Investigations on the conditional kinetic and thermodynamic stability of aquatic humic substance-metal complexes by means of EDTA exchange, ultrafiltration and atomic spectrometry

The conditional metal availability and the kinetic stability of humic subst ance-metal species in humic-rich waters (e g. bog water) was characterized by means of EDTA exchange. For this purpose a combined procedure consisting of time-controlled ligand exchange by EDTA, species differentiation by a f ast single-stage tangential-flow ultrafiltration (TF-UF) technique (cut-off I kDa) and sensitive atomic spectrometry methods (e.g. AAS, ICP-OES, TXRF) was developed. The kinetics and the yield of the EDTA exchange served as o perational parameters for assessing the kinetic stability and EDTA availabi lity of HS-metal species, respectively. Considerable fractions of natural H S-metal species studied were shown to be EDTA-inert (e.g. 31% of the total Fe, 44% of the total Al) even after long reaction times (48 h), in contrast to artificial ones formed in solutions of isolated HS. Moreover, the condi tional thermodynamic stability of HS-metal complexes formed by successive l oading of an aquatic reference HS (HO14) with a number of heavy metal ions (e.g. Cr(III), Cu(II), Fe(III), Mn(II), Zn(II)) was also evaluated discrimi nating the free metal concentrations by means of TF-UF. In addition, from t he loading isotherms obtained conditional complexation capacities could be derived for the studied HS exhibiting the order Fe(III) > > Cu(II) > Cr(III ) > Co(II) > Mn(II). (C) 2001 Elsevier Science B.V. All rights reserved.