CATALYTIC OXIDATION OF CYANIDES
Walter J. Weber, Jr., Professor
Ozden Corapcioglu, Research Assistant
Environmental and Water Resources Engineering
The University of Michigan
Ann Arbor, Michigan 48109
Cyanide is a common constitutent of a variety of wastes emanating from metal plating,
steel and petroleum operations. Classified as both hazardous and toxic, cyanide is an EPA-
designated priority pollutant [ 1 ].
Several different methods for treatment of cyanide wastes have been advanced [2-17],
most involving some form of chemical oxidation. It is thermodynamically possible to oxidize free cyanide with molecular oxygen to yield cyanate
2CN" + 02 ->■ 2CNO" (1)
which may then be hydrolyzed
CNO" + 2H30+->- C02 + NH4+ + H20 (2)
This presents an attractive possibility for cyanide treatment because it avoids the use of
reagents which may themselves constitute potential hazards, as do, for example, chlorine
and caustic in the alkaline chlorination process for cyanide destruction.
Unless catalyzed, however, the reaction of molecular oxygen with cyanide (Equation 1)
is impractically slow. Activated carbon has been demonstrated to be an effective catalyst
for this reaction [6,10,14]. Bernardin has further shown that divalent copper ions (Cu )
accelerate and increase the efficiency of the catalysis of cyanide oxidation by carbon [ 16].
Copper and cyanide are frequently coincident in metal plating wastes. In such cases-
indeed, even when copper is absent-a variety of other heavy metals may be present. The
effects of these metals on the function of carbon as a catalyst for oxidation of cyanides
must be given due consideration. Moreover, the potential during the process for removal
of heavy metals, including copper, by adsorption on the carbon is of substantial interest.
Lastly, the process involves heterogeneous catalysis; that is, it requires the adsorption
of cyanide from solution phase prior to its oxidation at the carbon surface. Analyses of mass
transport processes operative in the overall reaction are therefore required to evaluate
reaction rates, permit estimation of the effects of pertinent physical variables, and facilitate
specification and design of appropriate reactor systems.
EXPERIMENTAL
Waste Solutions
The cyanide solutions used throughout this work simulated plating wastes of the following composition:
Component Concentration
Cyanide (Cn) 10 mg/l
Cadmium (Cd) 1 mg/l
500