Appl Microbiol Biotechnol (2002) 58:409415DOI 10.1007/s00253-001-0887-2

O R I G I N A L PA P E R

K. Oudjehani G.J. Zagury L. Deschnes

Natural attenuation potential of cyanide via microbial activity in mine tailings

Received: 23 May 2001 / Revised: 5 October 2001 / Accepted: 19 October 2001 / Published online: 21 December 2001 Springer-Verlag 2001

Abstract Biological removal by indigenous microflora of cyanide, contained in old (69 years) and fresh tail-ings (3 months), was studied in order to assess its natural attenuation potential via biodegradation. To investigate the presence of indigenous microflora in tailings, total heterotrophic and cyanide resistant bacteria were count-ed using the spread-plate method. The free cyanide min-eralization potential was estimated using K14CN in the presence of various unlabeled cyanide concentrations (0, 5, and 10 mg CN/kg). The biodegradation of cyanide contained initially in the samples was also investigated by monitoring formate, formamide, ammonia and total cyanide (CNT) concentrations over 111 days. The enu-meration of total heterotrophic and cyanide-resistant bacteria in old tailings showed an average population of 105 cfu/g. However, no growth was detected in fresh tail-ings. Nevertheless, cyanide mineralization tests indicated the presence, in both old and fresh tailings, of a cyanide-degrading microflora. In old tailings, maximum mineral-ization percentages of free cyanide ranging from 85% to 100% were obtained after 65 days at all concentrations tested. A mineralization percentage of 83% after 170 days was also observed in fresh tailings. No de-crease of total cyanide concentration in old tailings was observed when the biodegradation of endogenous cya-nide was tested whereas a significant decrease was re-corded in fresh tailings after 96 days. The presence of strong metalcyanide complexes resistant to biodegrada-tion could explain the absence of biodegradation in old tailings. This study demonstrated the presence of an in-digenous free cyanide-degrading microflora in both old and fresh tailings, and suggests that natural attenuation of cyanide in gold mine tailings is likely to occur via mi-crobial activity.

K. Oudjehani G.J. Zagury () L. Deschnes Department of Chemical Engineering,cole Polytechnique de Montral, PO Box 6079,Station Centre-Ville, Montreal, Quebec, Canada H3C 3A7 e-mail: gzagury@site.polymtl.caFax: +-1-514-3405913

Introduction

The gold mining industries are among the largest con-sumers of cyanide due to its high affinity with gold. Af-ter the precious metal has been extracted from ore, cya-nides are discarded as effluents or as contaminated solid tailings. Cyanides are highly toxic and their toxicity is related to their physicochemical speciation. The free cya-nide form (HCN, CN) is classified as the most toxic be-cause of its high metabolic inhibition potential. The met-alcyanide complexes [e.g. Fe(CN)63, Fe(CN)64] are relatively less toxic (Shifrin et al. 1996). Many tech-niques for the detoxification of cyanide-contaminated mine effluents exist, but very few can be applied to solid tailings. Natural attenuation is of great interest to mine industries because of the low costs involved when com-pared to chemical oxidation processes (Smith and Mud-der 1991). A few studies concerning the natural attenua-tion of cyanide-contaminated tailings have been conduct-ed (Engelhardt 1985; Smith and Struhsacker 1987; Church and Boyle 1990). These studies all concluded that cyanide concentration decreases over time. The au-thors attributed this decrease to natural attenuation phe-nomena such as volatilization, precipitation, complexa-tion, adsorption, and biodegradation which involves the indigenous microorganisms. However, natural attenua-tion must be validated through experimental work in or-der to prove its efficiency. In the aforementioned studies, natural biodegradation of cyanide by indigenous micro-organisms was always hypothesized but was not directly tested.

Cyanide biodegradation under aerobic conditions has been investigated in several laboratory studies (Kang and Kim 1993; Suh et al. 1994; Dumestre et al. 1997). These studies, most often conducted in aqueous or sludge sys-tems, used either pure or mixed cultures and were gener-ally carried out for waste or wastewater treatment pur-poses. Microorganisms were acclimated, isolated and sometimes encapsulated for protection against cyanide toxicity. In this case, high concentrations of free cyanide in aqueous system (100400 mg/l) were degraded in410

120 h, and about 70% of the initial radiolabeled cyanide (Na14CN) was converted to 14CO2 and ammonium (Babu et al. 1992; Chapatwala et al. 1998). Recently, Silva and Beone (1999) reported cyanide degradation by indige-nous microorganisms contained in soil polluted by an ac-cidental spillage of acetone cyanohydrin. Radiolabeled cyanide (K14CN) was added to the soil, and up to 60% was mineralized to 14CO2 within 3060 days.

The purpose of the current work was to investigate the natural attenuation potential of cyanide in old and fresh mine tailings via microbial degradation. Firstly, the potential of indigenous microorganisms to degrade add-ed free cyanide was studied and, secondly, the biodegra-dation of cyanide initially contained in mine tailings was investigated.

Materials and methods

Tailing sampling

Tailing samples were collected from two gold mining sites, one operational and the other closed, located in northern Quebec. Two old tailing samples AO(0.2) and AO(1), generated 6 years ago, were collected from the closed site (A) at depths of 0.2 m and 1 m, respectively. The second sampling was conducted on the opera-tional site (B) where 9-year-old tailings [BO(0.2), BO(1)] and 3-month-old tailings [BF(0.2), BF(1)] were collected at depths of 0.2 and 1 m. In each location, a composite sample was taken, placed in a polyethylene container and sealed hermetically. Sub-samples were collected and placed in sterilized plastic bags for microbiological analysis. Samples were collected using an auger and stored immediately in the dark at 4C.

Chemicals

All chemicals used were ACS reagent grade: K14CN (684.5 MBq/ mmol) was purchased from Sigma Chemicals (St Louis, Mo.) and KCN (purity 99.3%) was obtained from Mallinckrodt Baker (Par-is, Ky.). Succinic acid was purchased from Sigma Ultra (Oakville, Canada). Sodium azide (NaN3), EDTA, NaOH and KOH were purchased from Anachemia (Lachine, Quebec, Canada).

Physical and chemical characterization

Tailing samples were characterized for pH, water content, volatile solids at 550C, and particle-size distribution (ASTM 1990). The surface tailings (0.2 m) were classified as sand (2 mm75 m), silt (75 m2 m), and clay (