Natural Attenuation Potential of Cyanide in Groundwater Near a SPL Landfill

Spent pot lining (SPL), a waste product containing cyanide, has been deposited in a landfill at an aluminum refining site in British Columbia (Canada). Since the capping of the landfill, a continuous decline of the cyanide level in the groundwater was observed for more than a decade, suggesting that natural attenuation might be taking place. The information gathered from a critical review of the existing literature along with available groundwater characterization data identified adsorption and biodegradation as two possible attenuation phenomena. Collection and analysis of additional groundwater samples showed that the majority of the cyanide in the groundwater was under the form of iron-cyanide complexes. Using the groundwater characterization data, geochemical modeling using Visual MINTEQ was performed in order to predict the concentration of dissolved cyanide species. The results were close to the field data and they confirmed the occurrence of significant concentrations of Fe(CN)6 and Na Fe(CN)6 along the contamination plume. exhibit little adsorption onto iron oxides (Theis & West 1986) but perhaps have greater adsorption on aluminum oxides and kaolin clay (Alesii & Fuller 1976), which can be important adsorbents in groundwater systems. Ghosh et al. (1999a) investigated fate and transport of cyanide at a manufactured-gas plant (MGP) site and their results showed that cyanide in groundwater was primarily in the form of iron-cyanide complexes which are transported as non-reactive solutes in the sand-gravel aquifer material. Also, chemical decomposition of iron cyanide complexes can occur, but the decomposition kinetics are extremely slow. Free cyanide has been shown to react with various forms of sulphur in the environment to form thiocyanate. The two forms of sulphur most likely to react with cyanide are polysulphides (S2) and thiosulphate (S2O3) (Smith & Mudder 1991). They react according to the following equations: Sx + CN → S(x-1) + SCN (1) S2O3 + CN → SO3 + SCN (2) In neutral to basic solutions, both polysulphides and thiosulphate are oxidation products of sulphides. As such, these products could possibly be present in oxidizing environments, such as the vadose zone in soils. The concentrations of polysulphides and thiosulphate in groundwater are strongly dependent on the sulphur content and the Eh-pH conditions in that groundwater. 1.3 Natural attenuation Natural attenuation phenomena such as volatilization, chelation and precipitation, chemical decomposition, adsorption, photolytic reactions, and biodegradation with indigenous microorganisms can occur in various environments. In groundwater, volatilization and photolytic reactions are less likely to occur and chemical decomposition has very slow kinetics. 1.3.1 Adsorption A key point in the case of cyanide is that most aqueous species are anionic (except HCN), and will therefore only be sorbed on soils with high anion exchange capacity. The surfaces of most soil particles have low anion exchange capacity. Retardation of cyanide release due to sorption processes is generally of minor importance in most soils (Kjeldsen 1999). However, some surfaces such as Fe, Al and Mn oxides, clay minerals and organic matter may provide anion exchange sites (McLean & Bledsoe 1992). 1.3.2 Precipitation Most of the cyanide at aluminum plant sites is originally in solid form as spent pot lining, which contains primarily solid cyanide complexes and thiocyanate compounds. Thiocyanates are quite soluble in water and can produce relatively high thiocyanate concentrations in pores and groundwater. Iron cyanide complexes are generally slightly soluble. The potassium and sodium ferrocyanides are readily soluble, but can reprecipitate in the presence of iron (III). The solubility of iron cyanide in soil is dependent on pH. Thus, re-precipitation of cyanide can occur if conditions change. The potential for the cyanide precipitation process to occur can be evaluated with the use of computer-based chemical speciation models. In this study, modeling of thermodynamic equilibrium conditions in groundwater samples collected from four wells will be performed using Visual MINTEQ software . Ghosh et al. (1999b) showed that precipitation of iron cyanide complexes using a reactive barrier has the potential to be used for in situ treatment of cyanide-contaminated groundwater. 1.3.3 Microbial degradation There are many examples of cyanide microbial degradation under both aerobic and anaerobic conditions (Wang et al. 1996; Barclay et al. 1997). Simple cyanides, in particular, are relatively easily degraded, especially under aerobic conditions. Degradation of iron cyanides under aerobic conditions also occurs, but at a slower rate than that of simple cyanide degradation. To our knowledge, bacterial degradation of iron cyanides in groundwater under anaerobic conditions has not yet been investigated. Aerobic degradation of CNis mediated by a number of bacteria, fungi, algae, and yeast (Young & Jordan 1995). The first step involves oxidation of cyanide to cyanate: CN + 1⁄2 O2 (aq) → OCN (3) This is followed by hydrolysis of the cyanate, which requires a pH below 7: OCN + 3 H2O → NH4 + HCO3 + OH (4) Aerobic degradation of CNSAD is mediated by a number of organisms, generally Pseudomonas bacteria (Young & Jordan 1995). The relevant reaction is: M(CN)x + 3xH2O + x/2 O2(aq) → M + x NH4 + x HCO3 + xOH (5) Thiocyanate is also subject to bacterially-mediated aerobic degradation: SCN + 3H2O + 2 O2(aq) → SO4 + NH4 + HCO3 + H (6) Anaerobic biodegradation of cyanide and hydrogen cyanide is restricted to the moderately to strongly reduced portions of the environment and can only occur if HS or H2S(aq) are present. The sulphur species present will depend on pH. At a pH value greater than 7, HS is the dominant species. At a lower pH, H2S(aq) will be present. These equations illustrate the anaerobic biodegradation of cyanide: CN + H2S(aq) → HCNS + H (7) HCN + HS → HCNS + H (8) The HCNS will be hydrolyze to form NH3, H2S and CO2. In comparison with the aerobic biodegradation of cyanide, anaerobic degradation is much slower and anaerobic bacteria have a cyanide toxicity threshold of only 2 mg/L compared to 200 mg/L for aerobic bacteria (Smith & Mudder 1991). Consequently anaerobic biodegradation would be a less effective cyanide removal mechanism. Presumably, natural attenuation may play an important role in cyanide reduction in groundwater. According to Kjeldsen (1999), studies on natural attenuation of cyanide compounds in groundwater at relevant contaminated sites must be carried out. Few investigations have been conducted in a realistic soil/groundwater environment. The presence of another substrate, or nutrients, as well as the accessibility of cyanide for the bacteria, can vary greatly in a natural soil/groundwater environment. Hence, the interest in conducting experiments under field conditions using indigenous material is evident. 2 CHARACTERIZATION OF WATER SAMPLES