Because the economics of soil extraction processes depend on conservation and reuse of costly chelating agents, the ability of various electrolytes to modify EDTA extraction of Pb from a grossly-contaminated soil (PbT=21%) was investigated using batch equilibration experiments. In the absence of added electrolyte, a single 5-hr. extraction with 0.04 M EDTA (corresponding to 1:1 PbT to EDTA ratio) released 65% of PbT over the pH 5 to 9 range. Under these conditions, Na+-, Li+-, and NH4ClO4 salts at 0.5 M increased Pb desorption to nearly 80%, probably from exchange displacement of soilbound Pb2+ and increased solubility of Pb-containing phases at higher ionic strength. Because Cl- and ClO4- salts were equally effective, chlorocomplex formation was insignificant. Under slightly acidic conditions, Ca(ClO4)2 and Mg(ClO4)2 at 0.167 M caused roughly the same elevation in Pb recovery as 0.5M of the monovalent electrolytes. However, with progressively higher pH, Ca, and to a lesser extent Mg, suppressed Pb solubilization by competitive chelation of EDTA. Pb recovery by EDTA soil washing could be enhanced by addition of Ca salts at pH 4 to 6. Subsequent pH elevation in the presence of Ca would promote decomposition of Pb-EDTA complexes and separation of Pb as a hydroxide precipitate.
All Science Journal Classification (ASJC) codes
- Environmental Engineering
- Environmental Chemistry
- Ecological Modeling
- Water Science and Technology