxide coated with fulvic acid) from positively charged inorganic ferric iron (Fe(III)′). By monitoring the acid-induced Fe(III)′ over an hour, it was found that iron complexed by fulvic acid dissociated rapidly to a large extent (86–92% at pH 2), whereas amorphous ferric oxyhydroxide particles associated with fulvic acid (AFO-L) dissociated very slowly with the first-order dissociation rate constants ranging from 6.1 × 10 − 5 for pH 3 to 2.7 × 10 − 4 s − 1 for pH 2. Therefore, a brief acidification followed by the AER treatment (acidification/AER method) was likely to be able to determine fulvic acid complexes and thus differentiate the complexes from the AFO-L particles (the dissolution of AFO-L was insignificant during the brief acidification). The acidification/AER method coupled with a simple filtration technique suggested that the iron–fulvic acid complexes exist in both the < 0.02 μm and 0.02–0.45 μm size fractions in our coastal water system. The truly dissolved iron (< 0.02 μm) was relatively long-lived with a life-time of 14 days, probably due to the complexation by strong ligands. Such an acid-labile iron may be an important source of bioavailable iron in coastal environments, as a significant relationship between the chemical lability and bioavailability of iron has been well recognised.

Effects of a dietary complex of humic and fulvic acids (FeedMAX 15™) on the health and production of feedlot cattle destined for the Australian domestic market

To examine the effects of a dietary humic and fulvic acid complex, FeedMAX 15, on the health, growth rate, feed conversion ratio, and carcase characteristics of feedlot cattle. Cattle, in eight pens of 125 animals each, were fed either a diet containing a humic and fulvic acid complex (FeedMAX 15, FeedMAX Industries, Toowoomba, Queensland) or the same diet without the additive. Control or FeedMAX 15 diets were allocated to each pen at random. Individual cattle were allocated alternately to control or treatment pens based on order of presentation. Comparisons of disease incidence, mortality, feed intake, growth rate, feed conversion ratio, fat depth, dressing percentage, meat colour, fat colour and marbling were made at the conclusion of the feeding period. No differences were found between cattle fed FeedMAX 15 and cattle not fed the additive in entry body weight (P = 0.99), exit body weight (P = 0.91), dressing percentage (P = 0.66), P8 fat depth (P = 0.57), meat colour (P = 0.67), marbling (P = 0.70), all diseases (P = 0.64), bovine respiratory disease (P = 0.91), or mortalities (P = 1.0). Cattle fed FeedMAX 15 reached the market specifications for body weight and fat depth in fewer mean days (P = 0.0001), had a greater average daily gain (P = 0.05), a lower feed conversion ratio (P = 0.05) and whiter fat (P < 0.0001). Feeding the humic and fulvic acid complex, FeedMAX 15, at 0.055 g per kg body weight per day, can increase growth rate and feed conversion efficiency in feedlot cattle.

Effect of pH and Fulvic Acid on the Sorption and Diffusion of Europium Ions in Compacted Bentonite as Studied by the Capillary Method

The effect of pH and fulvic acid on the diffusion and sorption of Eu(III) ions in compacted bentonite (ρb = 1120 ± 20 kg/m3) was studied via an in-diffusion method at an ionic strength of 0.1 M NaClO4. The results (Kd values derived from the first slice and theoretical calculations, apparent and effective diffusion coefficients, i.e. Da and De) derived from the capillary method were in good agreement with data obtained under similar conditions quoted in the literature and indicated that the diffusion and sorption of Eu(III) ions in compacted bentonite was dependent on the pH value. In the presence of fulvic acid (FA), EuIII–FA complexes were formed at the surface of the compacted bentonite. Hence, the presence of FA led to a reduction in the diffusion of Eu(III) ions in compacted bentonite. It may be concluded that humic substances act as a significant sink for Eu(III) ions in compacted bentonite. The value of Da derived experimentally was used to simulate the diffusion of Eu(III) ions in compacted bent...

Metal Logarithmic Scale Titration as a Tool for Complexing Ligand Distribution Determination : an Application by DPASV

A new logarithmic scale titration is proposed for the characterisation of natural organic matter-trace metals interactions in natural systems. The Suwannee River Fulvic Acid complexation with Cd and Pb ions has been analysed by this technique, using Differential Pulse Anodic Stripping Voltammetry (DPASV) for labile metal concentrations measurement. Results of titrations have been modelled by four ligands. Their complexing properties (CLi, Ki Cd, Ki Pb) have been determined by a non-linear optimisation based on the speciation program MINEQL, coupled with a simplex. These ligands were called "very weak, weak, strong and very strong" with respect to their complexing constants (Ki Cd, Ki Pb) ranging from 104 to 1011. The obtained ligands concentrations (CLi) are respectively 1.9 μM, 150 nM, 25.1 nM and 21.1 nM for a 7.9 ppm carbon content. Moreover, this model takes account of the pH dependency and metals competition respectively by experiments at pH 7.8 and 4.6, and by definition of stability constants for each ligand toward each analysed metal. As a conclusion, the authors suggest a systematic use of the logarithmic scale titrations when full characterisation of the metal-organic matter interactions is necessary.

Determination of neodymium–fulvic acid binding constants by capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS)

Capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS) has been used to study neodymium–Suwannee river fulvic acid (FA) interaction at pH 6–9 and 0.001–0.1 mol L−1 ionic strength. The use of a strong ligand (EDTA) competition method allowed us to succesfully manipulate a bimodal species distribution of NdFA and NdEDTA. These two species have sufficiently different charge and shape characteristics to be easily separable by CE, even in the presence of nebulizer induced siphoning. The NdFA and NdEDTA complexes were found to be stable on the timescale of a CE separation and quantitative artifacts due to complex dissociation were therefore negligible. A method development scheme for quantitative CE-ICP-MS addresses metal–ligand dissociation specifically and is oulined in this study. Siphoning induced laminar flow was optimized and used to our advantage by speeding up separations. Log values of conditional binding constants for Nd–FA complexation at 0.1 mol L−1 ionic strength varied gradually from 9.1 to 12.5 over the pH range 6–9. Detection limits for Nd speciation were 500 pM.

The effects of pH, ionic strength, and iron–fulvic acid interactions on the kinetics of non-photochemical iron transformations. I. Iron(II) oxidation and iron(III) colloid formation

Flow injection analysis was used to study the effect of a fulvic acid on the kinetics of iron(II) oxidation and iron colloid formation under conditions approximating fresh natural waters. While iron(II) oxidation in high-carbonate inorganic solutions is predicted well by a recently proposed homogeneous model, it overestimates the oxidation rate in low-carbonate solutions, possibly due to the formation of an intermediate iron(II) colloid or surface species. Results in fulvic acid solutions are consistent with the formation of an iron(II)–fulvic acid complex at both pH 6.0 and 8.0 which accelerates the overall oxidation rate relative to inorganic solutions. However, iron(III) complexation by fulvic acid greatly slows the formation of iron colloids, stabilizing dissolved iron(III). Decreased pH and increased ionic strength slow and decrease iron colloid formation. Evidence of a kinetic control on the distribution of iron(III) between organically complexed and colloidal forms is presented.

The effects of pH, ionic strength, and iron–fulvic acid interactions on the kinetics of non-photochemical iron transformations. II. The kinetics of thermal reduction

A combination of flow-injection analysis and kinetic analysis was used to examine the speciation of iron(II) and iron(III) in fulvic acid solutions as a function of pH, ionic strength, and time. This methodology was used to follow a shift in iron speciation from faster to slower reacting species over a timescale of several days. This speciation data shows that both iron(II) and iron(III)–fulvic acid complexes are important iron species in humic-containing natural waters and that their amounts and their rates of transformation to colloidal iron are controlled primarily by the kinetics of thermal (dark) reduction and iron(II) oxidation. The kinetic analysis methodology also yielded the rate constants for the thermal reduction of iron by the fulvic acid. These rate constants decrease with increasing pH and are independent of ionic strength. While thermal reduction was found to be too slow to produce large amounts of steady state iron(II) at circumneutral pH, it does provide a mechanism for iron redox cycling in the absence of photochemical or biochemical processes.

Lability of metal ion-fulvic acid complexes as probed by FIA and DGT: a comparative study

Two kinetic-based analytical techniques with very different time constants (flow injection analysis, FIA and diffusive gradients in thin films, DGT) have been compared in a study of metal ion lability in the systems Al 3+–FA and Cu 2+–FA (fulvic acid, FA). Flow injection analysis used an in-line micro-column of chelating ion exchanger to capture the labile metal fraction during the short contact time, 1–3 s, with the injected sample. The moderately labile and inert metal fractions were rejected by the ion exchanger but the former gave a colorimetric reaction down-line. The labile fraction was quantified by subsequent elution of captured metal ions and down-line analysis. The chelating resins used were oxine-derivatised fractogel (for Al 3+) and oxine-derivatised controlled-pore glass (for Cu 2+). Diffusive gradients in thin films utilised cross-linked polyacrylamide films as the diffusive barrier and Chelex-100 to capture the diffusing metal ions that are labile on the experiment time-scale (min). Diffusion coefficients, D, for metal–FA systems at different metal:FA ratios were measured independently in a diffusion cell. They indicated that for both the labile (Cu 2+–FA) and slowly labile (Al 3+–FA) systems the metal ion diffuses at a similar velocity to the FA, even when the total metal ion concentration exceeds the capacity of FA to bind metal ions in non-labile forms (FA complexation capacity, CC). Complexation capacity was used as a basis for comparison of the two techniques. It was observed that for the less labile Al 3+–FA system, the DGT–labile Al 3+ equated to the sum of the labile+moderately labile fractions determined by FIA. For the Cu 2+–FA system the CC determined by DGT was smaller than that determined by FIA (significant at 1S.D.). Further, the results indicated that D values measured for labile metal–FA complexes in a diffusion cell may not be appropriate for interpretation of diffusion processes that occur in the DGT experiment.

An Electrochemical Investigation of Complexation of Pb(II) by a Well‐Characterized Fulvic Acid in Model Systems – Effect of Competition with Major Cations and Trace Metals

AbstractEffect of competition of major cations, Ca2+, Mg2+, and trace metals, Cu2+, Ni2+ and Zn2+, on the stability of the Pb(II)‐fulvic acid (FA) complexes in model systems at constant pH and ionic strength was investigated by square‐wave anodic stripping voltammetry (SWASV). The results show that the trace metals compete with the target trace metal, Pb(II), for binding sites of the FA even when they are present in the same concentration as that of Pb(II), whereas the major cations compete with Pb(II) only when they are present in massive excess.

Analysis of copper and calcium–fulvic acid complexation and competition effects

The binding of Cu 2+ and Ca 2+ to a fulvic acid (FA), simulating naturally occurring conditions, was studied. Furthermore, copper–FA complexation in the presence and absence of calcium was compared. For this, potentiometric titrations were carried out using a solution of FA at a concentration of 100 mg L −1, and of ionic strength 0.1 M. The Ca 2+–FA complexation reaction was carried out at pH 6.5, 7.5 and 8.5 and the Cu 2+–FA complexation reaction at pH 5.5 and 6.5, in both the presence and absence of calcium ion. The calcium ion had a significant effect on copper binding at [Ca 2+]⩾2.5×10 −3 M. The experimental binding curves were analyzed using non-electrostatic discrete site and continuous distribution models. Competition by the calcium ion mainly affected the maximum binding capacity of the Cu 2+–FA complexation, whereas the slight effect observed on the binding constant appears to indicate that calcium only competes for specific copper sites, thus modifying the binding sites distribution function. It was also found that the number of sites occupied by the copper ion represented only 20% of the total concentration of acid groups ionized on FA at the pH values studied.

Analysis of the effect of pH on Cu2+-fulvic acid complexation using a simple electostatic model.

A simple electrostatic model was used to study the effect of pH on the binding of Cu2+ to fulvic acid in solutions containing similar amounts of dissolved organic carbon (DOC) as natural media, such as aquatic environments and soil solutions. Complexation behavior was affected by increased pH because of changes in the electrostatic interaction resulting from an increase in the negative charge on the fulvic acid molecule. Solutions of soil-extracted fulvic acid (FA), at concentrations of 25 and 35 mg L(-1), ionic strength 0.005 M, and pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5, conditions that simulate those of natural freshwaters, were titrated with copper ion using differential pulse anodic stripping voltammetry. Assuming the formation of 1:1 complexes, the conditional binding parameters (stability constant and binding capacity) were calculated for each pH value. Use of a 1:1 electrostatic model allowed estimation of the contribution of the electrostatic effect to the ion binding reaction, at each pH value, as well as calculation of a binding constant that was not dependent on pH and which thus represented the contribution of the chemical heterogeneity. Furthermore, it was found that only a small proportion of the FA acid functional binding sites are involved in the formation of copper complexes.

Low metal bioavailability in a contaminated urban site

Bioavailability of Cd, Cu, Ni, Pb, and Zn in a metal-enriched railway yard in Montréal, Québec, Canada was assessed using metal speciation, plant uptake, and microbial assays. Metal speciation of extracted soil solutions was estimated using the Windmere Humic Aqueous Model. In soil solutions, free Cd, Ni, and Zn ions represented as much as 80%, 72%, and 62%, respectively, of the total dissolved metals. Copper and Pb were strongly bound by dissolved organic matter, and metal-fulvic acid complexes represented as much as 99% of the total dissolved metals. Three field-collected plant species (dandelion, bladder campion, and chicory) varied in their tendency to accumulate metals in either their leaves or roots. Chicory grown in the greenhouse had significantly higher metal bioconcentration factors than wild chicory. Although the site studied is considered to be contaminated, no metal pool, such as free ions or dissolved or total soil metals, consistently predicted metal uptake by potted chicory. Regression analysis revealed that soil total metal concentrations could adequately predict tissue accumulations of Cd, Ni, Pb, and Zn in bladder campion but only Cd and Zn in dandelion. Data from microbial assays also showed that the soil respiration was not affected by the metal contamination, but that nitrification was inhibited for the most contaminated soils. These results indicate that the metal bioavailability in the railway yard is low, but they also suggest that nitrogen cycling may be affected.

Fluorescence and FT-IR spectroscopic studies of Suwannee river fulvic acid complexation with aluminum, terbium and calcium

In this study fluorescence emission and IR spectroscopy have been used to investigate the interaction of the class A (oxygen seeking ‘hard acid’) metal Al 3+, with Suwannee River fulvic acid. Addition of Al 3+ ion results in a significant enhancement in fulvic acid fluorescence emission (at λ em=424 nm) and significant red shift of the excitation wavelength (from λ ex=324 nm to λ ex=344 nm) at low pH values (pH∼4.0–5.0). At pH 4.0 (0.1 M ionic strength), where the predominant aluminum ion species is the ‘free’ (aquo) ion, the fulvic acid fluorescence reaches 142% of the value in the absence of added metal ion. Analysis of the pH 4.0 and pH 5.0 fluorescence enhancement data with the nonlinear (single site) model of Ryan and Weber indicated binding constants in the range of 4.67·10 4–2.87·10 6 M −1 and concentrations of ligand sites in the range of 18.6·10 −6–24.0·10 −6 M, both consistent with previous studies performed on both aquatic and soil fulvic acids. Companion fluorescence experiments performed on two other class A metal ions, Ca 2+ and Tb 3+, indicated no significant enhancement or quenching with Ca 2+ and only slight quenching with Tb 3+. Comparison of FT-IR spectra collected on fulvic acid alone and fulvic acid in the presence of the three class A metals (Al 3+, Ca 2+ and Tb 3+) provides strong evidence for the involvement of carboxyl carbonyl functions in the binding of all three metal ions, which is not unexpected. The spectra also reveal, however, a very pronounced difference in the 4000–2000 cm −1 IR spectral region between the Al 3+ spectrum and the Ca 2+ and Tb 3+ spectra. The –OH stretch spectral region in the Al 3+ spectrum has a major component shifted to higher energy (compared to fulvic acid alone or to fulvic acid in the presence of Ca 2+ or Tb 3+). Even more striking is the emergence of a pronounced IR band at 2407 cm −1, which is present only in the Al 3+ spectrum. The results of fluorescence and IR experiments with the model compounds salicylic acid and phthalic acid further confirm that both salicylic acid-like sites and phthalic acid-like sites are likely complexation sites for Al 3+ in fulvic acid and are major contributors to the observed spectroscopic changes associated with Al 3+ ion complexation. From a comparison of both the fluorescence and IR spectral results for all three class A metals, differing most strongly in the value of their ionic index, it seems clear that major sources of the deviation in spectral properties between Al 3+ and Ca 2+/Tb 3+ is the unusually high value of its charge density and relatively low propensity for involvement in covalent bonding interactions (very high ionic index and relatively low covalent index in the Nieboer and Richardson classification of environmental metals), as well as affinity for certain functional groups.

Analysis of the Cu2+-Soil Fulvic Acid Complexation by Anodic Stripping Voltammetry Using an Electrostatic Model

An electrostatic model has been used in the analysis of the fulvic acid-copper ion complexation with the aim of estimating the contribution of the electrostatic effect to metal binding and calculating the intrinsic complexation parameters in solutions with a dissolved organic carbon (DOC) analogous to that in natural media such as aqueous environments and soil solution. For this purpose solutions of a soil extracted fulvic acid (FA) at concentrations 30, 40, 50, and 60 mg L-1 and ionic strengths 0.001, 0.005, 0.01, 0.05, and 0.1 were titrated at pH 6.5 with copper ion by using differential pulse anodic stripping voltammetry. By means of a 1:1 nonelectrostatic model conditional complexation parameters (apparent stability constant and complexation capacity) were calculated, and their dependence on the FA concentration and ionic strength was demonstrated. Consideration of a second binding site and assumption of bidentate binding sites do not significantly affect the results. The application of a 1:1 electrostatic model with the geometric parameters derived from a previous study of the proton binding reaction of the fulvic acid allowed us for the obtention of the intrinsic stability constants which do not exhibit any dependence on the FA concentration nor on the ionic strength. Comparison of the apparent and intrinsic stability constants shows a significant contribution of the electrostatic effect on the copper complexation.