A kinetic-mechanistic study of cyanide degradation which can be contained in mining tailings dams using a divided electrolytic cell

Abstract

Cyanidation is considered the most employed process to recover precious metals such as silver and gold. However, cyanide containing wastes can be hazardous for the environment. Therefore, the soil electroremediation technique was systematically studied to eliminate/degrade the cyanide contained in mining tailing dams using an electrolytic cell with cationic separator and cheap-stable carbon electrodes. The extent and kinetics of cyanide degradation was firstly analyzed in synthetic solutions using the electrolytic cell at different current density, ionic conductivity, and cyanide concentration. The electrolytic system was also evaluated for the treatment of a sample obtained from a mining tailing dam containing cyanide. The results revealed that 100 % of cyanide can be degraded from synthetic solutions containing 200 ppm CN– at 60 A/m2, 0.25 M NaOH and room temperature. It was also found that 100 % of cyanide can be degraded from the real samples obtained from a Mexican mining tailing dam. The kinetic analysis revealed that cyanide degradation can occur through two pathways: a) in the case of tests containing 500 and 800 ppm NaCN, the rate determining step of the cyanide degradation to cyanate species follows a first order reaction with respect to the cyanide ions, and b) when the tests are carried out with 200 ppm NaCN, the same first order reaction controls the process kinetics from 0 to 30 min, while at t > 60 min, the rate determining step for cyanide degradation is modified and limited by the occurrence of parasite reactions. The proposed mechanism for cyanide degradation is also consistent with the thermodynamic calculations. The results also revealed that complete cyanide electro-degradation can be achieved with a current efficiency of 100 %.