Chemodynamics of heavy metals in long-term contaminated soils:Metal speciation in soil solution
Kwon-Rae Kim , Gary Owens
DOI:
Received January 16, 2009,Revised March 24, 2009, Accepted , Available online
Volume 21,2009,Pages 1532-1540
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The concentration and speciation of heavy metals in soil solution isolated from long-term contaminated soils were investigated. The
soil solution was extracted at 70% maximum water holding capacity (MWHC) after equilibration for 24 h. The free metal concentrations
(Cd2+, Cu2+, Pb2+, and Zn2+) in soil solution were determined using the Donnan membrane technique (DMT). Initially the DMT was
validated using artificial solutions where the percentage of free metal ions were significantly correlated with the percentages predicted
using MINTEQA2. However, there was a significant difference between the absolute free ion concentrations predicted by MINTEQA2
and the values determined by the DMT. This was due to the significant metal adsorption onto the cation exchange membrane used in
the DMT with 20%, 28%, 44%, and 8% mass loss of the initial total concentration of Cd, Cu, Pb, and Zn in solution, respectively.
This could result in a significant error in the determination of free metal ions when using DMT if no allowance for membrane cation
adsorption was made. Relative to the total soluble metal concentrations the amounts of free Cd2+ (3%–52%) and Zn2+ (11%–72%) in
soil solutions were generally higher than those of Cu2+ (0.2%–30%) and Pb2+ (0.6%–10%). Among the key soil solution properties,
dissolved heavy metal concentrations were the most significant factor governing free metal ion concentrations. Soil solution pH showed
only a weak relationship with free metal ion partitioning coefficients (Kp) and dissolved organic carbon did not show any significant
influence on Kp.
