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

The adsorption of free- and bound-metal ions (metal complexes) as well as o f ligands onto various hydrous oxide type sorbents have been extensively mo delled using EDTA as the model ligand. This type of modelling uses metal io n-EDTA mixture solutions containing stoichiometrically equivalent or excess ive amounts of either constituent. Consequently, for mixture solutions equi librated with the sorbent, the aim was to develop a suitable method for det ermining 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 exchangi ng different metal-EDTA complexes with Fe(NO3)(3) followed by HPLC using UV detection. The developed method utilizes Vis- and AA-spectrometry widespre ad 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 possibl e constituents of equilibrated solution (with the sorbent). AU EDTA (free o r 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 ar e determined by the thiocyanate spectrophotometric method at 480 nm. Then ' EDTA-bound iron (III)' is defined as added Fe minus colorimetrically (thioc yanate method) found Fe, and 'AAS-found metal' (lead or cadmium) correspond s 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-), a nd 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 su rface complexes on the hydrous oxide sorbent (similar to SOH) as bound meta l (similar to SOM), bound ligand (similar to SOL) or bound metal complex (s imilar to SOML) are much more difficult to desorb and analyze, the simple p rocedure developed here applicable to more conventional instruments carried out in sorbent equilibrated solutions (filtrates) may effectively aid heav y metal adsorption modelling in realistic environmental simulations. (C) 20 00 Elsevier Science B.V. All rights reserved.