Sorption of low levels of cyanide by granular activated carbon

Abstract

ABSTRACT: The sorption of cyanide at levels ≤ 1 mg/L in solution by granular activated carbon (GAC) was investigated in order to define the conditions required to reduce residual levels to below the maximum 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 activated carbon. In general, uptake was independent of pH except for a slight increase in the pH range of 8‐9, but increased linearly with increasing adsorbent density until the concentration of cyanide was lowered to approximately 0.3 mg/L; below 0.3 mg/L, the amount sorbed became proportional to a power‐law term of the adsorbent density. The effect of cyanide 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 (Ce)0.13. Addition of up to 10 mg/L nitrilotriacetic acid had little effect on the amount of cyanide adsorbed; however, humic acid, Ca(II), Mg(II), Al(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 sorption of cyanide as HCN molecules through formation of H‐bonds with oxygenated functional groups such as — COH.