Fate of Cyanide and Related Compounds
in Industrial Waste Treatment
S.F. RAEF, Graduate Student
W.G. CHARACKLIS, Assistant Professor
M.A. KESSICK, Assistant Professor
C.H. WARD, Professor and Chairman
Department of Environmental Science and Engineering
Rice University
Houston, Texas 77001
INTRODUCTION
Cyanide removal by biological oxidation has been the subject of considerable research
over the past forty years. Apparent removal has been demonstrated in both trickling filters
(1) and activated sludge units (2) although cyanide is toxic to biological systems (3). Reports
indicate that cyanide concentrations of about 2 mg/1 (as HCN) are toxic to unacclimated
bacterial cultures, but much higher cyanide concentrations can be tolerated and actually
removed from solution by acclimated cultures. Most researchers report that ammonia
production accompanies the apparent cyanide removal. In most instances, a supplemental
carbon source such as glucose, raw sewage, or soluble fish food was fed to the biological
oxidation units along with cyanide.
Recently, biochemists working with pure cultures of Bacillus megaterium have
demonstrated a pathway for bacterial metabolism of cyanide via an intermediate, /3-
cyanoalanine (4). Initial work indicates that cyanide probably reacts with an amino acid
such as serine to form /3-cyanoalanine and is eventually incorporated into asparagine or
other protein precursor. It is not clear whether cyanide enters the cell as HCN or as a
reaction product of some extracellular reaction.
Researchers investigating the biological oxidation of cyanide face a difficult task since
cyanide is highly volatile at neutral pH and highly reactive at alkaline pH. Failure to
account for stripping of cyanide from solution and for possible reactions of cyanide with
substrate could cause erroneous results. In certain instances, researchers have added caustic
to feed solutions containing both carbohydrates and cyanide in order to prevent
volatilization of the cyanide.
The objective of this paper is to determine the fate of cyanide in aerated biological
waste treatment. Four interrelated possibilities exist for cyanide removal in an aerated
biological system: 1) chemical reaction in solution with the substrate; 2) adsorption onto
biological floe; 3) stripping; and 4) biological metabolism. Each of these possibilities was
investigated to determine its relative importance with regard to overall cyanide removal.
CHEMICAL REACTION WITH SUBSTRATE
Hydrogen cyanide forms in water whenever soluble NaCN or KCN salts are added,
and dissociates as follows:
HCN ^ H+ + CN"
The ratio of HCN to cyanide ion is a function of pH. The equilibrium pK for cyanide at 25 C
is 9.21 (5); thus, below pH 8, cyanide is largely undissociated.
Although cyanide can hydrolyze or polymerize in aqueous solution, it has been shown
that for cyanide concentrations below 650 mg/1 and temperatures below 100 C, these
reactions are insignificant (6). For instance, at 30 C and pH 10, the half-life for hydrolysis of
832