Ultrafiltration as a technique for studying metal—humate interactions: studies with iron and copper

An ultrafiltration method was employed to study Fe(II)/Fe(III) and Cu(II) binding by Armadale Horizon fulvic acid. The results of the Fe—fulvate studies indicate the formation of “aggregates” between Fe and the fulvic acid molecule. The reduction of Fe(III) to Fe(II) by fulvic acid is also observed. Results for Cu(II)—fulvate electrode method. An algorithm developed previously for the prediction of metal ion binding by fulvic acid at different pH and salt concentration levels was tested further by comparing Cu²⁺ ion binding predictions with their measured values.     

The binding of strontium and europium by an aquatic fulvic acid--ion exchange distribution and ultrafiltration studies

The complexation of an aquatic fulvic acid, FA, with Sr(2+) and Eu(3+) was studied at 0.10 and O1.O1M NaClO(4) using trace levels of metal ([Sr(2+)] = 10(-9)M and [Eu(3+)] = 10(-11)M) and a constant FA concentration (0.12 g/l) by an ultrafiltration technique (UF) and an ion exchange distribution method (IEDS). The overall complex formation function, beta(OV) for the two metals was calculated and its dependence on pH, ionic strength and method was investigated. The absolute value of log beta(OV), the pH dependence and the influence of the ionic strength on the complexation differed depending on the metal ion and experimental technique employed. By considering the functional group heterogeneity of the FA molecule, it was possible to predict the most predominantly bound site (keto-enol) and resolve the complex formation function for this site and EU(3+) (IEDS: 9.43 +/- 0.29 l/eq at 0.10M and 10.58 +/- 0.72 l/eq at 0.01M; UF: 7.19 +/- 1.51 l/eq at 0.01M and 6.88 +/- 0.91 l/eq at 0.01M). The results are discussed in the light of possible intrinsic problems of the two experimental methods.     

Complexation of Eu and Tb with fulvic acids as studied by time-resolved laser-induced fluorescence

Complexation of Eu and Tb with fulvic acids (FA) has been studied by time-resolved laser-induced fluorescence. The lanthanide species are excited by a pulsed Xe/Cl excimer laser and the short-lived luminescence of the fulvic acids is eliminated by time gating. The conditional binding constants (K') and the total metal-binding capacities of the fulvic acids have been determined from the equilibrium titration curves. No significant variations of the log K' values (about 6) have been found in the pH range investigated (2.7, 5.5, 6.5) at ionic strengths of 0.1 and 1M (NaClO(4)). An inverse relationship of K' to total metal concentration is observed. Substitution of Eu for Al has been found to depress the fluorescence signal of the Eu-FA complex. The binding constants of Al(3+) and Eu(3+), derived from the competition experiments, are of the same order of magnitude. The effect of competition on metal binding and trace-metal transport is discussed.     

Accurate values of stability constant of Ca(2+)-murexide complex

In published reports, the values of stability constants of 1:1 complex of Ca(2+) and the dye ammonium purpurate (murexide) were not determined under controlled conditions and were not properly corrected for the binding of Ca(2+) with ions of buffer used to maintain pH and that of the background electrolyte used to maintain ionic strength. We report the molar absorptivities (epsilon) of murexide at pH 7.0, 7.5, 8.0, as well as the differential molar absorptivities (Deltaepsilon). Using these, we calculate the stability constants of the Ca-murexide complex at pH 5.0, 6.0, 6.5, 7.0, 7.5 and 8.0 at 15, 25 and 35 degrees C and 0.100 M ionic strength using KCl as background electrolyte. No buffer was used and the complication arising from buffer binding is thus avoided. These values are compared with those determined in the presence of buffers that bind metal ions negligibly (Tris at pH 7.5 and 8.0) or whose binding constant to Ca(2+) is reported and therefore can be corrected for (acetate at pH 5.0, Bistris at pH 6.5). Agreement is obtained within errors of measurement. The reported values are not true stability constants but can be used to calculate the concentration of free Ca(2+) ion in a metal-ligand mixture with high precision and accuracy. The effect of K(+) binding to murexide is considered and is found not to alter the calculated value of free calcium concentration in a mixture.     

Binding of cadmium to Laurentide fulvic acid. Justification of the functionalities assigned to the predominant acidic moieties in the fulvic acid molecule

The binding of cadmium to a fulvic acid (FA) extracted from peat was studied as a function of pH (4–8), ionic strength (0.01 and 0.10 M NaNo₃) and ratio of fulvic acid to metal ion concentrations (FA/Cd = 8, 4, and 2). An overall complex formation function of approximately 10^3.14 was determined to enable comparison with previous studies. Additionally, literature values of complex formation constants were employed to describe the interaction between Cd(II) and the individual acidic sites. A good prediction of the experimental results is considered as a justification of the functionalities assigned to the predominant acidic sites in the fulvic acid molecule.     

Vapor-pressure osmometric study of the molecular weight and aggregation tendency of a reference-soil fulvic acid

The molecular weight and aggregation tendency of a reference-soil fulvic acid in Armadale horizon Bh were determined by vapor-pressure osmometry using tetrahydrofuran and water as solvents. With tetrahydrofuran, number-average molecular weight values of 767 ± 34 and 699 ± 8 daltons were obtained from two separate sets of measurements. Two sets of measurements with water also yielded values within this range (754 ± 70 daltons) provided that the fulvic acid concentration in water did not exceed 7 mg ml^?1; at higher concentrations (9.1–13.7 mg ml^?1) a number-average molecular weight of 956 ± 25 daltons was resolved, providing evidence of molecular aggregation. Extension of these studies to 80% neutralized fulvic acid showed that a sizeable fraction of the sodium counter ion is not osmotically active.     

Sequestration of organometallic compounds by natural organic matter. binding of trimethyltin(IV) by fulvic and alginic acids

The binding capacity of fulvic and alginic acids towards trimethyl tin(IV) cation was quantitatively determined in order to evaluate the sequestering ability of toxic organometallic compounds by natural organic matter. Investigations were performed in the pH range of natural waters (5–8.5) where the carboxylate groups, largely present in both sequestering agents, are the main binding sites. A chemical interaction model, according to which both the protonation of polyelectrolyte ligands and the hydrolysis of the organotin cation in NaCl aqueous solution were considered, was used to define the speciation of the systems under investigation. Measurements performed at different ionic strength values (0.1, 0.25, 0.5 and 0.7 mol L^?1, NaCl) allowed us to consider the dependence of stability constants on the ionic strength, and to calculate the formation constants at infinite dilution. Results obtained show the formation of the complex species TMT(L), TMT(L)₂ and TMT(L)(OH) for L = fulvic acid and TMT(L) for L = alginic acid, respectively. In order to compare the strength of interaction of these natural poly electrolytes with other analogous synthetic polyelectrolytes, measurements were also carried out on the trimethyltin(IV)–polyacrylate (5.1 kDa) system, and in this case the formation of TMT(L), TMT(L)₂ and TMT(L)(OH) species was found. Results show the following trend of stability for the species TMT(L) in the systems investigated: TMT–fulvate ≈ TMT–polyacrylate > TMT–alginate. On the basis of the stability data obtained, the lowest concentration of fulvic and alginic acids, able to act as sequestering agents towards triorganotin(IV) cation in the conditions of natural waters, was also calculated. Copyright © 2006 John Wiley & Sons, Ltd.     

Calorimetric investigation of complex formation of some humic compounds

In the present paper we studied the complexation capacity towards copper ion of fulvic acids extracted from Arno River and Lake Como sediments, as well as Antarctic Sea water at different pHs in order to evaluate the interaction of different complexing groups of fulvic acids with copper ions. The binding capacity studies were carried out by means of titration with a copper-selective electrode and calorimetric measurements. For the same purpose, the heat of reaction in the complexation of copper ions with small molecules containing functional groups similar to fulvic acid was also determined. Titration results indicated that the fraction of bound metal increases with increasing pH (from 5 to 7). This may be accounted for by the increase in the ionisation of the fulvic acid molecule. Results obtained from calorimetric measurements show that the quantity of total heat involved in the metal ion-fulvate interactions determines a decreasing exothermic response with increasing pH values. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of the acid–base properties of fulvic acid-like substances extracted from senescent leaves of eucalyptus and oak

The acid-base properties of two fulvic acids (FA) extracted from senescent leaves of eucalyptus and oak were characterized by carrying out potentiometric titrations at two FA concentrations and four ionic strengths (0.005 M <[KNO(3)] <1.0 M). Experimental data were analyzed by means of the master curve approach, which includes an electrostatic spherical double layer model, and the Langmuir-Freundlich isotherm was used to fit the data. The contribution of the electrostatic effect to the proton binding reaction was lower than that observed for soil fulvic acids. The chemical heterogeneity of both samples was described by two acid sites with p Ks of about 4 and 7.5, the most abundant being the carboxylic site of p K = 4.     

Europium complexation by an aquatic fulvic acid - effects of competing ions

Effects of competing ions, Fe (2+)Fe (3+) and Al(3+), on Eu(3+) complexation with an aquatic fulvic acid (FA), have been investigated using an ion exchange technique. The influence of different concentrations (10(-6), 10(-4) M) of the competing ions on the distribution coefficient for Eu was measured, and the overall complex formation function, beta(ov), was resolved for the Eu systems with Fe and Al. All systems showed pH-dependent beta(ov)-functions. The presence of 10(-4) M concentration of competing ion reduced the resolved complex formation function (logbeta(ov)) for Eu complexation with fulvic acid by 0.6 and 0.4 log units at pH 5 for Fe and Al, respectively. this indicates that Fe has a more perturbing effect on Eu-FA complexation than Al. In similar competition studies Sr and Eu were found not to perturb each others complexation with fulvic acid, suggesting therefore that the two metals probably bind to different sites on the fulvic acid molecule.     

Effects of solution chemistry on the adsorption of ibuprofen and triclosan onto carbon nanotubes

Single-walled carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs), and oxidized MWCNTs (O-MWCNTs) were studied for the adsorption of ibuprofen (IBU) and triclosan (TCS) as representative types of pharmaceutical and personal care products (PPCPs) under different chemical solution conditions. A good fitting of sorption isotherms was obtained using a Polanyi-Manes model (PMM). IBU and TCS sorption was stronger for SWCNTs than for MWCNTs due to higher specific surface area. The high oxygen content of O-MWCNT further depressed PPCP sorption. The sorption capacity of PPCPs was found to be pH-dependent, and more adsorption was observed at pHs below their pK(a) values. Ionic strength was also found to substantially affect TCS adsorption, with higher adsorption capacity observed for TCS at lower ionic strength. In the presence of a reference aquatic fulvic acid (FA), sorption of IBU and TCS was reduced due to the competitive sorption of FA on carbon nanotubes (CNTs). Sorption isotherm results with SWCNTs, MWCNTs and O-MWCNTs confirmed that the surface chemistry of CNTs, the chemical properties of PPCPs, and aqueous solution chemistry (pH, ionic strength, fulvic acid) all play an important role in PPCP adsorption onto CNTs.     

Dynamics and heterogeneity of Pb(II) binding by SiO2 nanoparticles in an aqueous dispersion

Pb(II) binding by SiO(2) nanoparticles in an aqueous dispersion was investigated under conditions where the concentrations of Pb(2+) ions and nanoparticles are of similar magnitude. Conditional stability constants (log K) obtained at different values of pH and ionic strength varied from 4.4 at pH 5.5 and I = 0.1 M to 6.4 at pH 6.5 and I = 0.0015 M. In the range of metal to nanoparticle ratios from 1.6 to 0.3, log K strongly increases, which is shown to be due to heterogeneity in Pb(II) binding. For an ionic strength of 0.1 M the Pb(2+)/SiO(2) nanoparticle system is labile, whereas for lower ionic strengths there is loss of lability with increasing pH and decreasing ionic strength. Theoretical calculations on the basis of Eigen-type complex formation kinetics seem to support the loss of lability. This is related to the nanoparticulate nature of the system, where complexation rate constants become increasingly diffusion controlled. The ion binding heterogeneity and chemodynamics of oxidic nanoparticles clearly need further detailed research.     

Voltammetry of Plastocyanin at a Graphite Electrode: Effects of Structure, Charge, and Electrolyte

Comparative voltammetric studies on Anabaena variabilis plastocyanin (positively charged at neutral pH) and spinach and poplar plastocyanins (negatively charged at neutral pH) have been undertaken at an edge-plane graphite electrode as a function of ionic strength, pH, and Mg(2+) concentration at 3 degrees C. The aim was to provide a more detailed understanding of the influence of the electrode-protein (solution) interfacial characteristics, as well as the variation of the formal potential with both the nature of the plastocyanin species and the pH. As might be expected, some of the interfacial properties associated with the positive charge on A. variabilis plastocyanin are the opposite of those observed with the negatively charged plastocyanins. For example, the linear diffusion component of the mass transport process for A. variabilis plastocyanin under the conditions of cyclic voltammetry is decreased and the radial diffusion component is increased by the addition of Mg(2+), whereas the reverse occurs with poplar and spinach plastocyanins. The voltammetrically determined reversible potentials for A. variabilis plastocyanin are considerably less positive than those for spinach and poplar plastocyanins, in agreement with values calculated from chemically based redox studies. Ionic strength effects, as determined by addition of NaClO(4) over the concentration range 0.005-0.20 M, are negligible for all three proteins. The addition of Mg(2+) causes a significant shift in the reversible potential toward more positive values for spinach and poplar plastocyanin but only a small positive shift for A. variabilis plastocyanin. The difference is attributed to a specific binding effect. The addition of Mg(2+) also dramatically alters the pH dependence of the reversible potential, indicating that the equilibrium between the protonated and unprotonated forms of reduced plastocyanin is modified by binding of Mg(2+) to the protein. It is concluded that the effects of biologically relevant redox-inactive cations such as Mg(2+) or Ca(2+) have to be considered carefully in studies of the redox chemistry of metalloproteins.     

Influence of various monovalent cations and calcium ion on the colloidal fouling potential

The influence of various monovalent cations and of divalent calcium ions on colloidal fouling strength was investigated quantitatively on a bench-scale ultrafiltration device. A higher colloidal fouling potential (k) was consistently observed with lithium chloride compared to the same ionic strengths of chlorides of other monovalent cations (Na+, K+, and Cs+). This observation was attributed to the formation of an impervious layer around the colloidal particle by lithium ions that prevented the repulsive forces due to the interaction of the silica hairs formed on the particles in the presence of water. The impact of the divalent calcium ion on the fouling potential was more complex. The fouling potential first increased with calcium ion concentration and then decreased. The maximum value of fouling potential occurred at the ionic strength corresponding to the critical coagulation concentration, which decreased with increasing colloid concentration. The colloidal fouling potential was well correlated by a bilinear relationship with colloid concentration and ionic strength for all salts tested under the critical coagulation concentration.     

Factorial analysis of the trihalomethanes formation in water disinfection using chlorine

The factors that affect trihalomethane (THM) (chloroform, bromodichloromethane, chlorodibromomethane and bromoform) formation from the chlorination of aqueous solutions of hydrophobic fulvic acids (FA) were investigated in a prototype laboratorial simulation using factorial analysis. This strategy involved a fractional factorial design (16 plus 5 center experiments) of five factors (fulvic acids concentration, chlorine dose, temperature, pH and bromide concentration) and a Box Behnken design (12 plus 3 center experiments) for the detailed analysis of three factors (FA concentration, chlorine dose and temperature). The concentration of THM was determined by headspace analysis by GC-ECD. The most significant factors that affect the four THM productions were the following: chloroform-FA concentration and temperature; bromodichloromethane-FA concentration and chlorine dose; chlorodibromomethane-chlorine dose; and, bromoform-chlorine dose and bromide concentration. Moreover, linear models were obtained for the four THM concentrations in the disinfection solution as function of the FA concentration, chlorine dose and temperature, and it was observed that the complexity of the models (number of significant factors and interactions) increased with increasing bromine atoms in the THM. Also, this study shows that reducing the FA concentration the relative amount of bromated THM increases.     

Molecular structure of fulvic acids by electrospray with quadrupole time-of-flight mass spectrometry

Characterisation of the molecular structure of aquatic fulvic acids (FA) has been performed using a quadrupole time-of-flight (Q-TOF) mass spectrometer equipped with an electrospray ionisation interface. Molecular masses centred around 450 Da and sinusoidal spectral distributions have been obtained for all fulvic acids. Tandem mass spectrometry (MS/MS) experiments showed losses of 18 Da (H(2)O) and 44 Da (CO(2)), and possible molecular structures were determined for the first time to our knowledge. A methodology is reported for evaluating the average elemental composition of FA from high-resolution mass spectra by processing post-acquisition data calculations using molecular size distributions and atomic compositions of ions. The results are found to be consistent with elemental analysis data.     

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

Effect 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.     

Removal of Fulvic Acids from Aqueous Solutions via Surfactant Modified Zeolite

IntroductionNatural organic matter(NOM)is composed of acomplex mixture of organic compounds derived fromliv-ing and dead plants,animals and microorganisms,waste products of animals and microorganisms,andfrom the degradation of the above materials[1].Fulvi…     

Sorption of thorium(IV) from aqueous solutions by graphene oxide

Graphene oxide (GO) is one of the most important carbon nano-materials. In this paper, GO was synthesized from flake graphite and characterized by transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The sorption of Th(IV) on GO was investigated as a function of contact time, solid-to-liquid ratio, pH, ionic strength, and in the presence of fulvic acid (FA) and humic acid (HA) by batch experiments. The sorption percentage of Th(IV) on GO decreased with increasing ionic strength and decreasing solid-to-liquid ratio. The sorption edge of Th(IV) in the presence of FA/HA is much lower than that in the absence of FA/HA. Furthermore, the sorption processes of Th(IV) can be described by a pseudo-second order rate model. Based on the Langmuir model, the maximum sorption capacities (Cs_max) of Th(IV) were about 5.80 × 10^?4 mol/L. From thermodynamic investigation, sorption of Th(IV) on GO is spontaneous and endothermic in nature. The rapid sorption rate and high sorption capacity suggest that GO is a promising adsorbent for Th(IV).     

Copper release kinetics from a long-term contaminated acid soil using a stirred flow chamber: effect of ionic strength and pH

The effect of pH and ionic strength on copper release in a long-term Cu-polluted soil was studied using a stirred flow chamber. The presence of Ca(2+) and Na(+) was also evaluated. More copper was released as the ionic strength increased, and it was significantly higher in the presence of Ca(2+) than in the presence of Na(+). The maximum amount of Cu that could be released under experimental conditions increased logarithmically as the ionic strength increased, and the release rate parameters were not significantly correlated with ionic strength values. The maximum amount of Cu that could be released was similar for solutions with pH values between 5.5 and 8.5. For solutions with a pH value below 4.5, the amount of Cu released increased exponentially as the pH decreased. The release rate parameters and Cu release pattern were affected by pH, especially for more acidic solutions (pH values of 2.5 and 3.5).