Characterization and speciation modelling of cyanide in effluent from an active slimes dam

Abstract

The utilization of cyanide in the process of gold extraction is an environmental concern as this pollutant is discharged with the tailings. The distribution and fate of cyanide in the environment upon release from the tailings dumps depends on its physical-chemical speciation. The present study was conducted to assess the presence of cyanide species in drainage water from an active slimes dam that receives effluent from a gold reprocessing plant in the Central Rand goldfield (south of Johannesburg, South Africa). Water samples were collected from decant pipes draining water from the top of the slimes dam as well as from a solution trench constructed around the dam. Efflorescent salt crusts and algae were also sampled along the solution trench to assess the extent of cyanide contamination and its promulgation from the slimes. Water samples presented varying chemistry with samples collected from the pipes recording low pH (between 2 and 4) with concentrations of weak acid dissociable cyanide (CNWAD) varying between 5.635 mg L-1 and 8.525 mg L-1. Water samples from the trench were less acidic (pH ranged between 5 and 7) with an average concentration of CNWAD of 21.72 mg L-1. These values are far greater than the limit of 0.50 mg L-1 set by the authorities through the 'Best Practice guidelines' for any effluent exiting a metallurgical treatment facility. Copper and iron cyanide complexes were the most abundant cyanide complexes in the water samples. The pH and conductivity of the solution prepared by the dissolution of the salt crusts (10 g in 50 mL of deionized water) were 3.44 and 1.611 mScm-1, respectively. High concentrations (198.4mg kg-1) of CNT were obtained in the crusts and these were predominantly strong acid dissociable cyanides (CNSAD) of Fe and Co. The presence of iron cyanides was evident from the bluish-green crusts (Prussian blue) observed around the site, indicating the extent of cyanide contamination. A very low pH (2.39) was recorded for the algae, with elevated concentrations of SCN- and OCN- that are byproducts of chemical conversion of cyanide. The Visual MINTEQ geochemical modelling code was used to complement the analytical methods in characterizing the speciation of cyanide. The simulations predicted the presence of the following metal-cyanide complexes in water samples as well as in the solid materials: Fe(CN)63-, Fe(CN)6(aq), NaFe(CN)62-, KFe(CN)62-, Ni(CN)42-, Zn(CN)42-, CaFe(CN)6-, NiH(CN)4- and NiH3(CN)4+. This study revealed that cyanide remains persistent in its immediate environment following its release from slimes dams, an issue of concern as some residential areas have developed in the proximity of such facilities. The major highlight of this work has been the comprehensive characterization of cyanide speciation by using geochemical modelling to complement analytical techniques. This is important in understanding the potential risk posed by this pollutant.