SORPTION OF LOW-LEVELS OF CYANIDE BY GRANULAR ACTIVATED CARBON

The sorption of cyanide at levels less-than-or-equal-to 1 mg/L in solu tion by granular activated carbon (GAC) was investigated in order to d efine the conditions required to reduce residual levels to below the m aximum acceptable concentration value specified in the Guidelines for Canadian Drinking Water Quality. The sorption process proceeded slowly with uptake continuing over a 30-hour mixing period. The initial rate of adsorption increased with decreasing particle size of the activate d carbon. In general, uptake was independent of pH except for a slight increase in the pH range of 8-9, but increased linearly with increasi ng adsorbent density until the concentration of cyanide was lowered to approximately 0.3 mg/L; below 0.3 mg/L, the amount sorbed became prop ortional to a power-law term of the adsorbent density. The effect of c yanide concentration on the uptake could be described by a Freundlich isotherm with the amount sorbed per unit weight of granular activated carbon being proportional to (C(e))0.13. Addition of upto 10 mg/L nitr ilotriacetic acid had little effect on the amount of cyanide adsorbed; however, humic acid, Ca(II), Mg(II), AI(III), and Fe(III) reduced the cyanide uptake by 30-50%. Possible mechanisms for cyanide sorption by GAC and their implications for water treatment have been considered. The mechanisms considered include CN uptake by ion exchange, and sorpt ion of cyanide as HCN molecules through formation of H-bonds with oxyg enated functional groups such as -COH.