Section 6. METAL WASTES
CASE HISTORIES: REVIEWING THE USE OF SODIUM BOROHYDRIDE
FOR CONTROL OF HEAVY METAL DISCHARGE
IN INDUSTRIAL WASTEWATERS
Michael M. Cook, Group Leader
Joseph A. Lander, Sr. Technical Service Representative
Douglas S. Littlehale, Marketing Manager
Ventron Corporation
Division of Thiokol
Beverly, Massachusetts 01915
INTRODUCTION
The discharge of heavy metals is obviously a concern of both the public and the
industrial communities. As more information is developed regarding the effects of heavy
metals in our environment, more stringent metal discharge limits and more adequate
disposal methods will be required. The use of sodium borohydride provides an effective
and economical method for removal of a number of pollutants as well as offering the
potential for recycling a substantial portion of the presently discharged material. Use of
sodium borohydride treatment typically yields discharges having less than 0.1 mg/1 of the
borohydride reducible metal cations.
This presentation will review the industrial use of sodium borohydride for control of
heavy metal discharge, including the chemical aspects and overall advantages of this
technology. Case histories of three commercially proven borohydride systems will be
reviewed: the removal of mercury from the process effluent at the IMC Chemical Group,
Inc. chlor-alkali manufacturing plant in Orrington, Maine; the removal of dissolved organic
and inorganic lead species from wastewater at the E. I. duPont de Nemours and Company
Inc. tetraalkyllead manufacturing plant in Deepwater, New Jersey; and the removal and
recovery of soluble silver and cadmium complexes from the plant discharge at the Powers
Chemco, Inc. lithographic film production facility in Glen Cove, New York. Engineering
details of these borohydride treatment systems will be discussed.
CHEMICAL ASPECTS OF BOROHYDRIDE REDUCTION
In aqueous solution, sodium borohydride is a very effective reducing agent for conversion of soluble metal cations, weakly associated complexes, and many organometallic
compounds to the insoluble elemental metal   [ 1,2,3,4]:
NaBH4 + 80H"  -»■ NaB02 + 6H20 + 8e"
8M+ + 8e" -» 8M°
NaBH4 + 8M+ + 80H'  ■+ NaB02 + 8M° + 6H20
Since one mole of sodium borohydride can reduce eight moles of metal ion, low-level
borohydride usage results in substantial reduction of the dissolved metal. Table I illustrates
weight ratios of reduced metal per weight of sodium borohydride that can be theoretically obtained. For example, depending upon the oxidation state of soluble mercury, the
weight ratios are 21:1 for mercuric ions or 42:1 for mercurous ions.
In actual practice, the borohydride:metal ratio is substantially higher since other
reducible compounds may react with borohydride. Strongly complexed metals such as
cyanide complexes are not generally reducible. In these cases, it is necessary to first
destroy the complex (usually via hypochlorite or chlorine oxidation) prior to borohydride
treatment.
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