Simultaneous determination of Ca, Cu, Ni, Zn and Cd binding strengths with fulvic acid fractions by Schubert’s method Original Research Article

The equilibrium binding of Ca2+, Ni2+, Cd2+, Cu2+ and Zn2+ with unfractionated Suwannee river fulvic acid (SRFA) and an enhanced metal binding subfraction of SRFA was measured using Schubert’s ion-exchange method at pH 6.0 and at an ionic strength (μ) of 0.1 (NaNO3). The fractionation and subfractionation were directed towards obtaining an isolate with an elevated metal binding capacity or binding strength as estimated by Cu2+ potentiometry (ISE). Fractions were obtained by stepwise eluting an XAD-8 column loaded with SRFA with water eluents of pH 1.0 to pH 12.0. Subfractions were obtained by loading the fraction eluted from XAD-8 at pH 5.0 onto a silica gel column and eluting with solvents of increasing polarity. Schuberts ion exchange method was rigorously tested by measuring simultaneously the conditional stability constants (K) of citric acid complexed with the five metals at pH 3.5 and 6.0. The log K of SRFA with Ca2+, Ni2+, Cd2+, Cu2+ and Zn2+ determined simultaneously at pH 6.0 follow the sequence of Cu2+ > Cd2+ > Ni2+ > Zn2+ > Ca2+ while all log K values increased for the enhanced metal binding subfraction and followed a different sequence of Cu2+ > Cd2+ > Ca2+ > Ni2+ > Zn2+. Both fulvic acid samples and citric acid exhibited a 1 : 1 metal to ligand stochiometry under the relatively low metal loading conditions used here. Quantitative 13C nuclear magnetic resonance spectroscopy showed increases in aromaticity and ketone content and decreases in aliphatic carbon for the elevated metal binding fraction while the carboxyl carbon, and elemental nitrogen, phosphorus, and sulfur content did not change. The more polar, elevated metal binding fraction did show a significant increase in molecular weight over the unfractionated SRFA.