Microbial destruction of cyanide wastes in gold mining: process review

Equilibrium and kinetic studies on free cyanide adsorption from aqueous solution by activated carbon

Adsorption equilibrium and kinetics of free cyanide onto activated carbon were investigated in the batch tests, and the effects of contact time (1-72 h) and initial cyanide concentrations in the range of 102-532 mg/L were studied. Linear regression was used to determine the best fit of equilibrium and kinetics expressions. The two-parameter models including Freundlich, Dubinin-Radushkevich, Temkin and four different linearized forms of Langmuir and three-parameter models including Redlich-Peterson and Koble-Corrigan were employed for fitting the equilibrium data and it was found that, three-parameter models fitted the data better than the two-parameter models and among the three-parameter models the equilibrium data are best represented by Koble-Corrigan model. A number of kinetic models including fractional power, zero order, first order, pseudo-first order, Elovich, second order, intraparticle diffusion and four different linearized forms of pseudo-second order models were tested to fit the kinetic data. The latter was found to be consistent with the data. Intraparticle diffusion plots show that the adsorption process of free cyanide is a two steps process. In the first step, the adsorption of cyanide is fast while in the second step, cyanide adsorption slows down.

Biodegradation of cyanide containing effluents by Scenedesmus obliquus

Biological degradation of cyanide has been shown a viable and robust process for degrading cyanide in mining process wastewaters. Several algal cultures can effectively degrade cyanide as carbon and/or nitrogen source for their growth. In this study, cyanide effluent degradation by Scenedesmus obliquus was examined. Gold mill effluents containing WAD cyanide concentration of 77.9mg/L was fed to batch unit to examine the ability of S. obliquus for degrading cyanide. Cyanide was reduced down to 6mg/L in 77h. Microbial growth and metal uptake of Zn, Fe and Cu was examined during cyanide degradation. The cells well adapted to high pH and the effluent contained cyanide and the metals. It is important that Zn level reduced down 50%, of the starting concentration. pH was kept at 10.3 to prevent loss of cyanide as HCN, due its volatile nature. The bio treatment process was considered to be successful in degrading cyanide in the mine process water.

Application of ferrate(VI) in the treatment of industrial wastes containing metal-complexed cyanides : a green treatment

Ferrate(VI) was employed for the oxidation of cyanide (CN) and simultaneous removal of copper or nickel in the mixed/complexed systems of CN-Cu, CN-Ni, or CN-Cu-Ni. The degradation of CN (1.00 mmol/L) and removal of Cu (0.095 mmol/L) were investigated as a function of Fe(VI) doses from 0.3-2.00 mmol/L at pH 10.0. It was found that Fe(VI) could readily oxidize CN and the reduction of Fe(VI) into Fe(III) might serve efficiently for the removal of free copper ions. The increase in Fe(VI) dose apparently favoured the CN oxidation as well as Cu removal. Moreover, the pH dependence study (pH 10.0-13.0) revealed that the oxidation of CN was almost unaffected in the studied pH range (10.0-13.0), however, the maximum removal efficiency of Cu was obtained at pH 13.0. Similarly, treatment was carried out for CN-Ni system having the initial Ni concentration of 0.170 mmol/L and CN concentration of 1.00 mmol with Fe(VI) dose 2.00 mmol at various pH values (10.0-12.0). Results showed a partial oxidation of CN and partial removal of Ni. It can be observed that Fe(VI) can partially degrade the CN-Ni complex in this pH range. Further, Fe(VI) was applied for the treatment of simulated industrial waste/effluent waters treatment containing CN, Cu, and Ni.

Biological removal of cyanide compounds from electroplating wastewater (EPWW) by sequencing batch reactor (SBR) system

Biological treatment system especially, sequencing batch reactor (SBR) system could not be applied to treat the raw electroplating wastewater (EPWW) due to the low organic matter concentration of 10+/-3 mg-BOD5/L and toxic of high cyanide concentration of 23.0+/-2.2 mg-CN/L. However, EPWW could be used as the nitrogen source for the bio-sludge of SBR system. And 10% of EPWW (the final cyanide concentration of 2.3+/-0.2 mg/L) was most suitable to supplement into the wastewater as the nitrogen source. SBR system showed the highest COD, BOD5, TKN and cyanide removal efficiencies of 79+/-2%, 85+/-3%, 49.0+/-2.1% and 97.7+/-0.7%, respectively with 4-times diluted Thai-rice noodle wastewater (TRNWW) containing 10% EPWW and 138 mg/L NH4Cl (BOD5: TN of 100:10) at SRT of 72+/-13 days (under organic and cyanide loadings of 0.40 kg-BOD5/m3 d and 0.0023 kg-CN/m3 d, respectively). However, the effluent ammonia was still high of 22.6+/-0.4 mg-N/L while the effluent nitrate and nitrite was only 9.9+/-0.4 and 1.2+/-0.9 mg-N/L, respectively. And SVI and effluent SS of the system were higher than 95 and 75 mg/L, respectively.

New approach to optimize operational conditions for the biological treatment of a high-strength thiocyanate and ammonium waste: pH as key factor

Biological treatment of coke and steel-processing wastewaters has to satisfy both industrial economic needs and environmental protection regulations. Nevertheless, as some of the pollutants contained in these waters or produced during the treatment are highly toxic, an effective and safe treatment has proved to be difficult to obtain. This paper reports the study of a biological method for the treatment of wastewaters containing free cyanide, thiocyanate and ammonium (NH4). Laboratory-scale activated-sludge reactors were fed with a synthetic solution reproducing a steel-processing industrial wastewater and inoculated with the same industrial bacterial seeding used on-site (Ecosynergie Inc.). The results demonstrated that free cyanide and thiocyanate were efficiently degraded. Nevertheless, thiocyanate degradation and nitrification processes were actually inhibited by the free ammonia form (NH3) in place of the ionized NH4 form (NH4+) currently dosed and often unproperly named "ammonia" [IUPAC, 1997. In: McNaught, A.D., Wilkinson, A. (compilers). Compendium of Chemical Terminology. Royal Society of Chemistry, Cambridge, UK]. Optimum degradation rates were obtained for very narrow ranges of ammonia nitrogen (NH3-N) concentrations. This result can be explained by the role of pH, which mainly controls the NH3/NH4 equilibrium. Pollutants and NH3 concentrations influenced degradation rates of main pollutants. This influence was determined and expressed through elementary equations. Although the Michaelis-Menten equation could have been used to describe thiocyanate degradation, a Haldane-inhibition model was used to satisfactorily describe cyanide degradation. On the other hand, a slightly modified Haldane model was applied to describe both NH4 oxidation using NH3-N as substrate and thiocyanate degradation using NH3-N as inhibitor. These findings emphasize the role of pH on degradation rates and allow one to optimize operational conditions in the biological treatment of coke and steel industrial wastewaters.

A critical review of the effects of gold cyanide-bearing tailings solutions on wildlife

Wildlife deaths associated with cyanide-bearing mine waste solutions have plagued the gold mining industries for many years, yet there is little published data showing the relationship between wildlife mortality and cyanide toxicity. A gap of knowledge exists in monitoring, understanding the causal relationships and managing risks to wildlife from cyanide-bearing waste solutions and tailings. There is a need for the gold industry to address this issue and to meet the International Cyanide Management Code (ICMC) guidelines. The perceived extent of the issue varies, with one study finding the issue inadequately monitored and wildlife deaths grossly underestimated. In Nevada, USA during 1990 and 1991, 9512 carcasses were reported of over 100 species, although there was underestimation due to reporting being voluntary. Of these, birds comprised 80-91% of vertebrate carcasses reported annually. At Northparkes, Australia in 1995, it was initially estimated that 100 bird carcasses were present by mine staff following a tailings incident; when a thorough count was conducted, 1583 bird carcasses were recorded. Eventually, 2700 bird deaths were documented over a four-month period. It is identified that avian deaths are usually undetected and significantly underestimated, leading to a perception that a risk does not exist. Few guidelines and information are available to manage the risks of cyanide to wildlife, although detoxification, habitat modification and denying wildlife access have been used effectively. Hazing techniques have proven ineffective. Apparently no literature exists that documents accurate wildlife monitoring protocols on potentially toxic cyanide-bearing mine waste solutions or any understanding on the analysis of any derived dataset. This places the onus on mining operations to document that no risk to wildlife exists. Cyanide-bearing tailings storage facilities are environmental control structures to contain tailings, a standard practice in the mining industry. Cyanide concentrations below 50 mg/L weak-acid-dissociable (WAD) are deemed safe to wildlife but are considered an interim benchmark for discharge into tailings storage facilities (TSFs). Cyanide is a fast acting poison, and its toxicity is related to the types of cyanide complexes that are present. Cyanide in biota binds to iron, copper and sulfur-containing enzymes and proteins required for oxygen transportation to cells. The accurate determination of cyanide concentrations in the field is difficult to achieve due to sampling techniques and analytical error associated with loss and interferences following collection. The main WAD cyanide complexes in gold mine tailings are stable in the TSF environment but can release cyanide ions under varying environmental conditions including ingestion and absorption by wildlife. Therefore distinction between free, WAD and total cyanide forms in tailings water for regulatory purposes is justified. From an environmental perspective, there is a distinction between ore bodies on the basis of their copper content. For example, wildlife deaths are more likely to occur at mines possessing copper-gold ores due to the formation of copper-cyanide complexes which is toxic to birds and bats. The formation of copper-cyanide complex occurs preferentially to gold cyanide complex indicating the relative importance of economic vs. environmental considerations in the tailings water. Management of cyanide to a perceived threshold has inherent risks since cyanide has a steep toxicity response curve; is difficult to accurately measure in the field; and is likely to vary due to variable copper content of ore bodies and ore blending. Consequently, wildlife interaction needs to be limited to further reduce the risks. A gap in knowledge exists to design or manage cyanide-bearing mine waste solutions to render such facilities unattractive to at-risk wildlife species. This gap may be overcome by understanding the wildlife behaviour and habitat usage of cyanide-bearing solutions.

Biodegradation of Cyanide by a White Rot Fungus, Trametes versicolor

The cyanide degradation abilities of three white rot fungi, Trametes versicolor ATCC 200801, Phanerochaete chrysosporium ME 496 and Pleurotus sajor-caju, were examined. T. versicolor was the most effective with 0.35 g dry cell/100 ml degrading 2 mM KCN (130 mg/l) over 42 h, at 30 degrees C, pH 10.5 with stirring at 150 rpm.

The current and future applications of microorganism in the bioremediation of cyanide contamination

Inorganic cyanide and nitrile compounds are distributed widely in the environment, chiefly as a result of anthropogenic activity but also through cyanide synthesis by a range of organisms including higher plants, fungi and bacteria. The major source of cyanide in soil and water is through the discharge of effluents containing a variety of inorganic cyanide and nitriles. Here the fate of cyanide compounds in soil and water is reviewed, identifying those factors that affect their persistence and which determine whether they are amenable to biological degradation. The exploitation of cyanides by a variety of taxa, as a mechanism to avoid predation or to inhibit competitors has led to the evolution in many organisms of enzymes that catalyse degradation of a range of cyanide compounds. Microorganisms expressing pathways involved in cyanide degradation are briefly reviewed and the current applications of bacteria and fungi in the biodegradation of cyanide contamination in the field are discussed. Finally, recent advances that offer an insight into the potential of microbial systems for the bioremediation of cyanide compounds under a range of environmental conditions are identified, and the future potential of these technologies for the treatment of cyanide pollution is discussed.

Cyanide detoxification by recombinant bacterial rhodanese

Cyanide is a major environmental pollutant of the chemical and metallurgical industries. Although extremely toxic, cyanide can enzymatically be converted to the less toxic thiocyanate by rhodaneses (thiosulfate:cyanide sulfurtransferases, EC 2.8.1.1). We engineered a genetic system to express high levels of recombinant Pseudomonas aeruginosa rhodanese (r-RhdA) in Escherichia coli, and used this organism to test the role of r-RhdA in cyanide detoxification. Inducible expression of the rhdA gene under the control of the hybrid T7-lacO promoter yielded active r-RhdA over a 4-h period, though r-RhdA-expressing E. coli showed decreased viability starting from 1 h post-induction. At this time, Western blot analysis and enzymatic assay showed r-RhdA partition between the cytoplasm (95%) and the periplasm (5%). The accessibility of thiosulfate to r-RhdA was a limiting step for the sulfur transfer reaction in the cellular system, but cyanide conversion to thiocyanate could be increased upon permeabilization of the bacterial membrane. Specific r-RhdA activity was higher in the whole-cell assay than in the in vitro assay with pure enzyme (2154 vs. 816 micromol min-1 mg-1 r-RhdA, respectively), likely reflecting enzyme stability. The r-RhdA-dependent cyanide detoxification resulted in increased resistance of r-RhdA overexpressing E. coli to 5 mM cyanide. Bacterial survival was paralleled by release of thiocyanate into the medium. Our results indicate that cyanide detoxification by engineered E. coli cells is feasible under laboratory conditions, and suggest that microbial rhodaneses may contribute to cyanide transformation in natural environments.

Kinetic studies of adsorption of thiocyanate onto ZnCl2 activated carbon from coir pith, an agricultural solid waste

The adsorption of thiocyanate onto ZnCl2 activated carbon developed from coir pith was investigated to assess the possible use of this adsorbent. The influence of various parameters such as agitation time, thiocyanate concentration, adsorbent dose, pH and temperature has been studied. Adsorption followed second-order rate kinetics. Two theoretical adsorption isotherms, namely, Langmuir and Freundlich were used to describe the experimental results. The Langmuir adsorption capacity (Q0) was found to be 16.2 mg g(-1) of the adsorbent. The per cent adsorption was maximum in the pH range 3.0-7.0. pH effect and desorption studies showed that ion exchange and chemisorption mechanism are involved in the adsorption process. Thermodynamic parameters such as DeltaG0, DeltaH0 and DeltaS0 for the adsorption were evaluated. The negative values of DeltaH0 confirm the exothermic nature of adsorption. Effects of foreign ions on the adsorption of thiocyanate have been investigated. Removal of thiocyanate from ground water was also tested.

Heterogeneous catalytic degradation of cyanide using copper-impregnated pumice and hydrogen peroxide

The main objective of this research was to investigate the oxidative destruction of free cyanide with hydrogen peroxide and copper-impregnated pumice as a heterogeneous catalyst. Original or copper-impregnated pumices added alone were not effective adsorbents of negatively charged cyanide ions due to incompatible surface interactions. Peroxide and original pumices added together were also ineffective in removing cyanide. However, for all of the three natural pumices tested with various particle size fractions, the use of copper-impregnated pumices and peroxide together significantly enhanced both the initial rate and extent of cyanide removal. Although copper-impregnated specific surface area was the major factor affecting the rate and extent of cyanide destruction for a particular pumice source with similar surface chemistries, the type of surface chemistry (i.e., specific functional groups) within different pumice sources also appears to be a very important factor. Lower rates and extents of cyanide removals were observed at pH 11 compared to pH 8 probably because of the negative impacts of alkaline conditions in terms of scavenging peroxide and forming more negatively charged pumice surfaces. Both the initial rate and ultimate extent of cyanide removals were generally higher at a temperature of 20 degrees C compared with those found at 10 degrees C. The use of copper-impregnated pumice as a light, cheap, readily available, natural, and porous heterogeneous catalyst either in completely mixed/suspended or fixed-bed reactor configurations may be an effective treatment technology for cyanide removal from solution. This new approach may minimize downstream metal removal problems experienced in conventional cyanide oxidation technologies.

Destruction of cyanide in gold mill effluents: biological versus chemical treatments

In gold mining, cyanide has been the preferred lixiviant worldwide since 1887. Although cyanide can be destroyed and recovered by several processes, it is still widely discussed and examined due to its potential toxicity and environmental impact. Biological treatment of cyanide is a well-established process and has been commercially used at gold mining operations in North America. Biological treatment processes facilitate growth of microorganisms that are essential for the treatment. The present review describes the advances in the use of biological treatment for the destruction of cyanide in gold mill effluents.

Biosorption of americium-241 by immobilized Rhizopus arrihizus

Rhizopus arrihizus (R. arrihizus), a fungus, which in previous experiments had shown encouraging ability to remove 241Am from solutions, was immobilized by calcium alginate and other reagents. The various factors affecting 241Am biosorption by the immobilized R. arrihizus were investigated. The results showed that not only can immobilized R. arrihizus adsorb 241Am as efficiently as free R. arrihizus, but that also can be used repeatedly or continuously. The biosorption equilibrium was achieved within 2 h, and more than 94% of 241Am was removed from 241Am solutions of 1.08 MBq/l by immobilized R. arrihizu in the pH range 1-7. Temperature did not affect the adsorption on immobilized R. arrihizus in the range 15-45 degrees C. After repeated adsorption for 8 times, the immobilized R. arrihizus still adsorbed more than 97% of 241Am. At this time, the total adsorption of 241Am was more than 88.6 KBq/g, and had not yet reached saturation. Ninety-five percent of the adsorbed 241Am was desorbed by saturated EDTA solution and 98% by 2 mol/l HNO(3).