Kinetic study of speciation of nickel(II) bound to a fulvic acid

Abstract

The distinguishable species present In solutions of NI(II) equilibrated with a soil fulvic acid (FA) are studied by a kinetic method of analysis based on reaction with 4-(2pyrldylazo)resorclnol (PAR). An approximate Laplace transform Is used to assign the number of species and a nonlinear regression routine Is used to obtain final parameter values. Numerical methods are carefully evaluated by using simulated data Including synthetic noise. Four rate constants of 0.67, 0.15, 0.021, and 0.0026 s~1 consistently represent the samples Initially equilibrated at pH 4, 5, and 6.4. The first Is associated with NI(OH2)e2+. Species' concentrations vary In a reasonable way with pH and FA/Ni(II) ratio and seem to provide a realistic model. One important feature Is found at pH 6.4, where the weakest acid carboxylic groups of the fulvic acid have been deprotonated; 40% of the NI(II) Is then bound In a species which requires 10 days for complete reaction with PAR.

Study of the effects of concentration and pH on the dissociation kinetics of Fe(II)-fulvic acid complexes

“…As shown in Figure 3, an initial decrease in the dissociation rate constants between pH 4.5 (0.31) to Ϸ 5.1 (0.009) appears, followed by an almost flat plateau (pH from 5.3 to 6.5) and finally by a dramatic decrease at the highest pH value. Similar results have been obtained by other authors [3,5,6] and allow us to conclude that the dissociation of Fe(II) from Fe-FA complexes in the presence of stronger ligands (under the present experimental conditions) is controlled by the stability of the Fe-FA complexes.…”

“…According to the kinetic treatment used here, this means that the plot of H(k, t) against ln t may show several maxima, which would be equivalent to the number of complexes in the solution. The literature reports studies in which complexes with coordination numbers greater than one were found [3,5,6]. Under the experimental conditions used in the present work, the obtained plots at different concentration and pH values show only one peak (see Fig.…”

Speciation in Freshwaters

“…Plankey and Patterson, 1987;Cabaniss, 1990;Rate et al, 1993). Nevertheless, most of the results confirm the existence of a distribution of dissociation rates of fulvic and humic complexes, with dissociation increasing with increasing metal : humic ratio Lavigne et al, 1987;Cabaniss, 1990;Hering and Morel, 1990;Rate et al, 1993;Chakrabarti et al, 1994;Bonifazi et al, 1996;Lu et al, 1996). The pH also has an important influence on exchange kinetics (Li et al, 1998).…”

“…Typically a spectrophotometrically determinable ligand is used (e.g. 4-(2-pyridylazo)-resorcinol (PAR) for Ni II (Lavigne et al, 1987;Cabaniss, 1990) and Cu II (Rate et al, 1992;, 2,4,6-tri(2-pyridyl)-s-triazine (TPTZ) for Fe III (Sojo and de Haan,1991)). Fluorescence methods have also been applied by several workers (Plankey and Patterson, 1987;Cabaniss, 1992;Shuman, 1992).…”

“…Different data interpretation models have been applied: simple dissociation constants (Langford and Khan, 1975), discrete multi-component models (Lavigne et al, 1987;Plankey and Patterson, 1987;Sojo and de Haan, 1991;Langford and Gutzman, 1992), discrete kinetic spectra (Cabaniss, 1990), continuous kinetic spectra Nederlof et al, 1994) and log normal distribution (Rate et al, 1992;. It should be noted that for heterogeneous systems, analysis of rate constant distributions is a mathematically ill-posed problem and slight perturbations in the input experimental data can yield artefactual information (Stanley et al, 1994).…”