The Removal of Mercury from Industrial
Wastewaters by Metal Reduction
M. DAVID RICKARD, Section Manager
GORDON BROOKMAN, Chemical Engineer
Merck Sharp & Dohme Research Laboratories
Rahway, New Jersey
INTRODUCTION
Recently, a great deal of attention has been focused on the problem of
contamination of the aquatic environment by mercury. This element has been shown
to be subject to intensive bioconcentration in the aquatic foodchain and it has been
detected in relatively high concentrations in the flesh of both fresh and salt water
food fish. These findings have led to the imposition of stringent regulations governing
the amount of mercury which may be discharged in industrial wastes.
Because of the very recent realization of the existence of this problem, little
technology exists on methods of removing mercury from waste waters to the levels
now deemed necessary. Mercury recovery from chemical process liquors via caustic
precipitation has long been practiced. However, while treatment in this manner
permits recovery of nearly all mercury values, the solubility of mercuric oxide is such
that treatment by,this process cannot, in most instances, produce an effluent of the
quality required by the new standards. The precipitation of mercury by the addition
of sulfide is now being practiced by many plants (1). This process, however, while
effective in achieving the desired removal levels is beset with problems involved with
control and disposal or recovery of the sulfide sludges.
The potential magnitude of this problem led us to initiate a program to
evaluate various techniques of mercury removal from chemical process and
chlor-alkali wastes. A number of methods suggested in the literature, including
sulfide precipitation and borohydride treatment, were examined in some detail and
rejected as being either impractical or uneconomical. Work then centered on the
development of a different approach—reduction and complex formation with metals.
If a metal which is above mercury in the electromotive series of the elements is
placed in a solution containing mercuric ion, an oxidation/reduction couple is
formed with mercury being reduced to the elemental state. Further, if the metal
chosen is one which is capable of forming an amalgam with mercury and a
stoichiometric excess is present, the reduced mercury will combine with the metal
added forming a highly stable complex. This complex can then be easily removed by
sedimentation or filtration. Calculation of the equilibrium constants for the
reduction of mercuric ion by most common metals will illustrate that this reaction is
essentially irreversible and should therefore permit the removal of ionic mercury to
extremely low levels. This paper reports on the preliminary investigations into the
variables involved in this process and shows the results obtained from application of
metals reduction to a mercury containing chemical process waste.
713