61    TREATMENT OF METAL-EDTA WASTES USING
ELECTROCHEMICAL REDUCTION, CHEMICAL
OXIDATION, AND METAL PRECIPITATION
TECHNIQUES
Jeffrey S. Allen, Assistant Project Manager
Eckenfelder Inc.
Nashville, Tennessee 37228
Suzanne S. Fenton, Professor
James M. Fenton, Professor
Donald W. Sundstrom, Professor
Department of Chemical Engineering
The University of Connecticut
Storrs, Connecticut 06269
INTRODUCTION
Ethylenediaminetetraacetic acid (EDTA) is a common chelating agent and is used in chemical
cleaning solutions for scale removal from steam boilers and heat exchangers. The composition of
the chemical cleaning wastestream generated in the 1985 cleaning of the Northeast Utilities Millstone Unit No. 2 steam generators was used as a model for this study. The two generators
cleaned at Millstone produced approximately 567 pounds of corrosion products as iron, copper,
nickel, and zinc (complexed with EDTA) in 14.000 gallons of waste solvents.1
The treatment of these wastes requires breakdown of the metal-EDTA bonds, followed by separation and removal and/or destruction of metal and EDTA species. A diagram of a proposed
process for the treatment of chemical cleaning wastes is presented in Figure 1. There are three
major treatment steps in this proposed process. The first step is electrochemical reduction of the
copper- and iron-EDTA complexes at low pH to produce copper metal, precipitated EDTA, and a
solution containing iron (II), residual copper, and EDTA (as separate or complexed species).
Electrochemical reduction is followed by chemical oxidation of the residual EDTA by hydrogen
peroxide addition in order to destroy the remaining EDTA. The elimination of EDTA from the
waste permits the remaining free copper, iron, and other metal species (nickel and zinc) to be
precipitated out of solution at high pH in the third treatment step. The treatment system effluent
is then essentially free from the target pollutants of EDTA, copper, and iron. This processing
scheme allows for recovery of a high percentage of pure copper and nearly pure EDTA (via electrochemical reduction) for possible reuse. Since copper and EDTA are removed in the electrochemical reactor, a smaller amount of chemicals will be required for oxidation and precipitation
of the remaining wastes. In addition, destruction of the residual EDTA via hydrogen peroxide
oxidation will result in smaller, less hazardous volumes of final waste for ultimate disposal.
These features give the proposed system an advantage over currently available treatment alternatives, and promote the concept of "pollution prevention."
Several nondestructive and destructive technologies are currently available for the treatment of
EDTA-based chemical cleaning wastes.I_s The nondestructive technologies include direct solidification, precipitation, evaporation, ion exchange, membrane processes, and electrochemical reduction. Disadvantages associated with these technologies include: (1) no destruction of EDTA
(solidification, evaporation, membrane processes); (2) little or no volume reduction (solidification, precipitation, ion exchange, electrochemical reduction); (3) inability to treat EDTA-contain-
ing wastes (precipitation); and (4) inability to handle concentrated solutions (ion exchange). De-
51st Purdue Industrial Waste Conference Proceedings, 1996. Ann Arbor Press. Inc.. Chelsea. Michigan 48118.
Printed in U.S.A.
601