Cu(II) binding by a pH-fractionated fulvic acid

The relationship between acidity, Cu(II) binding and sorption to XAD resin was examined using Suwannee River fulvic acid (SRFA). The work was based on the hypothesis that fractions of SRFA eluted from an XAD column at various pH's from 1.0 to 12.0 would show systematic variations in acidity and poss ibly aromaticity which in turn would lead to different Cu(II) binding prope rties. We measured equilibrium Cu(II) binding to these fractions using Cu2 ion-selective electrode (ISE) potentiometry at pH 6.0. Several model Ligan ds were also examined, including cyclopentane-l,2,3,4-tetracarboxylic acid (CP-TCA) and tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THF-TCA), the la tter binding Cu(II) much more strongly as a consequence of the ether linkag e. The SRFA Cu(II) binding properties agreed with previous work at high ion ic strength, and binding was enhanced substantially at lower ionic strength , in agreement with Poisson-Boltzmann predictions for small spheres, Determ ining Cu binding constants (K-i) by non-linear regression with total ligand concentrations (L-(Ti)) taken from previous work, the fractions eluted at varying pH had K-i similar to the unfractionated SRFA, with a maximum enhan cement of 0.50 log units, We conclude that variable-pH elution from XAD doe s not isolate significantly strong (or weak) Cu(II)-binding components from the SRFA mixture. (C) 1999 Elsevier Science B.V. All rights reserved.