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

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 (pH_effluent = pH_influent) 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.