VOLTAMMETRIC INVESTIGATION OF THE DISTRIBUTION OF HYDROXO-, CHLORO-, EDTA AND CARBOHYDRATE COMPLEXES OF LEAD, CHROMIUM, ZINC, CADMIUM AND COPPER: POTENTIAL APPLICATION TO METAL SPECIATION STUDIES IN BREWERY WASTEWATER

Abstract

This paper reports results on complex formation reactions between OH^-, Cl^-, EDTA and carbohydrate ligands with Pb²⁺ ions at various [LT]:[MT] ratios and at different pH values (1.5-13.0). Differential pulse anodic stripping voltammetry (DPASV) employing an ex situ plated thin mercury film electrode (TMFE) was used to measure the shifts in peak potentials. Formation of simple, polyligand as well as mixed ligand complexes are reported. The reactions between the Pb(II) and the carbohydrate ligands showed pronounced pH dependency on metal forms compared to reactions with simple inorganic ions such as chloride. Modeling of the experimental data obtained with the DPASV method was done using computer software (3D-VISE). The calculated complex formation curves (CCFC) based on mass balance equations were fitted to the experimental complex formation curves (ECFC) and the goodness of the fit evaluated (RSD < 5%). These studies were applied to Pb, Cr, Zn, Cd and Cu speciation in brewery wastewater in which differences between total metal determined by flame atomic absorption spectrometry (FAAS) after acid digestion and labile fraction determined by DPASV were used to estimate the percentage of non-labile fraction (mainly metal-organic complexes). Up to 90% of the metal was found to exist as the “inert” fraction, implying that the effluent system from the brewery industry poses minimal health risks to the environment with regard to toxic forms of the metals as the organically bound metal forms are generally known to have low toxicity compared to the aquo or labile metal forms.



KEY WORDS: Metal speciation, Carbohydrates, Brewery wastewater, Differential pulse voltammetry


Bull. Chem. Soc. Ethiop. 2005, 19(1), 125-138