Removal of cyanide from wastewaters using hydrogen peroxide, activated carbon adsorption and ultrasonic waves

SUMMARY Removal of Cyanide from Waste Waters Using Hydrogen Peroxide, Activated Carbon Adsorption and Ultrasonic Waves Cyanide is currently used in the recovery of gold and silver and more extensively in electro plating, chemical and aluminium industries. Because of the slow degradation kinetics and dependency of natural degradation on seasonal changes the use of chemical destruction processes becomes increasingly important. Development of novel processes and/or refining the existing ones to improve their efficiency are also of prime importance for fullfilling the stringent environmental regulations. In this study the removal of cyanide from synthetic waste waters using hydrogen peroxide, activated carbon adsorption and ultrasonic waves was investigated. The decomposition kinetics of cyanide by hydrogen peroxide were found to be consistent with first order reaction model. With increasing the initial cyanide, hydrogen peroxide and copper sulphate concentration, the decomposition rate of cyanide was observed to increase. Optimum conditions for effective and rapid decomposition of cyanide (i.e. <1 mg/lt) by hydrogen peroxide were determined to be [H202]o/[CN"]o>8 and [H202]o/[CuSC>4]o>20. Increasing the temperature in the range of 13-50°C was found to improve the destruction rate of cyanide with the activation energy of 44,14 kJ/mol. High alkaline conditions adversely affected the decomposition of cyanide while the lime (Ca2+ ions) produced no discernable effect on the cyanide removal. Compared with free cyanide, weak acid dissociable cyanide (Zn(CN)2+CN") was effectively removed (<1 mg/lt) at a lower [H202]o/[CN2AA]o ratio of 6. Metal ions present were observed to precipitate presumably as hydroxides following the removal of cyanide at the end of decomposition period. Effects of pH, temperature, solids and metal ions on the decomposition/consumption of hydrogen peroxide were demonstrated. It was shown that activated carbons, rice husks and hazelnut shells could be used for the removal of cyanide from waste waters. The impregnation of activated carbon (AK) with silver (AK-AglO) and copper (AK-Cu) were found to significantly increase the capacity for the removal of cyanide (e.g. 19,7 mg CNVgr for AK c.f. 29,6 mg CNVgr for VIIIAK- AglO). The aeration of the solution was noted to exert a positive effect on the adsorption of cyanide on activated carbon. Kinetics of adsorption process were consistent with the pseudo-second order reaction model while Langmuir adsorption model was used to describe appropriately the adsorption isotherm. In accordance with the data of adsorption isotherm and kinetics, the breakthrough capacity of adsorbents was found to be in an increasing order of AK