Speciation of Aluminum, Cadmium, Copper, and Lead in Humic Soil Solutions-A Comparison of the Ion Exchange Column Procedure and Equilibrium Dialysis

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Abstract
Metal speciation of Al, Cd, Cu, and Pb was studied in soil solutions containing humic substances (HS). The ion exchange column procedure was optimized and compared with a modification of the equilibrium dialysis (ED). A soil solution, for which the metal speciation at equilibrium could be calculated, was obtained from the ED. The solution was processed through columns with pH-adjusted (pHeffluent = pHinfluent) and Na+ saturated strong cation exchange resins. Using a pH-adjusted exchanger, there was only a slight dissociation of Al-humic complexes (AIHS) in the column (x = 7%). The dissociation was independent of pH (4.0–5.5) and v Al (μmol Al bound/g HS) in the range 51–500. The kinetic lability of CdHS, CuHS, and PbHS in the pH-adjusted exchanger (Cd>Pb>Cu) was greater than for AlHS. Among the heavy metals, the ion exchange column procedure was most suitable for fractionating Cu into humic and non-humic forms. The CuHS concentration obtained with the Na+ exchanger was 90–109% of the one obtained with ED, the variability being independent of v Cu (μmol Cu bound/g HS), though slightly dependent of sample pH.