Cyanide and cyanide complexes in the goldmine polluted land in the East and Central Rand Goldfields, South Africa

摘要

The use of cyanide in gold extraction is of concern when it is not properlyudmanaged from the extraction process to the management of wastes. Theuddistribution and fate of cyanide in the environment upon release from theudtailings dumps depends on its physical-chemical speciation.udThis study presents results of distribution, speciation and fate of cyanide inudselected compartments, namely: tailings, sediments and water systems in goldudmine polluted land.udSampling of tailings in a facility that is being rehabilitated was done in 2006udand 2007 to assess the impact of AMD on cyanide release over that period.udDeposition of materials in the tailings dams stopped in 2004. The resultsudrevealed that the pH of the tailings decreased between 2006 and 2007. Elevatedudconcentrations of CNfree, SCN- and CNO- were observed for 2007 compared toud2006. Most cyanide species had degraded as a result, primarily, of decrease in pH due to generation of AMD, also the oxidation of CNfree and the reactionudwith active sulphur species such as S2O3.udThe decrease of cyanide total (CNT) with time is a consequence of naturaludattenuation of cyanides in tailings which may be attributed to physicalchemicaludand microbiological mechanisms.udCyanide and its metal complexes were found to be unstable followingudgeneration of AMD in the dump over a period of one year. The dissociation ofudmetal-cyanide complexes when the pH drops, releases CNfree which is eitherudvolatilised as HCN(g) or transported in solution with the contamination plumeudor converted to SCN- ,CNO- and NH4ud+.udHowever, in most of cases high concentrations of metal-cyanide complexesudwere found even at low pH values of the tailings suggesting that theseudcomplexes are very stable. This was substantiated by the geochemicaludmodelling which predicted the predominance of iron-cyanide complexes inudtailings at low pH. iiiudCyanide released from cyanide complexes flows into the central pond of theudtailings facility and partly leaches into the groundwater.udSalt crusts were observed along the capillary fringe of the central pond as welludas around other water bodies considered in the study. These crusts were foundudto contain elevated concentrations of heavy metals (e.g. 12940 mg kg-1 Fe andud186.1 mg kg-1 Co) and cyanide (e.g. 118.4 mg kg-1 CNT, 14.36 mg kg-1 CNWADudand 100.2 mg kg-1 CNSAD). This obviously has implications of secondaryudpollution as these crusts tend to be very soluble in water thus leading to theudrelease of heavy metals and cyanide into water systems during rainfall.udCharacterization of cyanide was also done in drainage water from an activeudslimes dam where deposition from a reprocessing plant takes place. The slimesuddam had drainage pipes and a solution trench around it that drained awayudexcess water. Low concentration of CNT was obtained in pipe water from theudpipe with low pH values (2 - 4) whilst this concentration was high in waterudfrom the trench with high pH values (5 -7). Copper and iron complexes wereudthe most abundant. High concentrations of SCN- and CNO- were obtained asudresult of conversion of CNfree as explained previously.udSalt crusts collected around the dam presented low pH (3) and highudconductivity, the evidence of high metals content. High concentrations (198.4udmg kg-1) of CNT were obtained in the crusts with predominance of CNSAD (Feudand Co). The bluish-green colour of the crusts and the elevated concentrationsudof CNSAD as well as those for iron could suggest the presence of Prussian blue.udAnalysis of the wetland sediments showed the transport of cyanide from theudtailings dumps to the wetland through the streams.udAn enrichment of cyanide was observed in the sediment with the enrichmentudfactor of 3 for CNT with predominance of strong complexes (Fe and Co). Theudsediment is rich in organic matter and cyanide is known to bind strongly withudorganic matter. Although other possible sources (e.g. bacterial or microbialudsources) could have contributed to the enrichment of cyanide in sediment, thisudwas not investigated. Cyanide can be transported from the tailings dams to natural streams and otherudsurface water bodies through groundwater. A natural stream within audreprocessing area was considered as a water system and cyanide in it wasudcharacterised. Three clusters were observed: water collected upstream withudhigh pH, water from downstream with low pH (4) and the groundwater withudlow pH (3). Low concentrations of CNfree were obtained downstream. Thisudcould be due by the lost of CNfree by volatilization due acidic pH conditions.udCNT was found to be lower downstream than upstream with the predominanceudof CNWAD. CNT concentrations were high at the seepage point, where theudgroundwater discharges to the surface. These concentrations were similar toudthose obtained in the groundwater.udCopper and iron complexes were dominant in the surface and groundwater andudthis was substantiated by modelling results as well.udSCN- was not detected in surface water as it is highly soluble in water and thenudleaches in the groundwater. The concentrations of CNO- were the same up and downstream.udThe results obtained from the study revealed that concentration of CNfree inudmost water bodies exceeded stipulated limits by bodies such as WHO, USEPAudand UE. For instance, concentrations of up to 0.304 mg l-1 of CNfree wereudobtained in some instances to compare with limits of 0.07 mg l-1 by WHO, 0.02udmg l-1 by DWAF/South Africa.udAdditional studies should be done to find out the impact of organic matter (e.g.udhumic and fulvic acids) on the fate of cyanide. Various natural attenuationudmechanisms of cyanide in tailings dams should be investigated. An assessmentudof the phytoremediation program vis-à-vis cyanide cyclisation is recommendedudand a monitoring of groundwater (borehole water) quality is required.