29    INHIBITION OF NICKEL PRECIPITATION BY
ORGANIC LIGANDS
Hsien-Lun Hu, Ph.D. Candidate
Nikolaos P. Nikolaidis, Associate Professor
Domenic Grasso, Associate Professor
Environmental Engineering Program
Environmental Research Institute
Department of Civil and Environmental Engineering
and
Department of Chemical Engineering
The University of Connecticut
Storrs, Connecticut 06269-2037
INTRODUCTION
Nickel is the most commonly used metal in the electroplating industry. The electroplating industry discharges more than 68% of the total nickel discharged from all industries in the United
States.1 In 1988 alone, 9,700 tonnes of nickel were released into the environment without recovery.2 The Environmental Protection Agency (EPA) has identified nickel as a high risk industrial
chemical because it poses significant risk to human health and the environment.3-4
Wastewaters from electroplating are very complex due to the composition of the plating baths.
A nickel plating bath typically consists of a nickel source (nickel chloride or nickel sulfate),
complexing agents to solubilize nickel ions controlling their concentration in the solution,
buffering agents to maintain pH, brighteners to improve brightness of the plated metal, stabilizers (inhibitors) to prevent undesired reactions, accelerators to enhance speed of reactions, wetting agents to reduce surface tension at the metal surface, and reducing agents (only for electro-
less nickel plating) to supply electrons for reduction of the nickel.56 Alkaline precipitation is the
most common method of recovering nickel from wastewaters. However, organic constituents
found in the wastewaters can mask or completely inhibit the precipitation of nickel. Strong lig-
ands like cyanide, EDTA, and NTA or less strong ligands like triethanolamine (TEA) and di-
ethonalamine (DEA) form strong nickel complexes and hinder or even prohibit the formation of
nickel precipitates.7"9 These complexes inhibit crystal growth by forming fine particles that do
not coagulate. Some organic ligands (hydroxyl carboxylate acids) such as tartrate and gluconate
form less strong nickel complexes in the lower pH range (acidic to neutral solution) and stronger
complexes in the higher pH ranges (highly alkaline solution).1011 This implies that the traditional alkaline precipitation method of treating nickel may not be feasible when these hydroxyl
carboxylate acids are present in the wastewaters. Unfortunately, their effect on nickel precipitation has yet to be studied in detail. In addition, nickel precipitation is a kinetically controlled
process that does not reach equilibrium within the typical detention time of the processing units
(minutes to hours). Longer reaction times should be used when studying such precipitation phenomena. Although various innovative technologies for nickel recovery other than alkaline precipitation exist (such as evaporation, electrolysis, reverse osmosis, ion exchange, and electrodial-
ysis), the recycling rate is still low due to the high cost of utilizing these technologies.12 This low
recycling rate not only represents a significant waste of resources, but also implies the possibility
for significant degradation of water quality.
The objective of this study was to conduct an equilibrium study to explore the inhibition behavior of various organic ligands on nickel precipitation. This will lay the groundwork for development of technologies efficacious in the treatment of complexed nickel. The organic ligands
used in this study are EDTA, triethanolamine (TEA), gluconate, and tartrate.
50th Purdue Industrial Waste Conference Proceedings, 1995. Ann Arbor Press, Inc.. Chelsea. Michigan 48118.
Printed in U.S.A.
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