Nouri M. Hassan

Kinetic studies of nickel speciation in model solutions of a well-characterized humic acid using the competing ligand exchange method

Abstract Publications on the binding characteristics of metals with humic acid (HA) are sparse. Here we investigated the release of nickel from Ni(II)–HA complexes using model solutions of three different [Ni(II)]/[HA] mole ratios at three different pH values; we also compared the results with those of [Ni(II)]/[FA] complexes from previous work in this laboratory. Ligand exchange kinetics using the competing ligand exchange method (CLEM) were studied using two different techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime as the competing ligand to measure the rate of dissociation of Ni(II)–HA complexes. The results of the kinetic studies showed that as the [Ni(II)]/[HA] mole ratio was decreased, the rate of dissociation of Ni(II)–HA complexes decreased, and the proportion of free Ni2+ ions plus very labile nickel complexes decreased while the proportion of the less labile kinetically distinguishable components increased. Generally, the rate of dissociation of Ni(II)–HA complexes was slower than that of Ni(II)–FA complexes. Studies on the validity of the kinetic model showed that the concentrations of chemical species varied in a reasonable way with pH and the [Ni(II)]/[HA] mole ratios, indicating that the kinetically distinguishable components have chemical significance and the kinetic model is valid.

A study of Nafion-coated and uncoated thin mercury film-rotating disk electrodes for cadmium and lead speciation in model solutions of fulvic acid

The speciation of lead and cadmium in model solutions of a well characterized fulvic acid (FA), Laurentian FA, was investigated by anodic stripping voltammetry (ASV) using a thin mercury film-rotating disk electrode (TMF-RDE) and a Nafion-coated TMF-RDE. Pseudopolarograms were fitted to a model that accounted for differences in the diffusion coefficients between the free metal ion, M, and the metal complexes, ML, and the heterogeneity of naturally-occurring organic complexants, L, such as fulvic acid. The pseudopolarograms were used for each metal system to determine the differential equilibrium function which is independent of the experimental conditions and allows the stability constants for other metal loading conditions to be predicted. The experiments were done using linear-scan anodic stripping voltammetry (LSASV), differential pulse anodic stripping voltammetry (DPASV), and square wave anodic stripping voltammetry (SWASV) with an uncoated TMF-RDE and a Nafion-coated thin mercury film-rotating disk electrode (NCTMF-RDE). The NCTMF-RDE was investigated to determine its usefulness in preventing adsorption of surface-active organic complexants such as fulvic acid on the mercury electrode surface. LSASV was found to be the least susceptible to adsorption. DPASV was affected by adsorption in the lead-FA model solutions, and SWASV was strongly affected by adsorption in both the lead-FA model solutions and the cadmium-FA model solutions. The Nafion-coated TMF-RDE did not prevent adsorption when DPASV or SWASV was used. However, the results suggest that the Nafion-coated TMF-RDE may be useful in preventing adsorption when LSASV is used.

Evaluation of semiquantitative analysis mode in ICP-MS.

Semiquantitative analysis mode in inductively coupled plasma mass spectrometry (ICP-MS) has been popularly used for fast screening purposes. Although the benefit of it has been studied by many researchers, its performance of application in real-world routine analyses has not been reported. In this study, we evaluated the reliability of semiquantitative analysis mode through inter-laboratory comparison using two different ICP-MS systems with one multi-element calibration standard. The suitability of semiquantitative analysis mode in routine analysis laboratory was demonstrated by evaluating its application in different laboratories and in real production laboratory practices. Twenty one elements were measured, namely, Be, B, Al, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, As, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Tl, and Pb in various fresh water reference samples. Good results concerning accuracy (relative percentage error within 10%) and reproducibility (relative standard deviation lower than 5%) were obtained in more than 90% analyzed samples at concentrations equal to or greater than 10 times the detection limit (DL). Semiquantitative analysis mode also enabled the determination of elements that are not present in the calibration standard. The results demonstrated the potential of semiquantitative analysis mode as a reliable approach in routine laboratory determination of simple matrices, where high throughput and cost-effectiveness are desired, as well as in emergency situations where speed of analysis is critical and quite often limited sample information is available.

Effect of the competition of copper and cobalt on the lability of Ni(II)–organic ligand complexes. Part I. In model solutions containing Ni(II) and a well-characterized fulvic acid

Abstract The competitive binding of Cu(II), Co(II) and Ni(II) ions by a well-characterized fulvic acid (FA) in model aqueous solutions has been investigated by employing the competing ligand exchange method (CLEM) with Chelex-100 as the competing ligand. The reaction rate between a metal ion and a ligand is dependent on the rate of exchange of the coordinated water, which varies greatly for the above metals. Fulvic acid is a polyfunctional, polyelectrolytical, physically-heterogeneous, organic complexant having binding sites that can be roughly classified into two categories: minor (∼1–10%), strong sites, and major (∼99–90%), weak sites. The strong binding sites are first occupied, and after all the strong binding sites are occupied, the weak binding sites are occupied. Experiments have been done using model aqueous solutions containing various concentrations of Cu, Co and Ni, and the above well-characterized FA. Graphite furnace atomic absorption spectrometry and inductively-coupled plasma mass spectrometry were employed to monitor the rate of uptake of the metals by Chelex-100. The above metals, in the metals/FA mole ratios used in this study, have been found to compete with one another for the relatively small number of strong binding sites of the FA, and Cu(II) and Co(II) which have much greater rate constant for water exchange than that of Ni(II) win the competition over Ni(II). In the absence of Cu(II) and Co(II), the strong binding sites of the FA are occupied by Ni(II), forming strong complexes, which are inert, whereas, in the presence of Cu(II) and Co(II), the strong binding sites of the FA are occupied by Cu(II); Co(II), Ni(II), and the remaining Cu(II) occupy the weak binding sites of the FA, forming weak complexes, which are labile. The enhanced lability of the Ni–FA complexes in the presence of Cu(II) and Co(II) indicates that Cu(II) and Co(II) successfully compete with Ni(II) for the strong binding sites of the FA.