3.3. Effects of EDTA extraction on the fractionation of the tested metals
Extraction with EDTA resulted in the significant reduction of exchangeable/acid soluble Zn,
exchangeable/acid soluble and reducible fraction of Pb, Cd and Cu (Fig. 4). It has been reported that
exchangeable/acid soluble fraction and reducible fraction are the main fractions that affected by chelating
agents during column extraction and batch experiment [21-23]. In present study, most of the extracted Zn
or Cd by EDTA was from exchangeable/acid soluble fraction, while most of the extracted Pb was from
reducible fraction. Both exchangeable/acid soluble fraction and reducible fraction contributed
significantly to the amount of EDTA extracted Cu. According to the modified BCR sequential extraction
procedure, metals liberated in step 1 (0.11M acetic acid) (B1) consisted of exchangeable ions and
carbonates, and reducible fraction (B2) comprised hydroxide or oxide related to Fe/Mn oxides [18, 24].
Of course, exchangeable ions can be desorbed by chelating agents. Solubility of the carbonates will
greatly increase at the presence of chelating agents [25]. Therefore, exchangeable/acid soluble fraction is
more easily extracted by EDTA. Metal oxide can also react with chelating agents, resulting in the release
of the associated metals [11]. Compared with B1 and B2 fraction, it is rather difficult for B3 fraction
(associated with the sulfide and the organics) and the residual fraction (combined in the matrix of the
crystal) to release at the presence of chelating agents, considering their combined phase.