A combined spectrophotometric-AAS method for the analysis of trace metal, EDTA, and metal-EDTA mixture solutions in adsorption modeling experiments

Abstract

The adsorption of free- and bound-metal ions (metal complexes) as well as of ligands onto various hydrous oxide type sorbents have been extensively modelled using EDTA as the model ligand. This type of modelling uses metal ion-EDTA mixture solutions containing stoichiometrically equivalent or excessive amounts of either constituent. Consequently, for mixture solutions equilibrated with the sorbent, the aim was to develop a suitable method for determining either metal complex + free ligand (MY2- + H2Y2-) or metal complex + free metal (MY2- + M2+) in the aqueous filtrate, the metal M being lead or cadmium. The conventional method of analyzing such filtrates is exchanging different metal-EDTA complexes with Fe(NO3)(3) followed by HPLC using UV detection. The developed method utilizes Vis- and AA-spectrometry widespread in common laboratories, eliminating the need for HPLC and UV techniques that require higher operational cost, expertise and contaminant-free media. The developed procedure is based on the following analyses for the possible constituents of equilibrated solution (with the sorbent). AU EDTA (free or bound, as H2Y2- or MY2-) species are converted into FeY- by adding Fe(NO3)(3), and heating at 80 degrees C for 1 h. All metal (free or bound, as M2+ or MY2-) species are determined by AAS. All unbound (free) Fe3+ species are determined by the thiocyanate spectrophotometric method at 480 nm. Then 'EDTA-bound iron (III)' is defined as added Fe minus colorimetrically (thiocyanate method) found Fe, and 'AAS-found metal' (lead or cadmium) corresponds to M2+ and/or MY2-, depending on the analyzed solution. If EDTA-bound Fe(III) is greater than AAS-found metal, then one has a (MY2- + H2Y2-) mixture where AAS-found metal is (MY2-), and foe EDTA, i.e. (H2Y2-), is calculated from the difference. If EDTA-bound Fe(III) is smaller than AAS-found metal, then one has a (M2+ + MY2-) mixture where EDTA-bound Fe(III) is (MY2-), and the foe metal, i.e. (M2+), is calculated from the difference. If the two compared quantities are equal? then one has a purl MY2- solution. Since surface complexes on the hydrous oxide sorbent (similar to SOH) as bound metal (similar to SOM), bound ligand (similar to SOL) or bound metal complex (similar to SOML) are much more difficult to desorb and analyze, the simple procedure developed here applicable to more conventional instruments carried out in sorbent equilibrated solutions (filtrates) may effectively aid heavy metal adsorption modelling in realistic environmental simulations. (C) 2000 Elsevier Science B.V. All rights reserved.