EFFECT OF CHELATING AGENTS ON THE REMOVAL OF HEAVY METALS BY SULFIDE PRECIPITATION.

The precipitation of metal sulfides has been studied under both batch and MSMPR precipitation conditions for various metals, pH conditions, and EDTA concentrations. EDTA forms very strong metal chelates which interfere with the precipitation of zinc sulfide. Weak chelating agents (citrate, tartrate, gluconic acid, etc. ) form weak metal chelates; formation of the metal sulfide precipitate predominates. Removal of copper is nearly complete even in the presence of EDTA. The metal sulfide reactions are very rapid; chemical equilibria was reached within 5 minutes reaction time. The zinc system indicates a phase transformation from an initial amorphous kinetically favored precipitate to a more crystalline thermodynamically favored precipitate (as the particles age) is likely; the K//s//p was corrected 2 log cycles to obtain agreement between the calculated and observed residual Zn concentrations. Microscope observations confirm this hypothesis.