Simultaneous determination of Ca, Cu, Ni, Zn and Cd binding strengths withfulvic acid fractions by Schubert's method

The equilibrium binding of Ca2+, Ni2+, Cd2+, Cu2+ and Zn2+ with unfractiona ted Suwannee river fulvic acid (SRFA) and an enhanced metal binding subfrac tion of SRFA was measured using Schubert's ion-exchange method at pH 6.0 an d at an ionic strength (mu) of 0,1 (NaNO3). The fractionation and subfracti onation were directed towards obtaining an isolate with an elevated metal b inding capacity or binding strength as estimated by Cu2+ potentiometry (ISE ), Fractions were obtained by stepwise eluting an XAD-g 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 me thod was rigorously tested by measuring simultaneously the conditional stab ility 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 subf raction and followed a different sequence of Cu2+ > Cd2+ > Ca2+ > Ni2+, Zn2 +. Both fufvic acid samples and citric acid exhibited a 1 : 1 metal to liga nd stochiometry under the relatively low metal loading conditions used here , Quantitative C-13 nuclear magnetic resonance spectroscopy showed increase s in aromaticity and ketone content and decreases in aliphatic carbon for t he elevated metal binding fraction while the carboxyl carbon, and elemental nitrogen, phosphorus, and sulfur content did not change. The more polar, e levated metal binding fraction did show a significant increase in molecular weight over the unfractionated SRFA. (C) 1999 Elsevier Science B.V. All ri ghts reserved.