TREATMENT OF METAL PLATING WASTEWATER
WITH A DISPOSABLE ION EXCHANGE MATERIAL
Vasiliki Keramida, Environmental Engineer
Water Pollution Control Division
Indiana State Board of Health
Indianapolis, Indiana 46206
James E. Etzel, Professor
School of Civil Engineering
Purdue University
West Lafayette, Indiana 47907
Treatment of metal plating wastewaters or other waste streams containing heavy metal
ions by means of ion exchange has long been practiced. The major drawback to the use of
ion exchange, however, is that it is really not a treatment system. Rather, it is a means of
concentrating the ions, because of the need to regenerate the ion exchange medium; thus,
the problem of ultimate disposal of the heavy metals remains.
It was the purpose of this study to develop a system that would use a disposable ion
exchange medium which could be low enough in cost yet high enough in capacity to be
economical and efficient. A native material called vermiculite (a clay-mineral) was found
and a process scheme evolved which was able to meet the criteria for effectiveness and
economy [1-3]. This paper details the results of column studies conducted to identify the
exchange behavior of vermiculite for copper, zinc and nickel removal from acid bath plating
wastewaters.
BACKGROUND
Clay minerals such as vermiculite have the property of being able to capture certain ions
and retain them in an exchangeable state. Vermiculite is a layer silicate and its structure
consists of two tetrahedral sheets composed of aluminosilicates and a central trioctahedral
layer (4-6). These groups of mineral sheets are separated by layers of water molecules
having magnesium and calcium ions associated therewith (Figure 1). These magnesium and
calcium ions are largely exchangeable and their function is to satisfy charge deficiencies
in the mineral layers.
Vermiculite shows large expansion (exfoliation) on heating. Thermal exfoliation at high
temperatures (over 550 C) ruptures the chemical bonds which hold the platelets together
and also partially decreases the available surface area of the silicate layers. In addition, some
of the mobile exchangeable cations become "fixed" to the surface |7). Exfoliated vermiculite is widely used commercially as insulating material as well as a lightweight aggregate for
plaster and concrete, and as packaging medium; it is also extensively used in home gardening
as a soil conditioner.
The largest domestic deposits of vermiculite are located at Libby, Montana, and Enoree,
South Carolina. Both mines are operated by W. R. Grace & Company which sells vermiculite
under the name Zonolite.
The term cation exchange capacity (CEC) will indicate the degree to which cations are
exchanged and held by the vermiculite. CEC is normally defined as the sum of the exchangeable cations in a material [8]. CEC is usually expressed in terms of milliequivalents of
exchange capacity per 100 grams of material (meq/100 g) and is determined at a pH of 7
(4]. There are many methods available for determining the CEC of clay minerals, although
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